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Continuous UV irradiation synthesis of ultra-small Au nanoparticles decorated Cu2O with enhanced photocatalytic activity
Composites Communications 9 (2018) 27–32
Contents lists available at ScienceDirect
Composites Communications journal homepage: www.elsevier.com/locate/coco
Continuous UV irradiation synthesis of ultra-small Au nanoparticles decorated Cu2O with enhanced photocatalytic activity Chuncai Konga, Bo Maa, Ke Liub, Weixin Zhanga,b, Zhimao Yanga, a b
T
⁎
School of Science, MOE Key Laboratory for Non-Equilibrium Synthesis and Modulation of Condensed Matter, Xi’an Jiaotong University, Xi’an 710049, Shaanxi, China Hubei Key Laboratory of Advanced Textile Materials & Application, College of Materials Science and Engineering, Wuhan Textile University, Wuhan 430200, China
A B S T R A C T
Noble metal nanoparticles (NPs) with controlled size and shape can be easily prepared by UV irradiation at room temperature. In this work, ultra-small Au NPs were successfully continuously synthesized via an UV irradiation approach, and the synthesis conditions of Au NPs with the size of 1.8 nm have been optimized to be an exposure time of 1.4 min, a HAuCl4 precursor concentration of 0.8 mM and a pH value of 10.4. The growth dynamics of Au NPs was investigated under different exposing time of the electron beam in transmission electron microscopy (TEM), revealing the formation of NPs via the oriented attachment of individual nanoparticles. In addition, well dispersed ultra-small Au NPs decorated Cu2O composites were obtained from this in-situ strategy and showed an enhanced photocatalytic activity compared with that of Au-free oxides and Au- Cu2O from direct replacement method.
1. Introduction Noble metal (Au, Ag, Pt, Pd) nanoparticles (NPs) have attracted intensive research interest because of the multitude of applications in catalysis, biomedicine, optics, and electronics [1–4]. For example, CeO2 catalysts with 1% Au NPs can selectively catalyze CO in H2 environment and remain stable in the presence of water vapor [5]. Au and Ag based nanocomposites are also used for the catalytic hydrogenation reaction [6], and nitrogen oxide reduction [7,8]. Consisting of over 80% surface atoms, ultra-small NPs with a size less than 2 nm have been increasingly investigated due to their high surface area which could enhance catalytic activity [9]. Hence, the controlled synthesis of ultrasmall noble metal nanoparticles based composites is very important and promising for their catalytic application. As one of the common methods, UV irradiation has been widely used for the preparation of ultra-small noble metal NPs with controlled sizes and shapes in solution, where the reductant agent is not necessary due to the direct generation of reduction sites by UV irradiation. The sizes and surface states of NPs can be easily adjusted by using surfactants or solvents. ZY Chen prepared Au NPs in the aqueous solution using polyvinyl alcohol (PVA) as the protective agent and a 30 W lowpressure mercury lamp as the irradiation source [10,11]. Anjali Pal synthetized Au NPs (3–5 nm) under the irradiation of a high-power (200 W) UV lamp, in which CTAC served as the surfactant [12]. By a continuous UV irradiation approach, Yang got coral-like Au
⁎
Corresponding author. E-mail address: [email protected] (Z. Yang).
https://doi.org/10.1016/j.coco.2018.04.003 Received 7 November 2017; Received in revised form 30 March 2018; Accepted 9 April 2018 2452-2139/ © 2018 Published by Elsevier Ltd.
nanostructures in citric acid electrolyte and ultimately formed a network structure [13]. Cuprous oxide (Cu2O) crystals as low-cost, environmentally friendly, direct and narrow bandgap semiconductors, have been extensively used in the photocatalytic degradation of organic materials [14–16]. The combination of noble metal NPs and Cu2O crystals has been demonstrated to significantly enhance the catalytic performance in comparison with the bare Cu2O [17–19]. In the metalmetal oxides heterojunction, the noble nanoparticles play two important roles, one is the enhanced absorption of light in the UV–vis region because of their surface plasmon resonance (SPR), the other one is the efficient electron-hole separation due to the formation of Schottky junction between Cu2O semiconductor and noble NPs [20]. Therefore, the controlled design and continuous synthesis of noble metal NPsCu2O heterojunction are significant and challenging. In this work, we designed a special continuous synthesis equipment for the preparation of ultra-small Au NPs based on the UV irradiation method, involving noble metal salt, citrate acid and PVP. The influences of the experimental conditions such as the concentration of the protecting reagent, the pH value of the precursor and the irradiation time are discussed in detail. In addition, the one-pot synthesis of Au NPsCu2O composite architectures is successfully achieved and their photocatalytic activity for the degradation of methyl orange is investigated.
Composites Communications 9 (2018) 27–32
C. Kong et al.
Fig. 1. TEM images and the size distributions of Au NPs obtained at different condition: (a) and (c) UV radiation time 1.4 min, pH value 10.4; (b) and (d) UV radiation time 20 min, pH value 3.0.
2. Results and discussion
peaks of Au NPs appeared again though the intensities were not obviously enhanced. All these results are compliant with previous reports, when the diameter of Au NPs is lower than 3.2 nm, the surface plasmon resonance absorption decreases drastically [21]. Moreover, theoretical calculation also showed that the particle diameter could influences significantly the absorption spectra, namely, when the diameter of Au NPs is smaller than 2 nm, the plasma resonance absorption peak disappears due to the free electron scattering of particles at the surface [22]. To highlight the growth mechanism of Au NPs, we have studied the effect of the HAuCl4 precursor concentration by changing the precursor (HAuCl4) concentration between 0.2 and 4 mM. And the obtained absorption spectra are displayed in Fig. S2. At the lowest HAuCl4 concentration (0.2 mM), the absorption spectrum shows an obvious characteristic absorption peak. With the increase of HAuCl4 concentrations (from 0.4 to 2 mM), no obvious characteristic peaks were observed which could be ascribed to the strong scattering of photons on the surface of NPs with ultra-small size. However, a broad absorption peak was clearly observed at around 528 nm when the concentration was 4 mM. In this case, the Au NPs prepared show a narrow size distribution but a rather inhomogeneous morphology. The effect of pH values on the Au NPs is investigated by adding various amounts of NaOH to adjust the pH values. As shown in Fig. 3ad, when the pH value is 10.4, the smallest Au NPs were obtained, suggesting that adjusting the pH of precursor solution is effective. Meanwhile, as shown in Fig. 3e-f, the resulting absorption is clearly visible at pH is 3.0 and 4.0, while if the pH value is above 5, the intensity of the characteristic absorption peak decreases with the increased pH value, which are in accordance with the former research [23]. When the pH value exceeded 10.4, obvious absorption peaks
2.1. Characterization of Au NPs Most of the noble metal compounds can be transformed into noble metals nanoparticles under the action of the light, and the sizes and morphologies of Au NPs can be controlled by changing the irradiation time, pH value and surfactants. Fig. 1 shows the TEM images of Au NPs with different sizes obtained from different synthesis conditions: NPs with an average diameter of 1.4 nm (Fig. 1a and c) are formed under the UV radiation for 1.4 min and pH value of 10.4, while larger Au NPs with an average diameter of 4.4 nm (Fig. 1b and d) are formed when the UV radiation time is 20 min and pH value is 3.0. The effect of the UV radiation time on the Au NPs is studied by changing the flow rate of the peristaltic pump, while all other conditions are kept the same. As shown in Fig. 2, when the irradiation time is lower than 1.4 min, the Au NPs shape and size are extremely irregular. While the obtained Au NPs show very uniform and small size under the irradiation for1.4 min. When the irradiation time exceeds 1.4 min, the Au NPs size increases rather uniformly. Fig. 2f-h shows the size distribution and the absorption spectra of Au NPs obtained at different UV irradiation time, which suggests that the irradiation time of 1.4 min results in the smallest and most uniform size, and the absorption spectra are directly related to the irradiation time. As far as the UV exposure time is less than 1.4 min, the increased irradiation time leads to a gradual, low and broad absorption peak. low and broad absorption peak. While there is no obvious absorption peak detected at 1.4 min radiation. With the increasing of irradiation time (more than 1.4 min), the characteristic absorption peaks showed blue shift with the irradiation time and standard absorption 28
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Fig. 2. TEM micrographs of Au NPs formed at different irradiation time: (a) 0.5 min; (b) 1.1 min; (c) 1.4 min; (d) 2.5 min; (e) 5.0 min; (f) relation curve of Au NPs average size and irradiation time; (g) and (h): the absorption spectra of Au nanoparticles obtained from different UV irradiation time (from 0.5 to 120 min).
gradually fuse under the action of the electron beam, finally leading to the formation of a single Au NP.
appeared (Fig. 3f) and NPs with smaller particle size are formed compared to that of pH 3 or 4 (Fig. 3d). Furthermore, we also have studied the growing process of Au NPs as a function of the irradiation time. It is well-known, that NPs in solution are always in constant motion resulting in the random collisions between them. When the NPs collide under a specific orientation, aggregation will occur to reduce the surface energy, namely, atoms will diffuse between the two particles until they reach a steady state in which the two particles fuse to a single particle. Particles colliding with an inconsistent orientation can also successively rotate to form a consistent orientation. When NPs are exposed to an electron beam, this process will be accelerated. This process can be observed shown in Fig. 4, where two gold NPs are observed under TEM at different times from 0 to 12 min. Starting form two separate NPs, within 12 min, NPs
2.2. In-situ formation of Au NPs-Cu2O nanocomposites Photodegradation of organic pollutants over nanocomposites has been widely studied due to the size effect and synergistic effect of their components [24,25]. Based on this view, Cu2O-Au NPs heterojunction crystals are further prepared through loading Au NPs on the surface of Cu2O by UV irradiation and direct substitution reduction, as shown in Fig. 5.a-b. The Au NPs prepared by UV irradiation for 1.4 min present a size less than 10 nm and uniform distribution on the Cu2O surface. On the contrary, Au NPs prepared by the direct substitution method exhibit large size and aggregated morphology. Particularly, the UV irradiation 29
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Fig. 3. TEM images of Au NPs obtained at different pH values: (a) 4.0; (b) 7.0; (c) 10.4; (d) 12.1; Absorption spectra of Au NPs obtained at different pH values: (e) pH values from 3.0 to 10.4; (f) pH values from 10.4 to 12.1.
Fig. 4. TEM images showing the growing process of Au NPs: (a) 0 min; (b) 1 min; (c) 2 min; (d) 4 min; (e) 6 min; (f) 8 min; (g) 10 min; (h) 12 min. 30
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Fig. 5. SEM images of Cu2O@Au NPs obtained (a) by UV radiation and (b) by direct substitution method. (c) XRD patterns and (d) EDS of the Cu2O@Au NPs made by UV radiation. (g) Photodegradation efficiency of MO with Cu2O, Cu2O@Au-substitution and Cu2O@Au-UV radiation. (f) Recycled photodegradation of MO over Cu2O@Au-UV radiation.
visible-light and electron-hole separation, and thus improve the photocatalytic performance [20,26]. However, overloading amount of Au NPs may serve as electron-hole recombination center or decrease the total effective surface area of Cu2O [20,27], and finally reduce the photocatalytic activity. Moreover, to evaluate the photocatalytic stability of Cu2O@Au-UV irradiation, the repetitive-use tests of MO degradation was performed and the results is shown in Fig. 5f. After three recycling runs, the photocatalytic performance of was maintained well with just slightly decrement, implying their high stability under visible light.
method using citric acid and other additives result in a certain etching of the Cu2O with the crystal corners passivated considerably. The crystalline phase of the Cu2O@Au NPs synthesized by UV irradiation was characterized by XRD (as shown in Fig. 5c). According to the standard monoclinic structure of Cu2O crystal (JCPDS no.0667), all the diffraction peaks can be indexed to (110), (1 1 1), (2 0 0), (2 2 0), (311) and (222) planes of crystalline Cu2O. Particularly, there are no obvious diffraction peaks of Au crystal (JCPDS no.0784), indicating the low content of Au. The EDS results (Fig. 5d) showed that the content of Au in the Cu2O@Au NPs obtained by UV irradiation was about 3.12%. The photocatalytic activity of as-prepared products were evaluated by photodegradation of MO dye under visible-light irradiation. As shown in Fig. 5e, the Cu2O@Au NPs made by UV irradiation displayed the best photocatalytic performance compared with bare Cu2O and Cu2O@Au NPs direct substitution. After 2 h irradiation, the decomposition of MO dye over Cu2O@Au-UV irradiation was about 68%, which was about 3.4 times and 1.7 times than that mediated by Cu2O (~20%) and Cu2O@Au-direct substitution (~40%), respectively. Au NPs dispersed on the surface of Cu2O crystals could lead to the formation of schottky barrier, which will enhance the absorption of
3. Conclusion In summary, well-dispersed Au NPs have been successfully prepared based on UV irradiation approach, and the different factors influencing the growing of Au NPs were investigated and discussed in detail. The best experimental conditions for ultra-small Au NPs with an average diameter of 1.8 nm are the following: an irradiation time of 1.4 min, a HAuCl4 precursor concentration of 0.8 mM, and a pH value of 10.4. In addition, the growing progress of Au NPs under electron beam was 31
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studied in TEM. The as-prepared Cu2O@Au NPs composites obtained by UV irradiation exhibited a higher photocatalytic activity for the degradation of MO than that of bare Cu2O and Au-Cu2O formed by direct replacement method. These results provide a new approach for the design and synthesis of noble metal-oxide heterojunctions with excellent activity.
Mater. 11 (9) (1999) 2310–2312. [11] Y. Zhou, L.Y. Hao, Y.R. Zhu, Y. Hu, Z.Y. Chen, A. Novel, Ultraviolet irradiation technique for fabrication of polyacrylamide–metal (M = Au, Pd) nanocomposites at room temperature, J. Nanopart. Res. 3 (5) (2001) 377–381. [12] A. Pal, S.K. Ghosh, K. Esumi, T. Pal, Reversible generation of gold nanoparticle aggregates with changeable interparticle interactions by UV photoactivation, Langmuir 20 (3) (2004) 575–578. [13] S. Yang, R. Zhang, Q. Wang, B. Ding, Y. Wang, Coral-shaped 3D assemblies of gold nuclei induced by UV irradiation and its disintegration, Colloids Surfaces A: Physicochem. Eng. Asp. 311 (1) (2007) 174–179. [14] M.A. Nguyen, N.M. Bedford, Y. Ren, E.M. Zahran, R.C. Goodin, F.F. Chagani, L.G. Bachas, M.R. Knecht, Direct synthetic control over the size, composition, and photocatalytic activity of octahedral copper oxide materials: correlation between surface structure and catalytic functionality, ACS Appl. Mater. Interfaces 7 (24) (2015) 13238–13250. [15] W.C. Huang, L.M. Lyu, Y.C. Yang, M.H. Huang, Synthesis of Cu2O nanocrystals from cubic to rhombic dodecahedral structures and their comparative photocatalytic activity, J Am. Chem. Soc. 134 (2) (2012) 1261–1267. [16] L. Tang, J. Lv, S. Sun, X. Zhang, C. Kong, X. Song, Z. Yang, Facile hydroxyl-assisted synthesis of morphological Cu2O architectures and their shape-dependent photocatalytic performances, New J Chem. 38 (10) (2014) 4656–4660. [17] S. Bai, J. Ge, L. Wang, M. Gong, M. Deng, Q. Kong, L. Song, J. Jiang, Q. Zhang, Y. Luo, Y. Xie, Y. Xiong, A unique semiconductor–metal–graphene stack design to harness charge flow for photocatalysis, Adv. Mater. 26 (32) (2014) 5689–5695. [18] S. Bai, J. Jiang, Q. Zhang, Y. Xiong, Steering charge kinetics in photocatalysis: intersection of materials syntheses, characterization techniques and theoretical simulations, Chem. Soc. Rev. 44 (10) (2015) 2893–2939. [19] L. Wang, J. Ge, A. Wang, M. Deng, X. Wang, S. Bai, R. Li, J. Jiang, Q. Zhang, Y. Luo, Y. Xiong, Designing p‐Type semiconductor–metal hybrid structures for improved photocatalysis, Angew. Chem. Int. Ed. 53 (20) (2014) 5107–5111. [20] Q. Zhang, D.Q. Lima, I. Lee, F. Zaera, M. Chi, Y. Yin, A highly active titanium dioxide based visible‐light photocatalyst with nonmetal doping and plasmonic metal decoration, Angew. Chem. Int. Ed. 123 (31) (2011) 7226–7230. [21] M.J. Hostetler, J.E. Wingate, C.J. Zhong, J.E. Harris, R.W. Vachet, M.R. Clark, J.D. Londono, S.J. Green, J.J. Stokes, G.D. Wignall, G.L. Glish, M.D. Porter, N.D. Evans, R.W. Murray, Alkanethiolate gold cluster molecules with core diameters from 1.5 to 5.2 nm: core and monolayer properties as a function of core size, Langmuir 14 (1) (1998) 17–30. [22] A.C. Templeton, J.J. Pietron, R.W. Murray, P. Mulvaney, Solvent refractive index and core charge influences on the surface plasmon absorbance of alkanethiolate monolayer-protected gold clusters, J Phys. Chem. B 104 (3) (2000) 564–570. [23] S. Yang, Y. Wang, Q. Wang, R. Zhang, B. Ding, UV irradiation induced formation of Au nanoparticles at room temperature: the case of pH values, Colloids Surfaces A: Physicochem. Eng. Asp. 301 (1) (2007) 174–183. [24] I.M. Sundaram, S. Kalimuthu, G. Ponniah, Highly active ZnO modified g-C3N4 Nanocomposite for dye degradation under UV and Visible Light with enhanced stability and antimicrobial activity, Compos. Commun. 5 (2017) 64–71. [25] Y. Bai, X. Mao, J. Song, X. Yin, J. Yu, B. Ding, Self-standing Ag2O@YSZ-TiO2 p-n nanoheterojunction composite nanofibrous membranes with superior photocatalytic activity, Compos. Commun. 5 (2017) 13–18. [26] M. Murdoch, G.I.N. Waterhouse, M.A. Nadeem, J.B. Metson, M.A. Keane, R.F. Howe, J. Llorca, H. Idriss, The effect of gold loading and particle size on photocatalytic hydrogen production from ethanol over Au/TiO2 nanoparticles, Nat. Chem. 3 (2011) 489. [27] H. Zhu, M. Du, D. Yu, Y. Wang, L. Wang, M. Zou, M. Zhang, Y. Fu, A new strategy for the surface-free-energy-distribution induced selective growth and controlled formation of Cu2O-Au hierarchical heterostructures with a series of morphological evolutions, J Mater. Chem. 1 (3) (2013) 919–929.
Acknowledgments This work was supported by National Natural Science Foundation of China (NSFC no. 51501140), China Postdoctoral Science Foundation (2016M602808), Natural Science Foundation of Jiangsu Province (BK20161250, BK20171235), Public welfare technology application research project of Zhejiang Province (2016C31G4181807) and the Fundamental Research Funds for the Central Universities. Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.coco.2018.04.003. References [1] K.Y. Chun, Y. Oh, J. Rho, J.-H. Ahn, Y.-J. Kim, H.R. Choi, S. Baik, Highly conductive, printable and stretchable composite films of carbon nanotubes and silver, Nat. Nanotechnol. 5 (2010) 853. [2] I.V. Lightcap, T.H. Kosel, P.V. Kamat, Anchoring Semiconductor and Metal Nanoparticles on a Two-Dimensional Catalyst Mat. Storing and Shuttling Electrons with Reduced Graphene Oxide, Nano Lett. 10 (2) (2010) 577–583. [3] C. Marambio-Jones, E.M.V. Hoek, A review of the antibacterial effects of silver nanomaterials and potential implications for human health and the environment, J. Nanopart. Res. 12 (5) (2010) 1531–1551. [4] A.S. Abreu, M. Oliveira, A. de Sá, R.M. Rodrigues, M.A. Cerqueira, A.A. Vicente, A.V. Machado, Antimicrobial nanostructured starch based films for packaging, Carbohyd Polym. 129 (2015) 127–134. [5] A. Luengnaruemitchai, S. Osuwan, E. Gulari, Selective catalytic oxidation of CO in the presence of H2 over gold catalyst, Int. J Hydrog. Energ. 29 (4) (2004) 429–435. [6] F. Cárdenas-Lizana, S. Gómez-Quero, H. Idriss, M.A. Keane, Gold particle size effects in the gas-phase hydrogenation of m-dinitrobenzene over Au/TiO2, J. Catal. 268 (2) (2009) 223–234. [7] L.Q. Nguyen, C. Salim, H. Hinode, Performance of nano-sized Au/TiO2 for selective catalytic reduction of NOx by propene, Appl. Catal. A: General. 347 (1) (2008) 94–99. [8] K. Sato, T. Yoshinari, Y. Kintaichi, M. Haneda, H. Hamada, Remarkable promoting effect of rhodium on the catalytic performance of Ag/Al2O3 for the selective reduction of NO with decane, Appl. Catal. B: Environ. 44 (1) (2003) 67–78. [9] K. Esumi, R. Isono, T. Yoshimura, Preparation of PAMAM− and PPI−Metal (Silver, Platinum, and Palladium) Nanocomposites and Their Catalytic Activities for Reduction of 4-Nitrophenol, Langmuir 20 (1) (2004) 237–243. [10] Y. Zhou, C.Y. Wang, Y.R. Zhu, Z.Y. Chen, A. Novel, Ultraviolet irradiation technique for shape-controlled synthesis of gold nanoparticles at room temperature, Chem.
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Contents lists available at ScienceDirect
Composites Communications journal homepage: www.elsevier.com/locate/coco
Continuous UV irradiation synthesis of ultra-small Au nanoparticles decorated Cu2O with enhanced photocatalytic activity Chuncai Konga, Bo Maa, Ke Liub, Weixin Zhanga,b, Zhimao Yanga, a b
T
⁎
School of Science, MOE Key Laboratory for Non-Equilibrium Synthesis and Modulation of Condensed Matter, Xi’an Jiaotong University, Xi’an 710049, Shaanxi, China Hubei Key Laboratory of Advanced Textile Materials & Application, College of Materials Science and Engineering, Wuhan Textile University, Wuhan 430200, China
A B S T R A C T
Noble metal nanoparticles (NPs) with controlled size and shape can be easily prepared by UV irradiation at room temperature. In this work, ultra-small Au NPs were successfully continuously synthesized via an UV irradiation approach, and the synthesis conditions of Au NPs with the size of 1.8 nm have been optimized to be an exposure time of 1.4 min, a HAuCl4 precursor concentration of 0.8 mM and a pH value of 10.4. The growth dynamics of Au NPs was investigated under different exposing time of the electron beam in transmission electron microscopy (TEM), revealing the formation of NPs via the oriented attachment of individual nanoparticles. In addition, well dispersed ultra-small Au NPs decorated Cu2O composites were obtained from this in-situ strategy and showed an enhanced photocatalytic activity compared with that of Au-free oxides and Au- Cu2O from direct replacement method.
1. Introduction Noble metal (Au, Ag, Pt, Pd) nanoparticles (NPs) have attracted intensive research interest because of the multitude of applications in catalysis, biomedicine, optics, and electronics [1–4]. For example, CeO2 catalysts with 1% Au NPs can selectively catalyze CO in H2 environment and remain stable in the presence of water vapor [5]. Au and Ag based nanocomposites are also used for the catalytic hydrogenation reaction [6], and nitrogen oxide reduction [7,8]. Consisting of over 80% surface atoms, ultra-small NPs with a size less than 2 nm have been increasingly investigated due to their high surface area which could enhance catalytic activity [9]. Hence, the controlled synthesis of ultrasmall noble metal nanoparticles based composites is very important and promising for their catalytic application. As one of the common methods, UV irradiation has been widely used for the preparation of ultra-small noble metal NPs with controlled sizes and shapes in solution, where the reductant agent is not necessary due to the direct generation of reduction sites by UV irradiation. The sizes and surface states of NPs can be easily adjusted by using surfactants or solvents. ZY Chen prepared Au NPs in the aqueous solution using polyvinyl alcohol (PVA) as the protective agent and a 30 W lowpressure mercury lamp as the irradiation source [10,11]. Anjali Pal synthetized Au NPs (3–5 nm) under the irradiation of a high-power (200 W) UV lamp, in which CTAC served as the surfactant [12]. By a continuous UV irradiation approach, Yang got coral-like Au
⁎
Corresponding author. E-mail address: [email protected] (Z. Yang).
https://doi.org/10.1016/j.coco.2018.04.003 Received 7 November 2017; Received in revised form 30 March 2018; Accepted 9 April 2018 2452-2139/ © 2018 Published by Elsevier Ltd.
nanostructures in citric acid electrolyte and ultimately formed a network structure [13]. Cuprous oxide (Cu2O) crystals as low-cost, environmentally friendly, direct and narrow bandgap semiconductors, have been extensively used in the photocatalytic degradation of organic materials [14–16]. The combination of noble metal NPs and Cu2O crystals has been demonstrated to significantly enhance the catalytic performance in comparison with the bare Cu2O [17–19]. In the metalmetal oxides heterojunction, the noble nanoparticles play two important roles, one is the enhanced absorption of light in the UV–vis region because of their surface plasmon resonance (SPR), the other one is the efficient electron-hole separation due to the formation of Schottky junction between Cu2O semiconductor and noble NPs [20]. Therefore, the controlled design and continuous synthesis of noble metal NPsCu2O heterojunction are significant and challenging. In this work, we designed a special continuous synthesis equipment for the preparation of ultra-small Au NPs based on the UV irradiation method, involving noble metal salt, citrate acid and PVP. The influences of the experimental conditions such as the concentration of the protecting reagent, the pH value of the precursor and the irradiation time are discussed in detail. In addition, the one-pot synthesis of Au NPsCu2O composite architectures is successfully achieved and their photocatalytic activity for the degradation of methyl orange is investigated.
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Fig. 1. TEM images and the size distributions of Au NPs obtained at different condition: (a) and (c) UV radiation time 1.4 min, pH value 10.4; (b) and (d) UV radiation time 20 min, pH value 3.0.
2. Results and discussion
peaks of Au NPs appeared again though the intensities were not obviously enhanced. All these results are compliant with previous reports, when the diameter of Au NPs is lower than 3.2 nm, the surface plasmon resonance absorption decreases drastically [21]. Moreover, theoretical calculation also showed that the particle diameter could influences significantly the absorption spectra, namely, when the diameter of Au NPs is smaller than 2 nm, the plasma resonance absorption peak disappears due to the free electron scattering of particles at the surface [22]. To highlight the growth mechanism of Au NPs, we have studied the effect of the HAuCl4 precursor concentration by changing the precursor (HAuCl4) concentration between 0.2 and 4 mM. And the obtained absorption spectra are displayed in Fig. S2. At the lowest HAuCl4 concentration (0.2 mM), the absorption spectrum shows an obvious characteristic absorption peak. With the increase of HAuCl4 concentrations (from 0.4 to 2 mM), no obvious characteristic peaks were observed which could be ascribed to the strong scattering of photons on the surface of NPs with ultra-small size. However, a broad absorption peak was clearly observed at around 528 nm when the concentration was 4 mM. In this case, the Au NPs prepared show a narrow size distribution but a rather inhomogeneous morphology. The effect of pH values on the Au NPs is investigated by adding various amounts of NaOH to adjust the pH values. As shown in Fig. 3ad, when the pH value is 10.4, the smallest Au NPs were obtained, suggesting that adjusting the pH of precursor solution is effective. Meanwhile, as shown in Fig. 3e-f, the resulting absorption is clearly visible at pH is 3.0 and 4.0, while if the pH value is above 5, the intensity of the characteristic absorption peak decreases with the increased pH value, which are in accordance with the former research [23]. When the pH value exceeded 10.4, obvious absorption peaks
2.1. Characterization of Au NPs Most of the noble metal compounds can be transformed into noble metals nanoparticles under the action of the light, and the sizes and morphologies of Au NPs can be controlled by changing the irradiation time, pH value and surfactants. Fig. 1 shows the TEM images of Au NPs with different sizes obtained from different synthesis conditions: NPs with an average diameter of 1.4 nm (Fig. 1a and c) are formed under the UV radiation for 1.4 min and pH value of 10.4, while larger Au NPs with an average diameter of 4.4 nm (Fig. 1b and d) are formed when the UV radiation time is 20 min and pH value is 3.0. The effect of the UV radiation time on the Au NPs is studied by changing the flow rate of the peristaltic pump, while all other conditions are kept the same. As shown in Fig. 2, when the irradiation time is lower than 1.4 min, the Au NPs shape and size are extremely irregular. While the obtained Au NPs show very uniform and small size under the irradiation for1.4 min. When the irradiation time exceeds 1.4 min, the Au NPs size increases rather uniformly. Fig. 2f-h shows the size distribution and the absorption spectra of Au NPs obtained at different UV irradiation time, which suggests that the irradiation time of 1.4 min results in the smallest and most uniform size, and the absorption spectra are directly related to the irradiation time. As far as the UV exposure time is less than 1.4 min, the increased irradiation time leads to a gradual, low and broad absorption peak. low and broad absorption peak. While there is no obvious absorption peak detected at 1.4 min radiation. With the increasing of irradiation time (more than 1.4 min), the characteristic absorption peaks showed blue shift with the irradiation time and standard absorption 28
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Fig. 2. TEM micrographs of Au NPs formed at different irradiation time: (a) 0.5 min; (b) 1.1 min; (c) 1.4 min; (d) 2.5 min; (e) 5.0 min; (f) relation curve of Au NPs average size and irradiation time; (g) and (h): the absorption spectra of Au nanoparticles obtained from different UV irradiation time (from 0.5 to 120 min).
gradually fuse under the action of the electron beam, finally leading to the formation of a single Au NP.
appeared (Fig. 3f) and NPs with smaller particle size are formed compared to that of pH 3 or 4 (Fig. 3d). Furthermore, we also have studied the growing process of Au NPs as a function of the irradiation time. It is well-known, that NPs in solution are always in constant motion resulting in the random collisions between them. When the NPs collide under a specific orientation, aggregation will occur to reduce the surface energy, namely, atoms will diffuse between the two particles until they reach a steady state in which the two particles fuse to a single particle. Particles colliding with an inconsistent orientation can also successively rotate to form a consistent orientation. When NPs are exposed to an electron beam, this process will be accelerated. This process can be observed shown in Fig. 4, where two gold NPs are observed under TEM at different times from 0 to 12 min. Starting form two separate NPs, within 12 min, NPs
2.2. In-situ formation of Au NPs-Cu2O nanocomposites Photodegradation of organic pollutants over nanocomposites has been widely studied due to the size effect and synergistic effect of their components [24,25]. Based on this view, Cu2O-Au NPs heterojunction crystals are further prepared through loading Au NPs on the surface of Cu2O by UV irradiation and direct substitution reduction, as shown in Fig. 5.a-b. The Au NPs prepared by UV irradiation for 1.4 min present a size less than 10 nm and uniform distribution on the Cu2O surface. On the contrary, Au NPs prepared by the direct substitution method exhibit large size and aggregated morphology. Particularly, the UV irradiation 29
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Fig. 3. TEM images of Au NPs obtained at different pH values: (a) 4.0; (b) 7.0; (c) 10.4; (d) 12.1; Absorption spectra of Au NPs obtained at different pH values: (e) pH values from 3.0 to 10.4; (f) pH values from 10.4 to 12.1.
Fig. 4. TEM images showing the growing process of Au NPs: (a) 0 min; (b) 1 min; (c) 2 min; (d) 4 min; (e) 6 min; (f) 8 min; (g) 10 min; (h) 12 min. 30
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Fig. 5. SEM images of Cu2O@Au NPs obtained (a) by UV radiation and (b) by direct substitution method. (c) XRD patterns and (d) EDS of the Cu2O@Au NPs made by UV radiation. (g) Photodegradation efficiency of MO with Cu2O, Cu2O@Au-substitution and Cu2O@Au-UV radiation. (f) Recycled photodegradation of MO over Cu2O@Au-UV radiation.
visible-light and electron-hole separation, and thus improve the photocatalytic performance [20,26]. However, overloading amount of Au NPs may serve as electron-hole recombination center or decrease the total effective surface area of Cu2O [20,27], and finally reduce the photocatalytic activity. Moreover, to evaluate the photocatalytic stability of Cu2O@Au-UV irradiation, the repetitive-use tests of MO degradation was performed and the results is shown in Fig. 5f. After three recycling runs, the photocatalytic performance of was maintained well with just slightly decrement, implying their high stability under visible light.
method using citric acid and other additives result in a certain etching of the Cu2O with the crystal corners passivated considerably. The crystalline phase of the Cu2O@Au NPs synthesized by UV irradiation was characterized by XRD (as shown in Fig. 5c). According to the standard monoclinic structure of Cu2O crystal (JCPDS no.0667), all the diffraction peaks can be indexed to (110), (1 1 1), (2 0 0), (2 2 0), (311) and (222) planes of crystalline Cu2O. Particularly, there are no obvious diffraction peaks of Au crystal (JCPDS no.0784), indicating the low content of Au. The EDS results (Fig. 5d) showed that the content of Au in the Cu2O@Au NPs obtained by UV irradiation was about 3.12%. The photocatalytic activity of as-prepared products were evaluated by photodegradation of MO dye under visible-light irradiation. As shown in Fig. 5e, the Cu2O@Au NPs made by UV irradiation displayed the best photocatalytic performance compared with bare Cu2O and Cu2O@Au NPs direct substitution. After 2 h irradiation, the decomposition of MO dye over Cu2O@Au-UV irradiation was about 68%, which was about 3.4 times and 1.7 times than that mediated by Cu2O (~20%) and Cu2O@Au-direct substitution (~40%), respectively. Au NPs dispersed on the surface of Cu2O crystals could lead to the formation of schottky barrier, which will enhance the absorption of
3. Conclusion In summary, well-dispersed Au NPs have been successfully prepared based on UV irradiation approach, and the different factors influencing the growing of Au NPs were investigated and discussed in detail. The best experimental conditions for ultra-small Au NPs with an average diameter of 1.8 nm are the following: an irradiation time of 1.4 min, a HAuCl4 precursor concentration of 0.8 mM, and a pH value of 10.4. In addition, the growing progress of Au NPs under electron beam was 31
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studied in TEM. The as-prepared Cu2O@Au NPs composites obtained by UV irradiation exhibited a higher photocatalytic activity for the degradation of MO than that of bare Cu2O and Au-Cu2O formed by direct replacement method. These results provide a new approach for the design and synthesis of noble metal-oxide heterojunctions with excellent activity.
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