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Au@Cu2O stellated polytope with core–shelled nanostructure for high-performance adsorption and visible-light-driven photodegradation of cationic and anionic dyes
Accepted Manuscript Short Communication Au@Cu2O stellated polytope with core-shelled nanostructure for high-performance adsorption and visible-light-driven photodegradation of cationic and anionic dyes Xueqing Wu, Jiabai Cai, Shunxing Li, Fengying Zheng, Zhanghua Lai, Licong Zhu, Tanju Chen PII: DOI: Reference:
S0021-9797(16)30064-9 http://dx.doi.org/10.1016/j.jcis.2016.01.064 YJCIS 21047
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
5 October 2015 4 January 2016 27 January 2016
Please cite this article as: X. Wu, J. Cai, S. Li, F. Zheng, Z. Lai, L. Zhu, T. Chen, Au@Cu2O stellated polytope with core-shelled nanostructure for high-performance adsorption and visible-light-driven photodegradation of cationic and anionic dyes, Journal of Colloid and Interface Science (2016), doi: http://dx.doi.org/10.1016/j.jcis.2016.01.064
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Au@Cu2O
stellated
polytope
with
core-shelled
nanostructure
for
high-performance adsorption and visible-light-driven photodegradation of cationic and anionic dyes† Xueqing Wu,a Jiabai Cai,a,b Shunxing Li,*,a,c Fengying Zheng,a,c Zhanghua Lai,a Licong Zhu,a Tanju Chena a
College of Chemistry and Environment, Minnan Normal University, Zhangzhou 363000, P. R.
China b
c
College of the Environment & Ecology, Xiamen University, Xiamen, 361005, P. R. China
Fujian Province Key Laboratory of Modern Analytical Science and Separation Technology,
Minnan Normal University, Zhangzhou, 363000, P. R. China *Corresponding author: [email protected]; [email protected].
Jiabai Cai and Xueqing Wu contributed equally to this work.
1
Abstract Au nanoparticles were covered by Cu2O nanoparticles shell and then Au@Cu2O stellated polytope was synthesized by a facile aqueous solution approach. The samples were characterized by Scanning electron microscopy, Transmission electron microscopy, X-ray diffraction patterns, X-ray photoelectron spectroscopy, Brunner-Emmet-Teller measurements, and Ultraviolet-visible spectroscopy analysis. With good aqueous dispersibility, surface positive charge, and high chemisorption capacity, Au@Cu2O could be used for anionic dyes removal. Compared with Degussa P25-TiO2, the adsorption of anionic dyes (acid violet 43 or methyl blue, 5.0 mg L-1) onto Au@Cu2O was increased by 90.12% and 50.8%, respectively. The photodegradation activity of methyl orange and methyl violet were in the declining order: Au@Cu2O > Cu2O-Au nanocomposites > Cu2O > P25-TiO2. The synergistic effect of coupling Au core with Cu2O shell on the dyes photodegradation was observed. The photoexcited electrons from Cu2O conduction band could be captured by Au nanoparticles, resulting in an improved electron-hole separation. Moreover, a Schottky barrier was assumed to form at the Cu2O-Au interface and Au NPs as electron sink could reduce the recombination of photoinduced electrons and holes, facilitating the photocatalytic interface reaction. The geometry of core-shell and stellated polytope is effective in the design of Cu2O-Au nanocomposites for adsorption and photocatalysis. Keywords: Core-shell nanomaterials; Stellated polytope; Sunlight; Photodegradation
2
1. Introduction Adsorption and photocatalysis play a significant role in the removal of organic pollutants from water
[1-3].
In
the
development
of
nanostructured
photocatalyst
and
absorbent,
metal@semiconductor nano-composites have recently attracted considerable attention on improving the photo-induced charge separation within the semiconductor [4-7]. Some metal oxide semiconductors [8] with large surface area, low density, and better light-harvesting ability have attracted much attentions because of their wide variety of applications, including drug-release systems [9], heterogeneous catalysts [10], waste removal [11], and protection of light-sensitive biological molecules, etc. Among various metal oxide semiconductors (i.e., TiO2 [12-14], CeO2 [15], ZnO [16], Cu2O [17, 18]), Cu2O with a suitable band gap of 2.2eV is widely used as environment-friendly and excellent photocatalyst for its large light absorption coefficient, low cost, high stability, and good electronic conductivity, etc. But it is still a challenge to suppress the electron-hole recombination on Cu2O. Therefore, tailored nanostructures offer a new way for achieving this goal by facilitating electron-hole separation. In order to improve the photo-induced charge separation of the semiconductor, the inorganic oxide-supported Au has been developed as novel and efficient photocatalyst for the separation of electron-hole [19, 20]. Unfortunately, the supported Au catalysts with small sizes are usually unstable and tend to sinter and grow into larger particles during photocatalytic reactions [21], leading to the loss of the unique properties of the original Au nanoparticles (NPs). Therefore, the photocatalytic activity of the supported Au catalysts decays rapidly, preventing their practical applications. To solve the bottleneck problem of Au NPs migration and aggregation, many methods have been attempted: (a) confinement of Au NPs into mesoporous material [22], (b) replacement of monocomponent Au NPs with Au-based alloy NPs [23], (c) modification of the nano-materials [24], 3
and (d) encapsulation of Au NPs into the supports with core-shell structure [25]. The former three methods may improve the photocatalytic performance to some extent, but cannot efficiently avoid the migration and aggregation of Au NPs. In contrast, the last one is a more effective way, due to the high thermal stability [26] and recyclability [27] of core-shell micro/nano-structures. Recently, Chen groups [28] reported that monodisperse octahedral Au@Cu2O nanocrystals with single-crystalline shells are useful in developing new photocatalysts. Au-Cu2O with core-shell heterostructures can be synthesized, using gold nanoplates, nanorods, octahedra, and highly faceted nanoparticles as the structure-directing cores for the overgrowth of Cu2O shells [29]. Herein, we design a general method to synthesize uniform core-shelled Au@Cu2O with stellated polytope. The purpose of this research is to design a new nanostructure, which can effectively avoid the migration and aggregation of Au NPs, absorbing organic pollutants, increasing light-harvesting and mass transfer, facilitating electron-hole separation (i.e., photo-exited electrons could move from Cu2O to Au NPs), and then enhancing visible-light-driven photodegradation activity (see Scheme 1). 2. Experimental section 2.1. Reagents and chemicals CuCl2 (Xilong, 98%), NaOH (Xilong, 96%), C12H25SO4Na (Xilong, 95%), NH2OH·HCl (Xilong, 99.99%), AuCl3·HCl·4H2O (Sinopharm, 99%), C6H12O7Na3·2H2O (Xilong, 99%), and C2H5OH (Xilong, 99.7%) were purchased in China and used without additional purification. Distilled water was employed as the polymerization medium. 2.2. Synthesis of Au nanoparticles [30] An aqueous solution of HAuCl4·3H2O (0.1 wt %, 10 mL) was added to deionized water (100 mL) and heated to boiling point, followed by the addition of a sodium citrate solution (1 wt %, 2 mL). The resulting mixture was kept for 15 minutes under stirring. Upon cooling down to room 4
temperature, the Au nanoparticles were centrifuged and then redispersed into water (100 mL). 2.3. Synthesis of core-shelled Au@Cu2O stellated polytope Au sol was used as Au seeds, Au@Cu2O stellated polytope was prepared by a one-step facile process. The deionized water (89 mL) was added into a beaker, which was placed in a water bath set at 33℃. The C12H25SO4Na powder (0.87 g) and CuCl2 solution (1 mL, 0.1 mol L-1) were added to the beaker with vigorous stirring. After complete dissolution of C12H25SO4Na powder, different volumes of the prepared Au NPs solution (2 mL, 4 mL, 8 mL, or 12 mL) were added to adjust the weight of Au NPs in Au@Cu2O. Subsequently, the NaOH solution (2.5 mL, 1 mol L-1) was introduced. Finally, the NH2OH·HCl solution (6.5 mL, 0.2 mol L-1) was quickly injected into the beaker. For aging process, the beaker was kept in the water bath and the mixture was stirred constantly for 2 h.
The resultant solid product was washed by three cycles of
centrifugation/redispersion in deionized water at 3000 rpm for 5 min to remove the surfactant and the precipitate was dispersed in 0.5 mL of ethanol. Cu2O octahedra nanomaterials were prepared by a modified method [31]. 2.4. Characterization Scanning electron microscopy (SEM) images were obtained on a Gemini microscope (Hitachi, S-4800, Japan), using an accelerating voltage of 10 kV. Prior to analysis, samples were coated with thin Pt layer to increase the contrast and quality of the images. Transmission electron microscopy (TEM) images were recorded on FEI America Tecnai G2 F20 U-TWIN microscope operated at 200 kV acceleration voltages. Samples were prepared by drying a drop of the dilute solutions of NPs on a carbon grid. In this analysis, when electron beam was incident into a specimen, a part of the electrons was inelastically scattered and lost a fraction of the energy. X-ray diffraction patterns (XRD, D/MAX-TTRIII(CBO), Rigaku corporation, Japan) were scanned by analyzing the samples 5
using Cu-Kα monochromatic beam at 40 kV and 30 mA over 20 min. The UV-vis absorption spectroscopy study was carried out using a UV-Vis 2550 spectrophotometer (Shimadzu Corporation). The system was referenced to a BaSO4 powder sample prior to collecting data on core-shelled Au@Cu2O stellated polytope. X-ray photoelectron spectroscopy (XPS, Thermal Scientific K-Alpha XPS spectrometer) was used to investigate the distribution of Cu, Au, and O in core-shelled
Au@Cu2O
stellated
polytope.
Brunner-Emmet-Teller
(BET)
surface
area
measurements were carried out via N2 adsorption at 77 K using an ASAP2020 instrument based on adsorption data in the partial pressure (P/P0) range of 0.01-0.99. 2.5. Adsorption and photocatalytic activity measurements Aromatic dyes (rhodamine B, methyl violet, methyl blue, methyl orange, or acid violet 43, 5.0 mg L-1, 50 mL, in a 100 mL of quartz reactor) were used for testing adsorptive activity and photocatalytic activity. After stirring the adsorbent (P25, Cu2O, or Au@Cu2O, 30 mg) with dye in the dark for different time intervals, the samples were collected and filtered, followed by the determination of the dye with spectrophotometry. The photocatalyst (P25, Cu2O, or Au@Cu2O, 30 mg) with dye was stirred in the dark for 1 h to reach its adsorption/desorption equilibrium and then the photocatalytic performance was tested under the irradiation with a 350 W Xenon Short Arc Lamp at room temperature. The wavelength of the visible light was controlled through a 400 nm cutoff filter [32]. After irradiation for different time intervals, the samples were collected, filtered, and used for the measurement of dye concentration. The degradation rate could be calculated using eq (1).
Degradatio n rate
(C0 - C) 100% C0
(1)
C0 and C are the initial and residual dye concentration before and after visible light irradiation, respectively. 6
The first-order kinetic equation (eq (2)) was used to fit the data:
ln(
C0 ) kapp t C
(2)
kapp is the reaction rate constant and t is the reaction time [33]. The adsorption rate can be calculated using eq (3).
Adsorption rate
(C0 - C) 100% C0
(3)
C0 and C are the initial and residual dye concentration before and after adsorption, respectively. 3. Results and discussion 3.1. Morphology and structure of core-shelled Au@Cu2O stellated polytope Au@Cu2O stellated polytope was prepared by a one-step facile process (see Scheme 2). Fig. 1a shows that the SEM image of uniform core-shelled Au@Cu2O stellated polytope. The average edge size of Au@Cu2O stellated polytope was 165 nm. Compared to the product prepared without Au NPs (Fig. S1, Supplementary data), the morphology of octahedra here was transformed into stellated polytope, confirmed by the TEM images (Fig. 1b). The TEM image (Fig. 1c) revealed the presence of individual nanoparticles with core-shell structure. In one stellated polytope, only one gold seed was captured. The SEM images and TEM image proved the existence of core-shelled Au@Cu2O stellated polytope. The polycrystalline nature of these individual NPs was also confirmed by the SAED measurements (Fig. 1d). A few diffraction spots of Cu2O are recognized after careful observation by enlarging the photo. The HRTEM image of the nanocomposite showed two types of intimately contacted lattice fringes, confirming the formation of Au core and Cu2O shell (Fig. 1e). The lattice spacing of 0.24 nm was the interplanar distance between adjacent (111) crystallographic plane of Cu2O, while the measured spacing (0.23 nm) matched the reported (111) lattice spacing for Au NPs. For further characterization of the Au@Cu2O stellated polytope, we carried out XRD and optical measurements (Fig. S2, Supplementary data). The presence of five 7
peaks were respectively indexed to the (110), (111), (200), (220), and (311) planes of Cu2O, in a good agreement with the standard pattern of Cu2O (JCPDS no. 65-3288) [34]. A weak peak at 38.2 showed the existence of a small amount of Au in the sample. V1 ρ (kR13) = V2 ρ (kR23)
(4)
V2 = V1 • (R1/R2)3
(5)
R2 = R1 • (V1/V2)1/3
(6)
According to the results above, one Au seed was only coated by Cu2O shell to form a stellated polytope. In addition, we proposed that the size of Au@Cu2O could be predicted and well-controlled by adjusting the added volume of Au seed solution. To demonstrate the above deductions, a series of Au seeds with different volumes were used for the preparation of Au@Cu2O with different Au contents, including 2.0 mL, 4.0 mL, 8.0 mL, and 12.0 mL, respectively. All of Au NPs solutions were from the same pot to ensure the identical concentration. The relevant SEM images of the products were shown in Fig. 2. As expected, all the samples were uniform stellated polytope. According to eq (4-6) [28] and our findings, Au@Cu2O stellated polytope was obtained and its edge sizes was 165 nm, 126 nm, 99 nm, and 85 nm, respectively, in a good agreement with the calculated value, and this result was confirmed by the size distribution histogram (inset Fig. 2). The standard deviation was calculated as 5.0 nm and the relative standard deviation was 4.1%. For easy comparison, Table 1 listed the experimental and calculated size values. These results proved the rationality of our calculations and deductions. For example, according to eq (5), if 99 nm of Au core in Au@Cu2O was required, 8.0 mL of Au seed solution should be used. Based on this experiment, size control could be easily realized according to eq (5) and eq (6). Consistent with SEM data (Fig. 2), the TEM images of Au@Cu2O stellated polytope confirmed that the produced particles were core-shell structure and the above conclusion was proved (Fig. 3). 8
The full range XPS spectrum of Au@Cu2O stellated polytope was shown in Fig. 4a. The XPS peaks of Cu2p3/2 were located at about 932.6 eV with a good symmetry (Fig. 4b). The chemical valence of Cu was +1 and copper was coordinated oxygen which peak was located at about 530.4 eV (Fig. 4c) [34]. The peak at 532.4 eV may be assigned to the oxygen in the Cu-O bond (Fig. 4c). Because of the thickness, it is possible to detect such weak Au peaks with Au particles located deep inside the Cu2O shells. The peaks of Au 4f7/2 and Au 4f5/2 were centered at 84.2 and 87.8 eV, respectively (Fig. 4d), and the spin energy separation was 3.6 eV, in a good agreement with previous studies [35]. The diffuse reflectance UV-vis spectra of the prepared Au@Cu2O were shown in Fig. 5. The absorption band at about 500 nm could be attributed to the band-edge absorption of Cu2O [36]. The absorption features of Au NPs encapsulated in the Cu2O was in the range of 530-580 nm, but such absorption was not obvious due to the band-edge absorption of Cu2O. The band at 700 nm is likely coming from Au. Au SPR peak is greatly red-shifted after coating with Cu2O [2]. The UV-vis spectrum of the Au cores and the size of the Au cores were checked this view (Fig. S3, Fig. S4, Supplementary data). The Cu2O absorption (the black line) looks different is because of their very large sizes which causes strong light scattering. The visible-light absorption of Au@Cu2O stellated polytope was stronger than that of Cu2O (see Fig. 5). 3.2. Evaluation of adsorption and photocatalytic activity The adsorption and photocatalytic activity of Au@Cu2O stellated polytope submicrospheres was evaluated in this work. Both cationic and anionic dyes with different molecular weights were used to test the adsorption efficiency of Au@Cu2O stellated polytope. Two kinds of anionic dyes (acid violet 43 and methyl blue) and cationic dyes (rhodamine B and methyl violet) were used. After adsorption for 60 min, 99.19% of acid violet 43, 98.8% of methyl blue, 35.72% of rhodamine B, 9
and 60.25% of methyl violet were removed by Au@Cu2O, respectively (see Fig. 6). The Zeta potential of the Au@Cu2O stellated polytope was 14.5 mv, which was shown in Fig. S5. The results demonstrated that anionic dyes had high adsorption efficiency due to the electrical attractions between Au@Cu2O and anionic dye, because the surface charge on Cu2O was positive [37]. In addition, the smaller dye molecules can easily penetrate into the internal pore structure of Au@Cu2O stellated polytope. Determine the surface facets for the stellated particles, because photocatalytic activity is highly dependent on the surfaces. The structure of stellated polytope for core-shelled Au@Cu2O (see Fig. 1) could enhance the affinity between Au@Cu2O and dyes and then the surface coverage and adsorption rate of dyes onto Au@Cu2O were increased. The specific surface area, pore volume, and average pore size of 4mL-Au@Cu2O stellated polytope were 16 m2g−1, 0.06 cm3g−1, and 18.3 nm, respectively (Fig. S6 and Table S1, Supplementary data). The specific surface area of Au@Cu2O was 1.57 times of Cu2O. After adsorption for 60 min, 9.07%, 80.24%, and 99.19% of acid violet 43 could be adsorbed by P25, Cu2O and Au@Cu2O, respectively (see Fig. 6). The adsorption activity of 4mL-Au@Cu2O was higher than that of P25 and Cu2O. The adsorption rate of 4mL-Au@Cu2O exhibited a 20% increase compared with Cu2O, because of an increase in surface area and pore size (Table S1, Supplementary data). For comparison, the same procedure was also performed for other aromatic pollutants (see Fig. 6). The adsorption rate of anionic dyes (acid violet 43 or methyl blue, 5.0 mg L-1) on Au@Cu2O stellated polytope with different Au NPs weight was listed in Fig. S7. The adsorption
rate
of
anionic
dyes
was
in
the
order
of
4mL-Au@Cu2O
>
8mL-Au@Cu2O/12mL-Au@Cu2O > 2mL-Au@Cu2O. The high specific surface area of Au@Cu2O stellated polytope (i.e., more than 10 m2g−1) could provide abundant adsorption sites for dyes. Pore size is also an important parameter, because the thermodynamic state of Kr within the narrow pores 10
at a temperature depended on the pore size [38]. The pore size of 4mL-Au@Cu2O was 18.3 nm, the largest one among the nanocrystalline Au@Cu2O stellated polytope (Table S1, Supplementary data), so the adsorption activity of 4mL-Au@Cu2O was the best. The major issue of heterogeneous photocatalysis is attributed to the poor surface coverage of targeted compounds on the catalyst [3]. 4mL-Au@Cu2O was selected for dye photodegradation. To investigate the effect of photocatalyst structure on photocatalytic activity, three kinds of photocatalysts, including P25, Cu2O, and 4mL-Au@Cu2O, were used for the photodegradation of methyl orange and methyl violet under simulated sunlight irradiation for 80 min. Fig. 7 showed only 9.64% of methyl orange could be photodegraded by P25 because of its photosensitization. The photocatalytic activity of 4mL-Au@Cu2O was higher than that of P25 and Cu2O. Using 4mL-Au@Cu2O, the photodegradation rate of methyl orange was 54.81%, which exhibited an increase of 45.17% and 37.57% compared with P25 and Cu2O. Furthermore, the rate constant k (min−1) of the 4mL-Au@Cu2O stellated polytope exhibited 3.4, 3.9, 2.3, and 1.2 times increase compared with the reported Cu2O-Au nanocomposites, Cu2O nanowire-Au nanoparticle assemblies, ZnO@Au@Cu2O nanorod films, and Au- mercaptoundecanoic acid-Cu2O, respectively (see Table 2 [39] and Table S3). Meanwhile, the linear relationship between ln(C0/C) and irradiation time indicated that the photodegradation reactions followed pseudo-first-order kinetics (see Fig. 7) with rate constants (0.0013, 0.0025, and 0.0106 min−1) for P25, Cu2O and 4mL-Au@Cu2O, respectively. For comparison, the same procedure was also performed for methyl violet (see Fig. 7, Table 3, and Table S2, Supplementary data). The photocatalytic activity was in the order of Au@Cu2O > Cu2O-Au nanocomposites > Cu2O, i.e., the synergies effect of Au core as cocatalysts for enhanced photocatalytic activity of Cu2O was observed (see Fig. 7 and Table 3). The stellated polytope structure could improve the affinity between photocatalyst and dyes, leading to increases in the 11
surface coverage and adsorption rate of dyes onto the photocatalyst. The back electron transfer from Cu2O conduction band to Au NPs was generated, because the work function of Au NPs (5.1 eV) was larger than that of Cu2O (4.84 eV). Thus, the energy band bend at the Au-Cu2O interface was higher, which promoted the separation of photo-induced electrons and holes [40]. Moreover, the unique structure of Au@Cu2O stellated polytope could also be facilitated the separation of photo-induced electrons and holes. According to the photodegradation rate of methyl orange, the efficiency of these prepared nanocatalysts in producing •OH radicals and hole was assessed. The visible-light photodegradation rate of 4mL-Au@Cu2O was 54.81%, which exhibited an increase of 37.57% compared with Cu2O. On the other hand, a Schottky barrier was assumed to form at the Cu2O-Au interface and Au NPs as electron sink could reduce the recombination of photo-induced electrons and holes [41]. 4. Conclusions Here, we have demonstrated a simple aqueous solution approach for the fabrication of core-shelled Au@Cu2O stellated polytope. Au nanoparticles are covered by Cu2O nanoparticles shell. With good aqueous dispersibility, surface positive charge, and high chemisorption capacity, Au@Cu2O can be used for anionic dyes removal. Compared with Degussa P25-TiO2, the adsorption of anionic dyes (acid violet 43 or methyl blue, 5.0 mg L-1) onto Au@Cu2O is increased by 90.12% and 50.8%, respectively. Furthermore, the photodegradation rate constant k (min−1) of the 4mL-Au@Cu2O stellated polytope was better than that the reported Cu2O-Au nanocomposites. The method affords the fine tuning of the core-shelled Au@Cu2O stellated polytope, toward an efficient photocatalyst in the photocatalytic decomposition of organic pollutant. The geometry of core-shell and stellated polytope is effective in the design of Cu2O-Au nanocomposites with remarkable adsorption and photocatalytic activity. 12
Acknowledgments This work is supported by the National Natural Science Foundation of China (21175115 and 21475055, S.X.L), the Program for New Century Excellent Talents in University (NCET-11 0904, S.X.L).
13
References [1] X. Xu, C. Randorn, P. Efstathiou, J.T. Irvine, A red metallic oxide photocatalyst, Nat. Mater. 11 (2012) 595-598. [2] Y.C. Yang, H.J. Wang, J. Whang, J.S. Huang, L.M. Lyu, P.H. Lin, S. Gwo, M.H. Huang, Facet-dependent optical properties of polyhedral Au-Cu2O core-shell nanocrystals, Nanoscale 6 (2014) 4316-4324. [3] S. Li, J. Cai, X. Wu, F. Zheng, X. Lin, W. Liang, J. Chen, J. Zheng, Z. Lai, T. Chen, Fabrication of positively and negatively charged, double-shelled, nanostructured hollow spheres for photodegradation of cationic and anionic aromatic pollutants under sunlight irradiation, Appl. Catal. B: Environ 160 (2014) 279-285. [4] J. Zhang, Y. Tang, L. Weng, M. Ouyang, Versatile strategy for precisely tailored core@shell nanostructures with single shell layer accuracy: the case of metallic shell, Nano Lett. 9 (2009) 4061-4065. [5] S.C. Hsu, S.Y. Liu, H.J. Wang, Facet-dependent surface plasmon resonance properties of Au-Cu2O core-shell nanocubes, octahedra, and rhombic dodecahedra, Small 11 (2015) 195-201. [6] S. Rej, H.J. Wang, M.X. Huang, S.C. Hsu, C.S. Tan, F.C. Lin, M.H. Huang, Facet–dependent optical properties of Pd–Cu2O core–shell nanocubes and octahedra, Nanoscale 7 (2015) 11135-11141. [7] D.Y. Liu, S.Y. Ding, H.X. Lin, B.J. Liu, Z.Z. Ye, F.R. Fan, Z.Q. Tian, Distinctive enhanced and tunable plasmon resonant absorption from controllable Au@Cu2O nanoparticles: experimental and theoretical modeling, J. Phys. Chem. C 116 (2012) 4477-4483. [8] J. Hu, M. Chen, X. Fang, L. Wu, Fabrication and application of inorganic hollow spheres, Chem. Soc. Rev. 40 (2011) 5472-5491. 14
[9] Y. Zhu, E. Kockrick, T. Ikoma, N. Hanagata, S. Kaskel, An efficient route to rattle-type Fe3O4@SiO2 hollow mesoporous spheres using colloidal carbon spheres templates, Chem. Mater. 21 (2009) 2547-2553. [10] X. Li, R. Huang, Y. Hu, Y. Chen, W. Liu, R. Yuan, Z. Li, A templated method to Bi 2WO6 hollow microspheres and their conversion to double-shell Bi2O3/Bi2WO6 hollow microspheres with improved photocatalytic performance, Inorg. Chem. 51 (2012) 6245-6250. [11] J. Cao, Y. Zhu, K. Bao, L. Shi, S. Liu, Y. Qian, Microscale Mn2O3 hollow structures: sphere, cube, ellipsoid, dumbbell, and their phenol adsorption properties, J. Phys. Chem. C 113 (2009) 17755-17760. [12] H. Li, Z. Bian, J. Zhu, Y. Huo, H. Li, Y. Lu, Mesoporous Au/TiO2 nanocomposites with enhanced photocatalytic activity, J. Am. Chem. Soc. 129 (2007) 4538-4539. [13] M. Murdoch, G. Waterhouse, M. Nadeem, J. Metson, M. Keane, R. 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) 489-492. [14] I. Lee, J.B. Joo, Y. Yin, F. Zaera, A yolk@shell nanoarchitecture for Au/TiO2 catalysts, Angew. Chem. Int. Ed. 123 (2011) 10390-10393. [15] J. Qi, J. Chen, G. Li, S. Li, Y. Gao, Z. Tang, Facile synthesis of core-shell Au@CeO2 nanocomposites with remarkably enhanced catalytic activity for CO oxidation, Energy Environ. Sci. 5 (2012) 8937-8941. [16] W. He, H.K. Kim, W.G. Wamer, D. Melka, J.H. Callahan, J.J. Yin, Photogenerated charge carriers and reactive oxygen species in ZnO/Au hybrid nanostructures with enhanced photocatalytic and antibacterial activity, J. Am. Chem. Soc. 136 (2013) 750-757. [17] L. Zhang, D.A. Blom, H. Wang, Au-Cu2O core-shell nanoparticles: a hybrid 15
metal-semiconductor heteronanostructure with geometrically tunable optical properties, Chem. Mater. 23 (2011) 4587-4598.. [18] M.A. Mahmoud, W. Qian, M.A. El-Sayed, Following charge separation on the nanoscale in Cu2O-Au nanoframe hollow nanoparticles, Nano Lett. 11 (2011) 3285-3289. [19] N.A. Heutz, P. Dolcet, A. Birkner, M. Casarin, K. Merz, S. Gialanella, S. Gross, Inorganic chemistry in a nanoreactor: Au/TiO2 nanocomposites by photolysis of a single-source precursor in miniemulsion, Nanoscale 5 (2013) 10534-10541. [20] J. Li, S.K. Cushing, J. Bright, F. Meng, T.R. Senty, P. Zheng, A.D. Bristow, N. Wu, Ag@Cu 2O core-shell nanoparticles as visible-light plasmonic photocatalysts, ACS Catalysis 3 (2012) 47-51. [21] W.C. Li, M. Comotti, F. Schüth, Highly reproducible syntheses of active Au/TiO2 catalysts for CO oxidation by deposition-precipitation or impregnation, J. Catal. 237 (2006) 190-196. [22] C. Chen, C. Nan, D. Wang, Q. Su, H. Duan, X. Liu, L. Zhang, D. Chu, W. Song, Q. Peng, Mesoporous multicomponent nanocomposite colloidal spheres: ideal high-temperature stable model catalysts, Angew. Chem. Int. Ed. 123 (2011) 3809-3813. [23] X. Liu, A. Wang, X. Wang, C.Y. Mou, T. Zhang, Au-Cu Alloy nanoparticles confined in SBA-15 as a highly efficient catalyst for CO oxidation, Chem. Commun. (2008) 3187-3189. [24] K. Zhao, B. Qiao, J. Wang, Y. Zhang, T. Zhang, A highly active and sintering-resistant Au/FeOx -hydroxyapatite catalyst for CO oxidation, Chem. Commun. 47 (2011) 1779-1781. [25] M. Cargnello, N.L. Wieder, T. Montini, R.J. Gorte, P. Fornasiero, Synthesis of dispersible Pd@CeO2 core-shell nanostructures by self-assembly, J. Am. Chem. Soc. 132 (2009) 1402-1409. [27] J.C. Park, J.U. Bang, J. Lee, C.H. Ko, H. Song, Ni@SiO2 yolk-shell nanoreactor catalysts: high temperature stability and recyclability, J. Mater. Chem. 20 (2010) 1239-1246. [28] L. Kong, W. Chen, D. Ma, Y. Yang, S. Liu, S. Huang, Size control of Au@Cu2O octahedra for 16
excellent photocatalytic performance, J. Mater. Chem. 22 (2012) 719-724. [29] C.H. Kuo, T.E. Hua, M.H. Huang, Au nanocrystal-directed growth of Au-Cu2O core-shell heterostructures with precise morphological control, J. Am. Chem. Soc. 131 (2009) 17871-17878. [30] M.C. Daniel, D. Astruc, Gold nanoparticles: assembly, supramolecular chemistry, quantum-size-related properties, and applications toward biology, catalysis, and nanotechnology, Chem. Rev. 104 (2004) 293-346. [31] 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 (2011) 1261-1267. [32] Y. Chen, C. Hu, X. Hu, J. Qu, Indirect photodegradation of amine drugs in aqueous solution under simulated sunlight, Environ. Sci. Technol. 43 (2009) 2760-2765. [33] J. Matos, J. Laine, J.M. Herrmann, Synergy effect in the photocatalytic degradation of phenol on a suspended mixture of titania and activated carbon, Appl. Catal. B: Environ 18 (1998) 281-291. [34] C.E. Dubé, B. Workie, S.P. Kounaves, A. Robbat, M.L. Aksub, G. Davies, Electrodeposition of metal alloy and mixed oxide films using a single-precursor tetranuclear copper-nickel complex, J. Electrochem. Soc. 142 (1995) 3357-3365. [35] J.B. Cai, X.Q. Wu, S. Li, F. Zheng, L. Zhu, Z, Lai, Synergistic effect of double-shelled and sandwiched TiO2@Au@C hollow spheres with enhanced visible-light-driven photocatalytic activity, ACS Appl. Mat. Interfaces 7 (2015) 3764-3772. [36] C.H. Kuo, Y.C. Yang, S. Gwo, M.H. Huang, Facet-dependent and Au nanocrystal-enhanced electrical and photocatalytic properties of Au-Cu2O core-shell heterostructures, J. Am. Chem. Soc. 133 (2010) 1052-1057. [37] W.C. Wang, L.M. Lyu, M.H. Huang, Investigation of the effects of polyhedral gold nanocrystal 17
morphology and facets on the formation of Au-Cu2O core-shell heterostructures, Chem. Mater. 23 (2011) 2677-2684. [38] K. Wessels, M. Minnermann, J. Rathousky, M. Wark, T. Oekermann, Influence of calcination temperature on the photoelectrochemical and photocatalytic properties of porous TiO2 films electrodeposited from Ti(IV)-alkoxide solution, J. Phys. Chem. C 112 (2008) 15122-15128. [39] Q. Hua, F. Shi, K. Chen, S. Chang, Y. Ma, Z. Jiang, G. Pan, W. Huang, Cu 2O-Au nanocomposites with novel structures and remarkable chemisorption capacity and photocatalytic activity, Nano Research 4 (2011) 948-962. [40] S.M. Majhi, P. Rai, S. Raj, B.S. Chon, K.K. Park, Y.T. Yu, Effect of Au nanorods on potential barrier modulation in morphologically controlled Au@Cu2O core-shell nanoreactors for gas sensor applications, ACS Appl. Mat. Interfaces 6 (2014) 7491-7497. [41] D. Jiang, W. Zhou, X. Zhong, Y. Zhang, X. Li, Distinguishing localized surface plasmon resonance and schottky junction of Au-Cu2O composites by their molecular spacer dependence, ACS Appl. Mat. Interfaces 6 (2014) 10958-10962.
18
Table captions Table 1 Experimental and calculated edge size values of the samples prepared from 2.0 mL, 4.0 mL, 8.0 mL, and 12.0 mL of Au NPs solutions. Table 2 Visible light photodegradation rate constant k (min−1) for P25, Cu2O-Au nanocomposites and Au@Cu2O stellated polytope. Table 3 Visible light photodegradation rate constant k (min−1) for P25, Cu2O and Au@Cu2O stellated polytope.
19
Table 1 Experimental and calculated edge size values of the samples prepared from 2.0 mL, 4.0 mL, 8.0 mL, and 12.0 mL of Au NPs solutions. (n=3) VAu(mL)a
Rexp(nm)b
Rcal(nm)c
2.0
165
4.0
126
129
8.0
99
102
12.0
85
89
a
VAu: the volume of Au NPs solution. bRexp: the average edge size of as-synthesized Au@Cu2O.
c
Rcal: edge size calculated according to (6) based on the reference experiment in which 2.0 mL of
Au NPs solution was used as seed.
20
Table 2 Visible light photodegradation rate constant k (min−1) for P25, Cu2O-Au nanocomposites and Au@Cu2O stellated polytope. k (min−1) Sample
Relative rate
Ref
methyl orange P25
0.0013
1
This work
Cu2O-Au nanocomposites
0.00315
2.42
[39]
Au@Cu2O stellated polytope
0.0106
8.15
This work
21
Table 3 Visible light photodegradation rate constant k (min−1) for P25, Cu2O and Au@Cu2O stellated polytope. k (min−1)
methyl orange
methyl violet
P25
0.0013
0.0057
Cu2O
0.0025
0.012
Au@Cu2O
0.0106
0.022
Sample
22
Figure captions Scheme 1. Charge-transfer process in the Au@Cu2O stellated polytope. Scheme 2. Illustrations of formation process of Au@Cu2O stellated polytope. Fig. 1. (a) SEM, (b and c) TEM, (d) SAED, and (e) HRTEM images of Au@Cu2O stellated polytope. Fig. 2. SEM images of Au@Cu2O stellated polytope with diverse average sizes prepared from different volumes of gold seed solution: (a) 165 nm, 2.0 mL; (b) 126 nm, 4.0 mL; (c) 99 nm, 8.0 mL; (d) 85 nm, 12.0 mL. Fig. 3. TEM images of Au@Cu2O stellated polytope with diverse average sizes prepared from different volumes of gold seed solution: (a) 165 nm, 2.0 mL; (b) 126 nm, 4.0 mL; (c) 99 nm, 8.0 mL; (d) 85 nm, 12.0 mL. Fig. 4. (a) Full range of the XPS spectrum, (b) XPS peaks of Cu2p, (c) O1s and (d) Au4f of core-shell Au@Cu2O stellated polytope. Fig. 5. Diffuse reflectance UV-vis spectra of Cu2O and Au@Cu2O samples. Fig. 6. Adsorption rate of acid violet 43 (an anionic dye, a), methyl blue (an anionic dye, b), rhodamine B (a cationic dye, c), and methyl violet (a cationic dye, d) on P25, Cu2O, and Au@Cu2O under dark. Fig. 7. Linear transform ln(C0/C) = f(t) of photodegradation kinetic curves of (a) methyl orange, or (b) methyl violet by P25, Cu2O, and Au@Cu2O under visible light irradiation.
23
Scheme 1. Charge-transfer process in the Au@Cu2O stellated polytope.
24
Scheme 2. Illustrations of formation process of Au@Cu2O stellated polytope.
25
Fig. 1. (a) SEM, (b and c) TEM, (d) SAED, and (e) HRTEM images of Au@Cu2O stellated polytope.
26
Fig. 2. SEM images of Au@Cu2O stellated polytope with diverse average sizes prepared from different volumes of gold seed solution: (a) 165 nm, 2.0 mL; (b) 126 nm, 4.0 mL; (c) 99 nm, 8.0 mL; (d) 85 nm, 12.0 mL.
27
Fig. 3. TEM images of Au@Cu2O stellated polytope with diverse average sizes prepared from different volumes of gold seed solution: (a) 165 nm, 2.0 mL; (b) 126 nm, 4.0 mL; (c) 99 nm, 8.0 mL; (d) 85 nm, 12.0 mL.
28
Fig. 4. (a) Full range of the XPS spectrum, (b) XPS peaks of Cu2p, (c) O1s and (d) Au4f of core-shell Au@Cu2O stellated polytope.
29
Fig. 5. Diffuse reflectance UV-vis spectra of Cu2O and Au@Cu2O samples.
30
Fig. 6. Adsorption rate of acid violet 43 (an anionic dye, a), methyl blue (an anionic dye, b), rhodamine B (a cationic dye, c), and methyl violet (a cationic dye, d) on P25, Cu2O, and Au@Cu2O under dark.
31
Fig. 7. Linear transform ln(C0/C) = f(t) of photodegradation kinetic curves of (a) methyl orange, or (b) methyl violet by P25, Cu2O, and Au@Cu2O under visible light irradiation.
32
Graphical abstract
33
34
S0021-9797(16)30064-9 http://dx.doi.org/10.1016/j.jcis.2016.01.064 YJCIS 21047
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
5 October 2015 4 January 2016 27 January 2016
Please cite this article as: X. Wu, J. Cai, S. Li, F. Zheng, Z. Lai, L. Zhu, T. Chen, Au@Cu2O stellated polytope with core-shelled nanostructure for high-performance adsorption and visible-light-driven photodegradation of cationic and anionic dyes, Journal of Colloid and Interface Science (2016), doi: http://dx.doi.org/10.1016/j.jcis.2016.01.064
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Au@Cu2O
stellated
polytope
with
core-shelled
nanostructure
for
high-performance adsorption and visible-light-driven photodegradation of cationic and anionic dyes† Xueqing Wu,a Jiabai Cai,a,b Shunxing Li,*,a,c Fengying Zheng,a,c Zhanghua Lai,a Licong Zhu,a Tanju Chena a
College of Chemistry and Environment, Minnan Normal University, Zhangzhou 363000, P. R.
China b
c
College of the Environment & Ecology, Xiamen University, Xiamen, 361005, P. R. China
Fujian Province Key Laboratory of Modern Analytical Science and Separation Technology,
Minnan Normal University, Zhangzhou, 363000, P. R. China *Corresponding author: [email protected]; [email protected].
Jiabai Cai and Xueqing Wu contributed equally to this work.
1
Abstract Au nanoparticles were covered by Cu2O nanoparticles shell and then Au@Cu2O stellated polytope was synthesized by a facile aqueous solution approach. The samples were characterized by Scanning electron microscopy, Transmission electron microscopy, X-ray diffraction patterns, X-ray photoelectron spectroscopy, Brunner-Emmet-Teller measurements, and Ultraviolet-visible spectroscopy analysis. With good aqueous dispersibility, surface positive charge, and high chemisorption capacity, Au@Cu2O could be used for anionic dyes removal. Compared with Degussa P25-TiO2, the adsorption of anionic dyes (acid violet 43 or methyl blue, 5.0 mg L-1) onto Au@Cu2O was increased by 90.12% and 50.8%, respectively. The photodegradation activity of methyl orange and methyl violet were in the declining order: Au@Cu2O > Cu2O-Au nanocomposites > Cu2O > P25-TiO2. The synergistic effect of coupling Au core with Cu2O shell on the dyes photodegradation was observed. The photoexcited electrons from Cu2O conduction band could be captured by Au nanoparticles, resulting in an improved electron-hole separation. Moreover, a Schottky barrier was assumed to form at the Cu2O-Au interface and Au NPs as electron sink could reduce the recombination of photoinduced electrons and holes, facilitating the photocatalytic interface reaction. The geometry of core-shell and stellated polytope is effective in the design of Cu2O-Au nanocomposites for adsorption and photocatalysis. Keywords: Core-shell nanomaterials; Stellated polytope; Sunlight; Photodegradation
2
1. Introduction Adsorption and photocatalysis play a significant role in the removal of organic pollutants from water
[1-3].
In
the
development
of
nanostructured
photocatalyst
and
absorbent,
metal@semiconductor nano-composites have recently attracted considerable attention on improving the photo-induced charge separation within the semiconductor [4-7]. Some metal oxide semiconductors [8] with large surface area, low density, and better light-harvesting ability have attracted much attentions because of their wide variety of applications, including drug-release systems [9], heterogeneous catalysts [10], waste removal [11], and protection of light-sensitive biological molecules, etc. Among various metal oxide semiconductors (i.e., TiO2 [12-14], CeO2 [15], ZnO [16], Cu2O [17, 18]), Cu2O with a suitable band gap of 2.2eV is widely used as environment-friendly and excellent photocatalyst for its large light absorption coefficient, low cost, high stability, and good electronic conductivity, etc. But it is still a challenge to suppress the electron-hole recombination on Cu2O. Therefore, tailored nanostructures offer a new way for achieving this goal by facilitating electron-hole separation. In order to improve the photo-induced charge separation of the semiconductor, the inorganic oxide-supported Au has been developed as novel and efficient photocatalyst for the separation of electron-hole [19, 20]. Unfortunately, the supported Au catalysts with small sizes are usually unstable and tend to sinter and grow into larger particles during photocatalytic reactions [21], leading to the loss of the unique properties of the original Au nanoparticles (NPs). Therefore, the photocatalytic activity of the supported Au catalysts decays rapidly, preventing their practical applications. To solve the bottleneck problem of Au NPs migration and aggregation, many methods have been attempted: (a) confinement of Au NPs into mesoporous material [22], (b) replacement of monocomponent Au NPs with Au-based alloy NPs [23], (c) modification of the nano-materials [24], 3
and (d) encapsulation of Au NPs into the supports with core-shell structure [25]. The former three methods may improve the photocatalytic performance to some extent, but cannot efficiently avoid the migration and aggregation of Au NPs. In contrast, the last one is a more effective way, due to the high thermal stability [26] and recyclability [27] of core-shell micro/nano-structures. Recently, Chen groups [28] reported that monodisperse octahedral Au@Cu2O nanocrystals with single-crystalline shells are useful in developing new photocatalysts. Au-Cu2O with core-shell heterostructures can be synthesized, using gold nanoplates, nanorods, octahedra, and highly faceted nanoparticles as the structure-directing cores for the overgrowth of Cu2O shells [29]. Herein, we design a general method to synthesize uniform core-shelled Au@Cu2O with stellated polytope. The purpose of this research is to design a new nanostructure, which can effectively avoid the migration and aggregation of Au NPs, absorbing organic pollutants, increasing light-harvesting and mass transfer, facilitating electron-hole separation (i.e., photo-exited electrons could move from Cu2O to Au NPs), and then enhancing visible-light-driven photodegradation activity (see Scheme 1). 2. Experimental section 2.1. Reagents and chemicals CuCl2 (Xilong, 98%), NaOH (Xilong, 96%), C12H25SO4Na (Xilong, 95%), NH2OH·HCl (Xilong, 99.99%), AuCl3·HCl·4H2O (Sinopharm, 99%), C6H12O7Na3·2H2O (Xilong, 99%), and C2H5OH (Xilong, 99.7%) were purchased in China and used without additional purification. Distilled water was employed as the polymerization medium. 2.2. Synthesis of Au nanoparticles [30] An aqueous solution of HAuCl4·3H2O (0.1 wt %, 10 mL) was added to deionized water (100 mL) and heated to boiling point, followed by the addition of a sodium citrate solution (1 wt %, 2 mL). The resulting mixture was kept for 15 minutes under stirring. Upon cooling down to room 4
temperature, the Au nanoparticles were centrifuged and then redispersed into water (100 mL). 2.3. Synthesis of core-shelled Au@Cu2O stellated polytope Au sol was used as Au seeds, Au@Cu2O stellated polytope was prepared by a one-step facile process. The deionized water (89 mL) was added into a beaker, which was placed in a water bath set at 33℃. The C12H25SO4Na powder (0.87 g) and CuCl2 solution (1 mL, 0.1 mol L-1) were added to the beaker with vigorous stirring. After complete dissolution of C12H25SO4Na powder, different volumes of the prepared Au NPs solution (2 mL, 4 mL, 8 mL, or 12 mL) were added to adjust the weight of Au NPs in Au@Cu2O. Subsequently, the NaOH solution (2.5 mL, 1 mol L-1) was introduced. Finally, the NH2OH·HCl solution (6.5 mL, 0.2 mol L-1) was quickly injected into the beaker. For aging process, the beaker was kept in the water bath and the mixture was stirred constantly for 2 h.
The resultant solid product was washed by three cycles of
centrifugation/redispersion in deionized water at 3000 rpm for 5 min to remove the surfactant and the precipitate was dispersed in 0.5 mL of ethanol. Cu2O octahedra nanomaterials were prepared by a modified method [31]. 2.4. Characterization Scanning electron microscopy (SEM) images were obtained on a Gemini microscope (Hitachi, S-4800, Japan), using an accelerating voltage of 10 kV. Prior to analysis, samples were coated with thin Pt layer to increase the contrast and quality of the images. Transmission electron microscopy (TEM) images were recorded on FEI America Tecnai G2 F20 U-TWIN microscope operated at 200 kV acceleration voltages. Samples were prepared by drying a drop of the dilute solutions of NPs on a carbon grid. In this analysis, when electron beam was incident into a specimen, a part of the electrons was inelastically scattered and lost a fraction of the energy. X-ray diffraction patterns (XRD, D/MAX-TTRIII(CBO), Rigaku corporation, Japan) were scanned by analyzing the samples 5
using Cu-Kα monochromatic beam at 40 kV and 30 mA over 20 min. The UV-vis absorption spectroscopy study was carried out using a UV-Vis 2550 spectrophotometer (Shimadzu Corporation). The system was referenced to a BaSO4 powder sample prior to collecting data on core-shelled Au@Cu2O stellated polytope. X-ray photoelectron spectroscopy (XPS, Thermal Scientific K-Alpha XPS spectrometer) was used to investigate the distribution of Cu, Au, and O in core-shelled
Au@Cu2O
stellated
polytope.
Brunner-Emmet-Teller
(BET)
surface
area
measurements were carried out via N2 adsorption at 77 K using an ASAP2020 instrument based on adsorption data in the partial pressure (P/P0) range of 0.01-0.99. 2.5. Adsorption and photocatalytic activity measurements Aromatic dyes (rhodamine B, methyl violet, methyl blue, methyl orange, or acid violet 43, 5.0 mg L-1, 50 mL, in a 100 mL of quartz reactor) were used for testing adsorptive activity and photocatalytic activity. After stirring the adsorbent (P25, Cu2O, or Au@Cu2O, 30 mg) with dye in the dark for different time intervals, the samples were collected and filtered, followed by the determination of the dye with spectrophotometry. The photocatalyst (P25, Cu2O, or Au@Cu2O, 30 mg) with dye was stirred in the dark for 1 h to reach its adsorption/desorption equilibrium and then the photocatalytic performance was tested under the irradiation with a 350 W Xenon Short Arc Lamp at room temperature. The wavelength of the visible light was controlled through a 400 nm cutoff filter [32]. After irradiation for different time intervals, the samples were collected, filtered, and used for the measurement of dye concentration. The degradation rate could be calculated using eq (1).
Degradatio n rate
(C0 - C) 100% C0
(1)
C0 and C are the initial and residual dye concentration before and after visible light irradiation, respectively. 6
The first-order kinetic equation (eq (2)) was used to fit the data:
ln(
C0 ) kapp t C
(2)
kapp is the reaction rate constant and t is the reaction time [33]. The adsorption rate can be calculated using eq (3).
Adsorption rate
(C0 - C) 100% C0
(3)
C0 and C are the initial and residual dye concentration before and after adsorption, respectively. 3. Results and discussion 3.1. Morphology and structure of core-shelled Au@Cu2O stellated polytope Au@Cu2O stellated polytope was prepared by a one-step facile process (see Scheme 2). Fig. 1a shows that the SEM image of uniform core-shelled Au@Cu2O stellated polytope. The average edge size of Au@Cu2O stellated polytope was 165 nm. Compared to the product prepared without Au NPs (Fig. S1, Supplementary data), the morphology of octahedra here was transformed into stellated polytope, confirmed by the TEM images (Fig. 1b). The TEM image (Fig. 1c) revealed the presence of individual nanoparticles with core-shell structure. In one stellated polytope, only one gold seed was captured. The SEM images and TEM image proved the existence of core-shelled Au@Cu2O stellated polytope. The polycrystalline nature of these individual NPs was also confirmed by the SAED measurements (Fig. 1d). A few diffraction spots of Cu2O are recognized after careful observation by enlarging the photo. The HRTEM image of the nanocomposite showed two types of intimately contacted lattice fringes, confirming the formation of Au core and Cu2O shell (Fig. 1e). The lattice spacing of 0.24 nm was the interplanar distance between adjacent (111) crystallographic plane of Cu2O, while the measured spacing (0.23 nm) matched the reported (111) lattice spacing for Au NPs. For further characterization of the Au@Cu2O stellated polytope, we carried out XRD and optical measurements (Fig. S2, Supplementary data). The presence of five 7
peaks were respectively indexed to the (110), (111), (200), (220), and (311) planes of Cu2O, in a good agreement with the standard pattern of Cu2O (JCPDS no. 65-3288) [34]. A weak peak at 38.2 showed the existence of a small amount of Au in the sample. V1 ρ (kR13) = V2 ρ (kR23)
(4)
V2 = V1 • (R1/R2)3
(5)
R2 = R1 • (V1/V2)1/3
(6)
According to the results above, one Au seed was only coated by Cu2O shell to form a stellated polytope. In addition, we proposed that the size of Au@Cu2O could be predicted and well-controlled by adjusting the added volume of Au seed solution. To demonstrate the above deductions, a series of Au seeds with different volumes were used for the preparation of Au@Cu2O with different Au contents, including 2.0 mL, 4.0 mL, 8.0 mL, and 12.0 mL, respectively. All of Au NPs solutions were from the same pot to ensure the identical concentration. The relevant SEM images of the products were shown in Fig. 2. As expected, all the samples were uniform stellated polytope. According to eq (4-6) [28] and our findings, Au@Cu2O stellated polytope was obtained and its edge sizes was 165 nm, 126 nm, 99 nm, and 85 nm, respectively, in a good agreement with the calculated value, and this result was confirmed by the size distribution histogram (inset Fig. 2). The standard deviation was calculated as 5.0 nm and the relative standard deviation was 4.1%. For easy comparison, Table 1 listed the experimental and calculated size values. These results proved the rationality of our calculations and deductions. For example, according to eq (5), if 99 nm of Au core in Au@Cu2O was required, 8.0 mL of Au seed solution should be used. Based on this experiment, size control could be easily realized according to eq (5) and eq (6). Consistent with SEM data (Fig. 2), the TEM images of Au@Cu2O stellated polytope confirmed that the produced particles were core-shell structure and the above conclusion was proved (Fig. 3). 8
The full range XPS spectrum of Au@Cu2O stellated polytope was shown in Fig. 4a. The XPS peaks of Cu2p3/2 were located at about 932.6 eV with a good symmetry (Fig. 4b). The chemical valence of Cu was +1 and copper was coordinated oxygen which peak was located at about 530.4 eV (Fig. 4c) [34]. The peak at 532.4 eV may be assigned to the oxygen in the Cu-O bond (Fig. 4c). Because of the thickness, it is possible to detect such weak Au peaks with Au particles located deep inside the Cu2O shells. The peaks of Au 4f7/2 and Au 4f5/2 were centered at 84.2 and 87.8 eV, respectively (Fig. 4d), and the spin energy separation was 3.6 eV, in a good agreement with previous studies [35]. The diffuse reflectance UV-vis spectra of the prepared Au@Cu2O were shown in Fig. 5. The absorption band at about 500 nm could be attributed to the band-edge absorption of Cu2O [36]. The absorption features of Au NPs encapsulated in the Cu2O was in the range of 530-580 nm, but such absorption was not obvious due to the band-edge absorption of Cu2O. The band at 700 nm is likely coming from Au. Au SPR peak is greatly red-shifted after coating with Cu2O [2]. The UV-vis spectrum of the Au cores and the size of the Au cores were checked this view (Fig. S3, Fig. S4, Supplementary data). The Cu2O absorption (the black line) looks different is because of their very large sizes which causes strong light scattering. The visible-light absorption of Au@Cu2O stellated polytope was stronger than that of Cu2O (see Fig. 5). 3.2. Evaluation of adsorption and photocatalytic activity The adsorption and photocatalytic activity of Au@Cu2O stellated polytope submicrospheres was evaluated in this work. Both cationic and anionic dyes with different molecular weights were used to test the adsorption efficiency of Au@Cu2O stellated polytope. Two kinds of anionic dyes (acid violet 43 and methyl blue) and cationic dyes (rhodamine B and methyl violet) were used. After adsorption for 60 min, 99.19% of acid violet 43, 98.8% of methyl blue, 35.72% of rhodamine B, 9
and 60.25% of methyl violet were removed by Au@Cu2O, respectively (see Fig. 6). The Zeta potential of the Au@Cu2O stellated polytope was 14.5 mv, which was shown in Fig. S5. The results demonstrated that anionic dyes had high adsorption efficiency due to the electrical attractions between Au@Cu2O and anionic dye, because the surface charge on Cu2O was positive [37]. In addition, the smaller dye molecules can easily penetrate into the internal pore structure of Au@Cu2O stellated polytope. Determine the surface facets for the stellated particles, because photocatalytic activity is highly dependent on the surfaces. The structure of stellated polytope for core-shelled Au@Cu2O (see Fig. 1) could enhance the affinity between Au@Cu2O and dyes and then the surface coverage and adsorption rate of dyes onto Au@Cu2O were increased. The specific surface area, pore volume, and average pore size of 4mL-Au@Cu2O stellated polytope were 16 m2g−1, 0.06 cm3g−1, and 18.3 nm, respectively (Fig. S6 and Table S1, Supplementary data). The specific surface area of Au@Cu2O was 1.57 times of Cu2O. After adsorption for 60 min, 9.07%, 80.24%, and 99.19% of acid violet 43 could be adsorbed by P25, Cu2O and Au@Cu2O, respectively (see Fig. 6). The adsorption activity of 4mL-Au@Cu2O was higher than that of P25 and Cu2O. The adsorption rate of 4mL-Au@Cu2O exhibited a 20% increase compared with Cu2O, because of an increase in surface area and pore size (Table S1, Supplementary data). For comparison, the same procedure was also performed for other aromatic pollutants (see Fig. 6). The adsorption rate of anionic dyes (acid violet 43 or methyl blue, 5.0 mg L-1) on Au@Cu2O stellated polytope with different Au NPs weight was listed in Fig. S7. The adsorption
rate
of
anionic
dyes
was
in
the
order
of
4mL-Au@Cu2O
>
8mL-Au@Cu2O/12mL-Au@Cu2O > 2mL-Au@Cu2O. The high specific surface area of Au@Cu2O stellated polytope (i.e., more than 10 m2g−1) could provide abundant adsorption sites for dyes. Pore size is also an important parameter, because the thermodynamic state of Kr within the narrow pores 10
at a temperature depended on the pore size [38]. The pore size of 4mL-Au@Cu2O was 18.3 nm, the largest one among the nanocrystalline Au@Cu2O stellated polytope (Table S1, Supplementary data), so the adsorption activity of 4mL-Au@Cu2O was the best. The major issue of heterogeneous photocatalysis is attributed to the poor surface coverage of targeted compounds on the catalyst [3]. 4mL-Au@Cu2O was selected for dye photodegradation. To investigate the effect of photocatalyst structure on photocatalytic activity, three kinds of photocatalysts, including P25, Cu2O, and 4mL-Au@Cu2O, were used for the photodegradation of methyl orange and methyl violet under simulated sunlight irradiation for 80 min. Fig. 7 showed only 9.64% of methyl orange could be photodegraded by P25 because of its photosensitization. The photocatalytic activity of 4mL-Au@Cu2O was higher than that of P25 and Cu2O. Using 4mL-Au@Cu2O, the photodegradation rate of methyl orange was 54.81%, which exhibited an increase of 45.17% and 37.57% compared with P25 and Cu2O. Furthermore, the rate constant k (min−1) of the 4mL-Au@Cu2O stellated polytope exhibited 3.4, 3.9, 2.3, and 1.2 times increase compared with the reported Cu2O-Au nanocomposites, Cu2O nanowire-Au nanoparticle assemblies, ZnO@Au@Cu2O nanorod films, and Au- mercaptoundecanoic acid-Cu2O, respectively (see Table 2 [39] and Table S3). Meanwhile, the linear relationship between ln(C0/C) and irradiation time indicated that the photodegradation reactions followed pseudo-first-order kinetics (see Fig. 7) with rate constants (0.0013, 0.0025, and 0.0106 min−1) for P25, Cu2O and 4mL-Au@Cu2O, respectively. For comparison, the same procedure was also performed for methyl violet (see Fig. 7, Table 3, and Table S2, Supplementary data). The photocatalytic activity was in the order of Au@Cu2O > Cu2O-Au nanocomposites > Cu2O, i.e., the synergies effect of Au core as cocatalysts for enhanced photocatalytic activity of Cu2O was observed (see Fig. 7 and Table 3). The stellated polytope structure could improve the affinity between photocatalyst and dyes, leading to increases in the 11
surface coverage and adsorption rate of dyes onto the photocatalyst. The back electron transfer from Cu2O conduction band to Au NPs was generated, because the work function of Au NPs (5.1 eV) was larger than that of Cu2O (4.84 eV). Thus, the energy band bend at the Au-Cu2O interface was higher, which promoted the separation of photo-induced electrons and holes [40]. Moreover, the unique structure of Au@Cu2O stellated polytope could also be facilitated the separation of photo-induced electrons and holes. According to the photodegradation rate of methyl orange, the efficiency of these prepared nanocatalysts in producing •OH radicals and hole was assessed. The visible-light photodegradation rate of 4mL-Au@Cu2O was 54.81%, which exhibited an increase of 37.57% compared with Cu2O. On the other hand, a Schottky barrier was assumed to form at the Cu2O-Au interface and Au NPs as electron sink could reduce the recombination of photo-induced electrons and holes [41]. 4. Conclusions Here, we have demonstrated a simple aqueous solution approach for the fabrication of core-shelled Au@Cu2O stellated polytope. Au nanoparticles are covered by Cu2O nanoparticles shell. With good aqueous dispersibility, surface positive charge, and high chemisorption capacity, Au@Cu2O can be used for anionic dyes removal. Compared with Degussa P25-TiO2, the adsorption of anionic dyes (acid violet 43 or methyl blue, 5.0 mg L-1) onto Au@Cu2O is increased by 90.12% and 50.8%, respectively. Furthermore, the photodegradation rate constant k (min−1) of the 4mL-Au@Cu2O stellated polytope was better than that the reported Cu2O-Au nanocomposites. The method affords the fine tuning of the core-shelled Au@Cu2O stellated polytope, toward an efficient photocatalyst in the photocatalytic decomposition of organic pollutant. The geometry of core-shell and stellated polytope is effective in the design of Cu2O-Au nanocomposites with remarkable adsorption and photocatalytic activity. 12
Acknowledgments This work is supported by the National Natural Science Foundation of China (21175115 and 21475055, S.X.L), the Program for New Century Excellent Talents in University (NCET-11 0904, S.X.L).
13
References [1] X. Xu, C. Randorn, P. Efstathiou, J.T. Irvine, A red metallic oxide photocatalyst, Nat. Mater. 11 (2012) 595-598. [2] Y.C. Yang, H.J. Wang, J. Whang, J.S. Huang, L.M. Lyu, P.H. Lin, S. Gwo, M.H. Huang, Facet-dependent optical properties of polyhedral Au-Cu2O core-shell nanocrystals, Nanoscale 6 (2014) 4316-4324. [3] S. Li, J. Cai, X. Wu, F. Zheng, X. Lin, W. Liang, J. Chen, J. Zheng, Z. Lai, T. Chen, Fabrication of positively and negatively charged, double-shelled, nanostructured hollow spheres for photodegradation of cationic and anionic aromatic pollutants under sunlight irradiation, Appl. Catal. B: Environ 160 (2014) 279-285. [4] J. Zhang, Y. Tang, L. Weng, M. Ouyang, Versatile strategy for precisely tailored core@shell nanostructures with single shell layer accuracy: the case of metallic shell, Nano Lett. 9 (2009) 4061-4065. [5] S.C. Hsu, S.Y. Liu, H.J. Wang, Facet-dependent surface plasmon resonance properties of Au-Cu2O core-shell nanocubes, octahedra, and rhombic dodecahedra, Small 11 (2015) 195-201. [6] S. Rej, H.J. Wang, M.X. Huang, S.C. Hsu, C.S. Tan, F.C. Lin, M.H. Huang, Facet–dependent optical properties of Pd–Cu2O core–shell nanocubes and octahedra, Nanoscale 7 (2015) 11135-11141. [7] D.Y. Liu, S.Y. Ding, H.X. Lin, B.J. Liu, Z.Z. Ye, F.R. Fan, Z.Q. Tian, Distinctive enhanced and tunable plasmon resonant absorption from controllable Au@Cu2O nanoparticles: experimental and theoretical modeling, J. Phys. Chem. C 116 (2012) 4477-4483. [8] J. Hu, M. Chen, X. Fang, L. Wu, Fabrication and application of inorganic hollow spheres, Chem. Soc. Rev. 40 (2011) 5472-5491. 14
[9] Y. Zhu, E. Kockrick, T. Ikoma, N. Hanagata, S. Kaskel, An efficient route to rattle-type Fe3O4@SiO2 hollow mesoporous spheres using colloidal carbon spheres templates, Chem. Mater. 21 (2009) 2547-2553. [10] X. Li, R. Huang, Y. Hu, Y. Chen, W. Liu, R. Yuan, Z. Li, A templated method to Bi 2WO6 hollow microspheres and their conversion to double-shell Bi2O3/Bi2WO6 hollow microspheres with improved photocatalytic performance, Inorg. Chem. 51 (2012) 6245-6250. [11] J. Cao, Y. Zhu, K. Bao, L. Shi, S. Liu, Y. Qian, Microscale Mn2O3 hollow structures: sphere, cube, ellipsoid, dumbbell, and their phenol adsorption properties, J. Phys. Chem. C 113 (2009) 17755-17760. [12] H. Li, Z. Bian, J. Zhu, Y. Huo, H. Li, Y. Lu, Mesoporous Au/TiO2 nanocomposites with enhanced photocatalytic activity, J. Am. Chem. Soc. 129 (2007) 4538-4539. [13] M. Murdoch, G. Waterhouse, M. Nadeem, J. Metson, M. Keane, R. 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) 489-492. [14] I. Lee, J.B. Joo, Y. Yin, F. Zaera, A yolk@shell nanoarchitecture for Au/TiO2 catalysts, Angew. Chem. Int. Ed. 123 (2011) 10390-10393. [15] J. Qi, J. Chen, G. Li, S. Li, Y. Gao, Z. Tang, Facile synthesis of core-shell Au@CeO2 nanocomposites with remarkably enhanced catalytic activity for CO oxidation, Energy Environ. Sci. 5 (2012) 8937-8941. [16] W. He, H.K. Kim, W.G. Wamer, D. Melka, J.H. Callahan, J.J. Yin, Photogenerated charge carriers and reactive oxygen species in ZnO/Au hybrid nanostructures with enhanced photocatalytic and antibacterial activity, J. Am. Chem. Soc. 136 (2013) 750-757. [17] L. Zhang, D.A. Blom, H. Wang, Au-Cu2O core-shell nanoparticles: a hybrid 15
metal-semiconductor heteronanostructure with geometrically tunable optical properties, Chem. Mater. 23 (2011) 4587-4598.. [18] M.A. Mahmoud, W. Qian, M.A. El-Sayed, Following charge separation on the nanoscale in Cu2O-Au nanoframe hollow nanoparticles, Nano Lett. 11 (2011) 3285-3289. [19] N.A. Heutz, P. Dolcet, A. Birkner, M. Casarin, K. Merz, S. Gialanella, S. Gross, Inorganic chemistry in a nanoreactor: Au/TiO2 nanocomposites by photolysis of a single-source precursor in miniemulsion, Nanoscale 5 (2013) 10534-10541. [20] J. Li, S.K. Cushing, J. Bright, F. Meng, T.R. Senty, P. Zheng, A.D. Bristow, N. Wu, Ag@Cu 2O core-shell nanoparticles as visible-light plasmonic photocatalysts, ACS Catalysis 3 (2012) 47-51. [21] W.C. Li, M. Comotti, F. Schüth, Highly reproducible syntheses of active Au/TiO2 catalysts for CO oxidation by deposition-precipitation or impregnation, J. Catal. 237 (2006) 190-196. [22] C. Chen, C. Nan, D. Wang, Q. Su, H. Duan, X. Liu, L. Zhang, D. Chu, W. Song, Q. Peng, Mesoporous multicomponent nanocomposite colloidal spheres: ideal high-temperature stable model catalysts, Angew. Chem. Int. Ed. 123 (2011) 3809-3813. [23] X. Liu, A. Wang, X. Wang, C.Y. Mou, T. Zhang, Au-Cu Alloy nanoparticles confined in SBA-15 as a highly efficient catalyst for CO oxidation, Chem. Commun. (2008) 3187-3189. [24] K. Zhao, B. Qiao, J. Wang, Y. Zhang, T. Zhang, A highly active and sintering-resistant Au/FeOx -hydroxyapatite catalyst for CO oxidation, Chem. Commun. 47 (2011) 1779-1781. [25] M. Cargnello, N.L. Wieder, T. Montini, R.J. Gorte, P. Fornasiero, Synthesis of dispersible Pd@CeO2 core-shell nanostructures by self-assembly, J. Am. Chem. Soc. 132 (2009) 1402-1409. [27] J.C. Park, J.U. Bang, J. Lee, C.H. Ko, H. Song, Ni@SiO2 yolk-shell nanoreactor catalysts: high temperature stability and recyclability, J. Mater. Chem. 20 (2010) 1239-1246. [28] L. Kong, W. Chen, D. Ma, Y. Yang, S. Liu, S. Huang, Size control of Au@Cu2O octahedra for 16
excellent photocatalytic performance, J. Mater. Chem. 22 (2012) 719-724. [29] C.H. Kuo, T.E. Hua, M.H. Huang, Au nanocrystal-directed growth of Au-Cu2O core-shell heterostructures with precise morphological control, J. Am. Chem. Soc. 131 (2009) 17871-17878. [30] M.C. Daniel, D. Astruc, Gold nanoparticles: assembly, supramolecular chemistry, quantum-size-related properties, and applications toward biology, catalysis, and nanotechnology, Chem. Rev. 104 (2004) 293-346. [31] 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 (2011) 1261-1267. [32] Y. Chen, C. Hu, X. Hu, J. Qu, Indirect photodegradation of amine drugs in aqueous solution under simulated sunlight, Environ. Sci. Technol. 43 (2009) 2760-2765. [33] J. Matos, J. Laine, J.M. Herrmann, Synergy effect in the photocatalytic degradation of phenol on a suspended mixture of titania and activated carbon, Appl. Catal. B: Environ 18 (1998) 281-291. [34] C.E. Dubé, B. Workie, S.P. Kounaves, A. Robbat, M.L. Aksub, G. Davies, Electrodeposition of metal alloy and mixed oxide films using a single-precursor tetranuclear copper-nickel complex, J. Electrochem. Soc. 142 (1995) 3357-3365. [35] J.B. Cai, X.Q. Wu, S. Li, F. Zheng, L. Zhu, Z, Lai, Synergistic effect of double-shelled and sandwiched TiO2@Au@C hollow spheres with enhanced visible-light-driven photocatalytic activity, ACS Appl. Mat. Interfaces 7 (2015) 3764-3772. [36] C.H. Kuo, Y.C. Yang, S. Gwo, M.H. Huang, Facet-dependent and Au nanocrystal-enhanced electrical and photocatalytic properties of Au-Cu2O core-shell heterostructures, J. Am. Chem. Soc. 133 (2010) 1052-1057. [37] W.C. Wang, L.M. Lyu, M.H. Huang, Investigation of the effects of polyhedral gold nanocrystal 17
morphology and facets on the formation of Au-Cu2O core-shell heterostructures, Chem. Mater. 23 (2011) 2677-2684. [38] K. Wessels, M. Minnermann, J. Rathousky, M. Wark, T. Oekermann, Influence of calcination temperature on the photoelectrochemical and photocatalytic properties of porous TiO2 films electrodeposited from Ti(IV)-alkoxide solution, J. Phys. Chem. C 112 (2008) 15122-15128. [39] Q. Hua, F. Shi, K. Chen, S. Chang, Y. Ma, Z. Jiang, G. Pan, W. Huang, Cu 2O-Au nanocomposites with novel structures and remarkable chemisorption capacity and photocatalytic activity, Nano Research 4 (2011) 948-962. [40] S.M. Majhi, P. Rai, S. Raj, B.S. Chon, K.K. Park, Y.T. Yu, Effect of Au nanorods on potential barrier modulation in morphologically controlled Au@Cu2O core-shell nanoreactors for gas sensor applications, ACS Appl. Mat. Interfaces 6 (2014) 7491-7497. [41] D. Jiang, W. Zhou, X. Zhong, Y. Zhang, X. Li, Distinguishing localized surface plasmon resonance and schottky junction of Au-Cu2O composites by their molecular spacer dependence, ACS Appl. Mat. Interfaces 6 (2014) 10958-10962.
18
Table captions Table 1 Experimental and calculated edge size values of the samples prepared from 2.0 mL, 4.0 mL, 8.0 mL, and 12.0 mL of Au NPs solutions. Table 2 Visible light photodegradation rate constant k (min−1) for P25, Cu2O-Au nanocomposites and Au@Cu2O stellated polytope. Table 3 Visible light photodegradation rate constant k (min−1) for P25, Cu2O and Au@Cu2O stellated polytope.
19
Table 1 Experimental and calculated edge size values of the samples prepared from 2.0 mL, 4.0 mL, 8.0 mL, and 12.0 mL of Au NPs solutions. (n=3) VAu(mL)a
Rexp(nm)b
Rcal(nm)c
2.0
165
4.0
126
129
8.0
99
102
12.0
85
89
a
VAu: the volume of Au NPs solution. bRexp: the average edge size of as-synthesized Au@Cu2O.
c
Rcal: edge size calculated according to (6) based on the reference experiment in which 2.0 mL of
Au NPs solution was used as seed.
20
Table 2 Visible light photodegradation rate constant k (min−1) for P25, Cu2O-Au nanocomposites and Au@Cu2O stellated polytope. k (min−1) Sample
Relative rate
Ref
methyl orange P25
0.0013
1
This work
Cu2O-Au nanocomposites
0.00315
2.42
[39]
Au@Cu2O stellated polytope
0.0106
8.15
This work
21
Table 3 Visible light photodegradation rate constant k (min−1) for P25, Cu2O and Au@Cu2O stellated polytope. k (min−1)
methyl orange
methyl violet
P25
0.0013
0.0057
Cu2O
0.0025
0.012
Au@Cu2O
0.0106
0.022
Sample
22
Figure captions Scheme 1. Charge-transfer process in the Au@Cu2O stellated polytope. Scheme 2. Illustrations of formation process of Au@Cu2O stellated polytope. Fig. 1. (a) SEM, (b and c) TEM, (d) SAED, and (e) HRTEM images of Au@Cu2O stellated polytope. Fig. 2. SEM images of Au@Cu2O stellated polytope with diverse average sizes prepared from different volumes of gold seed solution: (a) 165 nm, 2.0 mL; (b) 126 nm, 4.0 mL; (c) 99 nm, 8.0 mL; (d) 85 nm, 12.0 mL. Fig. 3. TEM images of Au@Cu2O stellated polytope with diverse average sizes prepared from different volumes of gold seed solution: (a) 165 nm, 2.0 mL; (b) 126 nm, 4.0 mL; (c) 99 nm, 8.0 mL; (d) 85 nm, 12.0 mL. Fig. 4. (a) Full range of the XPS spectrum, (b) XPS peaks of Cu2p, (c) O1s and (d) Au4f of core-shell Au@Cu2O stellated polytope. Fig. 5. Diffuse reflectance UV-vis spectra of Cu2O and Au@Cu2O samples. Fig. 6. Adsorption rate of acid violet 43 (an anionic dye, a), methyl blue (an anionic dye, b), rhodamine B (a cationic dye, c), and methyl violet (a cationic dye, d) on P25, Cu2O, and Au@Cu2O under dark. Fig. 7. Linear transform ln(C0/C) = f(t) of photodegradation kinetic curves of (a) methyl orange, or (b) methyl violet by P25, Cu2O, and Au@Cu2O under visible light irradiation.
23
Scheme 1. Charge-transfer process in the Au@Cu2O stellated polytope.
24
Scheme 2. Illustrations of formation process of Au@Cu2O stellated polytope.
25
Fig. 1. (a) SEM, (b and c) TEM, (d) SAED, and (e) HRTEM images of Au@Cu2O stellated polytope.
26
Fig. 2. SEM images of Au@Cu2O stellated polytope with diverse average sizes prepared from different volumes of gold seed solution: (a) 165 nm, 2.0 mL; (b) 126 nm, 4.0 mL; (c) 99 nm, 8.0 mL; (d) 85 nm, 12.0 mL.
27
Fig. 3. TEM images of Au@Cu2O stellated polytope with diverse average sizes prepared from different volumes of gold seed solution: (a) 165 nm, 2.0 mL; (b) 126 nm, 4.0 mL; (c) 99 nm, 8.0 mL; (d) 85 nm, 12.0 mL.
28
Fig. 4. (a) Full range of the XPS spectrum, (b) XPS peaks of Cu2p, (c) O1s and (d) Au4f of core-shell Au@Cu2O stellated polytope.
29
Fig. 5. Diffuse reflectance UV-vis spectra of Cu2O and Au@Cu2O samples.
30
Fig. 6. Adsorption rate of acid violet 43 (an anionic dye, a), methyl blue (an anionic dye, b), rhodamine B (a cationic dye, c), and methyl violet (a cationic dye, d) on P25, Cu2O, and Au@Cu2O under dark.
31
Fig. 7. Linear transform ln(C0/C) = f(t) of photodegradation kinetic curves of (a) methyl orange, or (b) methyl violet by P25, Cu2O, and Au@Cu2O under visible light irradiation.
32
Graphical abstract
33
34