International Journal of

Molecular Sciences Article

Property Characterization and Photocatalytic Activity Evaluation of BiGdO3 Nanoparticles under Visible Light Irradiation Jingfei Luan *, Yue Shen, Lingyan Zhang and Ningbin Guo State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, Nanjing 210093, China; [email protected] (Y.S.); [email protected] (L.Z.); [email protected] (N.G.) * Correspondence: [email protected]; Tel.: +86-135-8520-6718; Fax: +86-25-8968-0397 Academic Editor: Kevin D. Belfield Received: 13 July 2016; Accepted: 23 August 2016; Published: 8 September 2016

Abstract: BiGdO3 nanoparticles were prepared by a solid-state reaction method and applied in photocatalytic degradation of dyes in this study. BiGdO3 was characterized by X-ray powder diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, Brunauer-Emmett-Teller, UV-Vis diffuse reflectance spectroscopy and transmission electron microscopy. The results showed that BiGdO3 crystallized well with the fluorite-type structure, a face-centered cubic crystal system and a space group Fm3m 225. The lattice parameter of BiGdO3 was 5.465 angstrom. The band gap of BiGdO3 was estimated to be 2.25 eV. BiGdO3 showed a strong optical absorption during the visible light region. Moreover, the photocatalytic activity of BiGdO3 was evaluated by photocatalytic degradation of direct dyes in aqueous solution under visible light irradiation. BiGdO3 demonstrated excellent photocatalytic activity in degrading Direct Orange 26 (DO-26) or Direct Red 23 (DR-23) under visible light irradiation. The photocatalytic degradation of DO-26 or DR-23 followed the first-order reaction kinetics, and the first-order rate constant was 0.0046 or 0.0023 min−1 with BiGdO3 as catalyst. The degradation intermediates of DO-26 were observed and the possible photocatalytic degradation pathway of DO-26 under visible light irradiation was provided. The effect of various operational parameters on the photocatalytic activity and the stability of BiGdO3 particles were also discussed in detail. BiGdO3 /(visible light) photocatalysis system was confirmed to be suitable for textile industry wastewater treatment. Keywords: BiGdO3 ; solid state reaction; properties characterization; direct dyes; visible light irradiation; photocatalytic degradation

1. Introduction Photocatalytic degradation has emerged as an efficient method for purification and treatment of polluted water and air in recent years [1–12]. Compared with conventional methods including physical, chemical and biological processes, photocatalysis was considered to be the most efficient and have a market prospect for the degradation of persistent organic pollutants [13–15]. Therefore, research on all kinds of photocatalysts for efficient photocatalysis has become a hot subject. Titanium dioxide (TiO2 ) was widely accepted as one of the most promising photocatalysts owing to its high activity, low cost, non toxicity and chemical stability [16–19]. However, TiO2 could only absorb UV light and it was not responding to the visible light area, thus the efficiency of TiO2 was low for utilization of sunlight (4%) [20–22]. UV light occupies only 5% in the solar spectrum, while the visible light between 400 and 750 nm occupies 41%. If the UV light part and the visible light part of sunlight could be fully utilized at the same time, the light quantum efficiency would be greatly improved.

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One major direction for preparing efficient visible light photocatalysts is the use of non-metallic elements such as N, S, C, etc. to replace the oxygen element of TiO2 . Element doping could reduce the band gap of the materials to extend its responding area to the visible region of the spectrum, and improved the light quantum efficiency to some extent [23–31]. Another direction is the preparation of composite oxides and complex compounds, such as rGO/C-MoO3 , Cox Zn1−x Fe2 O4 -rGO, Fe3 O4 @C/Cu, Fe3 O4 @CuO, WO3 /ZnO, Pt/Au/WO3 , Ti/ZnO-Cr2 O3 , ZnO/CdS/TiO2 , CoO-TiO2 , CuO/SnO2 /TiO2 , BiPO4 /TiO2 /g-C3 N4 , AgInS2 , ZnS-AgInS2 , Ag2 Mo4 O13 , K6 Nb10.8 O30 , Y2 Sn2 O7 , Ca2 Nb-2 O7 , Bi2 GaTaO7 , ZnO-T, and ZnO nano- and microneedles [32–50] which showed photocatalytic activity under visible light irradiation. These metal oxides as photocatalysts have relatively high photocatalytic activity because of their unique arrangement of electronic structure, charge transport characteristics and light absorption properties, which could generate photoexcited charge carriers and show remarkable stability under varying conditions [51]. Bismuth oxide (Bi2 O3 ) is an important p-type semiconductor and has been extensively investigated for various applications owing to its unique optical and electrical properties [52–57]. The band gap of Bi2 O3 (2.58 eV) is narrower than the band gap of TiO2 (3.2 eV), thus Bi2 O3 has been investigated as a visible-light-driven photocatalyst. However, the rapid recombination of the photogenerated electron-hole pairs led to a low activity of Bi2 O3 [58–60]. Moreover, many bismuth-based mixed oxides such as BiVO4 , Bi2 WO6 , Bi2 Ti2 O7 , Bi12 TiO20 , and In2 BiTaO7 [61–69] have been reported to be excellent visible light photocatalysts. The Bi 6s2 lone pairs of electron might play an important role in these compounds [70,71]. Some previous reports have shown that Gd3+ , a rare earth ion, S doped on the photocatalysts could greatly enhance the photocatalytic activity under visible light irradiation. Guo et al. [72] evaluated the photocatalytic activity of Bi1−x Gdx FeO3 (x = 0, 0.05, 0.1, and 0.15) for the photodegradation of rhodamine B and the acquired results showed that the low concentration of Gd doping below x = 0.1 could significantly increase the photocatalytic activity of the photocatalysts compared with the pure BiFeO3 nanopowders. Luo et al. [73] reported that Gd-doped porous Bi2 O3 microspheres, accompanied with different Gd concentrations, 0%, 1%, 2%, 3% and 4%, prepared by a simple hydrothermal synthesis method, could degrade 95.7%, 98.2%, 97.1% and 91.1%, respectively of rhodamine B under visible light irradiation for 120 min, and also could degrade 97.0%, 99.3%, 98.1% and 80.5%, respectively, of methyl orange under visible light irradiation for 28 min, which showed higher photocatalytic activity than the pure β-Bi2 O3 catalyst. The main effect of rare earth ion Gd3+ doping on the photocatalytic activity of the photocatalysts was that Gd3+ could trap photoelectrons as efficient scavengers, which decreased the recombination probability of the electron-hole pairs [73–75]. Previous works indicated that photoexcitation of an electron in an O2p and Bi6s hybrid orbital could lead to a charge transfer occurring to a d orbital of the other metal in the composite oxide [76,77]. Gd had one occupied 4d orbital in the ground state and Gd–Bi composite oxide might be responding to the visible light irradiation. In the present paper, BiGdO3 nanoparticles were synthesized by a solid-state reaction method and tested to be efficient for photocatalytic degradation of dyes. The structural, optical and photocatalytic properties of BiGdO3 were studied in detail. This work concentrated on the photocatalytic properties of BiGdO3 for photodegradation of direct dyes in aqueous solution under visible light irradiation. The effect of various operational parameters on the photocatalytic degradation efficiency was examined in detail. The stability of both material and performance of BiGdO3 as a visible light photocatalyst was also investigated by material characterization and the repeated photocatalytic degradation tests. Finally, the degradation intermediates of DO-26 were observed and the possible photocatalytic degradation pathway of DO-26 was studied.

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2. Results and Discussion 2. Results and Discussion 2.1. Structural and Optical Properties 2.1. Structural and Optical Properties 2.1.1. XRD Analysis 2.1.1. XRD Analysis Figure 1 shows the powder X-ray diffraction pattern of BiGdO3 together with full-profile Figure 1 shows the X-ray data diffraction patternby of the BiGdO with full-profile structure structure refinements of powder the collected as obtained RIETAN™ [78] program, which was 3 together refinements the collected data obtained RIETAN™ [78] which was on the based on the of Rietveld analysis. It as can be seenby in the Figure 1 that all theprogram, diffraction peaks ofbased the sample Rietveld analysis. It can be seen in Figure all thecard diffraction of the samplethat were were sharp shape and were identical to the1 that standard (JSPDS peaks 27-1043). It meant thesharp asshape and were identical to theoxide standard card (JSPDS 27-1043). It meant that thepurity. as-prepared bismuth prepared bismuth gadolinium BiGdO 3 was single phase and was of high According to gadolinium oxide BiGdO phase and highitpurity. According high purity the high purity of the precursors that were used inwas this of study, was unlikely thatto thethe observed spaceof 3 was single the precursors that were used in this of study, it was unlikely the observed groups groups originate from the presence impurities. Figure that 2 shows the XRDspace patterns of originate BiGdO3 from the at presence of impurities. Figure 2 shows patterns BiGdO at different prepared different temperatures. It was clear the thatXRD BiGdO 3 wasof not formed completely when 3 prepared temperatures. It was clear BiGdO was not completely whenwere BiGdO was treated at BiGdO 3 was treated at 750 °Cthat for 10 h (as3shown in formed Figure 2a) because there small peaks which 3 ◦ C for 10 h (as shown in Figure 2a) because there were small peaks which indicated that other 750 indicated that other phases existed. The reason was that the melting point 820 °C of Bi2O3 or the ◦ of Bi O or the melting point 2330 ◦ C of phases existed. The°Creason thathigher the melting point melting point 2330 of Gdwas 2O3 was than 750 °C,820 as aCresult, 2the 3 solid particle of Bi2O3 and the ◦ Gd O was higher than 750 C, as a result, the solid particle of Bi O the solid particle of Gd solid2 particle of Gd2O3 were difficult to form single phase BiGdO23 by mutual diffusion of the solid 3 3 and 2 O3 were difficult formthey single phase by mutual diffusion solid phase particles when they phase particlesto when were putBiGdO in the3electric furnace with of thethe highest sintering temperature of were in characteristic the electric furnace with the highest sintering of 750 ◦at C. 1050 The characteristic 750 °C.put The diffraction peaks of the sample temperature which was treated °C for 12 h ◦ for 12 h (as shown in Figure 2b) were diffraction thewere sample treated with at 1050 (as shown inpeaks Figureof2b) not which so clearwas compared the C sample which was treated at 750 °C for ◦ C for 10 h, 850 ◦ C for 10 h, and then noth,so850 clear withthen the sample at 750 in 10 °C compared for 10 h, and at 1050which °C forwas 12 treated h (as shown Figure 2c) with an intermediate ◦ at 1050 C process. for 12 h (as shown in Figure an intermediate It showed that, regrinding It showed that, with2c) thewith three-step treatmentregrinding procedure,process. the sintering reaction with the three-step treatment procedure, the sintering reaction proceeded completely and a better proceeded completely and a better crystallized BiGdO3 could be obtained. The main reasons were crystallized obtained. main reasons thenot solid particle of phase Bi2 O3 and the that the solidBiGdO particle of Bibe 2O 3 and theThe solid particle of were Gd2Othat 3 did form single at the 3 could ◦ C, and then the grinding solid particle of Gd O did not form single phase at the temperature of 750 temperature of 750 °C, 2 3and then the grinding process increased the opportunity of high temperature process increased temperature andnot Gd2melt O3 . Secondly, diffusion betweenthe Bi2opportunity O3 and Gdof 2Ohigh 3. Secondly, Bi2Odiffusion 3 melted between and GdBi 2O 3 3did at the 2O ◦ C. In addition, high temperature Bi2 O3 melted Gd2In O3addition, did not melt the temperature of 850 temperature of and 850 °C. high at temperature diffusion between the molten liquid particle of between molten liquid particle of to Bi2form O3 and the phase solid particle Gd2 O3the was easier Bidiffusion 2O3 and the solid the particle of Gd 2O3 was easier single BiGdO3.ofFinally, molten to form singleof phase the molten liquid particle of Bi2 O3atand solid particle of liquid particle Bi2OBiGdO 3 and the solid particle of Gd 2O3, which remained thethe high temperature 3 . Finally, ◦ C) for a long time could diffuse evenly and Gd2 O°C) , which remained at the high temperature (1050 (1050 for a long time could diffuse evenly and form a purer single phase BiGdO 3 . Moreover, rapid 3 form a purer single phase BiGdO . Moreover, rapid heating process avoided the formation of the heating process avoided the formation of the single phase BiGdO3 in the heating intermediate process 3 single phase BiGdO heating intermediate andat guaranteed the single phase BiGdO and guaranteed the single BiGdO 3 to formprocess beginning 1050 °C. Furthermore, slow cooling 3 in thephase 3 to ◦ C. Furthermore, slow cooling process avoided the particles to become brittle form beginning at 1050 process avoided the particles to become brittle because rapid cooling would result in imperfect because rapidof cooling would result in imperfect crystallization of the3.single phase BiGdO crystallization the single phase BiGdO 3 or crystal defect of BiGdO In conclusion, the novelty of 3 or crystal defect of BiGdO . In conclusion, theof novelty of thissingle method wasBiGdO that the synthesisthree-step of the purely single this method was3 that the synthesis the purely phase 3 utilized sintering phase BiGdO grinding method for thethree-step first time.sintering grinding method for the first time. 3 utilized

0

20

40

60

80

422

511

331 420

400

311 222

220

200

Intensity (a.u.)

111

Experiment Simulation Difference Observed reflections

100

2 theta (degree)

Figure Figure 1.1. X-ray X-raypowder powderdiffraction diffractionpatterns patterns and and Rietveld Rietveld refinements refinements of of BiGdO BiGdO33.. AAdifference difference (observed-calculated) profile is shown beneath. The tic marks represent reflection (observed-calculated) profile is shown beneath. The tic marks represent reflectionpositions. positions.

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BiGdO3

Intensity (a.u.)

(a)

BiGdO3

(b)

BiGdO3

(c)

0

20

40 60 2 theta (degree)

80

100

Figure2.2.X-ray X-raypowder powderdiffraction diffractionpatterns patternsofofBiGdO BiGdO 3 samples prepared at different temperatures: Figure 3 samples prepared at different temperatures: ◦ sample prepared at 750 °C10 for (b) sample prepared for 12 and (c) sample (a)(a)sample prepared at 750 C for h; 10 (b)h; sample prepared at 1050 ◦atC 1050 for 12°C h; and (c) h; sample prepared ◦C ◦ Cat at10 750 for 10for h, 10 850h°C forthen 10 hatand then for 12 with an intermediate atprepared 750 ◦ C for h,°C 850 and 1050 for1050 12 h°C with an hintermediate regrindingregrinding process. process.

The crystal structure of BiGdO3 was successfully refined by using the Rietveld method from the crystal structure BiGdO wasfinal successfully refined by usingdisplayed the Rietveld method from the X-ray The diffraction data. Theofresult of3 the refinement for BiGdO a good agreement 3 X-ray diffraction data. The result of the final refinement for BiGdO 3 displayed a good agreement between the observed intensities and calculated intensities. As a result, BiGdO3 possessed the between thestructure, observedaintensities andcubic calculated Asa aspace result, BiGdO 3 possessed the fluoritefluorite-type face-centered crystalintensities. system and group Fm3m 225 (O atoms were type structure, a face-centered cubic crystal system and a space group Fm3m 225 (O atoms were included in the model). The lattice parameter α for BiGdO3 from the refined result was 5.465 angstrom. included in the model). The lattice parameter α for BiGdO 3 from the refined result was 5.465 According to the Bragg Equation (1) and the calculation formula for cubic crystal face spacing angstrom. Equation (2):According to the Bragg Equation (1) and the calculation formula for cubic crystal face spacing Equation (2): 2dsinθ = λ = 1.54056 (1)   1/2 (1) 2 dsinq = l = 1.54056 d = a/ h2 + k2 + l 2 (2) 1/2

2 2 2 (2) = a / (h + k + l ) where d was the face spacing, θ was thed diffraction angle, and h, k and l were the crystal indices. According to above equations, the lattice parameter α for BiGdO3 was calculated to be 5.472 angstrom, where d was the face spacing, θ was the diffraction angle, and h, k and l were the crystal indices. which was close to the refined result. All the diffraction peaks for BiGdO3 could be successfully According to above equations, the lattice parameter α for BiGdO3 was calculated to be 5.472 indexed according to the lattice constant and above space group. The atomic coordinates and angstrom, which was close to the refined result. All the diffraction peaks for BiGdO3 could be structural parameters of BiGdO3 are listed in Table 1. The outcome of the refinement for BiGdO3 successfully indexed according to the lattice constant and above space group. The atomic coordinates generated the R-factor of the Rietveld refinement. RP factor (unweighted residual error), one of the and structural parameters of BiGdO3 are listed in Table 1. The outcome of the refinement for BiGdO3 reliability index parameters, showed the difference between the observed and simulated powder generated the R-factor of the Rietveld refinement. RP factor (unweighted residual error), one of the diffraction patterns [79,80]. In our experiments, RP was 14.14% with space group Fm3m 225 for reliability index parameters, showed the difference between the observed and simulated powder BiGdO3 . This means that there was small difference between the observed and simulated powder diffraction patterns [79,80]. In our experiments, RP was 14.14% with space group Fm3m 225 for diffraction patterns. BiGdO3. This means that there was small difference between the observed and simulated powder diffraction patterns.

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Table 1. Structural parameters of BiGdO3. X Y Z Occupation Factor

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Atom Bi Gd Atom OBi

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Table 1. Structural parameters of BiGdO3 .

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Y Bi X 0.50000 0.50000 0.50000 Z 1.0Occupation Factor Table 1. Structural parameters of BiGdO 3. 0.50000 0.00000 0.50000 0.50000 1.0 Gd 0.00000 0.00000 1.0 O X 0.25000 0.00000 0.00000 0.5 0.00000 0.00000 0.00000 1.0Factor Occupation Y Z 0.25000 0.00000 0.00000 0.50000 0.50000 0.50000 1.00.5

Gd 0.00000 0.00000 0.00000 1.0 2.1.2. XPS Spectra 2.1.2. XPS Spectra O 0.25000 0.00000 0.00000 0.5 Figure 3 presents the XPS spectrum of BiGdO3 . It was obvious that the observed XPS spectra of Figure 3 presents XPS spectrum of BiGdO 3. It was obvious that the observed XPS spectra of BiGdO neither the shoulders nor widening peaks, suggesting (albeit not proving) the absence of 3 showed 2.1.2. XPS Spectra BiGdO 3 showed neither shoulders nor widening peaks, suggesting (albeit not proving) the absence any other phases. The sample showed the presence of bismuth (Bi 4f, Bi 4d, and Bi 5d), gadolinium Figurephases. 3 presents XPS spectrum of BiGdO 3. It of was obvious(Bi that observed of of any other Thethe sample showed the presence bismuth 4f,the Bi 4d, and BiXPS 5d),spectra gadolinium (Gd 3d, Gd34d, and Gd 4f), oxygen (Onor 1s)widening and carbon (C 1s). Moreover, thenot peak of C 1sthe was attributed BiGdO showed neither shoulders peaks, suggesting (albeit proving) absence (Gd 3d, Gd 4d, and Gd 4f), oxygen (O 1s) and carbon (C 1s). Moreover, the peak of C 1s was attributed to the surface adsorption Figure shows the XPS peaks of: Bi 4f (A); 5d), Gd 3d (B); O 1s (C); any other phases. Thepollution. sample showed the4 of bismuth (Bi 4f, to of the surface adsorption pollution. Figure 4presence shows the XPS peaks of:BiBi4d, 4f and (A);Bi Gd 3dgadolinium (B); O 1s (C); and the split of4d, O and 1s (D) in BiGdO Various peaks, which specific binding 3 .(O (Gd 3d,split Gd 1s) and elemental carbon (C 1s). Moreover, thecorrespond peak of C 1s to was attributed and the of O 1sGd (D)4f), in oxygen BiGdO 3. Various elemental peaks, which correspond to specific binding 5/2 , Bi 4f7/2 , Gd 3d3/2 and energies BiGdOadsorption , are provided in Table binding to theofsurface pollution. Figure2.4 The shows the XPSenergies peaks of:ofBiBi 4f4f (A); Gd 3d (B); O 1s (C); 3 energies of BiGdO3, are provided in Table 2. The binding energies of Bi 4f5/2, Bi 4f7/2, Gd 3d3/2 and Gd 5/2 in split and of O3 1s (D) 163.1, in BiGdO 3. Various elemental peaks, correspond to specific binding Gd3d3d BiGdO were 157.8, 1218.2 and 1185.9 eV,which respectively. can in2Table 5/2 inthe BiGdO 3 were 163.1, 157.8, 1218.2 and 1185.9 eV, respectively. It5/2can It be7/2 seenbeinseen Table that 2 3/2 and Gd energies of BiGdO 3 , are provided in Table 2. The binding energies of Bi 4f , Bi 4f , Gd 3d that there were two peaks with different binding energies for oxygen (O The 1s). split The split 1s peaks there were two peaks with different binding energies for oxygen (O 1s). of O of 1s O peaks in 3d5/2 in BiGdO 3 were 163.1, 157.8, 1218.2 and 1185.9 eV, respectively. It can be seen in Table 2 that in Figure Figure 4D was assigned to O 1s peaks of crystal lattice oxygen (528.8 eV) and surface adsorbed 4D was assigned to O 1s peaks of crystal lattice oxygen (528.8 eV) and surface adsorbed oxygen there were two peaks with different binding energies for oxygen (O 1s). The split of O 1s peaks in oxygen eV) [81]. Surface adsorbed oxygen might correspond to the hydroxyl oxygen, which was (530.4(530.4 eV) [81]. Surface adsorbed oxygen might correspond to the hydroxyl oxygen, which was Figure 4D was assigned to O 1s peaks of crystal lattice oxygen (528.8 eV) and surface adsorbed oxygen significant for the the improvement of the photocatalytic efficiency. Hydroxyl oxygen, which was adsorbed significant improvement the photocatalytic efficiency. Hydroxyl oxygen, which (530.4 eV)for [81]. Surface adsorbedof oxygen might correspond to the hydroxyl oxygen, which waswas onto the surface of the photocatalyst, was in favor of the generation of hydroxyl radicals. adsorbed onto the surface of the photocatalyst, was in favor of the generation of hydroxyl radicals. significant for the improvement of the photocatalytic efficiency. Hydroxyl oxygen, which was adsorbed onto the surface of the photocatalyst, was in favor of the generation of hydroxyl radicals. 45,000

Bi 4f

40,000

45,000

Gd 3d

35,000

Bi 4f

40,000

Gd 3d

30,000

Intensity Intensity (a.u.)(a.u.)

O 1s

35,000

O 1s Bi 4d

25,000 30,000

Bi 4d C 1s

20,000 25,000

C 1s

15,000 20,000

Gd 4d

10,000 15,000

Bi 5d

Gd 4d

5,000 10,000

Gd 4f Bi 5d

5,000 0

Gd 4f

-5,0000 1,400

-5,000 1,400

1,200

1,000

1,200

energy (eV) 1,000 Binding 800 600 400

800

600

400

200 200

0

-200

0

-200

Binding energy (eV)

Figure3.3.XPS XPS spectrum spectrum of Figure of BiGdO BiGdO3. . Figure 3. XPS spectrum of BiGdO3. 3 (A)

(B) 7/2

(A)

Bi 4f

(B)

7/2

3/2

Gd 3d

5/2

Gd 3d

Gd 3d

5/2

Gd 3d

5/2

Intensity (a.u.)

Bi 4f

Intensity (a.u.)

Intensity (a.u.)

Intensity (a.u.)

Bi 4f

3/2

Bi 4f

5/2

175175

170 170

165 165

160 160

155 155

150 150

1230 1230

Binding Bindingenergy energy(eV) (eV)

1220 1220

1210 1210

1200 1200

11901190 11801180 1170 1170

Binding energy Binding energy (eV)(eV) (D) (D)

(C)(C)

O 1sO 1s Adsorbed oxygen Adsorbed oxygen Lattice oxygen Lattice oxygen

Intensity (a.u.) Intensity (a.u.)

Intensity (a.u.)

Intensity (a.u.)

O1s 1s O

540

540

530

530 (eV) Binding energy

Binding energy (eV)

520

520

545

545

540

540

535

530

535 energy (eV) 530 Binding

525

525

520

520

Binding energy (eV)

Figure 4. XPS peaks of:BiBi4f4f(A); (A); Gd 3d 3d (B); O split of O 1s (D)(D) in BiGdO 3. Figure 4. 4. XPS O1s 1s(C); (C);and and split BiGdO . Figure XPSpeaks peaksof: of: Bi 4f (A);Gd Gd 3d (B); (B); O 1s (C); and split ofofOO1s1s(D) ininBiGdO 3. 3

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Table 2. Binding energies of Bi 4f, Gd 3d and O O 1s 1s in in BiGdO BiGdO3. Table 2. Binding energies of Bi 4f, Gd 3d and O 1s in BiGdO33.

Elements Bi 4f5/2 Elements Elements Bi 4f5/2 Bi 4f5/2 Binding energy (eV) 163.1 Binding energy (eV) 163.1 163.1 Binding energy (eV)

Bi 4f7/2 4f7/2 Bi Bi 4f7/2 157.8 157.8 157.8

Gd 3d3/2 1218.2 1218.2 1218.2

3/23/2 Gd Gd3d3d

Gd 3d5/2 1185.9 1185.9 528.8 1185.9

5/2 Gd 3d Gd 3d5/2 O 1s

O 1s O 1s 528.8 528.8

For BiGdO3, surface elemental analysis revealed that the average atomic ratio of Bi:Gd:O is For BiGdO3, surface elemental analysis revealed that the average atomic ratio of Bi:Gd:O is For BiGdO3The , surface analysis revealed that the theratio average atomic ratioofofoxygen Bi:Gd:O is 10.33:9.87:42.84. ratio ofelemental Bi:Gd in the sample was almost of 1:1. The value was 10.33:9.87:42.84. The ratio of Bi:Gd in the sample was almost the ratio of 1:1. The value of oxygen was 10.33:9.87:42.84. Thewas ratio of Bi:Gd in the sample almost the ratio of 1:1. The value of oxygen was high because there a lot of adsorbed oxygenwas on the surface. high because there was a lot of adsorbed oxygen on the surface. high because there was a lot of adsorbed oxygen on the surface. 2.1.3. TEM and SEM Analyses 2.1.3. TEM TEM and and SEM SEM Analyses Analyses 2.1.3. The morphology and microstructure of the as-prepared sample were examined by TEM, SEM The morphology morphology and and microstructure of of the as-prepared as-prepared sample sample were were examined examined by by TEM, SEM SEM The and EDS. Figure 5 displaysmicrostructure the TEM and SEMthe image of BiGdO 3. We could observe from TEM, the images and EDS. EDS. Figure 55 displays displays the the TEM TEM and and SEM SEM image image of of BiGdO3.. We could observe observe from from the the images images and 3 We couldappearance of BiGdO3Figure that the particles presented nanoscale and BiGdO irregular-shaped and that the of BiGdO 3 that the particles presented nanoscale and irregular-shaped appearance and that the of BiGdO that the particles presented nanoscale and irregular-shaped appearance and that the 3 was relatively uniform. The average particle size of BiGdO3 approached 750 nm. distribution distribution was relatively uniform. The average particle size of BiGdO 3 approached 750 nm. distribution relatively uniform. The average particle of BiGdO 750 nm. 3 approached Figure 6was shows the EDS spectrum of BiGdO 3. Thesize EDS spectrum that was taken from the Figure 66shows showsthethe EDS spectrum of BiGdO 3. The EDS spectrum that was taken from the Figure EDS spectrum of BiGdO . The EDS spectrum that was taken from the 3 prepared BiGdO3 displayed the presence of bismuth, gadolinium and oxygen. Moreover, theprepared peak of prepareddisplayed BiGdO3 displayed the presence of bismuth, gadolinium and oxygen. Moreover, the of BiGdO thesurface presence of bismuth, gadolinium and oxygen. Moreover, theidentified peak ofpeak Cfrom was C was 3attributed to adsorption pollution. Other elements could not be C was attributed to adsorption surface adsorption pollution. Other elements could not be identified from. attributed toratio surface pollution. Other could not be identified from with BiGdO BiGdO 3. The of Bi:Gd:O in BiGdO 3 sample waselements 12.57:12.88:39.53, which was accordant the3 BiGdO 3. The ratio of Bi:Gd:O in BiGdO3 sample was 12.57:12.88:39.53, which was accordant with the The ratio of Bi:Gd:O in BiGdO sample was 12.57:12.88:39.53, which was accordant with the ratio of 3 ratio of 1:1:3 expected from the BiGdO3 molecular formula. The results further indicated that the ratio expected of 1:1:3 expected from themolecular BiGdO3 molecular formula. The results further indicated that the 1:1:3 from the BiGdO formula. The results further indicated that the oxidation oxidation state of Bi, Gd and O3ions from BiGdO3 were +3, +3 and −2, respectively. According to the oxidation of Bi,OGd and O ions fromwere BiGdO +3,−+3 −2, respectively. According to the state ofavailable Bi,state Gd and from +3,3 were +3 and 2, and respectively. According to Bi the 3 above data, ions it could beBiGdO concluded that the sample which was prepared with 2Oabove 3 and above available data, it could be concluded that the sample which was prepared with Bi 2O3 and available data, it could be concluded that the sample which was prepared with Bi O and Gd 2 3 of XRD 2 Oand 3 in Gd2O3 in a solid state reaction was BiGdO3, which was in agreement with the results Gd 2O3 in a solid state reaction was BiGdO3, which was in agreement with the results of XRD and a solid state reaction was BiGdO3 , which was in agreement with the results of XRD and XPS analyses. XPS analyses. XPS analyses.

Figure 5. 5. (a) TEM TEM image of of BiGdO3;; and (b) SEM image of BiGdO3.. Figure Figure 5. (a) (a) TEM image image of BiGdO BiGdO33; and and (b) (b) SEM SEM image image of of BiGdO BiGdO33.

Figure EDS spectrum spectrum of Figure 6. 6. EDS of BiGdO BiGdO33.. Figure 6. EDS spectrum of BiGdO3.

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2.1.4. BET Analysis The specific surface area detected by the BET isotherm measurements was 3.25 m2 ·g−1 for BiGdO3 . The specific surface area of BiGdO3 was almost 14 times smaller than that of TiO2 , which was measured to be 46.24 m2 ·g−1 . Many catalysts that are produced by solid state methods often exhibit low surface area, but might own good photocatalytic activity. Photocatalytic materials LiBi4 M3 O14 (M = Nb, Ta) that were prepared by Muktha et al. [82] showed reasonable photocatalytic activity for degrading dyes and organic compounds, despite their low BET surface area (0.3 or 0.1 m2 ·g−1 ). 2.1.5. UV-Vis Diffuse Reflectance Spectra UV-Vis diffuse reflectance spectroscopy was carried out to investigate the optical properties of the photocatalyst sample. Figure 7a presents the UV-Vis diffuse reflectance spectrum of BiGdO3 , Bi2 O3 , Gd2 O3 and TiO2 . For a comparison, the UV-Vis diffuse reflectance spectra of Bi2 O3 , Gd2 O3 and TiO2 were also provided in Figure 7a. As we all know, the well-known TiO2 whose absorption edge was at less than 380 nm had no response to the visible light irradiation. It could be seen that the absorption edge of BiGdO3 was found to be at 585 nm, which belonged to the visible region of the spectrum. It was obvious that the as-prepared BiGdO3 exhibited absorption range extending from the UV light region to 600 nm, which was wider than the range of Bi2 O3 . We could draw a conclusion from the researches reported by Oshikiri et al. [83] and Zheng et al. [84] that the positions and width of the conduction band and the valence band of BiGdO3 could be investigated by calculating the electronic band structure of Bi2 O3 , Gd2 O3 and BiGdO3 with the plane-wave-based density functional method. Moreover, the band structure calculations of Bi2 O3 , Gd2 O3 and BiGdO3 might be performed with the program of Cambridge serial total energy package (CASTEP) and first-principles simulation [85]. The CASTEP calculation was composed of the plane-wave pseudopotential total energy method according to the density functional theory. The valence band of Bi2 O3 or Gd2 O3 was composed of 2p orbital of oxygen, while the conduction band of Bi2 O3 or Gd2 O3 was composed of 6p orbital of bismuth or 5d orbital and 6s orbital of gadolinium, respectively. Our previous works similarly explained [85,86] that the conduction band of the BiGdO3 photocatalyst consisted mainly of Gd 5d orbital component, while the valence band of BiGdO3 consisted of mainly O 2p orbital component, Bi 6p orbital component and Gd 6s orbital component. The contribution of the valence band on the value of band gap of the BiGdO3 photocatalyst was mainly owing to the hybridization among Bi 6p orbital component, Gd 5d orbital component and Gd 6s orbital component. The top of the value band of BiGdO3 increased and the distance between the top of the valence band of BiGdO3 and the bottom of the conduction band of BiGdO3 decreased. This change caused the decrease of band gap of the BiGdO3 photocatalyst, which was the main reason for the red-shift of the spectrum for BiGdO3 . Another possible reason was the different electronic structures of Bi2 O3 , Gd2 O3 and BiGdO3 .

Figure 7. (a) UV-Vis diffuse reflectance spectra of BiGdO3 , Bi2 O3 , Gd2 O3 and TiO2 ; and (b) plot of (αhν)1/n versus hν for BiGdO3 (inset).

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For a crystalline semiconductor, the optical absorption near the band edge followed the equation [87,88]:
n αhν = A hν − Eg (3) where A, α, Eg and ν were proportional constant, absorption coefficient, band gap and light frequency respectively. Within this equation, n determined the character of the transition in a semiconductor. Eg and n could be calculated by the following steps: (i) plotting ln(αhν) versus ln(hν-Eg ) by assuming an approximate value of Eg ; (ii) deducing the value of n based on the slope in this graph; and (iii) refining the value of Eg by plotting (αhν)1/n versus hν and extrapolating the plot to (αhν)1/n = 0. According to this method, the inset in Figure 7b shows the plot of (αhν)1/n versus hν for BiGdO3 . The band gap of BiGdO3 was calculated to be 2.25 eV according to above equation, while the value of n for BiGdO3 was 0.5, indicating that the optical transition for BiGdO3 is directly allowed. Furthermore, the band gap of Bi2 O3 was calculated to be 2.65 eV. Thus, the above results indicated an enhanced visible light absorption of BiGdO3 and a narrower band gap compared with Bi2 O3 . 2.2. Evaluation of Photocatalytic Activity The photocatalytic activities of the as-prepared BiGdO3 samples were evaluated by the degradation of Direct Orange 26, Direct Red 23 or phenol under visible light irradiation. Direct Orange 26, Direct Red 23 or phenol was degraded well by using BiGdO3 as a photocatalyst under visible light irradiation. The effect of various operational parameters, namely, catalyst loading, initial concentration of Direct Orange 26 or Direct Red 23, pH value, light intensity and coexisting ions on the photocatalytic degradation efficiency was investigated in detail. 2.2.1. Photocatalytic Degradation of Direct Orange 26 or Direct Red 23 or Phenol Figure 8 shows the comparison of removal efficiencies between Direct Orange 26 and Direct Red 23 under different conditions. As shown in Figure 8, the suspension had already reached adsorption/desorption equilibrium between the dye and the catalyst when stirred in the dark for 30 min, maintaining the decolorization rate of 31% for Direct Orange 26 (obtained from Figure 8A) and the decolorization rate of 23% for Direct Red 23 (obtained from Figure 8B) using BiGdO3 as a photocatalyst when stirred in the dark for 360 min. It could be concluded from the observation of the color of the catalyst that the adsorption process was just physical adsorption. The BiGdO3 particles changed to deep orange or red after the adsorption process while the particles still presented their original color after photocatalytic reaction, which meant that the adsorbed dye was also degraded during the reaction. It can be seen in Figure 8 that the photocatalytic degradation efficiency was 83.7% for Direct Orange 26 and 60.8% for Direct Red 23 using BiGdO3 as a photocatalyst under visible light irradiation within 360 min, while the photocatalytic degradation efficiency was 39.8% for Direct Orange 26 and 37.1% for Direct Red 23 using Bi2 O3 as a catalyst or 16.3% for Direct Orange 26 and 21.8% for Direct Red 23 using Gd2 O3 as a catalyst under visible light irradiation within 360 min. It can be seen in Figure 7a that Gd2 O3 had no visible light response, but Gd2 O3 did have a faint degradation efficiency on Direct Orange 26 or Direct Red 23 under visible light irradiation. The possible reasons of this phenomenon were the adsorption of the Gd2 O3 sample and the photosensitive effect of Direct Orange 26 or Direct Red 23. These catalysts could be arranged in order of increasing degradation efficiency during the same reaction time: Gd2 O3 < Bi2 O3 < BiGdO3 . The results in Figure 8 indicate that BiGdO3 had better photocatalytic activities for Direct Orange 26 or Direct Red 23 than Bi2 O3 or Gd2 O3 . The cost of 100 g Gd2 O3 and Bi2 O3 was 63.2 dollars and 64.5 dollars, respectively, while the synthesis of 100 g pure BiGdO3 catalyst totally cost about 71 dollars, indicating that BiGdO3 had higher cost performance than Bi2 O3 and Gd2 O3 . Additionally, there was almost no decrease for the UV-Vis absorbance signal of Direct Orange 26 or Direct Red 23 that was obtained under visible light irradiation in the absence of a photocatalyst.

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9 of26 26 understand the reaction kinetics of the degradation of Direct Orange or Direct Red 23, a pseudo first order was utilized to fit the experimentally obtained data. Figure 9 Figure 9 presents the reaction kinetics of the photocatalytic degradation of Direct Orange 26 or Direct presents the reaction kinetics of the photocatalytic degradation of Direct Orange 26 or Direct Red 23 by Red 23 by using BiGdO3 as a photocatalyst. The pseudo first order indicated a linear correlation using BiGdO3 as a photocatalyst. The pseudo first order indicated a linear correlation between ln(Co /C) between ln(Co/C) and the visible light irradiation time. In the above equation, C represents the dye and the visible light irradiation time. In the above equation, C represents the dye concentration at concentration at time t, and Co represents the initial dye concentration. On the basis of the correlation time t, and Co represents the initial dye concentration. On the basis of the correlation result between result between ln(Co/C) and the irradiation time, the first-order rate constant k was estimated−to be ln(Co /C) and the irradiation time, the first-order rate constant k was estimated to be 0.0046 min 1 for 0.0046 min−1 for Direct Orange 26. For Direct Red 23, the first-order rate constant k was estimated−to Direct Orange 26. For Direct Red 23, the first-order rate constant k was estimated to be 0.0023 min 1 , be 0.0023 min−1, which was smaller than that for Direct Orange 26. which was smaller than that for Direct Orange 26. Figure 10 shows the degradation rate of phenol by using BiGdO3 as catalyst under visible light Figure 10 shows the degradation rate of phenol by using BiGdO3 as catalyst under visible light irradiation with respect to time. It can be seen in Figure 10 that the photocatalytic degradation rate of irradiation with respect to time. It can be seen in Figure 10 that the photocatalytic degradation rate phenol was very low and could be ignored without adding BiGdO3 as a photocatalyst. Obviously, of phenol was very low and could be ignored without adding BiGdO3 as a photocatalyst. Obviously, we could observe that an improved activity was also obtained when colorless phenol was also we could observe that an improved activity was also obtained when colorless phenol was also selected selected as a contaminant model with BiGdO3 as catalyst. The photocatalytic degradation efficiency as a contaminant model with BiGdO3 as catalyst. The photocatalytic degradation efficiency of phenol of phenol was 59.8% by using BiGdO3 as a photocatalyst under visible light irradiation after 360 min, was 59.8% by using BiGdO3 as a photocatalyst under visible light irradiation after 360 min, indicating indicating that the catalyst BiGdO3 itself had photocatalytic activity and that the photodegradation that the catalyst BiGdO3 itself had photocatalytic activity and that the photodegradation process process of Direct Orange 26 or Direct Red 23 by using BiGdO3 as a photocatalyst was not mainly due of Direct Orange 26 or Direct Red 23 by using BiGdO3 as a photocatalyst was not mainly due to to photosensitive effect [89]. The above results revealed that the as-prepared catalyst BiGdO3 could photosensitive effect [89]. The above results revealed that the as-prepared catalyst BiGdO3 could decompose dye Direct Orange 26 or Direct Red 23 efficiently, at the same time, BiGdO3 could also decompose dye Direct Orange 26 or Direct Red 23 efficiently, at the same time, BiGdO3 could also degrade non-dye compounds, such as phenol. Ao et al. [90] used the same verification method to degrade non-dye compounds, such as phenol. Ao et al. [90] used the same verification method to prove that the photodegradation process of dye by using as-prepared photocatalyst was not mainly prove that the photodegradation process of dye by using as-prepared photocatalyst was not mainly due to the photosensitive effect. due to the photosensitive effect.

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Figure 8.8. Comparison Comparison of of removal removal efficiencies efficiencies between between Direct Direct Orange Orange 26 26 (A) (A) and and Direct Direct Red Red 23 23 (B) (B) Figure under different conditions. under different conditions.

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Figure photocatalyticdegradation degradationofofDirect DirectOrange Orange Direct Figure9.9.The Thefirst-order first-orderkinetic kineticplots plots for for the photocatalytic 2626 oror Direct Figure 9. The first-order kinetic plots for the photocatalytic degradation of Direct Orange 26 or Direct Red 23 using BiGdO as a photocatalyst. Red 23 using BiGdO3 3 as a photocatalyst. Red 23 using BiGdO3 as a photocatalyst. 1.0 1.0 0.8 0.8

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0.6 0.6 0.4 0.4 Photolysis without catalyst Photolysis without catalyst BiGdO 3+visible light BiGdO3+visible light

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Figure 10. Photocatalytic activity of BiGdO3 for degrading phenol under visible light irradiation. Figure 10. Photocatalytic activity of BiGdO3 for degrading phenol under visible light irradiation. Figure 10. Photocatalytic activity of BiGdO3 for degrading phenol under visible light irradiation.

2.2.2. Effect of Various Operational Parameters 2.2.2. Effect of Various Operational Parameters 2.2.2. Effect Operational Figureof11Various presents the effect ofParameters the catalyst loading on the photocatalytic degradation efficiency Figure 11 presents the effect of the catalyst loading on the photocatalytic degradation efficiency of Direct Orange 26 or Direct Red 23. catalyst loading from 0.4 to 1.6 g/L with theefficiency initial 11 presents effect of 23. theThe catalyst loading onvaried the photocatalytic degradation of Figure Direct Orange 26 orthe Direct Red The catalyst loading varied from 0.4 to 1.6 g/L with the initial concentration of 30 mg/L from Direct Orange 26 or Direct Red 23 and the original pH value 6.2 of concentration Direct Orangeof2630ormg/L Direct RedDirect 23. The catalyst loading 1.6 g/L with theof from Orange 26 or Directvaried Red 23from and 0.4 the to original pH value ofinitial 6.2 from Direct Orange 26 or Direct Red 23. It can be seen in Figure 11 that the photocatalytic degradation concentration of 30 mg/L from Direct or Direct Red and original pHdegradation value of 6.2 from Direct Orange 26 or Direct Red 23.Orange It can be26seen in Figure 1123 that thethe photocatalytic efficiency Direct Red slightly theincatalyst increased from 0.4 degradation to 1.6 g/L. from Direct of Orange or 23 Direct Redincreased 23. It canwhile be seen Figure loading 11 that the photocatalytic efficiency of Direct26Red 23 slightly increased while the catalyst loading increased from 0.4 to 1.6 g/L. The photocatalytic degradation efficiency of Direct Orange 26 increased while the catalyst loading efficiency of Direct Red 23 slightlyefficiency increasedofwhile catalyst increased from 0.4 to 1.6 g/L. The photocatalytic degradation Directthe Orange 26 loading increased while the catalyst loading increased from 0.4 to 1.2 g/L but decreased when the catalyst loading increased further to 1.6 g/L. The The photocatalytic degradation efficiency of Direct Orange 26 increased while the catalyst loading increased from 0.4 to 1.2 g/L but decreased when the catalyst loading increased further to 1.6 g/L. The decrease of degradation efficiency for above dyes with increasing catalyst loading could be attributed increased 0.4 to 1.2 g/L but decreased whenwith the increasing catalyst loading further to 1.6 g/L. decreasefrom of degradation efficiency for above dyes catalystincreased loading could be attributed to the increased opacity of the solution, which hindered the light transmission through the solution. The of degradation for above dyes the with increasing catalyst loading could be to decrease the increased opacity of theefficiency solution, which hindered light transmission through the solution. Owing to the decrease of the effective light intensity, the photo generation of electrons and positive Owing totothe of opacity the effective intensity, thehindered photo generation electrons and positive attributed thedecrease increased of thelight solution, which the light of transmission through the holes would be reduced and then the photocatalytic degradation efficiency of above dyes was also holes would betoreduced and then theeffective photocatalytic degradation of aboveof dyes was also solution. Owing the decrease of the light intensity, the efficiency photo generation electrons and reduced [91,92]. With the catalyst loading of 1.2 g/L, the photocatalytic degradation efficiency of reduced [91,92]. With the catalyst 1.2 g/L, the photocatalytic degradation efficiency of positive holes would be reduced and loading then theof photocatalytic degradation efficiency of above dyes was Direct Orange 26 was 87.1% which was a little higher than 83.7% with the catalyst loading of 1.0 g/L. Direct Orange 26 was 87.1% which was a little higher than 83.7% with the catalyst loading of 1.0 g/L. also reducedtaking [91,92].both With catalyst and loading of 1.2factors g/L, the efficiency Therefore, thethe economic efficient intophotocatalytic consideration,degradation the catalyst loading of of Therefore, taking both the which economic and efficient factors into consideration, the catalyst loading of Direct Orange 26 was 87.1% was a little higher than 83.7% with the catalyst loading of 1.0 g/L. 1.0 g/L was chosen as optimum loading content. 1.0 g/L was chosen as the optimum loading Therefore, taking both economic andcontent. efficient factors into consideration, the catalyst loading of 1.0 g/L was chosen as optimum loading content.

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Figure 11. Effect of the catalyst loading on the photocatalytic degradation efficiency Direct Orange Figure 11. 11. Effect Effect of ofthe thecatalyst catalystloading loadingon onthe thephotocatalytic photocatalytic degradation efficiency of Direct Orange Figure degradation efficiency of of Direct Orange 26 26 or Direct Red 23. 26 Direct or Direct or RedRed 23. 23.

Figure the effect of the initial concentration of Direct Orange 26 or Direct Red 23 Figure 12 12 describes describes Orange 2626 or or Direct Red 23 on on Figure 12 describes the the effect effectof ofthe theinitial initialconcentration concentrationofofDirect Direct Orange Direct Red 23 the photocatalytic degradation of Direct Orange 26 or Direct Red 23. With the initial catalyst loading the photocatalytic degradation of Direct Orange 26 or Direct Red 23. With the initial catalyst loading on the photocatalytic degradation of Direct Orange 26 or Direct Red 23. With the initial catalyst of and the pH value of within the dye the concentration of of 1.0 1.0 g/L g/L the original original valuepH of 6.2 6.2 within dye solution, solution, the initial initialthe concentration of Direct Direct loading ofand 1.0 g/L and the pH original value of 6.2the within the dye solution, initial concentration Orange 26 or Direct Red 23 varied from 10 to 100 mg/L. It was found from Figure 12 that with Orange 26 or Direct Red 23 varied from 10 to 100 mg/L. It was found from Figure 12 that with of Direct Orange 26 or Direct Red 23 varied from 10 to 100 mg/L. It was found from Figure 12 increasing the initial concentration of Direct Orange 26 or Direct Red 23, the degradation efficiency increasing the initial concentration of Direct Orange 26 or Direct Red 23, the degradation efficiency that with increasing the initial concentration of Direct Orange 26 or Direct Red 23, the degradation of Orange 26 Red decreased. The main reason was that with increasing the of Direct Direct of Orange 26 or or Direct Direct Red 23 23 decreased. The The main reason was that the efficiency Direct Orange 26 or Direct Red 23 decreased. main reason was thatwith withincreasing increasing the concentration of above dyes, the amount of organic species which would be degraded increased [93], concentration of of above above dyes, dyes, the the amount amount of of organic would concentration organic species species− which which would be be degraded degraded increased increased [93], [93], ++ pairs, dissolved oxygen concentration, but the photocatalyst loading, the generation amount of ee−/h but the photocatalyst loading, the generation amount of /h pairs, dissolved oxygen concentration, − + but the photocatalyst loading, the generation amount of e /h pairs, dissolved oxygen concentration, intensity of light illumination and light illumination time were constant during the photocatalytic intensity of of light light illumination illumination and and light light illumination illumination time time were were constant constant during during the the photocatalytic photocatalytic intensity degradation process. Another reason was that the high concentration of Direct Orange 26 or Direct degradation process. process. Another that the the high high concentration concentration of of Direct Direct Orange Orange 26 26 or or Direct Direct degradation Another reason reason was was that Red 23 reduced the light transmittance possibility towards the solution [94]. Thus, when increasing Red 23 reduced the light transmittance possibility towards the solution [94]. Thus, when increasing Red 23 reduced the light transmittance possibility towards the solution [94]. Thus, when increasing the initial concentration of Direct Orange 26 or Red 23, degradation the initial initialconcentration concentration Direct Orange or Direct Direct 23, the the photocatalytic photocatalytic degradation the of of Direct Orange 26 or26 Direct Red 23,Red the photocatalytic degradation efficiency efficiency of above dyes decreased. Comprehensively, considering the total pollutant removal efficiency of above dyes decreased. Comprehensively, considering the total pollutant removal of above dyes decreased. Comprehensively, considering the total pollutant removal efficiency and efficiency and suitable reaction time, the initial concentration of 30 mg/L for Direct Orange 26 or efficiency and suitable reaction time, the initial concentration of 30 mg/L for Direct Orange or suitable reaction time, the initial concentration of 30 mg/L for Direct Orange 26 or Direct Red 2326 was Direct Red 23 was selected in the following reactions. Direct Red 23 was selected in the following reactions. selected in the following reactions.

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Figure Figure 12. 12. Effect Effect of of the the initial initial concentration concentration of of Direct Direct Orange Orange 26 26 or or Direct Direct Red Red 23 23 on on the the photocatalytic photocatalytic Direct Orange 26 or Direct Red 23. degradation degradation of of Direct Direct Orange Orange 26 26 or or Direct Direct Red Red 23. 23.

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Solution with the initial concentration of 30 mg/L for Direct Orange 26 or Direct Red 23 and catalyst loading of 1.0 g/L was utilized to investigate the effect of the initial pH value on the photocatalytic degradation efficiency of Direct Orange 26 or Direct Red 23. Figure 13 shows the effect of the initial pH value on the photocatalytic degradation efficiency of Direct Orange 26 or Direct Red 23. The initial pH value of Direct Orange 26 solution or Direct Red 23 solution varied from 4 to 12 by utilizing the solution of HCl or NaOH. As shown in Figure 13, the degradation efficiency of Direct Orange 26 or Direct Red 23 increased with decreasing pH value within acidic pH value range and the degradation efficiency of Direct Orange 26 or Direct Red 23 achieved the highest at the pH value of 4. During alkaline pH value range, with the increase of pH value, the degradation efficiency of Direct Orange 26 or Direct Red 23 firstly decreased and then increased. The minimum degradation efficiency of Direct Orange 26 or Direct Red 23 was found to be at the pH value of 10. The effect of the initial pH value was significantly important and generally complex in photocatalytic degradation of organic pollutants [81]. It was reported that the effect of the initial pH value on the photocatalytic degradation efficiency of organic pollutants performed mainly on surface adsorption, the aggregation of the catalyst and the band position of the catalyst in the solution [95,96]. The initial pH value could directly affect the nature of the charge, which was carried by the catalyst surface and the adsorption behavior of the pollutant on the catalyst surface. In addition, direct dyes were easily adsorbed on to the surface of the catalysts under strong acidic condition. Therefore, strong acidic condition was better for excellent photocatalytic degradation of direct dyes according to above results. Moreover, there were a lot of OH− anions, which existed in the solution when the pH value was from 10 to 12. The h+ could easily react with a large amount of OH− for generating active ·OH species which could increase the photocatalytic degradation efficiency of Direct Orange 26 or Direct Red 23 [97–100]. Therefore, the degradation efficiency of Direct Orange 26 or Direct Red 23 could increase during strong alkaline pH value range. In order to investigate the effect of the visible light intensity on the photocatalytic degradation efficiency of Direct Orange 26 or Direct Red 23, two kinds of light source were applied in the reaction system. Figure 14 shows the effect of the light intensity on the photocatalytic degradation efficiency of Direct Orange 26 and the first-order kinetic plots for the photocatalytic degradation of Direct Orange 26 with different light intensity under visible light irradiation. As shown in Figure 14A, the photocatalytic degradation of Direct Orange 26 with a 500 W Xe arc lamp performed better than that with a 250 W Xe arc lamp. The photocatalytic degradation efficiency of Direct Orange 26 with a 500 W Xe arc lamp was 83.7% while that with a 250 W Xe arc lamp was only 56.9% after visible light irradiation of 360 min. The reaction kinetics of the photocatalytic degradation of Direct Orange 26 with BiGdO3 as a photocatalyst under different light irradiation conditions were clearly demonstrated in Figure 14B. According to the correlative result between ln(Co /C) and the light irradiation time, the first-order rate constant k was estimated to be 0.0019 min−1 with a 250 W Xe arc lamp as light source. While the first-order rate constant k was estimated to be 0.0046 min−1 with a 500 W Xe arc lamp as light source, indicating that the photocatalytic activity of the BiGdO3 sample was significantly improved by using a high-power light source. Figure 15 shows the effect of the light intensity on the photocatalytic degradation efficiency of Direct Red 23 and the first-order kinetic plots for the photocatalytic degradation of Direct Red 23 with different light intensity under visible light irradiation. Similarly, the photocatalytic degradation of Direct Red 23 with a 500 W Xe arc lamp performed better than that with a 250 W Xe arc lamp as shown in Figure 15. The photocatalytic degradation efficiency of Direct Red 23 with a 500 W Xe arc lamp was 60.8% and the first-order rate constant k was estimated to be 0.0023 min−1 after visible light irradiation of 360 min. While with a 250 W Xe arc lamp as light source, the photocatalytic degradation efficiency of Direct Red 23 was 48.4% and the first-order rate constant k was estimated to be 0.0016 min−1 after visible light irradiation of 360 min. The main reason of this observation which was reported by previous studies was that higher light intensity provided more photons within a given time, accelerated photolytic reaction ratio, and thus made it possible to achieve maximum degradation yield [101–103].

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8

10

pH 8value

6

12

10

12

pH value

Figure13. 13.Effect Effectofofthe theinitial initial pH value photocatalytic degradation efficiency of Direct Orange Figure pH value onon thethe photocatalytic degradation efficiency of Direct Orange 26 26 or Direct Red 23. or Direct Red 23. Figure 13. Effect of the initial pH value on the photocatalytic degradation efficiency of Direct Orange 26 or Direct Red 23. 1.0

Visible 500W Visible 250W Visible 500W Visible 250W

1.0 0.8

C/C0 C/C0

0.8 0.6 0.6 0.4 0.4 0.2 0.2

(A)

0.0

(A)

-1

Visible 500W-k=0.0046 min -1 Visible 250W-k=0.0019 min -1 Visible 500W-k=0.0046 min -1 Visible 250W-k=0.0019 min

Ln (C0/C) Ln (C0/C)

0.0

(B) 0 0

60 60

120 120

180

Time (min) 180

240 240

300 300

360 (B) 360

Time (min) Figure 14. Effect of the light intensity on the photocatalytic degradation efficiency of Direct Orange Figure 14. Effect of the light intensity onphotocatalytic the photocatalytic degradation efficiency ofOrange Direct 26 (A);14. and the first-order kinetic plotson forthe the photocatalytic degradation of Direct Orange 26 (B) with Figure Effect of the light intensity degradation efficiency of Direct Orange 26 (A); and the first-order kinetic plots for the photocatalytic degradation of Direct Orange 26 (B) different light intensity under visible light irradiation. 26 (A); and the first-order kinetic plots for the photocatalytic degradation of Direct Orange 26 (B) with with different light intensity under visible light irradiation. different light intensity under visible light irradiation.

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14 of 26 14 of 26

1.0

Visible 500W Visible 250W

C/C0

0.8

0.6

0.4

0.2

(A) 0.0 -1

Ln (C0/C)

Visible 500W-k=0.0023 min -1 Visible 250W-k=0.0016 min

(B) 0

60

120

180

240

300

360

Time (min)

Figure onon thethe photocatalytic degradation efficiency of Direct Red Red 23 (A); Figure15. 15.Effect Effectofofthe thelight lightintensity intensity photocatalytic degradation efficiency of Direct 23 and the first-order kinetic plots for the photocatalytic degradation of Direct Red 23 (B) with different (A); and the first-order kinetic plots for the photocatalytic degradation of Direct Red 23 (B) with light intensity visible light irradiation. different lightunder intensity under visible light irradiation.

2.2.3. 2.2.3. Effect Effect of of Coexisting CoexistingSalts Salts Composition Composition of of actual actual dye dye wastewater wastewater was was very very complicated complicated and and might might contain contain several several salts salts with with different different concentrations. concentrations. In In addition, addition, all all kinds kinds of of salts, salts, which which were were contained contained within within the the dye dye wastewater, might exert negative or positive influence on the photocatalytic degradation efficiency of wastewater, might exert negative or positive influence on the photocatalytic degradation efficiency dye pollutants. Figure 16 shows the effect of the presence of the coexisting salts on the photocatalytic of dye pollutants. Figure 16 shows the effect of the presence of the coexisting salts on the degradation efficiency of Direct Orangeof 26.Direct The amount variedoffrom 2.0 g/L. It can photocatalytic degradation efficiency Orangeof 26.each Thesalt amount each0.1 salttovaried from 0.1be to clearly seen in Figure 16 that the salts had different effect on the photocatalytic degradation efficiency 2.0 g/L. It can be clearly seen in Figure 16 that the salts had different effect on the photocatalytic of Direct Orange 26. Inof case of NaCl, the26. photocatalytic degradation efficiency of Direct Orange 26 degradation efficiency Direct Orange In case of NaCl, the photocatalytic degradation efficiency increased with increasing the with amount of NaClthe and obtained the maximum value an optimal of Direct Orange 26 increased increasing amount of NaCl and obtained thewith maximum value dosage 0.5 g/L. On the both Na2 SO4 both and of NaNa detrimental effect on the 2 CO 3 4had with anofoptimal dosage of contrary, 0.5 g/L. On theofcontrary, 2SO anda Na 2CO3 had a detrimental photocatalytic degradation efficiency of Direct Orange 26. With increasing amount the of Na 2 SO4 , effect on the photocatalytic degradation efficiency of Direct Orange 26. Withthe increasing amount the degradation efficiency of Direct Orange 26 decreased gently. Similar trend was observed for the of Na2SO4, the degradation efficiency of Direct Orange 26 decreased gently. Similar trend was photocatalytic degradation of diazo dyes [100,104]. As to Na CO , the degradation efficiency of Direct 3 observed for the photocatalytic degradation of diazo dyes 2[100,104]. As to Na2CO3, the degradation Orange 26of decreased suddenly when Nasuddenly g/L solution. 2 CO3 of 0.1 efficiency Direct Orange 26 decreased when Nawas 2COadded 3 of 0.1 to g/Lthe was added toThe thenegative solution. 2− and SO 2− anions 2− 2− effect could be attributed to the reason that CO inhibited the photocatalytic The negative effect could be attributed to the3 reason that4 CO3 and SO4 anions inhibited the • ●OH activity of BiGdO or h+ in the reaction [105–108]. The[105–108]. pH valueThe of the 3 by trapping photocatalytic activity of BiGdO3OH by trapping or h+ insystem the reaction system pH solution from varied 6.2 to 9.1 with the9.1 amount ofamount Na2 CO3ofwhich from 0 tofrom 2.0 g/L. It can value of varied the solution from 6.2 to with the Na2COvaried 3 which varied 0 to 2.0 g/L. be seen Figure 13 that the photocatalytic degradation efficiency of of Direct It can bein seen in Figure 13 that the photocatalytic degradation efficiency DirectOrange Orange26 26gradually gradually 2 − 2− ions decreased the pH pH value valuevaried variedfrom from4.2 4.2toto 10.2. Moreover, ions addition brought 3 decreased when when the 10.2. Moreover, CO3CO addition brought about about pH changes that weakened the adsorption of Direct Orange 26 on BiGdO and inhibited the pH changes that weakened the adsorption of Direct Orange 26 on BiGdO33 and the photocatalytic oxidation [109]. The following Equations (4)–(7) show the scavenging properties photocatalytic oxidation [109]. The following Equations (4)–(7) show the scavenging properties of of carbonate carbonateion ionand andsulfate sulfateion. ion.

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15 of 26

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2

-

15 of 26

-

CO3 + ⋅OH  OH + CO3 ⋅ -

(4)

-

HCO 3 + ⋅OH  H 2 O + CO 3 ⋅

CO3

2−

(5) (4) (6) (5)

−

+ ·OH → OH + CO3 − · 2-

+

-

SO + h  SO 4 ⋅ HCO3 − + 4·OH → H2 O + CO3 − · 2-

-

-

+ OH + SO 2− SOSO + h → SO4 − · 4 4 4+ ⋅OH

⋅

(7) (6)

− − SO4 2role + during ·OH →the OHphotocatalytic + SO4 − · (7) Though SO42− played a negative degradation process of Direct Orange 26, the SO2− 4−● which was produced in Equations (6) and (7) could increase the photocatalytic Though SO4 played a negative role during the photocatalytic degradation process of Direct degradation efficiency of Direct Orange 26 by trapping the photogenerated electrons and/or Orange 26, the SO4 −• which was produced in Equations (6) and (7) could increase the photocatalytic generating hydroxyl radical (Equations (8) and (9)). degradation efficiency of Direct Orange 26 by trapping the photogenerated electrons and/or generating hydroxyl radical (Equations (8) and (9)). 2(8) SO 4 ⋅ +e  SO 4

SO4 − · +e− → SO4 2− (8) (9) ⋅ +H 2 O  ⋅OH + SO 24 + H + SO4 − · +H2 O → ·OH + SO4 2− + H+ (9) According to Equations (6) and (7), we acquired a result that generating a certain amount of ● to Equations (6) amount and (7), of we●acquired result thatSO generating certainoxidant, amountthe of SO4−●According would consume an equal OH or h+a. Although 4− was aastrong −• • + −• ● ● SO would consume equal amount of OH h .the Although SO4 the was a strong oxidant, 4 activity and the amount an of product SO4− was less or than activity and amount of OH or h+ −• was less than the activity and the amount of • OH or ● the activity and the amount of product SO − 4 [110]. Thus, SO4 anions resulted in a mild decrease for the photocatalytic degradation efficiency of + [110]. Thus, SO −• anions resulted in a mild decrease for the photocatalytic degradation efficiency hDirect 4 − of Direct Orange 26 compared CO3 2−Cl anions. also reported have negative influence − was Cl Orange 26 compared with COwith 32− anions. also was reported to have to negative influence on the − was a reducing agent and on the photocatalytic degradation efficiency of dye pollutants because Cl − photocatalytic degradation efficiency of dye pollutants because Cl was a reducing agent and competed competed with with dye dye molecules molecules for for holes holes [109,111]. [109,111]. The The hole hole scavenging scavenging property property for for chloride chloride ion ion was was shown in the following Equations (10) and (11). shown in the following Equations (10) and (11). SO 4

−+ + Cl Cl + +hh →  Cl ⋅· -

(10) (10)

-

⋅ +ClCl− →  Cl Cl22−⋅· ClCl· +

(11) (11)

While While chlorine chlorine radicals radicals formed slowly, slowly, they they were were instantaneously instantaneously turned turned to to chloride chloride radical radical − − anions. anions. However, However, situation situation was was different differentthat thatthe theexisting existingof ofCl Cl increased the degradation efficiency of of Direct Direct Orange Orange 26 26 in in this this experimental experimental case. case. During the dyeing process of of direct direct dyes, dyes, sodium sodium chloride chloride and and sodium sodium sulfate were usually utilized to improve the dyeing efficiency of the direct dyes. 2− anions might promote the aggregation of It that the theexisting existingof ofCl Cl− − anions It was supposed that anions or or SOSO 42− 4 anions might promote the aggregation of dyes dyes the surface the catalysts. incase the case of NaCl, the increase of degradation the degradation efficiency to thetosurface of theofcatalysts. Thus,Thus, in the of NaCl, the increase of the efficiency for for Direct Orange 26 might be attributed to the effect of good adsorption. Direct Orange 26 might be attributed to the effect of good adsorption. 100

NaCl

Efficiency (%)

80

60

Na2SO4 40

20

Na2CO3 0 -0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

Amount of salt (g/L)

Figure 16. 16.Effect Effect of the coexisting salts the photocatalytic degradation of Direct Figure of the coexisting salts on theon photocatalytic degradation efficiency efficiency of Direct Orange 26. Orange 26.

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J. Mol.to Sci.remove 2016, 17, 1441 16 of 26 InInt. order negative effects which originated from the formation of sulfate and carbonate ions in the photocatalytic degradation process, we presented the desulfurization system or adjusted In order to remove negative effects which originated from the formation of sulfate and carbonate the pH value of the solution. Moreover, we also utilized BaCl2 to remove sulfate and carbonate ions in ions in the photocatalytic degradation process, we presented the desulfurization system or adjusted the photocatalytic system by using BiGdO as a photocatalyst under visible light irradiation. the pH value of the solution. Moreover,3we also utilized BaCl2 to remove sulfate and carbonate ions

in the photocatalytic system by using BiGdO3 as a photocatalyst under visible light irradiation.

2.2.4. Stability of BiGdO3

Stability of BiGdO 3 In2.2.4. order to study the stability of both material and performance for BiGdO3 as a visible light catalyst, property of BiGdO repeated photocatalytic tests of Direct In ordercharacterization to study the stability of both material and performance for degradation BiGdO3 as a visible light 3 and catalyst, property characterization of BiGdO 3 and repeated photocatalytic degradation tests of Direct Orange 26 were carried out. The structure of BiGdO3 after photocatalytic degradation of Direct 26 were carried out. Thewas structure of BiGdO 3 after photocatalytic degradation of Direct OrangeOrange 26 under different conditions examined by measuring the XRD patterns of every BiGdO3 Orange 2617 under different was examined by measuring theall XRD patterns of everythat BiGdO 3 sample. Figure presents theconditions X-ray powder diffraction patterns of BiGdO samples contain 3 sample. Figure 17 presents the X-ray powder diffraction patterns of all BiGdO 3 samples that contain the original BiGdO3 sample and the BiGdO3 sample that was obtained after photocatalytic degradation the original BiGdO3 sample and the BiGdO3 sample that was obtained after photocatalytic of Direct Orange 26. It can be seen in the XRD patterns (Figure 17) that the XRD pattern of the BiGdO3 degradation of Direct Orange 26. It can be seen in the XRD patterns (Figure 17) that the XRD pattern sampleofafter the photocatalytic degradation of Direct Orange 26 was the same as that of the original the BiGdO3 sample after the photocatalytic degradation of Direct Orange 26 was the same as that BiGdOof 3 sample. the original BiGdO3 sample.

Intensity (a.u.)

BiGdO3

(a)

BiGdO3

(b) 0

20

40 60 2 theta (degree)

80

100

17. X-ray powder diffraction patterns everyBiGdO BiGdO3 sample: sample: (a) BiGdO 3 sample; FigureFigure 17. X-ray powder diffraction patterns of of every (a)the theoriginal original BiGdO 3 3 sample; and (b) the BiGdO3 sample after photocatalytic degradation of Direct Orange 26. and (b) the BiGdO3 sample after photocatalytic degradation of Direct Orange 26.

Figure 18 presents the X-ray powder diffraction patterns of all BiGdO3 samples that are obtained

Figure 18 presents the X-ray powder diffraction patterns of all BiGdO that are obtained after photocatalytic degradation of Direct Orange 26 by adding salts or adding alkali or adding acid. 3 samples According to the characteristic diffraction peaks as shown Figure 18, changes not beacid. after photocatalytic degradation of Direct Orange 26 by addinginsalts or adding alkalicould or adding observed with the peaks of peaks the original BiGdO sample.18, Namely, thecould crystalnot structure of According to thecompared characteristic diffraction as shown in3 Figure changes be observed BiGdO 3 was stable when acid or alkali or salt was added to the dye solution. The above results clearly compared with the peaks of the original BiGdO3 sample. Namely, the crystal structure of BiGdO3 was indicated the excellent stability of the as-prepared BiGdO3. stable when acid or alkali or salt was added to the dye solution. The above results clearly indicated the Figure 19 presents the repeated photocatalytic degradation tests of Direct Orange 26 with excellent stability of the as-prepared BiGdO3 . BiGdO3 as photocatalyst under visible light irradiation. The photocatalytic degradation of Direct Figure 19 presents the repeated photocatalytic degradation tests of Direct Orange 26 with BiGdO Orange 26 with BiGdO3 as photocatalyst under visible light irradiation was repeated four times. It 3 as photocatalyst visible light The photocatalytic degradation Direct Orange can be seenunder in Figure 19 that theirradiation. removal efficiencies of Direct Orange 26 were of 82.9%, 79.7% and 26 with BiGdO under visible irradiation was repeated four times. It can 3 as 78.9% for thephotocatalyst second, third and fourth cycleslight respectively, after visible light irradiation of 360 min. It be seen incan Figure 19 in that the removal of Direct Orange 82.9%,which 79.7% and 78.9% for be seen Figure 19 that theefficiencies activity slightly decreased after26 thewere first cycle, was probably due to third the drop a smallcycles amount of BiGdO3 particles. Furthermore, the crystal structure of BiGdO 3 be the second, andoffourth respectively, after visible light irradiation of 360 min. It can was stable and the morphology of BiGdO 3 did not change after every repeated cycles. Although the seen in Figure 19 that the activity slightly decreased after the first cycle, which was probably due degradation Orange 26 for the repeated cyclesstructure slightly decreased to the photocatalytic drop of a small amount efficiency of BiGdOof3 Direct particles. Furthermore, the crystal of BiGdO3 compared with the photocatalytic degradation efficiency of 83.7% for Direct Orange 26 that was was stable and the morphology of BiGdO3 did not change after every repeated cycles. Although the obtained from the first-cycle result, BiGdO3 still showed excellent stability and was considered to be photocatalytic degradation efficiency of Direct Orange 26 for the repeated cycles slightly decreased an efficient photocatalyst. compared with the photocatalytic degradation efficiency of 83.7% for Direct Orange 26 that was obtained from the first-cycle result, BiGdO3 still showed excellent stability and was considered to be an efficient photocatalyst.

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1717ofof 2626

BiGdO

BiGdO33

(a)

Intensity (a.u.)

Intensity (a.u.)

(a)

BiGdO3

BiGdO3

(b)

(b) BiGdO3

BiGdO3 (c)

(c) 0 0

20

40

60

2 theta (degree)

20

40

60

80

100

80

100

2 theta (degree) Figure 18. X-ray powder diffraction patterns of all BiGdO3 samples after photocatalytic degradation

Figure 18. X-ray powder diffraction patterns of all BiGdO3 samples after photocatalytic degradation of of Direct Orange 26 under different conditions: (a) adding salts; (b) adding alkali; and (c) adding acid. Direct Orange 26 under different conditions: salts; (b) adding alkali; and (c) degradation adding acid. Figure 18. X-ray powder diffraction patterns(a) of adding all BiGdO 3 samples after photocatalytic of Direct Orange 26 under different conditions: (a) adding salts; (b) adding alkali; and (c) adding acid. 1.0

1.0

C/C0

C/C0

0.8

1st

1st

2nd

2nd

3rd

4th

3rd

4th

0.8

0.6

0.6 0.4

0.4 0.2

0.2 0.0 0 120 240 360 0 120 240 360 0 120 240 360 0 120 240 360

Time (min) 0.0 0 120 240 360 0 120 240 360 0 120 240 360 0 120 240 360 Figure 19. Repeated photocatalytic degradation tests of Direct Orange 26 with BiGdO3 as catalyst Time (min) under visible light irradiation.

Figure Repeatedphotocatalytic photocatalytic degradation tests 26 BiGdO 3 asas catalyst Figure 19.19. Repeated tests of of Direct Direct Orange Orange 26with withdegradation BiGdO catalyst 3 Figure 20 shows the change ofdegradation TOC concentration during photocatalytic of Direct under visible light irradiation. under visible light irradiation. Orange 26 and Direct Red 23 with BiGdO3 as photocatalyst under visible light irradiation. The TOC

measurements revealed the disappearance of organic carbon in the Direct Orange 26 solution or

Figure 20 shows the change of TOC concentration during photocatalytic degradation of Direct Direct 20 Redshows 23 solution which contained BiGdO3. The results indicated that 81.49% of TOC decrease Figure the change of TOC concentration during photocatalytic degradation of Direct Orange 26 and Direct Red 23 with BiGdO3 as photocatalyst under visible light irradiation. The TOC was during degradation of Direct Orange 26 after visible light irradiation forTOC Orange 26obtained and Direct Redphotocatalytic 23 with BiGdO as photocatalyst under visible light irradiation. The 3 measurements revealed the disappearance of organic carbon in the Direct Orange 26 solution or 360 min when BiGdO 3 was utilized as photocatalyst. In addition, it can also be seen in Figure 20 that measurements revealed the disappearance of organic carbon in the Direct Orange 26 solution or Direct Direct Redof 23TOC solution which BiGdOphotocatalytic 3. The results indicated that 81.49% of TOC decrease decrease wascontained obtained during degradation of Directof Red 23 after visiblewas Red 2359.19% solution which contained BiGdO indicated that 81.49% TOC decrease 3 . The results was obtained during photocatalytic degradation of Direct Orange 26 after visible light irradiation for light irradiation for 360 min with BiGdO3 as catalyst. Therefore, the above results showed that Direct obtained photocatalytic degradation of Direct Orange 26 after visible light irradiation for 360 Orange minduring when BiGdO 3 was utilized as photocatalyst. In addition, it can also be seen in Figure 20 that 26 was more easily to be mineralized compared with Direct Red 23 with BiGdO3 as catalyst 36059.19% min when BiGdO utilized as during photocatalyst. In addition, it can of also be seen in 20 that 3 was of TOC decrease was obtained degradation Direct 23 Figure after visible under visible light irradiation. According tophotocatalytic the results in Figures 8 and 20, it couldRed be found that the 59.19% of TOC decrease was obtained during photocatalytic degradation of Direct Red 23 after visible lightphotodegradation irradiation for 360 min with BiGdO 3 asof catalyst. Therefore, above results showedduring that Direct intermediate products Direct Orange 26 orthe Direct Red 23 appeared the light irradiation 360easily min with BiGdO Therefore, above showed Direct Orange 26 wasfor more toof beDirect mineralized compared with Red 23results with 3 asthat catalyst 3 as catalyst. photocatalytic degradation Orange 26 or Direct Red Direct 23 the under visible lightBiGdO irradiation. Orange was light moreirradiation. easily to beAccording mineralized compared Direct Red with BiGdO catalyst under26 visible to the results inwith Figures 8 and 20,23 it could be found that the 3 as under visible light irradiation. According results in Figures 8 andRed 20, it be found that photodegradation intermediate productsto of the Direct Orange 26 or Direct 23could appeared during thethe photocatalytic degradation of Direct Orange 26 or Direct Red visible light irradiation. photodegradation intermediate products of Direct Orange 2623orunder Direct Red 23 appeared during the photocatalytic degradation of Direct Orange 26 or Direct Red 23 under visible light irradiation.

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18 of 26 18 of 26 18 of 26 1.5 1.5

DR-23 DR-23 DO-26 DO-26

TOC (mmol/L) TOC (mmol/L)

1.2 1.2 0.9 0.9 0.6 0.6 0.3 0.3 0.0 0.0

0 0

80 80

160 160

240 240

Time (min) Time (min)

320 320

400 400

Figure The changeofofTOC TOCconcentration concentration during during photocatalytic Direct Orange 26 26 Figure 20.20. The change photocatalyticdegradation degradationofof Direct Orange Figure 20. The change of TOC concentration during photocatalytic degradation of Direct Orange 26 3 as catalyst under visible light irradiation. and Direct Red 23 with BiGdO and Direct Red 23 with BiGdO as catalyst under visible light irradiation. and Direct Red 23 with BiGdO3 3 as catalyst under visible light irradiation.

Figure 21. Suggested photocatalytic degradation pathway scheme for Direct Orange 26 under visible Figure 21. Suggested photocatalytic degradation pathway scheme for Direct Orange 26 under visible Figure Suggested degradation pathway scheme for Direct Orange 26 under visible light 21. irradiation withphotocatalytic BiGdO3 as catalyst. light irradiation with BiGdO3 as catalyst. light irradiation with BiGdO3 as catalyst.

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In our experiments, the identified photodegradation intermediate products of Direct Orange 26 with BiGdO3 as photocatalyst under visible light irradiation were identified as phenyldiazene (m/z: 105.98), phenol (m/z: 94.14), pyrocatechol (m/z: 112.42), 1,2,6-trihydroxy-3-naphthalene sulfonate (m/z: 257.12), oxalic acid (m/z: 90), 8-aminonaphthol (m/z: 154.02), naphtahlene (m/z: 127.01), C11 H10 O5 N2 S (m/z: 281.98), C11 H9 O5 NS (m/z: 265.97), and aniline (m/z: 93.02). Figure 21 shows the suggested photocatalytic degradation pathway scheme for Direct Orange 26 under visible light irradiation with BiGdO3 as catalyst. It can be seen in Figure 21 that the chromophore cleavage, opening-ring and mineralization should be the main photocatalytic degradation pathway of Direct Orange 26 in this work. Direct Orange 26 was turned to smaller organic species, and then mineralized together with other organic groups to inorganic products such as CO2 , SO4 2− , NH4 + , N2 and, ultimately, water. 3. Experimental Section 3.1. Preparation of BiGdO3 The BiGdO3 sample was prepared by a solid-state reaction method, as was reported previously [112]. Bi2 O3 and Gd2 O3 with purity of 99.99% (Sinopharm Group Chemical Reagent Co., Ltd., Shanghai, China) were utilized as starting materials. Owing to the volatility of Bi2 O3 , we finally decided to add 115.6% quantities of Bi2 O3 after 5 experiments. All powders were dried at 200 ◦ C for 4 h before synthesis. In order to synthesize BiGdO3 , the precursors were stoichiometrically mixed, then pressed into small columns and put into an alumina crucible (Shenyang Crucible Co., Ltd., Shenyang, China). After the raw materials calcining at 750 ◦ C for 10 h, we took the small columns out of the electric furnace, ground the mixed materials and then put into an electric furnace (KSL 1700X, Hefei Kejing Materials Technology Co., Ltd., Hefei, China). After the ground materials were calcined at 850 ◦ C for 10 h, we took the small columns out of the electric furnace, ground the mixed materials and then put them into the electric furnace again. The mixed materials were calcined at 1050 ◦ C for 12 h with an intermediate regrinding process in an electric furnace. Finally, pure BiGdO3 catalyst, which presented the color of light orange, was obtained after total grinding. The cost of 100 g Gd2 O3 or Bi2 O3 was 63.2 and 64.5 dollars, respectively, while the synthesis of 100 g pure BiGdO3 catalyst totally cost about 71 dollars. 3.2. Characterization of BiGdO3 The crystal structure of BiGdO3 was analyzed by the powder X-ray diffraction method (XRD, XRD 6000, Shimadzu Corporation, Kyoto, Japan) with CuKα radiation (λ = 1.54056 angstrom). The data were collected at 295 K with a step-scan procedure within the range of 2θ = 10◦ –100◦ . The step interval was 0.02◦ and the time per step was 1.2 s. The Bi3+ content, Gd3+ content and O2 − content of BiGdO3 were determined by X-ray photoelectron spectroscopy (XPS, PHI 5000 Versa Probe, ULVAC-PHI Corporation, Kanagawa, Japan). The chemical composition within the depth profile of BiGdO3 was detected by the argon ion denudation method when X-ray photoelectron spectroscopy was utilized. The particle morphology was detected by transmission electron microscope (TEM, JEM-200CX, JEOL Corporation, Tokyo, Japan). The chemical composition of BiGdO3 was examined by scanning electron microscope-X-ray energy dispersion spectrum (SEM-EDS, 1530 VP, LEO Corporation, Dresden, Germany). The surface area of BiGdO3 particle was measured by the Brunauer-Emmett-Teller method (BET, ASAP 2020, Micromeritics Corporation, Atlanta, GA, USA) with N2 adsorption at liquid nitrogen temperature. Totally, 0.6 g BiGdO3 was used to measure the surface area of BiGdO3 every time. UV-Vis diffuse reflectance spectroscopy measurement of BiGdO3 was carried out using an UV-Vis spectrophotometer (DRS, UV-2550, Shimadzu Corporation). 3.3. Photocatalytic Experiments The photocatalytic activity of BiGdO3 sample was evaluated by the degradation of Direct Orange 26 (DO-26, C33 H22 N6 Na2 O9 S2 ) or Direct Red 23 (DR-23, C35 H27 N7 O10 S2 ) (AR, Jiangsu Suzhou

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TLRLHX CO., Ltd., Suzhou, China) in aqueous solution under visible light irradiation (λ > 420 nm). The photocatalytic reaction system was composed of a 500 W Xe arc lamp (Jiangsu Nanjing XJJD Co., Ltd., Nanjing, China), a magnetic stirrer and a cut-off filter (λ > 420 nm, Jiangsu Nanjing XJJD Co., Ltd.). The Xe arc lamp was surrounded by an outer recycling water quartz jacket so that the reaction temperature was maintained at near 25 ◦ C by cooling water in the jacket. The cooling water jacket was surrounded by twelve quartz tubes (50 mL in volume), in which suspensions of direct dyes and photocatalysts were contained. The solution was continuously stirred with a low rate for better light irradiation by the operation of the magnetic stirrer below during the reaction. Prior to light irradiation, the suspension was magnetically stirred in the darkness for 30 min to reach adsorption/desorption equilibrium between the dye and the surface of the catalyst. The reactor was put in the air-saturated conditions to ensure enough oxygen in the reaction solution. The suspension pH values were adjusted to the desired level by using dilute NaOH and HCl, and then the pH values were measured with pH meter (Jiangsu Nanjing ASTKJFZ Co., Ltd., Nanjing, China). During visible light irradiation, one of the quartz tubes was sampled at certain time interval and centrifuged to remove solid particles. The filtrate was analyzed according to the absorption which was measured by a UV-Vis spectrophotometer (UV-2450, Shimadzu Corporation) at a certain wavelength in the UV-Vis spectra of the dyes. Using this method, the conversion percentage of dyes could be obtained at different intervals. The degree of degradation efficiency (DE, %) as a function of time was given as follows: DE/% = (C0 − Ct )/C0 × 100% (12) where C0 was the initial concentration of the dyes, and Ct was the instant concentration in the sample. The total organic carbon (TOC) concentration was determined with a TOC analyzer (TOC-5000, Shimadzu Corporation). Intermediate products of Direct Orange 26 were also identified by liquid chromatograph—mass spectrometer (LC-MS, Thermo Quest LCQ Duo, Silicon Valley, CA, USA, HPLC column: β Basic-C18 (150 mm × 2.1 mm × 5 µm), Finnigan, Thermo, Silicon Valley, CA, USA). Here, 20 µL of post-photocatalysis solution was injected automatically into the LC-MS system. The eluent contained 60% methanol and 40% water, and the flow rate was 0.2 mL·min−1 . MS conditions included an electrospray ionization interface and a capillary temperature of 27 ◦ C with a voltage of 19.00 V, a spray voltage of 5000 V and a constant sheath gas flow rate. The spectrum was acquired in the negative ion scan mode, sweeping the m/z range from 50 to 600. 4. Conclusions This work indicated the structural properties, optical properties and photocatalytic activity of BiGdO3 , which was prepared by a solid state reaction method. Structural characterization of the BiGdO3 sample demonstrated that BiGdO3 crystallized with the fluorite-type structure, face-centered cubic crystal system and space group Fm3m 225. The lattice parameter α for BiGdO3 was 5.465 angstrom. UV-Vis diffuse reflectance spectra of BiGdO3 displayed that BiGdO3 showed a strong optical absorption in the visible light region and the band gap of BiGdO3 was estimated to be 2.25 eV. BiGdO3 indicated efficient photocatalytic activity for degrading Direct Orange 26 or Direct Red 23 in aqueous solution under visible light irradiation. The photocatalytic degradation efficiency was 83.7% for Direct Orange 26 and 60.8% for Direct Red 23 using BiGdO3 as photocatalyst after visible light irradiation of 360 min. The photocatalytic degradation of Direct Orange 26 or Direct Red 23 followed the first-order reaction kinetics, and the first-order rate constant was 0.0046 or 0.0023 min−1 with BiGdO3 as catalyst. The degradation intermediates of Direct Orange 26 were observed and the possible photocatalytic degradation pathway of Direct Orange 26 under visible light irradiation was provided. The stability of both material and performance of BiGdO3 was investigated by material characterization and repeated photocatalytic degradation tests. The results showed that BiGdO3 possessed excellent stability and was considered to be an efficient catalyst. BiGdO3 /(visible light) photocatalysis system was confirmed to be suitable for textile industry wastewater treatment.

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The new photocatalyst BiGdO3 which was prepared in this present paper and some relevant technical means were beneficial to some engineers or scientists who were engaged in printing and dyeing industry for the purpose of further wastewater treatment. In order to remove organic pollutants from dyes in the actual printing and dyeing wastewater, more improved methods should be implemented as follows: First, a high temperature electric furnace with the maximum sintering temperature of 1400 ◦ C should be purchased. Second, it is important that three-step sintering grinding method be used to synthetize purely single phase BiGdO3 . During the whole process of preparation of BiGdO3 , we should pay attention to the increase of heating speed, the control of the maximum sintering temperature and holding time, and the reduction of cooling rate for obtaining the photocatalyst BiGdO3 that has perfect crystallinity. Third, for the sake of economizing cost and maximizing the output, we could effectively deal with the actual printing and dyeing wastewater using BiGdO3 as a new photocatalyst under the solar light irradiation. Finally, if the concentration of the organic pollutants that exist within actual printing and dyeing wastewater is too high (exceeded 1 mmol/L), we could diluted the printing and dyeing wastewater with pure water to obtain a low concentration of the organic pollutants and then utilized the photocatalyst BiGdO3 and the solar light as advanced wastewater treatment technique to degrade the organic pollutants from dyes that exist within the actual printing and dyeing wastewater. All in all, the new photocatalyst BiGdO3 that was prepared in this present paper could be applied to the advanced wastewater treatment of low concentration of the organic pollutants that existed within the actual printing and dyeing wastewater. Acknowledgments: This work was supported by a grant from the Natural Science Foundation of Jiangsu Province (No. BK20141312), by a Project of Science and Technology Development Plan of Suzhou City of China from 2014 (No. ZXG201440), and by a grant from China-Israel Joint Research Program in Water Technology and Renewable Energy (No. 5). Author Contributions: Jingfei Luan was involved with all aspects of the study including conception, design, data interpretation and writing the manuscript. Yue Shen, Lingyan Zhang and Ningbin Guo performed the experiments and analyzed data. Jingfei Luan and Yue Shen wrote the paper. All authors read and approved the manuscript. Conflicts of Interest: The authors declare no conflict of interest.

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