Photoconductivity–Lifetime Product Correlates Well with the

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Photoconductivity−Lifetime Product Correlates Well with the Photocatalytic Activity of Oxyhalides Bi4TaO8Cl and PbBiO2Cl: An Approach to Boost Their O2 Evolution Rates Hajime Suzuki,† Masanobu Higashi,‡ Hironobu Kunioku,‡ Ryu Abe,*,‡,§ and Akinori Saeki*,†,# †

Department of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan ‡ Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan § CREST, Japan Science and Technology Agency (JST), Kawaguchi, Saitama 332-0012, Japan # Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan S Supporting Information *

ABSTRACT: Owing to the shallow valence band maximum generated upon hybridization of Bi/Pb 6s and O 2p orbitals, layered oxyhalides of Bi4TaO8Cl and PbBiO2Cl are stable and promising O2-evolving photocatalysts for Z-scheme water splitting. However, some questions remain regarding how their synthesis parameter influences the charge carrier dynamics and overall photocatalytic process. Here, we show that the product of the transient pseudophotoconductivity maximum (φΣμmax) and half-lifetime (τ1/2), as evaluated by time-resolved microwave conductivity (TRMC) measurements, correlates well with the O2 evolution rates of such oxyhalides. Such correlation in Bi4TaO8Cl is rationalized by the increased crystallite size and decreased Cl/Bi atomic ratio with the increasing calcination temperature. On the basis of this, we examined the processing temperature of the rarely studied PbBiO2Cl by TRMC analysis and demonstrate a 3-fold enhancement of its O2 evolution rate (apparent quantum efficiency of 3%), which is among the best performances of oxyhalides prepared by a solid-state reaction. This study suggests that TRMC evaluation provides a facile means to examine the potential of semiconducting photocatalysts.

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ature and time) may also alter the physicochemical properties of the photocatalysts to a large extent, exerting also a great impact on their activity.14−19 Thus, researchers need to rigorously optimize the reaction conditions and perform tedious photocatalytic measurements. The speedy evaluation of the intrinsic photocatalytic activity will facilitate the development of novel photocatalysts. Time-resolved microwave conductivity (TRMC) analysis has been utilized to investigate the photogenerated charge carrier dynamics and predict the power conversion efficiency of organic photovoltaics.20 This technique allows facile evaluation without the

ater splitting using photocatalysts is a hot topic aiming at clean solar hydrogen production.1−11 Its development is highly encouraged by the 1.1% solar-to-hydrogen energy conversion efficiency of photocatalyst sheets,12 greater or comparable to the efficiency of natural photosynthesis. Despite the ever-growing number of publications, many issues remain, such as the improvement of the efficiency at low cost, the use of nontoxic abundant elements, and the long-term durability. Thus, in addition to the development of novel photocatalysts, gaining better understanding of the efficiency-limiting factors is of particular importance as it is known that the photocatalytic activity is influenced by many complex factors13 (e.g., band edge levels, surface area, crystallinity, defects, and charge carrier mobility). Similarly, the synthetic method (e.g., solid-state reactions and hydrothermal methods) and conditions (e.g., reaction temper© 2019 American Chemical Society

Received: April 12, 2019 Accepted: May 29, 2019 Published: June 17, 2019 1572

DOI: 10.1021/acsenergylett.9b00793 ACS Energy Lett. 2019, 4, 1572−1578

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ratios for Bi4TaO8Cl are provided in Figures S2−S4 and Table S1, all of which are consistent with those previously reported.32 Figure 2a shows the φΣμ transients of Bi4TaO8Cl powders measured by TRMC analysis under 355 nm excitation. The wavelength dependence of the photoconductivity maximum (Δσmax) of Bi4TaO8Cl(900) (the value in the parentheses represents the calcination temperature) is in a good agreement with the photoabsorption spectrum, confirming that the observed photoresponse is due to bandgap excitation (Figure S5). The φΣμ maxima (φΣμmax) and half-lifetimes (τ1/2) of the TRMC transients are plotted as a function of the calcination temperature in Figure 2b. With the increasing calcination temperature, the φΣμmax increases in a linear fashion, while τ1/2 rapidly decreases. The increased φΣμmax is reasonably attributed to the improved crystallinity, as evident from the trend observed for the calcination temperature vs crystallite size calculated from the diffraction peaks of the (0010) and (220) facets (Figure S3). The increased photoconductivity at high calcination temperatures has also been observed in the case of TiO2.22 In addition, high calcination temperatures for oxyhalides such as Bi4MO8X (M = Nb, Ta; X = Cl, Br) lead to volatilization of the halogen (i.e., a reduction of the halogen content),32 as observed for the Cl/Bi ratio in our Bi4TaO8Cl samples by XPS analysis (Table S1). The decrease in halogen content from Cl/Bi = 0.27 for Bi4TaO8Cl(700) to 0.20 for Bi4TaO8Cl(900) results in the formation of defects that generally act as charge carrier traps. Thus, the decay of τ1/2 is presumably ascribed to increased charge carrier trapping and/ or charge recombination. To gain deeper understanding of such decay, a differential equation considering first-order charge trapping and secondorder charge recombination processes was applied33,34

need to fabricate the actual device and is therefore applicable to photocatalysts in powder form. Several groups have reported the TRMC evaluation of oxide semiconductors,21−27 mainly focusing on TiO2 nanoparticles. However, the relationship between the TRMC photoconductivity and the photocatalytic performance has not yet been fully understood. In this study, we report the correlation between the photocatalytic activity of oxyhalides Bi4TaO8Cl and PbBiO2Cl and the kinetic parameters derived from the analysis of TRMC transients. Bi4TaO8Cl and PbBiO2Cl, whose crystal structures and diffuse reflectance spectra are shown in Figures 1 and S1,

φΣμ(t ) =

φΣμ0 ·k kt

(k + φΣμ0 γ )e − φΣμ0 γ

+ φΣμd ·e−(kdt )

β

(1)

The first term on the right side is the analytical solution of the differential equation, and the second term is the stretched exponential function to fit the long tail component, where φΣμ0 is the initial intensity of φΣμ, k is the first-order rate constant, and γ is the second-order rate constant (in V cm−2 units). φΣμd, kd, and β (fixed at 0.3) are the intensity, rate constant, and a power factor of the delayed component, respectively. As shown in Figure 2c, the experimental decays for the different excitation intensities (I0 = (1.2−9.1) × 1014 photons cm−2 pulse−1) match well the least-mean-square fitted analytical curves convoluted by a response function (all fitting results are shown in Figure S6). The dependence of k and other parameters (φΣμ0, γ, φΣμd, and kd) on the calcination temperature is shown in Figures 2d and Figure S7, respectively. Notably, k increases independently of I0 from ∼2 × 106 s−1 for Bi4TaO8Cl(700) to ∼5.5 × 106 s−1 for Bi4TaO8Cl(900), consistent with a decrease in τ1/2. In contrast, γ shows strong dependence on the laser intensity, opposite to that of k (Figure S7), where γ increases with I0 and decreases with the increasing calcination temperature. The distinct positive dependence of γ on I0 is explained by the increased charge carrier density at high I0 values, supporting the validity of the analysis. On the basis of this detailed kinetic analysis, we concluded that the drop in τ1/2 is caused by the increased k associated with carrier trapping due to the abundance of Cl vacancies.

Figure 1. Crystal structures of (a) Bi4TaO8Cl and (b) PbBiO2Cl. (c) Schematic diagram of the valence and conduction band structure for Bi4TaO8Cl.29

respectively, have been recently found to efficiently oxidize water into O2 under visible light without self-oxidation.28−30 In contrast to conventional oxyhalide photocatalysts such as BiOX (X = Cl, Br, I), the valence band maxima of Bi4TaO8Cl and PbBiO2Cl are predominantly occupied by destabilized O 2p orbitals instead of Cl 3p orbitals (Figure 1c), which accounts for their improved stability against self-oxidative deactivation by photogenerated holes during water oxidation.29,31 These photocatalysts were synthesized via a solidstate reaction (SSR) at different calcination temperatures (500−900 °C) to comprehensively examine the crystallinity, stoichiometry (Cl/Bi ratio), and morphology, as well as to validate such TRMC analysis as a robust tool providing a rapid and direct approach to optimize the performance of photocatalysts. We prepared Bi4TaO8Cl samples at varying SSR calcination temperatures (700−900 °C) as it is presumably one of the most important parameters influencing the photocatalytic performance. The XRD patterns, along with the derived crystallite sizes, SEM images, BET surface areas, and Cl/Bi 1573

DOI: 10.1021/acsenergylett.9b00793 ACS Energy Lett. 2019, 4, 1572−1578

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Figure 2. (a) TRMC transients of Bi4TaO8Cl samples calcined at different temperatures (λex = 355 nm, I0 = 9.1 × 1014 photons cm−2 pulse−1). (b) TRMC transient maxima (φΣμmax) and half-lifetimes (τ1/2). (c) φΣμ transients of Bi4TaO8Cl(700) at varying incident excitation densities (I0) of (1.2−9.1) × 1014 photons cm−2 pulse−1, along with the fitting curves (black lines). (d) First-order rate constants (k) obtained by analyzing the TRMC transients at different I0 values. (e) φΣμmax dependence on I0. (f) Correlation between φΣμmax × τ1/2 (solid bars) and the O2 evolution rate over Bi4TaO8Cl(700−900) in aqueous AgNO3 solution (striped bars). The O2 evolution rates were taken from a previous report.32

S1). Although small flakes are observed on the particle surface in the SEM images at calcination temperatures over 800 °C (Supporting Figure S4), it is difficult to quantitatively explain the reduced photocatalytic activity above such a temperature by the decreased surface area. In contrast, the TRMC analysis clearly suggests that both improving the crystallinity (related to φΣμmax) and minimizing the halogen defects (related to τ1/2) are key for good-performance oxyhalide photocatalysts. During the course of this investigation, it was reported that flux synthesis using chloride molten salts remarkably enhances the photocatalytic activity of Bi4MO8Cl (M = Nb,38 Ta28,39) through the realization of both high crystallinity and fewer Cl defects. On the basis of the good correlation of the product φΣμmax × τ1/2 with the photocatalytic activity found for Bi4TaO8Cl, we applied this concept to another oxyhalide photocatalyst: Sillen phase PbBiO2Cl. This oxyhalide has been reported as a stable photocatalyst for water oxidation in a two-step water-splitting system under visible light;30 however, the calcination was performed only at 700 °C following a common procedure.40 Therefore, there is likely much room for improvement in the photocatalytic activity of PbBiO2Cl using the present TRMC indicator. Figure 3a shows the XRD patterns of the PbBiO2Cl samples prepared via a SSR at calcination temperatures of 500−750 °C. The samples present a tetragonal phase with space group I4/mmm. All of the diffraction peaks were reasonably indexed by a Le Bail analysis41 (Figure S9). Moreover, the lattice constants (Table S2) are almost identical to those in the literature.42 In the same manner to that of Bi4TaO8Cl, high calcination temperatures resulted in increased crystallite sizes from 200−300 to 600−900 Å in the (110) and

The appearance of trap sites at high calcination temperatures was also examined by the φΣμmax dependence on the incident photon density, as shown in Figure 2e (all kinetic traces are provided in Figure S8). The Bi4TaO8Cl samples prepared at 700 and 750 °C exhibit a monotonous decrease of φΣμmax with the increasing I0 due to accelerated recombination at the end of pulse (i.e., a decrease of φ). On the other hand, convex curves are observed for the Bi4TaO8Cl samples prepared at high temperatures (800−900 °C). This phenomenon is caused by trap-filling effects (increase and saturation of Σμmax at the high I0) frequently observed in TiO2 nanoparticles.26,35−37 Thus, this result corroborates that calcination at high temperatures leads to the formation of halogen defect traps. The upstream photocatalytic reactions correspond to the photogeneration of charge carriers and their transport to the surface with minimal deactivation losses. Therefore, the product of φΣμmax and τ1/2 (φΣμmax × τ1/2), which encompasses the charge carrier generation yield, mobility, and lifetime, was envisioned as a good indicator of the performance of photocatalysts. Indeed, a good correlation between φΣμmax (or Δσmax) × τ1/2 and the performance of organic photovoltaic cells has been reported.20 Figure 2f displays the relationship between φΣμmax × τ1/2 and the O2 evolution rate of Bi4TaO8Cl prepared at different calcination temperatures. Most importantly, the product φΣμmax × τ1/2 presents its maximum at 800 °C, perfectly coinciding with the O2 evolution rate trend. The Pearson correlation coefficient was 0.97, indicating a strong linear correlation between φΣμmax × τ1/2 and the O2 evolution rate. Note that the BET surface areas, which often influence the photocatalytic performance, were similar across all of the samples (0.8−1.1 m2 g−1, Table 1574

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(Figure 4c; other fitting results are shown in Figure S12). The increased k values with the increasing temperature independent of I0 suggest that the initial decay is mainly due to trapping (Figure 4d). The other parameters obtained from the analysis are provided in Figure S13, where γ is dependent on I0 and decreases with the increasing temperature. Figure 4e shows the φΣμmax dependence of the PbBiO2Cl samples on I0, where such a trap-filling effect is more obviously observed than in Bi4TaO8Cl (all kinetic traces are shown in Figure S14). In addition, the maximum φΣμmax tends to shift to higher I0 values with the increasing calcination temperature, consistent with the increased number of halogen defects. Katoh et al. have reported that the maximum φΣμmax observed for commercially available TiO2 particles shifts to larger excitation intensities with the decreasing particle size as trap sites are dominantly located at the surface.26 In stark contrast, the trap-filling peak position for the PbBiO2Cl samples is governed by the halogen defects rather than the particle size. As a consequence, the φΣμmax × τ1/2 product exhibits a maximum at the calcination temperature of 600 °C (Figure 4f). Notably, this temperature deduced from the TRMC analysis is 100 °C lower than that from the previous report,30 suggesting the need to test lower calcination temperatures during the synthesis of this photocatalyst. The O2 evolution rates over the PbBiO2Cl samples in aqueous AgNO3 and Fe(NO3)3 solutions are shown in Figure 5. The Ag+ cation is a sacrificial electron acceptor commonly used to initially check the photocatalytic O2 evolution activity, while the Fe3+ cation is a nonsacrificial (i.e., reversible) electron acceptor. The latter nonsacrificial O2 evolution is compatible with a Z-scheme water-splitting system, where reversible redox couples such as Fe3+/Fe2+ and IO3−/I− transfer electrons from an O2-evolving photocatalyst to a H2evolving photocatalyst.5,43,44 Indeed, PbBiO2Cl(600) showed the highest O2 evolution rates of 26.8 μmol h−1 (Ag+/Ag) and 9.2 μmol h−1 (Fe3+/Fe2+) (Table S3). Moreover, a good correlation between the photocatalytic performance and φΣμmax × τ1/2 product was obtained over the examined temperature range (500−750 °C). The Pearson correlation coefficients were 0.77 and 0.95 for the Ag+ and Fe3 electron acceptor, respectively, confirming the good correlation. On the other hand, the more significant drop in the O2 evolution rate rather than in φΣμmax × τ1/2 at 700−750 °C is possibly caused by a gradual decrease of the BET surface area (Figure 3c). Despite the different surface condition of each measurement (dry in TRMC and wet in the photocatalytic reaction), it is interesting to obtain the good correlation between them. The TRMC transients of Bi4TaO8Cl and PbBiO2Cl samples in air were very similar to those under a nitrogen atmosphere, confirming the small impact of oxygen and humidity and the good stability of TRMC measurements (Figure S15). Thus, the photocatalytic activities of the present oxyhalides seem closely related to their intrinsic optoelectronic properties derived from TRMC analysis. The apparent quantum efficiency (AQE) of O2 evolution in the aqueous Fe(NO3)3 solution was measured after loading a RuO2 co-catalyst, which is known to remarkably enhance the O2 evolution rate of PbBiO2Cl photocatalysts.30 Surprisingly, the O2 evolution AQE using such RuO2-loaded PbBiO2Cl(600) was determined to be 3.0% at 400 nm, 3 times greater than those from previous reports using RuO2 -loaded PbBiO2Cl(700) (0.9% at 400 nm30) and RuO2-loaded Bi4NbO8Cl (0.9% at 420 nm38). It should be noted that

Figure 3. (a) XRD patterns of PbBiO2Cl samples calcined at different temperatures (°C). The diffraction peaks were indexed via Le Bail refinement, as shown in Figure S9. (b) Crystallite size and Cl/Bi atomic ratio of the different samples. The sizes were estimated using the Scherrer equation based on the (002) and (110) peaks in the XRD patterns. The Cl/Bi atomic ratios were determined from XPS analysis. (c) SEM images of PbBiO2Cl(500− 750). The BET surface area values are shown on the left bottom corner.

(002) directions (Figure 3b). The particle size increased from several hundred nanometers to a few micrometers with the increasing temperature (the SEM images are shown in Figures 3c and S10), accompanied by a considerable decrease of the BET surface area from 5.8 m2 g−1 for PbBiO2Cl(500) to 1.0 m2 g−1 for PbBiO 2Cl(750). In addition, the Cl/Bi ratio determined by XPS measurement progressively decreased with the increasing temperature, which is similar behavior to that of Bi4TaO8Cl (Figure 3b). Figure 4a shows the φΣμ transients of the PbBiO2Cl samples calcined at 500−750 °C. Bandgap excitation and subsequent charge carrier generation give rise to these TRMC signals, as confirmed by the identical photoconductivity maximum (Δσmax) from the photoabsorption spectrum (Figure S11). Similar to the Bi4TaO8Cl samples, the φΣμmax and τ1/2 values of PbBiO2Cl increase and decrease with the increasing calcination temperature, respectively, which is readily attributed to a trade-off between the crystallinity and abundance of halogen defects (Figure 4b). The kinetic traces were analyzed using eq 1, providing excellent fits to the experimental data 1575

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Figure 4. (a) TRMC transients of PbBiO2Cl samples calcined at different temperatures (λex = 355 nm, I0 = 9.1 × 1014 photon cm−2 pulse−1). (b) TRMC transient maxima (φΣμmax) and half-lifetimes (τ1/2). (c) φΣμ transients of PbBiO2Cl(500) at varying incident excitation densities (I0) of (2.4−9.1) × 1014 photons cm−2 pulse−1, along with the fitting curves (black lines). (d) First-order rate constants (k) obtained by analyzing the TRMC transients at different I0 values. (e) φΣμmax dependence on I0. (f) φΣμmax × τ1/2 product of the transients.

afforded an enhanced O2 evolution rate, boosting the AQE (3%) by 3 times compared to the results from previous studies. Oxyhalides are novel Z-scheme water-splitting photocatalysts,45 and thus, a large number of oxyhalides are likely efficient photocatalysts with great potential. Our work opens up the application of TRMC analysis to enable not only the speedy optimization of photocatalysts but also the efficient screening of other potential semiconducting photocatalysts.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.9b00793. Experimental details; BET surface area and Cl/Bi ratio of Bi4TaO8Cl; lattice constants; O2 evolution rates; diffuse reflectance spectra; XRD patterns; crystallite sizes; SEM images; photoconductivity transients; fitting of TRMC transients; parameters obtained by fitting; TRMC transients; and Le Bail refinements of XRD patterns (PDF)

Figure 5. Initial O2 evolution rates over PbBiO2Cl(500−750) in aqueous AgNO3 (solid bars) and Fe(NO3)3 (striped bars) solutions under visible light irradiation (λ > 400 nm). The data for PbBiO2Cl(700) were taken from a previous report.30

such a great improvement, in fact, the top performance among all oxyhalide photocatalysts prepared by SSR methods, was achieved by considering the TRMC optimization, thus demonstrating that the φΣμmax × τ1/2 product is a good indicator of photocatalytic activity. In conclusion, we report that the φΣμmax × τ1/2 product evaluated by TRMC analysis as a good indicator for the photocatalytic performance of oxyhalides. φΣμmax increases with the calcination temperature due to an increase in crystallinity, as evident from the crystallite size of the XRD patterns. On the other hand, τ1/2 was found to decrease with the increasing temperature, consistent with the XPSdetermined Cl defects and the first-order decay rate (k) and trap-filling dependence of the TRMC transients. On the basis of the established φΣμmax × τ1/2 correlation with the O2 evolution rate of Bi4NbO8Cl, we explored the optimal calcination temperature for PbBiO2Cl and found that 600 °C

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: +81-75-383-2478 (R.A.). *E-mail: [email protected]. Tel: +81-6-6879-4587 (A.S.). ORCID

Hajime Suzuki: 0000-0002-8891-2033 Masanobu Higashi: 0000-0002-1265-2191 Ryu Abe: 0000-0001-8592-076X 1576

DOI: 10.1021/acsenergylett.9b00793 ACS Energy Lett. 2019, 4, 1572−1578

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Akinori Saeki: 0000-0001-7429-2200 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the PRESTO program (Grant No. JPMJPR15N6) from the Japan Science and Technology Agency (JST), the Japan Society for the Promotion of Science (JSPS) through a KAKENHI Grant-in-Aid for Scientific Research (A) (Grant No. JP16H02285), and JST-CREST (Grant No. JPMJCR1421). H.S. acknowledges financial support of a JSPS scholarship (No. JP18J01488).

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