Influence of water content in mixed solvent on surface morphology, wettability, and photoconductivity of ZnO thin films
© Zhao et al.; licensee Springer. 2014
Received: 23 July 2014
Accepted: 7 September 2014
Published: 11 September 2014
ZnO thin films have been synthesized by means of a simple hydrothermal method with different solvents. The effect of deionized water content in the mixed solvents on the surface morphology, crystal structure, and optical property has been investigated by scanning electron microscopy, X-ray diffraction, and UV-Vis spectrophotometer. A large number of compact and well-aligned hexagonal ZnO nanorods and the maximal texture coefficient have been observed in the thin film, which is grown in the mixed solvent with x = 40%. A lot of sparse, diagonal, and pointed nanorods can be seen in the ZnO thin film, which is grown in the 40-mL DI water solution. The optical band gap decreases firstly and then increases with the increase of x. Reversible wettability of ZnO thin films were studied by home-made water contact angle apparatus. Reversible transition between hydrophobicity and hydrophilicity may be attributed to the change of surface chemical composition, surface roughness and the proportion of nonpolar planes on the surface of ZnO thin films. Photocurrent response of ZnO thin films grown at different solvents were measured in air. The response duration of the thin film, which is grown in the solvent with x = 40%, exhibits a fast growth in the beginning but cannot approach the saturate current value within 100 s. The theoretical mechanism for the slower growth or decay duration of the photocurrent has been discussed in detail.
ZnO have received increasing attention due to its wide band gap (3.37 eV) and large excitonic binding energy (60 meV), which is promising for fabricating nanoscale electronic, optical, optoelectronic, electrochemical, and electromechanical devices [1, 2]. Recently, surface wettability and photocurrent property of low-dimensional ZnO nanostructures have been studied by some research group [3–5]. A variety of methods have been employed to fabricate ZnO thin films, such as molecular beam epitaxy (MBE) , chemical vapor deposition (CVD) , sol-gel process , spray pyrolysis , magnetron sputtering , and hydrothermal synthesis [11, 12]. Among all these methods, the hydrothermal synthesis may be the most simple, low-cost, and effective way to control the surface morphology of ZnO at relatively low temperatures, while exempted from further annealing. For the hydrothermal synthesis, the solvents have a considerable influence on the final nanostructure size and morphology of the as-prepared ZnO thin films. So far, most of the studies on ZnO prepared by this method mainly employed single liquid (such as water, methanol, ethanol, and acetone) as solvent. However, studies on the surface morphology and photoelectric properties of ZnO by using two kind of liquid as a mixed solvent are still inadequate. Therefore, it is necessary to study the relationship between the ratio of solvents and surface morphology and properties of ZnO.
In this letter, ZnO thin films were synthesized using the mixed solvents of deionized (DI) water and ethylene glycol monomethyl ether by hydrothermal method. The effects of the DI water content in the mixed solvents on surface morphology, optical property, wettability, and photocurrent property were explored.
All chemicals were of analytical reagent grade and used as received without further purification. Polished silicon wafers (15 × 15 mm2), which were cleaned successively in acetone and DI water for 10 min in anultrasonic bath, were used as substrates. ZnO seed layer was prepared by sol-gel process. Ethylene glycol monomethyl ether [C3H8O2] and monoethanol amine (MEA) were used as the solvent and stabilizing agent, respectively. Zinc acetate dihydrate [Zn(CH3COO)2 2H2O] was dissolved in a mixture of ethylene glycol monomethyl ether and MEA at room temperature. The obtained solution was stirred using magnetic stirrer at 60°C for 2 h to obtain clear and homogeneous solution. The ZnO seed layer was prepared by spin-coating technology with a rate of 3,000 rpm for 30 s. After each spin coating, the seed layer was kept at 150°C for 10 min and this procedure was repeated two times. The seed layer was annealed at 600°C for 1 h and then cooled down to RT. Subsequently, ZnO thin films were synthesized by hydrothermal method on the ZnO seed layer. Zinc nitrate hexahydrate (Zn(NO3)2 · 6H2O) and equimolar hexamethylenetetramine (C6H12N4, HMT) dissolved in 40 mL-mixed solvents of DI water and ethylene glycol monomethyl ether (EGME). The concentration of zinc nitrate hexahydrate was 0.06 mol/L. The DI water content x in the mixed solvents was 0%, 40%, 80%, and 100%, respectively. The solution was transferred to a 50-mL Teflon-lined stainless steel autoclave (Machinery Factory of USTC, Hefei, China) and the ZnO seed layer was placed in the bottom of the autoclave. Growth of ZnO thin film was carried out at 115°C for 2 h in an oven. After growth, the as-prepared ZnO thin film was removed from the solution, rinsed with DI water, and dried at 60°C.
Surface morphology of the ZnO thin films were observed by scanning electron microscopy (SEM, SU1510). Microstructure of the ZnO thin films were measured by X-ray diffraction (XRD, MACM18XHF) with CuKα radiation (λ =0.15405 nm). Absorption spectra of the thin films were measured by UV-Vis spectrophotometer. Water contact angles (WCAs) were investigated by home-made WCA apparatus at room temperature. DI water droplet with a volume of 5 μL was used in the wettability test. WCAs were measured at four different positions of a sample, and the average values of the WCAs were reported. A 36-W UV lamp, centered at 254 nm, was used to study the effect of UV irradiation on the wettability of the samples. Light-induced hydrophilicity was evaluated in atmospheric air by irradiating the samples at certain time intervals. After each irradiation time interval, a 5-μL DI water droplet was placed on the irradiated area and the corresponding contact angle was measured. The reverse transition from the hydrophilic to hydrophobic states could be performed via the storage in dark conditions at room temperature. The photocurrent response of the ZnO thin films to UV light was measured at 5-V bias voltages by switching the 365-nm UV light source with an intensity of 1 μW/cm2 using a CHI 650 electrochemical workstation (CH Instruments, Chenhua Co., Shanghai, China) at room temperature in ambient conditions.
Results and discussion
Optical band gap and contact angle reduction rate of ZnO thin films
Contact angle reduction rate
It is known that UV irradiation can generate electron-hole pairs in the ZnO lattice. These electrons and holes can either recombine or move to the surface to react with species adsorbed on the surface . Some of the holes react with lattice oxygen to form surface oxygen vacancies, while the electrons can react with the lattice zinc ions (Zn2+) to form surface Zn+ defective sites . Water and oxygen may compete to dissociatively adsorb on these defective sites. The defective sites are kinetically more favorable for hydroxyl adsorption than oxygen adsorption. As a result, the surface hydrophilicity is improved . It has also been presented that the surface becomes energetically unstable after the hydroxyl adsorption. The oxygen adsorption is thermodynamically favored; thus, oxygen can form stronger bonds to the defective sites than the hydroxyl group. Consequently, the hydroxyl groups adsorbed on the defective sites can be replaced gradually by oxygen atoms when the samples are placed in the dark . Therefore, the surface evolves back to its original state before UV irradiation, and the wetting property is reconverted from hydrophilicity to hydrophobicity.
Wettability transition of the samples suggest that the wetting model switches from the Cassie-Baxter state to the Wenzel state , as the latter is the model predicting the possibility of superhydrophilicity of the thin film with very rough surfaces. This indicates that the surface chemical composition and surface roughness play an important role in the photo-induced process. The former provides a photosensitive surface, which can be used to control the wetting states, while the latter further enhances the wettability. In addition, the photo-induced hydrophilicity may be related to the proportion of nonpolar planes on the surface. On the nonpolar planes, both oxygen and zinc ions are terminated in the same plane. These surface oxygen ions are energetically more reactive than their surrounding atoms, and considered to act as reactive sites for increasing OH species on the surface. Photo-induced contact angle reduction rate of nonpolar plane-oriented ZnO thin films was faster than that of polar plane-oriented ZnO thin films . That is the reason of ZnO thin film grown in 40-mL DI water solution has the largest contact angle reduction rate, while the ZnO thin film grown in the solvent with x = 40% has the minimal contact angle reduction rate. Therefore, the reversible transition between hydrophobic and hydrophilic properties can be controlled by the surface chemical composition, the surface roughness, and the proportion of nonpolar planes on the surface.
In the present work, surface morphology, microstructure, optical absorption spectra, and WCA of ZnO thin films, which are grown in solutions with different DI water contents as solvents, were measured by SEM, XRD, UV-Vis spectrophotometer, and home-made WCA apparatus. The results indicate that the surface morphology, c-axis-preferred orientation, and optical band gap of the thin films are closely related to the DI water content. The maximal WCA before UV irradiation and the largest WCA reduction rate after 120-min UV irradiation have been observed in the thin film with x = 100%. The response duration of the thin film grown in the solvent with x = 40% attributed to oxygen absorption or desorption on the surface and interactions between the oxygen gas and the native defect states of ZnO thin film.
This work was supported by State Key Program for Basic Research of China (2013CB632705), National Natural Science Foundation of China (nos. 11334008, 61290301, 51472003, 51272001, 51002156, 51102072), China Postdoctoral Science Foundation (no. 2012 M520944), Natural Science Foundation of Anhui Higher Education Institution of China (no. KJ2012Z336), Shanghai Postdoctoral Science Foundation (no. 12R21416800), Funds for ‘136’ Talent of Hefei Normal University (no. 2014136KJB03).
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