Recent progress in the development of (oxy)nitride photocatalysts for water splitting under visible-light irradiation

Coordination Chemistry Reviews 257 (2013) 1957–1969

Contents lists available at SciVerse ScienceDirect

Coordination Chemistry Reviews journal homepage: www.elsevier.com/locate/ccr

Review

Recent progress in the development of (oxy)nitride photocatalysts for water splitting under visible-light irradiation夽 Yosuke Moriya a , Tsuyoshi Takata b , Kazunari Domen a,b,∗ a

Department of Chemical System Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan Research Network & Facility Service Division/GREEN/Solar Energy Conversion Field, National Institute of Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan b

Contents 1. 2. 3. 4.

5.

6.

7. 8.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1958 (Oxy)nitride photocatalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1958 Problems of the nitridation of (oxide) precursors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1960 Attempts to prepare high-quality (oxy)nitrides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1960 4.1. High-pressure high-temperature treatments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1960 4.2. Acid treatments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1961 4.3. Flux treatments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1961 Ta3 N5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1961 5.1. High-pressure high-temperature treatments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1961 5.2. Flux-assisted nitridation and post-treatments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1962 5.3. Other synthetic routes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1963 Perovskite oxynitrides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1964 6.1. Ammonothermal synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1964 6.2. Morphology control of LaTiO2 N by flux method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1965 6.3. Acid treatments on LaTiO2 N . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1966 Some developments in the preparation of d10 -type typical element oxynitrides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1966 Conclusions and future prospects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1967 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1968 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1968

a r t i c l e

i n f o

Article history: Received 24 October 2012 Accepted 13 January 2013 Available online 4 February 2013 Keywords: (Oxy)nitride Photocatalyst Water splitting Hydrogen production Sunlight

a b s t r a c t Hydrogen is considered to be a clean energy carrier alternative to exhaustible resources such as fossil fuels. To facilitate the transition to a hydrogen economy, hydrogen production using renewable energy is an important focus of attention. Splitting water into hydrogen and oxygen using a photocatalyst and sunlight is a potential candidate for future hydrogen production. To utilize sunlight efficiently, photocatalysts must be responsive to visible light with longer wavelengths, which makes up the majority of sunlight. Among transition metal (oxy)nitrides with d0 -electronic configurations, there are some photocatalysts with the potential to split water by absorbing light with wavelengths up to ∼600 nm. However, overall water splitting has not yet been achieved by these 600 nm-class photocatalysts. It is very likely that defects in photocatalysts that are inevitably formed during conventional preparation processes degrade their photocatalytic performance. In this review, we provide a summary of recent progress in the development of visible-light-responsive (oxy)nitride photocatalysts, especially from the aspect of syntheses and posttreatments to obtain high-quality crystals with few defects. © 2013 Elsevier B.V. All rights reserved.

夽 This is a contribution to the themed issue on metal nitride by invitation only – edited by W. Levason. ∗ Corresponding author at: Department of Chemical System Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan. Tel.: +81 3 5841 1148; fax: +81 3 5841 8838. E-mail address: [email protected] (K. Domen). 0010-8545/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ccr.2013.01.021

1958

Y. Moriya et al. / Coordination Chemistry Reviews 257 (2013) 1957–1969

1. Introduction Studies on splitting water (H2 O) into hydrogen (H2 ) and oxygen (O2 ) using light (photons) have their roots in the discovery of the Honda-Fujishima effect in 1967, which was published in 1972 [1]. Ultraviolet (UV) light irradiation of a titanium oxide (TiO2 ) photoanode that is electrically connected to a platinum (Pt) counter electrode in a cell comprising an aqueous solution of supporting electrolyte splits water and generates a current under an external bias. H2 is generated at the Pt electrode and O2 at the TiO2 photoanode. Since this discovery, water splitting using photocatalysts has been extensively studied along with photoelectrochemical techniques, and many metal oxide photocatalysts capable of splitting water under UV light irradiation have been reported [2]. In recent years, there has been growing concern about environmental destruction, such as so-called global warming caused by mass consumption of fossil fuels and radiation contamination due to nuclear-fuel-derived radioactive substances, e.g., released from a nuclear power plant by accident, and about the future depletion of these exhaustible resources. This situation highlights the need for renewable energy alternatives to exhaustible resources. Among various kinds of renewable energies, the direct utilization of solar energy calls for urgent attention. In the case of water splitting using a photocatalyst and sunlight, light energy is converted into chemical energy stored in H2 , and the generated H2 reverts back to H2 O by combustion. It can thus be considered the ultimate reaction, making a clean, storable, and transportable energy source available semipermanently without an adverse environmental load. In order to utilize sunlight, however, it is necessary to develop photocatalysts with the capability to split water stably in response to visible light, which makes up the majority of sunlight. Thus, it has become a major challenge to make photocatalysts responsive to visible light. Fig. 1 shows a schematic diagram illustrating the process of one-step water splitting. One-step water splitting is defined as water splitting using a single photocatalyst as opposed to twostep water splitting that uses two different photocatalysts for H2 and O2 generation, and requires a shuttle redox mediator between the two, which is also called Z-scheme in analogy with photosynthesis in chlorophyta [2,3]. All solid photocatalysts are materials with a forbidden band, i.e., semiconductors and insulators (widegap semiconductors). In these materials, electrons are excited from the valence band to the conduction band by irradiating light with energy larger than the band gap (BG), and consequently electron–hole pairs are formed. Taking advantage of this nature, photocatalysts oxidize and reduce other substances. In one-step water splitting, these excited electrons and holes directly reduce protons (H+ ) to generate H2 and oxidize H2 O to generate O2 , respectively. For this oxidation–reduction reaction to occur, the conduction band minimum (CBM) and the valence band maximum (VBM) of the intended photocatalyst must be lower and higher than the oxidation–reduction potential (ORP) of H+ /H2 (0 V vs. NHE) and the ORP of O2 /H2 O (+1.23 V vs. NHE), respectively. In many metal oxides, since the valence band is mainly composed of O 2p orbitals, the VBM takes its position at a potential much higher than +1.23 V vs. NHE. Therefore, the ORPs of H+ /H2 and O2 /H2 O are likely to be sandwiched between the VBM and the CBM, and metal oxides tend to have strong oxidizing power over H2 O. At the same time, the BG becomes too wide, and consequently, the materials can respond only to UV light. Hence, attempts have been made to make metal oxides responsive to visible light by dissimilar-metal doping [2]. One such example is strontium titanate (SrTiO3 ), which is a quantum paraelectric material with the perovskite structure ABX3 , doped with rhodium (Rh) [4]. In this case, the dopant exists as a trivalent ion (Rh3+ ) on the B-site (Ti4+ site), which forms a donor level at a lower potential than the VBM composed of O 2p orbitals,

and the apparent BG consequently gets narrowed. Rh-doped SrTiO3 thus becomes responsive to visible light. It is also known that nitrogen (N) doping makes TiO2 visiblelight responsive [5,6]. This is most likely because O 2p orbitals hybridize with N 2p orbitals drawing up the VBM, and the BG gets narrowed as well. It is thus expected that a metal oxide can be made visible-light-responsive by substituting N3− or S2− for O2− on the anion side. For example, considering the substitution of one N3− species per O2− anion in SrTiO3 , since the total valence on the anion side increases by 1, it is also necessary to increase the total valence on the cation side by 1 to maintain charge neutrality without forming anion vacancies. Accordingly, divalent Sr2+ occupying the A-site is substituted by trivalent La3+ . The perovskite oxynitride LaTiO2 N thus formed is able to absorb visible light. The perovskite structure is superior in diversity of cations included, and it is possible to make them in solid solution or as a complex (a crystallographically equivalent site is occupied by more than two kinds of ions with different valences and ionic radii). In the case of perovskite oxynitrides, a wide variety of compounds with metals in various ratios on the cation side can be formed by varying the ratio of N3− and O2− on the anion side, e.g., La(Mg2x/3 Ta1−2x/3 )O1+2x N2−2x , a solid solution between oxynitride LaTaON2 and complex oxide La(Mg2/3 Ta1/3 )O3 , regardless of whether they have photocatalytic activity [7]. In the last decade, we have devoted much effort to the development of such (oxy)nitrides and (oxy)sulfides.

2. (Oxy)nitride photocatalysts The following materials are (oxy)nitride [8] and (oxy)sulfide photocatalysts with the (potential) capability to split water by one-step photo-excitation: zinc oxide (ZnO)-based solid solutions such as (1−x)GaN–xZnO (wurtzite structure) [9–11] and (1−x)ZnGeN2 –2xZnO (wurtzite-related structure) [12,13], Tabased (oxy)nitrides such as TaON (baddeleyite structure) [14] and Ta3 N5 (anosovite structure) [15], and perovskite-related oxynitrides such as LaTiO2 N [16,17] and AETaO2 N (AE = Ca, Sr, and Ba) [18,19]. In (oxy)sulfides, there are adamantine-related sulfides [wurtzite/zinc blende-related sulfides such as CdS and ZnS, chalcopyrite-related sulfides such as (Cu, Ag)(Ga, In)S2 , and solid solutions of these] [2,20], which are also used as light-absorbing layers in photovoltaic cells, RE2 Ti2 S2 O5 (RE = Y and Sm) [21,22], and La5 Ti2 MS5 O7 (M = Cu, Ag) [23,24]. These are broadly classified into two categories: the first is d0 -type transition metal (Ti4+ , Zr4+ , Nb5+ , Ta5+ , etc.) compounds where the d-orbitals of metal ions are empty (d0 -electronic configuration), while the other is d10 type typical element (Ga3+ , In3+ , Ge4+ , Sn4+ , Sb5+ , etc.) compounds where the d-orbitals are fully filled with electrons (d10 -electronic configuration) [2,25]. Among the compounds enumerated above, (1−x)GaN–xZnO, (1−x)ZnGeN2 –2xZnO, and sulfides are of the d10 type, while all the others are considered to be the d0 -type. Their common feature is that the metal ions are in the highest oxidation state. There have been no reported cases of water splitting stably achieved by materials including metal ions with partially filled dorbitals (d1 –d9 ) regardless of whether they are oxides, nitrides, or sulfides, and it is empirically accepted that there are no promising candidates among such materials. Although some transition metal compounds with dx -electronic configurations (x is an integer from 1 to 9) such as ␣-Fe2 O3 have been extensively studied on their photoelectrochemical and photocatalytic properties [26], these materials mostly need considerable external bias to oxidize water in a photoelectrochemical cell and elaborate nano-architectural control on the photocatalytic matrix to split water in a photocatalytic powdersuspension system, and are considered to be essentially different from the d0 - and d10 -types that have the capability to split water in a bulk form.

Y. Moriya et al. / Coordination Chemistry Reviews 257 (2013) 1957–1969

1959

Fig. 1. Schematic diagram illustrating a sequence of photocatalytic water splitting reactions by one-step excitation in a single photocatalyst: (0) light irradiation and absorption, (1) photo-excitation of electron–hole pairs, (2) migration of excited carriers to the surface, and (3) oxidation–reduction reaction on the surface.

An example of overall water splitting (OWS) by (1−x)GaN–xZnO loaded with rhodium-chromium composite oxide (Rh2−y Cry O3 ) as an H2 -generating co-catalyst is shown in Fig. 2 [10]. It can be seen that H2 and O2 are stably generated under visible-light irradiation ( > 400 nm) over a relatively long time. What is important here is that the ratio of generated H2 and O2 is nearly 2:1, since H2 O → H2 + 1/2O2 . Even though H2 and O2 are generated simultaneously, if the ratio deviates considerably from 2:1, this implies that electrons or holes are also consumed in something other than water splitting. In many cases of (oxy)nitrides, holes are consumed in selfoxidation, and N2 is generated instead of O2 , which is generated less than expected or not at all. When oxides are used as co-catalysts, it is also likely that electrons are consumed in their self-reduction.

2.0

evac.

evac.

evac.

evac.

evac.

evac.

H2

n / mmol

1.5

1.0 O2

0.5

0.0 0

N2

5

10

15

20

25

30

35

t/h Fig. 2. Typical time course of the amounts (n) of generated hydrogen (H2 ; open circles), and oxygen (O2 ; black circles) in a photocatalytic overall water splitting reaction using (1−x)GaN–xZnO solid-solution photocatalyst modified with Rh1−y Coy O3 co-catalyst under visible-light irradiation (wavelength,  > 400 nm) [10]. The reaction was carried out for 35 h with evacuation of the reaction system every 5 h. H2 and O2 are stoichiometrically generated in a molar ratio of 2:1, while nitrogen (N2 ; grey circles), which is usually generated by self-oxidation of (oxy)nitride photocatalysts, is scarcely detectable.

Even though H2 and O2 are generated in the ratio of nearly 2:1, a photocatalyst is eventually deactivated if such self-decomposition progresses gradually [27]. As just described in d10 -type typical element oxynitrides, although problems still remain in achieving highly efficient (with less waste of absorbed light, i.e., with a high quantum yield) and long-term operation [9–13], OWS has already been achieved by elaborating co-catalysts loaded as active sites on the surface of a photocatalytic matrix. However, since each of these has an absorption edge at ∼500 nm (BG ∼2.5 eV; ∼20% of the sunlight can be utilized [28]), which is close to the UV-region ( < ∼380 nm) within the visible-region (∼380 nm <  < ∼780 nm), it is difficult to say that sunlight is utilized very efficiently (with less waste of incident sunlight). On the other hand, among d0 -type (oxy)nitrides and (oxy)sulfides, there are some with absorption edge wavelengths longer than 600 nm (BG < 2.1 eV; more than 35% of the sunlight can be utilized [28]), which is close to the infrared (IR)-region (∼780 nm < ). Therefore, it would be a route to more efficient utilization of sunlight if it becomes possible to split water using these 600 nm-class photocatalysts. To utilize sunlight efficiently with photocatalysts means that the BG must be narrower. As seen in Fig. 1, however, if the BG gets narrowed, there is an increasing possibility that the CBM becomes higher than the ORP of H+ /H2 , or the VBM becomes lower than the ORP of O2 /H2 O in regard to the potential. In such cases, one-step water splitting becomes impossible in principle. As a simple way to examine whether it is possible, there are H2 photo-generation (H+ photo-reduction) and O2 photo-generation (H2 O photo-oxidation) tests using suitable sacrificial agents. For H2 photo-generation, an organic material oxidized more easily than H2 O, such as methanol (CH3 OH), is used as an electrondonating sacrificial agent (hole scavenger). If the ORP of H+ /H2 is sufficiently higher than the CBM potential and the ORP of H2 CO3 /CH3 OH (+0.044 V vs. NHE) is sufficiently lower than the VBM potential, electrons reduce H+ to generate H2 , while holes oxidize CH3 OH instead of H2 O. The overall reaction is as follows: CH3 OH + H2 O → 3H2 + CO2 . The reaction terminates when all CH3 OH is consumed.

1960

Y. Moriya et al. / Coordination Chemistry Reviews 257 (2013) 1957–1969

For O2 photo-generation, a salt including ions reduced more easily than H+ such as Ag+ is used as an electron-accepting sacrificial agent (electron scavenger). If the ORP of Ag+ /Ag (+0.799 vs. NHE) is sufficiently higher than the CBM potential and the ORP of O2 /H2 O is sufficiently lower than the VBM potential, the holes oxidize H2 O to generate O2 , while electrons reduce Ag+ instead of H+ to deposit metallic Ag. The overall reaction is as follows: 4Ag+ + 2H2 O → 4Ag + O2 + 4H+ . The reaction terminates when all Ag+ ions are consumed or the photocatalysts are deactivated by Ag deposited on the surface. Materials promoting both reactions should be able to split water in principle by one-step photo-excitation, and it has been confirmed that both reactions actually occur with almost all the d0 -type transition metal (oxy)nitrides and oxysulfides referred to above (except for sulfides which are very unstable upon O2 generation) under visible-light irradiation. Nevertheless, OWS has not yet been achieved. Herein, although descriptions are confined to (oxy)nitrides for the purpose of this review and for simplicity, the situation is essentially the same for oxysulfides, which are mainly prepared by heating the corresponding oxide precursors under a hydrogen sulfide (H2 S) flow (sulfurization) at high temperatures. 3. Problems of the nitridation of (oxide) precursors Why can’t OWS, which should theoretically be possible, be achieved by the d0 -type transition metal (oxy)nitrides? It is nontrivial to pinpoint the exact cause in a sequence of photocatalytic reactions undergoing the following complex processes: (1) photoexcitation of electron–hole pairs in the bulk, (2) migration of the excited carriers to the surface to create the reaction field, and (3) oxidation–reduction reaction on the surface. Concerning Process 3, some loading methods such as impregnation and photo-electrodeposition have been examined for various kinds of co-catalysts to be active sites, but OWS has still not been attained even though the H2 /O2 generating activity was enhanced in the presence of sacrificial agents. This implies that Processes 1 and 2 that pertain directly to the photocatalytic matrix are more essential. What we focus our attention on as possible causes associated with these two processes are lattice and surface defects such as reduced species and anion vacancies. First, in Process 1, the presence of defects may affect the band structure itself, and it is also likely that it blocks the excitation of electron–hole pairs. Furthermore, in Process 2, excited electron–hole pairs are required that do not recombine before reaching the surface where the reaction occurs, but it is believed that most of them are lost in this process by defects acting as recombination centers. There is also a possibility that defects form barriers in the contact interface between the photocatalytic matrix and the co-catalysts. Unless the electron–hole pairs that could reach the surface without recombining migrate immediately to the active sites, they cannot contribute to water splitting since eventually they recombine or are consumed for selfdecomposition. It is thus regarded as critically important to reduce the defect density and improve the crystallinity of photocatalysts. The most common preparation method of metal (oxy)nitrides is nitridation of a metal oxide precursor by heating it under an ammonia (NH3 ) flow. NH3 thermally decomposes at high temperatures and then forms radicals such as NH and NH2 [29]. The nitridation occurs by the formed radicals extracting O from an oxide and introducing N instead. For example, in the case of Ta-based (oxy)nitrides, the oxide precursor is tantalum(V) oxide (Ta2 O5 ), and the nitridation proceeds by heating under an NH3 flow as follows: Ta2 O5 → TaON → Ta3 N5 .

When the nitridation is carried out under relatively mild conditions (e.g., NH3 flow rate, temperature, and duration of 20 mL min−1 , 1123 K, and 15 h, respectively), TaON is formed, but if these conditions become more severe (e.g., the NH3 flow rate is increased up to 500 mL min−1 ), it goes beyond TaON into Ta3 N5 . Up to this stage, the valence of the Ta ions is +5. However, upon further heating under more extreme conditions, e.g., at a higher temperature or for longer duration, decomposition of Ta3 N5 progresses as follows [30]: Ta3 N5 → Ta4 N5 → Ta5 N6

(→ TaN → Ta2 N).

The number of N atoms per Ta decreases as 1.67 → 1.25 → 1.2· · ·, and the valence of Ta ions decreases from +5 to +4 and +3. Under circumstances without radicals including N, e.g., under vacuum, the decomposition becomes more remarkable, but even in the process of nitridation, N vacancies and Ta reduced species inevitably form even though the structure of Ta3 N5 is still retained, because the sample is being continuously exposed to an atmosphere that is still reducing against (oxy)nitrides including metal ions in a higher oxidation state: virtually, the nitridation can be promoted only at high enough temperatures at which the decomposition also progresses gradually. The homogeneity of nitridation is also a major problem. In most cases, an oxide precursor is set in a container like a boat and then placed statically in a horizontal tubular furnace, where differences occur in the contact conditions with the radicals formed by the decomposition of NH3 between the interior and exterior of the statically placed lump of oxide powder. Accordingly, differences also occur in the degree of nitridation. To nitride the entire sample as homogeneously as possible, the following procedures can be taken: to repeat the nitridation with grinding and mixing halfway or to ensure as uniform NH3 flow as possible by reducing a single nitrided amount of precursor and lightly swathing it in an unreactive fibrous material like silica wool. As a way to nitride a large amount of precursor at once in a short time, usage of a rotary kiln, which can heat a sample with churning, is also regarded as an effective method [31]. A more essential problem is, however, the homogeneity of each particle in a powder sample. The nitridation naturally progresses from the surface to the core of the particles, and therefore, grain boundaries and lattice defects are probably formed due to the drastic transformation in the crystal structure from an oxide to a(n) (oxy)nitride. Additionally, the lattice becomes more rigid by the enhancement of covalent character with the substitution of N for O, making the thermal diffusion of atoms blunt. Therefore, it is most likely that many defects are formed in the vicinity of the surface by the time the nitridation reaches the core. This is likely not a favorable situation for photocatalytic reactions to occur promptly. 4. Attempts to prepare high-quality (oxy)nitrides As described above, the formation of defects seems inevitable especially in the vicinity of the surface in the conventional preparation method of transition metal (oxy)nitrides using NH3 . In an attempt to reduce the defect density in surface layers, we have examined various post-treatments. 4.1. High-pressure high-temperature treatments Although most transition metal (oxy)nitrides to be applied as photocatalysts start decomposing thermally at several hundred Kelvin at ambient pressure even under an inert atmosphere [32,33], it is considered possible to increase the decomposition temperature in a high-pressure environment including an N source like N2 or NH3 . Crystallinity could be enhanced by annealing at sufficiently

Y. Moriya et al. / Coordination Chemistry Reviews 257 (2013) 1957–1969

high temperatures and N vacancies may be compensated for by the dissociative adsorption of pressure medium under a high-pressure N2 atmosphere or high-pressure (or supercritical) NH3 . In addition, supercritical NH3 treatments are expected to have an effect similar to flux treatments as described below, since supercritical NH3 in general has strong dissolving power. 4.2. Acid treatments In order to expose the bulk (interior), which should be moderately nitrided, to the outermost part, the surface layers can be removed chemically using suitable acids. The type and concentration of acid, treatment temperature and duration, etc. are important factors in this strategy. 4.3. Flux treatments The flux method is a single-crystal growth method classified into solution techniques. As against the aqueous solution method where water is used as a solvent, inorganic materials or metals with a higher melting temperature are used as solvents, which are called flux. The material to be applied as a solute is dissolved into the flux, and then the crystals are grown from a solution oversaturated by lowering the solubility (soluble amount) by a gradual temperature decrease or by flux evaporation at a constant temperature. The following advantages are expected from the flux method as applied to preparation and post-treatment of photocatalysts: (1) High-quality crystals with few defects are obtained by undergoing a process of dissolution and subsequent recrystallization. (2) The crystals obtained are not affected by the precursors. (3) Crystals with clear crystal habits reflecting the crystal structure are obtained. Concerning the first point, although the flux method is somewhat unsuitable for growth of large-sized crystals, it is not necessary to obtain large crystals for photocatalytic reactions where the photocatalytic powder sample is suspended in water (aqueous solution) under light irradiation. It is thus most favorable to obtain highquality crystallites with a size of 1–10 ␮m. The second point is somewhat more specific. For instance, the precursor of TaON and Ta3 N5 is Ta2 O5 as mentioned previously, and it is directly nitrided as purchased from the supplier. A nitrided sample is susceptible to the precursor (actual purity, components of impurities, particle size and morphology, etc.), and often samples obtained by nitriding precursors from different suppliers show different photocatalytic performances even though the nominal purity is the same. In post-treatments using flux, an obtained sample becomes unaffected by the precursor since it undergoes a process of, so to speak, disassembly and reassembly. On the other hand, associated with the disadvantages to be described later, it is conceivable that an obtained sample is affected by the flux. The third point is significant in a different sense from reducing the defect density. In photocatalytic reactions, a co-catalyst is always required for H2 generation, whereas it is not always necessary for O2 generation. Even then, by introducing a suitable co-catalyst for the latter, electrons and holes are selectively consumed on H2 - and O2 -generating co-catalysts, respectively, and the enhancement of photocatalytic performance is expected in the overall system. However, as these co-catalysts are randomly loaded onto the surface, H2 -generating (electron-consuming) and O2 -generating (hole-consuming) sites coexist on the same surface. This is not a favorable situation in terms of the separation of electrons and holes (charge separation) and the suppression of the reverse reaction, i.e., H2 + 1/2O2 → H2 O. The high availability of photocatalytic crystallites with clear crystal habits can pave the way to discrete loading of each co-catalyst

1961

onto different surfaces (anisotropic modification). By consuming electrons and holes selectively on different surfaces, a charge separation is promoted by the formation of a concentration gradient, and recombination becomes less frequent. It is also expected that the reverse reaction will be suppressed if H2 - and O2 -generating faces are completely separated as on two sides of a membrane. The greatest disadvantage of the flux method is that flux components can be impurities in the target materials. An exception is the self-flux method wherein the flux itself is a component of the desired product, e.g., PbO flux in crystal growth of PbTiO3 . The existence of impurities in a photocatalyst holds risks for the formation of unfavorable impurity levels in addition to the impurities themselves behaving as defects. However, considering the fact that doping with dissimilar metals has been widely applied in the development of photocatalysts, it may be the case that the impurities themselves yield favorable effects. There have been few applications of flux growth of various (oxy)nitrides except for the so-called III–V nitrides of the d10 -type, especially gallium nitride (GaN) using materials with the capability to dissolve gaseous N2 as flux, e.g., metallic sodium (Na) [34], LiF–BaF2 –Li3 N [35], and calcium nitride (Ca3 N2 ) [36]. In d0 -type transition metal (oxy)nitrides, it is expected that more strict conditions are required for flux to be used, e.g., atmosphere or pressure, to inhibit decomposition. In addition to simple post-treatments using flux, the combination with high-pressure treatments as mentioned above should be applied as needed. We describe below recent progress in the syntheses and posttreatments on (oxy)nitride photocatalysts for water splitting to improve their photocatalytic performance, with a focus on Ta3 N5 and perovskite oxynitrides. 5. Ta3 N5 Tantalum(V) nitride (Ta3 N5 ) is a simple nitride with an orthorhombic anosovite (Ti3 O5 ) structure [37], which generates both H2 and O2 individually from H2 O in the presence of suitable sacrificial agents by absorbing light with wavelengths up to ∼600 nm (BG ∼2.1 eV) [15]. Since the H2 -generating activity is much lower than the O2 -generating one, it is considered more important to enhance the former. As mentioned previously, Ta3 N5 is obtained by heating a Ta2 O5 precursor under an NH3 flow, but it is also possible to prepare it from tantalum(V) chloride (TaCl5 ) as a precursor [38], since an O source is unnecessary unlike the case for TaON. 5.1. High-pressure high-temperature treatments Concerning syntheses or treatments under supercritical NH3 , i.e., ammonothermal syntheses or treatments, there have been many reports on d10 -type typical element nitrides, especially GaN [39], Si–Al-based phosphors [40], and transition metal nitrides except the d0 -type [41], while almost no reports can be found for d0 -type transition metal (oxy)nitrides. Ta3 N5 may be one of the very few reported examples of d0 -type transition metal (oxy)nitrides thermally treated under high-pressure environments. In thermogravimetry under an N2 flow, Ta3 N5 is clearly decomposed upon heating at ∼1300 K and further at ∼1500 K with a weight reduction of ∼2.5% and ∼0.5% (based on the original weight), which corresponds to the weight reduction from Ta3 N5 to Ta4 N5 (Ta3 N3.75 ) and further to Ta5 N6 (Ta3 N3.6 ), respectively. In fact, it has been confirmed by X-ray diffractometry that Ta3 N5 was decomposed into Ta4 N5 with a minority of Ta5 N6 and into Ta5 N6 after heat treatments at 1273 K and 1473 K, respectively, for 2 h under an N2 atmosphere at ambient pressure. To the best of our knowledge, Ta3 N5 can be structurally retained up to 1173 K at the lowest, but

1962

Y. Moriya et al. / Coordination Chemistry Reviews 257 (2013) 1957–1969

5

N2 atmosphere. The photocatalytic activity was confirmed to be considerably degraded after the treatments. An N2 atmosphere at such pressures of MPa at the highest may not be enough to inhibit the decomposition and enhance the crystallinity of Ta3 N5 . Watanabe et al. [42,43] reported that thermal treatments under high-pressure NH3 enhanced the H2 -generating activity of Ta3 N5 in the H2 photo-generation reaction in the presence of CH3 OH. The treatment temperature and duration were fixed at 823 K, which was between the decomposition temperatures of Ta3 N5 in vacuum (∼773 K) and at ordinary pressure (∼873 K), and 24 h, and the pressure was varied from 10 to 100 MPa. At 823 K, NH3 is in gaseous and supercritical states below and above 11.3 MPa, respectively, since the critical point of NH3 is at 405.6 K and 11.3 MPa. After the treatments, the absorption intensity above ∼600 nm got larger, which indicated that more Ta-reduced species were formed by the treatments. However, the H2 -generating activity was significantly enhanced under all treatment conditions. Watanabe et al. attributed this apparent contradiction to the difference between the lattice and surface defects: the former including reduced species increased, while the latter decreased by the treatments. No remarkable changes in the morphology, particle size, and specific surface area were observed after the treatments, suggesting that Ta3 N5 did not dissolve into supercritical NH3 . In fact, we have confirmed that acid treatments do not have much effect on Ta3 N5 , because it does not dissolve easily into most acids readily available, unlike perovskite oxynitrides to be described later. It is conceivable that supercritical NH3 only functions over a relatively long time to remove and reconstruct the atomic arrangements on the surface, just as acid treatments on perovskite oxynitrides act in a short time.

170 MPa N2

(a)

1273 K

0

4

I / 10 cps

5

1073 K

0 5

873 K

0 5

as-nitrided

0 reference ICSD #76460 0

20

30

40 2θ / deg.

50

60

1.2

(b)

170 MPa N2

fKM(Rd) / arb. units

1.0 0.8

1273 K

0.6

1073 K

5.2. Flux-assisted nitridation and post-treatments

0.4 873 K

0.2

as-nitrided 0 400

500

600

700

800

900

1000

λ / nm Fig. 3. (a) Powder X-ray diffraction patterns and (b) pseudo-absorption spectra [Kubelka–Munk function, fKM (Rd ), where Rd is diffuse reflectance] of Ta3 N5 samples as nitrided and post-treated at 873, 1073, and 1273 K for 2 h under an N2 atmosphere at 170 MPa. The as-nitrided sample was prepared by nitriding the Ta2 O5 precursor at 1123 K for 15 h under an NH3 flow at 500 mL min−1 . The pseudo-absorption spectra are normalized at the maxima for comparison.

the formation of Ta-reduced species (Ta4+ , Ta3+ , etc.) and N vacancies begins at ∼873 K upon heating even under a high-pressure N2 atmosphere up to ∼200 MPa. Fig. 3 shows powder X-ray diffraction (XRD) patterns and pseudo-absorption spectra [Kubelka–Munk function, fKM (Rd ) = (1 − Rd )2 /(2Rd ), where Rd is diffuse reflectance] of Ta3 N5 as nitrided and thermally post-treated at 873 K, 1073 K, and 1273 K for 2 h under an N2 atmosphere at 170 MPa. In the XRD patterns (Fig. 3(a)), it can be seen that the structure of Ta3 N5 is retained even after post-treatment at 1273 K, unlike treatments at ordinary pressure where Ta3 N5 is decomposed into Ta4 N5 . Therefore, a high-pressure environment clearly inhibits the complete decomposition of Ta3 N5 at higher temperatures. Visually, however, the original red color turns brownish with increasing treatment temperature. Looking at the absorption spectrum of an as-nitrided sample (Fig. 3(b)), besides a clear absorption edge at ∼600 nm, weak absorption with a broad peak at ∼725 nm is observed above 600 nm, which is attributed to a Ta-reduced species, and its intensity increases with increasing treatment temperature. This fact suggests that partial decomposition without the transformation of material, i.e., the formation of Ta-reduced species and N vacancies, is promoted above ∼873 K even under a high-pressure

The most readily performed treatment using flux is a thermal treatment under N2 or NH3 flow. A heating unit under a special gas flow (nitridation furnace) can be used as is without any modification, unless the used flux reacts with silica and alumina which are common materials of containers and combustion tubes. As an example, the results of preparation or post-treatments of Ta3 N5 using alkali metal salts as flux under an NH3 flow [44] are given below. Herein, either Ta2 O5 or TaCl5 was used as a precursor, and changes in the particle morphology and properties were studied in detail while varying the amount of precursor, use or non-use of flux (and the amount when it was used), treatment (nitridation) temperature and duration. Sodium chloride (NaCl; melting temperature 1074 K) and sodium carbonate (Na2 CO3 ; melting temperature 1131 K) were used as flux, and the flow rate of NH3 was fixed at 100 or 200 mL min−1 . Since the temperature was kept constant in all cases, the evaporation of flux in the open system contributed to the recrystallization. Fig. 4(a) and (b) show scanning electron microscope (SEM) images of Ta2 O5 precursor and Ta3 N5 prepared by a conventional method, where the precursor was directly nitrided at 1123 K for 20 h under an NH3 flow at 100 mL min−1 . Particles after the nitridation have irregular shapes similar to those of the precursor, and pore-like dips are observed on the surface as seen in many (oxy)nitrides prepared from oxide precursors. It is considered that such pores or voids are formed because oxide particles shrink locally with retention of the original sizes and shapes during the structural transformation (reduction in the coordination number of metal ions) from an oxide to a(n) (oxy)nitride, and diffusing atoms seek to take as short a route as possible when O is substituted with N. Fig. 4(c) shows a SEM image of Ta3 N5 (1 mmol) post-treated with NaCl (6 mmol) at 1123 K for 10 h under an NH3 flow at 100 mL min−1 . It can be confirmed that the porelike surface morphology seen in the as-nitrided sample (Fig. 4(b)) disappeared and irregular-shaped particles with a smooth surface

Y. Moriya et al. / Coordination Chemistry Reviews 257 (2013) 1957–1969

1963

Fig. 4. SEM images of (a) Ta2 O5 precursor, (b) Ta3 N5 prepared by nitriding Ta2 O5 precursor at 1123 K for 20 h under an NH3 flow of 100 mL min−1 , (c) Ta3 N5 post-treated with NaCl flux at 1123 K for 10 h under an NH3 flow of 100 mL min−1 , and (d) Ta3 N5 prepared by nitriding TaCl5 precursor at 1173 K for 20 h under an NH3 flow of 100 mL min−1 in the presence of NaCl flux. Reproduced with permission from Ref. [44]. Copyright 2011, American Chemical Society.

were formed. Hence, it was considered that, at minimum, dissolution and subsequent deposition of the surface layer took place. To further investigate the potential for morphology control using flux, the precursors were nitrided in the presence of flux under various conditions. Fig. 4(d) shows a SEM image of Ta3 N5 prepared by repeating the nitridation of the TaCl5 precursor (2 mmol) with NaCl flux (4 mmol) twice at 1173 K for 10 h under an NH3 flow at 100 mL min−1 with addition of the same amount of NaCl and mixing halfway into the treatment. It is seen that (sub)micronsized columnar crystallites reflecting the orthorhombic system are obtained with high dispersivity (without aggregation). This result demonstrates that it is possible to control the size, morphology, crystallinity, dispersivity, etc. of prepared Ta3 N5 depending on the kind and amount of precursor and flux, treatment (nitridation) temperature and duration. Ta3 N5 itself hardly dissolves into acids, flux, or even supercritical NH3 . When increasing the treatment temperature to increase the solubility, decomposition always occurs prior to dissolution. It seems that flux-assisted nitridation of a precursor where the dissolution and nitridation occur at the same time is more effective than post-treatments where already-formed Ta3 N5 is required to dissolve. More importantly, however, the photocatalytic performance is comparable to conventionally prepared samples, and there is as yet no remarkable improvement. In many cases, since the nitridation of a precursor under an NH3 flow is carried out at higher temperatures than the decomposition temperature of an objective (oxy)nitride under an inert atmosphere, it is believed that nitridation (introduction of N) and decomposition (extrication of N) are always in competition. Although the presence of flux may suppress some decomposition in flux-assisted nitridation and post-treatments, there is no change in the basic situation as long as the treatment temperature is higher than the decomposition temperature. After all, no remarkable improvement has been observed with respect to reduction in

defect density in surface layers, despite significant refinements in the particle and surface morphologies. Nevertheless, it can be considered as major progress to have found the means to control the morphology in transition metal (oxy)nitrides using some kind of flux. Significant improvement can be expected in both morphology control and reduction of defect density if a suitable flux is found that melts at a lower temperature than the decomposition temperatures of these (oxy)nitrides and that can dissolve and recrystallize them. 5.3. Other synthetic routes It has been reported by Fitzmaurice et al. [45] that lanthanide(III) nitrides with the rock salt structure, LnN (Ln = Y, La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb), can be synthesized from lanthanide(III) chlorides (LnCl3 ) and lithium nitride (Li3 N). In short, a 1:1 mixture of LnCl3 and Li3 N was sealed in a glass ampoule under vacuum and then heated at 673 K for 1–10 min. Consequently, LnN is obtained according to the following equation: LnCl3 + Li3 N → LnN + 3LiCl. Herein, lithium chloride (LiCl), which may function as either a flux or mineralizer, is the co-product. In this manner, as the reaction temperature is much lower than the decomposition temperature of Ta3 N5 , its crystals with few defects may be directly synthesized from TaCl5 and Li3 N by a similar route, if desired, with the addition of, e.g., LiCl–KCl flux, in order to make the reaction proceed moderately in a molten salt (liquid phase). Unfortunately, however, it has been already confirmed by the same group [46,47] that TaCl5 reacting with Li3 N, magnesium nitride (Mg3 N2 ), or Ca3 N2 does not form Ta3 N5 but forms TaN or Ta2 N instead with the release of N2 . This is probably caused by the readily reduced nature of Ta ions and high reactivity of alkali(-earth) metal nitrides.

1964

Y. Moriya et al. / Coordination Chemistry Reviews 257 (2013) 1957–1969

Fig. 5. SEM images of (a) Ta3 N5 prepared by nitriding Ta2 O5 precursor at 1123 K for 15 h under an NH3 flow of 1 L min−1 and (b) Ta3 N5 prepared by reaction between TaCl5 and NH4 Cl at 1123 K for 1 h in a quartz ampoule sealed at room temperature under vacuum. Reproduced with permission from Ref. [48]. Copyright 2009, American Chemical Society.

Ta3 N5 can also be prepared when ammonium chloride (NH4 Cl) is used instead of alkali(-earth) metal nitrides as an N source [48]. A mixture of TaCl5 and NH4 Cl with an excess of the latter (in a ratio of 3:10) was sealed in a quartz ampoule at room temperature under vacuum and then heated at various temperatures between 673 and 1123 K for 1 h. On heating, NH4 Cl is decomposed into NH3 and HCl at ∼610 K, above which the internal pressure of the sealed ampoule increases with increasing temperature. Therefore, the amount of mixture introduced into the ampoule is limited by the inner dead volume and the reaction temperature such that the ampoule does not burst in the process of heating. The reaction proceeds at temperatures above ∼873 K as follows: 3TaCl5 + 5NH4 Cl → Ta3 N5 + 20HCl. A SEM image of the obtained Ta3 N5 is shown in Fig. 5 together with that of Ta3 N5 prepared by a conventional method for comparison. The obtained Ta3 N5 (Fig. 5(b)) has a needle-like morphology completely different from the conventional Ta3 N5 (Fig. 5(a)) and hardly shows any photocatalytic activity, which is attributed to the limited number of active sites. The crystal growth is exclusively promoted in the [1 0 0] direction of the orthorhombic lattice [36] mainly due to the presence of chloride adsorbed onto the surface of growing crystallites, resulting in the needle-like morphology. It was experimentally confirmed that the active sites are located on the (100) faces perpendicular to the [1 0 0] direction, namely at both ends of the needle-like crystallites. This may explain the apparent degradation of the photocatalytic performance. In other words, if the crystal growth along the [1 0 0] direction is exclusively inhibited, it would be possible to obtain Ta3 N5 with more active sites, and consequently, higher performance. Such control of crystal growth directions is one of the important subjects in flux crystal growth. Additionally, many synthetic routes have been developed for Ta3 N5 such as the nitridation of TaS2 [49], solid state reactions between TaCl5 and lithium amide (LiNH2 ) or lithium dimethylamide [(CH3 )2 NLi] [50], reaction between TaCl5 and LiNH2 in the presence of NH3 under solvothermal conditions [51], heating a precursor prepared by letting NH3 flow through a chloroform (CHCl3 ) solution of TaCl5 under an NH3 flow [52], heating tantalum(V) ethoxide [Ta(OC2 H5 )5 ] embraced in a mesoporous carbon nitride (C3 N4 ) template under an NH3 flow [53], and colloidal syntheses using TaCl5 or pentakis(dimethylamino)tantalum(V) {Ta[N(CH3 )2 ]5 } as a Ta source and tris(trimethylsilyl)amine {[(CH3 )3 Si]3 N} or Li3 N as an N source [54]. Owing to the simplicity of composition, it appears to be easy to synthesize high-quality crystals of Ta3 N5 , but in reality, it is extremely challenging. This difficulty mainly arises from the

thermal instability of Ta3 N5 , the readily reduced nature of Ta ions, and the difficulty in creating an O-free environment, i.e., complete elimination of O sources such as moisture included in pressure media, solvents, and flux. 6. Perovskite oxynitrides Except for solid solutions, LaMO2 N (M = Ti, Zr), LaMON2 (M = Nb, Ta), and AEMO2 N (AE = Ca, Sr, Ba; M = Nb, Ta) are well-known perovskite oxynitrides [55]. Most of them can absorb light with wavelengths up to ∼600 nm or even longer, and moreover, the absorption edge, i.e., BG, is tunable in a wide range from the UV to the near-IR region by forming solid solutions with other perovskite oxides and oxynitrides. In addition to the chemical and thermal stabilities, which are both superior to nitrides, this nature makes them exceedingly attractive from the standpoint of application to nontoxic pigments [56] and visible-light-responsive photocatalysts for water splitting [8]. 6.1. Ammonothermal synthesis Perovskite oxynitrides can be also prepared by conventional nitridation, where an oxide precursor or a mixture of oxide and carbonate starting materials is heated under an NH3 flow. Recently, Watanabe et al. [57] have also reported a direct synthesis of LaTaON2 under supercritical NH3 , i.e., ammonothermal synthesis, instead of ammonothermal treatment on an already-prepared sample. The procedure is somewhat complicated, but there are some advantages over the conventional method: the reaction temperature can be lowered and the reaction occurs not in the solid state but rather in the liquid state. Therefore, a highly crystallized sample with few defects is expected. First, a La–Ti alloy precursor was prepared from metallic La and Ti by arc-melting under an argon (Ar) atmosphere and then grinding into powder. The precursor was mixed with sodium amide (NaNH2 ) as a basic mineralizer, which was necessary in this case, in a molar ratio of La:Na = 5:1 and then heated at 773–1073 K for 15–75 h under supercritical NH3 at 100 MPa. At reaction temperatures above 873 K, a single phase of LaTaON2 was obtained with a size of ∼1 ␮m and rectangular-parallelepiped particle morphology that did not depend significantly on either reaction temperature or duration. In this method, lanthanum hydroxide [La(OH)3 ] is formed as a byproduct, and the resultant powder must be washed with diluted hydrochloric acid to remove it. It follows that samples obtained in this manner are always treated with acid. Interestingly, the addition of an O source is unnecessary, because enough moisture to form La(OH)3 as a by-product can be supplied as an impurity from

Y. Moriya et al. / Coordination Chemistry Reviews 257 (2013) 1957–1969

air, NaNH2 , etc. This situation is critical when nitrides like Ta3 N5 are prepared or post-treated by this method. The photocatalytic performance of such obtained compounds has not yet been reported. 6.2. Morphology control of LaTiO2 N by flux method As described in Section 4.3, one of the advantages of applying the flux method to photocatalyst development is the availability of “crystals with clear crystal habits reflecting the crystal structure” opening the possibility of anisotropic modification. With crystallites having a rectangular parallelepiped shape including a cube and plate, it may become possible to form a “monoparticle layer,” where a single layer consists of a single crystallite in the layer thickness direction, by paving them closely like bricks on a substrate using suitable molecular linkers [58,59]. Since there are two sides in a layer (membrane), anisotropy can be vested in the surface modification of photocatalysts, as H2 and O2 generation on either side, by trying various membrane-forming and co-catalyst-loading techniques. The photocatalytic material of interest to us for such anisotropic modification is lanthanum titanium oxynitride (LaTiO2 N). This is a material composed only of relatively abundant, inexpensive, and nontoxic elements unlike Ta3 N5 , and moreover, can generate both H2 and O2 individually from H2 O in the presence of suitable sacrificial agents under light irradiation with wavelengths up to ∼600 nm (BG ∼2.1 eV; Fig. 6) like Ta3 N5 [15]. It would be an extraordinary outcome if OWS could be achieved using this material. LaTiO2 N is mainly prepared by nitriding lanthanum(III) titanium(IV) oxide (La2 Ti2 O7 or LaTiO3.5 ) as a precursor, which is a (1 1 0)-layered perovskite ferroelectric material with a ferroelectric phase transition temperature Tc ∼1773 K [60]. La2 Ti2 O7 itself is also known as a photocatalyst that splits water under UV-light irradiation (abs ∼325 nm, BG ∼3.8 eV) [61], and attempts have been made to make it visible-light responsive by dissimilar-metal doping [62]. As a precursor of LaTiO2 N, La2 Ti2 O7 is conventionally synthesized by either a solid-state reaction (SSR) technique using lanthanum oxide (La2 O3 ) and TiO2 as starting materials, or a polymerized complex (PC) method, where lanthanum nitrate [La(NO3 )3 ] and titanium isopropoxide [Ti(O-i-Pr)4 ] as starting materials are added into a mixture of CH3 OH, ethylene glycol [C2 H4 (OH)2 ], and citric acid [C3 H4 (OH)(COOH)3 ]. However, the size and morphology of the crystallites are not controllable using these methods. In contrast, using a flux method, rectangular parallelepiped (plate-like)

La 2Ti2O7 0.5NaCl-0.5KCl flux λabs ~ 325 nm, Eg ~ 3.8 eV molybdate flux

fKM(Rd) / arb. units

1.0

LaTiO 2N from La2Ti2O7 synthesized using 0.5NaCl-0.5KCl flux λabs ~ 600 nm, Eg ~ 2.1 eV

0.5

0

300

400

500

600 700 λ / nm

800

900

1000

Fig. 6. Pseudo-absorption spectra [Kubelka–Munk function, fKM (Rd ), where Rd is diffuse reflectance] of La2 Ti2 O7 and LaTiO2 N, which are normalized at the maxima for comparison. Solid and dashed lines are of La2 Ti2 O7 synthesized using 0.5NaCl–0.5KCl and a molybdate, respectively, as flux, and dashed-dotted line is of LaTiO2 N prepared by nitriding the former.

1965

crystallites reflecting the layered structure can be expected to form. In fact, there have been a few reports on the syntheses of La2 Ti2 O7 using a 0.5NaCl–0.5KCl flux with the goal of making a high-quality ferroelectric ceramic sample [63] and using a 0.5Na2 SO4 –0.5 K2 SO4 flux to examine the influence of size and morphology on the photocatalytic activity for water splitting [64], and in both cases, plate-like crystallites were obtained under certain conditions. There is a possibility of obtaining an oxynitride retaining the original size and morphology by nitriding an oxide precursor with plate-like particle morphology. Naturally, significant progress cannot be expected in comparison to the conventional synthetic methods from the aspect of reduction of defect density, because after all, a sample undergoes conventional nitridation using NH3 . In fact, in the absorption spectrum of LaTiO2 N (Fig. 6), absorpotion with a broad peak at ∼865 nm is observed above ∼600 nm just as in Ta3 N5 , which is attributed to a Ti reduced species (Ti3+ ), and it can be seen that there is no substantial change in the situation of defect formation even in the sample nitrided from the precursor synthesized by a flux method. Herein, La2 Ti2 O7 was synthesized by the method using 0.5NaCl–0.5KCl flux [63], where a few ␮m-sized crystallites formed, and then LaTiO2 N was prepared by nitridation. La2 O3 and TiO2 were mixed in a stoichiometric ratio with the subsequent addition of 0.5NaCl–0.5KCl in a molar ratio of (Na + K)/(La + Ti) ∼2 and then heated at 1423 K for 5 h, followed by cooling naturally down to room temperature. The resultant mass was washed with distilled water to dissolve and remove the flux, and the objective material, i.e., La2 Ti2 O7 , was finally isolated by filtration, followed by desiccation. Subsequently, heating at 1223 K for 15–45 h under an NH3 flow at 200 mL min−1 afforded LaTiO2 N. In Fig. 7, SEM images of the obtained La2 Ti2 O7 and LaTiO2 N are shown together with those derived by a PC method for comparison. It is often the case that a sample synthesized by a PC method is amorphous with an atypical form and a rough surface (Fig. 7(a)). In contrast, in the sample synthesized by a flux method (Fig. 7(b)), plate-like crystallites with a size up to ∼5 ␮m are formed, although they vary somewhat in size. The PC-methodderived LaTiO2 N (Fig. 7(c)) has irregular particles and pore-like surface morphology as does Ta3 N5 nitrided from Ta2 O5 (Fig. 3(b)). In contrast, the flux-method-derived LaTiO2 N (Fig. 7(d)) retains the plate-like particle morphology of its precursor with a few ␮m on a side without collapsing during the nitridation process, although it has pore-like surface morphology as do other samples nitrided by heating under an NH3 flow. This indicates that the particle morphology of oxynitrides can also be controlled by the flux method. In addition, it has been confirmed by recent transmission electron microscope (TEM) observations that individual plate-like particles are not aggregates of nanocrystallites (polycrystals), but single crystals by themselves, despite their pore-like surface morphology [65]. This suggests that there is very little interface resistance in a single particle, which is highly favorable for carrier transfer. Interestingly, the flux-method-derived LaTiO2 N shows higher activity in H2 /O2 photo-generation reaction in the presence of sacrificial agents than the PC-method-derived one [65]. When an appropriate amount of cobalt oxide (CoOx ) is loaded as an O2 generating co-catalyst, for instance, the former shows about twice the O2 -generation rate of the latter in the O2 photo-generation reaction using Ag+ as an electron scavenger. When an appropriate amount of Pt is loaded as an H2 -generating co-catalyst, the former shows only half the H2 -generation rate of the latter in the H2 photo-generation reaction using CH3 OH as a hole scavenger. When co-loading Pt and CoOx , however, the H2 -generation rate of the former becomes comparable to or larger than that of the latter. Although the details are still under consideration, these performance enhancements are closely associated with the crystallinity and the particle and surface morphologies including the loading

1966

Y. Moriya et al. / Coordination Chemistry Reviews 257 (2013) 1957–1969

Fig. 7. SEM images of La2 Ti2 O7 synthesized by (a) polymerized complex method and (b) flux method using 0.5NaCl–0.5KCl as flux, and LaTiO2 N prepared by nitriding (c) the former and (d) the latter at 1223 K for 15 and 40 h, respectively, under an NH3 flow of 200 mL min−1 . Note that only the magnification of (b) is different from the others. Partly reproduced with permission from Ref. [65]. Copyright 2012, American Chemical Society.

state of the co-catalysts, since there is no difference in the nitridation method between the two. It has also been found that it is possible to prepare La2 Ti2 O7 crystallites with a larger and more uniform size up to ∼100 ␮m and refined morphology using molybdate flux, as shown in Fig. 8 [66]. However, if the size becomes larger than a few 10 ␮m, a longer time is required to nitride completely, which results in the collapse of the original particle morphology. Furthermore, the photocatalytic performance of the obtained LaTiO2 N has been confirmed to be quite low. As shown by a broken line in Fig. 6, La2 Ti2 O7 synthesized using molybdate flux shows a slight absorption in the visible region between 400 and 800 nm. This suggests the possibility of commingling of Mo from the flux with the sample, which is believed to cause degradation of the photocatalytic performance. Currently, we are working on the fabrication of a monoparticle layer composed of LaTiO2 N with controlled morphology using an alkali halide flux [67] and its anisotropic modification, and new results will be reported in due course. 6.3. Acid treatments on LaTiO2 N It has been found that acid treatments effectively enhance the photocatalytic performance of LaTiO2 N, and the details will be reported elsewhere. The procedure is quite simple. A given amount of sample is placed into a given amount of suitable acid and then stirred for a predetermined time. After that, the sample is filtered out and washed with distilled water. Under prolonged treatment, the original surface layers with high defect density dissolve into the acid, and the moderately nitrided interior (bulk) is exposed to the outermost part as a new surface. This can be confirmed by the decrease in the absorption intensity above ∼600 nm. At the same time, however, long-time treatment tends to collapse the structure of the new surface and degrades the photocatalytic activity. In contrast, in short-time treatment, a large portion of the surface layers cannot be dissolved, so that there is no remarkable change in the

absorption intensity above ∼600 nm. Nevertheless, the photocatalytic activity is enhanced. In addition to the results of ammonothermal treatments on Ta3 N5 , this observation seems to suggest that the surface structure and state are more critical, and the presence of reduced species does not always negatively affect the photocatalytic performance. In fact, there are compounds showing metallic conduction including metal ions with dx -electronic configuration (x is an integer from 1 to 9), and it is not improbable that the surface layer including reduced species promotes the prompt migration of carriers from the matrix to the co-catalysts. In this sense, it may be necessary to design the photocatalytic matrix configurationally as has been accomplished for co-catalysts [11]. 7. Some developments in the preparation of d10 -type typical element oxynitrides As mentioned earlier, OWS has already been achieved by d10 type typical element oxynitrides such as (1−x)GaN–xZnO and (1−x)ZnGeN2 –2xZnO, but the absorption edges of these oxynitrides are located at wavelengths shorter than 500 nm, which are not long enough to utilize sunlight efficiently. (1−x)GaN–xZnO is conventionally prepared by nitriding a mixture of Ga2 O3 and ZnO [9–11] or ZnGa2 O4 and ZnO [68,69] at temperatures higher than 1073 K under an NH3 flow. In this method, however, the x value, i.e., Zn content, cannot exceed ∼0.4, because Zn2+ is reduced very easily to metallic Zn and then evaporates at high temperatures under a reducing atmosphere. There is still controversy over the reason why a solid solution between two UV-light responsive materials, i.e., GaN and ZnO with BG’s of ∼3.4 and ∼3.2 eV, respectively, becomes visible-light responsive, although many studies including density functional theory (DFT) calculations have been carried out on this subject [70–81]. Nevertheless, the BG should have a minimum at a certain value of x, and the absorption edge wavelength actually depends on the x value within a range of x available by conventional

Y. Moriya et al. / Coordination Chemistry Reviews 257 (2013) 1957–1969

1967

Fig. 8. SEM images of La2 Ti2 O7 synthesized by flux method using a molybdate as flux at (a) low and (b) high magnifications [66].

methods. Shifting the absorption edge towards a longer wavelength without losing the capability to split water would be a significant accomplishment comparable to OWS by d0 -type transition metal (oxy)nitrides and oxysulfides. Recently, a few groups have reported that (1−x)GaN–xZnO with x equal to or larger than 0.5 was successfully prepared, and furthermore, the absorption edge reached ∼600 nm by modifying the precursor. Han et al. [82] prepared a Ga–Zn oxide nanoprecursor by a sol–gel method, where given amounts of gallium nitrate hydrate (Ga(NO3 )3 ·xH2 O) and zinc acetate dihydrate [Zn(CH3 COO)2 ·2H2 O] mixed in a solution of ethanolamine [NH2 (C2 H4 OH)] were stirred at 338 K for 2 h, aged at 273 K for a week, and then calcined at 673 K for 1 h. The prepared precursor was nitrided at various temperatures between 823 and 1123 K for 10 h under an NH3 flow (the flow rate was not designated). At nitridation temperatures above 923 K, a single phase of wurtzite structure was obtained, which was confirmed as a nanocrystalline (1−x)GaN–xZnO solid solution. The x value determined by energy-dispersive X-ray (EDX) spectrometry decreased with increasing nitridation temperature from x = 0.482 at 923 K to x = 0.088 at 1123 K. With x = 0.482, the absorption edge wavelength became ∼560 nm (BG ∼2.21 eV). Lee et al. [83] used delicately prepared mixtures of nanocrystalline ZnO and ZnGa2 O4 as precursors, in which the mixing ratio was varied. About 100 mg of the precursors deposited on a glass substrate was nitrided at 923 K for 10 h under an NH3 flow of ∼100 mL min−1 , where the nitridation temperature was preliminarily optimized such that the x value in the precursor would be retained even after the nitridation. The x values of the nanocrystalline (1−x)GaN–xZnO solid solutions thus obtained ranged from 0.30 to 0.87, which were determined by inductively coupled plasma optical emission spectroscopy (ICP-OES), and the absorption edge wavelength shifted almost linearly from 460 nm (BG ∼2.7 eV) to 565 nm (BG ∼2.2 eV) with increasing x value. Interestingly, the minimum in the BG was not observed up to x = 0.87. This implies that the BG must show a sudden increase towards 3.2 eV (x = 1) at a certain value of x above 0.87. In contrast to the above two groups, Wang et al. [84] used quite different precursors. They prepared layered double hydroxides (LDH’s) containing Zn2+ , Ga3+ , and CO3 2− , where these ions were homogeneously mixed at the atomic level and the mixing ratio was varied over a wide range. By nitriding the LDH’s with different Zn contents at 1073 K for only 30 min under an NH3 flow at 300 mL min−1 , (1−x)GaN–xZnO solid solutions with x ranging from 0.46 to 0.81 were obtained. The absorption edge wavelength shifted not linearly but systematically up to ∼600 nm with increasing x value. No minimum was observed in the BG up to x = 0.81, as is the case with Lee et al. [83]. Although there is a discrepancy in the relationship between the x value (Zn content) and the absorption edge wavelength (or BG)

among these three results, Zn-rich (1−x)GaN–xZnO solid solutions can be prepared even by nitridation under an NH3 flow, and the absorption edge wavelength reaches ∼600 nm. However, the photocatalytic water splitting activity of these Zn-rich solid solutions has not yet been reported. A common feature of the nitridation in the above preparation methods is that the nitridation temperature and duration can be kept down compared to the conventional method, which probably results in a higher Zn content and suppression of grain growth. However, a low reaction temperature tends to result in low crystallinity. As seen in the example of LaTiO2 N, the crystallinity and the surface state and structure also have a considerable impact on the photocatalytic performance, and it is not always true that a photocatalyst only with large specific surface area and dimensions comparable to or smaller than the mean free path of electrons and holes shows high photocatalytic performance. 8. Conclusions and future prospects Using d10 -type typical element oxynitrides with absorption edge wavelengths (abs ) of ∼500 nm, OWS has been achieved under visible-light irradiation, only a small portion of which can be utilized. On the other hand, so-called 600 nm-class photocatalysts with abs exceeding 600 nm have been found only in d0 -type transition metal (oxy)nitrides and (oxy)sulfides. Despite the fact that most of these can generate both H2 and O2 individually from H2 O in the presence of suitable sacrificial agents under visible-light irradiation, suggesting that OWS is possible in principle, it has not yet been achieved. If OWS becomes attainable by even one of these photocatalysts, a clear route to H2 production by water splitting using sunlight can be envisioned. As the main cause for the inhibition of OWS by d0 -type transition metal (oxy)nitrides, we focus on the presence of defects inevitably formed during the conventional preparation process where a precursor is heated under an NH3 flow, and attempt various post-treatments on these photocatalysts aimed at the reduction of defect density and the improvement of crystallinity. In samples subjected to such post-treatments, the enhancement of photocatalytic activity has been definitely confirmed under some conditions in H2 /O2 photo-generation reactions in the presence of sacrificial agents and in two-step water splitting reactions. It is, however, still quite difficult to achieve OWS by d0 -type transition metal (oxy)nitride photocatalysts. This difficulty seems deep-rooted mainly in the thermal instability of these (oxy)nitrides compared to the corresponding oxides, which makes it extremely difficult to reduce the defect density and enhance the crystallinity without thermal decomposition involving the formation of reduced species and N vacancies. In fact, many post-treatments aimed at the reduction of high defect density in as-prepared samples seem to increase it further, contrary

1968

Y. Moriya et al. / Coordination Chemistry Reviews 257 (2013) 1957–1969

to expectations. Nevertheless, in some cases, the enhancement of photocatalytic activity has been confirmed. It is necessary to make careful observations of such cases. Much remains poorly understood about the details of photocatalytic reaction mechanisms on surfaces and the influence of individual defect species. To deepen our understanding, it is highly desirable to develop techniques to make it possible to evaluate individual defect species quantitatively and their influence on the photocatalytic reaction qualitatively, instead of making random attempts to improve the quality of photocatalysts. TEM is a powerful tool for that purpose. As explained in Section 4.3 and seen in the examples of LaTiO2 N and (1−x)GaN–xZnO, the characteristics of nitrided samples are considerably affected by the quality of their precursors. Considering the fact that the elaboration of precursors shows a positive influence on characteristics of samples nitrided even by the conventional method if the nitridation conditions are optimized, there is still room for improvement of precursors. In any case, there are certainly some problems in the photocatalytic matrix. We are at the cusp not only of finding nitridation or post-treatment conditions which fulfill all the requirements (i.e., to reduce the defect density, enhance the crystallinity, and inhibit decomposition, at the same time), but also to target more aggressively a direct preparation method to obtain high-quality crystals of (oxy)nitrides. Acknowledgments Some studies, including the syntheses of La2 Ti2 O7 by the flux method, have been conducted in collaboration with Profs. S. Oishi and K. Teshima at the Department of Environmental Science and Technology, Faculty of Engineering, and Ms. S. Suzuki at the Department of Materials Science and Engineering, Interdisciplinary Graduate School of Science and Technology, Shinshu University, Nagano, Japan. One of the authors (YM) has received much kind advice from Dr. Y. Inoue, a professor emeritus at Nagaoka University of Technology, Nagaoka, Japan. It is our great honor to show our gratitude to them. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]

A. Fujishima, K. Honda, Nature 238 (1972) 37. A. Kudo, Y. Miseki, Chem. Soc. Rev. 38 (2009) 253. R. Abe, K. Sayama, K. Domen, H. Arakawa, Chem. Phys. Lett. 344 (2001) 339. R. Konta, T. Ishii, H. Kato, A. Kudo, J. Phys. Chem. B 108 (2004) 8992. R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 293 (2001) 269. M. Ni, M.K.H. Leung, D.Y.C. Leung, K. Sumathy, Renew. Sustain. Energy Rev. 11 (2007) 401. Y.-I. Kim, P.M. Woodward, J. Solid State Chem. 180 (2007) 3224. K. Maeda, K. Domen, MRS Bull. 36 (2011) 25. K. Maeda, T. Takata, M. Hara, N. Saito, Y. Inoue, H. Kobayashi, K. Domen, J. Am. Chem. Soc. 127 (2005) 8286. K. Maeda, K. Teramura, D. Lu, T. Takata, N. Saito, Y. Inoue, K. Domen, Nature 440 (2006) 295. K. Maeda, K. Teramura, D. Lu, N. Saito, Y. Inoue, K. Domen, Angew. Chem. Int. Ed. 45 (2006) 7806. Y. Lee, H. Terashima, Y. Shimodaira, K. Teramura, M. Hara, H. Kobayashi, K. Domen, M. Yashima, J. Phys. Chem. C 111 (2007) 1042. Y. Lee, K. Teramura, M. Hara, K. Domen, Chem. Mater. 19 (2007) 2120. G. Hitoki, T. Takata, J.N. Kondo, M. Hara, K. Domen, Chem. Commun. (2002) 1698. G. Hitoki, A. Ishikawa, T. Takata, J.N. Kondo, M. Hara, K. Domen, Chem. Lett. 31 (2002) 736. A. Kasahara, K. Nukumizu, G. Hitoki, T. Takata, J.N. Kondo, M. Hara, H. Kobayashi, K. Domen, J. Phys. Chem. A 106 (2002) 6750. A. Kasahara, K. Nukumizu, T. Takata, J.N. Kondo, M. Hara, H. Kobayashi, K. Domen, J. Phys. Chem. B 107 (2003) 791. M. Higashi, R. Abe, K. Teramura, T. Takata, B. Ohtani, K. Domen, Chem. Phys. Lett. 452 (2008) 120. M. Higashi, R. Abe, T. Takata, K. Domen, Chem. Mater. 21 (2009) 1543. A. Kudo, Int. J. Hydrogen Energy 31 (2006) 197. A. Ishikawa, T. Takata, J.N. Kondo, M. Hara, H. Kobayashi, K. Domen, J. Am. Chem. Soc. 124 (2002) 13547.

[22] A. Ishikawa, Y. Yamada, T. Takata, J.N. Kondo, M. Hara, H. Kobayashi, K. Domen, Chem. Mater. 15 (2003) 4442. [23] M. Katayama, D. Yokoyama, Y. Maeda, Y. Ozaki, M. Tabata, Y. Matsumoto, A. Ishikawa, J. Kubota, K. Domen, Mater. Sci. Eng. B 173 (2010) 275. [24] T. Suzuki, T. Hisatomi, K. Teramura, Y. Shimodaira, H. Kobayashi, K. Domen, Phys. Chem. Chem. Phys. 14 (2012) 15475. [25] Y. Inoue, Energy Environ. Sci. 2 (2009) 364. [26] F.E. Osterloh, Chem. Soc. Rev., in press [27] T. Ohno, L. Bai, T. Hisatomi, K. Maeda, K. Domen, J. Am. Chem. Soc. 134 (2012) 8254. [28] Calculated from “ASTM G173-03 Standard Tables for Reference Solar Spectral Irradiances: Direct Normal and Hemispherical on 37◦ Tilted Surface” http://rredc.nrel.gov/solar/spectra/am1.5/ [29] I. Rahinov, N. Ditzian, A. Goldman, S. Cheskis, Appl. Phys. B 77 (2003) 541. [30] C. Stampfl, A.J. Freeman, Phys. Rev. B 71 (2005) 024111. [31] D. Logvinovich, S.G. Ebbinghaus, A. Reller, I. Marozau, D. Ferri, A. Weidenkaff, Z. Anorg. Allg. Chem. 636 (2010) 905. [32] A. Rachel, S.G. Ebbinghaus, M. Güngerich, P.J. Klar, J. Hanss, A. Weidenkaff, A. Reller, Thermochim. Acta 438 (2005) 134. [33] R. Aguiar, D. Logvinovich, A. Weidenkaff, A. Reller, S.G. Ebbinghaus, Thermochim. Acta 471 (2008) 55. [34] Y. Mori, M. Imade, K. Murakami, H. Takazawa, H. Imabayashi, Y. Todoroki, K. Kitamoto, M. Maruyama, M. Yoshimura, Y. Kitaoka, T. Sasaki, J. Cryst. Growth 350 (2012) 72. [35] B.N. Freigelson, R.L. Henry, J. Cryst. Growth 281 (2005) 5. [36] J.K. Jian, G. Wang, C. Wang, W.X. Yuan, X.L. Chen, J. Cryst. Growth 291 (2006) 72. [37] N.E. Brese, M. O’Keeffe, P. Rauch, F.J. DiSalvo, Acta Cryst. C 47 (1991) 2291. [38] P.E. Rauch, F.J. DiSalvo, J. Solid State Chem. 100 (1992) 160. [39] Q.-S. Chen, J.-Y. Yan, Y.-N. Jiang, W. Li, Prog. Cryst. Growth Charact. Mater. 58 (2012) 61. [40] M. Zeuner, S. Pagano, W. Schnick, Angew. Chem. Int. Ed. 50 (2011) 7754. [41] S. Desmoulins-Krawiec, C. Aymonier, A. Loppinet-Serani, F. Weill, S. Gorsse, J. Etourneau, F. Cansell, J. Mater. Chem. 14 (2004) 228. [42] Y. Lee, K. Nukumizu, T. Watanabe, T. Takata, M. Hara, M. Yoshimura, K. Domen, Chem. Lett. 35 (2006) 352. [43] K. Kishida, T. Watanabe, J. Solid State Chem. 191 (2012) 15. [44] T. Takata, D. Lu, K. Domen, Cryst. Growth Des. 11 (2011) 33. [45] J.C. Fitzmaurice, A. Hector, A.T. Rowley, I.P. Parkin, Polyhedron 13 (1994) 235. [46] J.C. Fitzmaurice, A.L. Hector, I.P. Parkin, J. Chem. Soc. Dalton Trans. (1993) 2435. [47] A.L. Hector, I.P. Parkin, Chem. Mater. 7 (1995) 1728. [48] D. Lu, M. Hara, T. Hisatomi, T. Takata, K. Domen, J. Phys. Chem. C 113 (2009) 17151. [49] R. Marchand, F. Tessier, F.J. DiSalvo, J. Mater. Chem. 9 (1999) 297. [50] I.P. Parkin, A.T. Rowley, Adv. Mater. 6 (1994) 780. [51] B. Mazumder, A.L. Hector, J. Mater. Chem. 18 (2008) 1392. [52] D. Choi, P.N. Kumta, J. Am. Ceram. Soc. 90 (2007) 3113. [53] L. Yuliati, J.-H. Yang, X. Wang, K. Maeda, T. Takata, M. Antonietti, K. Domen, J. Mater. Chem. 20 (2010) 4295. [54] C.-T. Ho, K.-B. Low, R.F. Klie, K. Maeda, K. Domen, R.J. Meyer, P.T. Snee, J. Phys. Chem. C 115 (2011) 647. [55] S.G. Ebbinghaus, H.-P. Abicht, R. Dronskowski, T. Müller, A. Reller, A. Weidenkaff, Prog. Solid State Chem. 37 (2009) 173. [56] M. Jansen, H.P. Letschert, Nature 404 (2000) 980. [57] T. Watanabe, K. Tajima, J.W. Li, N. Matsushita, M. Yoshimura, Chem. Lett. 40 (2011) 1101. [58] K.B. Yoon, Acc. Chem. Res. 40 (2007) 29. [59] J.S. Lee, J.H. Kim, Y.J. Lee, N.C. Jeong, K.B. Yoon, Angew. Chem. Int. Ed. 46 (2007) 3807. [60] S. Nanamatsu, M. Kimura, K. Doe, S. Matsushita, N. Yamada, Ferroelectrics 8 (1974) 511. [61] H.G. Kim, D.W. Hwang, J. Kim, Y.G. Kim, J.S. Lee, Chem. Commun. (1999) 1077. [62] D.W. Hwang, H.G. Kim, J.S. Lee, J. Kim, W. Li, S.H. Oh, J. Phys. Chem. B 109 (2003) 2093. [63] P.A. Fuierer, R.E. Newnham, J. Am. Ceram. Soc. 74 (1991) 2876. [64] D. Arney, B. Porter, B. Greve, P.A. Maggard, J. Photochem. Photobiol. A 230 (2008) 199. [65] F. Zhang, A. Yamakata, K. Maeda, Y. Moriya, T. Takata, J. Kubota, T. Teshima, S. Oishi, K. Domen, J. Am. Chem. Soc. 134 (2012) 8348. [66] By the courtesy of Profs. S. Oishi and K. Teshima at Shinshu University, Japan. [67] G. Ma, T. Takata, M. Katayama, F. Zhang, Y. Moriya, K. Takanabe, J. Kubota, K. Domen, Cryst. Eng. Commun. 14 (2011) 59. [68] X. Sun, K. Maeda, M.L. Faucheur, K. Teramura, K. Domen, Appl. Catal. A 327 (2007) 114. [69] H. Chen, W. Wen, Q. Wang, J.C. Hanson, J.T. Muckerman, E. Fujita, A.I. Frenkel, J.A. Rodriguez, J. Phys. Chem. C 113 (2009) 3650. [70] K. Maeda, K. Teramura, T. Takata, M. Hara, N. Saito, K. Toda, Y. Inoue, H. Kobayashi, K. Domen, J. Phys. Chem. B 109 (2005) 20504. [71] T. Hirai, K. Maeda, M. Yoshida, J. Kubota, S. Ikeda, M. Matsumura, K. Domen, J. Phys. Chem. C 111 (2007) 18853. [72] M.N. Huda, Y. Yan, S.-H. Wei, M.M. Al-Jassim, Phys. Rev. B 78 (2008) 195204. [73] L.L. Jensen, J.T. Muckerman, M.D. Newton, J. Phys. Chem. C 112 (2008) 3439. [74] C.D. Valentin, J. Phys. Chem. C 114 (2010) 7054. [75] M. Yoshida, T. Hirai, K. Maeda, N. Saito, J. Kubota, H. Kobayashi, Y. Inoue, K. Domen, J. Phys. Chem. C 114 (2010) 15510.

Y. Moriya et al. / Coordination Chemistry Reviews 257 (2013) 1957–1969 [76] M. Yashima, H. Yamada, K. Maeda, K. Domen, Chem. Commun. 46 (2010) 2379. [77] S. Wang, L.-W. Wang, Phys. Rev. Lett. 104 (2010), 065501/1–4. [78] H.-G. Yu, Chem. Phys. Lett. 512 (2011) 231. [79] L. Li, J.T. Muckerman, M.S. Hybertsen, P.B. Allen, Phys. Rev. B 83 (2011), 134202/1–6. [80] M. Dou, C. Persson, Phys. Stat. Sol. A 209 (2012) 75.

1969

[81] E.J. McDermott, E.Z. Kurmaev, T.D. Boyko, L.D. Finkelstein, R.J. Green, K. Maeda, K. Domen, A. Moewes, J. Phys. Chem. C 116 (2012) 7694. [82] W.-Q. Han, Z. Liu, H.-G. Yu, Appl. Phys. Lett. 96 (2010), 183112/1–3. [83] K. Lee, B.M. Tienes, M.B. Wilker, K.J. Schnitzenbaumer, G. Dukovic, Nano Lett. 12 (2012) 3268. [84] J. Wang, B. Huang, Z. Wang, P. Wang, H. Cheng, Z. Zheng, X. Qin, X. Zhang, Y. Dai, M.-H. Whangbo, J. Mater. Chem. 21 (2011) 4562.