Controlled Synthesis of CeO2NS-Au-CdSQDs Ternary

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Controlled Synthesis of CeO2NS-Au-CdSQDs Ternary Nanoheterostructure: A Promising Visible Light Responsive Photocatalyst for H2 Evolution S. Sultana,† S. Mansingh,† M. Scurrell,‡ and K. M. Parida*,† †

Centre for Nano Science and Nano Technology, SOA University, Bhubaneswar 751 030, Odisha, India Department of Civil & Chemical Engineering, University of South Africa, Florida, Johannesburg, Florida 1710, South Africa

‡

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S Supporting Information *

ABSTRACT: With the advancement of promising multifaceted powdered photocatalytic systems, problems related to environmental pollution and energy requirements have been addressed to a significant extent. The major reason for this great achievement lies in the combined effect of both structure modification and integration of different functional materials. Here, we report a ternary hybrid containing wide band gap CeO2 nanosheets with CdSQDs and Au nanoparticles, incorporated between this type II heterostructure through simple chemical reduction methods. Structural and morphological characterization of the fabricated samples was carried out by XRD, XPS, and TEM analysis. From a series of optical and photoelectrochemical measurements, it was found that the incorporation of Au nanoparticles into the interfaces of CeO2 and CdSQDs was the major cause of the enhancement in the photocatalytic activity. Au nanoparticles play a dual character by acting as a mediator and also inject hot electrons through LSPR (light-induced surface plasmon resonance) effects in the ternary hybrid. The photocatalytic activity of the fabricated samples was tested toward H2 evolution, where the ternary hybrid CeO2NS-Au-CdSQDs lead the activity sequence with 499 μmol/2 h followed by the binary and neat counterparts. From the Mott−Shottky and linear sweep voltammetry measurements, a heterostructure relay mechanism was predicted where electrons from CdSQDs flow to the surface of CeO2 via Au. The novelty of this work is that it provides useful information about the synergistic effect among three functional components, integrated in a nanosheet structured system, as the basic requirement for constructing good heterostructures in powdered photocatalytic systems.

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INTRODUCTION In order to address the increasing demand for sustainable fuels and the associated environmental problems of modern civilization, the research community has stimulated all its skills and experience to develop the best way of utilizing renewable solar light through photocatalysis.1,2 In this regard, photocatalytic water splitting not only serves as the current energy reservoir but also works toward a greener environment.3 Currently, the two methods of photoelectrochemical4−7 and powdered photocatalytic water splitting8−12 are hot topics for the research community. The photoelectrochemical approach needs an external applied bias, a complicated cell setup,and expensive platinum and silver electrodes. In addition to this, scale-up attempts are always limited by the exposed area of the photoelectrodes. In contrast, powdered photocatalytic systems © 2017 American Chemical Society

are cost-effective and are composed of a quartz batch reactor fitted with a light source that effectively carries out the water redox reaction toward energy production.2 To harness solar light more efficiently through powdered photocatalysis, the development of a stable, visible light responsive photocatalyst is the vital issue.3 This powder type is further classified into single photocatalytic systems and hybrids or heterostructural types. So far a large number of single photocatalytic systems such as metal oxides,13,14 metal sulfides,15 and (oxy) nitrides16 have been developed and tested toward photocatalytic water splitting and pollutant degradation; among them metal oxide based Received: July 10, 2017 Published: October 5, 2017 12297

DOI: 10.1021/acs.inorgchem.7b01751 Inorg. Chem. 2017, 56, 12297−12307

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Inorganic Chemistry

VB through the interface, extending the spectral range of the absorption without sacrificing recombination lifetime.45 Notwithstanding these advantages of QD and metal oxide heterostructures, the major problem lies with the slow electron injection rate and high charge recombination rates at interfaces. It is essential to promote efficient charge transfer from the QDs to the metal oxide to enable high photocatalytic efficiency.45 In this regard a noble metal such as Au or Ag can be incorporated between the metal oxide and CdS. Many reports suggest that the noble metal acts as a carrier conductor which can extend the visible light absorption capacity through LSPR (light surface plasmon resonance) and also efficiently promote interfacial charge transfer across the metal oxide/CdS junction.5,45−49 Herein, we report a three-component heterostructure composed of CeO2NS-Au-CdSQDs. In this ternary hybrid system, CeO2NSs are used as the fundamental building blocks, with the Au nanoparticles and CdS shells being deposited on the surface of CeO2. Previously, ternary hybrids containing CeO2 with CdSQDs and Au and their morphological variation (nanosheet and quantum dots) have not been reported for photocatalytic water splitting. Many reports suggest that the incorporation of CdS increases visible light absorption ability and the Au nanoparticles act as a mediator which efficiently enhances the interfacial charge transfer across the junction and further increases the absorption ability through LSPR. The asobtained ternary system has been examined for photocatalytic water splitting and found to exhibit enhanced performance relative to that of the parent system. The enhanced activity can be ascribed to the synergistic effects occurring between CdSQDs and Au nanoparticles by extended light absorption in the visible spectrum and the efficient interfacial charge carrier separation and transfer promoted by the sandwiched Au nanoparticles through type II band alignment between CeO2NSs and CdSQDs.

photocatalysts such as CeO2, TiO2, ZnO, and WO3 are used as trump cards because of their high chemical stability, low cost, and eco-friendly nature. However, they also suffer certain drawbacks, such as a narrow light responsive range, UV active nature, serious photocorrosion, and high recombination rates.17,18 Additionally, metal sulfides and nitrides have also been explored in this photocatalysis field due to their visible light absorbing ability. Again the photocorrosive nature of these metal sulfides and nitrides limits their applications in this field.3,19 Therefore, it is quite impossible to find all of the suitable properties in a single-material component for the water splitting reaction. Therefore, more interest has been given toward the construction of hybrid/heterostructure-based photocatalytic structures for potential application in the photocatalysis field. In this approach, wide band gap metal oxide semiconductors were combined with a narrow band gap semiconductor to form a heterostructure-oriented system. Regarding this concept, a number of articles have been published, combining metal oxides with narrow gap semiconductors, such as metal chalcogenides for many potential applications.20−23 Interestingly, the rare-earth species CeO2 has gained much popularity due to its wide variety of applications such as air and water purification, solid oxide fuel cells, carbon monoxide oxidation, and the water-gas shift reaction.24,25 Recently, more works have emphasized the use of 1D and 2D nanostructured CeO2 in comparison to the powdered or bulk material. A considerable amount of work has been carried out in the field of 1D rods,24,26 wires,27,28 and tubular29 CeO2, while only a limited amount of research has been reported for 2D CeO2.25,30−35Two-dimensional (2D) nano CeO2 is an ideal candidate for constructing potential hybrids due to the 2D structure-directing advantages that include confined atomic level thickness, large specific surface area, high surface to volume ratio, unique optical and electronic properties, and salient surface chemical states.35 These 2D nanostructures are expected to bridge the gap between three-dimensional species and the quantum world of zero- and one-dimensional nanomaterials.36 Though nanosheets such as CeO2 possess impressive properties, their UV light absorbing nature is still a concern and reduces its appeal toward photocatalysis. However, the activity of CeO2 nanostructures can be improved by doping with metallic or nonmetallic elements, depositing noble-metal nanoparticles (Ag, Au, or Pd), or combining with other lower band gap secondary semiconductors (e.g., CdS, MoS2, Fe2O3, Ag3PO4, ZnO).20,24,25,37 To date, the modification of CeO2 with a narrow band gap semiconductor such as CdS has been regarded as the most efficient approach toward improved photocatalysis.5,10,38−44 For example, Lu et al. reported the CeO2/CdS heterostructure for hydrogen evolution and they recorded rates of 782 μmol g−1 h−1 of H2.40 Subsequently, You et al. have recently reported 8.4 mmol g−1 h−1 of hydrogen being obtained, only by changing the morphology of the CeO2.42 Today, CdS QDs with size-dependent band gaps, high cross sections for multiple photon absorption, and large oscillator strengths have proven to be much more promising, in comparison with their bulk CdS counterparts.12,17 Energetically favorable band alignment is a necessary criterion in developing heterostructures between QD and metal oxide that facilitates efficient interfacial charge transfer and chemical stability. Upon illumination, electrons excited in the QD transfer to the conduction band of the metal oxide, while holes jump to the

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EXPERIMENTAL SECTION

Materials. Ce(NO3)3·6H2O, Cd(CH3COO)2·2H2O, HAuCl4· 4H2O, sodium carbonate, sodium sulfide, TGA, sodium borohydride, sodium sulfite, sodium sulfate, NaOH, and silver nitrate were obtained from Merck India. All of the reagents used in this work were of analytical grade (Merck) and were used as received without further purification. Deionized water was used in the experiments. Catalyst Preparation. The preparation of the ternary nanosheet hybrid was prepared through three main steps: i.e., preparation of CeO2 nanosheets and the subsequent deposition of Au and CdSQDs. Synthesis of CeO2 Nanosheet. Ceria nanosheets were prepared by a simple precipitation technique followed by calcination.31 First, 200 mL of an aqueous solution containing 16 mM Ce(NO3)3 and 52 mM sodium carbonate were prepared individually. Then the sodium carbonate solution was added dropwise to the cerium nitrate solution with stirring. Gradually a white precipitate was formed and the mixture was stirred for 2 h in an ice-cold water bath. Then the as-obtained precipitate was aged for another 15 h at 4 °C. The final product was collected by centrifugation, washed several times with DW, and then dried at 110 °C overnight in a hot air oven. Finally, the dried sample was calcined at 450 °C for 4 h in a muffle furnace. Preparation of CeO2-Au. The Au nanoparticles were loaded on the surface of CeO2NS through metal reduction impregnation. Initially 0.5 g of CeO2NSs was well dispersed in 50 mL of water. To the solution was added 4.2 mL of 0.01 M auric trichloride solution dropwise with constant stirring. The suspension was aged for 1 h in the dark to achieve the preferential adsorption of Au-based complex ions on the polar surface of CeO2NSs. Then 10 mL of an aqueous solution containing 0.5 g of NaBH4 was rapidly added to the aforementioned solution. The resulting precipitate was filtered, washed several times with warm water, and then dried at 150 °C. 12298

DOI: 10.1021/acs.inorgchem.7b01751 Inorg. Chem. 2017, 56, 12297−12307

Article

Inorganic Chemistry Preparation of CeO2-CdSQDs or CeO2-Au-CdSQDs. The calculated amount of Cd(CH3COO)2·2H2O was added to 100 mL of an aqueous suspension containing CeO2 or CeO2-Au. To the above suspension was added 20 μL of thioglycolic acid with stirring. During this period 0.1 M NaOH was added dropwise to adjust the solution pH to 11. After the pH was adjusted, the calculated amount of Na2S was added. Then the suspension was heated at 60 °C for 30 min and aged for another 90 min. The obtained final product was filtered and washed with deionized water and dried at 60 °C under vacuum. Similarly, the CeO2-CdSQDs-Au ternary hybrid was synthesized from the CeO2-CdSQDs precursor. Material Characterization. The XRD characterization was carried out on a Rigaku Miniflex XRD instrument with Cu Kα radiation (λ = 0.15418 nm) from 10 to 80° at a scan rate of 2°/min. By using a JASCO V-750 spectrophotometer, the UV−visible diffuse reflectance spectrum was obtained in the range of 200−800 nm. The photoluminescence properties were evaluated by using a JASCO FP8300 spectrofluorimeter with an excitation wavelength of 340 and 420 nm. XPS characterization was done by using a Kartos axis ultra X-ray photoelectron spectrometer system consisting of a charge neutralizer and Al Kα X-ray monochromating source. The transmission electron microscopy (TEM) images were carried out on a Philips TECNAI G2 electron microscope operated at an accelerating voltage of 200 kV. All photoelectrochemical studies were carried out on a IVIUMn STAT instrument, and the working electrode was prepared through the dropcasting technique on the surface of FTO. The respective counter and reference electrodes used were Pt and Ag/AgCl, and a 300 W Xe lamp was used as the light source. The Nyquist plot was done at 105−102 Hz at zero bias in 0.2 M Na2SO4 in the presence of light at open circuit potential. The Mott−Schottky measurement was made at 500 Hz under dark conditions in 0.2 M Na2SO4, whereas linear sweep voltammetry (LSV) plots were evaluated by sweeping the potential from −1 to 1 V in a mixed solution containing 0.25 M Na2S and 0.35 M Na2SO4 as the electrolyte. Photocatalytic Water Splitting Reaction. The photocatalytic H2 evolution reaction was carried out in a 100 mL sealed quartz batch reactor round-bottom flask with a 150 W xenon arc lamp (>400 nm) as the irradiating source followed by 1 M NaNO2 as the UV cutoff filter. The light source was kept 10 cm away from the bottom of the reactor and adjusted 8.9 cm above the aqueous suspension. The reaction was examined by dispersing 20 mg of as-synthesized powdered catalyst into 20 mL of an aqueous solution containing 0.1 M Na2S and 0.1 M Na2SO4 with constant stirring in order to maintain the uniformity of the suspension throughout the whole reaction. Before light irradiation, N2 gas was purged for 30 min through the reactor in order to completely remove all dissolved oxygen. The amount of hydrogen gas that evolved was determined thoroughly by collecting the gas via downward displacement of water and analyzing by gas chromatography. The apparent conversion efficiency of the synthesized ternary hybrid was calculated as follows. The apparent conversion efficiency was calculated by the formula apparent conversion efficiency = (stored chemical energy)/(incident light intensity). H 2O → H 2 +

1 O2 2

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conversion efficiency: 0.01980 W/150 mW = 13.2%

RESULTS AND DISCUSSION XRD analysis has been carried out to investigate the crystal structure and phase purity of the nanomaterials. The XRD profiles of all the synthesized samples are shown in Figure 1.

Figure 1. XRD spectra of CeO2NS and composites.

For parent CeO2NSs, all of the diffraction peaks indicate the formation of face-centered-cubic structures having space group Fm3m (No. 225) with a fluorite type structure (JCPDS 34394).31 It is clearly observed that the diffraction peaks are sharp and strong, without any impurity peaks, suggesting good crystallinity and phase purity of the as-prepared CeO2 materials. Typical diffraction peaks at 2θ = 28.5, 33.8, 47.5, and 56.4° can be indexed to 111, 200, 220, and 311 planes of pure CeO2. Similarly, the characteristic (111), (220), and (311) crystallographic planes for greenockite cubic blend structured CdSQDs were found in the case of neat CdS (S1).12 However, no diffraction peaks for cubic CdS were observed in the XRD patterns of CeO2-CdS, CeO2-Au-CdS, and CeO2-CdS-Au which might be due to small amounts and relatively low intensities of CdSQDs in comparison to the predominant bulk CeO2 NS peaks.17,44 The presence of CdSQDs was confirmed from the UV−visible spectra, XPS, and TEM analysis. In addition, the peak at 38.2° corresponds to the 111 plane of Au, indicating the successful decoration by AuNPs. However, the intensity of the plane was very low due to the uniform distribution and low incorporated amount.46 The above claim about the composite is well supported by TEM analysis, as described below. Further X-ray photoelectron spectroscopy has been carried out to unravel the surface redox properties of CeO2-AuCdSQDs, as shown in Figure 2. It gives detailed information about the nature of elements present at the surface, their percentages, and the chemical environment around them. The XPS wide spectrum shows peaks attributed to Ce, Cd, Au, S, and O in the ternary hybrid without any additional peaks of other elements. In order to explore the detail of the content elements, different core level peaks were deconvoluted by CASA XPS software. Figure 2b shows the deconvoluted core level Ce 3d peak that was fitted with eight peaks. The peaks labeled as v///, v//, v and u///, u//, u are attributed to Ce4+ 3d5/2 and Ce4+ 3d3/2 respectively. Similarly, peaks labeled v/ and u/ represent 3d5/2 and 3d3/2 spin states of Ce3+, respectively.50 The relative concentrations of Ce3+ and Ce4+ provide useful information about the interaction occurring between Ce and

ΔHC = 285.8 kJ/mol

Stored chemical energy = Y(H2)/(t × ΔHC); here Y(H2) is the number of moles of hydrogen produced during the reaction, t is the duration of the reaction (in s), and ΔHC is the combustion heat of H2 (in kJ/mol). Incident light energy density on the photocatalyst surface (De) = Qi/4πr2; here Qi = 150 W and r is the distance between the lamp and the catalyst surface. stored chemical energy = moles of hydrogen produced × ΔHC = 0.069 μmol/s × 285.8 kJ/mol = 0.01980 J/s or W

De = 150 W/4 × 3.141 × (8.9)2 = 150.7 mW/cm 2 12299

DOI: 10.1021/acs.inorgchem.7b01751 Inorg. Chem. 2017, 56, 12297−12307

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Figure 2. XPS spectra of CeO2NS-Au-CdSQDs nanocomposite: survey spectrum (a), Ce 3d (b), O 1s (c), Cd (d), S 2p (e), and Au 4f (f).

the surrounding atoms in the CeO2-based catalyst.51,52 The relative concentration of Ce3+ was thus estimated using the integrated area of each peak in the spectra through the following equation and was calculated to be 25%.

enhances the separation of induced electrons/holes, leading to improved photocatalytic performance.54 The two characteristic peaks at 405.3 and 412 eV indicating spin orbit states of Cd 3d5/2 and Cd 3d3/2 separated by 6.7 eV are consistent with the reported value and confirm the presence of Cd2+ ions as shown in Figure 2d. The spectra, after linear background subtraction, were deconvoluted into four different peaks as shown. The two high intense peaks correspond to the Cd atom bound to S atoms, whereas the other two peaks were due to the surface Cd ions bound to capping ligands and surface-adsorbed oxygen to form CdO. Subila et al. has observed same type of interaction in CdSeQD, which shows high quantum efficiency due to robust passivation of surface defects.55 Similarly Peterson et al. also suggested this abnormal Cd spectrum in efficient Cd-rich CdsQD catalysts.56 Meanwhile, the determined binding energies of S 2p as shown in Figure 2e were found for S 2p3/2 (161.4 eV) and S 2p1/2 (162.7 eV) correspond to the sulfide (S2−), which is nicely consistent with the reported values.38,41,42 The Au 4f spectrum was distinguished by two distinct spin−orbit components, Au 4f5/2 and Au 4f7/2, with a splitting energy of 3.7 eV, and the experimental curves were fitted with four peaks as shown in Figure 2f. Peaks at 84.0 and 87.7 eV are characteristic of

relative Ce3 + concentration (%) = [A(Ce3 +)]/[A(Ce3 +) + A(Ce 4 +)] × 100

For the composite, the data implies the presence of oxygen vacancies in the sample. In order to confirm this, the bonding environment of oxygen was deconvoluted, as shown in Figure 2c. Three peaks of oxygen atoms were fitted for (i) crystal lattice oxygen species O2− (OI, 529.3 eV), (ii) oxygen vacancy (OII, 531.2 eV), and (iii) surface-adsorbed molecular water (H2O or Oδ−) and hydroxyl-type oxygen species (OH−) (OIII, 532.9 eV).53,54 It has been acknowledged that the XPS measurement was carried out under ultrahigh vacuum and so physically absorbed hydroxyl groups on CeO2 can be easily removed under these conditions, but the presence of OII and OIII peaks can indeed originate from the species associated with the large amount of negatively charged surface oxygen vacancies and strongly bound surface adsorbed species on CeO2.54 The presence of these surface defects probably 12300

DOI: 10.1021/acs.inorgchem.7b01751 Inorg. Chem. 2017, 56, 12297−12307

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Inorganic Chemistry

deposition over CeO2NS. In the inset of Figure 3 it can be clearly seen that for the CeO2-Au-CdSQDs system the LSPR peaks was red-shifted to 570 nm from 540 nm (CeO2NS-Au) and the LSPR peak was shortened, which may be attributed to the relatively high refractive index of the CdSQDs layer around the Au nanoparticles. Thus, the loading of CdS and or Au greatly enhanced the light-harvesting capability of CeO2NS hybrids which may be able to facilitate the separation of photogenerated exciton pairs and ultimately improve the photocatalytic activity.45,46 Further, to examine the optical properties in detail, PL measurements of the synthesized samples were carried out at room temperature at two different excitation wavelengths, i.e. 340 and 420 nm, as shown in parts a and b of Figure 4, respectively. The PL spectra obtained with the excitation wavelength at 340 nm shows a strong and sharp blue emission band (427 nm) and a weak shoulder at 467 nm. These emissions are considered to be related to the defect levels localized between the O 2p and Ce 4f levels.24,25,39 The sharp blue emission became weaker, and there was a reduction in the fwhm value of the defect level emission band after the deposition of CdS and Au in binary and ternary hybrids in comparison to the parent CeO2NSs, which suggest a delay in charge recombination.18,46 However, CeO2NS-Au binary hybrid showed a lower PL intensity in comparison to the CeO2NS-CdSQDs binary system; this is due to the presence of a lower Fermi level of Au nanoparticles leading to effective charge separation. Thus, it was concluded that the loading of Au nanoparticles is a more effective way to reduce the recombination rate in comparison to CdS.58 The PL spectra recorded at an excitation wavelength of 420 nm display a emission band at 545 nm which is ascribed to the near band edge exciton emission of CdSQDs (Figure 4b). In addition, it was clearly seen that the PL intensity of the ternary hybrid was lower in comparison to the binary CeO2NS-CdSQDs sample. From this it was clear that the photogenerated exciton pair recombination was delayed more effectively due to the loading or incorporation of Au nanoparticles.18 Again, between the two ternary hybrid CeO 2 NS-CdSQDs-Au and CeO 2 NS-AuCdSQDs as shown in Figure 4, the Au-mediated system (CeO2NS-Au-CdSQDs) showed a reduced PL intensity. It was previously reported by Zhao et al. and Zhang et al. that noblemetal nanoparticles act as mediators at the interface of type II heterostructures, thereby providing an interior direct pathway,

metallic Au, and the other two shoulder peaks were due to the ionic gold species Auδ+. Liu et al. has reported that Au atoms can be oxidized by the surface Ce atoms (Au0 to Auδ+ and Ce4+ to Ce3+)57 and direct Ce−Au bonding occurred and partial positive charge on gold may arise because of their presence near the negatively charged oxygen vacancies.52 CeO2 is said to be an excellent support catalyst for Au nanoparticles. The presence of oxygen vacancies, Ce3+, Auδ+, and surface passivation of CdSQD are the key components for enhanced photocatalytic activity of the ternary hybrid. In order to understand the optical behavior, UV−visible and PL spectroscopy was conducted for the as-prepared samples. Figure 3 shows the UV−visible diffuse reflectance spectra of the

Figure 3. UV−vis diffuse reflectance spectra of neat CeO2NS, CeO2NS-CdSQDs, CeO2NS-Au, and CeO2NS-Au-CdSQDs ternary hybrid.

samples. The as-prepared CeO 2 NS displayed a sharp absorption edge at about 420 nm, due to its wide band gap nature (Eg = 3.1 eV). However, after the introduction of CdSQDs to CeO2NS, the absorption of CeO2-CdS was extended up to 530 nm, because of the strong absorption of CdSQDs through sensitization, which can be observed from the color change of the sample. Interestingly, the presence of Au nanoparticles further enhanced the light absorption range up to 700 nm. Generally, the peak positioned at 520 nm is attributed to the LSPR mode of Au nanoparticles (≥10 nm). A shift in this absorbance may be due to variation in shape, size, and surrounding environment.45 Correlating this, we observed the LSPR peak for Au in the binary hybrid (CeO2NS-Au) at 540 nm, which indicates the smaller size of Au nanoparticle

Figure 4. Photoluminescence (PL) emission spectra of neat CeO2 and various nanocomposites at excitation wavelengths of 340 nm (a) and 420 nm (b). 12301

DOI: 10.1021/acs.inorgchem.7b01751 Inorg. Chem. 2017, 56, 12297−12307

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Figure 5. TEM images of (a, b) CeO2NS-Au-CdSQDs and high-resolution TEM images of selected areas (c, d), SAED (e), and EDX pattern (f).

Figure 6. Histograms of H2 production rate under visible light irradiation for 2 h (a) and recyclibity study of CeO2NS-Au-CdSQDs ternary hybrid (b).

firm attachment between CeO2 and gold nanoparticles due to the negatively charged oxygen vacancies present on CeO2. These slightly positively charged Au nanoparticles were strongly attached to thioglycolic acid because gold has a high affinity toward sulfur groups present in thyoglycolic acid, and then Cd2+ ions were encapsulated through Coulombic interactions. Further, S2− ions migrate toward Cd ions and form CdSQDs. Thus, in the TEM picture (Figure 5c,d) it can be seen that CdSQDs are covering the Au nanoparticles. The

resulting in the efficient charge carrier transport and separation process.5,46,59 Figure 5a,b represents the TEM images of the CeO2NS-AuCdSQDs nanohybrid, and it can be clearly seen that 5−7 nm Au nanoparticles were uniformly deposited on the surface of CeO2NS. Pure CeO2 was composed of a lamellar nanosheet like morphology, as shown in Figure S2 in the Supporting Information. The lattice fringe spacing of 0.24 nm corresponds to the cubic Au nanoparticles as shown in Figure 5c. There is a 12302

DOI: 10.1021/acs.inorgchem.7b01751 Inorg. Chem. 2017, 56, 12297−12307

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Table 1. Comparison of H2 Evolution Rate of CeO2NS-Au-CdSQDs with Other Metal Oxide-Au-CdS Ternary Photocatalysts entry 1 2 3 4 5

sacrificial agent/light source

H2 evolution (mmol/(h g))

0.1 M Na2S + 0.1 M Na2SO3/300 W Xe lamp

6.08

0.1 M Na2S + 0.1 M Na2SO3/300 W Xe lamp 0.1 M Na2S + 0.1 M Na2SO3/300 W Xe lamp 20 mmol Na2S/Na2SO3/300 W Xe lamp 0.1 M Na2S + 0.1 M Na2SO3/150 W Xe lamp

1.81 3.56 1.7 12.5

catalytic system CdS/Au/ZnO-nanoflower (nanoplates are arranged into flowers) 3DOMTiO2-Au-CdS truncated octahedral TiO2-Au-CdS inverse opal WO3-Au-CdS CeO2NS-Au-CdSQDs

ref 8 18 48 49 present work

Figure 7. Electrochemical impedance spectra (EIS) (a) and Mott−Schottky plots for CeO2NS-CdSQDs and CeO2NS-Au-CdSQDs (b).

nanoparticle loaded CeO2NS-CdSQDs-Au ternary heterostructure and CeO2NS-Au were also tested toward photocatalytic H2 generation. It was observed that CeO2NS-CdSQDs-Au showed a high amount of H2 evolution activity in comparison to CeO2NS-Au due to the LSPR effect of Au and visible light harnessing ability of CdS. Additionally the hydrogen evolution rate for CeO2NS-CdSQDs-Au increases from 281 μmol/2 h (CeO2NS-CdSQDs) to 325 μmol/2 h, suggesting the cocatalytic role of Au nanoparticles. Hence, the high H2 generation in the CeO2NS-Au-CdSQDs ternary hybrid is mainly from Au mediator properties leading to effective charge separation and also some fraction from the cocatalytic effect of a small amount of exposed Au nanoparticles, which can be concluded from the decrease in the LSPR peak of CeO2NS-AuCdSQDs ternary system (Figure 3). Therefore, the incorporation of Au nanoparticles in the CeO2NS-CdSQDs interface should be mainly responsible for the significant improvement in the interfacial charge carrier transfer between CeO2 and CdS.8,18 This suggests that both CdS and Au are essential for CeO2 catalysts in H2 generation under visible light. To justify the efficiency of the designed ternary system, we have provided a comparison table containing other metal oxide-Au-CdS systems in water reduction (Table 1). In order to examine the photostability or longevity of the designed materials, three recycle tests were performed for durations of 6 h of 2 h each. Figure 6b represents the reusability plot of ternary CeO2NS-Au-CdSQDs photocatalyst toward hydrogen evolution under same experimental conditions. From the plot shown in Figure 6b(i) it was observed that there was a smooth decrease of H2 evolution rate with further recycling, which was due to the regular consumption of the sacrificial agents. In detail, as the quantity of S2−/SO32− is reduced with time, it is no longer possible to digest the photogenerated holes. Therefore, the holes start oxidizing CdS (photocorrosion) and as a result the catalytic activity of CdS is lost.18 When we collected the sample after the 2 h experiment and used it in further cycles, there was a regular decrease in the

lattice fringe spacings of 0.31 and 0.28 nm in the CeO2NS region match well with the spacing of the (111) and (101) planes of cubic CeO2, while that of 0.33 nm indicates the (002) planes of cubic CdS. Additionally, the selected area electron diffraction (SAED) pattern as displayed in Figure 5e shows good agreement with the XRD results. Similarly, Figure 5f depicts the EDAX pattern of the nanohybrid and confirmed the coexistence of Ce, Au, Cd, and S elements which are well matched with the XPS analysis. Further the uniform deposition of Au nanoparticles and CdSQDs are clearly seen from color electron mapping as given in Figure S3 in the Supporting Information. Photocatalytic Water Splitting and Mechanism. The photocatalytic activities of the prepared samples were investigated by monitoring hydrogen evolution from water under visible light irradiation with a mixed solution containing 0.1 M Na2SO3 and 0.1 M Na2S as the hole sacrificial agent under ambient conditions as shown in Figure 6a. It was observed that there was no evolution of H2 gas in the absence of light or catalyst. Figure 6a shows the hydrogen evolution rate of as-prepared photocatalysts. The parent CeO2NS sample showed extremely poor photocatalytic H2 evolution (proceeding through the use of a UV cutoff filter) due to the large band gap and fast recombination of photogenerated charge carriers. However, with the introduction of Au as well as CdSQDs, the H2 evolution rate significantly increased due to the higher visible light absorption and better charge separation efficiency. It was seen that, in comparison to CeO2NS-CdSQDs, the ternary heterostructure (CeO2NS-Au-CdSQDs) shows a remarkably improved photocatalytic hydrogen production, i.e. 499 μmol/2 h for 0.02 g of catalyst, which is nearly 1.7 times more than that for the binary nanohybrid. As demonstrated in the TEM images, a very small portion of Au nanoparticles is exposed without CdS coating. Therefore, it is quite necessary to evaluate the effect of the exposed Au nanoparticles on photocatalytic activity, to check whether the cocatalytic function of the noble metal dominated or not; 1 wt % Au 12303

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Inorganic Chemistry activity. In order to further confirm the stability of the ternary hybrid, we supplied Na2S/Na2SO3 solution at a regular interval for each cycle, as shown in Figure 6b(ii). Interestingly there was a steady increase in the H2 evolution rate for each cycle, which confirms that, for a sufficient amount of hole scavenger, the stability of ternary hybrid significantly increases. In order to gain more insights into CeO2NS-CdSQDs and CeO2NS-Au-CdSQDs heterostructures, different photoelectrochemical measurements were performed. Figure 7a represents the Nyquist plots of the obtained EIS data under light illumination under open circuit conditions. Generally, the impedance data interpret the charge carrier migration activities and the interfacial charge transfer resistance (Rct) of the materials.13,60 The obtained EIS plots with low Rct value of the CeO2NS-Au-CDsQDs in comparison to CeO2NS-CdSQDs indicate that the Au nanoparticles act as mediators and facilitate the charge transfer process across the junction.61 The lifetime of the excited electrons (τn) can be derived from the Bode phase plots with the help of an equation: i.e., τn= (1/2)f max. From the Bode phase plot it can be concluded that the single characteristic frequency peak f max is closely related to the photoinduced electron lifetime τn. The peak shift from high frequency to low frequency indicates a swifter electron transfer process. As shown in the inset of Figure 7a, the f max value of the CeO2NS-Au-CdSQDs electrode (τn = 42.95 μs) is shorter than that of CeO2NS-CdSQDs (τn = 68.08 μs), suggesting a greater lifetime (τn) of electrons and more efficient separation and migration of the exciton pair in the CeO2NS-Au-CdSQDs sandwich structures.13,61 Further, a Mott−Schottky (M-S) study was made to describe the carrier density upon the formation of the Schottky barrier between the electrodes and electrolytes, which further confirms the superior photoelectrochemical performance of the asprepared electrodes. Figure 7b depicts the Mott−Schottky plots for CeO2NS-CdSQDs and CeO2NS-Au-CdSQDs samples; both electrodes display a positive slope, indicating n-type characteristics. Mathematically, the carrier density of the electrode materials can be calculated via the equation

Figure 8. Polarization curve for neat CeO2NS-CdSQDs and CeO2NSAu-CdSQDs under visible light irradiation.

CeO2NS-CdSQDs. This high photocurrent may be due to the synergistic effect of Au nanoparticles and CdSQDs. Generally, when Au nanoparticles are decorated over a metal oxide semiconductor surface and are dipped in liquid electrolyte, a negative shift in the Fermi level of the Au-semiconductor heterostructure occurs due to charge equilibration. This results in a negative shift of the flat band potential as well as the onset potential in the J−V graph. 45 In this case some Au nanoparticles were not completely covered by CdSQDs and thus Au particles came in contact with liquid electrolyte, resulting in both a negative shift in onset potential and a flat band potential, which suggest a heterostructure type electron relay mechanism in CeO2NS-Au-CdSQDs.5,45 The charge transfer process and the photocatalytic water splitting mechanism in the CeO2NS-Au-CdSQD photocatalyst are illustrated in Figure 9. With regard to band gap structure, CeO2NS has a band gap of around 3.1 eV with the valence band and conduction band residing at 2.62 and −0.47 eV vs NHE, respectively. The conduction band position of CdSQDs is more negative than the CB of CeO2, ensuring that the photoexcited donor level is placed energetically above.38−44 Under visible light illumination, CdS in the binary hybrid absorbed the photons and produced electron−hole pairs and the photoexcited electrons from the CdS quantum dots migrated toward a low energetic CB of CeO2. At the surface of CeO2, adsorbed H+ ions are reduced to H2 by consuming photoelectrons, while holes accumulated at the surface of CdS, which was further quenched by SO32−/S2−. However, in the ternary system the photoexcited electrons were transferred through Au nanoparticles, where the Au would act as a mediator which carried the electrons from CdS to CeO2 and facilitate the charge transfer process. Au has a strong electronic interaction with both CeO2 and CdS, thus promoting the formation of a junction at the interface.62 It is known that the Fermi level of Au mainly resides at 0.5 eV on the NHE scale, whereas in the ternary hybrid system Au nanoparticles show an absorption band at about 570 nm, as shown in Figure 3. Therefore, the Au nanoparticles would generate electrons having an energy of 2.17 eV with respect to the Fermi level of −1.67 eV (NHE). The energetic photoexcited electrons of Au NPs have the ability to pass over the Schottky barrier as the ϕSB value is 0.8 eV between Au and CeO2.13,63 This Schottky barrier at the interface helps the transferred hot electrons accumulate in the CB of CeO2, preventing them from traveling back to the Au.60−62 Thus, the electrons of CdS as well as the hot electrons of Au tunneling through the Au nanoparticles

N = (2/εε 0e)[d(1/C 2)/dV ]−1

where ε denotes the dielectric constant of the material, ε0 denotes the permittivity of the vacuum (8.854 × 10−12 F m−1), e denotes the electronic charge unit (1.602 × 10−19 C), and V denotes the potential applied at the electrode. Moreover, the slope of the ternary hybrid is much smaller, in comparison to binary CeO2NS-CdSQDs, illustrating that the higher carrier density of CeO2NS-Au-CdSQDs is due to the introduction of Au nanoparticles between CeO2NS and CdSQDs.61,62 Figure 8 displays the linear sweep voltammogram (LSV) curves of the samples. The CeO2NS-CdSQDs exhibits a photocurrent density of 2.88 mA/cm2 at 1.0 V vsAg/AgCl (1.95 V vs RHE). The high photocurrent density of the CeO2NS-CdSQDs can be attributed to the good photoresponse in visible light, which leads to efficient generation of electron−hole pairs and their separation through type II band alignment. In addition, another factor may be the large effective surface area of the nanosheet-shaped CeO2 which provides enough space for uniform CdSQDs loading, which results in fast diffusive transport of the photogenerated electrons and holes. There is an increase in current density when Au nanoparticles are introduced between CeO2NS and CdSQDs, and this reached a maximum up to 5.71 mA/cm2 at 1.0 V vs Ag/AgCl (1.95 V vs RHE), which is about 1.98 times that of 12304

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Figure 9. Schematic illustration of proposed photocatalytic mechanistic pathway of charge separation and transfer in ternary CeO2NS-Au-CdSQDs for solar light H2 evolution.

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ACKNOWLEDGMENTS The authors are very thankful to the SOA University and its management for their support and encouragement and also to the SERB for their financial support.

transfer to the conduction band of CeO2, followed by migration to the surface in order to participate in the H2 evolution reaction. The process of internal electron transfer through Fermi level equilibration and the hot electron generation through Au nanoparticles could be the main cause of high photocatalytic activity. For the production of H2 the apparent conversion efficiency for the ternary hybrid was found to be 13.2% under visible light irradiation.

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(1) Zhou, P.; Yu, J.; Jaroniec, M. All-Solid-State Z-Scheme Photocatalytic Systems. Adv. Mater. 2014, 26, 4920−4935. (2) Iwase, A.; Ng, Y. H.; Ishiguro, Y.; Kudo, A.; Amal, R. Reduced Graphene Oxide as a Solid-State Electron Mediator in Z-Scheme Photocatalytic Water Splitting under Visible Light. J. Am. Chem. Soc. 2011, 133, 11054−11057. (3) Iwashina, K.; Iwase, A.; Ng, Y. H.; Amal, R.; Kudo, A. ZSchematic Water Splitting into H2 and O2 using Metal Sulfide as a Hydrogen-Evolving Photocatalyst and Reduced Graphene Oxide as a Solid-State Electron Mediator. J. Am. Chem. Soc. 2015, 137, 604−607. (4) Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238, 37−38. (5) Zhao, M.; Li, H.; Shen, X.; Ji, Z.; Xu, K. Facile Electrochemical Synthesis of CeO2@Ag@CdS Nanotube Arrays with Enhanced Photoelectrochemical Water Splitting Performance. Dalton Trans. 2015, 44, 19935−19941. (6) Tang, R.; Yin, R.; Zhou, S.; Ge, T.; Yuan, Z.; Zhang, L.; Yin, L. Layered MoS2 Coupled MOFs-derived Dual-phase TiO2 for Enhanced Photoelectrochemical Performance. J. Mater. Chem. A 2017, 5, 4962− 4971. (7) Su, J.; Guo, L.; Bao, N.; Grimes, C. A. Nanostructured WO3/ BiVO4 Heterojunction Films for Efficient Photoelectrochemical Water Splitting. Nano Lett. 2011, 11, 1928−1933. (8) Yu, Z. B.; Xie, Y. P.; Liu, G.; Lu, G. Q.; Ma, X. L.; Cheng, H. M. Self-assembled CdS/Au/ZnO Heterostructure Induced by Surface Polar Charges for Efficient Photocatalytic Hydrogen Evolution. J. Mater. Chem. A 2013, 1, 2773−2776. (9) Fang, J.; Xu, L.; Zhang, Z.; Yuan, Y.; Cao, S.; Wang, Z.; Yin, L.; Liao, Y.; Xue, C. Au@TiO2-CdS Ternary Nanostructures for Efficient Visible-light-Driven Hydrogen Generation. ACS Appl. Mater. Interfaces 2013, 5, 8088−8092. (10) Zhang, X.; Zhang, N.; Xu, Y. J.; Tang, Z. R. One-dimensional CdS Nanowires-CeO2 Nanoparticles Composites with Boosted Photocatalytic Activity. New J. Chem. 2015, 39, 6756−6764. (11) Mansingh, S.; Padhi, D. K.; Parida, K. M. Enhanced photocatalytic activity of Nanostructured Fe doped CeO2 for Hydrogen Production Under Visible light Irradiation. Int. J. Hydrogen Energy 2016, 41, 14133−14146. (12) Kandi, D.; Martha, S.; Thirumurugan, A.; Parida, K. M. Modification of BiOI Microplates with CdS QDs for Enhancing Stability, Optical Property, Electronic Behavior toward Rhodamine B Decolorization, and Photocatalytic Hydrogen Evolution. J. Phys. Chem. C 2017, 121, 4834−4849. (13) Huang, H.; Wang, J.; Dong, F.; Guo, Y.; Tian, N.; Zhang, Y.; Zhang, T. Highly Efficient Bi2O2CO3 Single-Crystal Lamellas with Dominantly Exposed {001} Facets. Cryst. Growth Des. 2015, 15, 534− 537.

CONCLUSION In summary, an Au-mediated CeO2NS and CdSQDs ternary heterostructure has been successfully designed through a simple and effective chemical route. The improved photocatalytic water splitting performance and high photocurrent density of the CeO2NS-Au-CdSQDs ternary heterostructure can be explained as follows. (i) The CeO2NS will provide high surface area and oxygen vacancy sites for the loading of Au nanoparticles and CdS quantum dots. The CdSQDs and Au particles extend the visible light harvesting ability of CeO2 by a sensitization mechanism and the LSPR process, respectively. (ii) The Au nanoparticles act as mediators which efficiently enhance the interfacial charge transfer between the CeO2 and CdS, thus reducing recombination by separating the exciton pair as confirmed from PL and EIS analysis. (iii) The equilibration of Fermi levels due to the exposure of Au nanoparticles to liquid electrolyte confirms the heterostructure electron relay mechanism. In addition, the surface plasmon effect of Au nanoparticles would generate hot electrons and inject these into CeO2 directly through the Schottky barrier, hence enhancing the photocatalytic activity of CeO2 in the visible light region.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01751. XRD data and TEM and color mapping images (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*K.M.P.: e-mail, [email protected] and [email protected]; tel, +91-674-2379425; fax, +91-6 74-2581637. ORCID

M. Scurrell: 0000-0002-6988-0785 K. M. Parida: 0000-0001-7807-5561 Notes

The authors declare no competing financial interest. 12305

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