Xiaoyong Xu

Photogenerated electron reservoir in hetero-p–n CuO–ZnO nanocomposite device for visible-light-driven photocatalytic reduction of aqueous Cr(VI)

The development of visible-light-responsive catalysts with efficient recyclability is important for solar energy conversion applications. Here, we report a three-dimensional (3D) heterohierarchical device consisting of two-dimensional (2D) p-type narrow bandgap semiconductor CuO nanosheets and one-dimensional n-type wide bandgap semiconductor ZnO nanorods that was fabricated via hydrothermal reaction after in situ crystallization on Cu foil. This heterostructured composite exhibited significantly enhanced visible light photocatalytic reduction capacity with stable recyclability to hexavalent chromium (Cr(VI)) compared with pure CuO nanosheets and ZnO nanorods. This improved photocatalysis was attributed to the synergistic actions of CuO and ZnO consisting of their formed p–n hetero-interface for the extension of solar absorption and the anti-recombination of photogenerated electron–hole pairs, demonstrating the potential for p–n heterostructures to be used for solar photocatalytic pollutant degradation or energy conversion. Moreover, the control of the photogenerated electron reservoir cultivated on ZnO nanorods over the photocatalytic reduction activity was demonstrated, further clarifying the photocarrier dynamics and the photocatalytic mechanism in the CuO–ZnO heterojunction. The preparation of the 3D CuO–ZnO p–n junction device could provide a brand new approach to design versatile devices for solar energy conversion.

Photogenerated Carriers Transfer in Dye–Graphene–SnO2 Composites for Highly Efficient Visible-Light Photocatalysis

The visible-light-driven photocatalytic activities of graphene-semiconductor catalysts have recently been demonstrated, however, the transfer pathway of photogenerated carriers especially where the role of graphene still remains controversial. Here we report graphene-SnO2 aerosol nanocomposites that exhibit more superior dye adsorption capacity and photocatalytic efficiency compared with pure SnO2 quantum dots, P25 TiO2, and pure graphene aerosol under the visible light. This study examines the origin of the visible-light-driven photocatalysis, which for the first time links to the synergistic effect of the cophotosensitization of the dye and graphene to SnO2. We hope this concept and corresponding mechanism of cophotosensitization could provide an original understanding for the photocatalytic reaction process at the level of carrier transfer pathway as well as a brand new approach to design novel and versatile graphene-based composites for solar energy conversion.

Well–Steered Charge–Carrier Transfer in 3D Branched CuxO/ZnO@Au Heterostructures for Efficient Photocatalytic Hydrogen Evolution

Multi-component hetero-nanostructures exhibit multifunctional properties or synergistic performance and are thus considered as attractive materials for energy conversion applications. There is a long-standing demand to construct more sophisticated heterostructures for steering charge-carrier flow in semiconductor systems. Herein we fabricate a large-scale quantity of three-dimensional (3D) branched CuxO/ZnO@Au heterostructure consisting of CuO nanowires (NWs) and grafted ZnO nanodisks (NDs) decorated with Au nanoparticles via sequential hierarchical assemblies. This treelike hetero-nanostructure ensures well-steered transfer of photogenerated electrons to the exposed ZnO NDs, while holes to the CuO backbone NWs with concerted efforts from multi-node p-n junctions, polar ZnO facets, and Au plasmon, resulting in the significantly enhanced photocatalytic hydrogen evolution performance.

Enriching Photoelectrons via Three Transition Channels in Amino-Conjugated Carbon Quantum Dots to Boost Photocatalytic Hydrogen Generation

Well-steered transport of photogenerated carriers in optoelectronic systems underlies many emerging solar conversion technologies, yet assessing the charge transition route in nanomaterials remains a challenge. Herein, we combine the photoinduced absorption, emission, and excitation properties in high luminescent carbon quantum dots (CQDs) with an amino-modified surface to identify the existence of three photoelectron transition channels, that is, near-band-edge transition, multiphoton active transition in CQDs, and transfer from amino groups to CQDs, and together they contribute to strong blue photoluminescence (PL) independent of the excitation wavelength. Moreover, the enriching electron reservoir via these three channels was demonstrated in a holes cleaning environment to efficiently trigger water splitting into hydrogen with excellent stability and recyclability.

Three electron channels toward two types of active sites in MoS2@Pt nanosheets for hydrogen evolution

Molybdenum disulfide (MoS2) has emerged as a promising electrocatalyst for the hydrogen evolution reaction (HER), but its activity generally suffers from limited edge active sites and stagnant conductivity. Here we report a controllable Pt decoration strategy on MoS2 nanosheet arrays (NSAs) anchored on three-dimensional (3D) conductive carbon fiber cloth (CFC) substrates to boost the electrocatalytic HER activity to an upper level comparable to commercial Pt foil catalysts in acidic media, with a low overpotential of −70 mV and a Tafel slope of 36 mV per decade as well as a large exchange current density of 0.43 mA cm−2. The results show that laden Pt makes the inert basal surface of MoS2 an attractive platform steering three electron channels toward two types of active sites, that is, electron transfer from CFC to MoS2 edge sites and surface Pt sites, as well as photo-excited electron injection from MoS2 domains to adjacent Pt sites, promising an excellent HER performance. This study presents a feasible approach to boost the HER activity of MoS2 through surface decoration activating the inert basal surface and provides deep insights for designing MoS2-based HER catalysts.

Coexistence of negative photoconductivity and hysteresis in semiconducting graphene

Solution-processed graphene quantum dots (GQDs) possess a moderate bandgap, which make them a promising candidate for optoelectronics devices. However, negative photoconductivity (NPC) and hysteresis that happen in the photoelectric conversion process could be harmful to performance of the GQDs-based devices. So far, their origins and relations have remained elusive. Here, we investigate experimentally the origins of the NPC and hysteresis in GQDs. By comparing the hysteresis and photoconductance of GQDs under different relative humidity conditions, we are able to demonstrate that NPC and hysteresis coexist in GQDs and both are attributed to the carrier trapping effect of surface adsorbed moisture. We also demonstrate that GQDs could exhibit positive photoconductivity with three-order-of-magnitude reduction of hysteresis after a drying process and a subsequent encapsulation. Considering the pervasive moisture adsorption, our results may pave the way for a commercialization of semiconducting graphene-based and diverse solution-based optoelectronic devices.

Self-assembly optimization of cadmium/molybdenum sulfide hybrids by cation coordination competition toward extraordinarily efficient photocatalytic hydrogen evolution

Combining photoharvesting semiconductors with MoS2 co-catalysts is an intriguing approach to develop inexpensive and efficient photocatalysts for visible-light-driven hydrogen (H2) evolution; however, how to coordinate light absorbers and catalytic sites still remains a great challenge. Here, we propose a facile strategy for optimizing assembly of CdS–MoS2 (CM) nanohybrids with controlled active edge exposure by cation coordination competition in one-pot solvothermal synthesis. With the involvement of Cd ions, more formed CdS nanocrystals coordinated preferentially with MoS2 edges self-optimize into unique CM hybrids, which enables the maximum performances of active area and photo-induced electron transfer and injection into the photocatalytic H2 evolution reaction (HER). As a result, the optimum CM hybrids exhibit an outstanding and stable photocatalytic activity with a H2 evolution rate of up to 1009 mmol h−1 g−1, more than 104-fold higher than that of pure CdS, which is the best among the state-of-the-art heterogeneous photocatalysts. This work provides a facile strategy for a controlled configuration of MoS2 edge active sites on photoharvesting semiconductors toward high-efficiency solar photocatalytic H2 generation.

Integrating Semiconducting Catalyst of ReS2 Nanosheets into P-silicon Photocathode toward Enhanced Solar Water Reduction

Loading electrocatalysts at the semiconductor-electrolyte interface is one of the promising strategies to develop photoelectrochemical water splitting cells. However, the assembly of compatible and synergistic heterojunction between the semiconductor and the selected catalyst remains challenging. Here, we report a hierarchical p-type silicon (p-Si)/ReS2 heterojunction photocathode fabricated through the uniform growth of vertically standing ReS2 nanosheets (NSs) on a planar p-Si substrate for the solar-driven hydrogen evolution reaction (HER). The laden ReS2 NSs not only serve as a high-activity HER catalyst but also render a suitable electronic band coupled with p-Si into a II-type heterojunction, which facilitates the photoinduced charge production, separation, and utilization. As a result, the assembled p-Si/ReS2 photocathode exhibits a 23-fold increased photocurrent density at 0 VRHE and a 35-fold enhanced photoconversion efficiency compared with the pure p-Si counterpart. The bifunctional ReS2 as a catalyst and a semiconductor enables multi-effects in improving light harvesting, charge separation, and catalytic kinetics, highlighting the potential of semiconducting catalysts integrated into solar water splitting devices.