Bismuth-Induced Integration of Solar Energy Conversion with

Oct 24, 2013 - For conventional photocatalysis, the energy threshold rather than merely the spectral response is always restricted that the infrared p...
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Bismuth-Induced Integration of Solar Energy Conversion with Synergistic Low-Temperature Catalysis in Ce1−xBixO2−δ Nanorods Dong Jiang, Wenzhong Wang,* Erping Gao, Ling Zhang, and Songmei Sun State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, P. R. China S Supporting Information *

ABSTRACT: For conventional photocatalysis, the energy threshold rather than merely the spectral response is always restricted that the infrared part (48% of solar energy) has never been efficiently utilized, undesirably elevating the temperature and damaging the photon-to-electron conversion. It remains challenging to conquer the IR-related contradiction and integrate the infrared energy into the solar energy conversion. Herein, we logically designed a Bi-induced synergistic photo/thermocatalyst (fluorite Ce1−xBixO2−δ nanorods), where the coupled ionic conductivity accompanying highly reductive Bi and concomitant oxygen vacancies helped bring about integration of photocatalysis with synergistic low temperature (20−80 °C, IR-driven) catalysis, promising for the effective utilization of infrared energy. More generally, through our results a feasible methodology is verified in detail that integration of semiconductor photocatalysis with solid state ionics may help design brand new catalysts, shedding light on the practical solar energy conversion.

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INTRODUCTION Under consideration of not only the environment but also the energy issues, solar energy conversion into chemical bonds using semiconductor photocatalysis should be promising.1−6 However, for conventional photocatalysis, the infrared part (IR) has never been efficiently utilized due to the energy restriction. Furthermore, temperature increase stemming from the infrared radiation is mostly repulsive, as it usually leads to severe lattice vibration, considerably decreasing the photon-toelectron conversion efficiency.7,8 It must be one milestone once the infrared energy can be integrated into solar energy conversion. Actually, without regard of optical excitation, temperature increment (20−80 °C) from the IR radiation is widespread and exactly favorable in thermocatalysis, broadly involved as a fairly mature technology.9−13 Therefore, incorporation of IR-driven low-temperature catalysis should be promising for the practical solar energy conversion. Up to now, such photo/thermo-related reports are scarce, and nearly all attempts involved noble metals as cocatalysts.14−18 Furthermore, in these cases, people focused on the general promotion of elevated temperatures on chemical reactions, which is easily intelligible with the empirical Arrhenius equation. However, the negative temperature effect on solar energy conversion was totally overlooked, let alone the incorporation of low-temperature catalysis.14−18 Moreover, introduction of noble metals inevitably made catalysts less competitive. Therefore, the challenge is whether based on the cost control a mild integration of IR-driven thermocatalysis into practical solar energy conversion can be achieved, primarily conquering the IR-related contradiction. Ceria comes into our view because of its unique capability of oxygen ion conductivity. As certified photocatalyst and © 2013 American Chemical Society

universal thermocatalyst, CeO 2 has an enviable report.11,13,19−23 With mixed ionic and electronic conductivity, ceria is promising for the suppression of negative temperature effect.24,25 That is in CeO2 electronic conductivity will be generated with coupling oxygen ion conduction, and the migration of hole-trapped oxygen ions can effectively suppress the recombination of carriers in spite of elevated temperatures.25 Yet just as our experience, CeO2 has a band gap of 3.0−3.2 eV and not enough light response; besides, ceria gets used to high temperatures and barely works under 100 °C. To overcome the above barriers, we take notice of bismuth oxide (cubic δ-Bi2O3), which is known as another oxide ionic conductor and can release oxygen at low temperatures.26−29 Therefore, it is logically associated with that Bi2O3 incorporating into CeO2, that is Bi-induced material (Ce1−xBixO2−δ), may help make a difference. Furthermore, through homogeneous incorporation of Bi3+: (1) a high thermodynamic stability can be guaranteed with the structural similarity that δ-Bi2O3 has an atomic configuration of cubic C-type rare earth sesquioxide (Scheme 1), which can be regarded as plentiful vacant sites within the fluorite lattice from which one-quarter of oxygen are extracted;26−28 (2) a consecutive energy band may be imported, avoiding the presence of enthetic localized states which act as recombination centers and achieving an efficient narrowing of the intrinsic gap, instead of just an additional shoulder-like absorption;30,31 (3) numerous vacancies will be produced within the ionic conductor due to charge Received: September 17, 2013 Revised: October 24, 2013 Published: October 24, 2013 24242

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under a pure H2 flow (80 mL min−1) at a heating rate of 10 °C min−1, using a thermal conductivity detector of a gas chromatography instrument. Raman spectra were recorded on a microscopic confocal Raman spectrometer (Renishaw 1000 NR) with an excitation of 514 nm laser light. X-ray photoelectron spectroscopy (XPS) was employed to study the chemical states of the prepared samples. Spectra were performed on ESCALAB 250, THERMO SCIENTIFIC Ltd. with a 320 mm diameter spot of monochromated aluminum Kα X-rays at 1486.6 eV under ultrahigh-vacuum conditions. The C 1s signal was used to correct the charge effects. Preparation of Ce1−xBixO2−δ Nanorods. Samples were synthesized by a facile one-step hydrothermal process under exactly the same conditions. In a typical hydrothermal procedure, Ce(NO3)3·6H2O (1.737 g) for CeO2 or Ce(NO3)3·6H2O (1.386 g) and Bi(NO3)3·5H2O (0.3881 g) for Ce0.8Bi0.2O2−δ were dispersed in diluted nitrite acid (HNO3, 5 mL, 4 M) by sonication first. Then the solution was added to deionized water (35 mL) dissolved with NaOH (10.24 g). This mixture was kept stirring for 30 min with the formation of a milky suspension and then added into a 50 mL Teflon-lined autoclave with a stainless steel tank. The autoclave was finally subjected to the hydrothermal treatment at 100 °C for 24 h, and afterward, the samples obtained were rinsed with deionized water and anhydrous ethanol for several times, respectively, and then oven-dried overnight in air at 60 °C. Catalytic Degradation Experiments. The catalytic activity of the as-prepared samples (100 mg) were evaluated by the gas-phase degradation of formaldehyde (200 ppm), operated in a gas-closed vitreous reactor (capacity 650 mL) with a quartz window and a double-walled jacket with water for temperature control. A 500 W Xe lamp and different temperatures were used to simulate the optical excitation of UV−vis part and thermal activation from the infrared radiation, respectively. The degradation process was monitored by the increment of CO2 in the reactor by GC analysis equipped with a CO, CO2 conversion furnace, and a FID detector, equivalent with a corresponding decrease of formaldehyde. After the experiment commenced, the variation of CO2 was plotted as a function of time to determine the catalytic performance. It is worth noting that prior to any catalytic oxidation, the relevant condition must be maintained for several hours until the measured concentration of CO2 remained unchanged to obtain equilibrium between adsorption and desorption, eliminating all adventitious interference factors. Electrochemical Analysis. The electrochemical measurements were performed on a CHI 660D electrochemical workstation (Shanghai Chenhua, China) using a standard three-electrode cell (a working electrode, a platinum wire counter electrode, and a saturated calomel electrode). To make a working electrode, as-obtained powders were deposited on a 15 × 25 mm fluorine-doped tin oxide (FTO) substrate by electrophoretic deposition. Briefly, the deposition was proceeded in an acetone solution (50 mL) containing catalyst powders (40 mg) and iodine (10 mg). Two parallel FTO electrodes were immersed in the solution with a 15 mm separation and were applied with a 10 V bias for 3 min. The asprepared electrodes were then rinsed with deionized water and dried at 60 °C overnight. The flatband potentials (Vfb) were estimated from Mott−Schottky plots by the electrochemical method, performed at a fixed frequency of 100 Hz with 10 mV amplitude at various applied potentials. The linear sweep voltammetry curve was achieved when the applied potentials

Scheme 1. Schematic Representation of the Relationship between the Cubic Fluorite Lattice (RO2) and the Cubic CType Rare Earth Sesquioxide (R2O3) (R = Rare Earths)

conservation, which may help promote the migration of oxygen ions and the low-temperature redox ability.32,33 Bearing the above issues in mind, we confidently believe the Bi-induced catalyst can help utilize the solar energy at large. In this present work, Ce1−xBixO2−δ (x = 0−0.5) were obtained with a facile one-pot solution method. With temperature control to simulate the thermal effect of the infrared radiation (80 °C), Ce0.8Bi0.2O2−δ did the best under the illumination (20−80 °C), presenting evident thermocatalytic as well as photocatalytic performance, rather than merely the general promotion of elevated temperatures from the empirical Arrhenius trend. The experimental results prove that this Biinduced material is promising for integrating the infrared irradiation into the solar energy conversion. With scientific arguments the origin of Bi-induced synergistically enhanced performance was unfolded, revealing the mystique lying in the integration of expanding light response and promoted ionic conductivity. So far as we can ascertain, this should be the first case of low-temperature integration of solar energy conversion with effective thermocatalysis. More generally, our results in detail suggest a feasible methodology that integration of semiconductor photocatalysis with solid state ionics may pave the way for further achievement in brand new catalysts design and practical solar energy conversion.

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EXPERIMENTAL SECTION Chemicals and Characterization. All the reagents were of analytical purity and used as received without further purification. NaOH, HNO3, and Bi(NO3 )3 ·5H2 O were purchased from Sinopharm Reagent Co. Ltd. Formaldehyde solution (37%) and Ce(NO3)3·6H2O were obtained from Aladdin Reagents, Shanghai. The purity and crystallinity of the as-prepared catalyst powders were characterized by powder Xray diffraction (XRD) with a Rigaku D/MAX 2250 V diffractometer using monochromatized Cu Kα (λ = 0.154 18 nm) radiation under 40 kV and 100 mA, over the range of 20° ≤ 2θ ≤ 80°. The morphologies and microstructures characterizations were performed on the transmission electron microscope (TEM, JEOL JEM-2100F, accelerating voltage 200 kV). High-resolution transmission electron microscopy analysis used the Digital Micrograph software (Gatan Inc.). UV−vis diffuse reflectance spectra (DRS) of the samples were obtained on a UV−vis spectrophotometer (Hitachi U-3010) using BaSO4 as the reference. The N2-sorption measurements were performed at 77 K using a Micromeritics Tristar 3000 analyzer, and the specific surface area was estimated with the Brunauer− Emmett−Teller (BET) method. Reduction behavior of samples was examined by temperature-programmed reduction (TPR) 24243

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ranged from −0.8 to 1.4 V. During all of the measurements, the electrolyte was 0.1 M Na2SO4 solution (pH ≈ 6).

CB samples. Obviously, under each condition, CB was much more efficient than C, presenting synergistic and overall enhancement. Moreover, CB achieved effective thermocatalysis under TC condition while pure ceria did not. That is when under illumination, the Ce0.8Bi0.2O2−δ sample will successfully integrate photocatalysis with IR-driven thermocatalysis (80 °C), rather than merely the promotion effect of elevated temperatures. To further verify the synergistic introduction of low-temperature catalysis and eliminate the interference from the general Arrhenius trend (eq 1), we studied the photocatalysis dynamics on the CB sample at different temperatures (20−80 °C, Figure S1).

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RESULTS AND DISCUSSION 1. Verifying the Methodology about Bi-Induced Synergistic Integration. Our proposal about the Bi-induced integration of solar energy conversion with synergistic lowtemperature catalysis will stand the trial when assuming a practical responsibility, once considerable thermocatalysis synergistically integrates into the photocatalytic process. Following logic of science, we confidently believe the Biinduced solid solution (Ce1−xBixO2−δ) incorporating Bi2O3 into CeO2 will be promising to make difference. In order to verify the illation, oxidative degradation of recalcitrant formaldehyde was carried out on Ce1−xBixO2−δ (x = 0−0.5). Here CeO2 (C for short) and Ce0.8Bi0.2O2−δ (CB for short) are taken for instance (for other samples, see Supporting Information). The elevated temperature stemming from infrared irradiation is detected to be 80 °C, and the samples were tested under three different conditions: at 25 °C under light irradiation, at 80 °C without light irradiation and under light irradiation without temperature control (80 °C), to simulate photocatalysis, thermocatalysis and photo/thermocatalysis, respectively. For convenience, hereinafter the above conditions are named PC, TC, and PTC. Figure 1a presents the CO2 increment averaged of three cycles during the catalytic oxidation of formaldehyde on C and

k = A exp(− Ea /RT ) or ln k = ln A − Ea /RT

(1)

where k and Ea represent reaction rate constant and apparent activation energy, respectively. According to the Arrhenius equation, the apparent activation energy (Ea) can be treated as a constant when undergoing a moderate temperature variation, and thus the reaction rate constant (k) will increase exponentially with temperature. The assignment of reaction rate constant series was performed through a linear estimation (Figure S1). As shown in Figure 1b, the −ln(k)−T−1 plot presents an obvious nonlinearity and indicates a successively increasing slope (Ea) with the temperature. The increased activation energy (Ea) unambiguously confirms the synergistic introduction of low-temperature catalysis rather than merely the promotion effect of the Arrhenius trend, because in that case the Ea will remain unchanged in spite of the increased rate constant (k), and the points should be located as marked along the dotted line (Figure 1b), corresponding to smaller rate constants (k) if the Ea remains the same. 2. Structural and Morphological Characterization of Catalyst. With repeatable tests (Figure S2) the suggested methodology is evidently feasible. Before probing the origin of the Bi-induced synergistic integration, comprehensive characterization together with justifiable analysis should be indispensable. With a smaller BET specific surface area (55.17 m2 g−1) than that of CeO2 (76.22 m2 g−1), the greatly enhanced performance of Ce0.8Bi0.2O2−δ cannot be considered as a matter of course. Crystal structures were investigated by powder X-ray diffraction. The XRD patterns (Figure 2a) only display diffraction peaks attributable to a standard cubic fluorite type structure, and the broadening peaks may suggest small size of nanostructures with numerous defects. With access of Bi3+ into the cubic lattice, the formed Ce0.8Bi0.2O2−δ presents nearly the same construction except a slight shift of diffraction pattern to lower angles (Figure S3). This variation can be intelligibly ascribed to the substitution of heteroatoms that Ce4+ ions with ionic radius of 0.097 nm were partially replaced by bigger Bi3+ ions (0.117 nm), also indicating the successful introduction of bismuth atoms into interior ceria. Obviously with a solid solubility of Bi3+ up to 20%, all the diffraction peaks remained highly symmetric and no structural revolution happened, indicating the high thermodynamics stability of the resulting material and confirming the assumption about the rule of similarity. X-ray photoelectron spectra (XPS) analysis was carried out to characterize the chemical states of cerium and bismuth ions. With six distinct characteristic peaks (u‴, u″, u and v‴, v″, v) in Figure 2b uniquely attributable to Ce4+ ions in as-prepared CeO2 and Ce0.8Bi0.2O2−δ, the main valence state of Ce ions in

Figure 1. Catalytic degradation experiments of Ce1−xBixO2−δ (x = 0, 0.2). (a) Time course of the CO2 increment during three diverse processes on CeO2 and Ce0.8Bi0.2O2−δ. (b) The −ln(k)−T−1 Arrhenius plot showing the temperature dependence of reaction rate constant for integrated catalysis. 24244

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Other than the structural and valent factor, the morphologies of CeO2 and Ce0.8Bi0.2O2−δ were ascertained using TEM images. Figure 3a clearly shows the pure ceria can be described

Figure 3. TEM images of (a) CeO2 and (b) Ce0.8Bi0.2O2−δ. Highresolution TEM (HRTEM) images of (c) CeO 2 and (d) Ce0.8Bi0.2O2−δ.

as gracile nanorods, 150−200 nm in length and 8−13 nm in diameter. After the introduction of Bi3+, Ce0.8Bi0.2O2−δ (Figure 3b) presents a similar morphology but gets shorter (25−30 nm) and a little fatter (13−20 nm). HRTEM images confirm the formation of solid solution that fringes with lattice spacing of ca. 0.310 nm (Figure 3c) and 0.315 nm (Figure 3d) respectively correspond to the (111) planes of the cubic CeO2 and Ce0.8Bi0.2O2−δ, and the variation tendency of lattice spacing coincides exactly with what the XRD patterns have told us. Furthermore, it can be clearly observed that there exist numerous dark spots on Ce0.8Bi0.2O2−δ, while there are fewer on CeO2 (Figure 3c,d), revealing that surfaces of Ce0.8Bi0.2O2−δ were much rougher than those of CeO2 and indicating more surface reconstruction and defects within the solid solution.38 With similar nanostructures, this difference may be associated with the promoted catalytic performance. 3. Probing the Origin of Bi-Induced Synergistic Integration. With detailed analysis of various characterizations, we reasonably attributed the synergistic enhancement in rod-like Ce0.8Bi0.2O2−δ to the introduction of Bi3+, eliminating the impetus from possible structural and morphology differences. Hereinafter, a deepgoing dissection probing the origin of Bi-induced integration will be carried out, and we believe this discussion will help delineate an accurate portrait for our model catalyst. 3.1. Enhanced Photocatalytic Performance. Light response must be one important evaluation factor for semiconductor photocatalysis. The improved photocatalytic performance on Ce0.8Bi0.2O2−δ is intelligibly attributed to the expanding spectral response with imported Bi3+. Figure 4a shows the UV−vis diffuse reflection spectra (DRS) of the two samples. According to the spectrum, with the introduction of Bi3+, the absorption onset of resulted Ce0.8Bi0.2O2−δ displays a considerable red shift

Figure 2. (a) X-ray powder diffraction patterns of CeO2 and Ce0.8Bi0.2O2−δ. (b) Ce 3d XPS spectra of CeO2 and Ce0.8Bi0.2O2−δ samples. (c) Bi 4f XPS spectrum of Ce0.8Bi0.2O2−δ sample.

both samples is indentified as +4.32,33 The presence of Ce3+ can be also revealed in both CeO2 and Ce0.8Bi0.2O2−δ, though most of the Ce ions are Ce4+.34,35 Furthermore, through the comparison of the four characteristic peaks (u′, u0 and v′, v0) attributable to Ce3+ ions in Figure 2b, especially the u0 peak, we are convinced of that, with the introduction of Bi ions into the ceria lattice, there produces more induced Ce3+ ions in the resulting Ce0.8Bi0.2O2−δ, which can be further confirmed by the following Raman analysis. Figure 2c is the representative core level spectrum of nanocrystalline solution Ce0.8Bi0.2O2−δ in the Bi 4f region. Displaying typical signals of broad doublets with high symmetry, the energy positions are located at 158.8 eV for Bi 4f7/2 and 164.1 eV for Bi 4f5/2. According to the corresponding spin−orbit splitting (5.3 eV) in good agreement with those observed in other bismuth oxides, we draw the conclusion that the bismuth ions in Ce0.8Bi0.2O2−δ are present as Bi(III).36,37 On the basis of such kind of surface characterization like XPS, together with above XRD results, the successful incorporation of Bi atoms into the CeO2 lattice and the formation of solid solution can be proved. 24245

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Figure 4. (a) UV−vis diffuse reflectance spectra (DRS) of Ce1−xBixO2−δ (x = 0, 0.2). Inset is the digital photograph of above two samples. (b) Band gap estimation of of Ce1−xBixO2−δ (x = 0, 0.2). (c) Mott−Schottky plots of CeO2 and Ce0.8Bi0.2O2−δ. (d) Energy level diagrams of CeO2 and Ce0.8Bi0.2O2−δ.

in comparison to that of pure CeO2. This revolution is easily discernible in the color change from yellowish to brilliant yellow as shown in the inset, and it can be supported by the band gap estimation with the Kubelka−Munk equation (eq 2) for indirect gap semiconductors that39 αhν = constant(hν ‐Eg )2

(2)

where α = (1 − R) /2R. Figure 4b shows that the imported Bi3+ caused a gap narrowing from 2.94 to 2.44 eV, approximately. This Bi-induced band gap narrowing initiated effective visiblelight-driven photocatalytic performance in Ce0.8Bi0.2O2−δ nanorods, which did not exist in pure ceria (Figure S4). In order to understand the role of imported Bi3+ in broadening the light response, we sought help from the flatband potentials (Vfb) estimation. Figure 4c displays the Mott−Schottky plots of CeO2 and Ce0.8Bi0.2O2−δ electrodes, presenting S-type curves which are consistent with typical plots for n-type semiconductors. The Vfb from the x-intercepts of the linear part were estimated to be 0.40 and 0.63 V vs NHE (0.16 and 0.39 V vs SCE), respectively, for CeO2 and Ce0.8Bi0.2O2−δ. It is generally acknowledged that the conduction band potential of one n-type semiconductor is 0−0.2 V more negative than its Vfb, depending on the carrier concentration and the effective mass of electron.40 Herein, the potential difference is set to 0.1 V; thus, the energy diagrams can be summarized (Figure 4d) that there being coexistence of a downshift of conduction band (from 0.3 V for C to 0.53 V for CB) and an upshift of the valence band (from 3.24 V for C to 2.97 V for CB). It is worth raising that the caused narrowing is something of expectation for exotic consecutive band insertion, and it may be ascribed to the mystical effect of Bi3+ that the Bi 6p and Bi 6s orbits participate in the construction of conduction band and valence band, respectively.41−43 Undoubtedly, this considerable revolution will contribute to the effective utilization of solar energy. 3.2. Enhanced Low-Temperature Catalytic Performance. The promoted thermocatalytic performance on Ce0.8Bi0.2O2−δ should be attributed to the introduction of Bi3+ accompanying with presence of oxygen vacancies. Figure 5a shows the Raman spectroscopy data, in which CeO2 and Ce0.8Bi0.2O2−δ are characterized by vibrational bands at 450−600 cm−1. For ceria the Raman mode at 456 cm−1 is assigned to the F2g mode of 2

Figure 5. (a) Raman spectra of CeO2 and Ce0.8Bi0.2O2−δ. (b) Temperature-programmed reduction profiles of CeO 2 and Ce0.8Bi0.2O2−δ. (c) Normalized linear sweep voltammetry curves of CeO2 and Ce0.8Bi0.2O2−δ.

CeO2 fluorite, and the weak signal at about 600 cm−1 can be ascribed to the intrinsic oxygen vacancies due to the existence of Ce3+ ions.44 Compared with pure ceria, the F2g mode in Ce0.8Bi0.2O2−δ shifts to a lower frequency (446 cm−1) and significantly broadens with imported Bi3+. The peak shift and broadening should be related to the sensitive F2g mode, representing the symmetrical stretching vibrations of CeO8 units.45 Additional mode of Ce0.8Bi0.2O2−δ located at 524 cm−1 can be indexed to the presence of Bi3+ ions and related extrinsic oxygen vacancies as charge-compensating defects.46 Furthermore, with the results of peak decomposition, the enhanced signal of Ce3+ ions located at about 600 cm−1 is obvious; therefore, we can get similar conclusion with the XPS characterization that the introduction of Bi atoms induces more Ce3+. In order to figure out what the imported Bi3+ and vacancies bring about in low-temperature catalysis, H2-TPR measurements and electrochemical analysis are persuasive.26,27,47,48 As 24246

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illustrated in Figure 5b, the TPR trace of pure ceria displays a faint reduction peak at 268 °C, ascribed to the reduction of Ce4+ ions. However, the profile of Ce0.8Bi0.2O2−δ presents two pronounced peaks at 248 and 304 °C. The two peaks are associated with the reduction of Ce4+ and Bi3+, respectively, and the downward shift of Ce related peak must be some sort of causal modification, attributed to the high reducibility of Bi3+ accompanying with the formed oxygen vacancies. Figure 5c displays the linear sweep voltammetry curves of CeO2 and Ce0.8Bi0.2O2−δ. Different from other general n-type semiconductors like TiO2, ceria and Ce0.8Bi0.2O2−δ both present an additional embossment, intelligibly ascribed to the electrochemical process of Ce and Bi related redox couples. After normalization with a mass activity to evaluate the oxidative activity of catalysts, it can be concluded that Ce0.8Bi0.2O2−δ electrode presents an anodic peak at about 0.520 V versus SCE of 5.615 × 10−6 A mg−1, having a lower peak position and a much higher mass activity than that of the ceria electrode (0.682 V versus SCE and 2.438 × 10−6 A mg−1). In summary, it can be firmly convinced that the Bi-induced synergistic photo/ thermocatalyst helps promote the redox activity under moderate conditions, and this remarkable feature can be attributed to exotic highly reductive Bi3+ accompanying with the generation of oxide anion vacancies. 3.3. Synergistic Integration of Photocatalysis with LowTemperature Catalysis. In general, the introduction of Bi atoms and abundant oxygen vacancies contribute to the synergistic integration of low-temperature catalysis into photocatalysis. Furthermore, inherited from ceria, Bi-induced Ce1−xBixO2−δ presents mixed ionic and electronic conductivity; thus, an enhanced integration will originate from the mutual promotion. More specifically, different from the n-type TiO2, Ce1−xBixO2−δ can be treated as combination of n-type semiconductor and oxygen ion conductor. Within this kind of generality: (1) As an n-type semiconductor, under the UV−vis irradiation it can be excited with the formation of localized Ce3+/Bi3+(e−)−O2−(h+) pairs. Transportation of hole-trapped oxygen ions can effectively help avoid the lattice scattering of phonons and thus contributes to the suppression of negative temperature effect.49,50 (2) As an oxygen ion conductor, abundant oxygen vacancies within the material on nanoscale have proved to be favorable for oxygen ions migration even at low temperatures. Undoubtedly with the introduction of Biinduced oxygen vacancies, the low-temperature redox ability can be greatly promoted.12,22,32,33 (3) There being a coupling connection between the mixed ionic and electronic conductivity. That is, once the lattice oxygen participates in the catalytic oxidization, a redox circulation of oxygen releasing and storage will set up according to the equation O(s) → Vo 2 +(s) + 2e−(s) + 1/2O2 ↑(g)

Scheme 2. Proposed Mechanism of Bi-Induced Integration of Solar Energy Conversion with Synergistic LowTemperature Catalysis

Introduction of Bi3+ ions will expand the response to visible light and enhance the redox activity at low temperatures; meanwhile, imported vacant sites considerably promote the migration of oxygen ions, further boosting the low-temperature integration. Under optical excitation of UV−vis irradiation and thermal activation of infrared radiation, the migration of holetrapped oxygen ions effectively suppresses the recombination of carriers, and the coupled ionic and electronic conductivity will be further enhanced by elevated temperatures originating from the IR-driven heat energy. With integration of all these correlative factors, the low-temperature catalysis or the infrared irradiation can be synergistically integrated into the solar energy conversion.

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CONCLUSIONS In summary, in order to make use of infrared energy, we have newly designed a novel Bi-induced photo/thermocatalyst, that is, the fluorite Ce1−xBixO2−δ. Involving coupled ionic and electronic conductivity, our catalyst successfully integrated effective low-temperature (IR-driven) catalysis into the photocatalytic process. Within Ce1−xBixO2−δ, the coupled ionic conductivity accompanying with highly reductive Bi and concomitant oxygen vacancies helped reverse the negative temperature effect, broaden the light response, and bring about the synergistic integration, promising for the effective utilize of infrared energy. On the basis of this study, a brand new strategy for catalyst design toward the utilization of infrared energy is suggested that integration of semiconductor photocatalysts with ionic conductors may be possible candidates for practical solar energy conversion.

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ASSOCIATED CONTENT

S Supporting Information *

(3)

Photocatalysis dynamics study, repeated experiments, XRD and XPS details, additional TEM images, systematical structural and performance study over other Ce1−xBixO2−δ (x = 0.1−0.5) samples. This material is available free of charge via the Internet at http://pubs.acs.org.

where V represents an oxygen tetrahedral site originating from the removal of O2− from the perfect lattice. Obviously, it can be discerned that with the transport of lattice oxygen or anion vacancies the electronic conductivity will be generated with coupled oxygen ion conduction. Therefore, the negative effect of high temperatures may be primarily reversed, as elevated temperature does promote the ionic conduction. With above arguments, a credible mechanism about the Biinduced integration of solar energy conversion with synergistic low temperature catalysis is proposed, as illustrated in Scheme 2. Within the lattice of Ce1−xBixO2−δ catalyst, there being abundant oxygen vacancies with the introduction of Bi3+.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Fax (+86)2152413122 (W.W.). Notes

The authors declare no competing financial interest. 24247

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ACKNOWLEDGMENTS We acknowledge the financial support from the National Basic Research Program of China (2013CB933203, 2010CB933503), National Natural Science Foundation of China (51272269, 51272303, and 51102262), and Science Foundation for Youth Scholar of State Key Laboratory of High Performance Ceramics and Superfine Microstructures (SKL201204).

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