Enhancement Photocatalytic Activity of the Heterojunction of Two-Dimensional Hybrid Semiconductors ZnO/V2O5

In this work, we report the fabrication of the new heterojunction of two 2D hybrid layered semiconductors—ZnO (stearic acid)/V2O5 (hexadecylamine)—and its behavior in the degradation of aqueous methylene blue under visible light irradiation. The optimal photocatalyst efficiency, reached at a ZnO (stearic acid)/V2O5 (hexadecylamine) ratio of 1:0.25, results in being six times higher than that of pristine zinc oxide. Reusability test shows that after three photocatalysis cycles, no significant changes in either the dye degradation efficiency loss, nor the photocatalyst structure, occur. Visible light photocatalytic performance observed indicates there is synergetic effect between both 2D nanocomposites used in the heterojunction. The visible light absorption enhancement promoted by the narrower bandgap V2O5 based components; an increased photo generated charge separation favored by extensive interface area; and abundance of hydrophobic sites for dye adsorption appear as probable causes of the improved photocatalytic efficiency in this hybrid semiconductors heterojunction. Estimated band-edge positions for both conduction and valence band of semiconductors, together with experiments using specific radical scavengers, allow a plausible photodegradation mechanism.


Introduction
The use of solar energy is increasingly needed to address many of the growing energy and environmental problems facing humanity. Processes like the production of clean fuels from abundant natural resources or the degradation of pollutants based on sunlight-driven reactions, catalyzed by nanostructured semiconducting metal oxides, have received much attention during the last decades [1,2]. These are heterogeneous processes where the role of the photocatalyst is like that of the and Aslam et al. reported the decomposition of nitrophenol with bare ZnO/V 2 O 5 mixtures [36]. The capability of V 2 O 5 to sensitize the ZnO nanocomposite makes the system a very attractive and new material for studied of heterogeneous photocatalysis.
In this work, we evaluated the photocatalytic activity of the organic-inorganic laminar ZnO (stearic acid) with a band gap (Eg) of 3.28 eV, combined with organic-inorganic hybrid V 2 O 5 (hexadecylamine), (Eg = 3.28) in the degradation of aqueous methylene blue solution in visible light. In addition, we have realized various combinations with V 2 O 5 and V 2 O 5 -xerogel to investigate the photocatalytic activity. The blend of both organic-inorganic hybrid semiconductors 2D/2D produces the expected synergy improving the degradation of the dye selected as organic contaminant model. Furthermore, tentative mechanism was also discussed based on the active species trapping experiments and estimated band edge positions.

Results and Discussion
The XRD pattern of synthesized nanocomposites are shown in Figure 1. The lamellar nature of ZnO (stearic acid) (ZnO(SA)), shown in Figure 1a, and V 2 O 5 (hexadecylamine) (V 2 O 5 (HDA)), shown in Figure 1b, which were constituted of single nanosheets of semiconductor sandwiched between self-assembled surfactants, is confirmed by the X-ray diffraction patterns. These display low angle reflections, characteristic of well-ordered laminar arrangements 2D, according to the positions of the 00l reflections in the diffraction pattern. The interlayer distances along the c-axis correspond to the first peak in the low angle XRD pattern, which correlate well with the molecular lengths of corresponding carboxylic acid and alkylamine, previously reported [37,38]. In Figure 1a also shows the reflections that indicate the inorganic moiety corresponding to ZnO single phase with the wurtzite structure (JCPDS . Figure 1c shows the XRD patterns of heterojunction ZnO(SA)/V 2 O 5 (HDA); two types of phases were detected, a phase well defined of ZnO(SA) and a 001 plane to V 2 O 5 (HDA), reveal the formation of layered structure with both composites. In this XRD pattern, no other possible impurities were detected which do suggest that nanocomposites ZnO(SA) and V 2 O 5 (HDA) remains unaltered.
The chemical states of the ZnO(SA)/V 2 O 5 (HDA) composite were investigated by X-ray photoelectron spectra (XPS) as shown in Figure 2. The XPS survey spectra ( Figure S1 Figure 2b, show the V 2p 3/2 spectrum of the composite, with two components centered at 516.8 eV and 515.5 eV, corresponding to the binding energies for the V 5+ and V 4+ species, in agreement with the presence of V 2 O 5 and VO 2 [39]. The O 1s spectrum ( Figure 2c) contains two peaks, at 530 eV and 531 eV with characteristic binding energies for O-oxidation state and the peak at 531-532 eV has been assigned to the oxygen of surfactants of the composite [40].
Scanning electron microscopy (SEM) were performed to investigate the morphology and microstructures of nanocomposite samples. The morphology of nanocomposites is illustrated in the micrographics in Figure 3. The SEM image shows in Figure 3a,b the layered nature of ZnO(SA) and V 2 O 5 (HDA) while its microstructured surface, with multi-layer laminas. Figure 3c presents the SEM images of the representative ZnO(SA)/V 2 O 5 (HDA) heterojunction and EDX patterns of confirm the presence of all constituent elements of the heterojunction. No other remarkable impurities were observed. In Figure S2 SEM images of ZnO(SA)/V 2 O 5 (HDA) and element mappings were further performed, and it can be seen clearly that Zn and V are evenly distributed, indicating the formation of hybrid nanostructure.
wurtzite structure (JCPDS 36-1451). Figure 1c shows the XRD patterns of heterojunction ZnO(SA)/V2O5(HDA); two types of phases were detected, a phase well defined of ZnO(SA) and a 001 plane to V2O5(HDA), reveal the formation of layered structure with both composites. In this XRD pattern, no other possible impurities were detected which do suggest that nanocomposites ZnO(SA) and V2O5(HDA) remains unaltered.   Scanning electron microscopy (SEM) were performed to investigate the morphology and microstructures of nanocomposite samples. The morphology of nanocomposites is illustrated in the micrographics in Figure 3. The SEM image shows in Figure 3a,b the layered nature of ZnO(SA) and V2O5(HDA) while its microstructured surface, with multi-layer laminas. Figure 3c presents the SEM images of the representative ZnO(SA)/V2O5(HDA) heterojunction and EDX patterns of confirm the presence of all constituent elements of the heterojunction. No other remarkable impurities were observed. In Figure S2 SEM images of ZnO(SA)/V2O5(HDA) and element mappings were further performed, and it can be seen clearly that Zn and V are evenly distributed, indicating the formation of hybrid nanostructure.
The UV-visible diffuse reflectance was measured for ZnO(SA), V2O5(HDA) and ZnO(SA)/V2O5(HDA) to determine their light absorption characteristics ( Figure S3). The wavelength distribution of the absorbed light is an important property of photocatalysts, irrespective of the quantum yield. Therefore, the high photoactivity was attributed to higher visible light absorbance, as indicated by the UV-visible diffuse reflectance spectroscopy. The absorption bands in the range of   In Figure 4a the infrared analysis of ZnO(SA) shows the peaks in the region 3000-2800 cm −1 assigned to stretching modes of the C-H bond of the methylene group, the band at 1705 cm −1 corroborates the presence of ν(C-O) band of the carboxylic acid in the sample and the bands at 1560 and 1315 cm −1 corresponding to the asymmetric and symmetric ν(C-O) modes, respectively, demonstrate that the organic moiety is found as a carboxylate mono-coordinated to the inorganic sheets [42]. In Figure 4b, the infrared analysis of V2O5(HDA) shows that the peaks in the region 1000 cm  The UV-visible diffuse reflectance was measured for ZnO(SA), V 2 O 5 (HDA) and ZnO(SA)/V 2 O 5 (HDA) to determine their light absorption characteristics ( Figure S3). The wavelength distribution of the absorbed light is an important property of photocatalysts, irrespective of the quantum yield. Therefore, the high photoactivity was attributed to higher visible light absorbance, as indicated by the UV-visible diffuse reflectance spectroscopy. The absorption bands in the range of 200-400 nm, observed in all spectra, suggest strong free exciton absorption at room temperature. The band gap of samples can be estimated from a plot of (αhυ) 2 versus the photon energy and the intercept of a tangent to the x-axis was recorded as shown in Figure S4 where energies, Eg, determined from the spectra are 3.28 and 2.28 eV for ZnO(SA) and V 2 O 5 (HDA) respectively [41]. The variation of the band gap with respect to that of ZnO(SA) is clearly seen, the absorption edge has a clear shift toward longer wavelengths.
In Figure 4a the infrared analysis of ZnO(SA) shows the peaks in the region 3000-2800 cm −1 assigned to stretching modes of the C-H bond of the methylene group, the band at 1705 cm −1 corroborates the presence of ν(C-O) band of the carboxylic acid in the sample and the bands at 1560 and 1315 cm −1 corresponding to the asymmetric and symmetric ν(C-O) modes, respectively, demonstrate that the organic moiety is found as a carboxylate mono-coordinated to the inorganic sheets [42]. In Figure

Photocatalytic Activity
The photocatalytic activity of nanocomposites and their heterojunctures was studied by visible light degradation of methylene blue as a pollutant model. We investigated the photocatalytic behavior a series of samples with different ZnO/V2O5 molar ratios prepared by mixing ZnO(SA) and V2O5 orthorhombic, V2O5-xerogel or V2O5(HDA) as described in Table S1. The photocatalytic properties of the as-prepared samples were evaluated under similar conditions and all curves were normalized after reaching the adsorption/desorption equilibrium ( Figure S5). The results in Figure  5a shows that the sample of the heterojunction ZnO(SA)/V2O5(HDA) 1:0.25 exhibits the highest photocatalytic efficiency in visible light irradiation. The comparison of catalytic activity of these samples indicated that 90% of the MB dye was degraded after 400 min of irradiation, the luminance of the light source was 0.5 Sun. Meanwhile, the hybrid heterostructure ZnO(SA)/V2O5(HDA) 1:0.5 shows lower photocatalytic ability than the other samples, which could be attributed to the higher concentration of V2O5 acting as a recombination center for photogenerated charge carriers and increasing the sample opacity, obstructing the light absorption and decreasing the photocatalytic

Photocatalytic Activity
The photocatalytic activity of nanocomposites and their heterojunctures was studied by visible light degradation of methylene blue as a pollutant model. We investigated the photocatalytic behavior a series of samples with different ZnO/V 2 O 5 molar ratios prepared by mixing ZnO(SA) and V 2 O 5 orthorhombic, V 2 O 5-xerogel or V 2 O 5 (HDA) as described in Table S1. The photocatalytic properties of the as-prepared samples were evaluated under similar conditions and all curves were normalized after reaching the adsorption/desorption equilibrium ( Figure S5). The results in Figure 5a shows that the sample of the heterojunction ZnO(SA)/V 2 O 5 (HDA) 1:0.25 exhibits the highest photocatalytic efficiency in visible light irradiation. The comparison of catalytic activity of these samples indicated that 90% of the MB dye was degraded after 400 min of irradiation, the luminance of the light source was 0.5 Sun. Meanwhile, the hybrid heterostructure ZnO(SA)/V 2 O 5 (HDA) 1:0.5 shows lower photocatalytic ability than the other samples, which could be attributed to the higher concentration of V 2 O 5 acting as a recombination center for photogenerated charge carriers and increasing the sample opacity, obstructing the light absorption and decreasing the photocatalytic activity [17]. Photocatalytic processes are very complex, and some aspects of the reaction kinetics are being studied. In this work, the photocatalytic degradation of methylene blue could be described by pseudo-first-order kinetics, ln(C 0 /C) = kt, where k is the corresponding kinetic constant and t the irradiation time Figure 5b. The apparent rate constants (k app , min −1 ) hybrid ZnO(SA)/V 2 O 5 (HDA) was about 6 time faster than for ZnO. The rate constants were calculated and the linear regression coefficients (R 2 ) for all samples under visible light. These results agree qualitatively with reports on the improvement of the photocatalytic efficiency of stratified hybrid ZnO [33,37]. The shape of the MB spectrum remains unchanged throughout the process and no organic by products were detected, indicating the degradation of the dye ( Figure S6). The photocatalytic performance of visible light observed indicates that there is a synergistic effect between both 2D nanocomposites used. In these heterojunctions the large face-toface contact between the sheets, which provide more reaction sites for the adsorption of contaminants exhibiting greater stability, should lead to the interfacial charge transfer efficiency [26,27]. the presence of the organic surfactant in the interlaminar spaces of ZnO and V2O5 favors the adsorption of dye on the surface of semiconductors thus the abundance of hydrophobic sites for the adsorption of dye appears as a probable cause of improved photocatalytic efficiency in this hybrid semiconductors heterojunction.
To further investigate the role of active species such as h + , • OH and O2 −• in the photocatalytic degradation of dye, active species trapping experiments were performed using the sample ZnO(SA), V2O5(HDA) and ZnO(SA)/V2O5(HDA) 1:0.25. For these experiments, oxalate of ammonium (OA) was used as hole (h + ) scavenger (2 mM), isopropanol (IPA) as a • OH scavenger (2 mM) and chloroform (CHCl3) as an O2 −• scavenger (2 mM) [44][45][46]. The impact of different scavengers is shown in Figure  6a, for ZnO(SA) (64%), when the CHCl3 was added into reaction solution, the photocatalytic degradation is almost invariable (60%). However, when the IPA and OA were added into solution, the degradation rate of MB was inhibited (44% and 47%, respectively). Thus, the • OH and h + radicals are the major reactive species in the ZnO(SA) reaction. For V2O5(HDA) (48%), when the IPA was added to the reaction solution photocatalytic degradation drops to 5% indicating that • OH is the major reactive species in the reaction. On the other hand, for the ZnO(SA)/V2O5(HDA) sample (90%), when the OA and IPA were added, the photocatalytic degradation rate was decreased (53% and 54%, respectively) indicating the h + and • OH are the predominant active species. When the CHCl3 was added to the reaction solution, the degradation rate of the inhibited slightly (68%), suggesting the O2 −• affects less in the photocatalysis process. Thus, from these experiments, we conclude • OH and These results agree qualitatively with reports on the improvement of the photocatalytic efficiency of stratified hybrid ZnO [33,37]. The shape of the MB spectrum remains unchanged throughout the process and no organic by products were detected, indicating the degradation of the dye ( Figure S6). The photocatalytic performance of visible light observed indicates that there is a synergistic effect between both 2D nanocomposites used. In these heterojunctions the large face-to-face contact between the sheets, which provide more reaction sites for the adsorption of contaminants exhibiting greater stability, should lead to the interfacial charge transfer efficiency [26,27]. the presence of the organic surfactant in the interlaminar spaces of ZnO and V 2 O 5 favors the adsorption of dye on the surface of semiconductors thus the abundance of hydrophobic sites for the adsorption of dye appears as a probable cause of improved photocatalytic efficiency in this hybrid semiconductors heterojunction.
To  Therefore, when the photon energy higher or equal to band gap energy of ZnO and V2O5, electrons can be excited from the VB to the CB with simultaneous generation of holes in the VB. Due to the CB of ZnO(SA) is more negative than that of the V2O5(HDA), electrons on the CB of ZnO(SA) can be easily injected into the CB of V2O5(HDA) by the interface. The photogenerated electrons on the CB of V2O5(HDA) could not produce O2 −• from dissolved O2, due to the position of CB is more positive than the potential of O2/O2 −• (−0.33 V vs. NHE). However, the reduction potential of O2/H2O2 is 0.695 eV vs. NHE, means electrons can react with O2 and H + to produce H2O2 which indirectly produce • OH [46]. This is also observed in heterojunctions with other semiconductors like Ag3VO4/WO3 [47], ZnO/MoS2 [45] and Ag3PO4/TiO2 [48]. Meanwhile the photoinduced holes in VB of both V2O5 and ZnO can oxidize adsorbed H2O molecules to produce • OH ( • OH/H2O is 2.72 eV vs. NHE).
The mechanism can be described as: • OH radicals are very reactive oxidative species to degrade of dye. As illustrated in Figure 6b, the prepared heterojunctions ZnO/V2O5 improve the photogenerated electron-hole pair's separation and transfer, favored by extensive interface area, as well as show an oxidation and reduction ability for efficient degradation of organic pollutants.
Concerning the stability of the photocatalyst, we have also investigated the stability and reusability of the ZnO(SA)/V2O5(HDA) photocatalyst over three cycles for a period of 400 min where χ is the electronegativity of semiconductors ZnO and V 2 O 5 (5.79 and 6.10 eV, respectively); E e is the energy of free electrons on the hydrogen scale (4.5 eV); and the band gap energy (E g ) of the semiconductor calculated from DRS data [40].  • OH radicals are very reactive oxidative species to degrade of dye. As illustrated in Figure 6b, the prepared heterojunctions ZnO/V 2 O 5 improve the photogenerated electron-hole pair's separation and transfer, favored by extensive interface area, as well as show an oxidation and reduction ability for efficient degradation of organic pollutants. Concerning the stability of the photocatalyst, we have also investigated the stability and reusability of the ZnO(SA)/V 2 O 5 (HDA) photocatalyst over three cycles for a period of 400 min irradiation [46,49]. Figure S7 shows a loss photocatalytic activity of approx. 10% in which might be due to the loss of the photocatalyst by centrifugation, washing and drying of sample, during each cycle of reusability. No significant changes in neither the dye degradation efficiency loss nor photocatalyst structure occur.

Materials
All the reagents in this study were commercial products purchased from Sigma-Aldrich (Darmstadt, Germany) and Merck (St. Louis, USA and were used without further purification. Nanopure water was obtained from a water purification system.

Synthesis of ZnO(Stearic Acid)-ZnO(SA)
ZnO-stearic acid nanocomposite was synthesized as described in a previous paper [36]. In a typical procedure, ZnSO 4 (1 mol L −1 ) was mixed with Na 2 CO 3 (1 mol L −1 )/NaOH (1 mol L −1 ) (1:1) to bear a ZnO hydrogel; then stearic acid (SA) 4.0 × 10 −1 mol L −1 aqueous solution was added to the ZnO hydrogel solution under stirring at room temperature; the suspension was stirred at 60 • C followed by an aging period of 24 h at room temperature. The sample was washed and dried at 80 • C for 72 h.

Synthesis of Compounds Vanadium Pentoxide
V 2 O 5 xerogel, a mixture of t-butyl alcohol and orthorhombic V 2 O 5 was refluxed for 6 h to form the xerogel. Water was added to the resulting dark yellow solid and the remaining t-butyl alcohol was removed with excess water under vacuum. Water was added to yield a suspension. The material was aged at room temperature yielding a red-brown colloidal V 2 O 5 [37]. V 2 O 5 (hexadecylamine)-V 2 O 5 (HDA). A solution of 10 −3 mol of hexadecylamine (HDA) in pure ethanol, previously degassed was mixed with 2 × 10 −3 mol of vanadium triisopropoxide (VOTPP) [37]. The yellow solution, obtained after vigorous stirring in an argon atmosphere for 1 h, was then hydrolyzed by adding 15 mL of water. The orange suspension obtained after stirring for 24 h. and subjected to a hydrothermal treatment in a Teflon lined autoclave at 180 • C for 6 days. From the results, an orange solid was separated, washed with pure ethanol and water, the sample was washed and dried at 80 • C for 72 h Analysis: % found (calculated) for V 2

Photocatalytic Experiments
The photocatalytic activity of nanocomposites was studied for the degradation in visible light irradiation of methylene blue as a pollutant model. For this study, the characteristic absorption peak of MB at 664 nm was monitored using a UV-vis spectrophotometer at regular intervals of time and the corresponding absorption spectra. The photocatalytic activity of the products was evaluated by measuring the degradation of methylene blue (MB) in water. The nanocomposite ZnO(SA) (10 mg) and compounds of V 2 O 5 in different proportions in 25 mL of a 1 × 10 −5 mol L −1 which solution phosphate buffer prepared mixing solutions of Na 2 HPO 4 and NaH 2 PO 4 . Prior to irradiation, the suspension was magnetically stirred for 30 min, to establish an adsorption/desorption equilibrium.
The suspensions were irradiated by a 300 W Xe arc lamp (Newport) with a UVIR-CUT filter at λ ≥ 400 nm, the luminance of the light source over the reactant solution was 0.5 Sun. All samples were constant magnetic stirring to ensure a higher level of homogeneity of the photocatalyst in the suspension. The MB concentration after equilibration was regarded as the initial concentration (C 0 ) and was monitored in the UV-vis spectra of the solution (Perkin Elmer Lambda 35, Shelton, USA), using nanopure water as a reference.

Characterization
X-ray diffraction (XRD) analyses of the products were performed using a Bruker D8 Advance (Cu Kα λ = 1.5418 Å). The Scanning Electron Microscopy (SEM) and images were obtained by using an EVO MA 10 ZEISS microscope. X-ray photoelectron spectra (XPS) were obtained in a STAIB system with a RQ-300 X-ray source, using monochromated Al Kα X-rays (1486.6 eV, 75 W). The charge referencing was done against adventitious carbon (C 1s 285 eV). Zn 2p, V 2p, O 1s and C1s energy regions were scanned with several sweeps until a good signal-to-noise ratio was observed. Chemical analysis was obtained by using (SISONS ES-1108). The diffuse reflectance UV-vis spectra were recorded in the range of 200-800 nm using a Perkin Elmer Lambda 35 spectrometer. Reflectance measurements were converted to absorption spectra using the Kubelka-Munk function. Fourier transform infrared (FTIR) spectra were recorded an FT/IR-4600 Jasco spectrometer (Tokyo, Japan).

Conclusions
In summary, we have reported the successfull fabrication of the new heterojunction of two 2D hybrid semiconductors, ZnO(SA)/V 2 O 5 (HDA) exhibiting improved photocatalytic activity than the pristine zinc oxide in the degradation of methylene blue under visible light. The efficacy of the products described as catalysts for the photooxidation of organic pollutants is associated with the surface properties of 2D hybrid nanocomposites, collection efficiency for visible light as well as the presence of organic surfactant in the interlaminar spaces of ZnO and V 2 O 5 that favors the adsorption of the dye on the surface of semiconductors. The nanocomposites of V 2 O 5 effectively inhibit the recombination of photogenerated electron/hole pairs. The products are recyclables and seen as potentially useful for environmental issues remediation.