Photodeposition of Pt on Colloidal CdS and CdSe/CdS Semiconductor Nanostructures

Abstract

Semiconductor photocatalysis has been identified as a promising avenue for the conversion of solar energy into environmentally friendly fuels, most notably by the production of hydrogen from water.[1-5] Nanometer-scale materials in particular have attracted considerable scientific attention as the building blocks for light-harvesting applications. [6,7] Their desirable attributes include tunability of the optical properties with size, amenability to relatively inexpensive low-temperature processing, and a high degree of synthetic sophistication leading to increasingly complex and multi-functional architectures. For photocatalysis in particular, the high surface-to-volume ratios in nanoscale materials should lead to an increased availability of carriers for redox reactions on the nanoparticle surface.Recombination of photoexcited carriers directly competes with photocatalytic activity.[3] Charge separation is often achieved with multi-component heterostructures. An early example is the case of TiO2 powders functionalized with Pt and RuO2 particles, where photoexcited electrons are transferred to Pt (the reduction site) and holes to RuO2 (the oxidation site).[8] More recently, many colloidally synthesized nanometer-scale metal-semiconductor heterostructures have been reported. [7,9,10] A majority of these structures are made by thermal methods. [7,10] We have chosen to study photochemical formation of metal-semiconductor heterostructures. The detailed understanding of the mechanisms involved in photodeposition of metals on nanometer-scale semiconductors is necessary to enable a high degree of synthetic control. At the same time, because the results of metal deposition can be directly observed by electron microscopy, it can be used to understand how factors such as nanocrystal composition, shape, carrier dynamics, and surface chemistry influence the photochemical properties of semiconductor nanocrystals.In this communication, we report on the photodeposition of Pt on colloidal CdS and CdSe/CdS core/shell nanocrystals. Among the II-VI semiconductors, CdS is of particular interest because it has the correct band alignment for water photolysis [2] and has been demonstrated to be photocatalytically active. [11][12][13][14][15][16] We have found that the photoexcitation of CdS and CdSe/CdS in the presence of an organometallic Pt precursor leads to deposition of Pt nanoparticles on the semiconductor surface. Stark differences are observed in the Pt nanoparticle location on the two substrates, and the photodeposition can be completely inhibited by the modification of the semiconductor surface. Our results suggest that tuning of the semiconductor band structure, spatial organization and surface chemistry should be crucial in the design of photocatalytic nanostructures.

Cu2ZnSnS4–Au Heterostructures: Toward Greener Chalcogenide-Based Photocatalysts

“…As noted in the Introduction above, metal heterostructures can be conveniently synthesized by either thermal or photochemical methods. Photochemical methods, 50−55 including whole flask lamp illumination methods, 18 offer many advantages such as control over nanoparticle loading through fine-tuning of the illumination or "irradiation" time, 56 high selectivity for surface-bound vs freestanding metal particles, 18 and selectivity for deposition along the length vs the tip of anisotropic semiconductor particles (rods, wires). 57−59 Further, we noted during our work with CdE-M heterostructures (E = S or Se, M = Pt, Pd, Au) 18,26,27 that pre-existing metal nanoparticles, particularly those made of gold (Au), have a tendency to adhere to chalcogenide surfaces.…”

Metal–Semiconductor Hybrid Nano-Heterostructures for Photocatalysis Application

“…This mainly depends on various factors, such as the selection of surface ligands, metal precursor concentration, and reaction temperature. For example, high temperature leads to the formation of facet selective Pt deposition on CdS nanorods (Figure 4e) [42], whereas photodeposition of Pt onto CdS favors no selectivity [48]. On the contrary, hierarchical one tip, two tip, and everywhere Au deposition onto CdSe/ CdS nanorod was demonstrated to depend on Au precursor concentration [41].…”

“…So far, the literature regarding site-selective deposition has been demonstrated on CdSe, CdS, and CdSe/CdS core/shell nanorods with the deposition of various metals, including Au [3,5,41,45,47], Pt [42,48,49], PtCo [42], PtNi [42], Pd, Ag 2 S [41], Co [50], PdO, and Pd 4 S [51]. Thermal-or photochemical-assisted metal deposition can lead to the formation of different metallic patterns on to the semiconductor surface.…”

Cu2ZnSnS4-Au heterostructures: Toward green photocatalytic materials active under visible light

“…As noted in the Introduction above, metal heterostructures can be conveniently synthesized by either thermal or photochemical methods. Photochemical methods, [50][51][52][53][54][55] including whole flask lamp illumination methods 18 offer many advantages such as control over nanoparticle loading through fine-tuning of the illumination or "irradiation" time, 56 high selectivity for surface-bound vs. freestanding metal particles, 18 and selectivity for deposition along the length vs. the tip of anisotropic semiconductor particles (rods, wires). 57 (Figure 1d).…”