Synthesis of Hierarchical ZnO/CdSSe Heterostructure Nanotrees

Summary

Here, we prepare and characterize novel tree-like hierarchical ZnO/CdSSe nanostructures, where CdSSe branches are grown on vertically aligned ZnO nanowires. The resulting nanotrees are a potential material for solar energy conversion and other opto-electronic devices.

Abstract

A two-step chemical vapor deposition procedure is here employed to prepare tree-like hierarchical ZnO/CdSSe hetero-nanostructures. The structures are composed of CdSSe branches grown on ZnO nanowires that are vertically aligned on a transparent sapphire substrate. The morphology was measured via scanning electron microscopy. The crystal structure was determined by X-ray powder diffraction analysis. Both the ZnO stem and CdSSe branches have a predominantly wurtzite crystal structure. The mole ratio of S and Se in the CdSSe branches was measured by energy dispersive X-ray spectroscopy. The CdSSe branches result in strong visible light absorption. Photoluminescence (PL) spectroscopy showed that the stem and branches form a type-II heterojunction. PL lifetime measurements showed a decrease in the lifetime of emission from the trees when compared to emission from individual ZnO stems or CdSSe branches and indicate fast charge transfer between CdSSe and ZnO. The vertically aligned ZnO stems provide a direct electron transport pathway to the substrate and allow for efficient charge separation after photoexcitation by visible light. The combination of the abovementioned properties makes ZnO/CdSSe nanotrees promising candidates for applications in solar cells, photocatalysis, and opto-electronic devices.

Introduction

ZnO is a II-VI semiconductor featuring a band gap (BG) of 3.3 eV, a high electron mobility, and a large exciton binding energy^1,2. It is an abundant semiconducting material with a plethora of present and future applications in optical devices, solar cells, and photocatalysis. However, ZnO is transparent, which limits its application in the visible spectral range. Therefore, materials absorbing visible light, such as narrow-gap semiconductors³, dye molecules⁴, and photosensitive polymers⁵, have frequently been employed for sensitizing ZnO to visible light absorption.

CdS (BG 2.43 eV) and CdSe (BG 1....

Protocol

1. Synthesis of Tree-like ZnO/CdSSe Nanostructures

1. Pretreatment and gold coating of sapphire substrates
   NOTE: The gold film acts as a catalyst in the growth of the ZnO nanowires.
   1. Clean sapphire slides (a-plane, 10 × 10 × 1 mm) in 99.5% ethanol with 5 min of sonication to prepare the substrate for Au sputtering.
   2. Deposit a 10-nm (± 2 nm)-thick film of gold onto the sapphire slides with a sputter coater and gold target.
2. Synthesis of ZnO nanowires
   NOTE: The sonication step 1.2.2 results in a homogeneous ZnO and carbon (ZnO/C) mixture. After mixing, the mixture changes ....

Results

Figure 1 shows the growth mechanism of ZnO/CdSSe NTs. The procedure involved a catalytic vapor-liquid-solid (VLS) process followed by a non-catalytic vapor-solid (VS) growth. In the first VLS step, ZnO and C react in the Ar atmosphere, resulting in metallic Zn and carbon oxide. Zn is subsequently dissolved in the gold precursor on the sapphire substrate. ZnO nanowires grow from the dissolved Zn and residual oxygen. In the second step, exposure to air results in the growth.......

Discussion

The vertical alignment of ZnO nanowires (stems) is based on epitaxial growth on the substrate. ZnO nanowires grow preferentially along the <0001> direction that matches with the periodicity of the a-plane of sapphire¹². Therefore, the type and the quality of the substrate are very important. Different thicknesses of the gold coating on the substrate, from 5 nm to 20 nm, have been tested and showed no significant difference in the growth of ZnO nanowires. The length of the ZnO nanowires can be adjusted .......

Materials

Name	Company	Catalog Number	Comments
ZnO	Sigma Aldrich	1314-13-2
Activated Carbon	Alfa	231-153-3
CdSe	Sigma Aldrich	1306-24-7
CdS	Sigma Aldrich	1306-23-6
Sapphire	MTI	2SP	a-plane, 10 × 10 × 1 mm
Furnace	Lindberg Blue M	SSP
Scanning electron microscope	Hitachi	S5700	assembled with an Oxford Inca X-act detector
X-ray powder diffractometer	Rigaku	MiniFlex	filtered Cu Kα radiation (λ=1.5418 Å)
Amplified Ti:sapphire oscillator	Coherent Mantis	Coherent Legend-Elite
Single photon detection module	ID Quantique	ID-100
Sputter coater	Cressington	308	assembled with gold target
Fiber probe spectrometer	Photon Control	SPM-002
Colored Glass Filter	Thorlabs	FGB37-A - Ø25 mm BG40	AR Coated: 350 - 700 nm
Compressed argon gas	Keen	7440-37-1