Photocatalytic Degradation of Methyl Orange over Single Crystalline

Feb 5, 2009 - Physics, UniVersity of South Florida, Tampa, Florida 33620 ... showed the highest activity for ZnO(10-10) followed by ZnO(0001)-Zn and t...
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Langmuir 2009, 25, 3310-3315

Photocatalytic Degradation of Methyl Orange over Single Crystalline ZnO: Orientation Dependence of Photoactivity and Photostability of ZnO Nikolai Kislov,† Jayeeta Lahiri,‡ Himanshu Verma,‡ D. Yogi Goswami,† Elias Stefanakos,† and Matthias Batzill*,‡ Clean Energy Research Center, UniVersity of South Florida, Tampa, Florida 33620, Department of Physics, UniVersity of South Florida, Tampa, Florida 33620

Langmuir 2009.25:3310-3315. Downloaded from pubs.acs.org by GEORGETOWN UNIV on 08/20/18. For personal use only.

ReceiVed NoVember 20, 2008. ReVised Manuscript ReceiVed January 8, 2009 The photocatalytic destruction of methyl orange in aqueous solution has been studied over single crystal ZnO surfaces under UV irradiation. Differences in the apparent reaction rates between the polar surfaces (first order) and the nonpolar ZnO(10-10) surface (zero order) were observed. Reaction rates for different crystallographic orientations showed the highest activity for ZnO(10-10) followed by ZnO(0001)-Zn and the lowest activity for ZnO(000-1)-O surfaces. In addition, the etching of surfaces by photolysis has been studied. For this process, strongly face-dependent behavior was also observed. Possible reasons for the face dependencies are discussed.

1. Introduction Metal oxide photocatalysts such as TiO2 or ZnO are promising materials for production of chemical fuels and for degradation of organic pollutants by utilizing solar and/or UV light. While enormous efforts have been devoted to tune the photocatalytic activity of TiO2, the properties of ZnO have been less thoroughly investigated, mainly because of its photoinstability in aqueous solution. Here we investigate the photostability and photocatalytic activity of different crystal orientations of ZnO in order to establish if all orientations face the same limitations for applications in photocatalysis. On TiO2 and SrTiO3, activity variations for different crystallographic orientations have been observed.1-3 In most cases, the fundamental origin for these variations could not been identified conclusively. Several mechanisms have been proposed to contribute to the face dependence of the photocatalytic activity. These are: (1) Different surface orientations exhibit different coordination and separation of cations and anions. This may have pronounced effects on the surface chemistry, i.e., adsorption, dissociation, and reaction of molecules at the surface. This is related to the structure-property relationship in heterogeneous catalysis. (2) Differently oriented surfaces may exhibit different space charge regions or flatband potentials.4 Large band bending at the surface increases electron-hole pair separation and thus surfaces with larger band bending may exhibit a lower electron-hole recombination and thus a potentially higher quantum yield. (3) Different surface structure or different surface composition of the same material, for example, different surface reconstructions of rutile TiO2(001)5 or SrO vs TiO termination of SrTiO3,6 may * Corresponding author. E-mail: [email protected]. † Clean Energy Research Center, University of South Florida. ‡ Department of Physics, University of South Florida. (1) Morris Hotsenpiller, P. A.; Bolt, J. D.; Farneth, W. E.; Lowekamp, J. B.; Rohrer, G. S. J. Phys. Chem. B 1998, 102, 3216. (2) Lowekamp, J. B.; Rohrer, G. S.; Morris Hotsenpiller, P. A.; Bolt, J. D.; Farneth, W. E. J. Phys. Chem. B 1998, 102, 7323. (3) Ohno, T.; Sarukawa, K.; Matsumura, M. New J. Chem. 2002, 26, 1167. (4) Hengerer, K. L.; Krtil, P.; Gra¨tzel, M. J. Electrochem. Soc. 2000, 147, 1467. (5) Wilson, J. N.; Idriss, H. J. Am. Chem. Soc. 2002, 124, 11284. (6) Matsumoto, Y.; Ohsawa, T.; Takakashi, R.; Koinuma, H. Thin Solid Films 2005, 486, 11.

result in different photocatalytic activity. It has been argued that this may be due to differences in charge separation or due to altered charge transfer from the catalyst to adsorbed molecules. (4) On ferroelectric materials, a dependence of the photocatalytic activity on the polarization of the material has been observed.7 The internal polarization of the material causes a directed drift of charge carriers with opposing directions for electrons and holes. This results in separation of oxidation and reduction reactions that coincide with the ferroelectric domain structure. Giocondi and Rohrer observed similar domain effects for nonferromagnetic SrTiO3 and concluded that this may be due to charged surface termination at the polar SrTiO3 surface.8 (5) Studies in solutions should also consider differences, not just in the catalyst but also in the liquid solution close to the surface, in particular in the Helmholtz layer, which may affect the rate at which molecules reach the surface. (6) Finally, it was proposed that electronic anisotropies of the material would allow photoexcited charge carriers to diffuse in preferred crystallographic directions. As a consequence of directed charge carrier diffusion, certain faces might be more active than others.9 In this contribution, we show that ZnO also exhibits markedly different photoactivity for different surface orientations. Our studies are consistent with studies that reported a higher activity for the ZnO(0001)-Zn surface compared to the ZnO(000-1)-O surface for photoreduction of Ag.10 In the studies reported here, we investigated the photodegradation of methyl orange as a model reaction for the degradation of organic pollutants over the two polar surfaces ZnO(0001)-Zn and ZnO(000-1)-O as well as the nonpolar ZnO(10-10) surface. The photo degradation rate differed by as much as 1 order of magnitude between the highest and lowest active crystal face, with the nonpolar ZnO(10-10) exhibiting the highest activity. The samples were illuminated from one side only, which enabled assessing the importance of the “dark side” of the 0.5 mm thick samples to the photoactivity. The photodegradation rate decreased multifold if the “dark side” (7) Giocondi, J. L.; Rohrer, G. S. Top. Catal. 2008, 49, 18. (8) Giocondi, J. L.; Rohrer, G. S. J. Am. Ceram. Soc. 2003, 86, 1182. (9) Giocondi, J. L.; Salvador, P. A.; Rohrer, G. S. Top. Catal. 2007, 44, 529. (10) Kawano, K.; Komatsu, M.; Yajima, Y.; Haneda, H.; Maki, H.; Yamamoto, T. Appl. Surf. Sci. 2002, 189, 265.

10.1021/la803845f CCC: $40.75  2009 American Chemical Society Published on Web 02/05/2009

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Figure 1. Ball-and-stick models of a cross-section through ZnO crystals. (a) the polar orientation of ZnO; the top illustrates the ZnO(000-1)-O surface and the bottom the ZnO(0001)-Zn surface. (b) Cross-section through the nonpolar crystal; both top and bottom have the same surface structure.

was masked by a Mylar tape. This may indicate an increase in the charge-recombination rate and thus suggests that charge carriers diffuse over long distances and can be scavenged on the “dark side” if accessible. We also found that the different ZnO surfaces showed different degrees of instability in aqueous solution under UV-irradiation.

2. Experimental Section 2.1. ZnO Samples. ZnO single crystals, epipolished on both sides and with dimensions of 5 mm × 5 mm × 0.5 mm, were used in the experiments. The crystals were hydrothermally grown and supplied by MTI Corporation and Goodwill. The c-axis of the ZnOwurtzite structure exhibits a polar stacking order, i.e., alternating Zn and O layers. Because of this structure, the two-sides of a single crystal expose different surfaces; one side is O-terminated (denoted as ZnO(000-1)-O and the opposite side is Zn-terminated (denoted as ZnO(0001)-Zn). The surface terminations of these polar surfaces are illustrated in Figure 1a. Because of the different surface structures, the photocatalytic activities of both sides of the crystals have to be evaluated separately. In addition to the c-axis oriented crystals, the nonpolar ZnO(10-10) surface was also considered. For such oriented single crystals, both sides of the crystal exhibit identical surface structures, shown in Figure 1b. 2.2. Photocatalytic Measurements. The decomposition of methyl orange (MO) was used in order to assess the photocatalytic activity of different crystallographic oriented crystals. The experimental setup consisted of a small reaction cell with the ZnO-crystal submerged in a 20 ppm MO solution with a pH of 6.8. In related studies of a ZnO suspension of 0.2 g/L, no change of the pH was observed during UV illumination. Therefore a constant pH value throughout the experiments is assumed. Oxygen was bubbled through the solution at a rate of one bubble per second (∼0.25 mL/min) in order to provide oxygen to the solution. Oxygen is commonly added in photocatalysis experiments in order to scavenge electrons from the catalyst surface. All the experiments were performed at a temperature of 30 ( 3 °C. The ZnO single crystals were illuminated from one side with UV lamps having an intensity of about 20 mW/cm2 for the 365 nm excitation line at the position of the sample. The UV source spectrum shown in Figure 2a indicates that approximately 90% of the UV lamp light has enough energy to create electron-hole pairs in ZnO, assuming a band gap of ZnO of 3.3 eV. To measure the decomposition of MO, samples were taken from the solution at regular time intervals and the absorption spectrum was measured. The absorption at a wavelength of 450 nm decreased with irradiation time, indicating the photodecomposition of methyl orange; see Figure 2b. Control experiments in the dark did not show any change in the concentration of MO. From the measured optical density of the analytical and measured solutions, the decomposition rate of MO is determined. This, together with the measured intensity of radiation of the light incident on the ZnO surface and the volume of the MO solution, enables us to determine the quantum yield of the photodecomposition process. In our measurements, the quantum yield is defined as the number of decomposed molecules per unit time, divided by the number of photons absorbed by the photocatalytic surface per unit time (i.e., photons having energy equal or higher than the energy gap).

Figure 2. Experimental results of MO decomposition over ZnO single crystal. (a) Light spectrum of UV source. (b) Absorption spectra of the MO solution after different time intervals of the photocatalytic degradation reactions over the ZnO(0001)-Zn surface. (c) Change in MO concentration for three different ZnO surfaces as a function of irradiation time.

The quantum yield was measured on the different ZnO samples. For the sample, with the polar surfaces exposed, one side of the sample is Zn- and the opposite side is O-terminated. Thus, by turning the sample around, both sides could be measured on the same crystal. To assess if the back side, i.e., the dark side, of the crystal contributes to the photo degradation of methyl orange, we also performed measurements with the back side covered with a Mylar tape. 2.3. Surface Morphology after Photoetching. Exposure of the samples to UV light in aqueous solution resulted in the dissolution of ZnO. Control experiments of samples in solution in the dark did not result in an etching of the surface. The surface morphologies after prolonged exposure to UV light were measured with an atomic force microscope (AFM). From these measurements, the density of etch pits and their depth could be established. AFM images were acquired with a Digital Instruments nanoscope in tapping mode.

3. Results 3.1. Photodegradation of Methyl Orange. Figure 2b shows the optical absorption spectra of the methyl orange solution for

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KisloV et al. Table 1 quantum yield (× 10-4) ZnO(0001) Zn-terminated (illuminated)

both sides of ZnO crystal are exposed to MO solution nonilluminated side of ZnO crystal is masked

ZnO(000-1) O-terminated (illuminated)

ZnO(10-10)

2.10 ( 0.05

1.40 ( 0.03

2.45 ( 0.04

0.56 ( 0.02

0.04 ( 0.0014

0.10 ( 0.007

Table 2 quantum yield (× 10-4)

after a series of photocatalytic experiments ZnO(0001) Zn-terminated (illuminated) ZnO(000-1) O-terminated (illuminated)

after ultrasonic cleaning in acetone, ethyl alcohol, DI water (10 min each)

1.11 ( 0.02 0.26 ( 0.03

after annealing in air at 360 °C, 60 min, followed by 400 °C, 15 min

after annealing in air at 600 °C, 60 min

0.950 ( 0.025 0.170 ( 0.014

different time intervals of the photodegradation reactions over the ZnO(0001)-Zn surface. For other samples, the degradation behaviors were similar but the time responses were different. The normalized concentration of the MO solution as a function of time for the different surfaces is shown in Figure 2c. The results show that the photocatalytic destruction of MO in aqueous solution can be described by a first-order kinetic model for both ZnO(0001)-Zn and the ZnO(000-1)-O surfaces. In contrast, for the ZnO(10-10) surface, the MO concentration against time is best fit by a linear dependence, indicating zero-order kinetics. Table 1 summarizes the quantum yields derived from the MO decomposition rates for the different surface orientations. First, we focused on the situation where both sides of the crystal were exposed to the solution. A face dependence of the photocatalytic activity was observed. The nonpolar ZnO(10-10) surface exhibited the largest activity, followed by the ZnO(0001)Zn and the ZnO(000-1)-O surface (see Table 1). The quantum yield differed by more than a factor of 1/2 between the highest and lowest active surface. If the back side of the crystal was masked, the photocatalytic activitydroppedmarkedlyforbothZnO(0001)-ZnandZnO(000-1)-O surfaces. For the ZnO(0001)-Zn surface, the activity decreased by about four times, while for the ZnO(000-1)-O surface, the photocatalytic activity almost completely disappeared. For practical purposes, it is worth mentioning that the photocatalytic activity of the ZnO(0001)-Zn surface decreased slightly over time. In contrast, the photocatalytic activity of the ZnO(000-1)-O surface decreased almost five times after a series of photocatalytic experiments. To re-establish the higher initial activity, different thermal and surface treatment processes were applied. Table 2 summarizes effects of different sample treatments on quantum yield of the photocatalytic decomposition of MO. The first treatment was ultrasonic cleaning in acetone (10 min), ethanol (10 min), and DI water (10 min). This procedure did not result in an increased activity. The treatment of the sample by annealing in air at 360-400 °C as well as 600 °C did not make any significant change in the quantum yield of the ZnO(000-1)-O surface. The best result was achieved by UV illumination of the samples in DI water for 3 h for each side with a 1.5 times increase in quantum yield for both Zn and O terminated surfaces. Although a clear improvement was achieved by UV irradiation in DI water, the treatment was unable to recover the activity to the level of their initial photocatalytic activities. The fact that neither ultrasonic cleaning nor air annealing recovers

0.260 ( 0.018

after UV irradiation in DI water, for 3 h for each side 1.64 ( 0.02

0.263 ( 0.014

0.356 ( 0.012

the photoactivity suggests that neither organic surface contamination nor surface etching (an effect that we will describe below) are responsible for the decrease in activity over time. However, we also do not have an explanation why UV irradiation in DI water recovers some of the photoactivity. Therefore, this is a phenomenological observation and further investigations of the deactivation mechanisms of ZnO are needed. 3.2. Photoetching/Photolysis of ZnO. A good catalyst should be stable under operation conditions. Therefore, the chemical stability of the ZnO photocatalyst was assessed. We investigated the surfaces of the ZnO crystals after prolonged exposure to UV light in aqueous solution. For all three crystal surfaces, we observed etching of the surface. Control studies in the dark did not show any alteration of the surfaces. Therefore, the surface etching of ZnO at the pH value of our solution is a photo-induced process. Figure 3 shows AFM images of the two polar surfaces before and after UV illumination. Localized hexagonal etch pit formation is observed on the ZnO(0001)-Zn, while on the ZnO(000-1)-O surface,a uniform etching of the surface is observed. The surface structure for the nonpolar ZnO(10-10) surface is shown in Figure 4. Again localized etch pits are formed, however, on this surface the crystal structure is anisotropic, causing the formation of strongly elongated pits. The fact that the ZnO(0001)-Zn and the ZnO(10-10) surfaces form etch pits suggests that only defect sites, such as dislocation lines, are being etched. Very similar surface etching structures have been known to form on ZnO in acidic solutions.11-14 For instance, dipping polar crystals in an acid is a common method for discriminating the O-polar surface from the Zn side. The O side reacts much stronger, allowing an unambiguous identification. Our observation shows that UV irradiation causes a facedependent photolysis of ZnO in a similar manner to etching in acidic environments. To further investigate the role of defects in the photoetching, we investigated ZnO films that have been grown with a preferential orientation of the c-axis normal to the substrate. Scanning electron microscopy (SEM) images of the surface of such films before and after 120 h UV irradiation in a MO solution are shown in (11) El-Shaer, V. A.-H.; Mofor, A.-C.; Waag, A. Electrochem. Solid State Lett. 2007, 10, H357. (12) Mariano, A. N.; Hanneman, R. E. J. Appl. Phys. 1963, 34, 384. (13) Fryar, J.; McGlynn, E.; Henry, M. O.; Cafolla, A. A.; Hanson, C. J. Physica B 2003, 340-342, 210. (14) Maki, H.; Ikoma, T.; Sakaguchi, I.; Ohashi, N.; Haneda, H.; Tanaka, J.; Ichinose, N. Thin Solid Films 2002, 411, 91.

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Figure 3. AFM images of polar ZnO: (a) Zn side illuminated for 20 h, (b) unilluminated Zn side, (c) O side illuminated for 20 h, (d) unilluminated O side. Note the differences in z-scale; a cross-section along the indicated line is shown to illustrate the height corrugation.

Figure 4. AFM image of the nonpolar ZnO(10-10) surface after illumination for 3 h with UV light in solution. Up to 50 nm deep, elongated etch pits have formed.

Figure 5. For the as grown film, grain boundaries are difficult to locate. After the photocatalytic experiments, etch pits decorate these grain boundaries, indicating the preferential dissolution of ZnO at these sites. Comparing the single crystal studies with the SEM images of the film suggest that the film has a preferential ZnO(0001)-Zn termination because of the localized nature of the attack. Thus the study on the film is further indication for the importance of defects in photolysis of ZnO.

Discussion We observed strongly face-dependent properties for ZnO crystals in photocatalytic processes in solution. The photodegradation of MO differes more than 1 order of magnitude when the unilluminated side of the ZnO single crystal is covered by a Mylar tape. The photostability of the surfaces also showed pronounced orientation dependence. A surface dependence for photoreactions has been observed previously for other materials, therefore our observations is another verification that the crystallographic orientations of photocatalysts influence the photoactivity. This observation may have more relevance for ZnO for which the growth of welldefined nanomaterials with defined surfaces orientations is well advanced and therefore materials with desirable surface orientations can be readily grown.

Figure 5. ZnO film with preferential c-axis orientation: (a) as grown and (b) after 120 h of photocatalysis in MO solution. The circles highlight grain boundaries that are decorated by etch pits after photocatalytic etching of the surface.

There are different possibilities for explaining the origin of the face dependence in the photocatalytic degradation of methyl orange. Assuming that the main decomposition of methyl orange comes from attack by OH and H2O2 radicals that are photocatalytically produced at the ZnO surface, the rate of production of radicals at the various orientation of ZnO has to be significantly different. This orientation dependence may have a combination of different origins. (1) The outermost atomic layer of crystals have been shown to cause differences in the photocatalytic activity5,6 Therefore, there is some precedent that the constellation and composition of the outermost surface layer affects the photoactivity. Similar effects may be relevant for our studies. The surface layers of the single crystal we examined are very different (see Figure 1). The polar surfaces, i.e., the ZnO(000-1)-O and the ZnO(0001)-Zn

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are completely O2- or Zn2+ terminated, respectively, while the nonpolar ZnO(10-10) surface has a mixed termination. (2) Bulk truncated polar surfaces are inherently instable. Conceptually, to stabilize these surfaces, charge needs to be transferred from one polar surface termination to the opposing surface termination.15 Under vacuum conditions, this usually results in a reconstruction of these surfaces to form a surface layer with an altered composition and thus with a surface charge that compensates for the bulk dipole moments. For the polar surfaces of ZnO,16 it has been shown that the Zn-terminated surface is Zn-cation deficient17 and the O-terminated side is O-anion deficient, i.e., the Zn-side is negatively and the O-side is positively charged. The clean, i.e., adsorbate free, O-terminated surface is, however, even under ultra high vacuum (UHV) conditions, difficult to obtain. Under almost any realistic conditions, this surface has been shown to be hydroxylated.16,18 Thus, in solution, it is likely that the necessary charge redistribution necessary to stabilize the polar surfaces is not achieved by the same mechanisms as under vacuum. Charged adsorbates at the surface can provide the same stabilizing effect as surface compositional changes. Ultimately, to explain hole or electron diffusion behavior to the surface, only the sign of the surface charge matters but not the kind of surface charge. Thus we can focus on the fact that the Zn-terminated side has to be negative and the O-terminated side has to be positive, i.e., hole diffusion to the Zn-terminated side may be expected, which is in agreement with the higher activity of this surface. The influences of charged adsorbates at the surface does, however, not only apply to polar faces and this is discussed further in general terms next. (3) Changed flatband potentials of surfaces in contact with aqueous solutions have been observed for different crystal faces of anatase TiO2.4 These differences in the flatband potentials due to different adsorption of water on the different crystallographic surfaces resulted in orientation-dependent charge transfer processes. We would also expect different flatband potentials for the different orientations of the ZnO crystals. The two polar surfaces are very different. The O-terminated surface is very likely hydrogen terminated, i.e., H+ adsorbed on lattice oxygen, which would induce a downward band bending, i.e., a space charge region that increases electron-hole pair separation and electron diffusion toward the O-terminated surface. The surface termination of the Zn side depends on the pH of the solution. Valtiner et al. showed that the likely termination of the ZnO(0001)-Zn surface is by hydroxides for solutions with a pH value between 4 and 11.19 This hydroxide layer consists of OH- and thus is different from the H+ termination of the hydroxylated ZnO(000-1)-O surface. Consequently, for the Zn side, an upward band bending induced by the OH- is expected. This is opposite to the expected band bending on the O side. This would enhance hole diffusion to the Zn-terminated surface and thus increase hole mediated production of radicals, including OH. Finally, for the nonpolar ZnO(10-10), a mixed dissociated and molecular water layer has been observed under vacuum conditions.20 Thus all three ZnO surfaces are (partially) terminated by hydroxides under our conditions. The ultimate effect of the interaction of ZnO with the aqueous solution on the flatband (15) Goniakowski, J.; Finocchi, F.; Noguera, C. Rep. Prog. Phys. 2008, 71, 016501. (16) Wo¨ll, C. Prog. Surf. Sci. 2007, 82, 55. (17) Dulub, O.; Diebold, U.; Kresse, G. Phys. ReV. Lett. 2003, 90, 016102. (18) Lahiri, J.; Senanayake, S.; Batzill, M. Phys. ReV. B 2008, 78, 155414. (19) Valtiner, M.; Borodin, S.; Grundmeier, G. Langmuir 2008, 24, 5350. (20) Meyer, B.; Marx, D.; Dulub, O.; Diebold, U.; Kunat, M.; Langenberg, D.; Wo¨ll, C Ang. Chem., Int. Ed. 2004, 43, 6642.

KisloV et al.

potential has not been measured in this work and therefore the influence of the flat band potential on the photoactivity remains speculation. (4) We observed photolysis of the ZnO catalyst, i.e., dissolution of the crystal under UV irradiation. This effect was most pronounced on the ZnO(000-1)-O side. This is also the surface with the lowest photoactivity. A correlation between photolysis and photocatalytic activity can be explained simply be the reasoning that holes are being consumed by photolysis reactions according to:

2h+ + ZnO w Zn2+ + O*

(1)

and consequently fewer charge carriers are available to produce OH radicals. The so formed intermediate atomic oxygen species O* can either attack MO or combine with another oxygen to form O2. Because the quantum yield for the ZnO(000-1)-O surface is lower than the Zn(0001)-Zn side, it appears that formation of inactive O2 is the dominant path. (5) For all crystal orientations, we observed a decrease in the photoactivity if the back side (dark side) of the crystal is masked by a tape. This indicates that the back side is important for the photocatalytic process. We explain this by charges drifting across the crystal to the “dark side” to contribute to surface chemical reactions. Such a macroscopic charge transfer is more likely for electrons in ZnO than holes because of the much higher electron mobility compared to hole mobility. Also, an upward shift of the bands on the illuminated side would favor electron transport to the opposite side. On the back side, these electrons may be scavenged by oxygen in the solution. This effective charge separation reduces charge recombination processes and therefore increases the overall photoactivity. In addition, the reduced by electrons O2*- species will result in formation of active oxidative species such as H2O2 that also promotes MO oxidation. In contrast, if the unilluminated back side is masked, no charges can be scavenged from this side and consequently bulk charge recombination is increased, causing the decrease in photodegradation of MO. Differences in the flatband potentials of the Zn- and O-terminated polar surfaces may add to the charge separation. (6) The different apparent reaction order between the polar surfaces (first order) and the nonpolar ZnO(10-10) surface (zero order) is intriguing. Zero-order reaction shows that the destruction of MO is independent from its concentration in the solution. This would be the case, for example, if the reaction sites on the surface are saturated and/or the reaction rate is limited by the supply of the charge carriers to the interface. A first-order reaction kinetics would be expected if surface reaction sites are not saturated. In this case, the kinetics follows the Langmuir-Hinshelwood law, which causes the reaction rate to be proportional to the MO concentration at low concentrations. The different apparent reaction rates on the nonpolar and polar surfaces implies an orientation dependence of the charge transfer processes on ZnO single crystal surfaces or different types and densities of reactive sites on the surface that affects photocatalytic reaction rates. The difference in the kinetic order of the destruction of MO demonstrates that kinetic phenomena can be important and need to be taken into account in assessing the differences in the photoactivity of different surface orientations in solution.

Summary and Conclusion ZnO has not been considered as an effective photocatalyst because it undergoes photolysis in aqueous solution. We showed that photolysis of ZnO is strongly orientation dependent, with only the ZnO(000-1)-O surface exhibiting strong photolysis while the ZnO(0001)-Zn and the ZnO(10-10) surfaces exhibited

Photocatalytic Degradation of Methyl Orange oVer ZnO

localized dissolution, most likely at defect sites only. Photolysis of the crystal correlates inversely with the photodecomposition of methyl orange, i.e., the ZnO(000-1)-O surface exhibits by far the lowest photocatalytic activity. This may be due to the consumptions of holes in the photolysis process of the ZnO catalyst. With these findings in mind, one may attempt to design ZnO micro/nanomaterials that exhibit photostable surface orientations or find ways to block the O-terminated polar surface and thus make ZnO a more feasible material for photocatalysis. On the other hand, our observation of orientation-dependent

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photolysis may also assist in solution-based growth of ZnO materials with specific orientation. For instance, Zn-terminated polar surfaces could be grown selectively by destabilizing the O-terminated polar face with UV irradiation during growth without the need of adjusting the pH value of the solution. Acknowledgment. Financial support from the FMMD-IERG initiative is acknowledged. LA803845F