etching route to controllable fabrication of zinc sulfide nanotube arrays for humidity sensing

Sensors and Actuators B 165 (2012) 62–67

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An inward replacement/etching route to controllable fabrication of zinc sulfide nanotube arrays for humidity sensing Weixin Zhang ∗ , Cheng Feng, Zeheng Yang School of Chemical Engineering, Hefei University of Technology and Anhui Key Laboratory of Controllable Chemical Reaction & Material Chemical Engineering, Hefei, Anhui 230009, China

a r t i c l e

i n f o

Article history: Received 26 September 2011 Received in revised form 15 January 2012 Accepted 7 February 2012 Available online 16 February 2012 Keywords: ZnS Nanotube arrays Photoluminescence Humidity sensing

a b s t r a c t Well-aligned arrays of polycrystalline ZnS nanotubes with close-tips have been successfully prepared by using ZnO nanorod arrays grown on zinc substrate as sacrificial precursors. The method is based on chemical conversion and inward etching of the ZnO sacrificial precursors. Field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) images show that the highly ordered ZnS nanotube arrays are 300 nm in average diameter, 6–8 ␮m in length and about 60 nm in wall thickness. A room temperature photoluminescence-type gas sensing device based on the nanostructured arrays has been established to investigate their humidity sensing properties. ZnS nanotube array-based sensor presents higher response and quicker response/recovery than the intermediate ZnO/ZnS nanorod arrays and the precursor ZnO nanorod arrays, respectively. Moreover, the ZnS nanotube array-based sensor exhibits good linearity in response to relative humidity (RH) and reliable reproducibility in a wide range of RH at room temperature. Compared with powder-form nanomaterials, the as-prepared nanotube arrays as the humidity sensing materials can provide much more open surfaces between neighboring nanotubes and inner surfaces inside the nanotubes and allow for easy diffusion and efficient transportation of sensed gas. As a prototype sensor, the nanotube arrays can avoid tedious process of fabricating a sensor from powder and may hold great potential applications in humidity sensing. © 2012 Elsevier B.V. All rights reserved.

1. Introduction It is becoming increasingly important to monitor and control relative humidity (RH) in the atmosphere for human health and comfort as well as in many industrial processes, such as in the quality control of manufacturing processes and products such as foodstuffs, paper, textiles and electronic devices [1,2]. Thus, the development of humidity sensors for precise and inexpensive control of water vapor concentration has been paid much attention [3]. As an important II–VI group semiconductor, zinc sulfide (ZnS) has attracted a great deal of interest due to their optical and optoelectronic properties during the past decades [4,5]. It has been reported that well-aligned one-dimensional (1D) arrayed semiconductor nanostructures not only have an enhanced collective response to external stimuli, but also build synergetic multifunctionalities into an integral system of a device [6–8]. In recent years, much effort has been made in the fabrication of well-aligned ZnS nanotube arrays because the appearance of inner surface increases the surface-to-volume ratio significantly could influence their optical and optoelectronic properties greatly [9–13]. For instances,

∗ Corresponding author. Tel.: +86 551 2901450; fax: +86 551 2901450. E-mail address: [email protected] (W. Zhang). 0925-4005/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2012.02.013

polycrystalline ZnS nanotube arrays with tens of micrometers in length have been synthesized within the channels of the porous anodic alumina (PAA) membranes by metal organic chemical vapor deposition (MOCVD) method at 400 ◦ C [9,10]. Comparatively, a solution-based approach has the advantages of mild synthetic conditions and simple manipulation. Qian and co-workers reported that polycrystalline ZnS nanotube arrays with a diameter of 1 ␮m were prepared through sulfuration conversion from ZnO microcolumn arrays on quartz substrate based on a hydrothermal method in thioacetamine (TAA) solution at 130 ◦ C for 48 h [11]. Shi and co-workers reported a solvothermal route for the synthesis of single-crystalline ZnS nanotube arrays with an average diameter of 200 nm and length of up to 3–5 ␮m on zinc foil in sulfur-dissolved hydrazine hydrate solution at 140 ◦ C for 8 h [12]. Xue and coworkers reported a thioglycolic acid-assisted hydrothermal route at 130–180 ◦ C for 4–12 h to first synthesize ZnO/ZnS nanorod arrays by using self-made ZnO nanorod arrays on zinc substrate as the templates and then change them into polycrystalline ZnS nanotubes with open tips of about 400 nm in diameter and 4 ␮m in length by removal of ZnO core with KOH [13]. It is still a challenge to fabricate ZnS nanotube arrays in an organic additive-free and aqueous solution-based approach at lower temperature and to investigate their other properties in addition to optical and optoelectronic properties.

W. Zhang et al. / Sensors and Actuators B 165 (2012) 62–67

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2.3. Characterization The X-ray diffraction (XRD) patterns were recorded on a Japan Riguku D/max-␥B X-ray diffractometer with Cu K␣ radia˚ operated at 40 kV and 80 mA. Field-emission tion ( = 1.54178 A), scanning electron microscopy (FESEM) was performed on a fieldemission microscope (JEOL JSM-6700F) at an acceleration voltage of 5 kV. Transmission electron microscopic (TEM) images were taken with a transmission electron microscope (Hitachi H-800)

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The as-prepared ZnO nanorod arrays grown on zinc foil were used as the precursors to prepare ZnS nanotube arrays. In a typical synthesis, 25.0 mL of Na2 S solution (1.0 M) was put into a 60 mL capped glass bottle. The zinc foil with ZnO nanorod arrays grown on it was immersed into the solution and the capped glass bottle was placed in a water bath at 60 ◦ C for 6 h. The ZnO nanorod arrays were changed into ZnO/ZnS nanorod arrays on the zinc foil and then taken out of the solution and rinsed with ethanol and distilled water for several times and put into 30 mL of ammonia solution (12.5 wt.%) for 5 h. The ZnO/ZnS nanorod arrays on the zinc foil were finally converted into ZnS nanotube arrays, which were then taken out of the solution and rinsed with distilled water and ethanol, then dried in air for characterization.

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2.2. Synthesis of ZnS nanotube arrays

The composition and phase purity of the products were examined by X-ray diffraction. The XRD patterns (Fig. 1) show the formation process of the ZnS nanotube arrays. Fig. 1a shows the XRD pattern of the ZnO nanorod arrays grown on the zinc substrate by a hydrothermal method, which has been used as the precursor for preparing ZnS nanotube arrays. Except for the peaks marked with * from the zinc substrate, all the other peaks can be well indexed to those of hexagonal wurtzite ZnO with lattice constants a = 0.3250 nm and c = 0.5207 nm, which are consistent with the reported data (PDF file No. 36-1451). Fig. 1b shows the XRD pattern of the ZnO/ZnS nanorod arrays as the intermediate when ZnO nanorod arrays react with Na2 S aqueous solution at 60 ◦ C for 6 h. All the diffraction peaks can be well indexed to cubic ZnS (PDF file No. 65-0309), except that the peaks marked with◦ from hexagonal wurtzite ZnO (PDF file No. 36-1451) and the peaks marked with * from the zinc substrate. Fig. 1c shows the XRD pattern of ZnS nanotube arrays as the final product when the ZnO/ZnS nanorod arrays have been immersed in the ammonia solution (12.5 wt.%) for 5 h. 002

The synthesis of ZnO nanorod arrays grown on a 10 mm × 10 mm × 0.25 mm zinc foil was referred to our previous report [19].

3. Results and discussion

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2.1. Synthesis of ZnO nanorod arrays

The humidity sensing measurements were referred to our previous report [20]. The relative humidity response was tested by an adapted photoluminescence-type setup, which comprised a gas flow control system, a colorimetric cuvette and a Hitachi F4500 fluorescence spectrophotometer at room temperature (18 ◦ C; information: ex = 300 nm, Exslit = 5 nm, Emslit = 10 nm). The experimental setup for the humidity sensing measurements was shown in supporting information (Fig. S1). The sensor was fabricated by putting the ZnS nanotube arrays on zinc substrate in the colorimetric cuvette directly, operated with nitrogen gas as a carrier gas for PL measurements. Measurements of the sensor’s response to different relative humidities were performed by introducing controlled amounts of water vapor in nitrogen into the system. After PL intensity was not changing, the analyte (water vapor) was subsequently blown off by purging the system with pure nitrogen gas. Relative humidity was controlled by adjusting the flow rate of the nitrogen gas through the two gas flow controllers. The total flow rate was maintained at 1000 mL min−1 . The relative humidity was calibrated by a commercial humidity meter.

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All reagents were analytical grade, purchased from Shanghai Chemical Reagents Company and used without further purification.

2.4. Humidity sensing measurements

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performed at an accelerating voltage of 200 kV. The photoluminescence (PL) measurements were recorded on a Hitachi F-4500 fluorescence spectrophotometer at room temperature.

Intensity (a. u.)

In our previous work, well-aligned arrays of single-walled or double-walled Cu7 S4 nanotubes and Cu2−x Se nanotube arrays have been respectively prepared by an chemical inward replacement/etching method based on Cu(OH)2 nanorod arrays grown on copper substrate [14,15]. Herein, we demonstrate that the method can be extended to the fabrication of ZnS nanotube arrays. Ordered polycrystalline ZnS nanotube arrays with about 300 nm in average diameter and 6–8 ␮m in length have been successfully prepared with ZnO nanorod arrays grown on zinc substrate as precursors. It has been reported that ZnO-based humidity sensors with different morphologies such as nanotetrapods [16], nanowires [17], and nanorods [18] have been prepared, which present good humidity sensing properties. ZnS has a larger bandgap of about 3.7 eV than ZnO (∼3.4 eV) and thus ZnS is more suitable for visible-blind ultraviolet (UV)-light based devices such as sensors and photodetectors [4]. Herein, a room temperature photoluminescence-type humidity sensing device based on ZnS nanotube arrays has been established to investigate their humidity sensing properties. The results indicate that ZnS nanotube arrays exhibit much better humidity sensing performance than the intermediate ZnO/ZnS nanorod arrays and the precursor ZnO nanorod arrays. Compared with nanomaterial powder as the humidity sensing material, nanotube arrays can provide much more surfaces for easy diffusion and transportation of sensed gas. Each nanotube has its own contact with the substrate at the bottom, which can ensure every nanotube participating in the sensing process. Moreover, the nanotube arrays can be directly used as a prototype sensor and avoid tedious process of fabricating a sensor from powder.

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Fig. 1. XRD patterns of the samples: (a) the precursor ZnO nanorod arrays; (b) the intermediate ZnO/ZnS nanorod arrays; (c) the final product ZnS nanotube arrays on zinc substrate.

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Fig. 2. (a, b) FESEM images of the precursor ZnO nanorod arrays: (a) top view; (b) cross-section view. (c) FESEM images of the intermediate ZnO/ZnS nanorod arrays. Inset: a high magnification FESEM image of a broken ZnO/ZnS nanorod tip. (d) High magnification FESEM image of the ZnO/ZnS nanorod arrays. (e) TEM image of an individual ZnO/ZnS nanorod.

Other than the peaks marked with * from the zinc substrate, all the other peaks can be indexed to cubic ZnS (PDF file No. 65-0309) with lattice constant a = 0.5400 nm. The field-emission scanning electron microscopy and transmission electron microscopy are used to observe the morphologies and structures of the as-prepared products. The top and cross-section views of the precursor ZnO nanorod arrays grown on zinc substrate in Fig. 2a and b show that ZnO nanorods with diameters of about 200–500 nm and lengths of 6–8 ␮m grow on the zinc substrate smoothly and compactly and almost align perpendicular to the zinc substrate. Fig. 2c shows the top view of the intermediate ZnO/ZnS nanorod arrays, which have kept the precursor’s morphology as well. But the surfaces of ZnO/ZnS nanorods become not so smooth compared with those of ZnO nanorods, which have been further confirmed in high-magnification image in Fig. 2d. Interestingly, inset of Fig. 2c reveals that the inner core of ZnO is enclosed by an outer wall of ZnS from a broken ZnO/ZnS nanorod and the ZnS wall seems to be composed of nanoparticles, which make the diameters of the ZnO/ZnS nanorods bigger. An energy-dispersive X-ray (EDX) spectrum of the nanorod arrays (Fig. S2a) exhibits the presence of Zn, O, and S elements, which confirms the composition of ZnO/ZnS nanorods. Fig. 2e presents a typical TEM image of an individual ZnO/ZnS nanorod scraped off the zinc substrate. The edge surfaces show lighter contrast, suggesting that ZnS is covered on the surface of ZnO nanorod to form a continuous coating layer. The ZnO/ZnS nanorod has a diameter of about 300 nm and wall thickness of about 60 nm. Shown in Fig. 3 are FESEM and TEM images of the final product ZnS nanotube arrays on zinc substrate. Fig. 3a shows a top view of the ZnS nanotube arrays with high order and compact similar to those of the ZnO nanorod array precursor. The side view of the ZnS nanostructures (Fig. 3b) shows that the thickness of the arrays is about 6–8 ␮m. A higher magnification top view image (Fig. 3c) of the ZnS nanotube arrays reveals that the diameters of the nanotubes are about 250–550 nm, which is a little bigger than those of

ZnO nanorod arrays. To examine that these tip-closed nanostructures have hollow interiors, the ZnS arrays are scraped slightly to cut off some of the close tips. Fig. 3d shows the image of some broken structures of the ZnS arrays. It can be seen that these tip-closed nanostructures have hollow interiors. An EDX spectrum (Fig. S2b) of the ZnS nanotube arrays reveals that the nanotubes mainly consist of elements of Zn and S, which confirms the composition of ZnS nanotube arrays. The morphologies of the nanotubes are further investigated by TEM. The TEM image of an individual ZnS nanotube is shown in Fig. 3e. Across the nanotube, the intensity profile shows a clear variation, and the edge surfaces show darker contrast, suggesting a hollow structure. The inset corresponding selected area electron diffraction (SAED) pattern of ZnS nanotube shows three diffraction rings, which can be characterized as (1 1 1), (2 2 0) and (3 1 1) planes of cubic ZnS. The strategy for the fabrication of ZnS nanotube arrays is illustrated in Scheme 1. ZnS has a relatively smaller Ksp value (2.5 × 10−22 ) compared with that of ZnO (6.8 × 10−17 ). This implies that the ZnO nanorods can act as both reactants to synthesize more stable chalcogenides and templates to obtain structures with a hollow interior morphology. When the ZnO nanorod arrays are immersed in the Na2 S solution, ZnO reacts with Na2 S and a thin ZnS layer can be formed on the surface of ZnO nanorods and ZnO/ZnS core/sheath nanorod arrays are formed. After the unreacted ZnO nanorod cores are dissolved in ammonia solution, ZnS nanotube arrays can be obtained. Cheng and coworkers have reported humidity sensing property of amorphous Al2 O3 nanotubes and demonstrated that tube-like nanostructures not only increase the sensing area and surface activity, but also promote the dissociation of water absorbed onto nanotube walls [21]. Moreover, Hummer and coworkers confirm that carbon nanotubes can be used as the simplest manifestation of a hydrophobic channel, which also provides an effective channel for vapor transportation [22]. Thus, prompted by the coarse surfaces and hollow interiors of the ZnS nanotube arrays, we expect

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Fig. 3. (a)–(d) FESEM images of the final product ZnS nanotube arrays grown on zinc substrate: (a) top view; (b) side view; (c) high-magnification FESEM image; (d) top view of some broken ZnS nanotubes. (e) TEM image of an individual ZnS nanotube. Inset of (e): SAED pattern of the ZnS nanotube.

that the ZnS nanotube arrays have a good performance in gas sensing. Photoluminescence (PL) spectra of as-prepared ZnS nanotube arrays are measured with an excitation wavelength of 300 nm at room temperature. Two strong emission peaks centered at 420 nm and 470 nm in the spectra can be observed, which are shown in Fig. S3. Based on the PL properties, a PL-type humidity sensor constructed of ZnS nanotube arrays have been fabricated to explore their humidity sensing properties. For comparison, ZnO nanorod arrays and ZnO/ZnS nanorod arrays on zinc substrate have also been used to fabricate other two PL-type humidity sensors. Here, the response is defined as IH2 O /IN2 , where IH2 O and IN2 denote the PL intensity of the samples exposed to RH and pure N2 , respectively [23]. The response and recovery times are, respectively, defined as the time reaching 90% and 10% of the maximum PL intensity [24,25]. Fig. 4 presents PL intensity profile of the ZnS nanotube arraybased sensor and ZnO/ZnS nanorod array-based sensor as the surrounding gas was switched between 65% and 0% RH at room temperature, with the excitation and emission wavelengths set at 300 and 470 nm, respectively. When each of the sensors is exposed to 65% RH in reference to dry N2 , the PL intensity promptly increases and then gradually reaches a relatively stable value. The PL intensity is fully reversible to its original value in a very short time after switching to dry N2 again. One can see that the PL intensity changes of each sensor are fully reversible. As is shown in Fig. 4, the response

ZnO nanorod arrays

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of the ZnS nanotube array-based sensor to 65% RH is about 1.125, whereas the ZnO/ZnS nanorod array-based sensor is about 1.049. The ZnS nanotube array-based sensor presents a higher response than the ZnO/ZnS nanorod arrays. Moreover, the response and recovery times of the ZnS nanotube array-based sensor to 65% RH are about 2.2 and 3.2 min, respectively. Fig. 4b shows that the response and recovery times of the ZnO/ZnS nanorod array-based sensor to 65% RH are about 2.4 and 3.3 min. The ZnS nanotube arraybased sensor exhibits quicker response/recovery than the ZnO/ZnS nanorod arrays. For comparison (Fig. S4), it takes much longer time for ZnO nanorod array-based sensor to increase to its stable value exposed to 65% RH, which is about 14.5 min. The ZnO nanorod array-based sensor presents a very low response to humidity at room temperature. The magnitude of PL enhancement is recorded by measurements of photoluminescence intensity in time when various amounts of analyte are introduced into and subsequently removed from the system. Shown in Fig. 5a, the ZnS nanotube array-based sensor can operate in a broad range of RH (0–65%) and the PL intensity changes proportionally with the RH change. Fig. 5b presents the response change of the ZnS nanotube array-based sensor with RH. Clearly, there is a linear relationship between the response and RH of the sensed gas. The correlation coefficient of the plot, R2 , is estimated to be 0.9951 by the least squares method, which indicates

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Scheme 1. Schematic illustration for the formation of ZnS nanotube arrays on zinc substrate.

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Fig. 4. PL intensity profile of the sensors based on: (a) ZnS nanotube array-based sensor and (b) ZnO/ZnS nanorod array-based sensor as the surrounding gas was switched between 65% and 0% RH at room temperature, with the excitation and emission wavelengths set at 300 and 470 nm, respectively.

that the ZnS nanotube array-based sensor shows excellent linearity between the response and RH. The ZnS nanotube array-based sensor may hold great potential in optical humidity sensing. Considering that organic vapors contained in the analyte carrier stream may influence the behavior of sensors, comparison tests have been conducted to evaluate their response to ethanol and acetone vapors in the absence of humidity under the same conditions by only replacing the distilled water bubbler with bubblers of ethanol and acetone, respectively. As presented in Fig. S5, very little PL responses could be detected when ethanol and acetone vapors were introduced into the gas sensing system. The results indicate that the ZnS nanotube array-based sensor shows a high response to humidity and the interference of organic vapors such as ethanol and acetone to the sensor can almost be neglected. The enhancement of PL intensity of the ZnS nanotube arrays exposed to nitrogen containing water vapor can be explained by an electron transfer mechanism as follows [26,27]. Li and coworkers have reported that ZnS is a weak p-type semiconductor grown under S-rich condition [28], hence holes are the main carriers. Thus H2 O molecules can act as electron donors to ZnS. Photo-excited ZnS nanotube arrays promote the formation of excitons. When the ZnS nanotube arrays are exposed to water vapor, oxygen atoms from the dipolar H2 O molecules will be preferentially adsorbed on the surfaces of ZnS nanotubes by donating their lone-pair electrons, which increases the amount of valence electrons in ZnS. The increased amount of valence electrons in ZnS attracts the holes, thus reduces the tendency of nonradiative transitions, and consequently results in the PL intensity enhancement [29]. When the tendency of radiative and nonradiative transitions keeps balance, the PL intensity reaches a relatively stable value. When the water vapor is switched

to pure nitrogen, H2 O molecules adsorbed on the surfaces of ZnS nanotubes desorb from ZnS nanotubes. The amount of valence electrons attracting the holes in ZnS decreases, thus the tendency of nonradiative transitions increases, and consequently results in the PL intensity quenchment. When all the H2 O molecules adsorbed on the surfaces of ZnS nanotubes desorb from ZnS nantubes, the PL intensity goes down to its original value. Different from the sensor constructed of ZnO/ZnS nanorod arrays, ZnS nanotube arrays can provide more efficient binding sites for gas adsorption. The walls of the ZnS nanotube arrays are composed of many small nanoparticles with coarse surfaces, which can be confirmed by the FESEM observation in Fig. 3d and the diffraction rings of ZnS nanotube shown in the SAED pattern (inset in Fig. 3e). The cavities resulted from the grain boundaries make the adsorbates penetrate the walls and adsorbed on the inside surfaces. Thus ZnS nanotubes with higher surface area can provide more binding sites for H2 O molecules combined with photogenerated holes and exhibit higher response to the humidity than the intermediate ZnO/ZnS. Similarly, our previous report [30] revealed that the double-walled Cu7 S4 nanobox sensor exhibited enhanced performances in ammonia gas sensing compared with the single-walled one. Compared with the coarse surfaces of ZnO/ZnS nanorods and ZnS nanotubes, the surfaces of ZnO nanorods are much smoother, and the surface area is also smaller, which cannot provide too much efficient sites for gas adsorption. Besides, as an n-type semiconductor, ZnO could not provide holes for oxygen atoms from the dipolar H2 O molecules. The ZnS nanotube array-based sensor presents quicker response/recovery time and higher response than both ZnO/ZnS nanorod array and ZnO nanorod array-based sensors.

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4. Conclusions In summary, ordered and tip-closed ZnS nanotube arrays have been synthesized on zinc substrate in large-scale with ZnO nanorod arrays as sacrificial precursors by an inward replacement/etching method under mild reaction conditions. The ZnS nanotubes are 300 nm in average diameter, 6–8 ␮m in length and about 60 nm in wall thickness. Based on their PL properties, room temperature PL-type humidity sensors based on ZnS nanotube arrays, the intermediate ZnO/ZnS nanorod arrays and the precursor ZnO nanorod arrays on zinc substrate have been respectively fabricated to explore their humidity sensing performances. The results indicate that ZnS nanotube array-based sensor exhibits higher response and quicker response/recovery than the ZnO/ZnS nanorod arrays and ZnO nanorod arrays. The good linearity in response, along with long lifetime, good reproducibility and no need for heat regeneration of the ZnS nanotube array-based sensor has been demonstrated. Compared with powder-form nanomaterials, the as-prepared nanotube arrays as the humidity sensing materials can provide much more open surfaces between neighboring nanotubes and inner surfaces inside the nanotubes and allow for easy diffusion and transportation of sensed gas. Since each nanotube has direct contact with the substrate, the nanotube arrays can ensure every nanotube participating in the sensing process efficiently. Furthermore, the nanotube arrays can be directly used as a prototype sensor and avoid tedious process of fabricating a sensor from powder. This kind of PL-type sensor based on the highly ordered ZnS nanotube arrays may hold great potential applications in humidity sensing. Acknowledgments This work has been supported by the National Natural Science Foundation of China (NSFC Grants 20871038, 20976033 and 21176054), the Fundamental Research Funds for the Central Universities (2010HGZY0012) and the Education Department of Anhui Provincial Government (TD200702). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.snb.2012.02.013. References [1] S.L. Yang, J.M. Wu, ZrO2 –TiO2 ceramic humidity sensors, J. Mater. Sci. 26 (1991) 631–636. [2] P.M. Faia, C.S. Furtado, A.J. Ferreira, Humidity sensing properties of a thickfilm titania prepared by a slow spinning process, Sens. Actuators B 101 (2004) 183–190. [3] J.M. Costa-Fernández, A. Sanz-Medel, Air moisture sensing materials based on the room temperature phosphorescence quenching of immobilized mercurochrome, Anal. Chim. Acta 407 (2000) 61–69. [4] X.S. Fang, T.Y. Zhai, U.K. Gautam, L. Li, L.M. Wu, Y. Bando, D. Golberg, ZnS nanostructures: from synthesis to applications, Prog. Mater. Sci. 56 (2011) 175–287. [5] X.S. Fang, L.M. Wu, L.F. Hu, ZnS nanostructure arrays: a developing material star, Adv. Mater. 23 (2011) 585–598. [6] W.X. Zhang, S.H. Yang, In situ fabrication of inorganic nanowire arrays grown from and aligned on metal substrates, Acc. Chem. Res. 42 (2009) 1617–1627. [7] M.H. Huang, S. Mao, H. Feick, H.Q. Yan, Y.Y. Wu, H. Kind, E. Weber, R. Russo, P.D. Yang, Room-temperature ultraviolet nanowire nanolasers, Science 292 (2001) 1897–1899. [8] Z.L. Wang, J.H. Song, Piezoelectric nanogenerators based on zinc oxide nanowire arrays, Science 312 (2006) 242–246. [9] X.P. Shen, M. Han, J.M. Hong, Z.L. Xue, Z. Xu, Template-based CVD synthesis of ZnS nanotube arrays, Chem. Vap. Deposition 11 (2005) 250–253. [10] T.Y. Zhai, Z.J. Gu, Y. Ma, W.S. Yang, L.Y. Zhao, J.N. Yao, Synthesis of ordered ZnS nanotubes by MOCVD-template method, Mater. Chem. Phys. 100 (2006) 281–284. [11] Z. Wang, X.F. Qian, Y. Li, J. Yin, Z.K. Zhu, Large-scale synthesis of tube-like ZnS and cable-like ZnS–ZnO arrays: preparation through the sulfuration conversion from ZnO arrays via a simple chemical solution route, J. Solid. State. Chem. 178 (2005) 1589–1594.

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Biographies Weixin Zhang studied chemistry and chemical engineering in Hefei University of Technology and received a B.S. in 1990 and a M.S. in 1993. She received her Ph.D. in Inorganic Chemistry in 2000 from the University of Science and Technology of China. From May 2001 to December 2002, she worked as a visiting scholar in Prof. Shihe Yang’s group at the Hong Kong University of Science and Technology. From November 2009 to April 2010, she visited Prof. Stephen Mann’s group at Bristol University. She has been a faculty member in Hefei University of Technology since 1993 and became a full professor in 2000. Her research interests include controlled synthesis of ordered nanostructured arrays and advanced inorganic functional materials and exploration of their potential applications in energy conversion, environmental protection, and micro/nanodevices. Cheng Feng studied materials chemistry and received his B.S. degree in 2009 from Anqing Normal College. He is currently studying chemical technology for M.S. degree at Hefei University of Technology. His current research interests include preparing semiconductor nanomaterials and exploring their applications in gas sensors and lithium-ion batteries. Zeheng Yang studied chemical engineering in Hefei University of Technology and received a B.S. in 1982 and a M.S. in 1991. He received his Ph.D. in inorganic chemistry in 2005 from the University of Science and Technology of China. From January 2008 to January 2009, he worked as a visiting scholar in Lab of Sustainable Technology with A/Prof. Andrew Harris at the University of Sydney, Australia. He has been a faculty member in Hefei University of Technology since 2000 and became a full professor in 2007. His research interests include preparation of advanced inorganic functional materials and exploration of their applications in sensors, catalysts, and lithium-ion batteries.