Cu/MoS₂/ITO based hybrid structure for catalysis of hydrazine oxidation

Abstract

We have successfully demonstrated large-area and continuous MoS₂ films grown on indium tin oxide (ITO) substrates by RF sputtering followed by a post-annealing process. Thin Cu layers were prepared on MoS₂/ITO films by thermal evaporation to enhance electron-transfer rates as well as the catalytic activity of hydrazine oxidation. High electrocatalytic activity towards hydrazine oxidation was observed in a Cu/MoS₂/ITO hybrid structure. A large residual current was also observed in the Cu/MoS₂/ITO hybrid due to the combination of MoS₂ and Cu film. The electrocatalytic activity was enhanced with the increase of the MoS₂ film thickness. The observed results showed that the Cu/MoS₂/ITO catalyst is indeed a valuable material for future applications in hydrazine fuel cells.

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Introduction

In recent years, layered transition metal dichalcogenides (LTMDs) have been the focus of many studies due to their distinctly unique properties such as high mobility and high current on/off ratio,¹ high mechanical strength and stability in inert atmospheres.^2–4 Molybdenum disulfide (MoS₂), a representative LTMD member, has many advantages as well, including low cost, earth-abundance, high chemical stability and outstanding photocatalytic activity as well as potential electrocatalytic properties in the hydrogen evolution reaction (HERs).^5–9 Basically, MoS₂ consists of two-dimensional sheets of vertically stacked S–Mo–S interlayers linked together by weak van der Waals forces. The most important feature in this LTMD family of materials is the body thickness scalability down from bulk (indirect band gap of 1.3 eV) to a monolayer (direct band gap of 1.8 eV). Much effort has been put forth to investigate the catalytic activities of MoS₂, and MoS₂/graphene heterostructures.^10,11 Both the theoretical and experimental results of previous studies have confirmed that the catalytic activity of MoS₂ originates from the reactive sulfur edges.^12–14 Electrocatalytic activity is highly dependent on the catalyst morphology, grain size, surface texture, crystalline structure and electrical conductivity. Thus, thin MoS₂ with exposed edges becomes more active for electrocatalysis than the bulk forms of the material.^12,13

Recently, many studies have been performed with the aim to enhance the electrocatalytic activity of liquid-feed fuel cells using fuel such as ethylene glycol,¹⁵ sodium borohydride¹⁶ and hydrazine.¹⁷ Among them, hydrazine (N₂H₄) is a high-energy fuel molecule that has received considerable attention. Many studies have been done to improve electrocatalytic activity using graphene oxide (GO) or carbon nanotubes (CNTS). Li et al.^18,19 have demonstrated high electrocatalytic activity toward hydrazine oxidation using Ni–Fe alloy on polyethyleneimine (PEI)-functionalized graphene oxide (GO) electrocatalyst. Ye et al.²⁰ have reported electrocatalytic activity of Pd–Ni/CNT. Graphene-like MoS₂ also can provide abundant active edges for potential catalytic performance and a large specific surface area for catalytic support. Zhong et al.²¹ have reported that Ni–Fe/MoS₂ hybrid by in situ growth of a Ni–Fe alloy, exhibited high electrocatalytic activity toward hydrazine oxidation.

In this work we used sputtered few-layer MoS₂ on indium tin oxide (ITO) and studied the catalytic activity of hydrazine oxidation. To enhance the electrocatalytic activity toward hydrazine oxidation, a thin Cu layer (∼2 nm) was coated onto MoS₂/ITO film by a thermal evaporation system in a high vacuum (an order of 10⁻⁷ torr) with a deposition rate of 1.0 Å s⁻¹ and the thickness was monitored during the deposition by a quartz oscillator. The thin Cu layer improves the electron-transfer rates and enhances the catalytic activity of hydrazine oxidation through the synergistic effects of the Cu and MoS₂/ITO. ITO substrate has many advantages such as low cost, excellent optical transparency, high electrical conductivity, wide electrochemical working window, stable electrochemical and physical properties.^22–26 Many previous reports based on ITO used as a substrate for electrocatalytic activity²⁷ are also available. The hybrid structure used is shown in Fig. 1(a). To date, many researchers have paid attention to growing MoS₂ film via RF magnetron sputtering on fluorine-doped tin oxide (FTO), silicon and glass sheet substrates.^28,29 However, to the best of our knowledge, our work is the first to study the electrocatalysis of hydrazine oxidation of continuous few-layer MoS₂ sputtered on ITO.

Fig. 1 (a) A schematic view of Cu/MoS₂/ITO hybrid structure; (b and c) optical images of MoS₂–ITO films. They were sputtered for 1 and 3 minutes onto ITO substrates and annealed in Ar and sulfur environment at 600 °C; (d and e) optical images of Cu/MoS₂/ITO hybrid structures prepared using MoS₂ films; (f) Raman spectra of MoS₂/ITO (black line) and Cu/MoS₂/ITO (red line).

Experimental and device fabrication details

ITO substrates were ultrasonically degreased by immersion in acetone, methanol, isopropyl alcohol (IPA) and deionized (DI) water and then baked at 120 °C for 5 minutes. Initially, MoS₂ thin films were deposited by RF magnetron sputtering. The high pure (99.96%) MoS₂ target was used for the deposition of films. The vacuum coating unit was initially evacuated to a base pressure of ∼1 × 10⁻⁷ torr and argon (Ar) gas was primarily allowed to flow into the chamber for 5 minutes in order to remove the oxide layer on the surface of the target. During the film depositions, the Ar flow ratio was maintained at 10 sccm and the RF power was fixed at 50 W. The films were sputtered initially at room temperature at four different deposition times (1, 3, 5 and 10 minutes). We have used a shadow mask to sputter MoS₂ only on a specific area of the whole substrate. The sputtered MoS₂ thin films were subjected to annealing by sulfurization process at 600 °C under an Ar environment for 1 hour. Sulfur powder (99.99% purity) of 0.3 g, placed upstream in the chamber, was evaporated at 120 °C with Ar (100 sccm) as a carrier gas and the pressure of the CVD chamber was kept at 2 × 10⁻² torr. The electrocatalytic activity of the Cu/MoS₂/ITO hybrid was systematically investigated through cyclic voltammetry in 75 mM NaOH with 0.1 mM hydrazine hydrate using a three-electrode setup which consists of saturated Ag/AgCl as the reference electrode, a platinum wire as the counter electrode and Cu/MoS₂/ITO hybrid as the working electrode.

Results and discussion

Fig. 1(b–e) shows the optical microscopy (OM) images of MoS₂/ITO and Cu/MoS₂/ITO. We have intentionally covered one portion of the films in order to enhance the contrast of the images from which the films were observed to be continuous and uniform over a large area.

Raman analysis was performed to investigate the structure of MoS₂ nanosheets and to identify the number of MoS₂ layers.^30,31 Raman spectra of MoS₂/ITO and Cu/MoS₂/ITO hybrid structure are shown in Fig. 1(f). The strong Raman peaks are observed at 383.9 and 405.3 cm⁻¹ for 1 minute-MoS₂ (∼2 nm) and they have associated with the in-plane vibrational (E_2g¹) and out-of-plane vibrational (A_1g) modes, respectively. The peak separation (Δk) between (E_2g¹) and (A_1g) mode is approximately 21.4 cm⁻¹, which is in good agreement with that of 2–3 layers of MoS₂.^30,32,33 The peak separation was also confirmed by Raman mapping analysis as shown in Fig. S1.† Raman mapping was performed over an area of 30 μm × 30 μm of the MoS₂ film (1 minute-sample). The E_2g¹ mode appeared at 382–384 cm⁻¹ and the A_1g mode at 404.5–406.5 cm⁻¹. The Δk values are located in the range of ∼20–22 cm⁻¹, confirming 2–3 layers of MoS₂.³² The Δk value increases to ∼25, ∼26.5 and ∼30.9 cm⁻¹, for the sputtering time of 3, 5 and 10 minutes, respectively in Fig. 2(e–g). Thus, Raman spectra clearly supports that the MoS₂ film becomes thicker with increasing sputtering time. It can be noted that E_2g¹ mode and A_1g vibrational modes decreases and broadens but no noticeable changes in peak position are observed after deposition of Cu film. For the thickness of the MoS₂ film, an AFM scan was taken from the corner of the MoS₂ film patterned using a shadow mask. The estimated film thickness is ∼2 nm for 1 minute-MoS₂ (sputtering time of 1 minute), as shown in Fig. 2(a), corresponding to 2–3 layers of MoS₂.³⁴ The thickness increases to ∼5, ∼7, and ∼10 nm for 3, 5, 10 minutes, respectively as shown in Fig. 2(b–d). Thus, the film thickness of MoS₂ is controllable by tuning the sputtering time. Surface morphological and topographical images of MoS₂/ITO and Cu/MoS₂/ITO hybrid structures (2 nm-thick MoS₂) are shown in Fig. 3(a–d). AFM topographical image represents the nanocrystalline structures of MoS₂/ITO.

Fig. 2 AFM height profiles for MoS₂ films with different thickness, (a) ∼2 nm (b) ∼5 nm (c) ∼7 nm and (d) ∼10 nm. Inset: AFM topological image of the corresponding film; (e–g) Raman spectra of MoS₂/ITO and Cu/MoS₂/ITO hybrid structures with different thickness of MoS₂ films (5 nm, 7 nm and 10 nm).

Fig. 3 SEM micrographs, 2D-AFM topographs and XRD patterns of MoS₂/ITO and Cu/MoS₂/ITO hybrid structures.

The X-ray diffraction (XRD) patterns of MoS₂/ITO and Cu/MoS₂/ITO hybrid structures are shown in Fig. 3(e–f). For the MoS₂/ITO structure, the MoS₂-related peaks are observed at 2θ = 14.18°, 33.57° and 70.01° which are consistent with bulk MoS₂ (JCPDS card no. 37-1492) having a hexagonal phase with P6₃/mmc (194) space group. And, substrate-related minor peaks are also observed at 22.5°, 32.5° and 45.5°, corresponding to the (222), (400) and (440) reflections, respectively. For the Cu/MoS₂/ITO hybrid structure, the diffraction peaks of Cu (2θ = 43.38 and 50.48) appeared. The MoS₂-related peaks of (002) and (201) and ITO-related peaks are highly suppressed and couple of Cu-related peaks are emerged in Cu/MoS₂/ITO hybrid structure as shown in Fig. 3(f).

To evaluate the electrocatalytic activity, cyclic voltammetry was investigated for MoS₂/ITO, Cu/ITO and Cu/MoS₂/ITO hybrid in 75 mM NaOH with 0.1 mM hydrazine aqueous solutions at the scan rate of 100 mV s⁻¹ (Fig. 4).

Fig. 4 (a) Cyclic voltammetry of MoS₂/ITO, Cu/ITO and Cu/MoS₂/ITO hybrid structures at a scan rate of 100 mV s⁻¹; (b) cyclic voltammetry of Cu/MoS₂/ITO hybrid structures with different thickness (2–10 nm) of MoS₂ layer at a scan rate of 100 mV s⁻¹.

It can be seen that the bare MoS₂ film on ITO (MoS₂/ITO) does not exhibit any hydrazine oxidation peak while the Cu-coated ITO electrode (MoS₂/ITO) exhibits a small electrocatalytic activity peak towards hydrazine oxidation (Fig. 4(a)). The observed current density value is 1.19 mA cm⁻² for Cu/ITO electrode. The maximum peak current is observed at 1.93 mA cm⁻² for the Cu/MoS₂/ITO hybrid, which represented the maximum electrocatalytic activity towards hydrazine oxidation. Yang et al.³⁵ reported up to 950 μA cm⁻² current density value for Ag/CNT composites in 0.1 M K₂SO₄ electrolyte solution with 10 mM hydrazine, which is quite low compared with our observed results. The catalytic mass activity for different types of electrodes is plotted as shown in inset Fig. 4(a). There is an overall negative shift of peak potential for Cu/MoS₂/ITO when compared to Cu/ITO, which reflects a fast electron-transfer reaction on Cu/MoS₂/ITO. Furthermore, the reverse scan gives no corresponding cathodic peak, suggesting a totally irreversible oxidation of hydrazine in alkaline solutions on Cu/MoS₂/ITO. The suggested mechanism of hydrazine oxidation on copper substrate is given as follows.³⁶

N₂H₄ + OH⁻ → N₂H₃ + H₂O + e⁻ (slow) (a)

N₂H₃ + OH⁻ → N₂ + 3H₂O + 3e⁻ (fast) (b)

This mechanism explains the irreversibility of the reaction and the absence of the cathodic peak because the formation of stable species such as nitrogen as an end product is difficult to reduce under the given voltages.

The cyclic voltammetry of the Cu/MoS₂/ITO hybrid with various thicknesses of MoS₂ is shown in Fig. 4(b). The current value increased with the increase of MoS₂ layer thickness. The current density increased to 2.3, 3.0 and 3.4 mA cm⁻² for 5, 7 and 10 nm-MoS₂, respectively. Also the catalytic mass activity values are plotted for different MoS₂ layer thicknesses as shown in inset Fig. 4(b). The highest mass catalytic activity of 555 mA g⁻¹ was obtained for Cu/MoS₂/ITO hybrid with 10 nm-thick MoS₂. Gao et al.³⁶ reported the current density of 14 mA cm⁻² using Cu nanotubes–graphene paper (Cu–GP) hybrid in 0.1 M KOH containing 10 mM hydrazine at a scan rate of 100 mV s⁻¹. The 106 μA were achieved for Pd/WO₃–ITO electrode by using 5 mM N₂H₄SO₄ solution.³⁷ Yi et al.³⁸ have reported significantly improved electrocatalytic activity (100 mA cm⁻²) with Au/Ti electrode for hydrazine oxidation in alkaline solutions, in which the electrode was modified using a hydrothermal method. In our work, sputtered-MoS₂ film thickness plays an important role in the electrocatalytic activity for the oxidation of hydrazine. As mentioned in the introduction, MoS₂ provides the active edges for potential catalytic performance and also provides a large specific surface area for catalytic support.^12–14 Our results indicate that the electrocatalytic activity gradually increases with the thickness of few-layer MoS₂ in this thickness range (2–10 nm). There is a report that sputtered MoS₂ on insulator most likely contains some Mo–O interfacial layer due to Mo atoms bound to the O atoms of oxide, while further deposition results in a MoS₂ film with a lower oxygen content.³⁹ We also noticed the same tendency in our films. The Mo–O interfacial layer may hinder the catalytic activity of MoS₂. The catalytic activity dependence of MoS₂ film thickness may be attributed to the fact that as the MoS₂ thickness increases, the effect of the Mo–O interfacial layer decreases in this thickness range. Cyclic voltammetry measurements were performed at different scan rates in order to investigate the reaction mechanism of hydrazine oxidation at the Cu/MoS₂/ITO electrode as shown in Fig. 5(a). The oxidation peak potential and peak current density as well as the catalytic mass activity increase with increasing scan rates as shown in Fig. 5(a). The oxidation peak current (I_p) for hydrazine is linearly proportional to the scan rate, which indicates that the electrochemical oxidation of hydrazine at the Cu/MoS₂/ITO electrode is an adsorption-limited process. The stability of the catalysts is of great importance for device fabrications. The multiple cycle voltammetry is presented in Fig. S2† for the stability confirmation. It shows a quite stable cyclic voltammetry up to 20 cycles. The chronoamperometric measurement was used to assess the durability of the catalysts. Fig. S3† shows the I–t curves of the Cu/MoS₂/ITO hybrid and the Cu/ITO electrode at a working potential of −0.4 V. The larger residual current is observed in Cu/MoS₂/ITO hybrid compared with Cu/ITO hybrid, which indicates that the MoS₂ with Cu film offers superior electrocatalytic activity for hydrazine electro-oxidation.

Fig. 5 (a) Cyclic voltammetry of Cu/MoS₂/ITO hybrid structures performed at different scan rates; (b) I_p against scan rate plot of Cu/MoS₂/ITO hybrid electrode.

Conclusion

We used few-layer MoS₂ on ITO and studied the catalytic activity of hydrazine oxidation. The MoS₂ film was sputtered at room temperature and then post-annealed to improve the crystallinity. The number of layers in MoS₂ was estimated by Raman spectra and AFM measurement. The Cu/MoS₂/ITO hybrid exhibited an excellent electrocatalytic activity and stability toward hydrazine oxidation. The electrocatalytic activity enhanced with the increase of MoS₂ film thickness up to 10 nm. This novel approach shows that graphene-like MoS₂ is a promising support material for a variety of applications in synergistic catalysis.

Acknowledgements

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2010-0020207, 2012R1A1A2007211).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra14048f

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