Efficient photoelectrochemical water splitting on ultrasmall defect-rich TaO_x nanoclusters enhanced by size-selected Pt nanocluster promoters

Abstract

Formation of nanoclusters has attracted a lot of attention in recent years because of their distinct properties from isolated atoms and bulk solids. Here, we focus on the catalytic properties of supported transition metal oxide nanoclusters, such as TaO, with a well-defined size distribution below 10 nm. We show that their catalytic performance can be greatly enhanced by introducing a reaction promoter such as Pt. Different combinations of precisely size-selected, defect-rich TaO and Pt nanoclusters are produced by a gas-phase aggregation technique in a special DC magnetron sputtering system. Argon flow rate and aggregation length are carefully optimized to control the sizes of these ultrasmall TaO and Pt nanoclusters by using a quadrupole mass filter, and TEM studies reveal the different crystalline nature of TaO (amorphous) and Pt (crystalline) nanoclusters. We have further demonstrated the size-dependent photoanode activity of (TaO, Pt) nanocluster systems in a photoelectrochemical water splitting reaction, where the Pt nanocluster promoters are found to provide a significant enhancement in the photocurrent density, approximately tripled that was observed from just TaO nanocluster catalysts alone. The photocurrent density and photoconversion efficiency tend to reduce when Pt nanoclusters become overpopulated due to blocking of the photosensitive TaO surface. Reducing the Pt nanocluster size resolves this problem by incorporating a greater number of smaller nanocluster promoters without blocking TaO, which leads to further enhancement in the photocurrent density. The enhanced photocatalytic activity is attributed to synergetic effects introduced by the Pt nanoclusters that act as temporary charge storage sites to facilitate effective separation of a large number of electron-hole pairs, generated from a large number of active sites on the defect-rich amorphous TaO nanoclusters upon illumination.

Controlling the Size and Pattern Pitch of Ni(OH)2 Nanoclusters Using Dip-Pen Nanolithography to Improve Water Oxidation

“…Reactions ( 4 ) and ( 5 ), below, are suggested for the nickel reduction and OER [ 27 , 28 , 29 ], respectively: …”

“…Part of the mechanism regards the relationship between the Ni(OH) 2 clusters. As a result that the clusters affect each other [ 27 , 29 ], the distances between them are significant and may affect reaction ( 5 ). Our findings prove, for the first time, to the best of our knowledge, that the distance between the clusters indeed affects the reaction.…”

“…This finding indicates that the MCS electrodes are more efficient than the ITO electrode and proves that the size of the clusters and the pattern pitch dimensions—two parameters that can be easily controlled by using DPN—dramatically affect the WSP efficiency. The clusters cannot be too close to each other (e.g., by coating the entire surface with the catalyst), since (a) the surface itself participated in some of the chemical reactions [ 29 , 46 ], and (b) the clusters must be in the nanoscale range, such that their surface area is sufficiently high.…”

“…Finally, because heterogeneous catalysis occurs on the surface of the catalyst, this surface area needs to be large, i.e., nano-sized catalysts are preferred over larger ones. To date, most processes that enable the formation of such nano-sized catalysts are expensive, involve a complicated production process (numerous steps are required to reach the nano-scale size), and do not enable full control over the size distribution of the nanoclusters or the distance between them (the pattern pitch), resulting in large variability that reduces the efficiency of the WSP [ 29 , 30 ].…”

Highly dispersed Pt nanoclusters supported on zeolite-templated carbon for the oxygen reduction reaction

“…To date, several synthesis strategies, such as wet-chemical, atomic-layer deposition, and photochemical methods, have been applied for the preparation of well-dispersed Pt nanoclusters on different types of support, such as dendrimer, metal oxide, and carbon materials. [13][14][15][25][26][27][28][29][30][31] An alternative approach to synthesize Pt nanocluster (PtNC) is the encapsulation of the cluster within nanosized pores, for example, by utilizing microporous (diameters less than 2 nm) carbon materials. 32 Among the microporous carbons, zeolitetemplated carbon (ZTC) has been attractive for supporting Pt clusters due to its ordered microporous structure.…”