- Email: [email protected]
Au nanocomposites through a mild wet-chemistry route
Materials Letters 140 (2015) 39–42
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Facile fabrication and enhanced photocatalytic properties of ZnO/Au nanocomposites through a mild wet-chemistry route Xianming Hou n School of Chemistry & Chemical Engineering, Taishan University, Taian 271021, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 15 October 2014 Accepted 31 October 2014 Available online 8 November 2014
Cobblestone-like ZnO/Au nanocomposites have been synthesized via a simple and practical nonaqueous synthetic strategy at room temperature. The structure, composition, morphology, and optical properties were characterized by X-ray diffraction, energy-dispersive X-ray spectroscopy, electron microscopy, and spectroscopic techniques. The results showed that the products were composed of metallic Au and ZnO components; spherical Au nanoparticles were attached to the ZnO supports; ZnO/Au hybrid nanostructures exhibited tunable optical properties. Moreover, these ZnO/Au nanohybrids were found to possess more efficient photocatalytic activity toward photodegradation of RhB than that of pure ZnO structure, which holds promise for applications in environmental remediation and water treatment. & 2014 Elsevier B.V. All rights reserved.
Keywords: Nanocomposites Optical materials and properties ZnO/Au Photodegradation
1. Introduction The development of an abundant, cost-effective and environment friendly photocatalyst is of prime importance in addressing the current energy and environmental issues [1,2]. Zinc oxide (ZnO) is one of the promising materials because of its distinct advantages, such as low cost, resource abundance, non-toxicity, and biocompatibility as well as physical and chemical stability [3,4], and has therefore received considerable attention for the applications in optoelectronics [3,5], energy conversion and storage [6], antimicrobial agents [7], sensors [8] and other areas [9,10]. However, main drawback of pure ZnO nanomaterials in practical applications is its relatively poor photocatalytic activity that is related to high recombination rate of photogenerated electron–hole pairs [11]. One of the strategies to improve the photocatalytic performance of ZnO photocatalysts is to build various heterogeneous nanocomposites consisting of ZnO and plasmonic metal components. For example, the coupling of ZnO nanostructures with gold nanocrystals was found to display improved photocatalytic activity [12–14]. Moreover, these ZnO/Au nanocomposites may possess novel properties and functionalities, which have been intensively studied and employed in diverse areas such as biological detection [15], gas sensing [16], solar cell [17], nonlinear optics [18], and environmental purification [19]. To this end, various preparative approaches and strategies, including spontaneous deposition [20], electrophoretic
n
Tel.: þ 86 538 6715156; fax: þ 86 538 6715536. E-mail address: [email protected]
http://dx.doi.org/10.1016/j.matlet.2014.10.164 0167-577X/& 2014 Elsevier B.V. All rights reserved.
deposition [21], ion beam sputtering [22], photochemical synthesis [23], hydrothermal process [24], and functional molecule-directed controllable assembly [25], have been developed to design and fabricate a wide variety of ZnO/Au nanohybrids during the last two decades. Although some important progress in constructing ZnO/Au hybrid nanostructures has been achieved, further extension of this work by exploring some simple and mild synthetic approaches is still desired. In this contribution, a facile and practical wetchemistry route is employed for the mass-productive preparation of ZnO/Au nanocomposites. Enhanced photocatalytic performance of as-prepared ZnO/Au nanohybrids was observed, demonstrating the active role of Au NPs in the photocatalytic reaction.
2. Experimental Preparation of ZnO/Au nanocomposites: in a typical procedure, 1.1 g of Zn(Ac)2 2H2O was dissolved in 50 mL of ethanol containing 4.5 mL of 50 mM HAuCl4 solution by ultrasonication. Then, 100 mL of 0.5 M NaOH ethanol solution was added into the above mixture under vigorous agitation. The solution immediately turned purple and the reaction was kept at ambient temperature for 30 min. Finally, the violet precipitates were collected by centrifugation, repeatedly washed with ultrapure water and ethanol for several times, and dried at 70 1C in vacuum for 12 h. Sample characterization: the structure, composition, and morphologies of the samples were characterized using X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS) and transmission electron microscopy (TEM). UV–visible spectra of the samples were recorded
40
X. Hou / Materials Letters 140 (2015) 39–42
using an α-Helios spectrophotometer. Photoluminescence (PL) spectra were measured on a Hitachi F-7000 fluorescence spectrophotometer using a Xenon lamp as the excitation source. The photocatalytic properties of ZnO and ZnO/Au samples were assessed by the degradation of Rhodamine B (RhB, 20 mg/L, pH¼6.5) under illumination of a 16 W UV light-tube (λ¼365 nm) with the light intensity of 12 mW cm 2. The photocatalytic degradation process was monitored by measuring the 553 nm optical absorption of RhB solution at regular intervals and final results were averaged out from at least 3 independent experiments.
3. Results and discussion The phase identification and structure analysis of the as-prepared sample were conducted by powder XRD. Fig. 1a shows a representative XRD pattern of ZnO/Au nanocomposites produced by an easy wetchemistry method. The XRD pattern of the composite consisted of two sets of diffraction peaks. Strong diffraction peaks marked with a hashmark (♯) matched well with the wurtzite hexagonal phase bulk ZnO (JCPDS Card No. 36-1451), whereas several weak peaks marked with an asterisk (n) could be indexed to the face-centered cubic (fcc)
b
a # (100) # (002) # (101)
40
50
60
Intensity (a.u.)
70
# (202)
∗
# (004)
(220)
∗
30
# (200) # (112) # (201)
# (110)
(200)
# (103)
# (102)
(111)
Intensity / a.u.
∗ Au
∗
20
Zn
# ZnO
Zn
O Au
Au
80
0
1
2
Au
3
4
2θ / deg.
5
6
7
8
9
10
keV
Fig. 1. (a) XRD pattern and (b) EDS image of ZnO/Au nanocomposites.
Fig. 2. TEM images of (a) ZnO and (b) ZnO/Au samples.
b
Abs
Intensity
a
b a
300
400
500
a b 600
700
Wavelength / nm
800
900
350
400
450
500
550
600
Wavelength / nm
Fig. 3. (A) Optical absorption spectra and (B) PL spectra of (a) pure ZnO nanoparticles and (b) ZnO/Au nanocomposites.
X. Hou / Materials Letters 140 (2015) 39–42
more than 80% of dye was decomposed after 1.5 h irradiation. These results demonstrated that the attachment of Au NPs on the ZnO supports induced significant improvement in the photodegradation efficiency. The stability and reusability of the ZnO/Au nanohybrid as a photocatalyst were also investigated by reusing them in fresh RhB solutions under UV irradiation. The exposure time for each test was 2.0 h. The catalyst particles were centrifuged and washed thoroughly before each cycle of use. Fig. 4c presents the RhB
a
2.87 2.46
0 h 0.5 h 1.0 h 1.5 h 2.0 h 2.5 h 3.0 h
Abs
2.05 1.64 1.23 0.82 0.41 0.00 375
450
525
600
675
750
Wavelength / nm
b
1.05 0.90 0.75
c/c0
0.60 0.45 0.30
No catalyst ZnO nanocrystals ZnO/Au nanocomposites
0.15 0.00 0.0
c
0.5
1.0
1.5
2.0
2.5
3.0
Irradiation Time / h 1.05 1 cycle 2 cycles 3 cycles
0.90 0.75 0.60
c/c0
phase gold (JCPDS Card No. 04-9748). No other crystalline phases arising from possible impurities were detected. The chemical composition of the products was further analyzed by EDS, which revealed that the ZnO/Au heterostructures consisted of only three elements Zn, O, and Au (Fig. 1b). The above results verified that the products were composite materials composed of metallic Au and ZnO components. In order to examine the morphology and microstructure of ZnO and ZnO/Au nanomaterials, TEM observations were carried out. Fig. 2a displays the typical TEM image of pure ZnO sample synthesized via a facile wet-chemistry approach. It was found that asprepared ZnO NPs having a relatively smooth surface exhibited a cobblestone-like appearance and possessed a narrow size distribution. And, these ZnO particles had a tendency to aggregate into clusters to reduce their free energy of newly-formed ZnO nuclei. TEM image of the ZnO/Au nanocomposites fabricated under similar conditions is shown in Fig. 2b. One can see that each Au nanoparticle was closely attached to a ZnO core and no support-free or isolated Au NPs were present. These spherical Au NPs with an average size of approximately 6 nm were randomly distributed on the ZnO substrates and no aggregated Au clusters were found under the repeated TEM observations, indicating that metallic Au NPs had a good dispersion on the ZnO supports. In addition, we also noticed that the attachment of Au NPs had no significant influence on the morphology and size of ZnO particles. Fig. 3A illustrates optical absorption spectra of ZnO/Au composite and ZnO NPs prepared under similar conditions but without addition of Au precursor. The ZnO NPs showed a sharp UV absorption maximum at 360 nm (labeled as line a), which can be attributed to free excitonic absorption of ZnO particles [26]. The absorption profile of the composite was similar to that of ZnO apart from broad absorption that appeared at approximately 540 nm (labeled as line b), corresponding to the characteristic absorption of surface plasmon of Au NPs [27]. Room-temperature PL emission spectra of ZnO and ZnO/Au samples dispersed in ethanol are shown in Fig. 3B. The spectrum of pure ZnO powder comprised two emission bands (labeled as line a): the narrow UV emission band at 376 nm was due to the direct radiative recombination of excitons [28], while the broad green emission band at approximately 540 nm was commonly assigned to the charge carrier relaxation via surface-related trap states [29]. The spectrum of ZnO/Au composites showed similar peak positions, but with significantly reduced PL intensity (labeled as line b). The fact that was responsible for the above phenomena may be interpreted as deposited Au NPs on ZnO surface acting as sinks for extracting electrons from the photoexcited ZnO, resulting in enhanced charge carrier separation within the ZnO NPs and thus decreasing the intensity of PL peaks [30]. To evaluate the photocatalytic performance of ZnO/Au heterostructures, the degradation of organic dye RhB (a typical pollutant in textile industry) under ultraviolet irradiation was tested as a model system. Fig. 4a shows a series of absorption spectra of RhB solution with time using ZnO/Au nanocomposite as photo- catalyst. It can be seen that the characteristics absorption of RhB at 553 nm decreased progressively with increasing irradiation time and completely disappeared after 2.5 h. No absorption peak in the whole spectrum was observed with further irradiation, indicating the complete decomposition of RhB in the presence of ZnO/Au nano-composites. Fig. 4b displays the RhB removal rate as a function of time with the assistance of different catalysts. The blank test (without any catalyst) showed that the photoinduced self-decomposition of RhB was insignificant. The photodegradation of RhB with the assistance of pure ZnO nanomaterials was relatively slow, removing approximately 55% of RhB from solutions after an illumination time of 3.0 h. In contrast, when the ZnO/Au hybrid nanostructure was employed as a photocatalyst, a significant enhancement in photocatalytic activity was observed and
41
0.45 0.30 0.15 0.00 0.0
0.5
1.0
1.5
2.0
Irradiation Time / h Fig. 4. (a) Time-dependent optical absorbance spectra for RhB solution in the presence of ZnO/Au nanocompostes after exposure to UV light for different durations, (b) kinetics of RhB photodegration using ZnO NPs and ZnO/Au nanocomposite as catalyst, and (c) photocatalytic activity of ZnO/Au photocatalyst on the degrdation of RhB under UV light for the three consecutive cycles.
42
X. Hou / Materials Letters 140 (2015) 39–42
degradation rate as a function of time for three consecutive cycles in the presence of ZnO/Au nanohybrids. It can be distinctly seen that there is no significant decrease in the photocatalytic efficiency after three recycles of operation, which confirms that the ZnO/Au photocatalysts are very stable and possess good reuse performance. There were several possible reasons for the remarkably enhanced photocatalytic performance of the ZnO/Au hybrid nanostructures. Firstly, these ZnO/Au nanocomposites had quite large surface area and possessed numerous interphase boundaries, providing more opportunity for the diffusion and mass transportation of reactant molecules, product molecules, and hydroxyl radicals generated in photochemical process. Secondly, the strong electronic coupling at the ZnO–Au intergranular interfaces promoted the charge transfer from ZnO to Au NPs and further reduced the recombination of photogenerated electron–hole pairs. Thirdly, the newly-formed ZnO–Au interfaces can lead to a substantial decrease in work function [31]. The rate-limiting step in the photocatalytic reactions, in general, is believed to be the electron transfer from the ZnO surface to adsorbed oxygen molecules. Also it is suggested that oxygen adsorbs at the interface between ZnO and Au [32]. Thus decreasing the local work function of ZnO by Au NPs in the proximity of such interfacial oxygen adsorption sites should help to increase the transfer rate of electrons from ZnO to O2 to yield highly active oxidizing species, such as superoxide, hydrogen peroxide, hydroxyl radical, and hydroperoxyl radical [33], which would substantially enhance the efficiency of photocatalytic reaction. 4. Conclusions We have synthesized cobblestone-like ZnO/Au nanocomposites via a simple and mild wet-chemistry method. These hybrid nanostructures exhibited more efficient photocatalytic activity toward photodegradation of RhB than that of pure ZnO structure. The improved photocatalytic activity of the ZnO/Au nanohybrids was attributed to their stable structure, special interphase boundaries, and increased charge separation as well as strong electronic interaction between neighboring Au NPs and ZnO supports. Using similar synthetic strategy, other novel hybrid nanoarchitectures with analogous structures and different components to explore more applications are expected to be prepared.
Acknowledgments This work was supported by NSFC, China (11174215), Natural Science Fund project of Shandong province (ZR2011BL004), and the Science and Technology Development Project of Taian Municipality (20122055).
References [1] [2] [3] [4]
Chen X, Shen S, Guo L, Mao S. Chem Rev 2010;110:6503–70. Qu Y, Duan X. Chem Soc Rev 2013;42:2568–80. Wang X, Summers C, Wang Z. Nano Lett 2004;4:423–6. Malandrino G, Blandino M, Fragala M, Losurdo M, Bruno G. J Phys Chem C 2008;112:9595–9. [5] He J, Chang P, Chen C, Tsai K. Nanotechnology 2009;20:135701. [6] Hochbaum A, Yang P. Chem Rev 2010;110:527–46. [7] Xie Y, He Y, Irwin P, Jin T, Shi X. Appl Environ Microb 2011;77:2325–31. [8] Jing Z, Zhan J. Adv Mater 2008;20:4547–51. [9] Huang M, Mao S, Feick H, Yan H, Wu Y, Kind H, et al. Science 2001;292: 1897 1899. [10] Saito N, Haneda H, Sekiguchi T, Ohashi N, Sakaguchi I, Koumoto K. Adv Mater 2002;14:418–21. [11] Chu F, Huang C, Hsin C, Wang C, Yu S, Yeh P, et al. Nanoscale 2012;4:1471–5. [12] He W, Kim H, Wamer W, Melka D, Callahan J. J Am Chem Soc 2014;136:750–7. [13] Li P, Wei Z, Wu T, Peng Q, Li Y. J Am Chem Soc 2011;133:5660–3. [14] Chen Y, Zeng D, Zhang K, Lu A, Wang L, Peng D. Nanoscale 2014;6:874–81. [15] Shan G, Wang S, Fei X, Liu Y, Yang G. J Phys Chem B 2009;113:1468–72. [16] Joshi R, Hu Q, Alvi F, Joshi N, Kumar A. J Phys Chem C 2009;113:16199–202. [17] Dhas V, Muduli S, Lee W, Han S, Ogale S. Appl Phys Lett 2008;93:243108. [18] Ozga K, Kawaharamura T, Umar A, Oyama M, Nouneh K, Slezak A, et al. Nanotechnology 2008;19:185709. [19] Roy P, Periasamy A, Liang C, Chang H. Environ Sci Technol 2013;47:6688–95. [20] Wang Q, Geng B, Wang S. Environ Sci Technol 2009;43:8968–73. [21] He H, Cai W, Liu Y, Chen B. Langmuir 2010;26:8925–32. [22] Cheng C, Sie E, Liu B, Huan C, Sum T, Sun H, et al. Appl Phys Lett 2010;96:071107. [23] Gu H, Yang Y, Tian J, Shi G. ACS Appl Mater Interfaces 2013;5:6762–8. [24] Chen L, Luo L, Chen Z, Zhang M, Zapien J, Lee C, et al. J Phys Chem C 2010;114:93–100. [25] Yang T, Huang L, Harn Y, Lin C, Chang J, Wu C, et al. Small 2013;9:3169–82. [26 Raula M, Rashid M, Paira T, Dinda E, Mandal T. Langmuir 2010;26:8769–82. [27] Eustis S, El-Sayed M. Chem Soc Rev 2006;35:209–17. [28] Hu Y, Jiang Z, Xu C, Mei T, Guo J, White T. J Phys Chem C 2007;111:9757–60. [29] Vanheusden K, Warren W, Seager C, Tallant D, Voigt J, Gnade B. J Appl Phys 1996;79:7983–90. [30] Udawatte N, Lee M, Kim J, Lee D. ACS Appl Mater Interfaces 2011;3:4531–8. [31] Sykes E, Williams H, Tikhov M, Lambert R. J Phys Chem B 2002;106:5390–4. [32] Hayashi T, Tanaka K, Haruta M. J Catal 1998;178:566–75. [33] Gaya U, Abdullah A. J Photochem Photobiol C 2008;9:1–12.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Facile fabrication and enhanced photocatalytic properties of ZnO/Au nanocomposites through a mild wet-chemistry route Xianming Hou n School of Chemistry & Chemical Engineering, Taishan University, Taian 271021, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 15 October 2014 Accepted 31 October 2014 Available online 8 November 2014
Cobblestone-like ZnO/Au nanocomposites have been synthesized via a simple and practical nonaqueous synthetic strategy at room temperature. The structure, composition, morphology, and optical properties were characterized by X-ray diffraction, energy-dispersive X-ray spectroscopy, electron microscopy, and spectroscopic techniques. The results showed that the products were composed of metallic Au and ZnO components; spherical Au nanoparticles were attached to the ZnO supports; ZnO/Au hybrid nanostructures exhibited tunable optical properties. Moreover, these ZnO/Au nanohybrids were found to possess more efficient photocatalytic activity toward photodegradation of RhB than that of pure ZnO structure, which holds promise for applications in environmental remediation and water treatment. & 2014 Elsevier B.V. All rights reserved.
Keywords: Nanocomposites Optical materials and properties ZnO/Au Photodegradation
1. Introduction The development of an abundant, cost-effective and environment friendly photocatalyst is of prime importance in addressing the current energy and environmental issues [1,2]. Zinc oxide (ZnO) is one of the promising materials because of its distinct advantages, such as low cost, resource abundance, non-toxicity, and biocompatibility as well as physical and chemical stability [3,4], and has therefore received considerable attention for the applications in optoelectronics [3,5], energy conversion and storage [6], antimicrobial agents [7], sensors [8] and other areas [9,10]. However, main drawback of pure ZnO nanomaterials in practical applications is its relatively poor photocatalytic activity that is related to high recombination rate of photogenerated electron–hole pairs [11]. One of the strategies to improve the photocatalytic performance of ZnO photocatalysts is to build various heterogeneous nanocomposites consisting of ZnO and plasmonic metal components. For example, the coupling of ZnO nanostructures with gold nanocrystals was found to display improved photocatalytic activity [12–14]. Moreover, these ZnO/Au nanocomposites may possess novel properties and functionalities, which have been intensively studied and employed in diverse areas such as biological detection [15], gas sensing [16], solar cell [17], nonlinear optics [18], and environmental purification [19]. To this end, various preparative approaches and strategies, including spontaneous deposition [20], electrophoretic
n
Tel.: þ 86 538 6715156; fax: þ 86 538 6715536. E-mail address: [email protected]
http://dx.doi.org/10.1016/j.matlet.2014.10.164 0167-577X/& 2014 Elsevier B.V. All rights reserved.
deposition [21], ion beam sputtering [22], photochemical synthesis [23], hydrothermal process [24], and functional molecule-directed controllable assembly [25], have been developed to design and fabricate a wide variety of ZnO/Au nanohybrids during the last two decades. Although some important progress in constructing ZnO/Au hybrid nanostructures has been achieved, further extension of this work by exploring some simple and mild synthetic approaches is still desired. In this contribution, a facile and practical wetchemistry route is employed for the mass-productive preparation of ZnO/Au nanocomposites. Enhanced photocatalytic performance of as-prepared ZnO/Au nanohybrids was observed, demonstrating the active role of Au NPs in the photocatalytic reaction.
2. Experimental Preparation of ZnO/Au nanocomposites: in a typical procedure, 1.1 g of Zn(Ac)2 2H2O was dissolved in 50 mL of ethanol containing 4.5 mL of 50 mM HAuCl4 solution by ultrasonication. Then, 100 mL of 0.5 M NaOH ethanol solution was added into the above mixture under vigorous agitation. The solution immediately turned purple and the reaction was kept at ambient temperature for 30 min. Finally, the violet precipitates were collected by centrifugation, repeatedly washed with ultrapure water and ethanol for several times, and dried at 70 1C in vacuum for 12 h. Sample characterization: the structure, composition, and morphologies of the samples were characterized using X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS) and transmission electron microscopy (TEM). UV–visible spectra of the samples were recorded
40
X. Hou / Materials Letters 140 (2015) 39–42
using an α-Helios spectrophotometer. Photoluminescence (PL) spectra were measured on a Hitachi F-7000 fluorescence spectrophotometer using a Xenon lamp as the excitation source. The photocatalytic properties of ZnO and ZnO/Au samples were assessed by the degradation of Rhodamine B (RhB, 20 mg/L, pH¼6.5) under illumination of a 16 W UV light-tube (λ¼365 nm) with the light intensity of 12 mW cm 2. The photocatalytic degradation process was monitored by measuring the 553 nm optical absorption of RhB solution at regular intervals and final results were averaged out from at least 3 independent experiments.
3. Results and discussion The phase identification and structure analysis of the as-prepared sample were conducted by powder XRD. Fig. 1a shows a representative XRD pattern of ZnO/Au nanocomposites produced by an easy wetchemistry method. The XRD pattern of the composite consisted of two sets of diffraction peaks. Strong diffraction peaks marked with a hashmark (♯) matched well with the wurtzite hexagonal phase bulk ZnO (JCPDS Card No. 36-1451), whereas several weak peaks marked with an asterisk (n) could be indexed to the face-centered cubic (fcc)
b
a # (100) # (002) # (101)
40
50
60
Intensity (a.u.)
70
# (202)
∗
# (004)
(220)
∗
30
# (200) # (112) # (201)
# (110)
(200)
# (103)
# (102)
(111)
Intensity / a.u.
∗ Au
∗
20
Zn
# ZnO
Zn
O Au
Au
80
0
1
2
Au
3
4
2θ / deg.
5
6
7
8
9
10
keV
Fig. 1. (a) XRD pattern and (b) EDS image of ZnO/Au nanocomposites.
Fig. 2. TEM images of (a) ZnO and (b) ZnO/Au samples.
b
Abs
Intensity
a
b a
300
400
500
a b 600
700
Wavelength / nm
800
900
350
400
450
500
550
600
Wavelength / nm
Fig. 3. (A) Optical absorption spectra and (B) PL spectra of (a) pure ZnO nanoparticles and (b) ZnO/Au nanocomposites.
X. Hou / Materials Letters 140 (2015) 39–42
more than 80% of dye was decomposed after 1.5 h irradiation. These results demonstrated that the attachment of Au NPs on the ZnO supports induced significant improvement in the photodegradation efficiency. The stability and reusability of the ZnO/Au nanohybrid as a photocatalyst were also investigated by reusing them in fresh RhB solutions under UV irradiation. The exposure time for each test was 2.0 h. The catalyst particles were centrifuged and washed thoroughly before each cycle of use. Fig. 4c presents the RhB
a
2.87 2.46
0 h 0.5 h 1.0 h 1.5 h 2.0 h 2.5 h 3.0 h
Abs
2.05 1.64 1.23 0.82 0.41 0.00 375
450
525
600
675
750
Wavelength / nm
b
1.05 0.90 0.75
c/c0
0.60 0.45 0.30
No catalyst ZnO nanocrystals ZnO/Au nanocomposites
0.15 0.00 0.0
c
0.5
1.0
1.5
2.0
2.5
3.0
Irradiation Time / h 1.05 1 cycle 2 cycles 3 cycles
0.90 0.75 0.60
c/c0
phase gold (JCPDS Card No. 04-9748). No other crystalline phases arising from possible impurities were detected. The chemical composition of the products was further analyzed by EDS, which revealed that the ZnO/Au heterostructures consisted of only three elements Zn, O, and Au (Fig. 1b). The above results verified that the products were composite materials composed of metallic Au and ZnO components. In order to examine the morphology and microstructure of ZnO and ZnO/Au nanomaterials, TEM observations were carried out. Fig. 2a displays the typical TEM image of pure ZnO sample synthesized via a facile wet-chemistry approach. It was found that asprepared ZnO NPs having a relatively smooth surface exhibited a cobblestone-like appearance and possessed a narrow size distribution. And, these ZnO particles had a tendency to aggregate into clusters to reduce their free energy of newly-formed ZnO nuclei. TEM image of the ZnO/Au nanocomposites fabricated under similar conditions is shown in Fig. 2b. One can see that each Au nanoparticle was closely attached to a ZnO core and no support-free or isolated Au NPs were present. These spherical Au NPs with an average size of approximately 6 nm were randomly distributed on the ZnO substrates and no aggregated Au clusters were found under the repeated TEM observations, indicating that metallic Au NPs had a good dispersion on the ZnO supports. In addition, we also noticed that the attachment of Au NPs had no significant influence on the morphology and size of ZnO particles. Fig. 3A illustrates optical absorption spectra of ZnO/Au composite and ZnO NPs prepared under similar conditions but without addition of Au precursor. The ZnO NPs showed a sharp UV absorption maximum at 360 nm (labeled as line a), which can be attributed to free excitonic absorption of ZnO particles [26]. The absorption profile of the composite was similar to that of ZnO apart from broad absorption that appeared at approximately 540 nm (labeled as line b), corresponding to the characteristic absorption of surface plasmon of Au NPs [27]. Room-temperature PL emission spectra of ZnO and ZnO/Au samples dispersed in ethanol are shown in Fig. 3B. The spectrum of pure ZnO powder comprised two emission bands (labeled as line a): the narrow UV emission band at 376 nm was due to the direct radiative recombination of excitons [28], while the broad green emission band at approximately 540 nm was commonly assigned to the charge carrier relaxation via surface-related trap states [29]. The spectrum of ZnO/Au composites showed similar peak positions, but with significantly reduced PL intensity (labeled as line b). The fact that was responsible for the above phenomena may be interpreted as deposited Au NPs on ZnO surface acting as sinks for extracting electrons from the photoexcited ZnO, resulting in enhanced charge carrier separation within the ZnO NPs and thus decreasing the intensity of PL peaks [30]. To evaluate the photocatalytic performance of ZnO/Au heterostructures, the degradation of organic dye RhB (a typical pollutant in textile industry) under ultraviolet irradiation was tested as a model system. Fig. 4a shows a series of absorption spectra of RhB solution with time using ZnO/Au nanocomposite as photo- catalyst. It can be seen that the characteristics absorption of RhB at 553 nm decreased progressively with increasing irradiation time and completely disappeared after 2.5 h. No absorption peak in the whole spectrum was observed with further irradiation, indicating the complete decomposition of RhB in the presence of ZnO/Au nano-composites. Fig. 4b displays the RhB removal rate as a function of time with the assistance of different catalysts. The blank test (without any catalyst) showed that the photoinduced self-decomposition of RhB was insignificant. The photodegradation of RhB with the assistance of pure ZnO nanomaterials was relatively slow, removing approximately 55% of RhB from solutions after an illumination time of 3.0 h. In contrast, when the ZnO/Au hybrid nanostructure was employed as a photocatalyst, a significant enhancement in photocatalytic activity was observed and
41
0.45 0.30 0.15 0.00 0.0
0.5
1.0
1.5
2.0
Irradiation Time / h Fig. 4. (a) Time-dependent optical absorbance spectra for RhB solution in the presence of ZnO/Au nanocompostes after exposure to UV light for different durations, (b) kinetics of RhB photodegration using ZnO NPs and ZnO/Au nanocomposite as catalyst, and (c) photocatalytic activity of ZnO/Au photocatalyst on the degrdation of RhB under UV light for the three consecutive cycles.
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X. Hou / Materials Letters 140 (2015) 39–42
degradation rate as a function of time for three consecutive cycles in the presence of ZnO/Au nanohybrids. It can be distinctly seen that there is no significant decrease in the photocatalytic efficiency after three recycles of operation, which confirms that the ZnO/Au photocatalysts are very stable and possess good reuse performance. There were several possible reasons for the remarkably enhanced photocatalytic performance of the ZnO/Au hybrid nanostructures. Firstly, these ZnO/Au nanocomposites had quite large surface area and possessed numerous interphase boundaries, providing more opportunity for the diffusion and mass transportation of reactant molecules, product molecules, and hydroxyl radicals generated in photochemical process. Secondly, the strong electronic coupling at the ZnO–Au intergranular interfaces promoted the charge transfer from ZnO to Au NPs and further reduced the recombination of photogenerated electron–hole pairs. Thirdly, the newly-formed ZnO–Au interfaces can lead to a substantial decrease in work function [31]. The rate-limiting step in the photocatalytic reactions, in general, is believed to be the electron transfer from the ZnO surface to adsorbed oxygen molecules. Also it is suggested that oxygen adsorbs at the interface between ZnO and Au [32]. Thus decreasing the local work function of ZnO by Au NPs in the proximity of such interfacial oxygen adsorption sites should help to increase the transfer rate of electrons from ZnO to O2 to yield highly active oxidizing species, such as superoxide, hydrogen peroxide, hydroxyl radical, and hydroperoxyl radical [33], which would substantially enhance the efficiency of photocatalytic reaction. 4. Conclusions We have synthesized cobblestone-like ZnO/Au nanocomposites via a simple and mild wet-chemistry method. These hybrid nanostructures exhibited more efficient photocatalytic activity toward photodegradation of RhB than that of pure ZnO structure. The improved photocatalytic activity of the ZnO/Au nanohybrids was attributed to their stable structure, special interphase boundaries, and increased charge separation as well as strong electronic interaction between neighboring Au NPs and ZnO supports. Using similar synthetic strategy, other novel hybrid nanoarchitectures with analogous structures and different components to explore more applications are expected to be prepared.
Acknowledgments This work was supported by NSFC, China (11174215), Natural Science Fund project of Shandong province (ZR2011BL004), and the Science and Technology Development Project of Taian Municipality (20122055).
References [1] [2] [3] [4]
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