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. 2020 Apr 22;5(17):9929–9936. doi: 10.1021/acsomega.0c00204

Photocatalytic Hydrogen Evolution over Exfoliated Rh-Doped Titanate Nanosheets

Wasusate Soontornchaiyakul †, Takuya Fujimura ‡, Natsumi Yano ‡, Yusuke Kataoka ‡, Ryo Sasai ‡,*
PMCID: PMC7203949  PMID: 32391480

Abstract

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Various amounts of Rh-doped titanate nanosheets (Ti3NS:Rh(x), where x is doped amount) were prepared to develop a new nanostructured photocatalyst based on metal oxide compounds that can split water to produce H2 under sunlight. Ti3NS:Rh(x) was obtained by acid exchange, intercalation, and exfoliation of Rh-doped layered sodium titanate compound (Na2Ti3–xRhxO7). A new energy gap was found in the diffuse reflection spectrum of the Ti3NS:Rh(x) colloidal suspension solution; this new energy gap corresponds to electrons in the 4d level of Rh3+ or Rh4+, which are doped in the Ti4+ site. A photocatalyst activity of Ti3NS:Rh(x) for H2 evolution in water with triethylamine (TEA) as an electron donor was investigated. The appropriate amount of Rh doping can improve the photocatalytic activity of Ti3NS for H2 evolution from water using triethylamine (TEA) as a sacrifice agent. The reason was related to the rich state of Rh3+ or Rh4+ doped in the Ti4+ site of Ti3NS. Doping Rh 1 mol % of Ti, Ti3NS:Rh(0.03) shows the H2 evolution rates up to 1040 nmol/h, which is about 25 times larger than that of nondoped Ti3NS under UV irradiation (>220 nm) and 302 nmol/h under near-UV irradiation (>340 nm). These results show that the development of new nanostructured photocatalyst based on Rh-doped titanate compounds that can produce H2 under near-UV irradiation present in sunlight was a success.

Introduction

Hydrogen has become an important source for a promising alternative energy instead of fossil fuel in many factories.1−3 Hydrogen can be produced by numerous chemical and electrochemical processes. Since Honda and Fujishima demonstrated the photoelectrochemical activity of TiO2 electrode with Pt as a counter electrode to split water into H2 and O2 gas under UV irradiation, the development of photocatalyst materials that can produce H2 and O2 from water-splitting reaction under solar light irradiation has been extensively researched.4−8

Due to the crystal structure, which has a distinct charge separation site, a layered metal oxide has become an interesting candidate for a new photocatalyst used in water-splitting reaction.9−11 The layered metal oxide contains stacks of charged sheets and the counterion (anion or cation) layer in the interspacing to neutral overall compounds. This interlayer becomes a water reduction site, while its edge of sheets becomes the water oxidation site.11 Over the past decades, many-layered metal oxide compounds (i.e., layered niobate; K4Nb6O17, layered tungsten Na2W4O13) have been reported for their photocatalytic H2 and/or O2 evolution activity from water under UV-light irradiation.12−14

Alkali metal titanate compounds such as Na2Ti3O7 or K2Ti4O9 are well-known for their exfoliation to single-phase titanate nanosheet (TiNS), which has a thickness of about 1–10 nm.15−19 Due to the conduction band formed by Ti 3p and the valence band formed by O 2p, TiNS is suitable for sacrificial H2 and O2 evolution or overall water-splitting photocatalyst.20,21 Due to its specific properties, TiNS has become an interesting photocatalyst material. However, TiNS shows very low photocatalyst activity under near-UV light and has an infirmity for activation under solar light irradiation. Thus, an improvement of TiNS photocatalyst activity is required.

Previously, our group has reported a synthesis of Rh-doped TiNS (Ti3NS:Rh(x), where x is the doped amount), which exfoliated from layered sodium titanate (Na2Tin–xRhxO2n+1). We found that Rh doping did not affect the conduction level of Ti3NS; however, the photocatalytic activity for the degradation of organic compounds under UV-light irradiation was improved. This improvement of photocatalytic decomposition activity can be explained by the photoinduced redox cycle of the Rh (Rh3+ and Rh4+) doped in the Ti site of Ti3NS:Rh(x) under UV-light irradiation.22,23 Nevertheless, the photocatalytic activity of Ti3NS:Rh(x) for H2 evolution has not yet been investigated.

In this study, we investigated the photocatalytic evolution of H2 in Ti3NS:Rh(x) from water under UV and visible light and tried to clarify the mechanism of Rh doping on this material. The photocatalytic H2 evolution was conducted in water solution (pH 11) using triethylamine (TEA) as a scarification electron donor. To investigate the disparity of Rh doping and Rh cocatalyst, we also prepared the Rh cocatalyst-loaded Ti3NS (Rh(x)-Ti3NS) by the photodeposition method.24−27 The photocatalytic H2 evolution was observed under UV- and visible-light irradiation. From these results, the effect of Rh doping on the photocatalytic activity of Ti3NS was discussed.

Results and Discussion

Characterization of Ti3NS:Rh(x) and Rh(x)-Ti3NS

The Rh/Ti ratios of Ti3NS:Rh(x) and Rh(x)-Ti3NS obtained from the inductively coupled plasma-atomic emission spectroscopy (ICP-AES) are shown in Table 1. These results suggest that Ti3NS:Rh(x) can be doped with various amounts of Rh using the present procedure. Moreover, we could prepare Rh(x)-Ti3NS, which has almost the same Rh/Ti ratio as that of Ti3NS:Rh(x). In Figure 1A, the photographs of the obtained Ti3NS:Rh(x) colloidal suspension are shown. The color of the Ti3NS:Rh(x) colloidal suspension gradually changed from white to dark brown with an increase in the Rh doping amount.

Table 1. Rh/Ti Molar Ratio, Chemical Formula, and Band Structure Data of All Prepared Ti3NS:Rh(x) and Rh(x)-Ti3NS Samples.

        energy level (V vs NHE, pH 7)
sample Rh/Ti molar ratio chemical formula band gap (eV) conduction band22 valence band
Ti3NS:Rh(0) 0 [Ti3.00O7]2– 3.61 –0.41 3.20
Ti3NS:Rh(0.001) 0.0003 [Ti2.999Rh0.001O7](2+δ)– 3.47 –0.41 3.06
Ti3NS:Rh(0.01) 0.0028 [Ti2.992Rh0.008O7](2+δ)– 3.44 –0.41 3.0
Ti3NS:Rh(0.03) 0.0101 [Ti2.970Rh0.030O7](2+δ)– 3.39 –0.41 2.98
Ti3NS:Rh(0.05) 0.0163 [Ti2.952Rh0.048O7](2+δ)– 3.32 –0.41 2.91
Rh(0.01)-Ti3NS 0.0026 [Ti3.00O7]2– 3.61 –0.41 3.20
Rh(0.03)-Ti3NS 0.0098 [Ti3.00O7]2– 3.61 –0.41 3.20
Rh(0.05)-Ti3NS 0.0155 [Ti3.00O7]2– 3.61 –0.41 3.20
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Figure 1.

Figure 1

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Photograph (A) and the diffuse reflection (DR) spectrum (B) of all prepared Ti3NS:Rhx colloidal suspensions at the same concentration (1 g/dm3). (C, D) DR spectrum of Ti3NS:Rh(0.03) compared with that of Rh(0.03)-Ti3NS and that of Ti3NS:Rh(0.05) compared with that of Rh(0.05)-Ti3NS, respectively.

Figure 1B shows the DR spectra in terms of the Kubelka–Munk (KM) function for the Ti3NS:Rh(x) colloidal suspension (1 g/dm3). In the DR spectrum of Ti3NS:Rh(0), the photoabsorption band originating from a band gap between O 2p and Ti 3d was observed at less than 350 nm. When Rh atoms are doped in the Ti site, a new absorption band appeared in the wavelength range from 350 to 600 nm. Thus, this new absorption band might be related to the transition of electrons from the impurity level formed by Rh in a band structure and/or the d–d transition of Rh. Moreover, this broad absorption band increases with increase in the Rh doping amount. The reason is related to the ionic state of Rh doped in the Ti site of Ti3NS. We will discuss this later in a section on the photocatalytic activity of Ti3NS:Rh(x).

In Table 1, the energy band gap (Eg) was determined by the Planck–Einstein equation from the main adsorption peak of Ti3NS:Rh(x). The Eg value of Ti3NS:Rh(x) decreases with an increase in the amount of Rh doping. The decrease in Eg value indicates that a new band above the O 2p band is formed by doping Rh in the Ti site of the Ti3NS crystal structure. According to Kudo et al., a new band above the O 2p band is the 4d level of the Rh3+ species.30 Meanwhile, the new broad absorption band observed from 350 to 600 nm, which can be seen in the DR spectrum of Ti3NS:Rh(0.01), Ti3NS:Rh(0.03), and Ti3NS:Rh(0.05), is explained by the existence of the 4d level of Rh4+ and the d–d transition of Rh3+ and/or Rh4+.

On the other hand, the DR spectrum and the band gap of Rh(x)-Ti3NS were similar to those of Ti3NS. This suggests that the Rh cocatalyst does not affect the band structure of Ti3NS. The reason was that the Rh cocatalyst was only loaded on the surface of Ti3NS. The DR spectra indicate a difference in Rh doping and Rh cocatalyst loading, and how Rh affects the energy band structure.

The morphologies of Ti3NS:Rh(x) and Rh(x)-Ti3NS were observed by X-ray diffraction (XRD) and transmission electron microscopy (TEM) as shown in Figures S1 and S2 in the Supporting Information, respectively. The XRD patterns of the films of Ti3NS:Rh(0.05) and Rh(0.05)-Ti3NS cast on a quartz substrate are shown in Figure S1. The XRD pattern of both Ti3NS:Rh(0.05) and Rh(0.05)-Ti3NS are originated from the laminated form of Ti3NS with TMA+ cations in the interlayer spacing, and diffraction peaks correspond to the stacking direction (d00l).31 Moreover, a higher angle shift of the diffraction peak of d002 was detected in the case of Rh doping. On the other hand, a broadening diffraction peak was observed in the case of Rh cocatalyst loading.

The difference between Rh doping and Rh cocatalyst loading was also observed in the TEM image of Ti3NS:Rh(0.03) and Rh(0.03)-Ti3NS. The TEM image of Ti3NS:Rh(0.03) shows a stack of several sheets of Ti3NS:Rh(0.03). However, small black dots were observed in the TEM image of Rh(0.03)-Ti3NS. According to Shimura et. al., these small black dots are nanoparticles of Rh with diameter of around 5–10 nm.25 The TEM image indicates that the XRD patterns of Ti3NS:Rh(0.03) and Rh(0.03)-Ti3NS are different. In the case of Rh(0.03)-Ti3NS, a lot of Rh nanoparticles were loaded on its surface. As a result, it prevented the separated Ti3NS sheets from stacking on each other; hence, the diffraction peak of the stacking direction (d002) was broadened.

Photocatalytic H2 Evolution Activity of Ti3NS:Rh(x) and Rh(x)-Ti3NS

The irradiation time dependence of the H2 evolution amount is shown in Figure 2. Ti3NS:Rh(x) was used as a colloidal suspension solution with TEA (pH 11). Ti3NS:Rh(x) was irradiated by UV and visible light (λ > 220 nm) with energy higher than the band gap energy of Ti3NS:Rh(x). Ti3NS:Rh(0) shows a very low activity for photocatalytic H2 evolution reaction. This low activity corresponds with the previously reported H2Ti3O7 results; it was not a good photocatalyst in the absence of cocatalyst loading.33,34 However, the photocatalytic activity of this Ti3NS:Rh(0) nanosheet in a suspension solution was seen for the first time.

Figure 2.

Figure 2

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Time course of H2 evolution over Ti3NS:Rh(x) from the water with TEA as an electron donor under UV light irradiation and with 500 W Xe lamp with a long pass filter (λ > 220 nm) as a light source.

By doping Rh from x = 0.001 to 0.03, we observed an increase in the photocatalytic activity for H2 evolution. In contrast, the photocatalytic activity of Ti3NS:Rh(0.05) drastically decreased compared to that of Ti3NS:Rh(0.03) and showed almost the same activity as Ti3NSRh(0).

Interestingly, Ti3NS:Rh(x) can evolve H2 from water at high basicity (pH 11). This indicates and supports the fact that the conduction level of Ti3NS varies with pH because the reduction potential of H2O to H2 rises to a higher level when the pH increases as described by Nernst’s equation.40 Ti3NS:Rh(x) shows good stability and repeatability for photocatalytic H2 evolution while using different samples of Ti3NS:Rh(x) colloidal suspension, as shown in Figure S3. The reason was because Ti3NS:Rh(x) are formed as purified crystal-phase sheetlike structures by exfoliation without any impurities. The impurities or nonexfoliated parts were removed by the centrifugal process. From its photocatalytic activity and stability in a high-pH solution, Ti3NS:Rh(x) is a promising photocatalyst material for industrial wastewater treatment without the requirement of pH adjustment.42,43

To understand the effect of Rh doping on the photocatalyst properties of Ti3NS:Rh(x), an irradiation light dependence of H2 evolution has been investigated by changing the wavelength region of incident light using various cutoff filters. Ti3NS:Rh(0.03) shows the highest photocatalytic activity for H2 evolution under UV irradiation. Moreover, Ti3NS:Rh(0.03) clearly shows an absorbance on the visible-light region. Thus, we used only Ti3NS:Rh(0.03) as a model photocatalyst to investigate the irradiation light dependence of photocatalytic H2 evolution activity. Figure 3A shows the time course and irradiation light dependence of H2 evolution from an aqueous TEA solution over Ti3NS:Rh(0.03). As a result, the photocatalytic H2 evolution was observed under irradiated light (λ > 220 and 340 nm). Thus, this indicates that our Ti3NS:Rh(0.03) is active only under UV light irradiation, although the DR spectrum shows an absorbance up to 600 nm. On the other hand, Ti3NS:Rh(0) did not show any activity for H2 evolution under the irradiation light with λ > 340 nm, as shown in Figure 3B. Although Rh doping does not contribute to the photocatalytic activity of Ti3NS under visible light (λ > 420 nm), it can improve the photocatalytic activity of Ti3NS for H2 evolution under the near-UV region (300 < λ < 400 nm).

Figure 3.

Figure 3

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(A) Time course of the H2 evolution rate from the water with TEA as an electron donor using Ti3NS:Rh(0.03) under 500 W Xe lamp with various cutoff filters. (B) Time course of H2 evolution rate from the water with TEA as an electron donor using Ti3NS:Rh(0) under 500 W Xe lamp with various cutoff filters. (C) Plot of H2 evolution rate from Ti3NS:Rh(0.03) as a function of the wavelength of irradiation light (wavelength of long pass filter, filled circle), and the DR spectrum of Ti3NS:Rh(0.03) (dash line). (D) Schematic diagram of the energy band structure of Ti3NS:Rh(0.03). The color of the filled circle in (C) corresponds to the color of the excitation pathways in (D) (solid line).

Likewise, the comparison between H2 evolution rate with each irradiation light and the DR spectrum of Ti3NS:Rh(0.03) shows a relation between the absorption band and the H2 evolution reaction (Figure 3C). Although Ti3NS:Rh(0.03) shows a high absorbance from 400 to 600 nm, this absorption band cannot be used for the photocatalytic activity. This is caused by the position of donor and acceptor of Rh in the energy band structure of Ti3NS. Kudo et. al. reported the formation of donor and acceptor levels of Rh 4d in the band structure of SrTiO3.30,32 The donor level of Rh3+ 4d is formed above the O 2p and acceptor level of Rh4+ 4d is formed between Ti 3d and Rh3+ 4d. Thus, this separates one excitation from O 2p to Ti 3d into five excitation pathways, which are O 2p to Ti 3d (main band gap of Ti3NS), O 2p to Rh3+ 4d, O 2p to Rh4+ 4d, Rh3+ 4d to Ti4+ 3d, and Rh3+ 4d to Rh4+ 4d.30,36Figure 3D shows the schematic of the energy band structure of Ti3NS:Rh(0.03), which corresponds with an absorption band observed in the DR spectrum. By combining the absorption spectra and the photocatalytic H2 evolution under various wavelengths of the different incident light, we could predict a mechanism of Ti3NS:Rh(0.03) for the photocatalytic H2 evolution under UV- and visible-light irradiation.

Under UV (λ > 220 nm) light irradiation, electrons from the valence band (O 2p) were excited to the conduction band (Ti4+ 3d). This excitation can be found in all Ti3NS:Rh(x). Alternatively, the photocatalytic activity of Ti3NS:Rh(x) under near-UV (λ > 340 nm) light irradiation was observed only in the Rh-doped Ti3NS. This means that the excited electrons did not come from O 2p but from a donor level of Rh3+ 4d to the conduction band (Ti4+ 3d) of Ti3NS. In the case of bulk or powder catalysts, the Rh3+ 4d to Ti4+ 3d will be active under visible light because of a general band gap of about 3.2 eV, which is narrow than that of Ti3NS (3.61 eV). This large band gap of Ti3NS makes the new energy gap of Rh3+ 4d to Ti4+ 3d become larger than the energy of the visible light. This large energy gap causes an inactive of Ti3NS:Rh(0.03) for photocatalytic H2 evolution under visible-light irradiation liked the doping in the bulk particle as Rh-doped TiO2 or SrTiO3. Meanwhile, the excitation from O 2p to Rh3+ 4d or O 2p to Rh4+ 4d cannot be used for H2O reduction to produce H2 because a potential of this Rh3+ 4d and Rh4+ 4d is lower than the potential level of H2O reduction. Conversely, the electrons from Rh4+ 4d cannot be excited to Ti4+ 3d due to the unstable Rh4+ state and the self-reduction to Rh3+.

Figure 4 shows a time course of the photocatalytic H2 evolution over Ti3NS:Rh(x) compared with Rh(x)-Ti3NS at each Rh/Ti ratio under UV irradiation (λ > 220 nm). This result shows that Rh cocatalyst also improves the photocatalytic activity of Ti3NS. Rh(0.03)-Ti3NS shows the highest photocatalytic activity. However, the photocatalytic activity dropped when more amount of Rh cocatalyst was loaded. This is caused by an incident light blocking due to a large amount of Rh cocatalyst particles on the surface of Ti3NS. Generally, this phenomenon can be found in most photocatalysts with cocatalyst loading.26,37−39

Figure 4.

Figure 4

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Time course of H2 evolution over Ti3NS:Rh(x) and Rh(x)-Ti3NS for different amounts of Rh (filled circle for Ti3NS:Rh(x) and open circle for Rh(x)-Ti3NS). A 500 W Xe lamp with a long pass filter (λ > 220 nm) was employed as a light source.

Figure 5 shows the x value dependence of the photocatalytic H2 evolution rate. Remarkably, the H2 evolution rate exhibits a different dependence on the molar ratios of Rh and Ti. In the case of Ti3NS:Rh(x), the H2 evolution rate was significantly increased by the small doping amount of Rh and then gradually increased with an increase in the doping amount of Rh up to x = 0.03. When the doping amount of Rh was x = 0.05, the H2 production could not be detected under UV irradiation. The H2 evolution rate shows maximum value at x = 0.03 in Rh(x)-Ti3NS. The maximum value of H2 evolution rate for Rh(0.03)-Ti3NS (1970 μmol/(g h)) is higher than that for Ti3NS:Rh(0.03) (1040 μmol/(g h)), but a higher efficiency for the photoinduced H2 evolution reaction was yielded by doping Rh atom on the Ti sites rather than Rh loading. Moreover, the H2 evolution rates of Rh(0.03)-Ti3NS and Ti3NS:Rh(0.03) at initiation were different. Ti3NS:Rh(0.03) shows a shorter induction period for photocatalytic H2 evolution than Ti3NS:Rh(0) and Rh(0.03)-Ti3NS, as shown in Figure 6. From these results, the effect of Rh doping is quite similar to Rh cocatalyst loading, which results in the improvement of the photocatalytic activity of Ti3NS for H2 evolution from water. Nevertheless, there are some differences between them that can be found when Rh doping amount was too low or too high. As we explained earlier, the low doping amount exhibits a Rh3+-rich state, while a high doping amount exhibits a Rh4+-rich state.

Figure 5.

Figure 5

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H2 evolution rate from the water with TEA as an electron donor plotted with the Rh/Ti ratios of Ti3NS:Rh(x) (filled circle) and Rh(x)-Ti3NS (open circle). A 500 W Xe lamp with a long pass filter (λ > 220 nm) was employed as a light source.

Figure 6.

Figure 6

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Time course of the H2 evolution rate from the water with TEA as an electron donor using Ti3NS:Rh0 (close circle), Ti3NS:Rh0.03 (filled circle), and Rh0.03-Ti3NS (open circle) under a 500 W Xe lamp with a long pass filter (λ > 220 nm).

Rh4+ basically acts as a hole pool, while Rh3+ acts as an electron pool. Consequently, when there is a high amount of Rh4+ in Ti3NS, the photogenerated electron will be consumed by Rh4+, causing the H2 evolution reaction to hardly occur or the reaction rate to decrease.31,32,35,36,41 Although Rh4+ will accept an electron and reduce to Rh3+, this does not increase the H2 evolution reaction. This is because Rh3+ can change to Rh4+ from the oxidation reaction between Rh3+ and h+ in the valence band. Hence, the H2 evolution rate does not rise when Rh doping amount increases. This explains why the H2 evolution rate of Ti3NS:Rh(0.001) and Ti3NS:Rh(0.01) do not differ much. In spite of this, the ratio of Rh3+/Rh4+ is not the only factor; another key factor is the redox reaction of Rh3+ and Rh4+.

Both Ti3NS:Rh(0.001) and Ti3NS:Rh(0.01) have a higher H2 evolution rate than Rh(0.01)-Ti3NS. Therefore, Rh doping improves the photocatalytic activity more than Rh loading does. The reason is that Rh3+ was only found in the adsorption spectrum of Ti3NS:Rh(0.001) and Ti3NS:Rh(0.01). Previous reports found that Rh3+ aids in the evolution of H2 in the photocatalytic activity. Furthermore, the H2 evolution rate of Rh doping is higher than that of Rh loading because the distribution of the electron pool site is totally different. By Rh doping, the Rh site is more diversely distributed than Rh cocatalyst loading. This is because Rh will replace Ti site but cannot stay close to each other as it will disrupt the crystal structure of Ti3NS. Meanwhile, Rh cocatalyst only sticks on the surface of the Ti3NS sheet. Accordingly, there is a possibility that particles of Rh cocatalyst will combine, resulting in a converse distribution.

On the other hand, Ti3NS:Rh(0.03) has a lower H2 evolution rate than Rh(0.03)-Ti3NS due to the effect of the redox reaction of Rh3+ and Rh4+. In case of Rh cocatalyst loaded Ti3NS, Rh either in the form of metal oxide acts as a cocatalyst for the accumulation of photogenerated electron as an electron pool.37−39 In addition, it also affects the photogenerated electron–hole to be spatially separated due to the electrons or holes localized on the surface of photocatalyst and cocatalyst. On the contrary, Rh doping enhances the photocatalytic activity of Ti3NS in a different manner. Rh can be doped in the form of Rh3+ and Rh4+ in the oxidation state. While Rh3+ acts as a reduction point for reducing water molecules, Rh4+ acts as an oxidation point. However, Rh4+ is unstable. It can take the electron in the Rh3+ site, and reduce to Rh3+. Thus, Rh4+ blocks the reduction of water molecules to evolve H2. Ti3NS:Rh(0.03) has both Rh3+ and Rh4+; therefore, it has H2 evolution rate lower than that of Rh0.03-Ti3NS. Figure S4 shows the DR spectra of Ti3NS:Rh(0.03) before and after the photocatalytic H2 evolution reaction. A little change in the absorption of the visible region was found. This relates to the change in the Rh3+/Rh4+ ratio. However, the overall absorption is not significantly changed. This indicated that Rh4+ may change into Rh3+ by taking a photogenerated electron then oxidize back to Rh4+, resulting in a cyclic redox loop of Rh3+/Rh4+. This redox reaction of Rh3+/Rh4+ inside the photocatalyst under light irradiation is still a crucial topic. However, the observation of this reaction in real time is too arduous due to the unstable Rh4+.

Conclusions

Ti3NS:Rh(x) with varied Rh doping amount can be synthesized according to our procedure. The DR spectrum of Ti3NS:Rh(x) shows a new absorption band in the visible region, and the absorbance of this new band increases with the increased Rh amount, which was not found in the case of Rh(x)-Ti3NS. The photocatalytic activity of Ti3NS:Rh(x) and Rh(x)-Ti3NS for H2 evolution using TEA as an electron donor was shown under UV light (λ > 220 nm) irradiation. A difference between the photocatalytic activities of Ti3NS:Rh(x) and Rh(x)-Ti3NS was found. This disparity is due to rich Rh3+ or Rh4+ doping in the Ti site, unlike a Rh cocatalyst. From the point of view of dependence on irradiation light, our Ti3NS:Rh(0.03) can be active under near-UV irradiation but inactive under visible-light irradiation. Nevertheless, Rh doping enhances the photocatalytic activity of Ti3NS for H2 evolution and contributes to the onset wavelength increasing to the near-UV region.

Experimental Section

Materials

Sodium carbonate Na2CO3 (99.9%) from Wako Pure Chemical Industries Co., anatase-type TiO2 (99.7%) from High Purity Chemicals Co., Rh2O3 (99.9%) and RhCl3·3H2O from Wako Pure Chemical Industries Co., methylamine (CH3NH2, 40 wt % solution) and tetramethylammonium hydroxide (TMAOH, 26 wt % solution) from Tokyo Chemical Industry Co., and triethylamine (N(CH2CH3)3, TEA) from Wako Pure Chemical Industries Co. were used without further purification.

Synthesis of Rh-Doped Sodium Titanate

The precursor of Ti3NS, Na2Ti3–xRhxO7 was synthesized using the solid-state reaction method described in our previous report.22 Na2CO3, anatase-type TiO2, and Rh2O3 were mixed and ground in a mortar. The ratio of Na/Ti/Rh was set to 1.05:3 – x:x. The mixture was calcined at 1173 K for 24 h in air. This operation was repeated after grinding. The added molar amount of Rh or x value was set from 0 to 0.05.

Preparation of Rh-Doped and Rh-Loaded Titanate Nanosheet Colloidal Suspension

The colloidal suspension of Ti3NS:Rh(x) ([Ti3–xRhxO7]2–) was prepared according to a modified procedure of method described by Miyamoto et al. and Sasaki et al.16,19 Na2Ti3–xRhxO7 powder (0.3 g) was dispersed in 30 cm3 of hydrochloric acid (1 mol/dm3) for exchanging the countercation from Na+ to H+. Subsequently, it was shaken for 3 days at room temperature. The hydrochloric acid was replaced every day to allow the protonation reaction to proceed effectively. The filtrated H2Ti3–xRhxO7 was washed with high-purity water to remove any remaining hydrochloric acid and dried under a reduced pressure condition at room temperature overnight. The collected H2Ti3–xRhxO7 was neutralized by methylamine aqueous solution at 333 K for 6 days to yield the (CH3NH3)2Ti3–xRhxO7 powder. Subsequently, (CH3NH3)2Ti3–xRhxO7 was dispersed in TMAOH solution ([Ti3–xRhxO72–]/[TMAOH] = 5) by sonication for 5 days. Nonexfoliated Ti3NS:Rh(x) was removed by centrifugation (at 4000 rpm or 1150 G for 15 min, IEC61010-2-020, KUBOTA). The Ti3NS:Rh(x) colloidal suspension was obtained as the supernatant with a concentration of about 1 g/cm3.

The Rh cocatalyst-loaded titanate nanosheets (Rh(x)-Ti3NS, where x is loading amount) were prepared by the photodeposition method reported by Shimura et al.24,25 The powder of protonate from nondoped Ti3NS (H2Ti3O7, 0.2 g) was dispersed into 10% vol methanol solution (10 mL) containing RhCl3·3H2O. The solution was stirred at room temperature and in the presence of oxygen from the atmosphere. The photodeposition was done by irradiating using a 300 W Xe lamp without a filter for 24 h while stirring continuously. The Rh cocatalyst-loaded photocatalyst powder was collected by filtration and washed with distilled water. Then, the powder was dried in vacuum overnight, and Rh cocatalyst-loaded H2Ti3O7 was obtained. The colloidal suspension of Rh(x)-Ti3NS was prepared through the same procedure as used for Ti3NS:Rh(x).

H2 Evolution Experiment

All photochemical reactions were performed in a closed gas circulation system with a Pyrex reaction cell.28,29 The reaction solution (10 cm3) containing TEA (0.36 mol/dm3, pH ∼ 10–11) and Ti3NS:Rh(x) or Rh(x)-Ti3NS colloidal suspension (1 mg) was degassed by repeated freeze pump–thaw cycles. The reaction solution was shifted into the reaction vessel with a glovebox under an Ar-saturated atmosphere. The reaction solution was irradiated at 300 K using a 500 W Xe lamp (USHIO Co.) equipped with optical cutoff filters (long pass: λ > 220, 340, 380, and 480 nm; Kenko Co.). The amount of evolved H2 gas was determined using a gas chromatograph (Shimadzu; GC-8A with TCD detector, a stainless steel column packed with molecular sieves 5A and ultrapure Ar as carrier gas). All measurements were performed under nonoxygen conditions.

Characterization

X-ray powder diffraction (XRD) patterns were obtained by an X-ray diffractometer (MiniFlex II, RIGAKU Co.) with Ni-filtered Cu Kα radiation (30 kV, 15 mA). The amount of Rh in Ti3NS:Rhx or Rhx-Ti3NS was determined by inductively coupled plasma-atomic emission spectroscopy (ICP-AES; Optima 2000, Perkin-Elmer Co.) after fully dissolving the prepared Ti3NS:Rh(x) or Rh(x)-Ti3NS in the mixed acid solution between nitric acid (1.0 mol/dm3) and sulfuric acid (0.5 mol/dm3). Absorption spectra of various samples were measured by a UV–vis spectrophotometer (V-670, JASCO Co.). Diffuse reflection (DR) spectra of the Ti3NS:Rh(x) or Rh(x)-Ti3NS colloidal suspension were measured using quartz cuvette and the UV–vis spectrophotometer (V-670, JASCO Co.) attached to an integrating sphere system (ISN-723, JASCO Co.).

Acknowledgments

This research was partly supported by KAKENHI 24651144 from MEXT, Japan. The synchrotron radiation experiments were performed at the BL02B2 of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2014B1744). Authors acknowledge the cooperation of the Interdisciplinary Center for Science Research, Shimane University, for providing the experimental facility of SPM. We thank Assoc. Prof. Kazuto Arakawa for taking an image of nanosheets and cocatalyst by TEM.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.0c00204.

  • XRD pattern and TEM image of Ti3NS:Rh(x) and Rh(x)-Ti3NS cast film; stability of Ti3NS:Rh(0.03) for H2 evolution is shown via a time course H2 evolution by different sample entry; and DR spectra of Ti3NS:Rh(x) before and after H2 evolution (PDF)

Author Present Address

§ Faculty of Science, Tokyo University of Science, 5th building, Kagurazaka, Shinjuku, Tokyo, Japan.

The authors declare no competing financial interest.

Supplementary Material

ao0c00204_si_001.pdf (321.9KB, pdf)

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Supplementary Materials

ao0c00204_si_001.pdf (321.9KB, pdf)

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