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Atomic-Layer Controlled Interfacial Band Engineering at Two Article www.acsnano.org Atomic-Layer Controlled Interfacial Band Engineering at Two-Dimensional Layered ffi PtSe2/Si Heterojunctions for E cient Photoelectrochemical Hydrogen Production △ △ Cheng-Chu Chung, Han Yeh, Po-Hsien Wu, Cheng-Chieh Lin, Chia-Shuo Li, Tien-Tien Yeh, Yi Chou, Chuan-Yu Wei, Cheng-Yen Wen, Yi-Chia Chou, Chih-Wei Luo, Chih-I Wu, Ming-Yang Li, Lain-Jong Li, Wen-Hao Chang,* and Chun-Wei Chen* Cite This: https://dx.doi.org/10.1021/acsnano.0c08970 Read Online ACCESS Metrics & More Article Recommendations *sı Supporting Information ABSTRACT: Platinum diselenide (PtSe2) is a group-10 two- dimensional (2D) transition metal dichalcogenide that exhibits the most prominent atomic-layer-dependent electronic behav- ior of “semiconductor-to-semimetal” transition when going from monolayer to bulk form. This work demonstrates an efficient photoelectrochemical (PEC) conversion for direct solar-to-hydrogen (H2) production based on 2D layered PtSe2/ Si heterojunction photocathodes. By systematically controlling fi the number of atomic layers of wafer-scale 2D PtSe2 lms through chemical vapor deposition (CVD), the interfacial band alignments at the 2D layered PtSe2/Si heterojunctions can be p- fi appropriately engineered. The 2D PtSe2/ Si heterojunction photocathode consisting of a PtSe2 thin lm with a thickness of 2.2 nm (or 3 atomic layers) exhibits the optimized band alignment and delivers the best PEC performance for hydrogen production with a photocurrent density of −32.4 mA cm−2 at 0 V and an onset potential of 1 mA cm−2 at 0.29 V versus a reversible hydrogen electrode (RHE) after post-treatment. The wafer-scale atomic-layer controlled band engineering of 2D fi ff PtSe2 thin- lm catalysts integrated with the Si light absorber provides an e ective way in the renewable energy application for Downloaded via Chun-Wei Chen on March 3, 2021 at 01:49:08 (UTC). direct solar-to-hydrogen production. KEYWORDS: atomic-layer-dependent electronic behavior, atomic layered catalyst, PtSe2/p-Si heterojunction, photoelectrochemical cell, solar-to-hydrogen conversion See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. hotoelectrochemical (PEC) conversion for direct solar- cathodes. Therefore, the effective integration of light absorbers to-hydrogen production has attracted great interest as a and catalysts at semiconductor/catalyst heterojunctions to renewable energy technology to store solar energy for achieve efficient carrier generation, charge transport, and P 1,2 hydrogen fuel. Silicon (Si) has been a promising photo- transfer is particularly important to fabricate an efficient PEC electrode candidate for photoelectrochemical (PEC) con- device for H2 generation using Si photocathode. version for direct solar-to-hydrogen because it consists of a Atomic-layer two-dimensional (2D) transition metal dichal- band gap value of 1.1 eV to enable sufficient solar light cogenides (TMDs) have emerged as promsing catalysts for harvesting and exhibits a suitable band energy level for H − 2 generation.3 7 Nevertheless, because the bare Si surface has solar-driven HER on Si photocathodes. For example, the relatively low activity for hydrogen evolution reaction (HER), an effective catalyst which can enhance charge transfer Received: October 27, 2020 ffi Accepted: February 24, 2021 e ciencies and reduce the overpotential for H2 production is usually required.8 In addition to intrinsic catalytic activities, the surface coverage, morphology, transparency, and energy band structure of catalysts on the Si surface play crucial roles in ffi determining the PEC H2 conversion e ciency of Si photo- © XXXX American Chemical Society https://dx.doi.org/10.1021/acsnano.0c08970 A ACS Nano XXXX, XXX, XXX−XXX ACS Nano www.acsnano.org Article fi MoS2/Si heterojunction photocathodes present an outstanding has also been examined by annular-dark- eld (ADF) scanning PEC H production, where the overpotential at electrolyte/ transmission electron microscopy (STEM). Figure 1a shows 2 − solid interfaces can be largely reduced.9 11 The thin coating fi catalyst of MoS2 lms provides both catalytic activity and corrosion protection on the Si photocathode for HER. Recently, a discovered group-10 two-dimensional TMD − layered material, platinum selenide (PtSe ),12 19 also shows 2 − superior electrocatalytic activity for HER,16,19 21 compared to the most intensively studied group-6 TMD catalyst of 22,23 fi MoS2. 2D atomic layer PtSe2 thin lms exhibit the most prominent atomic-layer-dependent electronic behavior of “semiconductor-to-semimetal” transition as the thickness is increased from monolayer to bulk.12,14,16,24,25 The electro- catalytic activity of 2D layered PtSe2 was also found to be enhanced with an increased number of atomic layers, mainly attributed to increased conductivity.19 Nevertheless, most of the studies on PtSe -based catalysts for HER were only limited 2 − to the investigation as standalone electrocatalysts.16,19 21 Until now, solar-driven PEC H2 production based on the PtSe2 catalyst has not been reported yet. To fabricate an efficient Si photocathode for HER, another critical issue for pursuing an effective catalyst is to consider a suitable band alignment at the semiconductor/catalyst heterojunction to ensure efficient charge transfer and transport across the multiple interfaces.26 ffi In this work, we demonstrate an e cient and stable PEC H2 production based on 2D PtSe2/Si heterojunctions by system- atically controlling the number of atomic layers of 2D PtSe2 films from monolayer to bulk using the wafer-scale synthesis by chemical vapor deposition (CVD) growth technique. The 2D layered PtSe2/Si heterojunction photocathodes exhibit promis- ing PEC performance for H2 generation through atomic band fi engineering of PtSe2 thin lm catalysts integrated with the Si light absorber. RESULTS AND DISCUSSION Figure 1. Wafer-scale PtSe2 synthesis via CVD process. (a) Preparations and Characterizations of Wafer-Scale Annular-dark-field (ADF) scanning transmission electron micros- ff fi 2D PtSe2 Thin Films with Di erent Atomic Layers. For copy (STEM) image of a PtSe2 thin lm. The Pt and Se sites were practical applications of PtSe /Si heterojunctions for PEC H marked by orange and red dots, respectively. (b) High-resolution 2 2 fi ∼ − fi TEM (HRTEM) image of PtSe2 thin lm with 10 20 nm domain generation, growing large-area and high-quality PtSe2 thin lms fi size. (c) Photographic image of the as-grown PtSe2 thin lms with with well-controlled layer thickness and uniformity is required. − fi various thicknesses ranging 0.8 3.0 nm. (d,e) Corresponding XPS To date, most reported PtSe2 lms were synthesized by direct spectra of the Pt 4f and the Se 3d core level peaks. selenization of predeposited Pt thin films via thermally assisted 27 fi conversion (TAC). Although large-area PtSe2 thin lms can be synthesized by direct selenization through the TAC the atomically resolved Z-contrast image of the PtSe2, where fi approach, it is still challenging to obtain PtSe2 lms thinner each Pt atom is surrounded by six Se atoms, indicating a 1T- than 4 nm, below which the “semiconductor-to-semimetal” phase octahedral structure with AA stacking. This is also the 24,25 24 transition is expected to occur. Accordingly, most PtSe2 most stable stacking structure as predicted theoretically and fi lms obtained from the direct selenization through the TAC consistent with the structure of PtSe2 bulk crystals synthesized approach usually exhibit semimetallic electronic behavior with by the CVT method.19 High-resolution TEM (HRTEM) 27 fl less optical transparency. Recently, PtSe2 akes with imaging shown in Figure 1b suggests that the CVD-grown ∼ fi ∼ − controlled thicknesses ranging from monolayer to 20 layers PtSe2 lms are polycrystalline, with a typical grain size of 10 can be directly synthesized by the modified chemical vapor 20 nm. The film thicknesses can be distinguished by their transport (CVT) method through adjusting the growth optical contrast and have been further identified by AFM 19 fi temperature and the amounts of reactants. However, the measurements (Figure S1). Large-area PtSe2 lms down to fl sizes of the CVT-grown PtSe2 atomic akes are still too small monolayer thickness with a root-mean-square (RMS) surface (∼10−20 μm) to form a continuous film, which may limit the roughness of only ∼0.2 nm can be grown by our CVD fabrications of large-area PtSe2/Si heterojunction photo- technique, which is not easy to achieve by using the fi cathodes for PEC H2 production. Here, wafer-scale PtSe2 conventional selenization of deposited Pt lms (Figure S2). fi fi lms were synthesized on c-plane sapphire substrates by Figure 1c shows the optical images of the PtSe2 lms on CVD using PtCl2 and Se powders as source precursors. By sapphire substrates with controlled thickness of 0.8, 1.4, 2.2, fi controlling the growth time and temperature, PtSe2 lms with and 3.0 nm, corresponding to monolayer (1L), two-layer (2L), ∼ fi thickness ranging from monolayer ( 0.8 nm) to bulk (21 nm) three-layer (3L), and four-layer (4L) PtSe2 thin lms, can be grown. The crystal structure of the CVD-grown PtSe2 respectively. The chemical compositions of the CVD-grown B https://dx.doi.org/10.1021/acsnano.0c08970 ACS Nano XXXX, XXX, XXX−XXX ACS Nano www.acsnano.org Article fi PtSe2 lms have been analyzed by X-ray photoelectron wavelength compared to those samples of 0.8 and 1.4 nm. The fi spectroscopy (XPS) shown in Figure 1d and e, where the Pt band gap values of these 2D PtSe2 lms were further evaluated α 1/2 4f and Se 3d core levels with the binding energies located at by using the Tauc plot method of ( hv) vs (hv-Eg) as shown α ffi 73.03 and 76.36 eV for the Pt 4f7/2 and 4f5/2, and 54.7 and 55.5 in Figure 2d and e, where is the absorption coe cient, hv is 28 eV for the Se 3d5/2 and 3d3/2, respectively, are consistent with the photon energy, and Eg is the optical band gap.
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