Surface Photovoltage Measurements Research Articles

Single-Crystal Tungsten Oxide Nanosheets: Photochemical Water Oxidation in the Quantum Confinement Regime

Here we investigate the structure, photophysics, and photocatalytic water splitting properties of single-crystalline WO3 nanosheets (0.75 nm × 90 ± 38 nm), obtained by exfoliation from Bi2W2O9. Upon delamination, the nanosheets undergo a structural change from tetragonal symmetry in the parent material to monoclinic, as confirmed by powder X-ray diffraction and electron microscopy. Diffuse reflectance optical spectra show band gap energies consistent with quantum confinement in nano-WO3 (EG = 2.88 eV) and Bi2W2O9(EG = 2.81 eV), relative to bulk WO3 (EG = 2.68 eV). Surface photovoltage measurements on nano-WO3 films on a F:SnO2 substrate demonstrate photochemical carrier formation under band gap excitation and irreversible trapping of holes. Photochemical oxygen formation is observed with 50 mg of the material in aqueous AgNO3 and (NH4)2Ce(NO3)6 solutions under full spectrum (>250 nm) or visible only (>400 nm) irradiation. The highest initial O2 evolution rates (69.7 μmol h–1 for bulk and 35.5 μmol h–1 for...

Overall photocatalytic water splitting with NiOx–SrTiO3 – a revised mechanism

NiOx (0 < x < 1) modified SrTiO3 (STO) is one of the best studied photocatalysts for overall water splitting under UV light. The established mechanism for this and many other NiOx containing catalysts assumes water oxidation to occur at the early transition metal oxide and water reduction at NiOx. Here we show that NiOx–STO is more likely a three component Ni–STO–NiO catalyst, in which STO absorbs the light, Ni reduces protons, and NiO oxidizes water. This interpretation is based on systematic H2/O2 evolution tests of appropriately varied catalyst compositions using oxidized, chemically and photochemically added nickel and NiO nanoparticle cocatalysts. Surface photovoltage (SPV) measurements reveal that Ni(0) serves as an electron trap (site for water reduction) and that NiO serves as a hole trap (site for water oxidation). Electrochemical measurements show that the overpotential for water oxidation correlates with the NiO content, whereas the water reduction overpotential depends on the Ni content. Photodeposition experiments with NiCl2 and H2PtCl6 on NiO–STO show that electrons are available on the STO surface, not on the NiO particles. Based on photoelectrochemistry, both NiO and Ni particles suppress the Fermi level in STO, but the effect of this shift on catalytic activity is not clear. Overall, the results suggest a revised role of NiO in NiOx–STO and in many other nickel-containing water splitting systems, including NiOx–La : KTaO3, and many layered perovskites.

Chemically assembled heterojunctions of SnO2 nanorods with TiO2 nanoparticles via “click” chemistry

SnO2 is a promising material for photovoltaic and photocatalytic applications because it exhibits high electron mobility, its conduction band lies at a convenient energy to act as an electron acceptor, and it can be easily grown in a variety of different nanostructures including nanoparticles, nanorods, and nanosheets. However, strategies for surface functionalization of SnO2 are much less well developed than alternative oxides. Here, we demonstrate the growth and subsequent chemical functionalization of SnO2 nanorods to enable the chemically directed assembly of SnO2 nanorod–TiO2 nanoparticle heterojunctions, and we characterize the charge-transfer properties using time-resolved surface photovoltage measurements. Vertically aligned SnO2 nanorods were grown via a high-pressure chemical synthesis method. The SnO2 nanorods were square in cross-section, exposing sidewalls consisting of {110}-type crystal planes. Functionalization via photochemical grafting with butenol yielded nanorods terminated with a high density of –OH groups that were converted to azide groups. The azide groups were linked with alkyne-modified TiO2 nanoparticles via the Cu(I)-catalyzed Azide–Alkyne Cycloaddition (CuAAC) reaction, a form of “click” chemistry, thereby covalently grafting the TiO2 nanoparticles to the SnO2 nanorods. Time-resolved surface photovoltage measurements of the resulting adducts showed that the covalent bonding of TiO2 nanoparticles to the SnO2 nanorods enhances the interfacial charge transfer compared to the unmodified SnO2 nanorods, leading to an increased accumulation of holes at the surface.

Investigation of the photovoltaic performance of the polycrystalline silicon p–n junction by a photothermal measurement

For establishing a new methodology for evaluating an effect of the grain boundaries, both the piezoelectric photo-thermal (PPT) and the surface photo-voltage (SPV) measurements of polycrystalline Si p–n junction samples with different volume fractions of grain boundaries were carried out. We could define the signal intensity ratio of SPV/PPT as the key indicator of photovoltaic performance. This is because the PPT signal implies the phonon emitting carrier loss, whereas the SPV denotes the photo-excited carrier accumulation at the surface and the junction interface. It was found that the SPV/PPT ratio and solar cell efficiency decreased with increasing volume fraction of the grain boundaries. Present experimental results demonstrated that one can directly estimate the photovoltaic performance of in-process polycrystalline Si p–n junction wafer by adopting the combination of the PPT and the SPV methodologies without electrodes. Since the PPT detects the non-radiative recombination process, present methodology and the laser-beam-induced current and the photoluminescence imaging methods are complementary. By complementary use of these methods, it becomes possible to investigate the characteristic of grain boundary.

Visible-Light-Assisted HCHO Gas Sensing Based on Fe-Doped Flowerlike ZnO at Room Temperature

In this work, Fe-doped flowerlike ZnO powders with various doping contents were successfully fabricated by a hydrothermal method. The results of X-ray diffraction and UV–vis DRS spectra revealed that the Fe ions have been successfully doped into the crystal lattice of the ZnO host structure, and the optical absorption response of Fe-doped ZnO was extended into the visible region for the incorporation of Fe ions. The room-temperature photoelectric gas sensing of formaldehyde (HCHO) based on the Fe-doped ZnO was also studied under 532 nm light irradiation provided by a green laser pointer. It was found that the as-prepared Fe-doped ZnO samples showed excellent sensitivity, in which the gas response to 5 and 100 ppm formaldehyde can reach to 22% and 287% under 532 nm light irradiation at room temperature, respectively. The sensing mechanism of the obvious visible-light-induced photoelectric gas sensing was discussed with the help of surface photovoltage measurement. Our results demonstrated that visible light irradiation was a promising approach to achieving a large response for gas sensors at room temperature. This work will pave a way for the development of a low-cost practical gas sensor.

Modular “Click” Chemistry for Electrochemically and Photoelectrochemically Active Molecular Interfaces to Tin Oxide Surfaces

We demonstrate the use of "click" chemistry to form electrochemically and photoelectrochemically active molecular interfaces to SnO(2) nanoparticle thin films. By using photochemical grafting to link a short-chain alcohol to the surface followed by conversion to a surface azide group, we enable use of the Cu(I)-catalyzed azide-alkyne [3 + 2] cycloaddition (CuAAC) reaction, a form of "click" chemistry, on metal oxide surfaces. Results are shown with three model compounds to test the surface chemistry and subsequent ability to achieve electrochemical and photoelectrochemical charge transfer. Surface-tethered ferrocene groups exhibit good electron-transfer characteristics with thermal rates estimated at >1000 s(-1). Time-resolved surface photovoltage measurements using a ruthenium terpyridyl coordination compound demonstrate photoelectron charge transfer on time scales of nanoseconds or less, limited by the laser pulse width. The results demonstrate that the CuAAC "click" reaction can be used to form electrochemically and photoelectrochemically active molecular interfaces to SnO(2) and other metal oxide semiconductors.

Surface photovoltage within a monolayer of CdSe quantum dots

AbstractLayers of pyridine terminated CdSe quantum dots (QDs) with thicknesses from a monolayer (ML) to 80 nm were deposited on ITO substrates by dip coating. Transient surface photovoltage measurements showed dominant charge separation within the first ML of QDs at short times and an increasing influence of layer thickness at long times. Auger recombination limited the maximum of separated charge carriers to about 1 × 1012 cm‐2.magnified imageSeparation of photo‐generated electron‐hole pairs within a ML of CdSe‐QDs. Electrons were either trapped on ITO surface states or separated due to a built‐in electrical field created by a transferred electron from the ITO to a trap state at the QD. (© 2011 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Electrical passivation and chemical functionalization of SiC surfaces by chlorine termination

We have developed a straightforward plasma-based method which yields chlorine-terminated n-type 6H-SiC surfaces. Atomic force microscopy shows that the surface roughness is not affected by the plasma processing. Additionally, x-ray photoelectron spectroscopy reveals a significant reduction in oxygen, and a corresponding rise of chlorine core level intensities, following halogen termination. Contact potential difference and surface photovoltage measurements show formation of negative surface dipoles and approximately flat band surface potentials after chlorine termination of (0001) n-type 6H-SiC (built-in voltage Vbi&amp;lt;20 meV). Starting from halogenated surfaces, we demonstrate both ultraviolet light-induced and thermally-induced functionalization with alkene-derived self-assembled organic monolayers.

Band lineup in amorphous/crystalline silicon heterojunctions and the impact of hydrogen microstructure and topological disorder

We determine the band offsets in amorphous/crystalline silicon [a-Si:H/c-Si{111}] heterojunctions using combined data from photoelectron spectroscopy and surface photovoltage measurements on structures comprising a-Si:H layers with device-relevant thickness ($10 \mathrm{nm}$). Altering the a-Si:H hydrogen (H) content ${C}_{\mathrm{H}}$ by the choice of deposition conditions, we observe a systematic retreat of the a-Si:H valence band edge leading to an increase of the band gap and the valence band offset $\ensuremath{\Delta}{E}_{V}$ with ${C}_{\mathrm{H}}$ by about $13 \mathrm{meV}/\mathrm{at}.\mathrm{ }\mathrm{%}\mathrm{ }\mathrm{H}$. The discrepancy with the $30\ensuremath{-}40 \mathrm{meV}/\mathrm{at}.\mathrm{ }\mathrm{%}\mathrm{ }\mathrm{H}$ predicted by theory can be consistently explained by the compensating effect of enhanced topological disorder imposed by the increasing density of microvoids as revealed by an analysis of the H microstructure. Thus we highlight the necessity of explicitly including the details of the H configuration in a theoretical treatment of the a-Si:H/c-Si heterojunction.

Electro-optic investigation of the surface trapping efficiency inn-alkanethiol SAM passivated GaAs(001)

The electro-optic characteristics of the semi-insulating andn + -type GaAs(001) surfacespassivated with n-alkanethiol self-assembled monolayers were investigated using Kelvin probe surface photovoltage(SPV) and photoluminescence (PL) techniques. Referencing the equilibrium surface barrierheight established in an earlier report, SPV measurements demonstrated a significant (>100 mV) increase in the non-equilibrium band-bending potential observed under low-levelphoto-injection. Modeling of the SPV accounts for these observations in terms of a large (>104) decreasein the hole/electron ratio of surface carrier capture cross-sections, which is suggested to result from theelectrostatic potential of the interfacial dipole layer formed upon thiol chemisorption. Thecross-section effects are verified in the high-injection regime based on carrier transportmodeling of the PL enhancement manifested as a reduction of the surface recombinationvelocity.

Photocatalytic degradation of gaseous toluene over Ag-doping TiO 2 nanotube powder prepared by anodization coupled with impregnation method

In this work, Ag-doping TiO 2 nanotubes were prepared and employed as the photocatalyst for the degradation of toluene. The TiO 2 nanotube powder was produced by the rapid-breakdown potentiostatic anodization of Ti foil in chloride-containing electrolytes, and then doped with Ag through an incipient wetness impregnation method. The samples were characterized by scanning electron microscope, high-resolution transmission electron microscopy, X-ray diffraction, surface photovoltage measurements, X-ray photoelectron spectroscopy and N 2 adsorption. The nanotubular TiO 2 photocatalysts showed an outer diameter of approximately 40 nm, fine mesoporous structure and high specific surface area. The photocatalytic activity of Ag-doping TiO 2 nanotube powder was evaluated through photooxidation of gaseous toluene. The results indicated that the degradation efficiency of toluene could get 98% after 4 h reaction using the Ag-doping TiO 2 nanotubes as the photocatalyst under UV light illumination, which was higher than that of the pure TiO 2 nanotubes, Ag-doping P25 or P25. Benzaldehyde species could be observed during the photocatalytic oxidation monitored by in situ FTIR, and the formed benzaldehyde intermediate during reaction would be partially oxidized into CO 2 and H 2O.

Synthesis of ordered multivalent Mn-TiO2 nanospheres with tunable size: A high performance visible-light photocatalyst

A facile and reproducible method has been developed for the synthesis of ordered anatase Mn-TiO2 nanospheres with controllable sizes in the range 200–300 nm by simply varying the amount of manganese(II) chloride added. The samples were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM), and transmission electron microscope (TEM) measurements, and it was found that the manganese exists in multivalent forms (Mn4+/ Mn3+) and substitutes for some of the Ti4+ in the anatase TiO2 lattice. The presence of Mn significantly influences the morphology and high-temperature stability of TiO2, and extends its light absorption range. Surface photovoltage and photocurrent measurements revealed that an electronic interaction between the Mn and TiO2 was present, in which Mn served as an electron acceptor and effectively inhibited the charge recombination in TiO2; this is thought to be responsible for the highly efficient photocatalytic activity of the material in the degradation of rhodamine B (RhB) under visible-light irradiation ( λ > 420 nm).

TiO2 nanotube/Ag–AgBr three-component nanojunction for efficient photoconversion

A TiO2 nanotube/Ag–AgBr three-component nanojunction with enhanced visible-light activity was synthesized by a two-step approach including an electrochemical anodization technique followed by an in situ photo-assisted dipping and deposition approach. Ag/AgBr nanoparticles with good dispersion were effectively deposited on both the inside and outside of the TiO2 nanotubes. Surface photovoltage measurements and luminescence spectra demonstrated that the Ag/AgBr loading enhanced the visible spectral absorption of TiO2 nanotube arrays, as well as their separation efficiency of photoinduced electron–hole pairs, which may due to the nanojunction built between Ag/AgBr and TiO2 in this system. The photoelectrochemical investigations verified that the TiO2 nanotube/Ag–AgBr nanojunction showed enhanced photocurrent generation efficiency and had a more effective photoconversion efficiency of 0.71% than the aligned TiO2 nanotube arrays alone. The enhanced photoelectrochemical properties can be attributed to the extended absorption in the visible light region resulting from Ag/AgBr nanoparticles, and the effective separation of photogenerated carriers driven by the photoinduced potential difference generated at the TiO2/Ag–AgBr nanojunction interface.

Surface Characterization of Ga-doped ZnO layers

ABSTRACTEpitaxial ZnO layers heavily doped with Ga (GZO) were grown at 400 °C under metaland oxygen-rich conditions in terms of metal-to-reactive oxygen ratio by plasma-assisted molecular beam epitaxy (MBE). Several atomic force microscopy (AFM) techniques were used to characterize the surface morphology and electrical properties of these GZO films in ambient conditions. Local I-V spectra indicate that layers grown under both O-rich and metal-rich conditions are highly resistive until a relatively high voltage sweep (±12 V) is used. After removal of an insulating surface layer, conduction is possible at lower voltages, but eventually the film resistivity increases and it again becomes insulating. In addition to local I-V spectra, local charge injection and subsequent surface potential measurements were used to probe surface charging characteristics. For charge injection experiments, a reverse-bias voltage is applied to the sample while scanning in contact mode with a metallized tip. The resultant change in surface potential due to trapped charge is subsequently observed using scanning Kelvin probe microscopy (SKPM). The layers deposited in a metal-rich environment demonstrate the expected behavior, but the O-rich layers show anomalous negative and positive charging. Finally, surface photovoltage (SPV) measurements using above-bandgap UV illumination were performed. The GZO layers produce SPV values of 0.4 to 0.5 eV, where the films deposited in an O-rich environment have slightly higher SPV values and faster restoration.

Photocatalytic degradation of gaseous toluene over ZnAl2O4 prepared by different methods: A comparative study

The development of a “green” treatment process for typical indoor pollutants such as toluene is greatly desirable. In this study, ZnAl2O4 nanoparticles were prepared via three different routes, i.e., solvothermal, citrate precursor and hydrothermal methods. Their structural properties were systematically investigated by X-ray powder diffraction (XRD), scanning electronic microscopy (SEM), energy-dispersive X-ray spectra (EDX), Brunauer–Emmett–Teller (BET), UV–vis diffuse reflectance spectroscopy (DRS), and Fourier transform infrared spectroscopy (FT-IR) techniques. The photo-induced charge separation in the samples was demonstrated by surface photovoltage (SPV) measurement. The photocatalytic performances of the ZnAl2O4 samples and nanostructured TiO2 samples were comparatively studied by the degradation of gaseous toluene under UV lamp irradiation in in situ FTIR reactor. The results indicated that the sample synthesized by facile solvothermal method exhibited about 90% photocatalytic efficiency of toluene. The toluene was mineralized into carbon dioxide and water as the major species. The photocatalytic oxidation of gaseous pollutant over UV-illuminated ZnAl2O4 is a promising technique for air purification.

The influence of the silicon/silicon oxide space charge region on excess charge carrier kinetics in silicon

AbstractExcess charge carrier kinetics in p‐doped crystalline silicon (Si) wafers covered by SiO2 films is investigated by transient photoconductance and surface photovoltage measurements. This makes it possible to distinguish between excess charge carriers collected in the Si/SiO2 heterojunction and those in the Si volume. The decay of the photoconductance signal in the millisecond time range could be unambiguously attributed to excess charge carriers stored in the space charge region. The decay of these charge carriers appears to be independent of the volume lifetime. The difference between the behaviours in Si/SiO2 and Si/SiNx is discussed.

In0.49Ga0.51P/GaAs 이종접합 구조의 표면 광전압 특성

Metal-organic chemical vapour deposition (MOCVD) 법으로 성장된 <TEX>$In_{0.49}Ga_{0.51}P$</TEX>/GaAs 이종접합 구조의 특성을 표면 광전압(surface photovoltage; SPV) 분광법으로 조사하였다. SPV 측정은 입사광의 세기, 변조 주파수, 온도의 함수로 수행하였다. 상온에서 시료의 띠간격 에너지(band gap energy)는 GaAs와 <TEX>$In_{0.49}Ga_{0.51}P$</TEX>는 각각 1.400 및 1.893 eV이었다. 광세기를 증가시킴에 따라 SPV 크기는 증가하는 반면에, 변조 주파수를 증가시킴에 따라 SPV 크기는 감소하였다. 그리고 SPV 스펙트럼의 온도 의존성으로부터 GaAs와 <TEX>$In_{0.49}Ga_{0.51}P$</TEX>의 띠간격 에너지의 변화를 Varshni 및 Bose-Einstein 표현에 의해 분석하였다. We report the surface photovoltage (SPV) properties of <TEX>$In_{0.49}Ga_{0.51}P$</TEX>/GaAs heterostructure grown by metal-organic chemical vapour deposition (MOCVD). The SPV measurements were studied as a function of modulation beam intensity, modulation frequency and temperature. From a line shape analysis of room temperature derivative surface photovoltage (DSPV) spectrum, the band gap energies for GaAs and <TEX>$In_{0.49}Ga_{0.51}P$</TEX> transitions were 1.400 and 1.893 eV respectively. The surface photovoltage (SPV) increases with increasing the light intensity and temperature, whereas the SPV decreases with increasing the modulation frequency. From the temperature variation of the energy gaps, we have analysis by both Varshni and Bose-Einstein type expressions.

Porous “brick-like” NiFe 2O 4 nanocrystals loaded with Ag species towards effective degradation of toluene

Porous and “brick-like” NiFe 2O 4 nanoparticles were synthesized by a modified chemical co-precipitation route with calcination temperatures of 773 K, 873 K and 973 K, respectively. The Ag/NiFe 2O 4 catalyst was prepared based on the porous NiFe 2O 4 by the incipient wetness impregnation strategy, which showed excellent photoelectric property and catalytic activity. The structural properties of these samples were systematically investigated by X-ray powder diffraction (XRD), scanning electronic microscopy (SEM), energy-dispersive X-ray spectra (EDX), UV–vis diffuse reflectance spectroscopy (DRS), and Fourier transform infrared spectroscopy (FT-IR) techniques. The photo-induced charge separation in the samples was demonstrated by surface photovoltage (SPV) measurement. The photocatalytic degradation of toluene by the Ag/NiFe 2O 4 and NiFe 2O 4 samples was comparatively studied under xenon lamp irradiation. The results indicate that the Ag/NiFe 2O 4 sample calcined at 773 K exhibited the highest efficiency for the degradation of toluene.

Synthesis and characterization of AgI thin films at low temperature

The growth of AgI (silver iodide) thin films has been performed in aqueous medium using simple chemical method at room temperature (25 °C). Silver nitrate and potassium iodide have been used as source materials. Thin films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), surface photovoltage (SPV) and optical absorption spectroscopy. Glass and indium doped tin oxide (ITO) coated glass was used as substrates. The thin films were surface homogeneous with mixed β and γ-phases with surface roughness value of 21 nm. Optical transmission on glass exceeds 80% for 150 nm thick film with the direct band gap value of 2.85 eV. The change in crystal phases after transition temperature is studied by SPV measurements.

Electrochemical Method for Synthesis of a ZnFe2O4/TiO2 Composite Nanotube Array Modified Electrode with Enhanced Photoelectrochemical Activity

AbstractAn electrode with intimate and well‐aligned ZnFe2O4/TiO2 composite nanotube arrays is prepared via electrochemical anodization of pure titanium foil in fluorine‐containing ethylene glycol, followed by a novel cathodic electrodeposition method. The deposition of ZnFe2O4 is promoted in the self‐aligned, vertically oriented TiO2 nanotube arrays but minimized at the tube entrances. Thus, pore clogging is prevented. Environmental scanning electron microscopy, energy‐dispersive X‐ray spectra, high‐resolution transmission electron microscopy, X‐ray diffraction patterns, and X‐ray photoelectron spectroscopy indicate that the as‐prepared samples are highly ordered and vertically aligned TiO2 nanotube arrays with ZnFe2O4 nanoparticles loading. The TiO2 nanotubes are anatase with the preferential orientation of &lt;101&gt; plane. Enhanced absorption in both UV and visible light regions is observed for the composite nanotube arrays. The current–voltage curve of ZnFe2O4‐loaded TiO2 nanotube arrays reveals a rectifying behavior. The enhanced separation of photoinduced electrons and holes is demonstrated by surface photovoltage and photocurrent measurements. Meanwhile, the photoelectrochemical investigations verify that the ZnFe2O4/TiO2 composite nanotube array modified electrode has a more effective photoconversion capability than the aligned TiO2 nanotube arrays alone. In addition, the photoelectrocatalytic ability of the novel electrode is found enhanced in the degradation of 4‐chlorophenol.