Photocatalytic performance of Pd decorated TiO2–CdO composite: Role of in situ formed CdS in the photocatalytic activity

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Photocatalytic performance of Pd decorated TiO2eCdO composite: Role of in situ formed CdS in the photocatalytic activity Sanjay B. Kokane a, S.D. Sartale a,*, K.G. Girija b, Jagannath c, R. Sasikala b,** a

Thin Films and Nanomaterials Laboratory, Department of Physics, Savitribai Phule Pune University, Pune 411 007, India b Chemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India c Technical Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India

article info

abstract

Article history:

We report an enhanced photocatalytic activity for TiO2eCdO composite for hydrogen

Received 2 June 2015

generation from aqueous solution of Na2S þ Na2SO3 compared to pure CdO and TiO2. An in

Received in revised form

situ conversion of CdO to CdS is observed during the photocatalytic reaction resulting in the

11 August 2015

formation of a CdOeCdSeTiO2 ternary composite, which play a role in the enhanced ac-

Accepted 12 August 2015

tivity. Presence of Pd as a co-catalyst in the composite increases the hydrogen production

Available online xxx

nearly ten times. As prepared composite exists as anatase phase of TiO2 and cubic phase of CdO. Characterization of the catalyst by powder XRD, SEM and TEM suggest that a nano-

Keywords:

sized TiO2 exists as a dispersed phase on aggregated CdO. UV-visible absorption spectrum

Titanium dioxide

of the composite shows features of both CdO and TiO2. Composite sample exhibits

Cadmium oxide

increased photocurrent response and has increased fluorescence lifetime for the charge

Cadmium sulfide

carriers compared to pure CdO and TiO2. A detailed characterization of the used catalysts

Nanocomposite

clearly shows the formation of CdS phase besides TiO2 and CdO. This is the first report on

H2 production

the photocatalytic activity of TiO2eCdO system for hydrogen generation from water due to in situ bulk formation of CdS during the photocatalytic reaction. The in situ formed CdS in the composite enhances the photocatalytic activity in two ways: (i) CdS, whose conduction band is at higher negative potential than CdO and TiO2, can reduce Hþ more efficiently and (ii) efficient separation of photogenerated charges can occur in this composite system, resulting in increased lifetime for the charge carriers. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

* Corresponding author. Tel.: þ91 20 25692678; fax: þ91 20 25691684. ** Corresponding author. Tel.: þ91 22 25592279; fax: þ91 22 25505151. E-mail addresses: [email protected] (S.D. Sartale), [email protected] (R. Sasikala). http://dx.doi.org/10.1016/j.ijhydene.2015.08.037 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Kokane SB, et al., Photocatalytic performance of Pd decorated TiO2eCdO composite: Role of in situ formed CdS in the photocatalytic activity, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.08.037

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Introduction Photocatalytic water splitting is a potential method for hydrogen generation as water and sunlight are renewable sources [1,2]. Development of photocatalyst having appreciable efficiency using solar radiation is required to make this process economically viable. Till now, many materials have been used as photocatalysts, but efficiency was found to be far from satisfactory [3,4]. In most of the cases, the visible light photocatalytic activity is poor as most of the oxides, such as TiO2 have a wide bandgap. Though sulfides are visible light active, they are prone to photocorrosion and the lifetime of the photogenerated charge carriers is not enough to carry out the redox reaction [5]. During the past decades, various strategies were employed to improve the efficiency by metal/ nonmetal doping in wide band gap semiconductors and making composite materials [6e10]. Recently, great attention is focused on the fabrication of nanocomposites, because these materials can enhance the lifetime of photogenerated charge carriers, improve visible light absorption, increase the effective surface area and minimize photocorrosion [10e13]. Cadmium oxide (CdO) is a narrow band gap semiconductor material and has good n-type electrical conductivity as a consequence of its cadmium interstitials or oxygen vacancies [14]. The attractive optical and electronic properties of CdO have made it a suitable candidate in optoelectronic devices, gas sensors and solar cells [14,15]. CdO is employed as a photocatalyst for dye degradation due to its favorable visible light absorption and high charge carrier mobility [16]. More recently, Van et al. have reported that a significant enhancement in the photoelectrochemical performance of CdS/ZnO nanorod array occurs by depositing a few layers of CdO, which suppress the charge recombination [17]. An enhanced photocatalytic hydrogen production is reported for CdO/CdS nanocomposite from an aqueous solution of Na2S and Na2SO3. Enhanced activity is ascribed to enhanced surface area, increased visible light absorption and improved separation of photoinduced charge carriers [18]. However, studies on TiO2eCdO composite are very few in literature for photocatalytic application. Electrospun nanofibers of TiO2eCdO is reported to show enhanced photocatalytic activity for methyl orange degradation due to better charge separation in the composite system [19]. TiO2 nanotube (TNT)-CdO composite synthesized by galvanostatic deposition technique showed enhanced photoelectrochemical properties [20]. The photoelectrochemical property was found to be dependent on the morphology of CdO. TNT with CdO in the nanocrystallite form was found to be the best among different structures. To the best of our knowledge, photocatalytic activity of TiO2eCdO composite for hydrogen production has not been reported so far. Herein, we report an interesting observation of enhanced photocatalytic activity of TiO2eCdO composite due to in situ bulk formation of CdS when Na2S and Na2SO3 are used as sacrificial reagents. Present work demonstrates that the photocatalytic activity of TiO2eCdO system for hydrogen generation from water greatly enhances due to in situ bulk formation of CdS during the photocatalytic reaction. Though it has been reported earlier that a surface formation of CdS

occurs on carbon doped CdO during photocatalytic hydrogen generation reaction [21], no detailed study exists on the surface and bulk characteristics of CdO during photocatalytic reaction. In the present work, the role of in situ formed CdS on the photocatalytic activity and its efficiency for repeated cycles are discussed. The role of palladium (Pd) as co-catalyst on CdO, TiO2 and composite on the photocatalytic activity for hydrogen production is investigated and discussed. A detailed characterization of the surface and bulk of the composite before and after photocatalysis is carried out and correlated the observed activity with its physico-chemical properties.

Experimental Synthesis of photocatalysts All chemicals were of analytical grade and used without any further purification. Titanium dioxide (TiO2) was synthesized by solegel method. A solution of distilled water in isopropanol was added drop wise to the solution of titanium tetraisopropoxide in isopropanol under constant stirring for 2 h (Ti:H2O molar ratio 1:100). The precipitate was washed by distilled water and ethanol three times each. Dried precipitate was annealed in air at 500  C for 5 h. TiO2eCdO composite was synthesized by simple precipitation method. Calculated amount of TiO2 powder was dispersed in required volume of 0.01 M CdCl2 aqueous solution (to get 30% (TC30), 50% (TC50) and 70% (TC70) by weight of TiO2) followed by ultrasonication for 20 min to obtain uniform dispersion of TiO2 particles. Aqueous solution of 0.25 M NaOH was added drop wise to the suspension containing Cd2þ ions, till the pH reached around 10 to 11 under constant stirring for 2 h. The precipitate was aged for 48 h followed by washing with distilled water and ethanol three times. The dried powder was annealed at 400  C for 2 h. For the synthesis of pure CdO, CdCl2 was treated with NaOH and followed the same procedure mentioned above. CdS was prepared by treating cadmium chloride with sodium sulfide (1:1 by mole) at room temperature under constant stirring. The precipitate obtained was washed with water and ethanol three times each followed by drying in an oven at 120  C for 5 h. 0.5% Pd (by weight) as co-catalyst was loaded on CdO, TiO2 and TC50 (which shows the highest photocatalytic activity) catalysts by a wet impregnation method. Catalyst was dispersed in aqueous palladium chloride solution containing calculated amount of PdCl2 and evaporated to dryness under constant stirring. This was followed by heating in air at 350  C for 2 h. No reductive treatment was given to these catalysts before photocatalysis experiments. Palladium oxide particles were reduced to Pd metal during the initial hours of irradiation [22].

Photocatalytic experiment Photocatalytic activity for hydrogen generation was studied in a tubular quartz reactor. 50 mg catalyst was suspended in 25 ml aqueous solution of Na2S (0.6 M) þ Na2SO3 (0.8 M) (1:1 by volume). Before irradiation, the reactor was flushed with argon gas for 45 min to remove the air from the reactor. The

Please cite this article in press as: Kokane SB, et al., Photocatalytic performance of Pd decorated TiO2eCdO composite: Role of in situ formed CdS in the photocatalytic activity, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.08.037

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catalyst suspension was irradiated under constant stirring in a circular irradiation chamber fixed with eight ordinary daylight fluorescent lamps (Wipro, 36 W each, light energy incident on the sample: 10.2 mW/cm2) symmetrically on the walls. Spectrum of the lamp consisted of fluorescent emission predominantly in the visible region along with a UV contribution of ~3% (Fig. S1, supporting information). Details of the irradiation chamber and photocatalytic reactor are given in our earlier publications [23]. After every 1 h, the gas mixture from the reactor was analyzed using a gas chromatograph (Chromatography and Instruments Company, GC 2011) equipped with a molecular sieve 5A column and thermal conductivity detector. Photocatalytic activity for repeated cycles of experiment was tested by flushing the system with argon gas after every cycle (of 7 h) followed by irradiation using the same catalyst.

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lifetimes of the charge carriers were measured with the same instrument using a nanosecond Hydrogen flash lamp as the excitation source and a Time Correlated Single Photon Counting (TCSPC) technique. Frequency of the lamp was 40 kHz. The samples were excited at 282 nm and the lifetime was measured for 570 nm emission. X-ray photoelectron spectroscopic (XPS) studies were carried out in a VG Microtech electron spectrometer using Mg-Ka X-rays (hn ¼ 1253.6 eV) as the primary source of radiation. Chamber pressure was maintained at 1  109 torr. Appropriate correction for charging effect was made with the help of C 1s signal appearing at 284.5 eV.

Results and discussion Structural and morphological studies

Photocurrent measurement For photocurrent measurements, coplanar interdigitated gold electrodes were deposited on CdO, TiO2 and TC50 pellets by thermal evaporation method. The CurrenteVoltage (IeV) characteristics were recorded for all the samples under ambient conditions using a Keithley 6517A Electrometer. To measure the photocurrent response, these pellets were irradiated using a day light fluorescent lamp (36 W, power density ¼ 1.3 mW cm2), which consisted of fluorescent emission predominantly in the visible region along with a UV contribution of ~3%. Photocurrent measurements under onoff conditions were performed by applying a bias of 1 V and using the same fluorescent lamp.

Characterization techniques Powder X-ray diffraction (XRD) patterns of these samples were recorded using a Philips PW1820-X-ray diffractometer coupled with a PW 1729 generator, which was operated at 30 kV and 20 mA. Graphite crystal monochromator was used for generating monochromatic CuKa radiation. Surface area of the samples was measured using BrunauereEmmeteTeller (BET) method using nitrogen adsorption at 77 K. Raman spectra were recorded using Renishaw InVia micro-Raman spectrometer using an excitation source of 532 nm laser with power below 0.6 mW. The scattered light was analyzed by using charge coupled device (CCD) detector. Scanning electron micrographs (SEM) were recorded using JEOL 6360A machine. Transmission electron microscopy (TEM) images and selective area diffraction patterns (SAED) were recorded using Tecnai G2 20 S-TWIN (FEI Company) instrument. UVvisible diffused reflectance (UV-vis DRS) spectra of samples were recorded in a JASCO V-670 spectrophotometer system equipped with an integrating sphere accessory. BaSO4 was used as reference. Photoluminescence (PL) spectra were recorded with an Edinburgh Instruments FLSP 920 system with a 450 W Xe arc lamp as the excitation source and a red sensitive Peltier element cooled Hamamatsu R2658 PMT as the detector. Emission spectra were recorded by exciting the samples at 282 nm. All the emission patterns were corrected for the detector response and were measured at 1 nm resolution. Excited state

Powder XRD patterns and Raman spectra of TiO2, CdO and TC50 are shown in Fig. 1. XRD patterns of TiO2 and CdO show characteristics peaks corresponding to anatase TiO2 (JCPDF Card No. 78-2486) and cubic CdO (JCPDF Card No. 65-2908), respectively. TC50 consists of anatase phase of TiO2 and cubic phase of CdO indicating that it is a composite. Raman spectrum of pure TiO2 shows bands with Raman shifts at 144, 395, 517 and 639 cm1 corresponding to Raman modes of Eg, B1g, A1g and Eg, respectively, which are characteristic of anatase phase of TiO2 [24]. Raman spectrum of CdO shows a broad vibrational band in the region, 200e500 cm1 centered around 285 cm1, which is characteristic of CdO [25]. The spectrum of TC50 shows bands corresponding to both CdO and TiO2, which is in conformity with the XRD results. SEM image of CdO is shown in Fig. 2a. This image indicates that CdO exists as aggregated rod like particles. TEM of TiO2 (Fig. 2b) reveals that the particle size of TiO2 is in the range of 20e40 nm. TEM image of the composite (Fig. 2c) shows the presence of an aggregated elongated structures and a dispersed nanosized particles. Thus, the TEM of the TC50 composite suggests that TiO2 and CdO are in a dispersed state. The composition of composite is confirmed by energy dispersive X-ray spectroscopy (Fig. 2d) studies and it shows the coexistence of Ti, Cd and O with calculated amount. To see the distribution of CdO and TiO2 in the composite, Raman mapping for TiO2 and CdO in TC50 has been done and is shown in Fig. 2d. These images clearly indicate that TiO2 and CdO are intimately mixed in the composite. Fig. 3a shows the TEM image and SAED pattern of Pd-TC50 composite after 10 h of photocatalytic reaction. Selected area electron diffraction pattern of Pd-TC50 after 10 h of irradiation exhibits the polycrystalline nature of the sample. The TEM image shows the presence of composite particles of size in between 20 and 50 nm range. High resolution TEM (HRTEM) images (Fig. 3b and c) clearly show distinct lattice fringes of 0.27, 0.35 and 0.32 nm corresponding to the most intense (111) plane of cubic phase of CdO, (101) plane of anatase phase of TiO2 and (111) plane of cubic phase of CdS, respectively. This result revealed that CdS is formed in the composite. The details of formation of CdS are discussed in the section ‘Characterization of Pd-TC50 after photocatalysis experiment’.

Please cite this article in press as: Kokane SB, et al., Photocatalytic performance of Pd decorated TiO2eCdO composite: Role of in situ formed CdS in the photocatalytic activity, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.08.037

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Fig. 1 e (a) Powder X-ray diffraction patterns and (b) Raman spectra of TiO2, CdO and TC50 composite.

Fig. 2 e (a) SEM image of pure CdO, (b) TEM image of pure TiO2, (c) TEM image of TC50 composite and (d) Raman mapping and EDS spectrum of TC50 composite.

Please cite this article in press as: Kokane SB, et al., Photocatalytic performance of Pd decorated TiO2eCdO composite: Role of in situ formed CdS in the photocatalytic activity, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.08.037

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Fig. 3 e (a) TEM image of Pd-TC50 composite after irradiated for 10 h (Inset: SAED pattern of Pd-TC50 composite after irradiated for 10 h), (b, c) HRTEM images of Pd-TC50 composite after irradiated for 10 h.

Optical properties

Absorption (K.M.)

Fig. 4 shows the KubelkaeMunk (KM) function of CdO, TiO2 and TiO2eCdO composites plotted against wavelength. Pure

2.5

CdO TiO2

2.0

TC30 TC50 TC70

1.5 1.0 0.5 0.0 300

400

500

600

700

Wavelenght (nm) Fig. 4 e Variation of the KubelkaeMunk (KM) function of CdO, TiO2 and TiO2eCdO composites with wavelength.

TiO2 shows the absorption onset around 420 nm and for CdO the onset at 750 nm. All composite samples show the features of both TiO2 and CdO and exhibit the absorption edge in between that of TiO2 and CdO. The blue shift in the absorption edge of CdO in the composite compared to pure CdO can be due to the strong electronic coupling at the interface and/or due to the decrease in particle size of CdO [26,27]. The band gap energies of TiO2 and CdO were determined form the plot (Fig. S2, supporting information) of modified KubelkaeMunk function [F(R) * hn]n (n ¼ 2 or ½ for direct or indirect transitions) vs photon energy (hn) [28]. The bandgap energies were found to be 3.2 and 2.2 eV for TiO2 (indirect) and CdO (direct), respectively and are comparable with the reported values [3,16]. PL spectra of CdO, TiO2 and TC50 were recorded to study the nature of emission occurring from these samples and to probe the presence of defects. The room temperature PL spectra of CdO, TiO2 and TC50 are depicted in Fig. 5a. PL spectrum of TiO2 is characterized by an emission centered at 410 nm. When compared to the bandgap of TiO2, it can be concluded that this emission occurs from the surface states/ defect levels. It is well known that, such surface states are located just below the conduction band (CB) giving rise to defect emission [29,30]. CdO shows broad and weak emission

Please cite this article in press as: Kokane SB, et al., Photocatalytic performance of Pd decorated TiO2eCdO composite: Role of in situ formed CdS in the photocatalytic activity, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.08.037

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Photocurrent response

(a)

30000

TiO2

Fig. 6 shows the IeV characteristics of CdO, TiO2 and TC50 composite under ambient conditions and fluorescent lamp (FL) irradiation. CdO exhibits high dark current and negligible photocurrent response. As the bandgap of CdO is 2.2 eV, an increased current is expected for the system when illuminated. The reason for not seeing an increased current can be attributed to the relatively higher current at the ambient conditions due to which a small difference produced by the light is not observed. Owing to the larger band gap, TiO2 shows lower dark current and the photocurrent response is minimal. The composite sample shows almost an order of increase in the photocurrent under FL irradiation indicating higher availability of electrons and improved charge separation due to illumination. The switching characteristics of TC50 composite (Fig. 7) was measured at 1 V by blocking the FL light intermittently. As seen in the figure, the composite sample responds with a sudden increase in the current when the light is on and decreased to the background current when the light is blocked suggesting a smooth movement of charge carriers in this system. The fast switching characteristics indicate low defect density and hence better photovoltaic properties of the composite system.

TC50 CdO

25000 20000 15000 10000

Intensity (a.u.)

5000 0 400

450 500 550 Wavelength (nm)

600

(b)

80

650

CdO TC50

60 40 20 0 22

24 26 Time (ns)

28

30

ambient FL

1E-7

Fig. 5 e (a) Room temperature PL spectra of TiO2, CdO and TC50 composite and (b) Fluorescence emission decay curves of CdO and TC50 composite.

1E-8 TC50 1E-9

Photocurrent (A)

-4 peak in the range of 530e640 nm, which can be ascribed to the band edge emission [31]. PL spectrum of TC50 shows emission peaks of both TiO2 (410 nm) and CdO (530e640 nm). In powder samples, as the intensity of the emission peak depends on the powder properties, a comparison of the intensities of the emission peaks need not give accurate information about the lifetime of photogenerated charge carriers. Hence, fluorescence lifetime of the charge carriers in the composite was measured and compared it with that of pure CdO, to study the effect of making a composite on the lifetime of the charge carriers. The time resolved fluorescence decay plots (for emission occurring at 570 nm) of CdO and TC50 are shown in Fig. 5b. Fluorescence emission decay at 570 nm was studied as CdO has emission peak maximum at this wavelength, which is the photocatalytically active material in this composite. Pure TiO2 did not show any photocatalytic activity under the present experimental conditions. It can be seen from the figure that the recombination is slightly slower in the composite samples as compared to pristine CdO. The fluorescence lifetimes obtained for CdO and TC50 are 310 and 550 ps, respectively. The increased lifetime of the charge carriers in the composite indicates that the separation of photogenerated charges is more efficient in the composite system.

-2

0

2

4

ambient FL

1E-7

1E-8

1E-9

TiO2 -4

-2

0

2

4

1E-4

1E-5

CdO ambient FL

1E-6 -1.0

-0.5

0.0

0.5

1.0

Voltage (V) Fig. 6 e IeV characteristics of CdO, TiO2 and TC50 composite under ambient conditions and fluorescent lamp (FL) irradiation.

Please cite this article in press as: Kokane SB, et al., Photocatalytic performance of Pd decorated TiO2eCdO composite: Role of in situ formed CdS in the photocatalytic activity, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.08.037

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Light off

7.0

-9

Photocurrent (x10 A)

7.5

6.5 6.0 5.5 5.0

Light on

0

100

200

300

400

500

600

Time (s) Fig. 7 e Photocurrent response of TC50 composite at a bias potential of 1 V.

Photocatalytic activity studies Fig. 8a shows the photocatalytic activity for H2 production of TiO2, CdO and TiO2eCdO composites in the presence of Na2S

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and Na2SO3 as sacrificial reagents. All composite samples exhibit significantly higher photocatalytic activity than its constituents. Photocatalytic activity increases with increase in CdO concentration and reaches the highest value when CdO concentration is 50% by weight of TiO2. Further increase in the concentration of CdO leads to a decrease in photocatalytic activity of the composite. The decreased activity with increased concentration can be attributed to an aggregation effect on the surface. Similar observation of decreased photocatalytic activity beyond an optimum concentration is reported for many composite systems [32,33]. The BET surface area and hydrogen generation rate of different samples are given in Table S1 of supporting information. It is seen from the table that the composites have higher surface area than TiO2 and CdO. Fig. 8b shows the photocatalytic activity of TiO2, CdO and TC50 in the presence of Pd as co-catalyst. Palladium cocatalyst brings out a large increase in the photocatalytic activity for CdO and TC50. Pd-TC50 generates hydrogen at the rate of 39 mmol h1, which is 12 times higher than that of TC50 sample. It is known that noble metals (e.g. Pd, Pt, Rh, Cu, etc.) acts as a sink for photogenerated electrons by forming a Schottky barrier at the interface between metal and n-type semiconductor junction leading to the efficient separation of photogenerated charges [34e37]. The photocatalytic performance of different catalysts cannot be directly compared because of different experimental parameters (like illumination condition, light source and the reactor design). Nevertheless, the photocatalytic performance of similar systems along with experimental conditions reported in the literature are presented here. Pt deposited CdS produced 300 mmol h1 g1 H2 gas in 10% lactic acid aqueous solution irradiated by 300 W xenon lamp with UV cutoff filter (l > 420 nm) [38]. Pt loaded mesoporous TiO2 produced H2 with rate of 6925 mmol h1 g1 in 10% methanol aqueous solution irradiated by 450 W xenon arc lamp with UV cutoff filter (l > 300 nm) [39]. Pt loaded CdOeCdS composite showed H2 evolution rate of 180 mmol h1 g1 in 0.6 M Na2S and 0.8 M Na2SO3 aqueous solution irradiated by 280 W fluorescent lamp having UV contribution about 3% [18]. Pt loaded CdSeTiO2 composite exhibited H2 activity of 2420 mmol h1 g1 in 40 mM Na2S and 40 mM Na2SO4 aqueous solution irradiated by 450 W xenon arc lamp with UV cutoff filter (l > 420 nm) [40]. Our system showed the H2 evolution rate of 780 mmol h1 g1 in 0.6 M Na2S and 0.8 M Na2SO3 aqueous solution irradiated by 280 W fluorescent lamp having UV contribution <3%. It is noteworthy that the activity of the above reported systems is higher might be due to application of UV light and/or higher photon flux, whereas in our case light source consisted of mostly visible light with minute presence of UV (<3%).

Characterization of Pd-TC50 after photocatalysis experiment

Fig. 8 e (a) Photocatalytic activity of TiO2, CdO and different composition of TiO2eCdO composite for hydrogen production and (b) Photocatalytic activity of 0.5% Pd loaded TiO2, CdO and TC50 composite.

Different phases present in TC50 catalyst were tested after photocatalytic reaction by powder XRD, Raman spectroscopy and XPS. XRD pattern of TC50 after irradiation (after photocatalysis experiment) for 2 h and 10 h in aqueous solution of Na2S and Na2SO3 along with that of the as prepared TC50 and CdS are shown in Fig. 9a. XRD pattern of TC50 irradiated for 2 h shows the presence of CdS suggesting that bulk in situ

Please cite this article in press as: Kokane SB, et al., Photocatalytic performance of Pd decorated TiO2eCdO composite: Role of in situ formed CdS in the photocatalytic activity, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.08.037

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Fig. 9 e (a) X-ray diffraction patterns and (b) Raman spectra of CdS, TC50 and TC50 after irradiation for 2 h and 10 h.

formation of CdS (cubic phase) occurs during the initial stage of photocatalytic reaction itself by ion exchange of O2- to S2[41]. The pattern recorded at the end of the experiment (after 10 h) showed an increase in the intensity of peaks corresponding to CdS and a significant decrease in the intensity of peaks corresponding to CdO is seen. Raman spectra of TC50 after irradiation for 2 h and 10 h are shown in Fig. 9b. Peaks due to CdS can be clearly seen for the sample irradiated for 2 h. The spectrum of pure CdS shows two strong Raman vibrational bands at 304 and 610 cm1 corresponding to 1LO (longitudinal optical) and 2LO phonon modes of CdS, respectively [42]. It may be noticed that the intensity of the bands due to CdS has increased for the sample irradiated for 10 h. These results are in good agreement with the XRD results of irradiated catalysts. X-ray photoelectron spectra of Pd-TC50 of as prepared, irradiated for 2 h and 10 h were recorded to see the surface compositions and changes occurring on the surface during photocatalytic reaction. Pd 3d spectra of different samples are shown in Fig. 10. The spectrum of as prepared sample shows peaks at binding energies (BE) of 336.2 and 341.7 eV corresponding to 3d5/2 and 3d3/2 spineorbit components of Pd2þ state [43]. These peaks of this sample after irradiation for 2 h show a shift towards lower BE and the spectrum could be fitted satisfactorily as two peaks with BE of 335.1 and 340.4 eV corresponding to the Pd0 state. The spectrum of the sample irradiated for 10 h is similar to that of the one irradiated for 2 h and shows peaks at 335.2 and 340.4 eV corresponding to Pd0 state. Thus, it is clear that Pd2þ impregnated on the surface of the composite gets reduced to Pd metal during the initial hours of irradiation. S 2p spectra of Pd-TC50 after irradiation are shown in Fig. 11. The spectrum of sample irradiated for 2 h shows a peak at 162.3 eV and another very less intense peak at 169.2 eV. The former can be assigned to the S2- of cadmium sulfide [44] and the latter peak due to the S6þ state of sulfur in

the sulfate species [45]. The spectrum of the sample irradiated for 10 h is more or less similar and exhibits both these peaks (161.7 and 168.7 eV). It may be noted that there is no significant change in the intensity of the sulfate peak after 10 h of irradiation suggesting that the cadmium sulfide formed is stable under the present experimental conditions. Cd 3d spectra of all samples are shown in Fig. S3 of supporting information. In all samples two peaks at around 405 and 411 eV are observed corresponding to the Cd 3d5/2 and Cd 3d3/2 [46,47]. There is no significant change in the peak positions before and after photocatalysis experiment. The Ti 2p spectra of all samples are similar and indicate that Ti is in Ti4þ state in these composites (Fig. S4, supporting information) [48]. O 1s spectra of all samples are shown in Fig. S5 of supporting information. The spectrum of the fresh sample is fitted as three peaks having BE at 528.4, 530.0 and 531.6 eV. These peaks are assigned to O2- of CdO [47], O2- of TiO2 [49] and the oxygen of surface OH groups [50], respectively. The spectrum of the irradiated samples could be fitted satisfactorily as only two peaks with BE of 529.4 and 531.5 eV corresponding to oxygen of TiO2 and OH groups, respectively. The peak at 528.4 eV is not seen due to the transformation of CdO to CdS during irradiation, which is in conformity with the XRD and Raman results. The spectrum of the sample irradiated for 10 h is similar and the corresponding peaks are placed at 529.5 and 531.5 eV, respectively.

Formation of PdeCdOeCdSeTiO2 composite and photocatalysis mechanism One of the major factors which influence the photocatalytic activity is the lifetime of photoexcited electrons (e) and holes (hþ). It is seen from the fluorescence lifetime and photocurrent studies that the lifetime of the charge carriers is higher for TC50 as compared to TiO2 and CdO suggesting improved charge separation in the composite system. The probable

Please cite this article in press as: Kokane SB, et al., Photocatalytic performance of Pd decorated TiO2eCdO composite: Role of in situ formed CdS in the photocatalytic activity, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.08.037

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Fig. 11 e XPS spectra of S 2p in Pd-TC50 catalyst after irradiation for 2 h and 10 h.

Fig. 10 e XPS spectra of Pd 3d in Pd-TC50 catalyst before and after irradiation for 2 h and 10 h.

photocatalysis mechanism is proposed based on energies of the band positions of CdO, CdS and TiO2 reported in the literature [3,18,51]. During the initial hours of irradiation, both CdO and CdS contribute to the photocatalytic activity. CdO, being a narrow bandgap material, gets excited by the visible light. The photogenerated electrons from the CB of CdO can get transferred to the defect levels/surface states of TiO2, which enhances the lifetime of the charge carriers as shown in Fig. 12a. This view is supported by the fluorescence lifetime studies of the as prepared TC50 sample. As these are shallow defect levels, the electrons can reach the CB of TiO2 and reduce water liberating hydrogen. Besides, the CdS produced initially also plays a role in enhancing the photocatalytic activity. When irradiated, CdS too gets excited and the CB of CdS is sufficiently energetic to reduce water to produce hydrogen. Photogenerated electrons from CdS can be injected into the CB of both CdO and TiO2, which increases the lifetime of the

charge carriers (Fig. 12b). Besides, the photogenerated electrons can be transferred to the Pd metal co-catalyst across the PdeCdO, PdeTiO2 or PdeCdS interface in Pd-TC50 system after irradiation resulting in enhanced photocatalytic activity. The schematic of the catalyst before and after irradiation is depicted in Fig. 12. Photocatalytic activity of Pd-TC50 for repeated cycles of experiment (7 h each) is shown in Fig. 13. It can be seen from the figure that the photocatalytic activity slightly decreases after the first cycle of 7 h of experiment. But, thereafter, there is no significant change in the activity for subsequent cycles. Though the activity is decreased after first cycle it is still higher than its constituents. The decrease in activity can be due to continuous changing the composition of the sample. With continued irradiation more and more CdS is produced as seen from the XRD pattern and Raman spectra of sample irradiated for long duration. Thus, a transformation of CdO to CdS occurs in situ during photocatalytic reaction and the CdSeTiO2 system formed generates hydrogen from water thereafter [41].

Conclusion TiO2eCdO composite exhibits enhanced photocatalytic activity for hydrogen generation from an aqueous solution of Na2S þ Na2SO3 compared to pure CdO and TiO2. The presence of Na2S leads to the transformation of CdO to CdS in TiO2eCdO composite when irradiated, resulting in increased photocatalytic activity for hydrogen generation. From a detailed surface and bulk characterization of the catalyst, it was concluded that the conversion of CdO to CdS occurs not only on the surface, but also in the bulk during photocatalytic

Please cite this article in press as: Kokane SB, et al., Photocatalytic performance of Pd decorated TiO2eCdO composite: Role of in situ formed CdS in the photocatalytic activity, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.08.037

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Fig. 12 e Energy band diagram, possible electron transfer process and schematic of formation of TiO2eCdO composite before and after irradiation.

Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2015.08.037.

references

Fig. 13 e Photocatalytic activity for repeated cycles of H2 production experiment using Pd-TC50 catalyst. reaction. The enhanced photocatalytic activity of TiO2eCdO containing CdS is due to the increased reductive power of the CB of CdS for the reduction of Hþ compared to that of CdO and TiO2 and due to the better charge separation in the tricomponent system. Palladium metal co-catalyst present in this system brings about a large increase in the photocatalytic activity due to the transfer of photogenerated electrons to the metal leading to efficient separation of charges.

Acknowledgment Sanjay B. Kokane gratefully acknowledges the financial support received from Bhabha Atomic Research Centre (BARC) (5(a)/02/TSC-2012), Mumbai, India under BARC-SPPU MoU collaborative project.

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