A facile “dark”-deposition approach for Pt single‐atom trapping on facetted anatase TiO2 nanoflakes and use in photocatalytic H2 generation
Graphical abstract
Introduction
Over the past few years, single atom (SA) catalysts have not only been of high interest in thermal heterogeneous catalysis but also entered strongly into the field of electrocatalysis where it is used for organic electrosynthesis, fuel cells, batteries, electrolyzers, and particularly focusing on oxygen reduction or evolution, hydrogen evolution or oxidation, carbon dioxide reduction or nitrogen reduction reactions) [1,2]. The reason to utilize SAs is mainly due to the maximized surface-to-volume ratio of such catalysts and novel reaction pathways that become available at the SA level [3], [4], [5], [6], [7], [8], [9], [10], [11]. Most recently, SAs became also attractive as co-catalysts in photocatalytic reactions [3,6,[12], [13], [14], [15], [16]]. In photocatalysis, the mobile charge carriers (electron-hole pairs) that are generated by illuminating a semiconductor, experience charge transfer reactions with the environment. To enhance the kinetics of the slow transfer reactions, typically noble metal co-catalysts are required for photocatalytic reactions and, in particular, for the photocatalytic H2 evolution reaction (HER). Noble metals such as Au, Pt, Pd, etc. are usually used in the form of nanoparticles (NPs) [15,[17], [18], [19], [20], [21]].
In contrary to the common methods, used to deposit noble metal co-catalyst as NPs, SA co-catalysts have several substantial advantages such as a minimal noble metal loading, i.e., reduced costs, and, at the same time, a maximal surface-to-volume ratio. One of the most crucial issues for using SA co-catalyst is its surface immobilization on the desired substrate. Typical approaches for SA loading include methods based on chemical trapping or molecular-beam-soft-landing [22], [23], [24].In general SA traps are based on specific electronic or geometric features of the substrates [25].
In the current state of research of the application of SA in photocatalysis, two semiconductors are primarily investigated, i.e., nanostructured C3N4 and anatase TiO2 [6]. The former has good visible light absorption characteristics. On the other hand, nanostructured anatase TiO2 is a benchmark photocatalytic material owing to its excellent photocorrosion stability and wide range of applications that rely on photocatalytic reactions [19,26]. It is worth pointing out that among the polymorphs of TiO2, anatase shows a higher reactivity to the red-ox potential of water and thus has more optimized energetics than rutile [26], [27], [28], [29].
In the field of electrochemistry and photoelectrochemistry, the formation of the anchoring points on the support materials’ surface, which can set up and sustain single atom (SA) states, is vital. Thus, the formation of suitable trapping sites is indispensable for granting stability to the SAs. So far, many approaches were investigated for the SA deposition including methods such as coprecipitation, atomic layer deposition, photochemical reduction, etc., which are based on metal-containing precursors that are placed, reduced, and anchored on the support by defects or vacancies [11]. Two points are further of significant importance for SACs in electrocatalysis, i.e., selectivity (since alternative reaction pathways compared to classic nanoparticles might be involved), and activity/stability (the strong interaction between SA and the support can significantly influence their electrochemical activity and stability) [11,30]. Yang et al. reported on a series of impregnation and annealing approaches to decorate TiO2 nanoparticles with Pt-O units that provide a remarkable reactivity [31].
In contrast to these approaches, concerning TiO2 and single-atom co-catalysts, we showed recently that surface defects (Ti3+-Ov) induced by reducing in Ar/H2 atmosphere (e.g., Pt SA deposition on anatase TiO2 sputtered layers [25], or Pd, Pt or Au on (001) TiO2 nanosheets [32]) or native defects in anodic TiO2 nanotubes (e.g., Ir or Pt SA deposition on anatase TiO2 nanotubes [33]), or on TiO2 nanoflakes (e.g. Pt SA [34]) allow a direct, simple immersion reaction of the TiO2 surface with a Pt4+ precursor to form Pt SAs with a nominal charge (d) of ≈2.
In view of single-crystalline (SC) anatase, facetted SC layers have attracted wide attention. The preferential faceting of anatase was shown to significantly influence the electron and hole transfer in various photocatalytic reactions. In the case of single-crystalline anatase with (101) and (001) faceting, the different surface energy of these two facets leads to the formation of an intrinsic electron junction, which under illumination (in an aqueous environment) results in electrons exiting from the (101) and holes from the (001) planes [25,26]. Yang et al [35]. reported the hydrothermal formation of single-crystal anatase TiO2 nanosheets (NSs) with a high percentage of (001) facets. The key parameter that was identified in this process was the presence of fluoride ions, acting as a capping agent that enables the preferential growth of (001) faceted crystals (the surface energy of the (001) facets is reduced via the fluoride termination) [35], [36], [37], [38].
To note is that such (001) faceted powder nanosheets have recently gained attention in photocatalytic and also in other TiO2 applications [39], [40], [41]. However, the application of such crystallite powder nanosheets in photoelectrochemistry requires their further coating on a conductive substrate, thus making the production process more complicated and less efficient. In this sense, direct growth of faceted nanosheets on an FTO substrate by hydrothermal synthesis is a straightforward approach, leading to nanoflake photoanodes [42], [43], [44].
Here, we explore such single-crystalline (001) faceted TiO2 nanosheets (or nanoflakes), grown directly on FTO substrates by a hydrothermal method, that we further use for Pt single-atom decoration and photocatalytic H2 evolution considering the feasibility to anchor Pt single-atoms. We show that a simple air annealing treatment is sufficient to create suitable trapping sites on the single crystal surface. These traps then react in an immersion treatment of the TiO2 nanosheets in a chloroplatinic acid solution without further UV illumination – we denote this process thus as “dark-deposition”. We then explore the SA decorated TiO2 nanoflakes for photocatalytic H2 evolution and find a remarkable reactivity compared to the classic anatase/Pt NP system. Such SAs decorated TiO2 nanoflakes were then also characterized by photoelectrochemistry in order to gain additional information on relevant charge transfer processes.
Section snippets
TiO2 nanoflakes fabrication
Before the TiO2 nanoflakes fabrication on FTO substrates (7 Ω•m−2, Pilkington, UK), the FTO glasses were cleaned by ultrasonication in acetone, ethanol, and DI water for 15 min each followed by drying under a N2 stream. To fabricate TiO2 nanoflakes (NFs) with preferential (001) faceting directly on the FTO substrates, a hydrothermal synthesis was carried out at 150 °C for 15 h using a Teflon-lined stainless steel autoclave (capacity 250 mL). The aqueous hydrogen chloride (HCl) solution was
Results and discussion
Fig. 1a, c shows high-resolution SEM images of as-formed (001) facetted TiO2 nanoflakes (NFs) at various magnifications, together with a schematic representation of the different NF facets (Fig. 1b). The TiO2 NF layers were formed by hydrothermal synthesis on fluorine-doped tin oxide (FTO) substrates as described in the experimental section. The TiO2 NF surface, as synthesized, is very smooth and the sheets show F-termination [43,47] resulting from the used growth process. Previously we
Conclusions
In conclusion, the current work introduces a strategy to use (001) faceted TiO2 nanosheets/nanoflakes as a substrate to "dark”-deposit Pt single-atoms (SAs). Traps are achieved by annealing of the nanoflakes and defluorination, whereas usually SA trapping on TiO2 is established on (Ti3+-Ov) TiO2 states (by annealing in Ar/H2 atmosphere) or native defects formed in anodic TiO2 nanotubes. Here we introduce single-crystalline TiO2 NF surfaces with enough defect sites to effectively reduce Pt
CRediT authorship contribution statement
Gihoon Cha: Investigation, Writing – original draft, Data curation, Formal analysis. Anca Mazare: Formal analysis, Visualization, Writing – original draft, Writing – review & editing. Imgon Hwang: Investigation, Data curation, Formal analysis. Nikita Denisov: Investigation, Data curation, Formal analysis. Johannes Will: Investigation, Formal analysis. Tadahiro Yokosawa: Investigation, Data curation, Formal analysis, Writing – review & editing. Zdeněk Badura: Formal analysis, Writing – review &
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors would like to acknowledge the DFG and the Operational research program, 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) for financial support. Fahimeh Shahvardanfard is acknowledged for her contribution to this work. J.W. and E.S. acknowledge the research training group GRK 1896 “In Situ Microscopy with Electrons, X-rays, and Scanning Probes”. A.B.T.
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Present address: Bavarian Center for Battery Technology (BayBatt), University of Bayreuth, 95447 Bayreuth, Germany.