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[Fumihiko Ichihara](https://orcid.org/0000-0002-0274-1342), [Hong Pang](https://orcid.org/0000-0002-9286-082X), [Tetsuya Kako](https://orcid.org/0000-0002-1891-6346), [Detlef W. Bahnemann](https://orcid.org/0000-0001-6064-6653), Jinhua Ye

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[Photogenerated charge carrier dynamics on Pt-loaded SrTiO<sub>3</sub> nanoparticles studied <i>via</i> transient-absorption spectroscopy](https://mdr.nims.go.jp/datasets/01e328ba-4cc6-4e1a-81af-7643f9b30990)

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Photogenerated charge carrier dynamics on Pt-loaded SrTiO3 nanoparticles studied via transient-absorption spectroscopyNanoscalePAPERCite this: Nanoscale, 2025, 17, 2567Received 12th November 2024,Accepted 8th January 2025DOI: 10.1039/d4nr04725grsc.li/nanoscalePhotogenerated charge carrier dynamics on Pt-loaded SrTiO3 nanoparticles studied via transient-absorption spectroscopy†Fumihiko Ichihara, a,b Hong Pang, *a,b Tetsuya Kako,a Detlef W. Bahnemann cand Jinhua Ye*a,bLoading cocatalysts on semiconductor-based photocatalysts to create active reaction sites is a preferablemethod to enhance photocatalytic activity and a widely adopted strategy to achieve effective photo-catalytic applications. Although theoretical calculations suggest that the broad density of states of noblemetal cocatalysts, such as Pt, act as a recombination center, this has never been experimentally demon-strated. Herein, we employed pico–nano and nano–micro second transient absorption spectroscopy toinvestigate the often overlooked photogenerated holes, instead of the widely studied electrons on Pt- andNi-loaded SrTiO3 to evaluate the effects of cocatalysts as a recombination center. It is demonstrated thatPt serves as the recombination center with no sacrificial agent; recombination can be suppressed by ahole scavenger, while recombination is not significant on Ni with localized density of states. It is alsofound that photo-generated holes in SrTiO3 tend to migrate to Pt within 400 ps, and photo-generatedholes generated in the bulk gradually migrate to Pt cocatalysts in a micro-second regime.1. IntroductionPhotocatalysis has garnered significant attention as a greentechnology since it can utilize sunlight to convert ubiquitousmolecules, such as water, carbon dioxide and nitrogen, intochemical energy.1–13 Despite extensive efforts, its efficiencystill remains far from industrial demands, and its practical usehas not been achieved yet. One approach to improving photo-catalytic efficiency is loading “cocatalysts” on photocatalysts.Certain cocatalysts are known as rectifiers for efficient photo-catalytic hydrogen evolution reactions (HERs) and oxygen evol-ution reactions (OERs). For example, Pt,14,15 Ni, NiO16–19 andCrOx/Rh20 are established cocatalysts for the HER, andIrO2,21,22 RuO2,21,23 CoPi (cobalt phosphate)24–27 andCoOx15,28,29 are frequently used as cocatalysts for the OER.There are two primary purposes of loading cocatalysts: (i) toreduce the overpotential required for the respective redoxprocess and (ii) to inhibit the recombination of photogene-rated charge carriers by securing and storing electrons orholes. While the former purpose has been verified usingelectrochemical measurements, there is limited experimentalevidence to confirm the latter purpose with regard to carriercollection behavior.In the field of theoretical calculations, cocatalyst loading ispredicted to act as a recombination center.30 When Pt isloaded on TiO2, the broad density of states of Pt overlaps withthe conduction band minimum and valence band maximumof TiO2. Thus, it is expected that both photo-generated elec-trons and holes are captured by Pt cocatalysts. Consequently,the recombination is promoted. Furthermore, even thoughPt is an excellent cocatalyst for hydrogen generation, the Ptcocatalyst is not suitable for complete water splitting owing toits backward reaction, in which the generated hydrogen andoxygen recombine and revert to water.31 Conversely, in thecase of transition metals such as Ni and Co, localized 3d orbi-tals do not serve as a bridge between the conduction band andvalence band.32,33 It is therefore expected to result in arelatively low recombination when such transition metalsare loaded as cocatalysts. As described above, the cocatalystplays a crucial but complicated role in the photocatalystdesign, and it is essential to obtain experimental insightsinto how cocatalyst loading influences charge carrierdynamics.Transient absorption spectroscopy is a very useful tech-nique for measuring and evaluating such recombination pro-†Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4nr04725gaResearch Center for Materials Nanoarchitectonics (MANA), National Institute forMaterials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan.E-mail: PANG.Hong@nims.go.jp, Jinhua.YE@nims.go.jpbGraduate School of Chemical Sciences and Engineering, Hokkaido University,Sapporo Hokkaido 060-0814, JapancInstitute for Technical Chemistry, Leibniz University Hannover, Hannover 30167,GermanyThis journal is © The Royal Society of Chemistry 2025 Nanoscale, 2025, 17, 2567–2576 | 2567Open Access Article. Published on 20 January 2025. Downloaded on 11/5/2025 7:09:06 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article OnlineView Journal  | View Issuehttp://rsc.li/nanoscalehttp://orcid.org/0000-0002-0274-1342http://orcid.org/0000-0002-9286-082Xhttp://orcid.org/0000-0001-6064-6653https://doi.org/10.1039/d4nr04725ghttps://doi.org/10.1039/d4nr04725ghttps://doi.org/10.1039/d4nr04725ghttp://crossmark.crossref.org/dialog/?doi=10.1039/d4nr04725g&domain=pdf&date_stamp=2025-01-24http://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d4nr04725ghttps://pubs.rsc.org/en/journals/journal/NRhttps://pubs.rsc.org/en/journals/journal/NR?issueid=NR017005cesses in semiconductors and reactions proceeding on thesurface of a photocatalyst.34–39 This method allows for theobservation of non-emission carrier dynamics in the semi-conductor, following photoexcitation. It has already beenapplied to investigate the carrier dynamics of Pt-TiO2,34NiO-NaTaO3,40 Pt-LaTiO2N,15 CoPi-Fe2O3,41,42 Pt, Au-CdS,43,44Ni-CdS,45 and MoS2-CdS.46 However, in many cases, themeasurements are focused on the carriers that are anticipatedto migrate, and the role of the cocatalyst as a recombinationcenter has not been sufficiently discussed. For instance, in thecase of Pt-TiO2 where electrons are highly expected to migrateto Pt cocatalysts, the migration behavior of the photogeneratedholes is often overlooked.In this study, we used transient absorption spectroscopy toobserve the interaction between photo-excited holes and thebroad density of states of Pt on SrTiO3, and compared theseresults with the situation using the Ni cocatalyst. By examiningthe carrier dynamics loading of Pt and Ni, the relationshipbetween the cocatalyst type and recombination dynamics isdiscussed.2. Experimental section2.1 Photocatalyst preparationSrTiO3 nanoparticles were synthesized through a polymeriz-able complex method.47 Specifically, Ti(OC4H9)4 (99.9%,Sigma-Aldrich) was dissolved in ethylene glycol, and themixture was stirred under N2 atmosphere for 30 min.Afterwards, Sr(NO3)3 (99.9%, Sigma-Aldrich) and citric mono-hydrate (99.5%, Carl Roth) were added, and the stirring wascontinued until the solution became completely transparent.The reaction mixture was subsequently stirred for anadditional 15 minutes to ensure full dissolution of thereagents, and then heated at 120 °C for 5 hours to promotepolymerization. During heating, the solvent evaporated andthe suspension transformed into a transparent brownish resin.This resin was further heated to 350 °C with a slow rampingtemperature rate (1 °C min−1) and kept for 3 hours. The result-ing cinders were ground to obtain fine nanoparticles, andfurther calcined at 750 °C for 6 hours.2.2 Photo-deposition of the cocatalyst and photocatalyticreactionPt and Ni photo-deposition was carried out in 300 ml of 10vol% methanol (MeOH) aqueous solution, and the evaluationof the photocatalytic hydrogen evolution was continued afterthe deposition. 50 mg SrTiO3 powder was placed in a Pyrexglass reactor, and subjected to ultrasonic treatment for30 minutes. Then, H2PtCl6·6H2O or Ni(NO3)2 aqueous solutionwas added with vigorous stirring for the Pt and Ni photo-depo-sition, respectively. The photocatalytic activity evaluation wasperformed with a gas-closed circulation system, as shown inFig. S1.† The Pyrex glass reactor and solution were degassed bylinking to a gas-closed circulation system connected to a rotarypump. The solution was irradiated for 3 hours with the full arcof a 300 W Xe lamp for the photo-deposition and photo-catalytic reaction. The produced H2 was analyzed by gaschromatography (GC) (GC-8A, Shimadzu, Japan; carrier gas,Ar) with a thermal conductivity detector (TCD), which was con-nected to the gas-closed circulation system.2.3 CharacterizationThe sample was characterized by X-ray diffraction (XRD: D8Advance; Bruker Corp, Germany) using Cu Kα radiation in the2θ range of 10–70° with a step width of 0.01°. The diffusedreflectance spectra of the sample were recorded on a UV-visible spectrometer (UV-2600; Shimadzu, Japan) with BaSO4as the reference. The reflectance spectra were converted toabsorption spectra by the Kubelka–Munk function. The photo-luminescence (PL) spectra of the sample were recorded on aFP-8550 (JASCO, Japan) with the excitation wavelength of355 nm. The microstructure observations were performed withtransmission electron microscopy (TEM, JEM-ARM200F; JEOLLtd, Japan) at an acceleration voltage of 60 kV. X-ray photo-electron spectroscopy (XPS) analysis was performed on aEnviroESCA (SPECS GmbH, Germany) equipped with Al Kαradiation, and the pass energy for all measurements was set to20 eV. I–V measurements were performed by potentiostat(SP-300; Bio-Logic SAS, France) with the voltage range of −6 to6 V.Charge carrier dynamics were measured by transientabsorption spectroscopy. The details of the set-up of the nano–micro48,49 and pico–nanosecond transient absorption aredescribed in Fig. S2 and S3,† respectively. For the transientabsorption experiments, the powder photocatalysts werepacked in a quartz cuvette, which allowed for the introductionof reactant gases. Here, O2 gas and MeOH vapor were appliedas electron and hole scavengers, respectively.34,353. Results and discussion3.1 The characterization of Pt, Ni-loaded SrTiO3Fig. 1(a and b) shows the XRD patterns of the synthesizedSrTiO3 and Pt and Ni-loaded SrTiO3. The synthesized SrTiO3shows a perovskite structure with the lattice constant of a =3.9233 Å, which is relatively consistent with the reported valueof a = 3.911 Å.50 In addition, no other diffraction patterns wereobserved after the Pt and Ni cocatalyst is loaded, indicatingthat the Pt and Ni cocatalyst was loaded as relatively small par-ticles. Fig. 1(c and d) shows the UV-Vis absorption spectra of(c) Pt and (d) Ni-loaded SrTiO3. SrTiO3 exhibits light absorp-tion in the UV range, as reported in the previous literature.49Fig. 1(c) shows an increase in absorption in the 400–700 nmrange with increasing loading amount of the Pt cocatalyst,which can be attributed to the absorption of Pt nanoparticles,as observed by TEM (Fig. S4†).51 In Fig. 1(d), the absorption ofNi-loaded SrTiO3 increased with increasing Ni loadingamount, as in the case of Pt. A slight change in the shapeof the shoulder of the absorption edge was observed when theNi loading amount was 1.0 wt%, indicating that a part ofPaper Nanoscale2568 | Nanoscale, 2025, 17, 2567–2576 This journal is © The Royal Society of Chemistry 2025Open Access Article. Published on 20 January 2025. Downloaded on 11/5/2025 7:09:06 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d4nr04725gthe Ni cocatalyst was oxidized. Fig. 1(e and f) shows the PLspectra of Pt and Ni-loaded SrTiO3. The broad photo-luminescence signal of SrTiO3 was derived from a radiativerecombination between electrons located in numerousnumbers of trapping states between the band gap and holes atthe valence band. The PL signal intensity was weakened whenthe Pt or Ni cocatalysts were loaded. This is probably due tothe non-radiative recombination pathway of the photoexcitedelectrons formed by cocatalyst loading. Fig. S5† shows the XPSspectra of the Pt and Ni 1.0 wt%-loaded SrTiO3. Fig. S5(a andb)† shows that the carbon species and –OH groups adsorbedon SrTiO3 were reduced by the loading process of the cocata-lyst. This is considered to be due to the removal of the surfaceadsorbed species by the photocatalytic effect of SrTiO3 irra-diated with light during the loading process. The chemicalstates of Ti and Sr shown in Fig. S5(c and d)† showed onlylimited changes due to the loading of the cocatalyst, indicatingthat the cocatalyst can be loaded without causing sidereactions, such as alloying and or elemental interdiffusion.The chemical states of Pt and Ni shown in Fig. S5(e and f)†indicate that Pt was metallic, while Ni was partially oxidized.This partial oxidation of Ni is considered to correspond to thechange in the shape of the absorption edge observed inthe UV-Vis spectrum (Fig. 1(d)). Fig. S6† shows the I–V curvesof SrTiO3 and Pt and Ni-loaded SrTiO3. The voltage–currentresponse shows no linear response components even afterthe addition of the Pt or Ni cocatalysts, indicating that thejunction between SrTiO3 and the cocatalyst is a Schottkyjunction.3.2 The photocatalytic activity of the Pt, Ni-loaded SrTiO3Fig. 2(a) shows the effect of Pt loading on the photocatalyticHER activities of the SrTiO3 nanoparticles. The results showthat the photocatalytic activity was significantly enhancedfrom 25.5 μmol g−1 h−1 to 851.3, 1149.4, 1386.3 μmol g−1 h−1with the increase of the Pt weight percent from 0.0 to 0.25, 0.5,and 1.0 wt%, respectively. This is reasonable as loading noblemetals such as Pt is expected to create active sites for HER.However, the photocatalytic activity gets saturated with furtherincreases in the Pt loading. Therefore, the loaded Pt contentsaffect the electron–hole recombination rate in SrTiO3 nano-particles. Fig. 2(b) shows the photocatalytic HER activities withrespect to the amount of Ni that is loaded. A similar trend wasobserved in the case of Ni loading up to 1.0 wt%, although theactivity at 1.0 wt% was as low as 205 μmol g−1 h−1, approxi-mately one-seventh of that achieved with Pt.3.3 Transient absorption of photogenerated charge carriersin SrTiO3Fig. 3(a) shows the transient absorption spectrum after theirradiation with a 355 nm Nd:YAG laser over SrTiO3. Theenergy of the Nd:YAG laser is a suitable excitation wavelengthto induce a band-to-band transition of SrTiO3. A broad absorp-tion band is observed around 765 nm, and a relatively narrowabsorption band is observed around 825 nm. To identify theobserved carriers, decay measurements were taken at 765 nmwith the presence of an electron scavenger (O2 gas) and a holescavenger (MeOH vapor) (Fig. 3(b)). As a reference, a decayFig. 1 XRD patterns of (a) Pt and (b) Ni-loaded SrTiO3. UV-vis absorption spectra of (c) Pt and (d) Ni-loaded SrTiO3. PL spectra of (e) Pt and (f ) Ni-loaded SrTiO3 with an excitation wavelength of 355 nm.Nanoscale PaperThis journal is © The Royal Society of Chemistry 2025 Nanoscale, 2025, 17, 2567–2576 | 2569Open Access Article. Published on 20 January 2025. Downloaded on 11/5/2025 7:09:06 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d4nr04725gmeasurement was also performed in a N2 gas atmosphere,where no reactive species are present, representing a simpleprocess of electron–hole recombination. In the measurementunder O2 atmosphere, the decay began to slow down slightlyafter 1 μs. This is because the electrons that tend to recombinewith holes are captured by O2.52O2ðgÞ þ e� ! O2�ðaÞ ð1ÞWhere (g) and (a) represent the gas phase and adsorbedphase, respectively. Interestingly, the decay was dramaticallyaccelerated when measuring in MeOH. This is because theholes were collected by MeOH-derived adsorbates:CH3O�ðaÞ þ hþ ! CH3O•ðaÞ ð2ÞFrom the above results, it can be concluded that the photo-excited carriers at 765 nm are transient absorption bandsderived from holes. As shown in Fig. S7,† the decay of thesignal was decelerated in O2 atmosphere and accelerated inMeOH vapor, similar to the signal seen at 765 nm, indicatinga hole-derived transient absorption signal.3.4 Transient absorption of photogenerated charge carriersover Pt, Ni-loaded SrTiO3Fig. 4(a and b) shows the transient absorption spectra of SrTiO3loaded with 0.25 wt% Pt and Ni, respectively. Both transientabsorption spectra of Pt and Ni-loaded SrTiO3 show broadabsorption peaks at around 765 nm and 825 nm, similar tothose observed on the pure SrTiO3. This suggests that even aftercocatalyst loading, the transient absorption signal reflects thetransient feature of SrTiO3. To further investigate the interactionbetween the cocatalyst loading and the trapping behavior ofphotogenerated holes in SrTiO3, the relationship between theamount of cocatalyst loading and the decay constant in the inertN2 atmosphere was determined (Fig. 5). The transient absorp-tion spectra of SrTiO3 loaded with 0.5 wt% and 1.0 wt% Pt or Nican be found in Fig. S8.† The linear increase in k2,f, which indi-cates the rate of decay with respect to the amount of cocatalyst,indicates the transfer of holes to the cocatalyst. This phenom-enon is also considered to be reasonable since holes can easilymove through the Schottky junction between n-type semi-conductors SrTiO3 and Pt or Ni cocatalysts, as shown in the I–VFig. 2 Effect of the loaded platinum weight percent on the photocatalytic activity of (a) Pt- and (b) Ni-loaded SrTiO3 nanoparticles for photo-catalytic hydrogen evolution.Fig. 3 (a) Transient absorption spectra of SrTiO3 particles irradiated by a UV (355 nm) pulsed laser under N2 gas. The pump energy was 1.3 mJcm−2, and the repetition rate was 10 Hz. (b) Decay curves of transient absorption of the SrTiO3 at 765 nm.Paper Nanoscale2570 | Nanoscale, 2025, 17, 2567–2576 This journal is © The Royal Society of Chemistry 2025Open Access Article. Published on 20 January 2025. Downloaded on 11/5/2025 7:09:06 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d4nr04725gcurves (Fig. S6†). The increase in the decay constant k2,f with Ptloading was much more pronounced than that with Ni,suggesting that the broad density of states of Pt act as a recombi-nation center.Fig. 6(a) exhibits the relationship between the amount of Ptloading and the decay constant of Pt-loaded SrTiO3 in differentatmospheres. As described in Fig. 5, an increasing decay con-stant in N2 atmosphere with respect to the increase of the Ptamount has been observed. This increase in decay constantwas further promoted in the presence of O2 gas and sup-pressed in MeOH vapor (Fig. 6(a)), suggesting that the adsorp-tion of O2 and MeOH molecules influences the decaydynamics. In contrast, the decay constant of Ni-loaded SrTiO3,shown in Fig. 6(b), was less affected by the increase in the Niamount and the various atmospheres.To further understand the effect of the adsorbed moleculeson the Pt cocatalyst, the decay curves of Pt-loaded SrTiO3 at765 nm in N2, O2 and MeOH atmospheres are compared, asshown in Fig. 7(a). As described in Fig. 6(a), the decay rateaccelerates in the O2 atmosphere and decelerates in MeOHvapor, compared to that of N2 atmosphere. Unlike SrTiO3(Fig. 3(b)), Pt loading causes the transient decay to be slowestin MeOH vapor. In the O2 atmosphere, the decay rate isslightly accelerated as the decay constant k2,f increases from0.395 to 0.449, compared with that in N2 atmospheres.Moreover, Fig. 8 illustrates the decay curves of SrTiO3 withdifferent loading amounts of Pt at 765 nm in N2 and MeOHatmospheres. In Fig. 8(a), the Pt loading dramaticallydecreases the signal intensity and increases the decay constantin a N2 atmosphere. Meanwhile, in a MeOH atmosphere(Fig. 8(b)), the decrease in signal intensity and increase indecay constant are not as significant as those observed in a N2atmosphere. These limited changes in MeOH indicate thatMeOH scavenges photogenerated holes in SrTiO3, thus miti-gating the impact of Pt cocatalyst loading on the decay curves.Conversely, as seen in Fig. 7(b) and Fig. S9,† for Ni-loadedSrTiO3, the decay was decelerated in an O2 atmosphere andaccelerated in a MeOH atmosphere compared with that in a N2atmosphere, similar to that of SrTiO3, confirming that the Nicocatalyst loading has a small effect on the carrier dynamics ofSrTiO3 and does not act as a recombination center.From the results discussed above, the dynamics shown inFig. 9 can be considered as follows: in an O2 atmosphere, O2adsorbed on Pt scavenges the electrons of the Pt cocatalyst toform charged oxygen species and produce holes.30,53 Thesecharged oxygen species and the holes generated at the Pt coca-talyst are expected to trap the electron–hole pairs generated inSrTiO3, accelerating the consumption of charge carriers in theO2 atmosphere compared to the N2 atmosphere. In the case ofMeOH vapor, MeOH molecules tend to scavenge the photo-generated holes in SrTiO3. Thus, there is no large effectobserved over the carrier dynamics from loading differentamounts of Pt cocatalyst. Therefore, the recombination at theFig. 4 (a) Transient absorption spectra of Pt 0.25 wt% SrTiO3 particles and (b) Ni 0.25 wt% SrTiO3 particles irradiated by a UV (355 nm) pulsed laserunder N2 gas. The pump energy was 1.3 mJ cm−2, and the repetition rate was 10 Hz.Fig. 5 The relationship between the loading amount of Pt and Ni coca-talysts and the decay constant k2,f.Nanoscale PaperThis journal is © The Royal Society of Chemistry 2025 Nanoscale, 2025, 17, 2567–2576 | 2571Open Access Article. Published on 20 January 2025. Downloaded on 11/5/2025 7:09:06 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d4nr04725gFig. 6 (a) The relationship between the loading amount of Pt cocatalysts, (b) Ni cocatalyst and decay constant k2,f in different atmospheres. Theinset figure shows the enlarged vertical axis of (b).Fig. 7 Decay curves of the transient absorption of (a) Pt 0.25 wt%-loaded SrTiO3 and (b) Ni 0.25 wt%-loaded SrTiO3 in different atmospheres at765 nm.Fig. 8 Decay curves of transient absorption of the Pt-loaded SrTiO3 in (a) N2 and (b) MeOH atmospheres at 765 nm.Paper Nanoscale2572 | Nanoscale, 2025, 17, 2567–2576 This journal is © The Royal Society of Chemistry 2025Open Access Article. Published on 20 January 2025. Downloaded on 11/5/2025 7:09:06 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d4nr04725gPt cocatalyst has been suppressed in the presence of hole sacri-ficial reagents.3.5 Hole behavior observed by femtosecond transientabsorption measurementThe photocatalytic reaction takes place in three steps, i.e.,carrier generation, charge transfer, and surface reaction.52 Inorder to investigate the time scale on which the photo-gener-ated holes transfer, we measured the transient absorptionspectrum at 765 nm on the pico-second scale, as shown inFig. 10(a). When Pt was loaded on SrTiO3, the sample itselfabsorbed light at 765 nm, and the signal intensity decreasedwith an increase in the Pt loading amount. After normalizingthe spectra (Fig. 10(b)), it was observed that the photo-excitation showed a faster decay than pure SrTiO3 as the Ptloading increased up to about 400 ps. This is similar to theaccelerated decay of LaTiO2N, which has the same perovskitestructure as SrTiO3, in the range of 2–350 ps due to Ptloading.15 However, the lifetime was extended after 400 ps.This phenomenon indicates that photoexcited holes near thesurface were effectively transferred to the Pt cocatalysts within400 ps. Beyond this time frame, hole trapping behavior fromthe bulk did not occur within the pico–nano second timerange of the transient absorption measurement. As describedabove, the holes were likely drawn and recombined in Pt coca-talysts in the micro-second range.4. ConclusionSrTiO3 samples loaded with Pt and Ni cocatalysts by photode-position were evaluated and compared using nano–micro andpico–micro second transient absorption spectroscopy.Focusing on the transient absorption signal derived fromholes, rather than the normally studied electron signals, it wasfound that the decay constant, representing the speed ofcarrier decay, increased linearly with the loading amount ofcocatalysts. When loading with the Pt cocatalyst, this increasein the decay constant was more significant than that with Nicocatalyst loading, indicating that the broad density of state ofPt may act as a recombination center, as suggested by theore-tical calculation. In MeOH vapor, this increasing recombina-Fig. 9 Schematic representation of the photoexcited electron and hole path over Pt-loaded SrTiO3 in the presence of (a) O2 gas and (b) MeOHvapor.Fig. 10 (a) Pico-second transient absorption spectra and (b) normalized spectra of Pt-loaded SrTiO3 nanoparticles irradiated by UV (355 nm) pulsedlaser under ambient conditions at 765 nm.Nanoscale PaperThis journal is © The Royal Society of Chemistry 2025 Nanoscale, 2025, 17, 2567–2576 | 2573Open Access Article. Published on 20 January 2025. Downloaded on 11/5/2025 7:09:06 AM.  This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d4nr04725gtion tendency was suppressed compared with that of a N2atmosphere, indicating that the Pt cocatalyst did not act as arecombination center in the presence of hole sacrificialagents. As a contrast, for the transition metal Ni-loadedsamples, the recombination was not accelerated. Therefore,this suggests that designing a cocatalyst with a localizeddensity of states is desirable, and an appropriate hydrogenabsorption/desorption feature is crucial for efficient overallwater splitting. Furthermore, by measuring the chargedynamics in the picosecond-to-nanosecond range, it wasobserved that photo-generated holes near the surface weretransferred to Pt cocatalysts within 400 ps following photo-excitation, while the holes generated in bulk migrated to Pt ina micro-second regime.Data availabilityThe data supporting this article have been included within thearticle and its ESI.†Conflicts of interestThere are no conflicts to declare.AcknowledgementsThis work received financial support from the World PremierInternational Research Center Initiative (WPI Initiative) onMaterials Nanoarchitectonics (MANA), MEXT, Japan, andPhotoexcitonix Project in Hokkaido University. 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