WSe2 Heterojunction from

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Functional Inorganic Materials and Devices

Electrophoretically Deposited MoSe2/WSe2 Heterojunction from Ultrasonically Exfoliated Nanocrystals for Enhanced Electrochemical Photoresponse Alkesh Patel, Hiren Kanchanbhai Machhi, Payal Chauhan, Som Narayan, Vijay Dixit, Saurabh Sureshchandra Soni, Prafulla K Jha, Gunvant Solanki, Kireetkumar Patel, and Vivek Pathak ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b18177 • Publication Date (Web): 03 Jan 2019 Downloaded from http://pubs.acs.org on January 4, 2019

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ACS Applied Materials & Interfaces

Electrophoretically Deposited MoSe2/WSe2 Heterojunction from Ultrasonically Exfoliated Nanocrystals for Enhanced Electrochemical Photoresponse

Alkesh B. Patel1,*, Hiren K. Machhi2, Payal Chauhan1, Som Narayan3, Vijay Dixit1, Saurabh S. Soni2,*, Prafulla K. Jha3, Gunvant K. Solanki1, Kireetkumar D. Patel1 and Vivek M. Pathak1 1Department 2

of Physics, Sardar Patel University, Vallabh Vidyanagar - 388 120, Gujarat, India.

Department of Chemistry, Sardar Patel University, Vallabh Vidyanagar - 388 120, Gujarat, India

3Department

of Physics, The M. S. University of Baroda, Vadodara - 390002 Gujarat, India

*Corresponding authors E-mail: Alkesh B. Patel ([email protected]) Saurabh S. Soni ([email protected])

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Abstract The solar response ability and low-cost fabrication of the photoanode is an important factor for the effective output of the photoelectrochemical system. Modification of photoanode by which its ability to absorb irradiation can be manipulated and gained tremendous attention. Here we demonstrated the MoSe2, WSe2, and MoSe2/WSe2 nanocrystals thin films prepared by liquid phase exfoliated and electrophoresis method. AFM and HR-TEM show that the liquid exfoliated nanocrystals have a few layered dimensions with good crystallinity. SEM demonstrated uniform distribution and randomly oriented nanocrystals having a homogeneous shape and size. XRD, XPS and Raman spectra confirm the equal contribution of MoSe2 and WSe2 nanocrystals in the formation of the MoSe2/WSe2 heterojunction. Due to superior absorption of MoSe2/WSe2 heterojunction in the visible region and type-II heterojunction band alignment, In-situ measurement of heterojunction electrode shows almost 1.5 times incident photo-to-current conversion (IPCE) efficiency and photoresponsivity in comparison to individual material electrodes. Our result clearly indicates the influence of heterojunction formation between liquid exfoliated nanocrystals on effective seperation of photogenerated exciton and enhance charge carrier transfer, which leads to the improvement in photoelectrochemical performance. Liquid exfoliated nanosheets based heterojunction are attractive as efficient photoanodes for the photoelectrochemical systems.

Keyword : Liquid phase exfoliation; electrophoresis; MoSe2; WSe2; heterojunction; selfpowered electrochemical photodetector.


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1. Introduction In recent time, nanomaterials including graphene, black phosphorus (BP) and transition metal chalcogenides (TMCs) are the foundation of present photoelectric conversion technology.1, 2 Photoelectric conversion efficiency (PCE) is still not satisfied as the large consumption of natural resources is now a serious issue. As photoanode is the principle component for the PCE, low cost and fabrication complicity of efficient photoanode cover the extensive research area. Many significant efforts have been made to modify the semiconductor photoanode in order to enhance the photoresponse ability. Transition metal dichalcogenides (TMDCs) is the group of materials with a common MX2 structure in which M atoms are sandwiched between two layers of X atoms, where a strong covalent interaction between in-plane atoms and weak Van der Waals interaction between layers are the prime binding mechanism to stacked layer type growth of these materials.3 TMDCs offering unique properties including high carrier mobility, strong spin-orbit coupling and broad light absorption which makes them advance material for the optoelectronics.4-6 Among the TMDC family, Molybdenum diselenide (MoSe2) and Tungsten diselenide (WSe2) are the best choice because of their layer number dependent high carrier mobility and finite band gap transition in the visible region allow alluring applications for flexible and lightweight optoelectronic devices.7 Heterojunction formation between these two materials modulates the band engineering for better charge transfer due to fundamental interaction within 2D planes bundled together by Van der Waals interaction. Further, the significant improvement in visible light spectrum could also be utilized to enhance the photoelectric response. Recently, Xiao et.al reported the photoelectrochemical behavior of monolayer MoS2/WSe2 heterojunction alkaline electrolyte.8 They concluded that heterojunction formation



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broadening the absorption spectra and leads to the better separation of photogenerated excitons. Among the MX2 and WX2 family, MoSe2 and WSe2 are the most favorable materials for the formation of type II heterojunction with the conduction band (CB) energy difference ΔEc = 0.30 eV and valence band (VB) energy difference ΔEv = 0.38 eV.9

Figure 1. Schematic diagram of (a) bulk material synthesis by DVT technique, (b) liquid phase exfoliation technique in order to obtain multi-layered nanocrystals, (c) Images of MoSe2 and WSe2 colloidal nanocrystals suspensions, (d) Schematic diagram of EPD method for the uniform deposition of nanocrystals on conducting substrate by electrophoresis and images of deposited films, (e) Schematic view of MoSe2/WSe2 heterojunction thin film, (f) Time-dependent photoresponce of MoSe2, WSe2 and MoSe2/WSe2 heterojunction electrode.

In the band alignment of MoSe2/WSe2 heterojunction, electron and hole transfer rapidly within the bands lead to the better separation of the photogenerated electron. Lee et.al studied the rectification and photovoltaic effect in the atomically thin MoSe2/WSe2 heterojunction and proposed the nanoscale p-n heterojunctions for the effective electronic and photonic devices.10 Mechanical exfoliation is the effective method to obtain good quality ultrathin layers of TMDC materials, but uncontrolled size and low yield restrict its practical applications.11 In


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contrast, sonication-assisted liquid phase exfoliation enables large-scale production by exfoliation of layered material into mono and multilayer nanosheets which are stabilized in solvent or surfactant.12 However, this technique require long time for production. Liquid exfoliated TMDC nanosheets are scale up over large area for the application in electronic and optoelectronic devices.13 Electrophoretic deposition (EPD) is being traditional and most effective technique for thin layer deposition. In EPD, at a threshold voltage the charge particles dispersed in a liquid medium transport under the influence of electric field for the film formation on an oppositely charged conductive surface.14 EPD provides low cost, binder free approach, short deposition time and reproducibility.15 Due to such advantages, EPD has been employed for the deposition of metal, semiconductor and insulator nanoparticles for photovoltaic15, catalytic16 and Li-ion battery17 application. Bulk and monolayer MoSe2/WSe2 heterostructure was extensively studied, but the electrochemical study of large area MoSe2/WSe2 heterojunction for the photoelectric conversion is still limited. In this study, we have opted the most suitable liquid exfoliation method for the large-scale preparation of MoSe2 and WSe2 nanocrystals. MoSe2, WSe2 and MoSe2/WSe2 electrode was prepared by the electrophoresis, which is an easy and clean technique for uniform deposition of nanoobjects.18 AFM and SEM clearly demonstrate the formation of few-layered nanocrystals and good quality films. Moreover, XPS confirms the composition and binding energies of elements in film. Absorption spectra clearly show that the heterojunction electrode exhibit the superior absorption in comparison to individual electrodes, which means more photons are absorbed for the photoelectric conversion. MoSe2/WSe2 demonstrates the superior photoelectrochemical (PEC) performance with the photocurrent density (Jsc) of 0.122 mA/cm2, which is increased by 2.5 times in comparison to WSe2 electrode. Heterojunction electrode achieved the highest IPCE at 560 nm. The heterojunction also exhibits the self-powered photodetection ability with fast ON/OFF ratio. 5 ACS Paragon Plus Environment


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Electron and hole transport mechanism of heterojunction based PEC characteristic is also discussed in brief. The result shows that large-scale transparent MoSe2/WSe2 film can be further utilized for another PEC system like dye-sensitized solar cell (DSSC) and hydrogen production. 2. Experimental Section 2.1 Materials Molybdenum metal powder (Alfa Aesar), Tungsten metal powder (Alfa Aesar), Selenium metal powder (Alfa Aesar), Fluorine-doped Tin Oxide (FTO) coated glass (8 Ω per square, thickness 2 mm, Sigma Aldrich, India), hot melt sealing tape (Solaronix Aubonne, Switzerland) and all other chemicals like acetonitrile, Chloroplatinic acid, tert.-butanol, lithium iodide, iodine, 3-methoxy propionitrile, 4-tert. butyl pyridine (4-TBP) and guanidinium thiocyanate are purchased from Sigma Aldrich, India and used as received without further purification. 2.2 Methods 2.2.1 Synthesis of MoSe2 and WSe2 nanocrystals via liquid phase exfoliation Elemental constituents for the growth of bulk MoSe2 and WSe2 are vapor phase reacted and transport to growth zone of a two-zone horizontal furnace by DVT technique as shown in Figure 1 (a).19,20 MoSe2 and WSe2 nanocrystals suspensions are obtained from this bulk material by a typical liquid phase exfoliation method shown in Figure 1 (b). For this, 0.7 g of reacted powder is well pulverized in a mortar and dispersed in 1:1 (v/v) ratio of ethanol/acetone at a concentration of 20 mg/ml. Sonochemical treatment was given at 40 kHz and 40 % amplitude in a bath sonicator for 4 h. Stable exfoliated nanosheets colloidal suspension of MoSe2 and WSe2 are obtained by centrifugation at 5000 rpm for 15 min to ensure removal of all unexfoliated bulk sediment at the bottom. The supernatant is collected which is used for further characterization and device fabrication. 6 ACS Paragon Plus Environment

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2.2.2 Preparation of MoSe2/WSe2 heterojunction PEC-type photodetector EPD technique was opted to obtain a continuous arrangement of nanocrystals over a large substrate area. The colloidal suspension of hybrid MoSe2/WSe2 was prepared by mixing MoSe2 and WSe2 nanocrystals suspensions with 1:1 (v/v) ratio followed by bath sonication for 3 h. MoSe2, WSe2, and MoSe2/WSe2 electrodes are prepared via EPD method in which 5V constant DC voltage is applied between two FTO glass substrates separated by 1 cm into exfoliated nanocrystals colloidal suspension as is shown in Figure 1 (d). Electrodes are then removed and dried at 80 oC in a hot air oven for 2 h. As prepared transparent electrodes are shown in supporting information (Figure. S1). Prepared electrodes were used as an anode and platinum coated FTO glass used as the cathode for the fabrication of sandwich-type PEC photodetector. A strip of hot melt tape used to seal this assembly. For electrolyte preparation, 0.5 M LiI, 0.05 M Iodine, 0.5 M 4-TBP, and 0.1 M guanidinium thiocyanate were dissolved in 3-methoxy propionitrile.21 The electrolyte was inserted from the predrilled holes onto the counter electrode. These holes are finally sealed with simple glass slide using adhesive. 2.2.3 Characterization and Measurements The morphological studies were made by using atomic force microscopy (AFM, NT-MDT, Ntegra Aura), High-resolution Transmission Electron Microscopy (TEM, JEOL JEM 2100 microscope operated at 200keV) and Scanning Electron Microscopy (SEM, JEOL JSM6010 LA). For structural identification standard X-ray diffraction (XRD, Rigaku Ultima IV with Cu/Kα radiation λ = 1.5406 Ao) and Micro Raman spectroscopy (recorded with 532 nm wavelength excitation laser) have been employed. Elemental confirmation was determined by Energy Dispersive Analysis X-rays (EDAX, Philips FESEM XL 30). UV-Visible absorbance spectrum was recorded under the diffused reflection mode within the range of 200 to 1000 nm (Ocean Optics, USB20000+XR1-ES). X-ray photoelectron spectroscopy (XPS) analysis was performed with SPECS GmbH spectrometer (Phoibos 100 MCD Energy Analyser) using Mg



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Kα radiation (1253.6 eV). Peaks were recorded with constant pass energy of 40 eV. The residual pressure inside the analysis chamber was in 10-10 m bar range. Electrochemical studies were performed under a solar simulator (Photo Emission Tech, Inc. U.S.A) and indigenously developed a computer-controlled LED source with different wavelengths. The electrical measurement was recorded using an electrochemical workstation (CH instrument, CHI660E) and source measure unit (Keithley, 2400 SMU).

3. Results and Discussion Sonochemically processed MoSe2 and WSe2 nanocrystals were initially characterized by AFM. Nanocrystals were transferred on to freshly cleaved mica sheets by drop casting. Figure 2(a) and 2(b) show well-segregated MoSe2 and WSe2 nanocrystals which are uniformly distributed on the mica sheet. Figure 2 (c) and (d) show the height profile that suggests the step height distribution of MoSe2 and WSe2 nanocrystals is ranging between 4050 nm and 35-40 nm, respectively. It is clearly resolved that exfoliated nanocrystals are composed of the multi-layered structure. Histogram of lateral step height distribution over a large area (10µm) is shown in supporting information (Figure S2). Nanocrystals were further studied by HR-TEM to investigate the structural properties. Figure 2(e) and 2(f) show the distribution of few-layered and nanometer size MoSe2 and WSe2 nanocrystals on HRTEM grid. Figure 2 (g-l) show the HRTEM images revealing that grown MoSe2 and WSe2 nanocrystals have hexagonal lattice fringes as observed under bright field imaging mode with an interlayer lattice spacing of 0.282nm and 0.27nm, which is in good agreement with (100) and (101) plane of hexagonal MoSe2 and WSe2, respectively.22,23 Corresponding SAED pattern shown on Figure (m) and (n), exhibiting the good crystalline structure of MoSe2 and WSe2 nanocrystals, respectively with 2H-phase even after following the liquid exfoliation process.


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Morphology appearance of MoSe2, WSe2, and MoSe2/WSe2 heterojunction thin films were examined using SEM. Figure 3 (a) shows the surface micrograph of MoSe2/WSe2 heterojunction film, demonstrating uniform distribution and randomly oriented nanocrystals having homogeneous shape and size. It may be due to well separated and non-agglomerated nanocrystals obtained by liquid exfoliation. It also supports the EPD technique for controlled

Figure 2. (a) and (b) AFM 2D scan images of MoSe2 and WSe2 nanocrystals distributed on mica sheet, (c) and (d) shows the height profile of MoSe2 and WSe2 nanocrystals correspond to selected line scan. Low magnified HR-TEM image of (e) MoSe2 and (f) WSe2 nanocrystals. Top view HR-TEM image of vertically grown (g) MoSe2 and (h) WSe2 nanocrystals. HR-TEM of horizontally grown (i) MoSe2 and (j) WSe2 nanocrystals. The magnified area under red line shows that the lattice fringes for (k) MoSe2 and (l) WSe2 nanocrystals are approximately 0.28nm and 0.27 nm, respectively. SAED pattern of (m) MoSe2 and (n) WSe2 nanocrystals exhibit the good crystalline structure. 9 ACS Paragon Plus Environment


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deposition of liquid exfoliated nanocrystals. Cross-sectional SEM image is shown in Figure 3 (b) confirms that the deposited heterojunction film has a thickness of ~ 0.307µm. Surface micrographs and cross-section images of MoSe2 and WSe2 films are shown in supporting information (Figure S1), indicating the same surface morphology and thickness as MoSe2/WSe2 heterojunction layer. Elemental estimation of MoSe2/WSe2 heterojunction film

Mo

(c)

Se

(e)

W

(a)

(b)

(d)

(g)

(f))

Figure 3. (a) SEM surface micrograph of MoSe2/WSe2 heterojunction electrode at a magnification of 1 µm. (b) Cross-sectional SEM image of MoSe2/WSe2 heterojunction electrode revealing the layer thickness of 0.307µm. EDAX elemental mapping area of MoSe2/WSe2 heterojunction surface for (c) Mo element, (d) W element, (e) Se element (f) all the element present on the surface. (g) EDAX spectrum of MoSe2/WSe2 heterojunction electrode.

is analyzed by EDAX analysis along with elemental mapping. Figure 3 (c-f) confirmed the coexistence and homogeneous distribution of Mo, W, and Se onto the surface of the heterojunction film by 30 %, 30 %, and 40 % respectively. Figure 3 (g) shows the EDAX spectrum in which the characteristic emission of Mo, W and Se peaks are depicted. For the 10 ACS Paragon Plus Environment

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vertical presence of MoSe2 and WSe2 in heterojunction film, cross section elemental mapping is shown in Figure S4, which again confirms the equal vertical distribution of Mo, W and Se elements.

Structural confirmation of WSe2, MoSe2 and MoSe2/WSe2 heterojunction films is made from the typical XRD pattern as shown in Figure 4 (a). Strong reflection peaks at 13.76o and 13.67o can be assigned

to (002) crystal plane for WSe2 and MoSe2 nanocrystals

respectively.24, 25 All other peaks are well indexed as (004), (100), (101), (102), (103), (006), (105), (106) and (008) which confirmed the mono hexagonal phase of as prepared MoSe2 and WSe2 nanocrystalline thin film (JCPDS#87-2419 & JCPDS#38-1388).

Figure 4. (a) XRD pattern (b) Raman spectra, (c) UV-Vis absorption spectra and (d) Tauc plot of pristine MoSe2, WSe2, and MoSe2/WSe2 heterojunction electrode.



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Weak intensity peaks confirm the exfoliation of bulk WSe2 and MoSe2 into a few layered nanocrystals by liquid phase exfoliation.26 XRD pattern of MoSe2/WSe2 heterojunction film shows all prominent peaks of MoSe2 and WSe2 with higher intensity suggesting an enhancement in crystallinity composition. Raman spectroscopy is one of the known nondestructive characterization tools used to study TMDC. Figure 4 (b) shows the Raman spectra of MoSe2, WSe2 and MoSe2/WSe2 heterojunction electrodes which are investigated using 532nm laser excitation. A1g, E12g and E1g modes of MoSe2 are identified at 241 cm-1, 281 cm-1 and 168 cm-1 are assigned to out-of-plane vibration of Se atoms and in-plane vibration of Mo and Se atoms.26 For WSe2 A1g, E12g and 2LA(M) modes are found at 250 cm-1, 247 cm-1, and 257 cm-1 - and they are assigned to the out-of-plane vibration of Se atoms and in-plane vibration of W and Se atoms similar to the case of MoSe2.27 Redshift in an A1g mode in both MoSe2 and WSe2 nanocrystals is found with respect to bulk shown in supporting information (Figure. S4), which confirms the synthesis of materials in nanocrystalline form. This may be attributed to the enhancement of Van der Walls interaction between layers and softening of different vibrational modes.28 Raman spectra of the MoSe2/WSe2 heterojunction shown in Figure 4 (b) clearly depicts all distinguished modes of MoSe2 and WSe2 that can be attributed to the formation of the MoSe2/WSe2 heterojunction. Further, Van der Waals-bounded layered systems show the combined functionality of individual layers.28 Hence, we observed red shift in modes of MoSe2/WSe2 heterojunction as compared to MoSe2 and WSe2. These shifting in modes can be attributed to the non-equality stacking pattern of MoSe2 and WSe2.28 Optical properties of the material are very crucial in case of an application for visible light detection. Figure 4 (c) shows the absorption spectra of MoSe2, WSe2, and MoSe2/WSe2 heterojunction electrodes. The absorption spectra of MoSe2 have two broad excitonic absorption peaks (A and B) at 807nm and 698nm. Similarly, two absorption peaks (C and D) are observed in absorption spectra of WSe2 at 756nm and 560nm.


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Peaks arise due to direct internal excitonic transition between bands and energy split in spinorbit at K-point of Brillouin zone.29 MoSe2/WSe2 heterojunction film shows a superior absorption with all excitonic absorption peaks of MoSe2 and WSe2, indicating the independent contribution of MoSe2 and WSe2 nanocrystals in the spectrum. The indirect band gap of MoSe2, WSe2, and MoSe2/WSe2 films are calculated from the Tauc’s equation αhʋ = A(hυ Eg)n/2.

Figure 5. XPS spectra of MoSe2/WSe2 heterojunction thin film. (a) XPS full survey spectrum, (b) Mo 3d core level peak region spectrum, (c) W 4f core level peak region spectrum, (d) Se 3d core level peak region spectrum.

Figure 4 (d) shows the Tauc’s equation plot from which the estimated indirect band gap energies of MoSe2, WSe2, and MoSe2/WSe2 heterojunction electrodes are 1.19 eV, 1.27 eV, and 1.10 eV, which is comparable with previously reported results for pristine MoSe2 and



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WSe2.30,31 Thus, the enhanced absorbance of heterojunction resulting in better absorption of photons under visible light for the generation more photogenerated exciton.32,33 X-Ray photoelectron spectroscopy (XPS) have been employed to further investigate the chemical composition and binding energy of MoSe2/WSe2 heterojunction thin film. Figure 5 (a) show an extended survey XPS spectrum of heterojunction electrodes with all the core levels peaks of Mo, W and Se, which confirmed the equal existence of MoSe2 and WSe2. 34 In Figure 5 (b), the Mo 3d 3/2 and 3d 5/2 core levels peaks are fitted (brown line) and centered at 233.85 and 230.70 eV, respectively in agreement with Mo4+ in MoSe2. In Figure 5 (c), the core level peaks for 5p 3/2, W 4f 5/2 and W 4f 7/2 binding energies are fitted (brown line) and centered at 39.49, 36.12 and 33.96 eV, respectively in agreement with W4+ in WSe2. Figure 5 (d) shows the single peak of Se fitted (brown line) and centered at 56.50 eV, which can be divided and attributed to the Se 3d 3/2 and Se 3d 5/2 binding energies with peak positions at 56.72 and 56.33 eV, respectively.35 All these results, expect the superior PEC performance of MoSe2/WSe2 heterojunction compared with pristine materials. Figure 6 (a) show the schematic diagram of MoSe2/WSe2 heterojunction based PEC photodetector and depicts the interlayer excitons and charge separation at MoSe2 and WSe2 nanocrystals interface in heterojunction electrode and collected to the respected sides.36 Figure 6 (b) shows J-V characteristic of MoSe2, WSe2, and MoSe2/WSe2 heterojunction in dark and under a solar simulator with illumination intensity of 95 mW/cm2. MoSe2 shows current density of 0.008 mA/cm2 and WSe2 demonstrated the current density of 0.054 mA/cm2. WSe2 show the bit higher PEC performance than MoSe2 because of WSe2 exhibit smallest peak-to-peak separation (∆Ep) than WS2, MoS2, and MoSe2 in electrocatalytic measurement.37 Smallest separation in anodic and cathodic peaks leads to the better electron transfer. Under the illumination of light, J-V curve of MoSe2/WSe2 heterojunction shows the current density increase up to 0.147 mA/cm2, which is 1.5 times


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larger than of WSe2, because of the increase in the visible light absorption and the appropriate band alignment between MoSe2 and WSe2. MoSe2/WSe2 heterojunction exhibit the forward turn–on voltage of 0.45 V, which is 5 times larger in comparison to WSe2 demonstrate the enhance photodiode behaviour and significant rectification characteristic. This gives the initial confirmation of type II heterojunction effect forming between MoSe2 and WSe2 nanosheets on PEC performance.38

Figure 6. (a) Schematic view of MoSe2/WSe2 heterojunction PEC photodetector and interlayer charge separation under illumination at the junction. (b) PEC J-V curves of MoSe2/WSe2 heterojunction (black), WSe2 (red) and MoSe2 (blue) under the illumination intensity of 95 mW/cm2. (c) Estimated bands alignment of MoSe2, WSe2 and I-/ I3- redox electrolyte. (d) Energy band diagram and charge carrier transfer in MoSe2/WSe2 heterojunction under illumination.

Under the influence of built-in electric field generated at heterojunction/electrolyte interface, photogenerated charges carriers are more productively separated and effectively collected to the respective terminals resulting into the enhancement of photocurrent in heterojunction



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photodetector.39 The valence band and conduction band edges of MoSe2 and WSe2 nanocrystals along with the redox potential of I-/ I3- redox electrolyte is shown in Figure 6 (c). Schematic view of MoSe2/WSe2 heterojunction film with energy band alignment and charge carrier transfer into the bands of MoSe2/WSe2 type II heterojunction is shown in Figure 6 (d). Under illumination, excitons are generated in WSe2 and MoSe2. The exciton lifetime (ps) is much larger than charge carriers transfer time (fs), leads to the more efficiently separation of excitons at the heterojunction interface.38 Then free electrons transfer to the conduction band of MoSe2 while holes drifted towards the valence band of WSe2. Electronic vacancy at WSe2 is reduced by the I- molecule present in electrolyte (h+ + 3I- → eo + I3-). Oxidized species of iodine are then again reduced at the platinum back electrode to form I- molecule (I3- +2e- → 3I-).40

Figure 7. EIS curves of MoSe2, WSe2 and MoSe2/WSe2 heterojunction layer PEC assembly.

Electrochemical Impedance spectroscopy carried out with an amplitude voltage signal at 750 mV using frequency range from 0.01 Hz to 1 MHz under dark at zero bias. Figure 7 shows the Nyquist plots of MoSe2, WSe2 and MoSe2/WSe2 heterojunctions by considering the inset equivalent circuit. Semicircle at low frequency indicates the charge transfer between 16 ACS Paragon Plus Environment

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thesemiconductor/electrolyte interface and arc diameters manifests the charge transfer resistance (Rct). Rct values for MoSe2/electrolyte interface is 1576.05 Ω/cm2 and WSe2/electrolyte interface is 926 Ω/cm2. While the MoSe2/WSe2/electrolyte interface has Rct value of 316 Ω/cm2. A smaller value of Rct indicates an enhancement in electron-hole pair separation and better charge transfer across the heterojunction/electrolyte interface in comparison to the bare semiconductor/electrolyte interface.41-43

Figure 8. (a) PEC transient photoresponse of MoSe2, WSe2, and MoSe2/WSe2 heterojunction under the illumination intensity of 95 mW/cm2, (b) A single cycle of transient photoresponse of MoSe2/WSe2 heterojunction photodetector, (c) Transient photoresponse of MoSe2/WSe2 heterojunction photodetector under different illumination intensities, (d) Variation of responsivity in MoSe2/WSe2 heterojunction as a function different light intensity.

The PEC transient photoresponse of the above-mentioned samples was carried out to investigate the self-powered photodetection ability under ON and OFF state of illumination



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with intensity of 95 mW/cm2 as shown in Figure 8 (a). Photocurrent generated by MoSe2/WSe2 heterojunction under same conditions is almost 5 times and 3 times larger in magnitude than that of MoSe2 and WSe2, respectively. A sudden increase in photocurrent when light switched ON is due to good separation of photogenerated excitons. Under illumination, a spike in photocurrent is observed due to separation of photo-generated electron-hole pair. Some decay is observed in photocurrent under ON state until it reaches the steady state, which attributes to recombination of photogenerated electrons and holes at interface states.44,45 Rise and decay time is also a very important parameter as far as the capability of material as a photodetector is concerned. Figure 8 (b) shows the single cycle of transient photoresponse of MoSe2/WSe2 heterojunction clearly indicating the fast photoresponse into the small interval on light illumination and restore after removal of illumination. MoSe2/WSe2 heterojunction demonstrates the fast ON/OFF switching behavior with rising time (τr) of 0.395 s and decay time (τd) of 0.275 s. It indicates the rapid generation of excitons and decay of electrons when subjected to light ON and OFF state that leads to the fast detection ability. The PEC transient photoresponse of MoSe2/WSe2 heterojunction under different light intensities is shown in Figure 8 (c). Figure 8 (d) shows the variation of responsivity of MoSe2, WSe2 and MoSe2/WSe2 heterojunction as a function of incident light intensity. Photocurrent gradually increases with light intensity but responsivity (Rλ) decreases under high-intensity light. Responsivity is defined as the electric output in terms of photocurrent per optical input is given by Rλ = Iph/Popt , Where Iph is the generated photocurrent and Popt is the incident light intensity. MoSe2/WSe2 heterojunction exhibit highest responsivity of 1.644 mA/W at 45mW/cm2 and decrease up to 1.293 mA/W at 90mW/cm2. This may be because, after a certain value of incident power, the quantity of the photogenerated holes is too large to be captured by the trap states. Photogenerated excitons undergo the recombination and decrease the photocarrier collecting efficiency. This makes


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responsivity (Rλ) as power-dependent quantity followed by the inverse power law Rλ α I-1/2.4547

PEC measurement of all electrodes was carried out under different wavelength from 400 -670 nm with 10 mW/cm2 intensity. In order to the generated photocurrent (Jsc) by the electrodes under the same intensity monochromatic light, the IPCE was calculated using the equation,

IPCE(%) =

(1)

Figure 9 (a) shows the calculated IPCEs of WSe2 and MoSe2/WSe2 heterojunction at a different wavelength. The IPCE graph of heterojunction electrode is in good agreement with the absorption spectra within the spectral range of 400-700 nm. Heterojunction of the heterojunction electrode is almost 1.5 times higher than that of the IPCE of WSe2 electrode at all wavelength. Highest IPCE of 0.239 % was achieved by the heterojunction electrode at the wavelength of 560 nm attributed to high absorbance in between 500-560 nm and the prominent absorption peak of WSe2 at 560 nm.

Figure 9. (a) IPCE of MoSe2/WSe2 heterojunction electrode and WSe2 electrode. (b) Photoresponsivity of MoSe2/WSe2 electrode and WSe2 electrode.



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Figure 9 (b) shows the variation of responsivity for WSe2 and MoSe2/WSe2 heterojunction electrode as a function of different wavelength. Heterojunction electrode exhibit the highest responsivity of 1.084 mA/W at the wavelength of 560 nm with 1.5 times higher value than that of the WSe2 electrode. Then all the parameters gradually decrease due to the low absorbance of the incident photon for electron excitation at a higher wavelength. Table 1 depicts the comparative data of PEC measurement of some composite electrodes. Our work shows that we can improve the photodetection ability of the device at zero bias by using I3-/I- redox couple electrolyte. The overall better PEC performance of MoSe2/WSe2 heterojunction than MoSe2 and WSe2 is attributed to; (i) The formation of Type II heterojunction between liquid exfoliated MoSe2 and WSe2 nanocrystals which efficiently separate the photogenerated electron-hole pair and enhance the charge carrier transfer; Table 1. Comparative data based on the different composite electrode for PEC based photodetector.

Materials

Bias

Electrolyte

Light intensity

Photocurrent

Ref.

MoS2/WS2/ITO

0.37 V

0.5 M H2SO4

~100 mW cm-2

~0.04 mA cm-2

49

MoS2/WS2/FTO

1V

0.5 M NaClO4

~100 mW cm-2

~0.35 mA cm-2

38

~100 mW cm-2

~0.06 mA cm-2

50

0.5 M H2SO4/ MoS2/In2S3/ITO

0.34 V

MoS2/Graphene/ITO

0V

0.5 M Na2SO4

~40 mW cm-2

0.025 mA cm-2

48

SnSe2/RGO/ITO

0.8 V

0.5 M Na2SO4

~100 mW cm-2

0.006 mA cm-2

51

CdS QDs/RGO/ITO

0.25 V

0.5 M Na2SO4

0.005 mA cm-2

52

SnS nanoribbons/FTO

0V

I3¯/I¯

~100 mW cm-2

0.087 mA cm-2

53

MoSe2/WSe2/FTO

0V

I3¯/I¯

~100 mW cm-2

0.122 mA cm-2

lactic acid

Xenon lamp (450 W)

This work

(ii) Significant improvement of absorption in the solar spectrum (400 nm-700 nm) by heterojunction turns into the superior utilization of irradiation.48, 8 20 ACS Paragon Plus Environment

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4. Conclusion In conclusion, we synthesized a few layered MoSe2, WSe2, and MoSe2/WSe2 heterojunction photoanodes via liquid phase exfoliation and electrophoresis methods. Comparing with the individual electrodes, MoSe2/WSe2 heterojunction electrode exhibit superior absorption in the visible region of the solar spectrum resulting in 1.5 times overall higher PEC performance. This enhanced PEC performance by heterojunction is due to under built-in electric field type II band alignment gives the better charge separation and reduce the charge transfer resistance between heterojunction/electrolyte interface. This work demonstrates the potential of solution processed MoSe2 and WSe2 nanocrystals for the formation of large area heterojunction for their extended applications in electrochemical-based structures. Conflicts of interest There are no conflicts to declare

Acknowledgment Authors ABP, HKM, SSS, KDP are thankful to Technology system group (TSG), Department of Science & Technology (DST), New Delhi for financial support. (Grant no. DST/TSG/PT/2012/125-G). Authors are also thankful to Prof. G. Mohan Rao, Department of Instrumentation and Applied Physics, IISc, Bangalore, India for providing the XPS facility for this work. Associate Content One Supporting Information file. SEM surface and cross-section images of MoSe2 and WSe2 electrodes. AFM 3D images and histogram shows the average size distribution of MoSe2 and WSe2 nanocrystals. Raman spectra of bulk and exfoliated nanocrystals for MoSe2 and WSe2.



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