Solar Energy and Electrocatalysis
Efficient Solar Energy Conversion Using Earth-Abundant (Nano)materials and Heterostructures
The success of solar photovoltaic (PV) or photoelectrochemical (PEC) technologies depends not only on achieving highly efficient devices, but also on dramatically reducing their cost. The scale and significance of the energy challenges facing us today demand earth-abundant and inexpensive materials prepared by less energy-intensive and costly processes. For efficient and practical PEC solar energy conversion, such as solar water splitting, we need earth-abundant and inexpensive electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) with catalytic performance rivaling that of the noble metal catalysts. We investigate various alternative semiconductor and catalyst materials, such as iron pyrite (cubic FeS2, “fool’s gold”),[9, 15, 25, 26] hematite (a-Fe2O3),[4, 7, 12] and cuprous oxide (Cu2O), as well as MoS2, WS2, WSe2 and other layered chalcogenides,[17, 20, 21, 27, 28] seeking to improve their properties toward potential high performance solar energy conversion and catalytic applications. For these alternative materials, significant challenges in materials synthesis, defect control, and doping control — all of which directly impact materials properties — must be overcome before their potential as solar materials can be realized. We have used a variety of electrochemical and surface science techniques and nanodevice studies to fundamentally understand the limitations of these earth-abundant materials in order to enable their applications in solar energy conversion and catalysis.
Metal-Pyrite Materials for Photovoltaics and Hydrogen Evolution
One example of an earth-abundant solar material we study is iron pyrite (cubic FeS2). [9, 15, 25, 26, 29] As the most abundant sulfide mineral in the earth’s crust, pyrite practically has unlimited material supply to satisfy our energy demand. It is a very interesting semiconductor for solar applications due to its band gap (0.95 eV), high absorption coefficient (6 × 105 cm-1) and other superior semiconducting properties (such as high carrier mobilities and lifetime) in single-crystalline pyrite. Despite its attractive properties, high solar energy conversion efficiency has never been achieved with pyrite. In order to enable this earth-abundant semiconductor for cost-effective solar energy conversion, we synthesize high-quality pyrite nanomaterials[9,15] and single crystals as ideal platforms to understand the defects in pyrite. Through a holistic evaluation of the surface and bulk properties of n-type pyrite single crystals, we revealed that despite a high density of surface states, the ionization of intrinsic bulk defect states creates a degenerate surface space charge region that reduces the usable barrier height and satisfactorily explains the limited photovoltage and poor photoconversion efficiency of pyrite single crystals. Currently we are developing strategies to passivate the intrinsic bulk defect states to enable pyrite as a high performance solar material.
Figure 1. Different morphologies of FeS2 crystals and nanostructures. Electrochemical and transport measurements of the FeS2 nanoplates and single crystals reveal the nature of the surface and bulk defect states, which help to explain the origin of the low photovoltage in iron pyrite.
In contrast to the significant challenges of utilizing iron pyrite as a solar absorber, we found that pyrite-phase transition metal disulfides (MS2, M= Fe, Co, Ni and their alloys), especially CoS2, are highly efficient electrocatalysts for hydrogen evolution reaction (HER).[22, 23] Their electrocatalytic activity toward the HER is correlated to their composition and morphology. Particularly, microwires or nanowires of CoS2 can be controllably synthesized with high surface areas to substantially boost their catalytic performance. The benefits of catalyst micro- and nanostructuring are further demonstrated by the increased electrocatalytic activity of CoS2 nanowire electrodes over planar film electrodes toward polysulfide and triiodide reduction, which suggests a straightforward way to improve the performance of quantum dot- and dye-sensitized solar cells, respectively.
Figure 2. (a) High performance HER catalysis by various CoS2 nano- and micro-structures. (b) SEM image of CoS2 NWs on graphite.
Layered Metal Dichalcogenides for Hydrogen Evolution Reaction Catalysis
Layered metal dichalcogenides, MX2, where M is a metal (Mo, W, Sn, Zr, Hf, Ta, Nb, etc.) and X is a chalcogen (S, Se, and Te), are a broad family of compounds that have layered two-dimensional (2D) crystal structures. Due to the high carrier mobility and fascinating optical and spin properties, MX2 are promising building blocks for electronic, photonic, and spintronic applications, and Group 6 MX2 (M = Mo, W) are also promising catalysts for electrocatalytic and photoelectrochemical hydrogen generation. We have discovered that chemical exfoliation of layered MX2 nanostructures and the simultaneous conversion of the as-synthesized MX2 material from its semiconducting 2H polymorph to the metallic 1T polymorph, grants greatly enhanced electrocatalytic activity for HER.[17, 20] The resulting 1T-MoS2 and 1T-WS2 nanosheets exhibit dramatically enhanced HER electrocatalytic performance as compared to their corresponding 2H polymorphs. Structural characterization and electrochemical studies confirm that the nanosheets of the metallic MoX2 polymorph exhibit facile electrode kinetics, low-loss electrical transport, and possess a proliferated density of catalytically active sites. These distinct and previously unexploited features of 1T-MX2 make these metallic nanosheets highly competitive earth-abundant HER catalysts.
Figure 3. (a) Intercalation of lithium into semiconducting 2H-MX2 causes a phase transition to the metallic 1T-MX2 polymorph and the subsequent exfoliation leads to nanosheets. (b) Polarization curves show a dramatic enhancement in the HER electrocatalytic activity after polymorph transition. (c) Direct growth of 1T-MoS2 nanosheets on a p-type Si photocathode enables (d) efficient hydrogen evolution driven by solar energy.
Furthermore, we have coupled 1T-MoS2 electrocatalyst to a p-type silicon photocathode to enable hydrogen evolution driven by solar light. Due to the excellent HER activity of 1T-MoS2 and the high-quality catalyst–semiconductor interface enabled by the direct CVD growth of MoS2 on silicon, a much better PEC performance was achieved as compared to 2H-MoS2/Si. Electrochemical impedance spectroscopy (EIS) and surface photoresponse (SPR) measurements further explain the superior performance of these 1T-MoS2/Si photocathodes, making them promising earth-abundant alternatives to noble metal-based systems for solar-driven HER.
Other than utilizing chemical exfoliation and polymorph manipulation to achieve good catalytic activity, we have also found that amorphous MoSxCly have HER catalytic activity similar to or even better than that of 1T-MoS2 with easier synthesis procedure and lower cost. This novel ternary electrocatalyst is synthesized via chemical vapor deposition at temperatures lower than those typically used to grow crystalline MoS2 nanostructures. It exhibits stable and high catalytic activity toward HER, as evidenced by large cathodic current densities at low overpotentials and low Tafel slopes (ca. 50 mV decade−1). Furthermore, MoSxCly can be directly deposited on p-type silicon photocathodes to enable efficient photoelectrochemical hydrogen evolution. 
Figure 4. Electron microscopy characterization of an amorphous MoSxCly grown on vertical graphene. (a) STEM-EDS mapping of a piece of graphene sheet partially covered by MoSxCly. The orange box indicates the region where EDS elemental mapping (b) was performed for C, Cl, S, and Mo. (c) Electrochemical characterization of MoSxCly for hydrogen evolution reaction.
Nanoscale Heterostructures for Solar Energy Conversion
All solar energy conversion technologies depend on the fundamental processes of light harvesting, charge carrier separation, charge carrier collection, and, in the case of PEC solar energy conversion, chemical transformations. Nanoscale heterostructures, such as quantum dot (QD) nanoscale heterostructures are potentially advantageous for efficient charge separation. We have developed methods to grow heteroepitaxial QDs directly on wide band gap metal oxide semiconductor nanowires. We are also investigating various methods of incorporating such heterostructures into solar cell devices, such as QD sensitized solar cells (Figure 5b and 5c). Using coherent multidimensional spectroscopy and scanning probe microscopy in collaboration with Profs. John Wright and Robert Hamers at UW–Madison,[5, 8, 11, 14] we are investigating coherent charge transport across quantum-confined nanoscale heterostructures, which mimic the coherent charge-transport processes in photosynthesis and dye-sensitized solar cells. We hope to develop a fundamental understanding of this process, which is necessary to make coherent charge transfer a ubiquitous feature of solar energy conversion, particularly in nanostructures of earth-abundant materials with complex shapes, compositions, and dimensionalities.
Figure 5. (a) Lattice-resolved HRTEM of lead selenide (PbSe) quantum dots (QD) epitaxially grown on a hematite (α-Fe2O3) nanowire, forming QD nanoscale heterostructures; (b) Schematic depiction of various donor-acceptor QD nanoscale heterostructures incorporated into a regenerative photovoltaic solar cell device; (c) Typical current photocurrent density-voltage characterization curve of a QD-sensitized solar cell (photograph shown in the inset).
Heterostructures of Layered Metal Dichalcogenides
Vertical heterostructures of MX2 materials are ideal platforms for electronic structure engineering of atomically thin 2D semiconducting heterostructures and could provide new opportunities for electronic, optoelectronic, and solar energy conversion devices. The realization of many of these envisioned applications critically depends on the controlled growth of 2D heterostructures and understanding of their structures and properties. We have developed the epitaxial growth of few-layer to monolayer MoS2, WS2, and WSe2 on SnS2 microplates using CVD under relatively mild temperature enabled by van der Waals epitaxy. The approach and examples of heterostructures reported herein open up the exploration of vertical heterostructures of diverse MX2 materials for physical property and spectroscopic investigation and device applications.
Figure 6. Confirmation and schematic illustration of the van der Waals epitaxy between MoS2 and SnS2 in the vertical 2D heterostructure.
Layered Double Hydroxides for Oxygen Evolution
Layered double hydroxides (LDHs) are another family of synthetic two-dimensional (2D) layered materials that consist of stacked brucite-like MII(OH)2 layers, where some of the MII ions are isomorphously substituted by MIII ions, giving positively charged host layers with charge-balancing anions between them. Several classes of LDHs have been shown to act as efficient catalysts for oxygen evolution reaction (OER), and are inexpensive alternatives to the conventional precious metal-containing OER catalysts such as RuO2 and IrO2. We have developed a controlled synthesis of NiCo layered double hydroxide (LDH) nanoplates using a newly developed high temperature high pressure hydrothermal continuous flow reactor (HCFR), which enables direct growth onto conductive substrates in high yield and, most importantly, better control of the precursor supersaturation and, thus, nanostructure morphology and size. The as-grown NiCo LDH nanoplates exhibit a high catalytic activity toward the oxygen evolution reaction (OER). By chemically exfoliating LDH nanoplates to thinner nanosheets, the catalytic activity can be further enhanced due to the increased surface area and more exposed active sites. This work presents general strategies to controllably grow nanostructures of LDH and ternary oxide/hydroxides in general and to enhance the electrocatalytic performance of layered nanostructures by exfoliation.
Figure 7. Schematic of a hydrothermal continuous flow reactor and the resulting NiCo LDH plates synthesized in the reactor. Exfoliation of the plates results in enhanced electrocatalytic activity toward the oxygen evolution reaction.
28) Hanfeg Liang, Fei Meng, Miguel Caban-Acevedo, Linsen Li, audrey Forticaux, Lichen Xiu, Zhoucheng Wang, and Song Jin; Hydrothermal Continuous flow synthesis and Exfoliation of NiCo Layered Double Hydroxide nanosheets for Enhanced Oxygen Evolution Catalysis, Nano Letters, 2015, pp. 1421-1427, DOI: 10.1021/nl504872s
27) Leith Samad, Miguel Caban-Acevedo, Melinda J. Shearer, Kwangsuk Park, Robert J. Hamers, and Song Jin, Direct Chemical Vapor Deposition Synthesis of Phase-Pure Iron Pyrite (FeS2) Thin Films, Chem. mater., 2015, 27(8), pp3108-3114
26) Miguel Caban-Acevedo, Nicholas S. Kaiser, Caroline R. English, Dong Liang, Blaise J. Thompson, Hong-En Chen, Kyle J. Czech, John C. Wright, Robert J. Hamers, and Song Jin; Ionization of High-Density Deep Donor Defect States Explains the Low Photovoltages of Iron Pyrite Single Crystals, J. Am. chem. Soc., 2014, 136 (49), pp 17163-17179, DOI: 10.1021/ja509142w
25) Dong Liang, Miguel, Caban-Acevedo, Nicholas S. Kaiser, and Song Jin; Gated Hall Effect of Nanoplate Devices reveals Surface-State-Induced Surface Inversion in Iron Pyrite Semiconductor, Nano Letters, 2014, 14(12), pp 6754-676, DOI: 10.1021/nl501942w
24) Matthew S. Faber and Song Jin; Earth-abundant inorganic electrocatalysts and their nanostructures for energy conversion applications, Energy. Environ. Sci., 2014, Accepted Manuscript , DOI: 10.1039/C4EE01760A
23) Matthew S. Faber, Mark A. Lukowski, Qi Ding, Nicolas S. Kaiser, and Song Jin; Earth-Abundant Metal Pyrites (FeS2, CoS2, NiS2, and Their Alloys) for Highly Efficient Hydrogen Evolution and Polysulfide Reduction, J. Phys. Chem. C, 2014, Just Accepted Manuscript, DOI:10.1021/jp506288w
22) Matthew S. Faber, Rafal Dziedzic, Mark A. Lukowski, Nicholas S. Kaiser, Qi Ding, and Song Jin; High-Performance Electrocatalysis Using Metallic Cobalt Pyrite (CoS2) Micro- and Nanostructures, J. Am. Chem. Soc., 2014, Article ASAP, DOI: 10.1021/ia504099w
21) Qi Ding, Fei Meng, Caroline R. English, Miguel Caban-Acevedo, Melinda J. Shearer, Dong Liang, Andrew S. Daniel, Robert J Hamers, and Song Jin; Effiecient Photoelectrochemical Hydrogen Generation Using Heterostructures of Si and Chemically Exfoliated Metallic MoS2, J. Am. Chem. Soc., 2014, 136(24) pp 8504-8507 DOI: 10.1021/ja5025673
20) Mark A. Lukowski, Andrew S. Daniel, Caroline R. English, Fei Meng, Audrey Forticaux, Robert Hamers, and Song Jin; Highly Active Hydrogen Evolution Catalysis from Metallic WS2 Nanosheets, Energy Environ. Sci., 2014, Accepted Manuscript DOI: 10.1039/C4EE01329H
19) Xingwang Zhang, Fei Meng, Jeffrey R. Christianson, Christian Arroyo-Torres, Mark A. Lukowski, Dong Liang, J. R. Schmidt, and Song Jin; Vertical Heterostructures of Layered Metal Chalcogenides by van der Waals Epitaxy, Nano Lett., 2014, 14(6), pp 3047-3054 DOI: 10.1021/nl501000k
18) Yizheng Tan, Song Jin, and Robert J. Hamers; Photostability of CdSe Quantum Dots Functionalized with Aromatic Dithiocarbamate Ligands, ACS Appl. Mater. Interfaces, 2013, 5 (24), pp 12975-12983
17) Mark A. Lukowski, Andrew S. Daniel, Fei Meng, Audrey Forticaux, Linsen Li, and Song Jin; Enhanced Hydrogen Evolution Catalysis from Chemically Exfoliated Metallic MoS2Nanosheets J. Am. Chem. Soc. 2013, Just Accepted Manuscript, DOI: 10.1021/ja404523s.
16) Faber, M. S.; Park, K.; Cabán-Acevedo, M.; Santra, P. K.; Jin, S.; Earth-Abundant Cobalt Pyrite (CoS2) Thin Film on Glass as a Robust, High-Performance Counter Electrode for Quantum Dot-Sensitized Solar Cells J. Phys. Chem. Lett. 2013, 4 (11), pp 1843–1849.
15) Cabán-Acevedo, M.; Liang, D.; Chew, K. S.; DeGrave, J. P.; Kaiser, N. S.; Jin, S.; Synthesis, Characterization, and Variable Range Hopping Transport of Pyrite (FeS2) Nanorods, Nanobelts, and Nanoplates. ACS Nano 2013, 7 (2), pp 1731–1739.
14) Tan, Y.; Jin, S.; Hamers, R. J.; Influence of Hole-Sequestring Ligands on Photostability of CdSe Quantum Dots. J. Phys. Chem. C, 2013, 117, 313–320.
13) Selinsky, R. S., Ding Q., Faber M., John C. Wright, and Song Jin; Quantum Dot Nanoscale Heterostructures for Solar Energy Conversion. Chem. Soc. Rev. 2013, 42, 2963-2985.
12) Franking, R.; Li, L.; Lukowski, M. A.; Meng, F.; Hamers, R. J.; Jin, S.; Facile Post-Growth Doping of Nanostructured Hematite Photoanodes for Enhanced Photoelectrochemical Water Oxidation. Energy Environ. Sci. 2013, 6, 500-512.
11) Block, S. B.; Yurs, L. A.; Pakoulev, A. V.; Selinsky, R. S.; Jin, S.; Wright, J. C.; Multiresonant Multidimensional Spectroscopy of Surface-Trapped Excitons in PbSe Quantum Dots. J. Phys. Chem. Lett. 2012, 3, 2707–2712.
10) Selinsky, R. S. ; Shin, S.; Lukowski, M. A.; Jin, S.; Epitaxial Heterostructures of Lead Selenide Quantum Dots on Hematite Nanowires. J. Phys. Chem. Lett. 2012, 3, 1649–1656.
9) Cabán-Acevedo, M.; Faber, M. S; Tan, Y.; Hamers, R. J.; Jin, S.; Synthesis and Properties of Semiconducting Iron Pyrite (FeS2) Nanowires. Nano Lett. 2012, 12, 1977-1982.
8) Yurs, L. A.; Block, S. B.; Pakoulev, A. V.; Selinsky, R. S.; Jin, S.; Wright, J. C.; Spectral Isolation and Measurement of Surface-Trapped State Multidimensional Nonlinear Susceptibility in Colloidal Quantum Dots.J. Phys. Chem. C. 2012,116, 5546-5553.
7) Li, L.; Yu, Y.; Meng, F.; Tan, Y.; Hamers, R. J.; Jin, S.; Facile Solution Synthesis of a-FeF3·3H2O Nanowires and Their Conversion to α-Fe2O3 Nanowires for Photoelectrochemical Application. Nano Lett., 2012, 12, 724-731.
6) Hacialioglu, S.; Meng, F.; Jin, S.; Facile Large-Scale Solution Synthesis of Cu2O Nanowires and Nanotubes Driven by Screw Dislocations. Chem. Commun., 2012, 48, 1174-1176.
5) Yurs, L. A.; Block, S. B.; Pakoulev, A. V.; Selinsky, R. S.; Jin, S.; Wright, J. C.; Multiresonant Coherent Multidimensional Electronic Spectroscopy of Colloidal PbSe Quantum Dots. J. Phys. Chem. C. 2011, 146, 22833–22844.
4) Lukowski, M.; Jin, S.; Improved Synthesis and Electrical Properties of Si-Doped a-Fe2O3Nanowires. J. Phys. Chem. C., 2011, 115, 12388-12395.
3) Morin, S. A.; Bierman, M. J.; Tong, J.; Jin, S., Mechanism and Kinetics of Spontaneous Nanotube Growth Driven by Screw Dislocations. Science 2010, 328, 476-480.
2) Bierman, M. J.; Jin, S.; Potential Applications of Hierarchical Branching Nanowires in Solar Energy Conversion. Energy Environ. Sci. 2009, 2, 1050-1059. (Invited Perspective, featured on inside front cover).
1) Albert Lau, Y. K.; Chernak, D. J.; Bierman, M. J.; Jin, S.; Epitaxial growth of hierarchical PbS nanowires. J. Mater. Chem. 2009, 19, 934-940.