Article pubs.acs.org/Langmuir
Atomic Layer Deposition of Ultrathin Nickel Sulfide Films and Preliminary Assessment of Their Performance as Hydrogen Evolution Catalysts Yasemin Ç imen,†,‡ Aaron W. Peters,‡ Jason R. Avila,‡,§ William L. Hoffeditz,‡ Subhadip Goswami,‡ Omar K. Farha,*,‡,§,∥ and Joseph T. Hupp*,‡,§ †
Department of Department of Sheridan Road, ∥ Department of ‡
Chemistry, Faculty of Science, Anadolu University, 26470 Eskişehir, Turkey Chemistry and §Argonne-Northwestern Solar Energy Research (ANSER) Center, Northwestern University, 2145 Evanston, Illinois 60208, United States Chemistry, Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia
S Supporting Information *
ABSTRACT: Transition metal sulfides show great promise for applications ranging from catalysis to electrocatalysis to photovoltaics due to their high stability and conductivity. Nickel sulfide, particularly known for its ability to electrochemically reduce protons to hydrogen gas nearly as efficiently as expensive noble metals, can be challenging to produce with certain surface site compositions or morphologies, e.g., conformal thin films. To this end, we employed atomic layer deposition (ALD), a preeminent method to fabricate uniform and conformal films, to construct thin films of nickel sulfide (NiSx) using bis(N,N′-di-tert-butylacetamidinato)nickel(II) (Ni(amd)2) vapor and hydrogen sulfide gas. Effects of experimental conditions such as pulse and purge times and temperature on the growth of NiSx were investigated. These revealed a wide temperature range, 125−225 °C, over which self-limiting NiSx growth can be observed. In situ quartz crystal microbalance (QCM) studies revealed conventional linear growth behavior for NiSx films, with a growth rate of 9.3 ng/cm2 per cycle being obtained. The ALD-synthesized films were characterized using X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) methods. To assess the electrocatalyitic activity of NiSx for evolution of molecular hydrogen, films were grown on conductive-glass supports. Overpotentials at a current density of 10 mA/ cm2 were recorded in both acidic and pH 7 phosphate buffer aqueous reaction media and found to be 440 and 576 mV, respectively, with very low NiSx loading. These results hint at the promise of ALD-grown NiSx materials as water-compatible electrocatalysts.
1. INTRODUCTION Concern over the environmental consequences of surplus greenhouse gases from the combustion of fossil fuels is driving new research into alternative renewable energy technologies.1−4 One example is the solar-driven electrolysis of water, an environmentally friendly method of electrochemically splitting water to produce O2 and H2 gases, of which H2 can be used as a high-gravimetric-density energy carrier. Selected noble-metalbased catalysts (e.g., Pt) show high catalytic activity for the production of hydrogen from water/acid; their high cost may limit their real world implementation, necessitating the development of new earth abundant catalysts.5 While many first-row transition metal oxide- and sulfide-based catalysts have shown great promise toward electrocatalytically reducing water to hydrogen,6−9 additional work is necessary to achieve high currents at low overpotentials to compete with state-of-the-art noble-metal-based catalysts. Metal sulfides, in particular, show great aptitude due to proximal proton-acceptor (i.e., sulfur/ sulfide) and hydride-acceptor sites (i.e., metal centers).5 These © XXXX American Chemical Society
properties manifest themselves in the auspicious potential metal sulfides exhibit toward energy-related applications, such as electrochemical catalysis,10−15 photoelectrochemical catalysis,16 lithium-ion batteries,17,18 and photovoltaics.19 Among the many metal sulfide materials, nickel sulfide, which appears in many different crystallographic forms (e.g., Ni3S2, NiS2, and NiS), exhibits encouraging electrocatalytic activity.5,20,21 Although nickel sulfide can be synthesized using many techniques such as electrochemical,20,22,23 hydrothermal,24 and solvothermal25,26 deposition, it is oftentimes difficult to prepare thin conformal films with these methods. Additionally, the requirement of conductive substrates for electrodeposition routes as well as typically harsh reaction conditions for thermal deposition methods point to the need for additional fabrication methods for nickel sulfide materials.27 Received: July 20, 2016 Revised: October 16, 2016
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DOI: 10.1021/acs.langmuir.6b02699 Langmuir XXXX, XXX, XXX−XXX
Article
Langmuir
depostited on the FTO surface using a literature preparation.52 ALD was carried out on a Savannah 100 ALD (Ultratech/Cambridge Nanotech, Inc.) under a constant 15 sccm flow of N2 with a baseline pressure of ∼0.6 Torr. In order to obtain sufficient vapor pressure, the nickel precursor reservoir was heated to 120 °C. Representative deposition conditions are as follows: the reactor chamber was heated to 125 °C, and the Ni precursor was pulsed into the reactor chamber for 1 s, evacuated for 45 s followed by a H2S pulse of 0.015 s, and then evacuated for 45 s. This pulsing sequence was repeated to obtain the desired film thickness. In situ QCM experiments were performed on a wall-mounted QCM incorporated into the lid of the ALD reactor53 and equipped with AT cut polished crystals (Inficon, SQM-160). QCM crystals were equilibrated at reactor temperature for ∼30 min to ensure a level baseline. The crystal was coated with Al2O3 via 20 ALD cycles trimethylaluminum (TMA) and water (steam) half-cycles. The timing sequence used 0.015−20−0.015−20 s (t1−t2−t3−t4, where t1 and t3 are the pulse times of TMA and water, respectively, and t2 and t4 are the purge times after pulses from TMA and water, respectively). The purpose of alumina predeposition was to create uniform hydroxoterminated surface suitable for initiation of NiSx growth. In this work, to convert mass growth data (ng/cm2) obtained by QCM data to film thicknesses (Å), we assumed an average density for NiSx of 5.2 g/ cm3.54 The reported growth rates have been adjusted for the roughness of the gold-coated crystals used by normalizing to the growth rate of Al2O3 (the roughness factor determined in this way was 0.62). Thicknesses of NiSx films grown on silicon wafers were measured ex situ by ellipsometry (J.A. Woollam M2000U). In order to obtain approximate thicknesses on surfaces other than silicon, witness silicon wafers were simultaneously deposited in the reactor chamber and thicknesses of the silicon wavers were measured ex situ using elliposmetry. Nickel sulfide films were also characterized by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Scientific) and grazing incidence X-ray diffraction (GIXRD, ATX-G, Rigaku). Raman spectra were collected on a 1000 ALD cycle NiSx film grown on FTO using a LabRAM HR Evolution Confocal RAMAN System (HORIBA Sci) with a 532 nm laser. Scanning electron microscopy (SEM) was used to image the films at an accelerating voltage of 2 keV on a Hitachi SU8030. Atomic force microscopy (AFM) images were collected on a Bruker Dimension FastScan. A Solartron Analytical Modulab potentiostat was used for electrochemical measurements. A platinum mesh counter electrode and a Ag/AgCl (saturated KCl) reference electrode were used in a standard three-electrode electrochemical cell utilizing the NiSx film as the working electrode. Surlyn was affixed to electrodes in order to precisely define electrode geometric surface areas (i.e., areas uncorrected for surface roughness). Electrocatalysis was carried out in aqueous 0.1 M HCl (pH 1) and 1 M aqueous phosphate buffer (pH 7) solutions. All electrochemical potentials are iR-corrected and have been converted to the reversible hydrogen electrode (RHE) as a reference as shown in eq 1.
One alternative synthetic route is atomic layer deposition (ALD), a self-limiting vapor-phase deposition process for producing conformal thin films of a wide variety of materials.28 The conventional ALD process involves the exposure of a pair of reactive vapors or gases to a surface in a sequential manner that ensures the deposition reaction occurs exclusively at the surface/vapor interface. Repeated exposures of a surface to these half-cycles allows for self-limited, layer-by-layer growth that can provide high purity and uniform thin films not accessible via other routes, making ALD an ideal technique for growing films of precise and predetermined thickness.28−30 The material grown is dependent upon the combination of precursor species used. For example, a metal-containing coordination compound or organometallic complex is used in conjunction with H2O or O328,31−34 or H2S35−37 to prepare various metal-oxide or metal-sulfide materials, respectively. Though most ALD research is focused on the growth of metal oxides,28,38−43 growing interest in sulfide materials has led to increased exploration of metal precursors reactive to various sulfur-containing sources.44−47 Because of the wide range of applications of nickel materials, many nickel ALD precursors exist such as Ni(acac)2, Ni(dmamb)2, NiCp2, and Ni(thd)2, which have been used in combination with H2, NH3, H2O, O3, or H2S.28,31−34,48 Given the literature precedent for many amidinate and bis-amidinate tranisition metal complexes35,37,49−51 reactivity with both water and hydrogen sulfide, we investigated NiSx thin-film growth by ALD using bis(N,N′-di-tert-butylacetamidinato)nickel(II) (Ni(amd)2) (Figure 1) and H2S hydrogen sulfide. In situ quartz crystal
Figure 1. Structure of bis(N,N′-di-tert-butylacetamidinato)nickel(II) (Ni(amd)2) used as an ALD precursor.
microgravimetry (QCM) was used to follow the progress of NiSx growth in real time. Additionally, ALD offers the unique opportunity to precicely and easily control thickness and/or the deposited amount of a material, thereby facilitating determination of the specific activity of a catalyst, an otherwise ambiguous or difficult-to-obtain figure of merit for many conventional fabrication methods. As an initial probe of the potential catalytic utility of the resulting films, their activity as electrocatalysts for hydrogen evolution from acidic and neutral (buffered) aqueous solutions was examined.
RHE = measured potential + 0.197 + 0.059 × pH
(1)
3. RESULTS AND DISCUSSION Deposition of NiSx on Silicon and FTO by ALD. ALD was used to deposit NiSx films on silicon, and growth rates were measured using ellipsometry. To determine the optimal conditions for the deposition of NiSx, growth rates were measured under a variety of conditions. Figure 2 summarizes the measured growth rates under different (A) reaction temperatures, (B) purge times, (C) Ni(amd)2 pulse times, and (D) H2S pulse times. A temperature-independent growth rate (Figure 2A) of 0.54 ± 0.04 Å/cycle was obtained for reactor temperatures from 125 to 225 °C, establishing that no thermal decomposition occurs within this window; going above 225 °C, however, led to loss of self-limiting behavior. Self-
2. EXPERIMENTAL SECTION Unless otherwise noted, all chemicals were purchased from SigmaAldrich and used as received. The nickel precursor, bis(N,N′-di-tertbutylacetamidinato)nickel(II) (Ni(amd)2) (98%), was obtained from Strem Chemicals, Inc. A gas mixture containing 1% H2S in N2 was purchased from Matheson Tri-Gas. Silicon wafers and fluorinated tin oxide (FTO) glass (Hartford Glass, Hartford City, Indiana) were cut into ca. 1.25 × 2.0 cm2 rectangles. Silicon wafers were cleaned by sonicating in a bath of soapy water, isopropyl alcohol, and methanol for 15 min each. Ethanol and acetone were used instead of isopropyl alcohol and methanol for FTO cleaning. Nanoparticulate ITO (Sigma-Aldrich,
