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 1.76 eV) are common II-VI narrow-gap semiconductors and have been intensively investigated. The BG and lattice parameters of the ternary alloy CdSSe can be adjusted by varying the mole ratios of the VI components^6,7. ZnO/CdSSe nanocomposites have been reported to result in efficient photovoltaic energy conversion^8,9.

Combining the efficient electron transport pathway of vertically aligned ZnO nanowires towards a substrate with the improved visible light absorption of the CdSSe branches led to efficient electron transfer between the stem and branches^9,10. Thus, we synthesized a new tree-like ZnO/CdSSe nanostructure, where vertically aligned ZnO nanowires are decorated with CdSSe branches. This composite material can act as a building block for novel solar energy conversion devices.

This protocol describes how ZnO nanowire arrays are grown on a sapphire substrate by one-step chemical vapor deposition (CVD) from ZnO and C powders, following a procedure that has previously been published¹¹. Following the growth of ZnO nanowires, a second step of CVD is employed to grow CdSSe branches on the ZnO nanowires. We employ X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS) to measure the crystal structures, morphology, and composition of the ZnO/CdSSe nanotrees (NTs). The optical properties and charge carrier transfer mechanism between the branches and stem have been investigated by photoluminescence (PL) spectroscopy and time-resolved PL lifetime measurements.

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 to a gray color. The compaction step 1.2.3 ensures that no air is present in the mixture and that the ZnO and carbon are in close contact. After CVD, a white film of ZnO nanowires should be deposited on the substrate, side facing down towards the boat.
   1. Mix 1 g of ZnO nanopowder and activated carbon (mass fraction of 50:50) in 10 ml of 99.5% ethanol and stir well with a spatula.
   2. Sonicate the mixture in a water bath at 20 ºC for 30 min, and then dry it in an oven for ~5 hr at 80 ºC.
   3. Place the ZnO/C mixture in an alumina boat and compact it well with a spatula.
   4. Place the gold-coated sapphire slides on top of the alumina boat, with the gold-coated side facing down. Place the alumina boat in the center of the quartz tube in a horizontal tube furnace.
   5. Purge the quartz tube for 1 hr with Ar at a flow rate of 40 sccm at room temperature (RT). Increase the temperature from RT to 900 ºC at a rate of 80 ºC/min and keep the argon flow rate constant.
   6. Hold the temperature at 900 ºC for 2 hr. Open the quartz tube on both ends by removing the rubber stopper gas inlets and let air enter the tube to provide oxygen for the reaction.
   7. Keep the reaction temperature at 900 ºC for 3 hr with the rubber stoppers removed. Cool down to RT at a rate of 10 ºC/min.
   8. Take the boat and the slide out of the furnace.
3. Deposition of CdSSe branches on ZnO nanowires
   NOTE: The alumina boat of CdS/Se was displayed in the center the quartz tube. The prepared ZnO nanowires were facing upward and were 10 cm downstream from the boat. After this second CVD, an orange/yellow film, which is the ZnO/CdSSe nanostructure, should be deposited on the slide.
   1. Mix 0.5 g of CdS and CdSe (CdS/Se) powder well (mass fraction of 50:50) and place the mixture in an alumina boat. Compact the mixture well.
   2. Place the alumina boat of CdS/Se and the previously prepared ZnO nanowire sample in the quartz tube.
   3. Purge the tube with Ar at a flow rate of 40 sccm at RT for 1 hr. Increase the reaction temperature to 820 ºC at a rate of 80 ºC/min. Hold the temperature at 820 ºC for 30 min.
   4. Cool down to RT at a rate of 10 ºC/min. Take the boat and the slide out of the furnace.
4. Synthesis of control samples: ZnO and CdSSe nanowires
   1. Synthesize ZnO nanowires as in section 1.2 under the same experimental conditions.
   2. Synthesize CdSSe nanowires as in section 1.3, under the same experimental conditions, with the same amount of CdS and CdSe composition, but with a clean, gold-coated sapphire slide as the substrate instead of the ZnO-deposited slide.

2. Morphological and Crystallographic Characterization

1. Mount the sample on the SEM stage with a clamp and place the sample in the vacuum chamber of the SEM instrument. Take SEM images on high-resolution mode with a working distance of 12.0 mm at a voltage of 3 kV and a magnification between 3,000X and 10,000X^11,12.
2. Take EDS data with the same sample using the X-ray detector at the same working distance of 12.0 mm. Set the instrument to analysis mode and adjust the voltage to 20 kV, resulting in a current of 20 to 40 µA¹³.
3. Collect XRD spectra on an X-ray powder diffractometer using filtered Cu Kα radiation (λ=1.5418 Å)^11,12.

3. PL Emission Spectroscopy and Time-resolved PL Lifetime Measurements

NOTE: PL spectra and time-correlated single photon counting (TCSPC) measurements at RT were carried out using an amplified Ti:sapphire oscillator after second harmonic generation (SHG), producing a train of 50 fsec pulses centered at a 400-nm wavelength and with an output power of 1.76 mW¹⁴.

1. Fix the sample into a sample holder that positions the sample face up to the laser and to the detector. Align the laser to focus on the sample. Measure the PL emission spectra of the samples from 500-nm to 900-nm wavelengths using a fiber spectrometer.
2. Use a single-photon detector (avalanche photo diode or phototube) to measure time-resolved PL lifetimes with a color glass filter and a 500- or 650-nm interference bandpass filter.
3. Insert ZnO, CdSSe, or ZnO/CdSSe slides into the sample holder. Measure the pure ZnO nanowires with the 500-nm bandpass filter and the CdSSe or ZnO/CdSSe samples with a 650-nm bandpass filter.
4. Use a time-correlated single photon counter or a fast oscilloscope to measure the time-resolved fluorescence decay lifetimes.

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