Recent Progress in Two-Dimensional Oxide Photocatalysts for Water

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Perspective

Recent Progress in Two-Dimensional Oxide Photocatalysts for Water Splitting Shintaro Ida, and Tatsumi Ishihara J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/jz5010957 • Publication Date (Web): 10 Jul 2014 Downloaded from http://pubs.acs.org on July 14, 2014

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The Journal of Physical Chemistry Letters

Recent Progress in Two-Dimensional Oxide Photocatalysts for Water Splitting Shintaro Ida,*,†,‡, § Tatsumi Ishihara†,‡ †

Department of Applied Chemistry, Faculty of Engineering, Kyushu University, 744 Motooka,

Nishi-ku, Fukuoka 819-0395, Japan, ‡International Institute for Carbon Neutral Energy Research (I2CNER),Kyushu University, Motooka 744, Nishi-ku, Fukuoka 819-0395, Japan, §PRESTO, Japan Science and Technology Agency (JST),4-1-8 Honcho Kawaguchi, Saitama 332-0012.

AUTHOR INFORMATION Corresponding Author [email protected].

ACS Paragon Plus Environment

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ABSTRACT.

This perspective focuses on the photocatalytic activity of two-dimensional (2D) oxide and nitrogen-doped oxide crystals and the effective use of 2D photocatalysts for understanding the mechanism of the water splitting reaction. Strategies for improving the activities of 2D photocatalysts are slightly different from those of bulk photocatalysts. Although it is well known that a photocatalyst without co-catalyst loading has low activity for hydrogen production from water, a certain type of 2D oxide nanosheet shows high activity without co-catalyst loading. It is difficult to determine what factors contribute to this separation of oxidation and reduction sites of water, since there are many factors on the reaction surface. A nanosheet p-n junction surface is an ideal surface for understanding the carrier transfer during the photocatalytic reaction. In this system, the driving force of the carrier transfer to the reaction sites was found to be the potential gradient generated by the nanosheet junction.

TOC GRAPHICS

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KEYWORDS. Nanosheet, exfoliation, hydrogen, pn-junction, potential gradient, water splitting.


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Hydrogen production from water using solar energy is the ultimate goal in renewable energy research. Various oxides and oxynitrides have been reported as potential photocatalysts capable of decomposing water.1-4 However, there are several issues preventing their practical application, such as low quantum efficiency in the visible light region. One factor that causes a decrease in the photocatalytic activity is a recombination reaction between the electron and hole. When a photocatalyst is illuminated by light with an energy greater than the bandgap energy, a photoexcited electron and hole are generated within the powder (diameter: 500-3000 nm). In order for these carriers generated within the powder to react with water, they must travel a long distance to the surface. During this trip, they may recombine or get trapped at defect sites. One ideal material that can suppress the recombination and provide a short travel distance might be a semiconducting nanocrystal without inner and surface defects. However, in order to accomplish four-electron oxidation of water into oxygen on a 1 nm-diameter nanocrystal, one nanocrystal particle must absorb four photons with sufficient energy in a remarkably short time. Actual photon flux density from solar is approximately 2000 µmol s-1 m-2 (400-700 nm).5 In the case of a nanoparticle (section area: 1 nm2), it takes at least 4 ms for the fine particle to collide with four photons, assuming that the particle absorbs all the photons passing through it. On the other hand, in general, the lifetime of the photo-excited carriers is less than 1 µs. Thus, it may be impossible to obtain sufficient photon flux density to meet requirements for solar energy (H2 and/or O2 cannot be obtained by a single electron and/or hole reaction) as shown in Fig. 1a.


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Figure 1. Reaction model of photocatalytic water splitting under solar irradiation with a low photon flux density for (a) nanocrystal photocatalyst with diameter of 1 nm, and (b) twodimensional (2D) photocatalyst with a thickness of 1 nm.

However, if the reaction intermediate has a longer lifetime (100 ms) or there is a solar concentrating system, it might be possible for the reaction to occur on the nanoparticle. A two-dimensional structure may satisfy the requirement described above. Specifically, a twodimensional crystal (nanosheet) prepared by exfoliation of a layered compound is a single crystal with a homogeneous ultrathin thickness, and has a lateral dimension of several hundred nanometers to several micrometers.6-17 Although there are many reports on two-dimensional materials, the 2D crystals prepared by exfoliation of layered compounds are defined as twodimensional crystals in this perspective. Therefore, the travel distance of the photo-excited carriers in the nanosheet is short, and many photons can be absorbed by the nanosheet in a remarkably short time under low photon flux density due to its large section area, as shown in Fig. 1b. Recently, there have been many reports on p-type and n-type semiconducting nanosheets


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and nanosheet photocatalysts with visible light response, and studies on nanosheet photocatalysts have been becoming more frequent. The characteristic performance of nanosheet photocatalysts has been reported. For instance, it is well known that a photocatalyst without co-catalyst loading has poor photocatalytic activity for water splitting. However, a particular type of oxide nanosheet shows a high photocatalytic activity for hydrogen evolution from water without co-catalyst loading.18

Graphene-based materials,19-25 layered double hydroxide nanosheet-based

materials,26-27 and MoS2 nanosheet-based materials28-30 are also candidate photocatalysts, and new insights on photocatalytic reactions have been reported. Thus, the two-dimensional structure is expected to contribute to the development of photocatalysts for water splitting in the future. The present Perspective provides a short overview of recent research activities related to oxide nanosheets,18,

31-32

nanosheet p-n junction photocatalysts,33-34 and attractive approaches for

understanding the reaction mechanism.

Nanosheet Photocatalyst without Co-Catalyst Loading. In general, to improve catalytic activity, a co-catalyst is deposited on the surface of a photocatalyst.1-4 Another method to improve the catalytic activity is by doping with transition metals such as Zr-doped KTaO3 35 and Rh-doped SrTiO3

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systems, where the transition metal doping is performed to control the

concentration of electrons and/or to improve the visible light response. However, simply doping a transition metal in a catalyst without co-catalyst loading does not generally cause a significant improvement in activity compared to that with co-catalyst loading. One reason for this phenomenon could be that most of the dopants in bulk materials such as Zr-doped KTaO3 and Rh-doped SrTiO3 have an indirect effect on the reaction on the surface, because almost all of the dopants are present within the catalyst, rather than on the catalyst surface. On the other hand, all


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dopants in a nanosheet are present very close to the surface, because the thickness of the nanosheet is only about 1 nm. Therefore, most of the dopants in a nanosheet can be expected to be directly involved in the catalytic reaction and cause a significant improvement in photocatalytic activity in the same manner as co-catalyst loading. In fact, a Rh-doped Ca2Nb3O10 nanosheet shows high catalytic activity18. Figure 2a shows the photocatalytic activity of various types of Ca2Nb3O10 nanosheets in the presence of a sacrificial reagent (CH3OH).

Figure 2. (a) Photocatalytic activity of various types of Ca2Nb3O10 nanosheets in the presence of a sacrificial reagent (CH3OH), where the Rh-doped Ca2Nb3O10 (Rh/Nb=1/99) nanosheet shows the highest activity, (b) appearance of nanosheet suspensions of the Rh-doped Ca2Nb3O10 and (b) Reaction model for photocatalytic reaction on the Rh-doped Ca2Nb3O10. (Reproduced from ref 18.)


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It was found that the Rh-doped nanosheet (red circle) shows higher activity than the parent layered compound, non-doped nanosheet, Rh-loaded nanosheet, and Rh-loaded Rh-doped nanosheet. The color of the nanosheet suspension of Rh-doped nanosheet is light yellow due to Rh3+ ion doped in the nanosheet (Fig. 2b). These results imply that Rh dopant (one RhO6 site) in the crystal lattice can act as a photocatalytic reaction site as shown in Fig. 2c. Thus, it should be noted that the strategy for improvement of the activity of 2D photocatalysts is slightly different from that for bulk photocatalysts. However, there is no clear evidence that Rh actually enters the nanosheet lattice, nor is there any information about whether Rh atoms exist singly or in clusters. This is because direct imaging of Rh atoms in such nanosheets is difficult using transition electron microscopy, as the atomic numbers of Nb (Z = 41) and Rh (Z = 45) are very similar. Knowledge of the environment surrounding the reaction center obtained by imaging would provide new insight for the design of nanosheet photocatalysts for water splitting. In the current study, it is reported that bi-metal doping such as Rh-Ir or Ir-Rh is effective for improvement of the activity. Figure 3 shows the expected reaction model for photocatalytic water splitting on a metal-ion doped oxide nanosheet. The MO6 and M’O6 (M and M’: doped element) octahedral sites in the lattice might function as co-catalysts for photocatalytic reduction and oxidation of water. In the future, such types of M and M’-doped oxide nanosheets may be reported as photocatalysts for water splitting.


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Figure 3. Reaction model for photocatalytic water splitting on metal-ion doped oxide nanosheet (M, and M’: doped elements).

Nitrogen-Doped Oxide Nanosheet with Visible Light Response for Water Splitting. Some oxide nanosheets show high catalytic activity for hydrogen production of water.37-40 However, these nanosheets have large band gaps and are thus not active under visible light irradiation. In the case of bulk photocatalysts, nitrogen-doped oxides, oxynitrides, and nitrides have been studied as photocatalysts with visible light response.41-43 However, there are few reports on such nanosheet photocatalysts. While N-doped TiO2 nanosheets

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and N-doped NbOx nanosheets

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have been

reported, their photocatalytic activity under visible light irradiation is much lower than that under UV-light irradiation. For example, N-doped AE2Ta3O10-δ (AE: Ca, Sr, Ba) 31 and Ca2Nb3O10-δ 32 nanosheets can absorb visible light. Figure 4 shows the appearance of suspensions of N-doped Ca2Ta3O10-δ and Ca2Nb3O10-δ nanosheets.

Figure 4. Appearance of nanosheet suspensions for (a) nitrogen-doped Ca2Ta3O10 and (b) nitrogen-doped Ca2Nb3O10. (Reproduced from ref 31 and 32.)


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These nanosheets can be prepared by exfoliating N-doped layered perovskite oxide. The suspensions of the N-doped Ca2Ta3O10-δ and N-doped Ca2Nb3O10-δ are orange and black, respectively. This is due to nitrogen doping and/or reduction of Nb5+. The photocatalytic activity of the N-doped nanosheet for H2 evolution under UV light in the presence of CH3OH is relatively high (400 µmol/h), while the activity under visible light is low (0.47 µmol/h). The possible reason for this recombination reaction might be the doping condition of nitrogen.43, 46-50 There are two main understandings on the state of nitrogen doped in the lattice: one is that the states of N 2p for the substitutional nitrogen hybridize with O 2p states, and the other is that the substitutional nitrogen forms isolated N 2p states slightly above the O 2p valence band. The decrease in the activity is believed to be due to the formation of isolated N 2p states above the O 2p valence band edge, which act as electron-hole recombination centers. The same state of nitrogen might be created in the N-doped nanosheet. Therefore, it is necessary to create the state of N 2p hybridized with O 2p states. For the formation of the N 2p state hybridized with the O 2p state, increasing the amount of nitrogen dopant might be an effective method. As described later, an increase in the amount of nitrogen dopant did result in improvement of the activity. With regard to overall water splitting, there are no reports of photocatalysts that accomplish perfect water splitting under visible light irradiation. The reasons for this might be that 1) perfect removal from the nanosheet surface of the organic reagent used for exfoliation is difficult, and 2) the amount of nitrogen in the N-doped nanosheet is not sufficient to absorb visible light for the photocatalytic reaction. In general, N-doped nanosheets have the potential to decompose water into hydrogen and oxygen under visible light irradiation. In the case of N-doped Ca2Ta3O10-δ and Ca2Nb3O10-δ nanosheets, H2 evolves from aqueous methanol solutions, and then O2 evolves from a 0.1 M AgNO3 solution under visible light irradiation. However, it often happens that only H2


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gas evolves from pure water under UV-light irradiation both under full arc and visible light irradiation, while no oxygen is generated from this system. In order to investigate why oxygen is not generated in the photocatalytic reaction in pure water, products other than H2, O2, and N2 were analyzed after the photocatalytic reaction in pure water. Small amounts of CH4 and CH3COO- and HCOO- ions were detected in the gas and liquid phases, respectively. These products may possibly have originated from tetrabutylammonium ion (TBA+) and/or ethylamine (EA) molecules utilized in the exfoliation and intercalation reactions. For example, in the FT-IR spectrum of the photocatalyst, small peaks at around 2700-2900 cm-1 are associated with the C-H stretching of the adsorbed organic species. Some organic amines may be strongly adsorbed onto the surface of the nanosheets. This implies that the washing process is not sufficient to remove all the organic amines from the nanosheet surfaces. It was, however, interesting to find that when the experiment was conducted over a longer period of time, oxygen was generated from the system.31 Figure 5 shows the photocatalytic hydrogen and oxygen generation of N-doped Ca2Ta3O9.7N0.2 nanosheets under UV-light irradiation in pure water for 82 hours, including the 42-hour pre-treatment of catalyst surfaces.


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Figure 5. Time course of photocatalytic hydrogen and oxygen generation in pure water over Rh (0.15 wt.%) loaded Ca2Ta3O9.7N0.2 nanosheets restacked with protons under UV-light irradiation (300 W Xe-lamp). (Reproduced ref. 31).

Both oxygen and hydrogen were generated after the pre-treatment process. In the period with no oxygen generation from the system, it is suspected that there are two main reactions occurring: the oxidation of organic amines and reduction of water into hydrogen. In the former reaction, the organic species absorbed on the nanosheet might be oxidized by holes. This oxidation may prevent the oxidation of water into oxygen until all the organic substances on the surface of the catalysts are decomposed. Thus, it was found that the removal of organic species such as organic amines from the surface of the nanosheet is an essential process for initiating the water splitting reaction. However, N-doped Ca2Ta3O9.7N0.2 nanosheets had no catalytic activity for overall water splitting under visible light irradiation. As described above, one reason might be that the amount of nitrogen in the N-doped nanosheet is not enough to absorb visible light for the photocatalytic reaction. In this material, although it was difficult to increase the amount of nitrogen, N-doped nanosheets (AE2Ta3O9.6N0.3, AE: Sr, Ba) with slightly larger amounts of nitrogen than Ca2Ta3O9.7N0.2 were prepared. Among these nanosheets, Sr2Ba0.5Ta3O9.6N0.3 showed the highest catalytic activity.51 In the current study, it was found that Sr2Ba0.5Ta3O9.6N0.3 had a catalytic activity for overall water splitting under visible light irradiation (Fig. 6), although pre-activation by UV-light irradiation for several hours was necessary to decompose the organic species adsorbed on the nanosheet before the test. If the amount of nitrogen can be increased in this system, the activity will also increase.


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Figure 6. Time course of photocatalytic hydrogen and oxygen generation in pure water over Rh (0.15 wt.%) loaded Sr1.5Ba0.5Ta3O9.6N0.3 under visible light irradiation (> 420 nm).

Photocatalytic Activity of Nanosheet p-n Junction Surface. The creation of a p-n junction is a strategy to improve photocatalytic activity, since the potential gradient generated by formation of a p-n junction can potentially act to suppress the recombination reaction.30, 52-59 The potential gradient on the surface is generated by the diffusion of charge carriers between n-type and p-type semiconducting materials, which is expected to function as a driving force for the photo-excited holes and electrons to move to their respective reduction and oxidation sites as shown in Fig. 7a


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Figure 7. (a) Reaction model for photocatalytic water splitting on pn-junction photocatalyst, (b) preparation of pn-junction from nanoparticles or nanosheets.

However, advancements in this field have been limited due to the difficulty in the preparation and evaluation of such junction structures. The preparation of ultrathin p-n junctions from two materials with a large lattice mismatch by chemical vapor deposition (CVD) is generally difficult, in that crystal lattice strain or a decrease in crystallinity is generated in the vicinity of the interface. Therefore, it is still unclear whether the creation of a p-n junction is effective for improvement of photocatalytic activity. Nanosheets are a potential material to provide an answer to the issue mentioned above. A p-n junction structure without an amorphous layer between different p-type and n-type semiconducting materials with different crystal phases can be prepared by lamination of p-type and n-type semiconducting nanosheets. In the case of nanoparticles, the connection between


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them is point-to-point, whereas in a nanosheet, the connection is face-to-face as shown in Fig.7b. This is one of the advantages of nanosheets in the preparation of p-n junctions For example, the p-n junction prepared by the lamination of p-type NiO and n-type Ca2Nb3O10 (CNO) nanosheets has been reported.34 Model structures of NiO and CNO nanosheets and the NiO/CNO junction are shown in Fig. 8.

Figure 8. Model structures of (a) p-type NiO, (b) n-type CNO crystals, (c) n-CNO/p-NiO (n-p) junction, and (d) p-NiO/n-CNO (p-n) junction. (Reproduced from ref 34.)

The NiO crystal has the same crystal structure as a one-NiO6-unit layer of the (111) plane in bulk NiO.33 The CNO crystal has a perovskite structure with A-site Ca2+ and B-site Nb5+, and is two perovskite units thick. The CNO nanosheet is prepared by exfoliation of HCa2Nb3O10, and the NiO sheet is prepared by the dehydration reaction of a Ni(OH)2-δ nanosheet that is obtained by exfoliation of a layered nickel hydroxide intercalated with dodecyl sulfate ion.13,

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Figure 9

shows the AFM image and cross-sectional profile of the p-n junction region of p-NiO crystals on n-CNO crystals. The hexagonal and polygonal shapes correspond to NiO and CNO nanosheets, respectively.


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Figure 9. (a) AFM image and (b) cross-sectional profile of AFM image of the p-n junction of pNiO crystals on n-CNO crystals. (Reproduced from ref 34.)

The thickness of the p-NiO crystal estimated from the AFM image is ca. 0.3-0.4 nm and that of the n-CNO crystal is 1.4-1.7 nm, such that the total thickness of the p-n junction is around 2.0 nm. The selected area electron diffraction pattern of the p-n (NiO/CNO) junction region has two types of diffraction patterns assigned to the [001]-oriented CNO crystal and [111]-oriented NiO crystal. An ideal p-n junction structure consisting of only single crystals can be prepared by lamination of the nanosheet. It has been found that the photocatalytic activity for hydrogen production from water of the nanosheet junction films is strongly influenced by the lamination structure. For example, the photocatalytic activity of different types of nanosheet lamination films such as n-p junction film (CNO surface), p-n junction (NiO surface), only NiO film, and only CNO film were investigated. These films were prepared by the Langmuir-Blodgett method. The CNO/NiO junction film is not a perfect film because some areas in the film are only CNO crystals, while other areas are CNO crystals with NiO crystals under the layer (n-p junction parts). The production rate of hydrogen from water of the n-p junction film (CNO surface) as shown in Fig. 8c is two times larger than


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that of the p-n junction (NiO surface) film as shown in Fig. 8d, while that of the p-n junction film is almost the same as that of the n-CNO film. The n-p junction film surface is mainly CNO crystals, as with the n-CNO film. However, the production rate of the n-p junction film is two times as large as that of the n-CNO film. Thus, it is possible to conclude that creation of the pn junction is effective for improvement of the photocatalytic activity. Furthermore, these results also indicate that the photocatalytic activity is influenced by not only the surface crystal structure and composition, but also the crystal structure and composition under the surface.

Contribution of Nanosheets toward Understanding of Photocatalytic Reaction Site. Nanosheet pn junction photocatalysts will make a major contribution to the understanding of the photocatalytic reaction mechanism. For example, it may be possible to determine the location of the oxidation and reduction sites in NiO-loaded KTaO3. It is believed that the hydrogen evolution site (reduction site of water) is on the NiO nanoparticle, while the oxygen evolution (oxidation site of water) is on KTaO3.35 With regard to the hydrogen evolution, the reaction site is considered to be on the surface of a co-catalyst such as Pt or NiO. The role of the co-catalyst might be to 1) introduce the adsorption site of water, 2) reduce the over-potential for reduction of water, 3) promote charge separation between electron and hole, and/or 4) introduce a difference in the surface potential. However, there have been no direct confirmations of where the hydrogen and oxygen evolution sites are. One reason for this is that hydrogen and oxygen are gases. If the product derived from the photocatalytic reaction is a solid material deposited by oxidation or reduction of metal ions, the reaction sites can be identified by observing the solid deposition sites after the photocatalytic reaction. Although such photodeposition is a good method for investigation of the reaction site, it should be noted that the photodeposition reaction is strongly


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influenced by the interactions between the metal ion and the deposition site, such as adsorption and electrostatic energy, and the crystal face of the deposition site as shown in Fig. 10.

Figure 10. (a) Reaction model for photodeposition and interaction model between the metal ion and the deposition site, (b) surface model of CNO/NiO n-p junction film.

Thus, it is difficult to distinguish which factors contribute to the separation of the reaction sites. Therefore, the surface composition and crystal face of the surface should be confirmed in order to evaluate the reaction sites using the photodeposition reaction of a metal ion. In this case, the co-catalyst should not be deposited on the surface of the photocatalyst, since the local conditions of the co-catalyst loaded sites and unloaded sites are different. However, if a co-catalyst is not deposited on the surface, there is no means to evaluate the reaction site of the active photocatalyst. Thus, in the case of a bulk photocatalyst, the preparation of an ideal surface to evaluate the reaction site with the photodeposition method is problematic. In contrast, nanosheets have the potential for preparation of an ideal surface for this type of evaluation.


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As described above, the photocatalytic activity of the n-CNO-sheet/p-NiO-sheet n-p junction surface showed higher catalytic activity than that of a CNO surface, indicating that the NiO sheet contributes to the activity. The CNO/NiO n-p junction film has one ideal surface for evaluation of the reaction site as shown in Fig. 10b. The film contains n-p junction and non-junction regions. In the n-p junction, the NiO sheet is located under the CNO sheet. Therefore, the surface condition of the CNO/NiO junction is exactly the same as that of the CNO part. In addition, the CNO surface has the same crystal face, because CNO is a single crystal. Figure 11 shows the results of a photodeposition reaction on the CNO/NiO junction film.

Figure 11. FE-SEM images of CNO/NiO junction regions after photodeposition reaction in (a) 0.1 M MnSO4 and (b) 0.1 M AgNO3 aqueous solutions, and (c) reaction model of the photodeposition reaction. (Reproduced from ref 34.)

Many NiO sheets (hexagonal shape, size: 300-500 nm) are covered with one CNO single crystal sheet (size: 5-10 µm). The photodeposition reaction of MnOx and Ag in 0.1 M AgNO3 and 0.1 M MnSO4 aqueous solutions was performed to confirm which parts of the junction films were


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photo-oxidation and photo-reduction sites. In this reaction, Mn2+ is oxidized to Mn2O3 or MnO2 by photogenerated holes, while Ag+ is reduced to Ag by photogenerated electrons. MnOx and Ag are deposited at the oxidation and reduction sites during the photocatalytic reaction as shown in Fig. 11c. The deposition parts of Mn2O3 and Ag are not always oxidation and reduction sites of water. However, we can discuss the tendency of the transfer of generated carriers in the photocatalytic reaction. Ag particles were deposited on CNO in the non-junction region, while MnOx particles were deposited on junctions and/or their edges. There was no material deposited on NiO in the non-junction region. These results indicate that the CNO/NiO junctions are the photo-oxidation sites, while the non-junction regions and/or their edges are the photo-reduction sites. We next turned our attention to the reaction site in this system. There is no special adsorption site such as a co-catalyst and no differences in the chemical composition and crystal face between CNO/NiO junction and non-junction regions. Thus, it is likely that the difference in the surface potential is the driving force to separate electrons and holes. The surface potential can be measured using Kelvin probe force microscopy (KPFM). Figures 12a and 12b show AFM and KPFM images of the CNO/NiO (n-p) junction parts under vacuum conditions.


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Figure 12. (a) AFM image, (b) KPFM image, and (c) cross-sectional profile between X and Y of the CNO/NiO (n-p) junction film under vacuum conditions. (Reproduced from ref 34.)

The CNO and NiO crystals were intentionally deposited with low coverage on the substrate to assist in clarification of the surface characteristics. The KPFM image shows light (high surface potential) and dark (low surface potential) areas, which indicate shallow and deep Fermi levels, respectively. Considering the difference in the surface potential between the CNO and NiO crystals, the sections consisting of only NiO crystals are darker than those of only CNO crystals. This image provides information regarding the relationship between the Fermi levels of n-CNO and p-NiO crystals. Figure 12c shows a cross-sectional profile of the junction in the AFM and KPFM images. The surface potential of the CNO/NiO junction is higher than that of the NiO crystals and lower than that of the CNO crystals. It should be noted that the surface potential is not affected by the height of the material, but is affected by the underlying material (junction structure). In general, the diffusion of carriers continues until the drift current balances the diffusion current. In the case of an ultrathin film, there is not enough space to form the depletion region, which is formed in a general p-n junction device. However, the electrons and holes diffuse across the junction into the p-type NiO/n-type CNO nanosheets to balance the Fermi level as long as carriers such as electrons and holes exist in n-type CNO and p-NiO nanosheets. Therefore, in the region of the ultrathin p-n junction (2 nm), it is assumed that all donors and acceptors are fully ionized. This means that the carrier concentration in the CNO and NiO sheets decreases in the ultrathin p-n junction region. It is known that the position of the Fermi level varies as a function of carrier concentration and obeys the following equations: n-type semiconductor : EF = EC + kbT ln(n0/NC)

(1)


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p-type semiconductor: EF = EV – kbT ln(p0/NV)

(2)

where EF is Fermi level, EC is conduction band level, EV is valence band level, kb is Boltzmann constant, NC is effective density of conduction band states, NV is effective density of valence band states, n0 is thermal-equilibrium concentrations of electrons, p0 is thermal-equilibrium concentrations of holes, and T is temperature. When the carrier concentration in the CNO/NiO junction part decreases, the position of the Fermi level in the ultrathin p-n junction becomes lower than that of CNO and higher than that of NiO. Thus, the position of the Fermi level in the ultrathin p-n junction identifies the positions between the Fermi levels of CNO and NiO nanosheets. The difference in the surface potentials (position of Fermi level) obtained from KFM images of CNO/NiO substrates as shown in Fig. 12b corresponds to the position of the Fermi level obtained from the theory on Fermi levels. In the case of the CNO/NiO junction film, most parts of the surface are CNO crystals. However, there are high and low surface potential areas in the same surface of CNO crystals, because there are junction and non-junction regions. Thus, potential gradients are generated on the same surface of the CNO crystals. It is possible that these potential gradients result in spatially separated reaction sites (oxidation and reduction sites), which may suppress the recombination reaction. Figure 13 shows a proposed mechanism for the photodeposition reactions of Ag and MnOx on the CNO/NiO junction surface.


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Figure 13. Proposed band structure model in horizontal direction to the CNO/NiO junction surface; (a) formation of depletion region in the CNO/NiO junction, (b) space charge density remaining in the CNO/NiO junction surface, (c) model of band structure for CNO sheet in the junction and non-junction regions. (Reproduced from ref 34.)

In the region of the ultrathin p-n junction (2 nm), it is possible to assume that all donors and acceptors are fully ionized. In addition, ionized donors remain in the narrow region of the nonjunction region close to the junction edge due to the diffusion of carriers as shown in Fig. 13a. These ionized donors and acceptors create a difference in the space charge density around the edge of the junction, while the space charge density of the junction in the direction horizontal to the substrate is zero because the charges of the ionized donors and acceptors in the junction spatially compensate each other as shown in Fig. 13b. The two regions of immobile positive and negative charges around the edge of the junction might result in an electric field and band


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bending in the same CNO nanosheet as shown in Fig. 13c. The photo-excited electrons and holes in the CNO nanosheet are separated by the intraband bending. The photo-excited electrons move to the non-junction region (photo-reduction site), and the photo-excited holes move to the junction (photo-oxidation site). The band structure proposed in Fig. 13c is in good agreement with the surface potential image (KPFM image) of the CNO/NiO junction (Fig. 12b). Although only one example is discussed here, effective use of nanosheets will aid in the elucidation of the mechanism of the photocatalytic reaction. These results indicate that material design to control the potential gradient on the catalyst surface is necessary to obtain highly efficient photocatalysts. It is expected that the evaluation method and results presented here will contribute to the development of photocatalysts and improve understanding of the reaction mechanism.

Future Direction of the 2D Photocatalyst. Although nanosheets are potential photocatalysts capable of decomposing water, they have some problem to be solved. One is in development of exfoliation method without organic exfoliation reagents such as organic amine. The organic amines block production of oxygen, since they work as sacrificial reagents. Mechanical exfoliation methods such as ball milling and ultrasonic treatment are might be one of the candidate approaches to obtain nanosheet photocatalyst without organic exfoliation reagents. Other problem is low photocatalytic activity under visible light irradiation. The N-doped oxide nanosheets have activity under visible light irradiation, whereas the efficiency is still quite low. One reason for this phenomenon could be that the amount of nitrogen dopant in the nanosheet is low (1 atom %). Probably, such low-density light absorption center in the N-doped nanosheet with a thickness of around 1 nm will not obtain sufficient photon flux density for 4-electron


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oxidation of water, although the nanosheet has an ideal structure to absorb solar light. Nitride nanosheet such as Ta3N5 is might be one of the candidate materials for nanosheet photocatalyst with visible light response for water splitting. Of course, oxide graphene and sulfide nanosheet are also candidate materials, since they have a strong absorption band in visible light region. However, they have problems that oxide graphene is self-decomposed to CO2 by the photoexited holes,23 and sulfide nanosheets are likely to be photo-dissolved. Thus, there is the need to develop a stable nanosheet toward the photo-exited electron and hole generated within it. With regard to the band structure of nanosheet, it is expected that the band structure of nanosheet is different from that of its parent layered compounds. The information of band structure is important in designing a photocatalyst for water splitting. Although Akatsuka et al. reported the detailed band structures for several types of oxide nanosheet as shown in Fig. 14,60 the detailed band structures of nanosheets upon doping and junction formation have yet to be determined. In the future, improvement of the basic information about the physical properties such as band structure, carrier density, and carrier mobility of nanosheets is also important for a further development of 2D-photocatalyst study.


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Figure 14. Energy diagram depicting the conduction-band edges, valence-band edges, and bandgap energy for (a) Ca2Nb3O10−, (b) TiNbO5−, (c)Ti2NbO7−, (d) Ti5NbO143−, (e) Ti0.91O20.36−, (f) Nb3O8− nanosheets. (Reprinted from ref 60 reported by Akatsuka et al.)

AUTHOR INFORMATION Corresponding Author *Email: [email protected] Biographies Shintaro Ida is an Associate Professor in Chemistry at Kyushu University. He received his BSc. (2001), MSc. (2003), and PhD. (2004) from Kumamoto University. His research is focused on two-dimensional materials, photocatalytic and photoelectrochemical water splitting, Li-air


25


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battery, luminescent materials, and solid oxide electrolysis cell using solar energy. See http://www.cstf.kyushu-u.ac.jp/~ishihara-lab/index2.html Tatsumi Ishihara is a Professor in Chemistry and International Institute for Carbon-Neutral Energy Research (I2CNER), Kyushu University. He received his BSc. (1984), MSc. (1986), and PhD. (1992) from Kyushu University. His research is focused on functional inorganic materials, fuel

cell,

and

environmental

catalyst.

See

http://www.cstf.kyushu-u.ac.jp/~ishihara-

lab/index2.html

ACKNOWLEDGMENT This work was supported by JST PRESTO program and JSPS KAKENHI 24685031.

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Most of the dopants in a nanosheet can be expected to be directly involved in the catalytic reaction and cause a significant improvement in photocatalytic activity in the same manner as cocatalyst loading.

The removal of organic species such as organic amines from the surface of the nanosheet is an essential process for initiating the water splitting reaction.

A p-n junction structure without an amorphous layer between different p-type and n-type semiconducting materials with different crystal phases can be prepared by lamination of p-type and n-type semiconducting nanosheets.

Nanosheet p-n junction photocatalysts will make a major contribution to the understanding of the photocatalytic reaction mechanism.


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Recent Progress in Two-Dimensional Oxide Photocatalysts for Water

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