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Novel growth of CuO-functionalized, branched SnO2 nanowires and their application to H2S sensors
This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2012 J. Phys. D: Appl. Phys. 45 205301 (http://iopscience.iop.org/0022-3727/45/20/205301) View the table of contents for this issue, or go to the journal homepage for more
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IOP PUBLISHING
JOURNAL OF PHYSICS D: APPLIED PHYSICS
J. Phys. D: Appl. Phys. 45 (2012) 205301 (8pp)
doi:10.1088/0022-3727/45/20/205301
Novel growth of CuO-functionalized, branched SnO2 nanowires and their application to H2S sensors Sang Sub Kim1 , Han Gil Na2 , Sun-Woo Choi1 , Dong Sub Kwak2 and Hyoun Woo Kim2 1 2
School of Materials Science and Engineering, Inha University, Incheon 402-751, Republic of Korea Division of Materials Science and Engineering, Hanyang University, Seoul 133-791, Republic of Korea
E-mail:
[email protected]
Received 19 January 2012, in final form 22 March 2012 Published 1 May 2012 Online at stacks.iop.org/JPhysD/45/205301 Abstract A novel growth method for CuO-functionalized, branched tin oxide (SnO2 ) nanowires was developed on the basis of a Cu-triggered tip-growth vapour–liquid–solid (VLS) process during annealing of Cu-coated SnO2 nanowires. The variation in annealing temperature changed the morphology, in which higher temperatures (500 ◦ C) are favourable for the formation of branches. From the observation of tip nanoparticles, we revealed that the growth of branches at 500 and 700 ◦ C was dominated by base-growth and tip-growth VLS processes, respectively. The tip nanoparticles at 700 ◦ C were mainly comprised of a CuO phase. We have demonstrated the potential applicability of the CuO-functionalized, branched SnO2 nanowires to H2 S sensors. CuO functionalization significantly enhanced the response to H2 S. In sharp contrast, it degraded the response to NO2 , suggesting their selective sensing performance to H2 S. (Some figures may appear in colour only in the online journal)
degradation of acid fuchsine [4]. Also, it is suggested that the chiral branched nanowires will provide novel material properties [5]. Since hydrogen sulfide (H2 S) is one of the most toxic and inflammable gases which is commercially used in industries and laboratories, its detection is very important for our safety. In particular, a person intoxicated with H2 S cannot recognize the smell before the dangerous threshold of 50–100 ppm is reached. Therefore, it is crucial to detect H2 S gas when present at a few parts per million [6]. For the detection of H2 S gas, SnO2 loaded with CuO provides the highest sensitivity [7]. Accordingly, a variety of material systems, including CuO–SnO2 bilayers [8], p-CuO/n-SnO2 heterostructures [9], CuO-doped SnO2 films [6], have been studied for enhancing H2 S sensing properties. CuO–SnO2 hybrid materials are likely to have a special sensing mechanism that is totally different from single-component semiconductor gas sensors. The exceptionally high H2 S sensing characteristics observed in CuO–SnO2 hybrid materials have been ascribed to the creation and disruption of p–n junctions built by p-type CuO and n-type SnO2 in the presence and absence of H2 S, respectively [10].
1. Introduction Since the sensing principles of semiconductors to gases are based on the adsorption and desorption of gas molecules, the increase in the surface-to-volume ratio is the most effective means to achieve excellent gas sensing properties. The branched nanowires are comprised of core nanowires and attached branches. Since they not only provide a large surfaceto-volume ratio but also combine different characteristics of core and branches, the branched nanowires will have a variety of industrial applications, including gas sensors. For example, semiconducting tin oxide (SnO2 ) branch/metallic core nanowire complex structures showed excellent ethanol sensing performance with high sensitivity, quick response and recovery time [1]. Apart from gas sensors, dye-synthesized solar cells with branched ZnO nanowires exhibited higher current density and energy conversion efficiency than those with normal ZnO nanowires [2]. The branched CdSe nanowires were employed to investigate the size- and shape-dependent properties of various one-dimensional (1D) materials [3]. Branched CdS nanowires exhibited high photocatalytic activity for 0022-3727/12/205301+08$33.00
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© 2012 IOP Publishing Ltd
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J. Phys. D: Appl. Phys. 45 (2012) 205301
S S Kim et al
Till now, a variety of techniques have been presented for the production of branched nanowires. First of all, branched nanowires were directly synthesized via a solution-based approach [3, 4, 11]. Second, several researchers have used a two-step approach, depositing intermediate nanoparticles or shell layers for subsequent growth of branches. For instance, Zn acetate dehydrate nanoparticles were coated on the as-grown Zn nanowires and subsequently annealed [12]. However, in most cases, catalytic metal shell layers were predeposited on the backbones and branched nanowires were obtained by thermal annealing. In most cases, Au has been used as a catalyst for the branch growth [1, 13, 14]. In this study, a novel growth method for CuOfunctionalized, branched SnO2 nanowires is developed on the basis of a Cu-triggered tip-growth vapour–liquid–solid (VLS) process. For the branch growth, core–shell nanowires were prepared by coating core SnO2 nanowires with Cu-shell layers by means of a sputtering technique. Subsequent thermal annealing generated CuO-related nanoparticles on the nanowire backbones and they not only played a catalytic role in growing branches, but also enhanced the sensing properties with regard to H2 S gas. We investigated the annealing-induced changes in structure and morphology of the composite structures. In addition, the correlation between the structural/morphological properties and the gas sensing characteristics was studied. Figure 1. Schematic outline of the fabrication of the CuO-functionalized, branched SnO2 nanowires. (a) Growth of SnO2 stem nanowires. (b) Deposition of Cu-shell layer. (c) Formation of Cu islands by thermal annealing. (d) Cu-triggered tip-growth VLS process.
2. Experimental As a source material, pure Sn powder (purity: 99.9%) was placed in a ceramic boat, which was inserted into a hightemperature furnace. A similar experimental procedure was previously reported [15]. A gold (Au) (about 3 nm-thick)coated Si plate was used. The temperature of the furnace was set to 900 ◦ C for 10 min and the stem SnO2 nanowires were grown (figure 1(a)). Cu coating on the stem SnO2 nanowires was carried out in a sputter coater with a Cu target (Emitech K575X, Emitech Ltd, Ashford, Kent, UK) [16] (figure 1(b)). At room temperature, the sputtering was carried out for 90 s under Ar plasma (base pressure = 2 × 10−4 Pa). The dc sputtering current was set to 65 mA. Following this, the as-prepared samples were annealed for 30 min at temperatures in the range 300 –700 ◦ C under flowing Ar gas. The schematic outline of the formation of Cu islands and subsequent growth of branches at a temperature of 700 ◦ C is described in figures 1(c) and (d), respectively. Scanning electron micrograph (SEM) images were obtained using a Hitachi S-4200 system. Glancing angle (0.5◦ ) x-ray diffraction (XRD; Philips X’pert MRD) and transmission electron micrography (TEM; Philips CM-200) were carried out for sample characterization. In addition, an energydispersive x-ray spectroscope (EDX) was used to characterize the chemical components of the samples. In the sensing experiments, Ti (∼200 nm) and Au (∼100 nm) were sequentially sputter-deposited on the specimens using an interdigital electrode mask to prepare double layer electrodes. The fabrication process of the sensors similar to the present CuO-functionalized, branched SnO2
nanowire sensors has been described in detail in the previous reports by the authors [17–21]. For measuring the sensing performances, the gas sensor was electrically connected to an electrical measuring system (Keithley 2400) interfaced with a computer. The sensors were placed in a horizontal-type tube furnace. The gas concentration was controlled by changing the mixing ratio of the target gas and dry air through mass flow controllers. The total flow rate was set to 500 sccm to avoid any possible variation in sensing properties. For NO2 and H2 S, 100 ppm NO2 and 200 ppm H2 S gases that were buffered with dry air were, respectively, mixed with dry air again to adjust the targeted gas concentrations. The responses of the fabricated sensors to H2 S were measured at 300 ◦ C. The sensitivity (S) was determined according to the following formula: S = Ra /Rg , where Ra is the initial resistance in the absence of H2 S and Rg is the resistance measured in the presence of H2 S. Meanwhile, for testing the sensing performance of the sensors to oxidizing gases, NO2 was selected. In the case of NO2 , the sensitivity was estimated to be S = Rg /Ra , in which Rg and Ra are the resistances measured with and without NO2 gas, respectively.
3. Results and discussion Figure 2(a) shows an SEM image of the as-synthesized SnO2 -core/Cu-shell nanowires, exhibiting 1D morphology 2
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Figure 2. Low-magnification SEM images of the Cu-sputtered core–shell nanowires (a) before and (b)–(d) after thermal annealing at (b) 300 ◦ C, (c) 500 ◦ C and (d) 700 ◦ C. The upper-right insets show enlarged images.
between the lattice planes are nearly 0.176 nm and 0.334 nm, corresponding to the d2 1 1 and d1 1¯ 0 spacings of tetragonal rutile SnO2 , respectively. Figure 3(f ) is the corresponding selected-area electron diffraction (SAED) pattern, showing a set of single-crystal electron diffraction spots corresponding to the tetragonal SnO2 -core. It clearly shows diffraction spots representing {1 1¯ 0}, {2 1 1} and {3 0 1} lattice planes of SnO2 . On the other hand, figure 3(g) is an SAED pattern of the squared region in figure 3(a), which corresponds to the tipcomprising part of the branch. In addition to the diffraction spots representing {0 0 1}, {2 0 0} and {2 0 1} lattice planes, the ring spots corresponding to the (1 0 1) and (2 0 1) lattice planes of SnO2 can be seen. Furthermore, it is noteworthy that the pattern exhibits the ring spots of (1 0 0), (1 1 1) and (2 0 0) lattice planes of a cubic CuO phase with lattice constants of a = 0.4245 nm (JCPDS card: No 78-0428). Accordingly, it is reasonable to conclude that the tip nanoparticles are comprised of a polycrystalline cubic CuO phase. Figure 4(a) shows an XRD pattern of the as-synthesized SnO2 -core/Cu-shell nanowires, in which all diffraction peaks correspond to the tetragonal rutile SnO2 phase (JCPDS card: No 41-1445). On the other hand, figures 4(b), (c) and (d) show the XRD patterns of the SnO2 -core/Cu-shell nanowires, which were annealed at 300 ◦ C, 500 ◦ C and 700 ◦ C, respectively, revealing the existence of Cux O-related reflections, in addition to the SnO2 -associated XRD peaks. The 300 ◦ C annealed core–shell nanowires exhibited a (1 1 0) reflection peak of the cubic Cu2 O phase with lattice constants of a = 0.4267 nm (JCPDS card: No 78-2076), whereas the 700 ◦ C annealed core– shell nanowires showed the (1 1 1) peak of a cubic CuO phase
with a smooth surface. On the other hand, figures 2(b), (c) and (d) show the SEM images of the SnO2 -core/Cu-shell nanowires, which were annealed at 300 ◦ C, 500 ◦ C and 700 ◦ C, respectively. A close examination by means of enlarged SEM images indicates that surface morphology changes on varying the subsequent annealing temperature. The surface became noticeably rougher after annealing, suggesting that morphological changes occurred in the shell layer. The inset of figure 2(b) indicates that the nanoparticles are formed in the shell region of the nanowires. Also, the insets of figures 2(c) and (d) reveal that the branches sprouted from the stem nanowires. The branches of 500 ◦ C annealed core– shell nanowires do not have nanoparticles at their tips. Instead, some nanoparticles are found at the bottom of the branches (see the arrowhead in the inset of figure 2(c)). On the other hand, the branches of 700 ◦ C annealed core–shell nanowires surely have nanoparticles at their tips (see the arrowhead in the inset of figure 2(d)). In order to further investigate the structure and composition of the 700 ◦ C annealed core–shell nanowires, we have carried out the TEM analysis. Figure 3(a) shows a TEM image which exhibits some branches and figures 3(b) and (c) correspond to the elemental maps of Sn and Cu, respectively. Obviously, the stem and tip parts of the branches reveal the presence of Sn and Cu elements, respectively. Figure 3(d) shows a TEM image of a branch, showing no noticeable structural defects such as dislocations and stacking faults, thereby demonstrating its single-crystalline nature. Figure 3(e) is a lattice-resolved TEM image, taken from the surface region of the branch of figure 3(d). The spacings 3
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Figure 3. (a) TEM image of some branches of the 700 ◦ C annealed nanowires and (b), (c) corresponding SAED patterns of (b) Sn and (c) Cu elements. (d) TEM image, (e) lattice-resolved TEM image and (f ) the corresponding SAED pattern of a branch. (g) SAED pattern of the region corresponding to the squared area in (a).
be expressed as G◦ (kJ mol−1 ) = 292 − 0.051T log T − 0.37T in the temperature range 25–1050 ◦ C [22]. At 300 ◦ C, 500 ◦ C and 700 ◦ C, the free energies are estimated to about −0.6 kJ mol−1 , −108.0 kJ mol−1 and −216.3 kJ mol−1 , respectively. Accordingly, the thermodynamic driving force for Cu2 O oxidation becomes larger at a higher temperature in the range 300–700 ◦ C. It results in the preferred formation of the CuO phase at 700 ◦ C. At an annealing temperature of 500 ◦ C, the CuO phase appeared and the SnO2 branches sprouted from the underlying Cu-related nanoparticles. Accordingly, it is revealed that the nucleation of branches is dependent on the annealing temperature, in which the temperature threshold exists between 300 and 500 ◦ C. We suppose that Sn atoms diffuse into the Cu-related particles at a sufficient temperature, forming Cu–Sn–O liquid alloy particles. Subsequently, the SnO2 nuclei sprout from the alloy droplets, via a base-growth VLS mechanism. On the other hand, at a lower temperature (i.e. 300 ◦ C), the diffusion of Sn atoms is not sufficient, not being able to form the alloy droplets. At 700 ◦ C, the SnO2 nuclei sprout from the alloy droplets, via a tip-growth VLS process.
(JCPDS card: No 78-0428). On the other hand, both the (1 1 0) Cu2 O peak and the (1 1 1) CuO were observed from the XRD spectrum of the 500 ◦ C annealed core–shell nanowires. Based on the results of the SEM (figure 2) and XRD (figure 4) investigations, we propose the associated growth mechanisms comparatively, for the samples annealed at 300, 500 and 700 ◦ C. The schematic of the associated mechanisms is outlined in figure 5. Since no crystalline phase was observed from the as-synthesized core–shell nanowires, it is surmised that the shell layer can be present as an amorphous Cu phase. At a lower annealing temperature of 300 ◦ C, the sputtered Cu-shell layer is likely to break into clusters, which agglomerate to form islands on the surface of the core, stem nanowires, and the islands turn into the cubic Cu2 O phase due to the oxidation process. Although we have used an Ar annealing ambient, the low-vacuum furnace contains a considerable amount of oxygen, oxidizing the Cu islands into ones of the Cu2 O phase. In contrast, at a higher annealing temperature of 700 ◦ C, instead of the Cu2 O phase, the CuO phase was observed. The Gibbs free energy for the reaction 2Cu2 O(s) + O2 (g) → 4CuO(s) can 4
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Figures 7(a) and (b) show the schematics of the H2 S sensing mechanisms without and with the CuO-functionalized branch structure, respectively. For the normal SnO2 nanowires, the sensing of H2 S gas by SnO2 is known to be accomplished by the two mechanisms; first, the chemisorbed oxygen on SnO2 surface reacts with H2 S gas to generate electrons, decreasing the resistivity (H2 S + 3O2− = SO2 + H2 O + 6e− ) [24]. In the case of nanocrystalline or polycrystalline structures, when the surface oxygen is consumed upon exposure to H2 S gas, electrons are donated back and the potential barriers between grains or nanocrystals will be decreased [25]. Second, H2 S gas directly react with SnO2 to generate SnS2 intermetallic compounds, which also decrease the overall resistivity (2H2 S + SnO2 = SnS2 + 2H2 O) [24]. In the CuO-functionalized, branched SnO2 nanowires, the increased surface area by adding the branches will increase the amount of H2 S reaction, further decreasing the resistivity. In this study, the thermal annealing of SnO2 –Cu core–shell nanowires generates a CuO phase, which was revealed by XRD and SAED investigations. The addition of CuO to SnO2 further enhances the sensitivity of H2 S gas (figure 6(b)). One possibility is that CuO reacts with H2 S to form the intermetallic CuS compound, which has much better conductivity than metal oxides, by following the chemical reaction CuO + H2 S = CuS + H2 O and, consequently, decreasing the resistivity of the CuOfunctionalized, branched SnO2 nanowires substantially. On the other hand, upon the removal of the H2 S gas, CuS will be oxidized in air and will change back to CuO reversibly by following the reaction CuS + O2 = CuO + SO2 [10]. Another possibility is that the CuO on the surface enables the spillover of H2 from the dissociated H2 S onto the uncovered SnO2 [26]. In the previous studies, CuO/SnO2 bilayers [8], formation of ultrathin CuO islands [7]/CuO nanoparticles [26] and doping of CuO into the SnO2 thin films [6] improved the sensing capability for H2 S gas. Meanwhile, figure 8 shows the response curves to NO2 gas for the sensors fabricated from normal SnO2 nanowires and CuO-functionalized, branched SnO2 nanowires. Three sensing cycles of the sensors are shown with the introduction of 1 ppm, 10 ppm and 50 ppm NO2 , respectively. For the normal SnO2 nanowire sensor, its resistance increases and decreases, respectively, upon exposure to and upon removal of NO2 . However, for the CuO-functionalized, branched SnO2 nanowires, the variation of resistance by NO2 gas becomes negligible, in comparison with the normal SnO2 nanowires, revealing that CuO functionalization considerably reduces the response to NO2 . The schematics of the NO2 sensing mechanisms without and with the CuO-functionalized branch structure, respectively, are outlined in figures 9(a) and (b). In normal SnO2 nanowires, the introduction of NO2 gas takes out a significant number of electrons, increasing the overall resistivity as follows. The adsorption of NO2 to the SnO2 surface can be accomplished in a variety of states, including + − NO− 2 , NO and NO [27]. These species contribute to the increase in resistance in SnO2 in various manners. First, by the chemisorption reaction; NO2 + e− → NO− 2 [28, 29]. Second, O− can be generated from the adsorbed NO2 species
Figure 4. XRD patterns of the Cu-sputtered core–shell nanowires (a) before and (b)–(d) after thermal annealing at (b) 300 ◦ C, (c) 500 ◦ C and (d) 700 ◦ C.
It is not clear why the growth of branches at 500 ◦ C and 700 ◦ C (annealing temperature) is operated by the base-growth and tip-growth VLS processes, respectively. We propose that the generation of SnO2 nuclei is thermodynamically and/or kinetically favourable on the top and bottom of the alloy droplets, respectively, at annealing temperatures of 500 ◦ C and 700 ◦ C [23]. Further close observation (figures 2(c) and (d)) reveals that the branch became thicker by increasing the annealing temperature. One possibility is that the alloy droplet became larger by means of the higher temperature annealing. The potential applicability of the CuO-functionalized, branched SnO2 nanowire sensors synthesized in this study to H2 S sensors was assessed. Figure 6(a) exhibits response curves to H2 S gas for the sensors fabricated from (1) normal SnO2 nanowires without CuO functionalization and the branch structure and (2) CuO-functionalized, branched SnO2 nanowires. It shows three sensing cycles with the introduction of 1 ppm, 10 ppm and 50 ppm H2 S, respectively. The resistance of the sensors decreases upon exposure to H2 S gas, whereas it increases upon the removal of H2 S gas, thus showing that the sensor responses clearly track the change in the H2 S gas. It is evident that the CuO-functionalized, branched SnO2 nanowire sensor reveals a much improved response to H2 S. Figure 6(b) shows the sensitivities as a function of H2 S concentration, definitely demonstrating that the H2 S gas response is enhanced by the CuO functionalization and the branched structure as well. 5
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Figure 5. Comparative diagrams of the evolution of the Cux O-related nanoparticles and SnO2 branches by means of thermal annealing at temperatures of 300, 500 and 700 ◦ C. − (NO− 2 ) [29] and dissociated O facilitates the filling of oxygen vacancies by the following reaction: O− + VO·· + e− ↔ OO . Third, oxygen adatoms generated from NO− reduce oxygen vacancies (VO·· ), by the following reaction: O + VO·· + 2e− ↔ OO [30]. In addition, NO+ is known to interact with the available Sn sites on the SnO2 surface, extracting electrons from free Sn sites and thus increasing the resistance [31]. All these processes resultantly trap electrons in the conduction band of SnO2 , eventually decreasing the conductivity of the SnO2 nanowires. By CuO functionalization, it is noteworthy that the increased amount of resistance by the introduction of NO2 is not significant, as shown in figure 8. It is known that CuO is intrinsically a p-type semiconductor mainly due to the Cu vacancies [32]. In air atmosphere, O2 gas molecules adsorb on the surface of CuO nanoparticles. During this process, electrons in the valence band of CuO are transferred to the adsorbed O2 gas molecules, consequently increasing the hole concentration in CuO [33]. The electron loss in CuO is more likely to be compensated by the transfer of electrons from the conduction band of the SnO2 branches. As shown in figure 10, the electrons existing in the conduction band of n-SnO2 readily occupy the holes in the valence band of p-CuO taking the energy levels into account. In this way, the SnO2 branches are fully depleted. When NO2 gas is introduced, additional
electron transfer is very limited because the SnO2 branches are already depleted totally by O2 in air. This means the resistance change by the adsorption and desorption of NO2 is marginal, agreeing well with figure 8. The novel method for the growth of the CuOfunctionalized, branched SnO2 nanowires, developed on the basis of a Cu-triggered tip-growth VLS process, is of significance. The CuO–SnO2 hybrid structure was realized in a single process using Cu as a catalyst for the growth of SnO2 branches and the source of CuO nanoparticles that eventually exist at the tips of the SnO2 branches. Moreover, it shows an outstanding selectivity and response to H2 S. According to the specifications provided by the manufacturer (Daeduk Gas Co., Korea), the contents of water vapour in the H2 S, NO2 and air containers were less than 1 ppm. Therefore, it is reasonable not to consider the effect of humidity in view of the negligible amount of water vapour. However, it is certain that the degree of humidity significantly influences the sensing performances of oxide semiconductors [34, 35]. The presence of water vapour usually lowers the sensitivity of oxide semiconductor sensors for the following reasons. Firstly, the reaction between the surface oxygen of oxide semiconductors and the water molecules leads to a decrease in the baseline resistance of the sensor, consequently resulting in a decrease in the sensor response [35]. Secondly, the adsorption of water 6
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Figure 7. Schematics explaining the change in the H2 S sensing behaviour (a) without and (b) with CuO functionalization and the branch structure.
Figure 6. (a) Typical response curves to H2 S gas at 300 ◦ C for the sensors fabricated from normal SnO2 nanowires and CuO-functionalized, branched SnO2 nanowires. (b) Change in sensitivity with varying H2 S concentration, for sensors fabricated from normal SnO2 nanowires and CuO-functionalized, branched SnO2 nanowires.
molecules leads to fewer adsorption sites for targeted gases on the surface of the sensors, thereby causing the decreased sensor response. In this study, the content of water vapour was negligible. It is thus reasonable to think that the effect of water vapour can be negligible in the sensing performances. To clarify the effect of water vapour, a systematic investigation needs to be performed at a later time.
Figure 8. Typical response curves to NO2 gas at 300 ◦ C for the sensors fabricated from normal and CuO-functionalized, branched SnO2 nanowires.
4. Conclusions
metal catalysts with a mixture of Cu2 O and CuO phases were situated at the bottom of the branches. At 700 ◦ C, the metal catalysts in a CuO phase were located at the tip of the branches. We investigated the potential applicability of the 700 ◦ C annealed sample in this study to chemical sensors. By investigating the H2 S sensing properties, we reveal that CuO functionalization and the branch structure considerably improved the sensing properties, presumably due to the generation of CuS compound and/or spillover effects of CuO nanoparticles. However, the NO2 sensing properties were degraded by CuO functionalization, presumably because the
We fabricated core–shell nanowires, in which outer Cu metal layers were deposited on SnO2 nanowires via sputter coating. For functionalization, we annealed the core–shell nanowires, at temperatures in the range 300–700 ◦ C. We investigated the effects of annealing temperature on the morphology and structure of the core–shell nanowires, respectively, by SEM and XRD. Annealing at 300 ◦ C generated Cu2 O-phased nanoparticles on the surface of core SnO2 nanowires. By annealing at 500 and 700 ◦ C, we obtained the SnO2 branches being attached to core SnO2 nanowires. At 500 ◦ C, the 7
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Figure 9. Schematics explaining the change in the NO2 sensing behaviour (a) without and (b) with CuO functionalization and the branch structure.
Figure 10. Energy band structure of p-CuO/n-SnO2 .
CuO-induced depletion region covered a considerable volume of the SnO2 branches.
Acknowledgment This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2011-0009946).
References [1] Wan Q, Huang J, Xie Z, Wang T, Dattoli E N and Lu W 2008 Appl. Phys. Lett. 92 102101
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