a As a visible light active p-type semiconductor, CuBi2O4 is of interest as a photocatalyst for the generation of hydrogen fuel from water. Here we present the first photovoltage and photocatalytic measurements on this material and DFT results on its band structure. Single crystalline CuBi2O4 nanoparticles (25.7 ± 4.7 nm) were synthesized from bismuth and cupric nitrate in water under hydrothermal conditions. Powder X-ray diffraction (XRD) confirms the CuBi2O4 structure type and UV-Vis spectroscopy observes a 1.75 eV optical band gap. Surface photovoltage (SPV) measurements on CuBi2O4 nanoparticle films on fluorine doped tin oxide yield 0.225 V positive photovoltage at >1.75 eV photon energy confirming holes as majority carriers. The photovoltage is reversible and limited by light absorption. When dispersed in 0.075 M aqueous potassium iodide solution, the CuBi2O4 particles support photochemical hydrogen evolution of up to 16 µmol h-1 under ultraviolet but not under visible light. Based on electrochemical scans, CuBi2O4 is unstable towards reduction at -0.2 V, but pH-dependent photocurrents of 6.45 A cm -2 with an onset potential of +0.75 V vs. NHE can be obtained with 0.01 M Na2S2O8 as a sacrificial electron acceptor. The photoelectrochemical properties of CuBi2O4 can be explained on the basis of the band structure of the material. DFT calculations show the valence and conduction band edges to arise primarily from the combination of O 2p and Cu 3d orbitals, respectively, with additional contributions from Cu 3d and Bi 6s orbitals just below the Fermi level. Trapping of photoelectrons in the Cu 3d band is the cause for reductive photocorrosion of the material, while the p-type conductivity arises from copper vacancy states near the VB edge. These findings provide an improved understanding of the photophysical properties of p-CuBi2O4 and its limitations as a proton reduction photocatalyst.A) The DFT+U electronic band structures of (left) pristine CuBi2O4 and (right) VCu -CuBi2O4. The Fermi level (EF) is set at 0 eV. The presence of VCu leads to partial occupation of the bands around the VB edge. This gives rise to p-type behavior in CuBi2O4. B) The DFT+U projected density of states of VCu -CuBi2O4.
We developed a simple and robust colloidal route for the installation of CdX2 (X = Cl, Br, I) ligands on the surface of CdSe nanocrystals, which effectively displace the native ligands and form stable suspensions. After colloidal ligand exchange, these nanocrystals can be easily solution cast into nanocrystal films. Photoelectrochemical measurements on solution-cast nanocrystal films reveal a striking influence of surface cadmium halide on photocurrent response, with mildly annealed, CdCl2-treated CdSe nanocrystals showing the greatest enhancement in photocurrent to above band gap illumination. The strong dependence of photoresponse on surface halide is thought to result from ligand-induced changes in the electronic structure of the nanocrystal samples. We arrive at this conclusion using a combination of ultrafast transient absorption, time-resolved photoluminescence, and surface photovoltage spectroscopies, which are being applied together for the first time to investigate nanocrystal trap states. From these measurements, we establish a trend for ligand-related sub-band gap states that accounts for electron and hole trapping at the nanocrystal surface. The nature of the electron and hole traps in the nanocrystal films are dependent on the thermal history of the sample as well as the specific halide surface treatment employed. After subjecting the nanocrystal films to mild thermal annealing, we find evidence that suggests a drastic reduction in electron trap states. Additionally, depending on the surface halide treatment employed, the energy of the hole trap states varies, with CdCl2 treatment resulting in energetically shallow hole trap states, and CdBr2 and CdI2 treatments leading to much deeper hole traps. Thus, judicious choice of cadmium halide surface treatment can be used to manipulate the trap state landscape of these ligand exchanged CdSe nanocrystals.
Nickel(II) oxide (NiO) is an important wide gap p-type semiconductor used as a hole transport material for dye sensitized solar cells and as a water oxidation electrocatalyst. Here we demonstrate that nanocrystals of the material have increased p-type character and improved photocatalytic activity for hydrogen evolution from water in the presence of methanol as sacrificial electron donor. NiO nanocrystals were synthesized by hydrolysis of Ni(II) nitrate under hydrothermal conditions followed by calcination in air. The crystals have the rock salt structure type and adopt a plate-like morphology (50-90 nm × 10-15 nm). Diffuse reflectance absorbance spectra indicate a band gap of 3.45 eV, similar to bulk NiO. Photoelectrochemical measurements were performed at neutral pH with methylviologen as electron acceptor, revealing photo-onset potentials (Fermi energies) of 0.2 and 0.05 eV (NHE) for nanoscale and bulk NiO, respectively. Nano-NiO and NiO-Pt composites obtained by photodepositon of H2PtCl6 catalyze hydrogen evolution from aqueous methanol at rates of 0.8 and 4.5 μmol H2 h(-1), respectively, compared to 0.5 and 2.1 μmol H2 h(-1) for bulk-NiO and NiO-Pt (20 mg of catalyst, 300 W Xe lamp). Surface photovoltage spectroscopy of NiO and NiO-Pt films on Au substrates indicate a metal Pt-NiO junction with 30 mV photovoltage that promotes carrier separation. The increased photocatalytic and photoelectrochemical performance of nano-NiO is due to improved minority carrier extraction and increased p-type character, as deduced from Mott-Schottky plots, optical absorbance, and X-ray photoelectron spectroscopy data.
Surface photovoltage spectroscopy (SPS) is used to measure the photopotential across a Ru-SrTiO:Rh/BiVO particle tandem overall water splitting photocatalyst. The tandem is synthesized from Ru-modified SrTiO:Rh nanocrystals and BiVO microcrystals by electrostatic assembly followed by thermal annealing. It splits water into H and O with an apparent quantum efficiency of 1.29% at 435 nm and a solar to hydrogen conversion efficiency of 0.028%. According to SPS, a photovoltage develops above 2.20 eV, the effective band gap of the tandem, and reaches its maximal value of -2.45 V at 435 nm (2.44 mW cm), which corresponds to 96% of the theoretical limit of the photocatalyst film on the fluorine-doped tin-oxide-coated glass (FTO) substrate. Charge separation is 82% reversible with 18% of charge carriers being trapped in defect states. The unusually strong light intensity dependence of the photovoltage (1.16 V per decade) is attributed to depletion layer changes inside of the BiVO microcrystals. These findings promote the understanding of solar energy conversion with inorganic particle photocatalysts.
ABSTRACT:The charge transfer properties of interfaces are central to the function of photovoltaic and photoelectrochemical cells and photocatalysts. Here we employ surface photovoltage spectroscopy (SPS) to study photochemical charge transfer at a p-silicon/n-BiVO 4 particle interface. Particle films of BiVO 4 on an aluminum-doped p-silicon wafer were obtained by drop-coating particle suspensions followed by thermal annealing at 353 K. Photochemical charge separation of the films was probed as a function of layer thickness and illumination intensity, and in the presence of methanol as a sacrificial electron donor. Electron injection from the BiVO 4 into the psilicon is clearly observed to occur and to result in a maximum photovoltage of 150 mV for a 1650 nm thick film under 0.3 mW cm −2 illumination at 3.5 eV. This establishes the BiVO 4 −p-Si interface as a tandem-like junction. Charge separation in the BiVO 4 film is limited by light absorption and by slow electron transport to the Si interface, based on time-dependent SPS measurements. These problems need to be overcome in functional tandem devices for photoelectrochemical water oxidation.
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