Synergically Improving Light Harvesting and Charge Transportation of TiO 2 Nanobelts by Deposition of MoS 2 for Enhanced Photocatalytic Removal of Cr ( VI )

Herein, MoS2/TiO2 nanobelts heterojunction have been successfully synthesized by in situ growth method for photocatalytic reduction of Cr(VI). TiO2 nanobelts (NBs) with rough surface were prepared firstly by acidic treatment process, which is beneficial for deposition and growth of MoS2 to form heterojunctions. As a result of special energy level offset and nanostructure, MoS2/TiO2 NBs composite were endowed with higher light-harvesting capacity and charge transportation efficiency, which are indispensible merits for excellent photocatalytic activity. The photocatalytic reduction of Cr(VI) reveals that the synthesized MoS2/TiO2 NBs composite have superior photocatalytic ability than other samples. Meanwhile, a photoreduction mechanism is proposed based on the systematic investigation, where the photogenerated electrons are demonstrated as the dominant reductive species to reduce Cr(VI) to Cr(III).


Introduction
Hexavalent chromium (Cr(VI)) is a common heavy metal pollutant in the wastewater, which has attracted considerable attention around the world owing to its high toxicity and strong carcinogenic activity for humans and living things in nature [1][2][3].Therefore, it is of great importance to explore how to effectively remove Cr(VI) in wastewater.Semiconductor-based photocatalytic reduction of Cr(VI) has received much attention recently due to its low cost, sustainability, and environmental friendliness without secondary pollution [4][5][6][7].Nevertheless, to date, developing a highly efficient, cost-effective and stable photocatalyst for removal of Cr(VI) with visible-light activity is still being pursued.
Among various metal oxide semiconductors, TiO 2 is probably one of the most studied oxide semiconductor materials, and is used in a broad range of applications such as paints [8], (photo)catalysis [9], photovoltaics [10], and hybrid light-emitting diodes [11], and, as aforementioned, alkaliion batteries [12].Owing to polymorphism richness of TiO 2 and its 3d 0 electronic configuration inducing exceptional sensitiveness of the optoelectronic properties to the introduction of point defects, it has been extensively studied and endowed with new properties [13][14][15].However, similar to many semiconductors, the poor harvesting of solar energy and charge carrier separation of pure TiO 2 leads to the low photocatalytic activity and thus cannot meet the demand of commercial applications.
To improve the photocatalytic performance of TiO 2 , coupling TiO 2 with other semiconductors for constructing a heterojunction system is an interesting method that has received more attention in the past decades [16][17][18][19][20]. Graphene-like molybdenum disulfide (MoS 2 ) can be a good candidate for tuning photoresponse and improving charge carrier transportation properties [21][22][23][24].In fact, layered MoS 2 is often used as an effective cocatalyst in photocatalytic or electrocatalytic hydrogen evolution reactions due to its large surface area and high electrical conductivity [25][26][27].These studies demonstrate that the incorporation of layered MoS 2 with a metal oxide can strongly promote visible light harvest ability and separation efficiency of excited charges and photocatalytic activity.
Based on the above strategy, herein, by means of coupling TiO 2 nanobelts with MoS 2 , we successfully fabricated MoS 2 /TiO 2 NBs composite to form a p-n heterojunction for improving efficiency of solar energy utilization and photoinduced charge transportation.The photocatalytic reduction of Cr(VI) reveals that the synthesized MoS 2 /TiO 2 NBs composite have superior photocatalytic ability than other samples.As a result of special energy level offset and nanostructure, MoS 2 /TiO 2 NBs composite were endowed with higher light-harvesting capacity and charge transportation efficiency, which are indispensible merits for excellent photocatalytic activity.Meanwhile, a photoreduction mechanism is proposed based on the systematic investigation, where the photogenerated electrons are demonstrated as the dominant reductive species to reduce Cr(VI) to Cr(III).

Synthesis and Characterizations of MoS 2 /TiO 2 NBs Heterojunction Composite
The crystallographic structure and phase of the as-obtained pristine TiO 2 NBs, MoS 2 , and MoS 2 /TiO 2 heterojunction samples were examined by XRD analysis, as shown in Figure 1.All the diffraction peaks of the TiO 2 NBs sample can be well matched with anatase phase of TiO 2 (Joint Committee on Powder Diffraction Standards (JCPDS) card no.21-1272) [28].No impurity peaks are detected, implying that the final TiO 2 product is of pure phase.The strong peaks at 25.2 • , 37.7 • , 48.0 • , 53.9 • , 55.0 • , and 62.7 • are attributed to the (101), (400), (200), (105), (211), and (204) crystal facets, respectively [29].In the case of MoS 2 nanotubes, the XRD pattern is agreement with hexagonal phase of MoS 2 (JCPDS card no.73-1508), whereas the crystallinity of MoS 2 nanotubes is relatively low, resulting in broadened diffraction peaks due to lack of high temperature annealing.As shown in Figure 1, three obvious peaks at 16.4 • , 32.7 • , and 56.9 • are ascribed to the characteristic (002), (100), and (110) facets, respectively [30,31].After in situ growth of MoS 2 by means of adding TiO 2 as a precursor, the characteristic diffraction peaks of MoS 2 and TiO 2 can be observed in the XRD pattern of the as-obtained composite in Figure 1, which indicates that MoS 2 and TiO 2 exist together in the composite.Meanwhile, we can find that the relative intensity of diffraction peaks due to MoS 2 is lower than that of TiO 2 despite designed molar ratio of TiO 2 and Mo element is 1:1, which results from the low amount of the formed MoS 2 in the composite.The morphologies of the as-synthesized TiO2 NBs, MoS2, and MoS2/TiO2 NBs samples are present in Figure 2. It is observed that the formation of H2Ti3O7 is uniform nanobelts with smooth surface in Figure 2a, whereas morphology and surface smoothness of the prepared TiO2 NBs were obviously changed because of dehydration at elevated temperature.Nevertheless, the rough surface of TiO2 NBs is beneficial for deposition of MoS2 precursors on the surface of TiO2 NBs.Meanwhile, in the absence of TiO2 NBs during the preparation procedure, it was found that uniform MoS2 nanotubes were formed, which does not agree well with the previous reports on prepared of layered MoS2 [32][33][34].We elucidate that octylamine and ethanol were chosen as combined solvent, which results in curling growth of MoS2 layers and formation of nanotubes.In the case of MoS2/TiO2 NBs composites, the change of TiO2 NBs morphology is negligible when MoS2 was formed by hydrothermal process, as displayed in Figure 2d.The MoS2 anchored on the surface of TiO2 NBs, which ensures efficient interaction between MoS2 and TiO2 NBs.Furthermore, TEM images of these samples were taken to further investigate morphologies and nanostructures, as shown in Figure 3.After annealed at 600 °C for 2 h, the dimension of TiO2 NBs was reduced by compared with the scale of H2Ti3O7 nanobelts owing to releasing of crystal water from the H2Ti3O7 lattice, results in shrinking of lattice frame and forming smaller nanobelt pieces with rough surface.In Figure 3c,d, it is observed that the MoS2 grew on the surface of TiO2 NBs and formed MoS2/TiO2 NBs heterojunction at the interface, which could promote excited charge transportation between MoS2 and TiO2 NBs.In addition, we found that when TiO2 NBs was added as a precursor, the MoS2 nanotubes were not formed in comparison with the pristine MoS2 nanotubes in Figure 3b, which indicates that the TiO2 NBs acts as a solid interface to reduce the curling trend of MoS2 layers deriving from different polar solvents.The morphologies of the as-synthesized TiO 2 NBs, MoS 2 , and MoS 2 /TiO 2 NBs samples are present in Figure 2. It is observed that the formation of H 2 Ti 3 O 7 is uniform nanobelts with smooth surface in Figure 2a, whereas morphology and surface smoothness of the prepared TiO 2 NBs were obviously changed because of dehydration at elevated temperature.Nevertheless, the rough surface of TiO 2 NBs is beneficial for deposition of MoS 2 precursors on the surface of TiO 2 NBs.Meanwhile, in the absence of TiO 2 NBs during the preparation procedure, it was found that uniform MoS 2 nanotubes were formed, which does not agree well with the previous reports on prepared of layered MoS 2 [32][33][34].We elucidate that octylamine and ethanol were chosen as combined solvent, which results in curling growth of MoS 2 layers and formation of nanotubes.In the case of MoS 2 /TiO 2 NBs composites, the change of TiO 2 NBs morphology is negligible when MoS 2 was formed by hydrothermal process, as displayed in Figure 2d.The MoS 2 anchored on the surface of TiO 2 NBs, which ensures efficient interaction between MoS 2 and TiO 2 NBs.Furthermore, TEM images of these samples were taken to further investigate morphologies and nanostructures, as shown in Figure 3.After annealed at 600 • C for 2 h, the dimension of TiO 2 NBs was reduced by compared with the scale of H 2 Ti 3 O 7 nanobelts owing to releasing of crystal water from the H 2 Ti 3 O 7 lattice, results in shrinking of lattice frame and forming smaller nanobelt pieces with rough surface.In Figure 3c,d, it is observed that the MoS 2 grew on the surface of TiO 2 NBs and formed MoS 2 /TiO 2 NBs heterojunction at the interface, which could promote excited charge transportation between MoS 2 and TiO 2 NBs.In addition, we found that when TiO 2 NBs was added as a precursor, the MoS 2 nanotubes were not formed in comparison with the pristine MoS 2 nanotubes in Figure 3b, which indicates that the TiO 2 NBs acts as a solid interface to reduce the curling trend of MoS 2 layers deriving from different polar solvents.In Figure 4, it is observed that the obvious peaks at 143, 397, and 515 cm −1 are attributed to the characteristic E g(1) , B 1g(1) , and A 1g + B 1g(2) vibration of anatase TiO 2 , respectively [26].In the case of MoS 2 , the peaks at 375 and 405 cm −1 are ascribed to the typical E 1 2g and A 1g vibration modes, respectively [31].It is well-known that the E 1 2g vibration mode associates with in-layer displacements of Mo and S atoms while A 1g is related to out of layer symmetric displacements of S atoms along c axis.The other three obvious peaks at 282, 146, and 336 cm −1 originate from E 1g and appearance of 1T-MoS 2 phase [35,36].After epitaxial growth of MoS 2 on the surface of TiO 2 NBs, the corresponding characteristic peaks of TiO 2 and MoS 2 in the composite were detected at 150 and 405 cm −1 , respectively, which demonstrates that MoS 2 /TiO 2 NBs heterojunction composite was successfully prepared.Furthermore, FTIR spectra of the obtained composite samples are displayed in Figure 5, where the surface organic groups of TiO 2 NBs, MoS 2 , and MoS 2 /TiO 2 samples have been investigated.The obvious peaks centered at 2920 and 2856 cm −1 , and 1502 cm −1 are attributed to stretching vibration of C-H and N-H bands from CH 3 -and NH 2 -because of usage of octylamine as a solvent [37,38].The characteristic peak at 920 cm −1 is assigned to vibration of Mo-S band, which was not found in the pristine TiO 2 NBs sample [39].In addition, the strong absorbance peak at 472 cm −1 was observed, which originates from vibration of Ti-O supported by the previous reports [40].The above results demonstrate that MoS 2 and TiO 2 phase exist together in the MoS 2 /TiO 2 composite.
Catalysts 2017, 7, 30 5 of 13 In Figure 4, it is observed that the obvious peaks at 143, 397, and 515 cm −1 are attributed to the characteristic Eg(1), B1g(1), and A1g + B1g(2) vibration of anatase TiO2, respectively[26].In the case of MoS2, the peaks at 375 and 405 cm −1 are ascribed to the typical E 1 2g and A1g vibration modes, respectively [31].It is well-known that the E 1 2g vibration mode associates with in-layer displacements of Mo and S atoms while A1g is related to out of layer symmetric displacements of S atoms along c axis.The other three obvious peaks at 282, 146, and 336 cm −1 originate from E1g and appearance of 1T-MoS2 phase [35,36].After epitaxial growth of MoS2 on the surface of TiO2 NBs, the corresponding characteristic peaks of TiO2 and MoS2 in the composite were detected at 150 and 405 cm −1 , respectively, which demonstrates that MoS2/TiO2 NBs heterojunction composite was successfully prepared.Furthermore, FTIR spectra of the obtained composite samples are displayed in Figure 5, where the surface organic groups of TiO2 NBs, MoS2, and MoS2/TiO2 samples have been investigated.The obvious peaks centered at 2920 and 2856 cm −1 , and 1502 cm −1 are attributed to stretching vibration of C-H and N-H bands from CH3-and NH2-because of usage of octylamine as a solvent [37,38].The characteristic peak at 920 cm −1 is assigned to vibration of Mo-S band, which was not found in the pristine TiO2 NBs sample [39].In addition, the strong absorbance peak at 472 cm −1 was observed, which originates from vibration of Ti-O supported by the previous reports [40].The above results demonstrate that MoS2 and TiO2 phase exist together in the MoS2/TiO2 composite.In Figure 4, it is observed that the obvious peaks at 143, 397, and 515 cm −1 are attributed to the characteristic Eg(1), B1g(1), and A1g + B1g(2) vibration of anatase TiO2, respectively [26].In the case of MoS2, the peaks at 375 and 405 cm −1 are ascribed to the typical E 1 2g and A1g vibration modes, respectively [31].It is well-known that the E 1 2g vibration mode associates with in-layer displacements of Mo and S atoms while A1g is related to out of layer symmetric displacements of S atoms along c axis.The other three obvious peaks at 282, 146, and 336 cm −1 originate from E1g and appearance of 1T-MoS2 phase [35,36].After epitaxial growth of MoS2 on the surface of TiO2 NBs, the corresponding characteristic peaks of TiO2 and MoS2 in the composite were detected at 150 and 405 cm −1 , respectively, which demonstrates that MoS2/TiO2 NBs heterojunction composite was successfully prepared.Furthermore, FTIR spectra of the obtained composite samples are displayed in Figure 5, where the surface organic groups of TiO2 NBs, MoS2, and MoS2/TiO2 samples have been investigated.The obvious peaks centered at 2920 and 2856 cm −1 , and 1502 cm −1 are attributed to stretching vibration of C-H and N-H bands from CH3and NH2-because of usage of octylamine as a solvent [37,38].The characteristic peak at 920 cm −1 is assigned to vibration of Mo-S band, which was not found in the pristine TiO2 NBs sample [39].In addition, the strong absorbance peak at 472 cm −1 was observed, which originates from vibration of Ti-O supported by the previous reports [40].The above results demonstrate that MoS2 and TiO2 phase exist together in the MoS2/TiO2 composite.The X-ray photoelectron spectroscopy (XPS) can probe chemical environment of element in composite, which is useful for investigating composition of MoS 2 /TiO 2 heterojunction system.As can be seen in Figure 6a, Mo, Ti, O, and S were obviously observed in survey spectrum of the MoS 2 /TiO 2 heterojunction composite, which indicates that the four elements exist in the sample.In the high resolution XPS spectrum of Ti 2p (Figure 6b), two strong peaks, appearing at 459.1 and 464.8 eV, are ascribed to Ti 2p 3/2 and Ti 2p 1/2 of Ti 4+ in the sample, respectively [41,42].In Figure 6c, we can observe that the high resolution XPS spectrum of Mo 3d reveals two strong peaks at 228.5 and 231.8 eV, corresponding to Mo 3d 5/2 and Mo 3d 3/2 , respectively, which evidently demonstrates the valence state of molybdenum element is +4 in the sample of MoS 2 /TiO 2 [31].Meanwhile, an apparent peak at 225.8 eV is assigned to the binding energy of S 2s, which strongly indicates the existence of MoS 2 .In Figure 6d, the peak at 161.4 and 162.5 eV can be assigned to S 2p 3/2 and S 2p 1/2 due to spin orbit separation of S element, which suggests the existence of S 2− in the final product [21].In addition, another peak at 169.2 eV was found, which is due to the residual of SO 4 2− in the product.
The X-ray photoelectron spectroscopy (XPS) can probe chemical environment of element in composite, which is useful for investigating composition of MoS2/TiO2 heterojunction system.As can be seen in Figure 6a, Mo, Ti, O, and S were obviously observed in survey spectrum of the MoS2/TiO2 heterojunction composite, which indicates that the four elements exist in the sample.In the high resolution XPS spectrum of Ti 2p (Figure 6b), two strong peaks, appearing at 459.1 and 464.8 eV, are ascribed to Ti 2p3/2 and Ti 2p1/2 of Ti 4+ in the sample, respectively [41,42].In Figure 6c, we can observe that the high resolution XPS spectrum of Mo 3d reveals two strong peaks at 228.5 and 231.8 eV, corresponding to Mo 3d5/2 and Mo 3d3/2, respectively, which evidently demonstrates the valence state of molybdenum element is +4 in the sample of MoS2/TiO2 [31].Meanwhile, an apparent peak at 225.8 eV is assigned to the binding energy of S 2s, which strongly indicates the existence of MoS2.In Figure 6d, the peak at 161.4 and 162.5 eV can be assigned to S 2p3/2 and S 2p1/2 due to spin orbit separation of S element, which suggests the existence of S 2− in the final product [21].In addition, another peak at 169.2 eV was found, which is due to the residual of SO4 2− in the product.The UV-vis absorption spectra of the samples are displayed in Figure 7a.The absorption edge of TiO2 NBs is about 380 nm, which indicates that the pristine TiO2 NBs only absorbs UV light part of solar light.When coupling with MoS2, the obtained MoS2/TiO2 NBs heterojunction system exhibits strong ability to absorb visible light.Meanwhile, it can be observed that pure MoS2 possesses excellent photoresponse ability for the entire solar spectrum, which is consistent with the previous report.The optical band gap energy (Eg) of the semiconductors can be calculated from the equation (αhν) n = A(hν − Eg) [31], where α, h, ν, Eg, and A are the absorption coefficient, plank constant, light frequency, band gap energy, and a constant, respectively.Among them, n depends on the characteristic of the transition in a semiconductor (n = 2 for direct transition or n = 1/2 for indirect The UV-vis absorption spectra of the samples are displayed in Figure 7a.The absorption edge of TiO 2 NBs is about 380 nm, which indicates that the pristine TiO 2 NBs only absorbs UV light part of solar light.When coupling with MoS 2 , the obtained MoS 2 /TiO 2 NBs heterojunction system exhibits strong ability to absorb visible light.Meanwhile, it can be observed that pure MoS 2 possesses excellent photoresponse ability for the entire solar spectrum, which is consistent with the previous report.The optical band gap energy (E g ) of the semiconductors can be calculated from the equation (αhν) n = A(hν − E g ) [31], where α, h, ν, E g , and A are the absorption coefficient, plank constant, light frequency, band gap energy, and a constant, respectively.Among them, n depends on the characteristic of the transition in a semiconductor (n = 2 for direct transition or n = 1/2 for indirect transition).Herein, n is 2 as the material is a direct gap semiconductor.From the plots of (αhν) 2 vs. (hν), the E g of the TiO 2 NBs, MoS 2 , and MoS 2 /TiO 2 NBs are about 3.18, 1.30, and 3.07 eV, respectively, as shown in Figure 7b.The result reveals that the light harvesting range of the heterojunction sample is enlarged after coupling TiO 2 NBs with MoS 2 .
Catalysts 2017, 7, 30 7 of 13 transition).Herein, n is 2 as the material is a direct gap semiconductor.From the plots of (αhν) 2 vs. (hν), the Eg of the TiO2 NBs, MoS2, and MoS2/TiO2 NBs are about 3.18, 1.30, and 3.07 eV, respectively, as shown in Figure 7b.The result reveals that the light harvesting range of the heterojunction sample is enlarged after coupling TiO2 NBs with MoS2.To evaluate effects of the morphologies of the as-obtained samples on adsorptive performance, N2 adsorption-desorption isotherm analysis was used to gain the surface area ratio and distribution of pore size.The BET specific surface areas of TiO2 NBs, MoS2 and MoS2/TiO2 heterojunction were calculated and equal to 46.8, 255.3 and 62.9 m 2 /g, respectively.The larger surface area of the pristine MoS2 sample is due to the unique nanotube structure, which could increase the surface area of TiO2 NBs after coupling MoS2 and TiO2.The corresponding pore size distribution are 2.2, 2.1, and 1.9 nm for the pristine TiO2 NBs, MoS2, and MoS2/TiO2 heterojunction system, respectively, which shows a similar pore distribution, resulting from the interstitial spaces between nanobelts.The results indicate that the coupling could enlarge the surface area and slightly change the pore size.

Photocatalytic Activity of MoS2/TiO2 NBs Heterojunctions
Owing to different redox potentials of Cr2O7 2− under different pH conditions [43], effects of pH values on photoactivity of MoS2/TiO2 heterojunction system for reducing of Cr(VI) were investigated (Figure 8).It was observed that the adsorption ability of MoS2/TiO2 composite for Cr(VI) under acid condition is the similar as under neutral and base condition during dark equilibrium process.However, when the solution was irradiated by visible light, the degradation efficiency of Cr(VI) under acidic condition is much higher than under neutral and alkaline condition, which indicates that acidic condition is beneficial for photoreduction of Cr(VI) over MoS2/TiO2 composite.It was found that 100% of Cr(VI) was reduced under acidic condition under irradiation for 1 h.We elucidate that the Cr2O7 2− ion under acidic condition possesses lower redox potential than under alkaline condition, which ensures photoreduction reaction of Cr(VI) carried out over the MoS2/TiO2 composites.To evaluate effects of the morphologies of the as-obtained samples on adsorptive performance, N 2 adsorption-desorption isotherm analysis was used to gain the surface area ratio and distribution of pore size.The BET specific surface areas of TiO 2 NBs, MoS 2 and MoS 2 /TiO 2 heterojunction were calculated and equal to 46.8, 255.3 and 62.9 m 2 /g, respectively.The larger surface area of the pristine MoS 2 sample is due to the unique nanotube structure, which could increase the surface area of TiO 2 NBs after coupling MoS 2 and TiO 2 .The corresponding pore size distribution are 2.2, 2.1, and 1.9 nm for the pristine TiO 2 NBs, MoS 2 , and MoS 2 /TiO 2 heterojunction system, respectively, which shows a similar pore distribution, resulting from the interstitial spaces between nanobelts.The results indicate that the coupling could enlarge the surface area and slightly change the pore size.

Photocatalytic Activity of MoS 2 /TiO 2 NBs Heterojunctions
Owing to different redox potentials of Cr 2 O 7 2− under different pH conditions [43], effects of pH values on photoactivity of MoS 2 /TiO 2 heterojunction system for reducing of Cr(VI) were investigated (Figure 8).It was observed that the adsorption ability of MoS 2 /TiO 2 composite for Cr(VI) under acid condition is the similar as under neutral and base condition during dark equilibrium process.However, when the solution was irradiated by visible light, the degradation efficiency of Cr(VI) under acidic condition is much higher than under neutral and alkaline condition, which indicates that acidic condition is beneficial for photoreduction of Cr(VI) over MoS 2 /TiO 2 composite.It was found that 100% of Cr(VI) was reduced under acidic condition under irradiation for 1 h.We elucidate that the Cr 2 O 7 2− ion under acidic condition possesses lower redox potential than under alkaline condition, which ensures photoreduction reaction of Cr(VI) carried out over the MoS 2 /TiO 2 composites.Meanwhile, we compared photoreduction efficiency of Cr(VI) over the MoS2/TiO2 NBs composite with the pure TiO2 NBs, pristine MoS2, and mechanically mixed TiO2 + MoS2 samples, as shown in Figure 9.The degradation efficiency of Cr(VI) reached to nearly 100% for the MoS2/TiO2 NBs composite, whereas other samples exhibited lower photocatalytic activities during the visible light illumination process.In the case of blank test, the concentration of Cr(VI) has almost no variation under visible light illumination for 1 h, which rules out the photolysis effect on the absorption peak of Cr(VI).Meanwhile, it was found that the mechanically mixed sample MoS2 + TiO2 displayed low photoreduction activity even though MoS2 was added, which demonstrates that the efficient heterojunction has not been formed at the interface between MoS2 and TiO2 NBs by mechanical mixing.In addition, the MoS2 nanotubes present less adsorptive ability of Cr(VI) under adsorption-desorption equilibrium process, which does not agree well with the result of BET specific surface area.We elucidate that although the prepared MoS2 possesses huge surface area, it cannot chelate with negative Cr2O7 2− ions, resulting in low adsorbing amount under dark.Meanwhile, we compared photoreduction efficiency of Cr(VI) over the MoS 2 /TiO 2 NBs composite with the pure TiO 2 NBs, pristine MoS 2 , and mechanically mixed TiO 2 + MoS 2 samples, as shown in Figure 9.The degradation efficiency of Cr(VI) reached to nearly 100% for the MoS 2 /TiO 2 NBs composite, whereas other samples exhibited lower photocatalytic activities during the visible light illumination process.In the case of blank test, the concentration of Cr(VI) has almost no variation under visible light illumination for 1 h, which rules out the photolysis effect on the absorption peak of Cr(VI).Meanwhile, it was found that the mechanically mixed sample MoS 2 + TiO 2 displayed low photoreduction activity even though MoS 2 was added, which demonstrates that the efficient heterojunction has not been formed at the interface between MoS 2 and TiO 2 NBs by mechanical mixing.In addition, the MoS 2 nanotubes present less adsorptive ability of Cr(VI) under adsorption-desorption equilibrium process, which does not agree well with the result of BET specific surface area.We elucidate that although the prepared MoS 2 possesses huge surface area, it cannot chelate with negative Cr 2 O 7 2− ions, resulting in low adsorbing amount under dark.Meanwhile, we compared photoreduction efficiency of Cr(VI) over the MoS2/TiO2 NBs composite with the pure TiO2 NBs, pristine MoS2, and mechanically mixed TiO2 + MoS2 samples, as shown in Figure 9.The degradation efficiency of Cr(VI) reached to nearly 100% for the MoS2/TiO2 NBs composite, whereas other samples exhibited lower photocatalytic activities during the visible light illumination process.In the case of blank test, the concentration of Cr(VI) has almost no variation under visible light illumination for 1 h, which rules out the photolysis effect on the absorption peak of Cr(VI).Meanwhile, it was found that the mechanically mixed sample MoS2 + TiO2 displayed low photoreduction activity even though MoS2 was added, which demonstrates that the efficient heterojunction has not been formed at the interface between MoS2 and TiO2 NBs by mechanical mixing.In addition, the MoS2 nanotubes present less adsorptive ability of Cr(VI) under adsorption-desorption equilibrium process, which does not agree well with the result of BET specific surface area.We elucidate that although the prepared MoS2 possesses huge surface area, it cannot chelate with negative Cr2O7 2− ions, resulting in low adsorbing amount under dark.Commonly, for n-type TiO 2 , the Fermi level is close to the conduction band, whereas for p-type MoS 2 , the Fermi level approaches to the valence band.When the TiO 2 NBs was coupled with the MoS 2 , the heterojunctions among these semiconductors were formed, resulting in the realignment of their valence and conduction bands due to the thermal equilibrium of different Fermi levels and the formation of built-in electric field [44,45].This allows the energy bands of TiO 2 and MoS 2 shift downward and upward, respectively, along the Fermi level, as shown in Figure 10.When the MoS 2 /TiO 2 heterojunction system was irradiated by visible light, the MoS 2 are excited to produce electrons and holes.The photoinduced electrons on the conduction band of the MoS 2 transfer to that of TiO 2 NBs, whereas the holes remain in the valence band of the MoS 2 , which could react with S 2− in the sample and to some degree undermine the photocatalytic performance of the sample, as shown in Figure S1.As discussed above, owing to the formation of the heterojunction at the interface between MoS 2 and TiO 2 NBs, the suppression of the recombination of photoinduced electron-hole pairs is realized, which allows more photogenerated electrons to participate in the reduction reactions and strongly enhances photocatalytic activity under visible light irradiation.

Photocatalytic Reduction Mechanism of MoS2/TiO2 NBs p-n Heterojunction
Catalysts 2017, 7, 30 9 of 13 for p-type MoS2, the Fermi level approaches to the valence band.When the TiO2 NBs was coupled with the MoS2, the heterojunctions among these semiconductors were formed, resulting in the realignment of their valence and conduction bands due to the thermal equilibrium of different Fermi levels and the formation of built-in electric field [44,45].This allows the energy bands of TiO2 and MoS2 shift downward and upward, respectively, along the Fermi level, as shown in Figure 10.When the MoS2/TiO2 heterojunction system was irradiated by visible light, the MoS2 are excited to produce electrons and holes.The photoinduced electrons on the conduction band of the MoS2 transfer to that of TiO2 NBs, whereas the holes remain in the valence band of the MoS2, which could react with S 2− in the sample and to some degree undermine the photocatalytic performance of the sample, as shown in Figure S1.As discussed above, owing to the formation of the heterojunction at the interface between MoS2 and TiO2 NBs, the suppression of the recombination of photoinduced electron-hole pairs is realized, which allows more photogenerated electrons to participate in the reduction reactions and strongly enhances photocatalytic activity under visible light irradiation.

Synthesis of TiO2 Nanobelts
First, 0.4 g of P25 powder was added into 80 mL of NaOH solution (10 M), and stirred vigorously for 30 min to obtain mean suspension.The mixture was transferred to 100 mL Teflon autoclave, which was heated to 180 °C and maintained for 48 h.After that, a white product, Na2Ti3O7, was collected and washed by deionized water.Then the white product was added into HCl solution (0.1 M) and stirred for 24 h to gain H2Ti3O7.The product was dispersed into 80 mL of 0.02 M H2SO4 and then transferred into Teflon autoclave, kept at 100 °C for 24 h.The white product was centrifuged, washed by purified water, and dried at 70 °C for overnight.Finally, the TiO2 nanobelt was produced after calcination at 600 °C for 2 h.

Synthesis of MoS2/TiO2 NBs Heterojunction
In typical procedure, 0.042 g of roughly TiO2 NBs, 0.265 g of ammonium molybdate tetrahydrate, and 0.11 g of sulfur powder were dispersed into 38 mL of absolute ethanol and 40 mL of octylamine, stirred vigorously for 30 min.Then the mixture was transferred into 100 mL Teflon autoclave, and kept at 180 °C for 24 h.After cooled to room temperature, the sample was obtained by centrifuging and washed by deionized water.The sample was dried at 70 °C for 24 h, denoted as MoS2/TiO2 heterojunction.

Synthesis of TiO 2 Nanobelts
First, 0.4 g of P25 powder was added into 80 mL of NaOH solution (10 M), and stirred vigorously for 30 min to obtain mean suspension.The mixture was transferred to 100 mL Teflon autoclave, which was heated to 180 • C and maintained for 48 h.After that, a white product, Na 2 Ti 3 O 7 , was collected and washed by deionized water.Then the white product was added into HCl solution (0.1 M) and stirred for 24 h to gain H 2 Ti 3 O 7 .The product was dispersed into 80 mL of 0.02 M H 2 SO 4 and then transferred into Teflon autoclave, kept at 100 • C for 24 h.The white product was centrifuged, washed by purified water, and dried at 70 • C for overnight.Finally, the TiO 2 nanobelt was produced after calcination at 600 • C for 2 h.

Synthesis of MoS 2 /TiO 2 NBs Heterojunction
In typical procedure, 0.042 g of roughly TiO 2 NBs, 0.265 g of ammonium molybdate tetrahydrate, and 0.11 g of sulfur powder were dispersed into 38 mL of absolute ethanol and 40 mL of octylamine, stirred vigorously for 30 min.Then the mixture was transferred into 100 mL Teflon autoclave, and kept at 180 • C for 24 h.After cooled to room temperature, the sample was obtained by centrifuging and washed by deionized water.The sample was dried at 70 • C for 24 h, denoted as MoS 2 /TiO 2 heterojunction.

Photocatalytic Activity Measurement
The photocatalytic activities of the samples were tested by the photocatalytic reduction of Cr(VI), and a 300 W Xe lamp with a 400 nm cut-off filter was used as the light resource.In a typical photocatalytic procedure, 0.05 g of the as-obtained sample was added into 100 mL of Cr(VI) solution (25 mg•L −1 ).The suspensions were stirred in the dark for 0.5 h to reach an adsorption-desorption equilibrium before exposed to irradiation.Then, the solution was exposed to light irradiation under magnetic stirring.At each given time interval, 3 mL suspension was sampled and centrifuged to remove the solid.The concentration of Cr(VI) during the degradation was monitored by colorimetry using a UV1100 spectrophotometer.All of the measurements were carried out at room temperature.

Conclusions
In this work, we successfully synthesized MoS 2 /TiO 2 nanobelt heterojunction by in situ growth of MoS 2 on the surface of TiO 2 NBs.The photocatalytic reduction of Cr(VI) reveals that the synthesized MoS 2 /TiO 2 NBs composite have superior photocatalytic ability than other samples.As a result of special energy level offset and nanostructure, MoS 2 /TiO 2 NBs composite were endowed with higher light-harvesting capacity and charge transportation efficiency, which are indispensible merits for excellent photocatalytic activity.Meanwhile, a photoreduction mechanism is proposed based on the systematic investigation, where the photogenerated electrons are demonstrated as the dominant reductive species to reduce Cr(VI) to Cr(III).

Figure 9 .
Figure 9. Degradation efficiency of Cr(VI) over the different samples under visible light irradiation.

Figure 10
Figure 10 exhibits a schematic diagram of the band structure of the pristine n-type TiO2 NBs and p-type MoS2.Commonly, for n-type TiO2, the Fermi level is close to the conduction band, whereas

Figure 9 .
Figure 9. Degradation efficiency of Cr(VI) over the different samples under visible light irradiation.

Figure 10
Figure 10 exhibits a schematic diagram of the band structure of the pristine n-type TiO2 NBs and p-type MoS2.Commonly, for n-type TiO2, the Fermi level is close to the conduction band, whereas

Figure 9 .
Figure 9. Degradation efficiency of Cr(VI) over the different samples under visible light irradiation.

2. 3 .
Figure10exhibits a schematic diagram of the band structure of the pristine n-type TiO 2 NBs and p-type MoS 2 .Commonly, for n-type TiO 2 , the Fermi level is close to the conduction band, whereas for p-type MoS 2 , the Fermi level approaches to the valence band.When the TiO 2 NBs was coupled with the MoS 2 , the heterojunctions among these semiconductors were formed, resulting in the realignment of their valence and conduction bands due to the thermal equilibrium of different Fermi levels and the formation of built-in electric field[44,45].This allows the energy bands of TiO 2 and MoS 2 shift downward and upward, respectively, along the Fermi level, as shown in Figure10.When the MoS 2 /TiO 2 heterojunction system was irradiated by visible light, the MoS 2 are excited to produce electrons and holes.The photoinduced electrons on the conduction band of the MoS 2 transfer to that of TiO 2 NBs, whereas the holes remain in the valence band of the MoS 2 , which could react with S 2− in the sample and to some degree undermine the photocatalytic performance of the sample, as shown in FigureS1.As discussed above, owing to the formation of the heterojunction at the interface between MoS 2 and TiO 2 NBs, the suppression of the recombination of photoinduced electron-hole pairs is realized, which allows more photogenerated electrons to participate in the reduction reactions and strongly enhances photocatalytic activity under visible light irradiation.

Figure 10 .
Figure 10.Schematic diagram for energy band of MoS2/TiO2 NBs p-n heterojunction and photocatalytic reduction mechanism of Cr(VI).

Figure 10 .
Figure 10.Schematic diagram for energy band of MoS 2 /TiO 2 NBs p-n heterojunction and photocatalytic reduction mechanism of Cr(VI).