Ultraviolet-Light-Assisted Formation of ZnO Nanowires in Ambient Air

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Ultraviolet-Light-Assisted Formation of ZnO Nanowires in Ambient Air: Comparison of Photoresponsive and Photocatalytic Activities in Zinc Hydroxide Jyh Ming Wu* and Yi-Ru Chen Department of Materials Science and Engineering, Feng Chia University, 100 Wenhwa Road, Seatwen, Taichung 40724, Taiwan

bS Supporting Information ABSTRACT: In ambient air, zinc acetylacetonate hydrate precursor was directly decomposed to fabricate large-area ZnO nanowires on a substrate using ultraviolet-light- (λ = 350-380 nm, I = 76 mW cm-2) assisted thermal decomposition processes at 200 °C. The growing process required 5 min. High-resolution transmission electron microscopy images revealed that the ZnO nanowires consisted of fine singlecrystal nanoparticles. The particle size was calculated to be ∼8-9 nm using the Debye-Scherrer equation. The photocatalytic activities of ZnOnonUV (without UV-light assistance) nanowires were found to be superior to those of ZnOUV (with UV-light assistance) nanowires and commercial TiO2 P25 nanoparticles. The oxygen defects (i.e., oxygen vacancies and interstitials) acted as key components for a photodegradation process in the ZnO nanowires. The oxygen defects are attributed to the presence of zinc hydroxide [Zn(OH)2] on the surface of the ZnO nanocrystallites. For the photoresponsive activities, no significant photocurrent-to-dark-current ratio was observed in ZnOnonUV nanowires using UV-light (λ = 365 nm, I = 2.33 mW cm-2), whereas the ratio of ZnOUV nanowires was high, reaching a maximum of 91. As-synthesized ZnOUV nanowires exhibited a relatively good crystallinity and superior photoresponsive properties when compared with ZnOnonUV nanowires based on the characterizations of the materials and sensor properties. Detailed mechanisms of the photoresponsive and photocatalytic properties were investigated.

1. INTRODUCTION Zinc oxide (ZnO) nanostructures have been considered promising materials for applications of (bio)gas sensors,1,2 ultraviolet (UV) photodiodes,3,4 UV lasers,5 and solar cells6 because of their high excitation binding energy of 60 meV with a wide direct band gap at 3.37 eV.7 There is great interest in developing a lowworking-temperature, large-area imprinting process for ZnO nanostructures for commercial applications using a low-cost and high-throughput process. The vapor transfer process is a straightforward method to approach high-crystallinity ZnO nanowires with high aspect ratios. However, high working temperatures and vacuum requirements limit the quality of electronics on alternative substrates and result in high facility investment costs. Many methods such as sol-gel, hydrothermal, and solution processing have been extensively studied, allowing approaches to low-temperature, large-scale, high-crystallinity, catalyst-free growth on various substrates. However, the main flaw of these methods is a long and complex process. Recently, rapid synthesis laser-induced methods have yielded high-crystallinity ZnO r 2011 American Chemical Society

nanowires, such as those developed by Fauteux et al.8 Unalan et al. reported a microwave heating process and hydrothermal synthesis of aligned ZnO nanowires on various substrates.9 This method provides useful information for applied research. However, to date, it is still challenging and of great interest to develop a mild process that includes low working temperature, large area, nontoxicity, short processing time, and low equipment investment for industrial applications. In this work, an ultraviolet-light-assisted (76 mW cm-2, λ = 350-380 nm) thermal decomposition process was employed to fabricate ZnOUV nanowires (nanowires synthesized by ultraviolet-light-assisted thermal decomposition) using ZnC10H14O4 3 H2O as a precursor under ambient air. A thermal decomposition process without UV-light assistance was also employed to synthesize ZnOnonUV nanowires (nanowires synthesized by thermal decomposition) for comparison samples. Received: October 28, 2010 Revised: December 7, 2010 Published: January 11, 2011 2235

dx.doi.org/10.1021/jp110320h | J. Phys. Chem. C 2011, 115, 2235–2243

The Journal of Physical Chemistry C As-synthesized ZnO nanowires were exposed to a UV lamp (2.33 mW cm-2, λ = 365 nm) to investigate their UV sensing properties. The ZnOUV nanowires exhibited a superior UV sensing property in comparison with ZnOnonUV samples. A photocurrent-to-dark-current ratio of ∼91 was achieved in ZnOUV nanowires, whereas no significant sensitivity was obtained in ZnOnonUV nanowires. However, excellent photocatalytic activities were observed in ZnOnonUV nanowires, which were superior to those in ZnOUV nanowires and commercial TiO2 P25 nanoparticles. Cathodoluminescence (CL) spectra revealed that the ZnOnonUV samples exhibited very high concentrations of oxygen defects (i.e., oxygen vacancies and oxygen interstitials) in comparison with ZnOUV nanowires. The defect states are attributed to the presence of zinc hydroxide [Zn(OH)2] layers positioned on the surface of ZnO nanocrystallites. Some reports have revealed that the oxygen-related defects in ZnO nanocrystals enhance photocatalytic activities.10,11 However, the influence of oxygen-related defects on photocatalytic activities is still complicated and requires further study. This work experimentally demonstrates how the lattice defects (i.e., oxygen vacancies or Zn interstitials and oxygen interstitials) affect photocatalytic and photoresponsive activities. As an oxygen vacancy can trap a photoinduced electron and act as a reactive center for a photocatalytic process, the photodegradation ratio of ZnOnonUV samples is therefore higher than that of ZnOUV samples. In contrast, defect states also act as trapping centers for electrons, resulting in poor UV sensing activities during UV irradiation. As-synthesized nanowires can be produced at low temperature and spin-coated with high transparency onto various substrates such as glass or flexible large-area substrates, thus allowing their integration into varying commercial applications of portable electronic devices (i.e., touch panels). A comprehensive mechanism of photoresponsive and photocatalytic properties in assynthesized ZnOUV and ZnOnonUV nanowires synthesized by two different processes is reported.

2. EXPERIMENTAL SECTION The detailed preparation process for the Zn(AcAc)2 3 H2O precursor has been described previously.8,12 As-prepared glass substrates (5 cm 5 cm) were cleaned with ethanol, acetone, and deionized water ultrasonically for ∼15 min. The Zn(AcAc)2 3 H2O precursor was spin-coated onto glass substrates. A UV light with a power intensity of 76 mW cm-2 (λ = 350380 nm) was used to assist in the decomposition of the Zn(AcAc)2 3 H2O slurry to fabricate ZnOUV nanowires using a thermal decomposition process. Another process for fabrication of the ZnO nanowires was a thermal decomposition process without UV-light assistance. In ambient air, the working temperature and decomposition time were controlled at 200 °C and 5 min, respectively. The structure of the sample was characterized by thin-film X-ray diffractometer (XRD, Bruker). The sample morphology and crystalline structure were characterized by field-emission scanning electron microscopy (FESEM, Hitachi, S-4800) and highresolution transmission electron microscopy (HRTEM, JEOL, JEM-3000F). As-synthesized products were evaluated by X-ray photoelectron spectroscopy (XPS, Perkin-Elmer, model PHI1600 system) to investigate their chemical states. As-synthesized ZnOUV and ZnOnonUV nanowires were investigated using a UV lamp (λ = 365 nm, I = 2.33 mW cm-2), which acted as the excitation source during photoelectric current measurements to investigate their

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photoresponsive properties. The Ag wires were bonded onto the device using silver glue to serve as metal electrodes.4 The distance between the two electrodes was 2 cm. Cathodoluminescence was employed to examine the luminescence characteristics of the as-synthesized products. Ultraviolet-visible spectrophotometry was performed (Hitachi U-3900) to investigate the photocatalytic activities of all as-synthesized products.

3. RESULTS AND DISCUSSION Material Characterizations and Chemical Reactions. ZnOUV nanowires were synthesized on a glass substrate by a UVlight-assisted thermal decomposition process, with irradiation for 5 min, as shown in the FESEM images in Figure 1a. ZnOUV nanowires with a length of about a few thousand micrometers were grown with high density over the entire substrate. Figure 1b shows that the ZnOnonUV nanowires were mesoporous in comparison with the ZnOUV nanowires. The length of the ZnOnonUV nanowires was also in the range of a few thousand micrometers. The insets in parts a and b of Figure 1 show that the nanowires consisted of fine nanoparticles. The diameter of the ZnOUV nanowires was 80-200 nm, whereas that of the ZnOnonUV nanowires was 50-150 nm. All as-synthesized nanowires were successfully fabricated with high transparency (around 500-850 nm, ∼70-80%) on the large-area (∼5 5 cm) glass substrate, as shown in Figure 1c and its inset. The XRD data in Figure 1d show that the ZnOUV and ZnOnonUV nanowires exhibited a single phase of crystalline structures and belonged to the wurtzite hexagonal structure with lattice constants of a = 0.325 nm and c = 0.52 nm (JCPDS 36-1451). The crystallinity of the ZnOUV nanowires is superior to that of the ZnOnonUV nanowires. The TEM images in Figure 2a,b show individual ZnOUV and ZnOnonUV nanowires, respectively. Consistent with the FESEM images, these TEM images show that the ZnOUV and ZnOnonUV nanowires consist of fine nanoparticles with a single-crystalline structure. In addition, the ZnOnonUV nanowires exhibited an amorphous layer formed on the surface of the nanoparticles, as indicated by black arrows in Figure 2b. The amorphous layers are attributed to the Zn(OH)2 (as discussed with respect to eqs 2-4 and XPS data below). Figure 2c,d shows HRTEM images of ZnOUV and ZnOnonUV nanowires, respectively. The certain lattice spacing is ∼0.52 nm, which corresponds to the [0001] axis, indicating that these nanoparticles were single-crystalline. The selected-area electron diffraction (SAED) patterns of the ZnOUV and ZnOnonUV nanowires are shown in Figure 2e,f, respectively, which were taken from multiple nanoparticles. The concentric-circle type of SAED pattern indicates that the as-synthesized ZnOUV and ZnOnonUV nanowires are polycrystalline. The SAED patterns again show that the crystallinity of the ZnOUV nanowires is better than that of the ZnOnonUV nanowires. Because the nanowires consisted of fine nanoparticles, according to the XRD data, the average particle sizes can be quantitatively evaluated using the Debye-Scherrer formula, which gives the relationship between peak broadening in the XRD and particle size 0:89λ ð1Þ D ¼ Bsize cos θ

where 0.89 is the Debye-Scherrer constant, λ represents the X-ray wavelength (Cu KR radiation, 0.15406 nm), Bsize is the full width at half-maximum, and θ is the Bragg angle.13 The particle sizes for ZnOnonUV and ZnOUV nanowires determined by the 2236

dx.doi.org/10.1021/jp110320h |J. Phys. Chem. C 2011, 115, 2235–2243

The Journal of Physical Chemistry C

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Figure 1. FESEM images of (a) ZnOUV and (b) ZnOnonUV nanowires. (c) Nanowires on a glass substrate with high transparency. Inset: Transmittance spectrum as a function of wavelength. (d) XRD patterns of ZnOUV and ZnOnonUV nanowires.

Debye-Scherrer equation were 7.8 and 8.6 nm, respectively. The ZnOUV nanowires exhibited a slightly larger particle size than the ZnOnonUV nanowires. Based on the TEM images and XRD data, we concluded that the as-synthesized ZnOnonUV nanowires exhibit a relatively poor crystallinity with smaller particle size in contrast to the ZnOUV nanowires. The as-synthesized ZnOUV and ZnOnonUV nanowires were also analyzed by cathodoluminescence (CL) as shown in Figure 3a. In comparison with the ZnOUV nanowires, it is clear that the ZnOnonUV nanowires exhibit a high-intensity emission in the broad visible wavelength interval. A near-band-edge peak was found at 382 nm for the ZnOnonUV nanowires and at 385 nm for the ZnOUV nanowires, The near-band-edge emission of the ZnOnonUV nanowires exhibited a significant blue shift in contrast to the ZnOUV nanowires because of the size effect.14 This is consistent with the evaluation of particle size using the DebyeScherrer equation. The broad emission band in the visible region for the as-synthesized nanowires (ZnOUV and ZnOnonUV) is ascribed to the superposition of green and yellow emissions.15 The CL spectrum of Figure 3b shows the intensity as a function of energy (in electronvolts) for the ZnnonUV nanowires. The broad visible emission bands can be deconvoluted into three main peaks. The peak at 2.16 eV is attributed to the complex of oxygen vacancies and Zn interstitials,16,17 whereas the peak at 2.96 eV can be ascribed to oxygen interstitials. The peak at 1.86 eV represents singly charged oxygen vacancies18,19 and exhibits a second harmonic emission in the form of an additional sharp peak at 1.63 eV.20 Generally, the green emission bands have been attributed to the presence of oxygen vacancies and Zn interstitials (∼2.16 eV), which are mainly located at the surface, and the

yellow emission has been associated with excess oxygen (i.e., Oi defects, ∼2.96 eV). We suggest that these visible emission bands are attributed to the presence of Zn(OH)2 on the surface of nanoparticles during the decomposition process, resulting in the surface defect states formed on the ZnO nanocrystallites.21-23 This is further demonstrated by the chemical reactions in eqs 2-4 below. In this work, the CL spectra showed that the ZnOUV nanowires obtain a relatively low intensity of visible emission (defectrelated) in contrast to the ZnOnonUV nanowires. The defect states such as Zn(OH)2 species can be expressed as ZnðC5 H7 O2 Þ2 ðsÞ þ 3H2 OðgÞ f ZnðOHÞ2 ðgÞ þ 2C5 H8 O2 ðgÞ þ H2 OðgÞ

ð2Þ

ZnðOHÞ2 f Zn2þ þ 2OH -

ð3Þ

Zn2þ þ 2OH - f ZnO þ H2 O

ð4Þ

Equation 2 demonstrates that the precursor of Zn(C5H7O2)2 3 H2O reacts with H2O to produce ZnO nanoparticles and Zn(OH)2 species. Ultraviolet light interacts with Zn(OH)2, which causes electronic transitions and promotion of electrons from the ground state to a high-energy state. The Zn(OH)2 (melting point ≈ 125 °C) further dissociates into Zn2þ and OHions by a UV-light-assisted thermal decomposition process, forming ZnO nuclei with H2O species, as indicated by eqs 3 and 4. Equation 4 also indicates that a dissociated water molecule can be adsorbed onto an oxygen bridging vacancy to form two hydroxyl groups during crystal growth,24,25 subsequently reducing the 2237



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Figure 2. TEM images of (a) ZnOUV and (b) ZnOnonUV nanowires, consisting of nanoparticles. The black arrows on the ZnOnonUV nanowires indicate the amorphous layer of Zn(OH)2. Corresponding HRTEM images of (c) ZnOUV and (d) ZnOnonUV nanowires, showing that the certain lattice spacing is ∼0.52 nm. SAED patterns of (e) ZnOUV and (f) ZnOnonUV nanowires.

concentration of oxygen defects. Therefore, the ZnOUV nanowires showed fewer defect states than the ZnOnonUV nanowires because bonding of Zn(OH)2 transited to antibonding by UV irradiation, and the Zn(OH)2 was consumed during the UVassisted thermal decomposition process. However, application of the thermal decomposition process without UV irradiation to the Zn(AcAc)2 precursor to produce the ZnOnonUV nanowires caused the Zn(OH)2 species with defect states to be positioned on the surface of the ZnOnonUV nanoparticles. Accordingly, we suggest that UV light can effectively assist the dissociation of Zn(OH)2 species, resulting in the aggregation of ZnOUV nuclei, the formation of ZnOUV nanoparticles, and an increase of the particle size. Meanwhile, the H2O species react with oxygen vacancies to form two hydroxyl groups that are incorporated into the ZnOUV lattices, reducing the concentration

of oxygen defects. This explains why the intensity of the green and yellow emissions for the ZnOUV nanowires was much lower than that for the ZnOnonUV nanowires. To investigate the surface states of the as-synthesized products, all samples were presputtered to remove surface contaminants before XPS analyses were performed. The Zn 2p3/2 corelevel spectra of the ZnOnonUV and ZnOUV nanowires are shown in Figure 4a. Both of the Zn 2p3/2 peaks for ZnOnonUV and ZnOUV nanowires can be deconvoluted into energies of 1021.5 eV ((0.2 eV) and 1022.5 eV ((0.2 eV), which correspond to ZnO and Zn(OH)2 species, respectively.26,27 The O 1s core-level spectra of the ZnOnonUV and ZnOUV nanowires are shown in Figure 4b. The O 1s peaks for the ZnOUV and ZnOnonUV nanowires can be deconvoluted into two peaks at 530.6 and 532.2 eV, which correspond to the ZnO and OH (H2O) peaks, 2238



Figure 3. (a) CL spectra of ZnOUV and ZnOnonUV nanowires. (b) Visible emission of the nanowires (i.e., ZnOnonUV), which can be deconvoluted into three peaks at 1.86, 2.16, and 2.96 eV.

respectively.28,29 Although a reasonable correlation regarding the concentration levels of Zn(OH)2 in ZnOnonUV versus those in ZnOUV nanowires was not obtained through the XPS spectra, the spectra demonstrate that both ZnOnonUV and ZnOUV had Zn(OH)2 layers on the surface of the ZnO nanocrystallites during the decomposition process. This also explains why ZnOnonUV and ZnOUV exhibited a broad visible emission. However, it is worth noting that a downward shift of binding energy occurs in the Zn 2p3/2 and O 1s peaks. The Zn 2p3/2 peak was broad and therefore shifted toward low binding energy in comparison with our previous synthesis of ZnO nanowires (average diameter larger than 100 nm; see Figure 4a).20 Tay et al. reported that the binding energy (BE) of the Zn 2p3/2 peak is located at 1022.0 eV for a submicrometer ZnO particle size of ∼202 nm.30 In our work, the average particle size of the ZnOnonUV and ZnOUV nanowires determined by Debye-Scherrer equation was less than 8.5 nm. It should be noted that the BE peaks of Zn 2p3/2 for ZnOnonUV nanowires shifted more significantly than those of ZnOUV nanowires as particle size was reduced (see Supporting Information Figures S1 and S2). The same phenomenon was also observed for the O 1s peaks.31 In addition to the effect of particle size on the Zn 2p3/2 peak shift toward low binding energy, a significant shifting effect of BE for the Zn 2p3/2 peak also occurred in the ZnOnonUV nanowires, which can be ascribed to the change in binding state of Zn ions because of a loss in the number of oxygen ions (i.e., oxygen vacancies effect) in ZnO nanocrystallites. Thus, the charge transfer from zinc to oxygen was reduced, increasing the shielding effect of the valence electrons in Zn ions, which decreased the binding energy of the core electrons in the Zn ion.31 This explains why the Zn 2p3/2 peak of the ZnOnonUV nanowires exhibited a much lower bonding energy in contrast to the ZnOUV nanowires

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Figure 4. XPS spectra of ZnOnonUV and ZnOUV nanowires: (a) Zn 2p3/2 and (b) O 1s.

as the ZnOnonUV nanowires have much higher concentrations of oxygen vacancies (as seen by the CL spectra above). Accordingly, the CL results are consistent with the XPS data. UV Sensing Properties and Effect of Defect States. The asprepared ZnOUV and ZnOnonUV nanowires were fabricated into UV photodetectors, which were placed under a dark box with a UV lamp (λ = 365 nm, I = 2.33 mW cm-2) to investigate their photoresponsive properties. The sensitivity is defined by the photocurrent (IUV, as the UV lamp switches on) divided by the dark current (Idark, as the UV lamp switches off). Figure 5a presents the current-voltage (I-V) characteristics of ZnOUV and ZnOnonUV nanowires. The I-V curves of the nanowires exhibit a well-behaved ohmic characteristic. It should be noted that the resistance of the ZnOnonUV nanowires is higher than that of the ZnOUV nanowires, which can be explained by oxygen defect effect. Figure 5b shows the UV response of the ZnOUV and ZnOnonUV nanowires. No significant photoresponse occurred for ZnOnonUV, whereas a photocurrent-to-dark-current ratio of ∼91 was obtained for ZnOUV nanowires. The average response time and recovery time for the ZnOUV nanowires were 27 and 32 s, respectively. The sensing activity of ZnOnonUV was much poorer than that of ZnOUV nanowires. This is because of the defect states such as oxygen vacancies and the amorphous layer [Zn(OH)2] positioned on the surface of the ZnOnonUV nanoparticles, forming many trapping centers, which trap the electrons, resulting in a poor sensitivity during the gas adsorption (UV off) and desorption (UV on) process. The interfacial reaction of ZnO nanocrystals strongly depends on the adsorption of gas molecules that redistribute the spatial density of conducting electrons through the interactions among surface-bonded O- and O2- and the incoming molecules.32 The UV sensitivity mechanisms have been reported previously.5,33 2239



Figure 5. (a) I-V characteristics of ZnOUV and ZnOnonUV nanowires. (b) Photoresponse of as-synthesized nanowires as a function of time during switching of UV on/off states.

This work proposes how the defect states [i.e., Zn(OH)2] influence the UV sensing properties. The sensitivity of the ZnOUV nanowires is 91 times that of the ZnOnonUV nanowires. This implies that the defect states such as Zn(OH)2 with water molecular species strongly influence the sensing properties. The poor sensitivity of the ZnOnonUV nanowires can be attributed to the amorphous layers of Zn(OH)2 that are accompanied by high concentrations of oxygen defects and form on the surface of the ZnOnonUV nanoparticles. The schematic diagram in Figure 6a depicts the formation of an amorphous layer of surface states [i.e., Zn(OH)2] on the surface or in the lattice sites of the ZnOnonUV nanocrystals. In either event (UV lamp on or off), the oxygen defects serve as trapping centers for the photogenerated electrons and reduce the charge and discharge of oxygen on the surface of the nanoparticles,34,35 resulting in poor sensing performance. In contrast, on the surface of the ZnOUV nanoparticles, the surface states of Zn(OH)2 are consumed during the UV irradiation of the thermal decomposition process. Thicker and thinner depletion layers formed on the surface of the nanoparticles during UV off and on states, respectively, as shown in Figure 6b. Therefore, the ZnOUV nanowires showed a very low conductivity with UV off and a high conductivity with a magnitude of ∼91 fold with UV on, so that ZnOUV nanowires exhibited a superior sensing property to the ZnOnonUV nanowires. To further investigate the effect of defect states on photoresponsive properties, the decomposition times of the ZnOnonUV and ZnOUV nanowires were controlled at 3, 5, and 10 min (see Supporting Information Figure S3). As the thermal decomposition time increased, the sensitivity of the ZnOnonUV and ZnOUV

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Figure 6. Schematic diagram of photoresponsiveness for (a) ZnOnonUV and (b) ZnOUV nanowires (under UV-lamp irradiation, I = 2.33 mW cm-2, λ = 365 nm).

nanowires increased. This can be attributed to the surface state of the Zn(OH)2 layers and the oxygen defects and how they react with ambient air. Consequently, the state of the surface defect was consumed as the annealing time increased. This demonstrates that the defect states acted as a key component in the UV sensing properties. The same phenomenon was also observed for the ZnOUV nanowires. Thus, these defect states act as huge trapping centers, trapping electrons at the surface of the nanoparticles or inside the ZnO lattices of defect sites during the gas adsorption and desorption process, making for poor photoresponsive properties in ZnOnonUV nanowires. In contrast, for the ZnOUV nanowires, the UV-light assistance to the thermal decomposition process can effectively decompose the Zn(OH)2 species to form ZnO nuclei with water molecules. These ZnOUV nuclei are consequently aggregated to enlarge the particle size while decreasing the concentration of oxygen vacancies by water molecules. This gives a reasonable explanation as to why the particle size of ZnOUV is larger than that of ZnOnonUV, whereas lower defect states appear in the ZnOUV nanowires than in the ZnOnonUV nanowires. Photocatalytic Activity: Effect of Oxygen Defects. The photocatalytic activities were investigated by immersing the nanowires into methylene blue (MB, C16H18ClN3S 3 H2O, 10 μM). Figure 7a-d shows the absorbance ratios as a function of wavelength under MB solution (50 mL), MB with TiO2 P25 nanoparticles, MB with ZnOnonUV nanowires, and MB with ZnOUV nanowires, respectively. The MB solution without nanowires exhibited a low photodegradation ratio from 0 to 60 min of UV irradiation (λ = 365 nm, I = 2.33 mW cm-2). Figure 7e compares the photodegradation ratios of the MB 2240



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Figure 7. UV lamp (I = 2.33 mW cm-2, λ = 365 nm) irradiation showing the evolution of a UV-visible absorption spectrum for the 1 μM MB solution: (a) MB solution without nanowires, (b) MB solution with TiO2 P25 nanoparticles, (c) MB solution with ZnOnonUV nanowires, (d) MB solution with ZnOUV nanowires, (e) photodegradation ratios for MB solution with as-synthesized nanowires and TiO2 P25 nanoparticles, (f) absorption spectra of all products.

solutions with UV exposure times for different kinds of products. The photodegradation ratio of the ZnOnonUV nanowires exhibited excellent photocatalytic activities in comparison with ZnOUV nanowires and TiO2 P25 nanoparticles, especially for irradiation times in the range of 0-20 min. It is clear that poor photocatalytic activity was obtained in the ZnOUV nanowires, which was only superior to the photocatalytic activity of MB solution alone. The UV-vis absorption spectra of the TiO2 P25, ZnOnonUV, and ZnOUV nanowires are shown in Figure 7f. The absorbance spectra were not correlated with the photodegradation ratio, which demonstrates that there are other more important factors that determine the photocatalytic activities. In this work, the CL spectrum of ZnOnonUV exhibited high concentrations of oxygen defects, such as oxygen vacancies and interstitials, in comparison with ZnOUV nanowires. The anion vacancies and Zn interstitials constitute the principal intrinsic defects in ZnO nanowires. The lattices of oxygen deficiencies • (V•• o ) and Zn interstitials (Zni ) can trap the electron carriers in

order to maintain the charge neutrality36 0 V •o f V •• o þe

ð5Þ

0 Zn•i f Zn•• i þe

ð6Þ

• • V •• o þ Oo þ H2 OðgÞ f OHO þ OHO

ð7Þ

V•• o

represents two positively Using Kr€oger-Vink notation, charged vacancies and are relative to a perfect lattice on an oxygen site. A schematic energy-band diagram, shown in Figure 8, depicts the lattice-defect mechanisms in photoreaction process. An electron-hole pair is generated when an excitation light energy (λ = 365 nm, I = 2.33 mW cm-2) is equal to or higher than the band gap absorbed in the material. Oxygen vacancies present on the surface of ZnO nanocrystals behave as active sites for the dissociative chemisorption of water. The dissociation of one water molecule can generate two hydroxyl groups, as presented in eq 7.37,38 2241



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Figure 8. Schematic drawing of the band structure of the photocatalyst for ZnOnonUV. (VB = valence band, CB = conduction band; under UVlamp irradiation, I = 2.33 mW cm-2, λ = 365 nm).

In addition to reaction 7, the defect chemical reactions accompanied by a chain reaction are shown in eqs 8-14. It is known that the ZnO nanocrystallites undergo an oxidation process (within ambient air) whereby extra oxygen is accommodated into the lattice as interstitial ions, with compensation by hole formation, as presented in eq 8.39 The holes diffuse to the surface of semiconductor and react with OH species to produce OH• radicals, as shown in eq 9 and Figure 8. The electrons react with oxygen to form oxygen ions (i.e., O2-, O-), as shown in eq 10. The oxygen ions further react with water molecules to produce the hydroperoxyl radicals (HO2•) with hydroxyl ions (OH-), as shown in eq 11. 1 00 O2 f Oi þ 2h• 2

ð8Þ

OH - þ hþ f OH•

ð9Þ

O2 þ e - f O2 -

ð10Þ

O2 - þ H2 O f HO2 • þ OH -

ð11Þ

2HO2 • f H2 O2 þ O2

ð12Þ

HO2 • þ H2 O þ e - f H2 O2 þ OH -

ð13Þ

H2 O2 þ e - f OH• þ OH -

ð14Þ

Equations 12 and 13 explain that there are two reaction routes to further dissociate the hydroperoxyl radicals (HO2•). Equation 12 presents that the hydroperoxyl radicals could dissociate into hydrogen peroxide (H2O2) with oxygen species (O2), and eq 13 presents that hydroperoxyl radicals can react with water molecules and electrons to produce the hydroperoxyls (H2O2) and hydroxyl ions (OH-). The hydrogen peroxide (H2O2) products further react with electron carriers to produce hydroxyl radicals (OH•) with hydroxyl ions (OH-), as presented in eq 14. The radicals of hydroperoxyls and hydroxyls are powerful oxidizing agents for decomposition of the MB solutions (as indicated in eqs 7, 9, and 11-14). In this work, the CL spectra revealed the high intensity of two defect levels such as oxygen vacancies (V•• o ) and oxygen interstitials (Oi00 ) in the ZnOnonUV nanowires. These defect levels effectively undergo reduction and oxidation reactions on the surface of the nanoparticles through reactions 5-14, consequently enhancing the photocatalytic activities of as-synthesized ZnO nanowires. Based on these chemical reaction processes, the lattice defects act as a key component in determining the photocatalytic activities of as-synthesized ZnO nanocrystallites. This is because the oxygen vacancies serve as electron acceptors

and can trap the photogenerated electrons temporarily, to reduce the surface recombination of electrons and holes, whereas oxygen interstitials serve as shallow trappers of the photogenerated holes, to hedge the recombination of photogenerated electrons and holes.40 In addition, the decrease in electron density within the semiconductors leads to an increase in the hydroxyl group acidity, which, in turn, improves the photocatalytic activity of the ZnO nanoparticles.38,41 The ZnOnonUV nanowires exhibited s high concentration of oxygen defects in comparison with the ZnOUV nanowires, which therefore contributed to their photocatalytic activity. All of these defect levels were the result of Zn(OH)2 layers, which were positioned on the surface of the ZnOnonUV nanoparticles, as evidenced by eqs 2-4 and XPS data. On the other hand, one should note the size effect. According to the TEM images with particle size evaluation by the DebyeScherrer equation, the mesoporous ZnOnonUV nanowires consisted of small particles with a size of ∼7.8 nm, which is smaller than the ∼8.6 nm for the ZnOUV nanowires. In the fine nanoparticles, the average distance that the electrons can move freely is very short. This might cause the oxygen defects to very easily bind electrons (or holes) to form excitations,42 allowing these binding carriers to be separated efficiently by UV light and to proceed to a highly effective photodegradation process in ZnOnonUV nanowires.

4. CONCLUSIONS In summary, ZnO nanowires can be fabricated by a UV-lightassisted thermal decomposition process at 200 °C, which requires only 5 min in ambient air. The nanowires consisted of fine particles with a size of 7.8 nm for the ZnOnonUV sample and 8.6 nm for the ZnOUV sample. The surface states [i.e., Zn(OH)2] accompanied by oxygen defects were demonstrated to act as key components to determine the photoresponsive and photocatalytic activities. The surface states not only serve as trapping centers and prolong the lifetime of the photogenerated electrons that enhance the photocatalytic activities; they also trap the electrons on the surface of the nanoparticles, resulting in poor sensing performance in the as-synthesized products. The ZnOnonUV nanowires, therefore, show better photocatalytic properties (especially during 0-20 min of UV irradiation) than ZnOUV nanowires and commercial TiO2 P25 nanoparticles because of the high concentration of oxygen defects with small particle size. The ZnOUV nanowires exhibited good UV sensing properties 91 times better than those of the ZnOnonUV nanowires. The assynthesized ZnO nanowires have high potential applications in UV photodetectors and good photocatalytic activities for ZnOUV and ZnOnonUV nanowires. ’ ASSOCIATED CONTENT

bS

Supporting Information. Processing explanation, XPS survey spectra, evaluation of particle size, and XRD pattern. This material is available free of charge via the Internet at http://pubs. acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel.: (þ886) 4-24517250 ext. 5316. Fax: (þ886) 4-24510014. 2242



’ ACKNOWLEDGMENT The authors thank the National Science Council of the Republic of China for financially supporting this research under Contract NSC 98-2221-E-035-008. ’ REFERENCES (1) Hsueh, T. J.; Chang, S. J.; Hsu, C. L.; Lin, Y. R.; Chen, I. C. Appl. Phys. Lett. 2007, 91, 053111. (2) Zhang, Y.; Xu, J.; Xiang, Q.; Li, H.; Pan, Q.; Xu, P. C. J. Phys. Chem. C 2009, 113, 3430. (3) Zhu, Z.; Zhang, L.; Howe, J. L.; Liao, Y.; Speidel, J. T.; Smith, S.; Fong, H. Chem. Commun. 2009, 18, 2568. (4) Wu, J. M.; Fang, C. W.; Lee, L. T.; Yeh, H. H.; Lin, Y. H.; Yeh, P. H.; Tsai, L. N.; Lin, L. J. J. Electrochem. Soc. 2011, 158, K6. (5) Yang, P.; Yan, H. Q.; Mao, S.; Russo, R.; Johnson, J.; Saykally, R.; Morris, N.; Pham, J.; He, R. R.; Choi, H. J. Adv. Funct. Mater. 2002, 12, 323. (6) Ku, C. H.; Wu, J. J. Appl. Phys. Lett. 2007, 91, 093117. (7) Yang, L.; Wang, G.; Tang, C.; Wang, H.; Zhang, L. Chem. Phys. Lett. 2005, 409, 337. (8) Fauteux, C.; Longtin, R.; Pegna, J.; Therriault, D. Inorg. Chem. 2007, 46, 11036. (9) Unalan, H. E.; Hiralal, P.; Rupesinghe, N.; Dalal, S.; Milne, W. I.; Amaratunga, G. A. J. Nanotechnology 2008, 19, 255608. (10) Bohle, D. S.; Spina, C. J. J. Am. Chem. Soc. 2007, 129, 12380. (11) Nakamura, I.; Negishi, N.; Kutsuna, S.; Ihara, T.; Sugihara, S.; Takeuchi, K.; Mol, J. Catal. A: Chem. 2000, 161, 205. (12) Wu, J. M.; Chen, Y. R.; Lin, Y. H. Nanoscale, 2011, DOI: 10.1039/C0NR00595A. (13) Klug, H. P. Alexander., L. E. X-ray Diffraction Procedures; John Wiley & Sons: New York, 1959. (14) Wang, X. D.; Ding, Y.; Summers, C. J.; Wang, Z. L. J. Phys. Chem. B 2004, 108, 8773. (15) Zheng, Y.; Chen, C.; Zhan, Y.; Lin, X.; Zheng, Q.; Wei, K.; Zhu, J.; Zhu, Y. Inorg. Chem. 2007, 46, 6675. (16) Djurisic, A. B.; Leung, Y. H. Small 2006, 2, 944. (17) Djurisic, A. B.; Choy, W. C. H.; Roy, V. A.L.; Leung, Y. H.; C. Kwong, Y.; Cheah, K. W.; Rao, T. K. G.; Chan, W. K.; Lui, H. F.; Surya, C. Adv. Funct. Mater. 2004, 14, 856. (18) Meng, X. Q.; Shen, D. Z.; Zhang, J. Y.; Zhao, D. X.; Lu, Y. M.; Dong, L.; Zhang, Z. Z.; Liu, Y. C.; Fan, X. W. Solid State Commun. 2005, 135, 179. (19) Ng, H. T.; Chen, B.; Li, J.; Han, J.; Meyyappan, M.; Wu, J.; Li, S. X.; Haller, E. E. Appl. Phys. Lett. 2003, 82, 2023. (20) Fang, C.-W.; Wu, J. M.; Lee, L.-T.; Hsien, Y.-H.; Lo, S.-C.; Chen, C.-H. Thin Solid Films 2008, 517, 1268. (21) Tam, K. H.; Cheung, C. K.; Leung, Y. H.; Djurisic, A. B.; Ling, C. C.; Beling, C. D.; Fung, S.; Kwok, W. M.; Chan, W. K.; Phillips, D. L.; Ding, L.; Ge, W. K. J. Phys. Chem. B 2006, 110, 208658. (22) Zhou, H.; Alves, H.; Hofmann, D. M.; Kriegseis, W.; Meyer, B. K.; Kaczmarczyk, G.; Hoffmann, A. Appl. Phys. Lett. 2002, 80, 210. (23) Norberg, N. S.; Gamelin, D. R. J. Phys. Chem. B 2005, 109, 20810. (24) Erdem, B.; Hunsicker, R. A.; Simmons, G. W.; Sudol, E. D.; Dimonie, V. L.; El-Aasser, M. S. Langmuir 2001, 17, 2664. (25) Wu, J. M. Jpn. J. Appl. Phys. 2008, 47, 383. (26) Battistoni, C.; Dormann, J. L.; Fiorani, D.; Paparazzo, E.; Viticoli, S. Solid State Commun. 1981, 39, 581. (27) Dake, L. S.; Baer, D. R.; Zachara, J. M. Surf. Interface Anal. 1989, 14, 71. (28) Zhdan, P. A.; Shepelin, A. P.; Osipova, Z. G.; Sokolovskii, V. D. J. Catal. 1979, 58, 8. (29) Yang, W. P.; Costa, D.; Marcus, P. J. Electrochem. Soc. 1994, 141, 2669. (30) Tay, Y. Y.; Li, S.; Sun, C. Q.; Chen, P. Appl. Phys. Lett. 2006, 88, 173118.

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