A high-throughput, solvent free method for dispersing metal atoms directly onto supports †

Atomically-dispersed metal catalysts (ADMCs) on surfaces have demonstrated high activity and selectivity in many catalytic reactions. However, dispersing and stabilising individual atoms in support materials in an atom/energy-e ﬃ cient scalable way still presents a signi ﬁ cant challenge. Currently, the synthesis of ADMCs involves many steps and further ﬁ ltration procedures, creating a substantial hurdle to their production at industrial scale. In this work, we develop a new pathway for producing ADMCs in which Pt atoms are stabilised in the nitrogen-interstices of a graphitic carbon nitride (g-C 3 N 4 ) framework using scalable, solvent-free, one-pot magnetron sputtering deposition. Our approach has the highest reported rate of ADMC production of 4.8 mg h (cid:1) 1 and generates no chemical waste. Deposition of only 0.5 weight percent of Pt onto g-C 3 N 4 led to improved hydrogen production by factor of ca. 3333 (cid:3) 450 when compared to bare g-C 3 N 4 . PL analysis showed that the deposition of Pt atoms onto g-C 3 N 4 suppressed the charge carrier recombination from the photogenerated electron – hole pairs of Pt/g-C 3 N 4 thereby enhance hydrogen evolution. Scanning transmission electron microscope imaging before and after the hydrogen evolution reaction revealed that the Pt atoms stabilised in g-C 3 N 4 have a high stability, with no agglomeration observed. Herein, it is shown that this scalable and clean approach can produce e ﬀ ective ADMCs with no further synthetic steps required, and that they can be readily used for catalytic reactions.


Introduction
5][6] Current methods for the synthesis of ADMCs are based on either wet-chemistry (i.e.][8][9][10][11][12] However, industrial scale-up of these synthetic methods is difficult because they require multiple steps and/or high temperatures, generate large amounts of chemical waste, and are not readily generalisable across supports and metal catalysts.In contrast, top-down physical methods 7,[13][14][15] for ADMC synthesis do not need high-temperatures and can be used for almost any type of solid support and transition metals without generating chemical waste.However, most physical methods are intrinsically limited to making small quantities of materials, and therefore are not scalable to industrial levels. 16agnetron sputtering has recently emerged as a promising technique for the production of metal nanoparticles (MNPs) on a wide variety of supports (e.g.8][19][20][21][22] It is one of the select few 'green' and scalable top-down methods, 17 and has already been applied on a large scale in the glass-coating and semiconductor industries.Crucially, this approach is carried out in ultraclean, high vacuum environments and can therefore generate extremely active metal species with clean surfaces not occluded by ligands or surfactants. 17,18,23In the magnetron sputtering process, accelerated argon (Ar) ions collide elastically with a highly pure metal target, which expels atoms onto a support material (Fig. 1a).Gohara & Yamazaki recently demonstrated that single Pt atoms can be dispersed and stabilised onto a single layer graphene deposited onto TEM grids using magnetron sputtering deposition. 24However, the deposition of ADMCs into bulk powder samples, suitable for use at scale and thus in catalysis, has not been demonstrated.
Herein we show, for the rst time, the deposition of ADMCs into bulk powder using magnetron sputtering.We stabilised Pt atoms into the nitrogen-interstices of graphitic carbon nitride (g-C 3 N 4 ).Furthermore, our approach showed the highest rate of ADMC production reported so far (at least 7.6 times higher than the previous benchmark 3,6,25 ), and does so without generating chemical waste (Table S5 †).The deposition and stabilisation of Pt atoms onto a g-C 3 N 4 framework was conrmed by aberration corrected scanning transmission electron microscopy (AC-STEM) and extended X-ray absorption ne structure (EXAFS) measurements, and was further supported by density functional theory (DFT) calculations.Furthermore, the photocatalytic performance of Pt/g-C 3 N 4 was tested for hydrogen evolution.Our Pt/g-C 3 N 4 shows outstanding photocatalytic activity when compared with similar reported systems (Table S6 †).AC-STEM and X-ray photoelectron spectroscopy (XPS) analysis before and aer the hydrogen evolution reaction conrmed the stability of Pt atoms on a g-C 3 N 4 framework.These results demonstrate that the scalable magnetron sputtering approach can generate effective ADMCs for catalytic processes such as hydrogen generation using only one clean synthetic step.

Results
g-C 3 N 4 was prepared via pyrolysis of melamine (10 g), heating under air at 300 C for 2 hours and then 520 C for 2 hours (Scheme S1 †). 26 The chemical composition and crystalline structure of the synthesised g-C 3 N 4 support was conrmed by Fouriertransform infrared spectroscopy (FTIR), powder X-ray diffraction (PXRD) and XPS measurements (see the ESI † for details).
The deposition of Pt atoms on g-C 3 N 4 via magnetron sputtering was carried out using a bespoke AJA International system (Fig. S1, see the ESI † for details).g-C 3 N 4 (1 g) was placed into a tailor-made stirring sample-holder, and then loaded into the magnetron sputtering chamber (reaching 3 Â 10 À8 torr background pressure in 40 min).Aer waiting 10 min for background pressure stabilisation, Ar gas (99.9999%) was introduced into the chamber, reaching 3 Â 10 À3 torr.Upon using an applied power (370 V and 16 mA), highly energetic ions were accelerated against the Pt target leading to a cascade of collisions at the surface of the Pt target, thus ejecting primarily individual neutral Pt atoms from the target 22,27,28 which were then deposited onto the g-C 3 N 4 framework (Fig. 1a).The g-C 3 N 4 powder was stirred constantly during the Pt atoms deposition to allow for atomic dispersion throughout the whole powder (Fig. 1a).In this fashion, Pt atoms deposition was carried out for 12 min, yielding Pt/g-C 3 N 4 (0.5 wt% of Pt onto 1 g of g-C 3 N 4 ) measured by inductively coupled plasma optical emission spectrometry (ICP-OES).This gave an ADMC production-rate of 4.8 mg h À1 , with a Pt loading on g-C 3 N 4 of 5 mg and a total 'feedstock-to-product' magnetron sputtering process time of 63 min, which is the highest rate of production reported so far (Table S5 †).Diverse characterisation techniques revealed no signicant differences aer deposition of the Pt atoms onto g-C 3 N 4 , including FTIR, PXRD, specic surface area and CHN elemental analysis (Fig. S2, S3 and Table S1 †).Notably, the PXRD data show no evidence of either Pt or PtO 2 in bulk or nanoparticulate form (Fig. S2b †).To demonstrate the generality of this method, Ni and Co atoms were atomically dispersed onto g-C 3 N 4 framework using similar parameters (Fig. S5 †).
AC-STEM measurements revealed highly dispersed Pt atoms in the g-C 3 N 4 framework, with no Pt NPs observed (Fig. 1b and S3 †).XAS measurements were performed to probe the local environment of Pt atoms (Fig. 1c and d).Fig. 1c shows the XANES spectrum of Pt/g-C 3 N 4 , and the standard spectra of Pt foil and PtO 2 .The white line intensities of these spectra revealed the oxidation state of the Pt atoms in each case, and the intensity for Pt/g-C 3 N 4 was intermediate between that of Pt foil and PtO 2 .0][31] The Pt K-edge EXAFS of Pt/g-C 3 N 4 showed only one peak at ca. 1.5 A (not phase-corrected), which can be associated to Pt-N/C coordination (Fig. 1d and Table S2 †). 10,30,31No peaks associated with Pt-Pt and/or Pt-O coordination were observed (Fig. 1d), which is consistent with the AC-STEM and PXRD analysis.To obtain further atomistic insight into the interaction between Pt atoms and g-C 3 N 4 , DFT calculations were performed (see the ESI † for details).For these calculations, the nitrogen-interstice of striazine and tri-s-triazine units of g-C 3 N 4 framework were considered as sites for stabilisation of single Pt atoms (Fig. S6 and Table S3 †). 32Therefore, we suggest that Pt atoms were stabilised in the threefold nitrogen-interstice (Fig. 1e), as the rst EXAFS peak appeared at a slightly shorter bond distance than would be expected for Pt-O, whereas the sixfold nitrogen Pt-N appeared at a slightly longer bond distance than Pt-O (Tables S2 and S3 †).This corroborated with our EXAFS results and previous reports on Pt atoms stabilisation onto g-C 3 N 4 . 10,31hotocatalytic hydrogen production reactions were performed to investigate the catalytic photoactivity of Pt/g-C 3 N 4 and g-C 3 N 4 (Fig. 2a and Table S4 †).As expected, pristine g-C 3 N 4 showed negligible hydrogen evolution and hydrogen was only detected in large enough quantities to be reliably measured aer 3 h.In contrast, Pt/g-C 3 N 4 showed a much higher activity, improving the hydrogen production 3333 AE 450 times aer 5 h of reaction.Moreover, our results compare favourably to similar systems previously reported, demonstrating the high photocatalytic activity of our Pt/g-C 3 N 4 system (Table S6 †).
To gain further insights into the high photocatalytic activity of Pt/g-C 3 N 4 , we carried out photoluminescence (PL) measurements to investigate the efficiency of charge transfer and separation (Fig. 2b).The PL spectrum of g-C 3 N 4 exhibits an intense peak at ca. 460 nm, which is ascribed to electron-hole recombination of g-C 3 N 4 .The deposition of Pt onto the g-C 3 N 4 surface led to a decrease of the PL intensity, implying suppressed charge carrier recombination (Fig. 2b) arising from the efficient dissociation of photogenerated electron-hole pairs of Pt/g-C 3 N 4 , in agreement with the photocatalytic results. 33,34AC-STEM images and XPS analysis of the Pt/g-C 3 N 4 aer 5 h under reactions conditions show no agglomeration and no signicantly changes in Pt electronic properties (Fig. 2c and S7, † respectively), the major mechanism responsible for catalyst deactivation, which demonstrates the high stability of Pt atoms in the g-C 3 N 4 framework.

Conclusion
In summary, we have demonstrated a new approach for producing ADMCs, using magnetron sputtering to deposit Pt atoms into a g-C 3 N 4 support.This approach has the highest reported rate of ADMC production out of all preparation routes.The Pt/g-C 3 N 4 photocatalyst generated using this approach is very effective for photocatalytic hydrogen production, which is attributed to the synergy between Pt atoms and g-C 3 N 4 .Our results suggest that Pt atoms were stabilised in the nitrogeninterstices of the g-C 3 N 4 framework.AC-STEM analysis aer hydrogen evolution reaction revealed the high stability of Pt atoms in g-C 3 N 4 .This approach is readily generalisable to other metals (i.e.Ni and Co), raising the possibility of straightforward and scalable synthesis of a wide range of ADMCs.As a result, we believe this strategy could therefore be a transformative method for the synthesis of ADMCs, potentially changing how to synthetise ADMCs at both the research and industrial scales.

Fig. 1
Fig. 1 (a) Schematic representation of Pt atom deposition onto g-C 3 N 4 using magnetron sputtering (not to scale).(b) Aberration correctedscanning transmission electron microscope (AC-STEM) image shows the high dispersion of Pt atoms in g-C 3 N 4 .(c and d) XANES and Fourier transformed EXAFS spectra of Pt/g-C 3 N 4 , Pt foil and PtO 2 , respectively (e) DFT optimised structure of Pt atoms stabilised onto threefold nitrogeninterstice between s-triazine units in g-C 3 N 4 framework.