In-Plane Axially Enhanced Photocatalysis by Re4 Diamond Chains in

Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

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In-Plane Axially Enhanced Photocatalysis by Re4 Diamond Chains in Layered ReS2 Perumalswamy Sekar Parasuraman,† Jhih-Hao Ho,‡ Min-Han Lin,† and Ching-Hwa Ho*,† †

Graduate Institute of Applied Science and Technology, National Taiwan University of Science and Technology, Taipei 106, Taiwan Department of Chemistry, National Central University, Zhong-Li, Taoyuan 320, Taiwan

‡

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S Supporting Information *

ABSTRACT: Two-dimensional (2D) semiconductors play a crucial role in high-efficiency photocatalysts because of their high surface-to-volume ratio. The surface property is a key part of photocatalysis. In this work, the enhanced photocatalytic behavior of the layered ReS2 with optical polarization along the Re4 nano-diamond-chain (DC) direction (b axis) has been demonstrated. The unpolarized photoconductivity (PC) response of ReS2 with an applied bias along the b axis is approximately 1 order higher than that of the applied bias perpendicular to the b axis. The polarization-dependent PC spectra of E ∥ b also reveal a higher photoresponsivity with respect to those measured along the E ⊥ b polarization for the layered ReS2. This result indicates that stronger polarization dipoles as well as a larger amount of photogenerated carriers and surface states can contribute to the C-plane ReS2 under the illumination of E ∥ b polarized photons. With the special axial effect, the layered ReS2 2D photocatalyst shows much faster degradation rates of 5.6 and 12.3 than the other transition-metal dichalcogenides of TaS2 and MoS2 for the degradation of methylene blue (MNB) solution. For the polarization-dependent photodegradation test, the degradation rate of illuminated E ∥ b polarized photons is also approximately 12 times faster than that of the illuminated E ⊥ b polarized light in a 25 μM MNB solution. The enhanced photocatalytic behavior of ReS2 along the DC also shows a peak photoreponse of ∼25 μV detected in the polarized photovoltaic spectrum of the 0.5 μM MNB dye-sensitized solar cell positioned at ∼1.99 eV. The formation of a nano-DC and a one-layer trigonal crystalline phase is beneficial for the versatile energy applications of ReS2.

1. INTRODUCTION Energy source and environmental pollution are two major issues for human, for which techniques should be developed for the creation of energy and prevention of pollutants.1−3 “Photocatalysts” are therefore important for hydrogen evolution reaction (HER), water splitting, organic pollutant degradation, and also for light harvesting into electricity.4,5 For an efficient semiconductor photocatalyst, it should possess the following characteristics: (1) it should have a longer carrier lifetime to prevent the recombination of electron−hole pairs (EHPs). (2) The photogenerated charge carriers should be easily created and then the redox reaction should be quickly activated. (3) It should have a suitable band gap (>1.2 eV) and a proper band position for the activation of the redox potential. (4) It should be easily separated and fast transfer of EHPs should take place across the surface and interface. To achieve the above points, the facet effect has become an important factor for a proficient photocatalyst. The photocatalytic behavior is highly sensitive to surface orientation, surface-tovolume ratio, and surface condition of the photocatalyst. Some wide band-gap semiconductors of nano-sized oxides such as TiO2 and ZnO have been reported for the application in HER and pollutant degradation to date.6,7 However, they fail to absorb the visible light because of the fact that their larger band © XXXX American Chemical Society

gap leads to poor visible performance. Only a few oxide photocatalysts, such as Ta2O5, can have the visible-light photocatalytic ability evaluated from the band-gap value (i.e., 2.4 eV).8 Two-dimensional (2D) semiconductors such as transitionmetal dichalcogenides (TMDCs) MX2 (M = Mo, W, Re and X = S, Se) are potential photocatalysts9 owing to their suitable band-gap value, large surface area, abundant reaction sites, and reduced migration distance for photogenerated EHPs under the reaction process. However, the band gaps of some 2D TMDCs are highly dependent on the number of layers, and precise control over the layered number may be a crucial factor in photocatalytic applications.10,11 MoS2 is therefore not applicable for all the photocatalytic reactions. Multilayered ReS2 is a direct semiconductor, which is currently used in various research studies owing to its specific in-plane anisotropy,12,13 polarization-sensitive property,14 high photoelectrochemical stability,15 and weak layer-to-layer coupling to keep the band edge unchanged for 2D exfoliation.16 Unlike most of TMDCs (e.g., MoS2, MoSe2, WS2, and WSe2) with a Received: June 21, 2018 Revised: July 25, 2018 Published: July 25, 2018 A

DOI: 10.1021/acs.jpcc.8b05946 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

ReS2, 960 °C → 900 °C (gradient of −3 °C/cm) for the growth of MoS2, and 900 °C → 800 °C (gradient of −5 °C/ cm) for the growth of TaS2, respectively. The reaction was maintained for 10 days for the production of large layered crystals. After the growth, the layered ReS2, MoS2, and TaS2 single crystals with a maximum area size up to 1.5 cm2 and thicknesses up to 100 μm that formed silver-colored and mirror-like crystalline surfaces were obtained. The layered TMDC crystals exhibited high crystallinity in the whole bulk piece and showed obvious C-plane surfaces.17 The layered crystals can be thinned out to nanoflakes (∼nanometer thick) along the c axis using a Scotch tape with mechanical exfoliation. The ReS2 nanoflakes were then transferred onto a SiO2/Si substrate for micro-thermoreflectance (μ-TR) experiments. The thickness of the layered single crystals used for the photocatalytic experiments was controlled to be about 800 nm and about 2.5 × 1.2 mm2 in area. Powder X-ray diffraction measurements showed that ReS2 has a triclinic structure (distorted 1T), MoS2 has a two-layer hexagonal (2H) structure, and TaS2 has a 2H structure, respectively. 2.2. Optical Characterization. For the PC measurement, the layered ReS2 was first thinned out and then separated along DC (b axis) to form a striplike sample with a longer crystal edge along the DC direction (i.e., in-plane mechanical anisotropy). The stripelike crystal was then cut into a specimen of area ∼2.5 × 1.2 mm2. The two contacts of the PC samples were made and arranged in bias ∥ b and bias ⊥ b configurations as displayed in the inset of Figure S1b in the Supporting Information. A loaded resistor RL (∼1 MΩ) was connected in series with the PC samples, and a 30 V bias was applied to the circuit. A 150 W tungsten halogen lamp filtered by a PTI 0.2-m monochromator provided the monochromatic light. A Glan−Taylor prism polarizer (wavelength 210−2300 nm) was employed for the polarization-dependent PC measurement. Phase-sensitive detection by using an optical chopper and a lock-in amplifier was employed. The chopped frequency of the incident light was set as 25, 50, 100, and 200 Hz, and the spectral photoresponse and PC intensities of ReS2 were also evaluated. For the μ-TR measurement, a light-guiding microscope (LGM) equipped with one Olympus objective lens (50×, working distance ≈ 8 mm) acted as the interconnection coupled medium between the ReS2/SiO2 nanosheet sample, incident, and reflected lights. The LGM was integrated in a RAMaker microscope integrated system. The same monochromatic light source as the PC system was guided by an optical fiber for the incident light. The reflected light of the nanosheet sample via an LGM was then sent to an EG&G HUV2000B Si photodetector. For thermal modulation of the multilayered sample, a gold-evaporated quartz substrate acted as the heating element. The ac heating modulation of the sample by using a phase-sensitive detection of the lock-in amplifier was employed for the μ-TR experiment. A Janis liquid-helium open-cycled cryostat equipped with a Lakeshore 335 digital thermometer controller facilitated the low-temperature measurement of the multilayered ReS2. A pair of dichroic sheet polarizers (in the visible-to-infrared range) was utilized for polarization-dependent measurements. 2.3. Chemical Reagent and Photoelectrochemical Experiments. MNB (C16H18ClN3S) powder was purchased from Kojima Chemical Co LTD. Deionized (DI) water, magnetic stirrer, copper foil, and Xe-arc lamp (with LPS-250 power supply) from photonics technology international (PTI)

uniform and isotropic C-plane, the 2D layered ReS2 possesses specific in-plane mechanical,17 optical,18 and electrical anisotropy19 owing to the formation of Re4 “diamond shape” clustered chains along the b axis also a distorted one-layer trigonal (1T) structure of triclinic symmetry.20−22 This consequence is due to the fact that the 1T-ReS2 phase has a nonbonding Re d3 electron number, which may easily exhibit the Re4 diamond-chain (DC) mode of structural distortion caused by the Re−Re bond interaction. However, some technical applications of oriented effects on the optical,23 electrical,24 and energy storage25 properties of ReS2 have been reported. The DC axial effect on the photogenerated carriers, photocatalytic behavior, and dye-sensitized function to power generation by ReS2 was rarely reported. One recent study showed highly efficient hydrogen evolution by ReS2 depending on the two-electron catalytic reaction caused by excitons (h+− e−) and tritons (h+−2e−).26 However, the excitonic transitions in ReS2 are highly polarization-dependent,12,17 and thus, a comprehensively polarized photocatalytic study of DC-ReS2 is needed to be explored for future energy-related applications. In this paper, the polarization-dependent photocatalytic behavior of ReS2 using methylene blue (MNB, C16H18N3ClS) solution as the target electrolyte was evaluated. The experiments were done with the incident linearly polarized photons along b (DC) and perpendicular to the b axis using a xenon (Xe)-arc lamp. For a 25 μM MNB solution, the photodegraded rate constant of E parallel to DC is 12 times faster than that of E perpendicular to DC for the DC-ReS2. It means that the photogenerated EHP number, transited interface carriers, and carrier lifetime along DC are much higher than those of E perpendicular to DC on the layered ReS2 surface. With this specific DC contribution, the photocatalytic ability of ReS2 is higher than that of other TMDCs, MoS2 and TaS2, for the photodegradation of MNB. The orientation-dependent polarized photoconductivity (PC) result showed that the photocurrent of E ∥ b and bias ∥ b is approximately 10 times greater than that of the other E ⊥ b and bias ⊥ b condition in the Cplane ReS2. This result shows the high efficiency of E ∥ DC polarization in the polarized photocatalytic behavior of ReS2. The Ex1 exciton with E ∥ DC polarization (at ∼1.495 eV) mainly dominates the polarized photocatalytic effect of the layered ReS2. The enhanced photocatalytic behavior by the Re4 clustering chain was also tested via the dye-sensitized solar cell (DSSC) structure using the MNB solution as the electrolyte. A peak photovoltaic response of ∼25 μV at ∼1.99 eV can be detected in the photoresponsivity spectrum of ReS2 DSSC (0.5 μM concentration) with E ∥ b polarization. The existence of Re4 DC makes ReS2 an axially enhanced photocatalyst for future energy and environmental applications.

2. EXPERIMENTAL SECTION 2.1. Crystal Growth of Layered TMDC Materials. Layered ReS2, MoS2, and TaS2 were grown by the chemical vapor transport (CVT) method using I2 as the transport agent. The compounds were prepared from the powdered elements (Re: 99.999% pure, Mo: 99.99% pure, Ta: 99.99% pure, and S: 99.999%) and sealed in evacuated quartz ampules with a pressure of ∼10−6 Torr. About 10 g of the stoichiometric mixture together with an appropriate amount of the transport agent (I2 of about 12 mg/cm3) was introduced into a quartz ampule (22 mm OD, 17 mm ID, and 20 cm length) and sealed. For crystal growth, the temperatures were set as 1050 °C → 1000 °C (gradient of −2.5 °C/cm) for the growth of B


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three Re atoms, and move toward each other to form a Re4 “diamond shape” cluster chain along the b axis.27 Such a structural distortion of forming a DC results in higher bond strength and also renders a shorter lattice constant along the DC direction of b axis. From the estimated HRTEM and SAED results in Figure 1a,b, the lattice constants of the Cplane ReS2 can be determined to be a = 6.45 Å and b = 6.39 Å, respectively. The DC structure and the values of lattice constants are denoted in the inset of Figure 1a for comparison. Next to this structural distortion, the in-plane optical anisotropy of the layered ReS2 will be shown. The in-plane optical anisotropy along and perpendicular to DC should be the key factor for achieving an axially enhanced photocatalytic property of ReS2. Figure 1c,d, respectively, shows the axially dependent PC spectra of unpolarized E ∥ b and E ⊥ b polarizations with the applied bias along DC and perpendicular to DC for the layered ReS2. The applied bias is 30 V, which are connected in series to the testing sample and a loaded resistor RL (∼1 MΩ). The contact electrodes and the sample arrangement of bias ∥ b and bias ⊥ b are, respectively, depicted in the inset of Figure S1b for comparison. The dark resistivity of the bias ⊥ b sample is ∼24.5 Ω cm (see in Figure 1d), which is about 4 times larger than that of the bias ∥ b sample shown in Figure 1c. The result shows the in-plane electrical anisotropy of ReS2 with a much higher carrier conductivity (mobility) along DC. For the unpolarized PC spectra in Figure 1c,d, it is clearly observed that the peak photoresponse in Figure 1c (i.e., 7.5 mV at ∼1.86 eV for bias ∥ DC) is higher than that detected in Figure 1d (i.e., 0.85 mV at ∼1.99 eV for the bias ⊥ DC sample). The blue shift of the PC peak position from bias ∥ b to bias ⊥ b can also be observed by the normalized and unpolarized PC spectra as depicted in Figure S1b. The lower-energy absorption peak of PC by the bias ∥ b operation is mainly attributed to the dipole excitonic effect coming from the direct band edge of ReS2 along DC with lower energy (i.e., exciton 1, Ex1). The excitonic effects of ReS2 will be evaluated and discussed by the polarized μ-TR experiment later. As shown in the polarizationdependent PC spectra in Figure 1c,d, the polarized PC peaks of E ∥ b (i.e., dominated by Ex1) also showed lower energy with respect to those of the other E ⊥ b polarized spectra in both of the bias ∥ b and bias ⊥ b conditions. However, all the PC peak intensities of unpolarized E ∥ b and E ⊥ b in the bias ∥ b condition in Figure 1c simultaneously showed ∼8.8 times larger than those detected by the bias ⊥ b operation in Figure 1d. The PC photoresponsivity change (ΔVPC) under dark and monochromatic light illumination is expressed by an expression related to the photocurrent change (ΔIph)

were used. For concentration-dependent measurements of MNB photodegradation and the DSSC spectral test, the MNB solutions were prepared from 0 μM (pure DI water) to 100 μM. For the MNB degradation experiment, a 150 W Xe-arc lamp was controlled at the power density of 10 mW·cm−2 via a variable neutral density filter. The transmission measurement of the MNB solution after photodegradation was carried out by a QE65000 spectrometer, where a tungsten halogen lamp was used as the light source. The ReS2 DSSC photovoltaic spectra were measured via a quartz window-equipped Teflon tank. The monochromatic light source was provided by a PTI-0.2m monochromator similar to that of the PC system. All the measured PC and DSSC spectral photoresponses are calibrated using a broadband thermal sensor ranging from 1.25 to 3.5 eV.

3. RESULTS AND DISCUSSION 3.1. Structure and Optical Properties. The highresolution transmission electron microscopy (HRTEM) image and selection-area electron diffraction (SAED) pattern of few-layered ReS2 with the zone axis along [001] are, respectively, shown in Figure 1a,b. The clear atomic sites in the HRTEM image and an obvious dotted pattern in the SAED picture exhibited a high crystal quality of the layered ReS2 grown by the CVT method. Rhenium is a transition-metal element of group VIIB with an electronic configuration of [Xe] 4f14·5d5·6s2. Re has a nonbonding d3 electron number; each Re atom in ReS2 will slip off its regular site, bond to the other

ΔVPC = ΔIph·RL = q·(Δn·μn + Δp ·μp ) ·F ·A ·RL

(1)

where Δn = Δp is the number of photogenerated EHPs, μn and μp are the electron and hole mobility, F is the applied electric field, and A is the cross-sectional area for the current flow. Under the same optical illumination condition in Figure 1c,d, the higher PC photoresponse of the bias ∥ DC condition (∼8.8×) indicates that much more EHPs are generated with the bias applied along Re4 DCs in the C-plane ReS2. The higher density of photogenerated EHPs along DC is a basic requirement for the rapid degradation of the organic pollutant using an efficient 2D layered photocatalyst. Furthermore, a longer lifetime of the photogenerated carrier is also a key factor for sustainability, and the organic dye was continuously

Figure 1. (a) HRTEM image of the C-plane ReS2 with an indication of Re4 diamond clustered chains along the b axis. (b) SAED pattern of the layered ReS2 with the zone axis along the [001] direction. The values of lattice constants estimated from (a,b) are a = 6.45 Å and b = 6.39 Å, respectively. The polarization-dependent PC spectra of unpolarized E ∥ b and E ⊥ b are, respectively, depicted in (c) bias ∥ b and (d) bias ⊥ b for the C-plane ReS2. C


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The Journal of Physical Chemistry C degraded for the photocatalytic process. For an intrinsic optical excitation (Δn = Δp) at t = 0, the concentration of photogenerated carriers followed an exponential decay rate equation as28 δn(t ) = Δn·exp( −t /τn), δp(t ) = Δp ·exp( −t /τp)

(2)

where τn and τp are, respectively, the recombination lifetimes of photogenerated electrons and holes. If τn (τp) is longer, the photogenerated EHPs will continue the photocatalytic reaction with organic pollutants in solution. To evaluate the carrier lifetime-related contribution in ReS2, the frequency-dependent PC measurement of E ∥ b was also implemented. The chopped frequency of the incident monochromatic light was changed from 200, 100, 50, and down to 25 Hz as shown in Figure S1a in the Supporting Information. It is obvious that the PC peak photoresponse of the lowest chopped frequency (25 Hz) is much higher than those of the other high-frequency results (50, 100, and 200 Hz) in the frequency-dependent PC spectra. This observation demonstrated that a longer lifetime (>40 ms) of the photogenerated carrier existed in the C-plane ReS2 along DC. Essentially, the recombination lifetime (τn, τp) of the band edge in a semiconductor is very short in the order of microsecond to nanosecond or lower. The longer lifetime (>40 ms) of frequency-dependent PC responses with bias ∥ b operation in Figure S1a indicated that the defect and surface states may exist in the C-plane ReS2 surface for enhancing the generation of the photocurrent along the b axis. For the formation of a PC spectral-line shape of the layered ReS2 single crystal (see Figure 1c), the direct−gap transition of ReS2 is about 1.5 eV observed from the photoluminescence result.18 At the lower energy part, the PC response is increased with hν > Eg and then it reaches a peak photoresponse of ∼1.86 eV as shown in Figure 1c. Over the PC peak, the photoresponse starts to decrease with increasing photon energy because of the surface recombination effect of the C-plane ReS2. Thus, a broadened PC peak spectrum is formed at 1.5−3.2 eV in Figure 1c. To further understand the polarized band-edge transitions that contributed to the axially enhanced photocatalytic behavior, the polarization-dependent μ-TR measurements of the multilayered ReS2 are implemented. Figure 2 shows the polarized μ-TR spectra of the multilayered ReS2 on the SiO2/ Si substrate using an objective lens of 50× at (a) 300 and (b) 10 K, respectively. The inset in Figure 2a shows the image of ReS2, where the DC direction of the b axis is shown. The thickness of the multilayered ReS2 is about 40 nm. The TR measurement was proven to be a very effective technique for measuring the direct band-edge transitions of TMDCs with different crystal structures.29 The derivative-like reflectance spectrum suppressed unintentional background and emphasized energies and line width of spectral features of interband transitions.30 In Figure 2a of the linearly polarized light along E ∥ DC, only the Ex1 exciton is detected at ∼1.495 eV, whereas the Ex2 exciton of ∼1.53 eV is merely observed with the linearly polarized light of E ⊥ DC polarization at 300 K. Both the Ex1 and Ex2 excitons simultaneously show energy blue shift and linewidth reduction as the temperature lowered down from 300 to 10 K shown in Figure 2b. The energy values of bandedge excitons Ex1 and Ex2 are ≈1.56 and 1.594 eV, respectively, at 10 K. For the higher series transitions of the ES excitons, ES1, ES2, and ES3 features are observed with E ∥ DC polarization, whereas the ES1 ′ , ES2 ′ , and ES3 ′ transitions are detected along E ⊥ DC. The polarized selection rule applied for the C-plane ReS2

Figure 2. Polarization-dependent μ-TR spectra of the multilayered ReS2 at (a) 300 and (b) 10 K to show polarized band-edge excitons. The experiments were done with E ∥ b,E ⊥ b and unpolarized conditions. The image of the multilayered ReS2 with a clear b axis was shown in the inset of (a). (c) Temperature dependences of energy shift of Ex1 and ES1 excitons measured at the E ∥ b polarized condition. (d) Temperature-energy shift of Ex2 and E′S1 measured at the E ⊥ b polarized condition. The corresponding inset respectively shows the angular dependence of polarized intensity change of Ex1 and Ex2 at 300 K. The result clearly indicates that the Ex1 dominates the E ∥ DC polarized condition.

clearly shows that the DC-oriented transitions are Ex1, ES1, ES2, etc.. They are predominated by the main band-edge transition of the Ex1 exciton with lower energy in ReS2. Figure 2c,d, respectively, shows the temperature-energy shift of the E ∥ DC (i.e., Ex1 and ES1) and E ⊥ DC (Ex2 and ES1 ′ ) transitions from 10 to 300 K. They show a temperature-energy variation of the general semiconductor behavior. The insets in Figure 2c,d clearly show angle-dependent amplitudes of polarized bandedge excitons from θ = 0° [E ∥ DC] varying steps to θ = 360° at 300 K. The analyzed results are I(θ) ≈ cos2(θ) = [1 + cos(2θ)]/2 for the Ex1 exciton and I(θ) ≈ sin2(θ) = [1 − cos(2θ)]/2 for the Ex2 exciton, respectively. The polarized photocatalytic activity of ReS2 with E ∥ DC polarization may be mainly correlated with the lower energy band-edge exciton, Ex1. It can efficiently produce more photogenerated carriers along DC that has been evident by the axially dependent PC experiment. The excitonic effect (h+−e−) along DC (promoted by surface states) can also sustain a longer carrier lifetime for the photocatalytic process in the anisotropic ReS2 photocatalyst. 3.2. Photocatalysis Study. The photocatalytic behaviors of CVT-grown layered TMDCs of ReS2, TaS2, and MoS2 are D


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Figure 3. Normalized absorbance spectra of MNB photodegradation of (a) MNB, (b) ReS2 + MNB, (c) TaS2 + MNB, and (d) MoS2 + MNB within 1 h. The concentration of the MNB solution is 50 μM.

100 to 10 μM as shown in Figure S2 of the Supporting Information. The concentration of 50 μM MNB shows the best observation scale of the C/C0 photodegradation change by the ReS2 photocatalyst within 1 h. From the analysis in Figure 4, the photodegradation rate of ReS2 is about 12.3 and 5.6 times faster than those of MoS2 and TaS2, but the photodegradation rates of MNB and TaS2 are comparable. From the C-plane X-ray diffraction measurements shown in Figure S3 in the Supporting Information, the as-grown layered TMDCs of MoS2 and TaS2 (by CVT) belong to a 2H structure and ReS2 is in a distorted 1T phase. The direct band gaps of MoS2 and ReS2 are, respectively, 1.8831 and 1.5 eV,18 and TaS2 is a metal.32 The similar rate constant k obtained for both TaS2 + MNB and MNB in Figure 4 is due to the fact that nearly no effective EHPs are generated in TaS2 under the light illumination for the supplementary enhancement in photocatalysis. The lowest rate constant k of MoS2 + MNB is due to the 2H-MoS2 surface that may present efficient recombination centers for eliminating the photogenerated EHPs. It can also be attributed to a short lifetime of the photogenerated EHPs in 2H-MoS2. The photocatalytic behavior affected by the stacking phase of layered TMDCs can also be found in WS2, where 2HWS2 presents surface recombination centers to diminish the photocatalytic activity when compared to the 1T-WS2.33 The high efficiency of the ReS2 2D photocatalyst may be related to its distorted 1T stacking phase caused by the Re4 DCs. ReS2 also forms an oriented-excitonic band edge to generate a high density of EHPs as well as to prolong the lifetime of carriers. To further evaluate the axially enhanced photocatalytic ability of Re4 DCs, polarization-dependent photodegradation of MNB with E ∥ DC and E ⊥ DC polarizations is carried out. The Xe-arc lamp was filtered by a Glan−Taylor prism polarizer, which acted as the linearly polarized light source of photocatalysis. To get a better sensitivity, a lower concentration of 25 μM MNB solution was used as the target source for photodegradation. Figure 5 shows the normalized

evaluated using the 50 μM MNB solution. The C-plane area of each layered sample was kept at ∼2.5 × 1.2 mm2, which was then put in solution and exposed to light. The volume of the MNB solution is about 4 mL, and a Xe-arc lamp was used as the light source, which was controlled at the power density of 10 mW·cm−2. Figure 3 respectively shows the normalized absorbance spectra of (a) MNB, (b) ReS2 + MNB, (c) TaS2 + MNB, and (d) MoS2 + MNB under the illumination of a Xearc lamp within 1 h. The absorbance spectra of the MNB solution revealed two absorption peaks (i.e., 610 and 663 nm), and the maximum peak-absorption wavelength was located at 663 nm. As the illumination time is increased, all the absorbance peaks decreased. Figure 4 shows the analysis of

Figure 4. Analysis of the photodegradation rate constant of MNB, ReS2 + MNB, TaS2 + MNB, and MoS2 + MNB in Figure 3.

the normalized photoreaction decay of the MNB solution (at 663 nm) using the rate equation C/C0 = exp{−k·t} by the Cplane MoS2, TaS2, ReS2, and the MNB itself, where k is the rate constant. The analyzing results are k = 0.003, 0.0066, 0.0069, and 0.037 min−1 for MoS2 + MNB, TaS2 + MNB, MNB, and ReS2 + MNB, respectively. The choice of the 50 μM MNB concentration for the testing layer TMDCs is based on the concentration-dependent photodegradation rate from E


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Figure 5. Polarization-dependent photodegradation results (absorbance spectra) of (a) E ∥ DC and (b) E ⊥ DC for the C-plane ReS2 in a 25 μM MNB solution.

Figure 6. Representative scheme for the polarized photodegradation of the MNB solution by DC-ReS2. The possible photocatalytic pathway after the optical excitation of the ReS2 polarized band gap (Eg∥ and Eg⊥) is also indicated.

absorbance spectra of MNB by photodegradation with (a) E ∥ DC and (b) E ⊥ DC polarizations using the ReS2 layered photocatalyst. The insets show the representative schemes for E ∥ Re4 DC and E ⊥ Re4 DC under light illumination. The experimental data in Figure 5 were recorded at the time interval of each 10 min. It is clearly observed that the 663 nm peak degraded very fast with the E ∥ DC polarized condition in Figure 5a, whereas the photodegradation rate of E ⊥ DC polarization was quite slow as depicted in Figure 5b. The obtained photodegradation rate constants are k = 0.058 for the E ∥ DC polarized condition and only k = 0.0048 for the E ⊥ DC polarization in the measurement condition of ReS2 + 25 μM MNB. A factor of ∼12 is enhanced when the polarized photocatalysis operation is performed along the E ∥ DC polarization as compared to that of E ⊥ DC. This special character can also make ReS2 a switching (i.e., on and off) 2D photocatalyst by using the linearly polarized light. The cycle test of the ReS2 2D photocatalyst was also implemented, and the plot of C/C0 versus illumination time is depicted in Figure S4 for illustration. In a 100 μM MNB solution, the cycle test of the ReS2 electrode showed repeatable and even better performance in the fourth round of photocatalysis in Figure S4. The test was continued for more than 40 cycles. The cycle test revealed ReS2 to be a very stable electrode for future electrochemical applications. The effectiveness of photocatalytic degradation of the organic dye by the ReS2 2D photocatalyst is caused by the excitonic dipole effect along DC (i.e., more photogenerated EHPs and prolonged carrier lifetime) and the permanent stability of the electrode in the redox reaction in the electrolyte.

Figure 6 shows the representative scheme for the polarized photocatalytic degradation of the MNB solution with the optical field parallel and perpendicular to the DC direction in ReS2. A Xe-arc lamp filter by a Glan−Taylor prism polarizer provided the linearly polarized light. Under the illumination of the E ∥ DC polarized white light, all the Eg∥ group transitions (Ex1, ES1, ES2, etc.) in ReS2 will generate a larger amount of EHPs for leaving h+ (hole) in the valence band (EV) and e− (electron) in the conduction band (EC). The OH− and O2 in the water solution near the ReS2 surface would combine with the photogenerated h+ and e− to form radicals. The hydroxyl reactive radicals (•OH) were formed by the combination of holes with water molecules and/or hydroxide anions. The reaction process also generates hydroperoxyl radicals (•O2−). Both the hydroxyl (•OH) and hydroperoxyl (•O2−) radicals are more active to degrade the organic pollutant such as MNB.34 The final product (FP) of the organic photodegradation is H2O. All the reaction processes of E ∥ DC photocatalytic behaviors of ReS2 + MNB can be described as follows: ReS2 + hν → ReS2 (h+ in E V and e− in EC along DC) (H 2O ↔ OH− + H+), h+ + OH− → •OH, e− + O2 → •O2− •

O2− + H+ → • HO2 , MNB + •OH → • MNB′ + H 2O (FP)

(3)

On the other hand, with the E ⊥ DC condition, the hν⊥ polarized excitation to the group Eg⊥ (i.e., Ex2, E′S1, E′S2, and so on) can only make one-twelfth photocatalytic degradation efficiency of the E ∥ DC polarization in the ReS2 2D photocatalyst. F


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Figure 7. (a) Photovoltaic response spectra of unpolarized E ∥ b and E ⊥ b in a 0.5 μM MNB solution with ReS2 as the photoanode and Pt wire as the cathode. (b) Concentration-dependent DSSC photoresponse spectra of the E ∥ b polarized condition. (c) Scheme and representation of the ReS2 MNB DSSC tank. The sample arrangement is also indicated. (d) Scheme of axially enhanced charge generation and transportation of the DCReS2-MNB DSSC. (e) Polarized EER spectra of the ReS2 DSSC tank to show the major contribution of Ex1 exciton along the E ∥ DC polarization.

was tested from pure DI water (0 μM) to 50 μM. The result showed that the concentration of 0.5 μM has the highest photovoltaic response peak (∼25 μV) at 1.99 eV. Compared to the E ⊥ DC polarized condition, Figure 7a shows that the photovoltaic response of E ∥ DC is still larger than that of the E ⊥ DC operation. The Re4 DC effect also dominates the open-circuit voltage output of the DC-ReS2 DSSC under the white-light illumination. As shown in Figure 7b, the peak photovoltaic response at 1.99 eV increases with the MNB concentration from 0, 0.1, to 0.5 μM (maximum) and then gradually decreases from 1 to 50 μM. When the concentration >5 μM, a 663 nm drop of absorbance was observed in the MNB solution. A more dense aqueous solution of MNB will prevent a large number of photons impinged on the interface of ReS2-MNB, thereby reducing the photovoltaic output. The highest photovoltaic response of E ∥ b polarization is thus acquired from the DC-ReS2 MNB DSSC with a 0.5 μM concentration. Figure 7d shows the energy diagram, excitation, and electron-transfer process of the polarized DC-enhanced

3.3. Investigation of DC-ReS2 Combined with the MNB DSSC. On the basis of the prominent photocatalytic performance of Re4 DCs, the solid-state DSSC behavior of ReS2 by using MNB as the sensitized dye was evaluated and tested. Figure 7a shows the polarization-dependent photovoltaic responses of unpolarized E ∥ b and E ⊥ b polarizations measured by the DC-ReS2 DSSC tank as depicted in the representative scheme of Figure 7c. As shown in the right part of Figure 7c, the area of ReS2 is ∼2.5 × 1.2 mm2 (i.e., b axis is the longer edge), and the sample is closely attached on a copper foil using silver paste. The sample edge and the copper part were daubed with Microstop insulation epoxy. An enameled insulating wire connected to the ReS2-attached copper foil was used as the photoanode, and a Pt wire was used as the cathode of the DSSC tank. The electrolyte is 0.5 μM MNB solution of ∼15 mL. The selection of a 0.5 μM concentration is based on the concentration-dependent photovoltaic responses of the DC-ReS2 DSSC with the E ∥ DC operation shown in Figure 7b. The MNB concentration G


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retains the higher photocatalytic efficiency of ReS2 along the E ∥ DC polarization. The polarization-dependent μ-TR measurement shows an Ex1 = 1.495 eV exciton transition that dominates the polarization dipole effect and occurs only in the E ∥ DC direction. The specific in-plane excitonic anisotropy of the Ex1 along DC renders ReS2 to possess a much higher photodegradation rate of MNB (12.3 and 5.6 times) than those of the other 2D MoS2 and TaS2. The exciton dipole (EHP) by the photoexcited Ex1 along DC can pave a smooth transferring pathway of electrons for enhancing the photovoltaic output of the ReS2 MNB DSSC. The existence of Re4 nano-DCs makes 2D ReS2 an efficient material for future energy applications in photocatalysis.

ReS2-MNB DSSC operation. ReS2 in the DSSC tank is a catalyst/cocatalyst to facilitate the charge transfer. The 2D ReS2 also gives a platform for the integration of light harvesting and carrier transportation for the photovoltaic output. The effect of the ReS2 cocatalyst is similar to that previously performed by ReS2-coated carbon fiber clothes for aiding carrier injection in the hydrogen evolution process.35 The axially enhanced charge generation and transportation of the DC-ReS2-MNB DSSC in Figure 7d can be described as (i) light illumination in MNB (C16H18N3ClS) to make MNB* excited state, the MNB* is oxidized to be MNB+ and one electron e−, and then (ii) the ReS2 polarized gap Eg∥ is excited by hν∥ (E ∥ DC) to create EHPs and build excitonic dipoles along DC. The excitonic dipole will attract and assist the electronic transport during the DSSC operation. +

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ASSOCIATED CONTENT

S Supporting Information *

−

(i) MNB + hν → MNB*, MNB* → MNB + e ,

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b05946. Frequency-dependent PC, concentration-dependent photodegradation, cycle test of the 2D ReS2 photocatalyst; and single-crystal X-ray diffraction patterns of 1T ReS2, 2H-MoS2, and 2H TaS2 (PDF)

(ii) ReS2 + hν → EHP (h+ → e−) to make dipole‐assisted e− transport, (iii) MNB+ + Cl− → MNB + 1/2 Cl 2, (iv) 1/2 Cl 2 + e− → Cl−

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(4) +

Some of the oxidized dye molecules (MNB ) can be recovered into MNB again by an electron donation from the halogen ion (usually Cl− in MNB) into the electrolyte (iii). Finally, (iv) the chloride ion Cl− can be restored by receiving one electron from the Pt cathode to the half dichloride 1/2 Cl2 in turn. The axially enhanced photocatalytic behavior of ReS2 by using the linearly polarized light should come from the formation of Re4 DCs in the C-plane. In the catalytic/ cocatalytic process, the Ex1 exciton of the lowest energy state should provide the main effort on photodegradation and carrier transportation of the catalytic reactions. Figure 7e shows the polarized electrolyte electroreflectance (PEER)36 spectra of E ∥ DC and E ⊥ DC polarizations for the ReS2 DSSC tank (see Figure 7c) for the verification of the bandedge contributions in ReS2. The modulation amplitude of the DSSC tank between ReS2 (+) and Pt wire (−) is about 100 mV. The PEER spectra of the ReS2 DSSC tank in Figure 7e clearly show that the exciton 1, Ex1 ≈ 1.5 eV, is only present along E ∥ DC, whereas the exciton 2, Ex2 ≈ 1.535 eV, merely appears in the E ⊥ DC polarization. The Ex1 and Ex2 exciton peaks of the DSSC tank are also observed in the polarized photovoltaic responses in Figure 7a. The axial enhancement of the photocatalytic ability of ReS2 is mainly caused by the Re4 diamond clustered chains with the polarized excitonic dipole coming from the band-edge exciton, Ex1.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +886-2-27303772. Fax: +886-2-27303733. ORCID

Ching-Hwa Ho: 0000-0002-7195-208X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the funding support from the Ministry of Science and Technology, Taiwan under the grant no. MOST 104-2112-M-011-002-MY3.

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REFERENCES

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4. CONCLUSIONS In conclusion, the axially enhanced photocatalytic and photococatalytic behaviors of 2D ReS2 are evaluated using the photodegradation of the MNB solution and the photovoltaic output test of a ReS2 MNB DSSC with E ∥ DC and E ⊥ DC polarized lights onto the C-plane. The photodegradation rate constant of E parallel to DC direction is 12 times faster than that of the E perpendicular to DC direction. It reveals that all the photocatalytic reactions almost occurred in the 2D ReS2 along with the E ∥ DC polarization. Polarized ac PC measurements show that more EHPs and a longer carrier lifetime (i.e., surface-state-assisted) can be detected using the bias ∥ DC and E ∥ DC polarized condition in ReS2. This result H


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In-Plane Axially Enhanced Photocatalysis by Re4 Diamond Chains in

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