# Single-Atom Catalysts on C 3 N 4: Minimizing Single Atom Pt Loading for Maximized Photocatalytic Hydrogen Production Efficiency

Nawres Lazaar, Siming Wu, Shanshan Qin, Abdessalem Hamrouni, Bidyut Bikash Sarma, Dimitry E Doronkin, Nikita Denisov, Hinda Lachheb, Patrik P Schmuki

► To cite this version:

Nawres Lazaar, Siming Wu, Shanshan Qin, Abdessalem Hamrouni, Bidyut Bikash Sarma, et al. Single-Atom Catalysts on C 3 N 4: Minimizing Single Atom Pt Loading for Maximized Photocatalytic Hydrogen Production Efficiency. Angewandte Chemie International Edition, 2025, 64 (6), pp.e202416453. 10.1002/anie.202416453. hal-04927966

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# Single-Atom Catalysts on C<sub>3</sub>N<sub>4</sub>: Minimizing Single Atom Pt Loading for Maximized Photocatalytic Hydrogen Production Efficiency

Nawres Lazaar<sup>+</sup>, Siming Wu<sup>+</sup>, Shanshan Qin, Abdessalem Hamrouni, Bidyut Bikash Sarma, Dimitry E. Doronkin, Nikita Denisov, Hinda Lachheb, and Patrik Schmuki*

**Abstract:** The use of metal single atoms (SAs) as co-catalysts on semiconductors has emerged as a promising technology to enhance their photocatalytic hydrogen production performance. In this study, we describe the deposition of very low amounts of Pt SAs (< 0.1 at %) on exfoliated graphitic carbon nitride (C<sub>3</sub>N<sub>4</sub>) by a direct Pt-deposition approach from highly dilute chloroplatinic acid precursors. We find that — using this technique—a remarkably low loading of highly dispersed Pt SAs (0.03 wt %) on C<sub>3</sub>N<sub>4</sub> is sufficient to achieve a drastic decrease in the overall charge transfer resistance and a maximized photocatalytic efficiency. The resulting low-loaded Pt SAs/C<sub>3</sub>N<sub>4</sub> provides a H<sub>2</sub> production rate of 1.66 mmol/h/mg Pt, with a remarkable stability against agglomeration; even during prolonged photocatalytic reactions no sign of light-induced Pt agglomerations can be observed. We ascribe the high performance and stability to the site-selective, stable coordination of Pt within the C<sub>3</sub>N<sub>4</sub> structure. Notably the H<sub>2</sub> production rate of the low-loaded Pt SAs surpasses the activity of Pt SAs deposited by other techniques or nanoparticles at comparable or even higher loading — the optimized Pt SAs decorated C<sub>3</sub>N<sub>4</sub> show ≈5.9 times higher rate than Pt NP decorated C<sub>3</sub>N<sub>4</sub>.

## 1. Introduction

Photocatalytic water splitting using semiconductor materials has gained significant attention as a promising approach for sustainable hydrogen production.[1,2,3,4] Most of the pioneering work has been performed on titanium dioxide (TiO<sub>2</sub>) due to its excellent stability, abundance, and favorable band structure for the water splitting reactions.[5,6] Meanwhile extensive exploration of a wide array of other semiconductors has taken place, and in recent years, particularly graphitic carbon nitride (C<sub>3</sub>N<sub>4</sub>) has attracted high attention for similar beneficial features as TiO<sub>2</sub> but with the crucial advantage of a band-gap in the visible light range. The latter allows for a much higher absorption of solar light and thus conceptually a much higher hydrogen production efficiency.[7,8,9,10] Nonetheless, the actual hydrogen production efficiency of many semiconductors including TiO<sub>2</sub> or C<sub>3</sub>N<sub>4</sub> falls short of expectations, mainly due to a slow charge transfer and reaction kinetics at the semiconductor-solution interface.[11,12,13]

This issue can be addressed by application of suitable charge-transfer co-catalysts. Particularly platinum (Pt), decorated or incorporated as nanoparticles, nanoclusters, or single atoms (SAs), has been extensively explored on many semiconductors. In the case of C<sub>3</sub>N<sub>4</sub>, the use of Pt nanoparticles anchored to the surface represents a most conventional approach to reach reasonable hydrogen evolution efficiencies.[14,15] However, the reliance on substantial amounts of this expensive noble metal (typically 2–5 wt %) has been a major barrier to practical deployment due to high costs and the inefficient use of Pt atoms.[16,17,18,19] This is a main driver for the use of nanoclusters and eventually SAs in photocatalysis[20,21]—SAs can provide a maximized atom utilization efficiency at a maximized catalytic efficiency.

[*] N. Lazaar,\* S. Wu,\* S. Qin, N. Denisov, Prof. P. Schmuki
Department of Materials Science WW4-LKO,
Friedrich-Alexander-University of Erlangen-Nuremberg,
Martensstrasse 7, 91058 Erlangen, Germany
E-mail: schmuki@ww.uni-erlangen.de

N. Lazaar,\* A. Hamrouni, H. Lachheb
Research Laboratory of Catalysis and Materials for the Environment and Processes LRCMEP (LR19ES08),
University of Gabès, Faculty of Sciences of Gabès (FSG),
University Campus Erriadh City, 6072, Gabès, Tunisia

A. Hamrouni
Laboratoire des Substances Naturelles,
Institut National de Recherche et d'Analyse Physico-chimique,
INRAP,
Pôle Technologique de Sidi Thabet, 2020, Tunisia

B. Bikash Sarma
Laboratoire de Chimie de Coordination (LCC), CNRS, Université de
Toulouse, INPT, UPR 8241,
205 route de Narbonne, 31077 Toulouse Cedex 4, France

D. E. Doronkin
Institute of Catalysis Research and Technology,
KIT,
Hermann-von Helmholtz Platz 1, 76344 Eggenstein-Leopoldshafen,
Germany

Prof. P. Schmuki
Regional Centre of Advanced Technologies and Materials,
Šlechtitelů 27, 78371 Olomouc, Czech Republic

[+] Equal Contribution

<figure><img src="image_2.png" /></figure>

© 2024 The Author(s). Angewandte Chemie International Edition 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.
In the case of C₃N₄, there is already a large body of synthesis approaches for the production of Pt nanoparticles and Pt SA-loaded materials. In situ approaches as well as post-synthesis techniques have been reported, namely using photodeposition or atomic layer deposition, that are often followed by calcination steps.[20,22,23,24,25,26] Such SA-loaded C₃N₄ photocatalysts provide an efficient enhancement of photocatalytic H₂ production, and remarkable light-to-H₂ conversion efficiencies have been achieved. Nevertheless, these approaches operate with a comparably high loading of Pt (often due to the fact that the deposition techniques lead to a mixed loading of SAs and nanoparticles). While a number of experiments and theoretical approaches point to a most effective metal loading at N4 coordination sites,[23,27,28,29] most deposition techniques provide a (someWHAT) random loading of Pt on C₃N₄ surfaces. It may be speculated that a significant amount of the Pt deposited is present in a non-active or non-ideal surface configuration on the C₃N₄ surface.[30] Therefore, it would be highly desirable to find an approach that leads to Pt SAs deposited at most active sites which then would minimize the Pt loading while maximizing the efficiency. This would optimize the photocatalytic reaction, not only in terms of H₂ production but would also yield a maximized Pt utilization.

In this context, it is noteworthy that recently it was shown on defective TiO₂ surfaces that a simple so-called “reactive” deposition approach from very dilute Pt-precursor solutions can lead to well anchored SAs with a remarkably high efficiency at a remarkably low Pt loading.[31,32,33,34,35] It has been postulated that the approach is self-guiding[36] i.e. dilute Pt-acid precursors react at most reactive sites which then leads “self-guided” to Pt SAs loaded (coordinated) in a most active surface configuration.[36] In the present work, we explore the feasibility to apply the very same principle to C₃N₄, i.e. to harvest from aqueous solutions minute amounts of Pt in the form of single atoms by a direct reaction of C₃N₄ with chloroplatinic acid. After reaction, we explore the functionality of the deposited Pt SAs as a co-catalyst in photocatalytic H₂ generation and we evaluate the conditions for a maximum H₂ production efficiency. We show that this approach of loading Pt SAs on exfoliated g-C₃N₄ (Pt SAs/C₃N₄) can provide a C₃N₄ photocatalyst that with a minimum of Pt loading of 0.03 wt % can yield remarkably high H₂ production efficiencies as well as Pt utilization efficiencies. If the activity is normalized to the Pt-loading, unprecedented H₂ production efficiencies (1.66 mmol/h/mg Pt) can be reached. Furthermore, the Pt SAs loaded by this approach show a very high stability of the Pt SAs – even after 24 h of reaction not any change in coordination or agglomeration of the Pt SAs could be observed. The high activity and stability of Pt SAs/C₃N₄ are attributed to the robust coordination of Pt SAs within the C₃N₄ structure, representing the most stable configuration and the most effective catalytic site. The findings are not only of a high scientific interest in terms of identifying ideal pairs of semiconductors and co-catalysts, and optimizing anchoring of SAs on substrates, but also in terms of the most cost-effective use of precious metals in photocatalysis.

## 2. Results and Discussion

We first prepare delaminated g-C₃N₄ powder according to well-established literature procedures by synthesis from an equimolar mixture of melamine and dicyandiamide followed by thermal delamination at 500 °C for 2 hours.[37,38,39] The decoration of Pt SAs on C₃N₄ is further carried out by a “spontaneous deposition” approach – that is, reaction of the C₃N₄ surface with an aqueous H₂PtCl₆ solutions in the concentration range of 10 mM-0.05 mM (more details are given in the experimental section).[31] Figure 1a shows an SEM image of the delaminated g-C₃N₄ and Figure 1b shows an SEM image of the delaminated g-C₃N₄ after Pt deposition– in this case loading was carried out in a 2 mM H₂PtCl₆ solution. The delaminated C₃N₄ and the Pt SA loaded C₃N₄ (Pt SAs/C₃N₄) are present as a sheet-like structure with a layer thickness of 18 nm (Figure 1a,b). No obvious difference of the morphology is observed after Pt deposition compared with the bare C₃N₄ sample. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images of Pt SAs/C₃N₄ with different magnifications are presented in Figure 1c,d. No Pt agglomeration nor nanoparticles can be observed in Figure 1c. Figure 1d, taken at a higher magnification, further highlights Pt atoms are evidently present as well-separated individual SAs, which are marked by yellow circles. EDX mapping confirms the uniform dispersion of Pt SAs on the C₃N₄ structure (Figure S1). The density of Pt SAs is approximately 9.6×10⁵ µm⁻², as estimated from HAADF-STEM images such as in Figure S2.

The XRD patterns of C₃N₄ and Pt SAs/C₃N₄ is shown in Figure 1e. Both samples (before and after Pt loading) show the characteristic peaks of g-C₃N₄, consistent with previous research works.[40] Specifically, two distinct peaks are visible at 2θ values around 12.9° and 27.7° that are attributed to the (100) repeating motif of tri-s-triazine and the (002) interlayer stacking of the conjugated aromatic system, respectively. Noteworthy, no diffraction peaks related to metallic Pt can be identified for the Pt SAs/C₃N₄ sample.

We further carried out X-rayabsorption spectroscopy (XAS)[41] in terms of X-ray absorption near-edge spectra (XANES) and X-ray absorption fine structure (EXAFS) and Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS)[42] experiments to investigate the oxidation state of Pt and its relevant coordination environment. The normalized XANES spectra measured at the PtL₃-edge (11.564 keV) suggest that Pt SAs/C₃N₄ sample has positively charged Pt centers (Figure 1i).[22] The extended EXAFS analysis and the corresponding Fourier transformed (FT) radial distribution function show a predominant feature at 1.6 Å (without phase correction) which originates from a light atom such as N in the Pt–N coordination (Figure 1j).[22,29] The CO-DRIFTS spectra of Pt SAs/C₃N₄ (Figure S3) show obvious CO vibrational frequency at 2120 cm⁻¹, which is characteristic of linearly bonded CO over Pt single sites.[43,44] These results confirm that Pt atoms are well-isolated within the C₃N₄ structure, with minimal agglomeration or clustering, attributed to the Pt–N coordination.
<figure><img src="image_3.png" /><figcaption>**Figure 1.** (a) SEM image of bare C₃N₄, (b) SEM image of Pt SAs/C₃N₄, (c), (d) HAADF STEM images of Pt SAs/C₃N₄, (e) XRD and XPS spectra of (f) C 1s, (g) N 1s of C₃N₄ and Pt SAs/C₃N₄, (h) Pt 4f XPS spectrum, (i) Normalized XANES spectra at the Pt L₃ edge and (j) Fourier transform EXAFS spectra (R-space) of Pt SAs/C₃N₄.</figcaption></figure>
X-ray photoelectron spectroscopy (XPS) was employed to obtain compositional information of the C₃N₄ and Pt SAs/ C₃N₄ samples (Figure 1f-h, Figure S4). As shown in Figure 1f, the C 1s spectrum of both samples can be fitted into three peaks at 288.1 eV, 286.7 eV and 284.7 eV, which are assigned to N=C-N, C–N and C–C, respectively.[22] Furthermore, the fitted N 1s spectrum contains two peaks at 398.9 and 401.1 eV, corresponding to C=N–C, and C–N–H, respectively.[22] No obvious difference is observed in the C 1s and N 1s spectrum after Pt SAs deposition. Figure 1h shows the XPS spectra of the Pt 4f region for the Pt SAs/C₃N₄ sample. The peak positions for Pt 4f<sub>7/2</sub> at ≈72.6 eV and Pt 4f<sub>5/2</sub> at ≈76 eV correspond well to a Pt oxidation state of δ⁺ (δ≈2). I.e. the Pt<sup>4+</sup> precursor, [Pt(Cl)<sub>6</sub>]<sup>2-</sup>, has been reduced to Pt<sup>δ+</sup> during the process.[45] The fact that no Cl is detected, as shown in the Cl 2p XPS in Figure S4, furthermore indicates that the Cl<sup>-</sup> coordination is completely lost during the surface reaction of Pt and its coordination to C₃N₄. These reaction characterisitics are well in line with the introduction of chloroplatinic acid and attachment of Pt SAs on TiO₂. I.e. a successful reductive coupling reaction of the Pt species with the C₃N₄ has taken place; i.e. Pt is not simply adsorbed on the surface, but the [Pt(Cl)<sub>6</sub>]<sup>2-</sup> has reacted with suitable surface sites (reduction

of Pt⁴⁺ to Pt⁵⁺ and loss of Cl-coordination).[46] Further evaluation of the relative concentrations from XPS shows 0.07 at% Pt for the deposition from the 2 mM H₂PtCl₆ precursor (Table S1). For comparison also, Pt nanoparticles were deposited following a classic photodeposition approach from literature.[43] For this sample SEM images presented in Figure S5b show distinct Pt nanoparticles which are clearly visible on the C₃N₄ surface, and from the corresponding XPS Pt 4f region (Figure S5a), we find Pt at a concentration of 0.14 at%. The position of the Pt 4f peaks at 70.6 eV and 73.9 eV are in good agreement with literature data for metallic Pt[0,43].

Figure 2a shows the Pt 4f region for C₃N₄ loaded with different concentrations of Pt precursor (H₂PtCl₆), ranging from 10 mM to 0.05 mM, used for SA deposition. Under all deposition conditions, no peaks of metallic Pt are observed. Instead, Pt SAs (Pt⁶⁺) are successfully decorated on all samples with Pt 4f peaks at 72.6 eV and 76 eV. Only at the highest concentration of 10 mM is there some concentration of Pt⁴⁺, evident from the Pt 4f peaks at 75 eV and 78.5 eV, which indicates presence of some adsorbed non-reacted precursor. With increasing Pt precursor concentration an increase in Pt loading is observed as summarized in Table S1. Notably, even at the highest Pt precursor concen-

<figure><img src="image_4.png" /><figcaption>**Figure 2.** (a) Pt 4f XPS spectra of Pt SAs/C₃N₄ prepared with different concentrations of Pt precursor, (b) Photocatalytic H₂ evolution of Pt SAs/C₃N₄ prepared with different concentrations of Pt precursor and Pt NPs decorated C₃N₄, (c) Evolved H₂ at different Pt SAs loading, and (d) Normalized H₂ evolution rates for the different samples.</figcaption></figure>
tration of 10 mM, which results in a loading of 0.23 at%, no Pt nanoparticles are observed on C₃N₄, as evident from the SEM image in Figure S6.

To additionally determine the bulk loading of Pt we used by atomic absorption spectroscopy (AAS) and the results are shown in Figure S7 and Table S2. An increasing Pt loading is observed from 0.05 mM to 2 mM, which is consistent with the XPS data. At higher precursor concentration (10 mM), the deposited amount does not further increase, indicating that saturation of reactive Pt SA loading on C₃N₄ is reached at a Pt loading of 0.03 wt %. This finding is in line with previous work on TiO₂, where also Pt SA loading reaches a plateau at a distinct loading concentration corresponding to a saturation of reactive anchoring points.[³³]

We then studied the photocatalytic H₂ generation of the C₃N₄ samples (using a LED 365 nm 65 mW/cm², as a light source, and 10 vol % triethanolamine as a sacrificial agent; further experimental details are described in the Experimental section). Results for all samples are shown in Figure 2b. Within the concentration range of 0.05 mM to 10 mM, the loading increases from 0.003 wt % to 0.03 wt % and the photocatalytic H₂ production activity is dependent on the Pt SA loading. Evidently a maximum H₂ production efficiency is already reached for sample synthesized with 2 mM H₂PtCl₆, which corresponds to very low loading amount of 0.03 wt % (from AAS data in Table S2). For those samples, the loading of Pt SAs is sufficient to provide a fully effective co-catalytic effect. In Figure 2c the photocatalytic activity is plotted vs the Pt SA loading to better assess the optimal amount of Pt SAs needed as cocatalyst on C₃N₄ for maximum co-catalytic effect. Initially, an increase in H₂ production is observed with increasing Pt loading (using Pt precursor from 0.05 mM to 2 mM), followed by a plateau where further increases in Pt loading do not significantly enhance the activity. Thus, a very low loading of ~0.03 wt % is sufficient to achieve the highest H₂ production amount. Importantly, this low loading for maximized efficiency is specific to the reactive deposition method we developed. Other methods either achieve similar Pt SA loadings with significantly lower activity or require much higher loadings to achieve comparable activity, which will be discussed later. Evidently also the Pt SAs outperform Pt NPs by photo-deposition, which have a much higher loading (0.15 wt %, Table S2). This observation clearly emphasizes the highly effective beneficial impact of the Pt SAs deposited by the present approach.

To further evaluate and compare the efficiency of Pt as a cocatalyst in the forms of SAs and NPs, we normalized the data in Figure 2b with the Pt loading (Table S2), and the results are shown in Figure 2d. Evidently, a maximum mass-specific photocatalytic efficiency, yielding an H₂ production rate of 1.66 mmol/h/mg Pt, is achieved with 2 mM Pt (0.03 wt %). This value is 5.9 times higher than that obtained by the classic photodeposition of Pt NPs on C₃N₄ (and significantly higher than any results obtained for common literature procedures, as we will show later). The turnover frequency (TOF) of Pt SAs decorated C₃N₄ (0.03 wt % Pt loading) reaches 324 h⁻¹. I.e. the Pt species deposited on

C₃N₄ by this simple dark deposition approach show a remarkable activity (and this without any further thermal treatments).

Particularly remarkable is the robustness of the Pt⁶⁺ state attained by reactive deposition. For many photocatalytic systems, illumination of surface coordinated Pt SA (e.g. oxygen coordinated Pt⁶⁺ on TiO₂) leads to rapid photoinduced reduction to Pt⁰ and agglomeration as metallic Pt nanoparticles.[²²] In our case Pt remains as SAs with δ≈2 after illumination (Figure 3a). This aligns with observations from SEM images (Figure 3b) where after illumination on the active surface no Pt nanoparticles can be observed. Notably, the Pt²⁺ state persists even after prolonged irradiation of 24 hours, with a slight decrease in activity possibly due to the loss of loosely bound Pt species and the depletion of the sacrificial agent. (Figure S8). This excellent stability of Pt SAs can be explained by a robust coordination of the Pt atoms within the C₃N₄ structure. In literature,[²³,⁴⁸] namely sites that provide an N₄ coordination in triazines can lead to very stable Pt (II) species due to chelation effects, aromatic stabilization and favorable electronic properties. For example, Xiong et al. investigated the coordination between Pt and N using XANES and XPS and found that g-C₃N₄ can form coordination bonds with Pt²⁺ through different N sites.[²⁹] Particularly the Pt²⁺-N₄ coordination state was identified as most stable and active, and as such advantageous for water splitting.[⁴⁹] Additionally, Zhang et al. proved the electron injection from Pt atom to N atom in Pt SAs decorated C₃N₄ upon light irradiation by in situ XPS.[²²] This may suggest that the high activity and stability of Pt SAs/C₃N₄ is due to dynamic evolution of Pt SAs within the C₃N₄ structure, and promote the efficiency of electron transfer to Pt SA. To elucidate the effect of Pt SA loading on the electrochemical nature of the C₃N₄ photocatalyst – namely the effects on the charge transfer characteristics, we additionally performed electrochemical impedance spectroscopy (EIS) measurements in 0.1 M Na₂SO₄. Figure 3c shows the Nyquist plots for electrode formed from bare C₃N₄, and C₃N₄ deorated with different amount of Pt SAs as well as for C₃N₄ decorated with Pt NPs (zoomed-in spectra in Figure S9). A classic Randles equivalent circuit was employed for fitting the Nyquist plots (Figure 3c, inset). Already a semiquantitative assessment shows the radius of the fitted curve for Pt SAs/C₃N₄ to be significantly smaller compared to bare C₃N₄, indicating strong drop in charge transfer resistance upon Pt SA loading. Quantitative fitting data are summarized in Table S4, confirming a 70 times drop in Rₑt. In Figure 3d, the Rₑt values are plotted against the Pt SA loading. Remarkably, even a very low loading of Pt SAs (0.02 wt %) can drastically improve the charge transfer from C₃N₄ to the electrolyte. This is remarkably well in line with the low loading to reach maximum efficiency in photocatalytic H₂ production. In contrast, for Pt nanoparticles (Pt NPs) a seven times higher loading of 0.15 wt % is needed to achieve a similar reduction in charge transfer resistance. These results indicate the high efficiency of minor amounts of Pt SAs in strongly mediating the charge transfer resistance of C₃N₄.
<figure><img src="image_5.png" /><figcaption>**Figure 3.** (a) Pt 4f XPS spectrum and (b) SEM image of Pt SAs/C₃N₄ after illumination, (c) EIS plots of C₃N₄, Pt SAs/C₃N₄ and Pt NPs/C₃N₄ at the voltage -0.5 V (vs.Ag/AgCl) in 0.1 M Na₂SO₄ aqueous electrolyte. The equivalent circuit model used for fitting is depicted in the inset of Figure 3c and (d) Rct vs. Pt loading plot of Pt SAs/C₃N₄ samples.</figcaption></figure>

Incident photon-to-current conversion efficiency (IPCE) measurement was also carried out to evaluate the photo-electrochemical properties of Pt SAs/C₃N₄. As shown in Figure S10, obvious photocurrent is observed for C₃N₄ and Pt SAs/C₃N₄ in the wavelength range of 300–450 nm, and the determined band gap is ≈2.7 eV (Figure S10, inset), which is typical for C₃N₄. Pt SAs/C₃N₄ show similar IPCE value with bare C₃N₄, indicating that the light absorption characteristic is hardly affected by the deposition of Pt SAs due to the low loading of Pt.We find well in line with literature that the layers show an n-type semiconductor behavior (Figure S11) with an increase in the photocurrent at potentials close to the flat-band potential, reflecting the strong beneficial effect of Pt SAs on the charge transfer also under illumination conditions.

The extraordinary performance of the dark deposition loaded Pt SAs on the exfoliated C₃N₄ structure is particularly apparent, when this structure is compared to other Pt-loaded C₃N₄ structures that were previously used and reported for photocatalytic H₂ generation. In order to exclude the effect of different reactor geometries, light sources, etc, the comparison of Pt SAs decorated C₃N₄ samples were tested under the same conditions as our C₃N₄ samples. A comparison of the photocatalytic H₂ evolution results of all samples are shown in Figure 4a. Not

only do Pt SAs decorated on C₃N₄ by our reactive deposition approach achieve the highest photocatalytic H₂ production activity but even more they achieve this with a remarkably low loading of Pt SAs (Table S3). An overall comparison of Pt loading and H₂ production rates is summarized in Figure 4b. In some studies with similar amount of Pt SAs loaded,[22,24,25] the H₂ production rates are much lower, indicating the higher efficiency of our Pt SAs or a higher efficiency of our Pt SAs with equally low amounts. Conversely, other studies achieve activity levels similar to ours, but require more than ten times the Pt SA loading.[23] I.e. our reactive deposition approach, achieves unprecedented efficiency in the use of Pt SAs on C₃N₄, demonstrating that a very low loading is sufficient to maximize photocatalytic H₂ production efficiency of such C₃N₄ based photocatalysts. This high efficiency of Pt SAs can be attributed to the nature of the deposition method: The reaction of the Pt-precursor occurs at suitable reactive sites of C₃N₄ – these Pt decorated sites then are most efficient for charge transfer and catalysis. In this sense the process is self-guiding: Pt is decorated where it is needed and where it is effective. Based on literature consideration some may consider Pt-N₄ configurations as this active site as they represent a most stable and effective catalytic coordination for Pt SAs within C₃N₄ structures.[23,27,28,29] These sites
<figure><img src="image_6.png" /><figcaption>**Figure 4.** Comparison of (a) Normalized H₂ evolution rates and (b) Comparison of Pt loading and H₂ production rates of Pt NPs decorated C₃N₄ and Pt SAs decorated C₃N₄ prepared with different procedures.</figcaption></figure>

become saturated at low Pt concentrations, indicating a comparably low site density. Nevertheless, this low amount of Pt SAs is sufficiently effective in reaching an optimal photocatalytic efficiency, thus eliminating the need for additional Pt.

### 3. Conclusions

In this study, we report a successful decoration of Pt SAs on exfoliated C₃N₄ through a reactive deposition approach, resulting in uniformly dispersed and highly stable Pt SAs. Remarkably, a very low loading of 0.03 wt% Pt SAs can lead to a maximized photocatalytic H₂ production efficiency. A maximized H₂ production rate of 1.66 mmol/h/mg Pt is achieved with Pt SAs/C₃N₄, far surpassing that of other SA deposition approaches on C₃N₄, as well as Pt nanoparticles decorated C₃N₄. Moreover, Pt SA deposited by our reactive deposition approach show superior stability in the photocatalytic H₂ production process – no light-induced SA agglomeration could be found even after prolonged illumination. Key effects of Pt SA loading is a strongly enhanced electron transfer even at very small loading by Pt SAs, together with their coordination within the C₃N₄ structure. Overall, the results show that reactive deposition, involving a self-guided anchoring of Pt SAs at reactive C₃N₄ sites provides exceptionally active and stable co-catalytic sites in C₃N₄ based photocatalysis.

### 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), the GA CR-EXPRO project (Grant No. 23-08019X) from the Czech Science Foundation for financial support. We

acknowledge DESY (Hamburg, Germany), a member of the Helmholtz Association HGF, for the provision of experimental facilities. Parts of this research were carried out at the light source PETRA III at DESY, a member of the Helmholtz Association (HGF). We would like to thank Dr. Edmund Welter for his assistance in using the beamline P65. Beamtime was allocated for proposal II-20230692. 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:** dark deposition · Pt single atoms · C₃N₄ · H₂ evolution · photocatalysis

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Manuscript received: August 27, 2024

Revised manuscript received: November 18, 2024

Accepted manuscript online: December 13, 2024

Version of record online: ■■, ■■
Communication

Photocatalysis

N. Lazaar, S. Wu, S. Qin, A. Hamrouni,
B. Bikash Sarma, D. E. Doronkin,
N. Denisov, H. Lachheb,
P. Schmuki* e202416453

Single-Atom Catalysts on C₃N₄: Minimizing
Single Atom Pt Loading for Maximized
Photocatalytic Hydrogen Production Effi-
ciency

<figure><img src="image_7.png" /></figure>

Pt single atoms are successfully anch-
ored onto C3N4 through a reactive
deposition approach, achieving optimal
positioning of Pt atoms in the most
stable and active configurations. This
strategy results in maximized photocata-
lytic H2 production efficiency at a low Pt
loading of approximately 0.03 wt%.