Abstract:a b s t r a c tWe report the well-controlled preparation of WS 2 nanosheets-CdS nanoparticle heterojunction for photoelectrochemical (PEC) water splitting application. The WS 2 nanosheets with an average thickness of $ 5 nm and lateral dimensions of $200 nm were synthesized via liquid phase exfoliation of bulk WS 2 in water/ethanol solution, followed by deposition onto ITO substrate via electrophoretic method. CdS nanoparticles were grown via facile successive ion layer absorption and reaction (SILAR) method. … Show more
“…6c) at ~2.88 eV (430 nm), which we ascribed to C transition, relative to the transition in the region of the Brillouin zone where the valence and conduction bands are nested 72,74,75 , in good agreement with previous data on exfoliated WS 2 sheets (see also Fig. S11) 41,61,63,65 .…”
Section: Resultssupporting
confidence: 91%
“…6b) was weakly structured, with a weak peak at ~630 nm and a shoulder at ~550 nm, resembling the A and B excitonic transitions (both transitions are labelled according to previous conventions, see schematic diagram in Fig. 6a) 61 . The observed broad absorption features were consistent with previous reports on colloidal semiconducting WS 2 flakes 23,62 , and can be explained as the convolution of the absorption signal arising from the ensemble of nanoflakes in the film 63 .Figure 6( a ) Schematic structure of the optical and electronic transitions in WS 2 ; ( b ) WS 2 thin film absorption spectrum; ( c ) Photocurrent response spectrum against incident photon energy.…”
Section: Resultsmentioning
confidence: 99%
“…The steep increase of photoconductivity at ~1.72 eV and then at ~1.94 eV (639 nm) can be assigned to the onset of optical absorption within the direct bandgap due to the presence of defects and/or of electron/hole-bound excitons (trions) 64,68–71 and to the direct band edge transition (Exciton A), respectively (see schematic diagram in Fig. 6a) 41,61,64,65,71,72 . The photocurrent reached a maximum at around 2.10 eV, and then it slowly decreased at higher energies 66,73 .…”
Two-dimensional transition-metal dichalcolgenides (2D-TMDs) are among the most intriguing materials for next-generation electronic and optoelectronic devices. Albeit still at the embryonic stage, building thin films by manipulating and stacking preformed 2D nanosheets is now emerging as a practical and cost-effective bottom-up paradigm to obtain excellent electrical properties over large areas. Herein, we exploit the ultrathin morphology and outstanding solution stability of 2D WS
2
colloidal nanocrystals to make thin films of TMDs assembled on a millimetre scale by a layer-by-layer deposition approach. We found that a room-temperature surface treatment with a superacid, performed with the precise scope of removing the native insulating surfactants, promotes in-plane assembly of the colloidal WS
2
nanoflakes into stacks parallel to the substrate, along with healing of sulphur vacancies in the lattice that are detrimental to electrical conductivity. The as-obtained 2D WS
2
thin films, characterized by a smooth and compact morphology, feature a high planar conductivity of up to 1 μS, comparable to the values reported for epitaxially grown WS
2
monolayers, and enable photocurrent generation upon light irradiation over a wide range of visible to near-infrared frequencies.
“…6c) at ~2.88 eV (430 nm), which we ascribed to C transition, relative to the transition in the region of the Brillouin zone where the valence and conduction bands are nested 72,74,75 , in good agreement with previous data on exfoliated WS 2 sheets (see also Fig. S11) 41,61,63,65 .…”
Section: Resultssupporting
confidence: 91%
“…6b) was weakly structured, with a weak peak at ~630 nm and a shoulder at ~550 nm, resembling the A and B excitonic transitions (both transitions are labelled according to previous conventions, see schematic diagram in Fig. 6a) 61 . The observed broad absorption features were consistent with previous reports on colloidal semiconducting WS 2 flakes 23,62 , and can be explained as the convolution of the absorption signal arising from the ensemble of nanoflakes in the film 63 .Figure 6( a ) Schematic structure of the optical and electronic transitions in WS 2 ; ( b ) WS 2 thin film absorption spectrum; ( c ) Photocurrent response spectrum against incident photon energy.…”
Section: Resultsmentioning
confidence: 99%
“…The steep increase of photoconductivity at ~1.72 eV and then at ~1.94 eV (639 nm) can be assigned to the onset of optical absorption within the direct bandgap due to the presence of defects and/or of electron/hole-bound excitons (trions) 64,68–71 and to the direct band edge transition (Exciton A), respectively (see schematic diagram in Fig. 6a) 41,61,64,65,71,72 . The photocurrent reached a maximum at around 2.10 eV, and then it slowly decreased at higher energies 66,73 .…”
Two-dimensional transition-metal dichalcolgenides (2D-TMDs) are among the most intriguing materials for next-generation electronic and optoelectronic devices. Albeit still at the embryonic stage, building thin films by manipulating and stacking preformed 2D nanosheets is now emerging as a practical and cost-effective bottom-up paradigm to obtain excellent electrical properties over large areas. Herein, we exploit the ultrathin morphology and outstanding solution stability of 2D WS
2
colloidal nanocrystals to make thin films of TMDs assembled on a millimetre scale by a layer-by-layer deposition approach. We found that a room-temperature surface treatment with a superacid, performed with the precise scope of removing the native insulating surfactants, promotes in-plane assembly of the colloidal WS
2
nanoflakes into stacks parallel to the substrate, along with healing of sulphur vacancies in the lattice that are detrimental to electrical conductivity. The as-obtained 2D WS
2
thin films, characterized by a smooth and compact morphology, feature a high planar conductivity of up to 1 μS, comparable to the values reported for epitaxially grown WS
2
monolayers, and enable photocurrent generation upon light irradiation over a wide range of visible to near-infrared frequencies.
“…A facile, well-known method to prepare MoS 2 few-layer nanoflakes is liquid exfoliation of layered bulk MoS 2 (and other TMDs such as WS 2 ) under ultrasonic wave irradiation. , This method has great potential for the scalable production of two-dimensional (2D) nanosheet-based materials by adjusting its important parameters. During ultrasonic exfoliation, the selected solvent plays an important role, since physical properties such as boiling point, surface tension, and energy along with solubility parameters affect the process and hence the resulting 2D flakes.…”
A facile, low-cost, and straightforward
procedure was developed for vertical deposition of MoS2 quantum dots (QDs)/nanoflakes (NFs) on fluorine-doped tin oxide
(FTO) substrate without any binder. Bulk MoS2 powder was
exfoliated in a bath sonicator followed by tip sonicator inside a
water–ethanol (0.5/0.5 volume ratio) solution. The obtained
MoS2 QDs/NFs are deposited on FTO substrate via a simple
electrophoretic technique which resulted in vertical deposition of
MoS2 nanoflakes. Field emission scanning electron microscopy
(FESEM) and atomic force microscopy (AFM) confirmed the formation
of MoS2 nanoflakes with thickness of ∼20 nm and
lateral dimension of ∼400 nm; transmission electron microscopy
(TEM) and AFM analysis revealed that many single- or two-layer MoS2 quantum dots exist with lateral size of ∼5 nm on average
on the basal plains of MoS2 nanoflakes. Electrochemical
measurements indicated that the vertical MoS2 QDs/NFs/FTO
electrode has a Tafel slope of 74 mV/decade and charge transport resistance
(R
ct) of 16 Ω which is ∼1.9
and 2.7 times smaller than the Tafel slope and R
ct of the nonvertical MoS2 NFs/FTO electrode, respectively.
This unique morphology exhibited excellent stability for the electrocatalytic
hydrogen evolution reaction (HER) after a repeating linear sweep voltammetry
(LSV) test for 1000 cycles.
“…TMDs have a general formula of MX 2 (M represents a transition-metal element and X represents a chalcogen), and there exist the planar layers of X–M–X, which are stacked together only by van der Waals interaction . Thus, they are easily exfoliated into thin-layer entities and exhibit excellent performance toward the electrocatalytic hydrogen evolution reaction (HER). − …”
WS 2 nanosheets−CdS nanorods with heterojunctions were prepared by an ultrasonic/exfoliation method using dimethylformamide as the dispersing agent. CdS nanorods were coupled with small WS 2 nanosheets as a result of exfoliation. An excellent hydrogen production rate of 1222 μmol h −1 (20 mg of catalyst) was achieved over the WS 2 −CdS composite with a WS 2 -to-CdS mass ratio of 1.6:1 under visible-light irradiation (λ ≥ 400 nm). The efficient evolution of hydrogen is attributed to promotion of the separation of photogenerated charge carriers due to the presence of heterojunctions created on the surface.
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