Synthesis of ZnO/Si Hierarchical Nanowire Arrays for Photocatalyst Application
- Dingguo Li†1,
- Xiaolan Yan†1,
- Chunhua Lin2,
- Shengli Huang1, 2, 3, 4Email author,
- Z. Ryan Tian2,
- Bing He1,
- Qianqian Yang1,
- Binbin Yu1, 3, 4,
- Xu He1,
- Jing Li1,
- Jiayuan Wang1,
- Huahan Zhan1,
- Shuping Li1 and
- Junyong Kang1
© The Author(s). 2017
Received: 29 September 2016
Accepted: 18 December 2016
Published: 5 January 2017
ZnO/Si nanowire arrays with hierarchical architecture were synthesized by solution method with ZnO seed layer grown by atomic layer deposition and magnetron sputtering, respectively. The photocatalytic activity of the as-grown tree-like arrays was evaluated by the degradation of methylene blue under ultraviolet light at ambient temperature. The comparison of morphology, crystal structure, optical properties, and photocatalysis efficiency of the two samples in different seeding processes was conducted. It was found that the ZnO/Si nanowire arrays presented a larger surface area with better crystalline and more uniform ZnO branches on the whole sidewall of Si backbones for the seed layer by atomic layer deposition, which gained a strong light absorption as high as 98% in the ultraviolet and visible range. The samples were proven to have a potential use in photocatalyst, but suffered from photodissolution and memory effect. The mechanism of the photocatalysis was analyzed, and the stability and recycling ability were also evaluated and enhanced.
Along with social development, the environmental pollution is becoming more and more critical. Finding a high efficient and cost-effective catalyst is one of the best ways to remove the pollutants such as organic dyes and toxic chemicals. Semiconductor photocatalysts are the key materials to complete mineralization of a wide range of dyes and organic compounds, as they offer tunable and enhanced photoresponse in ultraviolet (UV) and visible region and efficient electron-hole separation to create active radicals for photocatalysis . Among the semiconductor materials studied, hybrid nanowires with different materials have been of great interest due to the potential superior efficiency coming from the unique geometry and properties, unlimited combination, and effective integration at nanoscale to implement design and material integrity . In this regard, branched tree-like nanowire arrays may be the most favorite choice because they provide omnidirectional branches for effective photon absorption, enlarge surface area to react with the pollutants, maximize junction interface area for electron-hole separation, and enhance surface curvature for a high reaction kinetics [3–5].
Semiconductor materials that are currently applied in the building blocks of functional hybrid nanowire arrays includes Si [6–8], Ge , III–V [10–12], IV–VI [13, 14], II–VI [15, 16] and metal oxides (ZnO [2, 17–21], TiO2 , In2O3 , and Fe2O3 ). Among them, Si is the most fundamental material in current photovoltaic market and has demonstrated broad application as solar cell, sensor, and catalyst. However, the potential corrosion and high valence band maximum energy make it unsuitable alone in these fields. Another semiconductor material, ZnO, may be a good candidate to remedy these problems for its large bandgap (3.37 eV), large exciton binding energy (60 meV), and stable physical and chemical properties. Moreover, it is easily obtained in different types of nanostructures. Previously research on the ZnO/Si nanowire arrays found a very high photovoltaic current density and efficiency [17–20]. Study of the system currently concentrates on the photovoltaics , dye-sensitized solar cells , photoelectrochemical electrodes for water splitting [18, 20], and sensors , but literature about photocatalytic activity is rarely reported . In this research, the arrays were used as a catalyst to degrade methyl red and exhibited a good photocatalytic capability. However, the arrays were grown by both chemical etching and chemical vapor deposition. Photolithography was also applied in order to fabricate patterned Si arrays in a controlled geometry and density. All these costed much time and needed sophisticated equipments. In our experiment, except the seed layer, we used solution method to grow the hybrid nanowire arrays, owning the advantages of low synthetic temperature, simple equipment, and time saving.
In the fabrication of hybrid nanowire arrays, the seed layer is a vital factor, because it decides the position, diameter, orientation, and density of the nanowire arrays. In previous research, the seed layer of the branched ZnO nanowires was respectively deposited by magnetron sputtering (MS) , electrospinning , spin coating , dip coating , as well as atomic layer deposition (ALD) . All these reports are chiefly concerning on the final function of the ZnO/Si nanowire arrays. The optimization of the seed layer has yet to be resolved.
Therefore, in this article, two different seeding methods were used to figure out how it influence the growth of branched ZnO nanowires as well as the properties and function of tree-like ZnO/Si nanowire arrays. One seeding method was ALD, while the other one was MS. The photocatalytic activity of the as-grown tree-like ZnO/Si nanowire arrays was evaluated by the degradation of a typical organic dye methylene blue (MB) under UV light irradiation. The comparison of morphology, crystal structure, optical properties, and photocatalysis efficiency of the two samples was conducted. Moreover, the mechanism of the photocatalysis was analyzed, and the stability and recycling ability were also evaluated.
The fabrication process of ZnO/Si nanowire arrays includes substrate cleaning, wet chemical etching for Si backbones, ZnO seed layer deposition, and ZnO branches growth. First, P-type boron doped (100) Si wafers with resistivity of 1–10 Ω cm and thickness of 450 μm were used as substrates for the synthesis of the hybrid structure. The substrates were cut in a size of 10 × 15 mm2 and sequentially cleaned by ultrasonication in absolute toluene, acetone, ethanol, and piranha solution (H2SO4 and H2O2 in a volume ratio of 3:1) at 80 °C for 2 h, each of which was followed by ultrasonication in de-ionized water.
Second, after the substrates were dried with N2 flow, they were immersed in aqueous solution of 4 M HF and 0.02 M AgNO3 in a Teflon vessel for a galvanic displacement reaction at 50 °C for 30 min. The post-etched substrates were transferred to the solution of HCl/HNO3/H2O in a volume ratio of 1:1:1 overnight to remove the reduced Ag nanoparticles during the chemical etching. The substrates were then thoroughly rinsed with de-ionized water and dried in air.
Third, the substrates were divided into two equal groups to grow ZnO seed layer by applying the two different methods. One group, denoted as sample ALD, was deposited using TALD-100A ALD system (Keming Co. Ltd., China), while the other one, denoted as sample MS, was grown using JC500-3/D MS system (Chaomai Co. Ltd., China). In detail, to make sample ALD, the reactive chamber of the ALD system was preheated in nitrogen atmosphere at 100 °C for 24 h, then the substrates were placed into the chamber. Afterwards, the pressure of the chamber was pumped to 0.15 torr followed by heated to 150 °C. The 167 cycles of Zn(C2H5)2 dose for 0.02 s, N2 flow for 25 s, water vapor dose for 0.015 s, and N2 flow for 25 s were required to grow a ZnO seed layer with 30 nm thickness. As for sample MS, the substrates were transferred into a MS chamber in argon carrier gas. The background pressure was evacuated to be 10−4 Pa, and the working pressure was set at 1 Pa. The ZnO target was pre-sputtered for 10 min to gain a clean surface, then the seed layer was deposited on the substrates for 3 min at room temperature with sputtering power of 80 W. The film thickness was characterized to be about 30 nm by a planar Si substrate.
Fourth, ZnO nanowires were synthesized by hydrothermal method. A 500-ml aqueous solution of 25 mM Zn(CH3COO)2.2H2O and 25 mM C6H12N4 was filled into a glass beaker, which was agitated and heated in a magnetic stirring apparatus. After the temperature kept stable at 90 °C, the seeded substrates were soaked vertically in the solution for a period of 40 min. The as-grown samples were removed from the solution and rinsed with de-ionized water several times, then they were annealed at 420 °C for 30 min in an oven to obtain ZnO nanowires with better crystallinity.
Morphology of the as-prepared samples was characterized by a ZEISS Sigma scanning electron microscopy (SEM) with an accelerating voltage of 15.0 kV. Chemical composition of the seeded substrates was analyzed using an energy dispersive X-ray (EDX) spectroscopy as attached on the SEM. Structural quality of the nanowire arrays was evaluated by an X’Pert PRO X-ray diffraction (XRD) with Cu Kα radiation (λ = 1.54056 Å). Photoluminescence (PL) spectra were collected on a Hitachi F-7000 fluorescence spectrophotometer with an excitation wavelength of 325 nm. The parameters of PMT voltage, scan speed, and slit width were set at 700 V, 240 nm/min, and 5.0 nm for all the samples. Diffuse reflection spectra and absorption spectra were taken on an Agilent Carry-5000 UV-Vis-NIR spectrophotometer. X-ray photoelectron spectroscopy (XPS) was recorded in a PHI Quantum 2000 with Al Kα X-ray excitation source (hv = 1486.6 eV). All the measurements were carried out at room temperature in normal conditions.
The photocatalytic activity of the nanowire arrays was evaluated by the degradation of MB dye under UV light irradiation at ambient temperature. Aqueous suspension of MB (1 × 10−5 M) in a volume of 20 ml was poured into a glass petri dish with a diameter of 85 mm, then a piece of sample was immersed in the solution with the nanowires facing upwards. They were subjected to UV light in an intensity of ~4.5 mW/cm2. The photodegradation was performed 1 h per time, and three successive reaction periods on the MB solution with identical concentration were conducted for each sample. The photocatalytic efficiency was analyzed by measuring absorption spectra of the degraded MB solution.
For the enhancement of photocatalytic activity and recycling ability, the surface of sample ALD was coated by a layer of Ag particles and TiO2 film. The Ag particles were reduced by immersing the sample in a suspension of Na3C6H5O7.2H2O and AgNO3 in a mole ratio of 20/2 mM in 100 mL de-ionized water, then irradiated by sun light in an intensity of ~23.5 mW/cm2 for an hour . The sample was rinsed several times in de-ionized water and dried in air. Afterwards, a layer of TiO2 film with 10 nm thickness was deposited on it by ALD. The sample was finally annealed at 400 °C in nitrogen atmosphere for 3 min in an oven.
Results and Discussion
Morphology and Crystal Structure
Photoluminescence and Optical Reflection
ZnO/Si nanowire arrays with hierarchical structure were prepared by wet chemical etching and hydrothermal growth. The morphology, crystal structure, optical properties, and photocatalysis efficiency of the system were significantly influenced by the ZnO seed layer. In contrast to the seed layer deposited by MS, the more uniform seed layer deposited by ALD resulted in ZnO branches with better crystallinity grown on the whole sidewall of the Si backbones, in addition to the stronger light absorption and higher photodegradation efficiency of MB for its larger surface area and higher surface curvature effect. The photocatalytic efficiency of the system was declined under prolonging catalytic period, which might suffer from photodissolution and memory effect. A possible mechanism for the charge separation and organic dye pollutant degradation was proposed, and the stability and recycling ability of the system were improved by coating a layer of noble metal and metal oxide. All these suggested that the hybrid nanowires could find potential applications in photocatalysis and other fields, such as photovoltaics and sensors.
This work was supported by the National Key Research and Development Program of China (2016YFB0400801), 863 program (2014AA032608), National Natural Science Foundation of China (U1405253, 61227009, 61205051, 90921002), Natural Science Foundation of Fujian Province of China (2016 J01265, 2013 J05097), Fundamental Research Funds for the Central Universities (20720160015, 20720150032), and Fundamental Research Funds for Xiamen University (2016Y0589, 2016Y0591).
Availability of Data and Materials
Supporting information contains XPS spectra of sample ALD and sample MS before and after photocatalysis as well as their relative intensity of the deconvoluted peaks.
DL, XY, and CL performed the experiments, analyzed the data, and drafted the manuscript. SH designed the experiment, analyzed the data, and revised the manuscript. BH, QY, BY, and XH helped prepare and characterize the samples and analyze the data. ZRT, JL, JW, HZ, SL, and JK participated in the final data analysis and critical review of the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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