A Cu-Au bimetallic co-catalysis for the improved

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($21 nm) and Au ($25 nm) increased after the formation of core-shell ... change in color were observed with increasing the amount of Au and Cu, respectively.

Solar Energy 155 (2017) 1403–1410

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A Cu-Au bimetallic co-catalysis for the improved photocatalytic activity of TiO2 under visible light radiation Anila Monga, Aadil Bathla, Bonamali Pal ⇑ School of Chemistry and Biochemistry, Thapar University, Patiala 147004, Punjab, India

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Article history: Received 2 May 2017 Received in revised form 26 July 2017 Accepted 31 July 2017

Keywords: Au-Cu bimetallic co-catalyst [email protected]/TiO2 nanocomposite Co-Catalytic activity Visible light photo-catalysis

a b s t r a c t This study represents the co-catalytic activity of mono/bimetallic loaded titanium dioxide (TiO2) nanocomposite of copper (Cu) & gold (Au) for the photo-reduction of aromatic nitro compounds and the photo-oxidation of salicylic acid. It was observed that the average hydrodynamic size of Cu (21 nm) and Au (25 nm) increased after the formation of core-shell structure (36 nm and38 nm for [email protected] and [email protected] respectively). The morphological studies revealed the uniform coating of Cushell of thickness2.1 nm over Au core. Moreover, in optical analysis a considerable blue-shift in the absorption band of [email protected] (600 nm-559 nm) and a red-shift in [email protected] (528–580 nm) with significant change in color were observed with increasing the amount of Au and Cu, respectively. Under visible light irradiation these bimetallic/TiO2 nanocomposite showed higher activity for the reduction of 3nitroacetophenone and 1-chloro-3-nitrobenzene and oxidation of salicylic acid comparative to their monometallic counter parts with a higher rate constant k = 0.97  10 2 min 1 ([email protected]/TiO2), k = 0.65  10 2 min 1 ([email protected]/TiO2) relative to monometallic (k = 0.34  10 2 min 1 (Cu/TiO2) and k = 0.29  10 2 min 1)) for the oxidation of salicylic acid. The higher activity of bimetallic/TiO2 nanocomposite is ascribed to the decrease in work function (4.2 for [email protected]/TiO2 & 4.6 for [email protected]/TiO2) resulting in the proficient transfer of electrons at bimetallic-TiO2 interface. Ó 2017 Elsevier Ltd. All rights reserved.

1. Introduction Bimetallic core-shell nanostructure of two different metals attracts a lot of interest in comparison to their respective monometallic nanoparticles (NPs) because of their strong catalytic, electronic and fascinating optical property (Ma et al., 2010; Pande et al., 2007; Wang and Li, 2011). Since the catalytic performance is highly surface dependent (Kahraman et al., 2009; Yang et al., 2008), the addition of second metal on the surface of another metal alter the interface property and charge transfer process (Sangpour et al., 2010) due to the bifunctional effect (Zhou et al., 2013) (or synergistic effect) resulting in the higher selectivity and yield of a catalytic reaction. Moreover, the bimetallic nanocomposites (NCs) of noble metals (Au, Ag, Cu) (Chimentao et al., 2007; Gerber et al., 2004; Hai et al., 2014; Mancheño-Corvo and Martín-Duque, 2006; Zielin´ska-Jurek et al., 2011) have been broadly investigate because of another brilliant property possessed by these metals known as localized surface plasmon resonance (LSPR) i.e. the coherent oscillation with concurrent visible region (Gerber et al., 2004; Methadone et al., 2010). It has been reported ⇑ Corresponding author. E-mail address: [email protected] (B. Pal). http://dx.doi.org/10.1016/j.solener.2017.07.084 0038-092X/Ó 2017 Elsevier Ltd. All rights reserved.

that bimetallic core-shell of Au/Ag exhibit higher activity for glucose oxidation (Rasmussen et al., 2005) and for reduction of pnitrophenol(Dong and Zhang, 2014) relative to monometallic Au and Ag. Wang et al. (Wang and Li, 2011) reported the [email protected] core-shell nanostructure enhance the electrocatalytic activity for methanol oxidation compared to conventional monometallic Pt. The various physicochemical and catalytic properties of bimetallic nanocomposite can also be altered as a function of their size, shape and thickness of core and shell (Glaus et al., 2002; Jayakumar et al., 2010; Ma et al., 2010; Shankar et al., 2004). A recent report (Monga and Pal, 2015) depicted that the Au nanorodcore-Agshell exhibit higher selectivity for the reduction of 1,3-dinitrobenzene to 3nitroaniline relative to Au nanospherecore-Agshell and their respective monometallic counterparts. Similarly Ma and coworkers (Ma et al., 2010) studied the changes occur in optical properties of [email protected] nanocubes by varying the Ag shell thickness from 1.2 nm to 20 nm. Furthermore, these metal nanoparticles (NPs) show a good co-catalytic behavior when supported on certain metal oxide. Previous reports have suggested that the impregnation of these monometallic/bimetallic nanoparticles (NPs) on semiconductor (ZnO, WO3, TiO2 etc.) (Glaus et al., 2002; Li et al., 2017; Mittal et al., 2014; Primo et al., 2011; Shet and Vidya, 2016; Wang and Li, 2011) enhance the photocatalytic activity of semiconductor.

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Moreover, the metal–semiconductor heterojunction is the key factor for enhancing the photocatalytic activity of semiconductor. Previous reports are available determining that the properties of metal semiconductor interface depends on the relative position of the semiconductor band gap with respect to the metal work function leading to the formation of metal induce gap states at interface region. Hoffman et al. (Glaus et al., 2002) studied the interfacial properties of Ag-AgCl cluster and demonstrated that the charge transfer from Ag cluster to AgCl arise due to formation of metal induced gap states in energy range of -10 to -6 eV and metal induce d states at 15 eV. Similarly, Li-Zhu Wu et al. (Li et al., 2017) reported that the incorporation of Ag and AgCl nanoparticles on mpg-C3N4 leads to new absorption band in range from 450 to 600 nm relative to bare mpg-C3N4. As per this report the Ag nanoparticles deposited on the surface of AgCl provides empty level in the forbidden energy band of AgCl resulting in the new transition from the valance band (VB) of AgCl to the Fermi levels of Ag nanopaticles. The metal loaded on semiconductor act as photo-sensitizer to concentrate light which promotes the efficient transfer of electron from metal nanoparticle (NP) to conduction band of the semiconductor (Cushing et al., 2012; Zhang et al., 2013; Zhou et al., 2012; Zhu et al., 2009). Thus, metal (NP)-semiconductor heterojunction becomes an important factor for creation of visible active photocatalyst. The Au NPs loaded on TiO2 also promotes the aerobic oxidation under visible light (Tsukamoto et al., 2012). Zheng et al. (2011) reported the photo activity of [email protected] (M = Au, Pt, and Ag) by observing the oxidation of benzene to phenol under visible light. But there are few reports on the deposition of bimetallic NPs on semiconductor. Recently, bimetallic Pt&Au-TiO2 nanocomposites (NCs) has been reported to improve the catalytic activity of Pt for hydrogen production under UV-A and sunlight (Gallo et al., 2012). The bimetallic NPs loaded on semiconductor show higher activity relative to pure metal-semiconductor nanocomoposite. The bimetallic /semiconductor naocomposites show combining properties of the two metals and prevents the e /h+ recombination and increase the photocatalytic activity. In present study, we synthesized [email protected] ([email protected]) and inverse [email protected] ([email protected]) and their impregnation on TiO2 to study their comparative reduction of nitro-compounds and oxidation of salicylic acid relative to their monometallic-TiO2 nanocomposies (NCs) under visible light. The higher photocatalytic activity of bimetallic-TiO2 NCs relative to monometallic counterparts is probably due to the decrease in the barrier height formed at the bimetallic-TiO2 interface; resulting in the efficient electron transfer from the surface of metal NPs to the conduction band of TiO2 and improved the photocatalysis (Chimentao et al., 2007; Zielin´skaJurek et al., 2011).

2. Experimental

2.2. Preparation of core shell [email protected] bimetallic nanocomposites The as synthesized Cu nanosphere (NS) was used as a seed for the coating of Au to give rise to the [email protected] bimetallic (BM) type of nanostructures. These Cu (NS) (426 mL) were diluted with an aqueous solution of PVP (4 mL, 1 wt%) was treated with different volumes of HAuCl43H2O aqueous solution (49–197 mL, 0.01 M). After this, ascorbic acid (0.1 mL, 0.1 M) and NaOH (0.2 mL, 0.1 M) was added to initiate the reduction of Au on the surface of Cu (NS) resulting in the formation of [email protected] nanocomposites (NCs). 2.3. Preparation of inverse [email protected]@Cu bimetallic nanocomposites The inverse [email protected] nanocomposites (NCs) was prepared by taking 610 mL of Au nanosphere (NS) solution in an aqueous solution of PVP (4 mL, 1 wt%). Then different volumes of aqueous solution of CuSO45H2O (213–854 mL, 0.01 M) were added, followed by NaOH (11 mL, 1 M) and hydrazine (5 mL) to start the coating of the Cu shell on Au surface, giving rise to [email protected] NCs. The prepared [email protected] bimetallic NCs were then washed with deionized water using centrifugation (8000 RPM for 5 min) and re-dispersed in water. 2.4. Preparation of Monometallic-TiO2 and [email protected] BimetallicTiO2Nanocomposites The composites of monometallic as well as [email protected] bimetallic with TiO2 were prepared by the wet impregnation method in which the total metal mass percentage of metal NPs was kept at 1 wt% with respect to TiO2.The Cu (NS), Au (NS), [email protected] and [email protected] NCs was added to 500 mg of TiO2 separately in an aqueous medium under magnetic stirring at room temperature for 2 h. The solid mass was separated using centrifugation by repeated washings with deionized water and then, the sample was dried in an oven overnight at 50 °C followed by a heat treatment in a muffle furnace at 500 °C for 2 h. 2.5. Characterization The optical properties of monometallic and [email protected] bimetallic NCs were characterized using a UV–Vis spectrophotometer (Analytica Jena Specord 205). The morphology and composition were analyzed by transmission electron microscope (TEM) and energy dispersive X-ray (EDX) analysis and selected area electron diffraction (SAED) patterns using a FEI Technai F20 TEM. The hydrodynamic diameter of different NCs was determined by using a Brookhaven 90 plus Particle Size Analyzer. The optical properties of M-TiO2 samples were measured with a diffuse reflectance spectrophotometer (DRS) by Avantesusing BaSO4 as a reflectance standard. Photoluminescence (PL) spectra were measured on excitation with xenon lamp at 320 nm by Perkin-Elmer LS5 in ethanolic medium.

2.1. Materials 2.6. Photocatalytic activity Cupric sulfate (CuSO45H2O), aurochloric acid (HAuCl43H2O), ascorbic acid (C6H8O6), sodium hydroxide (NaOH), polyvinylpyrrolidone (PVP, (C6H9NO)n), hydrazine (N2H4), ethylene glycol (EG, C2H6O2) and salicylic acid (C7H6O3), were obtained from Loba Chemie, India. The 3-nitroacetophenone (C8H7NO3, Spectrochem Ltd), 1-chloro-3-nitrobenzene (C6H4ClNO2, Spectrochem Ltd), cetyltrimethylammoniumbromide (C19H42BrN, CDH Ltd), sodium borohydride (NaBH4 Rankem, India) and trisodium citrate (Na3C6H5O7 SDFCL, India), P25-TiO2 photocatalyst (Degussa Company, Germany) were used as-received. The water used throughout this study has been obtained from Millipore filtration system.

The photoactivity of the as prepared catalysts were examined by mixing 5 mL of 0.2 mM nitro-aromatic compounds (3nitroacetophenone, 1-chloro-3-nitrobenzene), 20 mg mono/ bimetallic-TiO2 in 50% isopropanol suspension and Argon atmospheres under visible light (Halogen lamp, 400–1100 nm) illumination. The reduction products were examined at regular interval of time by UV–Vis spectrophotometer after filtration with 0.22 lm cellulose filter and further quantify by high performance liquid chromatography (HPLC) equipped with C-18 column (250  4.6 mm, 5 mm). The wavelength used is 254 nm and solvent

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system consist of 70% methanol and 30% water with flow rate 1 ml min 1. Similarly, the photodegradation of salicylic acid (5 mL, 0.5 mM) was carried out in a Pyrex tube by the desired amount of catalyst under the visible light for different time periods. 3. Results and discussions 3.1. Optical properties of [email protected] and [email protected] bimetallic NCs Due to surface plasmon resonance (SPR) effect the reddish colored aqueous solution of Cu (NS) give a strong absorption band at 600 nm analogous to their spherical shape (Fig. 1a) and blueshifted to 587 nm with addition of Au3+ accompanied by the change in color from reddish to light brown. A significant change in color and absorption peak was found with increasing the amount of Au3+ as in Fig. 1c. This result indicated the coexistence of two elements (Cu and Au) in one composite and the Cu (NS) were entirely encapsulated with Au (NS) giving rise to Cucore-Aushell structure (Ma et al., 2010; Pande et al., 2007). On the other hand, the SPR band of Au was red-shifted to 545 nm with addition of Cu2+ on its surface accompanied with change in color from pink to light red. With progressive addition of Cu+2 the intensity of peak at 560–580 nm increases due to change in shell thickness as well as its composition (Fig. 1b). This observation is analogues to formation of Aucore-Cushell structure (Yang et al., 2008). The Cu shell on the surface of Au core leads to color transformation from pink to light red and to light orange as shown in Fig. 1d. 3.2. Morphology and particle size distribution 3.2.1. Dynamic light scattering (DLS) The changes occur in the absorption bands might be due to change in dielectric function and composition of resulting NCs and deposition of shell over the core surface, causes increase in size of NCs which was determined by DLS particle size distribution. It was found that the average hydrodynamic size of Cu (NS)

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(21 nm) increased to 36 nm after the deposition of Au0.1 on the surface of Cu (NS) (Fig. 2) (Kahraman et al., 2009). Similarly, the hydrodynamic size of Au (NS) (25 nm) consequently increased to 38 nm with the addition of Cu2+ ions (0.1) suggesting the coating of Au on Cu (NS). The morphology of the bare and bimetallic nanostructures has been presented in Fig. 3 and it was observed that the diameter of monometallic Au (NS) ranges in between 8–14 nm and the same Au (NS) were used as a seeds for the coating of Cu on its surface. With the addition of Cu2+ in the presence of ascorbic acid the core shell ([email protected], 15 nm) formation was witnessed, the shell thickness of homogenous Cu was measured to be 1.6–2.7 nm (Fig. 3). It was also observed that increasing the Cu2+ concentration leads to the increase in overall size (20 nm) and shell thickness (3 nm) of the [email protected] core shell nanostructure. 3.3. Optical properties of mono- and bimetallics-TiO2 NCs The absorption spectra of mono- and bimetallic-TiO2 NCs revels that the bare TiO2 give a strong absorption at 386 nm, which is correspond to (2p to 3d) electronic transition from valance shell of oxygen atom to the conduction band of TiO2 and with metal loading on TiO2, the absorption is slightly red shifted to 403 nm in case of Cu (NS)-TiO2 and characteristic SP band at 560 nm occur in case of Au (NS)-TiO2 due to charge transfer from metal ion to TiO2 (Fig. 4a). Because of strong coupling between core-shell bimetallic NCs and TiO2, the absorption band red-shifted to 550 nm and 634 nm for [email protected] and [email protected], respectively as compared to their monometallic counter parts, signifying the contraction of TiO2 band gap. The Tauc plot is used to determine band gap (Eg) of various NCs by extrapolating the plot of (ahm)2 versus hm and values comes out to be 3.2 eV, 3.1 eV and 3.06 eV for TiO2, Au/TiO2 &Cu/TiO2, respectively (Fig. ESI-S2, electronic supporting information). For bimetallic/TiO2 NCs band gap found to be narrowed as 2.95 eV and 2.77 eV for [email protected] and [email protected],respectively due to red shift in their absorbance spectra. The histogram showing comparison of band gaps of monometallic-TiO2 & bimetallic-TiO2 NCs relative to naked TiO2

Fig. 1. Effect of different amounts of (a–c) HAuCl4.3H20 (0.01 M) deposition onto Cu nanospheres and (b–d) CuSO45H2O (0.01 M) deposition onto Au nanospheres for the variation in the surface plasmon band and their respective color changes.

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Fig. 2. (a) Dynamic light scattering (DLS) based particle size distribution of Cu nanospheres and Au nanospheres after deposition of HAuCL43H2Oand CuSO45H2O (0.01 M), respectively.

is given in the Fig. 5. The elemental analysis (electron dispersive Xray spectroscopy (EDS)) of mono and bimetallic/TiO2 NCs is given in the supporting information (Fig. ESI–S1). In photoluminescence (PL, Fig. 6a) spectra TiO2 gives an emission band at 385 nm corresponding to excitation at 300 nm. The deposition of metal NPs (Cu & Au) on TiO2 led to quenching of PL intensity attributed to prevention of e -h+ recombination, the Cu (NS) shows a maximum reduction in PL intensity followed by Au (NS). On the other hand, the bimetallic [email protected] NCs displayed higher PL quenching comparative to their respective monometallic-TiO2 counterparts. This quenching of PL emission arise due to effective transfer of photogenerated charge carriers (e -h+) from the surface of TiO2 to bimetallic NCs (Hai et al., 2014).

The time resolved spectra of photocatalysts is presented in Fig. 6b and it has been observed that the lifetime of charge carriers was extended for TiO2 modified with bimetallic NPs relative to their monometallic ones and the calculated relaxation time found to be 1.04 ns, 1.5 ns, and 2.2 ns, 2.8 ns for TiO2, Cu (NS)-TiO2, [email protected] and [email protected] respectively. The photo-catalytic activity of synthesized mono&bimetallicTiO2 NCs has been determined by reducing 3-nitroacetophenone (NAP) and 1-chloro-3-nitrobenzene (CNB) under visible light. Fig. 7a shows changes occur in the absorption spectrum during the reduction of NAP to 3-aminoacetophenone (AAP) by using Au (NS)-TiO2 as a catalyst. It has been observed that band intensity corresponding to NAP at 235 nm decreased with the development of new peak at 280 nm, analogues to AAP formation, whose intensity progressively increased with time. The time course graph is given in Fig. 7b and it is observed that amount of product increases linearly with reaction time and bimetallic-TiO2 NCs [email protected] and [email protected] exhibited higher yield of AAP formationn 60% & 52% respectively within 60–80 min reduction time in contrast to their monometallic Cu (NS)-TiO2(36%) and Au (NS)-TiO2(29%). Similarly, the time course graph for the photo-reduction of CNB to CAB with mono and bimetallic-TiO2 NCs was shown in Fig. 8(a, b). The observed trend for the photo reduction of CNB was [email protected] > [email protected] > Cu (NS)-TiO2 > Au (NS)-TiO2 having a percentage yield of CAB 74%, 66%, 48%, 42% respectively. The oxidation of salicylic acid (SA) was also analyzed for mono&bimetallic-TiO2 NCs. Fig. 9a shows changes occur in absorption spectra during the degradation of SA under visible light, where absorbance band of high intensity at 296 nm corresponds to SA, which decreases as reaction proceeds from 30 to 150 min. The non-degradable nature of SA was analyzed under visible light without addition of any catalyst. Also, when the reaction was catalyzed by TiO2 and M-TiO2 NCs in dark, the substrate showed a negligible loss in the concentration that might be due to adsorption. A time course graph which show linear decrease in the concentration of salicylic acid using different catalyst displayed in Fig. 9b indicating different extent of degradation. The percentage (%) degradation of SA with mono and bimetallic-TiO2 NCs and bare

Fig. 3. (a–e) Transmission electron microscope (TEM) images of [email protected] nanosphere.

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Fig. 4. (a) Absorption spectra of monometallic and (b) bimetallic-TiO2 nanocomposites (NCs).

Fig. 5. (a) Comparative band gaps of mono and bimetallic-TiO2 nanocomposites (NCs) relative to bare TiO2 in the form of histogram.

TiO2 has been shown in Fig. 10a. By using Langmuir–Hinshelwood model i.e. r = dC/dt = k(KC)/(1 + KC) it was found that the photodegradation process follows the pseudo first order of kinetics and calculated rate constant (k) in Fig. 10b found to be highest for [email protected] (0.97  10 2 min 1) followed by [email protected] (0.65  10 2 min 1) as compared to Cu (NS)-TiO2 (0.34  10 2 min 1), Au (NS)-TiO2 (0.29  10 2 min 1) and bare TiO2 (0.13  10 2 min 1). The increase in photocatalytic activity of bimetallic NCs followed by monometallic-TiO2 composites relative to TiO2 in visible light can be explained as per mechanisms in Scheme 1.The metal loading on TiO2 absorb visible light because of coupling of resonant oscillation of surface electrons when light is incident on it, known as localized surface plasmon (LSP) resonance. This combined oscillation of electrons causes the transfer of electrons from the surface metal into the conduction band (CB) of TiO2 helps in the reducing the NAP and CNB substrates to their respective amines (Zhou et al., 2013). During oxidation reactions, the electron transfer from the surface of metal nanoparticle (NP) to the conduction band (CB) of TiO2 result in the formation of positive charge on metal surface. While the electron transferred in CB of TiO2 is consumed for the reduction of absorbed oxygen to superoxide radicals (Sangpour

Fig. 6. (a) Emission spectra and (b) Time resolved decay curve of monometallic and bimetallic nanocomposites modified TiO2.

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Fig. 7. (a) Changes in absorption spectra during the photoreduction of 3-nitroacetophenone (0.2 mM), (b) time course graph showing the amount of 3-aminoacetophenone produced from the photoreduction of 3-nitroacetophenone (0.2 mM) by Cu and Au nanoparticles and their [email protected] bimetallic nanocomposites under visible light.

Fig. 8. Time course graph showing the amount of 1-chloro-3-nitroenzene left and 1-chloro-3-aminobenzene produced by (a) Cu and Au nanospheres, (b) [email protected] Cu-Au bimetallic nanocomposites under visible light.

Fig. 9. (a) Changes in absorption spectra during oxidation of salicylic acid (0.5 mM) and (b) time course graph showing the extent of photo-oxidation by monometallic and [email protected] bimetallic-TiO2 nanocomposites under visible light.

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Fig. 10. (a) Percentage degradation of salicylic acid (0.5 mM) and (b) plot of –lnCt/C0 versus time by different monometallic and [email protected] bimetallic nanocomposites under visible light.

Scheme 1. Mechanism for the photo-oxidation and reduction process.

et al., 2010; Zhou et al., 2013). The superoxide radicals and positive charge on the surface of metal NP act as strong oxidant results in the degradation Salicylic acid which form organic molecules as intermediates and consequently to water and CO2. 4. Conclusions In summary, the bimetallic/TiO2 nanocatalyst found to be more effective than monometallic ones for the oxidation and reduction reaction under visible light. The higher activity of bimetallic catalyst arises due to prevention in the recombination rate of charge carriers corresponding to decrease in work function results in generation of e /h+ pairs even in the visible light. Therefore, this effective charge separation at bimetallic/TiO2 NCs and synergistic

effect between two metals makes effective photocatalyst for organic modification under visible light. Acknowledgement The authors are thankful to the Department of Science and Technology (DST), Government of India for providing the financial support under the DST-Nanomission scheme (Sanction order SR/ NM/NS-1471/2014). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.solener.2017.07. 084.

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