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Hexagonal Ceria Located at the Interface of Anatase/Rutile TiO2 Superstructure Optimized for High Activity under Combined UV and Visible-Light Irradiation Ranjana Verma,† S. K. Samdarshi,*,‡ and Jay Singh§ †

Solar and Energy Materials Laboratory, Department of Energy, Tezpur University, Tezpur, Assam, India Centre for Energy Engineering and Centre of Excellence in Green and Efficient Energy Technology (CoE-GEET), Central University of Jharkhand, Ranchi, Jharkhand, India § Department of Applied Chemistry & Polymer Technology, Delhi Technological University, Shahbad Daulatpur, Main Bawana Road, Delhi 110042, India ‡

S Supporting Information *

ABSTRACT: The photocatalytic activity of the system critically depends on the catalyst’s architecture and composition. In a ceria−titania system a proper ratio and location of ceria on titania might be crucial to have an optimum level of physicochemical properties for efficient charge carrier generation and separation in the system to initiate redox reaction at its surface. Herein, a novel architecture of the ceria-doped TiO2 system has been developed to study its photoactivity and the mechanism of charge transfer. The photocatalytic activity with optimum cerium content Ce/Ti = 0.2 wt % shows the highest degradation of methylene blue under combined (UV + Vis) irradiation compared to the pristine titania. The obtained result shows that catalyst composition is an important parameter in modeling and optimizing the system to have the desired impact on photoactivity. The proposed unique architecture and composition combination appears to be an ideal model for design of novel photoactive materials having activity in a wide range of irradiation wavelength. The study is supported with characterization tools such as X-ray diffraction (XRD), transmission electron microscope (TEM), energy-dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), UV−Vis absorption (UV−Vis, DRS), and fluorescence studies.

1. INTRODUCTION Solar energy is uniquely poised to solve both energy and environmental problems which are being faced by humanity. It is, therefore, pertinent to develop a suitable environmentally friendly technology which permits the use of the full range of the solar spectrum to solve energy and environmental related problems simultaneously. The challenge of solving these two problems with one technology is possible through solar photocatalytic conversion. A metal oxide semiconductor photocatalyst, such as TiO2, promises to provide a number of emerging solutions for enhancing energy efficiency and reducing environmental pollution with minimal carbon footprint. However, due to the large band gap, the practical application is restricted by limited photon utilization in the ultraviolet range which accounts for a small fraction (about 5%) of solar irradiance. Another major limitation of TiO2 is massive photogenerated electron−hole recombination which limits the efficiency of the photocatalyst. By tailoring/lowering the band gap, the extension of the absorption spectrum of TiO2 in the visible range can be achieved. Additionally, nanostructuring and band structure engineering employing materials with matching band potential can be done to increase the cumulative photon absorption, extend the effective visible spectrum, and improve the photogenerated charge carrier separation in TiO2.1,2 These © XXXX American Chemical Society

modifications in TiO2 have been accomplished by one/a combination of a number of different strategies such as doping, codoping, sensitization, phase coupling, hydrogenation, and coupling with semiconductors (compositization) as well.3−9 Among the composite system with TiO2 as the major component, CeO2 has been shown to be a promising candidate owing to the following reasons: First, CeO2, due to its suitable band edge positions, has been successfully used in a number of photocatalytic processes such as detoxification10 and hydrogen production.11 Second, the redox shift between Ce4+ and Ce3+ can impart high capacity to the system to store/release oxygen under oxidizing/reducing conditions.12−16 The band gap reduction enhances the number of photogenerated charge carriers, but their separation can only be ensured through mechanisms or lattice structure wherein (i) the easy mobility of charge carriers into or out of the nanostructure is favored by the architectural design of the lattice; (ii) suitable conditions/trap centers facilitate the photogenerated charge carriers to take part in the desired redox reaction at the interface; (iii) the charge carriers move in Received: June 13, 2015 Revised: September 29, 2015

A

DOI: 10.1021/acs.jpcc.5b05652 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

all the processes. The experiments were carried out at room temperature and humidity. 2.2. Catalyst Preparation. The mixed-phase titania photocatalyst was synthesized by the sol−gel method as reported earlier.7 Ceria-doped TiO2 nanocomposites were synthesized by the simple sol−gel method having a dopant concentration of 1, 0.5, 0.2, and 0.05 wt %, respectively, with titanium isopropoxide as the host and cerium nitrate hexahydrate as the dopant precursor. Initially, 3 mL of titanium isopropoxide was added to 15 mL of 2-propanol under constant stirring for 20 min at room temperature, followed by the addition of a few drops of water to initiate the hydrolysis process. Thereafter, cerium nitrate hexahydrate solutions were subsequently added dropwise to the above solution. Then the solution was left for aging for 12 h at room temperature under constant stirring. Finally, the resultant mixture was subjected to a heating ramp of 4 °C/min and calcined in air at 650 °C for 1 h. The samples were named as TiO2 (M) for the titania mixed phase and xCeTi for the composites, where x is the amount (in wt %) of Ce loaded [x wt % = Ce/(Ce + TiO2) × 100%]. 2.3. Characterization. X-ray diffraction (XRD) of the TiO2 and ceria-doped TiO2 samples was recorded with a Rigaku miniflex X-ray diffractometer equipped with Cu Kα radiation (λ = 1.54 Å) in a 2θ range of 10−70° at a scanning rate of 0.05° 2θ/s. Transmission electron microscope (TEM), highresolution TEM (HRTEM), and selected-area electron diffraction pattern (SAED) micrographs were taken by using Phillips, CM200 at an operating voltage of 200 kV with a resolution of 2.4 Å. The EDS and elemental mapping analysis of the sample were performed using a field emission energy filtering transmission electron microscope (FE-TEM) (JEM2200FS). An X-ray photoelectron spectroscopy (XPS) was performed using monochromated Al Kα irradiation using an Axis-Nova, Kratos Analytical Ltd., Manchester, UK. Diffuse reflectance spectra (DRS) were recorded in the range from 200 to 800 nm using a Shimadzu UV−vis spectrophotometer (VU 2200, Japan) equipped with an integrating sphere using BaSO4 as a reference. The fluorescence spectra were recorded on a PerkinElmer LS-55 spectrometer at room temperature with constant 5 nm excitation and emission slit width with an excitation wavelength of 320 nm. 2.4. Photocatalytic Degradation Experiments. The photocatalytic activity of each sample was studied by degradation of methylene blue under UV (λ < 400 nm), visible (λ > 400 nm), and UV + visible light. The UV and visible irradiance at the reactor surface was 0.15 W/m2 (Philips 15W/G15 T8, Holland) and 14.5 W/m2 (Philips 18W/54 1M7 India), measured by Research Radiometer International Light (USA) with detectors SD 005 and SD 033, respectively. The catalytic material loading of the experiment was kept at 0.5 g/L, and the average reactor temperature was maintained at 35 °C. The solution was kept in the dark for 2 h to achieve adsorption−desorption equilibrium in each case. The experiments were carried out by simultaneous exposure of the catalysts, each having 30 mL of methylene blue of 1 mM concentration under stirred conditions. The catalyst-loaded MB solution was illuminated under UV, visible, and UV + visible light for 60 min, respectively, and sampling was done at 15 min intervals. At given time intervals, the photoreacted solution of the centrifuged sample was analyzed by recording variations of the absorption band maximum (664 nm), using a UV−visible spectrophotometer (Shimadzu 1700, Japan).

two different directions under the influence of a potential barrier that exists in the interfacial region with precisely located band edges suitable for the intended photocatalytic activity; and (iv) the transfer of the charge particles from an excited metaloxide semiconductor to the coupled semiconductor across the interface is favored. These are the issues which need special attention in the development of a high activity photocatalytic system. Reports on the CeO2-doped TiO2 composite system as visible-light photocatalysts for selective organic transformations are negligible. Most of the authors have reported that the ceriadoped TiO2 system shows enhanced photoactivity only under UV irradiation. Xiao et al. prepared mesoporous cerium-doped TiO2 nanoparticles and showed the negative effect of cerium incorporation to the titania compared to undoped titania on the photoactivity even under UV light.17 Reli et al. prepared novel cerium-doped titania and showed that 1.2% Ce shows maximum photocatalytic activity in the decomposition of ammonia under UV light.18 Cao et al. prepared a hierarchical CeO2/TiO2 nanofibrous mat by a facile electrospinning and hydrothermal method. Their study showed that the CeO2/ TiO2 nanocomposite possessed a higher photocatalytic activity than the bare TiO2 for the degradation of RB dye under UV light irradiation.19 Xiao et al. prepared cerium-doped TiO2 mesoporous nanofibers using collagen fiber as the biotemplate and showed that the degradation degree of RhB using the Ce0.03/TiO2 nanofiber reached 99.59% in 80 min under visible light, which was high compared to the undoped TiO2 nanofiber and the commercial Degussa P25.20 Notably, the undoped titania nanofiber had more rutile, while doped titania samples had a higher anatase titania fraction; both showed high visiblelight activity compared to Degussa P25 which had a higher anatase fraction. Normally pristine mixed-phase titania with more rutile fraction than anatase shows high visible-light activity.21 Karunakaran et al. synthesized CeO2−TiO2 nanocomposites and found that under UV/visible light the nanocomposite with the smallest crystallite size is the most efficient photocatalyst to detoxify cyanide in alkaline solution.22 The photocatalytic reaction is a very complex process and is influenced by many parameters such as specific surface area, surface morphology, phase ratio, band gap, and band edge location of the photocatalyst material. The suppression of recombination of carriers by creating a differential mobility/ diffusivity environment in the material phase(s) and/or band offset at the interface has been demonstrated in the case of biphasic titania21 and the ZnO system,23 but the same has not been done in the case of the metal−oxide composite system to develop a general explanation. To the best of our knowledge, it was seldom studied in the case of CeO 2 -doped TiO 2 nanocomposites with the objective of analyzing both the positive and negative impact of cerium incorporation into the titania on the photoactivity.

2. EXPERIMENTAL SECTION 2.1. Materials. The chemicals used for the synthesis of CeO2-doped TiO2 were cerium(III) nitrate hexahydrate [Ce(NO 3 ) 3 ·6H 2 O], titanium isopropoxide [Ti(OCH(CH3)2)4], and 2-propanol [(CH3)2CHOH]. Titanium isopropoxide was purchased from Sigma-Aldrich. Cerium(III) nitrate hexahydrate was purchased from Himedia (India) and 2-propanol from Merck (India). All the reagents were of analytical grade, and double-distilled water was used in B


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3. RESULTS AND DISCUSSION 3.1. X-ray Diffraction (XRD) Analysis. Figure 1 shows the XRD diffraction pattern of CeO2-doped TiO2 samples calcined

Table 1. Crystallite Size, Phase Composition, and Band Gap Details for Undoped TiO2 (M) and CeO2-Doped TiO2 Samples sample

crystallite size (nm)

TiO2 (M) 1% CeTi 0.5% CeTI 0.2% CeTi 0.05% CeTi

(A) 19.2 (A) 14.6 (A) 15.9 (A)16.1 (A) 18.4

(A) (A) (A) (A) (A)

fraction (%)

A/R

band gap (eV)

56.56, 84.14, 84.13, 53.33, 52.51,

1.30 5.30 5.30 1.14 1.10

2.84 3.12 3.08 2.89 2.97

(R) (R) (R) (R) (R)

43.44 15.86 15.87 46.67 47.49

smaller crystallite size than those of undoped mixed phase TiO2 (M), and the crystal size decreases with the increase of Ce doping amount. Therefore, it can be concluded that doping leads to reduction in crystallinity and decrease of crystallite size.16 Doping cerium significantly influences the phase of TiO2. It is interesting to note that as we increase the doping concentration the main anatase peak corresponding to the [101] plane starts dominating over the rutile peak, and peak intensity of rutile TiO2 gradually decreases which indicates that cerium ion doping hinders phase transformation from anatase to rutile at high temperature (>400 °C) or high ceria concentration. 3.2. TEM Analysis. The TEM and HR-TEM images of the 0.2% CeTi sample are shown in Figure 2. Figure 2(b), the magnified view of the Figure 2(a), clearly identifies the CeO2 nanoparticles located at the interface of the anatase−rutile region as shown by pointed arrowheads. The average particle size of the 0.2% CeTi sample is 22 nm. HR-TEM images of 0.2% CeTi clearly show that the CeO2 nanoparticle, located at the anatase/rutile interface, has a well-defined contact surface area as shown in Figure 2(b and c). The regions of the image in Figure 2(b) having different phases are shown in higher magnification as indicated by white-colored dashed arrows. The lattice fringes in the high-resolution HR-TEM image (Figure 2b and 2c) have a spacing of 0.35 nm due to (101) planes and a spacing of 0.32 nm due to the (110) planes of anatase and rutile having CeO2 at the interface with a lattice spacing of 0.31 nm due to (111) planes, respectively. Figure 2(d and e) shows the corresponding fast Fourier transform (FFT) patterns of the selected area of anatase and rutile, respectively. The lattice spacing calculated through SAED and fast Fourier transform (FFT) patterns of the selected area of the corresponding HRTEM image indicate the presence of TiO2 (rutile, 0.32 nm) and CeO2 (cubic, 0.31 nm) phases (Figure S2) in the 0.2% CeTi nanocomposite. The small differences between nanocrystal atomic interplanar spacing can be caused by different concentrations of oxygen vacancies in the nanocrystals.29,30 The elemental energy-dispersive spectroscopy (EDS) mapping characterization was used to investigate the chemical composition and elemental distribution in 0.2% CeTi samples. The typical EDS spectrum (Figure 3a) for the sample 0.2% CeTi confirms the presence of titanium, cerium, and oxygen in them. The EDS elemental mapping of Ti, O, and Ce elements, shown in Figure 3b−e, shows that Ti, O, and Ce are homogeneously distributed in the 0.2% CeTi nanocomposites. 3.3. XPS Analysis. XPS measurement was carried out to understand the change of surface chemical bonding as well as the electronic valence band position of Ti and Ce in the TiO2 and 0.2% CeTi nanostructure. Figure 4(a) shows the XPS survey spectrum of typical TiO2 (M) and 0.2% CeTi nanoparticles, respectively. Photoelectron peaks of Ti, O, and

Figure 1. XRD diffraction of CeO2-doped TiO2 nanocomposites annealed at 650 °C with a peak identified for anatase (⧫), rutile (▲), and CeO2 (*).

at constant temperature 650 °C for 1 h. The diffraction peaks of the samples correspond to an anatase−rutile mixed phase with anatase as a dominating phase (JCPDS File No. 894203 and JCPDS File No. 894920). Minor peaks for [111], [200], and [222] planes indicate the presence of face centered cubic CeO2 (JCPDS File No.81-0792). The intensive peaks of CeO2 cannot be identified in all samples since the ionic radii of Ce3+/Ce4+ (1.03/1.02 Å) are bigger than that of Ti4+ (0.68 Å). It is thus difficult for all Ce3+ and Ce4+ ions to replace Ti4+ in the crystal lattice.24 In that situation, only a few Ce3+/Ce4+ ions penetrate the lattice, and the rest of the dopant ions are most likely to be located at the grain boundary/grain junction.20 However, these relatively large Ce3+/Ce4+ cations at the grain boundaries and grain junctions can inhibit crystallite growth of titania through the formation of Ce−O−Ti bonds as confirmed by XPS. Compared with undoped TiO2 (M), the shift of xCe/Ti samples is obvious, as shown in Figure S1. It is also observed that the width of the (101) plane diffraction peak of anatase becomes broader with the increase of Ce doping concentration. This shifting indicates that some of the dopant ions might have been incorporated in substitutional or interstitial sites of TiO2, leading to reduction in crystallinity.25 Also the broadening of the XRD peak is due to the contribution from the amorphous grain boundary region, which becomes more disordered if cerium sits on these regions, inhibiting crystal growth.26 In addition, the average crystallite size of each xCeTi sample is calculated from the broadening of XRD peak (101) for anatase by using the Scherrer equation:27 where d = kλ/β cos θ where d is the average crystallite size, k the shape factor taken as 0.9, β the full width at half-maximum (fwhm) of the most prominent peak [101], and θ the diffraction angle. Also, each of the anatase and rutile phase mass fractions and anatase-to-rutile ratio (A/R) of the samples are calculated using Spurr’s equation:28 fa = (1 + 1.26Ir/Ia)−1, f r = (1 − fa), and A/R = (fa/f r), where fa and f r are anatase and rutile mass fraction and Ia and Ir are the integrated intensities of the most intense peaks of anatase [101] and rutile [110], respectively. The crystallite size and phase composition of each sample are shown in Table 1. It is found that the xCe/TiO2 nanocomposites have relatively C


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Figure 2. TEM (a) and HR-TEM images of 0.2% CeTi nanocomposites (images b and c) showing the phases (marked by arrowheads). The images (d and e) show the corresponding FFT patterns of the selected area of anatase and rutile.

corresponds to an intermediate oxidation state of Ti from tetrato trivalent. The Ti 2p1/2 region in 0.2% CeTi is well fitted into two peaks of Ti3+ and Ti4+ (464.3 and 463.2 eV).34,35 Figure 4(c) shows the binding states of oxygen in TiO2 and 0.2% CeTi nanoparticles with the O 1s XPS peak fitted to three deconvoluted peaks. These peaks appear at 530.1, 531.4, and 528.7 eV for TiO2 (M).8 The peak at 530.1 eV is generally due to the O2− ion in the TiO2 crystal structure.36 In 0.2% CeTi the binding energy peaks at 531.2, 529.7, 529.2, and 528.4 eV are assigned to Ti−OH, Ti−O, Ti−O−Ce, and Ce−O, respectively.37,38 The binding energy of O 1s for surface oxygen is shifted from 530.1 to 529.7 eV in 0.2% CeTi. This O 1s peak shift suggests that Ti and Ce chemically interact with each other in the CeO2-doped TiO2 system.35 Figure 4(d) shows the

C are clearly seen in both the samples. The XPS survey spectrum of 0.2% CeTi shows a binding energy peak for Ce 3d in addition to the Ti 2p and O 1s orbitals. The peak at 284.5 eV signals the presence of adventitious elemental carbon as a reference with an accuracy of ±0.1 eV. The XPS spectra of the Ti 2p region of TiO2 and 0.2% CeTi are shown in Figure 4(b). The Ti 2p peak in TiO2 (M) appears as a single, well-defined, spin-split (5.7 eV) doublet which is assigned to Ti 2p1/2 and Ti 2p3/2 corresponding to Ti4+ in the tetragonal structure.31,32 The binding energies of the peaks are found to be at 464.5 eV for Ti 2p1/2 and 458.8 eV for Ti 2p3/2, which are in agreement with the binding energies of TiO2 reported earlier in the literature.7,33 However, the Ti 2p3/2 peak is shifted from 458.8 eV in TiO2 (M) to 458.3 eV in 0.2% CeTi. This shifting D


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Figure 3. EDS spectrum showing semiquantitative estimation of the elemental composition in 0.2% CeTi nanocomposites (a). Elemental mapping showing the presence of cerium (orange), titanium (yellow), and oxygen (pink) (b)−(e).

Figure 4. (a) XPS survey spectrum of 0.2% CeTi and TiO2 (M). (b) Ti 2p XPS spectra of TiO2 and 0.2% CeTi catalysts. (c) O 1s spectra of TiO2 and 0.2% CeTi. (d) Ce 3d region of 0.2% CeTi.

fitted deconvolution of the core level Ce 3d XPS spectrum of 0.2% CeTi into various states which are labeled according to the convention established by Burroughs et al., where v and u

indicate the spin−orbit coupling states of 3d5/2 and 3d3/2, respectively.39 The Ce 3d region is complicated because of the hybridization of Ce 4f and O 2p electrons. The spectrum E


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Figure 5. (a) Diffuse reflectance spectra of TiO2 (M) and CeO2-doped TiO2 nanocomposites. (b) Tauc’s plot showing optical band gap.

basically denotes a mixture of Ce3+/4+ valence states giving rise to various peaks, representing the coexistence of Ce3+ and Ce4+, respectively. The peaks assigned to 3d5/2 states (labeled v) containing five main peaks at 880.2 (v0), 882.3 (v), 884.9 (v′), 889.9 (v″), and 898.7 eV (v‴) and 3d3/2 spin−orbit states (labeled u) containing five main peaks at 899.5 (u0), 902.3 (u), 903.2 (u′), 907.8 (u″), and 917.1 eV (u‴) compare well with the results from earlier reported literature.13,24,40 Since the radii of Ce4+ (1.02°A) and Ce3+ (1.03°A) are bigger than Ti4+ (0.68°A), it is difficult to dope all Ce3+/Ce4+ ions into a TiO2 crystal lattice and substitute Ti4+. Therefore, it is expected that a Ce−O−Ti bond is formed at the interstitial sites or at the interface between CeO2 and TiO241 which is confirmed by the O 1s spectra of 0.2% CeTi. Thus, reduction of Ti4+ to Ti 3+ and the presence of Ce4+/Ce3+ at the interface of TiO2 indicates the presence of more oxygen defects at the interface of TiO2 nanoparticles. 3.4. Optical Properties. Optical properties are important to interpret photocatalytic behavior. The diffuse reflectance spectra (DRS) of the nanocomposites are shown in Figure 5. The DRS of CeO2-doped TiO2 shows strong visible-light absorption. A large absorption hump between 500 and 700 nm is seen for the undoped TiO2 mixed-phase samples in Figure 4(a). This hump in the visible zone may be due to a large concentration of defects at the phase interface and Ti4+. The band gaps of the nanocomposites have been calculated using Tauc’s method (Figure 5(b)), and their corresponding band gap42,43 values are shown in Table 1. The lowering of band gap in the undoped TiO2 mixed phase may be possibly due to defect states created by oxygen vacancies present at the interface which has a positive role in photocatalysis.7,44 The red shift is observed for 0.2% CeTi and undoped TiO2 (M) which is substantially high for the pure TiO2 (M) sample. This red shift is attributable to charge transfer between the conduction band of anatase and rutile via impurity band energy states. All samples exhibit strong absorption in both ultraviolet and visible regions which along with the multiple scattering processes enable harvesting of the wide range solar spectrum more efficiently. Thus, it is important to note that the 0.2% CeTi sample exhibits much stronger absorption in both ultraviolet and visible-light regions than TiO2 (M). This enhanced light trapping effect is attributed to the CeO2 nanoparticles.45 Because of the existence of an additional TiO2−CeO2 phase interface along with the TiO2 (anatase)−TiO2 (rutile) interface in the composite, the probability of separation of charge carrier increases. Thus, despite having higher band gap and the

resultant narrower absorption spectrum, the efficient charge separation in the 0.2% CeTi sample makes it more photoactive. 3.5. Fluorescence Spectra Analysis. Fluorescence (PL) spectra response of the samples was obtained at an excitation wavelength of 320 nm as shown in Figure 6. The PL emission

Figure 6. Fluorescence spectra of TiO2 (M) and CeO2-doped TiO2 nanocomposites at an excitation wavelength of 320 nm.

spectra results from the recombination of photoexcited electron and holes, wherein lower fluorescence intensity indicates less recombination between electron and holes.13 The undoped TiO2 (M) shows weaker PL intensity than that of ceria-doped TiO2 samples due to reduction in electron−hole recombination. Due to the close proximity of oxygen vacancy/defects at the interface which provides the nonradiative recombination path for electrons and holes and, hence, results in an increase in the transfer of emission energy between them and, subsequently, quenching of the PL peak.7,22,46 There is a close correlation between the oxygen vacancies and the anatase to rutile phase ratio (A/R) in the mixed phase titania.7 Accordingly, the quenching of the PL peaks may also be related to the A/R ratio. Thus, the lower A/R ratio shows higher reduction in recombination (given in Table 1). Moreover, CeO2-doped TiO2 exhibits emission spectra similar to that of the undoped mixed phase TiO2 (M). However, a difference lies in their emission intensity which is substantially lower for TiO2 (M). The variation in the intensity depends on the location of the dopant as well as on the separation between the dopant in the host system which increases the nonradiative transition. All samples exhibit strong UV emission peak at 358.5 nm, which F


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Figure 7. (a) Photocatalytic degradation of MB under UV light. (b) ln C/Co vs time plot for determination of rate of constant. (c) Photocatalytic degradation of MB under visible light. (d) ln C/Co vs time plot for determination of the rate constant. (e) Photocatalytic degradation of MB under combined (UV + visible) light. (f) ln C/Co vs time plot for determination of the rate constant.

3.6. Photocatalytic Detoxification of Methylene Blue (MB). Figure 7 shows the photocatalytic performance of CeO2doped TiO2 nanocomposites investigated through degradation of methylene blue irradiated with UV (λ < 400 nm), visible (λ > 400 nm), and a combination of both UV and visible (400 < λ > 400 nm) radiation, respectively. The photocatalytic degradation reaction is assumed to follow a pseudo-first-order reaction where ln(C/Co) = kt, and k is the apparent reaction rate constant (min−1) shown in Figure 6. The reaction rate constants are obtained by plotting ln C/Co vs time, and the results are shown in Table 2. The result indicates that 0.2% CeTi has much higher photoactivity than that of TiO2 (M) with the irradiation of UV light. However, under visible light 0.2% CeTi has lower photoactivity than that of undoped TiO2 (M). The variation in

arises due to direct transition in the X-zone of the brillouin zone, i.e., X1a to X1b, and the peak at 398 nm arises due to phonon-assisted indirect transition from the edge (X) to the center (Γ) of the brillouin zone, i.e., X1b to Γ3.47,48 However, the intensity of the peak is reduced for undoped TiO2 (M). The peak around 459 and 518 nm emissions may be attributed to the defect states and the surface oxygen vacancies in the surface region of TiO2.49 The observed broad band between 500 and 550 nm is attributed to radiative recombination of excitons of the shallow traps associated with oxygen vacancies and Ti4+ adjacent to oxygen vacancies.22,26,50 The peak around 585 nm is due to oxygen vacancy,51,52 and the peak around 539 nm is due to the F+ center.52 The peak around 560 nm is attributable to the light absorption coefficient known as the dual frequency peak.53 G


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electrons. If electrons are being transferred from the valence band to the new energy level, a highly oxidizing hvb+ will oxidize H2O to the hydroxyl radical (·OH) and H+. The ecb− will only be able to reduce any O2 to superoxide (O2 −) if its redox potential is greater than that of oxygen (E0 = +0.313 V). The TiO2 lattice structure is in intimate contact with CeO2 via Ti− O−Ce at the interface {shown in Figure 8(a)} which is duly supported by XPS spectra and TEM. Band bending will occur at the interface in accordance with the work function of the two systems.45,54 The work function of CeO2 (4.69 eV), being smaller than that of TiO2 (5.18 eV), enables the transfer of electrons from CeO2 to TiO2, giving rise to a higher carrier concentration in TiO2 nanoparticles.55,56 As a result the inner electric field will be developed which will be directed from TiO2 to CeO2. Under combined (UV + visible) irradiation, there will be an excitation of electrons in both the interfacial phases (anatase and rutile) as well as in CeO2. As the conduction band of CeO2 lies above TiO2 the photogenerated electrons in the conduction band of CeO2 will migrate to the anatase side which will subsequently diffuse to favorable electron trapping sites of rutile. Thus, the photoexcited electrons from CeO2 will migrate to the CB of TiO2. These electrons then diffuse to the catalyst surface where they easily scavenge the surface oxygen to produce the reactive superoxide radical ion O2−. The holes will migrate to CeO2 where they react with H2O to produce the active species hydroxyl radical ion OH*. Thus, a combination of both inner electric fields developed and light-induced separation of photogenerated charge contributes to the enhancement of the photocatalytic activity. Schematic illustration (Figure 8b) showing charge transfer from CeO2 at the interface TiO2 mixed phase for enhanced photocatalytic activity. The photoactivity of ceria-doped TiO2 gradually decreases with cerium doping concentration since the electrons in the new energy level do not have a higher redox potential than

Table 2. Shows the Rate Constant under UV (λ < 400 nm), Visible (λ > 400 nm), and on a Combination Effect of Both UV and Visible (400 < λ > 400 nm) Light by Calculating the Slope via Plotting ln C/Co vs Time rate constant (min−1) samples

kUV

kVisible

kUV+Visible

TiO2 (M) 1% CeTi 0.5% CeTi 0.2% CeTi 0.05% CeTi

0.0448 0.0171 0.0340 0.0651 0.0533

0.0081 0.0021 0.0025 0.0049 0.0038

0.0568 0.0282 0.0494 0.1105 0.0756

the result under different irradiation may be due to the shift of energy bands due to the composites formed at the interface. However, interestingly the synergy of the materials in optimized 0.2% Ce−Ti enables its photoactivity to surpass others under combined (UV + visible) irradiation. The highest rate constant for combined (UV + visible) photoactivity for the sample 0.2% CeTi is 0.1105 min−1. If the combined (UV + visible) photocatalytic reaction is assumed to be the sum of the two independent and simultaneous processes of photocatalytic degradation under visible and under UV radiation then dC/dt = kuvC + kvisC = k(UV+vis)C, where kUV is the rate constant only under UV, kvis only under visible, and kUV+vis under combined UV and visible radiation. It is seen that for all the samples k(UV+vis) is greater than kUV+ kvis. Therefore, a synergetic effect under visible and under UV irradiation does exist in the combined (UV + visible) photocatalytic degradation. This enhanced combined (UV + visible) photoactivity is attributed to unique structure and a variety of favorable properties which harvest the full solar spectrum more efficiently. 3.7. Kinetics Mechanism. The proper location of energy levels within the band gap of the host materials is an important parameter in determining the reducing strength of the

Figure 8. (a) Structure of CeO2-doped TiO2 nanocomposites showing Ti−O−Ce bonds. (b) Schematic showing charge transfer from CeO2 at the interface TiO2 mixed phase for enhanced photocatalytic activity. H


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(CSIR), India, for the award of a Senior Research Fellowship (SRF: 09/796/(0061)/2015-EMRI). SKS also acknowledges the support of CoE-GEET under FAST scheme of MHRD, New Delhi.

oxygen. Also the superoxide radicals cannot be formed since the electrons cannot be efficiently transferred. Visible light, however, is not energetic enough to promote electrons to the conduction band; as a result ceria-doped TiO2 has lower photocatalytic activity under visible light than under UV light. Thus, ceria-doped TiO2 has limited ability to utilize low energy visible-light photons. However, under optimized condition, CeO2-doped TiO2 surpasses the performance of undoped TiO2 under UV + visible light combination due to synergy of the materials.

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4. CONCLUSION A series of ceria-doped mixed-phase TiO2 composites were synthesized by the simple sol−gel route. It was found that doping cerium inhibits the onset of transformation of the metastable anatase phase to the most stable rutile phase of titania. This unique result helps in keeping the dominance of the anatase phase in the composite. The synthesized composite was analyzed for its photoactivity in the photodegradation of methylene blue under UV, visible, and combined (UV + Vis) irradiation. The conclusion from the analysis of the result provides quantitative evidence to interpret the photocatalytic behavior which is based on catalyst architecture. The prepared CeO2-doped TiO2 with optimized cerium content exhibited high activity under UV and low activity under visible irradiation compared to mixed-phase titania. However, it shows high activity under UV + visible light combination which is attributed to the synergistic effect of the junction formed between CeO2/anatase/rutile. The photocatalytic properties were promoted with CeO2 located at the interface of anatase/ rutile which behaves as the active site facilitating efficient charge separation for enhanced photocatalytic activity under combined (UV + visible) irradiance. The catalyst architecture (with optimized ratio of dopant) enables us to harvest the solar spectrum at a wide range of wavelengths more efficiently. Thus, it provides a new platform for the design and fabrication of a high-efficiency photocatalyst system.

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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.jpcc.5b05652. X-ray diffraction pattern of undoped TiO2 (M) and CeO2-doped TiO2 nanocomposites showing the enlarged (101) peak (Figure S1). SAED pattern of (a) TiO2 (M) mixed phase and (b) 0.2% CeTi nanoparticles. and The image (c, d, and e) shows the corresponding fast Fourier transform (FFT) patterns of the selected area of anatase (TiO2) and the cubic (CeO2) and rutile TiO2 structure in the 0.2% CeTi nanocomposite (Figure S2) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: drsksamdarshi@rediffmail.com. Mobile: +919431107270. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Authors acknowledge with thanks the support provided by DST, New Delhi, and AICTE, New Delhi (India). Author RV is thankful to the Council of Scientific and Industrial Research I


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