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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Visible-Light-Driven Water Oxidation Using Anatase Titania Modified with First Row Transition Metal Oxide Nanoclusters Megumi Okazaki, Yunan Wang, Toshiyuki Yokoi, and Kazuhiko Maeda J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b01222 • Publication Date (Web): 02 Apr 2019 Downloaded from http://pubs.acs.org on April 2, 2019

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The Journal of Physical Chemistry

Visible-Light-Driven Water Oxidation Using Anatase Titania Modified with First Row Transition Metal Oxide Nanoclusters Megumi Okazaki,1 Yunan Wang,2 Toshiyuki Yokoi,2 Kazuhiko Maeda,1,* 1

Department of Chemistry, School of Science, Tokyo Institute of Technology, 2-12-1-NE-2

Ookayama, Meguro-ku, Tokyo 152-8550, Japan 2

Nanospace Catalysis Unit, Institute of Innovative Research, Tokyo Institute of Technology,

4259-S2-5, Nagatsuta, Midori-ku, Yokohama 226-8503, Japan ABSTRACT: Composites comprising nanocrystalline anatase TiO2 and nanoclusters of first row transition metal oxides (MOx; M = Mn, Fe, Co or Ni), which can potentially function as water oxidation catalysts, were applied as photocatalysts for water oxidation under visible light (480 < λ < 900 nm). While TiO2 showed no visible light absorption, each of the MOx/TiO2 composites was capable of absorbing visible light. Activities during visible-light-driven water oxidation were examined using these materials in the presence of Ag+ as an electron acceptor. Co and Ni were found to be effective modifiers and promoted water oxidation with TiO2, while CoOx/TiO2 exhibited the highest activity. The catalytic activities of the MOx series during “dark” water oxidation were also investigated, employing an established photochemical water oxidation scheme with a Ru(II) trisdiimine complex as a redox photosensitizer in conjunction with weak visible light

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illumination. Under these conditions, the effect of light absorption by the MOx/TiO2 on the reaction could be neglected. This investigation revealed that CoOx/TiO2 had by far the highest activity among the MOx/TiO2 composites. The present work thus highlights the important role of the “dark” water oxidation process that occurs on the MOx surface in determining the overall efficiency of MOx/TiO2 during photocatalytic water oxidation. Introduction TiO2 is one of the most widely studied metal oxide semiconductors employed in heterogeneous photocatalysis.1–3 However, a serious drawback associated with TiO2 is its band gap (3.0–3.2 eV), which is too wide to efficiently absorb visible light, the main component of sunlight. Various techniques have been proposed to mitigate this problem, including photosensitization with organic dye/metal complexes,4–6 combination with metal (or metal oxide) nanoparticles7–19 and band engineering by doping with foreign cations/anions.20–27 In particular, hybrid photocatalysts constructed with a wide-gap semiconductor (such as a metal oxide) and an appropriate sensitizer have been extensively studied for applications in energy conversion and environmental purification powered by solar energy.28–30 Among these methods, the addition of metal (or metal oxide) species to TiO2 is the simplest means of achieving both visible light absorption and promotion of redox reactions, without requiring complicated multi-step synthetic procedures.7–19 Water oxidation is a kinetically difficult reaction that can represent the rate-determining step in water splitting, because four electrons are required to form the final product, O2.31,32 Photocatalytic water oxidation is essential when one closes artificial photosynthetic cycle by connecting the reductive half cycle of artificial photosynthesis, such as H2 evolution and CO2 reduction.33–36 It is especially important to develop catalytic water oxidation systems driven by longer-wavelength light. Kominami et al. reported the sensitization of TiO2 with plasmonic Au nanoparticles to achieve water oxidation under visible light.16 The resulting Au/TiO2 plasmonic photocatalyst was


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capable of splitting pure water into stoichiometric amounts of H2 and O2 in response to visible radiation.17 Recently, our group has reported that wide-gap titanium-based semiconductors (including TiO2 and SrTiO3) modified with nanoparticulate cobalt oxide (or hydroxide) exhibit visible light absorption over a wide range (up to 850 nm), and thus can be utilized for water oxidation employing an aqueous solution containing Ag+ as an electron acceptor.37–40 Photoexcitation of the TiO2 by the cobalt species in this system has been demonstrated by photoelectrochemical measurements.37 However, despite these promising results, and the use of earth-abundant metals in this cobalt-sensitized water oxidation system, overall water splitting to form H2 and O2 has not yet been achieved. Therefore, further research focusing on activity improvement and a better mechanistic understanding is necessary. In this cobalt-sensitized water oxidation system, electron transfer from the surface cobalt species to the semiconductor support, as well as the subsequent catalytic water oxidation over the electrondeficient cobalt species, is essential to promoting the water oxidation reaction. There have been several reports that describe interfacial electron transfer driven by visible light from first-row transition metal oxides (loaded on the support surface) to TiO2, which could be applied to the degradation of organic substrates. Tada et al. and Ohno et al. reported that FeOx/TiO2 can decompose organic substrates such as 2-naphthol, toluene and acetaldehyde.11,14,15 Tada et al. demonstrated that 2-naphthol is also degraded by NiO-modified TiO2 under visible light.12 Another important thing is that some first-row transition metal oxides will function as catalysts during water oxidation in the same manner as cobalt oxide. It has been reported that several oxides of first row transition metals, such as Mn, Fe, Co and Ni, act as water oxidation catalysts under electrochemical and/or photochemical conditions.41–43


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Based on this background, we considered that a similar photosensitized water oxidation system could be constructed by combining TiO2 with other first row transition metal oxides, as shown in Figure 1. To date, visible-light-driven water oxidation by MOx/TiO2 has only been reported for M = Co, although the photocatalytic activity of TiO2 modified with first row transition metal oxides during UV-driven water oxidation was investigated by Dong et al.44 In the present work, we prepared MOx/TiO2 composites (M = Mn, Fe, Co or Ni) as visible-light-driven photocatalysts for water oxidation to form O2 in the presence of Ag+ as an electron acceptor. To independently evaluate the intrinsic catalytic activities of these materials during water oxidation under “dark” conditions, we also performed photochemical water oxidation using a trisdiimine Ru(II) photosensitizer. In this case, the effect of light absorption by the MOx/TiO2 during the reaction is negligible due to the strong visible light absorption by the Ru photosensitizer. On the basis of our results, the factors affecting the activities of these MOx/TiO2 photocatalysts during visible light water oxidation are discussed herein. Ag+

Visible light

Ag

e-

C.B.

~ 850 nm

h+ H2O O2

V.B.

TiO2

Transition metal oxides Figure 1. Water oxidation driven by charge transfer from a surface transition metal oxide to TiO2 under visible light. V.B. and C.B. indicate valence and conduction bands, respectively. Experimental Section


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Materials and Reagents Anatase TiO2 (JRC-TIO-10) was obtained from the Catalysis Society of Japan. Mn(NO3)2·6H2O (Wako Pure Chemicals Co., ≥98.0%), Fe(NO3)3·9H2O (Kanto Chemicals Co., ≥99.0%), Co(NO3)2·6H2O (Kanto Chemicals Co., ≥99.95%), Ni(NO3)2·6H2O (Kanto Chemicals Co., ≥98.0%), AgNO3 (Wako Pure Chemicals Co., ≥99.8%) and Na2S2O8 (Wako Pure Chemicals Co., ≥97.0%) were used without further purification. [Ru(bpy)3]SO4 (bpy = 2,2'-bipyridine) was prepared by adding one equivalent of [Ru(bpy)3]Cl2 (Sigma Aldrich Co., used without further purification) to an aqueous solution of Ag2SO4 (Wako Pure Chemicals Co., ≥99.5%). La2O3 (Tokyo Chemical Industry Co. ≥99.9%) was calcined in air at 1273 K for 1 h before use to remove residual hydroxide and/or carbonate phases. An aqueous phosphate solution containing Na2HPO4·12H2O (Wako Pure Chemicals Co., ≥98%) and NaH2PO4·2H2O (Wako Pure Chemicals Co., ≥99%) and adjusted to pH 7.9 was used as a pH buffer. Preparation of MOx/TiO2 Modification of TiO2 with MOx was performed using a previously reported impregnation method based on work with CoOx/TiO2.38 Powdered TiO2 was first dispersed in 2 mL water containing an appropriate amount of the desired metal nitrate species in an evaporating dish. The resulting suspension was stirred using a glass rod until the water was completely evaporated, after which the resulting powder was collected and heated in air at 423 K for 1 h. In this work, the metal loadings were 2.0 wt% based on the metallic content of each sample. CoOx/Al2O3 for use as a reference material was prepared in a similar manner. Characterization of Materials The prepared samples were characterized by X-ray diffraction (XRD; Rigaku MiniFlex600; Cu Kα), scanning electron microscopy (SEM; Hitachi, SU9000), X-ray photoelectron spectroscopy (XPS; ESCA-3400, Shimadzu) and UV-Vis-NIR diffuse reflectance spectroscopy


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(DRS; JASCO, V-670 spectrophotometer). The binding energies determined by XPS were calibrated with respect to the C1s peak (285.0 eV) for each sample. The Brunauer-Emmett-Teller (BET) surface areas were determined at 77 K using a BELSORP-mini instrument (MicrotracBEL). Energy-dispersive X-ray spectroscopy (EDS) analysis was conducted using a JSM-IT100LA apparatus (Jeol). Photocatalytic Water Oxidation Reactions from Aqueous AgNO3 Water oxidation reactions were performed at room temperature using a closed gas circulation system described in a previous publication.45 Briefly, a top-irradiation type Pyrex cell was immersed in a cold water bath (ca. 293 K) connected to a closed gas system. The reaction took place in 140 mL of an aqueous solution containing 100 mg MOx/TiO2 and 10 mM AgNO3 buffered to a pH of 8.0–8.5, with 200 mg of La2O3.46 After degassing the reaction cell several times, a small amount of argon gas was introduced. The reaction cell was exposed to light from a 300 W xenon lamp (Cermax, PE300BF) fitted with a CM-1 cold mirror and cutoff filters (L42 and Y48) passed through a water filter, applying an output current of 15 A unless otherwise stated. The total light intensity (480 < λ < 900 nm) was measured using a calibrated silicon photodiode and found to be 2.6 W. The gases evolved in the reaction system were analyzed by on-line gas chromatography (Shimadzu GC-8A with a thermal conductivity detector and an MS-5A column, argon carrier gas). The turnover number (TON) for the O2 formation reaction was calculated according to the equation: TON = [O2] / [Metal], where [O2] and [Metal] represent the mole amounts of O2 produced and transition metal species introduced onto the TiO2 surface, respectively. Photochemical Water Oxidation Using [Ru(bpy)3]2+ as a Redox Photosensitizer


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Reactions were conducted in a Pyrex top-irradiation type reaction vessel connected to a glass closed gas circulation system. A 50 mg quantity of the as-prepared MOx/TiO2 powder was dispersed in a phosphate buffer solution (100 mL, 50 mM, pH 7.9) containing 5 mM Na2S2O8 and 0.25 mM [Ru(bpy)3]SO4, which served as a sacrificial electron acceptor and a redox photosensitizer, respectively.41–43 The solution was irradiated by a 300 W xenon lamp under a degassed atmosphere. The irradiation wavelength (480 < λ < 500 nm) was controlled using a CM2 cold mirror, a cutoff filter (Y48) and a neutral density filter (ND10). The evolved oxygen was analyzed by an on-line gas chromatograph (GC 3200, GL science). The total number of photons supplied was approximately 72 mW. Results and Discussion Physicochemical Properties of MOx/TiO2 MOx nanoparticles were deposited on the surface of anatase TiO2 by an impregnation method, using the corresponding metal nitrates as precursors. The preparation conditions were the same as those previously reported for the synthesis of CoOx/TiO2.38 The XRD patterns for the as-prepared MOx/TiO2 materials were all identical to that for anatase TiO2, without any peaks attributable to MOx (Figure S1). This result indicates that the MOx species were in the form of nanoparticles and/or an amorphous state, and did not react with the TiO2 support (such as to form M-Ti mixed oxides, as occurs during the high temperature calcination of CoOx/TiO238). The relatively broad peaks associated with the anatase TiO2 support are ascribed to the very small size of the particles (approximately 2–3 nm), as evidenced by SEM observations (Figure S2) and also reflected in the high specific surface area (220 m2 g–1). However, MOx species were not visible in the TiO2 in SEM images (Figure S2 shows typical images for M = Co and Ni), suggesting that the MOx was present as nanoclusters having a size equivalent to or smaller than the TiO2 support particles. The


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high degree of dispersion of the first row transition metal species on the TiO2 was confirmed by means of SEM/EDS analysis. As shown in Figure S3, for example, the loaded Co species were distributed uniformly on the surface of the TiO2. Previous structural analyses by X-ray absorption fine-structure spectroscopy have indicated that Co species (at a concentration of 3.0 wt% in the case of this prior work) loaded on anatase TiO2 were present as highly dispersed Co3O4-like nanoparticles approximately 1 nm in size, without noticeable aggregation. DRS analyses were used to assess the presence of transition metal species on the TiO2 (Figure 2). In contrast to the unmodified TiO2, all the MOx/TiO2 materials exhibited visible light absorption extending to the near-infrared region. Tada et al. reported the preparation of MnOx/TiO2, using P25 TiO2 and manganese(III) acetylacetonate as the precursors.14 The spectrum obtained from the present MnOx/TiO2 was similar to that reported by Tada et al., although a characteristic absorption peak at approximately 550 nm suggested the presence of bulky Mn2O3.14 The FeOx/TiO2 generated a spectrum in agreement with those reported by other groups.9–11 Similar to the MnOx/TiO2, the absorption shoulder at approximately 470 nm may have been due to bulky Fe2O3 species.9 The CoOx/TiO2 spectrum was also largely consistent with that reported in our previous work,38 with no peaks attributable to bulk CoOx species. The spectrum produced by the NiOx/TiO2, which had a greenish coloration, exhibited broad absorption over the range of 600–900 nm, which is typical of NiO-loaded wide gap metal oxide photocatalysts.47 XPS analyses also demonstrated the presence of MOx species on the TiO2 (Figure S4). A detailed discussion of the electronic states of the metal species loaded on the TiO2 is included in the Supporting Information.


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Figure 2. (A) UV-visible DRS data obtained from the MOx/TiO2 materials and from bare TiO2. (B) A magnified region of the spectra in (A). Photocatalytic Activities of MOx/TiO2 during Visible-Light-Driven Water Oxidation from Aqueous AgNO3 Water oxidation reactions using the MOx/TiO2 specimens were performed in aqueous AgNO3 solutions under visible light (480 < λ < 900 nm). While the TiO2 component did not undergo photoexcitation under the reaction conditions employed in this work,40 electron transfer took place from the MOx to the TiO2 support, followed by water oxidation, as has been previously observed in the CoOx/TiO2 system.38 However, despite the visible light absorption of these MOx/TiO2 materials, only the CoOx- and NiOx-loaded TiO2 samples were found to produce O2 (Figure 3). It is also notable that the activity of the CoOx/TiO2 was much higher than that of the NiOx/TiO2. The TON for O2 evolution over the CoOx/TiO2 was greater than 1 (Figure S5), confirming the catalytic nature of the reaction. The use of Al2O3 (which is an insulator and thus does not allow electron transfer) in place of TiO2 produced no O2 under the present conditions. This result indicates that


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both the CoOx and a suitable semiconductor support are necessary for the photocatalytic O2 evolution reaction to proceed.

Figure 3. O2 evolution rates over MOx/TiO2 photocatalysts under visible light (480 < λ < 900 nm). Reaction conditions: catalyst, 100 mg (2.0 wt % MOx-loaded); 10 mM aqueous AgNO3 solution, 140 mL; light source, 300 W xenon lamp with a CM-1 mirror and cutoff filters. Based on the above data, CoOx was determined to be the best modifier for TiO2 with the aim of obtaining visible-light-driven water oxidation. The results also suggest that various factors affect the water oxidation activity of MOx/TiO2 in addition to the extent of visible light absorption. The water photooxidation reaction was subsequently investigated, using a molecular photosensitizer that absorbed light to a significant extent, so as to minimize the contribution of light absorption by the MOx/TiO2 to the reaction. These trials allowed a comparison of the catalytic activities of each MOx for water oxidation. Photochemical Water Oxidation over MOx/TiO2 using [Ru(bpy)3]2+ as a Photosensitizer


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The metal oxide nanoparticles used as cocatalysts for water oxidation with semiconductor photocatalysts (such as MnOx and CoOx) are, of course, also able to function as catalysts for photochemical water oxidation in the presence of a suitable electron acceptor.41–43 The reaction scheme for an established photochemical water oxidation system using [Ru(bpy)3]2+ as the photosensitizer and S2O82– as the acceptor is shown in Figure 4. Following excitation of the [Ru(bpy)3]2+ by visible light, electron transfer to the Na2S2O8 occurs, generating [Ru(bpy)3]3+ (i.e., oxidative quenching). Following this, the [Ru(bpy)3]3+ receives an electron from a heterogeneous catalyst (such as a metal oxide) to regenerate [Ru(bpy)3]2+. Finally, the catalyst, which is now in an electron-deficient state, oxidizes water to form O2. [Ru(bpy)3]2+* S2O822SO4

hν

O2

[Ru(bpy)3]2+ Catalyst MOx/TiO2

2-

2H2O

[Ru(bpy)3]3+

Figure 4. Photochemical water oxidation on a metal oxide catalyst using [Ru(bpy)3]2+ and S2O82– as a redox photosensitizer and an electron acceptor, respectively. We applied a series of MOx/TiO2 materials as catalysts in this photochemical water oxidation system, in conjunction with weak visible light irradiation (480 < λ < 500 nm). The light intensity in these trials was approximately 36 times lower than that applied in earlier trials using the wavelength range 480 < λ < 900 nm for reactions employing AgNO3 as an acceptor. It should be noted that, under these conditions, the [Ru(bpy)3]2+ in the reactant solution efficiently absorbed the incident light, such that the MOx/TiO2 did not function even in the presence of AgNO3. Therefore, water oxidation using [Ru(bpy)3]2+ as the photosensitizer enabled assessment of the


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catalytic properties of each MOx/TiO2 combination during “dark” water oxidation, by minimizing the contribution of visible light absorption by the MOx/TiO2. Figure 5 shows the photochemical O2 evolution rates over the MOx/TiO2 specimens under visible light (480 < λ < 500 nm). The unmodified TiO2 alone produced O2 under the present conditions, but the activity was relatively low (< 1 µmol h–1). In contrast, modification of the TiO2 with most of the various MOx species improved the O2 evolution rate, with the extent of improvement depending on the metal. Among the samples examined, CoOx/TiO2 showed by far the highest activity, with a TON of approximately 25, and was relatively stable over three reuses (Figure 6). Loading MnOx or NiOx had some positive impact on the activity, but much less than that obtained with CoOx. Conversely, adding FeOx to the TiO2 actually lowered the activity. O2 evolution was negligible when either a MOx/TiO2 catalyst, [Ru(bpy)3]2+ photosensitizer or Na2S2O8 electron acceptor was absent (Table S1). Therefore, it was concluded that MOx/TiO2 (M = Mn, Co and Ni) materials were able to oxidize water in the presence of [Ru(bpy)3]2+ and Na2S2O8, with the highest activity obtained using the cobalt oxide.


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Figure 5. O2 evolution rates over MOx/TiO2 catalysts with a [Ru(bpy)3]SO4 photosensitizer under visible light (480 < λ < 500 nm). Reaction conditions: catalyst, 50 mg (2.0 wt % MOx-loaded); 100 mL of a 5.0 mM aqueous Na2S2O8 solution containing a 5.0 mM phosphate buffer (pH 7.9) and 0.25 mM [Ru(bpy)3]SO4 photosensitizer; light source, 300 W xenon lamp with a CM-2 mirror and cutoff filters.

Figure 6. A typical time course of O2 evolution over the CoOx/TiO2 catalyst with a [Ru(bpy)3]SO4 photosensitizer under visible light (480 < λ < 500 nm). Reaction conditions: catalyst, 50 mg (2.0 wt % CoOx-loaded); 100 mL of a 5.0 mM aqueous Na2S2O8 solution in a 5.0 mM phosphate buffer (pH 7.9) and 0.25 mM [Ru(bpy)3]SO4 photosensitizer; light source, 300 W xenon lamp with a CM2 mirror and cutoff filters. The reaction system was refreshed at 10 h intervals by replacing the reactant solution. The CoOx/TiO2 catalyst was reused for each run. It is evident that the trend exhibited by the activities for photochemical water oxidation are in qualitative agreement with those observed during visible-light water-driven oxidation using AgNO3 as an electron acceptor, in which case the visible light photoexcitation of the MOx/TiO2 may occur. As shown in Figure 2, each of the MOx/TiO2 materials was capable of absorbing visible


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light, although a quantitative comparison was not possible. However, the MOx-sensitized water oxidation was possible only when the loaded MOx possessed a minimum level of catalytic activity for water oxidation. Presumably, the state of charge separation between the MOx and TiO2 rapidly returned to the ground state if the subsequent water oxidation catalysis was relatively slow. Therefore, not only the visible light absorption capability but also the intrinsic catalytic activity of the MOx for water oxidation was important for obtaining high photocatalytic activity from the MOx/TiO2 system. In addition to the superior catalytic activity displayed by the CoOx during water oxidation, the strong electronic interaction between the CoOx and TiO2 would be expected to enhance photoexcitation, thus promoting the photocatalytic activity of the CoOx/TiO2 in association with water oxidation in the presence of AgNO3. This is evident from the observation that CoOx/Al2O3 did not work as a water oxidation photocatalyst in the presence of AgNO3 as an electron acceptor. Conclusions Visible-light-driven water oxidation over MOx/TiO2 (M = Mn, Fe, Co or Ni) composite materials was investigated. In the presence of Ag+ as an electron acceptor and relatively high-intensity visible light, CoOx/TiO2 and NiOx/TiO2 exhibited reasonable rates of O2 evolution, while the other materials did not, even though all specimens showed some level of visible light absorption. The activity of the CoOx/TiO2 was also found to be much higher than that of the NiOx/TiO2. The extent to which the water photooxidation reaction was promoted was greatly affected by the intrinsic catalytic capacity of the MOx species during water oxidation under “dark” conditions. That is, the relatively high water oxidation activity of CoOx species enhanced visible light water oxidation over the CoOx/TiO2 photocatalyst. The experimental results obtained in this work thus demonstrate that both the visible light absorption capability of the MOx/TiO2 composite and the intrinsic


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catalytic ability of the metal oxide species loaded on the support during water oxidation determine the level of performance obtained during visible-light-driven water oxidation. The present water oxidation system, promoted by charge transfer from the surface transition metal oxide (e.g., CoOx) to the wide gap metal oxide photocatalyst, could allow one to apply various wide gap semiconductors, as demonstrated by our previous works.37,39,40 We believe that the present findings will provide useful guidance in the future design of more active photocatalysts, because the catalytic activity of CoOx during water oxidation can evidently be tailored by varying the support material. ASSOCIATED CONTENT Supporting Information. Additional characterization and reaction data. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author [email protected] ACKNOWLEDGMENTS This work was supported in part by Grant-in-Aids for Young Scientists (A) (JP16H06130), for Scientific Research on the Innovative Area Mixed Anions (JP16H06441), for Challenging Research (Exploratory) (JP17K19169), and for Scientific Research (B) (JP19H02511). Reference 1. 2.

Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Environmental Applications of Semiconductor Photocatalysis. Chem. Rev. 1995, 95, 69–96. Ma, Y.; Wang, X.; Jia, Y.; Chen, X.; Han, H.; Li, C. Titanium Dioxide-based Nanomaterials


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