Native Defects in ZnO: Effect on Dye Adsorption and Photocatalytic

Article pubs.acs.org/JPCC

Native Defects in ZnO: Effect on Dye Adsorption and Photocatalytic Degradation Fangzhou Liu,† Yu Hang Leung,† Aleksandra B. Djurišić,*,† Alan Man Ching Ng,†,‡ and Wai Kin Chan§ †

Department of Physics, The University of Hong Kong, Pokfulam Road, Hong Kong Department of Physics, South University of Science and Technology of China, Shenzhen 518055, China § Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong ‡

ABSTRACT: We investigated the influence of native defects on the adsorption and photocatalytic degradation of anionic and cationic dyes for different ZnO nanoparticles. We found that there was no relationship between the dye adsorption onto ZnO nanoparticles and their photocatalytic activity. Fast photocatalytic degradation could be observed for samples having a low concentration of nonradiative defects (and thus low recombination losses of photogenerated carriers), regardless of the amount of dye adsorbed onto the surface. While the absorption of cationic dyes was not significantly affected by ZnO nanoparticle properties, dye adsorption of several anionic dyes was strongly affected by native defects in ZnO. The defects involved in the dye adsorption are likely shallow donor centers exhibiting an electron spin resonance peak at g ≈ 1.957, resulting in positively charged sites at the surface.

1. INTRODUCTION Organic dyes are commonly present in wastewater effluents of different industries, such as textile, leather goods, cosmetics, food, plastics, consumer electronics, etc.1−3 Therefore, there is considerable interest in the degradation of dyes, in particular photocatalytic degradation.1−3 Among different factors affecting the efficiency of photocatalytic degradation of dyes, the adsorption of the dye onto the photocatalyst is typically considered to play an important role.1,4 Numerous studies have reported that adsorption of the organic compound is necessary for its successful photocatalytic degradation,1,4,5 while the studies reporting that it does not have significant influence represent a minority view.1,6,7 Thus, it is of significant interest to examine the dye adsorption of different dyes on the photocatalyst material in detail. If the increased dye adsorption indeed results in more efficient photocatalytic dye degradation, then it is reasonable to concentrate on synthesis of photocatalysts with high surface area and high dye adsorption. However, if this is not the case, then the emphasis on development of photocatalysts with high surface area and high dye adsorption does not contribute to the development of novel efficient photocatalysts, as well as to the improvement of our understanding of the processes involved. For targeted design of efficient photocatalysts, as opposed to random trial and error, it is essential to understand which material properties are linked with different processes involved in photocatalytic degradation of organic compounds. In addition, dye adsorption on metal oxide materials is also of interest for potential applications in dye-sensitized solar cells.8−10 In terms of the choice of photocatalyst material, TiO2 is the most commonly studied, followed by ZnO.1 ZnO nanostructures can be prepared by very simple, low cost, and low © 2013 American Chemical Society

temperature techniques, which contributes to the attractiveness of this material.11 In addition, ZnO has frequently exhibited similar or higher photocatalytic activity compared to TiO2,12−14 and its main drawback is inferior stability compared to TiO2 and possible photocorrosion in some cases.13 Consequently, photocatalytic applications of ZnO have been extensively studied.12−31 Previous studies include studies of detailed photocatalytic degradation mechanisms of dyes on ZnO16 and the mechanism of photocatalytic activity,17 as well as the studies of the influence of native defects,18,19,21,25,27,28,31 cationic and anionic surface binding sites,26 terminating surfaces, morphology, particle size, and/or surface area,20−24,29,30 on ZnO photocatalysis. The role of native defects in visible light photocatalytic activity,19,25 as well as photocatalytic activity in general,18,21,27,28,31 has been previously studied. However, the influence of native defects on the dye adsorption has not been investigated. We have previously found that acid orange 7 (AO7) adsorbed strongly onto one ZnO nanoparticle sample (commercial nanoparticle sample from MK Impex Corp., Division MK Nano, labeled as C-ZnO here), and the degradation could not be completed for the dye molecules attached to the particle even after prolonged illumination.7,8 On the other hand, AO7 adsorption was lower on another ZnO nanoparticle sample (commercial nanoparticle sample from Nanostructured & Amorphous Materials Inc., labeled as BZnO), while the degradation was significantly faster.17,18 The difference in the degradation rates could be explained by the Received: April 9, 2013 Revised: May 20, 2013 Published: May 24, 2013 12218

dx.doi.org/10.1021/jp403478q | J. Phys. Chem. C 2013, 117, 12218−12228

The Journal of Physical Chemistry C

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Centre for Diffraction Data (ICDD, PDF-2 release 2004) database. All of the characterization measurements have been performed at room temperature. Electron spin resonance (ESR) was measured using a Bruker EMX ESR spectrometer to characterize defects in nanoparticles and determine the production of reactive oxygen species (ROS) under illumination. To characterize defects, ESR measurements were performed on nanoparticles alone. For ROS detection, a spin trap 5-(diethoxyphosphoryl)-5-methylpyrroline N-oxide (DEPMPO, Enzo Biochem, Inc.) solution was placed on a drop-cast nanoparticle film and exposed to UV illumination for 2 min, followed by immediate ESR measurement. To further alter the C-ZnO nanoparticle properties, three different treatments (O2 annealing, oxygen plasma, and hydrothermal treatment32,33) were performed. For O2 annealing, the samples were annealed in a tube furnace at 600 °C for 1 h at an oxygen pressure of ∼300 Torr. Oxygen plasma treatment was performed using a Plasma Prep III Solid State Low Temperature Etcher at a pressure 250 mTorr and power of 50 W. Hydrothermal treatment was performed by placing CZnO samples in a 65 mL autoclave with 15 mL of deionized water. The samples were 3 cm above the water level and sealed in the autoclave at 150 °C for 12 h.32 Dye Adsorption and Photocatalytic Dye Degradation. MO was purchased from Acros Organics, AR88 was obtained from International Laboratory U.S.A., and MB, AO7, AR27, and R6G were obtained from Sigma Aldrich Co. The dyes were dissolved in deionized water at a specified concentration (5 mg/L for MB, MO, AO7, and R6G; 10 mg/L for AR27 and AR88). To measure the dye adsorption on different nanoparticles, 50 mg of nanoparticles were mixed with 50 mL of the dye solution in a Petri dish, and the mixture was stirred with a magnetic stirrer in the dark at room temperature for 2.5 h to reach equilibrium. Absorption measurements of the blank dye solution (A1) and the mixture immediately after 2.5 h stirring with the nanoparticles removed by a pore size 0.45 μm MillexFH PTFE filter (A2) were performed using a PerkinElmer Lambda Bio 40 UV/vis spectrometer, in order to obtain the estimated amount of dye absorbed by (A1 − A2)/A1.7,8,34 The amount of adsorbed dye can also be expressed in terms of the amount of dye moles or quantity (mg/g) adsorbed, which equals the product of the solution volume and the change in the concentration, divided by the weight of the photocatalyst material.14,35 For the photocatalytic experiments, the mixture was exposed under 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 (to remove nanoparticles), and then the absorption was measured. All of the photocatalytic degradation measurements were performed using the same type of filters for filtering the solution since this may affect the absolute amount of adsorbed dye, and all the experiments were repeated at least once to verify reproducibility. Consequently, the use of different filters can affect dye concentration and as expected the absolute degradation rate even though trends (comparison between different nanoparticles) remain the same. Furthermore, in the case of the strongly adsorbed dyes on CZnO (such as AO7),7,8 final outcome, i.e., nanoparticles remaining colored after long UV illumination, also does not change. For the photocatalytic dye degradation at a different pH, 1 M NaOH solution was added to the dye solution to achieve the desired pH values immediately before the start of the dye adsorption and photocatalytic experiments.

different concentrations of nonradiative defects resulting in different losses of photogenerated carriers,18 but the reasons for the observed differences in the dye adsorption were not fully clear. Therefore, we will conduct a comprehensive investigation of the dye adsorption for several different anionic and cationic dyes to examine the relationship between the dye adsorption and photocatalyst physical properties, as well as photocatalytic degradation efficiency. Among dyes studied, several azo dyes are included since azo dyes are the dominant type of dyes in practical applications (∼50−70% of dyes produced), in addition to being commonly studied.1 The dyes we will investigate here are the anionic azo dyes methyl orange (MO), AO7, acid red 27 (AR27), and acid red 88 (AR88) as well as cationic dyes methylene blue (MB) and rhodamine 6G (R6G). For ZnO samples, we included samples with different UV-tovisible emission ratios (high indicating that UV emission is dominant and low indicating that visible emission is dominant) and different amounts of dye adsorption (high indicating over 50%, low indicating below 13%, and moderate for values in between), to cover different combinations of these two properties. We found that there was little variation in the adsorption of cationic dyes and MO dye among different ZnO nanoparticles, while large differences in the amount of adsorbed dye were observed for AO7, AR27, and AR88. The observed differences in anionic dye adsorption were attributed to the differences in the concentrations of positively charged sites attributed to shallow donors. Such defect sites were found to affect the adsorption of anionic dyes which do not contain any positively charged groups. However, dye degradation was not affected and significant adsorption of the dye on the ZnO surface was not required for efficient degradation.

2. EXPERIMENTAL SECTION Materials and Material Characterization. Four ZnO nanoparticles from three suppliers were studied: A-ZnO (99.5%, APS 20 nm) and B-ZnO (99.9%, APS 90−200 nm) from Nanostructured & Amorphous Materials Inc.; C-ZnO (99.9%, APS 20 nm) from MK Impex Corp., Division MK Nano; and D-ZnO (99%+, APS 35−45 nm) from US Research Nanomaterials Inc. The morphologies of ZnO nanoparticles were characterized by transmission electron microscopy (TEM) and selected area electron diffraction (SAED) using a FEI Tecnai G2 20 S-Twin TEM. Aggregation size measurements for different ZnO samples dispersed in deionized water were performed using a ZETASIZER 3000HSA from Malvern Instruments Ltd. Photoluminescence (PL) spectra were obtained from films consisting of ZnO nanoparticles dropcast onto quartz substrates (1.5 cm ×1.5 cm). PL measurements were performed in ambient conditions using a HeCd (325 nm) laser as the excitation source. For Fourier transform infrared spectroscopy (FTIR) measurements, ZnO nanoparticles were mixed with infrared grade KBr (Sigma Aldrich Co.) to prepare pellets for the FTIR spectra measurements using a PerkinElmer Spectrum Two IR spectrometer. FTIR measurements for ZnO samples with adsorbed dye were performed as follows: after equilibrium period for adsorption (2.5 h in the dark), ZnO nanoparticles were collected by centrifugation and dried in a vacuum oven at room temperature. Then, the pellets with KBr were prepared as previously described. X-ray diffraction (XRD) patterns were characterized by a Bruker D8 ADVANCE X-ray diffractometer (λ(Cu Kα) = 1.5418 Å, rated as 1.6 kW) in the 2θ range between 20 and 75. The detected diffraction peaks were indexed with International 12219



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3. RESULTS AND DISCUSSION The properties of different nanoparticle samples are summarized in Table 1, which shows the particle sizes, aggregation sizes Table 1. Summary of the Properties of Different ZnO Nanoparticlesa A-ZnO

B-ZnO

C-ZnO

D-ZnO

APS (nm) aggregation size (nm)

20 ∼245 (47%)

UV emission peak position (nm) visible emission peak position (nm) UV-to-visible emission ratio

383

90−200 ∼321 (28%), ∼1483 (14%) 383

20 ∼538 (15%), ∼2700 (32%) 378

45 ∼149 (34%), ∼594 (3%) 381

630

503

659

577

0.4

56.1

1.1

5.1

a

Figure 2. Photoluminescence spectra of different ZnO nanoparticles.

positions at 630 and 659 nm, respectively. Different peak positions could indicate the presence of different defects or simply a different proportion of green and orange-red defects since both types of defect emissions are broad. Thus, a superposition of two broad peaks at different intensities could result in a single broad peak with a shifted center wavelength. It was previously proposed that anion binding surface sites are involved in 40% of visible emission and 50% of photocatalytic activity.26 Surface modifications of ZnO have resulted in quenching of visible and UV emissions with different degrees of quenching depending on the emission wavelength (higher quenching for UV).26 However, the chemical origin of the defects responsible for visible emissions is still not fully clear.36,37 Although green emission is commonly attributed to oxygen vacancies, experimental data of different studies are not fully consistent with this hypothesis and other alternative explanations have been proposed.36 In particular, electron irradiation which should create oxygen vacancies was found to reduce green emission and result in an increase of the orangered emission.36 There have also been proposals that green emission originates from zinc vacancies38 and that neither zinc vacancies nor oxygen vacancies participate in the green emission.39 This claim that neither type of isolated vacancy point defects is responsible for green emission39 is in agreement with the hypothesis that green and orange-red emissions may both originate from defect complexes, and that yellow emission at ∼580−590 nm may be related to the presence of surface adsorbates.37 Defect complexes are also likely responsible for the nonradiative recombination in ZnO, and they may involve zinc vacancy containing complexes.40,41 Figure 3 shows FTIR spectra of the samples. Significant differences in the spectra of different ZnO samples can be observed, indicating variations in surface adsorbates present. It should be noted that our samples are commercially available nanoparticles which were stored in ambient conditions before being used as-received, to study a realistic situation of a practical photocatalyst application. From the TEM images (Figure 1), we can observe that the samples mainly have a spherical shape, with a small number of elongated particles present in A-ZnO samples, and B-ZnO samples exhibited more pronounced faceting. The majority of crystal facets (>80%) are expected to be the low energy facets.42 In the absence of significant differences in sample morphology, it can be expected that the main cause of differences in the adsorbate presence are surface and near-surface defects, since the adsorption of different molecules on defect containing surfaces will differ from the adsorption on perfect defect-free surfaces. As opposed

APS denotes average particle size.

in solution, and the summary of the optical properties (peak positions and UV-to-visible emission ratios). Figure 1 shows

Figure 1. Transmission electron microscopy and selected area electron diffraction (insets) images of different ZnO nanoparticles; (a) A-ZnO, (b) B-ZnO, (c) C-ZnO, and (d) D-ZnO.

the TEM images and SAED patterns of different nanoparticle samples. We can observe that that B-ZnO and D-ZnO samples consist of larger nanoparticles, compared to A-ZnO and CZnO. B-ZnO and D-ZnO also exhibit strong UV emission and weak defect emission, as observed in PL spectra of the samples shown in Figure 2. However, large particle size does not necessarily imply large aggregate size in the solution, as can be observed from Table 1. The overall emission from B-ZnO is considerably stronger, and UV-to-visible emission is higher compared to D-ZnO (all of the PL spectra are measured in the same geometry). Both B-ZnO and D-ZnO exhibit significantly more intense UV emission compared to C-ZnO and A-ZnO, which is consistent with more intense and narrow peaks in XRD spectra indicating higher crystalline quality of these samples. The defect emission in B-ZnO and D-ZnO contains a more prominent green emission component (defect emission peaks located at 503 and 577 nm, respectively), while for AZnO and C-ZnO orange-red emission dominates with peak 12220



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Figure 5. Electron spin resonance spectra for different ZnO nanoparticles.

Figure 3. Fourier transform infrared spectra for different as-received ZnO nanoparticles.

and more pronounced UV emission from these two samples. From the ESR spectra, in all of the samples a peak corresponding to a g-factor of g ≈ 1.96 can be observed. The peak intensity is low for A-ZnO and B-ZnO, high for D-ZnO, and very high for C-ZnO. This peak was attributed to different defects such as oxygen vacancies,46 as well as shallow donors and/or conduction band electrons.38,39,47−54 Multiple shallow donors can result in this ESR peak.38 The intensity of this ESR peak was reported to be correlated with green emission attributed to oxygen vacancies,46 as well as yellow emission attributed to donor doping.48 Furthermore, it has been clearly shown that although correlation between green emission intensity and ESR peak at g ≈ 1.96 may be observed in some samples, it is not the case for all samples.55 Considering the fact that singly ionized oxygen vacancies are associated with an ESR signal at g ≈ 1.995,36,38,49,51 the observed signal most likely originates from shallow donors. Also, since D-ZnO exhibits strong luminescence while C-ZnO exhibits weak luminescence, these samples may contain different shallow donor sites and/or shallow donors may not be the dominant defects involved in nonradiative recombination. The production of ROS was also investigated by ESR, and the obtained results are shown in Figure 6. Dye degradation

to the experiments in ultrahigh vacuum, samples in ambient (or water solutions during photocatalysis experiments) will inevitably contain adsorbates. The amount and type of adsorbates is expected to affect the dye adsorption since a significant dependence of dye adsorption onto the surface in the presence of coadsorbates is well established.1 From the FTIR spectra, we can observe several peaks in the spectral range 1000−1800 cm−1 corresponding to various C−H, −CH2, CC, C−C, C−O−C, and C−O vibrations,43−45 which indicate the presence of organic surface adsorbates. We can also identify a broad resonance corresponding to OH group vibrations at ∼3200−3600 cm−1,27,28,44 as well as a feature corresponding to a scissoring mode of molecular water at ∼1630 cm−1.28,45 A-ZnO samples have the largest amount of surface adsorbates, while B-ZnO have very low content of organic adsorbates based on absence of prominent adsorbate peaks in the range 1000−1800 cm−1. However, C-ZnO and DZnO have similar numbers and relative intensities (compared to OH group vibrations) of peaks in the spectral range 1000− 1800 cm−1, although peak positions are different indicating different chemical structures of organic adsorbates present. The adsorbates may originate from the residue from the synthesis process or from the adsorbed molecules from the atmosphere. No clear relationship can be observed between the amount of adsorbates and the PL emission. ESR spectra and XRD patterns are shown in Figures 4 and 5, respectively. All of the samples exhibit only peaks corresponding to ZnO in XRD patterns, and B-ZnO and D-ZnO exhibit superior crystallinity (sharp and intense peaks) compared to AZnO and C-ZnO. This is in agreement with larger particle size

Figure 6. Electron spin resonance spectra for different ZnO nanoparticles with 5-(diethoxyphosphoryl)-5-methyl-pyrroline Noxide spin trap. Solid vertical lines denote positions of peaks corresponding to hydroxyl radicals, while dashed lines indicate positions of peaks corresponding to superoxide ion radicals.

Figure 4. X-ray diffraction patterns for different ZnO nanoparticles. 12221



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Figure 7. Degradation curves of anionic dyes for different ZnO nanoparticles (a) methyl orange, (b) acid orange 7, (c) acid red 27, and (d) acid red 88. The insets show absorption spectra at different times corresponding to the fastest degradation curve and photos of C-ZnO nanoparticles after prolonged illumination.

curves for anionic and cationic dyes are shown in Figures 7 and 8, respectively, while dye adsorption data are summarized in Table 2. The chemical structure of different dyes is illustrated in Figure 9. It can be observed that the intensity of ROS production (predominantly hydroxyl radicals, with a small contribution from superoxide ions in some of the samples18) shows no obvious relationship with the photocatalytic dye degradation rate in agreement with our previous work.17,18 For anionic dyes, B-ZnO and D-ZnO exhibit very fast dye degradation, while degradation is much slower for C-ZnO and A-ZnO. For cationic dyes, B-ZnO results in significantly faster degradation than A-ZnO and D-ZnO which exhibit comparable degradation rates, while dye degradation for CZnO is very slow. It should also be noted that the dyes which adsorbed strongly onto C-ZnO (AO7, AR27, and AR88), have not been completely degraded even after prolonged exposure to UV illumination. While the solution turns colorless, the dye adsorbed on the particles does not degrade, as illustrated on the photos in insets of Figure 7. The four nanoparticle samples obviously have very different levels of native defects and exhibit very different behaviors. There are two issues to consider here: one is the effect of native defects on photocatalytic activity and the second is the effect of native defects on the dye adsorption onto the surface. Concerning the effect of defects on photocatalytic activity, it has been previously proposed that the presence of defects in ZnO is beneficial to photocatalytic degradation since defects could enhance charge separation and thus reduce recombination losses27,28 and serve as active sites for the reaction.27 However, it was also previously reported that ZnO samples with improved crystalline quality and/or fewer defects exhibit improved photocatalytic activity.18,22 This is because improved crystalline quality characterized by weak defect emission and

Figure 8. Degradation curves of cationic dyes for different ZnO nanoparticles (a) methylene blue and (b) rhodamine 6G. The insets show absorption spectra at different times corresponding to the fastest degradation curve.

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for different dyes. It can also be observed B-ZnO and D-ZnO both exhibit green defect emission. Green defect emission was previously attributed to surface defects,11,55 and it was also found that surface modifications with a molecule attaching to anionic binding sites affect both visible emission and photocatalytic activity in ZnO.26 We can observe that for cationic dyes D-ZnO and A-ZnO exhibit similar activity in spite of the fact that one exhibits green while the other exhibits orange-red defect emission. This is consistent with the possibility that there are anionic binding sites which contribute to the photocatalytic activity of ZnO (reported to account for 50% of photocatalytic activity26). In general, it is reasonable to expect that the effect of visible emission in ZnO, if any, would be smaller compared to the effect of UV emission intensity (or nonradiative recombination losses). This is due to the fact that PL decay time for weakly emitting (in UV) ZnO nanomaterials is usually very short,56 while the decay time of different defect emissions is typically longer by several orders of magnitude (of the order of μs).57,58 Consequently, since visible emission is much slower process compared to charge transfer across the interface, it is reasonable to expect that it will not have clear correlation with photocatalytic activity. On the other hand, a fast process such as nonradiative recombination would result in a loss of photogenerated carriers before they can be transferred to molecules at the surface of the photocatalyst. For most azo dyes, the dye adsorption is the highest on CZnO which emits orange-red defect emission. From Table 1, visible emission of C-ZnO is centered at 659 nm, and from Table 2 the adsorption of all azo dyes with exception of MO is over 50%. The amount of adsorbed dye on A-ZnO which also exhibits orange-red defect emission (centered at 630 nm) is significantly lower. Similarly, dye adsorption on B-ZnO is much lower than that on D-ZnO, despite the fact that both samples exhibit green emission. Thus, the visible defect emission is likely not directly related to the dye adsorption. Furthermore, dye adsorption is very different for four anionic dyes investigated (differences among the dyes as well as among the ZnO samples), while the adsorption of cationic dyes is lower and shows smaller differences from one sample to another. For anionic dyes, the adsorption of MO is low on all samples, while the adsorption of AR88 (which also exhibits a change in the absorption spectrum after as little as 1 min of UV light exposure) is high. The adsorption of AR88 is significantly higher on C-ZnO and D-ZnO, while AO7 and AR27 exhibit low adsorption on A-ZnO and B-ZnO and high adsorption on C-ZnO and moderate adsorption on D-ZnO. Obviously, the degradation rate is not directly related to the dye adsorption, in agreement with previous reports.6,7,17,18 From Figure 7, we can observe that in all cases the fastest dye degradation occurs for B-ZnO samples, which exhibits low adsorption of azo dyes, followed by D-ZnO, and then A-ZnO, and C-ZnO. Thus, the degradation rate B-ZnO > D-ZnO > A-ZnO > C-ZnO does not follow the dye adsorption trend C-ZnO > D-ZnO > A-ZnO,BZnO, while it matches the trend of PL intensity (Figure 2, BZnO > D-ZnO > A-ZnO > C-ZnO). As previously discussed and in agreement with our previous work,18 it is expected that lower recombination losses of photogenerated carriers would result in higher photocatalytic activity. Concerning the obvious differences in behavior among different anionic azo dyes, they are likely related to their chemical structure. For example, different degradation pathways have been reported in the literature for methyl orange and acid orange 7.1 While MO

Table 2. Summary of Dye Adsorption for Different ZnO Nanoparticlesa A-ZnO methyl orange acid orange 7 acid red 27 acid red 88 rhodamine 6G methylene blue

0.01 0.02 0.03 0.68 0.12 0.04

(0.01) (0.07) (0.25) (6.69) (0.63) (0.19)

B-ZnO 0.01 0.09 0.06 0.59 0.11 0.04

(0.03) (0.43) (0.55) (5.81) (0.57) (0.20)

C-ZnO 0.09 0.55 0.76 0.97 0.11 0.07

(0.44) (2.68) (7.60) (9.59) (0.57) (0.35)

D-ZnO 0.01 0.13 0.25 0.86 0.11 0.03

(0.04) (0.62) (2.43) (8.56) (0.62) (0.17)

a The estimates of the surface adsorption of dyes on different nanoparticles are given by (A1 − A2)/A1, where A1 is the absorption of the dye solution without nanoparticles and A2 is the absorption of the dye solution with the nanoparticles removed by filter after reaching equilibrium.7,8,34 The amount of adsorbed dye in mg/g (value in brackets) is also shown. This value is determined as (C0 − Ct)(v/w), where v = 50 mL is the solution volume, w = 50 mg is the amount of photocatalyst, while C0 and Ct are the starting dye concentration and the dye concentration after adsorption equilibrium (determined from calibration curves of absorption spectra).35

Figure 9. Chemical formulas of different dyes: methyl orange, acid orange 7, acid red 27, acid red 88, methylene blue, and rhodamine 6G.

high overall emission intensity (low concentration of both radiative and nonradiative defects) results in lower recombination losses.18 This is in particular the case for nonradiative defects,18 since nonradiative recombination results in decreased PL intensity as well as reduced photocatalytic activity.26 We can observe that samples exhibiting more intense PL emission (BZnO and D-ZnO) also have higher photocatalytic activity, indicating that lower loss of photogenerated carriers via nonradiative recombination is important for higher photocatalytic activity. Since the nonradiative defects in ZnO likely involve zinc vacancy containing defect complexes,40,41 reducing the overall defect density and the density of zinc vacancies may result in longer nonradiative PL lifetime41 which is expected to be beneficial to photocatalytic degradation efficiency. There are also additional factors affecting the activity, which is obvious from differences in relative performance of B-ZnO and D-ZnO 12223



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defect complexes,40,41 Since zinc vacancy is an acceptor type defect,36,38 nonradiative defects are likely unrelated to shallow donors, which affect the adsorption of azo dyes. In some samples with inferior crystal quality (such as C-ZnO), many different defects could be simultaneously present. D-ZnO on the other hand exhibits prominent and intense UV emission (Figure 2), despite significant presence of shallow donors (Figure 5). Due to the complex defect chemistry of ZnO, different combination of properties due to different types of native defects present could be achieved. It should also be noted that photocatalytic activity could be affected by both surface and bulk defects (since both types of defects can serve as recombination centers and result in recombination losses), while the dye adsorption is a surface process and thus it will be affected only by surface and near-surface defects. To further investigate the dye adsorption, we have selected C-ZnO nanoparticles (as samples exhibiting the strongest dye adsorption) and subjected them to different treatments (O2 annealing, oxygen plasma, hydrothermal treatment). AO7 was chosen as the test dye since its absorption estimate for asreceived nanoparticles is 0.55, so there is large range of possible change both up and down. Obtained results for photoluminescence, ESR, and photocatalytic degradation of AO7 are shown in Figure 11. Dye adsorption (estimated in the same manner as values given in Table 2) increases to 0.89 with O2 annealing, remains at a similar value for O plasma treatment (0.51), and it becomes significantly lower (0.23) for hydrothermal treatment. Similar to the comparison between different particles, there is no obvious correlation between PL spectra, photocatalytic dye degradation and dye adsorption (it should be noted that for O2 annealing large amount of dye is adsorbed to the particles and it does not degrade with prolonged illumination, i.e. particles remain orange while dye in the solution is degraded). We can observe that O2 annealing results in a significant increase in the intensity of ESR signal at g = 1.957, i.e., the same position as in as-received nanoparticles. For other two treatments, the peak position shifts to higher g factor value (still ∼1.96), indicating possible change in the dominant shallow donor type, since multiple shallow donors can be responsible for this signal.38,51−54 It was proposed that this signal originates from delocalized electrons,38 such as electrons in an impurity band rather than electron isolated at a single donor site.54 Due to the fact that multiple donors exhibit ESR peaks with g factor values in the range 1.957 ± 0.002, further information could be obtained from looking at the effects of treatment on the peak intensity.52 The hydrothermal treatment results in a significant decrease of dye adsorption, which can be attributed to the reactions between water vapor and surface defects.32,33 From this we can conclude that relevant shallow donors affecting the dye adsorption are likely located at or near the surface of the nanoparticles (which is to be expected since dye adsorption occurs at the surface), and these shallow donors result in an ESR signal at g = 1.957. Exact chemical identification of the shallow donor in question cannot be made on the basis of ESR spectrum (since different shallow donors result in similar peak positions at g = 1.96),38,51−54 but considering the details of the treatments the donor is likely not hydrogen. Oxygen annealing at 600 °C is above the temperature required for hydrogen desorption (∼420 °C)37 and thus would result in decreased concentration of hydrogen at the surface, while an increase in hydrogen presence would be reasonably expected in hydrothermal treatment which results in a different position of ESR peak and reduced dye adsorption.

degradation proceeded via progressive demethylation, various reaction intermediates were reported for AO7.1 The chemical structure of dye molecules can also affect the dye adsorption. In a study of degradation of acid blue dye, it was found that the dye adsorbs via positive diethylamino groups when ZnO surface is negatively charged, and negative sulfonyl groups when the surface is positively charged (ZnO surface charge is pH dependent).16 Zinc oxide surface was reported to contain both anionic and cationic binding sites.26 Unlike other dyes investigated here, MO contains an amino group and a sulfonyl group. If the anionic binding sites dominate on the surface, then a molecule containing both amino and sulfonyl group may have lower affinity to the surface, while those containing only sulfonyl groups would have higher affinity. Furthermore, if the surface charge of ZnO is changed, we would observe a change in the adsorption of dyes which adsorb strongly onto ZnO surface. This is indeed the case, as observed in Figure 10 which shows the comparison of

Figure 10. Degradation curves of acid orange 7 for C-ZnO and DZnO at different pH. The insets show corresponding photos of (from left to right) starting nanoparticles; nanoparticles after degradation at pH 6.4 and 8.

the degradation of AO7 at different pH values. When pH is increased to 8, significantly lower dye adsorption is observed for both C-ZnO and D-ZnO (reduction from 0.55 to 0.24 for C-ZnO and from 0.13 to 0.08 for D-ZnO). However, degradation rate is slower and in the case of C-ZnO we still do not observe complete degradation of the dye adsorbed onto the nanoparticles. C-ZnO and D-ZnO have one feature in common, and that is significant concentration of shallow donors, as observed from Figure 5. Since shallow donor sites are positively charged, if they are located at the surface they may serve as binding sites for anionic dyes. The shallow donor concentration in C-ZnO is much higher compared to D-ZnO based on the significantly higher intensity of ESR peak at g ≈ 1.96 (Figure 5), which is in agreement with much stronger dye adsorption onto C-ZnO (Table 2). However, the possibility that different types of shallow donors occur in these samples cannot be excluded, especially considering the fact that dye adsorbed onto D-ZnO particles degrades and particles remain white after degradation, while for C-ZnO dye adsorbed onto the nanoparticles does not completely degrade even after very long UV illumination, long after the solution became colorless due to degradation of the dye in solution, as shown in insets of Figures 7 and 10. Thus, from the obtained experimental results we can conclude that the photocatalytic activity is strongly dependent on nonradiative defects. Exact nature of those defects is not fully clear, but they probably involve zinc vacancy containing 12224



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Figure 12. Fourier transform infrared spectra of different ZnO nanoparticles before and after immersion into acid orange 7 solution. The Fourier transform infrared spectrum of the acid orange 7 dye is also shown for comparison.

likely require comprehensive theoretical and experimental study, preferably on single crystal surfaces for easier data analysis and interpretation, which is beyond the scope of this work. The information about AO7 adsorption on ZnO from FTIR spectra can be obtained by comparing the peak positions and intensities in the spectra of AO7, ZnO, and AO7+ZnO, looking for the shifting of the vibrational frequencies of the adsorbate, significant changes in the intensities of the bands, or the appearance of the new bands due to interaction.59 It was previously reported that the spectrum of AO7 on ZnO, unlike that of AO7 adsorbed onto TiO2, does not change significantly, and that sulfonate group is not affected by the adsorption.59 This would indicate weak interaction between ZnO and sulfonate group of AO7.59 In our work as well the intensity ratios of symmetric (∼1122 cm−1)59,61 and asymmetric (∼1212 cm−1)60,61 sulfonate adsorption bands are very similar in AO7 (0.957) and C-ZnO+AO7 (0.965), indicating weak interaction. This is different from AO7 adsoprtion on TiO259,62 and CeO263 where significant changes in sulfonate group vibrations were observed, and the adsorption of AO7 occurs via sulfonic group, forming a bidentate complex.59,62,63 Consequently, physisorption of the dye due to electrostatic interaction between negatively charged sulfonate group and positively charged shallow donor defects in the surface region of the nanoparticles likely occurs.

Figure 11. (a) Electron spin resonance spectra, (b) photoluminescence spectra, and (c) degradation curves of acid orange 7 for C-ZnO nanoparticles subjected to different treatments.

Finally, to obtain more insight into the mode of adsorption, FTIR measurements were performed on nanoparticles after dye adsorption, and the comparison of FTIR spectra of the particles before and after dye adsorption is shown in Figure 12. AO7 was used as a model dye for this investigation, since the adsorption of this dye to different materials has been extensively studied.59−63 It can be observed that significant changes occur for C-ZnO, while D-ZnO exhibits small and A-ZnO and B-ZnO exhibit negligible changes in agreement with estimates of significantly lower adsorption (Table 2). Therefore, AO7 adsorption on C-ZnO was examined in more detail, as shown in Figure 13, showing FTIR spectra in the spectral range relevant for the identification of interaction between AO7 and ZnO. It should be noted, however, that unique identification of vibrational frequencies and thus the identification of the adsorption modes is complicated by the fact that there can be multiple overlapping absorptions.64 Thus, conclusive identification of adsorption mode of AO7 would 12225



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ACKNOWLEDGMENTS

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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.

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Figure 13. Fourier transform infrared spectra of C-ZnO samples before and after immersion into acid orange 7 solution (magnified region relevant for discussion of dye adsorption modes). The Fourier transform infrared spectrum of the acid orange 7 dye is also shown for comparison.

4. CONCLUSIONS We investigated the influence of native defects on photocatalytic activity and dye adsorption onto the ZnO surface. We found that a lower concentration of nonradiative defects resulted in a higher photocatalytic activity. The adsorption of cationic dyes and an anionic dye (methyl orange), which also contained a positively charged group, was not significantly affected by ZnO native defects. Anionic dyes which contained no positively charged groups adsorbed strongly onto the ZnO samples which had a high shallow donor concentration, indicated by the ESR peak at g = 1.957. The high adsorption rate was not correlated with the high degradation rate. Samples with high defect concentration (C-ZnO) including high nonradiative defects as well as high shallow donor concentration will exhibit high anionic dye adsorption but low photocatalytic activity. Thus, for high photocatalytic activity low concentration of nonradiative defects is needed, which is not necessarily related to the concentration of shallow donors. In fact, an increased shallow donor concentration and consequently higher dye adsorption while maintaining high photocatalytic activity and bright light emission (D-ZnO) could also be achieved simultaneously, which may be beneficial for other applications of ZnO (for example dye-sensitized solar cells).

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Native Defects in ZnO: Effect on Dye Adsorption and Photocatalytic

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