Xiaoxia Chang

Enhanced Surface Reaction Kinetics and Charge Separation of p–n Heterojunction Co3O4/BiVO4 Photoanodes

Surface reaction kinetics and bulk charge separation are both critical to the efficiency of solar water splitting. In addition to the well-documented surface catalytic effect, the promotion of bulk charge separation upon loading of cocatalysts has rarely been reported. This paper describes the synergetic enhancement of surface reaction kinetics and bulk charge separation by introducing discrete nanoisland p-type Co3O4 cocatalysts onto n-type BiVO4, forming a p-n Co3O4/BiVO4 heterojunction with an internal electric field to facilitate charge transport. Being highly dispersed on the surface of photoanodes, the nanoisland cocatalysts could suppress the formation of recombination centers at the photoanode/cocatalyst interface. This cocatalyst-loading method achieved a charge separation efficiency of up to 77% in the bulk and 47% on the surface of catalysts. An AM 1.5G photocurrent of 2.71 mA/cm(2) at 1.23 V versus the reversible hydrogen electrode for water oxidation was obtained, which is the highest photocurrent yet reported for Co-catalyzed undoped BiVO4 photoanodes, with a photoconversion efficiency of 0.659%.

CO2 photo-reduction: insights into CO2 activation and reaction on surfaces of photocatalysts

Large amounts of anthropogenic CO2 emissions associated with increased fossil fuel consumption have led to global warming and an energy crisis. The photocatalytic reduction of CO2 into solar fuels such as methane or methanol is believed to be one of the best methods to address these two problems. In addition to light harvesting and charge separation, the adsorption/activation and reduction of CO2 on the surface of heterogeneous catalysts remain a scientifically critical challenge, which greatly limits the overall photoconversion efficiency and selectivity of CO2 reduction. This review describes recent advances in the fundamental understanding of CO2 photoreduction on the surface of heterogeneous catalysts and particularly provides an overview of enhancing the adsorption/activation of CO2 molecules. The reaction mechanism and pathways of CO2 reduction as well as their dependent factors are also analyzed and discussed, which is expected to enable an increase in the overall efficiency of CO2 reduction through minimizing the reaction barriers and controlling the selectivity towards the desired products. The challenges and perspectives of CO2 photoreduction over heterogeneous catalysts are presented as well.

Effective Charge Carrier Utilization in Photocatalytic Conversions

Continuous efforts have been devoted to searching for sustainable energy resources to alleviate the upcoming energy crises. Among various types of new energy resources, solar energy has been considered as one of the most promising choices, since it is clean, sustainable, and safe. Moreover, solar energy is the most abundant renewable energy, with a total power of 173 000 terawatts striking Earth continuously. Conversion of solar energy into chemical energy, which could potentially provide continuous and flexible energy supplies, has been investigated extensively. However, the conversion efficiency is still relatively low since complicated physical, electrical, and chemical processes are involved. Therefore, carefully designed photocatalysts with a wide absorption range of solar illumination, a high conductivity for charge carriers, a small number of recombination centers, and fast surface reaction kinetics are required to achieve a high activity. This Account describes our recent efforts to enhance the utilization of charge carriers for semiconductor photocatalysts toward efficient solar-to-chemical energy conversion. During photocatalytic reactions, photogenerated electrons and holes are involved in complex processes to convert solar energy into chemical energy. The initial step is the generation of charge carriers in semiconductor photocatalysts, which could be enhanced by extending the light absorption range. Integration of plasmonic materials and introduction of self-dopants have been proved to be effective methods to improve the light absorption ability of photocatalysts to produce larger amounts of photogenerated charge carriers. Subsequently, the photogenerated electrons and holes migrate to the surface. Therefore, acceleration of the transport process can result in enhanced solar energy conversion efficiency. Different strategies such as morphology control and conductivity improvement have been demonstrated to achieve this goal. Fine-tuning of the morphology of nanostructured photocatalysts can reduce the migration distance of charge carriers. Improving the conductivity of photocatalysts by using graphitic materials can also improve the transport of charge carriers. Upon charge carrier migration, electrons and holes also tend to recombine. The suppression of recombination can be achieved by constructing heterojunctions that enhance charge separation in the photocatalysts. Surface states acting as recombination centers should also be removed to improve the photocatalytic efficiency. Moreover, surface reactions, which are the core chemical processes during the solar energy conversion, can be enhanced by applying cocatalysts as well as suppressing side reactions. All of these strategies have been proved to be essential for enhancing the activities of semiconductor photocatalysts. It is hoped that delicate manipulation of photogenerated charge carriers in semiconductor photocatalysts will hold the key to effective solar-to-chemical energy conversion.

Synergistic Cocatalytic Effect of Carbon Nanodots and Co3O4 Nanoclusters for the Photoelectrochemical Water Oxidation on Hematite

Cocatalysis plays an important role in enhancing the activity of semiconductor photocatalysts for solar water splitting. Compared to a single cocatalyst configuration, a cocatalytic system consisting of multiple components with different functions may realize outstanding enhancement through their interactions, yet limited research has been reported. Herein we describe the synergistic cocatalytic effect between carbon nanodots (CDots) and Co3 O4 , which promotes the photoelectrochemical water oxidation activity of the Fe2 O3 photoanode with a 60 mV cathodically shifted onset potential. The C/Co3 O4 -Fe2 O3 photoanode exhibits a photocurrent density of 1.48 mA cm(-2) at 1.23 V (vs. reversible hydrogen electrode), 78 % higher than that of the bare Fe2 O3 photoanode. The slow reaction process on the single Co(III) -OH site of the Co3 O4 cocatalyst, oxidizing H2 O to H2 O2 with two photogenerated holes, could be accelerated by the timely H2 O2 oxidation to O2 catalyzed on CDots.

Gold Nanorod@TiO2 Yolk-Shell Nanostructures for Visible-Light-Driven Photocatalytic Oxidation of Benzyl Alcohol.

Fine gold nanorod@TiO2 yolk-shell catalysts are synthesized by an improved silica template method. With a hollow TiO2 shell and a unique tunable cylindrical gold core, the catalyst exhibits a high surface area and a wide range of photoabsorption, from ultraviolet to near infrared. The remarkable photochemical activity is obtained when such catalyst is utilized to oxidize benzyl alcohol.

Stable Aqueous Photoelectrochemical CO2 Reduction by a Cu2O Dark Cathode with Improved Selectivity for Carbonaceous Products

Photocatalytic reduction of CO2 to produce fuels is a promising way to reduce CO2 emission and address the energy crisis. However, the H2 evolution reaction competes with CO2 photoreduction, which would lower the overall selectivity for carbonaceous products. Cu2 O has emerged as a promising material for suppressing the H2 evolution. However, it suffers from poor stability, which is commonly regarded as the result of the electron-induced reduction of Cu2 O. This paper describes a simple strategy using Cu2 O as a dark cathode and TiO2 as a photoanode to achieve stable aqueous CO2 reduction with a high Faradaic efficiency of 87.4 % and a selectivity of 92.6 % for carbonaceous products. We have shown that the photogenerated holes, instead of the electrons, primarily account for the instability of Cu2 O. Therefore, Cu2 O was used as a dark cathode to minimize the adverse effects of holes, by which an improved stability was achieved compared to the Cu2 O photocathode under illumination. Additionally, direct exposure of the Cu2 O surface to the electrolyte was identified as a critical factor for the high selectivity for carbonaceous products.

Thin Heterojunctions and Spatially Separated Cocatalysts To Simultaneously Reduce Bulk and Surface Recombination in Photocatalysts

Efficient charge separation and light absorption are crucial for solar energy conversion over solid photocatalysts. This paper describes the construction of Pt@TiO2 @In2 O3 @MnOx mesoporous hollow spheres (PTIM-MSs) for highly efficient photocatalytic oxidation. TiO2 -In2 O3 double-layered shells were selectively decorated with Pt nanoparticles and MnOx on the inner and outer surfaces, respectively. The spatially separated cocatalysts drive electrons and holes near the surface to flow in opposite directions, while the thin heterogeneous shell separates the charges generated in the bulk phase. The synergy between the thin heterojunctions and the spatially separated cocatalysts can simultaneously reduce bulk and surface/subsurface recombination. In2 O3 also serves as a sensitizer to enhance light absorption. The PTIM-MSs exhibit high photocatalytic activity for both water and alcohol oxidation.

Surviving High-Temperature Calcination: ZrO2-Induced Hematite Nanotubes for Photoelectrochemical Water Oxidation

Nanotubular Fe2 O3 is a promising photoanode material, and producing morphologies that withstand high-temperature calcination (HTC) is urgently needed to enhance the photoelectrochemical (PEC) performance. This work describes the design and fabrication of Fe2 O3 nanotube arrays that survive HTC for the first time. By introducing a ZrO2 shell on hydrothermal FeOOH nanorods by atomic layer deposition, subsequent high-temperature solid-state reaction converts FeOOH-ZrO2 nanorods to ZrO2 -induced Fe2 O3 nanotubes (Zr-Fe2 O3 NTs). The structural evolution of the hematite nanotubes is systematically explored. As a result of the nanostructuring and shortened charge collection distance, the nanotube photoanode shows a greatly improved PEC water oxidation activity, exhibiting a photocurrent density of 1.5 mA cm-2 at 1.23 V (vs. reversible hydrogen electrode, RHE), which is the highest among hematite nanotube photoanodes without co-catalysts. Furthermore, a Co-Pi decorated Zr-Fe2 O3 NT photoanode reveals an enhanced onset potential of 0.65 V (vs. RHE) and a photocurrent of 1.87 mA cm-2 (at 1.23 V vs. RHE).

Synergism of Geometric Construction and Electronic Regulation: 3D Se‐(NiCo)Sx/(OH)x Nanosheets for Highly Efficient Overall Water Splitting

The exploration of highly efficient electrocatalysts for both oxygen and hydrogen generation via water splitting is receiving considerable attention in recent decades. Up till now, Pt-based catalysts still exhibit the best hydrogen evolution reaction (HER) performance and Ir/Ru-based oxides are identified as the benchmark for oxygen evolution reaction (OER). However, the high cost and rarity of these materials extremely hinder their large-scale applications. This paper describes the construction of the ultrathin defect-enriched 3D Se-(NiCo)Sx /(OH)x nanosheets for overall water splitting through a facile Se-induced hydrothermal treatment. Via Se-induced fabrication, highly efficient Se-(NiCo)Sx /(OH)x nanosheets are successfully fabricated through morphology optimization, defect engineering, and electronic structure tailoring. The as-prepared hybrids exhibit relatively low overpotentials of 155 and 103 mV at the current density of 10 mA cm-2 for OER and HER, respectively. Moreover, an overall water-splitting device delivers a current density of 10 mA cm-2 for ≈66 h without obvious degradation.

Collapsed polymer-directed synthesis of multicomponent coaxial-like nanostructures

Multicomponent colloidal nanostructures (MCNs) exhibit intriguing topologically dependent chemical and physical properties. However, there remain significant challenges in the synthesis of MCNs with high-order complexity. Here we show the development of a general yet scalable approach for the rational design and synthesis of MCNs with unique coaxial-like construction. The site-preferential growth in this synthesis relies on the selective protection of seed nanoparticle surfaces with locally defined domains of collapsed polymers. By using this approach, we produce a gallery of coaxial-like MCNs comprising a shaped Au core surrounded by a tubular metal or metal oxide shell. This synthesis is robust and not prone to variations in kinetic factors of the synthetic process. The essential role of collapsed polymers in achieving anisotropic growth makes our approach fundamentally distinct from others. We further demonstrate that this coaxial-like construction can lead to excellent photocatalytic performance over conventional core–shell-type MCNs.