Accepted Manuscript Boron-doped bismuth Oxybromide microspheres with enhanced surface hydroxyl groups: synthesis, characterization and dramatic photocatalytic activity ZhangSheng Liu, JinLong Liu, HaiYang Wang, Gang Cao, JiNan Niu PII: DOI: Reference:
S0021-9797(15)30273-3 http://dx.doi.org/10.1016/j.jcis.2015.10.028 YJCIS 20814
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
16 July 2015 11 October 2015 13 October 2015
Please cite this article as: Z. Liu, J. Liu, H. Wang, G. Cao, J. Niu, Boron-doped bismuth Oxybromide microspheres with enhanced surface hydroxyl groups: synthesis, characterization and dramatic photocatalytic activity, Journal of Colloid and Interface Science (2015), doi: http://dx.doi.org/10.1016/j.jcis.2015.10.028
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Boron-doped bismuth Oxybromide microspheres with enhanced surface hydroxyl groups: synthesis, characterization and dramatic photocatalytic activity ZhangSheng Liu ，JinLong Liu, HaiYang Wang, Gang Cao, JiNan Niu School of Material Science and Engineering, China University of Mining and Technology, XuZhou 221116, China Abstract B-doped BiOBr photocatalysts were successfully synthesized via a facile solvothermal method with boric acid used as boron source. As-obtained products consist of novel hierarchical microspheres, whose nanosheet building units were formed by nanoparticles splicing. They showed dramatic photocatalytic efficiency towards the degradation of Rhodamine B(RhB) and phenol under the visible-light irradiation and the highest activity was achieved by 0.075B-BiOBr. The enhanced photocatalytic activity could be attributed to the enriched surface hydroxyl groups on B-doped BiOBr samples, which not only improved the adsorption of pollutant on the photocatalyst but also promoted the separation of photogenerated electron-hole pairs. In addition, it was found that the main reactive species responsible for the degradation of organic pollutant were h+ and ∙O2 - radicals,
instead of ∙OH radicals.
Keywords: Solvothermal; BiOBr; Photocatalytic activity; Microstructure 1. Introduction Over the past decades, heterogeneous photocatalysis has attracted considerable attention in water treatment applications. It offers a way to eliminate organic
Corresponding author (Tel.:+86 516 83591979; fax:+86 51683591870. E-mail: [email protected]) 1
pollutants from the wastewater by using unlimited and green solar energy. TiO2 is the most widely investigated photocatalyst due to its high photoactivity, low cost, low toxicity and good chemical stability[1,2]. However, TiO2 only responses to solar ultraviolet irradiation because of wide band gap, and only less than 5% of the solar spectrum is exploited. Therefore, exploring new kinds of visible-light-driven photocatalysts has become one of the most hot research topics. So far, many visible light responsible photocatalysts, such as AgPO4[3], BiVO4[4,5], Bi12TiO20[6], Bi2 WO6[7,8], BiOBr[9,10] and g-C3N4[11], have been developed. Among them, BiOBr has been intensively investigated owing to its stability, suitable band gap and relatively superior photocatalytic ability[12,13]. As a p-type semiconductor, BiOBr belongs to a tetragonal PbFCl-type structure (space group P4/nmm; No. 129). It crystallizes in a layered structure, which consists of [Br-Bi-O-Bi-Br] sheets stacked together by van der Waals forces along the c-axis. The strong intralayer bonding and the weak interlayer van der Waals interaction lead to highly anisotropic electrical and optical properties, which reduce the recombination probability of the photogenerated electrons and holes [14,15]. Previously, BiOBr has exhibited considerable visible-light photocatalytic performance in the degradation of organic contaminants [16,17]. However, it is still far from efficient for practical applications and needs some further improvements. In general, doping is an effective way to enhance the photocatalytic performance. Doping BiOBr with metals, such as Al[18], Ag[19], Fe[20] and Ti[21], has been reported to improve the charge separation, reduce the band gap energy and thus improve the photocatalytic activity. However, to 2
the best of our knowledge, there have been limited reports on non-metal doped BiOBr so far [22]. Recently, elemental boron (B) has aroused wide interest because of its semiconducting property, and its use in photocatalysis (B-TiO2, B-Bi2 WO6 and B-BiVO4) has been examined [23-25]. B atoms can be located in the lattice oxygen positions to narrow the band gap due to the overlap of a p orbital on B with a p orbital on O [23], and they can also be located in interstitial positions to act as an electron trap, both of which favor the enhancement of the photocatalytic activity. In view of these, it is logical to anticipate that doping BiOBr with B will improve the photocatalytic performance of BiOBr. In this work, a series of B-doped BiOBr photocatalysts were prepared by a facile solvothermal method. The as-prepared samples were characterized by XRD, SEM, XPS, FT-IR, Raman, BET, DRS, PL and photocurrent measurement. The photocatalytic activity was evaluated by the photodegradation of Rhodamine B (RhB) and phenol under visible light irradiation. The results show that B-doped BiOBr samples exhibit higher photocatalytic performance than pure BiOBr. The mechanism of the enhanced photocatalytic activity is discussed in detail. 2. Experimental 2.1 Sample preparation All chemicals were used as received without further purification. The B-doped BiOBr photocatalysts were synthesized by a solvothermal method as follows: 0.97g of Bi(NO3)3.5H2O and 0.73g of cetyltriethylammonium bromide (CTAB) were orderly dissolved in 80 ml of ethylene glycol (EG), and then certain amount of boric acid 3
(HBO3) was added under magnetic stirring. After stirred for 30min, the mixture was transferred into a 100 ml Teflon-lined autoclave. The autoclave was sealed and heated at 160℃ for 16 h. After cooling down to room temperature naturally, the products were collected and repeatedly washed with absolute ethanol and distilled water, and then dried at 80℃. As-obtained samples were denoted as xB-BiOBr, where x represents the initial molar ratio of B to Bi (0, 0.025, 0.05, 0.075, and 0.1). 2.2 Characterization The crystal structures and phase compositions of the samples were examined by X-ray diffraction (XRD, Bruker D8 Advance). The morphology was observed using field emission scanning electron microscopy (FE-SEM, Hitachi S-4800) equipped with an energy dispersive X-ray spectrometer (EDS). The surface properties of the samples were examined by X-ray photoelectron spectroscopy (XPS:Thermo ESCALAB250, USA). FT-IR spectra were recorded by Vertex80 FTIR spectrometer. The Raman spectrum was obtained by a Bruker Senterra. Brunauer–Emmett–Teller (BET) surface area measurements were performed on a Quantachrome NOVA-4200E system. The optical property was analyzed by both UV–vis diffuse reflectance spectra (DRS, Varian Cary 300) and photoluminescence spectra (PL, Varian Cary-Eclipse 500). 2.3. Photocatalytic test The photocatalytic performance of the samples was evaluated by the degradation of RhB and phenol under visible light irradiation. 0.1 g of photocatalyst was added into 100 mL of 15 mg/L RhB/phenol aqueous solution. A 150W tungsten–halogen 4
lamp (Beijing Institute of Opto-Electronic Technology) with a 420 nm cutoff filter was acted as a visible light source. Before irradiation, the suspension was stirred for 10 min in the dark to reach the adsorption–desorption equilibrium. At given irradiation time intervals, about 3 ml suspensions were taken and centrifuged to remove the catalyst particles. The concentration of remnant RhB/phenol was evaluated by the UV–vis spectroscopy atλ=553/270 nm. 2.4 Photocurrent measurement Transient photocurrent measurements were performed on a CHI660B electro-chemical workstation with a standard three-electrode configuration. A platinum plate was used as counter electrode, and Ag/AgCl electrode (saturated KCl) as the reference electrode. For preparation of working electrodes, 2mg catalysts were suspended in 0.2 mL of ethanol and 0.2 mL of EG to produce slurry. 20μL of the suspension was dropped on 1.5×1.5 cm2 fluorine-tinoxide(FTO)glass. The obtained film electrode was dried under ambient conditions. The electrolyte was 0.5M Na2SO4 aqueous solution. A 300 W Xenon lamp was utilized as the visible light irradiation source. The measurements were carried out at room temperature. 3. Results and discussion 3.1 Characterization The phases and crystallinity of as-prepared samples was investigated by XRD. As shown in Fig.1a, the diffraction peaks of all the samples can match the standard tetragonal BiOBr phase (JCPDS 09-0393), and no impurity phase can be observed, indicating that B dopant does not change the phase structure. However, it is worth 5
Fig. 1 XRD patterns of as-prepared samples. noting that the positions of (012) and (110) peak shift gradually to a higher angle with the increase of the B content (Fig. 1b). According to Bragg’s law, the increase in the 2θ value means the decrease of the crystalline lattice parameters. In general, there are two doping modes for B dopant, i.e. interstitial mode and substitutional mode. The former will make the lattice parameters to increase, while the latter can cause an uncertain change in the lattice parameters, which depends on the differences of the ionic radius. Considered that the ionic radius of B3+ ions (23 pm) is much smaller than that of Bi3+ (96 pm), the shift of diffraction peaks may suggests that boron ions are doped into BiOBr lattice by substituting Bi3+ ions.
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Fig. 2 (a) Bi 4f, (b) Br3d, (c) O1s and (d) B1s XPS spectra of pure BiOBr and 0.075B-BiOBr sample The surface composition and chemical states of the elements were characterized by XPS. Fig. 2a–d shows high-resolution XPS spectra of the primary elements. In Fig.2a, two strong symmetrical peaks centered at 159.8 and 165.1 eV can be observed in pure BiOBr, which correspond to the Bi4f7/2 and Bi4f5/2 signals of the surface Bi3+ [26], respectively. Compared to pure BiOBr, there is little change in the binding energies of Bi4f spectra for 0.075B-BiOBr except for the broadened peaks, which may be related to similar Pauling electronegativity of B (2.04) and Bi (2.02). The Br3d spectra are displayed in Fig.2b. The binding energies of Br3d5/2 and Br3d3/2 of both BiOBr and 0.075B-BiOBr are 68.6 and 69.6 eV, respectively. Fig. 2c illustrates the O1s spectra of the samples. The O1s peak of BiOBr can be fitted by two peaks at 530.1 and 531.4 eV, which are related to the lattice oxygen (Ol) and hydroxyl oxygen (Oh), respectively[26]. However, the O1s spectra of 0.075B-BiOBr can be deconvoluted into three components in the case, which are Ol (529.9eV), Oh (531.2eV) and B–O (532.5eV) [27].The molar ratios of Oh/Ol in the pure BiOBr and B-doped BiOBr are evaluated as 0.15 and 1.27, respectively. Obviously, B-doped BiOBr 7
possesses much more surface hydroxyl groups than pure BiOBr, which is consistent with other B-doped systems [25]. Fig. 2d shows the B1s spectra of 0.075B-BiOBr sample. The peak centered at 189.1 eV is assigned to the B-O bond [28]. It may also suggests that B3+ ions is doped into BiOBr lattice in substitutional mode, since the binding energy of B1s locate between that of B1s in elemental boron (187.3eV) [27] and B1s in interstitial mode (192.6eV) [29]. In addition, there is no XPS characteristic peak of B2O3 at about 192.4eV [28], indicating the absence of B2O3 species in 0.075B-BiOBr.
Fig. 3 (a) FT-IR and (b) Raman spectra of pure BiOBr and 0.075B–BiOBr The FT-IR spectra of BiOBr and 0.075B-BiOBr sample are shown in Fig. 3a. The broad absorption peaks at about 3530cm-1 and 1630 cm-1 are indexed to the O-H stretching vibrations and the bending vibrations of free water molecules, respectively. Obviously, the two peaks in 0.075B–BiOBr sample are much stronger than those in BiOBr, suggesting more surface-adsorbed water and hydroxyl groups, which is consistent with the XPS results. The absorption peaks at 2870 and 2950 cm-1 are related to the remnant EG. The main absorption bands at 500-1200 cm−1 are attributed to stretching vibrations of Bi-O bonds [30]. Especially, the absorption peak at about 8
520 cm-1 is believed to be associated with the stretching vibration of the bonds in crystal tetragonal BiOX(X = Cl, Br and I) [31]. The absorption peak at around 1400cm−1 can be obviously found in 0.075B–BiOBr but not in pure BiOBr, which may be related to the B-O stretching vibration[24]. In addition, it is found that there is no characteristic absorption peaks of B2O3(about 1200 cm− 1) in 0.075B–BiOBr, suggesting that most of B3+ ions are doped into BiOBr lattice. Fig.3b displays the Raman spectra of pure BiOBr and 0.075B–BiOBr. Two peaks are distinguishable. The strong peak at 110 cm−1 can be assigned to the A1g mode of internal Bi-Br stretching, while the weak peak at about 155 cm−1 is attributed to the Eg mode of internal Bi-Br stretching [32]. Compared with BiOBr, no obvious shift of peaks is observed for 0.075B–BiOBr, suggesting that introduction of B3+ ions has little effect on the crystal structure, which is consistent with XRD results.
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Fig. 4 SEM images of the prepared samples: (a) pure BiOBr, (b) 0.025B–BiOBr, (c) 0.05B–BiOBr, (d) 0.075B–BiOBr and (e) 0.1B–BiOBr
The morphologies and microstructures of pure BiOBr and B-doped BiOBr samples were investigated by SEM and the results are shown in Fig.4a-e. Pure BiOBr consists of a lot of monodispersed microspheres with uniform size of ~3.5μm, which are further assembled by nanosheets. The thickness of nanosheet is about 20 nm. After different amounts of B were introduced, the obtained samples are still composed of microspheres assembled by nanosheets (Fig. 4b-e). However, when the molar ratio of B to Bi is larger than 0.05, the nanosheet building units exhibit a more elegant and finer structure. They are found to be formed by nanoparticle splicing, and the nanoparticles are about 30nm in average diameter. Such a microstructure is firstly reported, whose specific reason is not clear at this moment. However, it seems to indicate that the formation of nanosheet building units involves the oriented attachment process. The element composition of 0.075B–BiOBr was investigated by EDS. The sample is composed of Bi, O, Br and B, and the atomic ratio of B/Bi is close to 0.072, which basically agrees with the theoretical value.
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Fig. 5 N2 adsorption/desorption isotherm and pore size distribution (inset) of BiOBr and 0.075B–BiOBr BET measurements were conducted to detect the specific surface area and porosity of as-prepared samples. The surface areas of BiOBr, 0.025B–BiOBr, 0.05B–BiOBr, 0.075B–BiOBr and 0.1B–BiOBr are 8.9, 7.4, 7.9, 8.6 and 8.1m2/g, respectively. It seems that there is no obvious change in the surface area with the introduction of B dopant. Typical N2 adsorption/desorption isotherms are shown in Fig.5. Both the isotherms can be assigned as type IV according to the IUPAC classification, implying the mesoporous structure. The corresponding pore-size distribution is shown in the inset in Fig.5. Both samples show a bimodal pore size distribution. The large pores are centered at bout 8nm, which are attributed to the interspaces between the nanosheet building units, while the small pores may be related to the nanosheet building unit itself. Compared with pure BiOBr, the small pores for 0.075B–BiOBr are centered at less size, which corresponds to finer structure of the nanosheet building unit (Fig.4d). 3.2. Optical property Fig. 6a shows the UV-vis DRS of the obtained samples. It can be seen that although B-doped samples possess stronger absorption in the UV regions than pure 11
Fig.6 (a) UV-vis DRS spectra and (b) plots of (αhν)1/2 versus hν of as-prepared samples BiOBr, they exhibit same absorption curves in the visible region with an absorption edge of ~440nm, indicating little effect of B dopant on the visible-light response. The band gap energies can be evaluated from the UV-vis DRS by using the Kubelka–Munk function [13]. In Fig.6b, the extrapolated value (the straight line to the x-axis) of hν at α= 0 presents an absorption edge energy, which corresponds to the band gap energy. It is determined to be about 2.66eV for all the samples. Different from metal doped BiOBr, B dopant has no effect on the band gap, which is consistent with the previous reports [23,24].
Fig.7 (a) The temporal evolution of the absorption spectra of the RhB solution over 0.075B-BiOBr, (b) Photocatalytic degradation of RhB over different samples 12
The photocatalytic decomposition of RhB over as-obtained samples was investigated under visible light irradication. Fig.7a displays the temporal evolution of characteristic absorption spectra of RhB solution over 0.075B-BiOBr sample. It can be seen that the absorption peak intensity of RhB decreases gradually with the increase of irradiation time and tends to disappear after 30 min, accompanied by a hypsochromic shift from 553nm to 498nm. The hypsochromic shift suggests that the degradation of RhB undergoes a series of deethylation processes [33]. Based on experimental data, it is believed that RhB is deethylated into N,N,N¢-triethylated rhodamine(538nm),
N,N¢-diethylated
rhodamine(521nm),
N-ethylated
rhodamine(507nm) and rhodamine(498nm) in a stepwise manner during the degradation process, and then degraded through destruction of the conjugated structure. Fig. 7b shows the detailed photodegradation rate of RhB over different samples under visible light irradiation. After 30min of visible light irradiation, the photodegradation rate of RhB over BiOBr is 71.1%. After doping B, the photodegradation rates over 0.025B–BiOBr, 0.05B–BiOBr, 0.075B–BiOBr and 0.1B–BiOBr are 75.1%, 93.2%, 99.3% and 89.2%, respectively. Obviously, B-doped samples display higher photodegradation efficiency than pure BiOBr.
Fig.8 Photocatalytic degradation of phenol over as-prepared samples 13
Considering that the degradation of RhB often involves photosensitization, the photocatalytic activity was also evaluated by the degradation of phenol. Phenol is a colorless organic compound, which does not respond to the visible light. Therefore, its degradation can be attributed to photocatalysis instead of photosensitization. As shown in Fig. 8, B-doped BiOBr samples display higher activity than pure BiOBr in degradating phenol. It is found that the B content plays a crucial role in determining photoreactivity. Following the universal doping principle, excessive B may also act as the recombination centers of photogenerated electron-hole pairs, which is unfavorable to the photocatalytic activity. Therefore, 0.075B–BiOBr sample offers the highest activity, whose degradation rate of phenol is 78.3% after 120 min of visible light irradiation.
Fig.9 (a) Cycling degradation rate of RhB over 0.075B-BiOBr and (b) XRD pattern before and after recycling experiment The stability of photocatalyst is very important for practical applications. To investigate the stability of B-doped BiOBr, the recycling experiment for the photodegradation of RhB were carried out over the 0.075B-BiOBr sample. As shown in Fig.9a, after four consecutive cycles, the final photodegradation rate of RhB changes from 99.9% to 95.9%, indicating little loss of photocatalytic activity. The 14
recycled sample was re-examined by XRD, and the results are shown in Fig.9b. No detectable difference can be observed. Therefore, it can be concluded that B-BiOBr samples possess excellent stability. 3.3 Photocatalytic mechanisms
Fig.10 (a) Photoluminescence (PL) spectra and (b) transient photocurrent responses of as-prepared samples The above experimental results show that B-doped BiOBr samples possess the enhanced photocatalytic performance compared with pristine BiOBr. Based on the material characterization, the biggest difference between pure BiOBr and B-doped samples lies in the number of surface hydroxyl group. B-doped samples have more surface hydroxyl groups than BiOBr. Therefore, the enriched surface hydroxyl groups may be the main factor responsible for the enhanced photocatalytic activity. In fact, it has been well reported that the activity can be improved by surface hydroxyl groups [34,35]. On the one hand, the hydroxyl groups can result in effective trapping of photogenerated electrons by improving surface-adsorption of dissolved oxygen [36], and thus restrain the recombination of photogenerated electron-hole pair. The technique of photoluminescence (PL) spectra is an impactful technique to investigate 15
separation efficiency of photoinduced charge carriers. Fig.10a shows the PL spectra of as-prepared samples. It can be seen that the PL intensities of B-doped samples are obviously lower than that of pure BiOBr, suggesting the suppression of recombination of photo-induced electron–hole pairs. 0.075B-BiOBr displays the lowest emission peak, meaning the highest separation efficiency. The high separation efficiency for B-doped samples can be further confirmed by transient photocurrent measurement. Different from the PL spectra, which result from the recombination process of photogenerated carriers, photocurrent is directly related to the separation of photogenerated carriers. As shown in Fig.10b, the photocurrent density obtained over 0.075B-BiOBr is obviously enhanced comparing with that of pure BiOBr, indicating a more effective separation of photogenerated electron–hole pairs and faster interfacial charge transfer, which has to contribute to the enhanced photocatalytic activity. On the other hand, the hydroxyl layer can work as active sites to increase pollutant adsorption [37]. Take RhB molecules as an example, they can be strongly adsorbed on the hydroxylated surfaces via the diethyl amino group (–NHR2), and similar results are also reported in RhB/TiO2[38]. Therefore, although B-doped BiOBr samples have lower surface area than pure BiOBr, they show higher adsorbability toward RhB (Fig.7b), which has to contribute to the enhanced photocatalytic activity.
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Fig.11 Photodegradation of RhB over 0.075B-BiOBr in the presence of different scavengers To understand the photocatalytic process over B-doped BiOBr samples, the influence of various scavengers on the decomposition of RhB is investigated. Isopropanol (IPA) used as an ·OH scavenger, triethanolamine (TEOA) as the scavenger of h+, benzoquinone (BQ) as an ·O2- scavenger were respectively added to the photocatalytic reaction system, and the results are shown in Fig. 11. It can be seen that the photodegradation of RhB was greatly depressed with addition of BQ, and also inhibited after the addition of TEOA. The participation of IPA had nearly no inflence on the photocatalytic activity of RhB over 0.075B-BiOBr catalyst. It suggests that h+ and ·O2-, instead of ·OH, play the dominant role in the degradation of RhB over B-doped BiOBr catalysts under visible light irradiation. 4. Conclusion In summary, we have prepared B-doped BiOBr photocatalysts via a facile one-pot solvothermal route. The experimental results reveal that B dopant is doped into BiOBr lattice in substitutional mode. The obtained B-doped samples exhibit novel 17
hierarchical microspheric architectures, whose nanosheet building units are formed by nanoparticles splicing. Furthermore, these samples display enhanced surface hydroxyl groups with the introduction of B, as identified by FT-IR and XPS analysis. More importantly, all B-doped samples exhibit obviously enhanced photocatalytic performance for the degradation of RhB and phenol compared with pure BiOBr, and 0.075B-BiOBr possesses the maximum of the activity, indicating the optimum B content. It is demonstrated that the surface hydroxyl groups play crucial roles in the enhancement of the photocatalytic activity, which not only increase the adsorption of dye molecules on the photocatalyst, but also improve the separation of photogenerated electron-hole pairs. In addition, radical trapping experiments show that h+ and ∙O 2radicals are the main reactive species during the photocatalytic degradation. Inspired by the dramatic photocatalytic performance and novel architectures, the present B-doped strategy is expected to be extended to synthesize other bismuth-containing photocatalysts. Acknowledgements This work was supported by the Fundamental Research Funds for the Central Universities (2014QNA11). We would like to thank for the support of Advanced Analysis and Calculation Center of CUMT. Reference [1] S.J Bao, C Lei, M.W Xu, C.J Cai, C.J Cheng, C.M Li Environmentally-friendly biomimicking synthesis of TiO2 nanomaterials using saccharides to tailor morphology, crystal phase and photocatalytic activity, CrystEngComm, 15(2013)4694–4699 18
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Graphical Abstract
S0021-9797(15)30273-3 http://dx.doi.org/10.1016/j.jcis.2015.10.028 YJCIS 20814
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
16 July 2015 11 October 2015 13 October 2015
Please cite this article as: Z. Liu, J. Liu, H. Wang, G. Cao, J. Niu, Boron-doped bismuth Oxybromide microspheres with enhanced surface hydroxyl groups: synthesis, characterization and dramatic photocatalytic activity, Journal of Colloid and Interface Science (2015), doi: http://dx.doi.org/10.1016/j.jcis.2015.10.028
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Boron-doped bismuth Oxybromide microspheres with enhanced surface hydroxyl groups: synthesis, characterization and dramatic photocatalytic activity ZhangSheng Liu ，JinLong Liu, HaiYang Wang, Gang Cao, JiNan Niu School of Material Science and Engineering, China University of Mining and Technology, XuZhou 221116, China Abstract B-doped BiOBr photocatalysts were successfully synthesized via a facile solvothermal method with boric acid used as boron source. As-obtained products consist of novel hierarchical microspheres, whose nanosheet building units were formed by nanoparticles splicing. They showed dramatic photocatalytic efficiency towards the degradation of Rhodamine B(RhB) and phenol under the visible-light irradiation and the highest activity was achieved by 0.075B-BiOBr. The enhanced photocatalytic activity could be attributed to the enriched surface hydroxyl groups on B-doped BiOBr samples, which not only improved the adsorption of pollutant on the photocatalyst but also promoted the separation of photogenerated electron-hole pairs. In addition, it was found that the main reactive species responsible for the degradation of organic pollutant were h+ and ∙O2 - radicals,
instead of ∙OH radicals.
Keywords: Solvothermal; BiOBr; Photocatalytic activity; Microstructure 1. Introduction Over the past decades, heterogeneous photocatalysis has attracted considerable attention in water treatment applications. It offers a way to eliminate organic
Corresponding author (Tel.:+86 516 83591979; fax:+86 51683591870. E-mail: [email protected]) 1
pollutants from the wastewater by using unlimited and green solar energy. TiO2 is the most widely investigated photocatalyst due to its high photoactivity, low cost, low toxicity and good chemical stability[1,2]. However, TiO2 only responses to solar ultraviolet irradiation because of wide band gap, and only less than 5% of the solar spectrum is exploited. Therefore, exploring new kinds of visible-light-driven photocatalysts has become one of the most hot research topics. So far, many visible light responsible photocatalysts, such as AgPO4[3], BiVO4[4,5], Bi12TiO20[6], Bi2 WO6[7,8], BiOBr[9,10] and g-C3N4[11], have been developed. Among them, BiOBr has been intensively investigated owing to its stability, suitable band gap and relatively superior photocatalytic ability[12,13]. As a p-type semiconductor, BiOBr belongs to a tetragonal PbFCl-type structure (space group P4/nmm; No. 129). It crystallizes in a layered structure, which consists of [Br-Bi-O-Bi-Br] sheets stacked together by van der Waals forces along the c-axis. The strong intralayer bonding and the weak interlayer van der Waals interaction lead to highly anisotropic electrical and optical properties, which reduce the recombination probability of the photogenerated electrons and holes [14,15]. Previously, BiOBr has exhibited considerable visible-light photocatalytic performance in the degradation of organic contaminants [16,17]. However, it is still far from efficient for practical applications and needs some further improvements. In general, doping is an effective way to enhance the photocatalytic performance. Doping BiOBr with metals, such as Al[18], Ag[19], Fe[20] and Ti[21], has been reported to improve the charge separation, reduce the band gap energy and thus improve the photocatalytic activity. However, to 2
the best of our knowledge, there have been limited reports on non-metal doped BiOBr so far [22]. Recently, elemental boron (B) has aroused wide interest because of its semiconducting property, and its use in photocatalysis (B-TiO2, B-Bi2 WO6 and B-BiVO4) has been examined [23-25]. B atoms can be located in the lattice oxygen positions to narrow the band gap due to the overlap of a p orbital on B with a p orbital on O [23], and they can also be located in interstitial positions to act as an electron trap, both of which favor the enhancement of the photocatalytic activity. In view of these, it is logical to anticipate that doping BiOBr with B will improve the photocatalytic performance of BiOBr. In this work, a series of B-doped BiOBr photocatalysts were prepared by a facile solvothermal method. The as-prepared samples were characterized by XRD, SEM, XPS, FT-IR, Raman, BET, DRS, PL and photocurrent measurement. The photocatalytic activity was evaluated by the photodegradation of Rhodamine B (RhB) and phenol under visible light irradiation. The results show that B-doped BiOBr samples exhibit higher photocatalytic performance than pure BiOBr. The mechanism of the enhanced photocatalytic activity is discussed in detail. 2. Experimental 2.1 Sample preparation All chemicals were used as received without further purification. The B-doped BiOBr photocatalysts were synthesized by a solvothermal method as follows: 0.97g of Bi(NO3)3.5H2O and 0.73g of cetyltriethylammonium bromide (CTAB) were orderly dissolved in 80 ml of ethylene glycol (EG), and then certain amount of boric acid 3
(HBO3) was added under magnetic stirring. After stirred for 30min, the mixture was transferred into a 100 ml Teflon-lined autoclave. The autoclave was sealed and heated at 160℃ for 16 h. After cooling down to room temperature naturally, the products were collected and repeatedly washed with absolute ethanol and distilled water, and then dried at 80℃. As-obtained samples were denoted as xB-BiOBr, where x represents the initial molar ratio of B to Bi (0, 0.025, 0.05, 0.075, and 0.1). 2.2 Characterization The crystal structures and phase compositions of the samples were examined by X-ray diffraction (XRD, Bruker D8 Advance). The morphology was observed using field emission scanning electron microscopy (FE-SEM, Hitachi S-4800) equipped with an energy dispersive X-ray spectrometer (EDS). The surface properties of the samples were examined by X-ray photoelectron spectroscopy (XPS:Thermo ESCALAB250, USA). FT-IR spectra were recorded by Vertex80 FTIR spectrometer. The Raman spectrum was obtained by a Bruker Senterra. Brunauer–Emmett–Teller (BET) surface area measurements were performed on a Quantachrome NOVA-4200E system. The optical property was analyzed by both UV–vis diffuse reflectance spectra (DRS, Varian Cary 300) and photoluminescence spectra (PL, Varian Cary-Eclipse 500). 2.3. Photocatalytic test The photocatalytic performance of the samples was evaluated by the degradation of RhB and phenol under visible light irradiation. 0.1 g of photocatalyst was added into 100 mL of 15 mg/L RhB/phenol aqueous solution. A 150W tungsten–halogen 4
lamp (Beijing Institute of Opto-Electronic Technology) with a 420 nm cutoff filter was acted as a visible light source. Before irradiation, the suspension was stirred for 10 min in the dark to reach the adsorption–desorption equilibrium. At given irradiation time intervals, about 3 ml suspensions were taken and centrifuged to remove the catalyst particles. The concentration of remnant RhB/phenol was evaluated by the UV–vis spectroscopy atλ=553/270 nm. 2.4 Photocurrent measurement Transient photocurrent measurements were performed on a CHI660B electro-chemical workstation with a standard three-electrode configuration. A platinum plate was used as counter electrode, and Ag/AgCl electrode (saturated KCl) as the reference electrode. For preparation of working electrodes, 2mg catalysts were suspended in 0.2 mL of ethanol and 0.2 mL of EG to produce slurry. 20μL of the suspension was dropped on 1.5×1.5 cm2 fluorine-tinoxide(FTO)glass. The obtained film electrode was dried under ambient conditions. The electrolyte was 0.5M Na2SO4 aqueous solution. A 300 W Xenon lamp was utilized as the visible light irradiation source. The measurements were carried out at room temperature. 3. Results and discussion 3.1 Characterization The phases and crystallinity of as-prepared samples was investigated by XRD. As shown in Fig.1a, the diffraction peaks of all the samples can match the standard tetragonal BiOBr phase (JCPDS 09-0393), and no impurity phase can be observed, indicating that B dopant does not change the phase structure. However, it is worth 5
Fig. 1 XRD patterns of as-prepared samples. noting that the positions of (012) and (110) peak shift gradually to a higher angle with the increase of the B content (Fig. 1b). According to Bragg’s law, the increase in the 2θ value means the decrease of the crystalline lattice parameters. In general, there are two doping modes for B dopant, i.e. interstitial mode and substitutional mode. The former will make the lattice parameters to increase, while the latter can cause an uncertain change in the lattice parameters, which depends on the differences of the ionic radius. Considered that the ionic radius of B3+ ions (23 pm) is much smaller than that of Bi3+ (96 pm), the shift of diffraction peaks may suggests that boron ions are doped into BiOBr lattice by substituting Bi3+ ions.
6
Fig. 2 (a) Bi 4f, (b) Br3d, (c) O1s and (d) B1s XPS spectra of pure BiOBr and 0.075B-BiOBr sample The surface composition and chemical states of the elements were characterized by XPS. Fig. 2a–d shows high-resolution XPS spectra of the primary elements. In Fig.2a, two strong symmetrical peaks centered at 159.8 and 165.1 eV can be observed in pure BiOBr, which correspond to the Bi4f7/2 and Bi4f5/2 signals of the surface Bi3+ [26], respectively. Compared to pure BiOBr, there is little change in the binding energies of Bi4f spectra for 0.075B-BiOBr except for the broadened peaks, which may be related to similar Pauling electronegativity of B (2.04) and Bi (2.02). The Br3d spectra are displayed in Fig.2b. The binding energies of Br3d5/2 and Br3d3/2 of both BiOBr and 0.075B-BiOBr are 68.6 and 69.6 eV, respectively. Fig. 2c illustrates the O1s spectra of the samples. The O1s peak of BiOBr can be fitted by two peaks at 530.1 and 531.4 eV, which are related to the lattice oxygen (Ol) and hydroxyl oxygen (Oh), respectively[26]. However, the O1s spectra of 0.075B-BiOBr can be deconvoluted into three components in the case, which are Ol (529.9eV), Oh (531.2eV) and B–O (532.5eV) [27].The molar ratios of Oh/Ol in the pure BiOBr and B-doped BiOBr are evaluated as 0.15 and 1.27, respectively. Obviously, B-doped BiOBr 7
possesses much more surface hydroxyl groups than pure BiOBr, which is consistent with other B-doped systems [25]. Fig. 2d shows the B1s spectra of 0.075B-BiOBr sample. The peak centered at 189.1 eV is assigned to the B-O bond [28]. It may also suggests that B3+ ions is doped into BiOBr lattice in substitutional mode, since the binding energy of B1s locate between that of B1s in elemental boron (187.3eV) [27] and B1s in interstitial mode (192.6eV) [29]. In addition, there is no XPS characteristic peak of B2O3 at about 192.4eV [28], indicating the absence of B2O3 species in 0.075B-BiOBr.
Fig. 3 (a) FT-IR and (b) Raman spectra of pure BiOBr and 0.075B–BiOBr The FT-IR spectra of BiOBr and 0.075B-BiOBr sample are shown in Fig. 3a. The broad absorption peaks at about 3530cm-1 and 1630 cm-1 are indexed to the O-H stretching vibrations and the bending vibrations of free water molecules, respectively. Obviously, the two peaks in 0.075B–BiOBr sample are much stronger than those in BiOBr, suggesting more surface-adsorbed water and hydroxyl groups, which is consistent with the XPS results. The absorption peaks at 2870 and 2950 cm-1 are related to the remnant EG. The main absorption bands at 500-1200 cm−1 are attributed to stretching vibrations of Bi-O bonds [30]. Especially, the absorption peak at about 8
520 cm-1 is believed to be associated with the stretching vibration of the bonds in crystal tetragonal BiOX(X = Cl, Br and I) [31]. The absorption peak at around 1400cm−1 can be obviously found in 0.075B–BiOBr but not in pure BiOBr, which may be related to the B-O stretching vibration[24]. In addition, it is found that there is no characteristic absorption peaks of B2O3(about 1200 cm− 1) in 0.075B–BiOBr, suggesting that most of B3+ ions are doped into BiOBr lattice. Fig.3b displays the Raman spectra of pure BiOBr and 0.075B–BiOBr. Two peaks are distinguishable. The strong peak at 110 cm−1 can be assigned to the A1g mode of internal Bi-Br stretching, while the weak peak at about 155 cm−1 is attributed to the Eg mode of internal Bi-Br stretching [32]. Compared with BiOBr, no obvious shift of peaks is observed for 0.075B–BiOBr, suggesting that introduction of B3+ ions has little effect on the crystal structure, which is consistent with XRD results.
9
Fig. 4 SEM images of the prepared samples: (a) pure BiOBr, (b) 0.025B–BiOBr, (c) 0.05B–BiOBr, (d) 0.075B–BiOBr and (e) 0.1B–BiOBr
The morphologies and microstructures of pure BiOBr and B-doped BiOBr samples were investigated by SEM and the results are shown in Fig.4a-e. Pure BiOBr consists of a lot of monodispersed microspheres with uniform size of ~3.5μm, which are further assembled by nanosheets. The thickness of nanosheet is about 20 nm. After different amounts of B were introduced, the obtained samples are still composed of microspheres assembled by nanosheets (Fig. 4b-e). However, when the molar ratio of B to Bi is larger than 0.05, the nanosheet building units exhibit a more elegant and finer structure. They are found to be formed by nanoparticle splicing, and the nanoparticles are about 30nm in average diameter. Such a microstructure is firstly reported, whose specific reason is not clear at this moment. However, it seems to indicate that the formation of nanosheet building units involves the oriented attachment process. The element composition of 0.075B–BiOBr was investigated by EDS. The sample is composed of Bi, O, Br and B, and the atomic ratio of B/Bi is close to 0.072, which basically agrees with the theoretical value.
10
Fig. 5 N2 adsorption/desorption isotherm and pore size distribution (inset) of BiOBr and 0.075B–BiOBr BET measurements were conducted to detect the specific surface area and porosity of as-prepared samples. The surface areas of BiOBr, 0.025B–BiOBr, 0.05B–BiOBr, 0.075B–BiOBr and 0.1B–BiOBr are 8.9, 7.4, 7.9, 8.6 and 8.1m2/g, respectively. It seems that there is no obvious change in the surface area with the introduction of B dopant. Typical N2 adsorption/desorption isotherms are shown in Fig.5. Both the isotherms can be assigned as type IV according to the IUPAC classification, implying the mesoporous structure. The corresponding pore-size distribution is shown in the inset in Fig.5. Both samples show a bimodal pore size distribution. The large pores are centered at bout 8nm, which are attributed to the interspaces between the nanosheet building units, while the small pores may be related to the nanosheet building unit itself. Compared with pure BiOBr, the small pores for 0.075B–BiOBr are centered at less size, which corresponds to finer structure of the nanosheet building unit (Fig.4d). 3.2. Optical property Fig. 6a shows the UV-vis DRS of the obtained samples. It can be seen that although B-doped samples possess stronger absorption in the UV regions than pure 11
Fig.6 (a) UV-vis DRS spectra and (b) plots of (αhν)1/2 versus hν of as-prepared samples BiOBr, they exhibit same absorption curves in the visible region with an absorption edge of ~440nm, indicating little effect of B dopant on the visible-light response. The band gap energies can be evaluated from the UV-vis DRS by using the Kubelka–Munk function [13]. In Fig.6b, the extrapolated value (the straight line to the x-axis) of hν at α= 0 presents an absorption edge energy, which corresponds to the band gap energy. It is determined to be about 2.66eV for all the samples. Different from metal doped BiOBr, B dopant has no effect on the band gap, which is consistent with the previous reports [23,24].
Fig.7 (a) The temporal evolution of the absorption spectra of the RhB solution over 0.075B-BiOBr, (b) Photocatalytic degradation of RhB over different samples 12
The photocatalytic decomposition of RhB over as-obtained samples was investigated under visible light irradication. Fig.7a displays the temporal evolution of characteristic absorption spectra of RhB solution over 0.075B-BiOBr sample. It can be seen that the absorption peak intensity of RhB decreases gradually with the increase of irradiation time and tends to disappear after 30 min, accompanied by a hypsochromic shift from 553nm to 498nm. The hypsochromic shift suggests that the degradation of RhB undergoes a series of deethylation processes [33]. Based on experimental data, it is believed that RhB is deethylated into N,N,N¢-triethylated rhodamine(538nm),
N,N¢-diethylated
rhodamine(521nm),
N-ethylated
rhodamine(507nm) and rhodamine(498nm) in a stepwise manner during the degradation process, and then degraded through destruction of the conjugated structure. Fig. 7b shows the detailed photodegradation rate of RhB over different samples under visible light irradiation. After 30min of visible light irradiation, the photodegradation rate of RhB over BiOBr is 71.1%. After doping B, the photodegradation rates over 0.025B–BiOBr, 0.05B–BiOBr, 0.075B–BiOBr and 0.1B–BiOBr are 75.1%, 93.2%, 99.3% and 89.2%, respectively. Obviously, B-doped samples display higher photodegradation efficiency than pure BiOBr.
Fig.8 Photocatalytic degradation of phenol over as-prepared samples 13
Considering that the degradation of RhB often involves photosensitization, the photocatalytic activity was also evaluated by the degradation of phenol. Phenol is a colorless organic compound, which does not respond to the visible light. Therefore, its degradation can be attributed to photocatalysis instead of photosensitization. As shown in Fig. 8, B-doped BiOBr samples display higher activity than pure BiOBr in degradating phenol. It is found that the B content plays a crucial role in determining photoreactivity. Following the universal doping principle, excessive B may also act as the recombination centers of photogenerated electron-hole pairs, which is unfavorable to the photocatalytic activity. Therefore, 0.075B–BiOBr sample offers the highest activity, whose degradation rate of phenol is 78.3% after 120 min of visible light irradiation.
Fig.9 (a) Cycling degradation rate of RhB over 0.075B-BiOBr and (b) XRD pattern before and after recycling experiment The stability of photocatalyst is very important for practical applications. To investigate the stability of B-doped BiOBr, the recycling experiment for the photodegradation of RhB were carried out over the 0.075B-BiOBr sample. As shown in Fig.9a, after four consecutive cycles, the final photodegradation rate of RhB changes from 99.9% to 95.9%, indicating little loss of photocatalytic activity. The 14
recycled sample was re-examined by XRD, and the results are shown in Fig.9b. No detectable difference can be observed. Therefore, it can be concluded that B-BiOBr samples possess excellent stability. 3.3 Photocatalytic mechanisms
Fig.10 (a) Photoluminescence (PL) spectra and (b) transient photocurrent responses of as-prepared samples The above experimental results show that B-doped BiOBr samples possess the enhanced photocatalytic performance compared with pristine BiOBr. Based on the material characterization, the biggest difference between pure BiOBr and B-doped samples lies in the number of surface hydroxyl group. B-doped samples have more surface hydroxyl groups than BiOBr. Therefore, the enriched surface hydroxyl groups may be the main factor responsible for the enhanced photocatalytic activity. In fact, it has been well reported that the activity can be improved by surface hydroxyl groups [34,35]. On the one hand, the hydroxyl groups can result in effective trapping of photogenerated electrons by improving surface-adsorption of dissolved oxygen [36], and thus restrain the recombination of photogenerated electron-hole pair. The technique of photoluminescence (PL) spectra is an impactful technique to investigate 15
separation efficiency of photoinduced charge carriers. Fig.10a shows the PL spectra of as-prepared samples. It can be seen that the PL intensities of B-doped samples are obviously lower than that of pure BiOBr, suggesting the suppression of recombination of photo-induced electron–hole pairs. 0.075B-BiOBr displays the lowest emission peak, meaning the highest separation efficiency. The high separation efficiency for B-doped samples can be further confirmed by transient photocurrent measurement. Different from the PL spectra, which result from the recombination process of photogenerated carriers, photocurrent is directly related to the separation of photogenerated carriers. As shown in Fig.10b, the photocurrent density obtained over 0.075B-BiOBr is obviously enhanced comparing with that of pure BiOBr, indicating a more effective separation of photogenerated electron–hole pairs and faster interfacial charge transfer, which has to contribute to the enhanced photocatalytic activity. On the other hand, the hydroxyl layer can work as active sites to increase pollutant adsorption [37]. Take RhB molecules as an example, they can be strongly adsorbed on the hydroxylated surfaces via the diethyl amino group (–NHR2), and similar results are also reported in RhB/TiO2[38]. Therefore, although B-doped BiOBr samples have lower surface area than pure BiOBr, they show higher adsorbability toward RhB (Fig.7b), which has to contribute to the enhanced photocatalytic activity.
16
Fig.11 Photodegradation of RhB over 0.075B-BiOBr in the presence of different scavengers To understand the photocatalytic process over B-doped BiOBr samples, the influence of various scavengers on the decomposition of RhB is investigated. Isopropanol (IPA) used as an ·OH scavenger, triethanolamine (TEOA) as the scavenger of h+, benzoquinone (BQ) as an ·O2- scavenger were respectively added to the photocatalytic reaction system, and the results are shown in Fig. 11. It can be seen that the photodegradation of RhB was greatly depressed with addition of BQ, and also inhibited after the addition of TEOA. The participation of IPA had nearly no inflence on the photocatalytic activity of RhB over 0.075B-BiOBr catalyst. It suggests that h+ and ·O2-, instead of ·OH, play the dominant role in the degradation of RhB over B-doped BiOBr catalysts under visible light irradiation. 4. Conclusion In summary, we have prepared B-doped BiOBr photocatalysts via a facile one-pot solvothermal route. The experimental results reveal that B dopant is doped into BiOBr lattice in substitutional mode. The obtained B-doped samples exhibit novel 17
hierarchical microspheric architectures, whose nanosheet building units are formed by nanoparticles splicing. Furthermore, these samples display enhanced surface hydroxyl groups with the introduction of B, as identified by FT-IR and XPS analysis. More importantly, all B-doped samples exhibit obviously enhanced photocatalytic performance for the degradation of RhB and phenol compared with pure BiOBr, and 0.075B-BiOBr possesses the maximum of the activity, indicating the optimum B content. It is demonstrated that the surface hydroxyl groups play crucial roles in the enhancement of the photocatalytic activity, which not only increase the adsorption of dye molecules on the photocatalyst, but also improve the separation of photogenerated electron-hole pairs. In addition, radical trapping experiments show that h+ and ∙O 2radicals are the main reactive species during the photocatalytic degradation. Inspired by the dramatic photocatalytic performance and novel architectures, the present B-doped strategy is expected to be extended to synthesize other bismuth-containing photocatalysts. Acknowledgements This work was supported by the Fundamental Research Funds for the Central Universities (2014QNA11). We would like to thank for the support of Advanced Analysis and Calculation Center of CUMT. Reference [1] S.J Bao, C Lei, M.W Xu, C.J Cai, C.J Cheng, C.M Li Environmentally-friendly biomimicking synthesis of TiO2 nanomaterials using saccharides to tailor morphology, crystal phase and photocatalytic activity, CrystEngComm, 15(2013)4694–4699 18
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