Fabrication of ZnO nanowires array with nanodiamond as reductant

Abstract

The availability of well-aligned high quality ZnO nanowires will extend the potential applications of such materials. In this study, a new strategy for fabricating well-aligned high quality ZnO nanowire arrays via a lower temperature chemical vapor deposition method on un-resisted high temperature substrates was reported for the first time. The results indicated that the growth temperature can be decreased to 600 °C using nanodiamond as the reductant. The thermodynamic mechanism of nanodiamond in ZnO nanowire growth was discussed in detail. Finally, the sensitivity performance was confirmed by UV laser illumination, in which the ZnO nanowire grown at low temperature exhibited a comparable UV light response performance to that grown at high temperatures.

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1. Introduction

One-dimensional ZnO nanowire has attracted much interest for its potential applications in light emitting diodes, nanogenerators, nano resonators, solar cells and UV sensors, due to its wide band-gap, good electronic transportation and easy-fabrication etc.^1–7 To date, various kinds of ZnO nanostructures, including nanorods, nanowires, nanorings, and nanobelts, have been successfully fabricated on different substrates by different methods such as thermal evaporation method, metal–organic vapor-phase epitaxy, metal–organic chemical vapor deposition (MO-CVD), high temperature CVD, and hydrothermal method.^7–14 Among these growth techniques, CVD has been one of the most popular methods due to its simple process and ease of scale up. However, CVD technique normally requires high temperatures from 900 °C to 1400 °C. For example, Wang et al. heated ZnO powder to 1400 °C under a pressure of 300 Torr to fabricate a ZnO nanobelt;¹⁵ and Yang et al. fabricated high density ZnO nanowires with a Au catalyst at a high temperature around 1000 °C.¹⁶ The process temperatures of these methods were too high to use on mesothermal substrates such as transparent ITO and FTO because these substrates cannot bear very high temperatures. Therefore, developing a new method for well-aligned high quality ZnO nanowire growth at a lower temperature to match its application on mesothermal substrates was urgently needed.

Various kinds of carbon sources, such as graphite and active carbon, were usually used in the growth of ZnO nanostructures by high temperature CVD to reduce Zn vapor from a ZnO precursor at a lower temperature of 800 °C.^17–19 Moreover, Gundiah et al. reported that carbon nanotubes could be used to fabricate ZnO nanostructures. The results of this study indicated that the reaction temperature was still too high (around 900 °C) and the alignment was not good.²⁰ Therefore, a suitable material to replace the commonly used carbon reductant and a decrease in the Zn vapor reduction temperature were the key factors for fabrication of well-aligned high quality ZnO nanowires at lower temperature by CVD technique.

Nanodiamond is an attractive and novel carbon material that presents an alternative to carbon nanotubes and graphene. Since it was initially discovered, its high specific surface, high chemical activity, strong luminescence and biological compatibility have attracted broad attention in lubrication and biomedicine.^21–26 Li has previously discussed ZnO growth on substrates including Si, diamond, Fe, Al, and Mg.²⁷ With this single exception, nanodiamond has not been reported as a low cost and highly active reductant for the growth of ZnO nanowire. Herein, nanodiamond particles were used as the reductive carbon source with ZnO powder to fabricate a ZnO nanowire array by a lower temperature CVD method. The influence of different carbon sources on ZnO nanowire growth temperature was also discussed in detail using thermodynamic theory. Finally, the sensitivity performance of the low-temperature fabricated ZnO nanowire based UV sensor was further confirmed under the illumination of 375 nm wavelength UV laser.

2. Experimental

All chemicals in this experiment were analytical grade and used as received without further purification. A vacuum tube furnace chemical vapor deposition system was used to prepare the ZnO nanowire array. Firstly, GaN substrates (2 cm × 2 cm) were rinsed ultrasonically in acetone, ethanol and distilled water for 20 minutes. Then, a 5 nm thickness Au catalyst thin film was sputtered on the substrates by RF (13.56 MHz) magnetic sputtering machine. The catalyst layered substrates were then put into the center of furnace tube; the tube was pumped to about 2 Pa, and the working conditions of mixed gas ambient (1.5% O₂ in N₂ gas) and constant gas pressure of 300 mbar was settled down for 0.5–8 h. The growth temperature was ramped from 600 °C to 960 °C. The carbon reactants were 10 nm diameter nanodiamond, 100 nm diameter nanodiamond from Fang yuan Diamond Company, and graphite from Baichuan Graphite Company, China. The Zn reactant was analytical grade ZnO powder from Sigma-Aldrich Company. The schematic of the chemical vapor deposition set-up is shown in Fig. S1, ESI.†

The crystal structural properties of the ZnO nanowires were determined by X-ray diffraction with Cu Kα radiation at a scan speed of 2° min⁻¹. A field emission scanning electron microscope system (FE-SEM, JSM-7800F) and transmission electron microscope system (TEM, FEI Q200) were used to characterize the ZnO nanowires surface morphology.

A nitrogen adsorption–desorption isotherms test was applied to measure the specific surface area of nanodiamond and graphite. A thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) system were also used to obtain the thermodynamic properties of nanodiamond and graphite during the respective reactions. The measurement was carried out from room temperature to 1000 °C in the ambient mixing gas (1.5% O₂ in N₂ gas). The UV response test was applied with a 375 nm laser light and a power of 11.2 μW mm⁻² using the Keithley 2450 sourcemeter. All the measurements (except TGA-DSC) were carried out at room temperature.

3. Results and discussion

As shown in Fig. 1a, ZnO nanowire array on GaN substrate was successfully obtained at 600 °C with 10 nm diameter nanodiamond as the reactant. The average diameter and height of the nanowires were around 60 nm and 2 μm, respectively. Further XRD patterns of ZnO nanowires on GaN substrates, shown in Fig. 1b, indicated that all the ZnO nanowires had highly (002) preferred orientations. The microstructure of the ZnO nanowire array obtained from CVD process was also confirmed by TEM measurement, shown in Fig. 1c and d. It can be seen in the high-resolution image that the nanowire was highly ordered with c-axis orientation. The crystal constant c of 0.52 nm was confirmed (ZnO standard constant c is 0.52098 nm). The perfectly regular selected area electron diffraction (SAED) pattern of this nanowire in Fig. 1e also revealed that the ZnO nanowire had good crystallinity.

Fig. 1 Characterization of ZnO nanowire obtained from CVD process with 10 nm diameter nanodiamond as reactant at 600 °C. (a) SEM image of ZnO nanowire. (b) XRD pattern of ZnO nanowire. (c) Bright filed TEM image of single nanowire. (d) High-resolution TEM image of single nanowire. (e) SAED patterns of ZnO nanowire.

To understand the growth mechanism accurately, it was necessary to investigate the intermediate morphology involved in the formation of the ZnO nanostructures. Fig. 2 illustrates the SEM images of ZnO nanostructures obtained from the reactions of 10 nm nanodiamond particles and ZnO powder at 600 °C for different times.

Fig. 2 The growth processes of ZnO nanowires at 600 °C for different times, (a) 0.5 h, (b) 1 h, (c) 2 h, (d) 4 h, (e) 8 h; and (f) the TEM image of a ZnO nanowire with an Au droplet on its tip.

In this study, ZnO nanowires were synthesized in a vapor phase transport process via Au catalyzed epitaxial crystal growth.²⁸ This process involved the reduction of ZnO powder by nanodiamond/oxygen to form Zn and CO vapor at a high temperature. The Zn vapor was transported to, and reacted with, the Au catalyst on the substrate located downstream at a lower temperature to form alloy droplets. As the droplets became supersaturated, crystalline ZnO nuclei were formed (Fig. 2a), possibly by the reaction between Zn and O₂. A ZnO thin film was observed on the substrates surface with the ZnO nuclei becoming larger, as shown in Fig. 2b. The ZnO nanowires started to grow due to the catalysis of Au particles, resulting in the vertical ZnO nanowires that were clearly formed on the substrates, as shown in Fig. 2c and d. With the growth time gradually increasing to 8 h, the well-aligned, high quality ZnO nanowire array shown in Fig. 2e was obtained. In this process, the Au catalyst was the crucial factor to grow this high quality ZnO nanowires because the alignment and uniformity of the obtained ZnO nanowires were controlled by the Au catalyst. ZnO nanowires that were obtained without the Au catalyst layer exhibited failure to control the alignment and uniformity of the ZnO nanowires, as shown in Fig. S2.† The TEM image in Fig. 2f confirmed the reliability of Au catalyzed epitaxial crystal growth in this study by showing the Au alloy cap on the top of the obtained ZnO nanowires. The optical and low magnification SEM images of the ZnO nanowires in Fig. S3, ESI,† indicated that the nanostructures were uniform and well-distributed over the substrate.

In order to understand the effect of nanodiamond size on ZnO growth, a series of experiments were designed with different sizes of nanodiamond (10 nm, 100 nm) and graphite powder as the reductant to grow ZnO nanowires. According to the results, the ZnO nanowires could not be obtained at 600 °C when 100 nm nanodiamond particles and graphite powder were used as reductant. Furthermore, the effects of each reductant on the morphology of the ZnO nanowires was also investigated at the temperature of 960 °C. The effect of particle size on ZnO morphology was shown in Fig. S2, ESI.† The results indicated that nanodiamond as the reductant resulted in easily obtained nanowires. All the experimental results showed that nanodiamond was very important for the success of the ZnO nanowire growth at low temperature (i.e. 600 °C). It is reasonable that nanoscale diamond can quickly reduce ZnO power at this same temperature, resulting in atomic Zn. For further confirmation of this reasoning, the presence of carbon atoms (10 nm nanodiamond, 100 nm nanodiamond, graphite) in this reaction process was tracked in experimental atmosphere up to 1000 °C by TGA-DSC in Fig. 3. The results indicated that the triggering reaction temperature of 10 nm nanodiamond was significantly lower than the graphite powder material and 100 nm nanodiamond.

Fig. 3 (a) The TGA and (b) DSC curve of graphite, 100 nm diamond and 10 nm diamond.

The characterization of nanoparticle size is useful for understanding relative reactivity, catalytic efficiency and crystal growth; it also has an important implication on the properties of thermal systems, from bulk-thermodynamics to macro-thermodynamics, and even nanothermodynamics.¹⁵ In order to build on the investigations of morphology described above and the possible thermodynamic reactions in this system, the effect of size on the formation mechanism of ZnO patterns was further characterized by nitrogen adsorption/desorption isotherms measurement. The adsorption and desorption curves of graphite and nanodiamond with different sizes of 100 and 10 nm were test, as shown in Fig. S4, ESI.† Based on the adsorption and desorption data, the Brunauer–Emmett–Teller (BET) equation²⁹

was introduced to calculate specific surface area of graphite and nanodiamond. The results are shown in Table 1.

Table 1 The specific surface area of different samples

Parameters	Graphite	100 nm-diamond	10 nm-diamond
as,BET (m^2 g^−1)	2.97	55.57	275.63
rρ,peak (nm)	1.88	10.63	6.94
Total pore volume (cm^3 g^−1)	0.0112	0.2186	0.9768

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The calculated results of specific surface area exhibited an upward trend of the specific surface area in the order of graphite, 100 nm diamond and 10 nm diamond. As we know, the reaction between solid carbon (graphite or nanodiamond) and oxygen is a surface chemical reaction-controlled process. Based on the random pore model,^30,31 the rate expression can be represented as

(dX/dt: the reaction rate, S₀: the surface area, K: reaction rate constant, C: the concentration of oxygen, φ: the structure constant, ε₀: porosity). From eqn (2), it can be easily determined that the large surface area would greatly enhance the reaction. For example, the 10 nm diameter diamond should make ZnO nanowires grow faster than the other two reactants, which was confirmed by the Fig. S2, ESI.†

The surface area, Gibbs free energy, and chemical activity of the nanomaterial will affect the thermodynamics and dynamics of the reaction. For further understanding of exactly how the nanodiamond triggers the low temperature reaction, a theoretical and experimental investigation of the relationship between nanomaterial size, thermodynamics and dynamics were carried out,^32–34 including the size-dependence on the Gibbs free energy and enthalpy, etc. In this case, the 10 nm nanodiamond particles exhibited a greater surface area than the 100 nm particles, which was an important parameter leading to the growth process at a lower temperature.

Furthermore, the Debye characteristic temperature is an important physical quantity of solids, which not only reflects the crystal lattice dynamic distortion, but also characterizes the binding force between the atoms. The crystal atom bonding will be stronger if one material has higher Debye temperature because Debye temperature corresponds to the maximum frequency of the lattice vibration of the material, which reflects the strongest bonding in the crystal. If the binding force between the atoms was lower, the atomic amplitude would increase the activity of the materials. Our calculations were based on the physical model used for the Debye characteristic temperature calculation through the X-ray diffraction data of corresponding materials reported by Lu.³³ The Debye characteristic temperature of 10 nm and 100 nm nanodiamond were 304 K and 646 K (shown in Fig. S5, Tables S1 and S2, ESI†), whereas that of graphite was 1860 K. Therefore, 10 nm nanodiamond exhibited higher activity than 100 nm nanodiamond and graphite, resulting in the decrease of the reaction temperature of ZnO fabrication. In this case, using nanodiamond as reductant can reduce the reaction temperature to 600 °C, which allows a greater range of substrates to be used in our system, such as Si, SiO₂, and FTO. ZnO nanostructures were grown on these substrates at 600 °C with 10 nm diamond as reductant. The SEM images of obtained ZnO nanostructures are shown in Fig. S6, ESI.†

Lastly, a UV sensor based on the obtained ZnO nanowires was fabricated. The ultrafast response performance was investigated using a UV laser with 375 nm wavelength and 11.2 μW mm⁻² power density under 2 V bias, as shown in Fig. 4. The high sensitivity with an ultrafast rising response time of about 20 ms was obtained. This result indicated that the performance was similar to samples made with the high temperature method.³⁵

Fig. 4 The demo UV detector based on obtained ZnO nanowires.

4. Conclusions

In summary, a well aligned high quality crystalline ZnO nanowire array was fabricated via a lower temperature CVD method using nanodiamond as the reductant. The growth temperature can be greatly decreased to 600 °C, about 40% lower than in common CVD methods. The reason for this was discussed by analyzing the effect of nanodiamond size on nanothermodynamics. This method of ZnO nanowire grown at a lower temperature will offer a new way to grow other nanostructure materials using much smaller carbon as reductant.

Acknowledgements

This work was supported by the program for Chongqing Funds for Distinguished Young Scientists (No. CSTC2013JCYJJQ 50001), the Major Program for Application & Development of Chongqing (No. CSTC2013YYKFC50008), West Light Foundation of The Chinese Academy of Sciences for W. LU 2015, Scientific Research Foundation for the Returned Overseas Chinese Scholars for W. LU, State Education Ministry 2015 and National Natural Science Foundation of China (Grant No. 51402290 and 61605207).

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Footnotes

† Electronic supplementary information (ESI) available: Additional SEM images of obtained ZnO nanowires in 960 °C reaction with different reactants, the nitrogen adsorption–desorption isotherms of different allotropes of carbon, calculation of Debye characteristic temperature of 10 nm and 100 nm nanodiamond. See DOI: 10.1039/c6ra12398h

‡ Contributed equally.

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