TiO2 for

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Electronic Integration and Thin Film Aspects of AuPd/rGO/TiO2 for Improved Solar Hydrogen Generation Bijoy Tudu, Naresh Nalajala, Kasala Prabhakar Reddy, Pranjal Saikia, and Chinnakonda S. Gopinath ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b07070 • Publication Date (Web): 15 Aug 2019 Downloaded from pubs.acs.org on August 15, 2019

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ACS Applied Materials & Interfaces

Electronic Integration and Thin Film Aspects of Au-Pd/rGO/TiO2 for Improved Solar Hydrogen Generation Bijoy Tudu#, Naresh Nalajala¥, Kasala P. Reddy¥, Pranjal Saikia#,* and Chinnakonda S. Gopinath ¥,* #Department

of Applied Sciences (Chemical Science Division), Gauhati University, Guwahati 781014, Assam, India ¥Catalysis and Inorganic Chemistry Division, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411 008, India

ABSTRACT: In the present work, we have synthesized noble bimetallic nanoparticles (AuPd NPs) on carbon based support and integrated with titania to obtain Au-Pd/C/TiO2 and AuPd/rGO/TiO2 nanocomposites using an ecofriendly hydrothermal method. Here, 1:1 (w/w) Au-Pd bimetallic composition was dispersed on (a) high surface area (3000 m2g-1) activated carbon (Au-Pd/C), prepared from locally available plant source (in Assam, India), and (b) reduced graphene oxide (rGO) (Au-Pd/rGO); subsequently, they were integrated with TiO2. Shift observed in Raman spectroscopy demonstrates the electronic integration of bimetal with titania. The photocatalytic activity of the above materials for hydrogen evolution reaction (HER) was studied under one sun conditions using methanol as a sacrificial agent in powder form. The photocatalysts were also employed to prepare thin film by drop-casting method. Au-Pd/rGO/TiO2 exhibits 43 times higher hydrogen (H2) yield with thin film (21.50 mmol h1g-1)

compared to powder form (0.50 mmol h-1g-1). On the other hand, Au-Pd/C/TiO2 shows

13 times higher hydrogen (H2) yield with thin film (6.42 mmol h-1g-1) compared to powder form (0.48 mmol h-1g-1). While powder form of both catalysts show comparable activity, AuPd/rGO/TiO2 thin film shows 3.4 times higher activity than that of Au-Pd/C/TiO2. This can be ascribed to (a) an effective separation of photogenerated electron-hole pairs at the interface of Au-Pd/rGO/TiO2, and (b) the better field effect due to plasmon resonance of bimetal in thin film form. The catalytic influence of carbon based support is highly pronounced due to synergistic binding interaction of bimetallic nanoparticles. Further, large amount of hydrogen evolution in film form with both catalysts (Au-Pd/C/TiO2 and AuPd/rGO/TiO2) reiterates that charge utilization should be better compared to powder catalysts. Keywords: Bimetal, photocatalysis, water splitting, surface plasmon resonance, TiO2.

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INTRODUCTION Energy and fuel are indispensable for human beings to carry out day to day activities. Faster expansion and increase in industrialization necessitates the demand of large scale energy.1-5 Currently, the global energy supplies for various activities rely heavily on fossil fuel and to a minor extent on solar and other renewable energy sources.6 Due to limited reserve of fossil fuels and extensive emission of CO2 and other harmful gases from combustion of it, the global climatic condition is worsening day by day. This has brought an urgent need to find an alternative, clean, renewable and harmless option to environment energy resources.7,8 In this regard, hydrogen obtained from water splitting in sunlight may be considered as the fuel of the future.9,10 Harvesting solar light with semiconductor based photocatalyst has become promising strategy for photocatalytic water splitting into hydrogen and oxygen since Fujishima and Honda11 announced the photoelectrochemical (PEC) water splitting on a rutile titania electrode in 1972. Since then, various semiconductor based photo catalysts have been developed to increase the sunlight absorption and minimizing the charge carrier recombination in photocatalysis conversion.12-19 It is reported in literature that some noble metal nanoclusters on semiconductor enhance the absorption of visible light due to surface plasmon resonance (SPR), which facilitates transfer of electron from metal to semiconductor for photocatalysis reaction.1,20 In fact, the efficient strategy to enhance the photocatalytic hydrogen generation of TiO2 is to incorporate SPR active metals and suppress the electron hole pair recombination.21 In order to achieve an efficient photocatalyst, combining TiO2 with carbon based materials, such as graphene, is an effective mean and that has been immensely deployed in various energy conversion applications. The unique properties of graphene can facilitate the migration of photogenerated electron to active site, which can minimize the recombination of charge carriers and promote photocatalytic hydrogen generation.22-26 Nanocomposites containing noble metal nanoparticles, namely Au and Pd are already demonstrated to exhibit excellent co-catalyst activity in water splitting.13-16 The SPR of Au NPs can effectively increase the visible light absorption in the composites; also it can either directly transfer electrons to the semiconductor through metal-semiconductor junction or through field-effect it can transfer energy to nearby semiconductors for the formation of charge carriers in semiconductor. The non-plasmonic Pd, on the other hand, contributes in activity by forming Schottky barrier at the interface of metal-semiconductor. This Schottky 2 ACS Paragon Plus Environment

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barrier can serve as an effective electron trap causing high density of states at Fermi level (EF) of interface and minimizing the charge carrier recombination. This electronic factor contributes in charge separation and utilization.27,28 Interestingly, bimetal formation between Au-Pd (loaded on TiO2) leads to an increase in the water splitting activity due to a combination of more light absorption via SPR of Au and charge separation by Pd and its cocatalyst activity.1,16,29-33 The carbon based support has also significant influence on photocatalytic activity. Graphene based support, due to presence of sp2 hybridized hexagonal carbon network, can transport electrons rapidly across its 2D network to active site of hydrogen generation and hence enhancing charge utilization towards increasing the efficiency of photocatalyst.29,32 The reason behind using natural carbon is that it is always vital to find a cost-effective material with earth abundant material for any practical utility. Moreover, carbon with high surface area provide excellent site for anchoring metal NPs and its interaction in the pores of carbon surface and consequently exhibits better photocatalytic hydrogen evolution, despite lacking of graphene like delocalization property. Recently, it is well investigated that the thin film form of Pd/C catalyst shows higher catalytic activity in comparison to powder form of Pd/C catalyst when same amount of catalysts was used in hydrogen generation from hydrolysis of sodium borohydride.26 This might be due to surface morphology of carbon layer and inhibition against agglomeration.26 Although nano Au (alone or in bimetal form) with reduced graphene oxide (rGO) and titania and combination thereof has been employed by a number of groups,1,9,19,32-35 hydrogen yield reported under one sun conditions did not exceed few mmol/h.g. In spite of the presence of all relevant factors, such as light absorption (by SPR of nanogold and TiO2), charge separation (by graphene), low activity raises a question on the particulate form of catalyst and its relevance to photocatalysis. Indeed, when we made thin films out of particulate catalyst by simple drop cast method, 1-1.5 order of magnitude increase in activity was observed and reported in the current manuscript. This hints that, nanoscience is yet to be tapped to its fullest to reap the benefits of solar light to chemical energy conversion. On this background, we present synthesis of ternary composites Au-Pd/C/TiO2 and Au-Pd/rGO/TiO2 and investigated them for photocatalytic H2 evolution studies in thin film and powder form with aqueous methanol. EXPERIMENTAL SECTION



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Chemicals and reagents. The following chemicals were used for the experiments without further purification: Graphite powder (20 µm, Sigma-Aldrich), Degussa TiO2-P25 (Surface Area 55 m2g-1 and particle size 21 nm), sulfuric acid, hydrochloric acid and H2O2 (Qualigens), potassium permanganate (>99%), hydrogen tetrachloroaurate (HAuCl4.3H2O, 99.99%), sodium tetrachloropalladate (Na2PdCl4), hydrazine hydrate (N2H4.2H2O), NaNO3, NH3, NaOH and EtOH (Merck). Double distilled water was used in all experiments. Preparation of Activated Carbon from natural source. Activated carbon was prepared from a locally available natural source, Oleander Seeds (around Guwahati, Assam), following the methodology developed by Borah et. al.36 In a typical procedure, seeds were first ground into 2-3 pieces by jaw crusher to remove the kernel of the seed. It was then washed with water to remove the water soluble impurities and surface adhered particles followed by drying at 60 °C to remove any volatile impurities. It was then ground and sieved to obtain fine uniform powder of particle size 250 μ. The carbon precursor was impregnated with 85% phosphoric acid (H3PO4) using the chemical ratio for precursor: activating agent (w/v) of 1:4 and kept in oven at 60 ºC for 3 h. The acid impregnated carbon precursor was thermally treated at 500 ºC under nitrogen (N2) atmosphere at a heating rate of 5 °C/min. After cooling down to normal temperature, the carbonized sample was washed with 0.1 M HCl and distilled water until to get neutral supernatant solution. Hereafter, the prepared activated carbon sample will be represented as C. Preparation of graphene oxide (GO). Graphene oxide (GO) was prepared from graphite powder by adopting a modified Hummer and Offeman method.29,32 In a typical procedure, a mixture of graphite powder (1 g) and NaNO3 (1 g) was put into 40 mL conc. H2SO4 at 0 C in ice bath and then KMnO4 (3 g) was slowly added to the above solution with vigorous stirring keeping the reaction temperature below 20 C to obtain GO. After 30 min, the reaction mixture was warmed to 35 C and maintained at this temperature for 2 h under stirring until a brown color paste was formed. After that, the reaction was terminated slowly by adding 100 mL water and then temperature was increased to 95-98 C and it was maintained for 20 min. The reaction mixture was then diluted to approximately 250 mL by adding water followed by treatment with 10 mL H2O2 (30 %) and continue to stir until brownish yellow color to appear. At room temperature, the reaction mixture was centrifuged and washed with dilute hydrochloric acid (5%) and then with water many times. The solid obtained is graphite oxide. This graphite oxide was further exfoliated by sonication in water for 2 h and then the product 4 ACS Paragon Plus Environment

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was collected through centrifugation and dried in vacuum oven at 45 C overnight. The resultant product is designated as GO. Preparation of reduced graphene oxide (rGO). rGO was synthesized from GO by reduction with hydrazine hydrate. In typical procedure, a 0.3 g of GO was dispersed by sonication in 200 mL water and 10 mL hydrazine hydrate was added in presence of 3 mL 25% dilute NH3. The reaction mixture was continued to stir at 80 C for 2 h and allowed to cool down to room temperature. Then, the product was collected through centrifugation (3000 rpm) followed by washing with absolute ethanol and water several times. Finally, the product was dried in a vacuum oven at 45 C overnight. Synthesis of Au-Pd/C and Au-Pd/rGO nanocomposites. The nanocomposite Au-Pd/rGO (1:1 w/w) was synthesized by adopting eco-friendly hydrothermal method using hydrazine hydrate as reducing agent in alkaline medium. It is to be noted that no surfactant molecules are added in the preparation to retain the metallic nanoparticle and this makes the procedure simple. In a typical procedure, 0.028 g of rGO was well dispersed in 50 mL of water by sonication. Then 50 mL of aqueous solutions of metal precursors Na2PdCl4 (0.0277 g) and HAuCl4.3H2O (0.037 g) were added, while maintaining the molar ratio of Au:Pd (1:1). Subsequently, 10 mL of 1 M sodium hydroxide (NaOH) solution was added to the above solution to make it alkaline and allowed to stir for 10 min followed by the addition of 5 mL hydrazine hydrate. Then, the whole mixture was stirred vigorously for 2 h at room temperature and then it was transferred into an autoclave. The autoclave was put into a furnace at 140 0C for 6 h and allowed to cool to room temperature. The product is separated by centrifugation, washed with distilled water and absolute ethanol several times to remove impurities, and dried in vacuum oven at 45 0C. The same procedure was employed to synthesize Au-Pd/C nanocomposite. Synthesis of Au-Pd/C/TiO2 and Au-Pd/rGO/TiO2 nanocomposite. To synthesize AuPd/rGO/TiO2 nanocomposite, 1 wt% Au-Pd/rGO catalyst was well dispersed on the surface of TiO2 (5 mg of the catalyst and 495 mg of TiO2) by sonication in 30 mL ethanol. The solution was vigorously sonicated for 30 min and then kept for 12 h at 60 C in vacuum oven. The same procedure was employed to prepare Au-Pd/C/TiO2 nanocomposite. In both the cases, nominal bimetal weight percentage is 0.5% and that of rGO or C is also 0.5 %. Both composites will be denoted as Au-Pd/rGO/TiO2 and Au-Pd/C/TiO2 from hereafter. Following



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the same synthetic procedure, 0.5% Au-Pd/TiO2, 0.25% Au/TiO2 and 0.25% Pd/TiO2 were also prepared. Photocatalytic hydrogen evolution of prepared catalysts. Photocatalytic activity of the catalysts was evaluated using a Newport solar simulator equipped with light source (300 W xenon arc lamp) with AM 1.5 filter under one sun conditions (100 mW/cm2). In a 70 mL capacity quartz photo reactor, 25 mg catalyst was suspended in 40 mL of an aqueous methanol solution (25% v/v); methanol was used as a sacrificial agent. The reactor was closed with air tight silicone rubber septum. Before irradiation, the reaction mixture was sonicated for 15 min to form a uniform dispersion of catalyst. Then, N2 gas purging was continued for 30 min to remove any dissolved oxygen from the solution. Light intensity was confirmed by measuring the current (100 mw/cm2) with a digital Luxmeter. Evolved H2 gas was quantitatively measured in regular intervals up to 5 h with a gas chromatograph (Agilent 7890) equipped with a TCD detector. The average H2 evolution in 5 h duration is reported in the result and discussion section. Many reference experiments were made, such as, without the addition of catalyst, with no sacrificial agent, reaction under dark conditions; none of them show any significant or measurable H2 production. The photocatalytic hydrogen evolution was studied in thin film form as well. The thin film (of dimension 1.25 cm x 3.75 cm) was prepared on normal glass plate using drop casting method. 1 mg of Au-Pd/rGO/TiO2 or Au-Pd/C/TiO2 composite was dispersed in 1 ml ethanol by sonication and subsequently drop casted homogeneously over the glass plate.37 The dropcasted glass plate was dried at room temperature for an hour and then it was ready for measurements. In place of powder catalyst, thin film was used for activity measurements under solar simulator in one sun condition. Evolved hydrogen gas was quantitatively measured as described above. Materials characterization. Different analytical techniques have been employed to characterize the synthesized activated carbon and the nanocomposites. UV-visible spectral measurement was carried out using Shimadzu (Model UV-2550) spectrophotometer under diffuse reflectance mode for powder sample. Powder X-ray diffraction (PXRD) data were measured using a PAN analytical X’pert pro dual goniometer diffractometer. The radiation used was Cu K (1.5418 A) with a Ni filter. The data were collected with a step size of 0.02 at a scan rate of 0.5/min. The sample was rotated throughout the scan for better counting statistics. Raman scattering measurement were carried out using a Horiba JY LabRAM HR 6 ACS Paragon Plus Environment

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800 Raman spectrometer coupled with a microscope in reflectance mode with 633 nm excitation laser source and a spectral resolution of 0.3 cm-1. TEM analysis were carried out using a FEI TECNAI 300 electron microscope operating at 300 kV (Cs =0.6 mm, resolution 1.7 A). The SEM system equipped with an EDX attachment (FEI, model Quanta 200 3D) was used for morphological and chemical composition studies. X-ray photoelectron spectroscopy measurements were carried with custom built ambient pressure photoelectron spectrometer (Prevac, Poland), equipped with an Al K monochromator as a X-ray source (1486.6 eV) and VG-scienta R3000 as an electron analyzer.38 The Brunauer-Emmett-Teller (BET) surface area was obtained from the nitrogen gas sorption measurement at 77 K (Autosorb I/Quantachrome instrument, USA). Infrared spectra were recorded with a FT-IR spectrometer (Perkin Elmer) at ambient conditions. RESULTS AND DISCUSSION The BET surface area of the activated carbon was found to be 3000 m2g-1. FTIR spectroscopy was used to study the vibrational frequencies of the functional groups present in C and rGO (Fig. S1). The characteristic absorption band at 3392 cm-1 in IR spectrum is attributed to O-H stretching vibration mode of surface hydroxyl functional group. In addition, the stretching frequency at 2926 cm-1 is due to symmetric and asymmetric C-H bands and the peaks at 1573 and 1599 cm-1 can be assigned to stretching vibrations of C=C group. The stretching bands at around 1177 cm-1 and 1078 cm-1 indicate the presence of C-O and C-C stretching vibrations, respectively.36 As such, IR of C and rGO is very similar, except for a minor carbonyl feature observed around 1720 cm-1 and a C-H bending vibration mode observed at 1380 cm-1. FESEM study reveals porous nature of the material and EDXA study clearly shows formation of the activated carbon (Fig. S2). The structural features and crystalline phase analysis of the synthesized nanocomposites were carried out using X-ray diffraction unit. The crystalline phases were identified using the JCPDS database. Figure 1 represents the powder X-ray diffraction (PXRD) pattern of TiO2, Au-Pd, Au-Pd/TiO2, Au-Pd/rGO, Au-Pd/C/TiO2, and AuPd/rGO/TiO2. The diffraction peaks for pure TiO2 are found at 25.3, 37.8, 48.8, 53.9, 55.1, 62.7, 69.7, and 75.6 corresponding to the (101), (004), (200), (105), (211), (204), (220), and (215), crystal facets of anatase phase of TiO2 (JCPDS 21-1272).38-42 Sharp and intense peaks observed for all nanocomposites indicate high crystalline nature of TiO2. With pure Au-Pd and Au-Pd/rGO, the diffraction peaks appeared at 2 values 38.7, 44.5, 64.9 7 ACS Paragon Plus Environment


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and 77.8 which are assigned to Au (111), (200), (220) and (311) facets. The peaks appeared at 2 values 40.1 and 45.4 represent (111) and (200) facets of Pd NPs. These diffraction peaks are marginally shifted to higher angles from reported monometallic diffraction values indicating the formation of bimetal in alloy form.29,32 The X-ray diffraction peaks corresponding to bimetal appeared very less in intensity and practically difficult to distinguish in the final composite of photocatalyst, due to severe overlap in diffraction angle with titania features and small amount of bimetal. However, this indicates the deposition of small amount of metal NPs and well dispersion on support material TiO2.25 Also, this change in diffraction values indicates bimetal formation as well as homogeneous mixture of Au and Pd in the composites.

Figure 1. PXRD patterns of Au-Pd/rGO/TiO2, TiO2, Au-Pd, Au-Pd/TiO2, Au-Pd/rGO and Au-Pd/C/TiO2 nanocomposites. Due to overlap in the diffraction angle of Au-Pd bimetal with TiO2 features and very small content of Au-Pd in Au-Pd/rGO/TiO2, and Au-Pd/C/TiO2 no clear diffraction features of bimetal is observed. UV-Vis (DRS) spectroscopy is helpful in studying optical properties and band gap (Eg) calculation. As can be seen from the Figure S3, all the samples show absorption cut off at around 380 nm which clearly envisages that bimetals have no effect on the band gap of TiO2 nanoparticles. Nonetheless, there is definite colour change observed from pure white for TiO2 to cerulean blue + grey for both composites (inset in Figure S3). Careful observation 8 ACS Paragon Plus Environment

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shows that the colour of Au-Pd/rGO/TiO2 samples is somewhat darker than Au-Pd/C/TiO2. A linear increase in absorption for composites at higher wavelengths (>450 nm) is attributed to light scattering, and also there is no sharp SPR feature was observed. This may be due to possible bimetal (Au-Pd) formation and the same is known to reduce the SPR absorption intensity.1 Nonetheless, color changes fully underscore the presence of SPR effect in the catalyst, but it is subsided due to Pd. The bimetal formation is very crucial from the activity point of view which is discussed in the later part of the manuscript.

Figure 2. Raman spectra of Au-Pd/TiO2, Au-Pd/C/TiO2 and nanocomposites. Inset shows the Raman spectra of rGO and Carbon.

Au-Pd/rGO/TiO2

Raman spectroscopy being a powerful analysis technique was used to study the spectral and structural features, defect associated with carbon based supports and the electronic interactions in the nanocomposites. Figure 2 shows the Raman spectrum of TiO2, Au-Pd/TiO2,

Au-Pd/C/TiO2

and

Au-Pd/rGO/TiO2 nanocomposites.

The

electronic

environment surrounding TiO2 changes when the plasmonic metal NP Au and Pd bind with TiO2 and the same is reflected with frequency shift in the Raman spectra.27 Raman active fundamental mode of TiO2 for anatase phases appear at around 145 (Eg), 198 (Eg), 398 (B1g), 516 (A1g+B1g), and 640 (Eg) cm-1.27,43 The Raman feature for Eg mode is blue shifted from 147 to 150 cm-1 which is due to a favorable binding interaction of Au-Pd bimetal with the anatase (101) face of TiO2. However it shifts to 156 (152) cm−1 after integration with Au-Pd bimetal and rGO (Carbon) in the composites. It is also to be noticed that the line broadening occurs with all Raman features of composites. Significant shift and broadening observed in 9 ACS Paragon Plus Environment


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Raman features on composite formation necessitates the electronic integration between titania and other components. This electronic integration is expected to influence the activity of the bimetallic catalyst in comparison to their monometallic counterparts. Raman spectroscopy has widely been used to characterize carbonaceous materials and particularly for distinguishing the disorder in the crystal structures of carbon. In the Raman spectrum of rGO and carbon (inset of Figure 2), two prominent peaks are clearly visible corresponding to G band at 1576 cm-1 and the D band at 1333 cm-1. In addition, 2D peak in rGO appears at 2660 cm-1 indicating the regular structure of graphene layer. The G band could be assigned to the first order scattering of the E2g phonon and in plane stretching vibration of symmetric sp2 C-C bonds indicating the presence of graphite rings, whereas the D band results from the disruption of the symmetrical hexagonal graphitic lattice and breathing mode of A1g phonon.44,45 The intensity ratios of two bands (ID/IG) is a measure of the extent of disorder and the average size of the sp2 domains in graphite materials. The D and G band peaks in the spectrum of carbon are comparatively broad and weak indicating the presence of extensive disorder or very small crystal size.46 The ratios of areas of D and G bands between the two spectra also support the fact with the value of 2.04 and 1.17 for carbon materials and rGO respectively. The less intense peaks suggest the presence of small number of sp3 bonded carbon in the carbon materials, whereas the sharp and intense peaks appeared are the indication of more isolated graphene domains as well as sp2 character in rGO, implying in charge delocalization. To understand the electronic interactions between various components and oxidation state of elements/ions in the Au-Pd/rGO/TiO2 nanocomposite, XPS analysis was performed and the results obtained are shown in Figure 3. Core level Ti 2p spectra shows two intense peaks at binding energy (BE) 458.6 and 464.4 eV due to spin orbit splitting of Ti 2p core level into 2p3/2 and 2p1/2, respectively. The BE corresponding to Ti 2p3/2 appeared at 458.6 eV, indicating the presence of Ti4+ oxidation state.20,28 However, a small positive shift in the BE (0.2) compared to virgin TiO2, is attributed to the shift in EF (Fermi Level) due to EF level equilibrium between TiO2 and plasmonic metal NPs. This supports favorable electron transfer between noble metal and TiO2 conduction band. The C 1s spectrum shows peaks at 284.2, 284.9, 285.9 and 288.3 eV binding energy, that correspond to C=C, C-C, C-O-C and C=O groups, respectively. This clearly reveals integration of rGO in the composite. The core level spectra of Au appeared with low signal/noise ratio, due to presence of very small amount of gold (Au) on the surface of TiO2. Moreover overlapping of Pd 4s orbital peak at 10 ACS Paragon Plus Environment

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the same BE region further complicated the spectrum. However, deconvolution demonstrates the presence of Au and Pd (Fig. 3).

Figure 3. XPS spectra of Ti 2p, C 1s, Au 4f and Pd 3d for Au-Pd/rGO/TiO2 nanocomposite. Au 4f7/2 appears at 83.2 eV, which is significantly lower compared to standard binding energy for metallic Au that appears at 84 eV. It reflects that gold is in electron rich state or somewhat anionic in nature.1 This observation also reiterates the electronic interaction and integration of gold with titania and transfer of electron from semiconductor to gold, either directly or via rGO. Equal intensity of Au 4f and Pd 5s levels demonstrates the 1:1 ratio of Au and Pd on the catalyst surface. The high resolution spectra of Pd shows two intense peaks at binding 334.9 (3d5/2) and 340.2 eV (3d3/2), corresponding to Pd metal and the other peaks at 336.4 and 341.9 eV due to Pd2+.29,32,47 This result indicates the successful conversion of HAuCl4 and Na2PdCl4 into anionic Au and zero valent Pd due to formation of Au-Pd/rGO/TiO2 nanocomposite. It is in good correspondence with that of Raman results. 11 ACS Paragon Plus Environment


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Surface atom content was calculated from XPS results, and Au+Pd content found to be 0.69 atom % (see Fig. S4). Indeed this is nearly two times higher than the expected 0.38 atom %. This indicates a significant surface segregation of co-catalyst on the surface of titania, which in turn enhances the activity, discussed in last section.

Figure 4. UV-XPS spectra of Au-Pd/rGO/TiO2 nanocomposite and TiO2. Features appearing at 0-2 eV is attributed to bimetal and rGO. Further, we carried out the x-ray valence band studies to evaluate the onset valence band edge changes in the Au-Pd/rGO/TiO2 composite and compared with that of pure TiO2 and the results are shown in Figure 4. Onset of valence band photoelectron emission of TiO2 begins from 2.8 eV, whereas that of Au-Pd/rGO/TiO2 begins right from 0 eV and a broad feature is clearly observed between 0 and 2 eV. This broad feature is attributed to Au-Pd bimetal; significant overlap in Au-Pd bimetal and titania feature between 2 and 3 eV demonstrates a synergetic interaction and possible charge transfer between them. Main valence band that arise from titania is narrowed by 0.7 eV in the composite than pure titania. It is to be noted that though the amount of Au-Pd and rGO is 1 wt %, its contribution to the valence band is quite high and underscoring the change in nature of composite compared to virgin titania. TEM and HRTEM analysis was employed to know the details about the morphology and the interconnectivity (or junctions) of constituent components present in the AuPd/C/TiO2 and Au-Pd/rGO/TiO2 nanocomposites and the results are shown in Figure 5. The 12 ACS Paragon Plus Environment

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TEM images also provide information about dispersion and crystalline nature of NPs. Generally, carbon and rGO was found to be without any lattice fringes; however, it is clearly seen in grey color and distinct from the background grid. The TEM images of nanocomposites clearly display a uniform dispersion of spherical shaped titania particles decorated with Au-Pd bimetallic NPs. Figure 5b shows how all components of Au-Pd/C/TiO2 are interfaced nicely; a thin layer of carbon penetrated between titania and Pd-metal. Fig. 5c shows a highly interconnected (or good necking) particle network; inset in Fig. 5c shows a expanded view of thin layer of carbon interfacing with titania. The size distribution pattern of the dispersed metal NPs is in the range of 8 to 20 nm. Majority of the lattice fringes observed in Figure 5 (b and d-f) belongs to anatase titania with an interplanar d-spacing of 0.35 nm for (101), and 0.23 nm for (111) plane of Au and Pd.48,49 Fig. 5d (top right corner) shows rGO layers extending and interfacing with titania, which in turn making a Schottky junction with Pd (centre of Fig. 5d). Similarly, Fig. 5e also shows a highly interfaced network of titania, metal and rGO. Somewhat complicated looking HRTEM image demonstrates a large extent of interface and metal-semiconductor Schottky junctions in Fig. 5f. The hydrothermal method employed for the preparation of composites is worth highlighting here. Continuous agitation under autogenous pressure and temperature forces the constituent components of composite to grow on each other and make a highly interconnected composite. Energy dispersive X-ray spectroscopy (EDX) analysis was carried out to know the elemental composition of the AuPd/rGO/TiO2 nanocomposites (Fig. S4). The EDX spectra confirm the presence of Ti, O, Au and Pd elements in the nanocomposites. There is a similarity observed in elemental composition measured between EDX and XPS. The photocatalytic activity of Au-Pd/C/TiO2 and Au-Pd/rGO/TiO2 nanocomposites were evaluated for solar hydrogen evolution reaction in presence of methanol as a hole scavenger under simulated sunlight with AM 1.5 filter (one sun condition, which contain about 4% UV). The schematic illustration of the photocatalytic reaction mechanism is shown in Figure 6 and the activity results are shown in Table 1. Hydrogen evolution obtained in powder and thin film form is normalized to 1 g of catalyst weight for direct comparison. As can be seen from the data, significant amount of hydrogen yield (HY) was observed for C/TiO2 and rGO/TiO2 in thin film form (0.48 and 1.50 mmol h-1g-1, respectively); while powder form of C/TiO2 and rGO/TiO2 shows hydrogen generation values of 0.014 and 0.032 mmol h-1g-1, respectively. Commercial P25 TiO2 evaluated under similar condition was earlier shown to produce no significant hydrogen, mainly due to large extent of charge 13 ACS Paragon Plus Environment

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Figure 5: TEM and HRTEM images of (a-c) Au-Pd/C/TiO2, and (d-f) Au-Pd/rGO/TiO2. Thin layer of carbon interfacing with titania and Pd can be seen in panel b and c. Inset in panel c shows the interface between C and titania; panel c also shows the interconnectivity of the composite over large area. Similarly, Au-Pd/C/TiO2 images show the metal-titania Schottky junctions (d = 0.225-0.23 nm for Pd(111) and Au(111)) with rGO layers seen in grey (top right corner in panel d; center portion in panel e). Carbon and rGO is seen mostly without lattice fringes. 14 ACS Paragon Plus Environment

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recombination.1,34,50 Integration of 0.25 wt % Au or Pd on titania improved the HY significantly to 0.13 or 0.17 mmol/h.g; this demonstrates the role of SPR in Au/TiO2 and Pd in charge separation as well as a co-catalyst in Pd/TiO2. In the film form, both Au/TiO2 and Pd/TiO2 display 3-4 times higher HY than its powder counterpart. In a similar way, 0.5 wt % AuPd on TiO2 displays HY of 0.29 and 1.38 mmol/h.g for powder and film form, respectively. A combination of SPR due to Au and co-catalyst activity of Pd improves the activity significantly and it is evident from this experiment. It is also to be noted that Pd/TiO2 and rGO/TiO2 shows comparable activity in film form, very low HY was observed for rGO/TiO2 in powder form. This reiterates that even though all components might be present in a photocatalyst, it is necessary to evaluate them in the right form to achieve the best possible activity. Integration of 0.5 wt % Au-Pd bimetallic nanoparticles and 0.5 wt % of C or rGO with TiO2, remarkably enhanced the photocatalytic activity towards hydrogen production. Au-Pd/C/TiO2 and Au-Pd/rGO/TiO2 display almost same activity of 0.5 mmol h-1g-1 for powder form of catalyst. Nonetheless, when they are fabricated as film, activity increases by 13 and 43 times for Au-Pd/C/TiO2 and Au-Pd/rGO/TiO2, respectively, compared to their powder counterparts. Above results decisively support the role of rGO in film form (than C) with 2D nature in enhancing the activity, due to charge separation and charge diffusion towards the redox sites. Similarly, film form of Pd/TiO2, Au/TiO2, and Au-Pd/TiO2, also shows an enhancement in hydrogen generation by a factor of 3-5, compared to its powder counterparts. This enhanced photocatalytic activity of H2 evolution may be attributed to SPR nature of bimetallic nanoparticles, efficient electron trapping by Sckottky barrier at the interface of metals (as observed in HRTEM results in Fig. 5) and high conductivity of electrons through carbon network.9 Further, electron-rich (or anionic) gold observed in XPS underscores the integrated nature of gold with titania and carbon; it is likely that gold clusters are integrated through electron-rich defect sites, such as oxygen vacancy sites in titania. Due to unique feature of gold nanoparticles, such as SPR, the visible light absorption capability of the nanocomposite improves. Upon absorption of photon energy, the plasmonic metals get excited to its surface plasmon state inducing electric field around the metals. This triggers the transfer of electron to the semiconductor surface through carbon based support anchoring the photocatalytic reaction.27 In addition to that Pd acts as a co-catalyst and enhances charge separation and ultimately hydrogen generation. In fact, it is a combination of all effects, SPR of gold, co-catalyst activity by Pd, charge diffusion via C/rGO, synergetically enhances the 15 ACS Paragon Plus Environment


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solar hydrogen generation. Enhanced activity is attributed to enhanced light absorption, and better contact of catalyst nanoparticle in film form, which enhances charge separation and charge utilization. Powder form is known to exhibit large extent of charge recombination, which is minimized in film form. In addition, high surface area and highly conducting carbon assists in the diffusion of electrons towards reaction sites. A significant change in optical path length is also expected to influence the photocatalytic activity. To explore this aspect a simple comparison is made. Digital photograph of 70 ml capacity quartz reactor was recorded and shown with 40 ml solution with powder sample under dynamic spinning condition (Fig. S5-left), and with thin film under static condition (Fig. S5-right). It is presumed that the optical path length is 50 % of the liquid layer thickness due to dynamic stirring, which turns out to be 16 mm for powder sample. For the film size (1.25 cm x 3.75 cm) employed in our studies, optical path length is found to be 17 mm liquid layer thickness from top meniscus. Hence the optical path length for powder and thin film is comparable in the present study and the high activity observed with film is not due to difference in optical path length. Amount of light absorbed for the AuPd/rGO/TiO2 catalyst suspended solution (shown in Fig. S5 left) and clear aqueous methanol (Fig. S5 right) were also recorded and the result is shown in Fig. S6. Aqueous methanol shows no absorption above 300 nm. Indeed, aqueous methanol is transparent to visible light and no absorption is expected in the visible light region. Expectedly particulate catalyst suspended in aqueous methanol shows visible light absorption, due to the presence of catalyst. Interestingly, visible light absorption was observed between 400-550 nm due to gold nanoparticles and rGO in the suspended catalyst, and it is in good correspondence with earlier results.9,19,35 This observation reiterates that high HY observed in thin film is due to an effective light absorption and charge utilisation by the catalyst. In spite of more light absorption in suspended solution, poor HY suggests a large extent of light scattering and poor charge carrier utilization due to particulate form.51 Another experiment was carried out to measure the hydrogen generation from 1 mg of particulate form of Au-Pd/rGO/TiO2, since the thin film employed only 1 mg catalyst over the area of 1.25 x 3.75 cm2. Hydrogen yield observed from 1 mg each of thin film and particulate form are 2.15 x 10-2 mmol/h and 7.2 x 10-4 mmol/h, respectively. 30 times higher hydrogen production with thin film, compared to powder form, demonstrates its superiority in terms of performance due to several factors listed earlier.


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Table 1. Photo catalytic H2 evolution results from aqueous methanol solution under one sun condition

Catalyst C/TiO2 rGO/ TiO2 Au/TiO2 Pd/TiO2 Au-Pd/TiO2 Au-Pd/C/TiO2 Au-Pd/rGO/TiO2

H2 yield (mmol.h-1g-1) Powder form 0.014 0.032 0.13 0.17 0.29 0.48 0.50

Thin Film form 0.48 1.50 0.42 0.70 1.38 6.42 21.50

Although these aspects are similar to powder and thin film form of any material, 1-1.5 order of magnitude improvement in activity with thin film should necessarily involve increased absorption of light and subsequent increase in charge carriers. It is expected that internal light scattering within the thin film helps to improve the net light absorption, as in natural green leaf.51,52 This aspect is discussed in next paragraph. Plasmonic metals can transfer the energy through field effect, which enhances higher rate of formation of charge carrier in semiconductor and rGO. The non plasmonic Pd metals forms Schottky barrier at the interface of metal-semiconductor (Pd-TiO2) which enhance photoactivity of a reaction by trapping photo-induced electrons and contributing for larger amount of hydrogen production.1,53 The photoexcited electron in the conduction band (CB) of a semiconductor can be rapidly transferred and trapped at the Pd surface that shifts Fermi level (EF) toward the CB potential of the semiconductor.16 This electronic factor enhances in charge separation and utilization. It is also to be noted that thin film on non-conducting simple glass plate improves the connectivity among the catalyst particles, and facilitates the electron transfer to occur locally. The profound activities with thin film based catalysts suggests that light absorption is dominant and thus will produce more number of charge carriers instead of light scattering which is prevalent in powder counter parts. It is well known that light scattering is predominant with particulate matter than predominant light absorption with films has been reported by Osterloh.54 Light scattered within the film is subsequently absorbed. More light absorption also would lead to enhanced SPR effect and this would contribute to more charge



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generation. In addition, charge separation is expected to be much better in such large size films than in nanoparticulate catalyst. It is to be underscored that unlike solar cell, in

Figure 6. Possible mechanism of electron transfer during photocatalysis of hydrogen evolution under solar light. SPR induced electron of Au directly transfer to CB of TiO2 via rGO. which charge carriers necessarily diffuse to long distances of the order of several microns to reach the bottom conducting plate to produce sufficient current. However, water splitting is a molecular phenomenon and this can occur in a smaller area of few hundred nm2. Indeed, water splitting may be considered as local phenomenon with the current thin film approach to power production through solar cell or photoelectrochemical water splitting. On the other hand, under light irradiation with powder catalyst in suspension form under stirring conditions lead to predominant light scattering and hence small number of charge carriers and consequently lead to small amount of hydrogen. Moreover, at higher concentration of particulates activity reduction is reported, mainly due to in-equal exposure of light to all particles at any given point of time under irradiation conditions. Therefore, the activity comparison between powder and thin films forms of same catalyst, and better activity with later emphasizes that for better solar hydrogen production one might choose thin film form. The tabulated data clearly envisage the higher efficiency of Au-Pd/rGO/TiO2 in comparison to Au-Pd/C/TiO2 nanocomposite. The integration of bimetallic nanocomposite on the surface of C/rGO and TiO2 semiconductor improves the charge separation and diffusion 18 ACS Paragon Plus Environment

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and consequently enhances the HER activity. The carbon support obtained from locally abundant source has also been observed to influence the photocatalytic activity, by facilitating active adsorption site in the pores of carbon surface for electronic binding and thus improving the stability of bimetallic nanoparticles. Indeed, powder form of AuPd/rGO/TiO2 and Au-Pd/C/TiO2 gives the similar values of HY. Nonetheless, 2D nature of rGO is more advantageous than bulk carbon. The better H2 evolution in Au-Pd/rGO/TiO2 composite is due to high conductivity of reduced graphene oxide support. Being sp2 hybridized hexagonal carbon layer structure, reduced graphene oxide functions as an excellent electron acceptor and transporter for effective separation of photogenerated electron-hole pairs and therefore prolongs the lifespan of charge carrier. Also, the contribution of rGO in photoactivity is due to strong electronic interaction between bimetallic NPs and functional groups present in rGO. These characteristic properties are less pronounced in sp3 hybridized natural carbon. However, large surface area of the material can provide suitable adsorption sites for metallic NPs interactions in the pores available. It is worth mentioning that photocatalyst dispersed in thin film form produce significantly higher photocatalytic evolution than in powder form due to surface morphology of carbon layer, better dispersion of bimetallic nanoparticles and stability against agglomeration.26 Thus, the synergetic interaction between plasmonic Au-Pd nanoparticles and reduced graphene oxide sheet can effectively increase the light absorption capability, separate photogenerated electron-hole recombination and enlarge adsorption site which results in enhancing the photocatalytic activity for hydrogen evolution. CONCLUSION In the present work, we have successfully synthesized Au-Pd/C/TiO2 and Au-Pd/rGO/TiO2 photocatalysts by a simple and green solution-phase synthetic approach. The noble Au-Pd bimetallic NPs are uniformly dispersed on high surface area natural carbon and rGO sheet using an eco-friendly hydrothermal method which subsequently decorated on TiO2 and the catalytic influence of carbon support was evaluated. The catalytic activities of the synthesized nanocomposites were tested in photocatalytic water splitting to produce hydrogen (H2) under simulated sunlight in presence of methanol as a scavenger. The photocatalysts are used in the form of thin film and powder form and produces better H2 evolution in thin film than to powder form. An excellent photocatalytic activity was observed for Au-Pd/rGO/TiO2 nanocomposite with H2 evolution rate 21.50 mmol h-1g-1 which is due to improved solar light 19 ACS Paragon Plus Environment


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absorption, effective separation of charge recombination inducing from SPR active Au nanometals and Schottky barrier at the interface and high conductivity of rGO sheet. On the other hand, the photocatalytic activity of Au-Pd/C/TiO2 nanocoposite produces H2 evolution rate 6.42 mmol h-1g-1 due to high surface area containing suitable pores for effective interaction with bimetallic NPs. The critical novelty of the present work is in enhancing the solar hydrogen generation activity of film form of AuPd/rGO/TiO2 and AuPd/C/TiO2 catalyst between 1 and 1.5 order of magnitude compared to their powder form. This is attributed to enhanced light absorption (and hence minimum light scattering), and better contact of catalyst nanoparticle (in thin film form) which enhances charge separation and charge utilization. Powder form is known to exhibit large extent of charge recombination, which is minimized in film form. In addition, high surface area and highly conducting carbon assists in the diffusion of electrons towards reaction sites. However, deep systematic study is required for clear interpretation of significantly higher hydrogen evolution in the thin film form. ASSOCIATED CONTENT Supporting Information FT-IR Spectrum of activated carbon and rGO (S1), FESEM and EDXA patterns of activated carbon (S2), Normalized UV-Vis DRS patterns of TiO2, Au-Pd/rGO/TiO2 and Au-Pd/C/TiO2 nanocomposites (S3), EDXA patterns of Au-Pd/rGO/TiO2 and Au-Pd/C/TiO2 nanocomposites (S4), Digital photograph of quartz reactor with powder sample under spinning condition and with thin film under static condition (S5), UV-Visible absorption spectra recorded for the solutions shown in Fig. S5 in a typical cuvette (S6). AUTHOR INFORMATION Corresponding Authors. *Email: [email protected] *Email: [email protected]

ORCID Pranjal Saikia: 0000-0002-9708-3396 Chinnakonda S. Gopinath: 0000-0002-4525-3912


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Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS PS thanks Indian Academy of Sciences for providing summer research fellowship 2018 (FCHET 2). CSG thank CSIR, New Delhi for financial support through MLP034526 project. BT is highly grateful to the AICTE-TEQIP-III, MHRD, Govt. of India for research fellowship. We thank Ms. Kranti Salgaonkar and Mr. Inderjeet Chauhan for performing some of the experiment for the revised version of the manuscript. All the authors are also thankful to CSIR-NCL, Pune and Gauhati University for providing analytical facilities. REFERENCES (1) Melvin, A. A.; Illath, K.; Das, T.; Raja, T.; Bhattacharyya, S.; Gopinath, C. S. M-Au/TiO2 (M= Ag, Pd, and Pt) Nanophotocatalyst for Overall Solar Water Splitting: Role of Interfaces. Nanoscale 2015, 7, 13477-13488. (2) Takata, T.; Domen, K. Particulate Photocatalysts for Water Splitting: Recent Advances and Future Prospects. ACS Energy Lett. 2019, 4, 542-549. (3) Fang, S.; Hu, Y. H. Recent Progress in Photocatalysts for Overall Water Splitting. Int. J. Energy Res. 2018, 1-17. (4) Peng, Y.; Jiang, K.; Hill, W.; Lu, Z.; Yao, H.; Wang, H. Large-Scale, Low-Cost, and High-Efficiency Water-Splitting System for Clean H2 Generation. ACS Appl. Mater. Interfaces 2019, 11, 3971−3977. (5) Gao, M.; Zhu, L.; Ong, W. L.; Wang, J.; Wei Ho, G. Structural Design of TiO2-based Photocatalyst for H2 Production and Degradation Applications. Catal. Sci. Technol. 2015, 5, 4703-4726. (6) Rajaambal, S.; Sivaranjani, K. S.; Gopinath, C. S. Recent Developments in Solar H2 Generation from Water Splitting. J. Chem. Sci. 2015, 127, 33-47. (7) Sivaranjani, K.; Agarkar, S.; Ogale, S. B.; Gopinath, C. S. Toward a Quantitative Correlation between Microstructure and DSSC Efficiency: A Case Study of TiO2-xNx Nanoparticles in a Disordered Mesoporous Framework, J. Phys. Chem. C 2012, 116, 25812587. (8) Rathi, A. K.; Kmentova, H.; Naldoni, A.; Goswami, A.; Gawande, M. B.; Varma, R. S.; Zboril, R. Significant Enhancement of Photoactivity in Hybrid TiO2/g-C3N4Nanorod Catalysts Modified with Cu-Ni Based Nanostructures. ACS Appl. Nano Mater. 2018, 1(6), 2526-2535. (9) Melvin, A. A.; Bharad, P. A.; Illath, K.; Lawrence, M. P.; Gopinath, C. S. Is there any Real Effect of Low Dimensional Morphologies towards Light Harvesting? A Case Study of Au-rGO-TiO2 Nanocomposites. ChemistrySelect 2016, 5, 917-923.



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