Surface Photovoltage Measurements Research Articles

Enhanced visible-light photocatalytic activity of Fe/ZnO for rhodamine B degradation and its photogenerated charge transfer properties

Zinc oxide (ZnO) and iron doped zinc oxide (Fe/ZnO) nanoflowers were successfully synthesized via a simple hydrothermal process. The samples were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman scattering, ultraviolet–visible (UV–vis) diffuse spectroscopy and scanning electron microscopy (SEM), and it was found that the dopant ions replaced some of the crystal lattice zinc ions, and furthermore, Fe3+ and Fe2+ ions coexist. Photocatalytic activities of the catalysts were assessed based on the degradation of rhodamine B (RhB) in aqueous solution under both UV and visible light irradiation (λ>420nm), respectively. All Fe/ZnO samples showed enhanced photocatalytic activity under the irradiation of visible light. On the contrary, Fe/ZnO products displayed poorer performance than that of pure ZnO in the presence of UV light. This phenomenon can be attributed to the coexistence of Fe3+ and Fe2+ in the ZnO host. The photophysical mechanism of the UV and visible photocatalytic activity was investigated with the help of surface photovoltage (SPV) and PL measurements, respectively. The results indicated the influence of coexistence of Fe3+ and Fe2+ in ZnO host on the separation and transfer behavior of the photogenerated charges in the UV and visible regions, which are distinctly different: under the irradiation of UV light, the recombination of the photogenerated electrons and holes was promoted, whereas the separation and transfer of photogenerated electrons and holes was facilitated under the visible light irradiation. The detailed photocatalytic mechanism and the role of Fe on the photodegradation process are discussed.

Capturing photogenerated electrons and holes at the B/Cl co-modified rutile TiO2nanorods during organic pollutant degradation

Degrees for capturing photogenerated holes by residual chemically-adsorbed Cl as donators and electrons by increased amount of adsorbed O2 as receptors after H3BO3 modification in rutile nanorods are quantitatively controlled based on the surface photovoltage measurements, suggesting that capturing electrons is dominant compared to capture holes in colorless pollutant degradation.

Conditioning of Si-interfaces by wet-chemical oxidation: Electronic interface properties study by surface photovoltage measurements

The field-modulated surface photovoltage (SPV) method, a very surface sensitive technique, was utilized to determine electronic interface properties on wet-chemically oxidized and etched silicon (Si) interfaces. The influence of preparation-induced surface micro-roughness and un-stoichiometric oxides on the resulting the surface charge, energetic distribution Dit(E), and density Dit,min of rechargeable states was studied by simultaneous, spectroscopic ellipsometry (SE) measurements on polished Si(111) and Si(100) substrates.

Composites of small Ag clusters confined in the channels of well-ordered mesoporous anatase TiO2 and their excellent solar-light-driven photocatalytic performance

Small Ag clusters confined in the channels of ordered mesoporous anatase TiO2 have been fabricated via a vacuum-assisted wet-impregnation method, utilizing well-ordered mesoporous anatase TiO2 with high thermal stability as the host. The composites have been characterized in detail by X-ray diffraction, X-ray photoelectron spectroscopy, X-ray absorption fine structure (XAFS) spectroscopy, N-2 adsorption, UV-visible diffuse reflectance spectroscopy and transmission electron microscopy. The results indicate that small Ag clusters are formed and uniformly confined in the channels of mesoporous TiO2 with an obvious confinement effect. The presence of strong Ag-O interactions involving the Ag clusters in intimate contact with the pore walls of mesoporous TiO2 is confirmed by XAFS analysis, and favors the separation of photogenerated electron-hole pairs, as shown by steady-state surface photovoltage spectroscopy and transient-state surface photovoltage measurements. The ordered mesoporous Ag/TiO2 composites exhibit excellent solar-light-driven photocatalytic performance for the degradation of phenol. This is attributed to the synergistic effects between the small Ag clusters acting as traps to effectively capture the photogenerated electrons, and the surface plasmon resonance of the Ag clusters promoting the absorption of visible light. This study clearly demonstrates the high-efficiency utilization of noble metals in the fabrication of high-performance solar-light-driven photocatalysts.

Enhanced photo-induced charge transfer properties of vertically aligned nanorods arrays fabricated by thermal annealing approach

In this paper, vertically aligned CdSe/CdS nanorods multilayer was achieved via a novel and simple thermal annealing approach on silicon substrate. The high uniformity in morphology and structure of nanorods arrays was obtained by adjusting the annealing ramp and solution concentration. The mechanism of this self-assembly was explained by the thermally decomposed organics surrounding the nanorods through annealing process, which induces decreased interspace between nanorods and more freedom for nanorods self-assembly into ordered packing; the decrease in organic capping molecules was further confirmed by the thermogravimetric analysis and the fourier transform infrared spectroscopy results; and nanorods arrays showed effectively enhanced photo-induced charge transfer properties compared to that of random packing ones as tested in the surface photovoltage measurement. The results here may offer a way towards device-scale nanorods superlattices for application in photovoltaic devices.

Hall effect and transient surface photovoltage (SPV) study of Cu3BiS3 thin films Abstract

Here, we present the electrical properties of the compound Cu3BiS3 deposited by co-evaporation. This new compound may have the properties necessary to be used as an absorbent layer in solar cells. The samples were characterized by Hall effect and transient surface photovoltage (SPV) measurements. Using Hall effect measurements, we found that the concentration of n charge carriers is in the order of 10 16 cm -3 irrespective of the Cu/Bi mass ratio. We also found that the mobility of this compound (μ in the order of 4 cm 2 V -1 s -1 ) varies according to the transport mechanisms that govern it and are dependent on temperature. Based on the SPV, we found a high density of surface defects, which can be passivated by superimposing a buffer layer over the Cu3BiS3 compound.

Thiol-Capped Germanium Nanocrystals: Preparation and Evidence for Quantum Size Effects

Applications of Ge nanocrystals (NCs) are limited by the stability and air reactivity of the Ge surface. In order to promote stability and increase the diversity of ligand functionalization of Ge NCs, the preparation of thiol-passivated Ge NCs via a ligand exchange process was investigated. Herein a successful replacement of oleylamine ligands on the surface of Ge NCs with dodecanethiol is reported. The successful ligand exchange was monitored by FTIR and NMR spectroscopy and it was found that dodecanethiol provided a better surface coverage, leading to stable Ge NC dispersions. Dodecanethiol capping also enabled band gap determination of the NCs by surface photovoltage (SPV) spectroscopy. The SPV measurements indicated an efficient charge separation in the ligand-exchanged Ge NCs. On the other hand, oleylamine-terminated Ge NCs of similar sizes exhibited a very small photovoltage, indicating a poorly passivated surface.

Cold Gas Sprayed TiO2-Based Electrodes for the Photo-Induced Water Oxidation

Cold gas spraying (CGS) is presented as an innovative approach to deposit semiconductor particles onto substrates in order to form photoelectrodes for electrochemical applications. Thin layers of TiO2 (P25-20 by Evonik Industries) are deposited onto titanium substrates (TiO2-CGS films) at different temperatures of the gas carrier within the CGS process (300 - 1000 °C). Structural characterization reveals unchanged bulk properties of the TiO2 nanoparticles. Clearly, the short duration time of the CGS process hinders crystalline bulk changes of the TiO2 particles in the hot gas stream. However, surface photovoltage measurements indicate that the CGS process modified defect states at the surface when exposed to different gas temperature. In photoelectrochemical measurements TiO2-CGS films yield seven times higher photocurrents and IPCE values than comparable films prepared by the well-established doctor blade technique. The increased efficiency might be due to an enhanced particle to substrate bonding caused by particles welding to the metallic substrate during the cold gas spray process.

Surface photo-voltage characterization of GaAs/AlGaAs single quantum well laser structures grown by molecular beam epitaxy

We present surface photo-voltage (SPV) measurements on molecular beam epitaxy (MBE) grown single quantum well (SQW) laser structures. Each layer in the hetero-structure has been identified by measurement of the SPV signal after a controlled sequential chemical etching process. These results have been correlated with high resolution x-ray diffraction and photoluminescence (PL) measurements. Quantum confined Stark effect and the carrier screening of electric field have been taken into consideration both theoretically and experimentally to account for the differences observed in SPV and PL results. It is shown that SPV can be used as a very effective tool for evaluation of hetero-structures involving multiple layers.

Dual Mode Kelvin Probe: Featuring Ambient Pressure Photoemission Spectroscopy and Contact Potential Difference

We describe a novel dual-mode Kelvin probe featuring ambient pressure Photoemission Spectroscopy (PES), which yields information on the absolute work function (Φ) of a metal and the Ionisation Potential (IP) of a semiconductor, coupled with a high resolution Contact Potential Difference capability which can be extended to Surface Photovoltage measurements. The relative energy resolution are 50 meV for PES and 1-3 meV for CPD. To surmount the limitation of electron scattering in air the incident photon energy is rastered rather than applying a variable retarding electric field as is used UPS. We propose a mechanism of atmospheric ion generation and show that for the metal photoresponse obeys Fowler Theory. The relationship between CPD and photoelectric threshold is a useful tool in characterizing the electrical behavior of materials. We illustrate this with native oxide covered Cu and n-type Si. Further we show that the photoresponse can be used to generate the near Fermi-level Density of States (DOS) in Iron and Nickel-Phthalocyanine.

Dynamics in next-generation solar cells: time-resolved surface photovoltage measurements of quantum dots chemically linked to ZnO (101[combining macron]0).

The charge dynamics at the surface of the transparent conducting oxide and photoanode material ZnO are investigated in the presence and absence of light-harvesting colloidal quantum dots (QDs). The time-resolved change in surface potential upon photoexcitation has been measured in the m-plane ZnO (101[combining macron]0) using a laser pump-synchrotron X-ray probe methodology. By varying the oxygen annealing conditions, and hence the oxygen vacancy concentration of the sample, we find that dark carrier lifetimes at the ZnO surface vary from hundreds of μs to ms timescales, i.e. a persistent photoconductivity (PPC) is observed. The highly-controlled nature of our experiments under ultra-high vacuum (UHV), and the use of band-gap and sub-band-gap photoexcitation, allow us to demonstrate that defect states ca. 340 meV above the valence band edge are directly associated with the PPC, and that the PPC mediated by these defects dominates over the oxygen photodesorption mechanism. These observations are consistent with the hypothesis that ionized oxygen vacancy states are responsible for the PPC in ZnO. The effect of chemically linking two colloidal QD systems (type I PbS and type II CdS-ZnSe) to the surface has also been investigated. Upon deposition of the QDs onto the surface, the dark carrier lifetime and the surface photovoltage are reduced, suggesting a direct injection of charge carriers into the ZnO conduction band. The results are discussed in the context of the development of next-generation solar cells.

Glutathione-assisted hydrothermal synthesis of CdS-decorated TiO2 nanorod arrays for quantum dot-sensitized solar cells

Abstract We report a facile one-step hydrothermal route for the synthesis of efficient CdS-decorated TiO 2 nanorod photoanodes with the aid of glutathione (GSH). The CdS QDs are covalently linked to the surface of TiO 2 nanorods through the GSH bifunctional molecule which also acts as stabilizer and sulfur source in this one-step fabrication. The results from FESEM and HRTEM images reveal that the CdS QDs are well dispersed on the surfaces of TiO 2 nanorods. Surface photovoltage measurements demonstrate that the CdS QDs sensitization enhances the visible spectral absorption of TiO 2 nanorod arrays, as well as their separation efficiency of photo-induced electron–hole pairs. As a result, the CdS-decorated TiO 2 nanorod heterostructure shows a maximum power conversion efficiency (PCE) of 0.278% under illumination at 100 mW cm −2 by employing polysulfide electrolyte.

Measurement of the Band Bending and Surface Dipole at Chemically Functionalized Si(111)/Vacuum Interfaces

The core-level energy shifts observed using X-ray photoelectron spectroscopy (XPS) have been used to determine the band bending at Si(111) surfaces terminated with Si–Br, Si–H, and Si–CH3 groups, respectively. The surface termination influenced the band bending, with the Si 2p3/2 binding energy affected more by the surface chemistry than by the dopant type. The highest binding energies were measured on Si(111)–Br (whose Fermi level was positioned near the conduction band at the surface), followed by Si(111)–H, followed by Si(111)–CH3 (whose Fermi level was positioned near midgap at the surface). Si(111)–CH3 surfaces exposed to Br2(g) yielded the lowest binding energies, with the Fermi level positioned between midgap and the valence band. The Fermi level position of Br2(g)-exposed Si(111)–CH3 was consistent with the presence of negatively charged bromine-containing ions on such surfaces. The binding energies of all of the species detected on the surface (C, O, Br) shifted with the band bending, illustrating the importance of isolating the effects of band bending when measuring chemical shifts on semiconductor surfaces. The influence of band bending was confirmed by surface photovoltage (SPV) measurements, which showed that the core levels shifted toward their flat-band values upon illumination. Where applicable, the contribution from the X-ray source to the SPV was isolated and quantified. Work functions were measured by ultraviolet photoelectron spectroscopy (UPS), allowing for calculation of the sign and magnitude of the surface dipole in such systems. The values of the surface dipoles were in good agreement with previous measurements as well as with electronegativity considerations. The binding energies of the adventitious carbon signals were affected by band bending as well as by the surface dipole. A model of band bending in which charged surface states are located exterior to the surface dipole is consistent with the XPS and UPS behavior of the chemically functionalized Si(111) surfaces investigated herein.

Surface photovoltage analyses of Cu(In,Ga)S2/CdS and Cu(In,Ga)S2/In2S3 photovoltaic junctions

Spectrally dependent surface photovoltage measurements were performed with repetitive regime on Cu(In,Ga)S2 absorbers and on Cu(In,Ga)S2/CdS and Cu(In,Ga)S2/In2S3 junctions. The results are correlated to current-voltage investigations of the respective completed solar cells. The measurements show the presence of a space charge region in the bare Cu(In,Ga)S2 absorbers due to a high density of surface states. It is found that deposition of CdS increases the band bending whereas deposition of In2S3 does not change it.

Photocatalytic performances and activities of Ag-doped CuFe2O4 nanoparticles

In this work, CuFe2O4 nanoparticles were synthesized by a chemical co-precipitation route. The Ag/CuFe2O4 catalyst was prepared based on the CuFe2O4 nanoparticles 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), transmission electronic microscopy (TEM), UV–vis diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS) 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 4-CP by the Ag/CuFe2O4 and CuFe2O4 samples were comparatively studied under xenon lamp irradiation. The results indicate that the Ag/CuFe2O4 sample exhibited the higher efficiency for the degradation of 4-CP.

Photoelectrooxidation of Water on Hematite Thin Films

Hematite photoanodes for the oxygen evolution reaction (OER) were prepared by a cost-efficient sol-gel procedure. We show that annealing and plasma post-treatments improve significantly the photoelectrochemical properties for the OER. Annealing steps lead to a reorganization of the crystal structure and thereby to a pronounced shift of the photocurrent to negative potentials. Surface photovoltage measurements (SPV) indicate an enhanced separation of charge carriers originated from the band gap photoabsorption. Nevertheless a more pronounced effect on the catalytic efficiency was gained by a surface-treatment in oxygen plasma. X-ray photoelectron spectroscopy (XPS) analysis reveals an oxygen enriched surface layer with new oxygen species which might be responsible for the improved electrochemical activity. SPV measurements identify an increased fraction of transferred charge carriers from this newly produced surface defects. The combination of these post-treatments leads finally to an efficient hematite photoanode for the OER.

Surface Oxide Effect on Optical Sensing and Photoelectric Conversion of α-In2Se3 Hexagonal Microplates

The surface formation oxide assists of visible to ultraviolet photoelectric conversion in α-In2Se3 hexagonal microplates has been explored. Hexagonal α-In2Se3 microplates with the sizes of 10s to 100s of micrometers were synthesized and prepared by the chemical vapor transport method using ICl3 as a transport agent. Many vacancies and surface imperfection states have been found in the bulk and on the surface of the microplate because of the intrinsic defect nature of α-In2Se3. To discover physical and chemical properties and finding technological uses of α-In2Se3, several experiments including transmission electron miscopy (TEM), X-ray photoelectron spectroscopy (XPS), surface photovoltage (SPV), photoluminescence (PL), surface photoresponse (SPR), photoconductivity (PC), and thermoreflectance (TR) measurements have been carried out. Experimental results of TEM, XPS, SPV, PL, and SPR measurements show that a surface oxidation layer α-In2Se3-3xO3x (0 ≤ x ≤ 1) has formed on the crystal face of α-In2Se3 in environmental air with the inner layer content close to In2Se3 but the outermost layer content approaching In2O3. The near band edge transitions of α-In2Se3 microplates have been probed experimentally by TR and PC measurements. The direct band gap of α-In2Se3 has been determined to be 1.453 eV. The SPV result shows a maximum quantum efficiency of the surface oxide α-In2Se3-3xO3x (0 ≤ x ≤ 1) that presents a peak photoresponse near 2.18 eV. The analyses of SPV, SPR, PL, TR, and PC measurements revealed that the surface oxide layer facilitates the conversion of the ultraviolet to the visible range while the native defects (Se and In vacancies) sustain photoconductivity in the near-infrared region. On the basis of the experimental results a wide-energy-range photodetector that combines PC- and SPR-mode operations for α-In2Se3 microplate has been made. The testing results show a well-behaved function of photoelectric conversion in the near-infrared to ultraviolet region via the auxiliary forming of surface oxide on the crystalline face of the α-In2Se3 microplates.

Photoelectrochemical and Photovoltaic Properties of p–n Cu2O Homojunction Films and Their Photocatalytic Performance

The improvement of photoinduced charge separation is the key for light-harvesting systems in both photovoltaic and photoelectrochemical solar cells. In this study, the charge separation efficiency has been modulated through varying the magnitude of interfacial electric field in p–n Cu2O homojunction films prepared by simple electrodeposition method. The photoelectrochemical and surface photovoltage measurements were used to investigate the behaviors of photoinducded charge carriers in different p–n Cu2O homojunction films. The results confirmed that the p–n Cu2O homojunction film which exhibited the highest charge separation efficiency resulted in the highest activity in photocatalytic reduction of methyl viologen. These implied that it is possible to achieve high charge separation efficiency via constructing a large magnitude of interfacial electric field within a semiconductor using a simple electrodeposition method.

Organic Functionalization of 3C-SiC Surfaces

We demonstrate the functionalization of n-type (100) and (111) 3C-SiC surfaces with organosilanes. Self-assembled monolayers (SAMs) of amino-propyldiethoxymethylsilane (APDEMS) and octadecyltrimethoxysilane (ODTMS) are formed via wet chemical processing techniques. Their structural, chemical, and electrical properties are investigated using static water contact angle measurements, atomic force microscopy, and X-ray photoelectron spectroscopy, revealing that the organic layers are smooth and densely packed. Furthermore, combined contact potential difference and surface photovoltage measurements demonstrate that the heterostructure functionality and surface potential can be tuned by utilizing different organosilane precursor molecules. Molecular dipoles are observed to significantly affect the work functions of the modified surfaces. Furthermore, the magnitude of the surface band bending is reduced following reaction of the hydroxylated surfaces with organosilanes, indicating that partial passivation of electrically active surface states is achieved. Micropatterning of organic layers is demonstrated by lithographically defined oxidation of organosilane-derived monolayers in an oxygen plasma, followed by visualization of resulting changes of the local wettability, as well as fluorescence microscopy following immobilization of fluorescently labeled BSA protein.

Surface photovoltage measurements: A quick assessment of the photocatalytic activity?

Surface photovoltage (SPV) measurements can contribute to a better understanding of electronic properties of photocatalysts under illumination. Direct linking of SPV data to the actual photocatalytic activity remains troublesome. This work aims to discuss SPV measurements from a photocatalytic point of view. By means of several application-based scenarios we illustrate that the trend between SPV and photocatalysis strongly depends on parameters such as the crystal structure, surface modifications, morphology and humidity. This makes the interpretation far from straightforward.