ZnO nanorods as a plasmonic photocatalyst

Author’s Accepted Manuscript Synthesis and properties of Au/ZnO nanorods as a plasmonic photocatalyst Jia Lu, Huihu Wang, Daluo Peng, Tao Chen, Shijie Dong, Ying Chang www.elsevier.com/locate/physe

PII: DOI: Reference:

S1386-9477(15)30303-9 http://dx.doi.org/10.1016/j.physe.2015.11.035 PHYSE12218

To appear in: Physica E: Low-dimensional Systems and Nanostructures Received date: 31 August 2015 Revised date: 27 October 2015 Accepted date: 26 November 2015 Cite this article as: Jia Lu, Huihu Wang, Daluo Peng, Tao Chen, Shijie Dong and Ying Chang, Synthesis and properties of Au/ZnO nanorods as a plasmonic photocatalyst, Physica E: Low-dimensional Systems and Nanostructures, http://dx.doi.org/10.1016/j.physe.2015.11.035 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Synthesis and properties of Au/ZnO nanorods as a plasmonic photocatalyst Jia Lu, a,b Huihu Wang, a,b* Daluo Peng, a,b Tao Chen, a Shijie Dong, a,b Ying Chang b,c a School of Mechanical Engineering, Hubei University of Technology, Wuhan, P. R. China b Hubei Provincial Key Laboratory of Green Materials for Light Industry, Hubei University of Technology, Wuhan, P. R. China c School of Materials Science and Engineering, Hubei University of Technology, Wuhan, P. R. China

Corresponding author: Huihu Wang E-Mail Address: [email protected] Tel. +86-027-59750418 Fax. +86-027-59750418 Postal Address: School of Mechanical Engineering, Hubei University of Technology, Wuhan City, Hubei Province, P. R. China Postal Code: 430068

1

Abstract: It is of great interest to develop plasmonic photocatalysts with high activity and stability recently. In this paper, Au/ZnO nanorods were synthesized via a facile hydrothermal method and used as photocatalysts for methyl orange dye degradation. The results revealed an interesting phenomenon that photocorrosion cracks were produced specially along the c-axis of pure ZnO nanorods for five cycles photodegradation experiments under UV-Vis. light irradiation, while Au nanoparticles surface modification can effectively inhibit the occurrence of photocorrosion and improve its photocatalytic activity. The formation of photocorrossion cracks along the c-axis of pure ZnO nanorods verifies the photogenerated charges may follow the route that electrons migrate to Zn-terminated (0001) plane and holes to O-terminated



(0001)

plane. SPR effect of Au

nanoparticles enhances the light absorption ability and the electrons capture ability of Au/ZnO nanorods. Moreover, the surface adsorbed hydroxyl groups content is also increased due to Au nanoparticles modification. As Au nanoparticles can capture photogenerated electrons and hydroxyl groups are the favorable holes scavenger, the charges generation and separation in photocatalysis are strengthened. Especially, the charges separation path in Au/ZnO nanorods have changed, thus inhibiting the occurrence of photocorrosion along the c-axis of ZnO nanorods and improving the photocatalytic activity. Key Words: metal-semiconductor; ZnO; Au; plasmonic photocatalyst

2

1. Introduction In recent years, semiconductor photocatalysis technology for environment remediation has been widely studied.

[1, 2]

ZnO is one of the commonly used photocatalysts which has the

advantages of non-toxic and low cost. [3-5] However, it is confronted with low quantum efficiency [6, 7]

and photocorrosion [8] in its practical application. Many strategies have been used to address

these drawbacks of ZnO. Traditionally, modifying ZnO with noble metal nanoparticles has attracted attention due to promoting the separation of photogenerated charges in photocatalysts through the formation of Schottky barrier [9-12], and improving the photocatalytic stability [13]. Recently, basing on the surface plasmon resonance (SPR) effect of noble metal nanoparticles, the development of surface plasmon photocatalysts has been a hot research topic lately. It is commonly believed that there are two important aspects of SPR effect that affect the photocatalytic activity of semiconductors. First, noble metal nanoparticles surface modification would enhance the light photons absorption of wide gap semiconductor.

[14-16]

Secondly,

appropriate visible light irradiation may induce the SPR effect of noble metal nanoparticles and greatly improve its electrons capture ability.

[17, 18]

Both two reasons affect the charges generation

and separation in photocatalysis, resulting in the improvement of photocatalytic properties. On the other hand, as the photocatalytic reaction usually occurs at the surface of catalyst, the catalyst morphology may also play an important role on the photocatalytic properties. Owing to the higher ratio of length to width, larger specific surface area

[19]

, higher light photons capture

efficiency, as well as higher photoinduced charges generation and transfer rate compared to ZnO nanoparticles [20], one-dimensional (1D) ZnO nanostructures have received considerable attention. Especially, due to ZnO wurtzite crystal structure is composed by a large number of O2- ion plane and Zn2+ ion plane alternately stacking along the c axis, one-dimensional ZnO nanostructures possess the surfaces that terminated to the O2- plane



(0001)

and Zn2+ plane (0001).

[21, 22]

The two

polar crystal planes construct a built-in electric field in one dimensional ZnO nanostructure, which may undoubtedly affect the transfer path of photogenerated charges. When noble metal nanoparticles are used to modify one dimensional ZnO nanostructure, the as-prepared catalysts may obtain both noble metal nanoparticles SPR effect and one dimensional ZnO nanostructures morphology-dependent effect. Unlike most of previously reported researches in which the enhanced photocatalytic properties are ascribed to the formed Schottky barrier between noble 3

metal nanoparticles and semiconductors, the studies on SPR effect and morphology-dependent effect for the photocatalytic properties of catalysts are scarce. Therefore, more understanding about the two effects and their coupling interaction in photocatalysis may be helpful to comprehend the enhanced properties of surface plasmonic photocatalysts. In this paper, Au/ZnO nanorods were synthesized by hydrothermal method. The microstructures of Au/ZnO were characterized using XRD, FESEM, XPS, and DRS. Methyl orange dye was used as pollutant model to investigate the photocatalytic activity and stability of Au/ZnO nanorods. The influences of Au nanoparticles SPR effect and ZnO nanorods morphology-dependent effect as well as their coupling interaction have been well studied and discussed based on the photocatalytic mechanism and photocorrosion mechanism of ZnO.

2. Experimental 2.1 Preparation of ZnO nanorods All the chemicals were used without further purification. The detailed preparation process can be found in the referee elsewhere.

[23]

Typically, 1.1g of zinc acetate (Zn (CH3COO) 2) and

1.5g of polyvinyl alcohol ([C2H4O]n) were mixed and added to 40mL of deionized water. The mixtures were then stirred at 80ºC for 1h. A viscous sol was obtained and placed for 1 day at room temperature. Clean glass substrates were cut into pieces with certain size (25mm x 25mm) and placed into the SC-1B spin coater (Genesis Wiener, Beijing, China) for preparation of ZnO seed layer. The as-prepared ZnO seed layer were immersed in the mixture solution of 0.03mol/L zinc nitrate and 0.03mol/L hexamethylenetetramine at 60 ºC for 6 h to prepare ZnO nanorods. 2.2 Preparation of Au/ZnO nanorods Au nanoparticles surface modified ZnO nanorods were synthesized as follows: First, HAuCl4.4H2O solution was prepared and its pH value was adjusted to 8~9 using ammonia. 1mL methanol was also introduced to the solution. Secondly, ZnO film was immersed in this solution. The solution temperature was elevated to 70ºC and kept for 1h in order to make more Au ions adsorbed at the surface of ZnO nanorods. Then the film was removed from the solution and washed with deionized water for several times. Finally, the film was transferred into the tube furnace and annealed at 400 ºC for 2h in a nitrogen atmosphere to obtain Au/ZnO sample. For comparison, the pure ZnO nanorods were also treated under the same conditions. To study the influence of Au content on the photocatalytic performance of Au/ZnO nanorods, 4

the concentration of HAuCl4.4H2O solution was set as 0.25, 0.5, 0.75 and 1mmol/L. The as-prepared Au/ZnO samples were defined as 0.25Au/ZnO, 0.5Au/ZnO, 0.75Au/ZnO and 1Au/ZnO, respectively. 2.3 Characterization The crystal structure of samples was measured by XD-2 type X-ray diffractometer. The scanning range was 10º~90º and the scanning speed was 2º/min. The surface morphology analysis of Au/ZnO nanorods was performed with a Quanta 450 field emission scanning electron microscopy (FESEM) using an accelerating voltage of 15kV. The element composition was obtained using a VG Multilab X2000 X-ray photoelectron spectroscopy (XPS). The absorption spectra of Au/ZnO samples were acquired with a U-3900 UV-visible diffuse reflectance spectroscopy, and BaSO4 was used as the standard sample. 2.4 Photocatalytic test Methyl orange (MO) dye solution was used as pollutant model to evaluate the photocatalytic performance of different samples under UV-Vis. light irradiation which was provided by a 300W Xe lamp. In a typical photocatalytic reaction process, the as-prepared films were firstly immersed into 20mL of MO solution (5mg/L) for 30 min in the dark to attain the adsorption equilibrium. Then the Xe lamp was turned on. Aliquots of 2mL of MO solution were taken out for quantitative analysis at regular intervals during the photocatalytic process. The concentration of MO dye was tested using a 2102PC UV-Vis. spectrophotometer at an emission peak of 464nm. In order to investigate the influence of photolysis on the final experimental results, MO dye degradation without immersing photocatalysts film was also tested. Pure ZnO and 0.75Au/ZnO samples were selected for the photocatalytic stability test. The photocatalytic experiments were repeated five times using the same testing conditions above. After each photoreaction, the samples were washed with deionized water for several times and dried at 60 °C.

3. Results and Discussion Figure 1 shows the XRD patterns of different Au/ZnO samples. In all samples, three diffraction peaks at 31.57º, 34.2º, and 36.04ºare observed and can be indexed as ZnO hexagonal wurtzite structure, corresponding to ZnO crystal facet (100), (002), and (101). The diffraction peak at 34.2º is the strongest, indicating that the preferential growth direction of ZnO is [0001]. The 5

diffraction peak of Au nanoparticles is found at 38.25º in 0.5Au/ZnO, 0.75Au/ZnO, 0.25Au/ZnO, and 1Au/ZnO samples.[24] The diffraction peak intensity of Au is related to the concentration of HAuCl4.4H2O precursor solution. Figure 2 shows the FESEM images of different Au/ZnO samples. It can be seen that ZnO nanorods are uniformly distributed at the surface of substrate with a good orientation. The nanorods have an average diameter about 56nm. Many fine particles are homogeneously dispersed at the surface of ZnO nanorods. According to the results of XRD analysis, these particles may be Au nanoparticles. Figure 3 shows the XPS spectra of pure ZnO and 0.75Au/ZnO samples. In Fig. 3 (a), two peaks located at 1044.3eV and 1021.3eV are observed and attributed to Zn2p1/2 and Zn2p3/2, respectively. The difference on the binding energy of two Zn2p peaks is 23eV. [25]The Zn2p peaks of 0.75Au/ZnO shift by 0.2eV to the larger binding energy with respect to that of pure ZnO nanorods. This may be caused by the interactions between Au and ZnO

[26, 27]

, such as the

electrons transferring from ZnO to Au. Figure 3 (b) displays the Au4f peaks in 0.75Au/ZnO sample. Peaks located at 87.4eV and 83.5eV are corresponding to Au4f5/2 and Au4f7/2 in Au0 of Au nanoparticles. [28] The results further verify that the surface of the ZnO nanorods was successfully loaded with noble metal Au nanoparticles. Figure 3 (c) shows the deconvoluted O1s spectra of pure ZnO and 0.75Au/ZnO sample. The peak with binding energy centered at 530.2eV corresponds to the lattice oxygen in ZnO crystals. [29] The peak with a higher binding energy centered at 531.11eV is ascribed to the oxygen species in hydroxyl groups adsorbed at the surface of ZnO nanorods, while the peak located at 532.25eV with the highest binding energy is related to the ammonia acid molecules, namely the C=O bond.[25] With the surface modification of noble metal Au nanoparticles, the atomic percentage of different oxygen species in 0.75Au/ZnO nanorods has changed compared to pure ZnO nanorods. The detailed results are shown in Table 1. It can be seen the atomic percentage of oxygen species in hydroxyl groups adsorbed at the surface of ZnO increases from 13.98 at.% to 14.64 at.%, indicating noble metal Au nanoparticles modification obviously raise the hydroxyl groups content. As hydroxyl groups are the favorable holes scavenger, Au nanoparticles modification may be an effective mean to improve the photocatalytic stability of ZnO nanorods. Figure 4 shows the UV-Vis. diffuse reflectance spectra of ZnO and Au/ZnO nanorods. All the 6

samples are presented with typical ZnO absorption edge. Compared to pure ZnO, Au/ZnO samples exhibit a stronger light absorption in the near ultraviolet region (350nm~400nm). In addition, an obvious absorption peak in the visible light region (500nm~600nm) is also observed in Au/ZnO samples, which is corresponding to the SPR effect of Au nanoparticles. [30]With the increase of Au content in Au/ZnO samples, the SPR absorption peak intensity is enhanced. Figure 5a displays the degradation curves of MO using pure ZnO and different Au/ZnO samples. It can be observed the photolysis of MO dye solution is very weak. The efficiency of pure ZnO nanorods for MO photodegradation reaches nearly 66%. With Au nanoparticles surface modification, the photocatalytic activity of Au/ZnO nanorods is obviously improved, which is consistent with the previous reports. [9, 24, 28] In addition, the activity of Au/ZnO samples is related to the Au content. With the increase of HAuCl4.4H2O precursor solution from 0.25mmol/L to 0.75mmol/L, the efficiency for MO degradation is increased. Both 0.5Au/ZnO and 0.75Au/ZnO samples demonstrate nearly 100% photodegradation efficiency after 180min reaction. However, with the increase of precursor solution concentration to 1mmol/L, the photocatalytic activity of 1Au/ZnO begins to decrease. It may be due to the excessive Au content has reduced the active center of ZnO and hindered the incident light photons absorption of ZnO. Figure 5b shows the reaction kinetics profiles of ZnO and Au/ZnO nanorods. All the reactions obey the pseudo-first-order kinetics law which can be seen from the indicative of the slope in its reaction kinetics profile. It is known that ZnO is prone to photocorrosion, which has seriously hindered its practical application. Five cycles photodegradation of MO solution were used to test the photostability of pure ZnO and 0.75Au/ZnO nanorods. As shown in Figure 6 (a), the MO photodegradation efficiency using pure ZnO nanorods as the catalyst is not decreased, and even increased slightly. When 0.75Au/ZnO nanorods were used as the catalyst, shown in Figure 6 (b), the MO photodegradation efficiency remains unchanged after recycling. From this result, it can be seen that both pure ZnO nanorods and 0.75Au/ZnO nanorods demonstrate good photostability. However, photocorrosion cracks are observed in pure ZnO nanorods after five cycle reactions in Figure 7. It is interesting that the cracks extend along the c axis of ZnO nanorods, as shown in Figure 7a and Figure 7b. However, no corrosion cracks are formed at the surface of 0.75Au/ZnO nanorods, as shown in Figure 7c and Figure 7d. The experiment results verify that Au 7

nanoparticles surface modification not only makes Au/ZnO display a good cyclic activity, but also effectively inhibits the occurrence of photocorrosion of Au/ZnO. It should be pointed out that the unstable activity may obtain when photocorrossion happens. However, the pure ZnO nanorods show the good photostability may come from the slow photocorrosion process in this work. This phenomenon can be explained from Figure 7 that most of ZnO nanorods are kept perfectly. In the wurtzite structure of ZnO, Zn2+ and O2- ions stack alternatively along the c axis, resulting in the Zn-terminated (0001) plane and O-terminated



(0001)

plane become polarized and

form a built-in electric field in ZnO crystal. Compared to the polar planes, the nonpolar



(1010)

planes are composed of equivalent Zn2+ and O2- ions at the same planes. For ZnO nanorods, the surface dominating planes are mainly comprised of polar (0001),



(0001)

plane and nonpolar



(1010)

planes, as shown in Figure 8. This special surface atomic structure may affect the photogenerated charges behavior in photocatalysis. Under UV-Vis. light irradiation, light photons with energy greater than the band gap of ZnO will excite the electrons from valence band to conduction band. The positive holes can oxidize OH- or water molecules to produce OH radicals, while the ●

electrons in conduction band can be rapidly trapped by molecular oxygen to form superoxide radical anion. Both radicals contribute to the degradation of pollutant molecules. In photocatalysis, the transfer path of charges in ZnO nanorods may follow the route that electrons migrate to Zn-terminated (0001) plane, while the holes migrate to



(0001)

plane for two reasons. One is that

Zn-terminated (0001) plane has the higher chemsorption ability than the nonpolar



(1010)

planes

due to the nonsaturable oxygen coordination of Zn-terminated (0001) plane. The adsorbed molecular oxygen may attract the electrons migrating to



(0001)

plane. The other is that the built-in

electric field may accelerate the separation of electrons and holes following this route. According to Figure 7 (b), photocorrosion cracks along the c-axis of ZnO nanorods have already appeared at the surface. The possible mechanism of this phenomenon is presented in this paper. It is indicated by the previous research that the photocorrosion of ZnO mainly contains two processes. The first is that the holes are trapped by surface lattice oxygen to produce molecular oxygen escaping from the surface. The second is that the Zn2+ rapidly diffuses from the surface to solution. The typical photocorrosion process is explained by the following formula: [31] Osurface2-+h+ Osurface2-

(1)

Osurface2-+3O2-+3h+2(O-O2-)

(2) 8

O-O2-+2h+ O2

(3)

2Zn2+2Zn2+ (aq.)

(4)

As it has pointed out above, the holes are moving downward to



(0001)

plane in the reaction

process, thus leading to the development of corrosion along the c-axis, and eventually forming a longitudinal crack at the surface of ZnO nanorods. Modifications of ZnO with Au nanoparticles have effectively enhanced its photocatalytic activity and stability, as shown in Figure 5, 6, and 7. Usually, the Schottky barrier formed between noble metal nanoparticles and semiconductor has been widely discussed and ascribed to this enhancement through accelerating the separation of photogenerated charges. However, the coupling interaction of Au nanoparticles SPR effect and ZnO nanorods morphology-dependent effect in this experiment should be considered. According to the UV-Vis diffuse reflectance spectra of different ZnO samples, SPR effect of Au nanoparticles can strengthen the absorption ability of ZnO in the near ultraviolet region around 350nm~400nm. More electrons and holes will be produced for Au/ZnO samples in contrast to pure ZnO nanorods. Moreover, SPR effect of Au nanoparticles also effectively improves its electrons capture ability. Therefore, SPR effect of Au nanoparticles plays an important role for the separation of photogenerated electrons and holes. In this work, the photocatalytic activity of Au/ZnO samples for MO degradation under visible light irradiation was also analyzed using a 400nm UV-cut filter. However, the Au/ZnO samples don’t show any visible light photocatalytic activity. Therefore, the electric filed generated by the SPR of Au under the visible light irradiation could be neglected. On the other hand, the built-in electric field in ZnO nanorods may continue enhancing this separation of charges caused by Au nanoparticles surface modification, thus improving the quantum efficiency of ZnO. Due to most of Au nanoparticles are mainly distributed on the side of ZnO nanorods, the photogenerated electrons may migrate to the



(1010)

plane of ZnO nanorods and finally to Au

nanoparticles, as shown in Figure 9. It is quite different from that of pure ZnO nanorods. In addition, the surface of Au/ZnO nanorods adsorbs the higher content of hydroxyl groups than that of pure ZnO nanorods. Due to hydroxyl groups are the favorable hole scavengers, most of photogenerated holes has been trapped. Therefore, photocorrosion along the c axis of ZnO nanorods has been effectively inhibited and the stability of ZnO nanorods is enhanced.

9

4 conclusions In summary, the influence of SPR effect and morphology-dependent effect on the photocatalytic activity and stability of Au/ZnO nanorods was discussed in this paper. For pure ZnO nanorods, it displayed a stable photocatalytic activity for five cycles MO dye degradation under UV-Vis. light irradiation. However, the photocorrossion cracks along its c-axis were observed. For Au/ZnO nanorods, the photocorrosion was successfully inhibited. Moreover, the photocatalytic activity was obviously enhanced with respect that of pure ZnO nanorods. Based on these results, a possible mechanism was proposed. It was suggested that the formation of photocorrosion cracks along the c-axis of pure ZnO nanorods was attributed to the built-in electric field produced by polar Zn-terminated (0001) plane and O-terminated



(0001)

plane in ZnO

wurtzite structure. The enhanced photocatalytic activity of Au/ZnO nanorods was due to two reasons: strengthening the light absorption ability and electrons capture ability by Au nanoparticles SPR effect, and accelerating the photogenerated charges separation by the built-in electric field in ZnO nanorods. The change of charges separation path and the high content of hydroxyl groups at the surface of Au/ZnO nanorods are responsible for its improved stability.

Acknowledgements The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (No. 51202064), Natural Science Foundation of Hubei Province of China (No. 2013CFA085), Research Foundation for Talented Scholars of Hubei University of Technology (No. BSQD12119), and Open Foundation of Hubei Provincial Key Laboratory of Green Materials for Light Industry (No. [2013]2-22).

References [1] M. N. Chong, B. Jin, C. W. K. Chow, C. Saint, Water Res. 44(2010) 2997–3027. [2] C. Chen, W. Ma, J. Zhao, Chem. Soc. Rev. 39(2010)4206-4219. [3] S. Ma, J. Xue, Y. Zhou, Z. Zhang, J. Mater. Chem. A 2(2014) 7272-7280. [4] R. D. C. Soltani, A. Rezaee, M. Safari, A. R. Khataee, B. Karimi, Desalin. Water Treat. 53(2013)1613-1620.

10

[5] L. Zhang, L. Du, X. Yu, S. Tan, X. Cai, P. Yang, Y. Gu, W. Mai, ACS Appl. Mater. Inter. 6(2014)3623-3629. [6] Y. Yan, B. Zhang, J. Zhao, J. Alloy Compd. 586(2013)663–668. [7] J. Lu, H. Wang, S. Dong, F. Wang, Y. Dong, J. Alloy Compd. 617(2014)869–876. [8] J. Han, W. Qiu, W. Gao, J. Hazard. Mater. 178(2010)115–122. [9] W. He, H. K. Kim, W. G. Wamer, D. Melka, J. H. Callahan, J. J. Yin, J. Am. Chem. Soc. 136(2014)750-757. [10] X. Liu, J. Zhang, X. Guo, S. Wu, S. Wang, Nanoscale, 2(2010)1178-1184. [11] B. Divband, M. Khatamian, G. R. K. Eslamian, M. Darbandi, Appl. Surf. Sci. 284(2013)80–86. [12] Q. Deng, X. Duan, D. H. L. Ng, H. Tang, Y. Yang, M. Kong, Z. Wu, W. Cai, G. Wang, ACS Appl. Mater. Inter. 4(2012) 6030-6037. [13] W. Xie, Y. Li, W. Sun, J. Huang, H. Xie, X. Zhao, J. Photoch. Photobio. A: Chem. 216(2010)149–155. [14] L. Chen, T. Tran.T, C. Huang, J. Li, L. Yuan, Q. Cai, Appl. Surf. Sci. 273(2013):82–88. [15] Y. Wei, J. Kong, L. Yang, L. Ke, H. Tan, H. Liu, Y. Huang, X. Sun, X. Lu, H. Du, J. Mater. Chem. 16(2013)5045-5052. [16] C. Hsu, Y. Shen, Z. Wei, D. Liu, F. Liu, J. Alloy. Compd. 613(2014)117-121. [17] F. Wu, X. Hu, J. Fan, E. Liu, T. Sun, L. Kang, W. Hou, C. Zhu, H. Liu, Plasmonics, 8(2013)501-508. [18] H. Yin, K. Yu, C. Song, R. Huang, Z. Zhu, ACS Appl. Mater. Inter. 6(2014) 14851-14860. [19] J. Sin, S. Lam, K. Lee, A. R. Mohamed, Mater. Lett. 140(2015)51–54. [20] C. Li, T. Ahmed, M. Ma, T. Edvinsson, J. Zhu, Appl. Catal. B: Environ.138 (2013)175-183. [21] J. Yang, J. Wang, X. Li, J. Lang, F. Liu, L. Yang, H. Zhai, M. Gao, X. Zhao, J. Alloy. Compd. 528(2012)28-33. [22] G. Li, Z. Yi, H. Wang, C. Jia, W. Zhang, Appl. Catal. B: Environ. 158-159(2014)280-285. [23] J. Lu, H. Wang, Y. Dong, F. Wang, S. Dong, Chinese J. Catal. 35(2014)1113-1125. [24] Y. Qin, Y. Zhou, J. Li, J. Ma, D. Shi, J. Chen, J. Yang, J. Colloid Interf. Sci. 418(2014)171–177. [25] R. Al-Gaashani, S. Radiman, A. R. Daud, N. Tabet, Y. Al-Douri, Ceram. Int. 39(2013)2283-2292. [26] E. M. Rodríguez, G. Márquez, E. A. León, P. M. Álvarez, A. M. Amat, F. J. Beltrán, J. Environ. Manage. 127(2013)114–124. [27] H. Yang, G. Li, T. An, Y. Gao, J. Fu, Catal. Today 153(2010)200-207. 11

[28] P. Chen, G. Lee, S. Anandan, J. J. Wu, S Mat. Sci. Eng. B-Solid 177(2012)190-196. [29] A. Zwijnenburg, A. Goossens, W. G. Sloof, M. W. J. Crajé, A. M. van der Kraan, L. J. de Jongh, M. Makkee, J. A. Moulijn, J. Phys. Chem. B 106(2002) 9853-9862. [30] P. Zhang, C. Shao, X. Li, M. Zhang, X. Zhang, Y. Sun, Y. Liu, J. Hazard. Mater. 237(2012)331-338. [31] X. Zhang, J. Zhao, S. Wang, H. Dai, X. Sun, Int. J. Hydrogen Energ. 39(2014)8238-8245.

12

Figure Captions and Tables

Figure 1. XRD patterns of different Au/ZnO samples. Figure 2. FESEM images of different Au/ZnO samples. (a) 0.25Au/ZnO; (b) 0.5Au/ZnO; (c) 0.75Au/ZnO; (d) 1Au/ZnO. Figure 3. Zn 2p (a), Au 4f (b), and O 1s (c) XPS spectra of pure ZnO and 0.75Au/ZnO samples. Figure 4. UV-Vis. diffuse reflectance spectra of pure ZnO and different Au/ZnO samples. Figure 5. Degradation curves of MO (a) and their kinetics profiles (b) over pure ZnO and Au/ZnO samples. Figure 6. Degradation of MO over pure ZnO (a) and 0.75Au/ZnO (b) samples for five cycles. Figure 7. FESEM images of pure ZnO (a,b) and 0.75Au/ZnO (c,d) samples after five cycles degradation experiments. Figure 8. Proposed photocatalytic mechanism of pure ZnO sample. Figure 9. Proposed photocatalytic mechanism of Au/ZnO sample.

Table 1. Analysis of O 1s XPS spectra of pure ZnO and 0.75Au/ZnO samples.

13

ZnO -Au

Intensity/(a.u.)





 

0.25Au/ZnO



0.5Au/ZnO



0.75Au/ZnO



1Au/ZnO

10

20

30

40

50

60

70

80

90

o

2( ) Figure 1.

14

Figure 2.

15

Zn 2p Zn 2P3/2

(a)

Intensity/(a.u.)

Zn 2P1/2 ZnO

0.75Au/ZnO

1055

1050

1045

1040 1035 1030 1025 Binding Energy/(eV)

1020

Au 4f

Intensity/(a.u.)

(b)

Au 4f5/2 Au 4f 7/2

0.75Au/ZnO

100 98

96

94

92 90 88 86 84 Binding Energy/(eV)

82

80

78

76

O 1s

(c) C=O

Intensity/(a.u.)

1015

Zn-OH Zn-O

ZnO

0.75Au/ZnO

538

536

534 532 530 Binding Energy/(eV)

528

526

Figure 3.

16

Absorbance/(a.u.)

0.75Au/ZnO 1Au/ZnO 0.5Au/ZnO 0.25Au/ZnO ZnO

250

300

350

400

450 500 550 600 Wavelength/nm

650

700

750

Figure 4.

17

(a)

1.0

Photolysis ZnO 0.25Au/ZnO 0.5Au/ZnO 0.75Au/ZnO 1Au/ZnO

0.8

C/C0

0.6 0.4 0.2 0.0

0

30

60

90

120

150

180

80

100

120

t/min

2.5 (b)

lnC0/C

2.0

ZnO 0.25Au/ZnO 0.5Au/ZnO 0.75Au/ZnO 1Au/ZnO

1.5

1.0

0.5

0.0

0

20

40

60 t/min

Figure 5.

18

(a)

1.0

C/C0

0.8 0.6 0.4 0.2 0.0

0

150

300

450 t/min

600

750

900

150

300

450

600

750

900

(b)

1.0

C/C0

0.8 0.6 0.4 0.2 0.0

0

t/min

Figure 6.

19

Figure 7.

20

Figure 8.

21

Figure 9.

22

Table 1.

Analysis of O 1s spectra of pure ZnO and 0.75Au/ZnO samples.

Peak

Position BE(eV)

FWHM(eV)

Raw area(cps eV)

Atomic Conc.(%)

ZnO

\

\

\

\

O1s(1)

530.21

1.83

60975.7

9.88

O1s(2)

531.11

1.54

86251.96

13.98

O1s(3)

532.25

1.89

109082.9

17.68

0.75Au/ZnO

\

\

\

\

O1s(1)

530.21

2.13

38091.29

6.83

O1s(2)

531.16

1.56

81638.14

14.64

O1s(3)

532.07

1.88

69802.43

12.52

Highlights ► Photocorrosion cracks along the c-axis of ZnO nanorods were observed. ► Au/ZnO show enhanced photocatalytic activity and stability relative to ZnO nanorods. ► SPR effect enhances the light absorption and electrons capture ability of Au/ZnO. ► Au nanoparticles modification induces the formation of surface hydroxyl groups.

23