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Preparation of a novel bimetallic AuCu-P25-rGO ternary nanocomposite with enhanced photocatalytic degradation performance
Applied Catalysis A, General 549 (2018) 237–244
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Applied Catalysis A, General journal homepage: www.elsevier.com/locate/apcata
Research Paper
Preparation of a novel bimetallic AuCu-P25-rGO ternary nanocomposite with enhanced photocatalytic degradation performance ⁎
He Wena,b, Yu Longa,b, , Wei Hanc, Wei Wua,b, Yuanyuan Yanga,b, Jiantai Maa,b, a b c
MARK
⁎
State Key Laboratory of Applied Organic Chemistry (SKLAOC), Lanzhou University, Lanzhou, 730000, PR China Gansu Provincial Engineering Laboratory for Chemical Catalysis, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou, 730000, PR China Lanzhou Petrochemical Research Center, PetroChina, Lanzhou, 730060, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Photodegradation AuCu bimetallic nanoparticles Surface plasmon resonance effect Ternary photocatalyst Photocatalytic mechanism
In this work, a novel bimetallic ternary composite (AuCu-P25-rGO) had been designed and prepared, and been used as a high-performance photocatalyst. The morphology, chemical composition, physical structure and photoelectric property were confirmed by XRD, TEM, BET analysis, FT-IR, Raman, XPS, UV-vis DRS, Photoluminscence spectra and transient photocurrent responses techniques. The photocatalysts possessed higher catalytic activity than P25, P25-rGO, AuCu-P25, Cu-P25-rGO, Au-P25-rGO for photodegradation of organic pollutant (2-nitrophenol) under UV–vis light. Importantly, the molar ratio of Au:Cu exhibited significant influence on the photocatalytic property, and AuCu-P25-rGO with Au/Cu molar ratio = 1:1 showed higher activity than these with Au/Cu molar ratio = 1:3 and 3:1. In addition, Au1Cu1-P25-rGO could excellently photodegradation of organic dyes (Rhodamine B and methylene blue). The enhanced photocatalytic performance of AuCu-P25-rGO composites can be attributed to three reasons as follows: (1) surface plasmon resonance (SPR) effect of AuCu bimetallic nanoparticles; (2) outstanding conductivity and adsorptivity of rGO; (3) the polymorphs junction formed by mixture of anatase and rutile of P25.
1. Introduction The word photocatalysis had been designed in the 1970s, researchers from the University of Tokyo reported the photocatalytic ability of TiO2, which had opened ways towards using semiconductor photocatalysts for purification of water and air [1]. Particularly, TiO2 has been considered as one of the best and most potential photocatalysts, due largely to its high photocatalytic activity, relative nontoxicity, long-term thermodynamic stability, and low cost [2,3]. However, the fast recombination of photogenerated electron-hole pairs caused huge loss photocatalytic efficiency of TiO2. On the other side, because of its wide band-gap (3.2 eV), TiO2 can only be excited by ultraviolet, which occupies only 4–5% of the solar light spectrum. The above two points become the principal elements of limiting its practical applications [4]. In the past few years, various techniques have been attempted to modify the TiO2 photocatalysts to solve these problems. A popular approach for enhancing photocatalytic performance to employ the Degussa P25 TiO2 (P25). P25 is a commercial TiO2 nanopowder consisting of 80% anatase and 20% rutile. Their mixtures produce novel electronic states at two polymorphs junctions that can suppress the recombination of photogenerated electron-hole pairs and facilitate
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charge transfer [5]. Besides, incorporation of electron-accepting materials like graphene [6] and loading noble metal particles [7] are also two key methods to enhance the photocatalytic performance of TiO2. Graphene, a sp2-hybridized carbon atoms with a two-dimensional (2D) benzene-ring packed lattice structure [8], has attracted worldwide interest because of its outstanding electrical conductivity [9], electron mobility [8], optical transparency [10], thermal conductivity [11], and large theoretical surface area [12]. However, the current approaches to prepare graphene face the challenges of high cost and low yield. Therefore, reduced graphene oxide (rGO) with low cost and high output have been drawing increased attention. The incorporation of rGO and TiO2 can enhance the charge transport rate to suppress the recombination of charge carriers and enhance photocatalytic activity, due to the formation of TieOeC chemical bond narrowing the band gap of TiO2 [13] and the good conductivity of rGO. In addition, many researchers have demonstrated that the lack of visible light response could be improved by decorating metallic nanoparticles such as Au, Ag, Pt, Pb [14–17] etc. Recently, bimetallic alloy nanoparticles have been employed as a new attractive advanced material, to improve structure and electronic properties due to its synergistic effects [18,19]. Among them, alloying Cu with a noble metal
Corresponding authors at: State Key Laboratory of Applied Organic Chemistry (SKLAOC), Lanzhou University, Lanzhou 730000, PR China. E-mail addresses: [email protected] (Y. Long), [email protected] (J. Ma).
http://dx.doi.org/10.1016/j.apcata.2017.09.028 Received 30 July 2017; Received in revised form 20 September 2017; Accepted 24 September 2017 Available online 14 October 2017 0926-860X/ © 2017 Elsevier B.V. All rights reserved.
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2.1.2. Synthesis of AuCu-P25-rGO 50 mg of the prepared AuCu-P25 and 1 mg GO were dispersed in a mixed solution of distilled H2O (40 mL) and ethanol (20 mL) by ultrasonic treatment, then stirred for 1 h and the solution was transferred to a 100 mL Teflon-lined stainless steel autoclave, followed by a hydrothermal treatment at 120 °C for 2 h. The resultant AuCu-P25-rGO was centrifuged and dried at 50 °C. The bimetallic photocatalysts with different Au/Cu molar ratios (3/1, 1/1, 1/3) were synthesized by the same method and the ratios of Au/Cu and AuCu/Ti in as-synthesized samples are shown in Table S1.
has attracted tremendous research interests, such as AuCu alloy. Both Au and Cu nanoparticles have surface plasmon resonance (SPR) effect, they can absorb and utilize the visible light to promote photocatalytic reactions [20]. Besides, AuCu bimetallic nanoparticles have a low work function because of its unique optical and electronic abilities of Cu nanostructures [21]. It has been reported that AuCu bimetallic nanoparticles exhibited higher catalytic performance than their individual components for CO oxidation, propene epoxidation and benzyl alcohol oxidation [22]. However, only a few studies reported the photodegradation properties of AuCu nanoparticles. Herein, the bimetallic ternary composites (AuCu-P25-rGO) were successfully synthesized and employed for photocatalytic degradation of 2-nitrophenol (2-NP), methylene blue (MB) and Rhodamine B (RhB) which endanger the human health and aquatic ecosystems. So far our knowledge goes, less study has been carried for using bimetallic ternary composites as efficient photocatalysts for degradation of harmful phenolic compounds and other organic dyes. In addition, in the reported ternary composites, TiO2 were mostly in a single phase, principally anatase. Herein, we used P25 consisting of anatase and rutile as a high performance active ingredient.
2.2. Photocatalysts experiments The photocatalytic activity of AuCu-P25-rGO was evaluated by the photodegradation of 2-NP (0.5 mM, pH = 6.3), MB (10−5 mol L−1, pH = 6.8) and RhB (10−5 mol L−1, pH = 6.5) in a thermostatic photoreactor. Before irradiation, the suspension containing 0.3 g/L of the solid of AuCu-P25-rGO was sonicated and magnetically stirred for 30 min in the dark to achieve the adsorption and desorption equilibrium. Then, the mixture was irradiated with a 300 W halogen lamp (the distribution of wavelength was shown in Fig. S1). After a certain time interval, the suspension was taken and centrifuged to separate the catalyst to get supernatant liquid, the liquid was monitored by a UV–vis spectrophotometer, and the absorption peak of 2-NP, MB, RhB were 274 nm and 346 nm, 664 nm, 553 nm, respectively. In a similar way, AuCu-P25-rGO with different Au/Cu ratio, AuCu-P25, P25-rGO and P25 were used in the degradation of 2-NP.
2. Experimental 2.1. Photocatalysts preparation The synthesis routes of AuCu-P25-rGO are shown in Scheme 1. 2.1.1. Synthesis of AuCu-P25 Bimetallic Au1Cu3-P25, Au1Cu1-P25 and Au3Cu1-P25 photocatalysts with a total metal loading of 1 wt.% on P25 were synthesized by adopting Duff and Baiker Method [23]. In a 100 mL three-neck flask, 500 mg of P25 was dispersed in deionized water. During vigorous stirring at room temperature, 6 mL 0.2 M of NaOH and 4 mL 0.05 M of (hydroxymethyl) phosphonium chloride (THPC) were added into the mixed solution, 5 min was allowed between the addition of THPC and NaOH. Afterwards, HAuCl4 solution (0.01 M) and CuCl2 solution (0.1 M) were added to the mixture. Then the mixture maintained under vigorous stirring for 2 h at the room temperature. The as-prepared products were separated by centrifugation, and dried at 50 °C for 12 h. Finally, the solid was calcined at 550 °C in air for 6 h, and reduced at 550 °C in pure H2 for 1 h, then cooled down to room temperature under N2 flow to obtain the catalysts. The bimetallic photocatalysts with different Au/Cu molar ratios (3/1, 1/1, 1/3) and monometallic Au- and Cu-P25 were synthesized by the same method which described above, the only difference was the different addition volume of HAuCl4 solution (0.01 M) and CuCl2 solution (0.1 M).
2.3. Photoelectrochemical measurement The measurement was carried out in the electrochemical workstation (CHI 660E, shanghai chenhua) in Na2SO4 (0.5 M) solution containing 2-NP (0.5 mM) using a standard three-electrode system: Ag/ AgCl as the reference electrode, a Pt wire as the counter electrode and ITO glass (1 cm × 1 cm) deposited with the suspension of photocatalysts (1.0 g L−1) as the working electrode. The 300 W xenon lamp was used as the light source for photoelectrochemical measurement. 3. Result and discussion The crystalline structures of samples were acquired by XRD. Fig. 1 shows the XRD patterns of GO, P25, Au1Cu1-P25, P25-rGO and Au1Cu1P25-rGO. Except GO, all of the P25 nanocomposites (P25, Au1Cu1-P25, P25-rGO and Au1Cu1-P25-rGO) exhibited the similar XRD patterns, the diffraction peaks of anatase [JCPDS no. 21-1272] and rutile [JCPDS no. 77-0443] phases of P25 could be clearly observed. The diffraction peaks Scheme 1. The synthesis routes of AuCu-P25-rGO.
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measured interplanar spacing could demonstrate the formation of AuCu alloy nanoparticles. Meanwhile, HAADF-STEM was carried out to give powerful evidence about the good dispersity of AuCu alloy nanoparticles and morphology of nanostructure. Fig. 3c–g display the EDX mapping for the respective elements, Au and Cu maps were almost overlapped, suggesting that AuCu alloy nanoparticles were formed for the as-prepared AuCu-P25-rGO. And Ti, O and C were certainly existing. The presence of the elemental composition could be further confirmed with EDX. As displayed in Fig. S2, the X-ray peaks of Au, Cu, Ti, O and C met the element analysis of Au1Cu1-P25-rGO. Ni was generally influenced by the nickel network support films. Table 1 shows the physicochemical properties of P25 and Au1Cu1P25-rGO. It indicates that the pore size and pore volume obviously increased compared with pure P25. This might due to the incorporation of rGO. However, the specific surface area of Au1Cu1-P25-rGO decreased, which might due to the low loading amount of rGO (2 wt.%) and rGO covered some of the mesopores on P25. Hence, from the perspective of BET analysis, the relatively large pore size of Au1Cu1P25-rGO would favor the transportation of large molecular which involved in photocatalytic reaction. However, the contribution of specific surface area to the photocatalytic reaction was almost negligible. Fig. S3 shows the Nitrogen adsorption–desorption isotherms and corresponding pore size distribution curves of Au1Cu1-P25-rGO. The adsorption–desorption isotherms at high P/Po of Au1Cu1-P25-rGO were characteristic of a Type H3 loop, and the shape of isotherms indicated the presence of mesoporous and microporous [30]. The pore size distribution was calculated by BJH method from desorption branch of Nitrogen adsorption–desorption isotherms. And the pore size distribution exhibited wide range from 2 nm to 100 nm. This wide range might be ascribed to the crevice between hybrid interface [31]. Fig. 4a displays the Raman spectra of GO, P25, P25-rGO and Au1Cu1-P25-rGO. For the P25, P25-rGO and Au1Cu1-P25-rGO samples, the bands at 142, 396, 513 and 636 cm−1 were corresponded to the Eg(1), B1g, A1g and Eg(2) of anatase, respectively [32]. Two typical Raman bands could be observed in samples of GO, P25-rGO and Au1Cu1-P25-rGO. One band was D-band (∼1340 cm−1), corresponding to the existence of structural defects and disorder. The other band was G-band (∼1603 cm−1), which was the stretching of the sp2-hybridized CeC bonds. Thus, the D/G band intensity ratio (ID/IG) could reflect the structural changes from GO to rGO. The ID/IG ratio of P25-rGO (0.96) and Au1Cu1-P25-rGO (1.04) were higher than that of GO (0.88), which indicated the successful reduction of GO during the solvothermal experiment. And due to the introduction of P25 nanoparticles on the rGO, Au1Cu1-P25-rGO showed the highest ID/IG ratio. FTIR analysis was carried out to illustrate the organic group for the Au1Cu1-P25-rGO composite (Fig. 4b). From the GO spectrum, the broad peaks at 3420 cm−1 and 1216 cm−1 were ascribed to the hydroxyl groups stretching vibrations. Peaks at 1380 cm−1 and 1720 cm−1 were corresponded to the C]O groups on the surface of GO. Moreover, the
Fig. 1. XRD patterns of the GO, Au1Cu1-P25-rGO, P25-rGO, Au1Cu1-P25 and P25.
at 2Θ values of 25.3, 37.6, 47.9, 53.6, 55.0, 62.4, 68.4, 70.1 and 75.0° could be assigned to the crystal phases (101), (004), (200), (105), (211), (204), (116), (220) and (215) of anatase in P25, respectively. Moreover, the peaks at 2Θ values of 27.4, 36.0, 41.1 and 56.4° were corresponded to the rutile (110), (101), (111) and (220) planes. No significant characteristic peaks of AuCu alloy could be observed for Au1Cu1-P25 and Au1Cu1-P25-rGO, due to its low loading content (1 wt. %) [24,25]. Moreover, no characteristic peak for graphene at 2Θ value of 25° could be observed, which was presumably due to the low amount and the overlap of strong (101) diffraction peak of anatase TiO2 at 2Θ value of 25.3° [26,27]. Nevertheless, there was no significant characteristic peak at 2Θ value of 10.2° of GO could be observed in the XRD patterns of P25-rGO and Au1Cu1-P25-rGO, which conformed the completely reduction of GO to rGO through one-step hydrothermal process. In order to further investigate the morphology, microstructure and composition of photocatalysts, transmission electron microscopy (TEM), high resolution TEM (HRTEM) and energy dispersive X-ray analysis (EDX) were carried out. As the TEM photo shown in Fig. 2a, P25 with spherical AuCu alloy nanoparticles were successfully deposited on the surface of rGO. The obtained rGO still retained ultrathin 2D sheet structure with wrinkles after the hydrothermal treatment. Owing to the distribution of oxygen-containing functional groups on GO, the P25 nanoparticles were eager to deposit along the wrinkles and edge [28]. Fig. 2b and c are the HRTEM images of the Au1Cu1-P25-rGO. As shown in Fig. 2b, the interplanar spacing of 0.35 nm and 0.33 nm were corresponded to the (101) and (110) planes of anatase and rutile, respectively, indicating the presence of both anatase and rutile crystalline formed in P25. Fig. 2c shows the microstructure detail of AuCu bimetallic nanoparticles, fringe lines with 0.226 nm spacing could be attributed to (111) crystal planes of AuCu alloy [29], lying between the lattice spacing of monometallic Au (0.235 nm) and Cu (0.208 nm). The
Fig. 2. (a) TEM and (b, c) HRTEM images of the Au1Cu1-P25-rGO.
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Fig. 3. STEM of Au1Cu1-P25-rGO (a and b); EDX mapping of Au1Cu1-P25-rGO (c-g): mapping of C (c), mapping of O (d), mapping of Ti (e), mapping of Au (f), and mapping of Cu (g).
containing species could further prove that GO was reduced to rGO after solvothermal process. In Fig. 5c, two main peaks at 458.4 and 464.2 eV were attributed to the Ti 2p3/2 and Ti 2p1/2 in Ti 2p XPS spectra, which explained the Ti4+ chemical state. Besides, two weak peaks at 460.2 and 466.3 eV, might be due to a small amount formation of Ti5+ [36]. Fig. 5d exhibits two obvious peaks located at 529.7 eV and 530.9 eV, indicating the presence of TieOeTi and TieOeC chemical bonds, which conformed to the result of FT-IR spectra. The high-resolution XPS spectra also studied the chemical state of Au in the Au1Cu1-P25-rGO composite (Fig. 5e). The peaks at 83.8 and 87.5 eV were in agreement with Au0 chemical state. The others at 84.3 and 88.3 eV with broad band and weak intensity were ascribable to the surface oxidized gold (Auδ+) [37]. Then, for Cu binding energy (Fig. 5f), the peaks at 932.3, 932.7 and 934.1 eV could be distinguished in Cu 2p3/2, corresponding to Cu2O, Cu0 and CuO, respectively [38]. The existence of Cu2+ could be further confirmed by the broad satellite peak at 943 eV [29]. Besides, the peaks at 952.5 and 954.0 eV were ascribed to Cu2O and CuO. The optical properties of P25, Au1Cu1-P25, P25-rGO, Au-P25-rGO, Cu-P25-rGO, and Au1Cu1-P25-rGO were studied by diffuse reflectance spectroscopy (DRS) (Fig. 6a). All the samples of photocatalysts revealed the typical absorption with a distinct transition in the spectra, which was corresponded to the band gap absorption of P25 [39]. In addition, a red shift could be clearly observed for Au1Cu1-P25, P25-rGO, Au-P25rGO, Cu-P25-rGO and Au1Cu1-P25-rGO, indicating the introduction of rGO or (and) AuCu bimetallic nanoparticles could narrow the band gap energy of photocatalysts. For P25-rGO, the absorption was higher than
Table 1 Physicochemical properties of P25 and Au1Cu1-P25-rGO. Samples
Pore size (nm)
Pore volume (cm3 g−1)
Surface area m2 g−1
a
7.0 25.6
0.10 0.30
48.2 46.4
P25 Au1Cu1-P25-rGO a
Ref. [25].
1050 cm−1 band was for the CeO stretching vibrations [33]. These oxygen-containing functional groups built the covalent connection between TiO2 and GO. Notably, the intensity of CH2 groups (∼2855 and 2930 cm−1) increased and the intensity of oxygen-containing functional groups decreased in P25-rGO and Au1Cu1-P25-rGO compared with GO, which revealed the reduction of GO. Besides, the absorption peak at 1600 cm−1 was ascribed to the skeletal vibration of GO and rGO. For P25-rGO and Au1Cu1-P25-rGO, the wide and broad peaks at low frequency (below 1000 cm−1) were attributed to the bending and stretching vibration of TieOeTi and TieOeC bonds [34], indicating the successful combination of P25 and rGO. In order to investigate the chemical state of different elements, the XPS core level spectra was collected. Fig. 5a shows a XPS survey spectrum of the Au1Cu1-P25-rGO sample, indicating that the sample contained C, Ti, O, Au and Cu. In C 1 s core level XPS spectrum (Fig. 5b), the main peak at 284.6 eV was attributed to sp2 or sp3 carbon species from rGO, while the other peaks at 286.2 eV and 288.6 eV were assigned to CeO and C]O [35]. The weak intensity of oxygen-
Fig. 4. (a) Raman spectras of GO, P25, P25rGO, and Au1Cu1-P25-rGO; (b) FTIR spectras of GO, P25-rGO and Au1Cu1-P25-rGO.
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Fig. 5. (a) Survey XPS spectrum of Au1Cu1P25-rGO; Core level XPS spectra of (b) C 1s, (c) Ti 2p, (d) O 1s, (e) Au 4f and (f) Cu 2p of Au1Cu1-P25-rGO.
transient photocurrent responses, under the irradiation of 300 W xenon lamp with applied voltage 0.6 V. As Fig. 6d showed, all the samples exhibited obvious changes under the visible light on and off. Besides, the on–off cyclic curves had high reproducibilities, which indicated that a large proportion of photoelectrons were transferred to the cathode to produce photocurrent [44]. The photocurrents of P25, P25-rGO, Au1Cu1-P25, Cu-P25-rGO, Au-P25-rGO and Au1Cu1-P25-rGO were 1.8, 2.3, 4.2, 3.0, 8.3 and 10.5 μA, respectively. Obviously, the Au1Cu1-P25rGO ternary composite showed the highest transient photocurrent, which was 5 times more than pure P25, 2.5 times higher than Au1Cu1P25 and 1.3 times higher than Au-P25-rGO. In addition, the photocurrents of P25-rGO increased 30% compared to P25, but the photocurrents of Au1Cu1-P25 increased 130%, which revealed that the SPR effect and lower work function of AuCu bimetallic nanoparticles could enhance the photocatalytic activity. The rate of electron-hole recombination was characterized by measuring photoluminscence (PL) spectra of P25, P25-rGO, Au1Cu1P25, Au-P25-rGO, Cu-P25-rGO and Au1Cu1-P25-rGO. The lower intensity of the sample represents a correspondingly lower recombination rate. In Fig. 7a, pure P25 exhibited the highest intensity and Au1Cu1P25-rGO showed the lowest one, indicating that our modification of
pure P25 in the 400–800 nm region, because the formation of TieOeC bonds narrowed the band gap energy of P25 [40]. Furthermore, the more enhanced absorption of Au1Cu1-P25 and Au1Cu1-P25-rGO were attributed to the presence of AuCu bimetallic nanoparticles, with a broad absorption peak from 500 nm to 700 nm, which was ascribed to the SPR effect of the AuCu bimetallic nanoparticles [41,42]. Moreover, the broad absorption peak from 500 nm to 600 nm of Au-P25-rGO could also be ascribed to the SPR effect of Au nanoparticles, nevertheless Au1Cu1-P25-rGO still possessed higher visible light utilization and more obvious red shift. Therefore, Au1Cu1-P25-rGO nanocomposite showed not only a red shift of the absorption edge but also a further increased absorption in the 400–800 nm range. To acquire more information about the band gap of Au1Cu1-P25rGO and P25, the Kubelka–Munk transformation was utilized [43]. In Fig. 6b, the band gap of P25 was 3.04 eV, indicating its low photoactivity due to the wide band gap. Nevertheless, the band gap of Au1Cu1-P25-rGO reduced to 2.87 eV caused a red shift in the spectra (Fig. 6c). The results indicated that the incorporation of rGO and AuCu bimetallic nanoparticles could enhance the absorption of photocatalysts in the visible light range. Photoelectrochemical properties were characterized by measuring 241
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Fig. 6. (a) UV–vis DRS spectra of the P25, Au1Cu1-P25, P25-rGO, Cu-P25-rGO, AuP25-rGO and Au1Cu1-P25-rGO; modified Kubelka–Munk transformation of b) the P25 and c) the Au1Cu1-P25-rGO; (d) Transient photocurrent response curves of the P25, Au1Cu1-P25, P25-rGO, Cu-P25-rGO, AuP25-rGO and Au1Cu1-P25-rGO.
the photocatalytic property and adsorptivity compared with pure P25. Au1Cu1-P25-rGO took 7 min, while to reach the same level, the Au1Cu1P25, P25-rGO and pure P25 took 30 min, 85 min and more than 120 min, respectively, to achieve 50% of photodecomposition. Interestingly, all of the AuCu-P25-rGO composites with different Au/Cu molar ratios (3/1, 1/1, 1/3) showed higher photocatalytic activity than monometallic Au-P25-rGO, indicating that bimetallic nanoparticles successfully exhibited the combination of two different metals. And most importantly, Au1Cu1-P25-rGO showed the best photocatalytic activity, with the photodegradation rate of 95.5% after 120 min. Therefore, it could be inferred that the noble-metal/nonnoble-metal ratio played a significant role on its catalytic performance, and the lower noble-metal/nonnoble-metal ratio might be beneficial to improve catalytic activity. Besides, the photodegradation capability of Au1Cu1-P25-rGO was extended to two other organic dyes (MB and RhB) under the same condition. As Fig. S4a shows, the photocatalyst exhibited well adsorptivity and photocatalytic performance for the photodegradation of MB, with the photodegradation rate of 50% after 10 min, and 93% after
P25 was successful. In addition, we noticed that the intensity of P25rGO was obviously lower than that of Au1Cu1-P25, meaning the more photo-excited electrons were trapped and transferred through the P25rGO, due to the outstanding electrical conductivity and electron mobility of rGO. Thus, we could conclude that incorporation of rGO and P25 could enhance the charge transport rate to suppress the recombination of charge carriers more than incorporation of AuCu alloy and P25. Nevertheless, the test result of Au1Cu1-P25-rGO still significantly better than P25-rGO and Au-P25-rGO, suggesting that the catalyst could further inhibit electron and hole recombination owing to the SPR effect and low work function of AuCu alloy with the presence of rGO, that is the advantage of ternary composites. As a typical toxic aromatic compound, 2-nitrophenol was selected as the model organic contaminant to evaluate the photocatalytic activity of the AuCu-P25-rGO nanocomposites under UV–vis light, compared with monometallic Au-P25-rGO, monometallic Cu-P25-rGO, Au1Cu1P25, P25-rGO and pure P25. From Fig. 7b (with error bars), it could be seen that modification with metallic nanoparticles (∼1 wt.% of P25) and (or) rGO (∼2 wt.% of P25) exhibited an obvious enhancement in
Fig. 7. (a) Photoluminscence (PL) spectra of P25, P25-rGO, Au1Cu1-P25, Au-P25-rGO, Cu-P25-rGO and Au1Cu1-P25-rGO; (b) Evolution of 2-NP concentration versus irradiation time by (1) P25, (2) P25-rGO, (3) AuCu-P25, (4) Cu-P25-rGO, (5) Au-P25rGO, (6) Au3Cu1-P25-rGO, (7) Au1Cu3-P25rGO and (8) Au1Cu1-P25-rGO.
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Fig. 8. (a) Reusability of Au1Cu1-P25-rGO; (b) The degradation curve of 2-NP in the presence of different scavengers using Au1Cu1-P25-rGO as photocatalyst.
formed between AuCu and P25, and electrons would transfer from the higher Fermi level to the lower one. However, the Fermi energy of AuCu was lower than that of P25, which would impede electrons across the boundary from AuCu to P25. Despite this, the electron still could transfer from AuCu nanoparticles to P25 due to the SPR excitation of AuCu nanoparticles. One possible explanation suggested that when AuCu nanoparticles were excited by light, the conduction electrons collectively oscillated with the electromagnetic field of the incident photons, might lead to an interband excitation and generate energetic electrons. The SPR excited energetic electrons could possess enough energy to overcome the Schottky barrier between AuCu and P25, and thus transfered to the P25 conduction band. Then, a large proportion of the conduction band electrons consisted of these energetic electrons and the electrons generated in P25 would transfer into rGO sheets because of the outstanding conductivity of rGO. These electrons reduced the dissolved oxygen absorbed on the photocatalysis surface, to form superoxide anion radicals (%O2−) to participate in photocatalytic reactions. On the other hand, the separated holes (h+) could also participate in photocatalytic reactions. Thus, all of the rGO surface, P25 and AuCu nanoparticles could function as active sites. In general, the SPR effect of AuCu bimetallic nanoparticles, outstanding conductivity of rGO, and polymorphs junction formed by mixture of anatase and rutile of P25 could efficiently suppressed the recombination of the photogenerated electron-hole pairs, to improve the photocatalytic activity of AuCu-P25-rGO ternary composites. Futhermore, rGO possessed well adsorptivity and affinity with the substrate, and the well-dispersed P25 deposited with AuCu nanoparticles that were attached to rGO sheets offered homogeneous active sites for substrate adsorption, could also enhance photocatalytic degradation activity effectively. Consequently, the AuCu-P25-rGO ternary composites exhibited an enhanced photodegradation activity, compared to pure P25, P25-rGO, AuCu-P25, CuP25-rGO and Au-P25-rGO.
50 min. Although RhB is stable in aqueous solutions [45], RhB still experienced obvious photodegradation in the presence of Au1Cu1-P25rGO, and the photodecomposition achieved 95% after 150 min (Fig. S4b). The effective photodegradation of MB and RhB could further demonstrate the high photoactivity and wide suitability of AuCu-P25rGO ternary nanocomposites. In order to evaluate the photocatalytic stability of the Au1Cu1-P25rGO which is an essential parameter for practical applications, the experiment for recycling photocatalyst by photodegradation 2-NP was shown in Fig. 8a. Before each run, the catalyst was collected by centrifugation or filtration from reaction mixture, then washed with deionized water followed by ethanol and dried in air. Each cycle test was repeated three times. For the first four cycles, there was no obvious deactivation for the reused catalyst, all of the degradation efficiency achieved above 90%. And in the fifth cycle test, the catalyst activity was significantly reduced, the degradation efficiency fell to 75%. Thus, the experimental results indicated that the Au1Cu1-P25-rGO ternary composites possessed good reusability in the first four cycles. To explore the photocatalytic degradation mechanism of AuCu-P25rGO composites, the trapping experiments of active species were carried out in the photocatalytic degradation of 2-NP. Benzoquinone (BQ), tert-butyl alcohol (TBA), and ammonium oxalate (AO) were used as scavengers to trap superoxide radical (%O2−), hydroxyl radical (%OH) and holes (h+), respectively [46,47]. As Fig. 8b shows, the photocatalytic degradation of 2-NP was almost not affected by the addition of TBA, with just a slight decrease in degradation from 95.5% to 90% after 120 min. On the contrary, the addition of BQ and AO caused significant decrease. Therefore, we could infer that %O2− and h+ were the main active species rather than ·OH in 2-NP photodegradation process. In order to explain this enhanced photocatalytic activity, according to the reaction mechanisms proposed in the literature [14,48,49] and combining with the results in this work, the proposed photocatalytic mechanism of the AuCu-P25-rGO ternary composites was inferred in Fig. 9. For AuCu-P25 system, when metal (AuCu) and semiconductor (P25) combined with different Fermi level, a Schottky barrier would be
4. Conclusion In conclusion, we had successfully prepared the bimetallic AuCuP25-rGO ternary composites, which possessed a fine light response and exhibited higher photodegradation performance for 2-NP, compared to pure P25, P25-rGO, AuCu-P25, Cu-P25-rGO and Au-P25-rGO. The molar ratio of Au:Cu exhibited significant influence on the photocatalytic property, and AuCu-P25-rGO with Au/Cu ratio = 1:1 showed the highest activity. Meanwhile, the Au1Cu1-P25-rGO also exhibited good photodegradation performance for MB and RhB. Futhermore, it had been demonstrated that h+ and %O2− were the main active species rather than ·OH during the photodegradation process. Finally, the proposed mechanism of the photodegradation process of AuCu-P25rGO ternary composites was inferred according to the aforementioned results and combining with the reported mechanisms. Owing to the SPR effect of AuCu bimetallic nanoparticles; outstanding conductivity, affinity with the substrate and well adsorptivity of rGO; the polymorphs
Fig. 9. Proposed mechanism for the photodegradation process of AuCu-P25-rGO ternary composites for 2-NP.
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Applied Catalysis A, General journal homepage: www.elsevier.com/locate/apcata
Research Paper
Preparation of a novel bimetallic AuCu-P25-rGO ternary nanocomposite with enhanced photocatalytic degradation performance ⁎
He Wena,b, Yu Longa,b, , Wei Hanc, Wei Wua,b, Yuanyuan Yanga,b, Jiantai Maa,b, a b c
MARK
⁎
State Key Laboratory of Applied Organic Chemistry (SKLAOC), Lanzhou University, Lanzhou, 730000, PR China Gansu Provincial Engineering Laboratory for Chemical Catalysis, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou, 730000, PR China Lanzhou Petrochemical Research Center, PetroChina, Lanzhou, 730060, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Photodegradation AuCu bimetallic nanoparticles Surface plasmon resonance effect Ternary photocatalyst Photocatalytic mechanism
In this work, a novel bimetallic ternary composite (AuCu-P25-rGO) had been designed and prepared, and been used as a high-performance photocatalyst. The morphology, chemical composition, physical structure and photoelectric property were confirmed by XRD, TEM, BET analysis, FT-IR, Raman, XPS, UV-vis DRS, Photoluminscence spectra and transient photocurrent responses techniques. The photocatalysts possessed higher catalytic activity than P25, P25-rGO, AuCu-P25, Cu-P25-rGO, Au-P25-rGO for photodegradation of organic pollutant (2-nitrophenol) under UV–vis light. Importantly, the molar ratio of Au:Cu exhibited significant influence on the photocatalytic property, and AuCu-P25-rGO with Au/Cu molar ratio = 1:1 showed higher activity than these with Au/Cu molar ratio = 1:3 and 3:1. In addition, Au1Cu1-P25-rGO could excellently photodegradation of organic dyes (Rhodamine B and methylene blue). The enhanced photocatalytic performance of AuCu-P25-rGO composites can be attributed to three reasons as follows: (1) surface plasmon resonance (SPR) effect of AuCu bimetallic nanoparticles; (2) outstanding conductivity and adsorptivity of rGO; (3) the polymorphs junction formed by mixture of anatase and rutile of P25.
1. Introduction The word photocatalysis had been designed in the 1970s, researchers from the University of Tokyo reported the photocatalytic ability of TiO2, which had opened ways towards using semiconductor photocatalysts for purification of water and air [1]. Particularly, TiO2 has been considered as one of the best and most potential photocatalysts, due largely to its high photocatalytic activity, relative nontoxicity, long-term thermodynamic stability, and low cost [2,3]. However, the fast recombination of photogenerated electron-hole pairs caused huge loss photocatalytic efficiency of TiO2. On the other side, because of its wide band-gap (3.2 eV), TiO2 can only be excited by ultraviolet, which occupies only 4–5% of the solar light spectrum. The above two points become the principal elements of limiting its practical applications [4]. In the past few years, various techniques have been attempted to modify the TiO2 photocatalysts to solve these problems. A popular approach for enhancing photocatalytic performance to employ the Degussa P25 TiO2 (P25). P25 is a commercial TiO2 nanopowder consisting of 80% anatase and 20% rutile. Their mixtures produce novel electronic states at two polymorphs junctions that can suppress the recombination of photogenerated electron-hole pairs and facilitate
⁎
charge transfer [5]. Besides, incorporation of electron-accepting materials like graphene [6] and loading noble metal particles [7] are also two key methods to enhance the photocatalytic performance of TiO2. Graphene, a sp2-hybridized carbon atoms with a two-dimensional (2D) benzene-ring packed lattice structure [8], has attracted worldwide interest because of its outstanding electrical conductivity [9], electron mobility [8], optical transparency [10], thermal conductivity [11], and large theoretical surface area [12]. However, the current approaches to prepare graphene face the challenges of high cost and low yield. Therefore, reduced graphene oxide (rGO) with low cost and high output have been drawing increased attention. The incorporation of rGO and TiO2 can enhance the charge transport rate to suppress the recombination of charge carriers and enhance photocatalytic activity, due to the formation of TieOeC chemical bond narrowing the band gap of TiO2 [13] and the good conductivity of rGO. In addition, many researchers have demonstrated that the lack of visible light response could be improved by decorating metallic nanoparticles such as Au, Ag, Pt, Pb [14–17] etc. Recently, bimetallic alloy nanoparticles have been employed as a new attractive advanced material, to improve structure and electronic properties due to its synergistic effects [18,19]. Among them, alloying Cu with a noble metal
Corresponding authors at: State Key Laboratory of Applied Organic Chemistry (SKLAOC), Lanzhou University, Lanzhou 730000, PR China. E-mail addresses: [email protected] (Y. Long), [email protected] (J. Ma).
http://dx.doi.org/10.1016/j.apcata.2017.09.028 Received 30 July 2017; Received in revised form 20 September 2017; Accepted 24 September 2017 Available online 14 October 2017 0926-860X/ © 2017 Elsevier B.V. All rights reserved.
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2.1.2. Synthesis of AuCu-P25-rGO 50 mg of the prepared AuCu-P25 and 1 mg GO were dispersed in a mixed solution of distilled H2O (40 mL) and ethanol (20 mL) by ultrasonic treatment, then stirred for 1 h and the solution was transferred to a 100 mL Teflon-lined stainless steel autoclave, followed by a hydrothermal treatment at 120 °C for 2 h. The resultant AuCu-P25-rGO was centrifuged and dried at 50 °C. The bimetallic photocatalysts with different Au/Cu molar ratios (3/1, 1/1, 1/3) were synthesized by the same method and the ratios of Au/Cu and AuCu/Ti in as-synthesized samples are shown in Table S1.
has attracted tremendous research interests, such as AuCu alloy. Both Au and Cu nanoparticles have surface plasmon resonance (SPR) effect, they can absorb and utilize the visible light to promote photocatalytic reactions [20]. Besides, AuCu bimetallic nanoparticles have a low work function because of its unique optical and electronic abilities of Cu nanostructures [21]. It has been reported that AuCu bimetallic nanoparticles exhibited higher catalytic performance than their individual components for CO oxidation, propene epoxidation and benzyl alcohol oxidation [22]. However, only a few studies reported the photodegradation properties of AuCu nanoparticles. Herein, the bimetallic ternary composites (AuCu-P25-rGO) were successfully synthesized and employed for photocatalytic degradation of 2-nitrophenol (2-NP), methylene blue (MB) and Rhodamine B (RhB) which endanger the human health and aquatic ecosystems. So far our knowledge goes, less study has been carried for using bimetallic ternary composites as efficient photocatalysts for degradation of harmful phenolic compounds and other organic dyes. In addition, in the reported ternary composites, TiO2 were mostly in a single phase, principally anatase. Herein, we used P25 consisting of anatase and rutile as a high performance active ingredient.
2.2. Photocatalysts experiments The photocatalytic activity of AuCu-P25-rGO was evaluated by the photodegradation of 2-NP (0.5 mM, pH = 6.3), MB (10−5 mol L−1, pH = 6.8) and RhB (10−5 mol L−1, pH = 6.5) in a thermostatic photoreactor. Before irradiation, the suspension containing 0.3 g/L of the solid of AuCu-P25-rGO was sonicated and magnetically stirred for 30 min in the dark to achieve the adsorption and desorption equilibrium. Then, the mixture was irradiated with a 300 W halogen lamp (the distribution of wavelength was shown in Fig. S1). After a certain time interval, the suspension was taken and centrifuged to separate the catalyst to get supernatant liquid, the liquid was monitored by a UV–vis spectrophotometer, and the absorption peak of 2-NP, MB, RhB were 274 nm and 346 nm, 664 nm, 553 nm, respectively. In a similar way, AuCu-P25-rGO with different Au/Cu ratio, AuCu-P25, P25-rGO and P25 were used in the degradation of 2-NP.
2. Experimental 2.1. Photocatalysts preparation The synthesis routes of AuCu-P25-rGO are shown in Scheme 1. 2.1.1. Synthesis of AuCu-P25 Bimetallic Au1Cu3-P25, Au1Cu1-P25 and Au3Cu1-P25 photocatalysts with a total metal loading of 1 wt.% on P25 were synthesized by adopting Duff and Baiker Method [23]. In a 100 mL three-neck flask, 500 mg of P25 was dispersed in deionized water. During vigorous stirring at room temperature, 6 mL 0.2 M of NaOH and 4 mL 0.05 M of (hydroxymethyl) phosphonium chloride (THPC) were added into the mixed solution, 5 min was allowed between the addition of THPC and NaOH. Afterwards, HAuCl4 solution (0.01 M) and CuCl2 solution (0.1 M) were added to the mixture. Then the mixture maintained under vigorous stirring for 2 h at the room temperature. The as-prepared products were separated by centrifugation, and dried at 50 °C for 12 h. Finally, the solid was calcined at 550 °C in air for 6 h, and reduced at 550 °C in pure H2 for 1 h, then cooled down to room temperature under N2 flow to obtain the catalysts. The bimetallic photocatalysts with different Au/Cu molar ratios (3/1, 1/1, 1/3) and monometallic Au- and Cu-P25 were synthesized by the same method which described above, the only difference was the different addition volume of HAuCl4 solution (0.01 M) and CuCl2 solution (0.1 M).
2.3. Photoelectrochemical measurement The measurement was carried out in the electrochemical workstation (CHI 660E, shanghai chenhua) in Na2SO4 (0.5 M) solution containing 2-NP (0.5 mM) using a standard three-electrode system: Ag/ AgCl as the reference electrode, a Pt wire as the counter electrode and ITO glass (1 cm × 1 cm) deposited with the suspension of photocatalysts (1.0 g L−1) as the working electrode. The 300 W xenon lamp was used as the light source for photoelectrochemical measurement. 3. Result and discussion The crystalline structures of samples were acquired by XRD. Fig. 1 shows the XRD patterns of GO, P25, Au1Cu1-P25, P25-rGO and Au1Cu1P25-rGO. Except GO, all of the P25 nanocomposites (P25, Au1Cu1-P25, P25-rGO and Au1Cu1-P25-rGO) exhibited the similar XRD patterns, the diffraction peaks of anatase [JCPDS no. 21-1272] and rutile [JCPDS no. 77-0443] phases of P25 could be clearly observed. The diffraction peaks Scheme 1. The synthesis routes of AuCu-P25-rGO.
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measured interplanar spacing could demonstrate the formation of AuCu alloy nanoparticles. Meanwhile, HAADF-STEM was carried out to give powerful evidence about the good dispersity of AuCu alloy nanoparticles and morphology of nanostructure. Fig. 3c–g display the EDX mapping for the respective elements, Au and Cu maps were almost overlapped, suggesting that AuCu alloy nanoparticles were formed for the as-prepared AuCu-P25-rGO. And Ti, O and C were certainly existing. The presence of the elemental composition could be further confirmed with EDX. As displayed in Fig. S2, the X-ray peaks of Au, Cu, Ti, O and C met the element analysis of Au1Cu1-P25-rGO. Ni was generally influenced by the nickel network support films. Table 1 shows the physicochemical properties of P25 and Au1Cu1P25-rGO. It indicates that the pore size and pore volume obviously increased compared with pure P25. This might due to the incorporation of rGO. However, the specific surface area of Au1Cu1-P25-rGO decreased, which might due to the low loading amount of rGO (2 wt.%) and rGO covered some of the mesopores on P25. Hence, from the perspective of BET analysis, the relatively large pore size of Au1Cu1P25-rGO would favor the transportation of large molecular which involved in photocatalytic reaction. However, the contribution of specific surface area to the photocatalytic reaction was almost negligible. Fig. S3 shows the Nitrogen adsorption–desorption isotherms and corresponding pore size distribution curves of Au1Cu1-P25-rGO. The adsorption–desorption isotherms at high P/Po of Au1Cu1-P25-rGO were characteristic of a Type H3 loop, and the shape of isotherms indicated the presence of mesoporous and microporous [30]. The pore size distribution was calculated by BJH method from desorption branch of Nitrogen adsorption–desorption isotherms. And the pore size distribution exhibited wide range from 2 nm to 100 nm. This wide range might be ascribed to the crevice between hybrid interface [31]. Fig. 4a displays the Raman spectra of GO, P25, P25-rGO and Au1Cu1-P25-rGO. For the P25, P25-rGO and Au1Cu1-P25-rGO samples, the bands at 142, 396, 513 and 636 cm−1 were corresponded to the Eg(1), B1g, A1g and Eg(2) of anatase, respectively [32]. Two typical Raman bands could be observed in samples of GO, P25-rGO and Au1Cu1-P25-rGO. One band was D-band (∼1340 cm−1), corresponding to the existence of structural defects and disorder. The other band was G-band (∼1603 cm−1), which was the stretching of the sp2-hybridized CeC bonds. Thus, the D/G band intensity ratio (ID/IG) could reflect the structural changes from GO to rGO. The ID/IG ratio of P25-rGO (0.96) and Au1Cu1-P25-rGO (1.04) were higher than that of GO (0.88), which indicated the successful reduction of GO during the solvothermal experiment. And due to the introduction of P25 nanoparticles on the rGO, Au1Cu1-P25-rGO showed the highest ID/IG ratio. FTIR analysis was carried out to illustrate the organic group for the Au1Cu1-P25-rGO composite (Fig. 4b). From the GO spectrum, the broad peaks at 3420 cm−1 and 1216 cm−1 were ascribed to the hydroxyl groups stretching vibrations. Peaks at 1380 cm−1 and 1720 cm−1 were corresponded to the C]O groups on the surface of GO. Moreover, the
Fig. 1. XRD patterns of the GO, Au1Cu1-P25-rGO, P25-rGO, Au1Cu1-P25 and P25.
at 2Θ values of 25.3, 37.6, 47.9, 53.6, 55.0, 62.4, 68.4, 70.1 and 75.0° could be assigned to the crystal phases (101), (004), (200), (105), (211), (204), (116), (220) and (215) of anatase in P25, respectively. Moreover, the peaks at 2Θ values of 27.4, 36.0, 41.1 and 56.4° were corresponded to the rutile (110), (101), (111) and (220) planes. No significant characteristic peaks of AuCu alloy could be observed for Au1Cu1-P25 and Au1Cu1-P25-rGO, due to its low loading content (1 wt. %) [24,25]. Moreover, no characteristic peak for graphene at 2Θ value of 25° could be observed, which was presumably due to the low amount and the overlap of strong (101) diffraction peak of anatase TiO2 at 2Θ value of 25.3° [26,27]. Nevertheless, there was no significant characteristic peak at 2Θ value of 10.2° of GO could be observed in the XRD patterns of P25-rGO and Au1Cu1-P25-rGO, which conformed the completely reduction of GO to rGO through one-step hydrothermal process. In order to further investigate the morphology, microstructure and composition of photocatalysts, transmission electron microscopy (TEM), high resolution TEM (HRTEM) and energy dispersive X-ray analysis (EDX) were carried out. As the TEM photo shown in Fig. 2a, P25 with spherical AuCu alloy nanoparticles were successfully deposited on the surface of rGO. The obtained rGO still retained ultrathin 2D sheet structure with wrinkles after the hydrothermal treatment. Owing to the distribution of oxygen-containing functional groups on GO, the P25 nanoparticles were eager to deposit along the wrinkles and edge [28]. Fig. 2b and c are the HRTEM images of the Au1Cu1-P25-rGO. As shown in Fig. 2b, the interplanar spacing of 0.35 nm and 0.33 nm were corresponded to the (101) and (110) planes of anatase and rutile, respectively, indicating the presence of both anatase and rutile crystalline formed in P25. Fig. 2c shows the microstructure detail of AuCu bimetallic nanoparticles, fringe lines with 0.226 nm spacing could be attributed to (111) crystal planes of AuCu alloy [29], lying between the lattice spacing of monometallic Au (0.235 nm) and Cu (0.208 nm). The
Fig. 2. (a) TEM and (b, c) HRTEM images of the Au1Cu1-P25-rGO.
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Fig. 3. STEM of Au1Cu1-P25-rGO (a and b); EDX mapping of Au1Cu1-P25-rGO (c-g): mapping of C (c), mapping of O (d), mapping of Ti (e), mapping of Au (f), and mapping of Cu (g).
containing species could further prove that GO was reduced to rGO after solvothermal process. In Fig. 5c, two main peaks at 458.4 and 464.2 eV were attributed to the Ti 2p3/2 and Ti 2p1/2 in Ti 2p XPS spectra, which explained the Ti4+ chemical state. Besides, two weak peaks at 460.2 and 466.3 eV, might be due to a small amount formation of Ti5+ [36]. Fig. 5d exhibits two obvious peaks located at 529.7 eV and 530.9 eV, indicating the presence of TieOeTi and TieOeC chemical bonds, which conformed to the result of FT-IR spectra. The high-resolution XPS spectra also studied the chemical state of Au in the Au1Cu1-P25-rGO composite (Fig. 5e). The peaks at 83.8 and 87.5 eV were in agreement with Au0 chemical state. The others at 84.3 and 88.3 eV with broad band and weak intensity were ascribable to the surface oxidized gold (Auδ+) [37]. Then, for Cu binding energy (Fig. 5f), the peaks at 932.3, 932.7 and 934.1 eV could be distinguished in Cu 2p3/2, corresponding to Cu2O, Cu0 and CuO, respectively [38]. The existence of Cu2+ could be further confirmed by the broad satellite peak at 943 eV [29]. Besides, the peaks at 952.5 and 954.0 eV were ascribed to Cu2O and CuO. The optical properties of P25, Au1Cu1-P25, P25-rGO, Au-P25-rGO, Cu-P25-rGO, and Au1Cu1-P25-rGO were studied by diffuse reflectance spectroscopy (DRS) (Fig. 6a). All the samples of photocatalysts revealed the typical absorption with a distinct transition in the spectra, which was corresponded to the band gap absorption of P25 [39]. In addition, a red shift could be clearly observed for Au1Cu1-P25, P25-rGO, Au-P25rGO, Cu-P25-rGO and Au1Cu1-P25-rGO, indicating the introduction of rGO or (and) AuCu bimetallic nanoparticles could narrow the band gap energy of photocatalysts. For P25-rGO, the absorption was higher than
Table 1 Physicochemical properties of P25 and Au1Cu1-P25-rGO. Samples
Pore size (nm)
Pore volume (cm3 g−1)
Surface area m2 g−1
a
7.0 25.6
0.10 0.30
48.2 46.4
P25 Au1Cu1-P25-rGO a
Ref. [25].
1050 cm−1 band was for the CeO stretching vibrations [33]. These oxygen-containing functional groups built the covalent connection between TiO2 and GO. Notably, the intensity of CH2 groups (∼2855 and 2930 cm−1) increased and the intensity of oxygen-containing functional groups decreased in P25-rGO and Au1Cu1-P25-rGO compared with GO, which revealed the reduction of GO. Besides, the absorption peak at 1600 cm−1 was ascribed to the skeletal vibration of GO and rGO. For P25-rGO and Au1Cu1-P25-rGO, the wide and broad peaks at low frequency (below 1000 cm−1) were attributed to the bending and stretching vibration of TieOeTi and TieOeC bonds [34], indicating the successful combination of P25 and rGO. In order to investigate the chemical state of different elements, the XPS core level spectra was collected. Fig. 5a shows a XPS survey spectrum of the Au1Cu1-P25-rGO sample, indicating that the sample contained C, Ti, O, Au and Cu. In C 1 s core level XPS spectrum (Fig. 5b), the main peak at 284.6 eV was attributed to sp2 or sp3 carbon species from rGO, while the other peaks at 286.2 eV and 288.6 eV were assigned to CeO and C]O [35]. The weak intensity of oxygen-
Fig. 4. (a) Raman spectras of GO, P25, P25rGO, and Au1Cu1-P25-rGO; (b) FTIR spectras of GO, P25-rGO and Au1Cu1-P25-rGO.
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Fig. 5. (a) Survey XPS spectrum of Au1Cu1P25-rGO; Core level XPS spectra of (b) C 1s, (c) Ti 2p, (d) O 1s, (e) Au 4f and (f) Cu 2p of Au1Cu1-P25-rGO.
transient photocurrent responses, under the irradiation of 300 W xenon lamp with applied voltage 0.6 V. As Fig. 6d showed, all the samples exhibited obvious changes under the visible light on and off. Besides, the on–off cyclic curves had high reproducibilities, which indicated that a large proportion of photoelectrons were transferred to the cathode to produce photocurrent [44]. The photocurrents of P25, P25-rGO, Au1Cu1-P25, Cu-P25-rGO, Au-P25-rGO and Au1Cu1-P25-rGO were 1.8, 2.3, 4.2, 3.0, 8.3 and 10.5 μA, respectively. Obviously, the Au1Cu1-P25rGO ternary composite showed the highest transient photocurrent, which was 5 times more than pure P25, 2.5 times higher than Au1Cu1P25 and 1.3 times higher than Au-P25-rGO. In addition, the photocurrents of P25-rGO increased 30% compared to P25, but the photocurrents of Au1Cu1-P25 increased 130%, which revealed that the SPR effect and lower work function of AuCu bimetallic nanoparticles could enhance the photocatalytic activity. The rate of electron-hole recombination was characterized by measuring photoluminscence (PL) spectra of P25, P25-rGO, Au1Cu1P25, Au-P25-rGO, Cu-P25-rGO and Au1Cu1-P25-rGO. The lower intensity of the sample represents a correspondingly lower recombination rate. In Fig. 7a, pure P25 exhibited the highest intensity and Au1Cu1P25-rGO showed the lowest one, indicating that our modification of
pure P25 in the 400–800 nm region, because the formation of TieOeC bonds narrowed the band gap energy of P25 [40]. Furthermore, the more enhanced absorption of Au1Cu1-P25 and Au1Cu1-P25-rGO were attributed to the presence of AuCu bimetallic nanoparticles, with a broad absorption peak from 500 nm to 700 nm, which was ascribed to the SPR effect of the AuCu bimetallic nanoparticles [41,42]. Moreover, the broad absorption peak from 500 nm to 600 nm of Au-P25-rGO could also be ascribed to the SPR effect of Au nanoparticles, nevertheless Au1Cu1-P25-rGO still possessed higher visible light utilization and more obvious red shift. Therefore, Au1Cu1-P25-rGO nanocomposite showed not only a red shift of the absorption edge but also a further increased absorption in the 400–800 nm range. To acquire more information about the band gap of Au1Cu1-P25rGO and P25, the Kubelka–Munk transformation was utilized [43]. In Fig. 6b, the band gap of P25 was 3.04 eV, indicating its low photoactivity due to the wide band gap. Nevertheless, the band gap of Au1Cu1-P25-rGO reduced to 2.87 eV caused a red shift in the spectra (Fig. 6c). The results indicated that the incorporation of rGO and AuCu bimetallic nanoparticles could enhance the absorption of photocatalysts in the visible light range. Photoelectrochemical properties were characterized by measuring 241
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Fig. 6. (a) UV–vis DRS spectra of the P25, Au1Cu1-P25, P25-rGO, Cu-P25-rGO, AuP25-rGO and Au1Cu1-P25-rGO; modified Kubelka–Munk transformation of b) the P25 and c) the Au1Cu1-P25-rGO; (d) Transient photocurrent response curves of the P25, Au1Cu1-P25, P25-rGO, Cu-P25-rGO, AuP25-rGO and Au1Cu1-P25-rGO.
the photocatalytic property and adsorptivity compared with pure P25. Au1Cu1-P25-rGO took 7 min, while to reach the same level, the Au1Cu1P25, P25-rGO and pure P25 took 30 min, 85 min and more than 120 min, respectively, to achieve 50% of photodecomposition. Interestingly, all of the AuCu-P25-rGO composites with different Au/Cu molar ratios (3/1, 1/1, 1/3) showed higher photocatalytic activity than monometallic Au-P25-rGO, indicating that bimetallic nanoparticles successfully exhibited the combination of two different metals. And most importantly, Au1Cu1-P25-rGO showed the best photocatalytic activity, with the photodegradation rate of 95.5% after 120 min. Therefore, it could be inferred that the noble-metal/nonnoble-metal ratio played a significant role on its catalytic performance, and the lower noble-metal/nonnoble-metal ratio might be beneficial to improve catalytic activity. Besides, the photodegradation capability of Au1Cu1-P25-rGO was extended to two other organic dyes (MB and RhB) under the same condition. As Fig. S4a shows, the photocatalyst exhibited well adsorptivity and photocatalytic performance for the photodegradation of MB, with the photodegradation rate of 50% after 10 min, and 93% after
P25 was successful. In addition, we noticed that the intensity of P25rGO was obviously lower than that of Au1Cu1-P25, meaning the more photo-excited electrons were trapped and transferred through the P25rGO, due to the outstanding electrical conductivity and electron mobility of rGO. Thus, we could conclude that incorporation of rGO and P25 could enhance the charge transport rate to suppress the recombination of charge carriers more than incorporation of AuCu alloy and P25. Nevertheless, the test result of Au1Cu1-P25-rGO still significantly better than P25-rGO and Au-P25-rGO, suggesting that the catalyst could further inhibit electron and hole recombination owing to the SPR effect and low work function of AuCu alloy with the presence of rGO, that is the advantage of ternary composites. As a typical toxic aromatic compound, 2-nitrophenol was selected as the model organic contaminant to evaluate the photocatalytic activity of the AuCu-P25-rGO nanocomposites under UV–vis light, compared with monometallic Au-P25-rGO, monometallic Cu-P25-rGO, Au1Cu1P25, P25-rGO and pure P25. From Fig. 7b (with error bars), it could be seen that modification with metallic nanoparticles (∼1 wt.% of P25) and (or) rGO (∼2 wt.% of P25) exhibited an obvious enhancement in
Fig. 7. (a) Photoluminscence (PL) spectra of P25, P25-rGO, Au1Cu1-P25, Au-P25-rGO, Cu-P25-rGO and Au1Cu1-P25-rGO; (b) Evolution of 2-NP concentration versus irradiation time by (1) P25, (2) P25-rGO, (3) AuCu-P25, (4) Cu-P25-rGO, (5) Au-P25rGO, (6) Au3Cu1-P25-rGO, (7) Au1Cu3-P25rGO and (8) Au1Cu1-P25-rGO.
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Fig. 8. (a) Reusability of Au1Cu1-P25-rGO; (b) The degradation curve of 2-NP in the presence of different scavengers using Au1Cu1-P25-rGO as photocatalyst.
formed between AuCu and P25, and electrons would transfer from the higher Fermi level to the lower one. However, the Fermi energy of AuCu was lower than that of P25, which would impede electrons across the boundary from AuCu to P25. Despite this, the electron still could transfer from AuCu nanoparticles to P25 due to the SPR excitation of AuCu nanoparticles. One possible explanation suggested that when AuCu nanoparticles were excited by light, the conduction electrons collectively oscillated with the electromagnetic field of the incident photons, might lead to an interband excitation and generate energetic electrons. The SPR excited energetic electrons could possess enough energy to overcome the Schottky barrier between AuCu and P25, and thus transfered to the P25 conduction band. Then, a large proportion of the conduction band electrons consisted of these energetic electrons and the electrons generated in P25 would transfer into rGO sheets because of the outstanding conductivity of rGO. These electrons reduced the dissolved oxygen absorbed on the photocatalysis surface, to form superoxide anion radicals (%O2−) to participate in photocatalytic reactions. On the other hand, the separated holes (h+) could also participate in photocatalytic reactions. Thus, all of the rGO surface, P25 and AuCu nanoparticles could function as active sites. In general, the SPR effect of AuCu bimetallic nanoparticles, outstanding conductivity of rGO, and polymorphs junction formed by mixture of anatase and rutile of P25 could efficiently suppressed the recombination of the photogenerated electron-hole pairs, to improve the photocatalytic activity of AuCu-P25-rGO ternary composites. Futhermore, rGO possessed well adsorptivity and affinity with the substrate, and the well-dispersed P25 deposited with AuCu nanoparticles that were attached to rGO sheets offered homogeneous active sites for substrate adsorption, could also enhance photocatalytic degradation activity effectively. Consequently, the AuCu-P25-rGO ternary composites exhibited an enhanced photodegradation activity, compared to pure P25, P25-rGO, AuCu-P25, CuP25-rGO and Au-P25-rGO.
50 min. Although RhB is stable in aqueous solutions [45], RhB still experienced obvious photodegradation in the presence of Au1Cu1-P25rGO, and the photodecomposition achieved 95% after 150 min (Fig. S4b). The effective photodegradation of MB and RhB could further demonstrate the high photoactivity and wide suitability of AuCu-P25rGO ternary nanocomposites. In order to evaluate the photocatalytic stability of the Au1Cu1-P25rGO which is an essential parameter for practical applications, the experiment for recycling photocatalyst by photodegradation 2-NP was shown in Fig. 8a. Before each run, the catalyst was collected by centrifugation or filtration from reaction mixture, then washed with deionized water followed by ethanol and dried in air. Each cycle test was repeated three times. For the first four cycles, there was no obvious deactivation for the reused catalyst, all of the degradation efficiency achieved above 90%. And in the fifth cycle test, the catalyst activity was significantly reduced, the degradation efficiency fell to 75%. Thus, the experimental results indicated that the Au1Cu1-P25-rGO ternary composites possessed good reusability in the first four cycles. To explore the photocatalytic degradation mechanism of AuCu-P25rGO composites, the trapping experiments of active species were carried out in the photocatalytic degradation of 2-NP. Benzoquinone (BQ), tert-butyl alcohol (TBA), and ammonium oxalate (AO) were used as scavengers to trap superoxide radical (%O2−), hydroxyl radical (%OH) and holes (h+), respectively [46,47]. As Fig. 8b shows, the photocatalytic degradation of 2-NP was almost not affected by the addition of TBA, with just a slight decrease in degradation from 95.5% to 90% after 120 min. On the contrary, the addition of BQ and AO caused significant decrease. Therefore, we could infer that %O2− and h+ were the main active species rather than ·OH in 2-NP photodegradation process. In order to explain this enhanced photocatalytic activity, according to the reaction mechanisms proposed in the literature [14,48,49] and combining with the results in this work, the proposed photocatalytic mechanism of the AuCu-P25-rGO ternary composites was inferred in Fig. 9. For AuCu-P25 system, when metal (AuCu) and semiconductor (P25) combined with different Fermi level, a Schottky barrier would be
4. Conclusion In conclusion, we had successfully prepared the bimetallic AuCuP25-rGO ternary composites, which possessed a fine light response and exhibited higher photodegradation performance for 2-NP, compared to pure P25, P25-rGO, AuCu-P25, Cu-P25-rGO and Au-P25-rGO. The molar ratio of Au:Cu exhibited significant influence on the photocatalytic property, and AuCu-P25-rGO with Au/Cu ratio = 1:1 showed the highest activity. Meanwhile, the Au1Cu1-P25-rGO also exhibited good photodegradation performance for MB and RhB. Futhermore, it had been demonstrated that h+ and %O2− were the main active species rather than ·OH during the photodegradation process. Finally, the proposed mechanism of the photodegradation process of AuCu-P25rGO ternary composites was inferred according to the aforementioned results and combining with the reported mechanisms. Owing to the SPR effect of AuCu bimetallic nanoparticles; outstanding conductivity, affinity with the substrate and well adsorptivity of rGO; the polymorphs
Fig. 9. Proposed mechanism for the photodegradation process of AuCu-P25-rGO ternary composites for 2-NP.
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