Donald Keith Roper,1,2* Jeremy Dunklin,2 Alexander O’Brien2
1Microelectronics Photonics Graduate Program
2Ralph E Martin Department of Chemical Engineering (University of Arkansas) NANO 206, 731 W. Dickson St. Fayetteville, AR 72701
Nano-Micro Conference, 2017, 1, 01003
Published Online: 01 October 2017 (Article)
Photocatalysis of hydrogen from water is limited by incomplete absorption of solar radiation and by uncontrolled disposition of generated carriers. Nanoantenna (NA)-induced coupling of photons to excitons could enhance photocatalytic solar fuel generation by increasing broadband optical absorption and by injecting energetic electrons where NA interfaces with semiconductor catalyst. This work examined catalysis of hydrogen evolution reaction (HER) by monolayer (1L) transition metal dichalcogenide (TMD) with and without decoration by optical NA. Spectroscopic and microscopic characterization of heterostructures of 1L-TMD and NA self-assembled via exfoliation and redox chemistry was compared with discrete dipole simulation. Electrodes inked with 1L tungsten disulfide (WS2) onto which NA was electrochemically reduced exhibited higher HER relative to 1L-WS2 in neat or physically NA-decorated forms as measured by linear sweep and cyclic voltammetry. Coordinated simulation and measurement of supports improved design of 1L-TMD-NA photocatalysts and their implementation in chemical, biological, energy and water systems.
Two-dimensional (2D) transition metal dichalcogenide (TMD) semiconductor crystals offer optoelectronic inducibility with superior electron mobility and gate tunability. This enables enhanced photocatalytic activity of interest for chemical and biological catalysis, energy, sensing, desalination, and nanoelectromechanical systems. Heterostructures of monolayer (1L) TMD crystals decorated with optical nanoantenna (NA) could improve their catalytic efficiency. However, measured catalytic efficiency of such heterostructures relative to undecorated 1L-TMD is rare and characterization of opto-electronic effects at 1L-TMD-NA heterointerfaces is largely empirical. Integration of microscopic and spectroscopic analysis with simulation of 1L-TMD-NA has been limited by computational expense and complexity, particularly for dynamic interactions at nanometer (nm) scales. Compact, multi-scale, integrated analysis of optical, electronic and catalytic effects could identify extraordinary features to guide design and implementation.
Results and Discussion
Electrochemical measurements were carried out in a three-electrode cell with a 3-mm diameter glassy carbon (GC) working electrode, a graphite rod counter electrode, and an Ag/AgCl reference electrode (Pine Research Instrumentation, Durham, NC). The cell was filled with 8 milliliters (mL) of concentrated sulfuric acid, 0.5M H2SO4. The reference electrode was kept in a salt bridge of 3M sodium chloride (NaCl). A microliter of aqueous ethanol ink containing 6 micrograms (µg) liquid phase exfoliated (LPE) tungsten disulfide (WS2) powder (Sigma-Aldrich, St. Louis, MO), 3 µg Ketjen black (EC-600JD, AkzoNobel, Chicago, IL) and 1.5 µg Nafion ionomer (Sigma-Aldrich, St. Louis, MO) was drop cast onto a glassy carbon (GC) electrode. After allowing the ink to set for 10 minutes, the three-electrode system was placed in the cell. The cell and electrodes were contained in an unlit fume hood. Electrodes were connected to a WaveNow potentiostat (Pine Research Instrumentation, Durham, NC). Linear sweep voltammetry (LSV) was performed by measuring current as potential was varied at a rate of 5 mV/s over a range of -0 to -800 mV versus the Ag/AgCl reference electrode, i.e., reversible hydrogen electrode (RHE). Bubbles forming at the electrode tip indicated hydrogen evolution. LSV was performed in triplicate. To provide background measurement, voltammetry was run with the polished working electrode prior to adding catalyst. Cyclic voltammetry (CV) performed for twenty cycles to measure persistence of catalytic activity over time.
Values of peak current density measured by LSV and CV for catalysis by LPE 1LWS2 on a GC electrode ranged from -70 to -100 mA/cm2 at -0.5 eV vs. RHE. Figure 1 shows representative CV and controls. Adjusting the position of the electrode to facilitate bouyant removal of hydrogen bubbles improved the peak current density across this range. The onset potential was approximately 250 mV without buoyant removal of hydrogen and about -300 mV after electrode adjustment. This range of measured peak current densities was greater than values previously reported for comparable measures of hydrogen evolution from WS2. From a similar experimental setup, a current density of -25 mA/cm2 at approximately -0.45V vs SCE  or about -0.21V vs RHE was reported. That system consisted of three-electrodes; 25 µg of WS2 nanoflake catalyst was drop cast onto the GC electrode. Ketjen black was added to increase the conductivity of the solution. Other similar experiments reported current densities of -20 mA/cm2 at -0.5 V vs RHE and -2.2 mA/cm2 at -0.2 V vs RHE, respectively [2,3]. Scarce reports of of WS2 catalysis in GC systems suggests comparison with molybdenum disulphide (MoS2) in which disulfide edge sites similarly catalyze hydrogen evolution reaction (HER). One such study reported a current density of -13 mA/cm2 at -0.2 V vs RHE . Without the edge site enhancement, another study reported results for MoS2 of ca. -5 mA/cm2 at -0.5 V vs RHE .
Figure1. Catalytic activity of WS2 measured by cyclic voltammetry (CV;blue). Inset: CV of bare electrode (black) and Nafion/Ketjen matrix (red).
Use of TMDs as photocatalysts has been constrained by difficulty tuning the intrinsic optoelectronic excitation and damping mechanisms. Optical absorption in monolayer tungsten disulfide (WS2), for example, is limited to >2 eV (<620 nm). This keeps broadband UV-vis absorption below ca. 2%. Moreover, the photoluminscence (PL) response of WS2 is layer-number dependent . Decoration of 1L-TMD by plasmonic metal nanoparticle (NP) has enabled broadband tunability of resultant photon-exciton interactions in catalytic hydrogen generation [6,7]. The enhanced photon-exciton interactions have been attributed to hot electron transfer (HET) from plasmonic NA to TMD substrate. HET, which occurs at interfaces between non-insulating media, e.g., TMD and NA, increases plasmon damping beyond radiative and nonradiative mechanisms . Such damping has been reported to transfer plasmon energy to adjacent 1L semiconducting TMDs [9,10]. Moreover, NP decoration of WS2 was reported to enhance PL 11-fold via exciton coupling with plasmonic electric near fields . However, plasmonic HET had only been evaluated across interfaces of AuNP physically deposited on TMD; and characterization of HET from NA to WS2 has been empirical, lacking a computational framework.
To examine effect of NA on WS2 catalytic hydrogen evolution, Au was reduced onto edge sites of liquid phase exfoliated (LPE) 2H-WS2 from an aqueous Au(III) solution based on a previously reported method [12,13]. Thiols dangling at WS2 nanosheet edges underwent a redox reaction with AuCl3 to yield a covalent Au-S bond as confirmed by transmission electron microscopy (TEM; Figure 2) and X-ray photoelectron spectroscopy (XPS) . In contrast, covalent bond formation was not observed when AuCl3 was reacted with chemical exfoliated 1T-WS2. Oxidation of thiols to disulfides at edges of LPE-2H-WS2 coincident with reduction of Au (III) to Au(0) was anticipated to increase catalytic HER in Au-decorated 2H-WS2 [15,16]. Using LSV, AuNP-decorated 1L2H-WS2 catalyst exhibited a 5-fold enhancement relative to undecorated monolayer and >10-fold increase relative to other reported TMD-based electrodes when exchange current density was normalized by film thickness . For the comparison, equivalent mass of WS2 per unit area electrode was used and mean flake length was maintained at <L> =67 nm. All films exhibited 100 mV/decade slopes, consistent with reported nanolayered TMD HER electrocatalysts.
Figure 2. Transmission electron microscopy (TEM) of 1LTMD flake decorated with nanoantenna(NA).
To quantitate HET from plasmonic NA to catalytic WS2, discrete dipole approximation (DDA) of Maxwell’s equations using DDSCAT v7.3  was conducted based on a previously reported method for 1L graphene  and MoS2 . Computed far field spectra of 1L-WS2-NA heterostructures exhibited localized surface plasmon resonance (LSPR) and WS2 excitonic transitions . Decoration of 1LWS2 by AuNA enhanced broadband extinction, particularly at the 620 nm A exciton red-shifted from the LSPR. Tuning the LSPR to specific excitonic transitions is anticipated to increase enhancement . Edge decoration increased optical extinction efficiency relative to basal plane deposition. An 11±5% quantum efficiency of HET from ca. 20 nm AuNPs reduced on WS2 nanosheets was estimated by comparing electron energy loss spectroscopy (EELS) measured and DDA simulation .
In summary, reduction of plasmonic NA onto 1L-2H-WS2 edges formed covalent Au-sulfur bonds. Such 1L-WS2-NA heterostructures increased catalytic HER relative to 1L-2H-WS2 in physically decorated-NA or undecorated forms and enhanced plasmon damping attributable to direct electron transfer at the catalytic TMD-NA heterointerface. Application of this redox chemistry and integration of microscopic and spectroscopic measures and simulation enable improved photocatalysis using 1L-TMD-NA and implementation in a variety of chemical, biological, energy and water systems.
The authors acknowledge support from the National Science Foundation (NSF) Graduate Research Fellowship (awarded to J.R.D); The University of Arkansas Foundation; Walton Charitable Foundation; and Charles W. Oxford Professorship of Emerging Technologies. The authors thank Prashant Acharya and Lauren Greenlee for guidance and technical assistance.
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