Letter Cite This: J. Phys. Chem. Lett. 2019, 10, 1986−1991
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Complex Photoconductivity Reveals How the Nonstoichiometric Sr/ Ti Affects the Charge Dynamics of a SrTiO3 Photocatalyst Kento Yamada,† Hajime Suzuki,† Ryu Abe,‡ and Akinori Saeki*,†,§
J. Phys. Chem. Lett. Downloaded from pubs.acs.org by UNIV AUTONOMA DE COAHUILA on 04/09/19. For personal use only.
†
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 § 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: Strontium titanate (SrTiO3) is a perovskite that is important in watersplitting photocatalytic chemistry. Although excess Sr is known to improve the photocatalytic activity, its effect on charge dynamics remain largely unaddressed. Herein, we present a detailed analyses of gigahertz complex transient photoconductivity (Δσ) measured using time-resolved microwave conductivity (TRMC). We show that charge carrier trapping associated with the emergence of an anomalous positive imaginary part and the first-order rate constant of the normal positive real part of Δσ dramatically decreased with increasing Sr/Ti ratio. The second-order rate constant attributed to charge recombination simultaneously decreased, and these rate constants were well correlated with the improved hydrogen evolution rate of aqueous SrTiO3 suspensions with a Pt cocatalyst. These findings provide a fresh perspective on the stoichiometry−carrier dynamics relationship paramount for the optimization of composition-engineered photocatalysts and reveal the broad implications for mechanistic studies based on TRMC evaluation.
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regardless of preparation method (i.e., hydrothermal synthesis (HT), polymerizable complex method (PC), or solid-state reaction (SSR)). They suggested that the excess Sr decreases the amount of SrO defects that act as recombination centers for hole/electrons.27 The kinetics of single-carrier trapping in nondoped SrTiO3 crystals28 and charge carrier traps in surface defects of SrTiO3 powder29 have been determined by transient absorption (visible to mid-infrared) and photoluminescence spectroscopy. Nevertheless, correlating the trapping process with photocatalytic activity is difficult as trapped electrons/ holes exhibit long lifetimes owing to reduced charge recombination and could contribute to the photocatalytic process at the surface of the photocatalyst.29 This is in sharp contrast to photovoltaics, where the power conversion efficiency is directly decreased by charge traps.30,31 Therefore, different approaches to investigate carrier dynamics are essential for understanding traps and their effect on the photocatalytic activity. Herein, we present a systematic and comprehensive evaluation of the charge carrier trapping process in nonstoichiometric SrTiO3 (Sr/Ti = 0.9−1.5) powder prepared by SSR using flash-photolysis time-resolved microwave conductivity (TRMC). TRMC using a gigahertz electromagnetic wave
ater splitting using photocatalyst semiconductors is expected to provide a method for environmentally friendly hydrogen production from solar energy.1−3 Since the original interest in this topic was triggered by discovery of the TiO2 photocatalytic reaction,4 numerous oxide catalysts have been developed, including the conventional SrTiO3 (ref 5) and BiVO4,6 layered K4Nb6O17,7 and a complexed anionic compound of TaON.8 Because the photophysical and photochemical reactions involved in the photocatalytic cycle influence the gas production rate in a complex manner,9 the energetics (photoabsorption10−12 and band position13,14), morphology (particle size, surface area,15,16 and crystallinity17,18), and electronics (defect19−21 and carrier transport22,23) must be thoroughly characterized. In addition to the choice of co-catalyst, subtle changes in process conditions, including the calcination temperature24 and the feed ratio of reactants,25 have been shown to enhance photocatalytic activity by a few to tens of factors. However, the powder form of the photocatalyst unsuitable for electric characterization and optical spectroscopy often precludes the elucidation of efficiency-limiting factors. Thus, any direct evaluation of structure−property relationships has remained challenging. SrTiO3 doped with a transition metal (e.g., Rh) forming a donor level to absorb visible light is a representative photocatalyst used for Z-scheme water splitting.26 Kato et al. showed that the photocatalytic activities of SrTiO3:Rh catalysts are largely improved by controlling the Sr/Ti stoichiometry, © XXXX American Chemical Society
Received: March 28, 2019 Accepted: April 3, 2019
1986
DOI: 10.1021/acs.jpclett.9b00880 J. Phys. Chem. Lett. 2019, 10, 1986−1991
Letter
The Journal of Physical Chemistry Letters as the probe allows for direct measurement of photogenerated charge carriers without the need for electrodes and with nanosecond time resolution.32 Furthermore, the complex photoconductivity behavior contains a rich fingerprint reflecting the trapped charges and their evolution over time.33,34 We observed a pronounced decrease of trapped charges at the Sr/Ti stoichiometry that yielded improved photocatalytic activity, highlighting the effect of excess Sr. The SrTiO3 powder samples were prepared by SSR by mixing SrCO3 and anatase TiO2 at various molar stoichiometries (Sr/Ti = 0.9−1.5) and at 900, 1000, and 1100 °C for 10 h, with the samples named STO-900, STO-1000, and STO1100, respectively. Figures 1a shows the φ∑ μ transients of
Figure 2. (a) XRD patterns of STO-900 with changing Sr/Ti ratios. (b) Semilogarithmic plot of the spectra at 2θ = 31−34°. The baseline of each profile was shifted.
hand, two new representative peaks appeared at 31.0 and 31.5° at Sr/Ti ratios greater than 1.0, which could be ascribed to the (103) and (105) peaks35 of the Ruddlesden−Popper (RP) perovskites (Sr2TiO4 and Sr3Ti2O7),36,37 respectively (Figure 2b and S2). These RP perovskites also exhibited an intense diffraction peak near the main peak of the three-dimensional (3D) SrTiO3 perovskite (32.5°), making it difficult to directly quantify their composition from the XRD patterns. The Sr2TiO4 peak was observed even at Sr/Ti = 1.0 and gradually increased in intensity with increasing Sr/Ti, followed by growth of the Sr3Ti2O7 peak. The evolution of the RP perovskites is more pronounced in the STO-1000 and -1100 samples than that in STO-900 (Figure S3), consistent with the higher calcination temperature preferred for the formation of RP perovskites.37 From the XRD studies, excess Sr generated RP perovskites that exhibited smaller Ti content than that of 3D SrTiO3. Scanning electron microscopy (SEM) was used to visualize the crystal size and morphology, which were mostly similar regardless of Sr/Ti ratio (Figure S4). Therefore, the influence of morphological change on TRMC would be negligible. Accordingly, the increased φ∑ μmax with increasing Sr/Ti could be associated with the emergence of RP perovskites. We therefore prepared Sr2TiO4 and Sr3Ti2O7 according to their respective stoichiometries at a high calcination temperature (1350 °C) and performed TRMC while changing the excitation photon density (I0 = 1012−1016 photons cm−2) (XRD of these perovskites are provided in Figure S5, consistent with the calculation). As shown in Figure 3, Sr2TiO4 and Sr3Ti2O7 exhibited greater φ∑ μmax compared to that of SrTiO3 (calcined at 1350 °C, Sr/Ti = 1.00) and showed a convex-shaped dependence with maxima at 0.1 and 0.02 cm2 V−1 s−1 at I0 ≈ 1014 photons cm−2, respectively. This dependence can be explained by trap filling, as reported for TiO2 (refs 33, 38, and 39) and layered oxyhalide Bi4NbO8Cl (ref 40), where the increased photogenerated charge carrier density filled the trap sites and the effective ∑ μmax sublinearly increased. The maxima appeared as a result of this ∑ μmax dependence and monotonic decrease of φ due to the highorder charge carrier deactivation pathways at high excitation intensities. The photon excitation energy (355 nm = 3.49 eV) was above the bandgap energies of SrTiO3 (3.27 eV), Sr2TiO4
Figure 1. φ∑ μ transients of (a) STO-900 and (b) STO-1100 measured by TRMC at a 355 nm excitation (I0 = 9.1 × 1015 photons cm−2 pulse−1). The Sr/Ti ratio was changed from 0.9 to 1.5. (c) φ∑ μmax and (d) τ1/2 of STO-900 (red circles), STO-1000 (blue triangles), and STO-1100 (green rectangles) as a function of Sr/Ti.
STO-900 measured by TRMC upon 355 nm excitation, where φ is the charge carrier generation yield and ∑ μ is the sum of the charge carrier (holes and electrons) mobilities (∑ μ = μh + μe). It should be noted that φ∑ μ is converted from the real part of the transient photoconductivity (Δσ′), while the imaginary part (Δσ″) will be discussed later. The φ∑ μ decay rates of STO-900 greatly varied depending on the Sr/Ti ratio, though their maxima (φ∑ μmax) remained largely unchanged. In contrast, STO-1000 (Figure S1) and STO-1100 (Figure 1b) exhibited monotonic and considerable increases in both φ∑ μmax and lifetime. The φ∑ μmax and half-lifetime (τ1/2) of the STO-900, -1000, and -1100 samples are plotted as a function of Sr/Ti in Figure 1c, d, respectively. Despite the linearly increasing or constant value of φ∑ μmax, τ1/2 indicates a stepwise increase for STO-1000 and -1100 and a sharp peak for STO-900 at Sr/Ti = 1.15. The distinct change of decay rather than the constant or monotonic increase of φ∑ μmax likely relates to the improved photocatalytic H2 production (vide infra). Powder XRD experiments were performed for the Sr/Ticontrolled STO-900 to examine the coexistence of different phases (Figure 2a). All peaks observed in the Sr/Ti = 0.9 of STO-900 were readily attributed to SrTiO3.27 On the other 1987
DOI: 10.1021/acs.jpclett.9b00880 J. Phys. Chem. Lett. 2019, 10, 1986−1991
Letter
The Journal of Physical Chemistry Letters
However, the unusual dependence of τ1/2 on Sr/Ti observed for STO-900 cannot be explained in the same manner as those observed in STO-1000 and -1100. Thus, we performed detailed analyses of the φ∑ μ transients based on the following equation derived from the first- and second-order rate equations41,42 φ ∑ μ(t ) =
φ ∑ μ0 ·k (k + φ ∑ μ0 γ )ekt − φ ∑ μ0 γ + φ ∑ μd ·e−(kdt )
β
(1)
where φ∑ μ0 is the initial intensity of φ∑ μ; γ is the secondorder rate constant (in V cm−2, but it can be converted to a normal second-order rate constant, k2, in cm3 s−1 unit via k2 = γ × φ∑ μmax/n, where n is the charge carrier concentration in cm−3); and k is the first-order rate constant. The second term of the stretched exponential function is added to fit the longtailed decay, where φ∑ μd, kd, and β are the intensity, rate constant, and power factor (β = 0.3) of the delayed components of φ∑ μ transients, respectively. The leastmean-square fits of the STO-900, -1000, and -1100 transients yielded five fitting parameters (φ∑ μ0, γ, k, φ∑ μd, and kd; Figures 4 and S7) at Sr/Ti = 0.9−1.5. Interestingly, the ratio of φ∑ μd/φ∑ μ0 shown in Figure 4a reproduces the unusual dependence of τ1/2 in STO-900 (Figure 1d), indicating that the change of τ1/2 is caused by the intensity balance between the initial and delayed decays, rather than changes in their decay rates. In contrast, k rapidly decreased for all samples (STO900, -1000, and -1100) with increasing Sr/Ti and showed an impedance as small as 1.1 (Figure 4b). This result is clearly indicative of the decreased trap density with excess Sr. Simultaneously, γ gradually decreased with increasing Sr/Ti, reflecting the reduced charge recombination at high Sr/Ti ratios (Figure 4c), which would affect the photocatalytic reactions. Another notable change relating to the charge carrier trapping is the imaginary part of photoconductivity (Δσ″), which showed a positive and long decay at
Figure 3. (a) φ∑ μmax dependence on I0 and (b) φ∑ μ transients of SrTiO3 (blue), Sr3Ti2O7 (red), and Sr2TiO4 (green; λex = 355 nm, I0 = 9.1 × 1014 photons cm−2 pulse−1).
(3.47 eV), and Sr3Ti2O7 (3.49 eV) (Figure S6; note that the unsharp onsets of RP perovskites allow for excitation and charge carrier generation upon exposure to a 3.49 eV photon). In addition, the variation in valence band maxima was negligible (−6.87 to −6.94 eV vs vacuum level, Figure S6); therefore, the photophysical process (photoexcitation and charge separation) can be regarded as similar among the prepared samples. A more obvious difference in the φ∑ μ transients was the significant increase of τ1/2 from 1.3 × 10−7 s in SrTiO3 and 2.9 × 10−7 s in Sr3Ti2O7 to 4.5 × 10−6 s in Sr2TiO4, which is in line with the increased φ∑ μmax. Therefore, the emergence of the high photoconductivity in Sr3Ti2O7 and Sr2TiO4 with long-lived charge carriers at high Sr/Ti ratios is responsible for the increased φ∑ μmax and τ1/2 observed in STO-1000 and -1100.
Figure 4. (a) φ∑ μd (φ∑ μ0)−1, (b) k, and (c) γ obtained by analyzing the φ∑ μ transients (eq 1) of STO-900 (red circles), STO-1000 (blue triangles), and STO-1100 (green rectangles) as a function of Sr/Ti. (d) Imaginary photoconductivities (Δσ″) of STO-900 with changing Sr/Ti. (e) Plot of the maximum Δσ″ as a function of Sr/Ti for STO-900, STO-1000, and STO-1100. Δσ″ is regarded to be zero if it is always negative. 1988
DOI: 10.1021/acs.jpclett.9b00880 J. Phys. Chem. Lett. 2019, 10, 1986−1991
Letter
The Journal of Physical Chemistry Letters Sr/Ti = 0.9, while the polarity flipped to the negative with a short lifetime at a Sr/Ti of >1.1 (Figure 4d). Charge carriers in the Drude−Smith framework,43 such as TiO2 (refs 33 and 44) and conjugated polymers,33,45 exhibit a positive Δσ′ and negative Δσ″, corresponding to the increase in complex permittivity (real component: Δε′ > 0 and imaginary component Δε″ > 0).33,46 The opposite, anomalous polarity of Δσ (negative Δσ′ and positive Δσ″) was previously observed in an organic−inorganic lead−tin mixed halide perovskite.34 This can be rationalized by decreased complex permittivity (Δε′ < 0 and Δε″ < 0) caused by the trapped charges with the immobilized methylammonium cation dipole that is responsible for the dielectric response.34 Paraelectric SrTiO3 and ferroelectric BaTiO3 also exhibit positive anomalous Δσ″, presumably due to the insusceptibility of Ti and O ion displacement oriented to the trapped charges.34 It should be noted that such a trap-related positive Δσ″ completely disappears at high Sr/Ti ratios in SrTiO3. The positive Δσ″ remarkably decreased regardless of calcination temperature (Figure 4e), consistent with the decreased k associated with the trapping dynamics of charge carriers (Figure 4b). The kinetics of Δσ″ changed from long-lived positive to short-lived negative, the latter of which was slightly faster (Figure S8), similar to that observed in TiO 2 nanoparticles.33 This suggests the coexistence of shallowly trapped and mobile charge carriers. The H2 evolution rates of STO-900 with a Pt co-catalyst at Sr/Ti = 1.00−1.25 under white light illumination were measured in aqueous suspensions with methanol as a hole scavenger (Figure 5). The H2 production profiles of the
TRMC with reductive H2 production by electrons. Despite the pronounced change in k, γ, and Δσ″, φ∑ μmax remains unchanged by the Sr/Ti ratio (Figure 1c), indicating that the trapping process rather than the charge carrier generation and/ or mobility is the governing factor of photocatalytic reaction. It should be emphasized that positive Δσ″ is a more accurate and straightforward indicator of trapped charges than k because the positive Δσ″ is directly linked to the number of trapped charges. On the other hand, k is the rate constant of the trapping process derived from the transient analysis, and its deviation is larger than that observed for Δσ″. Therefore, the evaluation of complex photoconductivity is a powerful alternative approach for identifying trapped charges that significantly impact photocatalytic reactions. In conclusion, the TRMC complex photoconductivity of stoichiometry-varied SrTiO3 (Sr/Ti = 0.9−1.5) was measured and analyzed using first- and second-order rate equations. The first-order rate constant (k) of the positive real part of photoconductivity (Δσ′) and the anomalous positive imaginary photoconductivity (Δσ″) associated with the charge carrier traps progressively decreased with increasing Sr/Ti ratios. Simultaneously, the second-order rate (γ) of Δσ′ gradually decreased, indicative of reduced charge recombination. The H2 evolution rates of aqueous suspensions of Pt− SrTiO3 exhibited stepwise improvement with increasing Sr/Ti from 1.10 to 1.15, related to the decreased trapping revealed by the TRMC evaluation. Thus, excess Sr is concomitant with the formation of Sr-rich RP perovskites (Sr2TiO4 and Sr3Ti2O7), which impedes charge carrier traps and improves overall photocatalytic activity. Complex TRMC photoconductivity with detailed analyses is a versatile approach for characterizing photocatalysts and their activities, allowing for a deeper understanding of complicated photocatalytic systems.
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EXPERIMENTAL SECTION
Sample Preparation and Characterization. SrTiO3 powder was prepared by SSR from SrCO3 (99.9%) and anatase TiO2 (99.9%, 5 μm average particle size; Wako Chemical Industries Ltd.). SrCO3 was calcined at 300 °C for 1 h prior to the synthesis. The stoichiometry-controlled SrTiO3 was synthesized by mixing SrCO3 and TiO2 at various molar ratios (Sr/Ti = 0.9−1.5) and milling them in methanol (Kishida Chemical Co. Ltd.) with an alumina mortar for 30 min. The samples were then calcined in an electric furnace under an air atmosphere with the following temperature profile: elevation from room temperature to the calcination temperature (900, 1000, or 1100 °C) for 2 h, maintaining the calcination temperature for 10 h, and overnight cooling to room temperature. The RP perovskites (Sr2TiO4 and Sr3Ti2O7) were synthesized using the same method, where the calcination was performed at 1350 °C for 48 h to facilitate the crystal growth of their pure phase. The valence band maximum (VBM) was measured by photoelectron yield spectroscopy (PYS) using a Bunko Keiki Corp. BIP-KV201 instrument (accuracy: ±0.02 eV; extraction voltage = 10 V) in a vacuum (
