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Diane Haiber1 Peter Crozier1

1, Arizona State University, Tempe, Arizona, United States

Graphitic carbon nitrides (g-CNxHy’s) demonstrate immense potential for efficient photocatalytic hydrogen generation, attributed to their high surface area and ability to absorb visible light1. Due to kinetic limitations afforded during synthesis, involving calcination of N-rich precursors, g-CNxHy’s contain a range of residual hydrogen, perturbing structural condensation2. Efforts to improve the efficiency of g-CNxHy’s have focused on reducing H-content to give higher hydrogen evolution rates (HER’s), suggesting that amine (N-Hx) defects slow down charge transfer2. However, the undesirable need of Pt remains as g-CNxHy must be functionalized with ~2-3 wt.% Pt to achieve appreciable HER’s. Recently, g-CNxHy’s with supported single Pt atoms and very low loadings (<0.2 wt.%) have been shown to give improved HER’s on a per Pt-atom basis3. Yet, low-loading, highly-dispersed Pt co-catalysts still cannot surpass the high HER’s of the traditional photodeposition (PD), high-loading routes.

By combining annular dark field scanning transmission electron microscopy (ADF-STEM) and photoreaction data, we systematically determine the effect of co-catalyst dispersion and support structure on the photocatalytic performance of Pt/g-CNxHy’s. To capture a range of support structural disorder, three g-CNxHy samples are selected based on the broadening of the (002)-peak observed in powder x-ray diffraction. Here, g-CN:1, g-CN:2, and g-CN:3 refer to g-CNxHy’s with average domain sizes of 9.9, 4.5, and 3.2 nm, respectively, determined by Scherrer analysis. When loaded with 2 wt.% Pt via PD, the HER of g-CN:2 is ~2x higher than g-CN:1 and g-CN:3. ADF-STEM reveals that g-CN:2 also possesses the highest Pt dispersion quantified by a specific surface area (SSA) of 27.6 m2/g. By normalizing the HER’s by the number of exposed surface Pt atoms (based on measured SSA’s) the TOF of each support can be derived and are found to correlate with the average domain size.

With a TOF of 182/hr, g-CN:1 is the most active photocatalyst but suffers a low Pt dispersion. Based on this knowledge, we can predict the combination of support and Pt dispersion/loading to give desired HER’s, resulting in rational design strategies for reducing Pt consumption while maintaining high energy conversion efficiency. HER’s in excess of 5000 μmol/g/hr are predicted for 0.5 wt.% Pt on g-CN:1 with single atom stabilization. Current efforts confirm that the Pt particle size can be dramatically reduced on g-CN:1 by employing NaBH4-assisted chemical deposition4, giving a 60% improvement in HER for a 4x reduction in Pt loading.

[1] X. Wang et al. Nat. Mater. 2009, 8, 76-80. [2] D.J. Martin et al. Ange. Chem. Int. Ed. 2014, 53, 9240-45. [3] X. Li et al. Adv. Mater. 2016, 28, 2427-31. [4] Z. Chen et al. Adv. Funct. Mater. 2017, 1605785. [5] We gratefully acknowledge the support of DOE grant DE-SC0004954, ASU’s John M. Cowley Center for High Resolution Electron Microscopy and ASU’s Center for Solid State Science.

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