Solution Synthesis and Reactivity of Colloidal Fe₂GeS₄: A Potential Candidate for Earth Abundant, Nanostructured Photovoltaics

Iron chalcogenides, in particular iron pyrite, have great potential to be useful materials for cost-effective thin film photovoltaics. However, the performance of pyrite as an absorber material in photovoltaic devices has fallen far short of the theoretical efficiency. A potential cause of this may be the instability of the pyrite phase. An alternate class of iron chalcogenides, Fe₂MS₄ (M = Ge, Si) has been proposed as a possible alternative to pyrite, yet has only been studied for interesting magnetic properties. Herein, we report the first solution synthesis of colloidal Fe₂GeS₄ and report the optical properties, reactivity, and potential for use as a photovoltaic material

Enhanced Conductivity in CZTS/Cu_2–<i>x</i>Se Nanocrystal Thin Films: Growth of a Conductive Shell

Poor charge transport in Cu₂ZnSnS₄ (CZTS) nanocrystal (NC) thin films presents a great challenge in the fabrication of solar cells without postannealing treatments. We introduce a novel approach to facilitate the charge carrier hopping between CZTS NCs by growing a stoichiometric Cu₂Se shell that can be oxidized to form a conductive Cu_2–<i>x</i>Se phase when exposed to air. The CZTS/Cu₂Se core/shell NCs with varying numbers of shell monolayers were synthesized by the successive ionic layer adsorption and reaction (SILAR) method, and the variation in structural and optical properties of the CZTS NCs with varying shell thicknesses was investigated. Solid-phase sulfide ligand exchange was employed to fabricate NC thin films by layer-by-layer dip coating and a 2 orders of magnitude rise in dark conductivity (∼10^–3 S cm^–1 at 0 monolayer and ∼10^–1 S cm^–1 at 1.5 monolayers) was observed with an increase in the number of shell monolayers. The approach described herein is the first key step in achieving a significant increase in the photoconductivity of as-deposited CZTS NC thin films

Synthetic Control of Quinary Nanocrystals of a Photovoltaic Material: The Clear Role of Chalcogen Ratio on Light Absorption and Charge Transport for Cu_2–<i>x</i>Zn_1+<i>x</i>Sn(S_1–<i>y</i>Se_<i>y</i>)₄

Photovoltaic (PV) devices based on bulk polycrystalline Cu₂ZnSn­(S_1–<i>x</i>Se_<i>x</i>)₄ (CZTSSe) as the absorber material have historically shown the best efficiency with high Se compositions. The selenization process, which is employed in the formation of absorber layer, has been shown to result in maximum device efficiency at a lower than predicted optimal band gap (<i>E</i>_g= ∼1.1 eV as compared to the 1.34 eV predicted by the Shockley–Queisser detailed balance model). It is still not clear if this deviation is due to changes in the chalcogen composition, grain growth in the film, or increased order in the lattice. In contrast, CZTSSe nanocrystals (NCs) offer a unique opportunity to evaluate the effect of chalcogen ratio on light absorption, charge transport, and photovoltaic performance excluding the impact of the uncertain effects of the conventional selenization step and, importantly, offer a potential path to a dramatic reduction in PV manufacturing cost. Despite an abundance of literature reports on this compound, there is to date <i>no systematic study of the effects of controlled composition of the chalcogen on photocarrier generation and extraction at an optimal and constant cation ratio in a single system.</i> This is required to determine the interplay between light absorbance and transport without compositional convolution and, in turn, to identify the best chalcogen ratio for the unannealed NC PV devices. Here we show that the entire family of Cu_2–<i>z</i>Zn_1+<i>z</i>Sn­(S_1–<i>y</i>Se_<i>y</i>)₄ NCs can be made by a simple one-pot synthetic method with exquisite control over cation content and particle size across the entire range of chalcogen compositions. These NCs are then used to make solution-processed and electrically conductive CZTSSe NC films in the full range of S/(S + Se) ratios via ligand exchange without postdeposition annealing. The transport properties assessed by Hall-effect measurements revealed an intrinsic increase in film conductivity with selenium incorporation. These measurements are then correlated with the PV performance at the full range of band gaps (<i>E</i>_g = 1.0–1.5 eV), leading to an observed maximum in power conversion efficiency centered around <i>E</i>_g = 1.30 eV, which is much closer to the predicted Shockley–Queisser optimal band gap, an outcome predominantly dictated by the compromise between electrical conductivity and band gap

Copper Selenophosphate, Cu₃PSe₄, Nanoparticle Synthesis: Octadecane Is the Key to a Simplified, Atom-Economical Reaction

Nanoparticle syntheses are designed to produce the desired product in high yield but traditionally neglect atom-economy. Here we report that the simple, but significant, change of the solvent from 1-octadecene (1-ODE) to the operationally inert octadecane (ODA) permits an atom-economical synthesis of copper selenophosphate (Cu3PSe4) nanoparticles. This change eliminates the competing selenium (Se) delivery pathways from our first report that required an excess of Se. Instead Se0powder is dispersed in ODA, which promotes a formal eight-electron transfer between Cu3–xP and Se0. Powder X-ray diffraction and transmission electron microscopy confirm the purity of the Cu3PSe4, while 1H and 13C NMR indicate the absence of oxidized ODA or Se species. We utilize the direct pathway to gain insights into stoichiometry and ligand identity using thermogravimetric analysis and X-ray photoelectron spectroscopy. Given the prevalence of 1-ODE in nanoparticle synthesis, this approach could be applied to other chalcogenide reaction pathways to improve stoichiometry and atom-economy

Structure–Property Relationships in High-Rate Anode Materials Based on Niobium Tungsten Oxide Shear Structures

Nb16W5O55 emerged as a high-rate anode material for Li-ion batteries in 2018 [Griffith et al., Nature2018, 559 (7715), 556–563]. This exciting discovery ignited research in Wadsley–Roth (W–R) compounds, but systematic experimental studies have not focused on how to tune material chemistry and structure to achieve desirable properties for energy storage applications. In this work, we systematically investigate how structure and composition influences capacity, Li-ion diffusivity, charge–discharge profiles, and capacity loss in a series of niobium tungsten oxide W–R compounds: (3 × 4)-Nb12WO33, (4 × 4)-Nb14W3O44, and (4 × 5)-Nb16W5O55. Potentiostatic intermittent titration (PITT) data confirmed that Li-ion diffusivity increases with block size, which can be attributed to an increasing number of tunnels for Li-ion diffusion. The small (3 × 4)-Nb12WO33 block size compound with preferential W ordering on tetrahedral sites exhibits single electron redox and, therefore, the smallest measured capacity despite having the largest theoretical capacity. This observation signals that introducing cation disorder (W occupancy at the octahedral sites in the block center) is a viable strategy to assess multi-electron redox behavior in (3 × 4) Nb12WO33. The asymmetric block size compounds [i.e., (3 × 4) and (4 × 5) blocks] exhibit the greatest capacity loss after the first cycle, possibly due to Li-ion trapping at a unique low energy pocket site along the shear plane. Finally, the slope of the charge–discharge profile increases with increasing block size, likely because the total number of energy-equivalent Li-ion binding sites also increases. This unfavorable characteristic prohibits the large block sizes from delivering constant power at a fixed C-rate more so than the smaller block sizes. Based on these findings, we discuss design principles for Li-ion insertion hosts made from W–R materials

Microwave-Assisted Green Synthesis of Silver Nanoparticles Using Orange Peel Extract

Silver nanoparticles (AgNPs) were prepared in a one-step microwave-assisted synthesis guided by the principles of green chemistry. Microwave parameters were optimized using the Box–Benhken design for three factors (time, temperature, and pressure). Aqueous extracts from the peels of citrus fruits (orange, grapefruit, tangelo, lemon, and lime) were used for the synthesis of AgNPs using microwave technology, though the synthesis of AgNPs was only successful using the orange peel extract. Nanospheres of TEM mean diameter (with standard deviation) of 7.36 ± 8.06 nm were successfully synthesized in 15 min by reducing Ag⁺ ions (from AgNO₃) with orange peel extract, which also served as a capping agent. Creation of AgNPs was confirmed using UV–visible spectroscopy, fluorescence emission spectroscopy, powder X-ray diffraction, and transmission electron microscopy, while size analysis was gathered from both transmission electron microscopy as well as dynamic light scattering. Analysis of all citrus peel extracts by gas chromatography–mass spectrometry indicated that the putative compounds responsible for successful AgNP synthesis with orange extract were aldehydes. The creation of AgNPs using environmentally benign reagents in minimal time paves the way for future studies on AgNP toxicity without risking interference from potentially toxic reagents and capping agents

Investigation of Antibacterial Activity by Silver Nanoparticles Prepared by Microwave-Assisted Green Syntheses with Soluble Starch, Dextrose, and Arabinose

The objective of this study was to assess the antibacterial activity and inhibition of biofilm formation of silver nanoparticles (AgNPs) against Escherichia coli (MG1655), Bacillus subtilis, Klebsiella pneumoniae, Pseudomonas aeruginosa, Staphylococcus aureus, and Janthinobacterium lividum. The AgNPs utilized in this study were prepared through one-pot microwave-assisted syntheses guided by principles of green chemistry. The AgNPs were synthesized in three different schemes by reducing Ag⁺ ions (from AgNO₃) with reducing agents dextrose, arabinose, and soluble starch. Formation of AgNPs occurred in less than 15 min, and nanoparticles had diameters of 30 nm or less. Successful synthesis of AgNPs was confirmed using multiple orthogonal approaches, including UV–visible spectroscopy, fluorescence emission spectroscopy, powder X-ray diffraction, and transmission electron microscopy, while size analysis was gathered from transmission electron microscopy images and dynamic light scattering. All AgNPs prepared in this study exhibited antibacterial effects on a variety of organisms as determined by a well diffusion assay with no antibacterial effects observed in the control wells