Transient-state Surface Photovoltage Research Articles

Defect and Donor Manipulated Highly Efficient Electron-Hole Separation in a 3D Nanoporous Schottky Heterojunction.

Given the rapid recombination of photogenerated charge carriers and photocorrosion, transition metal sulfide photocatalysts usually suffer from modest photocatalytic performance. Herein, S-vacancy-rich ZnIn2S4 (VS-ZIS) nanosheets are integrated on 3D bicontinuous nitrogen-doped nanoporous graphene (N-npG), forming 3D heterostructures with well-fitted geometric configuration (VS-ZIS/N-npG) for highly efficient photocatalytic hydrogen production. The VS-ZIS/N-npG presents ultrafast interfacial photogenerated electrons captured by the S vacancies in VS-ZIS and holes neutralization behaviors by the extra free electrons in N-npG during photocatalysis, which are demonstrated by in situ XPS, femtosecond transient absorption (fs-TA) spectroscopy, and transient-state surface photovoltage (TS-SPV) spectra. The simulated interfacial charge rearrangement behaviors from DFT calculations also verify the separation tendency of photogenerated charge carriers. Thus, the optimized VS-ZIS/N-npG 3D hierarchical heterojunction with 1.0 wt % N-npG exhibits a comparably high hydrogen generation rate of 4222.4 μmol g-1 h-1, which is 5.6-fold higher than the bare VS-ZIS and 12.7-fold higher than the ZIS without S vacancies. This work sheds light on the rational design of photogenerated carrier transfer paths to facilitate charge separation and provides further hints for the design of hierarchical heterostructure photocatalysts.

Atomic-Level Regulated 2D ReSe2 : A Universal Platform Boostin Photocatalysis.

Solar hydrogen (H2 ) generation via photocatalytic water splitting is practically promising, environmentally benign, and sustainably carbon neutral. It is important therefore to understand how to controllably engineer photocatalysts at the atomic level. In this work, atomic-level engineering of defected ReSe2 nanosheets (NSs) is reported to significantly boost photocatalytic H2 evolution on various semiconductor photocatalysts including TiO2 , CdS, ZnIn2 S4 , and C3 N4 . Advanced characterizations, such as atomic-resolution aberration-corrected scanning transmission electron microscopy (AC-STEM), synchrotron-based X-ray absorption near edge structure (XANES), in situ X-ray photoelectron spectroscopy (XPS), transient-state surface photovoltage (SPV) spectroscopy, and transient-state photoluminescence (PL) spectroscopy, together with theoretical computations confirm that the strongly coupled ReSe2 /TiO2 interface and substantial atomic-level active sites of defected ReSe2 NSs result in the significantly raised activity of ReSe2 /TiO2 . This work not only for the first time realizes the atomic-level engineering of ReSe2 NSs as a versatile platform to significantly raise the activities on different photocatalysts, but, more importantly, underscores the immense importance of atomic-level synthesis and exploration on 2D materials for energy conversion and storage.

A Twin S-Scheme Artificial Photosynthetic System with Self-Assembled Heterojunctions Yields Superior Photocatalytic Hydrogen Evolution Rate.

Designing heterojunction photocatalysts imitating natural photosynthetic systems has been a promising approach for photocatalytic hydrogen generation. However, in the traditional Z-Scheme artificial photosynthetic systems, the poor charge separation, and rapid recombination of photogenerated carriers remain a huge bottleneck. To rationally design S-Scheme (i.e., Step scheme) heterojunctions by avoiding the futile charge transport routes is therefore seen as an attractive approach to achieving high hydrogen evolution rates. Herein, a twin S-scheme heterojunction is proposed involving graphitic C3 N4 nanosheets self-assembled with hydrogen-doped rutile TiO2 nanorods and anatase TiO2 nanoparticles. This catalyst shows an excellent photocatalytic hydrogen evolution rate of 62.37mmol g-1 h-1 and high apparent quantum efficiency of 45.9% at 365nm. The significant enhancement of photocatalytic performance is attributed to the efficient charge separation and transfer induced by the unique twin S-scheme structure. The charge transfer route in the twin S-scheme is confirmed by in situ X-ray photoelectron spectroscopy (XPS) and electron spin resonance (ESR) spin-trapping tests. Femtosecond transient absorption (fs-TA) spectroscopy, transient-state surface photovoltage (TPV), and other ex situ characterizations further corroborate the efficient charge transport across the catalyst interface. This work offers a new perspective on constructing artificial photosynthetic systems with S-scheme heterojunctions to enhance photocatalytic performance.

Plasmon-enhanced bulk charge separation via morphological and interfacial engineering in Au@carbon dots@CdS hybrid

The exploitation of morphological engineering strategies enables the design of semiconductor-based photocatalysts with a precision that significantly enhances their photocatalytic activity. In this study, a paradigm has been provided that embeds plasmonic nanostructures within the bulk of the semiconductor, which demonstrates an improvement in interfacial charge transfer and separation. It involves a distinct core-double shelled Au@carbon dots@CdS hybrid, exhibiting excellent photocatalytic H2 generation under the visible light radiation. The interfacial engineering between the plasmonic nucleus and the semiconductor shell has been performed by the incorporation of carbon dots as faster charge channels. The effective driving force for plasmon-enhanced bulk charge separation of semiconductors and the unhindered interfacial charge transmission derived from carbon dots have been demonstrated by the use of steady state surface photovoltage (SPV) and transient state surface photovoltage (TPV) methods, as well as some intrinsic kinetics information of interfacial photoexcited charge carriers transfer processes.

Mg-O-Bridged Polypyrrole/g-C3 N4 Nanocomposites as Efficient Visible-Light Catalysts for Hydrogen Evolution.

It is highly desired to improve the visible-light activity of g-C3 N4 for H2 evolution by constructing closely contacted heterojunctions with conductive polymers. Herein, a polymer nanocomposite photocatalyst with high visible-light activity is fabricated successfully by coupling nanosized polypyrrole (NPPy) particles onto g-C3 N4 nanosheets through a simple wet-chemical process, and its visible-light activity is improved further by constructing Mg-O bridges between the NPPy and g-C3 N4 . The amount-optimized bridged nanocomposite displays an approximately ninefold improvement in visible-light activity compared with g-C3 N4 . On the basis of transient-state surface photovoltage responses, photoluminescence spectra, . OH amount evaluation, and photoelectrochemical curves, it is concluded that the exceptional photoactivity can be attributed to the significantly promoted charge transfer and separation along with visible photosensitization from NPPy. Interestingly, it is confirmed that the promoted charge separation depends mainly on the excited high-level electron transfer from g-C3 N4 to NPPy by single-wavelength photocurrent action spectra. This work provides a feasible strategy for designing polymer nano-heterojunction photocatalysts with exceptional visible-light activities.

Ultrafine SnO2/010 Facet-Exposed BiVO4 Nanocomposites as Efficient Photoanodes for Controllable Conversion of 2,4-Dichlorophenol via a Preferential Dechlorination Path.

It is a great challenge for achieving efficiently controllable conversion of chlorinated organics through BiVO4-based photoelectrochemical methods by improving the selective adsorption of such organics and charge separation. Herein, we have successfully fabricated SnO2/010 facet-exposed BiVO4 nanocomposites by a series of hydrothermal processes and further used as efficient photoanodes. The resulting photoanode exhibits about 6.3 times higher photoelectrochemical activity than bulk-BiVO4, especially with the efficiently controllable conversion of 2,4-dichlorophenol (2,4-DCP) to the nontoxic valuable intermediates such as catechol and pyrogallol by preferential dechlorination. Based on the 2,4-DCP adsorption curves, in situ diffuse reflectance infrared spectra, transient-state surface photovoltage responses, and photocurrent action spectra, it was clearly confirmed that the exceptional performance could be mainly attributed to the promoted selective adsorption of 2,4-DCP for efficiently modulating holes by the strong coordination interactions between -Cl with lone-pair electrons in 2,4-DCP and Bi- with empty orbits on (010) facet-exposed BiVO4 nanoflakes and to the coupled nano-SnO2 for prolonging the charge lifetime of BiVO4 by acting as the high-energy-level electron-accepting platform. This work provides a feasible strategy to develop excellent BiVO4-based photoelectrochemical methods for efficiently controlling the conversion of chlorinated organics simultaneously with energy production and recovery.

Improved visible-light activities of g-C3N4 nanosheets by co-modifying nano-sized SnO2 and Ag for CO2 reduction and 2,4-dichlorophenol degradation

Graphitic carbon nitride (g-C3N4) usually exhibits weak photocatalytic activity for CO2 reduction and pollutant degradation. Herein, the visible-light activities of g-C3N4 nanosheets are successfully improved by co-modifying SnO2 and Ag, with ∼10-time and ∼8-time improvement respectively for CO2 conversion and 2,4-dichlorophenol (2,4-DCP) degradation as compared to the pure g-C3N4. Mainly based on the transient-state surface photovoltage responses, transient-state photoluminescence spectra and electrochemical analyses, it is confirmed that the improved photoactivities are attributed to the synergistic effect of the prolonged charge lifetime and the provided catalytic function by modifying SnO2 and Ag, respectively. In addition, the synergistic effect is also feasible by replacing Ag with Au. This work will provide an effective strategy for designing high-activity g-C3N4 based photocatalysts.

Surface co-modification with highly-dispersed Mn & Cu oxides of g-C3N4 nanosheets for efficiently photocatalytic reduction of CO2 to CO and CH4

Abstract In this work, an in-situ way has been developed to achieve the co-modification of highly-dispersed Mn and Cu oxides to g-C3N4 nanosheets for efficient CO2 reduction. The Mn & Cu amount-optimized nanosheets exhibit the improved photocatalytic activity for CO2 reduction to CO and CH4 both by 3 times compared with the generally-bulk g-C3N4, mainly attributed to the nanosheet structure and to the roles of modified Mn and Cu oxides. Based on the transient-state surface photovoltage responses, transient-state photoluminescence spectra, and electrochemical measurements, it is confirmed that the modified Mn and Cu oxides could respectively capture the photogenerated holes and electrons, and provide the catalytic functions for water oxidation and CO2 reduction. Remarkably, the simultaneous introduction of Mn and Cu oxides shows an obvious synergistic effect on the improvement of photocatalytic activity for CO2 reduction. This work provides a feasible way to synthesize high-activity g-C3N4-based photocatalysts for energy production.

Improved photoelectric properties of BiOBr nanoplates by co-modifying SnO2 and Ag to promote photoelectrons trapped by adsorbed O2

It is highly desired to improve the photoelectric property of nanosized BiOBr by promoting the photogenerated charge transfer and separation. Herein, SnO2 and Ag comodified BiOBr nanocomposites (Ag-SO-BOB) have been prepared through a simple one-pot hydrothermal method. Surface photovoltage response of BiOBr nanoplates has 4.1- time enhancement after being modified with SnO2 nanoparticles. Transient-state surface photovoltage (TS-SPV) and the atmosphere-controlled steady-state surface photovoltage spectroscopy (AC-SPS) confirmed that this exceptional enhancement of the photovoltage response can be ascribed to the coupled SnO2 acting as platform for accepting the photoelectrons from BiOBr so as to prolong the lifetime and enhance charge separation. Remarkably, the surface photovoltage response can be further enhanced by synchronously introducing Ag nanoparticles, which is up to 15.4-times enhancement compared with bulk BiOBr nanoplates. The enhancement can be attributed to the improved O2 adsorption by introducing Ag to further enhance charge separation. Finally, the synergistic effect of SnO2 and Ag co-modification enhances the surface photovoltage response due to the enhanced charge separation and promoted O2 adsorption, which is also confirmed through photoelectrochemistry and photocatalytic experiment.

Improved photocatalytic activities of porous In2O3 with large surface area by coupling with K-modified CuO for degrading pollutants

It is highly desired to improve the photocatalytic activities of In2O3 for degrading pollutants by increasing reactant adsorption, enhancing the charge separation and promoting oxygen activation. Herein, porous In2O3 with high photocatalytic activity for degrading pollutants has successfully been synthesized by an In(OH)3 precursor route with citric acid as the modifier, and its photoactivities could be further improved by coupling with CuO through a simple impregnation process, especially with the K-modified one. The amount-optimized K-modified CuO/In2O3 nanocomposite exhibits a rather high photocatalytic activity for degrading 2,4-dichlorophenol (2,4-DCP), with ∼3-time enhancement than the bare In2O3. It is attributed to the effective charge separation and the promoted O2 activation by coupled K-modified CuO mainly based on the surface photovoltage spectroscopy, transient state surface photovoltage, O2 temperature-programmed desorption and electrochemistry results. In addition, radical-trapping experiments show that the produced O2− is dominant to induce the photocatalytic degradation of 2,4-DCP, implying that the developed modulation strategy to photogenerated electrons is much feasible compared to holes. This work offers an effective strategy to design and synthesize high-activity In2O3-based nanophotocatalysts for degrading pollutants.

Exceptional visible-light activities of g-C3N4 nanosheets dependent on the unexpected synergistic effects of prolonging charge lifetime and catalyzing H2 evolution with H2O

It is highly desired to develop an efficient g-C3N4-based photocatalyst for energy production under visible-light irradiation. Herein, it is shown that the optimized g-C3N4 nanosheet-based photocatalyst could exhibit exceptional photocatalytic activities for H2 evolution under visible-light irradiation, by ∼14-time improvement compared to that of bare g-C3N4 one. It is confirmed by the methods of transient-state surface photovoltage responses, transient-state PL spectra and electrochemical measurements that the exceptional photocatalytic activities are attributed to the unexpected synergistic effects of prolonging the charge lifetime and catalyzing H2 evolution by coupling nanocrystalline anatase TiO2 as a proper-energy platform to accept visible-light-excited electrons from g-C3N4 and by decorating a nano-sized noble metal as the co-catalyst respectively. Among three decorated noble metals (Ag, Au and Pt), to prolong the photogenerated charge lifetime is much meaningful for the used noble metal cocatalyst with weak catalytic function like Ag in H2 evolution. Using nanocrystalline SnO2 to replace TiO2 is also applicable for the synergistic effect. Moreover, it is clarified by the designed experiment on photocatalytic CO2 conversion to CD4 in the presence of methanol in D2O that the resource of evolved H2 is mainly from the adsorbed H2O other than the disassociated H+ from methanol.

Visible-light induced electron modulation to improve photoactivities of coral-like Bi2WO6 by coupling SnO2 as a proper energy platform

It is essential to improve the activity of Bi2WO6 for the photocatalytic 2,4-dichlorophenol (2,4-DCP) degradation by modulating the visible-light excited photogenerated electrons. As for this, Bi2WO6 nanostructures have been synthesized by a mannitol assistant hydrothermal method. It is shown that the temperature-optimized Bi2WO6 exhibits high photocatalytic activity for 2,4-DCP degradation under visible-light irradiation, attributed to the coral-like structure, high crystallinity and large surface area. Interestingly, the photocatalytic activities of coral-like Bi2WO6 could be further improved by coupling ultrafine nanocrystalline SnO2. It is confirmed mainly from the steady-state and transient-state surface photovoltage responses and fluorescence spectra related to the formed hydroxyl radical amounts that the coupled SnO2 is taken as a proper energy platform to accept the visible-light excited high-energy-level photogenerated electrons from Bi2WO6, consequently enhancing the charge separation and prolonging the charge lifetime. Moreover, the optimized SnO2-coupled BWO exhibits higher photocatalytic activity under UV–vis light irradiation than the commercial P25 TiO2. This work will provide a new feasible route to prepare highly efficient Bi2WO6-based photocatalysts for purifying chlorophenol-polluted water environments.

Synthesis of ZnO/Bi-doped porous LaFeO3 nanocomposites as highly efficient nano-photocatalysts dependent on the enhanced utilization of visible-light-excited electrons

ZnO coupled Bi-doped porous LaFeO3 nanocomposites have successfully been fabricated via a wet-chemical method. It is confirmed that Bi3+ enters into the crystal lattice of PLFO and substitute La3+, while the ZnO with diameter of ∼15 nm is coupled to the Bi-doped PLFO. It is shown that the amount-optimized 5Zn/7Bi-PLFO nanocomposite exhibits greatly improved visible-light activities for 2,4-dichlorophenol (2,4-DCP) degradation and CO2 conversion, compared to the unmodified PLFO with rather high photoactivity due to its large specific surface area. Based on the measurements of valence band XPS spectra, steady-state surface photovoltage spectra, transient-state surface photovoltage responses, photoelectrochemical I–V curves, fluorescence spectra related to produced OH amount and photocurrent action spectra, it is clearly demonstrated that the significantly improved visible-light activities are attributed to the enhanced utilization of visible-light-excited high-level-energy electrons (HLEEs) by coupling with nanocrystalline ZnO to introduce a new energy platform for accepting electrons and to the extended visible-light absorption by doping Bi3+ to create surface states. Interestingly, it is proved that under UV–vis irradiation, the amount-optimized nanocomposite exhibit much higher photoactivity for 2,4-DCP degradation compared to the commercially available P25 TiO2. Moreover, it is confirmed by means of radical trapping experiments that the dominant radicals to decompose 2,4-DCP on PLFO could be modulated by doping Bi3+ and coupling ZnO. Furthermore, the possible decomposition pathways, respectively related to the OH and O2−, of 2,4-DCP over the amount-optimized Bi-doped PLFO and ZnO coupled Bi-doped PLFO samples are proposed by means of the liquid chromatography tandem mass spectrometry analysis of the intermediates, especially with the used isotopic D2O.

Exceptional photocatalytic activities for CO2 conversion on Al[sbnd]O bridged g-C3N4/α-Fe2O3 z-scheme nanocomposites and mechanism insight with isotopesZ

It’s highly desired to design and fabricate effective Z-scheme photocatalysts by promoting the charge transfer and separation. Herein, we firstly fabricated the ratio-optimized g-C3N4/α-Fe2O3 nanocomposites by adjusting the mass ratio between two components through a simple wet-chemical process. The resulting nanocomposites display much high photocatalytic activities for CO2 conversion and phenol degradation compared to bare α-Fe2O3 and g-C3N4. Noteworthily, the photocatalytic activities are further improved by constructing AlO bridges, by 4-time enhancement compared to those of α-Fe2O3. Based on the steady-state surface photovoltage spectra, transient-state surface photovoltage responses, photoelectrochemical I-t curves and the evaluation of produced OH amounts, the exceptional photoactivities of AlO bridged g-C3N4/α-Fe2O3 nanocomposites are attributed to the significantly promoted charge transfer and separation by constructing the g-C3N4/α-Fe2O3 heterojunctions and the AlO bridges. Moreover, the charge transfer and separation of this photocatalyst have been confirmed to obey the Z-scheme mechanism, as supported by the single-wavelength photocurrent action spectra and single-wavelength photoactivities for CO2 conversion. Furthermore, the mechanism of the photocatalytic CO2 conversion has been elaborately elucidated through the electrochemical reduction and the photocatalytic experiments especially with isotope 13CO2 and D2O, that the produced H atoms as intermediate radicals would dominantly induce the conversion of CO2 to CO and CH4.

Improved visible-light activities for degrading pollutants on TiO2/g-C3N4 nanocomposites by decorating SPR Au nanoparticles and 2,4-dichlorophenol decomposition path

It has been clearly demonstrated that the visible-light photocatalytic activities of g-C3N4 (CN) for degrading 2,4-dichlorophenol (2,4-DCP) and bisphenol A (BPA) could be improved by fabricating nanocomposites with a proper amount of nanocrystalline anatase TiO2. Interestingly, the visible-light activities of the amount-optimized nanocomposite could be further improved after decorating Au nanoparticles, with 5.11- and 3.1-time improvement respectively for 2,4-DCP and BPA compared to that of CN, even much higher than that of P25 TiO2 under UV–vis irradiation. Based on the transient-state surface photovoltage responses and photoelectrochemical measurements, it is confirmed that the exceptional visible-light activities of the fabricated Au-(TiO2/g-C3N4) nanocomposites are attributed to the extended visible-light response due to the surface plasmonic resonance (SPR) of decorated Au and its catalytic function, and to the enhanced charge separation by transferring electrons from CN and SPR Au to TiO2 in the nanocomposites. The highly promoted charge separation results in the effective availability of a large number of hydroxyl radicals (OH) participating in the photocatalytic oxidation process of 2,4-DCP. Furthermore, a possible mechanism of 2,4-DCP degradation is proposed according to the detailed analyses of produced intermediates. This work provides new idea for designing Au assisted nanocomposite photocatalysts for environmental remediation.

Prolonged lifetime and enhanced separation of photogenerated charges of nanosized α-Fe2O3 by coupling SnO2 for efficient visible-light photocatalysis to convert CO2 and degrade acetaldehyde

To develop efficient visible-light photocatalysis on α-Fe2O3, it is highly desirable to promote visible-light-excited high-energy-level electron transfer to a proper energy platform thermodynamically. Herein, based on the transient-state surface photovoltage responses and the atmosphere-controlled steady-state surface photovoltage spectra, it is demonstrated that the lifetime and separation of photogenerated charges of nanosized α-Fe2O3 are increased after coupling a proper amount of nanocrystalline SnO2. This naturally leads to greatly improved photocatalytic activities for CO2 reduction and acetaldehyde degradation. It is suggested that the enhanced charge separation results from the electron transfer from α-Fe2O3 to SnO2, which acts as a proper energy platform. Based on the photocurrent action spectra, it is confirmed that the coupled SnO2 exhibits longer visible-light threshold wavelength (~590 nm) compared with the coupled TiO2 (~550 nm), indicating that the energy platform introduced by SnO2 would accept more photogenerated electrons from α-Fe2O3. Moreover, electrochemical reduction experiments proved that the coupled SnO2 possesses better catalytic ability for reducing CO2 and O2. These are well responsible for the much efficient photocatalysis on SnO2-coupled α-Fe2O3.

Enhanced Cocatalyst-Free Visible-Light Activities for Photocatalytic Fuel Production of g-C3N4 by Trapping Holes and Transferring Electrons

We have successfully synthesized boron-doped g-C3N4 nanosheets (B-CN) and its nanocomposites with nanocrystalline anatase TiO2 (T/B-CN). The as-prepared T/B-CN nanocomposites with the proper amounts of boron and TiO2 exhibit rather high cocatalyst-free photoactivities for producing H2 from CH3OH solution (∼29× higher) and CH4 from CO2-containing water (∼16× higher) under visible-light irradiation, compared to those of bare g-C3N4. This is attributed to the greatly enhanced photogenerated charge separation after doping boron and subsequent coupling with TiO2, mainly based on the measurements of atmosphere-controlled steady-state surface photovoltage spectra, transient-state surface photovoltage responses, photoluminescence spectra, and fluorescence spectra related to the produced hydroxyl radical amount. It is suggested for the first time that the great charge separation enhancement results from the B-induced surface states near the valence band top to trap holes and the formed heterojunctions to transfer electrons from B-CN to TiO2. Moreover, the created surface states are also responsible for the visible-light extension from 450 nm of g-C3N4 to 500 nm of B-CN (T/B-CN) for solar fuel production. Interestingly, the obtained 6T/6B-CN exhibits much larger quantum efficiencies, which are 3.08% for hydrogen evolution and 1.68% for CH4 production at λ = 420 nm, respectively, with 5.1× and 7.6× enhancement as compared to CN, even superior to other works. This work will provide feasible routes to synthesize g-C3N4-based nanophotocatalysts for efficient solar fuel production.

Enhanced visible-light activities of porous BiFeO3 by coupling with nanocrystalline TiO2 and mechanism

In this work, different mole ratio percentage of nanocrystalline anatase TiO2/porous nanosized BiFeO3 (T/P-BFO) nanocomposites with effective contacts have been fabricated by putting the as-prepared P-BFO into the TiO2 sol, followed by drying at 80°C and then calcining at 450°C for 2h. The photoactivities of the obtained products for pollutant degradation and H2 evolution were measured. It is clearly demonstrated by means of the steady-state surface photo-voltage spectra, the transient-state surface photovoltage responses, and the photoluminescence spectra that the photogenerated charge carriers in the T/P-BFO nanocomposites with a proper mole ratio percentage of TiO2 (9%) display much long lifetime and high separation in comparison to the resulting P-BFO alone. This is well responsible for the enhanced activities for degrading gas-phase acetyldehyde, the liquid-phase phenol, and for producing H2 under visible-light irradiation. Based on the measurements of formed hydroxyl radical amount and photoelectrochemical behavior, it is suggested that the improved separation of photogenerated charges in the fabricated T/P-BFO nanocomposite is mainly attributed to the spatial transfer of visible-light-excited (λ≤500nm) high-energy electrons of P-BFO to TiO2. This work will provide a feasible route to enhance the photoactivities of visible-light responsive oxide composites as photocatalysts for efficient solar energy utilization.

Improved photoactivity of TiO2-Fe2O3 nanocomposites for visible-light water splitting after phosphate bridging and its mechanism.

In this work, we have successfully constructed phosphate bridges in a TiO2-Fe2O3 nanocomposite using wet-chemical processes. Based on FTIR, XPS and TEM measurements it is confirmed that phosphate groups form bridges that effectively connect TiO2 and α-Fe2O3. From steady-state surface photovoltage spectra (SS-SPS) and transient-state surface photovoltage (TS-SPV) measurements in N2, it is clearly demonstrated that the separation and lifetime of the photogenerated charge carriers in the TiO2-Fe2O3 nanocomposite are greatly enhanced by the introduction of the phosphate bridges. As a consequence, the visible light photocatalytic activity in water reduction by methanol and the photoelectrochemical water oxidation were obviously improved after phosphate bridging. It is concluded mainly on the basis of ultra-low-temperature EPR signals, EIS spectra, and the normalized photocurrent action spectra that the photogenerated electrons of α-Fe2O3 under irradiation with visible light would transfer to TiO2 in the nanocomposite, and the built phosphate bridges are favorable for charge transportation, leading to the greatly-increased separation and lifetime of visible-light excited charge carriers. This work provides a feasible route to improve the photoactivity of other visible-response nanocomposites for water splitting.

Effective visible-excited charge separation in silicate-bridged ZnO/BiVO4 nanocomposite and its contribution to enhanced photocatalytic activity.

It is highly desired to enhance the visible-excited charge separation of nanosized BiVO4 for utilization in photocatalysis. Here ZnO/BiVO4 nanocomposites in different molar-ratios are fabricated by simple wet-chemical processes, after synthesis of nanosized BiVO4 and ZnO by hydrothermal methods. It is shown by means of atmosphere-controlled steady-state surface photovoltage spectra and transient-state surface photovoltage responses that the photogenerated charges of resulting nanocomposite shows longer lifetime and higher separation than that of BiVO4 alone. This leads to its superior photoactivities for water oxidation to produce O2 and for colorless pollutant degradation under visible irradiation, with about three times enhancement. Interestingly, it is suggested that the prolonged lifetime and enhanced separation of photogenerated charges in the nanocomposite is attributed to the unusual spatial transfer of visible-excited high-energy electrons, by visible radiation from BiVO4 to ZnO on the basis of the ultralow-temperature electron paramagnetic resonance measurements and the photocurrent action spectra. Moreover, it is clearly demonstrated that the photogenerated charge separation of resulting ZnO/BiVO4 nanocomposite could be further enhanced after introducing the silicate bridges so as to improve the visible photocatalytic activity greatly, attributed to the built bridge favorable to charge transfer. This work would provide a feasible way to enhance the solar energy utilization of visible-response semiconductor photocatalysts.