2018, Vol.91, No.3


Co3O4-loaded TiO2 is a photocatalyst capable of oxidizing water into O2 by absorbing an entire range of visible light (400 < λ < 850 nm). In this work, the photocatalytic activity for water oxidation was investigated with respect to crystal phase, specific surface area, and surface morphology of TiO2 support. Results of photocatalytic reactions using six different TiO2 samples that possessed single-phase anatase or rutile structure indicated that the activity could be improved by applying a TiO2 support that had larger specific surface area, because it could accommodate larger amount of Co3O4 with minimal impact of undesirable aggregation. It was also suggested that when the specific surface area is similar, the activity is largely insensitive to crystal phase of TiO2, but is influenced by the surface morphology of TiO2, which can affect the dispersion of Co3O4.

In heterogeneous photocatalysis, sensitization of a wide-gap semiconductor (e.g., metal oxide) with a redox photosensitizer has been extensively studied as a means of utilizing visible light, which is the main component of the solar spectrum, for not only energy conversion such as water splitting but environmental remediation.13 So far, organic dyes,4,5 metal complexes,6,7 metal nanoparticles810 and metal ions1115 have been employed as photosensitizers so as to construct visible-light-driven photocatalytic systems. Among them, modification of a wide-gap metal oxide semiconductor with metal ions (or metal oxide nanoparticles) is a simple, but an effective method to prepare a visible-light-responsive photocatalyst for the degradation of organic pollutants.1115 In particular, the use of early-transition metals for making a visible-light-responsive photocatalyst is of interest from the viewpoint of “elements strategy” to minimize reliance on precious metals.

Water oxidation involving a four-electron process is regarded as the key step in artificial photosynthetic schemes for overall water splitting and CO2 fixation.16,17 Kominami et al. have reported sensitization of TiO2 with plasmonic Au nanoparticles to achieve water oxidation under visible light.9 They also succeeded in splitting pure water into stoichiometric H2 and O2.10 Li et al. very recently elucidated the active sites of visible-light water oxidation in the plasmonic Au/TiO2 system.18 However, water oxidation using early transition metals as sensitizers had not been realized until very recently.

Our group has developed a water oxidation photocatalyst driven by absorbing an entire range of visible light (λ < 850 nm), which consists of a wide gap metal oxide semiconductor (e.g., TiO2 and SrTiO3) and cobalt oxide (or hydroxide) nanoparticles.1921 As it is well known, the band gaps of most metal oxides including TiO2 and SrTiO3 are too wide to absorb visible light.13 Modification of such a wide gap metal oxide with the cobalt-based nanoparticles, however, resulted in generation of a new absorption band extending to the near infrared region. Importantly, this absorption band can be utilized for water oxidation to form O2, where photoexcitation occurs from the loaded cobalt species to TiO2, as revealed by photoelectrochemical measurements.19 Structural characterization by means of X-ray diffraction, UV-VIS-NIR diffuse reflectance spectroscopy, high-resolution transmission electron microscopy, X-ray absorption fine-structure spectroscopy and X-ray photoelectron spectroscopy showed that the activity of cobalt-modified rutile TiO2 depended strongly on the generation of Co3O4 nanoclusters with optimal distribution, which could interact with the rutile TiO2 surface to offer visible light absorption and active sites for water oxidation.20 For the Co3O4/SrTiO3 system, SrTiO3 particles as a support material had strong influence on the formation of Co3O4 nanoparticles, resulting in different photocatalytic activities for visible-light water oxidation.21 In these systems, it is considered that the loaded Co3O4 particles work as bifunctional units of visible-light absorption and water oxidation catalysis, as illustrated in Scheme 1.

TiO2 having several stable polymorphs is one of the most studied compounds as photocatalysts and components for visible-light-responsive photosystems, as mentioned above. It has been reported that crystal phase, particle size and the density of oxygen vacancies of TiO2 have a strong impact on photocatalytic water oxidation activity under band-gap photoexcitation of TiO2.2226 However, such structural effects on activity have not been investigated for visible-light water oxidation by Co3O4/TiO2 in detail.

In this work, effects of TiO2 support on photocatalytic activity of Co3O4/TiO2 for visible-light water oxidation were investigated with respect to physicochemical properties of TiO2. Guidelines to prepare an efficient photocatalyst using Co3O4 and TiO2 are discussed.

2.1 Preparation of TiO2 Powders.

Rutile TiO2 was supplied by the Catalysis Society of Japan (sample JRC-TIO-6). This rutile sample was heated in air at 1123–1273 K in order to make rutile TiO2 samples having different specific surface areas. TiO2 powder containing rutile as the main phase (99.995%) was also purchased from Aldrich Co. To obtain pure rutile phase, the sample was subject to heating in air at 1273 K for 2 h. A commercially available TiO2 having anatase phase (98.5%) was obtained from Kanto Chemicals Co. Anatase TiO2 was also supplied from the Catalysis Society of Japan (sample JRC-TIO-10). These six materials are referred to as R1, R2, R3, R4, A1 and A2, respectively (see Table 1).

Table 1. TiO2 samples used in this study
Table 1. TiO2 samples used in this study
Sourcea Calcination
Abbreviation Crystal phase SBET/
m2 g−1
JRC-TIO-6 - R1 Rutile 85
JRC-TIO-6 1123 R2 Rutile 14
JRC-TIO-6 1273 R3 Rutile 2
Aldrich 1273 R4 Rutile 1
Kanto - A1 Anatase 17
JRC-TIO-10 - A2 Anatase 225

aJRC-TIO-6 and JRC-TIO-10 were received from the Catalysis Society of Japan.

2.2 Modification of TiO2 with Co3O4 Nanoparticles.

Before photocatalytic reactions, modification of the TiO2 samples with nanoparticulate Co3O4 as a bifunctional unit of visible light absorption and water oxidation catalysis was conducted by an impregnation method.20 The powdered TiO2 was first dispersed in an aqueous solution (2–3 mL) containing an appropriate amount of Co(NO3)2·6H2O (Kanto Chemicals, ≥99.95%) using an evaporating dish. The suspension was stirred using a glass rod until the water was completely evaporated, and the resulting powder was collected and heated in air at 423 K for 1 h. In this paper, the cobalt loadings are reported based on the metallic cobalt contents of samples.

2.3 Characterization of Materials.

The prepared samples were characterized by XRD (Rigaku MiniFlex600; Cu Kα), high-resolution (HR)-TEM (JEOL, JEM-2010F), SEM (Hitachi, S-4700 and SU9000), and UV-Vis-NIR DRS (JASCO, V-565 spectrophotometer). The Brunauer-Emmett-Teller (BET) surface areas were determined at 77 K using a BELSORP-mini instrument (BEL Japan).

XAFS measurements were conducted on the BL01B1 beamline of the SPring-8 synchrotron facility (Hyogo, Japan) using a ring energy of 8 GeV and a stored current of 100 mA in the top-up mode (Proposal No. 2017B1040 and 2017B1438) to acquire Co-K edge spectra. XAFS spectra were acquired at room temperature in the fluorescence mode using a Si(111) double-crystal-monochromator and a Lytle detector filled with Kr gas. A pair of Rh-coated mirrors was used to eliminate higher harmonics. The XANES spectra were processed using the Athena software package.27

2.4 Photocatalytic Water Oxidation Reactions.

Water oxidation reactions were performed at room temperature using a closed gas circulation system described in a previous publication.28 Briefly, a top-irradiation 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 of Co3O4/TiO2 and 10 mM AgNO3 buffered to a pH of 8.0–8.5 using 200 mg of La2O3. After degassing the solution 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 cutoff filters (L42 + Y50) passed through a water filter, applying an output current of 10 A unless otherwise stated. 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).

3.1 Physicochemical Properties of TiO2 Materials.

It was confirmed by X-ray diffraction that the TiO2 samples used in this work had single-phase rutile or anatase phase (Figure S1). Specific surface areas (SBET) of these samples are listed in Table 1. Using the TiO2 samples as support materials, Co3O4 nanoparticles were loaded, and photocatalytic activities of Co3O4/TiO2 for water oxidation under visible light (λ > 500 nm) were investigated. It should be noted that loading Co3O4 onto these TiO2 supports did not change the original XRD patterns, and that no peak derived from any cobalt species could be identified, as confirmed by our previous work using R1.20 In this work, this was reconfirmed using A2 that had the largest surface area (Figure S2). It indicates that solid-state reaction between Co3O4 and TiO2 was negligible under the present preparation conditions.

3.2 Comparison of Rutile with Anatase.

Photocatalytic activity was first compared using rutile and anatase TiO2 particles (here R2 and A1, see Table 1) that have identical SBET (14–17 m2 g−1) and morphology (see Figure S3). As shown in Figure 1, TEM observation indicated that most of the deposited Co3O4 in both samples was in the form of nanoparticles a few nm in size, but some of them aggregated to form larger secondary particles (10–20 nm). Figure 2 shows that this modification resulted in the generation of a new absorption band in visible light region in both cases. Our previous study indicated that the 400–600 nm band results from electronic interaction between Co3O4 and TiO2, while a peak at around 700 nm could be assigned to bulky Co3O4.20

While no O2 was evolved without Co3O4 due to the lack of visible light absorption, the Co3O4-loaded TiO2 samples produced O2 upon visible light (λ > 500 nm). Note that Co3O4-loading onto SiO2, which is an insulator, did not yield O2 evolution. As listed in Table 2 (Entries 1–6), activities were improved in both cases with increasing the loading amount of Co3O4 to 1.0 wt%, then decreasing with further loading. This trend is consistent with our previous study, which showed that there is a trade-off between activity and Co3O4 loading in the visible-light water oxidation using rutile TiO2 powder (R1).20 More concretely, the appearance of an optimal loading amount indicates that the activity is governed by visible light absorption capability of surface water oxidation catalysis. As shown in Figure S4, more pronounced visible light absorption was provided by increasing the loading amount of Co3O4, which should also increase the density of active sites for water oxidation. Excess loading, on the other hand, decreases the active site density, leading to the drop in activity. Here, no significant difference in activity was observed between R2 and A1 samples at the same loading amount of Co3O4. These results suggest that the crystal phase of TiO2 support has little influence on the water oxidation activity of Co3O4/TiO2, most likely because it does not largely affect morphology of the deposited Co3O4 (Figure 1) and the resulting visible light absorption at least unless SBET and morphology of TiO2 are similar to each other (Figure 2).

Table 2. Photocatalytic activities of Co3O4-modified rutile and anatase TiO2 with different specific surface areas (SBET) for water oxidation under visible light (λ > 500 nm)a
Table 2. Photocatalytic activities of Co3O4-modified rutile and anatase TiO2 with different specific surface areas (SBET) for water oxidation under visible light (λ > 500 nm)a
Entry TiO2 Co3O4 loaded/wt% Activity/µmol h−1
1 R2 0.5 1.6
2 R2 1.0 3.7 ± 0.3
3 R2 2.0 1.1
4 A1 0.5 1.9
5 A1 1.0 4.2 ± 0.8
6 A1 2.0 1.3
7 R1 1.0 4.3 ± 0.1
8 R1 2.0 6.7 ± 0.3
9 R1 3.0 5.5 ± 0.1
10 R1 5.0 3.3 ± 0.1
11 A2 2.0 7.0
12 A2 3.0 10.3
13 A2 5.0 4.0
14 R3 0.5 0.9
15 R3 1.0 0.9 ± 0.3
16 R3 2.0 1.1
17 R4 0.5 0.8
18 R4 1.0 2.3 ± 0.1
19 R4 2.0 1.9

aReaction conditions: catalyst, 100 mg (with 200 mg La2O3); reactant solution, aqueous AgNO3 (10 mM, 140 mL); light source, 300 W xenon lamp with a cutoff filter. Data for Entries 7–10 was taken from ref. 20 with permission from the American Chemical Society.

3.3 Effects of Specific Surface Area of TiO2 on the Optimal Loading Amount of Co3O4 and the Water Oxidation Activity.

Using various TiO2 support materials that have different SBET values, how SBET of TiO2 support influences the optimal loading amount of Co3O4 as well as photocatalytic activity was then investigated in detail. Table 2 (Entries 7–19) displays the results. In all cases examined, there were certain optimal loading amounts of Co3O4, depending on the TiO2 support employed. For example, the optimal loading amount was 2.0 wt% for R1 (Entry 8), while 3.0 wt% for A2 (Entry 12). Results of photocatalytic reactions also indicated that the optimal loading amount tended to become lower when a lower surface area TiO2 was used.

Among TiO2 supports examined, A2 having the largest surface area (225 m2 g−1) was found to give the highest activity, which corresponded to an apparent quantum yield of ca. 0.15% at 500 nm.19,20 As shown in Figure 3, the best-performing material, 3.0 wt% Co3O4/A2, contained highly dispersed Co3O4 nanoparticles of ~1 nm in size without noticeable aggregation.

The highly dispersed state of Co3O4 nanoparticles on A2 was also confirmed by X-ray absorption fine-structure spectroscopy (XAFS) measurements. As discussed in our previous study using the R1 sample, the loaded cobalt species on R1 are Co3O4 nanoclusters, when the loading amount is 2.0 wt%.20 As shown in Figure 4, the EXAFS oscillation and the non-phase shift corrected FT-EXAFS spectra (k range: 3–12 Å−1) of the A2 samples (modified with 3.0 and 5.0 wt% Co3O4) are similar to those of the 2.0 wt% Co3O4/R2 sample, indicating that the loaded cobalt species in the A2 samples are identical to Co3O4 nanoclusters. The lack of shell peaks at longer distances clearly indicates the nanosized nature of the loaded Co3O4 in these samples. In the FT of EXAFS spectra, the peak at approximately 2.5 Å, which can be assigned to Co(oct)–Co(oct) shell,29 respectively, is stronger in the 5.0 wt% Co3O4/A2 sample than that in the 3.0 wt% analogue. This indicates the generation of larger Co3O4 nanoparticles at higher loadings. It is also noted that the possibility of the formation of CoTiO3 phase in Co3O4/TiO2 samples is excluded judging from the absorption edge of XANES spectra (Figure S5).

A TiO2 support that is able to accommodate larger amounts of Co3O4 without aggregation, here A2, appears to be important for the construction of an efficient water oxidation system using Co3O4 nanoparticles and TiO2. In this regard, a high surface area support would be useful. In the case of R1 that has lower SBET than A2, increasing the loading amount of Co3O4 from 2.0 to 3.0 wt% resulted in aggregation of Co3O4, leading to lower activity.20

Interestingly, however, R1 and A2, which exhibited almost the same activity with 2.0 wt% Co3O4 (Table 2, Entries 8–10), had different light-absorption profiles, as shown in Figure 5. Obviously, visible light absorption is more pronounced in 2.0 wt% Co3O4/R1 than 2.0 wt% Co3O4/A2 (even with 3.0 wt%). Therefore, the visible light absorption capability might be less important than the dispersion of Co3O4 in the Co3O4/TiO2 system.

3.4 Effects of the Surface Structure of TiO2.

It is known that surface structures of a semiconductor (e.g., exposed crystal facets) can affect photocatalytic activity, as individual surfaces may possess different reactivity, as reported by Ohno et al.30 As shown in Figure 6, R3 and R4 have different morphologies despite their identical SBET (1–2 m2 g−1). R3 consisted of aggregated primary particles of 100–400 nm, while R4 was well-crystallized particles 0.5–1 µm in size having exposed crystal facets. After modification of R3 and R4 with Co3O4, water oxidation reactions were conducted in the same manner. The results (as listed in Table 2, Entries 14–19) indicated that R4 worked better than R3, in particular at higher Co3O4 loadings (≥1.0 wt%).

TEM images of R3 and R4 modified with 1.0 wt% Co3O4 are shown in Figure 7. In both cases, Co3O4 nanoparticles were highly dispersed in both cases. However, the average size of Co3O4 on R4 (2.4 ± 0.1 nm) was smaller than that on R3 (3.5 ± 0.3 nm), as indicated by the Co3O4 particle size distribution (Figure S6). Even though SBET of these TiO2 samples are small (1–2 m2 g−1), it was possible to realize highly dispersed Co3O4 nanoparticles on these supports. It thus appears that distribution of Co3O4 is not simply governed by SBET of TiO2 support, but may differ with respect to the surface structure (e.g., crystal facets) of support.

UV-VIS-NIR diffuse reflectance spectra of these samples were also compared. As shown in Figure 8, visible light absorption was more pronounced in the case of R3 than in R4, even though the same amount of Co3O4 (1.0 wt%) was loaded in both cases. It suggests that surface structure of TiO2 can affect the light absorption properties of Co3O4/TiO2, most likely due to different electronic interaction between Co3O4 and the TiO2 surface. Despite the inferior visible light absorption of Co3O4/R4, R4 was a better-performing support than R3 (Table 2). Better light absorption of a photocatalyst should in principle contribute to an increase in the number of available photons to photocatalytic reactions. However, this was not the case. The present result strongly suggests that smaller particle size of Co3O4, which appears to be beneficial for water oxidation catalysis, on R4 is more important for enhancement of visible-light water oxidation activity, rather than the visible light absorption capability. According to a report by Li et al. on a plasmonic Au/TiO2 photocatalyst for water oxidation, it is claimed that the interface between Au and TiO2 is identified as the reaction site for water photooxidation.17 Because of the similarity in the reaction mechanism between Au/TiO2 and Co3O4/TiO2, active sites for water oxidation on the Co3O4/TiO2 photocatalyst may be the interface between Co3O4 and TiO2. However, identifying the real active site for water oxidation reaction in Co3O4/TiO2 still remains a challenge.

On the basis of these experimental results, important properties of TiO2 support for efficient water oxidation with Co3O4 could be summarized as follows. Increasing the loading amount of Co3O4 on TiO2 was essential for enhancing photocatalytic activity of Co3O4/TiO2 for water oxidation under visible light. At the same time, aggregation of Co3O4 needs to be avoided to minimize the negative impact for water oxidation catalysis. To satisfy these criteria, TiO2 having large specific surface area was useful as a support of Co3O4. In this work, nanoparticulate anatase TiO2 was found to be the best-performing support. However, no noticeable difference could be identified between anatase and rutile when their surface areas were similar (14–17 m2 g−1). It was also suggested that visible light absorption and the resulting photocatalytic activity were changed by interaction between Co3O4 nanoparticles and a specific crystal facet of TiO2.

This work was supported by Grants-in-Aid for Challenging Exploratory Research (project number JP15K14220), Young Scientists (A) (project number JP16H06130), and Scientific Research on Innovative Areas (project numbers JP16H06441; Mixed Anion). The authors would also like to acknowledge the support of The Hosokawa Powder Technology Foundation and The Noguchi Institute. The work presented herein was also funded in part by the “Chemical Conversion of Light Energy” program of PRESTO/Japan Science and Technology Agency (JST). The authors thank Ookayama Materials Analysis Division (Tokyo Institute of Technology) for assistance in SEM observations.

Additional characterization data (XRD, SEM, DRS, XANES, and particle size distributions). This material is available on http://dx.doi.org/10.1246/bcsj.20170373.

Kazuhiko Maeda

Kazuhiko Maeda received his PhD from The University of Tokyo (2007) under the supervision of Professor Kazunari Domen. During 2008–2009, he was a postdoctoral fellow at Pennsylvania State University where he worked with Professor Thomas E. Mallouk. In 2009, he joined The University of Tokyo as an Assistant Professor. Moving to Tokyo Institute of Technology in 2012, he was promoted to an Associate Professor. During 2010–2014, he was appointed as a research fellow of the PRESTO/JST program (Chemical Conversion of Light Energy). His research interests include water splitting and CO2 reduction using semiconductor materials as photocatalysts and photoelectrodes.