nanomaterials Article

Facile Synthesis of g-C3N4 Nanosheets/ZnO Nanocomposites with Enhanced Photocatalytic Activity in Reduction of Aqueous Chromium(VI) under Visible Light Xiaoya Yuan 1, *, Chao Zhou 1 , Qiuye Jing 1 , Qi Tang 1 , Yuanhua Mu 1 and An-ke Du 2, * 1 2

*

College of Materials Science and Engineering, Chongqing Jiaotong University, Chongqing 400074, China; [email protected] (C.Z.); [email protected] (Q.J.); [email protected] (Q.T.); [email protected] (Y.M.) Chongqing Academy of Science and Technology, Chongqing 401123, China Correspondence: [email protected] (X.Y.); [email protected] (A.-k.D.); Tel.: +86-23-6278-9154 (X.Y.); +86-23-6730-0072 (A.-k.D.)

Academic Editors: Hongqi Sun and Zhaohui Wang Received: 21 June 2016; Accepted: 2 August 2016; Published: 14 September 2016

Abstract: Graphitic-C3 N4 nanosheets (CN)/ZnO photocatalysts (CN/ZnO) with different CN loadings were successfully prepared via a simple precipitation-calcination in the presence of exfoliated C3 N4 nanosheets. Their morphology and structure were thoroughly characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), UV-Vis diffuse reflectance spectroscopy (DRS) and photoluminescence spectra (PL). The results showed that hexagonal wurzite-phase ZnO nanoparticles were randomly distributed onto the CN nanosheets with a well-bonded interface between the two components in the CN/ZnO composites. The performance of the photocatalytic Cr(VI) reduction indicated that CN/ZnO exhibited better photocatalytic activity than pure ZnO under visible-light irradiation and the photocatalyst composite with a lower loading of CN sheets eventually displayed higher activity. The enhanced performance of CN/ZnO photocatalysts could be ascribed to the increased absorption of the visible light and the effective transfer and separation of the photogenerated charge carriers. Keywords: photoreduction; aqueous Chromium(VI); graphitic C3 N4 nanosheets; ZnO; visible light

1. Introduction Hexavalent chromium (Cr(VI)) is the most toxic of the chromium species and has been listed as one of the priority pollutants by the US Environmental Protection Agency (EPA) due to its notoriously toxic, mutagenic and carcinogenic activity to human beings [1,2]. Until now, a number of techniques have been developed for Cr(VI) elimination from waste water, such as chemical precipitation, reverse osmosis, ion exchange, foam flotation, electrolysis, adsorption, and photocatalytic reduction [3,4]. Among them, photocatalytic reduction is an efficient, active, and clean technology for Cr(VI) removal because of its acceptable cost and easy operation as well as the inexhaustible solar energy [5–7]. Among the oxide-based photocatalysts, ZnO has been widely studied for environmental remediation and is believed to be an alternative photocatalyst to TiO2 due to their similar band-gaps and similar photocatalytic mechanisms [8,9]. Some research about Cr(VI) photoreduction activity using ZnO photocatalysts has also been reported [10–13]. However, the high recombination of photoinduced electron-hole pairs, poor response to visible light and photocorrosion have hindered the application of ZnO in photocatalysis [14,15]. One of the effective strategies to improve the charge carrier separation and the visible-light harvesting of ZnO-based photocatalysts is to couple ZnO with narrow-band-gap Nanomaterials 2016, 6, 173; doi:10.3390/nano6090173

www.mdpi.com/journal/nanomaterials

Nanomaterials 2016, 6, 173 Nanomaterials 2016, 6, 173

2 of 12

2 of 12

carrier separation and the visible-light harvesting of ZnO-based photocatalysts is to couple ZnO with semiconductors [16–21]. So far, a variety of semiconductors with narrow band-gaps including C3 N4 , narrow-band-gap semiconductors [16–21]. So far, a variety of semiconductors with narrow bandBi2 O3 , BiOI, V2 O5 , CdS, etc., have been used as sensitizers to design visible-light–driven ZnO-based gaps including C3N4, Bi2O3, BiOI, V2O5, CdS, etc., have been used as sensitizers to design visible-light– composite photocatalysts. Coupling ZnO with the visible-light sensitizer not only enhances the light driven ZnO-based composite photocatalysts. Coupling ZnO with the visible-light sensitizer not only harvesting, but also facilitates charge separation in the photoexcited electron-hole pairs. The key to enhances the light harvesting, but also facilitates charge separation in the photoexcited electron-hole achieve the desired coupling effects is a good interface connection between two semiconductors with pairs. The key to achieve the desired coupling effects is a good interface connection between two well-developed structures so that charge transferso can proceed across the interface semiconductors with well-developed structures that chargesmoothly transfer can proceed smoothly [16,22]. across Recently, graphite-like carbon nitride (g-C N ), a metal-free layered polymer semiconductor, 3 4 the interface [16,22]. exhibited excellent photocatalytic toward organic pollutants under visible-light Recently, graphite-like carbonactivities nitride (g-C 3N4), amany metal-free layered polymer semiconductor, irradiation and has attracted plenty of attention due to its appropriate band structure and visible-light exhibited excellent photocatalytic activities toward many organic pollutants under visible-light adsorption ability as well as its excellent chemical stability and ease of large-scale preparation using irradiation and has attracted plenty of attention due to its appropriate band structure and visiblevarious precursorsability [23–28]. about the exfoliated N4 shows that ultrathin g-C3 N4 light adsorption as Recent well as research its excellent chemical stability g-C and3ease of large-scale preparation using various precursors [23–28]. Recent research about thetoexfoliated 3N4 shows that ultrathin gnanosheets (CN) do exhibit many superior properties its bulk g-C analogy [29–33], very similar 3N4 nanosheets (CN) do exhibit many superior properties to its work bulk analogy [29–33], veryabout similarthe to Chotspot graphene [34,35]. Although there has been some recently reported to3 N hotspot [34,35]. Although there some work on recently reported aboutoxidation the gg-C composites [16,17,36–41], whichhas is been mainly focused the photocatalytic 4 /ZnOgraphene C3N4/ZnO composites [16,17,36–41], is mainly focused to onfabricate the photocatalytic oxidation property, to our knowledge, few reports which using CN as the substrate nanocatalysts for water property, to ourcoupling knowledge, fewwith reports CN as been the substrate to fabricate for water purification and ZnO CNusing has rarely reported so far fornanocatalysts Cr(VI) photoreduction. purification and coupling ZnO with CN has rarely been reported so far for Cr(VI) photoreduction. In a In this study, CN/ZnO photocatalysts with different CN loadings were successfully prepared via this study, CN/ZnO photocatalysts with different CN loadings were successfully prepared via a simple precipitation-calcination in the presence of exfoliated C3 N4 nanosheets. The photocatalysts simple precipitation-calcination in the presence of exfoliated C3N4 nanosheets. The photocatalysts were thoroughly characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), were thoroughly characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy diffuse reflectance spectroscopy (DRS) and photoluminescence spectra (PL) and their visible-light (XPS), diffuse reflectance spectroscopy (DRS) and photoluminescence spectra (PL) and their visiblephotocatalytic properties were evaluated by Cr(VI) photoreduction. light photocatalytic properties were evaluated by Cr(VI) photoreduction.

2. Results and Discussion 2. Results and Discussion 2.1. Charaterization of CN/ZnO Nanocomposites 2.1. Charaterization of CN/ZnO Nanocomposites Figure 1 illustrates the fabrication process and formation mechanism for CN/ZnO nanocomposites. Figure 1 illustrates the fabrication process and formation mechanism for CN/ZnO Firstly, H2 SO4 -C3 N4 prepared via H2 SO4 intercalation into bulk graphitic C3 N4 was exfoliated into nanocomposites. Firstly, H2SO4-C3N4 prepared via H2SO4 intercalation into bulk graphitic C3N4 was ultrathin CN nanosheets with few layers with the aid of ultrasonification, which was confirmed in exfoliated into ultrathin CN nanosheets with few layers with the aid of ultrasonification, which was previous studies [29,33]. As demonstrated in previous reports [23,25], CN sheets have triazine units on confirmed in previous studies [29,33]. As demonstrated in previous reports [23,25], CN sheets have thetriazine basal planes and groups located the edges. These groups act as groups anchoract sites units on theamino basal planes and aminoatgroups located at thefunctional edges. These functional 2+ could be adsorbed onto the surfaces of CN nanosheets due to the Van der Waals force and the and Zn as anchor sites and Zn2+ could be adsorbed onto the surfaces of CN nanosheets due to the Van der coordination between theinteraction metal ions between and the functional the nuclei of the Waals forceinteraction and the coordination the metal groups. ions andSecondly, the functional groups. Zn(HCO ) precursor were formed in a short time upon the addition of NH HCO [42] and gradually 3 2 4 3 Secondly, the nuclei of the Zn(HCO3)2 precursor were formed in a short time upon the addition of grew nanosheets at relatively temperature. Finally,high high-temperature treatment NHonto 4HCOCN 3 [42] and gradually grew onto high CN nanosheets at relatively temperature. Finally, high- of CN/Zn(HCO gave the product CN/ZnO. temperature3 )treatment ofN CN/Zn(HCO 3)2 under an N 2 atmosphere gave the product CN/ZnO. 2 under an 2 atmosphere

Figure schematicrepresentation representationofofthe thesynthetic synthetic route route to to graphitic-C graphitic-C3N 4 (g-C3N4) nanosheets Figure 1. 1. AA schematic 3 N4 (g-C3 N4 ) nanosheets (CN)/ZnO nanocomposites. CN: Graphitic-C 3N4. (CN)/ZnO nanocomposites. CN: Graphitic-C3 N4 .

Nanomaterials 2016, 6, 173 Nanomaterials 2016, 6, 173

3 of 12 3 of 12

Figure22shows showsthe theXRD XRD patterns of ZnO, C43N 4 and CN-ZnO different CN contents. For Figure patterns of ZnO, C3 N and CN-ZnO withwith different CN contents. For pure ◦ (100), pure ZnO, the peaks 31.7° (100), 34.4° (002), 36.2° (010), 47.3° (102), 56.5° (110), 62.7° and (103)67.9 and◦ 67.9° ZnO, the peaks at 31.7at 34.4◦ (002), 36.2◦ (010), 47.3◦ (102), 56.5◦ (110), 62.7◦ (103) (112) (112) matched well the with the standard data the hexagonal ZnO hexagonal wurtzite (JCPDS No. 36-1451) matched well with standard data of theofZnO wurtzite phasephase (JCPDS No. 36-1451) [36] [36] no andother no other crystallite phases the samples prepared were observed. Bulk and ZnO ZnO crystallite phases in all in theallsamples prepared hereinherein were observed. Bulk g-C 3 Ng4 ◦ and ◦ corresponding C3N4 exhibited two peaks 13.0°27.0 and 27.0° corresponding the and (100)(002) and crystal (002) crystal exhibited two peaks at 13.0at to the to (100) planesplanes of g-Cof 3 Ng4, C3N4, which were ascribed to the inter-planar peak of aromatic systems and thestructural in-plane which were ascribed to the inter-planar stacking stacking peak of aromatic systems and the in-plane structural packing motif, respectively [29,33]. Nopeaks diffraction peaks of the g-C 3 N 4 phase packing motif, respectively [29,33]. No diffraction of the crystalline g-Ccrystalline N phase were detected 3 4 were detected in CN-2/ZnO with a lower content of CN, indicating CN nanosheets were in CN-2/ZnO with a lower content of CN, indicating CN nanosheets were totally exfoliated at totally lower exfoliatedand at lower loadings and uniformly disperseddue in the due to the effective loadings uniformly dispersed in the CN-2/ZnO to CN-2/ZnO the effective prevention of the prevention restacking of the the CN restacking layered by the ZnOto particles, similar to other (2D) two lamellar dimensional (2D) of layeredofbythe theCN ZnO particles, similar other two dimensional material lamellar material embedded in the composites [43–45]. With higher CN loadings, however, the asembedded in the composites [43–45]. With higher CN loadings, however, the as-prepared CN-10/ZnO preparedexhibited CN-10/ZnO samples g-C exhibited a crystalline 3N4 peak, which was [17]. similar to recent samples a crystalline which wasg-C similar to recent reports Meanwhile, 3 N4 peak, reports Meanwhile, theCN/ZnO ZnO crystal phase inwas the detected, CN/ZnO which composites was which the ZnO[17]. crystal phase in the composites favored thedetected, photocatalytic favored theofphotocatalytic properties of the as-prepared photocatalyst. properties the as-prepared photocatalyst.

Figure2.2.X-ray X-raydiffraction diffraction(XRD) (XRD)patterns patternsofofZnO, ZnO,CCN 3N4, CN-2/ZnO and CN-10/ZnO. Figure 3 4 , CN-2/ZnO and CN-10/ZnO.

Figure 3 shows the typical SEM images of g-C3N4, H2SO4-C3N4, CN-2/ZnO and ZnO. g-C3N4 was Figure 3 shows the typical SEM images of g-C3 N4 , H2 SO4 -C3 N4 , CN-2/ZnO and ZnO. g-C3 N4 composed of a large number of irregular aggregated particles about several micrometers in size and was composed of a large number of irregular aggregated particles about several micrometers in size plenty of small pores resulting from gas discharged from the melamine decomposition could be and plenty of small pores resulting from gas discharged from the melamine decomposition could detected in these particles (Figure 3a). After acid treatment, the H2SO4-C3N4 sample exhibited surfacebe detected in these particles (Figure 3a). After acid treatment, the H2 SO4 -C3 N4 sample exhibited smooth particles (Figure 3b). From Figure 3c,d, CN-2/ZnO exhibited an ordered sheet-like structure surface-smooth particles (Figure 3b). From Figure 3c,d, CN-2/ZnO exhibited an ordered sheet-like composed of aggregated spherical ZnO nanoparticles with a mean size of around 25 nm on the structure composed of aggregated spherical ZnO nanoparticles with a mean size of around 25 nm on surface of the CN sheets, indicating that the CN sheets acted as the substrate for the formation of the the surface of the CN sheets, indicating that the CN sheets acted as the substrate for the formation of the ZnO precursor during the chemical precipitation process. Compared to the CN-2/ZnO nanocomposite, ZnO precursor during the chemical precipitation process. Compared to the CN-2/ZnO nanocomposite, as seen from Figure 3e,f, pure ZnO displayed more disordered agglomerates composed of a larger as seen from Figure 3e,f, pure ZnO displayed more disordered agglomerates composed of a larger diameter size (ca. 30–50 nm) and this phenomena was also found in graphene-based composites diameter size (ca. 30–50 nm) and this phenomena was also found in graphene-based composites because the planar 2D material provided more substrates for the nuclei formation and the subsequent because the planar 2D material provided more substrates for the nuclei formation and the subsequent special confinement of the crystalline growth [46,47]. special confinement of the crystalline growth [46,47]. Figure 4 shows the typical HRTEM images of ZnO and CN-2/ZnO. The pure ZnO nanoparticles Figure 4 shows the typical HRTEM images of ZnO and CN-2/ZnO. The pure ZnO nanoparticles exhibited an irregular spherical morphology with a size diameter in the range of 20–100 nm (Figure exhibited an irregular spherical morphology with a size diameter in the range of 20–100 nm (Figure 4a) 4a) and the lattice fringes were measured to be 0.26 nm (Figure 4b), which could be assigned to the and the lattice fringes were measured to be 0.26 nm (Figure 4b), which could be assigned to the (002) (002) facets of the hexagonal wurtzite ZnO phase [36]. From Figure 4c–e, it was seen that ZnO facets of the hexagonal wurtzite ZnO phase [36]. From Figure 4c–e, it was seen that ZnO nanoparticles nanoparticles were randomly and densely distributed on the surface of the CN sheets and showed a were randomly and densely distributed on the surface of the CN sheets and showed a smaller particle smaller particle size compared to that of pure ZnO. Two different kinds of lattice fringes were clearly size compared to that of pure ZnO. Two different kinds of lattice fringes were clearly exhibited exhibited (Figure 4f), one of d = 0.31 nm matching the (002) crystallographic plane of C3N4 [33], the (Figure 4f), one of d = 0.31 nm matching the (002) crystallographic plane of C3 N4 [33], the other of other of d = 0.255 nm identical to the pure wurtzite ZnO nanoparticles as presented in Figure 4b, d = 0.255 nm identical to the pure wurtzite ZnO nanoparticles as presented in Figure 4b, further further indicating that the prepared sample consisted of CN and ZnO and the CN did not destroy the ZnO crystalline structure in our synthetic process. Moreover, the junction interface between the CN and ZnO phase in the composite was easily detected from Figure 4f, where the CN sheets acted as

Nanomaterials 2016, 6, 173

4 of 12

indicating that the prepared sample consisted of CN and ZnO and the CN did not destroy the ZnO crystalline Nanomaterialsstructure 2016, 6, 173in our synthetic process. Moreover, the junction interface between the CN and4ZnO of 12 Nanomaterials 2016, 6, 173 4 of 12 phase in the composite was easily detected from Figure 4f, where the CN sheets acted as bridges for the connection the ZnObetween nanoparticles, and consequentlyand the CN sheets were densely decorated bridges for between the connection the ZnO nanoparticles, consequently the CN sheets were bridges for the connection between the ZnO nanoparticles, and consequently the CN sheets were by the ZnO nanoparticles. The close connection between the two components was favorable for the densely decorated by the ZnO nanoparticles. The close connection between the two components was densely decorated by the ZnO nanoparticles. The close connection between the two components was increased of photogenerated carriers and would enhance the photocatalytic of favorable separation for the increased separation of photogenerated carriers and wouldperformance enhance the favorable for the increased separation of photogenerated carriers and would enhance the the photocatalysts. photocatalytic performance of the photocatalysts. photocatalytic performance of the photocatalysts.

Figure 3. 3. Scanning electron electron microscopy(SEM) (SEM) imagesofof(a) (a) g-CN 3N4; (b) H2SO4-C3N4; (c,d) CN-2/ZnO; Figure Scanning electron microscopy microscopy (SEM)images images of (a)g-C g-C (b)HH2 SO 2SO -C33N N44; (c,d) CN-2/ZnO; CN-2/ZnO; 3 3N 4 ;4;(b) 44-C and (e,f) ZnO. and (e,f) ZnO.

Figure 4. High-resolution transmission electron microscopy (HRTEM) images of (a,b) ZnO; and (c–f) Figure 4. electron microscopy (HRTEM) images of (a,b) ZnO;ZnO; and (c–f) Figure 4. High-resolution High-resolutiontransmission transmission electron microscopy (HRTEM) images of (a,b) and CN-2/ZnO. CN-2/ZnO. (c–f) CN-2/ZnO.

Figure 5 provides the XPS survey spectra and Zn2p of the CN-2/ZnO composite and the ZnO as Figure 5 provides the XPS survey spectra and Zn2p of the CN-2/ZnO composite and the ZnO as 5 provides the spectrafor and of the CN-2/ZnO composite the ZnO well Figure as high-resolution C1sXPS andsurvey N1s spectra theZn2p CN-2/ZnO composite. The surveyand spectrum in well as high-resolution C1s and N1s spectra for the CN-2/ZnO composite. The survey spectrum in as well as high-resolution C1s and N1s spectra for the CN-2/ZnO composite. The survey spectrum Figure 5a clearly indicated that the composition of CN-2/ZnO included Zn, O, C and N elements. The Figure 5a clearly indicated that the composition of CN-2/ZnO included Zn, O, C and N elements. The in Figure 5a clearly that the composition CN-2/ZnO included Zn, O, Cspectrum and N elements. peak located at 531.0indicated eV was associated with the O2−ofions in the ZnO [48]. The Zn2p of pure peak located at 531.0 eV was associated with the O2− ions2− in the ZnO [48]. The Zn2p spectrum of pure The peak located at 531.0 eV was associated with the O ions in the ZnO [48]. The Zn2p spectrum ZnO (Figure 5b) exhibits two peaks at 1021.2 eV and 1044.3 eV assigned to Zn2p3/2 and Zn2p1/2, ZnO (Figure 5b) exhibits two peaks at 1021.2 eV and 1044.3 eV assigned to Zn2p3/2 and Zn2p1/2, of pure ZnO which (Figurewere 5b) exhibits twowith peaks 1021.2 binding eV and 1044.3 to Zn2p and respectively, consistent theattypical energy eV of assigned Zn2+ in the ZnO [49,50]. respectively, which were consistent with the typical binding energy of Zn2+ 2+ in the ZnO 3/2 [49,50]. Zn2p were with the 1/2 typical binding energy of Zn inwere the ZnO [49,50]. However, the bindingwhich energies ofconsistent Zn2p3/2 and Zn2p for the CN-2/ZnO composite 1021.6 and 1/2 , respectively, However, the binding energies of Zn2p3/2 and Zn2p1/2 for the CN-2/ZnO composite were 1021.6 and 1044.8 eV, slightly higher than those for pure ZnO. Such a chemical shift of the Zn2p spectrum in the 1044.8 eV, slightly higher than those for pure ZnO. Such a chemical shift of the Zn2p spectrum in the CN/ZnO composite could be ascribed to the formation of the N–Zn bonds [36,50]. The C1s spectra CN/ZnO composite could be ascribed to the formation of the N–Zn bonds [36,50]. The C1s spectra showed three deconvoluted peaks located at 284.6, 286.3 and 288.9 eV. The peak at 284.6 eV was showed three deconvoluted peaks located at 284.6, 286.3 and 288.9 eV. The peak at 284.6 eV was exclusively assigned to surface adventitious carbon while the peaks at 286.3 and 288.9 eV were exclusively assigned to surface adventitious carbon while the peaks at 286.3 and 288.9 eV were attributed to the sp22-hybridized carbon bonded to N inside the triazine rings and bonded to the –NH2

Nanomaterials 2016, 6, 173

5 of 12

However, the binding energies of Zn2p3/2 and Zn2p1/2 for the CN-2/ZnO composite were 1021.6 and 1044.8 eV, slightly higher than those for pure ZnO. Such a chemical shift of the Zn2p spectrum in the CN/ZnO composite could be ascribed to the formation of the N–Zn bonds [36,50]. The C1s spectra showed three deconvoluted peaks located at 284.6, 286.3 and 288.9 eV. The peak at 284.6 eV was exclusively to surface adventitious carbon while the peaks at 286.3 and 288.9 eV 5were Nanomaterials 2016, 6,assigned 173 of 12 2 attributed to the sp -hybridized carbon bonded to N inside the triazine rings and bonded to the –NH2 of CN-2/ZnO CN-2/ZnO could group, respectively [51]. The high-resolution N1s spectrum of could be be fitted into three 2 2-bonded N atoms in different peaks. The main signals at 398.0 and 399.8 eV were assigned to the sp different peaks. The main signals at 398.0 and 399.8 eV were assigned to the sp -bonded N atoms in the the triazine andbridging the bridging N in atoms in the N–(C)3 [29,51], groupswhile [29,51], the weak peak triazine ringsrings and the N atoms the N–(C) the while weak peak associated 3 groups associated with terminal amino functions much energy higher atbinding at due 407.3to eV, with terminal amino functions exhibited muchexhibited higher binding 407.3 eV,energy probably the probably due to the formation of the Zn–N bonds in the composite material [36,40,49]. formation of the Zn–N bonds in the composite material [36,40,49].

Figure 5. (XPS) survey survey spectra spectra and and(b) (b) Zn2p Zn2p peaks peaksof of CN-2/ZnO CN-2/ZnO Figure 5. (a) (a) X-ray X-ray photoelectron photoelectron spectroscopy spectroscopy (XPS) composite and ZnO; (c) high-resolution C1s and (d) N1s spectra for CN-2/ZnO composite. composite and ZnO; (c) high-resolution C1s and (d) N1s spectra for CN-2/ZnO composite.

The UV-Vis UV-Vis DRS DRS spectra spectraof ofg-C g-C3N N4, ZnO and different mass ratios of the CN/ZnO photocatalysts The 3 4 , ZnO and different mass ratios of the CN/ZnO photocatalysts are shown in Figure 6. As expected, a are shown in Figure 6. As expected, a sharp sharp fundamental fundamental absorption absorption edge edge for for ZnO ZnO rose rose at at ca. ca. 400 400 nm, nm, attributable to the 3.11 eV bandgap. The main absorption edge of the pure g-C 3N4 occurred at ca. 470 attributable to the 3.11 eV bandgap. The main absorption edge of the pure g-C3 N4 occurred at nm,470 corresponding to a typical band-gap of 2.63 eV.eV. Compared with ca. nm, corresponding to a typical band-gap of 2.63 Compared withpure pureZnO, ZnO,the the absorption absorption wavelength range of both CN-2/ZnO and CN-10/ZnO extended towards the visible-light wavelength range of both CN-2/ZnO and CN-10/ZnO extended towards the visible-light region region and and the absorption intensity also increased, which demonstrated the effective surface coupling of ZnO the absorption intensity also increased, which demonstrated the effective surface coupling of ZnO with CN CN sheets sheets and and therefore therefore an an improved improved photocatalytic photocatalytic activity activity of of the the prepared prepared photocatalysts photocatalysts with could be expected under visible-light irradiation. The red-shifted absorption of the composites in the the could be expected under visible-light irradiation. The red-shifted absorption of the composites in visible-light region region was was clearly clearly enhanced enhanced with with the the increasing increasing CN CN amount. amount. These implied that that visible-light These results results implied well-bonded interfaces between the CN nanosheets and ZnO phase could be formed [36,40] and thus well-bonded interfaces between the CN nanosheets and ZnO phase could be formed [36,40] and thus make CN/ZnO CN/ZnO photocatalysts make photocatalystsshift shift to to the the lower lower energy energy region. region. PL spectral analysis was conducted to demonstrate the migration, transfer, and recombination processes of photoinduced electron-hole pairs in the photoinduced system. Figure 7 shows PL spectra of the g-C3 N4 , ZnO and CN/ZnO photocatalysts. A main strong emission peak at 498 nm and a shoulder-peak at around 417 nm for pure ZnO were observed, which was consistent with other reports [40,52]. The peak at 463 nm of the bulk g-C3 N4 was almost not detected in the CN/ZnO composites due to the low content of CN. Compared to pure ZnO, the PL peak intensity at 498 nm

Figure 5. (a) X-ray photoelectron spectroscopy (XPS) survey spectra and (b) Zn2p peaks of CN-2/ZnO composite and ZnO; (c) high-resolution C1s and (d) N1s spectra for CN-2/ZnO composite.

The UV-Vis DRS spectra of g-C3N4, ZnO and different mass ratios of the CN/ZnO photocatalysts are shown in Figure 6. As expected, a sharp fundamental absorption edge for ZnO rose at ca. 400 nm, Nanomaterials 2016, 6, 173 6 of 12 attributable to the 3.11 eV bandgap. The main absorption edge of the pure g-C3N4 occurred at ca. 470 nm, corresponding to a typical band-gap of 2.63 eV. Compared with pure ZnO, the absorption wavelength range photocatalysts of both CN-2/ZnO CN-10/ZnO extended towards thethe visible-light region and for both CN/ZnO was and significantly reduced, indicating that modification of ZnO the absorption intensity also increased, demonstrated surface of coupling of ZnO with few-layered CN nanosheets couldwhich effectively suppress the theeffective recombination photoinduced with CN sheets andintherefore an improved photocatalytic activity of prepared photocatalysts electron-hole pairs the composites and thus efficiently separate thethe photoinduced electrons in could be expectedprocesses under visible-light irradiation.with The red-shifted absorption of the in the electron-transfer [53,54]. Combined the enhanced absorption incomposites the visible-light visible-light region enhanced the increasing CN amount. results implied that region presented inwas the clearly UV-DRS analysis,with therefore, the incorporation of These CN nanosheets into ZnO well-bonded interfaces nanosheets and ZnO phase be formed [36,40] and thus was greatly expected tobetween enhancethe theCN photocatalytic performance of could the CN/ZnO composites under make CN/ZnO photocatalysts shift to the lower energy region. visible-light irradiation. Nanomaterials 2016, 6, 173

6 of 12

PL spectral analysis was conducted to demonstrate the migration, transfer, and recombination processes of photoinduced electron-hole pairs in the photoinduced system. Figure 7 shows PL spectra of the g-C3N4, ZnO and CN/ZnO photocatalysts. A main strong emission peak at 498 nm and a shoulder-peak at around 417 nm for pure ZnO were observed, which was consistent with other reports [40,52]. The peak at 463 nm of the bulk g-C3N4 was almost not detected in the CN/ZnO composites due to the low content of CN. Compared to pure ZnO, the PL peak intensity at 498 nm for both CN/ZnO photocatalysts was significantly reduced, indicating that the modification of ZnO with few-layered CN nanosheets could effectively suppress the recombination of photoinduced electron-hole pairs in the composites and thus efficiently separate the photoinduced electrons in electron-transfer processes [53,54]. Combined with the enhanced absorption in the visible-light region presented in the UV-DRS analysis, therefore, the incorporation of CN nanosheets into ZnO was greatly expected to enhance the photocatalytic performance of the CN/ZnO composites under Figure 6. UV-Vis diffuse reflectance reflectancespectra spectraofofg-C g-CN 3N4, ZnO, CN-2/ZnO and CN-10/ZnO photocatalysts. visible-light Figure 6.irradiation. UV-Vis diffuse 3 4 , ZnO, CN-2/ZnO and CN-10/ZnO photocatalysts.

Figure 7. of of g-C 3N4,N andand CN/ZnO photocatalysts. The Figure 7. Photoluminescence Photoluminescencespectra spectra(PL) (PL)spectra spectra g-C CN/ZnO photocatalysts. 3 ZnO 4 , ZnO insetinset shows the PL of ZnO, CN-2/ZnO and CN-10/ZnO composites. The shows thespectra PL spectra of ZnO, CN-2/ZnO and CN-10/ZnO composites.

2.2. Photocatalytic Chromium(VI) under Light 2.2. Photocatalytic Reduction Reduction of of Aqueous Aqueous Chromium(VI) under Visible Visible Light Figure 88 presents presents the the photocatalytic photocatalytic reduction reduction of of aqueous aqueous Cr(VI) Cr(VI) over over the the g-C g-C33N N44,, ZnO ZnO and and Figure CN/ZnO photocatalysts under visible light. The control experiments indicated that aqueous Cr(VI) CN/ZnO photocatalysts under visible light. The control experiments indicated that aqueous Cr(VI) was very very stable stable under under visible visible light light (blank (blank experiments) experiments) and and the the adsorption adsorption removal removal ratio ratio of of aqueous aqueous was Cr(VI) was negligible (about 4%) for the CN-ZnO composites (dark experiments). The Cr(VI) was negligible (about 4%) for the CN-ZnO composites (dark experiments). The photoreduction photoreduction of aqueous was 18% and 34% for pure g-C3N4 and ZnO under 240 min rate of aqueousrate Cr(VI) was 18%Cr(VI) and 34% for pure g-C3 N 4 and ZnO under 240 min irradiation. irradiation. However, after some CN nanosheets were incorporated the reduction However, after some CN nanosheets were incorporated into the into ZnO,the theZnO, reduction rate of rate the of the Cr(VI) was increased to 70% and 47% for CN-2/ZnO and CN-10/ZnO, strongly suggesting that Cr(VI) was increased to 70% and 47% for CN-2/ZnO and CN-10/ZnO, strongly suggesting that the modification photocatalytic reduction activity of the modification with with CN CNnanosheets nanosheetscould couldgreatly greatlyenhance enhancethe the photocatalytic reduction activity CN/ZnO composites. Compared to other C 3 N 4 [55,56] and ZnO-based photocatalysts for visibleof CN/ZnO composites. Compared to other C3 N4 - [55,56] and ZnO-based photocatalysts for light-driven Cr(VI) Cr(VI) photoreduction [42,57], the the CN/ZnO prepared enhanced visible-light-driven photoreduction [42,57], CN/ZnO preparedherein hereinshowed showed enhanced photoreduction activity, modification with CN CN was effective for improving the photophotoreduction activity,indicating indicatingthat that modification with was effective for improving the activity of ZnO. Furthermore, the lower loading of the CN content for CN-2/ZnO exhibited better photo-activity of ZnO. Furthermore, the lower loading of the CN content for CN-2/ZnO exhibited activity than that for CN-10/ZnO and this was because the excess CN nanosheets may act as a recombination center, covering the active sites on the surface of ZnO particles and therefore reducing the efficiency of charge separation. The photostability of the CN-2/ZnO in the circulating runs for the photocatalytic reduction of Cr(VI) under visible-light irradiation was further examined (Figure 9). After 240 min of irradiation in

Nanomaterials 2016, 6, 173

7 of 12

under the studied conditions, indicating that the photocorrosion effect of ZnO was effectively suppressed by CN hybridization [14,58]. The improved photoreduction activity could be ascribed to Nanomaterials 2016, 6, 173 7 of 12 the increased absorption in the visible-light range and the enhanced charge separation efficiency at the interface of ZnO and CN nanosheets [16,36]. Furthermore, compared to those reports where better activities werethan obtained with the use ofand additional [11,55,57,59], this method described better activity that for CN-10/ZnO this wasscavengers because the excess CN nanosheets may act as a herein will be highly desirablethe foractive environmental due particles to no additional scavengers in recombination center, covering sites on theremediation surface of ZnO and therefore reducing this process. of charge separation. the efficiency

Nanomaterials 2016, 6, 173

7 of 12

under the studied conditions, indicating that the photocorrosion effect of ZnO was effectively suppressed by CN hybridization [14,58]. The improved photoreduction activity could be ascribed to the increased absorption in the visible-light range and the enhanced charge separation efficiency at the interface of ZnO and CN nanosheets [16,36]. Furthermore, compared to those reports where better activities were obtained with the use of additional scavengers [11,55,57,59], this method described herein will be highly desirable for environmental remediation due to no additional scavengers in Figure of aqueous Cr(VI)Cr(VI) over g-C 3N4, g-C ZnO3 N and CN/ZnO photocatalysts under visible Figure 8.8.Photoreduction Photoreduction of aqueous over 4 , ZnO and CN/ZnO photocatalysts this process. light. 0 and C light. are theCinitial concentration and solution concentration of scheduled irradiation intervals. underCvisible 0 and C are the initial concentration and solution concentration of scheduled irradiation intervals.

The photostability of the CN-2/ZnO in the circulating runs for the photocatalytic reduction of Cr(VI) under visible-light irradiation was further examined (Figure 9). After 240 min of irradiation in each cycle, the photocatalyst was separated from the aqueous suspension by centrifugation and washed with deionized (DI) water. As shown in Figure 9, there was no significant decrease of the photocatalytic activity after five cycles. Compared to the recycled property reported by Wang [17], where bulk g-C3 N4 /ZnO showed dramatically reduced activity in the first cycle of Cr(VI) photoreduction, apparently the CN/ZnO composite had good photostability and chemical stability under the studied conditions, indicating that the photocorrosion effect of ZnO was effectively suppressed by CN hybridization [14,58]. The improved photoreduction activity could be ascribed to the increased absorption in the visible-light range and the enhanced charge separation efficiency at the interface Figure 9. Cycling runs for the photoreduction of Cr(VI) by CN-2/ZnO composite under visible light. of ZnO and CN nanosheets [16,36]. Furthermore, compared to those reports where better activities 8. Photoreduction of aqueous Cr(VI) over g-C3[11,55,57,59], N4, ZnO and CN/ZnO photocatalysts under visible were Figure obtained with the use of additional scavengers this method described herein will be On the basis of the above results presented in this work, a synergistic mechanism between ZnO light. C 0 and C are the initial concentration and solution concentration of scheduled irradiation intervals. highly desirable for environmental remediation due to no additional scavengers in this process. and CN nanosheets for the improved photoreduction of Cr(VI) is proposed as illustrated in Figure 10. Under visible-light irradiation, both CN sheets and ZnO can be excited and produce photogenerated electron-hole pairs. Since the conduction band (CB) edge of C3N4 (−1.12 eV) [60] is more negative than that of ZnO (−0.5 eV) [58], the photogenerated electrons on the surface of CN sheets can readily transfer to the CB of ZnO via the well-developed interface and thus reduce Cr2 O2− 7 to Cr3+ on the surface of the ZnO particles and the holes oxidize water to form O2, as illustrated in the following equations [61]. As depicted in Figure 10, this transfer favors electron-hole separation and leads to large numbers of electrons on the ZnO surface and holes on the g-C3N4 surface, respectively, thus promoting the photocatalytic reduction of aqueous Cr(VI).

Cr2O72− + 14H+ + 6e → 2Cr3+ + 7H2O

(1)

2H2O + 4h+ → O2 + 4H+

(2)

Figure 9. 9. Cycling Cycling runs runs for for the the photoreduction photoreduction of of Cr(VI) Cr(VI) by by CN-2/ZnO CN-2/ZnO composite Figure composite under under visible visible light. light.

On the basis of the above results presented in this work, a synergistic mechanism between ZnO On the basis of the above results presented in this work, a synergistic mechanism between ZnO and CN nanosheets for the improved photoreduction of Cr(VI) is proposed as illustrated in Figure and CN nanosheets for the improved photoreduction of Cr(VI) is proposed as illustrated in Figure 10. 10. Under visible-light irradiation, both CN sheets and ZnO can be excited and produce Under visible-light irradiation, both CN sheets and ZnO can be excited and produce photogenerated photogenerated electron-hole pairs. Since the conduction band (CB) edge of C3N4 (−1.12 eV) [60] is more negative than that of ZnO (−0.5 eV) [58], the photogenerated electrons on the surface of CN sheets can readily transfer to the CB of ZnO via the well-developed interface and thus reduce Cr2 O2− 7 to Cr3+ on the surface of the ZnO particles and the holes oxidize water to form O2, as illustrated in the following equations [61]. As depicted in Figure 10, this transfer favors electron-hole separation and

Nanomaterials 2016, 6, 173

8 of 12

electron-hole pairs. Since the conduction band (CB) edge of C3 N4 (−1.12 eV) [60] is more negative than that of ZnO (−0.5 eV) [58], the photogenerated electrons on the surface of CN sheets can readily transfer to the CB of ZnO via the well-developed interface and thus reduce Cr2 O27− to Cr3+ on the surface of the ZnO particles and the holes oxidize water to form O2 , as illustrated in the following equations [61]. As depicted in Figure 10, this transfer favors electron-hole separation and leads to large numbers of electrons on the ZnO surface and holes on the g-C3 N4 surface, respectively, thus promoting the photocatalytic reduction of aqueous Cr(VI). Cr2 O7 2- + 14H+ + 6e → 2Cr3+ + 7H2 O Nanomaterials 2016, 6, 173

2H2 O + 4h+ → O2 + 4H+

(1) 8 of 12

(2)

10. Schematic Figure 10. Schematic illustration illustration of of the mechanism of electron-hole separation and transport and photocatalytic activity of CN/ZnO photocatalyst under under visible-light visible-light irradiation. irradiation. CN/ZnO photocatalyst

3. Materials Materialsand andMethods Methods 3.1. Preparation of SO44-Intercalated -Intercalated CC33NN44(H (H2SO 2 SO 4 -C 34N 3.1. Preparation of H H2SO 4-C 3N ) 4) All chemicals chemicals were were used used as as received receivedwith withany anypurification. purification.The The bulk g-C prepared 4 was bulk g-C 3N34 N was prepared by by direct pyrolysis thermal polycondensation of melamine in the semi-closed system. In a typical direct pyrolysis thermal polycondensation of melamine in the semi-closed system. In a ◦C synthesis, 10 with a cover and then heated to 550 10 gg of ofmelamine melaminewas wasplaced placedininananalumina aluminacrucible crucible with a cover and then heated to 550 ◦ − 1 with a heating raterate of 3ofC3·°C· minmin, −1and kept at at this temperature forfor 4 h4 in static air.air. °C with a heating , and kept this temperature h in static The grey-white grey-whiteHH2SO SO -C N was obtained by stirring bulk g-C N powder in concentrated obtained by stirring bulk g-C3N4 powder in concentrated H2SO4 2 4-C 4 3N 3 4 was 4 3 4 H room temperature as described in our previous report [33]. Briefly, the as-prepared (98%) roomattemperature as described in our previous report [33]. Briefly, the as-prepared yellow 2 SO4at(98%) yellow C3 N4 powder was mixed concentrated H42 SO stirred at room C3N4 powder (1.0 g) (1.0 wasg)mixed with with concentrated H2SO (104 (10 mL)mL) andand stirred forfor8 8hh at temperature for its intercalation, during which the solution color changed from yellow to grey. Then the temperature for its intercalation, during which the solution color changed from yellow to grey. Then mixture waswas slowly poured into into deionized water (200 (200 mL).mL). The obtained light-grey suspension was the mixture slowly poured deionized water The obtained light-grey suspension filtrated, washed repeatedly with DI water to remove the residual acid and finally freeze-dried under was filtrated, washed repeatedly with DI water to remove the residual acid and finally freeze-dried vacuum to give to thegive grey-white H2 SO4 -C The SO4H content in this in sample was estimated to 30% under vacuum the grey-white H32N SO 3NH 4. 2The 2SO4 content this sample was estimated 4 .4-C by thermal analysis. to 30% by thermal analysis. 3.2. Preparation of 3.2. Preparation of CN/ZnO CN/ZnO Nanocomposites Nanocomposites CN/ZnO nanocomposites with with different different theoretical (2% and and 10%) 10%) and and pure pure ZnO ZnO CN/ZnO nanocomposites theoretical content content of of C C33N N44 (2% nanoparticles used in this work were prepared by a reaction similar to that described by Pan [62]. nanoparticles used in this work were prepared by a reaction similar to that described by Pan [62]. In In brief, H SO -C N (60 mg) was ultrasonicated in DI water (100 mL) for 5 h to give homogeneous 2 4-C43N43 (60 4 mg) was ultrasonicated in DI water (100 mL) for 5 h to give homogeneous brief, H2SO colloidal suspensioncontaining containinglarge large amount of ultrathin few-layered CN nanosheets colloidal suspension amount of ultrathin few-layered CN nanosheets [29,33].[29,33]. Then Then ZnSO · 7H O (6.9 g) was added and ultrasonicated for another 30 min to ensure the uniform ZnSO4·7H2O4 (6.92 g) was added and ultrasonicated for another 30 min to ensure the uniform 2+ onto the surface of CN nanosheets. Aqueous solution (100 mL) containing adsorption adsorption of of Zn Zn2+ onto the surface of CN nanosheets. Aqueous solution (100 mL) containing ◦ C within 10 min and this NH HCO (2.9 g) was added dropwise the above solution at 60 4 3 NH4HCO3 (2.9 g) was added dropwise intointo the above solution at 60 °C within 10 min and this mixture

solution was kept stirring at this temperature for 3 h, during which a lot of pungent NH3 was produced. The white precipitate (CN/Zn(HCO3)2) was collected by filtration, washed for three times with distilled water and then dried in vacuum oven at 60 °C for 24 h. Finally, this product was calcined at 550 °C for 2 h under N2 atmosphere to obtain the grew-yellow powder CN/ZnO nanocomposites with 2% theoretical content of CN. The as-synthesized CN/ZnO samples with 2.0%

Nanomaterials 2016, 6, 173

9 of 12

mixture solution was kept stirring at this temperature for 3 h, during which a lot of pungent NH3 was produced. The white precipitate (CN/Zn(HCO3 )2 ) was collected by filtration, washed for three times with distilled water and then dried in vacuum oven at 60 ◦ C for 24 h. Finally, this product was calcined at 550 ◦ C for 2 h under N2 atmosphere to obtain the grew-yellow powder CN/ZnO nanocomposites with 2% theoretical content of CN. The as-synthesized CN/ZnO samples with 2.0% and 10% theoretical CN content were named as CN-2/ZnO and CN-10/ZnO. For comparison, pure ZnO was prepared with the same procedure without the use of H2 SO4 -C3 N4 . 3.3. Characterization SEM images were obtained with a Hitachi SU8020 scanning electron microscope (Hitachi Ltd., Tokyo, Japan) with acceleration voltage of 20 kV. HRTEM was performed on a FEI Tecnai G2 F20 field-emission transmission electron microscopy (Hillsboro, OR, USA) at an accelerating voltage of 200 kV. XRD were recorded on a PANalytical X’pert Pro powder diffractometer (Almelo, The Netherland) with Cu-Ka radiation (λ = 1.5418 Å) with a scan step of 0.013◦ . XPS measurements were performed on a Thermo Fisher ESCALAB 250Xi photoelectron spectrometer (Waltham, MA, USA) using monochromatic Al Kα X-ray source (hv = 1486.6 eV). DRS was carried out on a Shimadzu UV-2450 UV-Vis spectrophotometer (Kyoto, Japan) using BaSO4 as the reference sample. PL was measured with a Hitachi F-7000 fluorescence spectrophotometer (Tokyo, Japan) using a Xe lamp as excitation source with optical filters. 3.4. Photocatalytic Tests The photocatalytic reduction of aqueous Cr(VI) were carried out in a homemade photochemical reactor under visible-light irradiation. The visible-light source was provided by a 500 W Xe lamp with maximum wavelength emission at 470 nm (Shanghai Jiguang Special Lighting Factory, Shanghai, China). The lamp source was laid in the empty chamber of the annular quartz condensing tube with a circulating water jacket outside to immediately remove the heat released from the lamp. The distance between the light source and the tube containing the reaction mixture was set to be 15 cm. In each experiment, the photocatalyst (50 mg) was dispersed in 50 mL aqueous Cr(VI) solutions of different concentrations. Prior to irradiation, the mixture solution was magnetically stirred for 60 min in the dark to establish the adsorption-desorption equilibrium. During the irradiation, 4 mL of the reaction solution was withdrawn at certain time intervals and centrifuged to remove the photocatalyst from the solution. The Cr(VI) content in the supernatant solution was determined spectrophotometrically at 540 nm using the diphenylcarbazide method [41] (Evolution 600 UV-Vis spectrophotometer, Thermo Fisher Scientific Inc., Waltham, MA, USA). 4. Conclusions In summary, CN/ZnO photocatalysts were successfully prepared via a facile precipitation-calcination in the presence of exfoliated C3 N4 nanosheets. A well-bonded interface was formed between ZnO and CN nanosheets in the as-prepared photocatalysts. CN/ZnO exhibited better photocatalytic Cr(VI) reduction activity than pure ZnO under visible-light irradiation. The enhanced photoreduction performance of the CN/ZnO photocatalyst was be ascribed to the increased absorption of the visible light and the effective transfer and separation of the photogenerated charge carriers at the interface. The CN/ZnO photocatalyst prepared herein is being further evaluated by removing other pollutants and has great potential in water remediation. Acknowledgments: The authors would like to gratefully acknowledge the financial supports from the National Science Foundation of China (No. 51402030) and the Municipal Science Foundation Project of CQEC (No. KJ110421) and the Natural Science Foundation Project of CQ CSTC (cstcjjA50016). Author Contributions: X.Y. conceived and designed the experiments; Q.Y. performed the experiments; X.Y. and C.Z. analyzed the data; Q.T., Y.M. and A.-k.D. contributed partial reagents and materials. X.Y. wrote the paper.

Nanomaterials 2016, 6, 173

10 of 12

Conflicts of Interest: The authors declare no conflict of interest.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

12.

13. 14. 15. 16. 17. 18. 19. 20.

21.

22.

Langard, S. Chromium carcinogenicity: A review of experimental animal data. Sci. Total Environ. 1988, 71, 341–350. [CrossRef] Katz, S.A.; Salem, H. The toxicology of chromium with respect to its chemical speciation: A review. J. Appl. Toxicol. 1993, 13, 217–224. [CrossRef] [PubMed] Barrera-Diaz, C.E.; Lugo-Lugo, V.; Bilyeu, B. A review of chemical, electrochemical and biological methods for aqueous Cr(VI) reduction. J. Hazard. Mater. 2012, 223, 1–12. [CrossRef] [PubMed] Kabra, K.; Chaudhary, R.; Sawhney, R. Treatment of hazardous organic and inorganic compounds through aqueous phase photocatalysis: A review. Ind. Eng. Chem. Res. 2004, 43, 7683–7696. [CrossRef] Daghrir, R.; Drogui, P.; Robert, D. Modified TiO2 for environmental photocatalytic applications: A review. Ind. Eng. Chem. Res. 2013, 52, 3581–3599. Jing, L.; Zhou, W.; Tian, G.; Fu, H. Surface tuning for oxide-based nanomaterials as efficient photocatalysts. Chem. Soc. Rev. 2013, 42, 9509–9549. [CrossRef] [PubMed] Wang, Y.; Jiang, L.; Feng, C. Photocatalytic reduction of Cr(VI). Prog. Chem. 2013, 25, 1999–2010. Hernandez-Alonso, M.D.; Fresno, F.; Sareza, S.; Coronado, J.M. Development of alternative photocatalysts to TiO2 : Challenges and opportunities. Energy Environ. Sci. 2009, 2, 1231–1257. [CrossRef] Zhang, H.; Chen, G.; Bahnemann, D. Photoelectrocatalytic materials for environmental applications. J. Mater. Chem. 2009, 19, 5089–5121. [CrossRef] Khalil, L.B.; Mourad, W.E.; Rophae, M.W. Photocatalytic reduction of environmental pollutant Cr(VI) over some semiconductors under UV/visible light illumination. Appl. Catal. B 1998, 17, 267–273. [CrossRef] Jin, Z.; Zhang, Y.X.; Meng, F.L.; Jia, Y.; Luo, T.; Yu, X.Y.; Wang, J.; Liu, J.H.; Huang, X.J. Facile synthesis of porous single crystalline ZnO nanoplates and their application in photocatalytic reduction of Cr(VI) in the presence of phenol. J. Hazard. Mater. 2014, 276, 400–407. [CrossRef] [PubMed] Chakrabarti, S.; Chaudhuri, B.; Bhattacharjee, S.; Ray, A.K.; Dutta, B.K. Photo-reduction of hexavalent chromium in aqueous solution in the presence of zinc oxide as semiconductor catalyst. Chem. Eng. J. 2009, 153, 86–93. [CrossRef] Qamar, M.; Gondal, M.A.; Yamani, Z.H. Laser-induced efficient reduction of Cr(VI) catalyzed by ZnO nanoparticles. J. Hazard. Mater. 2011, 187, 258–263. [CrossRef] [PubMed] Van Dijken, A.; Janssen, A.H.; Smitsmans, M.H.P.; Vanmaekelbergh, D.; Meijerink, A. Size-selective photoetching of nanocrystalline semiconductor particles. Chem. Mater. 1998, 10, 3513–3522. [CrossRef] Daneshvar, N.; Salari, D.; Khataee, A.R. Photocatalytic degradation of azo dye acid red 14 in water on ZnO as an alternative catalyst to TiO2 . J. Photochem. Photobiol. A 2004, 162, 317–322. [CrossRef] Wang, Y.; Shi, R.; Lin, J.; Zhu, Y. Enhancement of photocurrent and photocatalytic activity of ZnO hybridized with graphite like C3 N4 . Energy Environ. Sci. 2011, 4, 2922–2929. [CrossRef] Liu, W.; Wang, M.; Xu, C.; Chen, S. Facile synthesis of g-C3 N4 /ZnO composite with enhanced visible light photooxidation and photoreduction properties. Chem. Eng. J. 2012, 209, 386–393. [CrossRef] Balachandran, S.; Swaminathan, M. Facile fabrication of heterostructured Bi2 O3 -ZnO photocatalyst and its enhanced photocatalytic activity. J. Phys. Chem. C 2012, 116, 26306–26312. [CrossRef] Jiang, J.; Zhang, X.; Sun, P.B.; Zhang, L.Z. ZnO/BiOI heterostructures: Photoinduced charge-transfer property and enhanced visible-light photocatalytic activity. J. Phys. Chem. C 2011, 115, 20555–20564. [CrossRef] Zou, C.W.; Rao, Y.F.; Alyamani, A.; Chu, W.; Chen, M.J.; Patterson, D.A.; Emanuelsson, E.A.C.; Gao, W. Heterogeneous lollipop-like V2 O5 /ZnO array: A promising composite nanostructure for visible light photocatalysis. Langmuir 2010, 26, 11615–11620. [CrossRef] [PubMed] Wang, X.W.; Yin, L.C.; Liu, G.; Wang, L.Z.; Saito, R.; Lu, G.Q.; Cheng, H.-M. Polar interface-induced improvement in high photocatalytic hydrogen evolution over ZnO-CdS heterostructures. Energy Environ. Sci. 2011, 4, 3976–3979. [CrossRef] Chen, X.B.; Shen, S.H.; Guo, L.J.; Mao, S.S. Semiconductor-based photocatalytic hydrogen generation. Chem. Rev. 2010, 110, 6503–6570. [CrossRef] [PubMed]

Nanomaterials 2016, 6, 173

23.

24. 25. 26. 27. 28.

29. 30. 31. 32. 33.

34. 35. 36.

37. 38.

39. 40.

41. 42. 43. 44.

45.

11 of 12

Wang, Y.; Wang, X.; Antonietti, M. Polymeric graphitic carbon nitride as a heterogeneous organocatalyst: From photochemistry to multipurpose catalysis to sustainable chemistry. Angew. Chem. Int. Ed. 2012, 51, 68–89. [CrossRef] [PubMed] Zhao, Z.; Sun, Y.; Dong, F. Graphitic carbon nitride based nanocomposites: A review. Nanoscale 2015, 7, 15–37. [CrossRef] [PubMed] Cao, S.; Low, J.; Yu, J.; Jaroniec, M. Polymeric photocatalysts based on graphitic carbon nitride. Adv. Mater. 2015, 27, 2150–2176. [CrossRef] [PubMed] Zheng, Y.; Lin, L.; Wang, B.; Wang, X. Graphitic carbon nitride polymers toward sustainable photoredox catalysis. Angew. Chem. Int. Ed. 2015, 54, 12868–12884. [CrossRef] [PubMed] Dong, F.; Wu, L.; Sun, Y.; Fu, M.; Wu, Z.; Lee, S.C. Efficient synthesis of polymeric g-C3 N4 layered materials as novel efficient visible light driven photocatalysts. J. Mater. Chem. 2011, 21, 15171–15174. [CrossRef] Liu, J.; Zhang, Y.; Lu, L.; Wu, G.; Chen, W. Self-regenerated solar-driven photocatalytic water-splitting by urea derived graphitic carbon nitride with platinum nanoparticles. Chem. Commun. 2012, 48, 8826–8828. [CrossRef] [PubMed] Xu, J.; Zhang, L.; Shi, R.; Zhu, Y. Chemical exfoliation of graphitic carbon nitride for efficient heterogeneous photocatalysis. J. Mater. Chem. A 2013, 1, 14766–14772. [CrossRef] Zhao, H.; Yu, H.; Quan, X.; Chen, S.; Zhao, H.; Wang, H. Atomic single layer graphitic-C3 N4 : Fabrication and its high photocatalytic performance under visible light irradiation. RSC Adv. 2014, 4, 624–628. [CrossRef] Niu, P.; Zhang, L.; Liu, G.; Cheng, H.M. Graphene-like carbon nitride nanosheets for improved photocatalytic activities. Adv. Funct. Mater. 2012, 22, 4763–4770. [CrossRef] Han, Q.; Zhao, F.; Hu, C.; Lv, L.; Zhang, Z.; Chen, N.; Qu, L. Facile production of ultrathin graphitic carbon nitride nanoplatelets for efficient visible-light water splitting. Nano Res. 2015, 8, 1718–1728. [CrossRef] Yuan, X.; Zhou, C.; Jin, Y.; Jing, Q.; Yang, Y.; Shen, X.; Tang, Q.; Mu, Y.; Du, A.K. Facile synthesis of 3D porous thermally exfoliated g-C3 N4 nanosheet with enhanced photocatalytic degradation of organic dye. J. Colloid Interface Sci. 2016, 468, 211–219. [CrossRef] [PubMed] Geim, A.K. Graphene: Status and prospects. Science 2009, 324, 1530–1534. [CrossRef] [PubMed] Guo, S.; Dong, S. Graphene nanosheet: Synthesis, molecular engineering, thin film, hybrids, and energy and analytical applications. Chem. Soc. Rev. 2011, 40, 2644–2672. [CrossRef] [PubMed] Sun, J.X.; Yuan, Y.P.; Qiu, L.G.; Jiang, X.; Xie, A.J.; Shen, Y.H.; Zhu, J.F. Fabrication of composite photocatalyst g-C3 N4 -ZnO and enhancement of photocatalytic activity under visible light. Dalton Trans. 2012, 41, 6756–6763. [CrossRef] [PubMed] Zhou, J.; Zhang, M.; Zhu, Y. Preparation of visible light-driven g-C3 N4 @ZnO hybrid photocatalyst via mechanochemistry. Phys. Chem. Chem. Phys. 2014, 16, 17627–17633. [CrossRef] [PubMed] Li, X.; Li, M.; Yang, J.; Li, X.; Hu, T.; Wang, J.; Sui, Y.; Wu, X.; Kong, L. Synergistic effect of efficient adsorptiong-C3 N4 /ZnO composite for photocatalytic property. J. Phys. Chem. Solids 2014, 75, 441–446. [CrossRef] Liu, W.; Wang, M.; Xu, C.; Chen, S.; Fu, X. Significantly enhanced visible-light photocatalytic activity of g-C3 N4 via ZnO modification and the mechanism study. J. Mol. Catal. A 2013, 368–369, 9–15. [CrossRef] Jo, W.K.; Selvam, N.C.S. Enhanced visible light-driven photocatalytic performance of ZnO-g-C3 N4 coupled with graphene oxide as a novel ternary nanocomposite. J. Hazard. Mater. 2015, 299, 462–470. [CrossRef] [PubMed] Yuan, X.; Wang, Y.; Wang, J.; Zhou, C.; Tang, Q.; Rao, X. Calcined graphene/MgAl-layered double hydroxides for enhanced Cr(VI) removal. Chem. Eng. J. 2013, 221, 204–213. [CrossRef] Yang, G.C.C.; Chan, S.W. Photocatalytic reduction of Chromium(VI) in aqueous solution using dye-sensitized nanoscale ZnO under visible light irradiation. J. Nanopart. Res. 2009, 11, 221–230. [CrossRef] Rafiee, M.A.; Rafiee, J.; Wang, Z.; Song, H.; Yu, Z.; Koratkar, N. Enhanced mechanical properties of nanocomposites at low graphene content. Acsnano 2009, 3, 3884–3890. [CrossRef] [PubMed] Liang, J.; Huang, Y.; Zhang, L.; Wang, Y.; Ma, Y.; Guo, T.; Chen, Y. Molecular-level dispersion of graphene into poly(vinyl alcohol) and effective reinforcement of their nanocomposites. Adv. Funct. Mater. 2009, 19, 2297–2302. [CrossRef] Manias, E.; Touny, A.; Wu, L.; Strawhecker, K.; Lu, B.; Chung, T.C. Polypropylene/Montmorillonite Nanocomposites: Review of the Synthetic Routes and Materials Properties. Chem. Mater. 2001, 13, 3516–3523. [CrossRef]

Nanomaterials 2016, 6, 173

46.

47.

48. 49. 50. 51. 52. 53.

54. 55.

56. 57.

58. 59.

60.

61. 62.

12 of 12

Gao, Z.; Wang, J.; Li, Z.; Yang, W.; Wang, B.; Hou, M.; He, Y.; Liu, Q.; Mann, T.; Yang, P.; et al. Graphene nanosheet/Ni2+ /Al3+ layered double-hydroxide composite as a novel electrode for a supercapacitor. Chem. Mater. 2011, 23, 3509–3516. [CrossRef] Zhang, L.; Zhang, X.; Shen, L.; Gao, B.; Hao, L.; Lu, X.; Zhang, F.; Ding, B.; Yuan, C. Enhanced high-current capacitive behavior of graphene/CoAl-layered double hydroxide composites as electrode material for supercapacitors. J. Power Sources 2012, 199, 395–401. [CrossRef] Deng, Z.W.; Chen, M.; Gu, G.X.; Wu, L.M. A facile method to fabricate ZnO hollow spheres and their photocatalytic property. J. Phys. Chem. B 2008, 112, 16–22. [CrossRef] [PubMed] Futsuhara, M.; Yoshioka, K.; Takai, O. Structural, electrical and optical properties of zinc nitride thin films prepared by reactive rf magnetron sputtering. Thin Solid Films 1998, 322, 274–281. [CrossRef] Chen, S.; Wei, Z.; Zhang, S.; Liu, W. Preparation, characterization and photocatalytic activity of N-containing ZnO powder. Chem. Eng. J. 2009, 148, 263–269. Zhou, Z.; Wang, J.; Yu, J.; Shen, Y.; Li, Y.; Liu, A.; Liu, S.; Zhang, Y. Dissolution and liquid crystals phase of 2D polymeric carbon nitride. J. Am. Chem. Soc. 2015, 137, 2179–2182. [CrossRef] [PubMed] Chen, C.; Amini, A.; Zhu, C.; Xu, Z.; Song, H.; Wang, N. Enhanced photocatalytic performance of TiO2 -ZnO hybrid nanostructures. Sci. Rep. 2014, 4. [CrossRef] Hotchandani, S.; Kamat, P.V. Charge-transfer processes in coupled semiconductor systems: Photochemistry and photoelectrochemistry of the colloidal cadmium sulfide-zinc oxide system. J. Phys. Chem. 1992, 96, 6834–6839. [CrossRef] Robel, I.; Bunker, B.; Kamat, P.V. Single-walled carbon nanotube-CdS nanocomposites as light-harvesting assemblies: Photoinduced charge-transfer interactions. Adv. Mater. 2005, 17, 2458–2463. [CrossRef] Zhang, Y.; Zhang, Q.; Shi, Q.; Cai, Z.; Yang, Z. Acid-treated g-C3 N4 with improved photocatalytic performance in the reduction of aqueous Cr(VI) under visible-light. Sep. Purif. Technol. 2015, 142, 251–257. [CrossRef] Dong, G.; Zhang, L. Synthesis and enhanced Cr(VI) photoreduction property of formate anion containing graphitic carbon nitride. J. Phys. Chem. C 2013, 117, 4062–4068. [CrossRef] Shirzad-Siboni, M.; Farrokhi, M.; Soltani, R.D.C.; Khataee, A.; Tajassosi, S. Photocatalytic reduction of hexavalent Chromium over ZnO nanorods immobilized on Kaolin. Ind. Eng. Chem. Res. 2014, 53, 1079–1087. [CrossRef] Fu, H.; Xu, T.; Zhu, S.; Zhu, Y. Photocorrosion inhibition and enhancement of photocatalytic activity for ZnO via hybridization with C60. Environ. Sci. Technol. 2008, 42, 8064–8069. [CrossRef] [PubMed] Wang, L.; Wang, N.; Zhu, L.; Yu, H.; Tang, H. Photocatalytic reduction of Cr(VI) over different TiO2 photocatalysts and the effects of dissolved organic species. J. Hazard. Mater. 2008, 152, 93–99. [CrossRef] [PubMed] Wang, X.C.; Maeda, K.; Thomas, A.; Takanabe, K.; Xin, G.; Carlsson, J.M.; Domen, K.; Antonietti, M. A metal-free polymeric photocatalyst for hydrogen production from water under visible light. Nat. Mater. 2009, 8, 76–82. [CrossRef] [PubMed] Yang, Q.L.; Kang, S.Z.; Chen, H.; Bu, W.; Mu, J. La2 Ti2 O7 : An efficient and stable photocatalyst for the photoreduction of Cr(VI) ions in water. Desalination 2011, 266, 149–153. [CrossRef] Liu, X.; Pan, L.; Zhao, Q.; Lv, T.; Zhu, G.; Chen, T.; Lu, T.; Sun, Z.; Sun, C. UV-assisted photocatalytic synthesis of ZnO-reduced graphene oxide composites with enhanced photocatalytic activity in reduction of Cr(VI). Chem. Eng. J. 2012, 183, 238–243. [CrossRef] © 2016 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC-BY) license (http://creativecommons.org/licenses/by/4.0/).