Atomic Layer Deposition of MoS₂ Decorated TiO₂ Nanotubes for Photoelectrochemical Water Splitting

Abstract

MoS 2 /CdS, [15] have demonstrated a high PEC performance. It was proven that the formation of such heterostructures is beneficial for the separation of photo-generated excitons (hole-electron pairs). [13,14,16,17] In particular, MoS 2 /TiO 2 heterostructures have attracted significant interest, as MoS 2 can be used as a photosensitizer for chemically stable but wide-bandgap TiO 2 semiconductor. [18] The respective bandgap edge positions of the two semiconductors form a charge-transfer cascade, which significantly increases the separation of the photogenerated charge carriers. [19,20] Overall, both the photosensitizing properties of MoS 2 and the electronic structure of the heterojunction create a suitable platform for UV and visible light utilization resulting in high PEC performance.It should be noticed that the electronic and catalytic properties of MoS 2 are highly dependent on the film thickness and morphology, [21] as well as the architecture of the substrates. [22,23] Therefore, various nanostructured architectures of TiO 2 (e.g., TiO 2 nanorods and nanotubes) have been applied as a scaffold to construct MoS 2 /TiO 2 heterostructures because the increased surface area can efficiently promote their (photo)electrochemical performance. [7,[16][17][18][19][20] However, these general synthesis methods (e.g., hydrothermal synthesis, chemical vapor deposition, chemical exfoliation, etc.) suffer from the limitation of the complicated synthesis process, poor film thickness control and uniform coverage on high aspect ratio structures, [17,19,20,24,25] which seriously suppress the investigation and promotion of A thermal atomic layer deposition (ALD) process to fabricate MoS 2 thin films is successfully demonstrated by using cycloheptatriene molybdenum tricarbonyl (C 7 H 8 Mo(CO) 3 ) and H 2 S as precursors at an ALD temperature below 300 °C. The process is systematically investigated, showing a typical selflimiting characteristic within an ALD temperature window of 225-285 °C and a high growth-per-cycle of 0.11 nm. The as-deposited films are amorphous while they can be crystallized in situ by sulfurization with H 2 S at a low temperature of 300 °C. A prototypical application of the developed ALD process is demonstrated by constructing a MoS 2 /TiO 2 heterostructure through depositing MoS 2 onto anodized TiO 2 nanotubes for photoelectrochemical water splitting. The MoS 2 /TiO 2 heterostructures exhibit approximately three times superior photoelectrochemical performance than the pristine TiO 2 nanotubes. This is attributed to an enhanced visible light-harvesting ability of MoS 2 and an improved separation of the photo-generated charge carriers at the heterostructure interface, which is affirmed by a staggering gap (type II) between MoS 2 and TiO 2 as probed by ultraviolet photoelectron spectroscopy.

MoS2 thin film decorated TiO2 nanotube arrays on flexible Ti foil for solar water splitting application

“…The maximum photocurrent density was obtained as 135.7 μA cm −2 at 0.8 V versus Ag/AgCl for MoS 2 decorated TiO 2 /Ti foil which was two times larger than TiO 2 /Ti foil (450 °C) and TiO 2 /Ti foil (700 °C) for which photocurrent density was obtained to be ∼70 μA cm −2 and ∼67.1 μA cm −2 , respectively. The obtained photocurrent density value of MoS 2 /TiO 2 photoelectrode in this study is several times higher than the previous report photocurrent density (30 μA cm −2 ) by Shen et al (2022) [24]. The enhanced photocatalytic activity of CVD-grown MoS 2 /TiO 2 heterostructure in comparison to Shen et al (2022) study could be explained due to the few (2 or 3) layers growth of MoS 2 as confirmed by Raman spectrum (figure 4(c)).…”

“…In context to TiO 2 , which absorbs only UV light, coupling with MoS 2 offers to absorb a much wider range of sunlight. Thus, the photosensitization and band structure of MoS 2 create a suitable approach for enhancement in PEC performance [24]. It is also reported that the performance of TiO 2 -based heterostructure mostly depends on the surface morphology of the film, and the thickness of the film [25].…”

“…As per the literature, various TiO 2 nanostructure morphologies (e.g., nanoflower, nanotube, nanorod, etc) have been used as a platform to construct MoS 2 /TiO 2 . Shen et al (2022) deposited ∼ 9 nm thick MoS 2 on TiO 2 nanotube on Ti substrate via thermal atomic layer deposition (ALD) process and obtained the enhancement in photocurrent density of ∼30 μA cm −2 in comparison to bare TiO 2 photocurrent density (∼10 μA cm −2 ) in PEC application [24]. Li et al (2018) prepared MoS 2 /TiO 2 nanoflowers on fluorine-doped tin oxide (FTO) coated glass by using the one-step hydrothermal method and found an improvement in photocurrent density ∼0.58 mA cm −2 in PEC [25].…”

Recrystallization of MBE‐Grown MoS2 Monolayers Induced by Annealing in a Chemical Vapor Deposition Furnace

“…[23] A second doublet at 233.5 and 236.6 eV can be assigned to the Mo 6þ of MoO 3 due to the oxidation at the near surface as the sample exposed to air before the measurement. [24] At the low BE side (Figure 4a), two S 2s peaks are fitted, where one belongs to basal S 2À of MoS 2 and the other belongs to unreacted elemental S 0 or amorphous MoS 3 (S 2 2À ), which is further confirmed from the S 2p spectrum (Figure 4g). [25,26] The lower BE doublet located at 162.7 and 163.9 eV is attributed to MoS 2 , while another doublet located at 164.0 and 165.2 eV is attributed to unreacted elemental S or amorphous MoS 3 , indicating a significant amount of S 0 or S 2 2À species in the as-grown MBE-MoS 2 layer.…”

“…[ 23 ] A second doublet at 233.5 and 236.6 eV can be assigned to the Mo 6+ of MoO 3 due to the oxidation at the near surface as the sample exposed to air before the measurement. [ 24 ] At the low BE side (Figure 4a), two S 2s peaks are fitted, where one belongs to basal S 2− of MoS 2 and the other belongs to unreacted elemental S 0 or amorphous MoS 3 (S 2 2− ), which is further confirmed from the S 2p spectrum (Figure 4g). [ 25,26 ]…”