Table of contents

A tandem two photoelectrode cell separated by a proton exchange membrane for simultaneous conversion of solar energy into both chemical and electrical energy was investigated by illumination of both a large bandgap photoanode, to produce a highly oxidized species, and a narrow bandgap photocathode, to produce a highly reduced species. The two photoelectrode configuration provides higher photovoltages than a single semiconducting material while also absorbing complementary portions of the solar spectrum leading to potentially higher energy conversion efficiencies. Utilization of kinetically fast redox couples overcomes the overpotential barriers required for water splitting as well as the difficulties associated with gas collection and transport. Additionally, the photopotentials obtained show that not only does this configuration not require external bias but excess electrical power could also be produced in addition to the storage of chemical energy. These experiments demonstrate the feasibility for highly efficient solar energy conversion by directly photocharging a redox flow battery.

TiS₂ nanotubular structures were synthesized by a high temperature H₂S treatment of anodized TiO₂ nanotube layers, and their electrochemical activity for a use as counter electrodes in dye sensitized solar cells (DSSCs) was evaluated. During conversion to TiS₂ compositional, morphological and structural transformations were monitored. The fully converted TiS₂ nanostructures show a high electrocatalytic activity for the I⁻/I₃⁻ oxidation comparable to a nanoparticular platinum layer. For a simplified model a DSSC solar cell efficiency of 6.1% was obtained using the TiS₂ nanotube layer as counter electrode, which is very close to values obtained for a Pt reference (6.2%).

CuWO₄ is a medium bandgap (2.3 eV) n-type semiconductor capable of photoelectrochemical water oxidation under applied electrical bias. Here, we show for the first time that suspended microcrystals CuWO₄ evolve oxygen photocatalytically under visible illumination from solutions of 0.05 M AgNO₃ (10.8 μmol/hour; AQE of 0.56% at 400 nm) and 0.0002 M FeCl₃ (1.5 μmol/hour). No oxygen is detected with 0.002 M [Fe(CN)₆]³⁻ as sacrificial agent. The activity dependence on the redox potential of the acceptors is due to the presence of Cu²⁺ based electron trap states in CuWO₄. According to surface photovoltage spectroscopy and electrochemistry, these states are located on the particle surface, 1.8 eV above the valence band edge of the material. Controlling the chemistry of these states will be key to uses of CuWO₄ particles in tandem catalysts for overall water splitting.

Photoelectrochemical (PEC) water splitting has the potential to significantly reduce the costs associated with electrochemical hydrogen production through the direct utilization of solar energy. Many PEC cells utilize liquid electrolytes that are detrimental to the durability of the photovoltaic (PV) or photoactive materials at the heart of the device. The membrane-electrode-assembly (MEA) style, PEC cell presented herein is a deviation from that paradigm as a solid electrolyte is used, which allows the use of a water vapor feed. The result of this is a correspondent reduction in the amount of liquid and electrolyte contact with the PV, thereby opening the possibility of longer PEC device lifetimes. In this study, we demonstrate the operation of a liquid and vapor-fed PEC device utilizing a commercial III-V photovoltaic that achieves a solar-to-hydrogen (STH) efficiency of 7.5% (12% as a PV-electrolyzer). While device longevity using liquid water was limited to less than 24 hours, replacement of reactant with water vapor permitted 100 hours of continuous operation under steady-state conditions and diurnal cycling. Key findings include the observations that the exposure of bulk water or water vapor to the PV must be minimized, and that operating in mass-transport limited regime gave preferable performance.

This study presents two strategies to modify the thermally stable crystalline UiO-66(Zr) metal-organic framework (MOF) structure, in which the organic struts have been functionalized to modulate the electronic band structure and the catalytic activity toward selective oxidation of benzyl alcohol to benzaldehyde. The two strategies include the functionalization of the organic struts with branched ligands and manually creating structural defects with unsaturated organic linkers. The computational and experimental results show that functional groups such as -NH₂ and -NO₂ attached to the main organic strut modify the electronic environment of the photoactive aromatic carbon and thereby reduce the optical bandgap by 1 eV, improving the photocatalytic activity. Whereas the introduction of structural defects by the organic linker desaturation provides a shift in the highest occupied molecular orbital (HOMO), resulting from a decrease in the strut coordination with the inorganic knots, modulating the catalytic activity without light illumination.

Cobalt telluride (CoTe) thin films were electrodeposited for the first time using alkaline solutions containing nitrilotriacetic acid (NTA), CoCl₂, and K₂TeO₃. NTA was employed in order to shift the reduction potential of cobalt in the negative direction and to stabilize Co²⁺ ions in alkaline electrolytes via the formation of Co-NTA complexes. The electrodeposition mechanism was investigated by linear sweep voltammetry (LSV) combined with electrochemical quartz crystal microgravimetry (EQCM). The formation of CoTe is proposed to occur through the reaction of Co²⁺ with HTe⁻, which is generated by the reduction of TeO₃²⁻ via a 6-electron reduction pathway. A concurrent pathway consists of the stepwise reduction of TeO₃²⁻ to Te by a 4-electron process, followed by the 2-electron process reduction of Te to HTe⁻. The electrodeposited CoTe films were characterized by a variety of physical methods including scanning electron microscopy, energy dispersive X-ray, X-ray diffraction, and X-ray photoelectron spectroscopy. An optical energy bandgap of ∼1.86 eV was obtained by diffuse reflectance spectroscopy.

Fast and straightforward characterization of semiconductors is crucial for the efficient progress of new generation light-harvesting materials. Photoelectrochemical characterization, in particular, is a powerful method to understand the relative quality of semiconductors. Cu₂ZnSnS₄ (CZTS) is a promising eco-friendly and earth-abundant absorber for photovoltaic applications. The efficiency of CZTS, however, can be improved by rational manipulation of crystal, morphology, and doping. In this work, we study the incorporation of bismuth into CZTS films in order to improve its crystal quality. Electrodeposited Cu-Zn-Sn-Bi (CZTB) metallic precursors were grown from a single deposition bath and were sulfurized, resulting in CZTS layers containing < 1 at% bismuth. Investigation of crystallographic features of resulting CZTS-Bi layers provided insight into the doping role of Bi. Composition was determined by EDS and structural properties, the degree of ordering was analyzed with XRD and Raman Spectroscopy. Finally, we investigated the correlation between phase purity and photoelectrochemical response of materials. Structural analyses suggest that the Zn-rich sample presents better phase purity and improved crystallinity compared to Sn-rich samples. The grain size of relatively phase pure samples is larger in Zn-rich CZTS than the plain CZTS layers. The highest photoresponse of 2.65 mA/cm² is observed at a Zn-rich CZTS-Bi layer.

A generalized theoretical model of intensity modulated photocurrent spectroscopy (IMPS) for the random morphology in a dye sensitized solar cell (DSSC) under uniform illumination is developed. The generalized IMPS expression for the disordered semiconducting/conducting glass interface of a DSSC is obtained in term of power spectral density of roughness. Influence of surface roughness on the dynamic response of DSSC originate due to the coupling of characteristic phenomenological and morphological length scales. A detailed analysis of IMPS response is performed over finite self- affine fractals to highlight roughness induced anomalies and cause of photocurrent enhancement. The IMPS of a rough DSSC exhibit three characteristic frequency regimes: lifetime of charge carrier dependent low frequency regime, surface irregularity dependent intermediate power-law regime and diffusion controlled high frequency regime. Finally, our theory facilitates the understanding of dynamics and kinetics of charge carriers under the influence of ubiquitous surface disorder.

We report a simple, low temperature and solution-processable approach to prepare a composite film of copper sulfide/graphene (CuS-G) as a transparent conducting oxide (TCO) and platinum (Pt)-free CE for Dye-Sensitized Solar Cells (DSSCs). We find that CuS with 3.3 vol% of graphene (CuS-3G) yields the highest power conversion efficiency (PCE) of 4.83%, which is about 12% higher than DSSCs based on CEs made of pristine CuS. After optimizing the graphene concentration, the PCE of the DSSC assembled with the optimized CuS-3G is comparable to that based on Pt CE. The similar performance of the CuS-3G CE compared with Pt CE is mainly attributed to the small series resistance and high electrocatalytic activity of the CuS-3G CE; this is confirmed by cyclic voltammetry and electrochemical impedance spectroscopy. These results indicate a straightforward methodology for the low cost and easy synthesis of an alternative CE in DSSCs.

Indium oxide (In₂O₃) doped zinc oxide (ZnO) nanocomposites were successfully synthesized through a facile microwave hydrothermal method. The products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), N₂ adsorption–desorption isotherms (BET) and UV-Vis diffuse reflectance spectroscopy. The morphology of In₂O₃-ZnO composites was observed to be like flowers, and the diameter of particles constituting the porous petal was about 30 nm. The photoelectrocatalytic test results showed that the photoelectrocatalytic methylene blue (MB) degradation efficiency using In₂O₃-ZnO nanocomposites as photocatalysts under visible light irradiation and a certain voltage could reached above 95.3% after 60 min, much higher than that of In₂O₃ particles and ZnO particles. The enhanced photoelectrocatalytic activity was attributed to the doping of In₂O₃ and applied voltage, which beneficially reduced the recombination of electrons and holes in the photoelectrocatalytic process, therefore, it promoted the production of active species (•OH and •O⁻₂).

The Sn₂TiO₄ phase is a small-bandgap (E_g ∼ 1.6 eV) semiconductor with suitable band energies to drive photocatalytic water-splitting. A new fast flux reaction can be used to prepare high purity Sn₂TiO₄ in reaction times of down to 5 minutes. Shorter reaction times (5 and 15 min) lead to nanosized particles while longer reaction times (24 hours) yield micron-sized particles. The nanoparticles show an increased bandgap size owing to quantum size effects in the weak confinement regime (r >> a_B), increasing by ∼0.3 eV from 1.60 eV to 1.89 eV (indirect). From Mott-Schottky analyses, the conduction band edge is found to shift to slightly more negative potentials while the valence band edge exhibits a relatively larger positive shift. Calculations show this arises from the more disperse Sn s-orbital bands at the top of the valence band, compared the large Ti-based d-orbital band at the bottom of the conduction band. The photocatalytic activities of the Sn₂TiO₄ nanoparticles for molecular hydrogen and oxygen production showed higher rates than the equivalent micron-sized particles as a result of both higher surface areas and higher overpotentials to drive each of the half reactions.

The vast majority of semiconductors photocatalysts reported for artificial nitrogen fixation have a large bandgap at around 3.0 eV, thus photocatalytic nitrogen reduction is driven mainly by ultraviolet light. In contrast, this report demonstrates for the first time that bismuth iron molybdate (Bi₃FeMo₂O₁₂) with a bandgap of 2.25 eV exhibits visible-light photocatalytic activity toward nitrogen-to-ammonia conversion. Furthermore, introduction of oxygen vacancy to this photocatalyst increases the ammonia production rate remarkably. Density functional theory (DFT) calculation reveals that the oxygen vacancies help adsorb and stabilize the N-H intermediate species, and lower the energy barrier of intermediate reactions. This work has an implication in design of semiconductor photocatalysts for sustainable ammonia synthesis under the ambient condition using solar energy.

Anodization of n-InP electrodes was carried out over a range of temperatures and KOH concentrations. Scanning electron microscopy showed <111>A aligned pore growth with pore width decreasing as the temperature was increased. This variation is explained in terms of the relative rates of electrochemical reaction and hole diffusion and supports the three-step model proposed earlier. As temperature is increased, both the areal density and width of surface pits decrease resulting in a large increase in the current density through the pits. This explains an observed decrease in porous layer thickness: pits sustain mass transport at the necessary rate for a shorter time before precipitation of etch products blocks the pores. As the concentration of KOH is increased, both pore width and layer thickness decrease to minima at ∼9 mol dm⁻³ after which they again increase. This variation of pore width is also explained by the three-step model and the variation in layer thickness is explained by mass transport effects. Layer porosity follows a similar trend to pore width, further supporting the three-step model. A transition from porous layer formation to planar etching is observed below 2 mol dm⁻³ KOH, and this is also explained by the three-step model.

Cu₂SnSe₃ (CTSe) thin films were fabricated by selenization of a Cu-Sn alloy, which was electrochemically co-deposited from a stirred citrate solution. Photoelectrical and optical properties of the obtained CTSe thin films were investigated by using only the electrochemical (we applied the cyclic voltammetry for determination the band gap of Cu₂SnSe₃ thin film semiconductor), photoelectrochemical and electrochemical impedance spectroscopy methods. New method is suggested, which allows to estimate the relationship between the etching time of secondary copper selenide phase and the photoelectrical properties (open circuit photopotential and photocurrent) of the CTSe thin films. The Cu₂SnSe₃ thin film with the best photo-response was obtained at a selenization temperature of 500°C. A significant improvement in photoelectrical behavior was observed on the CTSe thin film after 30 s of chemical etching in a 0.75 M KCN aqueous solution with the aim to remove the residual copper selenide phase. This film has a good photoelectrical and optical properties for solar cell formation: a p-type conductivity, a band gap of 0.92 eV, a carrier concentration N_α = 1.45 × 10¹⁷cm⁻³ and flat band potential E_FB of 0.297 V vs. saturated hydrogen electrode and a photocurrent density of 0.44 mA × cm⁻² at cathodic potential of −600 mV vs. saturated hydrogen electrode. A diagram of energy bands of the CTSe/electrolyte junction has been presented.

Nanoscale architecturing of photoanodes with the improved photoelectrochemical activities for water splitting has been demonstrated by introduction of gold (Au) and reduced graphene oxide (rGO) to cadmium-sulfide (CdS). We optimized CdS thin film by an inert atmosphere heat-treatment, on which Au-rGO-coated CdS (Au-rGO@CdS) could be obtained by sequential deposition of rGO and Au. The morphology, structure and optical and vibrational properties of the CdS and Au-rGO@CdS samples were found to be in good correlation with the photoelectrochemical analysis. The Au-rGO@CdS thin film showed a current density of 5 mA/cm² at 0 V (vs SCE), which is approximately 3.5 times higher than that of the as-prepared CdS thin film. The enhancement mechanism has been discussed elaborately through the extensive photoelectrochemical characterization.

Zn_0.80Mg_0.17Al_0.03O (AZMO) thin film of 180–200 nm in thickness was produced by radio frequency (r. f.) argon-ion sputtering from commercial ZnO:Al and Mg-metal targets with 150 and 200 W of applied r. f. power. AZMO film is of bandgap 3.62 eV, compared with 3.25 eV for Zn_0.96Al_0.04O (AZO) film. The electrical conductivity (σ) of AZO film is 80 Ω⁻¹ cm⁻¹, which reduces to 0.2 Ω⁻¹ cm⁻¹ when the film was heated in air at 300°C. For AZMO film these values are, 10 and 10⁻⁴ Ω⁻¹ cm⁻¹, respectively. Thus, the AZMO film is converted to a high resistive transparent n-type film for solar cell. Such films on F-doped SnO₂ (FTO) film was used in antimony sulfide solar cell: FTO/AZMO/Sb₂S₃/C-Ag. This cell showed notable increase in the short circuit current density (10.86 mA/cm²) with respect the solar cell, FTO/CdS/Sb₂S₃/C (6.28 mA/cm²). The increase is due to improved external quantum efficiency toward the ultraviolet region. The open circuit voltage of these cells are 0.478 V and 0.627 V, respectively, with conversion efficiencies of 1.3% and 1.13%.

The theory of intensity-modulated photocurrent spectroscopy (IMPS) is extended to consider the two-electron two-proton hydrogen evolution reaction at p-type semiconductor photocathodes. A generalized reaction scheme is described, and two limiting cases are analyzed. The first involves photoelectrochemical desorption of hydrogen by electron/proton transfer and the second chemical desorption by reaction of adsorbed hydrogen atoms. Expressions are derived for the IMPS responses in the two cases as well as for the modulated surface electron concentration, which is related to the quasi Fermi level at the interface. The surface electron concentration expressions can be used to interpret electrical, optical, or microwave experiments that detect the excess electron concentration. The IMPS responses calculated for the two limiting cases are fitted to a simplified IMPS model that gives rate constants k_t nd k_r for charge transfer and recombination respectively. k_t and k_r are then related to the rate constants describing the two limiting schemes for the light-driven hydrogen evolution reaction.

Charge transfer at the semiconductor/electrolyte interface is an important process that dictates the efficiency of quantum dots solar cells. Hole transfer kinetics remains a sluggish reaction for metal chalcogenide electrodes. In this work we examine the beneficial effect of Cu_xS nanoparticles on the photoelectrochemical performance of Cu-In-Zn-S (CIZS) quantum dots sensitized solar cells as they promote hole transfer to the S²⁻/S_n²⁻ redox couple. Cu_xS nanoparticles if deposited without a protecting layer undergo compositional transformation in sulfur rich electrolytes. Addition of a protecting ZnS layer improves the stability and efficiency of the resulting solar cells. Establishing the hole transport property of Cu_xS in solar cell offers new ways to improve photovoltaic performance of QDSSC.

Gold nanoparticles (∼24 nm) were successfully loaded on the surface of BiVO₄ nanopyramidal arrays by simple electrostatic self-assembly approach. By decorating gold nanoparticles on the surface of BiVO₄ films, The photocurrent of as-prepared Au/BiVO₄ photoanode increased from around 0.42 mA/cm² for pristine BiVO₄ to nearly 0.93 mA/cm² at 1.23 vs. RHE under AM 1.5 illumination. The improved photoresponse of the Au/BiVO₄ was found as a result from higher carrier generation and enhanced charge separation with the decoration of Au nanoparticles. The successful deposition provide a new route for desigining plasmonic photoanode and offers better understanding the photochemistry of noble metal-complex oxide heterostructures for photoelectrichemical water splitting.

Ni₃S₂/CdS nanocomposites based on zero-dimensional Ni₃S₂ nanoparticles (NPs) and one-dimensional CdS nanowires (NWs) were synthesized via a two-step solvothermal approach. These nanocomposites were characterized using a combination of experimental techniques, including scanning electron microscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, UV-Vis diffuse reflectance spectroscopy, and photoelectrochemistry method. The Ni₃S₂/CdS nanocomposites exhibit much higher photocatalytic hydrogen evolution activity than individual CdS NWs or Ni₃S₂ NPs under visible light irradiation (λ ≥ 400 nm) from water. Photoelectrochemical (PEC) and photocatalytic properties of the Ni₃S₂/CdS nanocomposites were investigated as a function of the molar ratio between Ni₃S₂ and CdS. In addition, ultrafast transient absorption (TA) spectroscopy was employed to probe the charge carrier dynamics, which is about four times shorter in 10%-Ni₃S₂/CdS than pure CdS NWs. These results suggest efficient charge transfer between CdS and Ni₃S₂ at the interface, which is desired for PEC and photocatalytic applications.

A representative metal–organic framework, NU-1000, was functionalized with MoS_x. The previously determined crystal structure of the material, named MoS_x-SIM, consists of monometallic Mo(IV) ions with two sulfhydryl ligands. The metal ions are anchored to the framework by displacing protons presented by the –OH/–OH₂ groups on the Zr₆ node. As shown previously, the MOF-supported complexes are electrocatalytic for hydrogen evolution from acidified water. The earlier electrocatalysis results, together with the nearly ideal formal potential of the Mo(IV/II) couple (i.e., nearly coincident with that of the hydrogen couple), and the physical proximity of UV-absorbing MOF linkers to the complexes, suggested to us that the linkers might behave photosensitizers for catalyst reduction, and subsequently, for H₂ evolution from water. To our surprise, MoS_x-SIM, when UV-illuminated in an aqueous buffer at near-neutral pH, displays a biphasic photocatalytic response: an initially slow rate of reaction, i.e. 0.56 mmol g⁻¹ h⁻¹, followed by an increase to 4 mmol g⁻¹ h⁻¹. Ex-situ catalyst examination revealed that nanoparticulate MoS_x suspended within the reaction mixture is the actual catalyst. Thus, photo-assisted restructuring and detachment of the catalyst or pre-catalyst from the MOF node appears to be necessary for the catalyst to reduce water at neutral pH.

In this paper, we report on results of a systematic study of porous morphologies obtained using anodization of HVPE-grown crystalline GaN wafers in HNO₃, HCl, and NaCl solutions. The anodization-induced nanostructuring is found to proceed in different ways on N- and Ga-faces of polar GaN substrates. Complex pyramidal structures are disclosed and shown to be composed of regions with the degree of porosity modulated along the pyramid surface. Depending on the electrolyte and applied anodization voltage, formation of arrays of pores or nanowires has been evidenced near the N-face of the wafer. By adjusting the anodization voltage, we demonstrate that both current-line oriented pores and crystallographic pores are generated. In contrast to this, porosification of the Ga-face proceeds from some imperfections on the surface and develops in depth up to 50 μm, producing porous matrices with pores oriented perpendicularly to the wafer surface, the thickness of the pore walls being controlled by the applied voltage. The observed peculiarities are explained by different values of the electrical conductivity of the material near the two wafer surfaces.

Owing to the exceptional properties of graphene and the crucial role of substrate on the performance of electrochemical biosensors, several graphene-based hybrid structures have recently emerged, yielding improved selectivity and sensitivity. To date, most of the reported biosensors utilize solution-driven graphene flakes with drawbacks of low conductivity (due to high inter-junction contact resistant) and structural fragility. Herein, we present a conductive three-dimensional CeO₂ semiconductor nanoparticles/graphene nanocomposite, as a platform for sensitive detection of hydrogen peroxide, an important molecule in fundamental biological processes. The 3D conductive graphene architecture is fabricated by chemical vapor deposition on nickel foam. The fabricated biosensor displays high sensitivity (60 μA.mM⁻¹) at a low negative potential of −0.25 V, a low detection limit (<1.0 μM at S/N = 3), and a fast response (<5 s) in the range of 2.8 to 160 μM. Furthermore, density functional theory simulations show that the improved detection is not only related to the catalytic effect of ceria nanoparticles, but also to more efficient charge transfer from nanoparticles to the 3D graphene network. Moreover, it is established that the amperometric response of the biosensor is insensitive to interfering molecules such as glucose, sucrose, and potassium chloride, indicating its potential for practical applications.

What is the best approach for estimating standard electrochemical potentials, E⁽⁰⁾, from voltammograms that exhibit chemical irreversibility? The lifetimes of the oxidized or reduced forms of the majority of known redox species are considerably shorter than the voltammetry acquisition times, resulting in irreversibility and making the answer to this question of outmost importance. Half-wave potentials, E^(1/2), provide the best experimentally obtainable representation of E⁽⁰⁾. Due to irreversible oxidation or reduction, however, the lack of cathodic or anodic peaks in cyclic voltammograms renders E^(1/2) unattainable. Therefore, we evaluate how closely alternative potentials, readily obtainable from irreversible voltammograms, estimate E⁽⁰⁾. Our analysis reveals that, when E^(1/2) is not available, inflection-point potentials provide the best characterization of redox couples. While peak potentials are the most extensively used descriptor for irreversible systems, they deviate significantly from E⁽⁰⁾, especially at high scan rates. Even for partially irreversible systems, when the cathodic peak is not as pronounced as the anodic one, the half-wave potentials still provide the best estimates for E⁽⁰⁾. The importance of these findings extends beyond the realm of electrochemistry and impacts fields, such as materials engineering, photonics, cell biology, solar energy engineering and neuroscience, where cyclic voltammetry is a key tool.

To provide a viable alternative for counter electrodes used in dye sensitized solar cells, polypyrrole (PPy) based films have been synthesized via electrochemical deposition in the presence of the ionic liquid 1-butyl-3-methylimidazolium bis-(trifluoromethanesulfonyl) imidate (NTf₂) and incorporated with gold nanoparticles (Au_nanop). The films were analyzed by SEM, UV-Vis-NIR, Raman, Electrochemical impedance spectroscopy, Cyclic voltammetry and Conductivity measurements. The presence of the ionic liquid is found to result in a more conductive film, to improve catalytic reduction of I₃⁻ and the electrochemical reversibility of the electrode. In addition to increase conductivity, impedance spectroscopy has shown that incorporating Au_nanop in the PPy/NTf₂ film helps improving the interfacial charge transportation, the electrocatalytic properties and solar energy conversion efficiency. DSSCs assembled with PPy based CE presented nearly the same J-V characteristic parameters as observed from conventional Pt based device.

We have studied the effects of structural symmetry related distortions on the band gaps and overall energetics of SbMO₄ compounds (M = Ta, Nb). Evolution of the electronic structures was studied as one transitions gradually from a system of 4d bands to 5d bands, and whether this has any effect on the Sb oxidation state or its lone pair s electrons. From the atomic point of view, electronic correlation energy is higher in Ta 5d than in Nb 4d, and Nb 4d bands are higher in energy than Ta 5d. These were examined in terms of oxidation states in their respective solids and mixed alloys. We look at the effects of increasing Nb concentration on the electronic structures of SbTa_1-xNb_xO₄ alloys with emphasis on polyhedral interactions and overall cell distortions. We decouple the effects caused by the presence of different nd cations from those caused by distortions and reorientation of various polyhedra.

The ongoing search for new photoelectrode materials generated interest toward semiconductors containing multiple anions. In this work, three different antimony oxide iodides (Sb₃O₄I, Sb₈O₁₁I₂, and Sb₅O₇I) were synthesized by anhydrous synthesis. Scanning electron microscopy revealed mainly needle-shaped particles for Sb₃O₄I, elongated plate-shaped ones for Sb₈O₁₁I₂ and well-crystallized hexagonal particles for Sb₅O₇I. The isoelectric point of the antimony oxide iodides (pH∼3) was independent of the chemical composition. With increasing pH particles became negatively charged to different extents, depending on the relative amount of oxygen in the samples, through the presence of ≡Sb−O⁻ surface functional groups. The optical properties were heavily affected by the composition as well: bandgap energies related to the direct transitions in Sb₃O₄I, Sb₈O₁₁I₂, and Sb₅O₇I were 2.16 and 2.74 eV, 2.85 eV, and 3.25 eV, respectively. Photoelectrochemical analysis proved that all samples behave as n-type semiconductors, but the performance in water oxidation showed large variation for the different compositions. The band energy diagram was constructed for all compounds and the composition dependent optoelectronic properties were rationalized on this basis.

The (100) p-Si | n⁺-Si | Cu interface is investigated with respect to the electronic band structure and the electrochemical performance. Thin layers of metallic copper were deposited stepwise by E-beam deposition and analyzed in-line by XPS after each deposition step. For this purpose, different silicon surface terminations were prepared: hydrogen termination, thermal oxide (7 Å) and native oxide (3 Å). After contact formation the initial flatband situation of the n⁺-Si layer changes to an upward band bending depending on the Si surface termination: 0.45 eV for the H termination, 0.35 eV for the native oxide and 0.27 eV for the thermal oxide. The electrochemical performance measured by cyclic voltammetry for each junction correlates to the respective energy band alignment. While the H terminated surface with the highest upward band bending leads to the worst electrochemical performance, the surface passivated with thermal oxide and the lowest upward band bending results in the best electrochemical performance. For the two passivated surfaces, the thickness of the passivating oxide layer may also be an issue. The Si surface with a thicker thermal oxide (7 Å) shows better band alignment and electrochemical performance compared to the Si surface with a thinner native oxide (3 Å).

A new kind of Al photoanode composite based on photocatalytic material and sacrificial anode was proposed and designed to provide new ideas for cathodic protection in marine environment. Fabricated with the traditional Al-Zn-In-Mg-Ti sacrificial anode and Co(OH)₂ modified anatase TiO₂ nanotubes, a new Al photoanode composite with good photoelectrochemical cathodic protection property was obtained. To compare with the traditional sacrificial anode, the cathodic protective properties of both the sacrificial anode and this Al photoanode composite were evaluated by electrochemical measurements in aqueous 3.5 wt.% NaCl solution. By means of scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and UV-Vis spectrophotometer morphologies, crystal structures, surface compositions and light response range of the Al photoanode composite during the immersion were characterized, respectively. The generated protective current density of the Al photoanode composite is found to be greater under illumination than that of the traditional Al-Zn-In-Mg-Ti sacrificial anode. Co(OH)₂@TiO₂ plays an important role in supplemental protection for Q235 carbon steel and improves the current efficiency.

ZnO nanoparticles and nanowires decorated with platinum nanoparticles in different loading concentrations were prepared to detect low concentrations of NO₂ under 365-nm ultraviolet-light emitting diode (UV-LED) irradiation at room temperature. Solution precipitation, self-assembly crystallization, and photo-deposition techniques were used to synthesize the sensing material. The synthesized sensors at different stages of development were characterized by XRD, HRTEM and FE-SEM analyses. Although the sensing response of the pristine ZnO nanoparticles was 0.41 for NO₂ detection, the response improved significantly to 1.5 using ZnO nanowires in the identical photo-activation settings of 365 nm UV wavelength and 25 mW/cm² irradiance. The decoration of the surface of the ZnO nanowires with Pt nanoparticles further enhanced the sensing performance, whereas the 0.1wt% Pt-decorated ZnO nanowire sensor exhibited a response of 4.33 with a 140-s response time, which is more than one order of magnitude higher and 50s faster than the response generated by the ZnO nanoparticles. This improved performance is attributed to the role of Pt active sites in promoting NO₂ adsorption on the ZnO nanowires' surface as well as enhancing the layer electron utilization.

The oil-produced water generated during drilling of oil wells and gas extraction has been a cause for great concern since it contains a complex mixture of different organic and inorganic compounds, large amount of CO₂, grease, salts, minerals, oils, and many hazardous compounds. The present work investigates the efficiency of photocatalysis (PC), photoelectrocatalysis (PEC), ozonation (O₃), and photoelectrocatalysis coupled with ozonation (PEC+O₃) in the removal of organic and inorganic contaminants in the oil-produced water monitored by Gas Chromatography-Mass Spectrometry (GC-MS) and Ionic Chromatography (IC) techniques. Parameters such as toxicity, which was investigated using Zebrafish embryos, color, turbidity, pH, quantity of dissolved solids, conductivity, chemical oxygen demand (COD), and concentrations of organic and inorganic carbon were also investigated. The best results were obtained by coupling PEC and O₃ techniques, which presented superior reduction in color (98%), turbidity (100%), inorganic carbon (99%), COD (73%), and a decrease of 96% in the fluoride and 35% in the chloride detected previously in the real oil-produced water. Among the 12 organic compounds identified in the oil-produced water, the PEC+O₃ treatment reached complete oxidation in eight of them and a lower Zebrafish embryo mortality occurred with 12.5% of dilution after 2 h treatment.

Titanium oxide-based photocatalytic filters were produced by Fused Deposition Modelling (FDM) using biopolymers obtained from renewable biomass resources. The thermoplastic route allows shaping composites through the immobilization of photoactive TiO₂ nanoparticles in an environmentally friendly bioplastic such as the polylactic acid (PLA). Composites with an inorganic charge of 30 wt% of TiO₂ nanoparticles (NPs) exhibit a 100% methyl orange (MO) degradation after 24 h of light exposition due to the extremely uniform dispersion of the nanophase within the polymer matrix in the FDM feedstock. Surface modification of TiO₂ NPs allows the optimization of the colloidal dispersion and stabilization of the inorganic charge in a PLA solution and hence, the optimal distribution of nano-photoactive points in the TiO₂/PLA filaments and scaffolds. The proposed new route of processing improves the dispersion of nano-charges comparing with the traditional thermo-pressing routes used for mixing thermoplastics based composites, avoiding the thermal degradation of the polymer and providing a customised product. In this manuscript the evolution of photodegradation with the increase of TiO₂ content in the composite and the variation of the filter geometry was evaluated.

Deposition of thin film compound semiconductors with good control over stoichiometry, crystallinity and thickness is essential for a range of diverse applications such as solar cells, photocatalysis, thermoelectrics, photodetectors, etc. In this work, nanometer-thick cadmium telluride (CdTe) films with exceptional control over its stoichiometry were electrodeposited onto Au substrates using a novel potential pulse atomic layer deposition (PP-ALD) process. The films were electrodeposited from an acidic aqueous solution of CdSO₄ and TeO₂ at room temperature using a flow cell electrodeposition set-up. Deposition potential and effect of solution flow were first investigated to optimize the codeposition process. Potential pulses were then subsequently introduced to achieve atomic layer deposition of CdTe. X-ray diffraction (XRD), Scanning electron microscope (SEM), Electron probe microanalysis (EPMA), Energy Dispersive X-Ray Analyzer (EDX) were used to characterize resulting CdTe films. XRD studies of the films showed the evolution of a much sharper peak corresponding to CdTe cubic (111) than is customarily observed for CdTe thin films grown at room temperature. The optimized PP-ALD method was also used to deposit CdTe on Au nanowire-array electrodes. Initial results show high-quality CdTe deposits conformally coated on Au nanowire arrays, which could open pathways for ultrathin light absorber solar energy conversion devices.

Transient IR measurements of photo-excited bismuth oxychloride (BiOCl) were performed by a step-scan technique at a resolution of 2.5 nanoseconds. Six types of faceted BiOCl particles, which differed also in their average size, were used. Following excitation, an increase in the intensity of the Bi-O signal, which lasted for 70–130 nanoseconds was observed. A negative correlation between the characteristics of the transient signal (duration of signal and intensity) versus the photocatalytic activity toward the reduction of Cr(VI) was found. In parallel, a positive correlation was found between the duration of the transient signal and the specific surface area of the various types of particles. The increased transient intensity is explained in terms of formation of deep traps, most likely at the surface of the particles. The results demonstrate that transient vibrational spectroscopy may serve as an important tool for studying photocatalytic materials.

Nickel oxide (NiO) is often used as a hole-transporter material in both photovoltaic and photoelectrochemical solar cells. As a result of the reversible nickel(II)/(III) transformation, it is also electrochromic. These potential-dependent optoelectronic properties of this intriguing material, however, are yet to be fully understood. In this article, we show that the picture is more complicated than the generally discussed nickel(II)/(III) transformation, because of the presence of trap states. We reveal that the density of states is directly influenced by the applied potential in nanoporous NiO films; and show how it manifests in the electrical properties and Raman spectral features. We demonstrated that the population/depopulation of shallow trap states has an important role in dictating these changes. The presented insights can also contribute to the better understanding of the optoelectronic properties of different semiconductor electrodes under charging conditions.

Unique hybrid systems for electroreduction of CO₂ under both conventional and visible-light-induced conditions are proposed and designed here by over-coating copper(I) oxide with tungsten(VI) oxide nanowires. When Cu₂O and WO₃ nanostructures are sequentially deposited on glassy carbon substrate, the resulting system has exhibited high electrocatalytic activity toward reduction of CO₂ in phosphate buffer of pH = 6.1. By introducing WO₃, the Cu-based system becomes more selective against the competitive hydrogen evolution and exhibits higher CO₂-reduction currents relative to the performance of single components. During electroreduction, highly catalytic Cu sites are generated or intercalated within WO₃ nanowires partially reduced to mixed-valence hydrogen-absorbing tungsten(VI,V) oxide bronzes, H_xWO₃, co-existing with sub-stoichiometric tungsten(VI,IV) oxides, WO_3-y. Strong adsorption and activation of CO₂ molecule has been demonstrated at the WO₃-decorated Cu₂O interface. Under photoelectrochemical conditions involving illumination with sun-light, the proposed hierarchical system of Cu₂O semiconductor deposited onto the transparent fluorine-doped conducting glass electrode and decorated with WO₃ nanowires is well-behaved and active toward CO₂-reduction in the Na₂SO₄ neutral medium. In addition to the Cu₂O stabilization effect, the heterojunction formed by p-type Cu₂O and n-type WO₃ semiconductors seems to facilitate charge distribution and separation at the photoelectrochemical interface.

Reduced graphene oxide (rGO) films for application as electrocatalyst have been deposited using an inkjet materials printer, with excellent control over the film morphology and properties. The dispersions were prepared by the ultrasound-assisted liquid exfoliation method using polyvinylpyrrolidone (PVP) as stabilizer. The environmentally friendly ink formulation exhibits a stability for at least 6 months. The inkjet-printed rGO/PVP composite films were thermally treated at different temperatures, and it was found that the fraction of remaining PVP considerably affects the final structural, textural, optical and electrocatalytic characteristics. After annealing at 400°C, the rGO film exhibits a morphology of interconnected flakes and a very good transparency with a transmittance >90% in the entire visible spectrum. The inkjet-printed rGO films have been applied as electrocatalytic coating on fluorine-doped tin oxide (FTO) and tested for activity for the reduction of Co(bpy)₃³⁺ in acetonitrile. Dye-sensitized solar cells (DSSCs) fabricated with an rGO film on FTO annealed at 400°C as counter-electrode achieved 87% of the efficiency with respect to DSSCs with a Pt-catalyzed counter electrode. Hence, the fabrication of active and inexpensive rGO films using a high-throughput technique such as inkjet printing is an interesting approach to accelerate the development of competitive electrocatalytic nanomaterials.

Understanding reaction processes at the semiconductor/electrolyte interface is critical to developing (photo)electrochemical energy conversion systems. For the water oxidation reaction at a photoanode in a solar water splitting cell, there are multiple reaction steps that produce surface adsorbed reaction intermediates, such as OH^• and H₂O₂. Little is known about how the reaction intermediate surface concentrations evolve during the water oxidation reaction. Here we use single-molecule fluorescence microscopy to track the time-dependent surface concentration of Ti-OH^• species on TiO₂ photoanodes under chopped light illumination. We correlate the reaction intermediate surface concentration with the net rate of photoelectrochemical water oxidation (i.e., the photocurrent). The reaction intermediate dynamics follow different time scales in its temporal evolution from the photocurrent dynamics. By fitting the temporal evolutions of Ti-OH^• species and photocurrent over a range of light on/off conditions and applied potentials, we observed that the rate constants for interfacial hole transfer and O-O bond formation depend on the applied potential under illumination. The variation in the rate constants could be attributed to the presence of surface states and/or a change in the chemical nature of Ti-OH^• species on the photoanode surface. Our findings provide insight into water oxidation kinetics under intermittent solar irradiation conditions.

The surfactant-assisted synthesis of tantalum oxide nanoparticles (Ta₂O₅), its subsequent nitridation to form tantalum nitride (Ta₃N₅), and the evaluation of the photoactivity of these nanoparticles, are presented. The surface, optical, and compositional characterization of the in-house synthesized photocatalysts indicate spherical nanoparticles with ϕ = 30 nm Ta₂O₅ and ϕ = 30 nm Ta₃N₅ with the latter photocatalysts showing absorbance onset at ∼ 630 nm. Photoelectrochemical analysis of the various photocatalyst films using chronoamperometry, linear sweep voltammetry, and impedance measurements attribute an improved photoactivity with the in-house catalysts to better charge generation, transport, and utilization. Initial results using Ta₂O₅ and Ta₃N₅ demonstrate photocatalytic activity toward conversion of colored pollutants.