Non-oxide materials have increasingly attracted the interest of researchers, as many advanced applications in optoelectronics, catalysis, energy and other fields require properties that are difficult or impossible to achieve with oxides. While these materials can be air-sensitive, the improvement in performance offsets the difficulties in handling and protecting the compounds. Metal sulfides have received attention because of their catalytic, electronic and optical properties. Many applications depend on the quality of the sulfide materials. Homogeneity, low oxidation state, glassy structure, or high surface area are often desirable. Traditional solid-state methods require high temperatures to overcome diffusion-related kinetic barriers, restricting products to thermodynamically stable phases, and prohibiting control over grain size or surface area. The growing demand for advanced materials has led to the exploration of a variety of alternative low temperature methods. This proposal introduces a novel low temperature route by extending non-hydrolytic sol-gel (NHSG) chemistry to the synthesis of metal sulfides. The specific objectives of the proposed research are: 1) To gain a thorough understanding of synthetic variable space of non-hydrolytic sol-gel processes for selected binary metal sulfide systems, 2) to explore phase space by NHSG chemistry to discover new binary metal sulfides, and 3) to explore NHSG routes for the synthesis of ternary or quarternary sulfides. Students will be trained in novel low temperature routes, synthesis of air-sensitive materials, and a number of solid-state characterization methods applied to air-sensitive materials. Naturally occurring sulfide mineral crystals will be used in outreach projects to expose the public to basic crystallography, as well as to the importance of sulfides. Solar cell materials or geomarkers like troilite will be used as examples to connect with non-scientists. This research is supported by the NSF Solid-State and Materials Chemistry/Division of Materials Research program.
NON-TECHNICAL SUMMARY: Most metals readily form oxides when heated in air. This has made them the most widely explored class of solids, and resulted in many applications. However, some applications require properties that are difficult or impossible to achieve with oxides, which has spurred interest in the synthesis and characterization of non-oxide materials like metal sulfides. Metal sulfides have found use as catalysts in oil cracking, solid lubricants, solar cells, electrodes in battery materials, light emitting diodes, and many other technologically important fields. Some of these applications require materials with very high purity, controlled oxidation state, crystal structure, and morphology. This proposal will explore a novel synthetic method, which will allow the preparation of metal sulfides at low temperatures, and can offer control over oxidation state, crystal structure, surface area, and the resulting properties of the materials. In addition, it is likely that new metal sulfides with unexplored properties will be obtained by this approach. These materials will be characterized in detail, and their properties may lead to novel applications. Graduate and undergraduate students will be trained in synthesis and characterization of solid-state materials, with particular emphasis on handling air-sensitive materials. Women and minority students will be actively recruited. Exchange students will work on the project on six month internships, adding to the diversity of the research group. In addition, outreach programs related to naturally occurring sulfide minerals and technologically important sulfides will be designed to give the general public an appreciation for crystallography and materials chemistry, with particular emphasis on the less well known non-oxide materials. This research is supported by the NSF Solid-State and Materials Chemistry/Division of Materials Research program.
Advanced materials have significantly contributed to many amenities available to modern society. Examples include applications like photovoltaics, lithium ion batteries, ever faster computers or catalytic converters in cars. However, Earth’s growing population, increased industrialization, and shrinking natural resources pose significant global challenges. To maintain or enhance our current standard of living in industrialized countries, and to improve the standard of living worldwide, better performing materials will be necessary for many applications. This is likely to include optimization of already known materials, different "forms" of known materials (e.g., nanotechnology, thin film applications) and completely new materials. Metal sulfides have found use as catalysts in oil cracking, solid lubricants, solar cells, electrodes in battery materials, light emitting diodes, and many other technologically important fields. Some of these applications require materials with very high purity, controlled oxidation state, crystal structure, and morphology. In addition, devices that contain multiple materials pose limitations with respect to temperature and processing conditions based on the most sensitive component present. This proposal explored a novel synthetic method that can produce metal sulfides with controlled crystal structure and morphology at low temperatures. As the properties of materials depend strongly on structure, which is evident in the metal sulfide systems we investigate, we also developed a teaching unit on "Minerals and Crystals" targeted at middle and high school students. Intellectual Merit: The research conducted through this grant has resulted in 3 published peer reviewed scientific papers, with 2 additional manuscripts in the final stages before submission, and 1 additional manuscript in preparation. During the duration of this grant, we developed an understanding of the synthetic variables for new solution-based low temperature approaches to metal sulfides. Thorough explorations of several systems containing a single type of metal have been completed, and resulted in excellent control over crystal structure and particle morphology. In the copper sulfide system, we managed to access more different phases (five) than any other reports of a single synthetic method. This included a-chalcocite, which is the copper sulfide phase most active in solar energy harvesting. However, especially at the nanoscale, this phase is generally not very stable, and usually rapidly converts to a different structure within days to a few weeks. While our a-chalcocite also showed a tendency towards this transformation, our most stable samples still contained 35% a-chalcocite after 2 years. In the tantalum sulfide system, we found that the metal halide starting material significantly influenced the tendency of the recovered samples to oxidize during heating. Under specific conditions, we obtained nanocrystalline TaS2 directly from a reaction at 150 °C. This is much lower than any previously reported synthesis temperatures, and could make this material compatible with devices that contain temperature-sensitive components. In the tin sulfide system, nanostructured materials were recovered. This included interesting 3D hierarchitectures, which show promising electrochemical performance. In addition, very thin 2D platelets were obtained not only in the tin sulfide system, but also for the tantalum and molybdenum sulfide systems. This suggests that our synthetic approach may be applicable to the preparation of high quality 2D layered transition metal sulfides. Broader Impacts: Two graduate students, six German exchange students (enrolled at the Master’s level) and one undergraduate researcher contributed to this project and were trained in synthesis and characterization of air-sensitive compounds. This included a large number of women (5) and several minorities (2 Hispanic). The inclusion of exchange students broadened the perspectives of not only the exchange students, but all students in our research group. Most of the students also participated in outreach to middle and high school students. An outreach unit on "Minerals and crystals" was specifically developed under this grant, as the properties that make metal sulfides useful are intimately related to their structures. This popular unit has been requested for a variety of outreach programs every summer since we developed it, and has also been transferred to Lawrence Tech University in Michigan by a summer student in our group.
