CNTs Supported Pyrolysis Derivatives of [Mo3S13]2- Clusters as

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rGO/CNTs Supported Pyrolysis Derivatives of [Mo3S13]2- Clusters as Promising electrocatalyts for Enhancing Hydrogen Evolution Performances Yanan Shang, Xing Xu, Zihang Wang, Bo Jin, Rui Wang, Zhongfei Ren, Baoyu Gao, and Qinyan Yue ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00763 • Publication Date (Web): 25 Mar 2018 Downloaded from http://pubs.acs.org on March 25, 2018

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ACS Sustainable Chemistry & Engineering

rGO/CNTs Supported Pyrolysis Derivatives of [Mo3S13]2- Clusters as Promising electrocatalyts for Enhancing Hydrogen Evolution Performances Yanan Shanga, Xing Xua∗, Zihang Wanga, Bo Jinb, Rui Wanga, Zhongfei Rena, Baoyu Gaoa, and Qinyan Yuea a

Shandong Key Laboratory of Water Pollution Control and Resource Reuse, School of Environmental Science and Engineering, Shandong University, No. 27 Shanda south Road, Jinan 250100, PR China b

School of Chemical Engineering, The University of Adelaide, Adelaide SA 5005, Australia

ABSTRACT: Reduced graphene oxide/carbon nanotube (rGO/CNTs) supported [Mo3S13]2- clusters and [Mo3S13]2- pyrolysis derivatives were synthesized as electrocatalysts for hydrogen production. We investigated the physio-chemical characteristics and electro-catalytic abilities of the [Mo3S13]2- clusters and their pyrolysis derivatives. TEM images of pyrolysis derivatives of [Mo3S13]2- clusters indicated that some crystalline derivatives were surrounded by the amorphous derivatives at annealing temperature of 200-270 oC, and some well-crystallized MoS2 with diameters of 50-100 nm were observed in the pyrolysis derivatives at 500 oC. Both the structure transition and the HER performance of [Mo3S13]2- pyrolysis derivatives were mapped in terms of temperature. Atomic ratio of S: Mo significantly decreased from 3.48 to 1.89 as annealing temperature increased, which indicated the multiple transition forms in pyrolysis derivatives. XPS, XRD, Raman spectra also indicated the decreased density of edge sites and a peer extent of ordering in the layers of pyrolysis derivatives as the annealing temperature increased. These results corresponded well to the HER activities of the rGO/CNTs macrostructures anchored with different pyrolysis derivatives. The rGO/CNTs anchored with pyrolysis derivatives (annealed at 270 oC) of [Mo3S13]2- exhibited an overpotential of ~178 mV (10 mA cm−2) with Tafel slope value located at 64.2 mV/dec, which showed relatively higher HER performances than most analogous single-metal molybdenum sulphide nanocomposites. They also exhibited a close performance to those multi-metal nanocomposites. ∗

Corresponding author: Tel.: +86-531-88361912; Fax: +86-531-88364513 E-mail: [email protected] 1

ACS Paragon Plus Environment

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KEYWORDS: Hydrogen Evolution Reaction (HER); Reduced graphene oxide; Carbon nanotube; macrostructures; [Mo3S13]2- clusters; Pyrolysis derivatives INTRODUCTION The joint challenges of climate change and dwindling fossil fuel reserves are driving intense research into the development of alternative clean and renewable energy sources. Hydrogen (H2) is universally recognized as an environmentally friendly energy source, which has the highest energy density of the known fuels.1-3 Virtually most H2 in the world is produced today by reforming fossil fuels (natural gas, crude oil and coal). These processes are, however, highly energy intensive and not always environmentally benign. Therefore, H2 currently is neither renewable nor carbon neutral. Instead, H2 manufacturing has a large greenhouse-gas footprint.4-8 Nowadays, H2 evolution from splitting water by either electric or photocatalytic processes has attracted a great attention considering its efficient, stable, and economical properties.9-12 The electrochemical H2 synthesis occurs through the electrochemical hydrogen evolution reaction (HER), which is an essential step in water splitting via a reaction of 2H+ + 2e-→ H2.13-14 Achieving the goals of high H2 production rate and low electricity costs, the most critical research needs are to develop novel and cost-effective electrocatalysts to replace platinum (Pt) catalysts.15-17 Therefore, it is highly attractive to explore cost-effective electrocatalysts deriving from earth-abundant metal compounds. Recently, the two-dimensional transition metal chalcogenides, e.g. MoS2, MoSe2, CoS2, WS2, and WSe2, have been extensively investigated.15, 18-21 Among them, MoS2 nanoparticles showing excellent electrocatalytic HER performance has received a great interest in recent studies.22-26 High electrocatalytic activity of amorphous molybdenum sulphide has been reported in a series of recent studies.21, 27-29 The high activity of amorphous MoS2 is attributed by the coordinatively unsaturated sulfur atoms.22-23 Furthermore, the amorphous MoS2 comprises of unsaturated and/or terminal S22- ligands as well as apical and/or bridging S22- ligands sites, which is analogous to the thiomolybdate [Mo3S13]2- clusters.23-24, 30 The potential high electrocatalytic activity of anionic clusters [Mo3S13]2− is partially assigned to their diverse sulfur ligands in the forms of bridging S22-, apical S2- ligand and terminal S22- ligand with a ratio of 6:1:6, which expose numerous active sulfur complexes on the molecular edges.24, 31 However, there is lack of reported 2


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studies on the structure-function correlations in [Mo3S13]2- derivatives and their HER performances. It was reported that the [Mo3S13]2- clusters were instable as they were heated at high temperature (>200 oC).32 The pyrolysis decomposition of the anionic cluster [Mo3S13]2- is associated with the rupture of Mo-Mo, Mo-S, and S-S bonds,32 and also generates different derivatives with unknown amorphous, crystalline or poorly crystalline forms. Yet, studies on the electrocatalytic H2 evolution performances of these pyrolysis derivatives decomposed from [Mo3S13]2- clusters were rare reported. The mapping of the HER performance of these derivatives across temperatures and evaluating how the catalytic activity towards HER evolves with different transformations seemed to be important to understand the structure-function correlations of [Mo3S13]2- derivatives. This would also be of guiding significance to make clear the structure-function correlations of other molybdenum sulfide nanocomposites and their pyrolysis derivatives across temperatures. Because catalytic activity of these semiconductor for HER is limited by their low electrical conductivity and inevitable agglomeration in the bulk catalysis.33 Thus, the particles need to be loaded on a conductive support, which can further improve their catalytic activity by exposing more catalytic sites. To ensure high electrical conductivity and large specific surface area, rGO is a promising electrode material for hydrogen

evolution

reaction.34-35

Scientists

found

that

electrodes

with

three-dimensional (3D) porous structures can improve the catalytical activity due to their enlarged electroactive surface area.36-38 Herein, we hypothesized that hybridized rGO and CNTs particles with a stable 3D macrostructure could be a functional catalytic support. Hybrid [Mo3S13]2- clusters and pyrolysis derivatives of [Mo3S13]2clusters were then prepared and anchored onto the rGO/CNTs macrostructure materials as new electrocatalysts. In this work, the bare rGO/CNTs macrostructures were prepared by coupling rGO and CNTs with different ratios. The pyrolysis decomposed derivatives from [Mo3S13]2- clusters were fabricated at different temperature intervals (200-500 oC). Morphology characterizations of the [Mo3S13]2− derivatives including [Mo3S13]2− clusters as well as their electrode materials were determined. The electrocatalytic activities of the [Mo3S13]2−−rGO/CNTs electrodes were evaluated by their HER performances in a laboratory scale H2 production system. In addition, we mapped the transformation of the structure of these derivatives together with their HER performance, which provided a clear clue for perceiving the relationship between the crystal structure of transition metal dichalcogenide electrocatalysts and their HER 3



catalytic activities; this was the key novelty of the work. EXPERIMENTAL SECTION Materials. Graphite (99.95 wt%), ammonium polysulfide solution (25 wt%), ammonium molybdate ((NH4)6Mo7O24·4H2O), and ascorbic acid (99.7 wt%) were supplied by Sinopharm Chemical Reagent Co., Ltd. The carbon nanotubes were obtained from Chengdu Organic Chemical Co., Ltd, China. Graphene oxide (GO) was prepared following a modified Hummer’s method (Supporting Information). Ultra-pure water (18.2 MΩ cm−1, Millipore) was used throughout the experiments. Synthesis of (NH4)2Mo3S13nH2O and its pyrolysis derivatives. The (NH4)2Mo3S13·nH2O was synthesized based on the method reported by Müller et al.39 Briefly, 4.0 g of (NH4)6Mo7O24·4H2O was first dissolved in 20 ml ultra-pure water, followed by adding 120 ml of ammonium polysulfide. The solution was heated on an oil bath stewing for 5 d until the deep red primary (NH4)2Mo3S13·nH2O separating from the solution. The product was rinsed by ultra-pure water, ethanol, carbon disulfide and hot toluene in sequence for further purification. To convert diverse pyrolysis decomposed derivatives, (NH4)2Mo3S13·nH2O (500 mg) was annealed under N2 gas in a quartz tube furnace (SK2, LongKou City Factory Furnace CO., Ltd) at 200, 240, 270, 300, 350 and 500 °C for 5 h at 10 °C·min-1. These pyrolysis derivatives of [Mo3S13]2- clusters were in turn named as [Mo3S13]2--200, [Mo3S13]2--240, [Mo3S13]2--270, [Mo3S13]2--300, [Mo3S13]2--350, [Mo3S13]2--500. The preparation procedures of pyrolysis derivatives of [Mo3S13]2- are shown in Figure 1. Synthesis of rGO/CNTs macrostructures. The ice crystal template method was employed for fabricating rGO-CNTs microstructure.40 In brief, rGO (1 g/L) and CNTs (1 g/L) solutions were separately ultra-sonicated and then mixed for another ultrasonicating process for 2 h followed by 24 h of magnetic stirring to form a homogeneous suspension. During magnetic stirring, the ascorbic acid (99.7 wt%, 1 g/100 mg rGO) was add to reduce excessive oxygen-containing group of GO. The mixture was free-dried to form the final rGO/CNTs macrostructures. Different composite rGO/CNTs macrostructures were prepared by controlling the volume ratios of rGO and CNTs solution (1/1, 2/1, 4/1), which were named as rGO/CNTs (1/1), rGO/CNTs (2/1), rGO/CNTs (4/1) macrostructures, respectively. Synthesis of rGO/CNTs supported [Mo3S13]2- cluster and pyrolysis derivatives of [Mo3S13]2- cluster. To synthesize [Mo3S13]2--rGO/CNTs clusters, 20 4


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mg (NH4)2Mo3S13·nH2O together with 40 mg rGO/CNTs were added into a 20 ml hydropyrolysis reactor containing 10 ml N, N dimethylformamide (DMF). The hybrid was under ultrasonic treatment till a well-distributed solution formed. Then the tightly sealed hydropyrolysis reactor was kept at 180 oC for 10 h with spontaneously ramping and cooling rate. After cooling, the DMF was removed by centrifugation (10000 rpm, 10 min). Finally, the sample was centrifuged at least 5 times with Ultra-pure water for a further purification and dried by freeze drying. The loading processes are illustrated in Figure 1. The (NH4)2Mo3S13·nH2O anchored with different rGO/CNTs macrostructures were named as [Mo3S13]2--rGO/CNTs (1/1), [Mo3S13]2--rGO/CNTs (2/1), and [Mo3S13]2--rGO/CNTs (4/1), respectively. The same method was used for loading the pyrolysis derivatives of [Mo3S13]2clusters onto the rGO/CNTs (1/1) macrostructure. Macrostructures anchored with different pyrolysis derivatives at annealing temperatures of 200, 240, 270, 300, 350, 420

and

500

°C

were

named

as

[Mo3S13]2--200-rGO/CNTs,

[Mo3S13]2--240-rGO/CNTs, [Mo3S13]2--270-rGO/CNTs, [Mo3S13]2--300-rGO/CNTs, [Mo3S13]2--350-rGO/CNTs,

[Mo3S13]2--420-rGO/CNTs,

and

[Mo3S13]2--500-rGO/CNTs macrostructures, respectively. Different amounts of [Mo3S13]2--270 (5, 20, 200 mg) were anchored on 40 mg rGO/CNTs (1/1) macrostructure, using the same loading method mentioned above; they were named [Mo3S13]2--270-5-rGO/CNTs, [Mo3S13]2--270-20-rGO/CNTs and [Mo3S13]2--270-200-rGO/CNTs, respectively.

Figure 1. Schematic description of (I) Preparation of pyrolysis derivatives of [Mo3S13]2-; (II) anchoring the pyrolysis derivatives of [Mo3S13]2- onto the rGO/CNTs macrostructures.

Electrochemical measurements. A CHI 760D electrochemical workstation (Shanghai Chenhua Instrument Co., China) was employed for all electrochemical experiments. All electrochemical measurements were conducted in a typical 5



three-compartment cell using sulfuric acid (H2SO4, 0.5M) as electrolyte, and a Pt foil electrode (10 mm×10 mm×0.3 mm), Ag/AgCl electrode and glassy carbon electrode (GCE, 3 mm) were included as counter electrode, reference electrode and working electrode. The working electrode was modified as follows: 5 mg of macrostructure and 40 µL Nafion solution (5 wt %) were mixed with 1ml DMF and then sonicated till a homogeneous black ink formed. Afterwards, 5 µL of ink was drop-casted on the GCE. The electrode was dried at room temperature. Linear sweep voltammetry (LSV) was carried out with a scan rate of 5 mV s−1 in a range from −0.1 V to −0.8 versus Ag/AgCl electrode. The cyclic voltammetry (CV), scanning from −0.1 to −0.8 versus Ag/AgCl electrode for 1000 cycles at a scan rate of 50 mV s−1, was conducted to measure the stability. The electrochemical impedance spectroscopy (EIS) measurements were carried out from 1 MHz to 0.1 Hz. Time dependence of current density was under −200 mV (vs. RHE) for 20000 s. It is noted the potential data presented in this paper without special indication were versus Reversible Hydrogen Electrode (RHE) (ERHE = EAg/AgCl + 0.059 × pH + 0.1988) and not corrected for iR losses. Furthermore, the Faradaic efficiency was also estimated. A gas chromatography (GC-7820, Lunan analysis instrument CO., Ltd, Tengzhou, China) was used to measure the practically generating amount of H2. Faraday law was used to calculate the theoretical amount of H2 expected based on a chronopotentiometric response at the constant current density of 20 mA cm-2 for 120 min. Characterizations. The microstructures of all samples were characterized by transmission electronic microscopy (TEM, FEI Tecnai G2 F20 S-TWIN) and field-emission scanning electronic microscopy (FE-SEM, QUANTA430). The element composition was tested by energy dispersive X-ray spectroscopy (EDX). Raman spectrometer, X-ray powder diffraction (XRD) and thermogravimetric analyzer (TGA) were also used for further characterizations. All details are given in Supporting Information. RESULTS AND DISCUSSIONS Properties of [Mo3S13]2- and [Mo3S13]2--rGO/CNTs prepared with various rGO/CNTs ratios. The crystalline states of (NH4)2 [Mo3S13]·nH2O, pure rGO/CNTs and [Mo3S13]2--rGO/CNTs were investigated by XRD and the results are presented in Figure S1. Two strong peaks appearing at 2θ =10.1°, 16.4° are discovered in the pure (NH4)2 [Mo3S13]·nH2O sample, which are due to the (110) and (111) lattice plane of (NH4)2[Mo3S13]·nH2O.31 As (NH4)2 [Mo3S13]·nH2O was anchored on the 6


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rGO/CNTs macrostructures, all [Mo3S13]2--rGO/CNTs samples prepared with different ratio rGO/CNTs (1:1, 2:1 and 4:1) show the characteristics peaks of anchored [Mo3S13]2- clusters at 2θ = 12.1°, and 17.0°. These two peaks appear to be significantly weakened towards the non-ignorable shifts, suggesting that the anchored [Mo3S13]2- clusters on the rGO/CNTs macrostructure are semi-crystallized and transferred into new states.23, 41 Zeta potential of [Mo3S13]2- clusters is located at -50.9 mV (Figure S2) and all [Mo3S13]2--rGO/CNTs macrostructures are centered at about -33.2 ∼ -32.1 mV (Figure S3). These results indicate that the surface charge of [Mo3S13]2--rGO/CNTs macrostructures is insensitive to the rGO/CNTs ratios. FTIR data of different [Mo3S13]2--rGO/CNTs

indicate

that

the

anchored

[Mo3S13]2-

on

the

[Mo3S13]2--rGO/CNTs corresponded well to the bare [Mo3S13]2- particles (Figure S4). Raman shift of all [Mo3S13]2--rGO/CNTs samples shows the distinct D (∼1348 cm-1) and G (∼1575 cm-1) bands (Figure S5). It is known that the D band is attributed to the breathing mode of A1g, and the D band corresponds to the first-order scattering of E2g vibrations of aromatic carbon. However, the specific Raman peaks of [Mo3S13]2clusters are less defined in these [Mo3S13]2--rGO/CNTs samples (Figure S6). The

microsturture

and

micromorphology

of

[Mo3S13]2--rGO/CNTs

macrosturtures were characterized by TEM, SEM and element distribution mapping measurements (Figure 2). The TEM and SEM images of [Mo3S13]2--rGO/CNTs macrosturture exhibit the sheet-like structure coupled with abundant crumpled filiform texture, which reveal the formation of rGO/CNTs based macrostructure.42-43 Such macrostructure can provide enough space for anchoring the high amounts of [Mo3S13]2- nanoclusters. In addition, the SEM element distribution of Mo was consistent with the S distribution (Figure 2e). These microscope results reveal that the observed intimate coupling between [Mo3S13]2- and rGO/CNTs macrosturture could greatly intensify the e- transfer in the [Mo3S13]2--rGO/CNTs, and therefore the HER activities of the [Mo3S13]2--anchored macrosturtures would be strengthened.44-45 The morphology of pure (NH4)2Mo3S13·nH2O was presented in Figure 2(f), which showed that the (NH4)2[Mo3S13]·nH2O remained the nano structure.

7



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Figure 2. TEM (a-c), SEM (d) and element distribution mapping (e) of 2-

[Mo3S13] -rGO/CNTs macrostructure; SEM imagine (f) (NH4)2[Mo3S13]·nH2O nanoclusters.

Figure 3 presents XPS spectra of Mo 3d and S 2p of (NH4)2Mo3S13·nH2O and various [Mo3S13]2--rGO/CNTs. The three doublets (227.6, 229.1 and 233.2 eV) in the Mo 3d spectra correspond to the S 2s, Mo 3d 5/2 and Mo 3d 3/2 features (Figure 3a) 31. The binding energy at 226.8 eV (S 2s) is assigned to the sulfur elements located with various chemical states.31, 40 The binding energies at 229.1 eV (Mo 3d 5/2) and 233.2 eV

(Mo

3d

(NH4)2Mo3S13·nH2O

2/2)

represent

the oxidation

[IV]

state 2−

46-47

as

Mo4+

in 2−

. S 2p XPS data show that the bridging S2 and apical S of

(NH4)2Mo3S13·nH2O are located at higher binding energies at 163.7 and 164.6 eV. The lower binding energies at 161.9 and 163.1 eV are assigned to the terminal S22−. These specific peaks in (NH4)2Mo3S13·nH2O was also observed in the multiple [Mo3S13]2--rGO/CNTs. It was reported that the amorphous MoSx comprises of unsaturated and/or terminal S22- ligands as well as apical and/or bridging S22- ligands sites which was analogous to the (NH4)2Mo3S13·nH2O and anchored [Mo3S13]2- in rGO/CNTs.23 As a result, the anchored [Mo3S13]2- nanoclusters could expose enough S atoms at the edge. In addition, the new XPS peaks located at 168.0 eV is assigned to the S-O bond in [Mo3S13]2--rGO/CNTs macrostructures, which indicated that the fastening of [Mo3S13]2- nanoclusters onto the rGO/CNTs macrostructures was substantially in terms of S-O bindings.

8


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Figure 3. XPS spectra of (a) Mo 3d for (NH4)2Mo3S13·nH2O; (b) Mo 3d for [Mo3S13]2--rGO/CNTs (4/1); (c) Mo 3d for [Mo3S13]2--rGO/CNTs(2/1); (d) Mo 3d for [Mo3S13]2--rGO/CNTs(1/1); and (e) S 2P for (NH4)2Mo3S13·nH2O; (f) S 2P for [Mo3S13]2--rGO/CNTs(4/1); (g) S 2P for [Mo3S13]2--rGO/CNTs(2/1); (h) S 2P for [Mo3S13]2--rGO/CNTs(1/1). The 1/1, 2/1 and 4/1 mean the different ratios of rGO/CNTs in the rGO/CNTs macrostructures.

Our characterization results reveal clearly that the anchored [Mo3S13]2- exhibit almost the same properties in all [Mo3S13]2--rGO/CNTs, indicating that the effect of rGO/CNTs ratios on the interaction between the rGO/CNTs macrostructures and anchored [Mo3S13]2- could be negligible. As a result, rGO/CNTs (1/1) macrostructures were applied as the supporters for all pyrolysis derivatives of [Mo3S13]2- clusters.

Characterization of pyrolysis derivatives of [Mo3S13]2- clusters and their rGO/CNTs macrostructures. A necessary TGA measurement was 9



conducted

for

a

clear

comprehension

on

the

Page 10 of 29

calcination

process

of

(NH4)2[Mo3S13]·nH2O. The weight loss curves of the (NH4)2[Mo3S13]·nH2O sample are presented in Figure 4. The first weight loss stage occurred at 218 oC constitute about 4.3% of weight loss, which may be associated with the loss of bound water in (NH4)2[Mo3S13]·nH2O. The second and third continuous weight loss occurred successively at a temperature range of 218 to 280 oC and 280 to 395 oC. The ammonia and hydrogen sulfide may be separated in this range, contributing to the 13.8% of weight loss. The last stage of weight loss is beyond 412 oC. As a sharp step, it led to a weight loss of about 21.3%, which may be attributed to the loss of S atoms.

Figure 4. TGA curves for the pyrolysis decomposition of [Mo3S13]2- clusters

Figure 5 presents the TEM images of pyrolysis derivatives (200-500 oC) of (NH4)2[Mo3S13]·nH2O. Figure 5a show that a few sparse crystalline derivatives of (NH4)2[Mo3S13]·nH2O can be observed at annealing temperature of 200 oC and the samples exist as agglomerated nanoclusters.32 As the annealing temperature increased to 270 oC, the relative abundance of the crystalline derivatives was increased and these sparse crystalline derivatives can be observed with a few nanometers in length (Figure 5b).23 The HRTEM images also indicated that the ultra-thin domains of crystalline derivatives were in nuclei which were surrounded by the amorphous matrix (crystalline MoSx@amorphous MoSx), which showed a resemblance to the semi-crystalized (NH4)2[Mo3S13]·nH2O (Figure S7).23 The relative amount of the crystalline area is greatly aggrandized upon annealing at 350

o

C and some

nanoparticles with diameters of 50-100 nm are observed in the pyrolysis derivatives at 500 oC (Figure 5f). These were most likely to be the crystalline MoS2.23, 48-49 The TEM images of rGO/CNTs (1/1) particles anchored with different pyrolysis derivatives are shown in Figure S8. The pyrolysis derivatives annealed at 200-270 oC were bonded on the rGO/CNTs particles separately with amorphous form or less 10


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crystalline forms, which indicate the need of the rGO/CNTs support to avoid the catalyst agglomeration, contributing an effectively catalytic activity.43 After loading the annealed samples with rGO/CNTs, these samples with little crystal texture change (200-270

o

C) had excellent dispersivity. In contrast, pyrolysis derivatives of

(NH4)2[Mo3S13]·nH2O at higher temperatures (350-500 oC) were attached onto the rGO/CNTs in the forms of crystal morphologies.

Figure 5. TEM images of derivatives of (NH4)2[Mo3S13]·nH2O annealed in the temperature range from 200 to 500 oC under a high purity nitrogen atmosphere. The annealing temperatures are as follows: (a) 200 oC; (b) 270 oC; (c) 300 oC; (d) 350 oC; (e) 500 oC (f) 350 o

C

To further investigate the transformation of the crystalline state of (NH4)2[Mo3S13]·nH2O, the X-ray diffraction patterns were recorded at different annealing temperatures (Figure 6). The specific diffraction of (NH4)2[Mo3S13]·nH2O is presented in Figure S9, which is in agreement with the simulated powder XRD patterns of (NH4)2[Mo3S13]·nH2O (JCPSD 76-2038). As observed in Figure 6a, pure (NH4)2[Mo3S13]·nH2O exhibits distinct (110) and (111) peaks, suggesting orientation preference of the sample’s crystal growth. When the annealing temperature is up to 200 oC, the XRD pattern shows no obvious change. After being calcinated at 270 oC, the (110) and (111) peaks still remain. The new broad peak appearing between these two peaks suggests that transform of the crystalline state of (NH4)2[Mo3S13]·nH2O occurs, and some amorphous matrix can be formed among the crystals.23, 50 Upon 11



annealing to 350 oC, the (110) and (111) peaks almost disappear, and the broad peak becomes sharp. Besides, a series of peaks arises at 2θ = 33.7, 39.9, 59.1°. These results reveal that a new crystal state is formed.51 Up to 500 oC, it is suggested that the sample could be further crystallized as the more obvious peaks of the XRD patterns are observed. When the annealed samples were loaded on rGO/CNTs, the samples annealed at 200 and 270 oC show no characteristic peaks, while the pyrolysis derivatives annealed at 350 and 500 oC still exhibit obvious characteristic peaks. These results further demonstrate that the [Mo3S13]2--200, and [Mo3S13]2--270 may be existed in poor crystal state, but the [Mo3S13]2--350, and [Mo3S13]2--500 are stable in crystal state. FTIR spectra of [Mo3S13]2- clusters, [Mo3S13]2- pyrolysis derivatives and their anchored rGO/CNTs were presented in Figure 6c. FTIR spectra of [Mo3S13]2nanoclusters exhibit the Mo-S binding at peaks of 508.0, 1393.9 and 1635.5 cm-1. After the pyrolysis treatment, the specific peak at 508.0 cm-1 is weakened and some new peaks located at 945.9, and 1156.5 cm-1 are observed, which are assigned to the new states of Mo-S and S-S bindings in the pyrolysis derivatives.

Figure 6. XRD (a, b) and FTIR (c) patterns of pyrolysis derivatives (200-500 oC) of (NH4)2[Mo3S13]·nH2O as well as their anchored rGO/CNTs macrostructures

Raman spectra of [Mo3S13]2- clusters and their pyrolysis derivatives are presented 12


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in Figure 7a and Figure S10. The specific peak of [Mo3S13]2- clusters at 235 cm-1 indicates the molybdenum–molybdenum bindings (v(Mo–Mo)). The bands at 284– 383 cm-1 and 450 cm-1 are assigned to the molybdenum-sulfide bonds (v (Mo–S) and v(Mo3–S))

23

. In addition, the specific terminal disulfides (v(S–S)t) and

bridging/shared disulfide (v(S–S)br/sh) are found to be located at 520 and 553 cm-1 (Figure S6). This finding is consistent with previous reports.23, 31 As the annealing temperature increased, the bridging/shared disulfide (v(S–S)br/sh) at 553 cm-1 was less defined. In contrast, two strong features of in-plane E2g (383 cm-1) and out-of plane A1g (406 cm-1) vibrational mode reflecting the crystalline MoS2 are greatly intensified in these pyrolysis derivatives, which suggest that some intermediate-range order occurs in the pyrolysis derivatives23. The pyrolysis derivatives of [Mo3S13]2- clusters at 500 oC only show the same features of the well-crystallized MoS2 (Figure S10). In contrast, the pyrolysis derivatives of [Mo3S13]2- clusters annealed at lower temperatures were characterized as both the coexisting amorphous and crystalline phases in frameworks of these pyrolysis derivatives.

47

The Raman spectra of

pyrolysis derivatives anchored rGO/CNTs are presented in Figure 7b. The distinct D (∼1350 cm-1) band is attributed to the breathing mode of A1g, and the G (∼1580 cm-1) band corresponds to the first-order scattering of E2g vibrations of aromatic carbon.51-53 It is obvious that the ratio of A1g to E2g was gradually increased as the annealing temperature increased from 200 to 500 oC. The raise in the A1g/E2g ratio at higher annealing temperature indicates a decreased density of edge sites and a peer extent of ordering in the layers of pyrolysis derivatives, which would greatly inhibit their HER performance.23 Zeta potentials of [Mo3S13]2- clusters and their pyrolysis derivatives at 200-500 oC are shown in Figure S11. The surface charges of pyrolysis derivatives show an increasing trend as the annealing temperature varies from 200 to 500 oC. As a result, more crystal morphologies with less negative charges are formed at a higher temperature.

13



Figure 7. Raman spectra of (a) pyrolysis derivatives of [Mo3S13]2- clusters at different annealing temperatures; and (b) pyrolysis derivatives anchored rGO/CNTs macrostructures

XPS results in Figure 8 presents chemical bonding characters of these derivatives. At 200-500 oC, the Mo 3d spectra of these derivative samples show obvious doublets with similar binding energies at ca. 229.2 and 232.2 eV (Figure 8a-d). These doublets reveal that a 4+ oxidation state could be the principal chemical state of molybdenum in these samples. The small peak at ca. 226.70 eV is attributed to S 2s. It can be observed that the area of the S 2s peak decreases as the annealing temperature increases which reveals a downtrend in the amount of S. Quantitative analysis of the XPS results (Figure 8) shows that the atomic ratios of S:Mo significantly decrease from 3.48 to 1.89 as annealing temperature increases from 200 to 500 oC. This indicated some different forms of structures were produced. The apical triply-bridging sulfur is first decomposed from the [Mo3S13]2- clusters with annealing temperature above 200 oC, forming the Mo3S12.54 From the binding energies at ca. 229.2 and 232.2 eV of Mo 3d, it can be revealed that the MoS2 has a 2H phase instead of 1T phase (ca. 228 and 231.0 eV).55-57 The further decomposition results in the formation of amorphous Mo3S6 (270 oC), and in the end, forms the well-crystallized MoS2 (350-500 oC).23, 54 As a result, the pyrolysis derivatives comprise of multiple transition forms of Mo3S12, Mo3S6 (or 3MoS2), and MoS2 as the annealing temperature increases.32 The pyrolysis derivatives mainly comprise of Mo3S12 and amorphous Mo3S6 at annealing temperature of 200-270 oC, while they are mainly composed of MoS2 at higher annealing temperature. For further analysis, we examined the S 2p region and the results are shown in Figure 8e-h. Interestingly, the spectra of S 2p show strong temperature dependence. As annealing temperature increases, the 14


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bridging S22− and apical S2− located at higher binding energies at 163.7 and 164.6 eV are weakened, while the terminal S22− at lower binding energies at 161.9 and 163.1 eV maintain strong signals. It was reported that the bridging S22− and apical S2− are the main active sites for HER,31 hence the decrease of these species at a high annealing temperature may lead to a low HER activity of the catalysts. A series of HER performances would be conducted to get a better understanding of these results. When the derivatives of [Mo3S13]2- clusters were loaded on rGO/CNTs, the C 1s XPS patterns show negligible difference (Figure S12, 13, 14, 15). The C 1s region of [Mo3S13]2--rGO/CNTs clusters could be deconvoluted into three components, ascribed to chemical bond of C=C, C−O, and C=O (Figure S12).58 These oxygen-containing functional groups would be beneficial for anchoring of [Mo3S13]2- as well as its derivatives onto the framework of the rGO/CNTs support. The O 1s XPS patterns were presented at Figure S13, which could also be deconvoluted into three peaks, presenting C−O, S−O and C=O respectively (Figure S13). The chemical bond of S−O suggested the [Mo3S13]2- clusters may be anchored on the macrostructure through chemical action.

15



Figure 8. XPS spectra of (a) Mo 3d for Mo3S13-200 (b) Mo 3d for Mo3S13-270; (c) Mo 3d for Mo3S13-300; (d) Mo 3d for Mo3S13-350; (e) Mo 3d for Mo3S13-420; (f) Mo 3d for Mo3S13-500; and (g) S 2P for Mo3S13-200; (h) S 2P for Mo3S13-270; (i) S 2P for Mo3S13-300; (j) S 2P for Mo3S13-350; (k) S 2P for Mo3S13-420; (l) S 2P for Mo3S13-500.

HER Performance of pyrolysis derivatives of [Mo3S13]2- clusters. The HER activities of [Mo3S13]2- clusters and multiple pyrolysis derivatives of [Mo3S13]216


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clusters anchored rGO/CNTs were evaluated in 0.5 M H2SO4 solution with a typical three-electrode configuration. HER results of the [Mo3S13]2--rGO/CNTs clusters are presented in Figure S16. The [Mo3S13]2- clusters anchored onto different rGO/CNTs (prepared at different rGO/CNTs ratios, 1/1, 2/1, and 4/1) exhibit the high HER electrocatalytic performance with similar onset potential of 123 mV (Figure S16a). Meanwhile, to achieve a current density of 10 mA cm−2, the [Mo3S13]2- based macrostructures need similar energy with an overpotential of ∼189 mV. In contrast, the bare rGO/CNTs showed the negligible catalytic activity. All [Mo3S13]2- anchored rGO/CNTs prepared at different rGO/CNTs ratios (1/1, 2/1, 4/1) exhibit similar Tafel slopes in the range of 62.0-66.6 mV/dec (Figure S16b). The polarization curves as well as Tafel slopes of all [Mo3S13]2- pyrolysis derivatives anchored rGO/CNTs (1/1) are presented in Figure 9a and b. To achieve a current density of 10 mA cm−2, the [Mo3S13]2--200-rGO/CNTs (pyrolysis derivatives of [Mo3S13]2- at 200 oC) need an overpotential of ~205 mV and its Tafel slope value is located at 72.9 mV/dec. As the annealing temperature rose to 270

o

C, the

overpotential is decreased from ~205 mV to ~178 mV, whereas Tafel slope is reduced from 72.9 to 64.2 mV/dec. These results appear to be even lower than those (overpotential of ~189 mV, and Tafel slope of 66.6 mV/dec) of [Mo3S13]2--rGO/CNTs. As a result, the pyrolysis derivatives of [Mo3S13]2- at 270 oC exhibit a higher HER capacity than the bare [Mo3S13]2- nanoclusters without pyrolysis treatment (Figure 9a). However,

the

pyrolysis

derivatives

prepared

at

higher

temperatures

([Mo3S13]2--350-rGO/CNTs and [Mo3S13]2--500-rGO/CNTs) achieved the significantly higher overpotential and Tafel slope values, indicating a poor HER capacity imposed by these pyrolysis derivatives (350-500 oC). This result is consistent with their crystalline morphologies, which restrict the expose of sulfur atoms at the edges. To determine the electrocatalytic reaction kinetics of all pyrolysis derivatives, electrochemical impedance spectroscopy (EIS) measurements were performed at applied potential of -300 mV (vs RHE). The equivalent electrical circuit is given in Figure S17. Nyquist plots shown in Figure 9c indicate low Rct values (200 Ω). Considering that all pyrolysis derivatives were anchored onto the same supporting

materials,

the

small

Rct

in

[Mo3S13]2--200-rGO/CNTs

and

[Mo3S13]2--270-rGO/CNTs macrostructures were most likely due to the increased catalytically active sites in pyrolysis derivatives of [Mo3S13]2- annealed at 200-270 oC. 17



An important factor in considering the ECSA values of different pyrolysis derivatives is that the non-faradaic current could provide a measure of exposed surface sites in all these pyrolysis derivatives samples. The ECSA of rGO/CNTs samples anchored with different pyrolysis derivatives were measured from the capacitance of the double layer Cdl using a sample CV method (Figure S18). Results indicated that the Cdl in [Mo3S13]2--500-rGO/CNTs was 5.1 mF cm-2, which was greatly lower than those (10.2-19.8 mF cm-2) of rGO/CNTs anchored with pyrolysis derivatives at 200-350 oC (Figure 9d). This result corresponded well with the pyrolysis derivatives of [Mo3S13]2- at 500 oC, which were mainly composed of the crystal line MoS2 with less surface sites exposed.

Figure 9. (a) Linear sweep voltammogram curves (iR uncorrected) for HER in 0.5 M H2SO4 of [Mo3S13]2--200-rGO/CNTs, [Mo3S13]2--240-rGO/CNTs, [Mo3S13]2--270-rGO/CNTs, [Mo3S13]2--300-rGO/CNTs, [Mo3S13]2--350-rGO/CNTs, and [Mo3S13]2--500-rGO/CNTs. Potential sweep rate is 5 mV s-1; (b) Correlative Tafel plots derived from (a); (c) AC impedance spectra; (d) Variation of double-layer charging currents at 0.05 V with potential scan rate. (e), (f) Long-term cycling stability of the pyrolysis derivatives anchored macrostructure. The rGO/CNTs was prepared by the rGO/CNTs ratio of 1/1.

The long-term cycling stability of pyrolysis derivative samples was accessed using chronoamperometry at a scan rate of 50 mV s−1 (Figure 9e and f). The polarization curve of [Mo3S13]2--200-rGO/CNTs after 1000 CV cycles almost 18


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overlapped with the initial curve. The observation of negligible current loss of [Mo3S13]2--270-rGO/CNTs also indicates the good electrochemical stability of pyrolysis derivatives (200-270

o

C) in acidic electrolyte. HER schematics of

(NH4)2[Mo3S13]·nH2O as well as pyrolysis derivatives of (NH4)2[Mo3S13]·nH2O in acidic conditions was given in Figure 10. The rGO/CNTs macrostructures bridged freeway for electron transfer at a high rate of speed during HER. A comparison of these rGO/CNTs macrostructures with other HER electrocatalysts in the HER performance was presented in Table S1. [Mo3S13]2- cluster and its pyrolysis derivatives (270

o

C) based electrocatalysts showed relatively higher HER

performances as compared with most analogous single-metal molybdenum sulphide nanocomposites. They also exhibited a close performance to those multi-metal nanocomposites.

Figure 10. HER schematics of (NH4)2[Mo3S13]·nH2O as well as pyrolysis derivatives of (NH4)2[Mo3S13]·nH2O in acidic conditions. The rGO/CNTs macrostructures bridged freeway for electron transfer at a high rate of speed during HER

Figure 11 maps the correlation between annealing temperature, HER performance, and S:Mo atom ratio of all [Mo3S13]2- pyrolysis derivatives. As the temperature increase from 200 to 270 oC, the atom ratio of S:Mo has a clear declining trend from 3.48 to 3.01. Meanwhile, the increasing temperature from 200 to 270 oC also resulted in a decreased overpotential of [Mo3S13]2- pyrolysis derivatives. The [Mo3S13]2- pyrolysis derivative annealed at 270

o

C comprised a crystalline

MoSx@amorphous MoSx, which contributed to the HER performance. Comparison 19



Page 20 of 29

between the theoretical amount of H2 and the generating amount of H2 under test conditions was exhibited in Figure S19. The experimental values observed were greatly close to the theoretical values. This result demonstrated the Faradaic efficiency for the HER by [Mo3S13]2--270-rGO/CNTs is close to 100%. It is a favorable evidence of water splitting, which suggested that the cathodic currents caused by the hydrogen evolution.

Figure 11. 3D map revealing the correlation between annealing temperature, overpotential, and atomic ratio of S: Mo in pyrolysis derivatives of [Mo3S13]2-

Characteristics and HER performance of [Mo3S13]2--270-rGO/CNTs anchored with different amounts of [Mo3S13]2--270. The XRD patterns of rGO/CNTs anchored with different amounts of Mo3S13-270 was presented in Figure S20a. After being anchored on the rGO/CNTs macrostructure, the obvious (110) (at 10.1°) and (111) (at 16.4°) peaks of Mo3S13-270 become vague; this suggests a poor crystal structure of Mo3S13-270 in anchored samples. Raman spectra of Mo3S13-270 and Mo3S13-270-macrosturctures was presented in Figure S20b. The distinct peak (at ~819.8

cm-1)

of

Mo3S13-270

became

vague

in

the

patterns

of

Mo3S13-270-5-macrosturctures and Mo3S13-270-20-macrosturctures, but it was still distinct in the patterns of Mo3S13-270-200-macrosturctures. This implied the excess amount of Mo3S13-270 cluster in Mo3S13-270-200-macrosturcture. FTIR spectra of 20


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three different Mo3S13-270-macrosturctures were also conducted (Figure S20c), which exhibit similar characteristic peaks. XPS spectra of these Mo3S13-270-macrosturctures was presented at Figure S21, S22 and S23, which also show high similarity in Mo 3d and S 2p. The HER activities of rGO/CNTs anchored with various amounts of [Mo3S13]2--270 clusters (5, 20 and 200 mg) were presented in Figure 12. It is obvious that [Mo3S13]2--270-20-rGO/CNTs exhibited better HER electrocatalytic performance than [Mo3S13]2--270-5-rGO/CNTs and [Mo3S13]2--270-200-rGO/CNTs. This means that an appropriate ratio between the [Mo3S13]2--270 nanoclusters and carbon support (rGO/CNTs) was required. Tafel slope values of all samples were presented in Figure 12b. [Mo3S13]2--270-5-rGO/CNTs exhibited the smallest Tafel slope of 64.2 mV/dec among all macrosturctures, which was consistent with the polarization curves. The electrochemical impedance spectroscopy (EIS) was used to further investigate the electrode kinetics of hydrogen evolution performance. As can be seen in Figure 12c, Rct of rGO/CNTs anchored various amount [Mo3S13]2--270 clusters (5, 20 and 200 mg) is 15.7, 5.9 and 20.7 Ω, respectively. ECSA of all these samples were also measured from the capacitance of the double layer Cdl, using a sample CV method (Figure S24).51,59 The Cdl of [Mo3S13]2--270-5-rGO/CNTs, [Mo3S13]2--270-20-rGO/CNTs and [Mo3S13]2--270-200-rGO/CNTs was 10.3 mF cm-2, 15.3 mF cm-2 and 3.2 mF cm-2. The smallest Rct and largest ECSA suggest [Mo3S13]2--270-20-rGO/CNTs balanced the catalysis cites and conductivity of these catalysts. The durability test of various [Mo3S13]2--270-rGO/CNTs was conducted utilizing CV measurements for scanning 1000 cycles from −0.1 to −0.8 versus Ag/AgCl electrode. The negligible current loss (Figure S25a) of these samples indicates that the [Mo3S13]2--270-rGO/CNTs have superior stability during electrochemical catalysis process. To measure the durability of the samples during the practical operation, continuous HER at fixed overpotential (-0.2V vs. RHE) was conducted. As suggested in Figure S25b, after a long period of 8 h,

the

current

density

shows

tiny

falloff,

[Mo3S13]2--270-rGO/CNTs has good stability.

21


which

suggests

that


Figure 12 (a) Linear sweep voltammogram curves (iR uncorrected) for HER in 0.5 M H2SO4 of [Mo3S13]2--270-5-rGO/CNTs, [Mo3S13]2--270-20-rGO/CNTs [Mo3S13]2--270-200-rGO/CNTs and bare [Mo3S13]2-. Potential sweep rate is 5 mV s-1; (b) Correlative Tafel plots derived from (a); (c) AC impedance spectra; (d) Variation of double-layer charging currents at 0.05 V with potential scan rate.

CONCLUSION rGO/CNTs supported [Mo3S13]2- clusters and pyrolysis derivatives of [Mo3S13]2clusters were used as promising electrocatalyts for enhancing hydrogen evolution. [Mo3S13]2- anchored composite prepared at different rGO/CNTs ratios exhibited the similar stable physicochemical properties and enhanced HER capacities. Quantitative analysis of the XPS results shows that the atomic ratio of S:Mo significantly decreased from 3.48 to 1.89, which indicates the transitions of Mo3S12, Mo3S6, and 22


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MoS2 in the multiple pyrolysis derivatives at different annealing temperature. XPS, XRD, and Raman spectra of the pyrolysis derivatives of [Mo3S13]2- clusters annealed at 200-500 oC exhibited the decreased edge sites density and a peer extent of ordering in the layers of pyrolysis derivatives as the annealing temperature increased. In addition, the pyrolysis derivatives annealed at temperature of 200-270 oC was bonded on the rGO/CNTs separately with amorphous form or less crystalline forms. The rGO/CNTs macrostructure anchored with pyrolysis derivatives annealed at 270 oC exhibited an excellent HER capacity, which was even higher than that of rGO/CNTs anchored with bare [Mo3S13]2-. In contrast, pyrolysis derivatives (350-500 oC) of [Mo3S13]2- exhibited poor HER capacity as the annealing temperature was higher at 350 oC. ASSOCIATED CONTENT Supporting Information Additional characteristics (XPS, EDS, FTIR, XRD, Zeta potential) of pyrolysis derivatives of [Mo3S13]2-, HER activities of [Mo3S13]2- clusters and multiple pyrolysis derivatives of [Mo3S13]2- clusters.

AUTHOR INFORMATION Corresponding Author *Tel: +86-531-88361912, E-mail: [email protected] (Xing Xu) Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The research was supported by the National Natural Science Foundation of China (51178252, 51508307, 51508308), China Postdoctoral Science Foundation funded project (2014M560556, 2015T80721). This work was also supported by grants from Tai Shan Scholar Foundation (No. ts201511003) and the Fundamental Research Funds of Shandong University (2015GN006).

23



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For Table of Contents Use Only

Synopsis

[Mo3S13]2- pyrolysis derivatives obtained by the removal of S atoms via heat treatment exhibited diverse electrochemical HER activities.


CNTs Supported Pyrolysis Derivatives of [Mo3S13]2- Clusters as

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