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MoS Nanoflowers as a Gateway for Solar-Driven CO Photoreduction Anne Meier, Anita Garg, Brad Sutter, John N. Kuhn, and Venkat R. Bhethanabotla ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03168 • Publication Date (Web): 03 Dec 2018 Downloaded from http://pubs.acs.org on December 5, 2018
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ACS Sustainable Chemistry & Engineering
MoS2 Nanoflowers as a Gateway for Solar-Driven CO2 Photoreduction Anne J. Meier*,†, ∥, Anita Garg‡, and Brad Sutter§, John N. Kuhn∥, Venkat R. Bhethanabotla∥ * Correspondence
should be addressed to:
[email protected], +1 321-861-9315. Laboratories Branch, NASA Kennedy Space Center, FL 32899, USA. Mail Stop NE-L3. ‡ Advanced Metallics Branch, NASA Glenn Research Center, Cleveland, OH 44135, USA. Mail Stop GRC-LM-A. § Jacobs Engineering Science, NASA Johnson Space Center, Houston, TX 77058, USA. Mail Stop JSC-XI3. ∥ Department of Chemical & Biomolecular Engineering, University of South Florida, Tampa, FL 33620, USA. † Analytical
Abstract The layering of transition metal dichalcogenides (TMD) has revealed unprecedented engineering opportunities for optoelectronics, field emitter and photocatalysis applications. Precise and controlled intrinsic material property combinations is the crucial demand needed for visible light photocatalysis optimization, which we demonstrate in this work with MoS2 nanoflowers containing abundant edge plane flakes for CO2 photoreduction optimization. This is the first time controlled imperfections and flake thickness through facile CVD synthesis was demonstrated on the nanoflowers, revealing the tuning ability of flake edge morphology, nanoflower size, stacked-sheet thickness, optical band gap energy (Eg) and catalytic function. These influences facilitated Eg tuning from 1.38 to 1.83 eV and the manifestation of the 3R phase prompting improvement to the catalytic behavior. The ‘sweet spot’ of higher catalytic activity during photoreduction experiments was found in those with plentiful nanoflower density and thick edge-site abundance. Ample edge-sites with dangling bonds, and crystal impurities assisted in lowering the Eg to achieve reduced recombination for improved photocatalytic reactions, including those found on what would have been a typical chemically inert basal plane. The production rates of CO improved two-fold after a calculated post-treatment reduction step. This reliable CVD technique for nanoflower synthesis paves the way for enhanced understating of synthetic parameters for defect-laden 2D TMD nanoflower structures. We also note that photocatalysis should consider Mars applications, as deep space humans exploration will be require harvesting of the CO2 rich atmosphere to generate fuel from sustainable resources, such as the sun. Keywords: visible light catalyst, transition metal dichalcogenide, band gap tuning, molybdenum disulfide
Introduction Environmental CO2 reduction using sustainable technologies has been infiltrated with exhaustive research on carbon capture and sequestration, and nevertheless requires much attention as we still remain in our global climate change conundrum. Despite efforts over the last several decades to apply a sustainable photocatalysis solution for H2O splitting and CO2 reduction to synthetic fuels, it has not reached practical applicability in industry Environmental CO2 reduction using sustainable technologies has been infiltrated with exhaustive research on carbon capture and sequestration, and nevertheless requires much attention as we still remain in our global climate change conundrum. Despite efforts over the last several decades to apply a sustainable photocatalysis solution for H2O splitting and CO2 reduction to synthetic fuels, it has not reached practical applicability in industry1,2. Large scale implementation of photocatalysis has been afflicted by poor conversion rates, selectivity challenges, and low quantum efficiencies with expensive materials such platinum. This demands sincere efforts towards fundamental understanding of the photocatalyst material design, including optical and elemental properties, charge transport, stability, catalytic function and scalability. Here we explore not only the morphology effects, but also band gap tuning and edge plane effects on catalytic function. The organic approach does not involve doping or initiating a co-catalyst, so as to further explore and understand the subtle changes and the affects they insue on photoreaction experiments. Electrochemical photolysis was first reported over 43 years ago by Honda and Fujishima using TiO2, and is now one of the most studied UV-light activated photocatalysts3. Photocatalysis involves light irradiation onto a material, followed by photogenerated charge carrier formation of holes (h+) at the valence band (vb), and electrons (e-) in the conduction band (cb) which then become active sites for chemical reactions. Photocatalyst studies have explored preparation techniques, reactants, light sources and desired product formation for H2O splitting, CO2
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conversion, and pollution reduction4,5. Heterostructures and stacked transition metal dichalcogenides (TMDs) have opened a new door of exploration, leading to exciting properties with applications linked to visible light activated materials. TMD materials with stacked sheets, and uniform layers with thin open edges, such as nanoflowers, are ideal for enhanced photoreaction experiments. These heterostructures have improved light absorption and charge separation properties, aiding in the e-/h+ transport process6,7. MoS2 is low cost and has outstanding optical, catalytic, and interlayer properties. Moieties containing unsaturated and dangling bonds exist at the stacked sheet edges, as opposed to the basal planes that exists on the sheet surfaces and internal bulk locations, making the nanoflowers ideal for catalytic reactions.8–11 Chalcogenides are also inexpensive photosensitizers with ideal band positions and facile nanoparticle synthesis. The overwhelming potential and applications for MoS2 display their flexibility in future areas of research growth and application. Recently, MoS2 composites12,13, films/layers14,15, and quantum dot16 coated photocatalysts have received extensive attention. MoS2 improves charge transfer and promotes separation of e-/h+ pairs and suppresses recombination for H2 evolution12. Chai etc. al. utilized visible light activation with a MoS2/ZnIn2S4 composites formed to assist as charge carrier through microspheres. Layering combinations with MoS2 and TiO2 demonstrated increased surface area and reaction sites to improve electron flow from TiO2 to graphene, to MoS2. This particular work yielded 14.5 times more CO production compared with pure TiO215. A similar layering combination demonstrated H2 production from ethanol using TiO2 nanoparticles on layers of stacked MoS2 and graphene17. The MoS2 and graphene layer suppressed charge recombination and provided more adsorption sites for photocatalytic reactions. MoS2, with its ease of synthesis, has also demonstrated further morphology and shape applicability through double hollow spherical structures with nanoparticle18 and nanorod19 applications. The chemical vapor deposition (CVD) synthesis parameters for two-dimensional (2D) MoS2 TMDs was explored in this work to demonstrate controllable edge-rich nanoflower morphology, optical response of the band gap energy (Eg) and crystal phase stability. The materials show promise as an effective substitute for noble metals to support photocatalytic activity through p-n junction repair and enhancement of the photo-generated e-/h+ alignment to accelerate the charge carriers7,9,10. The reliable CVD synthesis was encouraged from previous efforts that aimed to control TMD monolayer thicknesses7,8,20–23, vertical layers24,25, flowers26 and nanosheets27–29. Intrinsic structural defects such as atomic point vacancies and crystallinity distortions that form during 2D growth were also exploited here to control sheet thickness, crystal phase stability, and Eg tuning properties. MoS2 nanoflowers with multiple edge sites can increase conductive sites while still remaining nanosized30. Introducing lattice defects were also explored with synthesis effects, as they may have an important role of encouraging light adsorption behaviors and delaying recombination31,32. Materials activated by visible light can support the hydrogen evolution reaction (HER) (Eg of 1.2 eV to 2.4 eV)33–35 to enable a possible proton source during solar-driven CO2 reduction experiments. Single-layer MoS236 (1.0 to 1.96 eV7,11) and bilayer/bulk values (1.3 to 1.6 eV37,38) were ideal for these expectations and further investigated. This is the first time an in depth analysis of the controlled introduction of imperfections or crystal defects through synthesis variations was demonstrated on the nanoflowers, revealing the regulated growth on flake edge morphology, nanoflower diameter, stacked-sheet thickness, optical band gap energy (Eg) and catalytic activity. These exciting influences facilitated the Eg range from 1.38 to 1.83 eV, and provide a strategy for solar-driven photoreduction of CO2 for CO production, including two-fold production after post treatment in a reduction step. The facile CVD synthesis for the controlled nanoflower formations pave the wave for repeatability and tunability, as well as aiding in enhanced understating of synthetic parameters for defect-laden 2D TMD nanoflower structures. This work contributes to thoughtful material design for future catalytic applications in the field of photocatalysis and field emitter applications39–42. Longer-term visionary goals include developing photocatalyst technologies that can be used in space applications. Technologies such as photocatalysis can be developed to produce fuels (i.e. CO, H2, CH4) in-situ on a Mars surface with available resources such as the Sun (specifically visible light)43, atmospheric CO244 (95% abundant) and subsurface ice.
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ACS Sustainable Chemistry & Engineering
Experimental Section Materials and Methods The CVD process in this work utilized a single zone tube furnace (SZF) and a three zone tube furnace (TZF) to produce the bulk powder MoS2 nanoflowers and stacked sheets of MoS2 flakes grown on downstream Si substrates. Commercially purchased MoS2 powder (Sigma-Aldrich 99% pure molybdenum (IV) sulfide, 5 °C/min) yielded some undeveloped flake formations in the bulk and amorphous particles on the nanoflower surface that gave the impression of surface contamination. Ultra-sonication for 10 min in deionized H2O, followed by drying, did not eliminate contamination off of the nanoflower surface, and the nanoflower remained intact. This surface material of undeveloped 2D flake formation was more prevalent in the TZF than the SZF powders. The SZF-20 powders produced the smallest yield of overall nanoflower formations, accompanied by amorphous bulk solids. Sigma-Aldrich powder contained unorganized amorphous solids and no organized stacked sheets or nanoflower products.
Fig. 3. Top: MoS2 nanoflower illustration depicting the flake edges that are comprised of stacked sheets that contain rich edge sites on both the flake edges and flake surfaces. Middle: SEM micrographs showing the MoS2 particle
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morphology at 2,500X and 10,000X for SZF and TZF nanoflowers. Also displayed is the SZF substrate growth and the TZF average cell volume and lattice parameter calculations. The EDS mapping confirmed an even distribution of S and Mo in the bulk powder (Fig. 4 A-C), while elemental analysis confirmed the average Mo:S atomic ratio reduction as ramp rate increased (Fig. S5). Trace Cl was identified with EDS at inconsistent locations throughout the bulk powders of the TZF and high ramp rates of SZF synthesized powders, but was not a significant presence classified in the underdeveloped nanosheets. Higher ramp rates enabled rapid layer formation and precursor material (MoCl5) to linger, reducing the overall MoS2 nanosheet density, and had an increased presence of underdeveloped amorphous MoS2 clusters that developed on and around the nanosheets. Thicker flakes at high ramp rates were in the diffusion controlled region, which facilitated homogeneous nucleation, causing crystal structure deformations. Slower ramp rates (
