Spectroelectrochemical Analysis of the Water Oxidation Mechanism on Doped Nickel Oxides.

Metal oxides and oxyhydroxides exhibit state-of-the-art activity for the oxygen evolution reaction (OER); however, their reaction mechanism, particularly the relationship between charging of the oxide and OER kinetics, remains elusive. Here, we investigate a series of Mn-, Co-, Fe-, and Zn-doped nickel oxides using operando UV-vis spectroscopy coupled with time-resolved stepped potential spectroelectrochemistry. The Ni2+/Ni3+ redox peak potential is found to shift anodically from Mn- < Co- < Fe- < Zn-doped samples, suggesting a decrease in oxygen binding energetics from Mn- to Zn-doped samples. At OER-relevant potentials, using optical absorption spectroscopy, we quantitatively detect the subsequent oxidation of these redox centers. The OER kinetics was found to have a second-order dependence on the density of these oxidized species, suggesting a chemical rate-determining step involving coupling of two oxo species. The intrinsic turnover frequency per oxidized species exhibits a volcano trend with the binding energy of oxygen on the Ni site, having a maximum activity of ∼0.05 s-1 at 300 mV overpotential for the Fe-doped sample. Consequently, we propose that for Ni centers that bind oxygen too strongly (Mn- and Co-doped oxides), OER kinetics is limited by O-O coupling and oxygen desorption, while for Ni centers that bind oxygen too weakly (Zn-doped oxides), OER kinetics is limited by the formation of oxo groups. This study not only experimentally demonstrates the relation between electroadsorption free energy and intrinsic kinetics for OER on this class of materials but also highlights the critical role of oxidized species in facilitating OER kinetics.

Impact of Oxygen Vacancy Occupancy on Charge Carrier Dynamics in BiVO4 Photoanodes.

Oxygen vacancies are ubiquitous in metal oxides and critical to performance, yet the impact of these states upon charge carrier dynamics important for photoelectrochemical and photocatalytic applications remains contentious and poorly understood. A key challenge is the unambiguous identification of spectroscopic fingerprints which can be used to track their function. Herein, we employ five complementary techniques to modulate the electronic occupancy of states associated with oxygen vacancies in situ in BiVO4 photoanodes, allowing us to identify a spectral signature for the ionization of these states. We obtain an activation energy of ∼0.2 eV for this ionization process, with thermally activated electron detrapping from these states determining the kinetics of electron extraction, consistent with improved photoelectrochemical performance at higher temperatures. Bulk, un-ionized states, however, function as deep hole traps, with such trapped holes energetically unable to drive water oxidation. These observations help address recent controversies in the literature regarding oxygen vacancy function, providing new insights into their impact upon photoelectrochemical performance.

An anthraquinone-based bismuth-iron metal-organic framework as an efficient photoanode in photoelectrochemical cells.

Metal-organic frameworks (MOFs) are appealing candidate materials to design new photoelectrodes for use in solar energy conversion because of their modular nature and chemical versatility. However, to date there are few examples of MOFs that can be directly used as photoelectrodes, for which they must be able to afford charge separation upon light absorption, and promote the catalytic dissociation of water molecules, while maintaining structural integrity. Here, we have explored the use of the organic linker anthraquinone-2, 6-disulfonate (2, 6-AQDS) for the preparation of MOFs to be used as photoanodes. Thus, the reaction of 2, 6-AQDS with Bi(iii) or a combination of Bi(iii) and Fe(iii) resulted in two new MOFs, BiPF-10 and BiFePF-15, respectively. They display similar structural features, where the metal elements are disposed in inorganic-layer building units, which are pillared by the organic linkers by coordination bonds through the sulfonic acid groups. We show that the introduction of iron in the structure plays a crucial role for the practical use of the MOFs as a robust photoelectrode in a photoelectrochemical cell, producing as much as 1.23 mmol H2 cm-2 with the use of BiFePF-15 as photoanode. By means of time-resolved and electrochemical impedance spectroscopic studies we have been able to unravel the charge transfer mechanism, which involves the formation of a radical intermediate species, exhibiting a longer-lived lifetime by the presence of the iron-oxo clusters in BiFePF-15 to reduce the charge transfer resistance.

Dynamic restructuring of nickel sulfides for electrocatalytic hydrogen evolution reaction.

Transition metal chalcogenides have been identified as low-cost and efficient electrocatalysts to promote the hydrogen evolution reaction in alkaline media. However, the identification of active sites and the underlying catalytic mechanism remain elusive. In this work, we employ operando X-ray absorption spectroscopy and near-ambient pressure X-ray photoelectron spectroscopy to elucidate that NiS undergoes an in-situ phase transition to an intimately mixed phase of Ni3S2 and NiO, generating highly active synergistic dual sites at the Ni3S2/NiO interface. The interfacial Ni is the active site for water dissociation and OH* adsorption while the interfacial S acts as the active site for H* adsorption and H2 evolution. Accordingly, the in-situ formation of Ni3S2/NiO interfaces enables NiS electrocatalysts to achieve an overpotential of only 95 ± 8 mV at a current density of 10 mA cm-2. Our work highlighted that the chemistry of transition metal chalcogenides is highly dynamic, and a careful control of the working conditions may lead to the in-situ formation of catalytic species that boost their catalytic performance.

Light-Driven Nitrogen Fixation to Ammonia over Aqueous-Dispersed Mo-Doped TiO2 Colloidal Nanocrystals.

Photocatalytic nitrogen fixation to ammonia and nitrates holds great promise as a sustainable route powered by solar energy and fed with renewable energy resources (N2 and H2O). This technology is currently under deep investigation to overcome the limited efficiency of the process. The rational design of efficient and robust photocatalysts is crucial to boost the photocatalytic performance. Widely used bulk materials generally suffer from charge recombination due to poor interfacial charge transfer and difficult surface diffusion. To overcome this limitation, this work explores the use of aqueous-dispersed colloidal semiconductor nanocrystals (NCs) with precise morphological control, better carrier mobility, and stronger redox ability. Here, the TiO2 framework has been modified via aliovalent molybdenum doping, and resulting Mo-TiO2 NCs have been functionalized with charged terminating hydroxyl groups (OH-) for the simultaneous production of ammonia, nitrites, and nitrates via photocatalytic nitrogen reduction in water, which has not been previously found in the literature. Our results demonstrate the positive effect of Mo-doping and nanostructuration on the overall N2 fixation performance. Ammonia production rates are found to be dependent on the Mo-doping loading. 5Mo-TiO2 delivers the highest NH4+ yield rate (ca. 105.3 µmol g-1 L-1 h-1) with an outstanding 90% selectivity, which is almost four times higher than that obtained over bare TiO2. The wide range of advance characterization techniques used in this work reveals that Mo-doping enhances charge-transfer processes and carriers lifetime as a consequence of the creation of new intra band gap states in Mo-doped TiO2 NCs.

Impact of NiCo2O4/SrTiO3 p-n Heterojunctions on the Interface of Photoelectrochemical Water Oxidation.

Forming semiconductor heterojunctions is a promising strategy to boost the efficiency of solar-driven photoelectrochemical (PEC) water splitting by accelerating the separation and transport of photogenerated charge carriers via an interfacial electric field. However, there is limited research considering the influence of electrolytes on the band alignment of the heterojunction under PEC conditions. In this work, we use a single crystal NiCo2O4/SrTiO3 (NCO/STO) heterojunction with atomic-precision controlled thickness as a model photoelectrode to study the band structure modulations upon getting in contact with the electrolyte and the correlation with the PEC activity. It is found that the band alignment can be tuned by the control of p-n heterojunction film thickness and regulated by the water redox potential (Eredox). When the Fermi level (EF) of the heterojunction is higher/lower than the Eredox, the band bending at the NCO/STO-electrolyte interface will increase/decrease after contacting with the electrolyte. However, when the band bending width of the NCO layer is thinner than its thickness, the electrolyte will not influence the band alignment at the NCO/STO interface. In addition, PEC characterization results show that the 1 nm NCO/STO heterojunction photoanode exhibits superior water-splitting performance, owing to the optimum band structure of the p-n heterojunction and the shorter charge transfer distance.

Highly Durable Nanoporous Cu2-xS Films for Efficient Hydrogen Evolution Electrocatalysis under Mild pH Conditions.

Copper-based hydrogen evolution electrocatalysts are promising materials to scale-up hydrogen production due to their reported high current densities; however, electrode durability remains a challenge. Here, we report a facile, cost-effective, and scalable synthetic route to produce Cu2-xS electrocatalysts, exhibiting hydrogen evolution rates that increase for ∼1 month of operation. Our Cu2-xS electrodes reach a state-of-the-art performance of ∼400 mA cm-2 at -1 V vs RHE under mild conditions (pH 8.6), with almost 100% Faradaic efficiency for hydrogen evolution. The rise in current density was found to scale with the electrode electrochemically active surface area. The increased performance of our Cu2-xS electrodes correlates with a decrease in the Tafel slope, while analyses by X-ray photoemission spectroscopy, operando X-ray diffraction, and in situ spectroelectrochemistry cooperatively revealed the Cu-centered nature of the catalytically active species. These results allowed us to increase fundamental understanding of heterogeneous electrocatalyst transformation and consequent structure-activity relationship. This facile synthesis of highly durable and efficient Cu2-xS electrocatalysts enables the development of competitive electrodes for hydrogen evolution under mild pH conditions.

Laser-Reduced BiVO4 for Enhanced Photoelectrochemical Water Splitting.

The present study proposes a laser irradiation method to superficially reduce BiVO4 photoelectrodes and boost their water oxidation reaction performance. The origin of this enhanced performance toward oxygen evolution reaction (OER) was studied using a combination of a suite of structural, chemical, and mechanistic advanced characterization techniques including X-ray photoelectron (XPS), X-ray absorption spectroscopy (XAS), electrochemical impedance spectroscopy (EIS), and transient absorption spectroscopy (TAS), among others. We found that the reduction of the material is localized at the surface of the sample and that this effect creates effective n-type doping and a shift to more favorable energy band positions toward water oxidation. This thermodynamic effect, together with the change in sample morphology to larger and denser domains, results in an extended lifetime of the photogenerated carriers and improved charge extraction. In addition, the stability of the reduced sample in water was also confirmed. All of these effects result in a two-fold increase in the photocurrent density of the laser-treated samples.

Push-Pull Electronic Effects in Surface-Active Sites Enhance Electrocatalytic Oxygen Evolution on Transition Metal Oxides.

Sustainable electrocatalysis of the oxygen evolution reaction (OER) constitutes a major challenge for the realization of green fuels. Oxides based on Ni and Fe in alkaline media have been proposed to avoid using critical raw materials. However, their ill-defined structures under OER conditions make the identification of key descriptors difficult. Here, we have studied Fe-Ni-Zn spinel oxides, with a well-defined crystal structure, as a platform to obtain general understanding on the key contributions. The OER reaches maximum performance when: (i) Zn is present in the Spinel structure, (ii) very dense, equimolar 1 : 1 : 1 stoichiometry sites appear on the surface as they allow the formation of oxygen vacancies where Zn favors pushing the electronic density that is pulled by the octahedral Fe and tetrahedral Ni redox pair lowering the overpotential. Our work proves cooperative electronic effects on surface active sites as key to design optimum OER electrocatalysts.

TiO2 Nanotubes for Solar Water Splitting: Vacuum Annealing and Zr Doping Enhance Water Oxidation Kinetics.

Herein, we report the cooperative effect of Zr doping and vacuum annealing on the carrier dynamics and interfacial kinetics of anodized TiO2 nanotubes for light-driven water oxidation. After evaluation of different Zr loads and different annealing conditions, it was found that both Zr doping and vacuum annealing lead to a significantly enhanced light harvesting efficiency and photoelectrochemical performance. The substitution of Zr4+ by Ti4+ species leads to a higher density of surface defects such as oxygen vacancies, facilitating electron trapping on Zr4+, which reduced the charge recombination and hence boosted the charge transfer kinetics. More importantly, vacuum annealing promoted the presence of surface defects. Furthermore, the mechanistic study through impedance spectroscopy revealed that both charge transfer and surface conductivity are significantly enhanced due the presence of an oxygen-deficient TiO2 surface. These results represent an important step forward in the optimization of nanostructured TiO2-based photoelectrodes, with high potential in photocatalytic applications, including solar fuel production.

WO3/BiVO4: impact of charge separation at the timescale of water oxidation.

The four hole oxidation of water has long been considered the kinetic bottleneck for overall solar-driven water splitting, and thus requires the formation of long-lived photogenerated holes to overcome this kinetic barrier. However, photogenerated charges are prone to recombination unless they can be spatially separated. This can be achieved by coupling materials with staggered conduction and valence band positions, providing a thermodynamic driving force for charge separation. This has most aptly been demonstrated in the WO3/BiVO4 junction, in which quantum efficiencies for the water oxidation reaction can approach near unity. However, the charge carrier dynamics in this system remain elusive over timescales relevant to water oxidation (µs-s). In this work, the effect of charge separation on carrier lifetime, and the voltage dependence of this process, is probed using transient absorption spectroscopy and transient photocurrent measurements, revealing sub-µs electron transfer from BiVO4 to WO3. The interface formed between BiVO4 and WO3 is shown to overcome the "dead-layer effect" encountered in BiVO4 alone. Moreover, our study sheds light on the role of the WO3/BiVO4 junction in enhancing the efficiency of the water oxidation reaction, where charge separation across the WO3/BiVO4 junction improves both the yield and lifetime of holes present in the BiVO4 layer over timescales relevant to water oxidation.

Photocatalytic and Photoelectrochemical Degradation of Organic Compounds with All-Inorganic Metal Halide Perovskite Quantum Dots.

Inspired by the outstanding optoelectronic properties reported for all-inorganic halide perovskite quantum dots (QDs), we have evaluated the potential of these materials toward the photocatalytic and photoelectrochemical degradation of organic compounds, taking the oxidation of 2-mercaptobenzothiazole (MBT) as a proof-of-concept. First, we determined electrochemically the energy levels of dispersions of perovskite QDs with different band gaps induced by the different ratios between halides (Br and I) and metallic cations (Pb and Sn). Then, we selected CsPbBr3 QDs to demonstrate the photocatalytic and photoelectrochemical oxidation of MBT, confirming that hole injection takes place from CsPbBr3 QDs to MBT, resulting in the total degradation of MBT as evidenced by electrospray mass spectrometry analyses. Although the stability and toxicity of these QDs are major issues to address in the near future, the results obtained in the present study open promising perspectives for the implementation of solar-driven catalytic strategies based on these fascinating materials.