ARTICLE pubs.acs.org/JPCC
Effect of Native Defects on Photocatalytic Properties of ZnO Mu Yao Guo,† Alan Man Ching Ng,† Fangzhou Liu,† Aleksandra B. Djurisic,*,† Wai Kin Chan,‡ Huimin Su,§ and Kam Sing Wong§ †
Department of Physics and ‡Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong § Department of Physics, The Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong ABSTRACT: We have studied the photocatalytic activity of ZnO tetrapods, ZnO powder, and different ZnO nanoparticles. While nanoparticles with small particle size exhibited higher BrunauerEmmettTeller (BET) surface area, tetrapods exhibited significantly higher photocatalytic activity for degradation of different dyes. Degradation of four different dyes has been achieved for at least 5 times shorter time for tetrapods compared to nanoparticles (20 nm average particle size). Different ZnO samples have been characterized by variable-temperature photoluminescence and time-resolved photoluminescence measurements, and the relationship between optical properties, native defects, production of the reactive oxygen species, and photocatalytic dye degradation has been discussed.
1. INTRODUCTION Metal oxide materials, such as TiO2, ZnO, etc., are of significant interest for photocatalytic applications. Although it is less commonly studied and has inferior stability compared to TiO2,1 ZnO is more efficient for degradation of various organic compounds.13 Consequently, there is an interest in the study of photocatalytic activity of ZnO, which has been investigated for a range of different morphologies.119 It has been shown that both activity and stability could be affected by changing the morphology of ZnO.1,2 Improvements in photocatalytic activity and/or stability have been attributed to different terminating crystal facets,1,4,9,11 preparation method,15 higher crystallinity,2,3,8 crystallite size,8 lower defects,2,8 increased surface defects,47,14,19 and increased surface area.5,18,19 Higher surface area would typically lead to an increased photocatalytic activity, although there have been demonstrations of increased activity of different morphologies of ZnO in spite of the lower surface areas.4,11 In addition, contradictory effects of native defects have been reported, with higher47,14 and lower2,8 concentrations having beneficial effects. In addition, it was proposed that there is an optimal surface oxygen vacancy concentration that would result in an increased photocatalytic activity, while lower and higher concentrations compared to an optimal one would result in reduced photocatalytic activity.13 Photoluminescence (PL) measurements are a convenient method to study the native defects in ZnO and investigate their effect on the photocatalytic activity.14,17 However, the relationship between the PL, defects, and photocatalytic activity is likely to be inherently complex, especially in metal oxides, which have rich defect chemistry. The presence of different defects could result in either improvement or worsening of the photocatalytic activity, depending on the type and location of native defects. r 2011 American Chemical Society
Different ZnO nanostructured morphologies can have very different optical properties and native defect types and concentrations. Among various ZnO morphologies, ZnO tetrapods are of significant interest since they can be synthesized by relatively simple procedures in large quantities. ZnO tetrapods have been shown to be efficient for the degradation of different dyes, such as methyl orange (MO),11 methylene blue (MB),12 and rhodamine B.19 In addition, their optical properties have been extensively studied.2023 Thus, here we studied the photocatalytic activity of ZnO tetrapods, commercial ZnO nanoparticles (NPs), and ZnO powder. ZnO tetrapods have significantly lower defect density compared to nanoparticles with small average particle size (APS), and depending on the growth temperature, different ratios of UV to defect emission could be obtained.23 Optical properties of the samples, reactive oxygen species (ROS) generation in aqueous solutions, and photocatalytic degradation of four different dyes have been studied. The relationship between native defects and photocatalytic activity is discussed.
2. EXPERIMENTAL SECTION Materials and Material Characterization. 1-ZnO (99.5%, APS 20 nm) and 2-ZnO (99.9%, APS 90200 nm) were obtained from Nanostructured & Amorphous Materials Inc. 3-ZnO nanoparticles (99.9%, APS 20 nm) were obtained from MK Impex Corp., Division MK Nano. ZnO powder (99.99%), labeled as 4-ZnO, was obtained from SigmaAldrich. Tetrapods Received: January 28, 2011 Revised: April 25, 2011 Published: May 13, 2011 11095
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The Journal of Physical Chemistry C were synthesized in a tube furnace by evaporating 0.2 g of Zn powder (99.995%, SigmaAldrich) in the flow of Ar gas (bubbled through the water before introduction into the furnace) at a flow rate of 0.2 Lpm.23 The evaporation was performed in ambient at a temperature of 950 °C for 15 min. After deposition, the white material (tetrapods) was collected from the inner surface of a quartz tube (∼3 cm inner diameter). For photocatalytic degradation experiments MB, acid orange 7 (AO7), and rhodamine 6G (R6G) were purchased from Sigma Aldrich Co., while MO was obtained from B&T. Spin traps 5,5Dimethyl-1-pyrroline N-oxide (DMPO) and 5-(diethoxyphosphoryl)-5-methylpyrroline N-oxide (DEPMPO) were purchased from SigmaAldrich Co. and Enzo Biochem Inc., respectively. The morphology of ZnO nanomaterials was examined by transmission electron microscopy (TEM) and selected areas electron diffraction (SAED) using a Phillips Tecnai G2 20 S-Twin TEM. BrunauerEmmettTeller (BET) surface area of the samples was determined on a Micromeritics ASAP 2020 surface area and porosity analyzer. The aggregation size in aqueous solution was determined with a Zetasizer 3000HSA from Malvern Instruments Ltd. Electron paramagnetic resonance (EPR) spectra were recorded at room temperature on a Bruker EMX EPR spectrometer. The solution for the measurement was prepared by adding 0.02 M DMPO to 1 mg/mL nanomaterial suspension or by adding 0.02 M DEPMPO to 5 mg/mL nanomaterial suspension. The solution was then transferred to an EPR tube and illuminated with 365 nm UV (30 mW/ cm2) for 2 min, followed by immediate EPR measurement with the following instrument settings: microwave power 20 mW, microwave frequency 9.74 GHz, modulation frequency 100 kHz, modulation amplitude 2 G, time constant 655 ms, and sweep time 84 s. For photoluminescence measurements, ZnO samples were deposited onto silicon wafers. Second harmonic output at 355 nm from a femtosecond titaniumsapphire oscillator operating at a repetition rate of 76 MHz with 150 fs pulse width was used as the excitation source. Low-temperature and roomtemperature photoluminescence were measured by use of a liquid-helium cryostat and a spectrometer coupled to a liquidnitrogen-cooled charge-coupled device (CCD) for high spectral resolution measurements. An Ocean Optics fiber-coupled spectrometer was used for low spectral resolution measurements. Time-resolved photoluminescence was carried out with a Hamamatsu model C4334 streak camera. PL measurements at room temperature were also performed with a HeCd (325 nm) laser as an excitation source, and the spectra were collected on a PDA512USB (Control Development Inc.) fiber optic spectrometer. Photocatalysis Experiments. The dye solutions were prepared by dissolving 5 mg of the dye in 1 L of deionized (DI) water. Suspension of nanomaterial (tetrapods, powder, or nanoparticles, 50 mg) in dye solution (50 mL) was placed in a Petri dish and stirred with a magnetic stirrer in the dark for 1 h to achieve equilibrium. The solution was then placed under UV illumination (365 nm, 66.2 mW/cm2, Blak-Ray B-100 AP lamp) for a specified time. Absorption spectrum of the solution was measured immediately before the start of UV exposure and at fixed time intervals (2.5 mL of the solution was withdrawn for the absorption measurements at each time; the solution was passed through a filter to remove the nanoparticles or tetrapods before the measurement). The absorption measurements were performed on a PerkinElmer Lambda Bio 40 UV/vis spectrometer.
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Figure 1. TEM images of ZnO nanoparticles and powder: (a) 1-ZnO, (b) 2-ZnO, (c) 3-ZnO, (d) 4-ZnO, (e) ZnO tetrapod, and (f) highmagnification TEM image of ZnO tetrapod. (Insets) Corresponding SAED patterns.
3. RESULTS AND DISCUSSION Figure 1 shows TEM images of different ZnO samples. Commercial NPs and powder consist of particles of different sizes and shapes, but most of the individual particles are smaller than 50 nm for 1-ZnO and 3-ZnO. The tetrapods are significantly larger, with the length of tetrapod legs in the range of micrometers. Tetrapod legs typically grow along [0001] direction, resulting in (1010) terminating facets for the tetrapod legs.11 From the high-resolution TEM image and the corresponding SAED pattern, the growth direction of tetrapods is in agreement with the literature. Obtained BET surface areas and aggregation sizes in water suspensions for different ZnO samples are summarized in Table 1. Different mechanisms and different effects of native defects have been proposed in the literature for photocatalytic dye degradation by use of ZnO. Photocatalytic activity of ZnO nanostructures has been attributed to a high concentration of surface donor defects (oxygen vacancies,3,6 zinc interstitials3), and degradation of MB has been attributed to the electron transfer between donor states and MB.3,6 Photodegradation of dyes (methyl red, methyl orange) by photooxidation reaction with hydroxyl radicals5,9 and hydrogen peroxide9 as oxidating 11096
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Table 1. BET Surface Area, Aggregation Size, PL Decay Times, and Estimated Surface Adsorption of Dyes on Different Nanoparticles a tetrapod 2
1-ZnO
2-ZnO
3-ZnO
4-ZnO
BET surface area (m /g)
2.2
45.9
7.0
60.7
10.0
aggregation size (nm) PL decay time τ1 (ns)
>5000 6.4 ( 0.9
220, 870 0.028 ( 0.001
340, 860 2.9 ( 0.2
>5000 1.3 ( 0.1
350 (80%), 1000 1.6 ( 0.1
PL decay time τ2 (ns)
0.5 ( 0.05
0.24 ( 0.03
0.2 ( 0.02
0.17 ( 0.02
MOb
0.01
0.03
0.06
0.09
0.01
AO7b
0.03
0.01
0.04
0.87
0.07
MBb
0.06
0.05
0.10
0.09
0.14
R6Gb
0.10
0.07
0.16
0.10
0.08
Estimated surface adsorption is (A1 A2)/A1, where A1 is the maximum absorption of the dye solution without nanoparticles. After addition of the nanoparticles, stirring for 1 h in the dark to achieve equilibrium, and filtering to remove nanoparticles (together with the dye adsorbed onto them), absorption maximum A2 is obtained. b Dyes: MO, methyl orange; AO7, acid orange 7; MB, methylene blue; R6G, rhodamine 6G. a
intermediates was also proposed, and in this case higher activity in the presence of more surface defects (oxygen vacancies) was attributed to lower recombination between photogenerated electrons and holes with oxygen vacancies serving as electron traps.5 Higher activity in degradation of rhodamine B and MO was attributed to increased concentration of both electron traps (oxygen vacancies) and hole traps (oxygen interstitials).7,14 Methyl orange degradation was attributed to a reaction with both hydroxyl radicals and superoxide ions10 or superoxide ions only,11 and it was proposed that silver nanoparticles deposited on ZnO surface served to reduce recombination of photogenerated carriers.10 Thus, while different mechanisms of photocatalytic dye degradation have been proposed, it is generally accepted that recombination of photogenerated carriers needs to be suppressed to achieve increased photocatalytic efficiency. Native defects can serve both as recombination sites and as active sites participating in surface photocatalytic reactions. Thus, we have examined the optical properties, ROS generation, and photocatalytic activity of the tetrapods and nanoparticles. Figure 2 shows the PL spectra (at 4 K and room temperature) of different ZnO samples, as well as PL decay curves obtained from TRPL measurements. It can be observed that the tetrapod, 2-ZnO, and 4-ZnO samples exhibit prominent UV emission. For 3-ZnO samples, UV emission is stronger than defect emission, but overall emission intensity is considerably weaker. The lowest emission intensity and the most pronounced defect emission is obtained for 1-ZnO samples. In addition, 1-ZnO NPs exhibit redorange defect emission, different from green observed in tetrapods and other ZnO samples. The difference in shape and position of the visible emission peak indicates that different types of defects are likely present in these samples. Different hypotheses have been proposed in the literature on the origin of different defect emissions,20,2325 and the exact chemical identity of defects responsible for visible emissions in ZnO is still controversial. For example, proposed hypotheses for the green emission involve oxygen vacancies, copper impurities, surface defects/defect complexes, zinc vacancies, oxygen antisites, oxygen vacancies, and zinc interstitials,24 while for orange-red emission, oxygen interstitials24 and defect complexes possibly involving zinc vacancies25 have been proposed. Consequently, it is difficult to establish conclusively what type and concentrations of defects are present in the samples. Nevertheless, from the differences in peak positions and intensities, it is obvious that the samples investigated contain different types and concentrations of native defects, which can
also be concluded from different positions of bound exciton peaks in the low-temperature spectra shown in Figure 2a,b. In the case of 1-ZnO, 3-ZnO, and 4-ZnO samples, we can resolve only a single broad peak located at ∼3.3583.360 eV, while for tetrapods and 2-ZnO samples, three peaks (3.363, 3.361, and ∼3.3573.358 eV) can be clearly resolved. The exciton peak close to ∼3.3628 eV likely originates from an exciton bound to hydrogen donor, while lines at ∼3.357, ∼3.360, and ∼3.361 eV could be assigned to indium, gallium, and aluminum donors, respectively.23,24 The most significant difference between the tetrapods and the nanorods is in the PL decay time, as shown in Figure 2d. It can be clearly observed that the slowest decay occurs for tetrapods, followed by 2-ZnO, 4-ZnO, 3-ZnO, and 1-ZnO samples. The obtained decay time constants (single exponential for 1-ZnO and biexponential for the other samples) are summarized in Table 1. The long PL decay time for ZnO tetrapods has been assigned to a low concentration of nonradiative defects in these nanostructures.21,23 The smallest size samples, 1-ZnO and 3-ZnO, exhibit the fastest decay. The faster decay of luminescence in smaller ZnO nanoparticles has been previously attributed to nonradiative quenching at surface defects/impurities.26 We also investigated ROS generation for different samples and their photocatalytic activity. EPR spectra for two different spin traps are shown in Figure 3. It can be observed that ZnO NPs exhibit a considerably stronger signal compared to tetrapods. EPR spectra of a dilute H2O2 solution, which produces OH• radicals under illumination, and a more concentrated H2O2 solution, which produces both OH• radicals and superoxide ions,27 are also shown for comparison. For all samples, peaks due to OH• radicals can be observed. With DEPMPO trap we can observe a superoxide ion signal for 2-ZnO and 4-ZnO samples, while for other samples there is negligible signal corresponding to superoxide ion peaks. For both spin traps, the weakest signal is observed from tetrapods and the strongest from 4-ZnO samples. 1-ZnO and 2-ZnO samples exhibit similar peak intensity for OH• radicals, while the signal for 3-ZnO samples is lower. It was reported that ZnO tetrapods exhibited higher production of superoxide ion radicals upon illumination compared to ZnO nanoparticles and thus exhibited higher photocatalytic activity.11 However, no superoxide ion signal from tetrapods was detected in our work. This could possibly occur due to small differences in the experimental procedures, resulting in differences in native defects concentrations, or due to different method of reactive oxygen species detection. Nitroblue tetrazolium probe (NBT) in 11097
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Figure 2. (a, b) Low-temperature (4 K) PL spectra of different ZnO samples; (c) room-temperature PL spectra of different ZnO samples; (d) PL decay curves of different ZnO samples.
a spectroscopic method has been previously used,11 while in our work we used EPR to detect reactive oxygen species. Spin trapping EPR measurements are typically more selective and sensitive methods for detecting superoxide ions (2040 times compared to cytochrome c).28 Spectroscopic probes such as NBT have the advantage of being simple methods but are less sensitive, more qualitative rather than quantitative, and can be subject to autoxidation in presence of oxygen.28 Thus, we can conclude that tetrapods are actually producing fewer ROS compared to nanoparticles and that the main ROS species produced is the OH• radical. Tetrapods exhibited higher photocatalytic activity compared to all nanoparticles and powder for all the dyes investigated in spite of their lower surface area, as shown in Figures 4 and 5 and in agreement with the literature.11 No correlation is observed between BET surface area, aggregation size, and dye degradation.29 The smallest nanoparticles, 1-ZnO and 3-ZnO, exhibit the slowest dye degradation in all cases. For 3-ZnO particles,
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Figure 3. EPR spectra of (a) H2O2 with DMPO and DEPMPO, (b) ZnO samples with DMPO, and (c) ZnO samples with DEPMPO.
decrease in the absorption of the AO7 solution is observed, but the dye adsorbed on the particle surface does not entirely degrade even for prolonged illumination (>30 min), as shown in the inset of Figure 4. This is the only sample/dye combination where significant dye adsorption onto the sample surface is found, as summarized in Table 1. This fact, together with low overall PL emission intensity and lower UV-to-visible emission ratio compared to ZnO samples with larger particle size, possibly indicates that 3-ZnO samples likely have different surface properties/ defects, which would also affect their photocatalytic activity. It was previously reported that tetrapods exhibited improved photocatalytic activity compared to titania nanoparticles and ZnO powders.19 One possible reason for this is the tetrapod morphology, which results in a higher proportion of (1010) terminating facets. It was proposed that nanostructured morphologies that have higher proportions of (1010) surfaces have advantages in terms of both efficiency1,11 and stability,1 since the 11098
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Figure 4. Dye degradation curves for ZnO samples for (a) MO and (b) AO7. (Inset) Photograph of samples 3-ZnO and 4-ZnO.
Figure 6. (a) PL spectra of tetrapods grown in different temperature zones. (b, c) EPR spectra of tetrapods grown in different temperature zones with (b) DMPO or (c) DEPMPO.
Figure 5. Dye degradation curves for ZnO samples for (a) MB and (b) R6G.
(1010) surface has been shown to be more photocatalytically active for single-crystal ZnO samples.9 However, it should be noted that tetrapod morphology is not necessarily inherently
beneficial for photocatalysis. It was shown that tetrapods exhibited only a small improvement in degradation efficiency compared to nanoparticles, but in this case tetrapods had irregular morphology and the growth method was different compared to our work.15 Furthermore, we observe relatively fast dye degradation in samples 2-ZnO and 4-ZnO, although the degradation is slower compared to tetrapods. What these samples have in common is slow decay of photoluminescence, indicating good crystalline quality and low concentration of nonradiative defects. Therefore, we can conclude that native defects are most likely a critical factor in the photocatalytic efficiency of ZnO tetrapods, since lower concentration of nonradiative defects would result in lower losses of photogenerated carriers. From the comparison of all the different samples, we can conclude that while other factors (surface area, surface properties) may play a role in photocatalysis, low nonradiative defect/ recombination center concentration enhances photocatalytic activity. Consequently tetrapods, as the samples with the lowest 11099
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defects,23 it is not related to the defects involved in the OH• radical generation. In terms of dye degradation, we can observe different trends of dye degradation for different dyes, as shown in Figure 7. In the case of MO, which is less sensitive to OH• radicals,29 the fastest degradation is obtained for MT samples, while for AO7, which is more sensitive to OH• radicals,29 the slowest degradation is obtained for the MT sample. Thus, we can conclude that overall reduction of the nonradiative defects is beneficial for photocatalytic activity, while the effect of ROS radical production will be dependent on the material undergoing degradation. It should be noted that the differences between tetrapods grown in different temperature zones are smaller than the differences between individual nanoparticles and tetrapods. This is likely because even tetrapods with the fastest PL decay (for detailed optical characterization, see ref 23) still have longer decay times compared to all other samples. We can also conclude that once nonradiative decay kinetics is sufficiently slow so that charge transfer at the photocatalyst surface can be efficient, further improvements in the sample quality would result in less dramatic improvements in activity. Nonradiative recombination centers have significant negative impact on the photocatalytic activity, while the defects responsible for green emission have no significant effect on ROS production or photocatalytic activity.
Figure 7. Dye degradation curves for tetrapods grown in different temperature zones for (a) MO and (b) AO7.
BET surface area, highest aggregation size in aqueous solution, and lowest ROS generation, still exhibit the highest photocatalytic activity. It should be noted that tetrapods exhibited higher photocatalytic activity for dye degradation in spite of producing fewer ROS upon illumination compared to NPs. This is likely due to the fact that dye degradation can occur either via direct transfer of photogenerated carriers or via reactions with generated ROS as intermediate species. It was reported that the quantum yield of OH• radicals is several orders of magnitude smaller than that of photogenerated holes,30 so that oxidation by photogenerated holes is likely the primary mechanism of dye degradation. Contributions of photogenerated carriers and ROS to the dye degradation are different for different dyes, but we have shown previously that, for all four dyes used, direct reaction with photogenerated carriers is the dominant degradation mechanism.29 Consequently, we can conclude that for more efficient photocatalytic degradation of compounds where the dominant degradation mechanism is direct reaction with photogenerated charge carriers, reduction of nonradiative defects that lead to recombination losses is necessary. To study the influence of defects in more detail, we have separated the tetrapods on the basis of growth temperature, since this significantly affected their optical properties.23 The tetrapods grown in high-temperature zone are labeled HT, those from the middle-temperature zone (close to optimal growth temperature) are labeled MT, and the tetrapods grown in low-temperature zone are labeled LT. Obtained PL and EPR spectra are shown in Figure 6. It can be observed that MT tetrapods have the lowest defect emission and also the lowest OH• radical production. However, HT samples exhibit higher signal due to OH• radicals but lower green emission compared to LT samples. Thus, although green emission has been in some cases linked to surface
4. CONCLUSIONS We have investigated the photocatalytic activity of ZnO tetrapods and nanoparticles and the relationship between photocatalytic activity for dye degradation, ROS production, and optical properties. We found that there is no relationship between green emission and photocatalytic activity in samples with good optical quality, while reduction in the nonradiative defect concentration resulted in increased photocatalytic activity of tetrapods. We also found that the samples exhibiting low production of OH• radicals (MT tetrapods) still exhibited efficient photocatalytic activity, which was attributed to the dominance of direct reactions with photogenerated charge carriers. ’ AUTHOR INFORMATION Corresponding Author
*Tel þ852 2859 7946; fax þ852 2559 9152; e-mail
[email protected].
’ ACKNOWLEDGMENT Financial support from the project RGC CRF CityU6/CRF/ 08 and the Strategic Research Theme, University Development Fund, and Small Project Funding of the University of Hong Kong is acknowledged. We thank Professor K. Y. Chan, University of Hong Kong, for BET surface area measurements. ’ REFERENCES (1) Chu, D. W.; Masuda, Y.; Ohji, T.; Kato, K. Formation and photocatalytic application of ZnO nanotubes using aqueous solution. Langmuir 2010, 26, 2811–2815. (2) Li, Y. Z.; Xie, W.; Hu, X. L.; Shen, G. F.; Zhou, X.; Xiang, Y.; Zhao, X. J.; Fang, P. F. Comparison of dye photodegradation and its coupling with light-to-electricity conversion over TiO2 and ZnO. Langmuir 2010, 26, 591–597. (3) Warule, S. S.; Chaudhari, N. S.; Kale, B. B.; More, M. A. Novel sonochemical assisted hydrothermal approach towards the controllable synthesis of ZnO nanorods, nanocups and nanoneedles and their photocatalytic study. CrystEngComm 2009, 11, 2776–2783. 11100
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