Engineering the Inhomogeneity of Metal-Insulator-Semiconductor Junctions for Photoelectrochemical Methanol Oxidation.

Si-based inhomogeneous metal-insulator-semiconductor (MIS) junctions with a discontinuous metal nanostructure on the Si/insulator layer are expected to be efficient photoelectrodes for solar energy conversion. However, the formation of a metal nanostructure with an optimized arrangement on semiconductors for efficient charge carrier collection is still a big challenge. Herein, we report a method for the in situ formation of an n-Si inhomogeneous MIS junction with well-dispersed metal nanocontacts through a self-assembly process during photoelectrochemical (PEC) methanol oxidation. The photovoltage shows a strong dependence on the inhomogeneity of the n-Si MIS junction, which can be precisely tuned by the applied electrode potential and operation time. The appropriate inhomogeneity of the Schottky junction as well as the high barrier regions induced by the metal oxide/(oxy)hydroxide layer synergistically produces a large photovoltage of 500 mV for the n-Si inhomogeneous MIS junction. Finally, the n-Si-based photoanode is coupled with a CO2-to-formate reaction to realize the production of formate at both electrodes, resulting in a high faradic efficiency (FE) of 86 and 93% for anode and cathode reactions at an operational current of 30 mA/cm2, respectively. These findings provide important insights into the design of highly efficient inhomogeneous MIS junctions through an in situ self-assembly route for solar energy conversion and storage.

Coupling CO2-to-Ethylene Reduction with the Chlor-Alkaline Process in Seawater through In Situ-Formed Cu Catalysts.

The overall commercial value of a CO2 electroreduction system is hindered by the valueless product and high energy consumption of the oxygen evolution reaction (OER) at the anode. Herein, with an in situ-formed copper catalyst, we employed the alternative chlorine evolution reaction for OER, and high-speed formation of both C2 products and hypochlorite in seawater can be realized. The EDTA in the sea salt electrolyte can trigger an intense dissolution and deposition of Cu on the surface of the electrode, resulting in the in situ formation of dendrites of Cu with high chemical activity. In this system, a faradaic efficiency of 47% can be realized for C2H4 production at the cathode and a faradaic efficiency of 85% can be realized for hypochlorite production at the anode with an operation current of 100 mA/cm2. This work presents a system for designing a highly efficient coupling system for the CO2 reduction reaction and alternative anodic reactions toward value-added products in a seawater environment.

Tuning the Selectivity of Liquid Products of CO2RR by Cu-Ag Alloying.

The combination of Cu and Ag presents a promising way to steer the CO2 reduction products through regulating the surface active sites. However, the difficulty in forming the CuAg alloy with a controllable atomic ratio impedes the in-depth understanding of the structure-activity relationship of CuAg catalysts. Herein, we use E-beam evaporation to synthesize a series of CuAg films with uniform distribution and controllable stoichiometry to reveal the real reaction mechanism on CuAg for the electrochemical CO2 reduction process. Compared with Cu, the Cu1-xAgx (x = 0.05-0.2) alloy showed an apparent suppression of HCOOH and the ratio between C2 liquid products (e.g., ethanol and acetate) and C1 liquid product (HCOOH) is also increased. Operando synchrotron radiation Fourier transform infrared spectroscopy results suggest that the introduction of Ag into the Cu phase can significantly strengthen the absorbed *CO and *OCCO intermediates and suppress the O-C-O intermediates. This research provides a reliable way to inhibit the generation of HCOOH and enhance the production of liquid C2 products during CO2RR and presents a guideline for the future manipulation of copper catalysts by alloying.

In Situ Construction of Flexible VNi Redox Centers over Ni-Based MOF Nanosheet Arrays for Electrochemical Water Oxidation.

Atomic-level design and construction of synergistic active centers are central to develop advanced oxygen electrocatalysts toward efficient energy conversion. Herein, an in situ construction strategy to introduce flexible redox sites of VNi centers onto Ni-based metal-organic framework (MOF) nanosheet arrays (NiV-MOF NAs) as a promising oxygen electrocatalyst is developed. The abundant redox VNi centers with flexible metal valence states of V+3/+4/+5 and Ni+3/+2 enable NiV-MOF NAs excellent oxygen evolution reaction (OER) activity and a long-term stability under high current densities, achieving current densities of 10 and 100 mA cm-2 at recorded overpotentials of 189 and 290 mV, respectively, and showing ignorable decay of initial activity at 100 mA cm-2 after 100 h OER operation. Operando synchrotron radiation Fourier transform infrared combined with quasi in situ X-ray absorption fine structure spectroscopies reveal at atomic level that the flexible V sites can continuously accept electrons from adjacent active Ni sites to accelerate OER kinetics for NiV-MOF NAs during the reaction process, accompanied by a self-optimized structural distortion of VO6 octahedron for promoting the electrochemical stability.

Electrolysis Can Be Used to Resolve Hydrogenation Pathways at Palladium Surfaces in a Membrane Reactor.

For common hydrogenation chemistries that occur at high temperatures (where H2 is adsorbed and activated at the same surface which the substrate must also adsorb for reaction), there is often little consensus on how the reactions (e.g., hydro(deoxy)genation) actually occur. We demonstrate here that an electrocatalytic palladium membrane reactor (ePMR) can be used to study hydrogenation reaction mechanisms at ambient temperatures, where the catalyst does not necessarily undergo structural reorganization. The ePMR uses electrolysis and a hydrogen-selective palladium membrane to deliver reactive hydrogen to a catalyst surface in an adjacent compartment for reaction with an organic substrate. This process forms the requisite metal-hydride surface for hydrogenation chemistry, but at ambient temperature and pressure, and without a H2 source. We demonstrate the utility of this analytical tool by studying the hydrogenation of benzaldehyde at palladium nanocubes with dimensions of 13-24 nm. This experimental design enabled us to resolve that the alcohol product forms at the facial sites, whereas the hydrodeoxygenation step occurs at edge sites. These observations enabled us to develop the first site-specific definition of how a carbonyl species undergoes hydro(deoxy)genation.

Stabilizing Copper for CO2 Reduction in Low-Grade Electrolyte.

We demonstrate herein a CO2 reduction electrocatalyst regeneration strategy based on the manipulation of the Cu(0)/Cu2+ equilibrium with high concentrations of ethylenediaminetetraacetic acid (EDTA). This strategy enables the sustained performance of copper catalysts in distilled and tap water electrolytes for over 12 h. The deposition of common electrolyte impurities such as iron, nickel, and zinc is blocked because EDTA can effectively bind the metal ions and negatively shift the electrode potential of M/M n+. The Cu/Cu2+ redox couple is >600 mV more positive than the other metal ions and therefore participates in an equilibrium of dissolution and redeposition from and to the electrode in high concentrations of EDTA. These dynamic equilibria serve to further regenerate the surface copper catalyst to prevent the deactivation of catalytic sites. On the basis of this strategy, we show that >95% of initial hydrocarbon production activity can be maintained for 12 h in KHCO3 (99% purity) enriched distilled water and 6 h in KHCO3 (99% purity) enriched tap water.

Electrocatalytic Alloys for CO2 Reduction.

Electrochemically reducing CO2 using renewable energy is a contemporary global challenge that will only be met with electrocatalysts capable of efficiently converting CO2 into fuels and chemicals with high selectivity. Although many different metals and morphologies have been tested for CO2 electrocatalysis over the last several decades, relatively limited attention has been committed to the study of alloys for this application. Alloying is a promising method to tailor the geometric and electric environments of active sites. The parameter space for discovering new alloys for CO2 electrocatalysis is particularly large because of the myriad products that can be formed during CO2 reduction. In this Minireview, mixed-metal electrocatalyst compositions that have been evaluated for CO2 reduction are summarized. A distillation of the structure-property relationships gleaned from this survey are intended to help in the construction of guidelines for discovering new classes of alloys for the CO2 reduction reaction.

Brass and Bronze as Effective CO2 Reduction Electrocatalysts.

Electrochemically reducing CO2 into fuels using renewable electricity is a contemporary global challenge that requires significant advances in catalyst design. Photodeposition techniques were used to screen ternary alloys of Cu-Zn-Sn, which includes brass and bronze, for the electrocatalytic reduction of CO2 to CO and formate. This analysis identified Cu0.2 Zn0.4 Sn0.4 and Cu0.2 Sn0.8 to be capable of reaching Faradaic efficiencies of >80 % for CO and formate formation, respectively, and capable of achieving partial current densities of 3 mA cm-2 at an overpotential of merely 200 mV.

Photodecomposition of Metal Nitrate and Chloride Compounds Yields Amorphous Metal Oxide Films.

UV light is found to trigger the decomposition of MClx or M(NO3)x (where M = Fe, Co, Ni, Cu, or Zn) to form uniform, amorphous films of metal oxides. This process does not elevate the temperature of the substrate and thus conformal films can be coated on a range of substrates, including rigid glass and flexible plastic. The formation of the oxide films were confirmed by a combination of powder X-ray diffraction, X-ray photoelectron spectroscopy, X-ray fluorescence spectroscopy, Fourier transform infrared spectroscopy and scanning electron microscopy techniques. Amorphous oxide films of iron, nickel and a combination of iron and nickel demonstrated oxygen evolution reaction electrocatalytic activities commensurate with films of the same compositions prepared by widely used electrodeposition and sputtering methods. These results illuminate a potential route to amorphous oxides at scale using simple metal precursors without vacuum or heat.

Photoelectrochemical oxidation of organic substrates in organic media.

There is a global effort to convert sunlight into fuels by photoelectrochemically splitting water to form hydrogen fuels, but the dioxygen byproduct bears little economic value. This raises the important question of whether higher value commodities can be produced instead of dioxygen. We report here photoelectrochemistry at a BiVO4 photoanode involving the oxidation of substrates in organic media. The use of MeCN instead of water enables a broader set of chemical transformations to be performed (e.g., alcohol oxidation and C-H activation/oxidation), while suppressing photocorrosion of BiVO4 that otherwise occurs readily in water, and sunlight reduces the electrical energy required to drive organic transformations by 60%. These collective results demonstrate the utility of using photoelectrochemical cells to mediate organic transformations that otherwise require expensive and toxic reagents or catalysts.Photoelectrochemical water splitting is a promising method for H2 fuel production, but the O2 by-product generated has little economic value. Here, Berlinguette and colleagues demonstrate that BiVO4 photoanodes immersed in organic media can instead perform valuable alcohol oxidation and C-H functionalization reactions.

Electrolytic CO2 Reduction in Tandem with Oxidative Organic Chemistry.

Electrochemical reduction of CO2 into carbon-based products using excess clean electricity is a compelling method for producing sustainable fuels while lowering CO2 emissions. Previous electrolytic CO2 reduction studies all involve dioxygen production at the anode, yet this anodic reaction requires a large overpotential and yields a product bearing no economic value. We report here that the cathodic reduction of CO2 to CO can occur in tandem with the anodic oxidation of organic substrates that bear higher economic value than dioxygen. This claim is demonstrated by 3 h of sustained electrolytic conversion of CO2 into CO at a copper-indium cathode with a current density of 3.7 mA cm-2 and Faradaic efficiency of >70%, and the concomitant oxidation of an alcohol at a platinum anode with >75% yield. These results were tested for four alcohols representing different classes of alcohols and demonstrate electrolytic reduction and oxidative chemistry that form higher-valued carbon-based products at both electrodes.

High-Throughput Synthesis of Mixed-Metal Electrocatalysts for CO2 Reduction.

The utilization of CO2 as a feedstock requires fundamental breakthroughs in catalyst design. The efficiencies and activities of pure metal electrodes towards the CO2 reduction reaction are established, but the corresponding data on mixed-metal systems are not as well developed. In this study we show that the near-infrared driven decomposition (NIRDD) of solution-deposited films of metal salts and subsequent electrochemical reduction offers the unique opportunity to form an array of mixed-metal electrocatalyst coatings with excellent control of the metal stoichiometries. This synthetic method enabled us to develop an empirical structure-property correlation to help inform the development of optimized CO2 catalyst compositions.

Exposure of WO3 Photoanodes to Ultraviolet Light Enhances Photoelectrochemical Water Oxidation.

Exposure of WO3 photoanodes to sustained irradiation by ultraviolet (UV) light induces a morphology change that enhances the photoelectrochemical (PEC) activity towards the oxygen evolution reaction (OER). A 30% enhancement in photocurrent density at 1.23 V vs RHE was measured despite a nominal change in onset potential. A structural and electrochemical analysis of the films before and after exposure to UV irradiation indicates that a higher film porosity and correspondingly higher specific surface area is responsible for the enhancement in PEC activity. The effect of prolonged UV irradiation on the WO3 films is fundamentally different to that which was previously observed for BiVO4 films.

Curing BiVO4 Photoanodes with Ultraviolet Light Enhances Photoelectrocatalysis.

Exposure of BiVO4 photoanodes to ultraviolet (UV) radiation for extended time periods (e.g., 20 h) produces a morphological change and concomitant improvement in photo-electrocatalytic (PEC) efficiency for driving water splitting directly by sunlight. The ∼230 mV cathodic shift in onset potential and doubling of the photocurrent at 1.23 V vs. RHE after UV curing are comparable to the effects engendered by the presence of a secondary catalyst layer. PEC measurements and absorption spectra indicate that the cathodic shift after UV curing corresponds to a suppression of charge recombination and a greater photovoltage generation caused by the shift of the flat-band potential, and not an improvement in electrocatalytic activity or light absorption. Spectroscopic surface analysis suggests that surface defect sites, which are eliminated by UV curing, for the differences in observed charge recombination.

CoOOH Nanosheets with High Mass Activity for Water Oxidation.

Endowing transition-metal oxide electrocatalysts with high water oxidation activity is greatly desired for production of clean and sustainable chemical fuels. Here, we present an atomically thin cobalt oxyhydroxide (γ-CoOOH) nanosheet as an efficient electrocatalyst for water oxidation. The 1.4 nm thick γ-CoOOH nanosheet electrocatalyst can effectively oxidize water with extraordinarily large mass activities of 66.6 A g(-1), 20 times higher than that of γ-CoOOH bulk and 2.4 times higher than that of the benchmarking IrO2 electrocatalyst. Experimental characterizations and first-principles calculations provide solid evidence to the half-metallic nature of the as-prepared nanosheets with local structure distortion of the surface CoO(6-x) octahedron. This greatly enhances the electrophilicity of H2O and facilitates the interfacial electron transfer between Co ions and adsorbed -OOH species to form O2, resulting in the high electrocatalytic activity of layered CoOOH for water oxidation.

Vacancy-induced ferromagnetism of MoS2 nanosheets.

Outstanding magnetic properties are highly desired for two-dimensional ultrathin semiconductor nanosheets. Here, we propose a phase incorporation strategy to induce robust room-temperature ferromagnetism in a nonmagnetic MoS2 semiconductor. A two-step hydrothermal method was used to intentionally introduce sulfur vacancies in a 2H-MoS2 ultrathin nanosheet host, which prompts the transformation of the surrounding 2H-MoS2 local lattice into a trigonal (1T-MoS2) phase. 25% 1T-MoS2 phase incorporation in 2H-MoS2 nanosheets can enhance the electron carrier concentration by an order, introduce a Mo(4+) 4d energy state within the bandgap, and create a robust intrinsic ferromagnetic response of 0.25 µB/Mo by the exchange interactions between sulfur vacancy and the Mo(4+) 4d bandgap state at room temperature. This design opens up new possibility for effective manipulation of exchange interactions in two-dimensional nanostructures.

Aligned Fe2TiO5-containing nanotube arrays with low onset potential for visible-light water oxidation.

There remains a pressing challenge in the efficient utilization of visible light in the photoelectrochemical applications of water splitting. Here, we design and fabricate pseudobrookite Fe2TiO5 ultrathin layers grown on vertically aligned TiO2 nanotube arrays that can enhance the conduction and utilization of photogenerated charge carriers. Our photoanodes are characterized by low onset potentials of ~0.2 V, high photon-to-current efficiencies of 40-50% under 400-600 nm irradiation and total energy conversion efficiencies of ~2.7%. The high performance of Fe2TiO5 nanotube arrays can be attributed to the anisotropic charge carrier transportation and elongated charge carrier diffusion length (compared with those of conventional TiO2 or Fe2O3 photoanodes) based on electrochemical impedance analysis and first-principles calculations. The Fe2TiO5 nanotube arrays may open up more opportunities in the design of efficient and low-cost photoanodes working in visible light for photoelectrochemical applications.

Graphene activating room-temperature ferromagnetic exchange in cobalt-doped ZnO dilute magnetic semiconductor quantum dots.

Control over the magnetic interactions in dilute magnetic semiconductor quantum dots (DMSQDs) is a key issue to future development of nanometer-sized integrated "spintronic" devices. However, manipulating the magnetic coupling between impurity ions in DMSQDs remains a great challenge because of the intrinsic quantum confinement effects and self-purification of the quantum dots. Here, we propose a hybrid structure to achieve room-temperature ferromagnetic interactions in DMSQDs, via engineering the density and nature of the energy states at the Fermi level. This idea has been applied to Co-doped ZnO DMSQDs where the growth of a reduced graphene oxide shell around the Zn(0.98)Co(0.02)O core turns the magnetic interactions from paramagnetic to ferromagnetic at room temperature, due to the hybridization of 2p(z) orbitals of graphene and 3d obitals of Co(2+)-oxygen-vacancy complexes. This design may open up a kind of possibility for manipulating the magnetism of doped oxide nanostructures.

Half-unit-cell α-Fe2O3 semiconductor nanosheets with intrinsic and robust ferromagnetism.

The synthesis of atomically thin transition-metal oxide nanosheets as a conceptually new class of materials is significant for the development of next-generation electronic and magnetic nanodevices but remains a fundamental chemical and physical challenge. Here, based on a "template-assisted oriented growth" strategy, we successfully synthesized half-unit-cell nanosheets of a typical transition-metal oxide α-Fe2O3 that show robust intrinsic ferromagnetism of 0.6 µB/atom at 100 K and remain ferromagnetic at room temperature. A unique surface structure distortion, as revealed by X-ray absorption spectroscopy, produces nonidentical Fe ion environments and induces distance fluctuation of Fe ion chains. First-principles calculations reveal that the efficient breaking of the quantum degeneracy of Fe 3d energy states activates ferromagnetic exchange interaction in these Fe(5-co)-O-Fe(6-co) ion chains. These results provide a solid design principle for tailoring the spin-exchange interactions and offer promise for future semiconductor spintronics.

Realizing ferromagnetic coupling in diluted magnetic semiconductor quantum dots.

Manipulating the ferromagnetic interactions in diluted magnetic semiconductor quantum dots (DMSQDs) is a central theme to the development of next-generation spin-based information technologies, but this remains a great challenge because of the intrinsic antiferromagnetic coupling between impurity ions therein. Here, we propose an effective approach capable of activating ferromagnetic exchange in ZnO-based DMSQDs, by virtue of a core/shell structure that engineers the energy level of the magnetic impurity 3d levels relative to the band edge. This idea has been successfully applied to Zn(0.96)Co(0.04)O DMSQDs covered by a shell of ZnS or Ag2S. First-principles calculations further indicate that covering a ZnS shell around the Co-doped ZnO core drives a transition of antiferromagnetic-to-ferromagnetic interaction, which occurs within an effective depth of 1.2 nm underneath the surface in the core. This design opens up new possibility for effective manipulation of exchange interactions in doped oxide nanostructures for future spintronics applications.