Chemical Treatment of Sn Containing Transparent Conducting Oxides for the Enhanced Adhesion and Thermal Stability of Electroplated Metals

A surface treatment process, named ReTreat, is presented, and is shown to enhance the adhesion of electroplated metals on Sn containing transparent conducting oxides TCOs . The ReTreat process uses Zn powders, FeSO4 and glycine buffered aqueous solutions pH 3 to 5 in order to regulate a controlled and uniform conversion of SnO2 surfaces to SnO, Sn metal, and FexSny alloys. These surface metallic and intermetallic layers selectively enrich the electroplating of metallic films including Ni, Au, and Ag . Subsequently, the process has been used to fabricate thermally stable metal films on rigid FTO coated glass and flexible ITO coated PET substrates. Standardized testing confirms that the metallic coatings exhibit sufficient adhesion to the underlying TCO with high thermal stability and tolerance to flexural strain. A reaction mechanism for the heterogenous surface treatment is deduced from X ray diffraction, X ray photoelectron spectroscopy, and in situ transmittance measurements. These investigations show how the process parameters e.g., Zn powders, FeSO4 concentration, pH, and TCO type impact the reaction rate, morphology, and composition of the treated TCO surface. This report provides detailed insights necessary for the future implementation of this innovative surface treatment, which has the prospect to be a customary process for electroplating onto Sn containing TCO

CO2 electroreduction activity and dynamic structural evolution of in situ reduced nickel indium mixed oxides

In the field of CO2 electroreduction CO2ER , tuning the selectivity among diverse products remains a major challenge. Mixed metal catalysts offer possible synergetic effects which can be exploited for tuning product selectivity. We present a simple wet chemical approach to synthesize a range of nickel indium mixed oxide NiAInBOx thin films with homogeneous metal distribution. CO2 electroreduction results indicate that the NiAInBOx mixed oxide thin films can achieve high CO selectivity gt;70 in contrast with the single metal oxides NiO H2 gt;90 and In2O3 formate gt;80 . The relative composition Ni40In60Ox attained the best CO selectivity of 71 at moderate cathodic bias of amp; 8722;0.8 VRHE, while a higher cathodic bias E lt; amp; 8722;0.9 V promoted a decrease of CO in favor of formate. A detailed investigation of the Ni40In60Ox thin films following progressive stages of reduction during CO2ER revealed dynamic structural transformations strongly dependent on applied bias and electrolysis time. For the CO selective catalyst composition, at moderate cathodic bias E lt; amp; 8722;0.8 V and short electrolysis times 1 h , the catalyst is composed of nickel indium alloy grains embedded in amorphous Ni In mixed oxide as observed by electron microscopy. Extending electrolysis time at amp; 8722;0.8 V for 10 h, or increasing the applied reductive bias to amp; 8722;1.0 V, result in a complete reduction of the residual oxide film into an interconnected array of multicomponent In, Ni, Ni3In7 nanoparticles which display significantly lower CO selectivity lt;50 . Our results indicate that the persistent amorphous NiInOx oxide alloy composite material preserved in the early stages of reduction at amp; 8722;0.8 V plays a key role in CO selectivity. The highly dynamic structure observed in this catalytic system demonstrates the importance of conducting detailed structural characterization at various applied potentials to understand the impact of structural changes on the observed CO2ER selectivity trends; and thus be able to distinguish structural effects from mechanistic effects triggered by increasing the reductive bia

Extending the Absorption Limit of BiVO4 Photoanodes with Hydrogen Sulfide Treatment

Bismuth vanadate is a promising photoanode material for photoelectrochemical water splitting due to its relative stability, low cost, and nontoxic properties. However, its performance is limited by the large bandgap Eg of 2.4 amp; 8201;eV, and the record photocurrent is already within 90 of its theoretical limit. Further photocurrent enhancement could only be obtained by increasing its optical absorption, for example, by reducing Eg. Herein, sulfur incorporated bismuth vanadate S BiVO4 thin films are synthesized via spray pyrolysis combined with post treatment in hydrogen sulfide environment. Under optimal H2S treatment conditions, sulfur can be incorporated successfully into the BiVO4 lattice, without the formation of any secondary phases. The use of reactive H2S, instead of solid sulfur powders, allows us to decrease the required annealing temperature and increase the kinetics for sulfur incorporation into BiVO4. The Eg of the resulting S BiVO4 films is decreased by gt;200 amp; 8201;meV vs. pristine BiVO4 , which theoretically corresponds to a 20 increase in the theoretical photocurrent limit. Finally, the stability limitation of S BiVO4 is overcome by introducing pulsed laser deposited NiOx protection layers. The modified S BiVO4 NiOx film exhibits higher photocurrent density with no reduction of photocurrent during the 9 amp; 8201;h stability test with AM1.5 illuminatio

Role of Gd in Enhancing the Charge Carrier Mobility of Spray Deposited BiVO4 Photoanodes

The emergence of bismuth vanadate BiVO4 as one of the most promising photoanodes for solar water splitting is largely driven by the successful efforts of dopant introduction and optimization to improve its photoelectrochemical PEC performance. To this end, although less commonly used, several trivalent ions e.g., Sm3 , In3 , Gd3 that substitute Bi3 have also been demonstrated to be effective dopants, which can increase the photocurrent density of BiVO4 photoanodes. However, the main factor behind such improvement is still unclear, as various explanations have been proposed in the literature. Herein, Gd3 is introduced to substitute Bi3 in spray deposited BiVO4 films, which enables up to a 2 fold increase in the photocurrent density. Further PEC analysis suggests that Gd doping enhances the charge carrier separation in the BiVO4 films and does not affect the catalytic and optical properties. Indeed, time resolved microwave conductivity TRMC measurements reveal that the charge carrier mobility of BiVO4 is increased by 50 with the introduction of Gd while the charge carrier lifetime is unaffected. This increase of mobility is rationalized to be a result of a higher degree of monoclinic lattice distortion in Gd doped BiVO4, as evident from the X ray diffraction and Raman spectroscopy data. Overall, these findings provide important insights into the nature and the underlying role of Gd in improving the photoelectrochemical performance of BiVO4 photoanode

Interfacial Oxide Formation Limits the Photovoltage of Alpha SnWO4 NiOx Photoanodes Prepared by Pulsed Laser Deposition

alpha SnWO4 is a promising metal oxide photoanode material for direct photoelectrochemical water splitting. With a band gap of 1.9 eV, it ideally matches the requirements as a top absorber in a tandem device theoretically capable of achieving solar to hydrogen STH efficiencies above 20 . It suffers from photoelectrochemical instability, but NiOx protection layers have been shown to help overcome this limitation. At the same time, however, such protection layers seem to reduce the photovoltage that can be generated at the solid electrolyte junction. In this study, an extensive analysis of the alpha SnWO4 NiOx interface is performed by synchrotron based hard X ray photoelectron spectroscopy HAXPES . NiOx deposition introduces a favorable upwards band bending, but also oxidizes Sn2 to Sn4 at the interface. By combining the HAXPES data with open circuit potential OCP analysis, density functional theory DFT calculations, and Monte Carlo based photoemission spectra simulation using SESSA, the presence of a thin oxide layer at the alpha SnWO4 NiOx interface is suggested and shown to be responsible for the limited photovoltage. Based on this new found understanding, suitable mitigation strategies can be proposed. Overall, this study demonstrates the complex nature of solid state interfaces in multi layer photoelectrodes, which needs to be unraveled to design efficient heterostructured photoelectrodes for solar water splittin

Different Photostability of BiVO4 in Near pH Neutral Electrolytes

Photoelectrochemical water splitting is a promising route to produce hydrogen from solar energy. However, corrosion of photo electrodes remains a fundamental challenge for their implementation. Here, we reveal different dissolution behaviors ofBiVO4photoanode in pH-buffered borate, phosphate, and citrate (hole-scavenger)electrolytes, studied in operandoemploying an illuminated scanning flow cell. We demonstrate that decrease in photocurrents alone does not reflect the degradation of photo electrodes. Changes in dissolution rates correlate to the evolution of surface chemistry and morphology. The correlative measurements on both sides of the liquid−semiconductor junction provide quantitative comparison and mechanistic insights into the degradation processes

Nature of Nitrogen Incorporation in BiVO4 Photoanodes through Chemical and Physical Methods

In recent years, BiVO4 has been optimized as a photoanode material to produce photocurrent densities close to its theoretical maximum under AM1.5 solar illumination. Its performance is, therefore, limited by its 2.4 amp; 8201;eV bandgap. Herein, nitrogen is incorporated into BiVO4 to shift the valence band position to higher energies and thereby decreases the bandgap. Two different approaches are investigated modification of the precursors for the spray pyrolysis recipe and post amp; 8208;deposition nitrogen ion implantation. Both methods result in a slight red shift of the BiVO4 bandgap and optical absorption onset. Although previous reports on N amp; 8208;modified BiVO4 assumed individual nitrogen atoms to substitute for oxygen, X amp; 8208;ray photoelectron spectroscopy on the samples reveals the presence of molecular nitrogen i.e., N2 . Density functional theory calculations confirm the thermodynamic stability of the incorporation and reveal that N2 coordinates to two vanadium atoms in a bridging configuration. Unfortunately, nitrogen incorporation also results in the formation of a localized state of amp; 8776;0.1 amp; 8201;eV below the conduction band minimum of BiVO4, which suppresses the photoactivity at longer wavelengths. These findings provide important new insights on the nature of nitrogen incorporation into BiVO4 and illustrate the need to find alternative lower amp; 8208;bandgap absorber materials for photoelectrochemical energy conversion application

Determining Structure Activity Relationships in Oxide Derived Cu Sn Catalysts During CO2 Electroreduction Using X Ray Spectroscopy

The development of earth abundant catalysts for selective electrochemical CO2 conversion is a central challenge. Cu amp; 63743;Sn bimetallic catalysts can yield selective CO2 reduction toward either CO or formate. This study presents oxide derived Cu amp; 63743;Sn catalysts tunable for either product and seeks to understand the synergetic effects between Cu and Sn causing these selectivity trends. The materials undergo significant transformations under CO2 reduction conditions, and their dynamic bulk and surface structures are revealed by correlating observations from multiple methods X ray absorption spectroscopy for in situ study, and quasi in situ X ray photoelectron spectroscopy for surface sensitivity. For both types of catalysts, Cu transforms to metallic Cu0 under reaction conditions. However, the Sn speciation and content differ significantly between the catalyst types the CO selective catalysts exhibit a surface Sn content of 13 at. predominantly present as oxidized Sn, while the formate selective catalysts display an Sn content of amp; 8776;70 at. consisting of both metallic Sn0 and Sn oxide species. Density functional theory simulations suggest that Sn amp; 948; sites weaken CO adsorption, thereby enhancing CO selectivity, while Sn0 sites hinder H adsorption and promote formate production. This study reveals the complex dependence of catalyst structure, composition, and speciation with electrochemical bias in bimetallic Cu catalyst

The role of selective contacts and built in field for charge separation and transport in photoelectrochemical devices

Direct photoelectrochemical PEC solar water splitting has the potential to be a key element in a sustainable energy supply chain. However, integrated PEC systems based on metal oxides still lack the high efficiencies required for large scale, economically feasible applications. A main obstacle for the realization of higher solar to hydrogen efficiencies is the appropriate design of the semiconductor catalyst and semiconductor electrolyte interfaces. Thus, a more accurate understanding of the energy loss mechanisms and the driving forces that determine the charge separation, transport and recombination of electrons and holes in a PEC device would be instrumental for the selection of the most appropriate design routes. In this context we highlight a common misconception within the PEC research community, which is to consider the built in electrical field at the solid liquid interface as essential for charge separation. We subsequently emphasize the established viewpoint within the photovoltaic research community that the gradient of the electrochemical potential is the principle driving force for charge separation and efficient solar energy conversion. Based on this realization, we argue that improved contact design in PEC devices should be one of the main research directions in the design of PEC devices. To address this challenge, we take a closer look at how optimized contacts have been constructed so far and present potential design approaches which can be used to urther improve the performance of PEC device