# Minuscule Amounts of Pt Single Atoms Selectively Loaded on Minor (101) Facet of Anatase Crystallites Enables Outstanding Utilization Efficiency for Photocatalytic H2 Production

Shanshan Qin, Nikita Denisov, Benedict Osuagwu, Jan Kolařík, Bidyut Bikash Sarma, Zdeněk Baďura, Patrik Schmuki

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Shanshan Qin, Nikita Denisov, Benedict Osuagwu, Jan Kolařík, Bidyut Bikash Sarma, et al.. Minuscule Amounts of Pt Single Atoms Selectively Loaded on Minor (101) Facet of Anatase Crystallites Enables Outstanding Utilization Efficiency for Photocatalytic H2 Production. Advanced Functional Materials, 2025, 35 (24), pp.2423088. 10.1002/adfm.202423088. hal-05141811

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# Minuscule Amounts of Pt Single Atoms Selectively Loaded on Minor (101) Facet of Anatase Crystallites Enables Outstanding Utilization Efficiency for Photocatalytic H₂ Production

Shanshan Qin, Nikita Denisov, Benedict Osuagwu, Jan Kolařík, Bidyut Bikash Sarma, Zdeněk Baďura, and Patrik Schmuki*

Pt single atoms (SAs) on TiO₂ have been identified as effective co-catalysts in solar photocatalytic H₂ production. In this study, Pt SAs is deposited on single crystal anatase nanosheets (NSs) with minor (101) and major (001) facets and expose them to various thermal treatments in air, Ar, and Ar-H₂ environments. It is found that through Ar-H₂ annealing Pt-species can be accumulated exclusively on the minor (101) facets, i.e., they can be concentrated selectively on the facet where electron exit occurs. This optimally establishes a high photocatalytic H₂ evolution efficiency for an extremely low overall Pt SA loading of ≈0.005 wt.% on the anatase crystallites. For this most active SA placement, 11.7 mmol g⁻¹ h⁻¹ H₂ production is obtained with a turnover frequency (TOF) up to 253 #H₂ site⁻¹ s⁻¹. Note that the corresponding Pt SAs density of 3.1 × 10⁴ atoms µm⁻² is established by loading from only 0.2 µm Pt solution. These findings demonstrate that placing a very low density of Pt SAs as an electron transfer co-catalyst onto the electron exit sites of a light-harvesting structure (i.e., where there it is needed) can provide a most active photocatalytic structure with an outstanding Pt utilization.

## 1. Introduction

In 1972, Fujishima and Honda introduced a groundbreaking approach to H₂ generation through photocatalytic water splitting, utilizing TiO₂ as a semiconductor.[1] In this process, light is absorbed in the semiconductor, and photogenerated charge carriers

migrate to the semiconductor surface, where they can be transferred to redox species in the surrounding environment.[2] In photocatalytic H₂ generation, electrons from the conduction band of the semiconductor can react with protons (H⁺) in the aqueous medium, yielding H₂, while the resulting holes in the valence band participate in oxidation reactions, either with water or other hole scavengers.[3,4] Despite the exploration of numerous semiconductors for photocatalytic H₂ generation,[5-7] TiO₂ remains the focus of intense investigation, mainly due to its remarkable resistance to photo-corrosion and its economic benefits.[3,4,8] To provide ample surface area and reactive sites, various nanoscale morphologies of TiO₂ have been employed in photocatalytic H₂ generation, including nanotubes, nanoparticles, and notably, single crystal nanosheets (NSs) of anatase.[9-14] Such anatase TiO₂ NSs with predominant (001) facets, and a minor amount of (101)

facets, had a large impact on photocatalytic work due to the formation of an internal electronic junction that provides facet-selective charge carrier separation.[15-17] This is, photogenerated electrons and holes are spatially separated and as a result, electrons preferentially exit the crystallite on the (101) planes and holes exit on the majority (001) planes.[18-24] It should be noted

S. Qin, N. Denisov, B. Osuagwu, P. Schmuki
Department of Materials Science and Engineering
Chair for Surface Science and Corrosion (WW4-LKO)
Friedrich-Alexander-Universität Erlangen-Nürnberg
Martensstraße 7, 91058 Erlangen, Germany
E-mail: schmuki@ww.uni-erlangen.de

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.202423088

© 2025 The Author(s). Advanced Functional Materials published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

DOI: 10.1002/adfm.202423088

B. Osuagwu, J. Kolařík, Z. Baďura, P. Schmuki
Regional Centre of Advanced Technologies and Materials
Czech Advanced Technology and Research Institute
Palacky University
Olomouc 779 00, Czech Republic

B. B. Sarma
Laboratoire de Chimie de Coordination (LCC)

CNRS
Université de Toulouse
INPT, UPR 8241, 205 route de Narbonne, Toulouse, Cedex 4 31077, France

Z. Baďura
Nanotechnology Centre
Centre for Energy and Environmental Technologies

VSB-Technical University of Ostrava
17. listopadu 2172/15, Ostrava, Poruba 708 00, Czech Republic
that this facet-selective carrier separation [induced by the differ-
ence in surface energy of the (101) and (001) facets][¹²,¹⁸⁻²⁰,²⁵]
only holds for clean oxide surfaces, not for crystallites that carry
a F-termination (generally the crystallites are synthesized hy-
drothermally in fluoride solutions where F⁻ serves as a capping
agent).[¹⁹,²⁶]

Nevertheless, as for all titania used in photocatalytic H₂ pro-
duction, also these facetted crystallites show a sluggish electron
transfer from TiO₂ surfaces to the aqueous reactant.[³,⁴,⁸] Typi-
cally, this issue is addressed by decorating TiO₂ surfaces with
suitable co-catalysts.[²⁷⁻²⁹] Frequently, these co-catalysts are no-
ble metal nanoparticles (NPs) such as platinum (Pt) that pro-
mote the electron transfer reaction and additionally catalyze the
hydrogen evolution reactions, thus significantly enhance the H₂
production.[¹⁴,²⁹⁻³²]

Conventionally, Pt as a co-catalyst is deposited in the form of metallic NPs using a range of precipitation/reduction methods and a variety of Pt precursors.[¹⁹,³²,³³] More recently, significant research has been directed to the shrinkage of such NPs to increase utilization efficiency (and optimize reaction rates) when using costly Pt.[³⁴⁻³⁹] This has led to the application of increasingly dispersed Pt entities, down to few-atom clusters and ultimately to the use of Pt single atoms (SAs).[⁴⁰⁻⁴⁵] Some recent work on Pt SA decorated TiO₂ in photocatalysis has emphasized the fact that for titania layers under solar irradiation already a comparably low density of SA decoration (≈2 × 10⁵ SAs µm²) is adequate to reach a fully sufficient co-catalytic efficiency.[³⁴,⁴³] That is, on titania layers above such a threshold, not the SA surface density but photon harvesting and electron collection in the semiconductor become rate-determining.[³⁴,⁴⁶,⁴⁷]

In the present work, we describe how to even more drastically
optimize the utilization of Pt SAs; that is by selectively placing
the SAs on electron-exit-sites of a semiconductor. For this, we
deposit Pt SAs on the minor (101) facets of anatase crystallites
that otherwise consist of major (001) facets; i.e., Pt SAs are only
placed on locations where they are needed. We achieve this SA
placement by the gathering effect of an Ar-H₂ annealing treat-
ment that leads to the accumulation of Pt SAs on the (101) facets
of single crystal anatase. The resulting photocatalyst combines
the excellent electronic properties of anatase single crystal sheets
(single crystalline carrier harvesting) with their unique charge
separation feature and highly localized Pt SA (maximum effi-
ciency) co-catalysis. We show that by this strategy a minuscule
Pt SA loading of 0.005 wt.% localized on (101) facets can provide
a maximized photocatalytic H₂ evolution activity combined with
a most outstanding Pt utilization efficiency.

**2. Results and Discussion**

We first synthesized anatase nanosheets (NSs) by a classic hy-
drothermal method described in the Supporting Information.
**Figure 1a** shows a scanning electron microscope (SEM) image
of the resulting anatase NSs with roughly a 80:20% facet ratio
of large (001) to small (101) facets–this is well in line with the
literature.[²⁸,⁴³,⁴⁴,⁴⁸] After the hydrothermal synthesis, the facets
of the sheets are fluorine terminated and in the next step, we
used a fluorine removal treatment that consists of water soaking
(at 70 °C) followed by a thermal treatment (annealing at 450 °C
in air), as described in detail in the Supporting Information. No-

tably, direct air annealing without prior water washing leads to se-
vere sintering of NS, as shown in Figure S1 (Supporting Informa-
tion). The F-removal effect of water rinsing and thermal anneal-
ing is evident from X-ray photoelectron spectroscopy (XPS) mea-
surements. Water cleaning results in a partial reduction of resid-
ual fluorine on NS by ≈50% (Figure S2, Supporting Information).
After air annealing, the samples remain in a well-separated flake
powder (Figure 1b) and show no detectable fluorine peak any-
more in XPS (Figure S2, Supporting Information).

X-ray diffraction (XRD) patterns of the NSs (Figure 1g) reveal a principal peak at 2θ = 25.4°, corresponding to the anatase (101) plane (PDF#21-1272). Without the cleaning treatment, the NSs exhibit additional peaks at 2θ = 23.5° and 2θ = 33.4° in XRD that can be attributed to TiOF₂ species (PDF#08-0060), formed during synthesis in HF. Notably, the diffraction peak corresponding to TiOF₂ disappears upon washing at 70 °C in deionized water–indicating the dissolution of the TiOF₂ phase. Based on XRD and XPS, NSs subjected to water and air annealing treatments represent highly crystalline anatase without detectable fluorine species.

These fluorine-eliminated NSs were then used for reactive
deposition[³⁸,⁴³,⁴⁴,⁴⁶,⁴⁹,⁵⁰] of Pt single atoms (SAs) using H₂PtCl₆
solutions with various Pt concentrations ranging from 0.02 µM
to 2 mm. After Pt deposition, no changes in the SEM morphol-
ogy are apparent (see for example Figure 1c)—this is typical for
Pt SA reactive decorated TiO₂.[³⁴,³⁸,⁴⁴,⁵⁰] For such “as-deposited”
samples, we screened different annealing treatments and noticed
that annealing in Ar-H₂ can induce modifications of Pt arrange-
ment (Figure S3, Supporting Information). For high Pt loading
(from mm H₂PtCl₆ solutions), agglomeration of Pt nanoparticles
(NPs) can be observed even in SEM images. An example is shown
in Figure 1d, for a Pt SA loaded sample that was treated for 1 h
at 450 °C in Ar-H₂. For this sample one can see Pt agglomer-
ated to ≈1 nm-size clusters, importantly they are almost exclu-
sively attached on the (101) facets (see also Figures S3e and S4,
Supporting Information). For lower Pt loading (deposited from
5 µM H₂PtCl₆ or lower concentrations), SEM examination does
not reveal any discernible deposition features on the NS surface
following the thermal treatment at 450 °C in Ar-H₂ atmosphere
(Figures S3f and S5, Supporting Information).

For these samples, high-angle annular dark-field scanning
transmission electron microscopy (HAADF-STEM) images show
prior to the Ar-H₂ treatment well-dispersed Pt SAs on the entire
surface of the anatase NS (Figure 1e; Figure S6, Supporting Infor-
mation). Elemental mapping further confirms a uniform distri-
bution of Pt on the surface of the anatase sheet (Figure S7, Sup-
porting Information). The lattice spacing from the top surface
is 0.35 nm (Figure 1f), well in line with a (101) plane distance,
and 0.24 nm on the edge surface (inset 2 in Figure 1e), in line
with the (001) orientation of the ≈9 nm thick single crystals. Af-
ter Ar-H₂ annealing, HAADF-STEM images (Figure 1f; Figures
S8 and S9, Supporting Information) reveal that Pt SAs are not
observed any longer on the (001) facets; instead, Pt SAs (inset 2
and Figure S8, Supporting Information) and few-atom-clusters
(inset 1 and Figure S9, Supporting Information) are present on
the edges of the sheets, that is the (101) facets or defects of the
TiO₂ single crystal that expose (101) facets. At these sites, either
individual SAs are present or some few-atom-agglomerates–, i.e.,
very small aggregates in the order of 10–20 atoms (<<1 nm) are
<figure><img src="image_2.png" /><figcaption>**Figure 1.** SEM images of a) as-formed anatase NS, b) anatase NS removing F⁻ by water treated at 70 °C and then annealing at 450 °C in air, c) Pt SA-NS using 0.02 mm H₂PtCl₆ solution by reactive deposition, and d) Pt SA-NS (0.02 mm) annealed at 450 °C in Ar-H₂. Red dotted circles indicate Pt clusters that are attached to the (101) facets. HAADF-STEM images of e) Pt SA-NS using 0.2 µm H₂PtCl₆ solution by reactive deposition, and (f) Pt SA-NS (0.2 µm) annealed at 450 °C in Ar-H₂. Inset image in (e) is the side-view of TiO₂ NS. Yellow and red dotted circles indicate Pt SAs and few-atom-agglomerates, respectively. Inset images in (f) are the enlarged Pt SAs and few-atom-clusters situated at (101) facets. g) XRD patterns of as-formed NS and anatase NS removing F⁻. h) Time dependence of H₂ evolution for Pt SA-NS (not annealed) prepared using 0.2 µm H₂PtCl₆ solution and Pt SA-NS post Ar-H₂ annealed at 450 °C.</figcaption></figure>

present as shown in Figure 1f. Moreover, in HAADF-STEM images (see Figure S10, Supporting Information), Pt SAs are no longer detectable on the predominant (001) facets of the Ar-H₂-annealed Pt SA-NS (0.2 µm). These results suggest that during the Ar-H₂ treatment, Pt SAs diffuse or migrate over the surface and get trapped and accumulate on the (101) planes.

Most remarkable is the effect of the SA accumulation on the (101) facets of photocatalytic H₂ production activity. Figure 1h shows the photocatalytic H₂ production measured using illumination with a 365 nm LED (65 mW cm⁻²) for a sample with a uniform SA deposition on both facets (not annealed) and for the sample with accumulated Pt SAs on the (101) facet (Ar-H₂-annealed). Evidently, agglomeration leads to a remarkable 5 times improvement in H₂ evolution (in this example, deposition of Pt took place from a 0.2 µm H₂PtCl₆ solution). We then produced a series of samples where Pt was deposited from precursor solutions of different concentrations in the range of 0.02 µm-2 mm. The Pt loading on the anatase NSs was then analyzed

by atomic absorption spectroscopy (AAS) as shown in Figure 2a and Table S1 (Supporting Information) (for comparison, we also give Pt loading data from EDX in Figure S11, Supporting Information). Evidently, as the concentration of the Pt precursor increases, the loading of Pt steadily increases, until saturation occurs at a Pt loading of ≈1 wt.% for a Pt precursor concentration of ≈0.02 mm and loading stays saturated for higher Pt precursor concentrations.

If we evaluate the photocatalytic H₂ production for all samples (Figure 2b), then we note that for a precursor concentration of 0.02 µm (=20 nM!), no co-catalytic effect is detectable for a non-annealed sample, but a clear co-catalytic effect can be detected after Ar-H₂ annealing. Furthermore, the evolution of H₂ exhibits an almost linear correlation with time (see Figure S12, Supporting Information). Note that a plain treatment in Ar-H₂ (referred to 0 µm) leads to a H₂ production increase from 0.14 to 0.43 mL g⁻¹ h⁻¹ (in Figure 2b), which may be attributed to some beneficial effects of defects introduced by the Ar-H₂ treatment.[51] Please
<figure><img src="image_3.png" /><figcaption>**Figure 2.** a) Pt loading amount on Pt SA-NS using various concentrations (using 0.02 µM–2 mM) of H₂PtCl₆ solution by reactive deposition. b) Rates of H₂ evolution by Pt SA-NS not annealed and annealed at 450 °C in Ar–H₂. Reference sample without Pt decoration is indicated as “0 µM.” c) H₂ evolution in 4 h for Pt SA-NS not annealed and annealed at 450 °C in Ar–H₂ with various Pt loading amount. d) Improvement of H₂ evolution by Pt SA-NS annealed at 450 °C in Ar–H₂ with various Pt loading amount compared with photocatalysts not annealed. e) TOF of Pt SA-NS with various Pt loading amount.</figcaption></figure>

note that this effect is 73 times smaller than the effect of deposition from the 20 nM solution, followed by Ar–H₂ treatment.

With increasing concentrations of Pt precursor in the deposition solution and corresponding increase in Pt loading, the photocatalytic H₂ production rate increases, until it reaches a maximum at 5 µM (an effective Pt loading of 0.16 wt.%). For higher Pt loading the activity does not increase any further. This behavior can be ascribed to reaching a critical amount of Pt sites,[34] i.e., a sufficient Pt loading to lead to a saturated co-catalytic effect. Most remarkable is however the effect of the Ar–H₂ treatment on the H₂ production for low Pt loading (Figure 2b,c). For the non-Ar–H₂ treated samples, Pt loading at low concentrations

(in the range of 0.00026–0.014 wt.%) hardly affects the H₂ evolution performance—a significant increase occurs only for higher loading >0.03 wt.%, finally reaching a maximum at 0.16 wt.%. In stark contrast, the Ar–H₂ treated samples show a significant H₂ production already for loadings of 0.002 wt.%, i.e., at a 10 times lower total Pt loading than the non-treated samples. Particularly in the low Pt concentration range, the effect of Ar–H₂ treatment is strongly evident (see the highlighted area of Figure 2c); for concentrations of 0.005 to 0.04 wt.% an amplification of 5–10 times is achieved by the Ar–H₂ treatment. At high Pt concentrations >0.08 wt.% this beneficial effect is increasingly lost. For all samples, at very high Pt loading, the H₂
production efficiency drops slightly, possibly due to enhanced catalysis of the counter-reaction, i.e., recombination of ($H_2 + 0.5 O_2 \rightarrow H_2O$).[49,52] Figure 2d presents a comparative analysis of the improved $H_2$ evolution performance by post Ar–$H_2$ treatment at different Pt loadings, which shows a maximum beneficial effect for Pt loading of 0.005 wt.%. Please note that this Pt loading is 200–400 times lower than the typically used decoration of ≈1–2 wt.% Pt for decoration for photocatalytic $H_2$ generation but provides already a 50% maximum $H_2$ evolution activity, and at 0.03 wt.% the Ar–$H_2$ treatment is able to fully activate the samples.

Particularly in view of the very low loading concentrations used in this work, also interesting is the evaluation of the turnover frequency (TOF) for the deposited Pt species. The TOF for the different loading was calculated as described in *Supporting Information* and the results are given in Figure 2e. Evidently, the TOF increases with a lower loading up to ≈125 $\text{#H}_2$ site⁻¹ s⁻¹. Such a correlation of TOF with the number of active sites is in line with reported work.[53] Naturally, due to the equal influx of electrons to the surface in all cases, reducing the number of electron transfer sites simply leads to more intense utilization of each site, until the intrinsic TOF limitation for Pt co-catalyst is reached (TOF ≈300 $\text{#H}_2$ site⁻¹ s⁻¹).[34] If the TOF is evaluated using even higher light intensities (using a light saturation technique as described in the literature[34]), then TOFs of up to >250 $\text{#H}_2$ site⁻¹ s⁻¹ can be reached (Figure S13, Supporting Information). This is in the range of maximum TOFs obtained for Pt in electrochemistry.[53] Furthermore, Figure S14 (Supporting Information) presents a comparative analysis of the mass-normalized photocatalytic activity, specifically the $H_2$ production per mg of Pt loaded, as determined through AAS, for previously reported Pt SA decorated $TiO_2$ photocatalysts.[43,49] Notably, the selective loading of only 0.005 wt.% Pt onto the (101) facet of anatase crystallites demonstrates an exceptionally high performance in comparison to earlier methods presented in the literature (Figure S14, Supporting Information).[43,49]

To obtain information on the chemical state of the surface-loaded Pt after Ar–$H_2$ treatment, XPS measurements were conducted on NSs. **Figures 3a** and S15 (Supporting Information) depict the XPS spectra of the Pt 4f and Cl 2p regions for various samples. For the SA-decorated $TiO_2$ NSs prepared from 2 mM to 5 µM solution, without Ar–$H_2$ treatment, clear Pt 4f$_{7/2}$ and 4f$_{5/2}$ doublet peaks at 72.5 and 75.4 eV are observed (see Figure 3a), for lower concentrations the Pt peaks are below the detection limit of XPS. The peak position of the Pt doublet is well in line with literature values for Pt SAs coordinated with surface oxygen atoms from $TiO_2$ (Pt⁶⁺ with δ ≈2) (see Figure 3a; Figure S16, Supporting Information).[38,43,48,50] Note that minor contributions from Pt⁴⁺ peaks (74.6 and 78.0 eV) are observed at the highest precursor concentration (2 mM) with a corresponding Cl 2p peak (see Figure S15, Supporting Information) suggesting for this highest loaded sample the presence of some unreacted precursor species.[38,41] Nevertheless, any Cl contribution is absent for lower concentrations, indicating a completed reactive deposition reaction.[38,49]

After Ar–$H_2$ treatment, XPS data of the Pt 4f region (Figure 3b) show significant changes, fitting is shown in Figure 3c, Figures S17 and S18 (Supporting Information). Notably, the Ar–$H_2$ treatment causes a shift of the Pt main peak contribution for Pt

4f$_{7/2}$ from 72.5 to 70.8 eV, indicating conversion of Pt⁶⁺ species to Pt–Pt bonds (Figure 3c; Figures S17 and S18, Supporting Information).[44,46,48] For high concentrations this is in line with the formation of metallic NPs from SEM (see Figure 1d; Figures S3e and S4, Supporting Information). Nevertheless, it is noteworthy that treated Pt-NS samples still show a considerable contribution from the Pt⁶⁺, i.e., 19.4% at a loading of 0.16 wt.% Pt (see Figure S17, Supporting Information). Figure 3d shows a compilation of the contributions of various species, namely Pt⁶⁺ and Pt⁰, as a function of the Pt loading (after Ar–$H_2$ treatment). Note that the contribution of the SA signal to total Pt increases with a lower Pt loading, see Figure 3d.

This finding is consistent with the formation of metallic NPs for the samples with higher Pt loading (see Figure S19, Supporting Information). But it is important to note that few-atom clusters (evident from HAADF-STEM) yield also a XPS signature at ≈71 eV, i.e., these features represent the main contribution to the XPS signal at low surface densities (Figure S17, Supporting Information).[46]

We further conducted a diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) experiment with CO to investigate the state of Pt on the $TiO_2$ surface. The CO-DRIFTS spectra of Pt SA-NS following Ar–$H_2$ annealing (refer to Figure S20, Supporting Information) display distinct CO vibrational peaks at 2109 and 2123 cm⁻¹, characteristic to linearly adsorbed CO on Pt single-atom sites[49,54,55] (or Pt²⁺ sites on $TiO_2$).[56] Notably, over the course of 1 h under 1% CO/Ar gas flow (which led to a gradual increase in CO concentration), neither of these peaks showed any visible shift associated with dipole-dipole interactions between neighboring co-adsorbed CO molecules (e.g., such shift could be expected during CO adsorption on Ptₓ or PtₓOᵧ clusters, where x ≥ 2)[57] – this further confirms the isolated state of Pt SAs, which are coordinated solely with the $TiO_2$ surface. Besides the dominant peaks of CO adsorption over Pt SAs, a weak shoulder is also present at ≈2060 cm⁻¹, which corresponds to CO adsorption over metallic Pt,[56] consistent with the presence of few Pt clusters in the sample, as observed by HAADF-STEM (Figure S9, Supporting Information) and XPS (Figure 3c; Figures S17 and S18, Supporting Information). Notably, this shoulder is significantly weaker than the main Pt SA peaks, indicating a significantly lower surface area of Pt clusters available for CO adsorption (i.e., lower surface density), as compared to that of Pt SAs. Based on previous studies,[58–61] we suggest that variously coordinated Pt SAs, few-atom (Ptₙ) clusters, and Pt NPs present on the surface could produce a broad shoulder at 2050 and 2100 cm⁻¹. Furthermore, upon purging the chamber with a pulse of pure Ar (see Figure S21, Supporting Information), these peaks at 2109 and 2123 cm⁻¹ show nearly unchanged intensity along with increased sharpness, in contrast to the significantly weakened signal of metallic Pt at ≈2070 cm⁻¹.

To elucidate a possible effect of Ar–$H_2$ treatment on the electronic properties of the titania, we carried out electron paramagnetic resonance (EPR) spectroscopy. Figure 3e shows powder X-band EPR envelopes of plain NSs before (black line) and after the Ar–$H_2$ annealing (blue line). After fluorine removal, in the EPR spectrum of the NSs sample, we observe an axial resonant line with g⊥ = 1.985 and g‖ = 1.952 corresponding to $Ti^{3+}$ crystalline defects in the anatase phase.[62] After annealing the NSs in Ar–$H_2$, we do not observe any increase in EPR inten-
<figure><img src="image_4.png" /><figcaption>**Figure 3.** XPS spectra of Pt 4f for Pt SA-NS a) not annealed, b) annealed at 450 °C in Ar-H₂ using various concentrations (1 µM-2 mM) of H₂PtCl₆ solution by reactive deposition. c) Fitted XPS spectra of Pt 4f for Pt SA-NS not annealed and annealed at 450 °C in Ar-H₂ (using 2 mM of H₂PtCl₆ solution by reactive deposition). d) Relative content (from the total Pt content) of Pt⁰ and Pt⁶⁺ species for the Pt SA-NS annealed at 450 °C in Ar-H₂ with various Pt loading amount. e) X-band EPR spectra for NS (removing F⁻) and it annealed at 450 °C in Ar-H₂, f,g) SEM images of Pt NPs and PbO₂ NPs deposition on of (101) and (001) facets of anatase NS by illumination, respectively. Yellow dotted circles and red dotted irregular shape indicate Pt NPs that are attached to the (101) facets and PbO₂ NPs that are attached to the (001) facets, respectively.</figcaption></figure>

sity, indicating that the Ar-H₂ treatment does not yield a sig-
nificant amount of new defect sites, but only slight changes in
the line widths point toward the reorganization and migration
of already present defects from the subsurface to the surface of
NSs. As part of the Ti³⁺ defects reach the surface, a new res-
onant line with a noticeably larger linewidth at g<sub>avg</sub> ≈ 1.95 is

observed, which is typically assigned to surface-localized Ti³⁺
defects.[63-65]

Furthermore, to confirm the role of the two facets as electron
exit [(101) plane] and hole exit sites [(001) plane] under our pre-
treatments, we conducted Pt and PbO₂ photodeposition.[19,26] (To
verify this on our NSs is also very important, as the energetics of
<figure><img src="image_5.png" /><figcaption>**Figure 4.** SEM images of a) Pt SA-NS using 0.02 mM H₂PtCl₆ solution by reactive deposition, and b) Pt SA-NS (0.02 mM) annealed at 450 °C in Ar-H₂. c,d) corresponding SEM images for (a,b) after 4 h of illumination. e) Schematic illustration of Pt SA moves to (101) facets of TiO₂ NS by Ar-H₂ annealing for improving photocatalytic H₂ evolution.</figcaption></figure>

F-terminated and F-free NS surfaces greatly differ.) Figure 3f,g only show preferential Pt and PbO₂ deposition on the (101) and (001) facets, respectively. In other words, the charge carrier separation effects by the facets are operative also in the NS used in this work.

An important aspect in terms of the photocatalyst application is the stability of the Pt configuration after they have been concentrated on the (101) facets. Remarkably, the resultant SA configuration exhibits notably enhanced stability against photoinduced agglomeration effects compared with the non-Ar-H₂ annealed samples, which is evident from the significantly decreased density of Pt NPs after photocatalytic H₂ evolution reaction. Specifically, only minimal agglomeration was observed after 4 h of H₂ evolution (see Figure 4d, Pt NPs density is 849 number per µm²), in comparison with a sample without subsequent Ar-H₂ annealing (depicted in Figure 4c, Pt NPs density is 2836 number per µm²). Additionally, remarkable long-term photocatalytic H₂ evolution stability is observed for Ar-H₂ annealed sample (see Figure S22, Supporting Information). The results show 86% retention of the stability of the H₂ evolution rate over the entire test period of 168 h, while non-Ar-H₂ annealed sample lead to a significant loss of activity over time. It should be also pointed out that the present finding

of a preferred anchoring of SAs or few-atom-clusters on (101) facets and its stabilization on this facet is very well in line with theoretical work that investigated the stability of Pt on different anatase facets by density functional theory (DFT).[66] The work clearly concludes that anatase (101) facets provide a superior stability for highly dispersed Pt, in comparison to anatase (001) facets. The accumulation of Pt SAs on the (101) facets during Ar-H₂ annealing is primarily due to the tendency of the system to minimize surface energy.[24,66] Thermal activation mobilizes Pt SAs and the (101) facets are a more anchoring Pt host under the reducing conditions provided by the Ar-H₂ atmosphere.[66] Additionally, the hydrogen in the atmosphere further facilitates Pt migration by destabilization of the SA Pt species.[44]

Overall, Figure 4e illustrates the findings of this work. Highly dispersed Pt species can be concentrated on the minority facet of anatase crystallites by a thermal Ar-H₂ treatment. These facets represent the electron exit sites of the crystal due to the inherent heterojunction between (101) and (001) facets.[18–20,26] In order to be an effective photocatalyst for H₂ generation, a minimum (critical) Pt loading (or loading density) is required.[34] This critical density however has only to be present at the electron exit sites of a photon harvester structure–thus allowing
for a strong increase in Pt utilization on the photo absorber structure.

**3. Conclusion**

The present work deals with very small amounts of Pt (0.0003–0.03 wt.%) placed selectively on minority (101) crystal planes of anatase nanocrystallites. We describe that these minuscule amounts of Pt SA decorated on anatase crystals with dominant (001) and minority (101) planes after annealing in an appropriate Ar–H₂ atmosphere show a strongly enhanced photocatalytic H₂ evolution efficiency–in fact, a maximum photocatalytic activity can be reached with a Pt SA loading of 0.03 wt. %. The effect of the Ar–H₂ treatment is that it leads to a concentration of Pt SAs selectively on the low surface energy of (101) facets. Due to the internal junction present in these 2-facet crystallites, the (101) facets represent the exit sites for photogenerated electrons. Due to facet-selective Pt deposition, the co-catalytic effect of Pt SAs is focused where it is needed–, i.e., at the electron exit sites. In a wider context, the work illustrates the importance of such an optimization in terms of photon and charge carrier management not only to optimize the performance of a photocatalyst but also its economy. In our example, the combination of the charge separation effect and Pt SA placements provides a so far unprecedented Pt utilization efficiency in photocatalytic hydrogen evolution.

**Supporting Information**

Supporting Information is available from the Wiley Online Library or from the author.

**Acknowledgements**

The authors would like to acknowledge DFG and the Operational Program Research, Development and Education (European Regional Development Fund, Project No. CZ.02.1.01/0.0/0.0/15_003/0000416 of the Ministry of Education, Youth and Sports of the Czech Republic) for financial support. P.S. would particularly like to acknowledge the support of the Czech Science Foundation projects GA CR-EXPRO, 23–08019X. Z.B. acknowledge the support by the European Union under the REFRESH – Research Excellence For Region Sustainability and High-tech Industries project number, CZ.10.03.01/00/22_003/0000048 via the Operational Programme Just Transition. The authors would like to acknowledge the support of the Center for Nanoanalysis and Electron Microscopy (CENEM, Friedrich-Alexander-Universität Erlangen-Nürnberg).

Open access funding enabled and organized by Projekt DEAL.

**Conflict of Interest**

The authors declare no conflict of interest.

**Data Availability Statement**

The data that support the findings of this study are available from the corresponding author upon reasonable request.

**Keywords**

hydrogen evolution, photocatalysis, platinum, single-atom catalysts, TiO₂ facets

Received: November 29, 2024

Revised: December 29, 2024

Published online: January 31, 2025

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