Effect of Plasma Treatment on Native Defects and Photocatalytic

Article pubs.acs.org/JPCC

Effect of Plasma Treatment on Native Defects and Photocatalytic Activities of Zinc Oxide Tetrapods Fangzhou Liu,† Yu Hang Leung,† Aleksandra B. Djurišić,*,† Alan Man Ching Ng,†,‡ Wai Kin Chan,§ Ka Long Ng,∥ Kam Sing Wong,∥ Changzhong Liao,⊥ Kaimin Shih,⊥ and Charles Surya# †

Department of Physics, §Department of Chemistry, and ⊥Department of Civil Engineering, The University of Hong Kong, Pokfulam Road, Pokfulam, Hong Kong ‡ Department of Physics, South University of Science and Technology of China, Shenzhen, China ∥ Department of Physics, Hong Kong University of Science and Technology, Clearwater Bay, Hong Kong # Department of Electronic and Information Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong S Supporting Information *

ABSTRACT: ZnO tetrapods, as a ZnO nanostructure with a low defect concentration, were subjected to plasma treatments using different gases to induce different types of defects in the samples and examine their effects on sample properties. Hydrogen and argon plasmas resulted in significant oxygen deficiencies, but no defect emission appeared. Oxygen plasma had only a minor effect on the sample composition, but it resulted in the appearance of defect emission and a significant overall reduction in luminescence intensity. On the other hand, the photocatalytic activity was significantly reduced in samples exhibiting oxygen deficiencies. It can thus be concluded that oxygen vacancies do not participate in the green emission of ZnO and also that their presence at the sample surface does not enhance the photocatalytic activity.

1. INTRODUCTION As a common semiconducting material, zinc oxide has been extensively studied in recent years.1−27 The intrinsic advantages of ZnO, such as a wide band gap (∼3.37 eV), a large exciton binding energy (∼60 meV), and piezoelectric properties, make it suitable for a variety of applications, such as solar cells, lightemitting diodes (LEDs), sensors, catalysts, and antibacterial applications.1−3 Although ZnO has been studied extensively, a number of issues concerning ZnO still remain unresolved, because of the complexity of its native defect chemistry. For example, contradictory hypotheses have been raised regarding the controversial origins of the commonly existing visible emissions in ZnO.2−4 Native defects significantly affect the properties of ZnO in various respects, including photoluminescence, surface properties, and the photocatalytic efficiencies of various dyes in aqueous solutions.5−7 Thus, if tuning of the defect chemistry in a desirable way can be realized, it will be possible to obtain ZnO materials with selected properties of interest. A variety of ZnO nanostructures can be achieved by rather simple and low-cost methods.1−3 However, these nanostructures have distinctive optical properties and native defect species, and the properties of the synthesized morphologies depend in a complicated way on the fabrication conditions. Postfabrication treatments also play a critical role in modifying the properties of ZnO nanostructures. A number of studies © 2014 American Chemical Society

have reported modifications of the defect chemistry and, hence, the properties of bulk and nanostructured ZnO by postfabrication plasma treatments.4,9−14 For example, hydrogen plasma can passivate both surface and bulk defects.4 Green emission in ZnO nanoshells is strongly suppressed by hydrogen plasma.4 Moreover, it was reported that oxygen plasma can effectively reduce oxygen vacancies and oxygen adsorption on the ZnO surface.25 A lower surface defect concentration after oxygen plasma treatment was achieved in ZnO nanorods.14 However, studies comparing the effects of plasma treatments using different gas species on ZnO nanostructures have been scarce. Among various ZnO nanostructures, tetrapods are of special interest because of their excellent optical properties and significantly lower density of native defects compared to other morphologies.15 The properties of tetrapods are largely dependent on the substrate temperature during synthesis. In a previous work, tetrapods with negligible defect emission and intense band-edge UV emission were achieved in a very narrow temperature range.15 High crystallinity, a relatively low concentration of native defects, and the absence of visible emissions make these materials an ideal subject for studying the Received: June 30, 2014 Revised: September 10, 2014 Published: September 10, 2014 22760

dx.doi.org/10.1021/jp506468r | J. Phys. Chem. C 2014, 118, 22760−22767

The Journal of Physical Chemistry C

Article

illumination. For ROS detection, a solution of the spin trap 5,5dimethyl-1-pyrroline N-oxide (DMPO, Sigma-Aldrich) was dropped onto the tetrapod pellets and exposed to UV illumination for 2 min, after which ESR measurements were performed immediately at room temperature. 2.2. Photocatalytic Dye Degradation. MB (SigmaAldrich) was dissolved in deionized water at a concentration of 5 mg/L. One tetrapod pellet with a weight of 50 mg was placed in 50 mL of the dye solution in a Petri dish, and the dye solution was stirred with a magnetic stirrer in the dark at room temperature for 1 h to reach equilibrium. Absorption spectra of the dye solution were measured immediately after 1 h of stirring using a Perkin-Elmer Lambda Bio 40 UV/vis spectrometer. For the photocatalytic experiments, the dye solution was exposed to UV illumination (365 nm, 66.2 mW/ cm2, Blak-Ray B-100 AP lamp). At specified time intervals, 3 mL of solution was withdrawn and filtered with a Millex-FH polytetrafluoroethylene (PTFE) filter with a pore size of 0.45 μm, and then the absorption was measured.

effects of plasma treatment on optical characteristics, native defects, and other related properties. In this work, we have conducted a comprehensive investigation on the influence of different postfabrication plasma treatments on ZnO tetrapods. Tetrapods with exceptional optical properties, namely, dominant UV emission and no defect emission, were prepared by vapor phase deposition, as described previously.15 A series of characterization experiments were performed to study the effects of plasma treatments on tetrapods. Photocatalytic efficiencies in the degradation of the common cationic dye methylene blue (MB) were also investigated for the as-grown and plasma-treated tetrapod samples.

2. EXPERIMENTAL SECTION 2.1. Materials and Material Characterization. ZnO tetrapods were synthesized by evaporating 0.2 g of Zn powder (99.995%, Sigma-Aldrich) in a quartz tube (∼3-cm inner diameter) inside a horizontal tube furnace under humid Ar gas flow at a flow rate of 0.2 L/min. The evaporation was conducted in the ambient at a temperature of 950 °C. After 15 min, a white material (tetrapods) was deposited on the inner wall of the quartz tube. Tetrapods with intense UV emission and no defect emission were collected from the quartz tube in the temperature range from 920 to 940 °C. The collected materials were then made into pellets (each 50 mg in weight) using a pellet die of 13-mm inner diameter for further treatment and characterization. Plasma treatment was performed on tetrapod samples for three different gases (hydrogen, argon, and oxygen) using an Oxford Plasmalab 80 Plus Reactive Ion Etcher. The gas flow rate, radio-frequency (RF) power, pressure, and treatment time were 100 sccm, 150 W, 700 mTorr, and 5 min, respectively. The morphologies of ZnO tetrapods were characterized by scanning electron microscopy (SEM) using a JEOL JSM-7001F scanning electron microscope. High-resolution transmission electron microscopy (HRTEM) and selected-area electron diffraction (SAED) were performed with a JEOL JEM-2100F field-emission electron microscope and an FEI Tecnai G2 20 STWIN scanning transmission electron microscope (STEM) system. Photoluminescence (PL) measurements were performed both at low temperatures and at room temperature. The low-temperature PL measurements were conducted using a liquid-helium cryostat system. The samples were excited at a wavelength of 360 nm using the second harmonic of a femtosecond Ti-sapphire laser at 720 nm. A pulse picker was used to reduce the repetition rate to 4.75 MHz with the power set at around 48 μW. High-resolution time-integrated PL spectra were measured by a lock-in amplifier system. Roomtemperature PL measurements were performed in the ambient using a HeCd (325 nm) laser as the excitation source, and the spectra were collected using a PDA-512USB (Control Development Inc.) fiberoptic spectrometer. Fourier transform infrared (FTIR) spectroscopy measurements were performed using a Perkin-Elmer Spectrum Two IR spectrometer. For FTIR measurements, the samples were prepared by pressing mixtures of tetrapods and infrared-grade KBr (Sigma-Aldrich Co.) into pellets using an evacuable pellet die (13-mm inner diameter). X-ray photoelectron spectroscopy (XPS) was performed using a Physical Electronics PHI 5600 XPS system. Electron spin resonance (ESR) spectra of the samples were obtained using a Bruker EMX ESR spectrometer to determine the production of reactive oxygen species (ROS) under UV

3. RESULTS AND DISCUSSION Figure 1 shows an SEM image of a tetrapod pellet without plasma treatment, from which one can observe the typical

Figure 1. Scanning electron microscopy (SEM) image of ZnO tetrapod pellets.

tetrapod structure with four legs growing from a common core; this morphology remained unchanged after the materials were pressed into pellets. Tetrapod structures have previously been reported for a number of materials, including ZnO.28−37 Two models have been proposed for the growth of ZnO tetrapods, one involving a zinc-blende core33,37 and the other involving a wurtzite core,34−36 called the octa-twin model.35 XRD patterns of the tetrapods were found to exhibit only peaks corresponding to the wurtzite phase (see Supporting Information, Figure S1), and no evidence of zinc-blende phase was observed from TEM images and SAED patterns (see Supporting Information, Figure S2). Therefore, tetrapod formation is consistent with the octa-twin model.35 Whereas SEM images obtained after plasma treatment did not reveal any significant morphology changes (not shown), from the HRTEM images shown in Figure 2, one can observe that the plasma treatments resulted in surface roughening. The roughening due to plasma damage appeared to be most extensive for hydrogen plasma, which is expected given that hydrogen is capable of etching ZnO and also has the recognized effect of defect passivation.26 Nevertheless, the depth of the damaged layer for the plasma treatment conditions used did not exceed 10 nm. Other plasma treatments, using oxygen and argon plasmas, caused less damage, with damaged-layer 22761



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Figure 2. High-resolution TEM images of (a) as-grown and (b−d) plasma-treated tetrapod samples: (b) hydrogen plasma, (c) oxygen plasma, and (d) argon plasma.

1LO peak for the HP and OP samples compared to the AP and as-grown samples at 50 K. This might indicate variations in donor-bound excitonic states among the samples with different treatment conditions. Significant broadening and merging of the LO and DBE emission lines were observed as the temperature rose to 100 K. The PL spectra of all samples taken at room temperature are shown in Figure 4. The intensity of the OP spectrum was magnified by a factor of 30 for a clearer presentation, because the emission from this sample was very weak. For the as-grown sample, a sharp near-band-edge ultraviolet emission peak dominated the spectrum, no defect emission was observed in the visible region, which is as expected given that we selected the tetrapods grown in the optimized temperature range with enhanced UV emission and no defect emission. The HP and AP samples exhibited spectra similar to that of the as-grown sample, with the absence of visible emission as well. The asgrown sample exhibited the strongest UV emission, followed by the HP and AP samples. On the other hand, a contrasting result was observed for the OP sample. In this case, the overall intensity of the PL emission was considerably reduced, whereas the green emission band was increased significantly in comparison to the near-band-edge emission. This can also be observed from the photographs of the luminescence taken from different samples, as shown in the inset of Figure 3. For all three types of plasma treatment applied, green emission was introduced only by oxygen plasma. As a commonly existing defect emission in various ZnO morphologies, green emission has been extensively investigated through both experimental and theoretical approaches, yet a sound conclusion regarding its chemical origins has not been reached.2−4,17 Oxygen vacancies are usually considered as a possible cause of green emission in ZnO, which is favored by a number of studies in the literature.2−4,17 Some studies have reported that oxygen plasma treatment is an effective approach to reducing the concentration of oxygen vacancies and the adsorption of oxygen on the ZnO surface, resulting in the quenching of green emission.18−20 However, a significantly increased green emission after oxygen plasma treatment was observed in this work, indicating that oxygen vacancies are unlikely to be the chemical origin of the

thicknesses of several nanometers, in agreement with previous work.18 To examine the effects of plasma treatment on the optical and excitonic properties of ZnO tetrapod samples, PL measurements at various temperatures were performed. For the PL measurements at cryogenic temperatures, spectra were recorded in the near-band-edge region from 3.2 to 3.4 eV at 10, 20, 30, 50, 75, and 100 K. The normalized spectra of all samples at each temperature measured are summarized for comparison in Figure 3. At 10 K, sharp emission lines with the peak located at ∼3.36 eV, which are commonly assigned to bound exciton (BE) recombinations,1,16 were observed for all samples. The emission peaks of the samples after plasma treatment were slightly blue-shifted compared to those of the as-grown sample, with a slight variation in the exact peak position among different samples [hydrogen-plasma-treated (HP), ∼3.3637 eV; oxygen-plasma-treated (OP), ∼3.3655 eV; argon-plasmatreated (AP), ∼3.3637 eV; and as-grown, ∼3.3600 eV], and more accurate identification was prevented by limitations on the experimental resolution. Nevertheless, the prominent emissions were located in the region that is predominantly attributed to donor-bound excitons (DBEs).1,16 It should be noted that the emission peaks for the HP and AP samples were close to the I4 line at ∼3.3628 eV, which has been reported to have a convincing connection with shallow donor- and hydrogen-related impurities in ZnO.10,16 In addition to the major BE emissions, minor emission peaks located at ∼3.309 eV were also observed at 10 K; they are likely due to the firstorder longitudinal-optical (LO) phonon replicas of the freeexciton peak located at ∼3.375 eV.1,16 As the temperature was raised, a red shift of the peak position commonly occurred for all samples; the dominant DBE emissions quenched gradually, whereas the relative intensities of the first LO phonon replica of free-exciton recombination increased as a result of the thermal dissociation of bound excitons.1,16 A shoulder at the higherenergy side of the dominant DBE emission was observed at 50 K and above corresponding to the free-exciton emission peak, which can also be interpreted by thermal dissociation of bound excitons.1 Note that the quenching of the major DBE emission peaks was slower with a relatively more intense DBE peak than 22762



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Figure 3. Low-temperature photoluminescence (PL) spectra of as-grown and plasma-treated tetrapod samples.

also reported recently that green emission occurs due to zinc vacancies on nonpolar (101̅0) surfaces or, more specifically, oxygen dangling bonds associated with zinc vacancies.27 Despite its controversial chemical origins, it is generally agreed that the green emission is attributable to surface defects, as its quenching can be achieved by various surface modifications.4,17 Considering that the effective penetration depth of oxygen plasma is approximately within the range of a few nanometers,18 whereas the lengths of tetrapod legs are usually on the micrometer scale, it is likely that the green emission introduced by oxygen plasma treatment is related to modified defect species present near the surface of the OP sample. In the cathodoluminescence spectra obtained at different acceleration voltages (see Supporting Information, Figure S3), we observed no significant difference in the spectra of as-grown, argonplasma-treated, and hydrogen-plasma-treated samples. This indicates that the plasma-damaged layer was indeed very shallow, in agreement with the HRTEM images. In the case of oxygen plasma treatment, there was an initial increase followed by a decrease in the UV-to-visible emission ratio, indicating that, in this case, defects were induced in the tetrapods, likely

Figure 4. Room-temperature photoluminescence (PL) spectra of asprepared and plasma-treated tetrapod samples. The OP spectrum was magnified by a factor of 30 for better presentation.

green emission in this case. One of the possible assignments of the green emission is defect complexes containing zinc vacancies, as demonstrated in our previous work.4,6 It was 22763



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extending deeper than the damage observable in the TEM images. Because of the complex shape of the tetrapods, it is not possible to estimate the depth from the acceleration voltage dependence, but in general, for lower acceleration voltages, the penetration depth of the electrons into the sample would be lower. Figure 5 shows FTIR spectra of different tetrapod samples. In general, relatively low contents of organic adsorbates were

Table 1. Adsorption of MB and XPS Characterization of Different ZnO Tetrapod Samples HP AP OP as-grown

MB adsorptiona

O/Zn

O1/Zn

0.07 0.07 0.07 0.03

1.02 1.10 1.09 1.04

0.30 0.41 0.66 0.72

O2/Zn O3/Zn O1/Ototal 0.69 0.67 0.43 0.32

0.03 0.02 − −

0.29 0.37 0.61 0.69

a

Surface adsorption of MB on different tetrapod samples estimated as (A1 − A2)/A1, where A1 is the absorption of the blank dye solution and A2 is the absorption of the dye solution with the tetrapod pellets placed in a Petri dish after reaching equilibrium.

respectively) compared to the OP and as-grown samples. In addition, a minor contribution at ∼533.8 eV due to water40−42 was identified only for the AP and HP samples. The XPS results indicate variations in the compositions of the samples after treatment by different types of plasma, which might be linked to differences in defects and surface adsorbates. No clear correlation between the oxygen composition in the XPS results and the defect emission was observed, which can also be attributed to the complex nature of the origin of green emission, rather than oxygen vacancies in ZnO tetrapods. The ROS generation of different samples was examined by ESR. From our previous work, no signal corresponding to superoxide ions was detected from the tetrapods;43 hence, we measured the ESR spectra using a DMPO spin trap to detect the production of OH• radicals. The results obtained are shown in Figure 6. An increase in the OH• radical production was observed in all samples with plasma treatment compared to the as-grown sample. In addition, plasma-treated samples also exhibited increased adsorption of MB compared to the asgrown sample, as summarized in Table 1. The dye degradation curves of methylene blue for different tetrapod samples are shown in Figure 7. However, the OP sample exhibited the fastest degradation, followed by the as-grown, AP, and HP samples. Both the HP and AP samples showed reductions in photocatalytic activity, but in various degrees. In particular, considerable suppression of the photocatalytic activity was observed for the HP sample, where less than 10% of the MB was degraded even after 90 min of UV illumination. No correlation between dye adsorption and photocatalytic activity was found, which is in agreement with our previous work.7,8,43 It should also be noted that no correlation was observed between ROS generation and photocatalytic dye degradation, in agreement with previous work,43 especially for the case of the as-grown sample, which exhibited a significantly higher degradation rate compared to the HP and AP samples, whereas it had the lowest ROS generation. This indicates that OH• radicals are not responsible for the predominant mechanism of MB photodegradation, although it has been reported in the literature that OH• is contributed in the photodegradation pathway of methylene blue in aqueous environments.44,45 This result is in good agreement with our previous work, which reported direct charge separation as the dominant role regarding the photocatalytic activities of various ZnO nanostructures.7,8,43 Increases in both dye adsorption and ROS production were commonly observed in the plasmatreated samples, and the increases were at similar levels regardless of the gas species used in the plasma treatment. This can be explained by a general effect of plasma treatment on the surface properties of tetrapods. Some studies reported surface damage due to hydrogen plasma in ZnO,10 and such damage

Figure 5. Fourier transform infrared spectra of as-grown and different plasma-treated ZnO tetrapod samples.

observed on the surface of all samples. Several peaks can be identified for all samples, including a broad resonance in the range of 3200−3600 cm−1 due to OH-group vibrations and a peak at ∼1630 cm−1 due to scissoring vibrations of molecular water, as well as peaks in the range of 650−1200 cm−1 corresponding to various vibrations.21−24 Nevertheless, several marked differences can be identified. For the HP sample, there is a noteworthy increase in the intensities of the peaks at ∼2954, ∼2923, and ∼2852 cm−1, which are assigned to asymmetric and symmetric stretching of alkane C−H.37 A sharp peak at ∼1384 cm−1 was observed only for the OP sample, as well as a shoulder neighboring the molecular-water scissoring at ∼1714 cm−1, which can be assigned to carbonate-related vibrations.38,39 The FTIR results indicate a change in surface adsorbates due to the effects of plasma treatment on the tetrapod surface, although to a less significant degree. Still, it is possible to identify oxygen-related surface species in OP samples and hydrogen-related species in HP samples that possibly resulted from the oxygen and hydrogen plasma treatments, respectively. XPS measurements were performed to obtain information on the surface compositions of the samples. The spectra were calibrated with the carbon 1s peak located at 285 eV as a reference position. The estimated atomic composition of each sample was obtained from the integrated areas of the O 1s and Zn 2p3/2 peaks, with the relative ratios of different O and Zn components summarized in Table 1. The O 1s spectra were resolved as a superposition of two predominant Gaussian components (denoted as O1 and O2) corresponding to different oxygen environments. The O1 peak located at lower binding energy (∼530.0 eV) is attributed to oxygen in the ZnO lattice, whereas the O2 peak centered at ∼531.6 eV is usually assigned to loosely bound oxygen on the surface (OH groups or O2− ions in oxygen-deficient regions).40−42 The as-grown sample had the highest O1/Ototal ratio of ∼0.69, followed by OP (∼0.61), whereas the AP and HP samples exhibited significantly lower O 1/Ototal ratios (∼0.37 and ∼0.29, 22764



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Figure 7. Degradation curves of MB for as-grown and different plasma-treated ZnO tetrapod samples.

plasma species. Furthermore, the distinctions in the effects of plasma treatments triggered by different plasma species were also revealed by the variations in photodegradation rates of different samples, since the initial properties were identical before plasma treatment. It is worth noting that a clear relationship was found between the photocatalytic activities and the surface composition obtained by XPS: The O1/Ototal ratios were in positive correlation with the photodegradation rates. A lower O1/Ototal ratio indicates a higher composition of oxygenrelated defects, as well as loosely bound oxygen and hydroxyl species near the tetrapod surface. These increased defect species primarily resulted from both hydrogen and argon plasmas, but were less significant for oxygen-plasma-treated tetrapods. The role of defects in photocatalytic activity has been extensively studied.46−54 Bulk defects can result in the reduction of photocatalytic activity, through the loss of photogenerated carriers at nonradiative defects, which serve as recombination centers.50 Because all plasma-treated samples exhibited lower UV emission intensities than the as-grown samples, it can be concluded that the former likely contained more nonradiative defects. Oxygen vacancies have commonly been proposed to result in increased photocatalytic activity.46−49,52−54 More specifically, surface defects and surface oxygen deficiencies are commonly associated with increased photocatalytic activity, with the rationale that charge trapping at oxygen vacancies or surface hydroxyl groups on the catalyst surface reduces recombination losses.46−52 Unlike surface oxygen vacancies, bulk oxygen vacancies have been reported to lead to a reduction of photocatalytic activity.52 However, in the majority of those studies, ZnO samples were obtained in a reduced environment with the presence of both surface and bulk oxygen defects, while XPS has seldom been facilitated to obtain information on surface oxygen deficiency.46−48 In the case where XPS was available, only two samples were compared,49 or XPS has been used to determine the presence of surface hydroxyl groups.51 Contrary to a previous report,51 the increased photocatalytic activity was not associated with the increased higher-energy XPS O 1s peak. In fact, the samples that required three-peak fitting, HP and AP, exhibited the lowest photocatalytic activities. In this work, we investigated the influence of plasma treatments, which more significantly affect surface defects compared to annealing (annealing can affect both bulk and surface defects), and performed a detailed analysis of the sample properties. From the obtained results, we conclude that surface stoichiometry plays a significant role in the photocatalytic dye degradation and the high oxygen deficiency at or near the surface is not necessarily beneficial to the photocatalytic activity, contrary to common assumptions that oxygen vacancies or surface hydroxyl groups play a critical role in photocatalytic degradation.46−53

might lead to an increased surface area and, consequently, an increased number of available active sites on the sample surface, which would give rise to increased ROS production and dye adsorption. This is in agreement with the observed increase in surface roughness from HRTEM images shown in Figure 2. On the other hand, differences in the effects of plasma treatments can be deduced from the XPS results, which show a marked variation in surface stoichiometry, in particular, the compositions of different oxygen environments. FTIR spectra also provided evidence for modification of the surface properties of ZnO tetrapods by plasma treatments, simultaneously indicating the distinctions in the effects due to different

4. CONCLUSIONS We have studied the effecs t of plasma treatment by different gas species on ZnO tetrapods. A series of characterizations of the optical and excitonic properties, surface composition, ROS production, and photocatalytic activities of the as-grown and plasma-treated tetrapod samples were performed. We found that there is a general effect of plasma treatment on the surface of tetrapod samples that leads to increased OH• generation and surface adsorption of MB. In addition to this general modification, changes in native defects resulting from plasma treatment were found to be dependent on the plasma species, resulting in the variations in photocatalytic activities. Oxygen

Figure 6. Electron spin resonance spectra of as-grown and different plasma-treated ZnO tetrapod samples with 5,5-dimethyl-1-pyrroline N-oxide as a spin trap.

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Ren, F.; Zavada, J. M. Hydrogen Plasma Treatment Effects on Electrical and Optical Properties of n-ZnO. J. Appl. Phys. 2003, 94, 400−406. (12) Ohashi, N.; Ishigaki, T.; Okada, N.; Sekiguchi, T.; Sakaguchi, I.; Haneda, H. Effect of Hydrogen Doping on Ultraviolet Emission Spectra of Various Types of ZnO. Appl. Phys. Lett. 2002, 80, 2869− 2871. (13) Ra, H.-W.; Im, Y.-H. Effect of Chemically Reactive Species on Properties of ZnO Nanowires Exposed to Oxygen and Hydrogen Plasma. Nanotechnology 2008, 19, 485710. (14) Zhao, S.-L.; Wang, Y.-S.; Gao, S.; Yang, Y.-F.; Xu, Z. The Effect of the Oxygen Plasma Treatment for ITO and ZnO Nanorods on the Electroluminescence of ZnO Nanorod/MEH-PPV Heterostructure Devices. Chin. Phys. Lett. 2013, 30, 037802. (15) Zhong, Y.; Djurišić, A. B.; Hsu, Y. F.; Wong, K. S.; Brauer, G.; Ling, C. C.; Chan, W. K. Exceptionally Long Exciton Photoluminescence Lifetime in ZnO Tetrapods. J. Phys. Chem. C 2008, 112, 16286−16295. (16) Jiang, S.; Ren, Z.; Gong, S.; Yin, S.; Yu, Y.; Li, X.; Xu, G.; Shen, G.; Han, G. Tunable Photoluminescence Properties of Well-Aligned ZnO Nanorod Array by Oxygen Plasma Post-Treatment. Appl. Surf. Sci. 2014, 289, 252−256. (17) Meyer, B. K.; Alves, H.; Hofmann, D. M.; Kriegseis, W.; Forster, D.; Bertram, F.; Christen, J.; Hoffmann, A.; Straβburg, M.; Dworzak, M.; Haboeck, U.; Rodina, A. V. Bound Exciton and Donor−Acceptor Pair Recombinations in ZnO. Phys. Status Solidi B 2004, 241, 231− 260. (18) Bohle, D. S.; Spina, C. J. Cationic and Anionic Surface Binding Sites on Nanocrystalline Zinc Oxide: Surface Influence on Photoluminescence and Photocatalysis. J. Am. Chem. Soc. 2009, 131, 4397− 4404. (19) Faber, H.; Hirschmann, J.; Klaumünzer, M.; Braunschweig, B.; Peukert, W.; Halik, M. Impact of Oxygen Plasma Treatment on the Device Performance of Zinc Oxide Nanoparticle-Based Thin-Film Transistors. ACS Appl. Mater. Interfaces 2012, 4, 1693−1696. (20) Kim, M. S.; Kim, T. H.; Kim, D. Y.; Kim, G. S.; Choi, H. Y.; Cho, M. Y.; Jeon, S. M.; Kim, J. S.; Kim, J. S.; Lee, D. Y.; Son, J. S.; Lee, J. I.; Kim, J. H.; Kim, E.; Hwang, D.-W.; Leem, J. Y. Improvement in Crystallinity and Optical Properties of ZnO Epitaxial Layers by Thermal Annealed ZnO Buffer Layers with Oxygen Plasma. J. Cryst. Growth 2009, 311, 3568−3572. (21) Du, P. F.; Song, L. X.; Xiong, J.; Li, N.; Xi, Z. Q.; Wang, L. C.; Jin, D. L.; Guo, S. Y.; Yuan, Y. F. Coaxial Electrospun TiO2/ZnO Core−Sheath Nanofibers Film: Novel Structure for Photoanode of Dye-Sensitized Solar Cells. Electrochim. Acta 2012, 78, 392−397. (22) Inamdar, A. I.; Mujawar, S. H.; Ganesan, V.; Patil, P. S. Surfactant-Mediated Growth of Nanostructured Zinc Oxide Thin Films via Electrodeposition and Their Photoelectrochemical Performance. Nanotechnology 2008, 19, 325706. (23) Noei, H.; Qiu, H. S.; Wang, Y. M.; Löffler, E.; Wöll, C.; Muhler, M. The Identification of Hydroxyl Groups on ZnO Nanoparticles by Infrared Spectroscopy. Phys. Chem. Chem. Phys. 2008, 10, 7092−7097. (24) Zheng, Y. H.; Chen, C. Q.; Zhan, Y. Y.; Lin, X. Y.; Zheng, Q.; Wei, K. M.; Zhu, J. F.; Zhu, Y. J. Luminescence and Photocatalytic Activity of ZnO Nanocrystals: Correlation between Structure and Property. Inorg. Chem. 2007, 46, 6675−6682. (25) Lee, S.; Bang, S.; Park, J.; Park, S.; Jeong, W.; Jeon, H. The Effect of Oxygen Remote Plasma Treatment on ZnO TFTs Fabricated by Atomic Layer Deposition. Phys. Status Solidi B 2010, 207, 1845− 1849. (26) Kim, W. S.; Kwak, G. J.; Jung, M. B.; Jo, S. K.; Miller, J. B.; Gellman, A. J.; Yong, K. J. Surface and Internal Reactions of ZnO Nanowires: Etching and Bulk Defect Passivation by H Atoms. J. Phys. Chem. C 2012, 116, 16093−16097. (27) Fabbri, F.; Villani, M.; Catellani, A.; Calzolari, A.; Cicero, G.; Calestani, D.; Calestani, G.; Zappettini, A.; Dierre, B.; Sekiguchi, T.; Salviati, G. Zn Vacancy Induced Green Luminescence on Non-Polar Surfaces in ZnO Nanostructures. Sci. Rep. 2014, 4, 5158.

plasma treatment was found to significantly reduce the overall photoluminescence and increase defect emission, whereas it enhanced the photocatalytic activity. Hydrogen and argon plasmas, however, resulted in significant reductions of the photocatalytic activity, despite an increase in oxygen deficiency/ oxygen-related defects near the surface.

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ASSOCIATED CONTENT

S Supporting Information *

XRD patterns, TEM images and SAED patterns, and cathodoluminescence (CL) spectra of different ZnO tetrapod samples. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +852 2859 7946. Fax: +852 2559 9152. E-mail: dalek@ hku.hk. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Strategic Research Theme, University Development Fund, Seed Funding Grant (of the University of Hong Kong) is acknowledged.

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REFERENCES

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