Surface Photovoltage Response Research Articles

Pore walls as high-way for efficient bulk charge transfer in porous SrTiO3 single crystals boosting photocatalytic overall water splitting

Suppressing carrier recombination in bulk and facilitating carrier transfer to surface via rational structure design is of great significance to improve solar-to-H2 conversion efficiency. We demonstrate a facile hydrothermal method to synthesize porous SrTiO3 single crystals (SrTiO3-P) with exposed (001) facets by introducing carbon spheres as templates. The obviously increased surface photovoltage and photocurrent response indicate that the interconnected pore walls act as enormous charge transfer “highways”, accelerating carrier transport from bulk to surface. Furthermore, the absence of grain boundaries and high crystallinity could also lower the carrier recombination rate. Thus, the SrTiO3-P photocatalyst loaded with Rh/Cr2O3 as cocatalyst exhibits 1.5 times higher overall water splitting activity than that of solid SrTiO3, with gas evolution rate of 19.99 μmol h−1 50 mg−1 for H2 and 11.37 μmol h−1 50 mg−1 for O2. Additionally, SrTiO3-P also shows superior stability without any decay during cycling testing. This work provides a new insight into designing efficient multicomponent photocatalysts with a single-crystal porous structure.

A novel method for synthesizing specific surface area modulable g-C3N4 photocatalyst with maize-like structure

The synthesis of g-C3N4 with a large specific surface area is highly desirable for developing g-C3N4-based photocatalysts with good activity. Herein, the specific surface area of g-C3N4 was enlarged by preconditioning its raw material via a preassembly method, and the prepared g-C3N4, which has an enlarged specific surface area, exhibited a special maize-like structure. Interestingly, the specific surface area of this maize-like g-C3N4 can be easily modulated by simply changing the amount of the special preconditioned precursor and the volume of the reaction cell used during the synthesis process, and the specific surface area can be enlarged to at least 224.11 m2/g. Several techniques, including transient-state photoluminescence spectroscopy, surface photovoltage response, and electrochemical measurements, confirmed that the charge separation property of g-C3N4 is enhanced by enlarging its specific surface area, and it also exhibited increased photocatalytic degradation activities. This work provides a feasible strategy for designing high-performance g-C3N4-based photocatalytic systems.

A new multi-cobalt substituted molybdovanadate [{Co(H2O)5}4{Co8(OH)2(H2O)2V10Mo30O132}]12-: Synthesis, crystal structure and surface photovoltage

A novel multi-cobalt substituted molybdovanadate, (NH4)12[{Co(H2O)5}4{Co8(OH)2(H2O)2V10Mo30O132}]·68H2O (1), was isolated via in situ self-assembly reaction of NaVO3·2H2O, (NH4)6Mo7O24·4H2O, and CoCl2·6H2O in the aqueous solution. Structural analysis reveals that compound 1 displays a zero-dimensional (0D) framework constructed from the Z-shaped {V10Mo30Co8} cluster surrounded by four {Co(H2O)5} units. The morphology of compound 1 features a lamellar construct, smooth surface, and regular edges with sizes 50–150 μm. Photophysical measurements indicate that compound 1 shows solid-state semiconductor-like characteristics of both the surface photovoltage response and the optical band gap of ca. 2.04 eV.

Unraveling photo-induced charge transfer properties at 3D/2D perovskite interfaces via in-situ surface photovoltage spectroscopy

As a non-destructive and contactless characterization technique, the surface photovoltage (SPV) method based on a lock-in amplifier was applied to characterize charge transfer at the nanoscale of 3D/2D perovskite interface. The SPV response of the 3D/2D perovskite was found to be stronger than that of the 3D perovskite, suggesting improved separation and transfer of photo-induced electron-hole pairs by the 2D overlayer. The photovoltaic process can be presented as a vector diagram with the magnitude and the phase angles to unravel charge transfer direction. The magnitude of the overall photovoltaic vector increases with an anticlockwise rotation and confirms the hole transfer at 3D/2D perovskite interfaces. Moreover, light-chopping-frequency dependent measurements were performed to study photogenerated electron transfer kinetics at the 3D/2D interface. The frequency of peak SPV for 3D/2D MAPbI3 perovskite was higher than that of 3D MAPbI3 demonstrated a faster charge separation in 3D/2D interface. The results provide valuable information about the in-situ charge transfer properties of photogenerated charge carriers at the 3D/2D perovskite interfaces.

Dynamic study of photo-generated charge transport in BiI3 and Cs3Bi2I9

The article examined the optoelectronic properties of thin films made from BiI3 and Cs3Bi2I9, which were potential materials for use in solar cells. The films' photocurrent density-time responses were measured and compared, revealing that BiI3 generated higher photocurrent than Cs3Bi2I9. To investigate the differences in optoelectronic properties further, IMPS, PEIS, and SPS measurements were conducted. Results from the IMPS and PEIS measurements showed that BiI3 exhibited much higher charge separation, transfer, and collection compared to Cs3Bi2I9, which could explain the higher photocurrent in BiI3. However, a faster trapping and detrapping process and a quicker charging/discharging process in BiI3 increased the possibility of interface electron/hole recombination. The surface photovoltage (SPV) phase spectroscopy results indicated that a significant increase in the SPV response intensity in the BiI3 film further demonstrated improved surface charge concentration due to better separation and transfer of photo-induced electron-hole pairs.

Improved photocatalytic H2O2 production of boron doped g-C3N4 by controllably co-modifying nanosized PDI and Ag

Herein, boron-doped g-C3N4 (BCN) is first modified with perylene diimide (PDI) nanosheets driven by the π−π interactions, and subsequently decorated with nanosized Ag through the photodeposition method. The optimal PDI/BCN-Ag photocatalyst achieves a H2O2 production rate of 143 μmol g−1 h−1 without any sacrificial agents under visible light irradiation, which is about13-fold improvement compared with pristine g-C3N4. The boosted photoactivity is mainly ascribe to the enhanced charge separation due to the cascade S-scheme charge transfer between the coupled PDI and BCN, and the promoted oxygen activation from the introduced nanosized Ag by means of the time-resolved surface photovoltage responses and fluorescence spectra, kelvin probe measurement analyses, O2 temperature-programmed desorption and electron paramagnetic resonance spectroscopy. Notably, there is a dual channel for H2O2 production on PDI/BCN-Ag. The two-electron oxygen reduction dominated by the produced •O2− radicals occurs on the Ag nanoparticles, whereas the two-electron water oxidation occurs on the PDI.

Surface Photovoltage Response of ZnO to Phosphate-Buffered Saline Solution with and without Presence of Staphylococcus aureus.

Nano- and microscale zinc oxide (ZnO) exhibits significant potential as a novel antibacterial agent in biomedical applications. However, the uncertainty regarding the underlying mechanisms of the observed antimicrobial action inhibits the realization of this potential. Particularly, the nature of interactions at the free crystalline surface and the influence of the local bacterial environment remains unclear. In this investigation, we utilize ZnO particles synthesized via tunable hydrothermal growth method as a platform to elucidate the effects of interactions with phosphate-rich environments and differentiate them from those with bacteria. This is achieved using the time- and energy-dependent surface photovoltage (SPV) to monitor modifications of the surface electronic structure and surface charge dynamics of the ZnO particles due to these interactions. It is found that there exists a dramatic change in the SPV transients after exposure to phosphate-rich environments. It also presents differences in the sub-bandgap surface electronic structure after these exposures. It can be suggested that these phenomena are a consequence of phosphate adsorption at surface traps corresponding to zinc deficiency defects. This effect is shown to be suppressed in the presence of Staphylococcus aureus bacteria. Our results support the previously proposed model of the competitive nature of interactions between S. aureus and aqueous phosphates with the free surface of ZnO and bring greater clarity to the effects of phosphate-rich environments on bacterial growth inhibition of ZnO.

Accelerated charge transfer of g-C3N4/BiVO4 Z-scheme 2D heterojunctions by controllably introducing phosphate bridges and Ag nanocluster co-catalysts for selective CO2 photoreduction to CO

Artificial Z-scheme heterojunctions by mimicking photosynthesis have been widely investigated for photoreduction of CO2, yet the activity is hindered by the weak interfacial interactions and insufficient CO2 activation sites. Here, a cascade Z-scheme g-C3N4/BiVO4 (CN/BVO) heterojunction has been tailored by the dual modification of phosphates and Ag nanoclusters for CO2 reduction in pure water, in which phosphates are modified in the interface of CN/BVO by a facile impregnation method, while Ag nanoclusters are anchored on the surface of CN by a light induced in-situ deposition strategy under a low-temperature environment created by liquid N2. The optimal Ag-CN/PO-BVO heterojunction delivers a ca.48 μmol g−1h−1 CO generation rate with 97 % selectivity, which exhibits a 24-fold increment in CO production rate compared with that of pristine BVO. The improved photoactivity is mainly ascribed to the accelerated Z-scheme charge transfer from the built PO-bridged dimension-matched 2D interfaces and from the introduced Ag nanoclusters with preferable catalytic functions for CO2 reduction mainly by means of the time-resolved surface photovoltage responses and fluorescence spectra. Moreover, temperature-programmed desorption curves and in-situ DRIFTS results demonstrate that the introduced Ag is favorable for CO2 activation and CO desorption with COOH intermediates, responsible for the high CO selectivity.

Rational design of BiFeO3 nanostructures for efficient charge carrier transfer and consumption for photocatalytic water oxidation

Crystal surfaces play significant role in controlling the photocatalytic properties of a semiconductor material. In this work, we report the controlled synthesis of various BiFeO3 nanostructures and validate the influence of various crystal surfaces towards photocatalytic properties for water oxidation. The synthesis of BiFeO3 nanostructures including hexagons, rectangular cuboids and nanoplates is achieved in NaOH aqueous solutions with various concentrations via a facile hydrothermal method. High-resolution transmission electron microscopy (HRTEM) revealed NaOH concentration dependent growth direction of various BiFeO3 nanostructures. BiFeO3 rectangular cuboids dominated with (102) crystal facets exhibited higher surface photovoltage (SPV) response confirming efficient charge carrier separation compared to hexagons and nanoplates. Furthermore, the photoelectrochemical measurements also confirmed the improved photocurrent density and positive shift in flat band potential for BiFeO3 rectangular cuboids studied via Mott-Schottky plots. Resultantly, BiFeO3 rectangular cuboids demonstrated superior photocatalytic O2 evolution activity of 82.2 µmol h−1 g−1 compared to 42.6 µmol h−1 g−1 and 53 µmol h−1 g−1 for hexagons and nanoplates, respectively, without any cocatalyst under visible light irradiation.

Direct measurement of surface photovoltage by AC bias Kelvin probe force microscopy.

Surface photovoltage (SPV) measurements are a crucial way of investigating optoelectronic and photocatalytic semiconductors. The local SPV is generally measured consecutively by Kelvin probe force microscopy (KPFM) in darkness and under illumination, in which thermal drift degrades spatial and energy resolutions. In this study, we propose the method of AC bias Kelvin probe force microscopy (AC-KPFM), which controls the AC bias to nullify the modulated signal. We succeeded in directly measuring the local SPV by AC-KPFM with higher resolution, thanks to the exclusion of the thermal drift. We found that AC-KPFM can achieve a SPV response faster by about one to eight orders of magnitude than classical KPFM. Moreover, AC-KPFM is applicable in both amplitude modulation and frequency modulation mode. Thus, it contributes to advancing SPV measurements in various environments, such as vacuum, air, and liquids. This method can be utilized for direct measurements of changes in surface potential induced by modulated external disturbances.

Improved visible-light activities of ultrathin CoPc/g-C3N4 heterojunctions by N-doped graphene modulation for selective benzyl alcohol oxidation

The interface modulation is the key factor affecting the photoactivity of metal phthalocyanine (MPc)/g-C3N4 heterojunctions for aerobic selective alcohol oxidation. Here, we have successfully fabricated the N-doped graphene-modulated CoPc/g-C3N4 nanosheets (CoPc/NG/CN) heterojunctions. Optimal CoPc/NG/CN photocatalyst demonstrates approximately fourfold photoactivity improvement for benzyl alcohol oxidation with ∼99% selectivity toward benzyl aldehyde and of approximately ninefold 2,4-dichlorophenol degradation rate compared with bare CN, using O2 as oxidant. By means of steady-state surface photovoltage responses in N2 atmosphere, time-resolved photoluminescence spectra, single-wavelength photocurrent action spectra, and electrochemical O2 reduction measurements, it is confirmed that the exceptional photoactivities of resulting CoPc/NG/CN heterojunction are attributed to the facilitated interfacial charge transfer between CoPc and CN by the modulation of NG with favorable electron-induced capacity. In addition, more enriched hydroxyl groups of NG by N doping could induce high dispersion of CoPc molecules via H-bonding effect, resulting in the increase of exposed number of CoPc as visible-light absorption units and single Co2+ sites as catalytic centers for O2 activation so as to further benefit the photocatalytic oxidation processes. This work provides a feasible interfacial modulation strategy to fabricate efficient supported MPc-based heterojunction photocatalyst nanomaterials.

Organic Bilayer Photovoltaics for Efficient Indoor Light Harvesting

AbstractIndoor organic photovoltaics (OPVs) are a potential niche application for organic semiconductors due to their strong and well‐matched absorption with the emission of indoor lighting. However, due to extremely low photocurrent generation, the device parameters critical for efficient indoor OPVs differ from those under 1 Sun conditions. Herein, these critical device parameters—recombination loss and shunt resistance (Rsh)—are identified and it is demonstrated that bilayer OPVs are suitable for indoor PV applications. Compared to bulk‐heterojunction (BHJ), the open‐circuit voltage loss of bilayer devices under low light intensities is much smaller, consistent with a larger surface photovoltage response, indicating suppressed recombination losses. The bilayer devices show a higher fill factor at low light intensities, resulting from high Rsh afforded by the ideal interfacial contacts between the photoactive and the charge transport layers. The high Rsh enables bilayer devices to perform well without a light‐soaking process. Finally, the charge carriers are extracted rapidly in bilayers, which are attributed to strongly suppressed trap states possibly induced by isolated domains and non‐ideal interfacial contacts in BHJs. This study highlights the excellent suitability of bilayer OPVs for indoor applications and demonstrates the importance of device architecture and interfacial structures for efficient indoor OPVs.

Direct evidence of Z-scheme effect and charge transfer mechanism in titanium oxide and cadmium sulfide heterostructure

Although Z-scheme effect plays a significant role in explaining the transfer mechanism of photogenerated electrons and holes between semiconductor heterostructure, the existence of Z-scheme effect can only be proved via limited photocatalytic experimental means as an indirect evidence at present. Our research confirms the direct evidence of Z-scheme effect according to the experimental results from the view of thermodynamics and kinetics, and substantiates the typical competitive relation for transfer of photogenerated electrons and holes between Z-scheme effect and traditional semiconductor heterostructure effect respectively. The surface photovoltage (SPV) spectroscopy, photoacoustic (PA) spectroscopy and transient photovoltage (TPV) are firstly applied in the study of Z-scheme effect between titanium oxide and cadmium sulfide (TiO 2 /CdS) heterostructure. SPV and PA responses are generally acknowledged as classic technologies to analyze the excitation, separation, transfer and recombination properties on photogenerated electrons and holes, and TPV responses are an indispensable method to study the behavior of photogenerated electrons and holes. • The existence of Z-scheme effect is found by SPV spectroscopy, PA spectroscopy and SPV transient. • The competitive relation of charge transfer is confirmed between Z-scheme effect and traditional type II transfer. • The balance of Z-scheme effect is revealed from the view of thermodynamics and kinetics.

Ultrathin phosphate-modulated zinc phthalocyanine/perylenete diimide supermolecule Z-scheme heterojunctions as efficiently wide visible-light photocatalysts for CO2 conversion

Ultrathin phosphate-modulated zinc phthalocyanine/perylene diimide supermolecule (ZnPc/P-PDI) heterojunctions have been successfully constructed as efficient wide visible-light photocatalysts for converting CO2. The key to realize this construction is to establish a facile phosphate-induced assembly of ZnPc on PDI with the improved dispersion by strengthening H-bonding connection. The amount-optimized ZnPc/P-PDI heterojunction shows an exceptional photoactivity by ~30-fold enhancement compared with PDI. Based on the time-resolved surface photovoltage responses, low-temperature electron paramagnetic resonance and in-situ diffuse reflectance infrared Fourier transform spectroscopy spectra, etc., it is confirmed that the excellent photocatalytic performance mainly depends on the greatly-enhanced charge transfer and separation in the fabricated closely-contacted Z-scheme heterojunction with the increased amount of ZnPc, along with the formed negative field on resulting PDI by the modified phosphate anions for trapping photogenerated holes. Moreover, it is proved that the excited electrons of ZnPc could continuously transfer from the ligand to the central Zn2+ with good catalytic function for CO2 reduction.

Surface photovoltage response of zinc oxide microrods on prismatic planes: effect of UV, temperature and oxygen ambience

In this study, we report the time evaluation of work function measurements on ZnO microrods in Kelvin probe method under photoexcitation, called Surface Photovoltage (SPV). The SPV measurements are one of the most important techniques for characterizing both surface and bulk of the semiconductors. The ZnO microrods synthesized by hydrothermal method are highly crystalline with typical length of 50–100 µm and hexagonal cross section of about 2 µm. Upon drop-casting, most of the rods lie flat on the surface making the basal (002) plane vertical and the prismatic (101) and (100) plane on top. Thus, the SPV measurements are performed on these prismatic planes which are non-polar in nature. The SPV response of microrods has been studied for UV laser of photon energy 3.45 eV which is higher than that of the observed bandgap of 3.3 eV of ZnO microrods. The response is studied in varying oxygen ambience and temperatures. The average work function of the material at room temperature (25 °C) and ambient atmosphere is found to be 4.52 ± 0.3 eV. The work function was found to decrease with increasing temperature and upon photoexcitation. The photovoltage recovery time depicts that the presence of oxygen in atmosphere greatly accelerates the photocurrent recovery through trapping of photo-generated charge carriers by the surface states. The minority carrier diffusion length thus estimated from variable energy photoexcitation is found to be 160 nm.

Porous two-dimension MnO2-C3N4/titanium phosphate nanocomposites as efficient photocatalsyts for CO oxidation and mechanisms

Herein, two-dimension (2D) δ-MnO2-modified porous g-C3N4/micro-mesoporous titanium phosphate (MO-CN/TiP) nanocomposites have been controllably fabricated by a two-step solvothermal process. The amount-optimized MO-CN/TiP nanocomposite exhibits greatly improved photocatalytic CO oxidation performance (87%) compared with corresponding bare TiP and CN/TiP, even superior to the commercial P25 TiO2. Based on the experimental results of time-resolved surface photovoltage responses, photocurrent action spectra, and electron paramagnetic resonance signals, it is clearly confirmed that the exceptional photoactivity is mainly attributed to the great promotion in the Z-scheme charge transfer in resultant dimension-matched CN/TiP nanocomposite and in the O2 activation via the modified MO. The porous structure favorable to increase the specific surface areas and to facilitate the mass diffusion and the extended visible-light absorption also contribute to the photoactivity enhancement. Moreover, it is uncovered by the in-situ diffuse reflectance fourier transform infrared spectra that CO is previously adsorbed by the formed OHOC intermediate with the surface hydroxyls and then oxidized.

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.

Synthesis of nanosized Ag-modified 2D/2D hydroxylated g-C3N4/TS-1 Z-scheme nanocomposites for efficient photocatalytic CO2 reduction

It is significant to enhance the charge separation and extend the visible-light range for the TS-1 molecular sieve-based photocatalysts to improve the photocatalytic performance for CO2 reduction reaction (CO2RR). Here, dimension-matched 2D/2D hydroxylated g-C3N4/TS-1 (hCN/TS-1) nanocomposites have been successfully fabricated and further modified by nanosized Ag by the photoreduction method. The optimum Ag-modified hCN/TS-1 exhibits ∼ 7-time CO2 conversion than that of 2D TS-1. Based on the surface photovoltage responses, monochromatic photocurrent action spectra and electrochemical reduction curves, it is confirmed that the exceptional photoactivity is attributed to the greatly-enhanced charge separation via the Z-scheme mode between hCN and TS-1 at intimately-connected interfaces, extended visible-light absorption endowed by hCN, and Ag nanoparticles as the effective electron capturers with favorable catalytic function for activating CO2 molecules. This work showcases an effective and feasible design strategy to fabricate highly effective TS-1 molecular sieve-based photocatalysts for practical application.

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.