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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.
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.
ZnO is a II-VI semiconductor featuring a band gap (BG) of 3.3 eV, a high electron mobility, and a large exciton binding energy1,2. 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 components6,7. ZnO/CdSSe nanocomposites have been reported to result in efficient photovoltaic energy conversion8,9.
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 branches9,10. 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.
1. Synthesis of Tree-like ZnO/CdSSe Nanostructures
2. Morphological and Crystallographic Characterization
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.
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...
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 ...
Data and figures in this article are cited from the literature in nanotechnology by Li et al.17.
The authors thank Svilen Bobev for his help with the XRD spectra and K. Booksh for assistance with the sputter coater equipment.
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 |
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