Heterointegration of Pt/Si/Ag Nanowire Photodiodes and Their Photocatalytic Properties

Nanostructured silicon photocatalysts for solar-driven fuel production

Solar-driven production of fuels such as hydrogen, hydrocarbons, and ammonia using semiconducting photocatalysts has the potential to be a sustainable alternative to current chemical processes. In recent years, silicon (Si) nanostructures have been recognized as a promising photocatalyst for hydrogen generation and organic oxidation reactions owing to its abundance, biocompatibility, and cost. While bulk Si has been studied extensively, on the nanoscale, plenty of opportunities exist to understand and engineer optimally performing Si photocatalysts. This perspective will highlight key results on the use of Si nanostructures for photocatalytic H₂ production, CO₂ reduction via light and heat-driven chemical looping, and current challenges in utilizing it for fuel-forming reactions. A brief guide on how these challenges can be addressed in the future and other unexplored questions that remain in the field are also discussed.

Design of a Silicon Photocatalyst for High-Efficiency Photocatalytic Water Splitting

Metallurgical silicon was studied for photocatalytic H₂ evolution activity. It has been found that metallurgical silicon with large particle size (above 800 nm) possesses poor photocatalytic activity because of the deteriorating photoelectric performance of the low-purity silicon. After size reduction (around 400 nm) and metal nanoparticle decoration, the photocatalytic performance was significantly enhanced to 1003.3 μmol·g^-1·h^-1. However, the photocatalytic performance of the Cu-, Ag-, and Pt-decorated silicon is degraded with the increase of time. Moreover, the degradation is independent of the metal. Electrochemical test and X-ray photoelectron spectroscopy suggested that the Mott-Schottky effect in the metal-silicon contact should be responsible for the degradation. After forming a heterojunction by vulcanizing the Ag-decorated silicon, the degradation was suppressed. Upgradation of the metal-silicon contact to form a heterojunction was a promising way to suppress the degradation and retain the high photocatalytic performance.

Maskless Spatioselective Functionalization of Silicon Nanowires

Spatioselective functionalization of silicon nanowires was achieved without using a masking material. The designed process combines metal-assisted chemical etching (MACE) to fabricate silicon nanowires and hydrosilylation to form molecular monolayers. After MACE, a monolayer was formed on the exposed nanowire surfaces. A second MACE step was expected to elongate the nanowires, thus creating two different segments. When monolayers of 1-undecene or 1-tetradecyne were formed on the upper segment, however, the second MACE step did not extend the nanowires. In contrast, nanowires functionalized with 1,8-nonadiyne were elongated, but at an approximately 8 times slower etching rate. The elongation resulted in a contrast difference in high-resolution scanning electron microscopy (HR-SEM) images, which indicated the formation of nanowires that were covered with a monolayer only at the top parts. Click chemistry was successfully used for secondary functionalization of the monolayer with azide-functionalized nanoparticles. The spatioselective presence of 1,8-nonadiyne gave a threefold higher particle density on the upper segment functionalized with 1,8-nonadiyne than on the lower segment without monolayer. These results indicate the successful spatioselective functionalization of silicon nanowires fabricated by MACE.

Turbostratic carbon nitride enhances the performance and stability of cadmium sulfide nanorod hydrogen evolution photocatalysts

The t-CN_x layer functions as (1) a protection layer to isolate CdS from the electrolyte and (2) as a tunnel junction to promote charge separation.

Semiconductor-Metal Nanofloret Hybrid Structures by Self-Processing Synthesis

We present a synthetic strategy that takes advantage of the inherent asymmetry exhibited by semiconductor nanowires prepared by Au-catalyzed chemical vapor deposition (CVD). The metal-semiconductor junction is used for activating etch, deposition, and modification steps localized to the tip area using a wet-chemistry approach. The hybrid nanostructures obtained for the coinage metals Cu, Ag, and Au resemble the morphology of grass flowers, termed here Nanofloret hybrid nanostructures consisting of a high aspect ratio SiGe nanowire (NW) with a metallic nanoshell cap. The synthetic method is used to prepare hybrid nanostructures in one step by triggering a programmable cascade of events that is autonomously executed, termed self-processing synthesis. The synthesis progression was monitored by ex situ transmission electron microscopy (TEM), in situ scanning transmission electron microscopy (STEM) and inductively coupled plasma mass spectrometry (ICP-MS) analyses to study the mechanistic reaction details of the various processes taking place during the synthesis. Our results indicate that the synthesis involves distinct processing steps including localized oxide etch, metal deposition, and process termination. Control over the deposition and etching processes is demonstrated by several parameters: (i) etchant concentration (water), (ii) SiGe alloy composition, (iii) reducing agent, (iv) metal redox potential, and (v) addition of surfactants for controlling the deposited metal grain size. The NF structures exhibit broad plasmonic absorption that is utilized for demonstrating surface-enhanced Raman scattering (SERS) of thiophenol monolayer. The new type of nanostructures feature a metallic nanoshell directly coupled to the crystalline semiconductor NW showing broad plasmonic absorption.

High-performance nanowire oxide photo-thin film transistor

A gate-modulated nanowire oxide photosensor is fabricated by electron-beam lithography and conventional dry etch processing.. The device characteristics are good, including endurance of up to 10(6) test cycles, and gate-pulse excitation is used to remove persistent photoconductivity. The viability of nanowire oxide phototransistors for high speed and high resolution applications is demonstrated, thus potentially expanding the scope of exploitation of touch-free interactive displays.

Bimetallic Ag-Au nanowires: synthesis, growth mechanism, and catalytic properties

Silver-gold (Ag-Au) bimetallic nanowires were controllably synthesized by a newly developed wet-chemical method at room temperature. The Ag nanowires and Au nanoparticles were sequentially formed by reduction with vanadium oxide (V2O3) nanoparticles so as to form Ag-Au bimetal, in which the Ag nanowires show a diameter of ~20 nm and length up to 10 μm. A few unique features were noted in our new approach: it was rapid (within a few minutes), controllable in shape and size, reproducible, and there was no need for any surface modifiers. The formation and growth mechanisms of these Ag-Au bimetallic nanostructures driven by lattice match and a unique reducing agent (V2O3) have been proposed in this study. Moreover, the application of such bimetallic nanoparticles for catalytic reduction of 4-nitrophenol to 4-aminophenol was performed, and they exhibit catalytic properties superior to those of the Ag nanowires, Au nanoparticles, and Ag-Pd bimetallic nanostructures prepared under the reported conditions. These Ag-Au bimetallic nanoparticles have potential to be highly efficient catalysts for the reduction of 4-nitrophenol. This study may lead to new path for the generation of other bimetallic nanostructures with excellent catalytic efficiency.

Porous silicon nanowires

In this mini-review, we summarize recent progress in the synthesis, properties and applications of a new type of one-dimensional nanostructures-single crystalline porous silicon nanowires. The growth of porous silicon nanowires starting from both p- and n-type Si wafers with a variety of dopant concentrations can be achieved through either one-step or two-step reactions. The mechanistic studies indicate the dopant concentration of Si wafers, oxidizer concentration, etching time and temperature can affect the morphology of the as-etched silicon nanowires. The porous silicon nanowires are both optically and electronically active and have been explored for potential applications in diverse areas including photocatalysis, lithium ion batteries, gas sensors and drug delivery.

Plasmonic enhancements of photocatalytic activity of Pt/n-Si/Ag photodiodes using Au/Ag core/shell nanorods

We report the plasmonic enhancement of the photocatalytic properties of Pt/n-Si/Ag photodiode photocatalysts using Au/Ag core/shell nanorods. We show that Au/Ag core/shell nanorods can be synthesized with tunable plasmon resonance frequencies and then conjugated onto Pt/n-Si/Ag photodiodes using well-defined chemistry. Photocatalytic studies showed that the conjugation with Au/Ag core/shell nanorods can significantly enhance the photocatalytic activity by more than a factor of 3. Spectral dependence studies further revealed that the photocatalytic enhancement is strongly correlated with the plasmonic absorption spectra of the Au/Ag core/shell nanorods, unambiguously demonstrating the plasmonic enhancement effect.