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Microwave hydrothermal synthesis of K+ doped ZnO nanoparticles with enhanced photocatalytic properties under visible-light
Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Microwave hydrothermal synthesis of K þ doped ZnO nanoparticles with enhanced photocatalytic properties under visible-light Dan Li, Jian-Feng Huang n, Li-Yun Cao, Hai-Bo OuYang, Jia-Yin Li, Chun-Yan Yao School of Material Science and Engineering, Shaanxi University of Science and Technology, Xi'an 710021, China
art ic l e i nf o
a b s t r a c t
Article history: Received 17 October 2013 Accepted 11 December 2013
Potassium doped ZnO crystallites were successfully synthesized via a microwave hydrothermal method with Zn(NO3)2 6H2O and KNO3 used as source materials. The phase and microstructure of the asprepared Zn1 xKxO crystallites were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Ultraviolet–visible (UV–vis) spectrum and photochemical reaction instrument were used to analyze the photocatalytic properties of the particles. Results show that the diffraction peaks of the K þ doped ZnO crystallites shifted slightly toward higher angle when increasing K þ doping concentration was increased from 0% to 0.5%. Moreover, morphology of these ZnO crystallites transforms from lamellar structure to small granules with higher absorption in UV region and narrower band gaps (Eg). Finally, comparing the products to pure ZnO particles exhibits obviously improved photocatalytic properties of the K þ doped ZnO crystallites. & 2013 Elsevier B.V. All rights reserved.
Keywords: ZnO Photocatalytic activity Defects Microstructure Nanoparticles
1. Introduction ZnO is one of the most promising photocatalysts due to its desirable properties, like high activity, low-toxicity and low-cost [1]. However, it also presents some drawbacks like fast recombination rate of the photogenerated electron–hole pairs and low quantum yield in the photocatalytic reaction in aqueous solutions, highly restraining its photocatalytic activity under visible light [2]. Therefore, enhancing the photocatalytic activity of ZnO has drawn much attention from researchers all over the world. They have found that doping is an effective and facile method to improve the photocatalytic properties since it may cause the variation of the surface area, the incorporation of dopant ions is able to generate lattice defects and variation of band gap energy [3,4]. Many dopants such as Ag [5], Ta [6], Cr [7] and La [8] have been used to show better photocatalytic performance. However, using these dopants may be a very expedient way to improve the photocatalytic activity of ZnO materials. Therefore, group-I elements (Li, Na and K) have active physical and chemical properties. They have been used as dopants to adjust the performance of ZnO in some researches and they may behave both as an acceptor and as a donor in ZnO [9]. The p-type ZnO with group-I elements as dopants were prepared to improve its optoelectronic and ferroelectric properties [10]. Recently, Huang et al. [11] studied the p-type doping for many elements in wurtzite ZnO by first-principle. Based on their calculation results, they found that K is calculated
n
Corresponding author. Tel./fax: þ 86 29 86168802. E-mail addresses: [email protected], [email protected] (J.-F. Huang).
to be the best candidate for p-type doping in wurtzite ZnO structure. Nevertheless, the researches of K þ doped ZnO and its photocatalytic activity under visible-light irradiation have rarely been reported. Microwave-assisted hydrothermal (M-H) method has been considered to be one of significant methods to prepare doped metal oxide crystallites in recent years. Microwaves energy has been demonstrated to enhance organic chemical reaction, increasing the net rate early in the heating process [12]. This method could be attributed to the difficulties in controlling the simultaneous growth of the crystal and recombination of interparticles by the microwave heating [13]. Therefore, in the present work, we try to use a novel microwave hydrothermal approach to prepare K þ doped ZnO (Zn1 xKxO) crystallites and try to reveal their photocatalytic activities by degrading RhB in water under visible-light irradiation.
2. Experimental Preparation of pure ZnO and K þ doped ZnO: The ZnO crystallite was prepared by a novel microwave hydrothermal approach. Firstly, 30 mL NaOH (3.2 mol/L) solution was slowly added into 30 mL Zn(NO3)2 6H2O (1.6 mol/L) solution. This mixed solution was transferred to a Teflon-lined autoclave of 100 mL capacity and then put into a MDS-10 Microwave Hydrothermal System (Shanghai Sineo Microwave chemistry Co., Ltd. Power 1000 W). Secondly, the solution was heated at 160 1C for 30 min (detailed experimental procedure is shown in Supplementary data). For preparing different concentration K þ doped ZnO crystallites, KNO3 with K þ
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.12.052
Please cite this article as: Li D, et al. Microwave hydrothermal synthesis of K þ doped ZnO nanoparticles with enhanced photocatalytic properties under visible-light. Mater Lett (2013), http://dx.doi.org/10.1016/j.matlet.2013.12.052i
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D. Li et al. / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
from 0.1 to 0.5% were put into 30 mL Zn(NO3)2 6H2O (1.6 mol/L) solution separately without changing other conditions Characterization: The powder X-ray diffraction patterns of asprepared samples were measured with an X-ray diffractometer (XRD, D/max-2200PC, Rigaku, Japan) with Cu Kα radiation at a scanning rate of 81 min 1 in the 2θ range from 151 to 701. The morphology of as-prepared samples was observed by a scanning electron microscopy (SEM, HITACHI Japan). UV–vis spectra were obtained on a Lambda 950 spectrophotometer (PerkinElmer, USA). Measurement of photocatalytic activity: Photocatalytic activity of the catalysts was evaluated by degradation of 1 10 5 mol/L RhB aqueous solution. The loading concentration of the catalysts was 0.5 g/L. Before illumination, the suspension of RhB with catalysts was magnetically stirred in dark for 30 min until an adsorption– desorption equilibrium was achieved. Photocatalytic experiments were carried out under a 500 W Xe lamp with a 420 nm cutoff filter. At given intervals, a certain amount of the suspension was
sampled and centrifuged. RhB in the supernatant was measured by a UV–vis spectrophotometer (UV-2550, Shanghai) at 664 nm wavelength.
3. Results and discussion Structural analysis: Fig. 1(a) shows the XRD patterns of the pure ZnO and K þ doped ZnO crystallites. The XRD patterns showed that K2O occurred when the K þ content is 0.5%. The major phase observed in the XRD pattern is the ZnO hexagonal wurtzite structure. In Fig. 1(b), there was an obvious shift to a lower 2θ angle of the peak (100), (002) and (101), indicating the substitution of K þ ion partly doped into the ZnO lattice. This may result from that the ionic radius of K þ (1.51 Å) was much larger than that of Zn2 þ (0.74 Å), causing an expansion of the lattice parameter in the K þ doped ZnO crystallites. In order to confirm whether K þ
Fig. 1. (a) XRD patterns of the Zn1 xKxO crystallites and (b) XRD patterns enlarged. SEM images of the Zn1 xKxO crystallites with different doping concentration of K þ : (c) [K þ ] ¼0%, (d) [K þ ] ¼ 0.1%, (e) [K þ ] ¼ 0.3%, and (f) [K þ ] ¼0.5%.
Please cite this article as: Li D, et al. Microwave hydrothermal synthesis of K þ doped ZnO nanoparticles with enhanced photocatalytic properties under visible-light. Mater Lett (2013), http://dx.doi.org/10.1016/j.matlet.2013.12.052i
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doped in ZnO or not, we did XPS test and the result showed that the K þ doped in ZnO successfully (the detail test result is shown in Supplementary data). Fig. 1 also shows the morphology of the as-synthesized crystallites. As is shown in Fig. 1(c), the structure of ZnO crystallites exhibited a morphology evolution from lamellar-like to granulelike structures with particle sizes decreased (from 300 nm to 100 nm). This evolution indicates that higher specific surface area may be obtained by increasing K þ doping concentration. Photocatalytic activity: The UV–vis absorption spectra of the prepared crystallites are shown in Fig. 2(a). It can be seen that the enhanced absorption in the visible-light region ranges from 400 to 700 nm for the K þ doped ZnO (indicated by arrows), which can be attributed to the K þ ions dope into ZnO to produce defect levels. In addition, the defects in ZnO crystallinity can act as the active center to capture photogenerated electrons and the recombination of electron–hole pairs can be effectively suppressed [14]. In order to calculate the band gap of Zn1 xKxO crystallites, the K-M model is employed to treat the absorption data of the samples
3
[15]. Fig. 2(b) presents the direct band-gap energy estimated from the equation (αhv)2 ¼A(hv Eg) according to the K-M model. The band gap decreases slightly when K þ doping concentrations are increased. This may result from the band tail, which is composed of defect localized states both at the bottom of conduction band and the top of the valance band [16]. When the K þ doping concentration is 0.3%, the ZnO has the smallest band gap of 3.196 eV to exhibit the best absorption ability in visible-light region. The photocatalytic activity of the as-prepared products is studied by the degradation of RhB under visible-light, as is shown in Fig. 3(a). As can be seen from Fig. 3(a), the K þ doped ZnO exhibits higher photocatalytic activity than that of pure ZnO. The 0.3% K þ doped ZnO crystallite exhibits the highest photocatalytic decolorization efficiency with RhB concentration reduced as much as 93.7% after 80 min irradiation. The detailed results of the adsorption spectra during the photocatalytic degradation process of 0.3% K þ doping concentration is displayed in Fig. 3(b). The photocatalysis of K þ doped ZnO can be explained in Fig. 4. The K þ ions as impurities dope into the ZnO lattices to increase
Fig. 2. (a) UV–vis absorption spectra of the Zn1 xKxO crystallites with different doping concentration of K þ . (b) The relationship between (αhν)2 and hν of the Zn1 xKxO crystallites with different doping concentration of K þ .
Fig. 3. (a) Photocatalytic properties of the Zn1 xKxO crystallites with different doping concentration of K þ . (b) Visible-light degradation of RhB by Zn1 xKxO prepared at the K þ doping concentration 0.3%.
Please cite this article as: Li D, et al. Microwave hydrothermal synthesis of K þ doped ZnO nanoparticles with enhanced photocatalytic properties under visible-light. Mater Lett (2013), http://dx.doi.org/10.1016/j.matlet.2013.12.052i
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crystallite may be a potential candidate for its practical application in visible-light photocatalytic degradation of organic contaminant.
Acknowledgments The authors are grateful to National Key Technology R&D Program (No. 2013BAF09B02), International Science and Technology Cooperation Project Funding of Shaanxi Province (No. 2011KW-11), Innovation Team Assistance Foundation of Shaanxi Province (2013KCT-06), Innovation Team Assistance Foundation of Shaanxi University of Science and Technology (No. TD09-05), and Graduate Innovation Fund of Shaanxi University of Science and Technology. Fig. 4. The mechanism of the photocatalytic reaction in Zn1 xKxO crystallites.
the specific surface area and decrease the Eg, which can increase the reaction area and produce more hole–electron pairs under visible-light irradiation. Moreover, defects are considered to be formed when K þ ions dope in ZnO lattices. The defects, including oxygen defects, hydrogen-related, and so forth, could form defect levels to benefit the efficient separation of the hole–electron pairs [17]. This is believed to effectively improve the crystallites photocatalytic properties. Furthermore, too much K þ ions may also lead to the recombination of photogenerated electrons and holes, decreasing the photocatalytic activity of ZnO [18]. 4. Conclusion ZnO crystallites were successfully doped with different amount of K þ via a microwave hydrothermal method. Results show that the K þ doping led to the increase of ZnO crystal lattice with the morphology changes from lamellar into a granule structure. These K þ doped ZnO crystallites exhibit higher specific surface area, absorptions in visible-light region and lower optical band gaps than the pure ZnO. Moreover, defects are considered to be formed when K þ ions dope in ZnO lattices. The K þ doped ZnO crystallites also present an enhanced photocatalytic activity than pure ZnO under visible-light irradiation. Our research suggests that K þ doped ZnO
Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.matlet.2013.12.052. References [1] Cai YY, Wang YR, Liu Y, Guo CF, Hu Y. Mater Lett 2013;100:278–81. [2] Sun JH, Dong SY, Feng JL, Yin XJ, Zhao XCJ. Mol Catal A: Chem 2011;335:145–50. [3] Ahmad M, Hong ZL, Ahmed E, Khalid NR, Elhissi A, Ahmad W. Ceram Int 2013;39:3007–15. [4] Suwanboon S, Amompitoksuk P, Sukolral A, Muensit N. Ceram Int 2013;39:2811–9. [5] Ahmad M, Ahmed E, Zhang YW, Khalid NR, Xu JF, Ullah M, et al. Curr Appl Phys 2013;13:697–704. [6] Ji ZK, Ai DL, Hai FZ, You PG, Hui L, Di WJ. Solid State Chem 2009;182:2061–7. [7] Chang LW, Li S, Yong CZ, Qing LH. Mater Lett 2011;65:1794–6. [8] Zhang Y, Zhang WF, Zheng HW. Scr Mater 2007;57:313–6. [9] Fan JC, Sreekanth KM, Xie Z, Chang SL, Rao KV. Prog Mater Sci 2013;58:874–985. [10] Gupta MK, Nidhi S, Singh BK, Binay K. Mater Lett 2010;64:1825–8. [11] Huang GY, Wang CY, Wang JT. Physica B 2010;405:158. [12] Chao X, Li XC, Ge S, Wei L, Xiao FQ, Ya QY. J Alloys Compd 2010;14:373–6. [13] Kappe CO. Angew Chem Int Ed 2004;43:6250. [14] Fu H, Pan C, Yao W, Zhu YJ. Phys Chem B 2005;109:22432. [15] Bae SY, Na CW, Kang JH, Park JJ. Phys Chem B 2005;109:2526–31. [16] Jian GL, Xue MC, Jian BZ, Kai H, Xue PS, Zhao QS. J Synth Cryst 2010;39:1494–8. [17] Fatiha BB, Brigitte S, Nacer B, Josef B, Pascal R, et al. J Mater Chem 2011;9:10982–9. [18] Stengl V, Bakardjieva S, Murafa N. Mater Chem Phys 2009;114:217–26.
Please cite this article as: Li D, et al. Microwave hydrothermal synthesis of K þ doped ZnO nanoparticles with enhanced photocatalytic properties under visible-light. Mater Lett (2013), http://dx.doi.org/10.1016/j.matlet.2013.12.052i
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Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Microwave hydrothermal synthesis of K þ doped ZnO nanoparticles with enhanced photocatalytic properties under visible-light Dan Li, Jian-Feng Huang n, Li-Yun Cao, Hai-Bo OuYang, Jia-Yin Li, Chun-Yan Yao School of Material Science and Engineering, Shaanxi University of Science and Technology, Xi'an 710021, China
art ic l e i nf o
a b s t r a c t
Article history: Received 17 October 2013 Accepted 11 December 2013
Potassium doped ZnO crystallites were successfully synthesized via a microwave hydrothermal method with Zn(NO3)2 6H2O and KNO3 used as source materials. The phase and microstructure of the asprepared Zn1 xKxO crystallites were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Ultraviolet–visible (UV–vis) spectrum and photochemical reaction instrument were used to analyze the photocatalytic properties of the particles. Results show that the diffraction peaks of the K þ doped ZnO crystallites shifted slightly toward higher angle when increasing K þ doping concentration was increased from 0% to 0.5%. Moreover, morphology of these ZnO crystallites transforms from lamellar structure to small granules with higher absorption in UV region and narrower band gaps (Eg). Finally, comparing the products to pure ZnO particles exhibits obviously improved photocatalytic properties of the K þ doped ZnO crystallites. & 2013 Elsevier B.V. All rights reserved.
Keywords: ZnO Photocatalytic activity Defects Microstructure Nanoparticles
1. Introduction ZnO is one of the most promising photocatalysts due to its desirable properties, like high activity, low-toxicity and low-cost [1]. However, it also presents some drawbacks like fast recombination rate of the photogenerated electron–hole pairs and low quantum yield in the photocatalytic reaction in aqueous solutions, highly restraining its photocatalytic activity under visible light [2]. Therefore, enhancing the photocatalytic activity of ZnO has drawn much attention from researchers all over the world. They have found that doping is an effective and facile method to improve the photocatalytic properties since it may cause the variation of the surface area, the incorporation of dopant ions is able to generate lattice defects and variation of band gap energy [3,4]. Many dopants such as Ag [5], Ta [6], Cr [7] and La [8] have been used to show better photocatalytic performance. However, using these dopants may be a very expedient way to improve the photocatalytic activity of ZnO materials. Therefore, group-I elements (Li, Na and K) have active physical and chemical properties. They have been used as dopants to adjust the performance of ZnO in some researches and they may behave both as an acceptor and as a donor in ZnO [9]. The p-type ZnO with group-I elements as dopants were prepared to improve its optoelectronic and ferroelectric properties [10]. Recently, Huang et al. [11] studied the p-type doping for many elements in wurtzite ZnO by first-principle. Based on their calculation results, they found that K is calculated
n
Corresponding author. Tel./fax: þ 86 29 86168802. E-mail addresses: [email protected], [email protected] (J.-F. Huang).
to be the best candidate for p-type doping in wurtzite ZnO structure. Nevertheless, the researches of K þ doped ZnO and its photocatalytic activity under visible-light irradiation have rarely been reported. Microwave-assisted hydrothermal (M-H) method has been considered to be one of significant methods to prepare doped metal oxide crystallites in recent years. Microwaves energy has been demonstrated to enhance organic chemical reaction, increasing the net rate early in the heating process [12]. This method could be attributed to the difficulties in controlling the simultaneous growth of the crystal and recombination of interparticles by the microwave heating [13]. Therefore, in the present work, we try to use a novel microwave hydrothermal approach to prepare K þ doped ZnO (Zn1 xKxO) crystallites and try to reveal their photocatalytic activities by degrading RhB in water under visible-light irradiation.
2. Experimental Preparation of pure ZnO and K þ doped ZnO: The ZnO crystallite was prepared by a novel microwave hydrothermal approach. Firstly, 30 mL NaOH (3.2 mol/L) solution was slowly added into 30 mL Zn(NO3)2 6H2O (1.6 mol/L) solution. This mixed solution was transferred to a Teflon-lined autoclave of 100 mL capacity and then put into a MDS-10 Microwave Hydrothermal System (Shanghai Sineo Microwave chemistry Co., Ltd. Power 1000 W). Secondly, the solution was heated at 160 1C for 30 min (detailed experimental procedure is shown in Supplementary data). For preparing different concentration K þ doped ZnO crystallites, KNO3 with K þ
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.12.052
Please cite this article as: Li D, et al. Microwave hydrothermal synthesis of K þ doped ZnO nanoparticles with enhanced photocatalytic properties under visible-light. Mater Lett (2013), http://dx.doi.org/10.1016/j.matlet.2013.12.052i
67 68 69 70 71 72 73 74 75 76 77 Q4 78 79 80 81 82 83 84 85 86 87 88 89 Q5 90 91 92 93 94 95 96 97 98 99 100
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D. Li et al. / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
from 0.1 to 0.5% were put into 30 mL Zn(NO3)2 6H2O (1.6 mol/L) solution separately without changing other conditions Characterization: The powder X-ray diffraction patterns of asprepared samples were measured with an X-ray diffractometer (XRD, D/max-2200PC, Rigaku, Japan) with Cu Kα radiation at a scanning rate of 81 min 1 in the 2θ range from 151 to 701. The morphology of as-prepared samples was observed by a scanning electron microscopy (SEM, HITACHI Japan). UV–vis spectra were obtained on a Lambda 950 spectrophotometer (PerkinElmer, USA). Measurement of photocatalytic activity: Photocatalytic activity of the catalysts was evaluated by degradation of 1 10 5 mol/L RhB aqueous solution. The loading concentration of the catalysts was 0.5 g/L. Before illumination, the suspension of RhB with catalysts was magnetically stirred in dark for 30 min until an adsorption– desorption equilibrium was achieved. Photocatalytic experiments were carried out under a 500 W Xe lamp with a 420 nm cutoff filter. At given intervals, a certain amount of the suspension was
sampled and centrifuged. RhB in the supernatant was measured by a UV–vis spectrophotometer (UV-2550, Shanghai) at 664 nm wavelength.
3. Results and discussion Structural analysis: Fig. 1(a) shows the XRD patterns of the pure ZnO and K þ doped ZnO crystallites. The XRD patterns showed that K2O occurred when the K þ content is 0.5%. The major phase observed in the XRD pattern is the ZnO hexagonal wurtzite structure. In Fig. 1(b), there was an obvious shift to a lower 2θ angle of the peak (100), (002) and (101), indicating the substitution of K þ ion partly doped into the ZnO lattice. This may result from that the ionic radius of K þ (1.51 Å) was much larger than that of Zn2 þ (0.74 Å), causing an expansion of the lattice parameter in the K þ doped ZnO crystallites. In order to confirm whether K þ
Fig. 1. (a) XRD patterns of the Zn1 xKxO crystallites and (b) XRD patterns enlarged. SEM images of the Zn1 xKxO crystallites with different doping concentration of K þ : (c) [K þ ] ¼0%, (d) [K þ ] ¼ 0.1%, (e) [K þ ] ¼ 0.3%, and (f) [K þ ] ¼0.5%.
Please cite this article as: Li D, et al. Microwave hydrothermal synthesis of K þ doped ZnO nanoparticles with enhanced photocatalytic properties under visible-light. Mater Lett (2013), http://dx.doi.org/10.1016/j.matlet.2013.12.052i
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doped in ZnO or not, we did XPS test and the result showed that the K þ doped in ZnO successfully (the detail test result is shown in Supplementary data). Fig. 1 also shows the morphology of the as-synthesized crystallites. As is shown in Fig. 1(c), the structure of ZnO crystallites exhibited a morphology evolution from lamellar-like to granulelike structures with particle sizes decreased (from 300 nm to 100 nm). This evolution indicates that higher specific surface area may be obtained by increasing K þ doping concentration. Photocatalytic activity: The UV–vis absorption spectra of the prepared crystallites are shown in Fig. 2(a). It can be seen that the enhanced absorption in the visible-light region ranges from 400 to 700 nm for the K þ doped ZnO (indicated by arrows), which can be attributed to the K þ ions dope into ZnO to produce defect levels. In addition, the defects in ZnO crystallinity can act as the active center to capture photogenerated electrons and the recombination of electron–hole pairs can be effectively suppressed [14]. In order to calculate the band gap of Zn1 xKxO crystallites, the K-M model is employed to treat the absorption data of the samples
3
[15]. Fig. 2(b) presents the direct band-gap energy estimated from the equation (αhv)2 ¼A(hv Eg) according to the K-M model. The band gap decreases slightly when K þ doping concentrations are increased. This may result from the band tail, which is composed of defect localized states both at the bottom of conduction band and the top of the valance band [16]. When the K þ doping concentration is 0.3%, the ZnO has the smallest band gap of 3.196 eV to exhibit the best absorption ability in visible-light region. The photocatalytic activity of the as-prepared products is studied by the degradation of RhB under visible-light, as is shown in Fig. 3(a). As can be seen from Fig. 3(a), the K þ doped ZnO exhibits higher photocatalytic activity than that of pure ZnO. The 0.3% K þ doped ZnO crystallite exhibits the highest photocatalytic decolorization efficiency with RhB concentration reduced as much as 93.7% after 80 min irradiation. The detailed results of the adsorption spectra during the photocatalytic degradation process of 0.3% K þ doping concentration is displayed in Fig. 3(b). The photocatalysis of K þ doped ZnO can be explained in Fig. 4. The K þ ions as impurities dope into the ZnO lattices to increase
Fig. 2. (a) UV–vis absorption spectra of the Zn1 xKxO crystallites with different doping concentration of K þ . (b) The relationship between (αhν)2 and hν of the Zn1 xKxO crystallites with different doping concentration of K þ .
Fig. 3. (a) Photocatalytic properties of the Zn1 xKxO crystallites with different doping concentration of K þ . (b) Visible-light degradation of RhB by Zn1 xKxO prepared at the K þ doping concentration 0.3%.
Please cite this article as: Li D, et al. Microwave hydrothermal synthesis of K þ doped ZnO nanoparticles with enhanced photocatalytic properties under visible-light. Mater Lett (2013), http://dx.doi.org/10.1016/j.matlet.2013.12.052i
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crystallite may be a potential candidate for its practical application in visible-light photocatalytic degradation of organic contaminant.
Acknowledgments The authors are grateful to National Key Technology R&D Program (No. 2013BAF09B02), International Science and Technology Cooperation Project Funding of Shaanxi Province (No. 2011KW-11), Innovation Team Assistance Foundation of Shaanxi Province (2013KCT-06), Innovation Team Assistance Foundation of Shaanxi University of Science and Technology (No. TD09-05), and Graduate Innovation Fund of Shaanxi University of Science and Technology. Fig. 4. The mechanism of the photocatalytic reaction in Zn1 xKxO crystallites.
the specific surface area and decrease the Eg, which can increase the reaction area and produce more hole–electron pairs under visible-light irradiation. Moreover, defects are considered to be formed when K þ ions dope in ZnO lattices. The defects, including oxygen defects, hydrogen-related, and so forth, could form defect levels to benefit the efficient separation of the hole–electron pairs [17]. This is believed to effectively improve the crystallites photocatalytic properties. Furthermore, too much K þ ions may also lead to the recombination of photogenerated electrons and holes, decreasing the photocatalytic activity of ZnO [18]. 4. Conclusion ZnO crystallites were successfully doped with different amount of K þ via a microwave hydrothermal method. Results show that the K þ doping led to the increase of ZnO crystal lattice with the morphology changes from lamellar into a granule structure. These K þ doped ZnO crystallites exhibit higher specific surface area, absorptions in visible-light region and lower optical band gaps than the pure ZnO. Moreover, defects are considered to be formed when K þ ions dope in ZnO lattices. The K þ doped ZnO crystallites also present an enhanced photocatalytic activity than pure ZnO under visible-light irradiation. Our research suggests that K þ doped ZnO
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Please cite this article as: Li D, et al. Microwave hydrothermal synthesis of K þ doped ZnO nanoparticles with enhanced photocatalytic properties under visible-light. Mater Lett (2013), http://dx.doi.org/10.1016/j.matlet.2013.12.052i
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