Polyaniline Aerogel for Enhanced

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Conductive MoS2 Quantum Dot/Polyaniline Aerogel for Enhanced Electrocatalytic Hydrogen Evolution and Photoresponse Properties Sujoy Das, Radhakanta Ghosh, Parimal Routh, Arnab Shit, Sanjoy Mondal, Aditi Panja, and Arun K. Nandi ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00373 • Publication Date (Web): 03 May 2018 Downloaded from http://pubs.acs.org on May 4, 2018

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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ACS Applied Nano Materials

Conductive MoS2 Quantum Dot/Polyaniline Aerogel for Enhanced Electrocatalytic Hydrogen Evolution and Photoresponse Properties Sujoy Das, Radhakanta Ghosh, Parimal Routh,a Arnab Shit, Sanjoy Mondal, Aditi Panja, and Arun K. Nandi* Polymer Science Unit, Indian Association for the Cultivation of Science, Jadavpur, Kolkata-700032, India *For Correspondence: A. K. Nandi, email: [email protected], Telephone No. 913324734971 a Present address : Charuchandra College, Kolkata-29 ABSTRACT The low conductivity and poor active sites of MoS2 sheet present a huge barrier for it’s exploitation of catalytic applications in the hydrogen evolution reaction (HER). To alleviate this difficulty, we have synthesized MoS2 quantum dots (QDs) having greater quantity of catalytic edge sites by breaking up the bulk MoS2 sheet using solvent exfoliation technique. The synthesized MoS2 QDs are embedded into polyaniline (PANI) - N,N′-Dibenzoyl-L-cystine (DBC) hydrogel matrix by in situ polymerization of aniline where DBC acts as a gelator, dopant as well as a crosslinker. The hybrid conducting aerogels (DBC-MoS2-PANI) thus produced act as an efficient electrocatalyst showing lower HER overpotential in comparison to MoS2 QDs. It exhibits an optimum overpotential value of 196 mV at 10 mA cm−2, a favorable Tafel slope of 58 mV/dec, and an excellent cyclic stability. Also, DBC-MoS2-PANI aerogel is used in photoresponding devices. The DBC-MoS2-PANI hybrid aerogel exhibits better photo response compare to the DBC-PANI aerogel and MoS2 QDs upon white light illumination of one sun. The hybrid aerogel exhibits a maximum enhancement of photocurrent to the value of 3.95 mA at 2V bias, and the time-dependent photo illumination shows much faster rise and decay of photocurrent compared to those of DBC-PANI aerogel and MoS2 QDs. KEYWORDS: MoS2 QDs, aerogel, electrocatalyst, Tafel plot, photocurrent 1

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INTRODUCTION To meet the overwhelming energy demand of modern civilization, recently electrocatalytic water splitting is playing a vital role in the area of renewable energy,1,2 due to the crisis of coal and fossil fuels as principal energy sources. One way to solve the issue is sustainable hydrogen production from water splitting in our daily lives. Hydrogen energy generation in fuel cell is thought to be a promising candidate for replacing fossil fuels.3-5 Platinum (Pt)-based materials are the renowned and most-active electro catalysts for hydrogen evolution reaction (HER),7,8 but due to the high cost, these materials are restricted from their widespread application. Further, it is very difficult to produce a highly active new noble metal-free catalyst for an efficient HER.9 In last few years, numerous works have been attempted to synthesize low cost efficient non-noblemetal based catalysts, such as metal oxides, sulfides, selenides, phosphides, nitrides, and heteroatom-doped

nanocarbons.10-15

Amongst transition

metal

sulfides,

especially 2-

dimensionally (2D) molybdenum sulfides (MoS2) based materials, have attracted considerable attention towards the HER catalyst.16,17 Also, 2D- MoS2 based materials are interesting candidates for optical applications such as photocurrent, photovoltaics, light emitters, photodetectors, and optoelectronic memory devices.18-20 However, HER catalyst and photocurrent enhancement application based on MoS2 is still challenging, due to low active side and low responsivity for their low carrier mobility. The electrocatalytic HER activity is closely related to the number of the active sites of MoS2 21,22 and the intrinsic exchange current density (j0) which play a crucial role for HER activity of MoS2 catalysts according to Butler-Volmer equation.23,24 For optoelectronics and photoresponsivity, the combination of MoS2 with conducting materials can also be an effective approach.25 To 2


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improve the electrocatalytic HER activity and photo current, different strategies have been proposed. Liao et al. have synthesized MoS2 dots onto mesoporous graphene for hydrogen evolution.26 Gong et al. have prepared multiwalled carbon nanotube core MoS2 for photocurrent emission.25 Also, the 3D conductive conjugated polymers have been invoked as excellent materials for their dual functionalities of adhesive and conductive nature.27,28 Furthermore, conducting polymers exhibit different kinds of morphology, which can be tuned further for enhanced performances in different applications.29,30 Recently, conductive polymer gel31 matrix with a 3D hierarchical structure is found to be useful for versatile applications, such as supercapaciator,32 lithium ion batteries,33 solar cell,34 photo current device etc.35 Thus, it would be very interesting to fabricate a new electrical conducting polymeric aerogel matrix with uniform dispersion of MoS2 QDs for better electrocatalytic HER activity and photocurrent generation. In this work, we present the synthesis of a conductive polymer aerogel to improve electrocatalysis and photocurrent properties. Here, we have used N,N′-Dibenzoyl-L-cystine (DBC)-PANI aerogel as a framework (Scheme S1) and MoS2-QDs prepared by simple solvent exfoliation method (Scheme S2) as an electrocatalyst material. DBC-PANI aerogel matrix prepared by polymerization of aniline in presence of DBC cross-linker as well as gelator, is highly conducting and DBC- MoS2-PANI aerogels exhibit good overpotential value for HER catalysis (196 mV at 10 mA cm-2), which is lower than that of the MoS2 QDs. In DBC-MoS2PANI aerogel framework, the electron transfer process is spontaneous, due to the high surface area of the network favoring the HER. Also, DBC-MoS2-PANI aerogel shows better photoresponsivity compare to that of DBC-PANI aerogel.

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EXPERIMENTAL SECTION Materials N,N′-Dibenzoyl-L-cystine (DBC), molybdenum disulfide (MoS2) (Aldrich Chemical. Co., USA) and ammonium persulphate (APS) (MERCK Chemicals, Mumbai) were used as-received. Aniline (Ani) monomer (MERCK Chemicals, Mumbai) was distilled under reduced pressure prior to use. Preparation of MoS2 QDs The MoS2 QDs were prepared from MoS2 powder by simple solvent exfoliation method in N , N’-dimethylformamide (DMF), as shown in Scheme S2.16 2 g of MoS2 powder was added in 50 ml of DMF and was sonicated for 6 hr. Then, the resultant suspension was centrifuged for 20 minutes and a transparent light yellow solution was produced. Afterwards, to remove the DMF, the solution was evaporated under vacuum at a certain temperature and the obtained residue was redispersed in water by sonication yielding a light yellow solution. Preparation of DBC-Anix, DBC-PANI1, DBC-PANI2, DBC-MoS2-PANI1 and DBC-MoS2PANI2 hydrogels and aerogels A stock solution of aniline was prepared by dissolving 0.6 ml of aniline in 20 ml water by votexing. Then 10 mg of DBC was separately dissolved in 1.5, 2 ml of stock aniline solution by mild heating and was subsequently sonicated at 30 oC to produce white DBC-Ani1 and DBCAni2 hydrogel, respectively. 10 ml of APS solution was added to DBC-Anix hydrogel and was kept at 5 oC for 24 hr. to produce the conductive polymer by in situ polymerization within the hydrogel matrix. On polymerization the white DBC-Anix hydrogels were transferred to deep 4


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green DBC-PANI1 and DBC-PANI2 hydrogels, respectively. The hydrogel was repeatedly dipped into water to remove the oligomer, APS and unreacted residue present as impurity. At the same procedure 10 mg of DBC, 5 mg of MoS2-QDs and 1.5 ml of aniline solution were mixed together, followed by heating to dissolve the DBC and was sonicated at 30 oC to produce light yellowish DBC-MoS2-Ani1 hydrogel. Next, 10 mg of DBC, 2, 3, 5, 7 mg of MoS2 QD and 2 ml of aniline solution were mixed together, followed by heating to dissolve the DBC and was sonicated at 30 oC to produce light yellowish DBC-MoS2X-Ani2 hydrogel. Afterwards, all hydrogels were kept at 5 oC for 24 hr in the APS solution to accomplish polymerization of aniline within the hydrogel matrix. During the polymerization the light yellow hydrogel turned into green colored DBC-MoS2-PANI1, DBC-MoS22-PANI2, DBC-MoS23-PANI2, DBCMoS25-PANI2 (represented as DBC-MoS2-PANI2 in the manuscript) and DBC-MoS27-PANI2 gels. The gels were immersed in water for 3 days with intermittent change of water after every 12 hr. to remove APS, oligomeric and other impurities. Afterwards, all gels were frozen in a liquid nitrogen bath and subsequently lyophilized in a freeze drier (Eyela, FDU-1200) at -50 oC at ~10 Pa pressure resulting the formation of the aerogels (Scheme 1). Characterization The morphology of the MoS2-QDs, DBC-PANI2 aerogel and DBC-MoS2-PANI2 aerogel was investigated by transmission electron microscopy (TEM). The composition of the DBC-MoS2PANI2 aerogel was carried out by ICP-OES (Perkin-Elmer Optima 2100 DV). The UV-vis spectra of all the aerogels were measured with a UV-vis spectrophotometer (Hewlett-Packard, model 8453) in a cuvette of 0.1-cm path length. Fluorescence study of the MoS2 QDs was carried out with a Horiba Jobin Yvon Fluoromax 3 instrument. X-ray scattering (WAXS) experiments 5



on MoS2 QDs, DBC-PANI2 aerogel and DBC-MoS2-PANI2 aerogels were performed with a Bruker AXS diffractomer (model D8 Advance) using a Lynx Eye detector. The instrument was operated at a 40 kV voltage and at a 40 mA current. MoS2 QDs and aerogel were placed on glass slides and were scanned in the range of 2θ = 10-70° at a scan rate of 0.4 s/step with a step width of 0.02°. The DBC-MoS2-PANI2 aerogel was characterized by X-ray photoelectron spectroscopy (XPS). The thermal stabilities of the DBC-MoS2-PANI2 was measured using a Perkin-Elmer TGA instrument (Pyris Diamond TG/DTA, model SDT Q600) under atmospheric condition at a heating rate of 10 oC min-1. To understand the mechanical properties of all the gels rheological experiments were performed with advanced rheometer (AR 2000, TA Instruments) using cone plate geometry on a peltier plate, with the diameter of the plate 40 mm and the cone angle is 4° with plate gap of 121 µm. Also, the mechanical properties of the aerogels were measured using a Universal Testing Machine (Zwick Roell, Z005), fitted with a 10 N load cell. For the compression tests, the xerogel samples (column, with a diameter of 10 mm and height of 10 mm) were placed between the self-leveling plates. The gels were compressed at a rate of 12 mm min-1 until the compression ratio reached to 80%. The dc-conductivity of all the aerogels was measured by two-probe method at 25 0C by casting a drop on indium-titanium oxide (ITO, 1 mm) strips, dried and sandwiched with another ITO. The conductivity of the sandwiched samples were measured by an electrometer (Keithley, model 2401) at 25 oC using the equation: (1)

where ‘R’ is the resistance, ‘a’ is the area and ‘d’ is the thickness of the sandwiched samples. 6


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Electrochemical Measurements Electrochemical experiments of the MoS2 QDs and aerogels were performed using a three electrode system at room temperature with the help of an electrochemical station (CHI600E). Here, aerogel modified glassy carbon electrode (GCE) was used as working electrode, saturated Ag/AgCl as reference electrode, Pt wire as counter electrode and 0.5 M H2SO4 as an electrolyte. All samples were coated on GCE with 3 mm diameter and 2 µL of nafion solution (5 wt %) was coated after the suspensions was dried. Linear sweep voltamperometry experiments were made in 0.5 M H2SO4 solution deaerated with N2 with a scan rate of 5 mV s−1. Photocurrent measurement For photocurrent measurement, the DBC-PANI1, DBC-PANI2, DBC-MoS2-PANI1 and DBCMoS2-PANI2 aerogel were dispersed in a 6:4 mixture of m-cresol and chloroform and were sonicated for 5 hr while the MoS2 QDs were dispersed in DMF. The suspension mixture was spin-coated at 2000 rpm for 60 s over the FTO and was dried under vacuum at 70 °C for 4 hr. Thin films over the FTO was coated with silver, and was used as another electrode. This aerogels and MoS2 QDs based devices were used for I-V characteristics and photocurrent measurement. The Keithley model 2401 source meter was used for I-V and time sweep measurements where 150 W xenon lamp source (Newport Corp., Springfield, OH; model 67005) was used as light source. RESULT AND DISCUSSION The MoS2 QDs are synthesized by simple solvent exfoliation method of MoS2 powder in DMF, and the prepared MoS2 QDs are well dispersed in H2O to produce yellow colour solution,which emits strong blue light. DBC and Ani from a gel (DBC-Ani), and DBC-PANI hydrogel are prepared by insitu polymarization of Ani using ammonium persulfate (APS) solution into the 7



DBC-Ani hydrogel. MoS2 dots are dispersed in deionized water, afterwards aniline monomers and DBC were added followed by in situ polymarization. Here, DBC acts as a supramolecular crosslinker as well as a dopant. The in-situ produced PANI is in the doped state due to radical cations of PANI interact with the carboxylate anion of DBC. The MoS2 QDs are present at the surface as well as at the bulk of the PANI nanofibers (Scheme 1). Scheme 1. Schematic Illustration of DBC-MoS2-PANI Aerogel Preparation and Their Electrocatalytic Properties Showing Hydrogen Evaluation Reaction.

TEM images illustrate that MoS2 QDs are produced uniformly (Figure 1a and Figure S1a) and the size distribution curve (inset of Figure 1a) shows that the average size of MoS2 QDs is 6.1±0.9. nm. The fringe pattern in HRTEM images (Figure 1b and inset figure 1b) suggest that the MoS2 QDs are crystalline, with d-spacing value of 0.26 nm, corresponding to the (100) planes of MoS2 crystals.21 The DBC-PANI2 aerogel consists of branched nanofibers (Figure 1c) and the TEM images of DBC-MoS2-PANI2 (Figure 1d) aerogel reveals that the MoS2 QDs (dark 8


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spots) are present at the surface as well as at the bulk of the PANI nanofibers (PANI-NFs) due to van der Walls interaction between MoS2 and PANI.36,37 d = 6.1±0.9

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Figure 1. (a) TEM images of MoS2 QDs (Inset : size distribution of MoS2 QDs); (b) MoS2 QD at higher magnification (Inset : fringe pattern of MoS2 QD (top); SAED pattern of MoS2 QD (bottom); (c), (d) TEM images of DBC-PANI2 and DBC-MoS2PANI2 aerogel, respectively; (e) TEM image DBC-MoS2-PANI2 aerogel at higher magnification, (Inset : SAED pattern of MoS2 QD); (f) EDX spectra of DBC-MoS2PANI2 aerogel; (g) dark-field STEM image of DBC-MoS2-PANI2 aerogel; corresponding elemental mapping of DBC-MoS2-PANI2 aerogel (h-j) clearly showing the uniform distribution of molybdenum (cyan), sulfur (yellow), and nitrogen (green).

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Figure 1e and figure S1b (enlarged image) show that the MoS2 QDs are present around the NFs surface, and the fringe pattern of the DBC-MoS2-PANI2 matches well with that of the MoS2 QDs. Also the selected-area electron diffraction (SAED; inset, Figure 1e) matches well with that of MoS2 QDs (Figure 1b, inset). The energy dispersive X-ray spectroscopy (EDX) elemental mapping analysis of DBC-MoS2-PANI2 (Figure 1f) aerogel indicates the presence of the elements C, N, Mo, and S and the atomic % are 3.44 for sulphur and 0.79 for Mo. So, the ratio of S and Mo is approximately 4.35. The higher value of the ratio from that of the MoS2 can be attributed to the presence of sulphur in N,N′-dibenzoyl-L-cystine (DBC) in the PANI gel. Furthermore, the presence of different elements in DBC-MoS2-PANI2 aerogel are evaluated by dark-field STEM image (Figure 1g) and the corresponding elemental mappings are presented in Figure 1(h-j) which unambiguously verify the presence of Mo, S and N. The composition of the DBC-MoS2-PANI2 aerogel is also determined by ICP-OES, which yields S/Mo atomic ratio of 4.41, close to the value of EDX analysis. Spectroscopy The UV-Vis spectrum of MoS2 QDs, presented in Figure 2a, exhibits four characteristic absorption bands; the two peaks at 385 and 447 nm are assigned to transition from the valence band to the conduction band, and the other two peaks at 605 and 668 nm are attributed to the K point of the Brillouin zone.38 The photoluminescence excitation (PLE) and photoluminescence emission (PL) spectra of MoS2 QDs are presented in Figure 2b, and it exhibits a strong emission maximum at 440 nm for excitation at 365 nm. The MoS2 QDs exhibit excitation-dependent photoluminescence emission (Figure 2c) and it showes the bathochromic shift in emission peaks from 415 to 500 nm. when the excitation wavelength ranges from 300 to 440 nm. This may be 10


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attributed to the exciton recombination at the electron (hole) trap produced from uncompensated positive (negative) charge at the dangling bonds.39

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Figure 2. (a) UV-Vis spectra of MoS2 QDs, DBC-PANI2 and DBC-MoS2-PANI2 aerogel; (b) excitation and emission fluorescence spectra of MoS2 QDs; (c) Fluorescence spectra of MoS2 QDs for excitation at different wavelength. The UV-Vis spectrum (Figure S2) of DBC-Ani gel shows two peaks at 230 and 280 nm, corresponding to π-π* and n-π* transition, respectively. UV-Vis spectra of DBC-PANI1 (Figure S2), and those of DBC-PANI2 and DBC-MoS2-PANI2 aerogels (Figure 2a) exhibit three peaks. DBC-PANI2 have peaks at 339, 442 and 842 nm and DBC-MoS2-PANI2 aerogel shows the three peaks at 342, 435 and 851 nm, which are attributed to π-π*, polaron to π* band and π- band to the polaron-band transition peak of the PANI chain, respectively. This result confirms that PANI produced is in the dopped emaldine salt (ES) state becasue DBC dopes PANI well. It is evident from the data that π-band to the polaron-band transition band shows a 9 nm red shift in DBC-MoS2-PANI2 aerogel indicating the increase of conjugation length of PANI decreasing the π band to polaron band gap, and in turn, the gap of the polaron band to π* band increases showing a blue shift. The π-π* band exhibits a red shift of 3 nm, indicating an interaction between the MoS2 and π-orbitals of PANI chains. In DBC-MoS2-PANI2 aerogel the UV-Vis

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peaks of MoS2 QDs are absent, probaly due to lower amount of MoS2 present in large quantity of PANI. The crystaline structure of the MoS2 QDs, investigated by WAXS study (Figure 3a), exhibits a very strong diffraction peak at 2θ = 14.4° assigned to (002) plane and three small peaks at 2θ = 32.6°, 39.6° and 49.8° which are assigned to (100), (103) and (105) planes, respectively [JCPDF(37–1492) and (84-1398)].40 DBC-PANI2 aerogel shows two peaks at 2θ = 20.4o and 25.5° ascribed to the periodicity in the directions parallel and perpendicular to the PANI chains, respectively.32 However, DBC-MoS2-PANI2 aerogel exhibits one sharp crystalline peak, three small crystalline peaks for MoS2 and two broad peaks for PANI. (a)

(b) MoS2 QDs

DBC-MoS2-PANI2

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Figure 3. (a) WAXS pattern of MoS2 QDs, DBC-PANI2 and DBC-MoS2PANI2 aerogel; (b) XPS spectra of DBC-MoS2-PANI2 aerogel; Enlarged XPS spectra of (c) molybdenum, and (d) nitrogen. 12


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XPS spectra of DBC-MoS2-PANI2 aerogel (Figure 3b) exhibit strong signals at 167.5, 232, 286.5, 399, and 532.5 eV which are assigned to S (2p), Mo (3d), C (1s), N (1s), and O (1s), respectively. The binding energy of Mo (3d) in high-resolution XPS spectrum are 229 and 232.2 eV (Figure 3c) which corresponds to Mo (3d5/2) and Mo (3d3/2) energy levels, respectively. Similarly, for S atom, the high-resolution XPS spectrum are 163.4 and 167.8 eV (Figure S3), corresponding to S (2p1/2) and S (2p3/2) energy levels, respectively.25 Also, the high resolution XPS spectrum of N 1s is shown in Figure 3d. The N1s core-level spectrum of DBC-MoS2-PANI aerogel are fitted into three peaks with quinonoid imine (-N=) at 396.9 eV, benzenoid amine (NH-) at 398.9 eV and positively charged radical nitrogens (-N+.-) at 400.3 and 401.7 eV, respectively, indicating that PANI nitrogen is positively charged radical signifying it’s ES state.41 The TGA data of DBC-MoS2-PANI2 aerogel is shown in Figure S4 and it shows mainly three losses of weight with heating. There is some loss of moisture till ~100 oC and then loss of the dopant cum gelator (DBC) may occur making a total loss about ~20%. At 314 oC the weight loss of PANI starts showing a sharp loss and at 370 oC a hump is noticed and it might be attributed to the loss of sulphur from MoS2 together with PANI.42 Finally the loss remains almost leveling (~10 wt %) at 500 oC. The mechanical properties of all gels and aerogels are measured by rheological and compressive stress-strain experiments. Storage modulus (G′) and loss modulus (G″) are measured from rheological experiment. The frequency sweep experiments of DBC-Ani2, DBC-PANI2 and DBC-MoS2-PANI2 gels are presented in Figure 4a and in all cases G′>G″ characterizing the gel state of the samples. DBC-PANI2 and DBC-MoS2-PANI2 gels have higher G′ and G″ value than those of the DBC-Ani2 gel indicating that polymerization of aniline improve the modulus values. Also DBC-MoS2-PANI2 gel exhibits higher G′ value compare to DBC-PANI2 gel due to the 13



reinforcing effect of MoS2 QDs which facilitate better energy storage under applied frequency representing better elastic behaviour of the visoelastic material. The tanδ values are less than one indicating less dampening in all the hydrogels. Furthermore, the higher mechanical properties of the DBC-PANI2 and DBC-MoS2-PANI2 aerogel are confirmed by compressive stress-strain experiment presented in Figure 4b. All aerogels display linear elastic deformations during small compressive strain followed by inelastic hardening and densification. The compressive stress at 70% strain of DBC-Ani2, DBC-PANI2 and DBC-MoS2-PANI2 aerogels are 4.3, 17.8 and 23.5 kPa, respectively, indicating increase of compressive stress after PANI formation. The higher stress for each strain in DBC-MoS2-PANI2 aerogel compare to that of DBC-PANI2 aerogel may also be attributed to the reinfocing effect of MoS2 QDs arising from its high surface area.

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Strain (%) ang. frequency (rad/s) Figure 4. (a) Plots of storage modulus (G′) loss modulus (G″) and tanδ vs angular frequency of the DBC-Ani2, DBC-PANI2 and DBC-MoS2-PANI2 hydrogels; (b) corresponding compressive stress vs strain plot of DBC-Ani2, DBC-PANI2 and DBCMoS2-PANI2 aerogels.

It is necessary to discuss here the origin of reinforcing effect arising from the MoS2 QDs in the aerogel. The π-π* band in UV-Vis spectra of DBC-MoS2-PANI2 aerogel exhibits a red shift of 3 nm, indicating presence of Van der Walls interaction between the MoS2 with the π-orbitals of 14


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PANI chains. Also the π- band to polaron band transition band shows a red shift, attributed to the increase of conjugation length of PANI chain compared to that of DBS-PANI chain which occurred due to decoiling of the PANI chain arising from the interaction of polarons (radical cations of doped PANI) with the electrons (nonbonding) of MoS2 QDs. This generates good interfacial interaction between the components and the large surface area of the QDs augments this interfacial interaction. These electronic interactions cause a good mixing of MoS2 QDs with the PANI chains present in the fibrils of the DBC-PANI2 gel. Thus easier energy transfer occurs between the components, yielding the reinforcement property. Electrical properties The dc-conductivity of

DBC-PANI1, DBC-PANI2 DBC- MoS2-PANI1 and DBC- MoS2-

PANI2 aerogels are 4.1 × 10-3, 0.02, 4.02 × 10-3 and 0.021 S/cm, respectively. The higher conductivity in DBC-PANI2 aerogel compare to DBC-PANI1, is due to higher content of PANI in DBC-PANI2 aerogel. These values are comparable with the other conducting polymer gels.4345

MoS2 has dc conductivity of 10

-4

S/cm46 and DBC-PANI2 and DBC- MoS2-PANI2 aerogels

have dc-conductivity values of 0.02 and 0.021 S/cm, respectively. In the DBC- MoS2-PANI2 aerogel since polaron band (Figure 2a) shows 9 nm red shift from that of DBC-PANI2 aerogel, so an increase of dc-conductivity of PANI is expected. But due to lower conductivity of the component MoS2 and also due to interaction beteen polarons of PANI and MoS2, the increment of conductivity of DBC- MoS2-PANI2 aerogel does not occur. This is also true for DBC- MoS2PANI1 aerogel. The electrocatalytic properties of the exfoliated MoS2 sheet, MoS2 QDs, DBC-PANI1, DBCPANI2 aerogel and hybrid DBC-MoS2-PANI1, DBC-MoS2-PANI2 aerogel are studied for HER 15



via a three electrode system in 0.5 M H2SO4 solution at a scan rate of 0.5 mV/s (Figure 5a). MoS2 sheet is a poor HER catalyst, due to low conductivity and lesser active edge sites availability, however, MoS2 QD is an effective material for HER because, it possess more active edge sites.16 The polarization curves after IR correction (Figure 5a) show that the MoS2 sheet possessed negligible catalytic activity with sluggish current response to the applied potential. Figure 5b displays the onset potentials and the potentials producing a current density of 10 mA cm−2. MoS2 QDs, DBC- MoS2-PANI1 and DBC- MoS2-PANI2 aerogel show better HER catalytic activity with considerably smaller overpotential (262 mV, 223 mV and 196 mV) at 10 mA cm−2. Tafel slope is an essential property of the electrocatalyst materials, to know the kinetics of the HER process. Tafel slopes are determined by linear fit to the Tafel equation [η = b × log j + a, where η is overpotential, j is the current density, b is the Tafel slope and a is the intercept related to the exchange current density j0 (A cm-2)]. Tafel slope of MoS2 QDs is 69±1.2 mV/decade (Figure 5c), which is much lower than that of MoS2 sheet (152±1.6 mV/decade) at 10 mA cm−2. Furthermore, DBC-MoS2-PANI1 and DBC-MoS2-PANI2 aerogel exhibit good HER catalytic activity showing Tafel slope values 64 ±1.2 and 58±1.1 mV/decade, respectively, which are lower than that of the MoS2 QDs and closer to the value of commercial Pt catalyst (32 mV/decade).

Also among the two hybrid aerogel the DBC-MoS2-PANI1 exhibits lower

efficiency than DBC-MoS2-PANI2 aerogel, probably arising from the lower conductivity of the former. The small Tafel slope of DBC-MoS2-PANI aerogel catalysts arise for the coupling between the MoS2 QDs and PANI interface of the aerogels, allowing highly efficient electrical communication between the catalytic MoS2 edge sites and the electrode substrate, which improves the reaction speed, considerably. The optimum amount of MoS2 QDs in DBC-PANI2 aerogel for HER catalytic activity has been measured by varying MoS2 QD concentration in the 16


Page 16 of 36

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DBC-PANI2 aerogel (Figure S5a.) The catalytic activity of DBC-MoS2-PANI2 electrodes follows the order DBC-MoS25-PANI2 > DBC-MoS27-PANI2 > DBC-MoS23-PANI2 > DBCMoS22-PANI2 as evident from the corresponding Tafel slopes presented in Figure S5b. These results indicate that the DBC-MoS25-PANI2 is the optimum composition in the aerogel showing the highest electrocatalytic activity. Henceforth, in the paper we have presented the results of the optimum composition and it is denoted simply as DBC-MoS2-PANI2, as denoted in experimental section. It is now necessary to discuss the sensitivity of HER evoluation using the Tafel slope with with different composites and this is presented as a bar diagram in Figure 5d. Dalla Corte et al. have obtained a Tafel slope137 mV/decade for another Ni-PANI system.46 Feng et al. have observed an almost unchanged Tafel slope for Co(OH)2@PANI system under different bending conditions.48 The MoS2-CNT exhibits a Tafel slope of 36 mV/decade, indicating it has electocatalytic efficiency very near to that of commercial PT electrode (32 mV/decade). The higher catalytic value for MoS2-CNT than that of present work is due to higher conductivity of CNT (104 S/cm)56 from that of DBC-PANI2 aerogel (0.02 S/cm). So the present DBC-MoS2PANI aerogel is a reasonably efficient material for HER catalysis. In the system MoS2 QDs is the effective material for HER because, it possess the active edge sites. Tafel slope of MoS2 QDs is 69 mV/decade and that of DBC-MoS2-PANI2 aerogel is 58 mV/decade. So PANI network augments the catalytic activity of MoS2 QD due to conducting nature of the network causing easier flow of electrons required for the HER process.

17



(b) 400

(a) 0

Potential / mV

-2

j (mA cm )

MoS2 QD DBC-PANI1 aerogel DBC-PANI2 aerogel DBC-MoS2-

-40 -60

300

200

100

PANI1 aerogel DBC-MoS2-

-80

Onset potential

Potential at 10 mA cm-2

Pt MoS2 sheet

-20

PANI1 aerogel

0

-100 0.0

MoS2 Sheet

DBC-MoS2-

MoS2 QD

(d) 152 mv/dec

120

0.0

0.5

1.0

Ref.48

0

1.5

-2

30

Ref.47

Different catalysts

log j (mA cm ) 500

(f)

(e) 0

400

Initial After 1000 cycles

-10 -2

-ZIm

-30

DBC-MoS2-PANI1

MoS2 sheet

150

MoS2 QDs

100

DBC-MoS2-PANI1 DBC-MoS2-PANI2

300

-20

DBC-MoS2-PANI2

50 0 0

50

100

ZRe

150

200 100

-40 -50 -0.4

Ni/PANI-20/5

0.0

60

3D MoS2-G-Ni

32 mv/dec

Ref.51

Co(OH)2@PANI HNSs/NF

58 mv/dec

Ref.50 Ref.49

-ZIm

69 mv/dec

0.2

Ref.52

MoS2/MoO2

64 mv/dec

PANI2 Aerogel

Ref.54 Ref.53

90

MoS2-BP

PANI1 Aerogel DBC-MoS2-

This work

DBC-MoS2-

0.4

Ref.55

TMS6

Overpotential (V)

MoS2 QD

150

1T-MoS2/CNT

Pt MoS2 Sheet

Tafel Slope (mV/dec)

(c) 0.6

DBC-MoS2-

PANI1 Aerogel PANI2 Aerogel

E (V vs RHE)

BCF/Mo2C-0.4

-0.2

Au25/MoS2

-0.4

j (mA cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 36

0

-0.3

-0.2

-0.1

0.0

0.1

0

200

400

600

ZRe

E (V vs RHE)

Figure 5. (a) Polarization curves of MoS2 Sheet, MoS2 QDs, DBC-PANI1, DBC-PANI2, DBC-MoS2-PANI1, DBC-MoS2-PANI2, aerogel and high-quality commercial Pt catalyst; (b) Onset potential and potential at 10 mA cm−2 of MoS2 Sheet, MoS2 QDs, DBC-MoS2-PANI1, DBC-MoS2-PANI2, aerogel; (c) Corresponding Tafel plots of MoS2 Sheet, MoS2 QDs, DBC-MoS2-PANI1, DBC-MoS2-PANI2 aerogel and Pt catalyst; (d) Comparison of HER performance between DBC-MoS2-PANI2 and other electrocatalysts in the literature, (e) Stability after 1000 times of CV cycles; (f) Nyquist plots of all components (Inset : Enlarged Nyquist plots of DBC-MoS2-PANI1 and DBCMoS2-PANI2. 18


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The high dispersion and small size MoS2 QDs around PANI nanofibers provide lots of available catalytic active sites. Besides, the strong coupling of MoS2 QDs to the PANI(ES) nanofibers, as discussed above, in an interconnected conductive network provides fast electron transfer from the electrodes to the MoS2 QDs.. The MoS2 QD has large surface area and some portion may be blocked by the PANI chains. Though available surface area for the adsorption become slightly decreased the efficient electronic coupling between the catalytic MoS2 edge sites and electrode substrate causes the electronic transfer more rapid through the doped PANI fibrillar network, thus causing beneficial for HER. Cyclic stability of the electrocatalyst is tested to understand the long-term operating stability. Cyclic voltammetry (CV) in the cathodic potential at a scan rate of 50 mV/s has been performed over 1000 cycles to investigate their long-term stability (Figure 5e). After 1000 cycles, the lowering of current density is negligible and can be ignored for any practical purposes. This suggests that the DBC- MoS2-PANI2 aerogel is a stable electrocatalyst for HER throughout the long-term repeated cycling in acidic electrolyte.

The quick charge transfer from the high

conducting PANI(ES) nanofibers to active sites of MoS2 QDs

is also identified by

electrochemical impedance spectroscopy (Figure 5f). The smaller radius of curvature (Figure 5f, inset) of Nyquist plot at higher frequency region qualititively, indicates that the DBC-MoS2PANI2 aerogel has smaller charge transfer resistance (Rct) from that of MoS2 sheets and also from that of MoS2 QDs. The lower Rct value of DBC-MoS2-PANI2 aerogel significantly increases the HER catalyst performance affording markedly faster HER kinetics. The mechanism of HER evolution may be ascribed in a similar fashion to that reported for SWNT/ MoS2 system.47 The DBC-MoS2-PANI aerogel exhibits Tafel slope of of 58 mV/dec 19



indicating a possible Volmer- Heyrovsky reaction path, i.e desorption of H2 produced in

Page 20 of 36

the

electrochemical process is the rate determining step. The synergistic effects of MoS2 QD embedded PANI NFs for electrocatalytic reaction in acid solution is attributed to the high conductive PANI network as well as high surface area of MoS2 QD. The better catalytic activity of hetero nanostructured DBC-MoS2-PANI aerogel compare to MoS2 can be ascribed to the following reasons. (i) The aerogels provide a large surface area, causing fast electrolyte diffusion because of the porous network structure, which is beneficial to adsorption of H+ on MoS2 edges where a rapid reduction to H-atom occurs due to easier flow of electrons to MoS2 surface through the PANI network (Scheme-1). And (ii) the synergistic coupling between MoS2 QD and PANI in aerogels results in electron delocalization between the MoS2 and the PANI π-electrons, which also lowers the adsorption energy of hydrogen in the aerogels,47 facilitating the product hydrogen to be desorbed to form molecular hydrogen augmenting the HER activity. So, the MoS2 QD embedded PANI aerogel shows better catalytic activity compare to that of MoS2 QD.53 Photocurrent The photocurrent and photoresponse behavior of the aerogels and MoS2 QDs based devices are examined by illuminating it with a visible light using 150 W xenon lamp. Figure 6a,b and Figure S6a,b and S7a displays the current-voltage plots of the DBC-PANI1, DBC-MoS2-PANI1, DBCPANI2, DBC-MoS2-PANI2 aerogel and MoS2 QD respectively, under dark and illuminating conditions. The photocurrent increment is higher in case of DBC-MoS2-PANI2 aerogel compared to that of the DBC-PANI2 aerogel and MoS2 QD. The photocurrent increament 3.95 mA for DBC-MoS2-PANI2 (Figure 6b), while DBC-MoS2-PANI1, DBC-PANI1, DBC-PANI2 and MoS2 QD show enhancement of 0.8, 0.37, 1.56 mA and 0.26 µA (Figure 6b, Figure S6a,b, 20


Page 21 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


and Figure S7a), under radiation of one sun at 2V bias. The lower increment in photocurrent of DBC-MoS2-PANI1 may be attributed to the lower PANI content attributing to lesser conductivity than that of DBC-MoS2-PANI2 aerogel. In case of DBC-PANI2 aerogel under white light illumination, the PANI electrons move from the HOMO to LUMO, and these electrons are stabilized through conjugation with the π orbitals of

DBC molecules, thus

generating a considerable photocurrent. The remarkable (153%) enhancement of photocurrent in the DBC-MoS2-PANI2 aerogel compared to that of DBC-PANI2 aerogel requires a significant discussion, and it is mainly attributed to the electronic coupling between MoS2 QDs and PANI nanofibers. Upon white light illumination, electron-hole pairs are generated inside MoS2 QDs,25 afterwards the electrons

are transferred to the DBC-PANI nanofibers. These transferred

electrons increase the charge carrier density in PANI, resulting in the remarkable increase of photoresponse propery. There are some reports of photoresponse property of the PANI hybrids and also in their gel / xerogel states. In 2012, Yang et al. have synthesized Bi2O3-PANI and V2O5-PANI nanocomposites showing photocurrent enhancement of 10-2 and 10-4 mA under UV light irradiation at 1 V, respectively.57 Recently, from our laboratory, Bairi et al. have reported 5,5′(1,3,5,7-tetraoxopyrrolo[3,4-f ]isoindole- 2,6-diyl)- diisophthalic acid (PMDIG)-PANI hydrogel, which has exhibited photocurrent rectification property under white-light illumination.35 The Fmoc-protected phenylalanine with polyaniline co-assembled xerogel exhibit photocurrent increment of ~0.1 mA58 at 5V bias under one sun irradiation and folic acid-silver nanoparticlesPANI hybrid hydrogel has shown photocurrent enhancement ~3.8 mA at 2V bias under the one sun illumination.32

21



18

(a)

(b)

15

6 0 -6

5 0 -5

-10

-12

-15 -20

-18 -2

-1

0

1

-2

2

-1

Voltage (V) (c)

15.0

2V

DBC-PANI2

14.5

14.0

13.5

(d)

18

Current (mA)

Current (mA)

Light Dark

DBC-MoS2-PANI2

10

Current (mA)

Current (mA)

20

Light Dark

DBC-PANI2

12

0

Voltage (V)

1

2

DBC-MoS2-PANI2

2V

16

14 13.0 0

100

200

300

400

500

600

0

100

Time (Sec) 15.0

(e)

16.2s

DBC-PANI2

200

300

400

500

600

Time (Sec) 18

20.4s

Current (mA)

Data Fitting

Current (mA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 36

14.5

14.0

13.5

(f)

4.7s DBC-MoS2-PANI2

5.1s Data Fitting

17 16 15 14

13.0 100

200

Time (Sec)

300

400

0

50

100

150

200

250

Time (Sec) Figure 6. I–V curve of the DBC-PANI2 (a) and DBC-MoS2-PANI2 (b) aerogel based device under dark and illuminating conditions; On-off photoresponse cycles of the aerogel based device at 2 V in white-light illumination for (c) DBC-PANI2 and (d) DBC-MoS2PANI2; Rise and decay time constants obtained from fitting with bi exponential function of the device for (e) DBC-PANI2 and (f) DBC-MoS2-PANI2.

22


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The large increment in the latter case has been attributed to the coupling of Ag plasmons with the polarons of doped PANI chains, however, in the present DBC-MoS2-PANI2 system the electron/hole pair in MoS2 being separated on illuminations interact with the polarons of PANI chains causing a similar hike (3.95 mA) in photocurrent under the same condition. To understand the photo responsivity in details, we have measured photocurrent on-off cyclic stability test of DBC-PANI1, DBC-PANI2, DBC-MoS2-PANI1, DBC-MoS2-PANI2 aerogel and MoS2 QDs by recording the photocurrent growth (for 50 s) and decay (for 50 s) at a time gap of 1 ms with one sun illumination at 2V bias (Figure S6c,d, Figure 6c,d and Figure S7b). These figures show that the aerogels and MoS2 QDs can be reversibly turned on-off by switching the white-light illumination on-off, respectively. We have also examined photocurrent on-off cycles of DBCPANI2 and DBC-MoS2-PANI2 aerogel at other bias voltages, 1 and 4 V (Figure S8a-d). It is clear that with increase of bias voltage at the same one sun illumination, the current increment is higher due to greater potential drop between the electrodes, and the electron-hole pairs are betterseparated by these strong electric fields, causing an increase in photocurrent increment from the dark current (Figure S9). It is important to note that the presence of MoS2 enhances the photocurrent increment more because the electron-hole separation in MoS2 becoming higher on illumination at higher voltages. Response time is the most important parameter for photosensing devices, and it is illustrated in Figure S6e,f, Figure 6e,f and Figure S7c for DBC-PANI1, DBC-MoS2-PANI1, DBC-PANI2, DBC-MoS2-PANI2 aerogel and MoS2 QDs based devices, respectively. During white light illumination (the “on” state) the dark current rises quickly, followed by a slower rise until it is saturated. When light is removed (the “off” state) the current drops quickly, followed by a slow decrease until saturation. To evaluate the growth and decay time of aerogels and MoS2 QD, 23



exponential functions were used to fit both the processes. The fitting equations are I(t)rise = ID + A1 exp(t/τg1) + A2 exp(t/τg2) for growth and I(t)decay = ID + A1 exp(-t/τd1) + A2 exp (-t/τd2) for decay, where the parameter τgi and τdi are the growth and decay time constants, respectively. All parameters of DBC-PANI1, DBC-PANI2, DBC-MoS2-PANI1, DBC-MoS2-PANI2 aerogels and MoS2 QDs based devices are presented in Table-1. The required time for the current rise from 10 to 90% (off-on state) for MoS2 QDs, DBC-PANI1, DBC-PANI2, DBC-MoS2-PANI1 and DBCMoS2-PANI2 aerogel are 68.8, 22.4, 16.2, 8.5 and 4.7 sec, respectively at 2V bias. Similarly, at 2V bias the decay time constant of current from 90 to 10% (on-off state) of the MoS2 QDs, of DBC-PANI1, DBC-PANI2, DBC-MoS2-PANI1 and DBC-MoS2-PANI2 aerogel are 64.7, 26.1, 20.4, 8.7 and 5.1 sec, respectively. So, from table-1 it is apparent that DBC-PANI2 and MoS2 QDs based device shows a slower rise and fall but, the DBC-MoS2-PANI2 based device shows faster response in the rise and fall processes. The rise time constants (τri) calculated from the above equations are found to be three times lower in case of DBC-MoS2-PANI2 aerogel from that DBC-PANI2 aerogel and thirteen times from that of MoS2 QDs. Similarly, the decay time constants (τdi) calculated as above are found to be much lower in DBC-MoS2-PANI2 aerogel compare to DBC-PANI2 aerogels and MoS2 QDs, indicating the presence of MoS2 QDs into the DBC-PANI2 aerogel increases the photosensitivity sharply due to the synergistic effect of the QDs on the DBC-PANI gel network. The enhanced photoresponsivity, depicted from the photoresponse times of the DBC-MoS2-PANI2 aerogel is ascribed to the fruitful electronic coupling between 2D MoS2 QDs and the polaronic band of doped conjugated PANI.

24


Page 24 of 36

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Table 1. Fitting Parameters for Growth and Decay Process for MoS2 QDs DBC-PANI1, DBC-PANI2, DBC-MoS2-PANI1 and DBC-MoS2-PANI2.

samples

growth time (s)

decay time(s)

τg1 (s)

τg2 (s)

τd1 (s)

τd2 (s)

MoS2 QDs

68.8

64.7

4.6

64.2

7.1

57.6

DBC-PANI1

22.4

26.1

4.2

18.1

5.3

20.8

DBC-PANI2

16.2

20.4

3.1

13.1

4.8

15.6

DBC-MoS2PANI1

8.5

8.7

1.3

7.2

1.6

7.1

DBC-MoS2PANI2

4.7

5.1

0.9

3.8

1.1

4.0

CONCLUSION We have synthesized conducting aerogels (DBC-MoS2-PANI1 and DBC-MoS2-PANI2) based on MoS2 QDs, N,N′-Dibenzoyl-L-cystine (DBC) and PANI where DBC acts both as gelator, dopant as well as a crosslinker to the PANI chains. Uv-Vis spectra shows that PANI is in the ES state in the aerogels while WAXS and XPS spectra demonstrate the co-existence of MoS2 QDs and PANI in the aerogel matrix. Also, the rheology of the DBC-MoS2-PANI2 hydrogel and compressive stress-strain experiment on the aerogel indicate that they are mechanically robust suitable for different applications. Varying the concentration of MoS2 QDs in the DBC-PANI2 aerogel indicates that DBC-MoS25-PANI2 (represented as DBC-MoS2-PANI2) is the optimum composition for electrocatalytic activity of HER process. The DBC-MoS2-PANI2 aerogel displays lower HER overpotential from that of MoS2 QDs and exhibits overpotential of 196 mV at 10 mA cm−2. Tafel slope of MoS2 QDs is 69 mV/decade, and it decreases in DBC-MoS225



PANI2 to 58 mV/decade showing better HER catalytic property. DBC-MoS2-PANI1 aerogel exhibits an overvoltage potential of 223 mV at 10mA/cm2 and the corresponding Tafel slope is 64 mV/decade showing lower efficiency than that of DBC-MoS2-PANI2 aerogel. The DBCMoS2-PANI aerogel is also good for fabricating optoelectronic devices. DBC-PANI2 aerogel shows moderate photoresponse, but DBC-MoS2-PANI2 hybrid aerogel exhibits faster photoresponse under one sun illumination. The photocurrent enhancement is 3.95 mA for DBCMoS2-PANI2 at 2V bias. The rise and decay time constants in MoS2 QDs, DBC-PANI1, DBCPANI2, DBC-MoS2-PANI1 and DBC-MoS2-PANI2 aerogel decreases gradually in order showing lowest value for DBC-MoS2-PANI2 aerogel indicating that DBC-MoS2-PANI2 has better photoresponse property than the others. So, embedding MoS2 QDs into DBC- PANI2 network not only significantly enhance the electro-catalytic effect for HER but also dramatically improves the photoresponse property. ACKNOWLEDGEMENT We gratefully acknowledge CSIR grant No. 02(0241)/15/EMR-II for financial support. S.D. R.G. A.S. S.M. and A.P. acknowledge CSIR and DST (Inspire), New Delhi for the fellowship. Supporting Information TEM images, Uv-Vis and XPS spectra, TGA thermogram, Polarization curves, Tafel slope, photoluminescence I-V and on/off diagrams etc. This information is available free of charge via the internet at http://pubs.acs.org.

26


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35



TOC

H 2O H2

light photo current

HER

DBC-MoS2-PANI2

0

DBC-MoS2PANI2 Aerogel DBC-MoS2PANI1 Aerogel MoS2 Sheet DBC-PANI2 Aerogel

-40 -60 -80

DBC-PANI2

18

Current (mA)

MoS2 QDs

-20

-2

j (mA cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 36

16

14

-100 -0.4

-0.2

0.0

0

200

400

Time (Sec)

E (V vs RHE)

36


600

Polyaniline Aerogel for Enhanced

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