Light-Driven Photocatalytic Hydrogen Evolution on Spindle-like MoSx

Subscriber access provided by Kaohsiung Medical University

Article

Light-Driven Photocatalytic Hydrogen Evolution on Spindle-like MoSx Nanostructures Grown on Poly-salicylic Acid Synthesized through Bipolar Electrochemistry Somaye Lotfi, Aso Navaee, and Abdollah Salimi ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00847 • Publication Date (Web): 07 Jun 2018 Downloaded from http://pubs.acs.org on June 7, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

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.

Page 1 of 31 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

ACS Sustainable Chemistry & Engineering

Light-Driven Photocatalytic Hydrogen Evolution on Spindle-like MoSx Nanostructures Grown on Poly-salicylic Acid Synthesized through Bipolar Electrochemistry

Somaye Lotfi,a Aso Navaee,b Abdollah Salimia,b*

a

Research Centre for nanotechnology, University of Kurdistan, 66177-15175, Sanandaj- Ira b

Department of Chemistry, University of Kurdistan, 66177-15175, Sanandaj- Iran,

Electronic supplementary information (ESI) available includes of materials and instruments, Figures S1-S11 and Table S1.

*corresponding author: Tel. Fax +988733624008, e-mail: [email protected], [email protected]

ACS Paragon Plus Environment

1

ACS Sustainable Chemistry & Engineering 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

Abstract Transition metal chalcogenides (TMCs) such as molybdenum sulfide are scientifically and economically considered because of their abundance and capability in electronic, catalysis and so on. On the other hand, conducting polymers have displayed the unique properties and have been widely used in chemistry and materials science. Here, a facile electrochemical route, called bipolar electrochemistry (BPE), is employed to fabricate the MoSx catalyst integrated with a poly-salicylic acid (PSA), as a catalyst support, to enhance the electron transfer characteristics. Electro polymerization was used to prepare PSA on the anodic pole of BP gold microfilm, followed by the BPE electrodeposition of MoSx on the first-designed PSA substrate. Characterization of the asprepared integrated system reveals a thin layer of spindle-like MoSx nanostructures arranged on a gold microfilm. The PSA offers a synergistic effect in the integrated system of MoSx/PSA, which significantly decreases the hydrogen reduction overpotential with onset potential of -0.04 V vs. RHE and Tafel slope of 0.62 V dec-1 in 0.5 M H2SO4. More importantly, the prepared electrode displays the characteristic of dye-sensitized photocathode, applicable for sustainable energy conversion, since under light irradiation the hydrogen evolution reaction

(HER) begins at +0.16 V vs. RHE.

Keywords: Bipolar electrochemistry, Poly-salicylic acid, Molybdenum sulfide, Hydrogen evolution reaction, Dye synthesized photocathode.


2

Page 2 of 31

Page 3 of 31 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


Introduction

Developing alternatives to fossil fuels has become an urgent need to decrease disastrous consequences for our climate.1 This has motivated the development of sustainable processes to generate fuels and chemical feedstocks from water splitting or CO2 conversion to chemicals,2-4 which the latter is sometimes referred to as artificial photosynthesis. There are large scientific and technical challenges involved in storage of H2 and even more for CO2 capturing. Beyond that, catalyst provision to proceed these processes is more challenging. Hydrogen evolution reaction (HER) is more attracted attention than CO2 reduction because of simplex operation and pathway with longer faradaic efficiency.4,5 Platinum (Pt) derivatives are among the most popular catalysts to accelerate HER from electrolysis of water or acidic aqueous solution; however, the high cost of Pt catalyst limited its widespread application.6 Recent research has demonstrated that Nano sized molybdenum sulfide (MoSx, x is chiefly 2 or 3) has unique features like low cost, high stability and catalytic activity toward HER and low cytotoxicity (even compared to graphene analogues).8,9 Thin films MoSx has been considered as catalyst since two decade ago,10-13 and more recently, great attention has been paid to investigate the ratio of active edge sites per unit area aimed at enhancing its electrochemical and catalytic properties.3,4,13-30 Alongside the various production methods such as solvothermal synthesis, chemical or physical vapor deposition and chemical or physical exfoliation,16,26,27 electrodeposition process has appeared as a promising way to fabricate the thin films,10-12,14 few layers or mono-layers14,20,23,29,30 as well as hollow shape21 and nanowires.28 However, the conventional electrochemical system suffers from some drawbacks such as being time consuming and complexity. Recently, it has been demonstrated that amorphous MoSx possess higher catalytic activity compared to crystalline MoSx because of abundant active edge sites,31,32 typically originating from the building blocks of [Mo3S13]2− clusters.32-34 However, the electrocatalytic activity of MoSx can be more improved through combination with a conductive or photoactive substrates. In this regard, reduced graphene oxide,35,36 porous carbon aerogel,37 mesoporous carbon38 or other carbon based structures,39,40 polymers,41,42 mesoporous organosilica nanosheets30,43 and so on have been used to enhance the ACS Paragon Plus Environment

3


applicability of MoSx. Interestingly, integration of

transition metal chalcogenides (TMCs) with

different metal-organic dyes, metal-free organic dyes, quantum dots, carbon substrates and so on, have displayed the characteristic of dye-synthesized photodevices.30,44,45 Generally, a dye component connected with a metal derivative catalyst, which is able to convert photon to electrical energy and fuels, is termed as dye-synthesized photocathode.44,45 Extensive absorption band of sensitizers in combination with nanocrstalline transition metal derivatives have significantly harvested a large segment of light from the UV to the near IR region. Organic materials with electrical conductivity are named the conducting polymers and have attracted great attention in various areas of research due to their very unique properties,46 especially in electrochemistry and electrocatalysis.44-49 They are well deposited on a surface by chemical, physical, or electrochemical process through monomers conjugation.46,50 Most of researches have focused on the derivatives of pyrrole, butanesulfonic acid, aniline and thiophene,51 where polypyrrole has also been used as a support for MoSx and showed significant improvement in catalytic activity.41,42 The polymerization of green organic chemical can be a fascinated prospect in chemical or biological sciences. In a recent research, phenolic derivative species such

as salicylic acid (SA) have been used as pH sensor, since proton transfer between the water molecules and the redox active quinone moieties are facilitated by inter and intra molecular hydrogen bonding.52 And more recently, electropolymerization and its possible mechanism have been described on a carbon fiber.53 Thus, the great interest in MoSx/organic composite needs to find a rapid and simple fabrication procedure as well as a profitable organic scaffold to achieve highly active integrated system. BPE is a technique in electrochemistry with a rather young history. The basic principles of BPE dates back to 1970s54,55 and more recently has been carefully understood and introduced as a rapid and costeffective technique in fabricating different conducting polymers or inorganic metal catalyst.56-67 The BP electrode is a wireless conductive object located between two driving electrodes in an electrolyte solution. By applying a sufficient driving voltage (∆Eelec) to driving electrodes, the extremities of conductive BP object is polarized to anodic and cathodic poles, which consequently induced anodic and cathodic reactions. Compared to conventional electrochemistry, BPE needs a simple direct current (DC) power supply in which the electrochemical reaction of interest can take place. Furthermore, concurrent ACS Paragon Plus Environment

4

Page 4 of 31

Page 5 of 31 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


screening over many electrodes with gradient structures can be rapidly obtained with a single DC power supply. Some interesting examples of BPE model in polymer synthesis and nanoscience are the fabricating of Janus structures,60,63,66 polypyrrole in ionic liquied,61 gradient Cu nanostructures,64 gradient polymer,65 polymer light emitter.66 Fosdick et. al.60 have deposited bi- and trimetallic combinations through BPE for HER. Recently, Tan and Pumera67 have introduced this concept in fabrication of compositiongraded MoWSx hybrids with tailored catalytic activity toward HER. To the best of our knowledge, there are no reports for combination of polymer light emitter and MoSx for photocatalytic HER.

Herein, amorphous MoSx catalyst anchored on a polymer substrate is synthesized through BPE. SA, a green organic chemical, is chosen to prepare a polymerized substrate in order to develop an integrated system aimed at enhancing the electron transfer of MoSx as well as photoactivity in HER. Following the salicylic acid polymerization on an anodic pole of BP gold microfilm, the BP electrodeposition of MoSx is performed. The as-prepared materials were comprehensively characterized by different voltammetric, spectroscopic and microscopic techniques. The prepared catalyst was used as efficient photo and electroactive material in HER.

Experimental Section General procedure of BPE: The bipolar electrochemical system consists of two stainless steel slips as driving electrodes (1×2 cm2) in a BP channel (3×1×3 cm) connected to the power supply (MASTECH DC Power Supply HY3005F-3) to provide the desired driving potential. The gold microfilm (1.0×0.5 cm with 100 µm thickness) and carbon paper (CP, Toray, TGP-H-090, 280 µm thickness, 78% porosity) or indium thin oxide (ITO, 1 mm thickness) as BP electrodes were placed between the driving electrodes in a deaerated solvent containing electrolyte. Preparation of PSA: The BP electrode and the driving electrodes were immersed in an aqueous solution containing 0.05 M of SA. To adjust the power supply and time of powering, the BPE was performed in several potentials (10-30 V) for different times (5-25 min) and based on the HER activity of the resulted catalysts the optimum condition was chosen as 30 V for 30 min in water and 30 V for 20 min in ethylene glycol (EG). After BP procedure, the colorless SA solution turned to purple. These ACS Paragon Plus Environment

5


modified electrodes were rinsed with water and ethanol to remove organic residues and dried in ambient temperature. Preparation of MoSx: To prepare amorphous MoSx catalyst, the pristine Au or PSA/Au microfilm was immersed in 2 mM of ammonium tetrathiomolibdate aqueous or ethylene glycol solution in a designed BP channel. As described for PSA polymerization, based on the HER results, the power magnitude and time of applied potential for MoSx synthesis were optimized to be 30 V for 15 min. The modified electrodes were rinsed with water and ethanol for further applications. The electrochemical active surface area, A, of resulted catalysts were estimated based on the Randles–Sevcik equation which was measured by cyclic voltammetry (CV) in presence of 1 mM Fe(CN)64-/3- redox probe. RESULTS AND DISCUSSION Materials preparation and characterization The BP setup for electrodeposition of SA is shown in Figure 1. Overall procedure includes two typical steps: at first; by applying the proper voltage, a thin layer derived from SA is grown on Au BP electrode. SA derivative(s) can be anodically obtained over a possible mechanism pathway presented in Figure 1, which has been suggested by Park and Eun on a carbon-fiber electrode using a conventional three electrode system.53 Here, for electrochemical characterization, SA is polymerized on CP electrode, but it did not show a significant HER after MoSx deposition. SA was not polymerized on Au electrode in conventional three electrode system, which may be due to the necessity of high overvoltage and absence of possible interaction between smooth Au surface and SA derivative(s). However, regarding to the concept of BPE in a higher overvoltage, electropolymerization of SA monomer is initiated at the edge of anodic pole of BP electrode, when the acrossed potential over BP electrode reaches the potential difference between cathodic and anodic reactions (∆Vmin) of electroactive species. Then, it gradually grows toward the center of the BP electrode53-66 with a gradient surface concentration.64 During BP electrodeposition, the Au electrode can be simultaneously dissolved and deposited on the BP electrode,leads to a rough electrode surface.61 By the comparison of AFM images of Au surfaces after CV cycling and BPE in an aqueous electrolyte solution (Figure S1), almost a smooth surface of Au is ACS Paragon Plus Environment

6

Page 6 of 31

Page 7 of 31 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


seen after CV, whereas a highly rough surface is observed after BPE. The resulted surface roughness of Au electrode helps to more stabilize the SA derivative(s). The mechanism of electropolymerization may include several steps. At first, SA loses one electron and one proton and the radical species at meta-, para- or ortho-positions can be formed. Following that step, a dimer can be produced by coupling of two radical species. Continuously, the possible π-conjugated CP is generated by linking other radicals to the dimers. Water reduction at cathodic pole is the auxiliary reaction to complete the entire electrochemical reaction. Polymer growth has been significantly affected by magnitude of the electric field (∆Eelec) between two stainless steel driving electrodes, where ∆Eelec=Etot.(LBPelec/Lchannel), Etot is the strength of the external electric field, LBPelec is the length of the BP substrate and Lchannel is the distance between driving electrodes. However, potential and time of BP procedure were adjusted based on the HER activity of resulted catalyst in different potentials for different times. Accordingly, the optimum conditions for SA deposition were obtained to be 30 V for 30 min (Figure S2). Under this condition, ∆Eelec could be 10 V. It should be noted that with increase in the conductivity of solution in BP channel, the generated current mostly crossed over the solution instead of BP electrode, so the electrochemical reactions cannot proceed appropriately. On the other hand, at low concentration of electroactive species, material synthesis cannot proceed in a desired way. Therefore, the concentration of electroactive species or electrolyte should be experimentally optimized (here 0.05 M SA is the optimum concentration). Here Figure 1 Afterwards, MoSx is electrodeposited on the resulted PSA through BPE in a solution containing 2 mM ammonium tetrathiomolibdate. In this step, electropolymerization was carried out in both of aqueous and organic EG solvents to investigate the effect of solvent on structure and activity of the resulted MoSx. Similar to SA polymerization, the optimum conditions of BP setup were obtained, based on the HER (Figure S3 and S4), as 30 V for 30 min in water and 30 V for 20 min in EG. It should be mentioned that MoSx was also synthesized by conventional CV according to the procedure reported by


7


Ambrosi and Pumera21 (Figure S5), and its electrocatalytic activity was compared to that of BP electrode. To investigate the electrochemical behavior of resulted SA derivative(s) film, a slice of carbon paper sheet was applied as BP electrode. For comparison, BPE was also performed on another CP in the 0.05 M LiClO4 aqueous solution and absence of SA, referred to as bare CP in the following discussion. After BPE, the resulted modified electrode was first tested by CV. Figure 1 represents the I-E curves of bare CP electrode (top curve) and after BP performance (bottom curve). Compared to the bare CP, a lowintense pair of anodic peaks is appeared at 0.71 and 1.18 V in the forward sweep, assigned to SA oxidation,53,68 and corresponding cathodic peaks in the reverse sweep at 0.53 and 0.93 V, respectively. These observations along with significant increase in capacitive current suggested the presence of new structure(s) such as polymerized SA on the CP electrode. In order to evaluate the nature of electrodeposited film, FT-IR and UV-Vis spectroscopy were also achieved. BP electropolymerization of SA was performed on a transparent ITO electrode. A well-defined absorption peak in UV-Vis spectra can be seen with a red-shift compared to pristine SA, signifying the alteration of SA structure (Figure S6). Moreover, compared to the low intense peaks of pristine ITO, FT-IR spectra of PSA/ITO (Figure 2) displays much intense peaks, braded from 3500 to 2900 and 2300 to 400, attributed to O-H stretching vibration of conjugated carboxylic acid and C=O stretching vibration beside C=C in anhydrides or conjugated structures grafted ITO, respectively. Moreover, a doublet peak at 2969 and 2931 attributed to the C-H stretching vibration properly confirms the presence of organic material. Here Figure 2 Further confirmation by electrochemical techniques is obtained when the BPE modified Au and CP electrodes were assessed in presence of Fe(CN)64-/3- redox probe using CV and electrochemical impedance spectroscopy (EIS). Covering the Au surface by PSA leads to decrease in current signals of CV curve of Fe(CN)64-/3- redox probe (Figure 3A) and increase in charge transfer barrier (the diameter of the semicircles of Nyquist plot represents the Rct) of EIS (Figure 3B, curve b). In contrary, the reverse response is seen for the case of CP, where the current signal of CV is increased after SA


8

Page 8 of 31

Page 9 of 31 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


deposition, and consequently, the charge transfer barrier is significantly decreased (Figure 3C,D). These phenomena could be attributed to the intrinsic properties of Au and CP. It is worth noting that pristine metallic Au naturally has a much greater conductivity than CP or other bulky carbon-based materials. Therefore, it is predictable that conducting polymer has a lower conductivity than Au and the barriers against interfacial electron transfer are increased. Whereas, coating of CP with a conducting material incidentally decreases the electron transfer barriers. These results reflect the fact that a conductive polymer is derived through BPE at the surface of BP electrodes. Once more decreasing in electron transfer barrier of Fe(CN)64-/3- redox probe is observed when MoSx is electrodeposited on CP through BPE. Moreover, the capacitive current is significantly increased at both of electrodes, representing the stacking of another substance on the BP electrodes. Compared to the Au and PSA/Au electrodes, MoSx/PSA/Au electrode displays a much greater capacitive current, due to increasing of surface area. On one hand, the negative charges of MoSx/PSA on the electrode surface have decreased the redox response of Fe(CN)64-/3- due to the repulsive effect of negative charges, which impedance and voltammetric data designate these phenomena. Whereas, in HER the increasing of surface area and negative charges on the electrode surface increase the adsorption of H+ ions, and consequently enhance the catalytic current of proton reduction. On the other hand, for an electrode material with higher capacitive current, the higher ability to reserve the charges is expected. This electrode can be discharged in presence of the electroactive species. Therefore, the resulted MoSx/PSA/Au electrode can offer the higher HER activity (data are coming in the next section)

Here Figure 3 and 4 Figure 4A shows the scanning electron microscopy (SEM) images of the anodic side of Au BP electrode after BPE running in presence of SA. Higher magnification image (Figure 4B) reveals a cloudy-like material, indicating that the anodic edge plane of BP electrode is covered with a layer(s) of SA derivative(s) as it is confirmed by energy dispersive X-ray spectroscopy (EDX) and X-ray ACS Paragon Plus Environment

9


photoelectron spectroscopy (XPS) analysis. The SEM images of resulted MoSx derived in H2O (Figure 3C,D) reveal few physical structure categories, with spindle-like nanostructures as the dominant structure. Such nanostructures can display higher edge sites than the bulk, nanoparticles or nanosheets. In contrary, the SEM image of resulted MoSx by conventional three electrode system reveals the amorphous hollow nanostructures consistent with the previous work21 (Figure S7). Magnified SEM image from anodic edge of BP electrode exhibits a dark fragment, which covers about ~35% of BP electrode (Figure 4C and 5A). The presence of carbon, molybdenum and sulfur atoms and their distribution on the 0.5 mm distance from anodic edge of BP electrode is confirmed by EDX analysis (Figure S8) and map analysis (Figure 5B-D), respectively. However, elemental analyses of electrode surface by EDX in several points of anodic pole have revealed the gradient concentrations of Mo and S similar to the previous works63,64,67 (Table S1). There are highest concentrations of Mo and S in 10% of BP electrode in the anodic side with a drastically decreased population toward the center of BP electrode. It may be because of: 1) the growth-limitation of MoSx by probable side reactions such as water oxidation at that anodic pole, which now even has a higher catalytic activity than intrinsic Au microfilm, or, 2) decreasing the density of voltage from the side to center of BP electrode. On the other hand, when EG was used as solvent for preparation of MoSx, the subsequent structure is nanoparticles with size of 50-100 nm (Figure 4E,F), which less edge sites is conceivable than that of aqueous solvent.

Here Figure 5 The XRD patterns of as-prepared MoSx on Au microfilm or PSA/Au microfilm as well as their substrates are presented in Figure S8. Compared to the bare Au microfilm, there is no diffraction peak for PSA, showing the growth of amorphous polymer. Some tiny diffraction peaks, because of low concentration, were seen at 2θ values of 32.7° and 38.9° assigned to the (100) and (103) planes of MoS2. Other peaks at 26.0°, 36.9° and 53.7° correspond to (200), (210) and (031) reflections of MoO2, which can be indexed as monoclinic MoO2 (JCPDS 65-5758).69 The absence of (002) reflection peak in the XRD pattern demonstrates the formation of few layer MoS2.70,71 These results indicate the fact that


10

Page 10 of 31

Page 11 of 31 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


beside amorphous MoS3 which is a main outcome of anodic MoSx synthesis, a small amount of MoS2 and MoO2 is found. XPS from the edge of anodic pole was taken to investigate the elemental composition as well as their oxidation states (Figure 6). Sulfur, molybdenum, carbon, oxygen and trace amount of aurum (attributed to the substrate) are detected in survey spectra. Fitting outcomes from the narrow scan reveal three carbon species including carbon bonded carbon at 284.6 eV, carbon bonded hydroxyl at 286.3 eV and carbon bonded carboxyl groups at 288.6 eV, confirming the existing of SA derivative(s) at the Au substrate. Curve fitting around 224 to 238 eV displays S 2s photoemission peak at 226.0 eV together with three doublet peaks attributed to Mo 3d photoemission signals. A doublet peak at 227.3 and 230.0 eV assigned to Moδ+ 3d5/2 and 3d3/2, respectively, where δ is ascribed to intermediate oxidation states of Mo species between 0 and 4 (0

Light-Driven Photocatalytic Hydrogen Evolution on Spindle-like MoSx

Recommend Documents