Efficient visible light photocatalyst fabricated by depositing plasmonic Ag nanoparticles on conductive polymer-protected Si nanowire arrays for photoelectrochemical hydrogen generation

Enhancing photoelectrochemical CO2 reduction with silicon photonic crystals

The effectiveness of silicon (Si) and silicon-based materials in catalyzing photoelectrochemistry (PEC) CO2 reduction is limited by poor visible light absorption. In this study, we prepared two-dimensional (2D) silicon-based photonic crystals (SiPCs) with circular dielectric pillars arranged in a square array to amplify the absorption of light within the wavelength of approximately 450 nm. By investigating five sets of n + p SiPCs with varying dielectric pillar sizes and periodicity while maintaining consistent filling ratios, our findings showed improved photocurrent densities and a notable shift in product selectivity towards CH4 (around 25% Faradaic Efficiency). Additionally, we integrated platinum nanoparticles, which further enhanced the photocurrent without impacting the enhanced light absorption effect of SiPCs. These results not only validate the crucial role of SiPCs in enhancing light absorption and improving PEC performance but also suggest a promising approach towards efficient and selective PEC CO2 reduction.

Structuring of Si into Multiple Scales by Metal-Assisted Chemical Etching

Structuring Si, ranging from nanoscale to macroscale feature dimensions, is essential for many applications. Metal-assisted chemical etching (MaCE) has been developed as a simple, low-cost, and scalable method to produce structures across widely different dimensions. The process involves various parameters, such as catalyst, substrate doping type and level, crystallography, etchant formulation, and etch additives. Careful optimization of these parameters is the key to the successful fabrication of Si structures. In this review, recent additions to the MaCE process are presented after a brief introduction to the fundamental principles involved in MaCE. In particular, the bulk-scale structuring of Si by MaCE is summarized and critically discussed with application examples. Various approaches for effective mass transport schemes are introduced and discussed. Further, the fine control of etch directionality and uniformity, and the suppression of unwanted side etching are also discussed. Known application examples of Si macrostructures fabricated by MaCE, though limited thus far, are presented. There are significant opportunities for the application of macroscale Si structures in different fields, such as microfluidics, micro-total analysis systems, and microelectromechanical systems, etc. Thus more research is necessary on macroscale MaCE of Si and their applications.

Nanoarray Structures for Artificial Photosynthesis

Conversion and storage of solar energy into fuels and chemicals by artificial photosynthesis has been considered as one of the promising methods to address the global energy crisis. However, it is still far from the practical applications on a large scale. Nanoarray structures that combine the advantages of nanosize and array alignment have demonstrated great potential to improve solar energy conversion efficiency, stability, and selectivity. This article provides a comprehensive review on the utilization of nanoarray structures in artificial photosynthesis of renewable fuels and high value-added chemicals. First, basic principles of solar energy conversion and superiorities of using nanoarray structures in this field are described. Recent research progress on nanoarray structures in both abiotic and abiotic-biotic hybrid systems is then outlined, highlighting contributions to light absorption, charge transport and transfer, and catalytic reactions (including kinetics and selectivity). Finally, conclusions and outlooks on future research directions of nanoarray structures for artificial photosynthesis are presented.

Dispersibility and Size Control of Silver Nanoparticles with Anti-Algal Potential Based on Coupling Effects of Polyvinylpyrrolidone and Sodium Tripolyphosphate

In nearly all the cases of biotoxicity studies of silver nanoparticles (AgNPs), AgNPs used often have general dispersibility and wide size distribution, which may inevitably generate imprecise results. Herein, a kind of synthesis method by coupling effects of polyvinylpyrrolidone (PVP) and sodium tripolyphosphate (STPP) was proposed, in order to prepare AgNPs with better dispersibility and a stable size. Based on this, the preparation mechanism of AgNPs and the potential anti-algae toxicity were analyzed. UV-vis analysis showed that the particle size distribution of AgNPs prepared by co-protective agents was more uniform. X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), and energy dispersive X-ray (EDX) were used to confirm that the obtained nano silver was of a high purity and stable size (~30 nm in diameter). Zeta potential and Fourier transform infrared spectroscopy (FTIR) analysis results indicated the synthesis mechanism of AgNPs by co-protective agents, more precisely, PVP limited the polynegative effect and prevented the linear induction of P₃O₁₀^5- produced by STPP during the growth of silver nuclei. Subsequently, Chlorella and Scenedesmus obliquus were utilized to test the toxicity of AgNPs, confirming that AgNPs synthesized through co-protective agents have potential inhibitory ability on algae, but not severe. This study provides a basic theory for the induction of synthetic AgNPs by various factors in the natural environment and a scientific reference for the environmental risk assessment.

Large-Scale Fabrication of Silicon Nanowires for Solar Energy Applications

The development of silicon (Si) materials during past decades has boosted up the prosperity of the modern semiconductor industry. In comparison with the bulk-Si materials, Si nanowires (SiNWs) possess superior structural, optical, and electrical properties and have attracted increasing attention in solar energy applications. To achieve the practical applications of SiNWs, both large-scale synthesis of SiNWs at low cost and rational design of energy conversion devices with high efficiency are the prerequisite. This review focuses on the recent progresses in large-scale production of SiNWs, as well as the construction of high-efficiency SiNW-based solar energy conversion devices, including photovoltaic devices and photo-electrochemical cells. Finally, the outlook and challenges in this emerging field are presented.

Effective silicon nanowire arrays/WO3 core/shell photoelectrode for neutral pH water splitting

We report the first demonstration of a high-efficiency photoelectrochemical (PEC) water splitting reaction using a novel Si NWs/WO₃ core/shell photoanode prepared by a mild and inexpensive metal-catalyzed electroless etching process followed by dip-coating, airing and annealing methods. The dense and vertically aligned Si NWs/WO₃ core/shell nanostructure were characterized by scanning electron microscopy, transmission electron microscopy and x-ray diffraction. In comparison to planar n-Si, Si NWs and planar Si/WO₃, the Si NWs/WO₃ samples showed significantly enhanced photocurrent over the entire potential sweep range. More significantly, the Si NWs/WO₃ samples have an exceptionally low photocurrent onset potential of -0.6393 V versus reversible hydrogen electrode (RHE), indicating very efficient charge separation and charge transportation processes. The as-prepared electrode also has a photocurrent density of 2.7 mA cm^-2 at 0.6107 V versus RHE in 0.5 M Na₂SO₄ solution under simulated solar light irradiation (100 mW cm^-2 from 300 W Xenon lamp coupled with an AM 1.5 G filter). An optimal solar-to-hydrogen efficiency of about 1.9% was achieved at 0.2676 V versus RHE. Electrochemical impedance spectroscopy was conducted to investigate the properties of the charge transfer process, and the results indicated that the enhanced PEC performance may due to the increased charge separation. The x-ray photoelectron spectroscopy measurements indicated the chemical composition of the Si NWs/WO₃ nanostructure. Our work has provided an efficient strategy to improve the energy conversion efficiency and photocurrent of water splitting materials.

Tuning Band Structure of Cadmium Chalcogenide Nanoflake Arrays via Alloying for Efficient Photoelectrochemical Hydrogen Evolution

Owing to their high extinction coefficient and moderate band gap, cadmium chalcogenides are known as common semiconductors for photoelectric conversion. Nevertheless, no ideal cadmium chalcogenide with proper band structure is available yet for photoelectrochemical hydrogen evolution. In this work, we modified the band structure of CdTe via alloying with Se to achieve a ternary compound (CdSe_0.8Te_0.2) with n-type conduction, a narrower band gap, and a more negative band position compared to those of CdSe and CdTe. This novel material exhibits strong light absorption over a wider spectrum range and generates more vigorous electrons for hydrogen reduction. As a result, a photoelectrode based on nanoflake arrays of the new material could achieve a photocurrent density 2 times that of its CdSe counterpart, outperforming similar materials previously reported in the literature. Moreover, the quick transfer of holes achieved in the novel material was found to depress photocorrosion processes, which led to improved long-term working stability.

Stabilized and Improved Photoelectrochemical Responses of Silicon Nanowires Modified with Ag@SiO2 Nanoparticles and Crystallized TiO2 Film

Semiconductor nanostructure photoelectrochemical (PEC) cells for sunlight conversion have the potential to supply continuous, low-cost, high-efficiency and carbon-free energy; however, in order to provide economically competitive energy, long-time stability and high-efficiency are requisite. In this work, the saturation photocurrent density (J_sat) and stability of the silicon nanowires (SiNWs) in a PEC cell are significantly improved via surface modification with Ag@SiO₂ nanoparticles and conformal TiO₂ ultrathin film. Compared to the bare SiNWs, a 40% enhancement in J_sat, a cathodic shift of 0.8 V in the applied potential for J_sat, and over 10-h continuous operation with a high photocurrent density without decay are realized by using the properly modified SiNWs. Comparison of minority carrier lifetime indicates that the surface recombination of the modified SiNWs has been effectively suppressed. External quantum efficiency measurements reveal that the superstable and improved PEC responses of the modified SiNWs can be maintained for a broadband spectrum. This work provides an alternative route to the industrial applications for the semiconductor nanostructure PEC cells.

The diameter-dependent photoelectrochemical performance of silicon nanowires

We demonstrate the first systematic study of the diameter-dependent photoelectrochemical performance of single silicon nanowires within a broad size range from 200 to 2000 nm.

Metal Semiconductor Heterostructures for Photocatalytic Conversion of Light Energy

For fast separation of the photogenerated charge carriers, metal semiconductor heterostructures have emerged as one of the leading materials in recent years. Among these, metal Au coupled with low bandgap semiconductors remain as ideal materials where both can absorb the solar light in the visible region. It is also established that on excitation, the plasmonic state of gold interacts with excited state of semiconductor and helps for the delocalization of the photogenerated electrons. Focusing these materials where electron transfer preferably occurs from semiconductor to metal Au on excitation, in this Perspective, we report the latest developments in the synthetic chemistry in designing such nano heterostructures and discuss their photocatalytic activities in organic dye degradation/reduction and/or photocatalytic water splitting for generation of hydrogen. Among these, materials such as Au-CZTS, Au-SnS, Au-Bi2S3, Au-ZnSe, and so forth are emphasized, and their formation chemistry as well as their photocatalytic activities are discussed in this Perspective.

C/Si/core–shell structured TiO2@TiO2−x nanocomposites with excellent visible-light photocatalytic performance

C/Si/core–shell structured TiO₂@TiO_2−x nanocomposites were obtained by combining C, Si and black TiO₂.

Construction of CuInS2/Ag sensitized ZnO nanowire arrays for efficient hydrogen generation

Due to its smaller and tunable band gap, CuInS₂ has attracted much attention for clean energy applications. With the synergistic effect of Ag and CuInS₂ NPs, the PEC performance and hydrogen evolution rate of ZnO NWs are significantly enhanced.

Quantitative analysis of photons' decaying pathways in Si nanowire arrays for highly efficient photoelectrochemical solar hydrogen generation

Quantitative investigation of decaying pathways of photon-excited SiNW array's exciton.

Synthesis of Ni/Au/Co trimetallic nanoparticles and their catalytic activity for hydrogen generation from alkaline sodium borohydride aqueous solution

Alloyed Ni/Au/Co nanoparticles with negatively-charged Au, Co atoms, and positively-charged Ni atoms were prepared as effective catalysts for hydrolysis of NaBH₄.