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j.matdes.2018.08.038

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Accepted Manuscript
Green and efficient exfoliation of ReS2 and its photoelectric
response based on electrophoretic deposited photoelectrodes
Xiang Xu, Yaohui Guo, Qiyi Zhao, Keyu Si, Yixuan Zhou,
Jingyao Ma, Jintao Bai, Xinlong Xu
PII:
DOI:
Reference:
S0264-1275(18)30653-1
doi:10.1016/j.matdes.2018.08.038
JMADE 7336
To appear in:
Materials & Design
Received date:
Revised date:
Accepted date:
29 May 2018
11 August 2018
19 August 2018
Please cite this article as: Xiang Xu, Yaohui Guo, Qiyi Zhao, Keyu Si, Yixuan Zhou,
Jingyao Ma, Jintao Bai, Xinlong Xu , Green and efficient exfoliation of ReS2 and its
photoelectric response based on electrophoretic deposited photoelectrodes. Jmade (2018),
doi:10.1016/j.matdes.2018.08.038
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Green and efficient exfoliation of ReS2 and its photoelectric response based on
electrophoretic deposited photoelectrodes
Xiang Xua, Yaohui Guoa, Qiyi Zhaoa, Keyu Sia, Yixuan Zhoua, Jingyao Maa, Jintao Baia,*, Xinlong Xua,b,*
a
Shaanxi Joint Lab of Graphene, State Key Lab Incubation Base of Photoelectric Technology and Functional Materials,
International Collaborative Center on Photoelectric Technology and Nano Functional Materials, Institute of Photonics &
Photon-Technology, Northwest University, Xi'an 710069, China.
Guangxi Key Laboratory of Automatic Detecting Technology and Instruments, Guilin University of Electronic Technology,
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Guilin 541004, People’s Republic of China
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Abstract: Environmentally friendly synthesis and scalable photoelectrode fabrication are important for the photoelectric conversion
in photocurrent generation. We find that 28% ethanol in 72% deionized water are the best mixed solvent for efficient exfoliation of
layered ReS2 by absorption spectra and the enthalpy of mixing theory. Electrophoretic deposition method is used to fabricate
photoelectrodes with different thicknesses by controlling the deposition time. These photoelectrodes demonstrate fast and stable
photoelectric response and enhanced photocurrent with the thickness of the nanosheet films controlled by the deposition time. We
also discuss the photoelectric mechanism as well as the influence of the effective mass of the monolayer, two-layer, and bulk ReS2
via Vienna ab initio Simulation Package (VASP). This work paves the way for the solvent selection in exfoliation of ReS2 nanosheets
as well as the scalable fabrications for the photodetectors, photoelectrodes, and other solar energy conversion devices.
Keywords: Mixed-solvent, ReS2, Electrophoretic deposition, Photoelectric response, First-principle calculations
Introduction
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1.
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With high mobilities, novel optical properties, and intrinsic band-gaps in visible region transition metal dichalcogenides (TMDs)
have emerged as a significant class of materials for solar energy conversion[1-3]. For this purpose, photoelectrodes have been
fabricated with different kinds of TMDs such as MoS2[4], MoSe2[5], Bi2S3[6] for photocurrent generation as well as for the
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photoelectrolysis of water. The photoelectric conversion efficiency is dependent on several issues such as the band-structure of the
TMDs materials, environmentally friendly synthesis, and scalable photoelectrode fabrications. Significant exploration has been
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made in this direction to develop photoactive TMDs films with good photoelectric response.
Rhenium disulphide (ReS2), a new member of TMDs family, also have the strong in-plane covalent interaction and weak out-ofplane van der Waals interaction like other traditional TMDs such as MoS2[7], WS2[8], WSe2[9]. However, ReS2 demonstrates unique
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distorted octahedral (1T) crystal structure with triclinic symmetry, which renders many distinctive features from MoS2 and WS2. As
such, ReS2 remains a layer independent direct bandgap, which does not show the transition from indirect bandgap to direct
bandgap[10]. The band-gap of the bulk ReS2 is 1.35 eV and the band-gap of the monolayer is 1.43 eV, which is in the visible region
with excellent environment stability[10]. This direct band-gap in visible region suggests that few-layered ReS2 could have efficient
locking lasers[11],
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photoelectric conversion compared with few-layered MoS2 and WS2. This optical property also shows broad applications in modetransistors[12], photodetectors[13] and hydrogen evolution reaction catalysis[14]. Most of these applications
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are based on few-layered ReS2 due to high absorption and large current on/off ratio[15].
Green, efficient, and large-scale synthesis method is also important for future photoelectrodes based on few-layered ReS2.
Mechanical exfoliation method can get clean ReS2 layers with low efficiency and low scalibity[16]. Chemical vapor deposition
(CVD) is a controllable method for large-scale ReS2 synthesis[17]. Nonetheless CVD is still complex and expensive for massive
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production at harsh experimental conditions with many influence factors. Liquid-phrase exfoliation (LPE) method is a low-cost
method with high yield compared with mechanical exfoliation and CVD[4, 6]. Even though LPE has been used for exfoliation of
ReS2 in N-methyl-pyrrolidone (NMP)[18], green and efficient solvent is desirable for the LPE as organic solvents (such as NMP)
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is toxic and nonvolatile. As efficient exfoliation is dependent on the solvent-solution interaction, a mixed-solvent exfoliation
method[6, 19] can tune the Hansen solubility parameters (HSP) by the change of the two-component solvents with better solubility
than single-component solvent[20]. However, to our best knowledge, there is still no report about the LPE synthesis with a proper
mixed solvent to exfoliate ReS2 effectively and environment friendly.
Scalable photoelectrode fabrication based on ReS2 films is also of importance for photoelectric devices. Vacuum filtration is one
of the common method for photoelectrode fabrication based on TMDs[4]. However, transfer progress is time-consuming and
contaminated by the filtration membrane removal with acetone. Electrophoretic deposition (EPD) is a versatile technology used for
a wide range of applications such as energy storage[21], super capacitors[22], sensors[23], and optical modulators[24]. In EPD
process, nanosheets or nanoparticles in solution would move to the substrate due to the applied electric field and then they would
lose the charges at the electrode after possible electrochemical reactions or physical aggregation and finally the deposited layer is
formed[25]. EPD demonstrates fast deposition rate, short processing time, large deposition area, high packing density, controllable
thickness, and flexibility of the deposition apparatus for low-cost and efficient photoelectrode fabrication[26, 27]. This suggests that
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EPD would be an appropriate method to fabricate high quality photoelectrodes based on ReS2 films.
In this paper, we report a mixed-solvent strategy for the facile and green preparation of few-layered ReS2 nanosheets by
exfoliating bulk ReS2 in ethanol-water mixture. The surface tension of ReS2 is proved to be approximately 40.73 N.m-1 by analyzing
the absorbance spectra with the enthalpy of mixing theory. ReS2 based photoelectrodes with different thicknesses are fabricated by
EPD method. ReS2 based photoelectrodes demonstrate that few-layered ReS2 is a stable and reliable optoelectronic material. The
photocurrent is a few times larger than that in previous reports[4, 6, 28]. The electronic structure and effective masses of ReS2 have
been calculated to analyze the physical process. Our work afford an easy and friendly fabrication method for the scale and stable
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photoelectrodes based on TMDs materials for both photodetectors and other solar energy conversion devices.
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2.
Experimental
2.1. Liquid-phrase exfoliation method
A mixed-solvent liquid-exfoliation method to fabricate ReS 2 film is as follows. Firstly, 50 mg ReS 2 powder (Alfa Aesar, 99%
powder) is added into 500 mL solvents, which are composed of ethanol and DI water with different volume ratios. Secondly, the
suspension is sonicated with 500 W for 90 min. by a supersonic machine (Qsonica Q700) to exfoliate ReS 2 into nanosheets in a
water bath. Thirdly, the mixed suspensions are stored in serum bottles for 10 days to wipe out residual nanoparticles. Figure 1 shows
photographs of suspensions containing ReS2 nanosheets in different volume ratio of ethanol/DI water.
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2.2. ReS2 photoelectrode preparation
To fabricate ReS2 photoelectrodes, we use the 28% ethanol to obtain suspension with a concentration of 0.1 mg/mL. The
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mixture solution is then transferred into 10 mL centrifuge tubes and centrifuged at a rate of 3000 rpm for 10 min. by a centrifuge
(Centurion Scientific K241) to obtain layered ReS2 with good uniformity. The 75% supernatant with a semitransparent brown color
is then collected for subsequent characterization and EPD fabrication. We use a typical EPD system as shown in Fig. 1(a), in which
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two electrodes are immersed into the above centrifuged supernatant. An F-doped SnO2 (FTO) glass is used as the anode and a Ti
plate is employed as the cathode. Before that, FTO substrates have been ultrasonically cleaned in acetone, ethyl alcohol and DI
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water for 10 min. in sequence. The distance between FTO and Ti plate is kept at 2 cm. A direct current (DC) power source with a
constant voltage of 100 V is employed to provide the electric field. The ReS2 platelets migrate toward the positive electrode when
a DC voltage is on. Three factors determine the deposition rate: the concentration of ReS2 suspension, the applied DC voltage, and
the conductivity of substrate[27]. We control the deposition time of 1 min., 3 min., 5 min., 7 min., 10 min., 15 min. and 20 min.,
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respectively. Then films are dried in vacuum oven at 60 ℃ for 30 minutes and then annealed in Ar gas for 60 min. at 300 ℃ for
compact films on FTO substrates. ReS2 films with different thickness are obtained as shown in Fig. 1(b). It is found that the color
of ReS2 films gets darker with the deposition time increasing. It is found that the color of ReS2 films gets darker with the deposition
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time increasing.
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Fig. 1. (a) Schematic of EPD setup. (b) Photography of ReS2 films with different electrophoretic deposition time. (c) Schematic of photoelectric
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measurement setup. Pt and saturated calomel electrode (SCE) are used as counter electrode and reference electrode, respectively. The aqueous
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electrolyte is 1 mol/L Na2SO4.
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2.3. Characterization
A visible spectrometer (Idea optics Company) is employed to take the absorption spectroscopy of the supernatants and ReS2
based photoelectrodes for further concentration investigation quantitatively.
We have performed the transmission electron microscopy (TEM, FEITF-20) study for the as-prepared ReS2 supernatant.
To judge the thickness of ReS2 nanosheets more accurately, atomic force microscopy (AFM, Dimension Icon (Bruker)) under a
tapping mode.
The Raman spectrums of ReS2 photoelectrodes with different thickness are measured by a Micro-Raman spectroscopy (Renisha
Win Via) excited at 633 nm.
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Crystalline properties of the ReS2 based photoelectrodes are investigated via X-ray diffraction (XRD) spectroscopy (DX-2700,
Hao Yuan Instrument with Cu Kα radiation source).
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2.4. Photoelectric response
Photoelectric measurement is carried out by using a 3-electrode photoelectrochemical cell (Fig. 1(c)), with a platinum counter
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electrode, a saturated calomel electrode (SCE) reference electrode, under a xenon lamp with exciting power of 100 mW/cm2 (Zolix
SS150) and a bias voltage of 0.8 V. The experiment data are taken by an electrochemical workstation (Shanghai Chenhua CHI660E).
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ReS2 films are used as the working electrode with an area of approximately 1.5 cm2. Electrolyte is 1 mol/L Na2SO4.
2.5. Electronic band structures
The electronic band structures of monolayer ReS2, 2-layers ReS2, and bulk ReS2 have been made via the Vienna ab initio
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Simulation Package (VASP)[29, 30]. The Green-Wannier (GW) version of Perdew, Burke, and Ernzerhof (PBE) parametrization
has been used within the framework of generalized gradient approximation (GGA). In order to ensure the energy and structural
convergence sufficiently, the cut off energy of the plane-wave basis set is 650 eV. The energy relaxation is taken as 1.0 × 10-5 eV
with the Hellmann–Feynman force between each atom is set to less than 0.01 eV/Å. Besides, 7×7×7 Monkhorst–Pack meshes are
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used to sample the Brillouin zones.
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3.
Results and discussion
3.1. Solvent choice
Ethanol is a green, volatile, and common solvent. Our work is based on the ethanol and de-ionized (DI) water of different
volume fraction. In the mixed-solvent strategy, it is significant to acquire the optimum mixing ratio of water and ethanol. The
dispersion of nanomaterials in liquid can be predicted by the following equation[31]:
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 s   NS
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(1)
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H Mix 
Where ΔHMix is the enthalpy of mixing. γs and γNS are the total surface energies of solvent and nanosheet respectively. TNS is the
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nanosheet thickness and φ is the dispersed nanosheet volume fraction. To obtain the maximum dispersed concentration, ΔHMix
should be as low as possible. This means that the surface energy of solvents should be very close to the surface energy of the
   s  TSs
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nanosheets. In this case, we work in term of solvent surface tension, which can be expressed as:
(2)
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Where Γ is solvent surface tension and Ss is the surface entropy. The value of TSs is approximate 29 mJ/m2 for almost all liquids at
room temperature[32]. The closer solvents and ReS2 nanosheets in surface tension are, the higher absorbance of suspension is. We
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experimentally find the best mixing ratio of ethanol and DI to exfoliate ReS2 and predict the surface tension of ReS2 nanosheets.
3.2. Effective exfoliation solvent
Fig. 2(a) shows photographs of suspensions containing ReS2 nanosheets in different volume ratio of ethanol/DI water. ReS2
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exhibits obviously different dispersion capacity in ethanol/DI water mixtures with different volume ratios. As the volume ratios of
ethanol reaches 28%, the darkest dispersion of ReS2 was obtained. A visible spectrometer is employed to take the absorbance
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spectroscopy of the supernatants for further concentration investigation quantitatively. Fig. 2b shows the absorbance of supernatants
with 28% ethanol volume fraction. In order to verify the dispersed concentration in Fig. 2(a), we take the absorbance at 750 nm of
these supernatants which is shown in Fig. 2(c). We find that the concentrations of ReS 2 dispersions are strongly dependent on the
volume fraction of ethanol and DI. The maximum dispersed concentration appears at the point with 28% ethanol volume fraction,
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which is consistent with the results in Fig. 2(a). This suggests that 28% ethanol is the best solvent to exfoliate ReS 2. In order to
explore the surface tension of ReS2 nanosheets, we first find the relationship between surface tension and Mole fraction of ethanol
according to the report from Makoto et al[33]. As shown in Fig. 2(d), they show the exponential dependence, which can be fitted
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by an exponential function. We also plot the absorbance of ReS 2 supernatant as a function of surface tension as shown in Fig. 2(e),
which implies the best surface tension of ReS2 to be 40.73 N.m-1 for high exfoliation.
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Fig. 2. (a) Photograph of ReS2 dispersions in different volume ratio of ethanol/DI water, which have been stored under ambient conditions for 10
days. (b) The absorbance spectra of ReS2 dispersions with 28% ethanol volume fraction. (c) The absorbance of ReS 2 dispersions in ethanol/DI
water mixture at 750 nm with different ethanol volume fraction. (d) Surface tension of different Mole fraction of ethanol (black triangle). Red line
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is exponential function fitting line. (e) Absorbance of ReS 2 supernatant as a function of surface tension at 750 nm.
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3.3. Characterization
Fig. 3(a) shows the absorption spectra of these ReS2 films. It is found that the absorbance increases with the increasing of
deposition time. The energy band-gap of the semiconductors can be calculated by following equation 3[34, 35]:
Ahν=A (hν-Eg)n/2
(3)
Here, A and hν stand for absorbance and the energy of
irradiation, respectively. Furthermore, the value of n depends on the characteristic of the transition in a semiconductor: for direct
transition n=1 and for indirect transition n=4[36, 37]. Due to ReS2 are direct transition semiconductor, the value of n is 1. Fig. 3(b)
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is the linear transformation hν- (Ahν)1/2 curves of absorption spectra of ReS2 photoelectrodes. The optical band gap energy of ReS2
photoelectrodes with 10 min., 15 min., and 20 min. deposition time are calculated to be 1.27 eV, 1.29 eV, and 1.35 eV, respectively.
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The band gaps of other samples are also calculated to be 1.25-1.33 eV.
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Fig. 3. (a) Absorption spectra of ReS2 photoelectrodes with different deposition time. (b) Plots of (Ahν)1/2 versus (hν) of 10 min., 15 min., and 20
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min. samples.
Fig. 4(a) and (b) show the typical TEM images of ReS2 nanosheets at different magnifications. The ReS2 nanosheets are randomly
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folded and wrinkled (Fig. 4(a)) and these ReS2 films are few-layered ReS2 nanosheets. Fig. 4(b) shows that ReS2 films have an
ordered crystal structure with a good crystalline quality. The thickness of ReS2 nanosheet obtained is ~3.9 nm, ~4.5 nm and ~5 nm,
respectively by the analysis of height profiles as shown in Fig. 4(d), (e) and (f) (across the red①, blue② and green③ lines in Fig.
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4(c)). The nanosheets have smooth surface and very well-defined edges.
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Fig. 4. (a) and (b) TEM images of layered ReS2 nanosheets at different magnifications. (c) AFM image of ReS 2 nanosheets. (d), (e), (f) height
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profiles along the red (①), blue (②) and green (③) lines in (c), respectively.
Fig. 5(a) shows the Raman spectrum of ReS2 photoelectrodes with different thickness. There is no significant peak shift in the
spectrum among different thickness ReS2 photoelectrodes since ReS2 is electronically and vibrationally decoupled[10]. Fig. 5(b)
shows the case of 20 min. deposition time, several modes can be observed in the frequency range from 100 cm-1 to 300 cm-1 due to
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the low crystal symmetry of ReS2. The four Eg-like Raman peaks locate at 150.91 cm-1, 160.85 cm-1, 211.11 cm-1 and 233.76 cm-1
corresponding to the in-plane vibrations of Re atoms[5, 13]. And two Ag-like Raman peaks locate at 133.23 cm-1 and 142.07 cm-1
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corresponding to the out-of-plane vibrations of S atoms[5, 13]. The spacing between first peaks of Ag-like and Eg-like is 17.68 cm1
, which manifests the monolayer structure[38]. Although ReS2 photoelectrodes are accumulated by ReS2 sheets with different
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layered numbers via EPD method, it could be inferred that the majority of the ReS2 photoelectrodes are monolayer.
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Fig. 5. (a) and (b)Typical Raman spectrum of the ReS2 film. The dash lines point at the vibration modes. (c) XRD patterns of ReS2 films with
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different deposition times.
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In order to further investigate the crystalline properties of the ReS 2 based photoelectrode, we conducted XRD spectroscopy as
shown in Fig. 5(c). The peaks at 26.61, 33.89, 37.95, and 51.78 are from the diffraction of FTO glass (JCPDS Card No. 41-
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1445). The 2θ of 14.59 are corresponded to the (0 0 1) planes, which are in good
agreement with the 2H-type ReS2 (JCPDS Card No. 52-0818), implying that ReS2 nanosheets are successfully deposited on the
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surface of FTO. Meanwhile, the intensity of (0 0 1) peaks increases with the increasing of the deposition time. According to the
Scherrer Equation, the higher intensity represents larger grain size and better crystallinity[39]. We also measure the
photoluminescence (PL) spectroscopy of the ReS2 based photoelectrodes. However, there is no obvious PL signal in all samples at
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room temperature. We consider that there are some defects in our liquid-phrase exfoliation samples. If electrons or holes are trapped
by the defects, the PL signal can not be obtained. In fact, these defects have been reported and used to explain the sub-bandgap
absorption of TMDs. The electronic and optical properties of TMDs materials will be renovated due to the defect-state theory[39,
40]. However, these may contribute to the photoelectric experiment. As we know, the recombination rate of photoinduced charge
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carriers is proportional to the PL peak intensity, and the low recombination rate is the key point of photoelectric experiment.
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3.4. Photoelectric response
To reveal the photoelectric response performance with different thickness of the samples, the voltammogram by a linear sweep
voltammtry is shown in Fig. 6(a). Firstly, it can be seen that there is almost no photocurrent for all photoelectrodes at bias potentials
less than 0.2 V. Secondly, the photocurrent is relatively low but with an increasing trend at bias potentials from 0.2 to 0.5 V. Finally,
the photocurrent increases conspicuously at the bias potential from 0.5 to 0.9 V. The photocurrent is also growing with the thickness
increasing of the photoelectrodes. The ReS2 photoelectrode with the deposition time of 1 min. generates a photocurrent of 0.11 mA
at 0.8 V under visible light irradiation, while ReS2 photoelectrode with the deposition time of 20 min. generates a photocurrent of
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0.92 mA under the same condition.
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Fig. 6. (a) Linear sweep voltammograms measured from photoelectrode with different deposition time. (b) Photocurrent switching
changes of the ReS2 based photoelectrodes. (c) Deposition time dependent photocurrent. Red line is an exponential function fitting line. (d)
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Schematic of ReS2 photoelectrode response with light illumination.
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Within on/off period light illumination, I-t curves are collected from the ReS2 samples with different deposition time to examine
the photoresponse of these photoelectrodes as shown in Fig. 6(b). It is found that the fast and uniform photocurrent shows a periodic
increasing and decreasing by switching on and off light in the photoelectrodes. At the beginning, there is not any photocurrent
response in the dark state. Then when the light is switched on, a rapid photocurrent response can be acquired from all the
photoelectrodes. At last, the photocurrent gradually gets steady. It is noteworthy that the photocurrent response of FTO glass is
pretty small compared with ReS2 photoelectrodes. This means that the strong photocurrent response can mainly be attributed to the
photo-activity from the ReS2 nanosheets. The transient photocurrent density of these photoelectrodes with different times (1 min.,
3 min., 5 min., 7 min., 10 min., 15 min., 20 min.) are approximately 0.35 μA, 0.62 μA, 0.89 μA, 0.95 μA, 1.15 μA, 1.18 μA, 1.25
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μA, respectively. The value of 1.25 μA is higher compared with previous works[4, 6, 28] with the vacuum filtration method. This
may be attributed to not only the ReS2 with a direct band-gap, but also the EPD method with high packing density.
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Fig. 6(c) shows the photocurrent as a function of deposition time, it can be seen that with the increasing of deposition time, the
photocurrent increases gradually and finally saturates with the deposition time. This suggests that increasing the thickness of the
ReS2 will not lead to the further increasing of photocurrent. The photoelectric processes are shown in Fig. 6(d). Firstly electron-hole
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pairs in the ReS2 films are induced by illumination with energies larger than that of the band gap. Secondly, the electrons are collected
by FTO with an applied bias while the holes are captured on the surface of ReS2. Finally the electrons transfer to external circuit
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and the holes are gradually consumed by the oxidation reaction in electrolyte. The rising of photocurrent with the deposition could
be attributed to more absorbance of the light with the thickness of the films. However, after the absorbance depth of the ReS2 films,
the number of ReS2 nanosheets that actually participate in the photoelectric response may gradually decrease, which result in
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saturation finally.
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Fig. 7. Rising and decay response time of photocurrents from (a) 1 min., (b) 10 min., (c) 20 min. deposition time.
Fig. 7 presents the time-dependent responses of different ReS2 films with 1 min., 10 min. and 20 min. deposition time,
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respectively. It can be seen that the rising time is ~ 0.4 s, ~ 0.9 s, and ~ 1.6 s for 1 min., 10 min. and 20 min. deposition time,
respectively. And the decay response time for these photoelectrodes is 1.2 s, 1.2 s and 1.3 s, which are almost constant. In our
experiments, the time-dependent responses are related to the diffusion and drift of photo-generated carriers near the depletion layer
as well as the the capacitor and inductor of the whole circuit. With the increasing of the thickness of ReS2 films, the drift time of
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carriers and time constant of circuit increasing could lead to the rising of response time.
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Fig. 8. ESI spectra of ReS2 based photoelectrodes with different thicknesses. (a) Nyquist plots. (b) Bode phase plots.
In order to investigate the charge transportation process and recombination properties in these ReS 2 based photoelectrodes, we
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measured the EIS of these photoelectrodes in a frequency range from 0.01 Hz to 10000 Hz. The radius of the semicircle is related
to the charge transfer resistance at electrode/electrolyte
interface. Normally, the smaller the radius, the lower the charge
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transfer impedance at the interface[37, 41, 42]. According to the Fig. 8(a), the radii of the semicircles at middle-frequency decrease
with the increasing of the deposition time. In addition, the Bode phase plots (Fig. 8(b)) correspond to the electron life time (τe) via
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τe=1/(2πfmax)[43]. The comparison of samples with the deposition time of 1min. and 20 min. is inserted in Fig. 8(b). We can find
that peak frequencies shift to lower value while the deposition time is increasing from 1 min. to 20 min., which indicate that there
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is a lower recombination of e-h pairs for longer deposition time.
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3.5. Band structures of ReS2
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Fig. 9. Band structures of (a) monolayer ReS2, (b) 2-layer ReS2, (c) bulk ReS2. The Fermi level is indicated as the dash line at E=0.0 eV.
As shown in Fig. 9, ReS2 always display a direct band gap of approximately 1.46 eV, 1.32 eV, and 1.22 eV from monolayer to
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bulk state, which suggest that ReS2 can absorb visible light and have the potential to transfer solar energy into electric energy with
high efficiency. The carrier mobility plays a significant role in the photoelectric processes. It has been experimentally demonstrated
inversely proportional to its effective mass as
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that the mobility of ReS2 generally increases with an increasing of the sample thickness[44]. It is known that the carrier mobility is
  e /  m *  [45]. The effective mass m* can be easily derived from the band
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dispersion Ek by using the following formula[45]:
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(4)
d 2 / dk 2
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Where ћ is the reduced Planck’s constant. The effective masses of monolayer ReS2, two-layer ReS2, and bulk ReS2 are 1.94 m0, 1.53
m0, and 0.71 m0, respectively, where m0 is free electron mass. This suggest that the mobility of ReS2 generally increases with an
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increasing of the thickness and further lead to the increasing of photocurrent.
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4.
Conclusion
In summary, we have demonstrated a mixed-solvent strategy for the green and efficient preparation of few-layered ReS2 films
by exfoliating bulk ReS2 in the ethanol-water mixture. The surface tension of ReS2 is suggested to be 40.73 N.m-1. TEM, AFM,
Raman spectroscopy and absorption spectroscopy measurement suggest that the ReS2 nanosheet films could maintain the properties
of few-layered materials with good quality. ReS2 based photoelectrodes are fabricated by an EPD method, which manifests stable
and sensitive photoelectric response with fast and high photocurrent. The calculations of electronic structures of monolayer, 2-layers,
and bulk ReS2 by first-principle give a theoretical explanation for the photoelectric response. The results could be a guidance for
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efficient and scalable photoelectrode fabrication based on ReS2 for solar energy conversion devices.
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Acknowledgement
This work was supported by National Natural Science Foundation of China (No. 11774288, 61605160), Key Science and
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Technology Innovation Team Project of Natural Science Foundation of Shaanxi Province (2017KCT-01), and Guangxi Key
Laboratory of Automatic Detecting Technology and Instruments (YQ17201).
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Graphical abstract
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Highlights
A green and scalable liquid-phrase exfoliation of layered ReS2 with a mixed-solvent
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strategy is used for the fabrication of ReS2 nanosheets.
The surface tension of ReS2 is obtained by analyzing the absorbance spectra with the
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enthalpy of mixing theory.
Different thicknesses of ReS2 photoelectrodes are fabricated via an efficient
electrophoretic deposited method by controlling the deposition time.
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The ReS2 based photoelectrodes exhibit stable and sensitive photoelectric response with
fast and high photocurrent.
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The electronic structures and effective mass of monolayer, 2-layers, and bulk ReS2 are
calculated by first-principle calculations via VASP.
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