Fabrication and superior performance of Pt/epoxy-rich graphene sub-nanocatalysts. It is well-known that the activity and stability of metal sub-nanocatalysts is highly depended on the structure and stability of anchoring sites on supports. For graphene, among various oxygen-containing functional groups (OCGs), the in-plane epoxy groups have excellent thermal stability, which exist stably even at high temperatures above 1000 K,33 as proved by the DFT calculation results (Figure S1a and S1b). Clearly, the in-plane epoxy shows a higher binding energy than the out-of-plane epoxy, suggesting that the in-plane epoxy has a better stability. In addition, structurally, the single metal atom bonded with in-plane epoxy is closer to graphene basal plane than that bonded with out-of-plane epoxy or phenol. So it is reasonable to expect that the metal atoms introduced in the following ALD cycle would simultaneously bond with the single metal atom and graphene, achieving more stable metal cluster containing metal-metal bond and peculiar coordination environment. Accordingly, our study began by precisely engineering the thermally and chemically stable in-plane epoxy groups on graphene as the anchoring sites to enable the growth of sub-nanometric Pt species with controllable size, good stability and unique complex structure. As schematically shown in Scheme 1, a new strategy based on the pretreatment of multiple O3 pulses is developed to predominantly yield abundant epoxy groups on graphene denoted as epoxy-rich graphene, distinguished from extensively reported acid etching method,33–35 which yields graphene oxides with multi-kind OCGs (such as ester, phenol, carbonyl and carboxyl groups). To minimize the adverse effects of the initial OCGs on the pretreatment process, OCGs-deficient graphene prepared by thermal reduction of graphene oxide at 1600 °C,36–38 i.e., G1600, was selected as starting material (see details in Supplementary Information). The epoxy groups were then selectively introduced on the G1600 by multiple O3 pulses pretreatments with the pulse, exposure, and purge times of 2, 30 and 60 s, respectively. Their contents were precisely tailored by adjusting the cycle number (y) of O3 pulses designated as G1600-O3-y, as confirmed by X-ray photoemission spectroscopy (XPS) and Raman measurements (Fig. 1a and Figure S1, S2 and S3), providing ideal platforms to construct Pt sub-nanocatalysts with tunable density of isolated Pt species.
Using the well-controlled epoxy-rich graphene as the substrate, we then sought to precisely synthesize Pt sub-nanocatalysts from single atom to cluster by tuning the cycle number (x) of Pt ALD (Scheme 1), denoted as xPt/G1600-O3-y. It is obviously seen that based on inductively coupled plasma atomic emission spectrometer (ICP-AES) measurements, the Pt loadings of xPt/G1600-O3-y (y = 30, 60, 90 and 120) are much higher than those of xPt/G1600 prepared with the same cycle number of Pt ALD (x = 1, 2, 5 and 8) (Table S1). Interestingly, the Pt loading and the content of epoxy group are observed to follow similar trends with respect to the cycle number of O3 pulse (Fig. 1a), indicating a linear relation of the Pt loading with the content of epoxy group. Meanwhile, at a certain O3 cycle number (i.e., y = 60), the Pt loading of xPt/G1600-O3-y increases quite linearly with the cycle number of Pt ALD (Fig. 1a). In contrast, the Pt loading of xPt/G1600 is observed to increase non-linearly with the Pt ALD cycle number owing to the unselective deposition of Pt species over the G1600 support (Table S1 and the following HAADF-STEM and XAS measurements). These results demonstrate that the pretreatment process with the controllable O3 pulses is of crucial significance to create predominantly abundant epoxy groups for guaranteeing the precise fabrication of Pt/graphene sub-nanocatalysts by the ALD technique.
The as-prepared Pt/graphene sub-nanocatalysts were further evaluated for AB hydrolysis to explore their unique catalytic behaviors. As clearly shown in Figure S4, the 1Pt/G1600 catalyst prepared without the O3 pulse pretreatment is almost inactive, while the 1Pt/G1600-O3-y (y = 0, 30, 60, 90, 120) catalysts with the O3 pulse pretreatment show significantly enhanced hydrogen generation rate, strongly indicating positive effects of the O3 pulse pretreatment. Especially, the 1Pt/G1600-O3-60 catalyst achieves the highest hydrogen generation rate among these five catalysts. More interestingly, as the Pt ALD cycle number increases, the resultant sub-nanometric xPt/G1600-O3-60 catalysts give rise to remarkably increased hydrogen generation rate followed by a decline (Fig. 1b). Considering that increasing the Pt ALD cycle number leads to the increase in the Pt loading, a fair comparison was made to normalize the hydrogen generation activity based on the Pt loadings. All the ALD-Pt/graphene sub-nanocatalysts are found to show linear hydrogen evolution curves in the initial reaction periods, suggesting pseudo-zero order kinetics for the reaction, and thus the turnover frequency (TOF) would be easily calculated according to our previous method.39–41
Figure 1a summarizes the TOF values of xPt/G1600-O3-60 catalysts. Clearly, the TOF almost double increases as the cycle number increases from one to two, followed by a steady increase until the cycle number up to 5 (5Pt/G1600-O3-60) and then a decline with the further increase in the cycle number. In other words, the xPt/G1600-O3-60 catalysts display a volcano-shape curve with respect to the Pt ALD cycle number, which is remarkably different from the linear relation of their Pt loadings with the Pt ALD cycle number mentioned above. This implies that these xPt/G1600-O3-60 catalysts exhibit different geometric and/or electronic structures of catalyst active sites, which will be shown below. Meanwhile, the most active 5Pt/G1600-O3-60 catalyst exhibits the TOF of 37134 h− 1, which is approximately 2.6 times that of 1Pt/G1600-O3-60 catalyst, and such catalyst also possesses much higher durability than the Pt/C nanoparticles (NPs) catalyst. Specifically, at the sixth recycling run, the 5Pt/G1600-O3-60 catalyst maintains 88% of the hydrogen generation rate at the first run, while the Pt/C NPs with only 29% of the hydrogen generation rate (Fig. 1c and Figure S5).30 In addition, the durability of this catalyst is better than that of 1Pt/G1600-O3-60 catalyst. These strongly indicate that downsizing Pt nanocatalysts to appropriate sub-nanometric ones is a promising strategy for delivering simultaneously high hydrogen generation activity and catalyst durability. Notably, at the sub-nanometric scale, all the Pt atoms almost locate at the surfaces, which are all accessible to the reactants. Therefore, this TOF is contributed by the intrinsic activity of the Pt species, calling for more fundamental understanding of sub-nanometric active Pt species and active site structures from the viewpoint of catalyst active sites.
In addition, the graphene control sample with higher contents of OCGs and phenols as dominant OCGs (Figure S6a) was also prepared by traditional Hummers method and vacuum-promoted exfoliation of graphene oxide at 600 °C (denoted as G600) and used for Pt deposition. As shown in Figure S6b and 6c, the xPt/G600 exhibit much lower catalytic activity than that of xPt/G1600-O3-60. Moreover, the durability of these catalysts (1Pt/G600 and 5Pt/G600, Figure S6d) is much worse than that of 5Pt/G1600-O3-60 catalyst. Specifically, at the sixth recycling run, the 1Pt/G600 and 5Pt/G600 catalysts show only 20% and 11% of the initial hydrogen generation rate, respectively (Figure S6d). These results strongly indicate that epoxy groups play a key role on the catalytic activity and durability of Pt sub-nanocatalysts.
Unique structural and kinetics insights into Pt/epoxy-rich graphene sub-nanocatalysts. To obtain unique atomic, electronic and kinetics insights into the highly active and durable 5Pt/G1600-O3-60 sub-nanocatalyst, we resort to advanced microscopic, spectroscopic and synchrotron radiation techniques together with kinetics analysis. Aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (AC-HAADF-STEM) measurements were first carried out, and the results are shown in Fig. 2. Clearly, over the well-designed epoxy-rich G1600-O3-60 support, the one Pt ALD cycle yields uniform Pt single atoms (Fig. 2c), whose loading is 1.1 wt%, while increasing the Pt ALD cycle number to two gives rise to the formation of Pt dimers (Fig. 2d). In contrast, over the G-1600 support without O3 pretreatment, the one Pt ALD cycle yields the Pt single atoms of low loading of 0.1 wt% (Fig. 2a), while such single atomic Pt is easily grown to Pt clusters (~ 1 nm) in the second Pt ALD cycle (Fig. 2b). More interestingly, increasing the Pt ALD cycle number from 2 to up to 5 is observed to still facilitate the formation of Pt dimers (Fig. 2d-g,) as confirmed by the observed Pt-Pt bond lengths of 2.60 ± 0.05 Å (Figure S7), which are smaller than the Pt-Pt bond length of 2.80 Å in bulk Pt.42, 43 However, further increasing the Pt ALD cycle leads to the appearance of Pt clusters (Fig. 2h). These results demonstrate that employing the created abundant epoxy groups on the G1600-O3-60 as the nucleation sites of Pt ALD is a simple yet effective strategy for the precise fabrication of Pt/graphene sub-nanocatalysts at the atomic level from Pt single atoms, dimers to clusters by tailoring the Pt ALD cycle numbers.
The X-ray absorption near-edge spectroscopy (XANES) and X-ray photoelectron spectroscopy (XPS) measurements were further conducted to reveal the unique electronic structure of Pt sub-nanocatalysts. Figure 3a shows the Pt L3-edge XANES profiles of xPt/G1600-O3-60, 2Pt/G1600, and the reference samples Pt foil and PtO2. The white line intensities of xPt/G1600-O3-60 were all higher than that of Pt foil, indicating that the Pt species are partially positively charged (Ptδ+, 0 < δ < 4).44, 45 In contrast, the white-line intensity of 2Pt/G1600 is clearly lower than those of Pt sub-nanocatalysts, suggesting lower oxidation state of Pt determined by its nanometric size (Fig. 2b). More interestingly, in the sub-nanometric size regime, the white-line intensities fluctuate (inset of Fig. 3a), i.e., the white line intensity first decreases when the Pt ALD cycle number increases from 1 to 4, followed by an abnormal increase in 5Pt/G1600-O3-60 and then a decline in 6Pt/G1600-O3-60. In line with this phenomenon, a size-dependent oscillation in binding energy for Pt sub-nanocatalysts was also found by XPS analysis (Fig. 3b and Table S2),46, 47 which is probably a result of the combination of various effects such as size effect and charge transfer between Pt and graphene support. These results further reliably confirm the precise tailoring of the sub-nanometric Pt/graphene catalysts and their size-depended electronic and geometric properties. In addition, The AC-HAADF-STEM, XAFS and XPS results for the used 5Pt/G1600-O3-60 (Figure S8) showed that the Pt dimer structure is stable and not prone to migrate and agglomerate to nanocluster or nanoparticles under reaction conditions, consistent with above catalytic results.
Further kinetics analysis of xPt/G1600-O3-60 (x = 1–8) sub-nanocatalysts with the unique atomic and electronic structures identified above was performed to obtain mechanistic insights into the Pt single atoms-, dimers- against clusters-catalyzed AB hydrolysis to generate hydrogen. As obviously shown in Figure S9, these catalysts retain zero-order reaction kinetics, and thus the corresponding reaction rate constants are yielded based on the slope of the linear part for each curve. Further combining with the Arrhenius equation, the activation energy (Ea) and the logarithm of pre-exponential factor (lnA) values were obtained and shown in Table S3. Among all these catalysts, the 5Pt/G1600-O3-60 catalyst demonstrates the lowest Ea and lnA, while the highest values for the 1Pt/G1600-O3-60 catalyst. Based on the transition state theory, the Ea and lnA represent the indications of activation and adsorption of reactants, respectively. Hence, the 5Pt/G1600-O3-60 with the lowest lnA corresponds to the strongest interaction with the reactants in terms of their adsorption, which would facilitate the bond cleavage of reactants in terms of their activation, thus paving an explanation of its highest catalytic activity. Interestingly, the results of kinetics parameters Ea versus lnA of these catalysts demonstrate a remarkable compensation effect as shown in Fig. 3c.48, 49 This can be further divided into three kinetics regimes: the single atom, dimer and cluster catalysts locate in the regimes of high, low and medium Ea and lnA values, respectively. In light of the previous DFT study,50 this could be interpreted as the change in the binding energy of reaction species, which induces a switch in the kinetic regime. To be more specifically, the adsorption of AB and H2O could be weak for the single atom catalyst, in which the catalyst surfaces are mainly covered by reaction products. With the increase of Pt size, the AB hydrolysis slowly switches from the product coverage-limited regime to the reactant activation regime. However, further increasing the Pt size would switch back to the product coverage-limited regime, due to the decreased adsorption strengthen of reactants (Fig. 3d).
Identification of sub-nanometric Pt active sites and kinetics analysis. The above-mentioned size-depended electronic and geometric properties of sub-nanometric Pt catalyst augur the specific active site structures of Pt species in xPt/G1600-O3-60. To clarify the atomic coordination and structural signature of Pt active site, the extended X-ray absorption fine structure (EXAFS) measurements were carried out. As shown in Fig. 4a, all the xPt/G1600-O3-60 samples show a major scattering peak at ~ 1.65 Å (without phase shift) from the Pt-O/C contributions and a very weak peak at ~ 2.42 Å from either the satellite peak of Pt-O/C or the Pt-Pt contribution, which need be further identified through the EXAFS curve fitting. In contrast, the peak at ~ 2.70 Å (without phase shift) for 2Pt/G1600 is similar to that of Pt foil and corresponds to Pt-Pt bonds, indicating that it is mainly composed of Pt clusters or Pt nanoparticles. According to the EXAFS curve fitting results of 1Pt/G1600-O3-60 (Fig. 4a and Table S4), Pt-O/C coordination peak (average coordination number of 4.5 at a distance of 2.00 Å) is the dominant one, while the Pt-Pt contribution is significant weak with an average coordination number of only 0.5, implying that the Pt species mainly exist in the form of single atom in 1Pt/G1600-O3-60. For xPt/G1600-O3-60 (x = 2–5), besides the Pt-O/C coordination, a Pt-Pt contribution at a distance of ~ 2.62 Å with an average coordination number between 1.1 and 1.7 is also observed, indicating Pt dimer are predominantly formed and their contents gradually increases with the increase of Pt ALD cycles. However, for 6Pt/G1600-O3-60, a longer Pt-Pt bond length of 2.72 Å and larger average coordination number of 2.0 are observed, indicating that a large number of Pt clusters exist in 6Pt/G1600-O3-60. Furthermore, the wavelet transforms (WT) of Pt L-edge EXAFS also well demonstrate the different forms of Pt species in xPt/G1600-O3-60 (x = 1–5). As shown in Fig. 4b-f, as the cycle number of Pt ALD increases, the peak of Pt-Pt bond near 2.4 Å (without phase shift) gradually moves to the high-k portion in k-space, indicating that the bonding forms of the Pt atoms change from single atom to dimer in xPt/G1600-O3-60 (x = 1–5).
From the above-mentioned results, we can know that the specific active site structures of Pt species can be produced using the abundant epoxy groups as the anchoring sites for Pt ALD with the controllable cycle numbers. Based on the XPS and XAFS results, density function theory (DFT) calculations were conducted to determine the optimal structure of Pt single atom, dimer and cluster catalysts (Figure S10). The simulated coordination structure of the Pt single atom and dimer are C2PtO and C5Pt2O (Fig. 4g and Figure S10a, b), respectively. Interesting, the Pt-Pt bond exists in Pt dimer. And the calculated bond length of Pt-Pt in Pt dimer is 2.621 Å, consistent with the XAFS result (~ 2.61 Å), but it is shorter than the Pt-Pt bond length of 2.80 Å in bulk Pt. Because the lattice of the cluster was not observed by HAADF-STEM, it is not reasonable to use Pt(111) for calculation. Moreover, the calculation time and complexity will increase exponentially for each additional Pt atom as for cluster. Therefore, Pt4 with better stability is selected to replace cluster for DFT calculation (Figure S10c). Then the charge density of Pt atoms in all three configurations was calculated (Fig. 4g and Figure S11). It is revealed that the Pt electron loss on three configurations are − 0.1402 (Pt single atom), -0.0558 (Pt dimer) and − 0.1545 (Pt cluster). So, the Pt cluster catalyst has the most Pt electron loss, suggesting that there exist different electronic interactions between graphene and Pt species from single atom, dimer to cluster. This also explains the size-dependent oscillation in above XANES and XPS results (Fig. 3a and 3b).
Low-temperature scanning tunneling microscopy (LT-STM) was further employed to identify the atomic and electronic structure of Pt active sites. For convenient characterization, xPt/HOPG (highly oriented pyrolytic graphite)-O3-60, which were synthesized using the same method as xPt/G1600-O3-60, were applied for LT-STM analysis (see details in Supplementary Information). Figure 4h and 4i display the typical atomic-resolution STM and 3D atomic structure images of 1Pt/HOPG-O3-60, respectively. The small protrusion in zone 1 of Fig. 4h is attributed to the in-plane epoxy O atoms produced by O3 pretreatment, whose neighboring C atoms also appear to be brighter than the C atoms further away. In addition, the single Pt center is resolved as a bright spot (zone 2 in Fig. 4h), whose neighboring C and O atoms also appear brighter and exhibit a higher apparent height than other C atoms (Fig. 4i), arising from the electronic interaction between Pt single atom and neighboring C and O atoms.51, 52 This indicates that the Pt single atoms are bound with O and C atoms on the surface of epoxy-rich HOPG-O3-60. For 5Pt/HOPG-O3-60, Pt single atom (Zone 2), dimer (Zone 1) and cluster (Zone 3) are all observed, which is consistent with the AC-HAADF-STEM and XAFS results of 5Pt/G1600-O3-60 (Fig. 4j). Specially, for Pt dimer, two adjacent bright dots were observed, being attributed to the Pt dimer (Fig. 4k). Note that the neighboring O and C atoms of one Pt atom (Atom 1 in Fig. 4k) appear brighter and exhibit a higher apparent height than those next to another Pt atom (Atom 2 in Fig. 4k), suggesting that the two Pt atoms of Pt dimer have different electronic interaction with neighboring atoms. These results are consistent with the above DFT results (Fig. 4g).
Based on the identified structures of the sub-nanometric Pt active sites, we further carried out DFT calculations to explore their impacts on the activation of water, which has been suggested involved in the rate-determining step for this reaction. The optimized most stable adsorption configurations of water molecule on the Pt single atom, dimer and cluster as well as the corresponding potential energy profiles are displayed in Fig. 5a and Figure S12. Obviously, the Pt dimer exhibits the highest water adsorption energy with the largest enlarged O-H bond length (-0.87 eV, 0.979 Å) compared with the Pt single atom (-0.78 eV, 0.977 Å) and cluster (-0.50 eV, 0.976 Å). As a result, the activation barrier for the water dissociation was calculated as 1.28, 1.13 and 1.40 eV for the Pt single atom, dimer and cluster, respectively. Hence, from the point view of theoretical calculations, the Pt dimer exhibits the lowest activation barrier for the water dissociation. To verify this, kinetic isotope experiments by replacing H2O with D2O as the reactant were conducted to probe the kinetic isotope effect as an indication of the capacity of water dissociation, and the results are shown in Fig. 5b and Figure S13. Obviously, the kinetic isotope effect (KIE) value follows the trend of Pt dimer < Pt cluster < Pt single atom, which is quite consistent with the DFT calculations.
The adsorptions of AB on Pt single atom, dimer and cluster catalysts were comparatively studied by DFT calculations. The optimized most stable adsorption configurations of the involved species on Pt single atom, dimer and cluster are listed in Figure S14. It can be obviously observed that AB does not dissociative adsorb on the Pt single atom, and AB dissociative adsorbs on the Pt dimer and cluster. Moreover, from the bond length of the B-H bond, the B-H bond of AB adsorbed on the Pt dimer is elongated (1.338 Å, the B-H bond length of AB is 1.218 Å). Hence, a combination of theoretical and experimental study suggests the Pt dimer, especially the 5Pt/G1600-O3-60, demonstrates the highest capacity for the water dissociation and activation of AB, paving an explanation for its highest catalytic activity. Moreover, based on the above results, we have proposed the correlation between the Pt species (single atom, dimer, and cluster) and TOF or activation energy (Ea) for the AB hydrolysis as shown in Fig. 5c. The TOF and activation energy exhibit a volcanic relationship with the cycle number of Pt ALD, in which the higher content of Pt dimer within the catalyst gives rise to the higher reaction activity owing to the appropriate Pt electronic properties.