Surface Photocurrent Research Articles

SrTiO3 nanoparticles and nanofibers: synthesis and comparison of photocatalytic properties

SrTiO3 nanofibers and nanoparticles were respectively synthesized by electrospinning process and sol–gel method. The obtained SrTiO3 photocatalysts were characterized by X-ray diffractometer, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, Barrett–Emmett–Teller technique, and spectrophotometer. The X-ray diffraction patterns show that SrTiO3 samples have the pure perovskite-type crystalline phase. The scanning electron microscopy and transmission electron microscopy images indicate that SrTiO3 nanofibers and nanoparticles were respectively obtained by electrospinning process and sol–gel method. SrTiO3 nanofibers show the higher specific surface area and photocurrent than those of SrTiO3 nanoparticles. The photocatalytic activities of SrTiO3 nanofibers and nanoparticles were evaluated by photocatalysis degradation of Rhodamine B and Methyl orange. The SrTiO3 nanofibers exhibit higher photocatalytic activity than SrTiO3 nanoparticles, which is induced by the smaller crystallite size and hierarchical structure of SrTiO3 nanofibers.

Preparation of a Leaf-Like BiVO4-Reduced Graphene Oxide Composite and Its Photocatalytic Activity

We prepared a unique leaf-like BiVO4-reduced graphene oxide (BiVO4-rGO) composite with prominent adsorption performance and photocatalytic ability by a single-step method. Multiple characterization results showed that the leaf-like BiVO4with average diameter of about 5 um was well dispersed on the reduced graphene oxide sheet, which enhanced the transportation of photogenerated electrons into BiVO4, thereby leading to efficient separation of photogenerated carriers in the coupled graphene-nanocomposite system. The characterization and experiment results also indicated that the outstanding adsorption ability of such composite was closely associated with the rough surface of the leaf-like BiVO4and doped rGO. The surface photocurrent spectroscopy and transient photocurrent density measurement results demonstrated that the doped rGO enhanced separation efficiency and transfer rate of photogenerated charges. As a result, the BiVO4-rGO exhibited higher photocatalytic capacity toward the degradation of rhodamine B dye under visible-light irradiation compared with pure BiVO4and P25.

Enhanced photocatalytic degradation of aqueous phenol and Cr(VI) over visible-light-driven TbxOy loaded TiO2-oriented nanosheets

A visible-light-driven TbxOy loaded TiO2-oriented nanosheets (Tb-TNSs) of 3.6nm thick and specific surface area of 240–350m2/g provided with highly dispersed TbxOy nanoparticles of 1–2nm over the surfaces were synthesized using a one-pot hydrothermal route. Loaded TbxOy nanoparticles influenced morphology, structure, and optical properties of the TNSs. The XPS results showed that Tb3+ and Tb4+ co-exist in Tb-TNSs, and loaded TbxOy resulted in changes in binding energies of Ti and O. The amount of Ti3+ increased gradually with the amount of loaded TbxOy nanoparticles. FL and surface photocurrent spectra results indicated that an appropriate amount of TbxOy (≤1.0at.%) loading can effectively improve the separation efficiency of charge carriers. The visible-light-driven photocatalysis could be measured by the removal of phenol or dichromate separately along with synergistic degradation in phenol-Cr(VI) coexistence system. The optimum photocatalytic activity was obtained at Tb/Ti=1.0at.% under degradation of aqueous phenol or Cr(VI) solution. Besides, the desorbed Tb-TNSs could be easily regenerated by remedying the damaged structure and reused with excellent performance. High regeneration and stability of photocatalysts were confirmed by seven cyclic tests. An alternant mechanism for the enhanced visible-light-driven photocatalytic activity was also considered.

Synergistic effect of Fe2O3/Ho2O3 Co-modified 2D-titanate heterojunctions on enhanced photocatalytic degradation

TiO2-based nanosheets (TNSs) co-modified by Fe2O3 and Ho2O3 were synthesized by one-pot hydrothermal method using Fe(NO3)3 and Ho(NO3)3 as precursors compositing with TiO2. The Fe2O3/Ho2O3-TNSs heterojunctions possessed a thickness of approximately 3–4 nm, large specific surface area of 210–310 cm2/g, with Fe2O3 and Ho2O3 nanoparticles highly dispersed over the surface of the nanosheets. The crystallization of the samples gradually increased with the amount of Fe2O3 nanoparticles, which was confirmed by the XRD, BET and Raman spectra, indicating that Ho2O3 and Fe2O3 influenced the crystallinity and structure evolution of the TNSs, besides, led to an improved the visible-light absorption. Surface photocurrent and fluorescence spectral studies revealed that the photo-generated charge carrier separation efficiency could be efficiently improved by an appropriate amount of modification. The Fe2O3/Ho2O3-TNSs exhibited synergistic effect on photocatalytic degradation of RhB as well as MO under visible light. The highest efficiency was obtained by 0.05%-Fe2O3/Ho2O3-TNSs (Fe:Ho:Ti = 0.05:1:100), which was 8.86 and 6.72 times than that of individual 1.0%-Ho2O3-TNSs (Ho:Ti = 1:100) and 0.05%-Fe2O3-TNSs (Fe:Ti = 0.05:100), respectively. The possible mechanism for enhanced visible-light-induced photocatalytic activity was proposed. Ho2O3 introduced in the photocatalysts may act as the hole capture while Fe2O3 may share the same Fermi levels with TNSs and serve as the electron capture center in the n-n-p system, which reduced the recombination rate of photo-induced electron-hole pairs.

Two-dimensional TiO2-based nanosheets co-modified by surface-enriched carbon dots and Gd2O3 nanoparticles for efficient visible-light-driven photocatalysis

Two-dimensional TiO2-based nanosheets (TNSs) co-modified by surface-enriched carbon dots (CDs) and Gd2O3 nanoparticles: (Gd-C-TNSs), capable of exhibiting visible-light-driven photo catalysis were synthesized using a two-pot hydrothermal route. The samples had a sheet-like structure, thickness of approximately 3.6nm, large specific surface area of 240–350cm2/g. The CDs (2–3nm) and Gd2O3 nanoparticles (1–2nm) were highly dispersed over the surface of the nanosheets. The co-modification by Gd2O3 nanoparticles and CDs influenced the crystallinity, crystal structure, and surface area of the TNSs, and improved the visible-light absorption. Surface photocurrent and fluorescence spectral studies revealed that the photo-generated charge carrier separation efficiency could be improved by an appropriate amount of modification. A very high efficiency was obtained using 0.5 at% Gd/Ti and 3.0g/L of CDs. The visible-light-induced photocatalytic activity is enhanced under the isolated Cr(VI) system, isolated Rhodamin B (RhB) system, and the synergism between RhB degradation and Cr(VI) reduction for the Gd-C-TNSs photocatalysts. Initially, the photocatalytic activity gradually increased with an increase in the amount of CDs, and then decreased after attaining a maximum, in the case where 0.5 at% Gd/Ti and 3.0g/L of CDs were used. The enhancement in the photocatalytic activity was attributed to the synergetic effect of the Gd2O3 nanoparticles, TNSs, and CDs in the Gd-C-TNSs composites. The effect led to a fast separation and slow recombination of photo-induced electron–hole pairs. An alternate mechanism for enhanced visible-light photocatalytic activity was also considered.

Influence of N,N-Dimethylformamide Annealing on the Local Electrical Properties of Organometal Halide Perovskite Solar Cells: an Atomic Force Microscopy Investigation

Organometal halide perovskites have been recognized as a new class of materials for photovoltaic application. Solvent annealing introduced during crystallization of bulk or thin-film materials can improve the performance of perovskite solar cells. Herein, we present Kelvin probe force microscopy and conductive atomic force microscopy (c-AFM) measurements to investigate the local optoelectronic properties of a perovskite film after N,N-dimethylformamide (DMF) annealing. AFM results show that DMF annealing induces recrystallization, yielding a large-size polycrystalline perovskite film. Uniform and higher photocurrent is distributed on the film. However, lower surface potential enhancement and photocurrent are observed at grain boundaries (GBs), illustrating that GBs acting as recombination sites are detrimental to photocurrent transport and collection. Our observation provides a nanoscale understanding of the device performance improvement after DMF annealing.

Competition between the reaction medium and nanostructured ZnO in the photocatalytic degradation of anthracene. Toward an optimal process for polycyclic aromatic hydrocarbons remediation

Contamination with polycyclic aromatic hydrocarbons (PAHs) is considered an important health issue due to the toxicity of these compounds. Photocatalytic degradation of anthracene, a representative molecule of PAHs, using the high quantum yield semiconductor ZnO, has been reported. The solubility of anthracene in water makes necessary to use mixtures with organic solvents in fundamental degradation studies. It is well known that some organic solvents participate in the photochemical transformation of this molecule. In the PAHs photocatalysis, the competition between a semiconductor and solvents has not reported. Therefore, in this work, we decided to study the photocatalytic degradation of anthracene with two common reaction media and nanostructured ZnO. The semiconductor was obtained by a one pot method which consists in an alkaline hydrolysis of Zn(CH3COO)2⋅2H2O in ethanol. Nanoparticles size in colloidal dispersion was calculated using UV-Vis spectroscopy and High Resolution Transmission Electron Microcopy (HR-TEM). ZnO powder was isolated and characterized by X-ray diffraction to be used in photocatalytic experiments. Surface area determination and photocurrent spectroscopic experiments were also carried out. Linear sweep voltammetries under darkness and UV-Vis irradiation indicate a charge separation due to photoexcitation. Photocatalytic experiments in ethanol:water pH 12 (1:1) and acetone:water pH 12 (1:1), with and without ZnO was explored. The results demonstrated that ethanol:water and acetone:water promotes the photo-transformation of anthracene to 9,10-anthraquinone. Meanwhile, ZnO transformed anthracene to benzoic acid and to 9,10-anthraquinone in ethanol:water and acetone:water, respectively. A faster photochemical kinetic is observed when acetone was used as solvent in the presence and in the absence of ZnO.

Photovoltaic properties of titanium dioxide nanowires with different crystal structures

Titanium dioxide(TiO2) nanowires with different crystal structures were successfully synthesized, and their charge transfer properties were further investigated by surface photovoltage(SPV), transient photovoltage(TPV) and surface photocurrent(SPC) techniques. The results reveal that both the surface states and the charge transfer rate of different TiO2 nanowires are highly dependent on their crystal structures.

One-pot hydrothermal route to fabricate nitrogen doped graphene/Ag-TiO2: Efficient charge separation, and high-performance “on-off-on” switch system based photoelectrochemical biosensing

Charge separation is crucial for increasing the performances of semiconductor-based materials in many photoactive applications. In this paper, we designed novel nanocomposites consisting of TiO2 nanocrystals, Ag nanoparticles (NPs) and nitrogen doped graphene (NGR) via a facile one-pot hydrothermal route. The as-prepared ternary nanocomposites exhibited enhanced photoelectrochemical (PEC) performances owing to the introduction of Ag NPs and NGR, which increase the excitons’ lifetime and improve the charge transfer. In particular, it is shown by means of the transient-state surface photocurrent responses that the photocurrent intensity of the as-fabricated composites exhibited 18.2 times higher than that of pristine TiO2. Based on the robust photocurrent signal, a new kind of “on-off-on” PEC aptasensor was established with the assistance of Pb2+ aptamer, which integrates the advantages of low background signal and high sensitivity. Under optimal conditions, a wide linear response for Pb2+ detection was obtained from 1pM to 5nM as well as a detection limit down to 0.3pM. With its simplicity, selectivity, and sensitivity, this proposed strategy shows great promise for Pb2+ detection in food and environment analysis.

Effect of specific surface area on photoelectrochemical properties of TiO2 nanotubes, nanosheets and nanowires coated with TiC thin films

It is recommended in the exploitation of catalyst for hydrogen generation from photoelectrochemical (PEC) water splitting to use stable, non-toxic and chemically inert electrodes. The objective in this work is to combine the merits of large surface area and absorption in ultraviolet light–visible light range of hybrid structure of titanium carbide (TiC) thin film deposited on titanium dioxide (TiO2) nanowires (NWrs), nanosheets (NShts) and nanotubes (NTbs). TiO2 NTbs were prepared by rapid breakdown anodic oxidation technique, while, TiO2 NWrs and TiO2 NShts were prepared by hydrothermal method. TiC thin film is deposited on TiO2 NWrs, TiO2 NShts and TiO2 NTbs by radio frequency (RF) magnetron sputtering. Consequently, the absorption threshold of TiO2 is moved into visible spectrum range and led to a significant improvement in the photocurrent in comparison to bare TiO2. The samples were characterized by XRD, SEM, UV/Vis and Raman spectroscopy. The chemisorption (active) surface area of the synthesized nanostructures was measured by BET technique and found to be in the order of TiO2 NTbs < TiO2 NShts < TiO2 NWrs. The influence of the specific surface area on the PEC properties of TiC/TiO2 NWrs, NSts and NTbs is investigated, and a direct proportion between surface area and photocurrent is identified. The impedance measurement has been correlated with the photocurrent-potential properties, so, the hydrogen generation efficiency. The reaction mechanism is briefly discussed.

Visible light induced photocatalytic removal of Cr(VI) over TiO2-based nanosheets loaded with surface-enriched CoOx nanoparticles and its synergism with phenol oxidation

Visible-light-driven (VLD) CoOx loaded TiO2-based nanosheets (Co-TNSs) with surface-enrich CoOx nanoparticles were successfully synthesized by using a facile one-pot hydrothermal method. The photocatalysts were characterized by SEM, AFM, HRTEM, XRD, Raman, XPS, FL and the surface photocurrent (SPC) technique, etc. The as-synthesized samples exhibit sheet-like structure with large specific surface area (260–350cm2/g) and small thickness (3–4nm). The CoOx nanoparticles (1–2nm) are highly dispersed on the surface of TNSs. Loaded CoOx nanoparticles not only have influenced the crystal structure, crystallinity and surface area of the TNSs, but also resulted in considerable enhancement of visible-light (VL) absorption and a red-shift of the band gap of the TNSs. XPS results reveal that cobalt mainly exists in the form of Co2+ and Co3+, and the binding energies of TiO bonds are changing with the loaded amount of CoOx nanoparticles. FL and SPC results indicate appropriate amount of CoOx (2.5at.%) loading can effectively inhibit the recombination of photo-generated electron-hole pairs, thus improve the separation efficiency of charge carriers.The photocatalytic activity of the samples was evaluated by the dichromate (Cr(VI)) solution under VL irradiation. It can be found that the Co-TNSs photocatalysts showed excellent VL photocatalytic activity for the removal of Cr(VI). Initially the photocatalytic activity increased with the content of CoOx, and then decreased after attaining a maximum value at an optimal content (2.5at.%) for degradation of aqueous Cr(VI) solution. The addition of Ca2+ promotes photocatalysis owing to its ionic bridging function in the form of TiOH+Cr(VI)–Ca2+Cr(VI) linkages, while SO42− slightly inhibits the photo-reduction of Cr(VI), indicating good synergy of photocatalysis and adsorption even at higher ionic strength of electrolyte. The photocatalytic reduction of Cr(VI) is also significantly promoted by phenol and synergism between Cr(VI) reduction and degradation of phenol which is demonstrated by measuring the effect of multiple usage of Co-TNSs on its photocatalytic efficiency. Desorbed Co-TNSs (Co-TNSs-Des) are easily regenerated and reused for Cr(VI) removal with excellent performance. A possible alternate mechanism for the enhancement of photocatalytic activity under visible light irradiation is also proposed.

Probing the Photovoltage and Photocurrent in Perovskite Solar Cells with Nanoscale Resolution

In this work, by coupling scanning Kelvin probe force microscopy (SKPM) and photoconductive atomic force microscopy (pcAFM), the variation of the surface potential, photogenerated voltage, and photocurrent networks of the perovskite solar cells (PSCs) with different film topography is studied. The nanoscale photovoltaic reaction of different perovskite capping layers with three different perovskite crystalline sizes is first studied by using the SKPM technique. The performance of the overall device is correlated with the local nanostructure of the perovskite film. Photocurrent maps under various applied voltages are also presented. The pcAFM measurements on three different morphology positions determine that the defect region on the capping layer can induce the charge recombination process in the complete PSCs and thus suppress the Voc in the complete device. These results suggest that the performance of PSCs can still be improved through better control of morphology. Henceforth, SKPM coupled with pcAFM techniques has the potential to become a routine characterization tool for perovskite organic and hybrid photovoltaics.

Facile wet chemical method for fabricating p-type BiOBr/n-type nitrogen doped graphene composites: Efficient visible-excited charge separation, and high-performance photoelectrochemical sensing

The development of highly efficient visible-excited charge separation photo active materials has attracted great attention for solving the global energy crisis and environmental problems. In this paper, p-type BiOBr nanoplates decorated n-type nitrogen doped graphene (BiOBr-NGR) composites are constructed by a facile wet chemical method. The formation of p-n junctions was beneficial to the efficient visible-excited charge transfer, and effectively restrains the recombination of photoinduced electron–hole pairs, resulting enhanced photoelectrochemical (PEC) performance. In particular, it is shown by means of the transient-state surface photocurrent responses that the photocurrent intensity of the as-fabricated composites exhibited 2.6 times higher than that of pristine BiOBr. Based on the robust photocurrent signal, a novel PEC sensor based on BiOBr-NGR was established for sensitive and selective detection of chlorpyrifos. The as-fabricated PEC sensor demonstrates many advantages such as wide linear range (5 pg mL−1–11.6 ng mL−1), low detection limit (1.67 pg mL−1, S/N = 3), and remarkably convenient, which provides a general format for chlorpyrifos detection in food and environment analysis.

Cross-Sectional Investigations on Epitaxial Silicon Solar Cells by Kelvin and Conducting Probe Atomic Force Microscopy: Effect of Illumination.

Both surface photovoltage and photocurrent enable to assess the effect of visible light illumination on the electrical behavior of a solar cell. We report on photovoltage and photocurrent measurements with nanometer scale resolution performed on the cross section of an epitaxial crystalline silicon solar cell, using respectively Kelvin probe force microscopy and conducting probe atomic force microscopy. Even though two different setups are used, the scans were performed on locations within 100-μm distance in order to compare data from the same area and provide a consistent interpretation. In both measurements, modifications under illumination are observed in accordance with the theory of PIN junctions. Moreover, an unintentional doping during the deposition of the epitaxial silicon intrinsic layer in the solar cell is suggested from the comparison between photovoltage and photocurrent measurements.

Surface photocurrent in an electron gas over liquid He subjected to a quantizing magnetic field

The photogalvanic effect is studied in electron gas over the liquid He surface with the presence of quantizing magnetic field. The gas is affected by the weak alternating microwave electric field tilted towards the surface normal. Both linear and circular photogalvanic effects are studied. The current occurs via indirect phototransition with the participation of ripplons emission or absorption. The photogalvanic tensor has strong resonances at the microwave frequency $\omega$ approaching to the frequencies of transitions between size-quantized subbands. The resonances are symmetric or antisymmetric, depending on a tensor component. Other resonances appear at $\omega\approx n \omega_c$, where $n$ being integer and $\omega_c$ is the cyclotron frequency. It is found that the latter resonances split to two peaks connected with emission or absorption of ripplons. The calculated photogalvanic coefficients are in accord with the experimental observed values.

Effect of solution concentration on surface morphology, chemical composition and photoresponse of CuO/Cu2O composite thin films grown by hydrothermal synthesis

Crystal structure, surface morphology, surface chemical composition and photocurrent response curves of the thin films were investigated by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy and electrochemical workstation. The results show that the composite thin films are composed of CuO and Cu2O phase. The relative content of copper (I) oxide on the composite thin films increases with the increase of solution concentration. Leaf-like CuO particles have been observed on the thin film grown in the 0.5 mol/L solution. Some regular octahedral and truncated octahedral Cu2O particles have been observed on the surface of thin films grown in the 0.6 and 0.8 mol/L solution, respectively. The best photocurrent response and the fastest growth and decay have been observed in the thin film grown in 0.6 mol/L solution.

Enhanced photocatalytic H₂ generation on cadmium sulfide nanorods with cobalt hydroxide as cocatalyst and insights into their photogenerated charge transfer properties.

Cobalt hydroxide/cadmium sulfide composite was prepared using an easy coprecipitation strategy. The field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) confirmed that Co(OH)2 nanometer particles were modified on CdS. Even without noble-metal cocatalyst, the photocatalytic H2 evolution over CdS after Co(OH)2 loaded was evidently increased. The most excellent Co(OH)2 of 6.8 mol %, resulted in a H2 generation rate of 61 μmol h(-1) g(-1), which exceeded that of pure CdS by a factor of 41 times. Surface photovoltage (SPV) and surface photocurrent (SPC) investigations revealed that the photogenerated electrons could be captured by the loaded Co(OH)2 nanoparticles. The interface formed between Co(OH)2 and CdS is vital to the enhancement of photocatalytic H2 generation. Electrochemical measurement results indicated that another reason for the enhanced photocatalytic activity of Co(OH)2/CdS catalyst is that Co(OH)2 has outstanding H2 generation activity.

Anomalous photoelectric effect of a polycrystalline topological insulator film.

A topological insulator represents a new state of quantum matter that possesses an insulating bulk band gap as well as a spin-momentum-locked Dirac cone on the surface that is protected by time-reversal symmetry. Photon-dressed surface states and light-induced surface photocurrents have been observed in topological insulators. Here, we report experimental observations of an anomalous photoelectric effect in thin films of Bi2Te3, a polycrystalline topological insulator. Under illumination with non-polarised light, transport measurements reveal that the resistance of the topological surface states suddenly increases when the polycrystalline film is illuminated. The resistance variation is positively dependent on the light intensity but has no relation to the applied electric field; this finding can be attributed to the gap opening of the surface Dirac cone. This observation of an anomalous photoelectric effect in polycrystalline topological insulators offers exciting opportunities for the creation of photodetectors with an unusually broad spectral range. Moreover, polycrystalline topological insulator films provide an attractive material platform for exploring the nature and practical application of topological insulators.

In-situ Cr doped anodized TiO2 nanotubes with increased photocurrent response

Low-content chromium (Cr) doped titania nanotubes were grown by electrochemical anodization of Ti-Cr alloys in ethylene glycol electrolyte at varying time of anodisation to control the length of the tubes. Surface properties and photocurrent response of anodized layers after annealing at 450-750°C were evaluated. Nanotubes with 0.02 at% Cr doping activated at 450°C show a considerably increased photocurrent in comparison to pure anatase titania. This photocurrent increase of the Cr doped titania nanotubes is ascribed to higher anatase crystallinity and lower photoelectrochemical recombination losses.

Photogalvanic current in electron gas over a liquid helium surface

We study the stationary surface photocurrent in 2D electron gas near the helium surface. Electron gas is assumed to be attracted to the helium surface due to the image attracting force and an external stationary electric field. The alternating electric field has both vertical and in-plane components. The photogalvanic effect originates from the periodic transitions of electrons between quantum subbands in the vertical direction caused by a normal component of the alternating electric field accompanied by synchronous in-plane acceleration/deceleration due to the electric field in-plane component. The effect needs vertical asymmetry of the system. The problem is considered taking into account a friction caused by the electron-ripplon interaction. The photocurrent resonantly depends on the field frequency. The resonance occurs at field frequencies close to the distance between well subbands. The resonance is symmetric or antisymmetric depending on the kind (linear or circular) of polarization.