Solution phase treatments of Sb2_2Se3_3 heterojunction photocathodes for improved water splitting performance

Antimony selenide (Sb$_2$Se$_3$) is an auspicious material for solar energy conversion that has seen rapid improvement over the past ten years, but the photovoltage deficit remains a challenge. Here, simple and low-temperature treatments of the p–n heterojunction interface of Sb$_2$Se$_3$/TiO$_2$-based photocathodes for photoelectrochemical water splitting were explored to address this challenge. The FTO/Ti/Au/Sb$_2$Se$_3$ (substrate configuration) stack was treated with (NH$_4$)$_2$S as an etching solution, followed by CuCl$_2$ treatment prior to deposition of the TiO$_2$ by atomic layer deposition. The different treatments show different mechanisms of action compared to similar reported treatments of the back Au/Sb$_2$Se$_3$ interface in superstrate configuration solar cells. These treatments collectively increased the onset potential from 0.14 V to 0.28 V vs. reversible hydrogen electrode (RHE) and the photocurrent from 13 mA cm$^{−2}$ to 18 mA cm$^{−2}$ at 0 V vs. RHE as compared to the untreated Sb$_2$Se$_3$ films. From SEM and XPS studies, it is clear that the etching treatment induces a morphological change and removes the surface Sb$_2$Se$_3$ layer, which eliminates the Fermi-level pinning that the oxide layer generates. CuCl$_2$ further enhances the performance due to the passivation of the surface defects, as supported by density functional theory molecular dynamics (DFT-MD) calculations, improving charge separation at the interface. The simple and low-cost semiconductor synthesis method combined with these facile, low-temperature treatments further increases the practical potential of Sb$_2$Se$_3$ for large-scale water splitting

Crystal orientation-dependent etching and trapping in thermally-oxidised Cu₂O photocathodes for water splitting

Ammonia solution etching was carried out on thermally-oxidised cuprous oxide (TO-Cu2O) in photocathode devices for water splitting. The etched devices showed increased photoelectrochemical (PEC) performance compared to the unetched ones as well as improved reproducibility. -8.6 mA cm-2 and -7 mA cm-2 photocurrent density were achieved at 0 V and 0.5 V versus the reversible hydrogen electrode (VRHE), respectively, in the champion sample with an onset potential of 0.92 VRHE and a fill factor of 44 %. An applied bias photon-to-current efficiency of 3.6 % at 0.56 VRHE was obtained, which represents a new record for Cu2O-based photocathode systems. Capacitance-based profiling studies showed a strong pinning effect from interfacial traps in the as-grown device, and these traps were removed by ammonia solution etching. Moreover, the etching procedure gave rise to a diverse morphology of Cu2O crystals based on the different crystallographic orientations. The distribution of crystallographic orientations and the relationship between the crystal orientation and the morphology after etching were examined by electron backscatter diffraction (EBSD) and scanning electron microscopy (SEM). The high-index crystal group showed a statistically higher PEC performance than the low-index group. X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) revealed metallic copper at the Cu2O/Ga2O3 interface, which we attribute as the dominant trap that limits the PEC performance. It is concluded that the metallic copper originates from the reduction of the CuO impurity layer on the as-grown Cu2O sample during the ALD process, while the reduction from Cu2O to Cu is not favorable

Solution-Processed Cu2_2S Nanostructures for Solar Hydrogen Production

Cu$_2$S is a promising solar energy conversion material due to its suitable optical properties, high elemental earth abundance, and nontoxicity. In addition to the challenge of multiple stable secondary phases, the short minority carrier diffusion length poses an obstacle to its practical application. This work addresses the issue by synthesizing nanostructured Cu$_2$S thin films, which enables increased charge carrier collection. A simple solution-processing method involving the preparation of CuCl and CuCl$_2$ molecular inks in a thiol-amine solvent mixture followed by spin coating and low-temperature annealing was used to obtain phase-pure nanostructured (nanoplate and nanoparticle) Cu$_2$S thin films. The photocathode based on the nanoplate Cu$_2$S (FTO/Au/Cu$_2$S/CdS/TiO$_2$/RuO$_x$) reveals enhanced charge carrier collection and improved photoelectrochemical water-splitting performance compared to the photocathode based on the non-nanostructured Cu$_2$S thin film reported previously. A photocurrent density of 3.0 mA cm$^{–2}$ at −0.2 versus a reversible hydrogen electrode (V$_{RHE}$) with only 100 nm thickness of a nanoplate Cu$_2$S layer and an onset potential of 0.43 V$_{RHE}$ were obtained. This work provides a simple, cost-effective, and high-throughput method to prepare phase-pure nanostructured Cu$_2$S thin films for scalable solar hydrogen production

Post-Synthetic Silver Ion and Sulfurization Treatment for Enhanced Performance in Sb2Se3 Water Splitting Photocathodes

In the past decade, antimony selenide (Sb2Se3) has made significant progress as a solar energy conversion material. However, the photovoltage deficit continues to pose a challenge and is a major hurdle that must be overcome to reach its maximum solar conversion efficiency. In this study, various post-synthetic treatments are employed, of which the combination of a solution phase silver nitrate treatment and sulfurization has shown to be the most effective approach to mitigate the photovoltage deficit in this Sb2Se3-based device. A significant enhancement in the photovoltage is observed after the treatments, as evident by the increase in the onset potential from 0.18 to 0.40 V versus reversible hydrogen electrode. Multiwavelength Raman shows that combining these two treatments removes amorphous Se and metallic Sb from the surface and yields a high-quality surface layer of Sb2(S1−x, Sex)3 on the bulk Sb2Se3 photoabsorber layer. X-ray photoelectron spectroscopy with depth profiling reveals extensive incorporation of silver into the film. Density functional theory calculations suggest that silver ions can intercalate between the [Sb4Se6]n ribbons and remain in the Ag+ state. This effective treatment combination brings the practicality of the Sb2Se3 photocathode for water splitting one step closer to large-scale applications

Single-cell profiling of lncRNA expression during Ebola virus infection in rhesus macaques

Long non-coding RNAs (lncRNAs) are involved in numerous biological processes and are pivotal mediators of the immune response, yet little is known about their properties at the single-cell level. Here, we generate a multi-tissue bulk RNAseq dataset from Ebola virus (EBOV) infected and not-infected rhesus macaques and identified 3979 novel lncRNAs. To profile lncRNA expression dynamics in immune circulating single-cells during EBOV infection, we design a metric, Upsilon, to estimate cell-type specificity. Our analysis reveals that lncRNAs are expressed in fewer cells than protein-coding genes, but they are not expressed at lower levels nor are they more cell-type specific when expressed in the same number of cells. In addition, we observe that lncRNAs exhibit similar changes in expression patterns to those of protein-coding genes during EBOV infection, and are often co-expressed with known immune regulators. A few lncRNAs change expression specifically upon EBOV entry in the cell. This study sheds light on the differential features of lncRNAs and protein-coding genes and paves the way for future single-cell lncRNA studies

Comparative Transmissibility of SARS-CoV-2 Variants Delta and Alpha in New England, USA

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) Delta variant quickly rose to dominance in mid-2021, displacing other variants, including Alpha. Studies using data from the United Kingdom and India estimated that Delta was 40-80% more transmissible than Alpha, allowing Delta to become the globally dominant variant. However, it was unclear if the ostensible difference in relative transmissibility was due mostly to innate properties of Delta\u27s infectiousness or differences in the study populations. To investigate, we formed a partnership with SARS-CoV-2 genomic surveillance programs from all six New England US states. By comparing logistic growth rates, we found that Delta emerged 37-163% faster than Alpha in early 2021 (37% Massachusetts, 75% New Hampshire, 95% Maine, 98% Rhode Island, 151% Connecticut, and 163% Vermont). We next computed variant-specific effective reproductive numbers and estimated that Delta was 58-120% more transmissible than Alpha across New England (58% New Hampshire, 68% Massachusetts, 76% Connecticut, 85% Rhode Island, 98% Maine, and 120% Vermont). Finally, using RT-PCR data, we estimated that Delta infections generate on average ∼6 times more viral RNA copies per mL than Alpha infections. Overall, our evidence indicates that Delta\u27s enhanced transmissibility could be attributed to its innate ability to increase infectiousness, but its epidemiological dynamics may vary depending on the underlying immunity and behavior of distinct populations

Comparative transmissibility of SARS-CoV-2 variants Delta and Alpha in New England, USA.

The SARS-CoV-2 Delta variant rose to dominance in mid-2021, likely propelled by an estimated 40%-80% increased transmissibility over Alpha. To investigate if this ostensible difference in transmissibility is uniform across populations, we partner with public health programs from all six states in New England in the United States. We compare logistic growth rates during each variant\u27s respective emergence period, finding that Delta emerged 1.37-2.63 times faster than Alpha (range across states). We compute variant-specific effective reproductive numbers, estimating that Delta is 63%-167% more transmissible than Alpha (range across states). Finally, we estimate that Delta infections generate on average 6.2 (95% CI 3.1-10.9) times more viral RNA copies per milliliter than Alpha infections during their respective emergence. Overall, our evidence suggests that Delta\u27s enhanced transmissibility can be attributed to its innate ability to increase infectiousness, but its epidemiological dynamics may vary depending on underlying population attributes and sequencing data availability

Enhancing the Efficiency of Antimony Selenide Photocathodes Towards Solar Hydrogen Production

The global imperative to transition from conventional energy sources to green and renewable alternatives has become increasingly evident in the face of escalating environmental challenges. The key motivation for this transition includes the urgent need to mitigate climate change by reducing greenhouse gas emissions, obtaining energy security and independence, fostering economic growth and increasing employment opportunities. Hence, there is a necessity for ongoing advancements and innovations in green energy technologies to drive global collaborations towards sustainable developments. One example of such advancement is the use of the vast solar energy available on the surface of our planet. Solar technologies, characterised by their renewable and inexhaustible nature, present a compelling solution to the challenges posed by conventional energy sources. One promising avenue within solar technology is photoelectrochemical water splitting, which holds a unique potential in directly converting solar energy into storable chemical energy by splitting water into hydrogen and oxygen. This process provides a clean and abundant source of hydrogen fuel and addresses the issues associated with the intermittent nature of solar power by enabling energy storage. The current challenge surrounding photoelectrochemical water splitting includes searching for abundant and low-cost semiconductor materials with efficiencies and stability comparable to those containing precious metals, the benchmark materials currently used in solar technologies. Antimony selenide, Sb2Se3, as a semiconductor material, is a promising candidate for a wide range of applications in photovoltaics and photoelectrochemical water-splitting. This is due to its favourable properties, including high electrical conductivity, optimum optical characteristics, reasonable cost and abundance. Scientists have dedicated over six decades to transforming silicon into the commercially prevalent solar cells available today, and with time, Sb2Se3 can achieve comparable strides in solar-to-hydrogen (STH) efficiencies. This thesis focuses on enhancing the efficiency of Sb2Se3 photocathode for photoelectrochemical water-splitting purposes. It employs a simple and cost-effective synthesis technique with the objective of mitigating the photovoltage deficiency inherent to this material, a fundamental impediment constraining its overall efficiency.The first publication within this thesis endeavours to address the surface defects found in Sb2Se3. This study delves into innovative approaches for enhancing the performance of this photoabsorber material by subjecting it to treatments involving (NH4)2S etching and CuCl2 passivation. These treatments yielded a substantial increase in onset potential and photocurrent, surpassing the untreated Sb2Se3 films. The treatments induced morphological changes, eliminated surface Sb2O3 layers, and improved charge separation at the interface. The second publication extends upon the preceding one by conducting a comprehensive screening to identify other viable surface treatment candidates capable of augmenting efficiency. The study sought to elucidate the influence of metal treatments and ascertain whether the enhancement could be attributed to specific metal characteristics, including oxidation state, ionic radius, or electronegativity. Furthermore, given the prior performance improvements due to bulk passivation via sulphurisation, this investigation aimed to explore the combined impact of surface and bulk treatments. Interestingly, the most efficacious post-synthetic treatment proved to be the combination of silver nitrate and sulphurisation. These treatments substantially increased the photovoltage, leading to a remarkable onset potential increase of more than 200 mV. However, it was intriguingly revealed that the enhancement did not stem from the initially hypothesised causes. Instead, the treatments were found to eliminate amorphous Se and metallic Sb from the surface, forming a high-quality Sb2(S1–x, Sex)3 surface layer. These improvements instigate favourable morphological alterations, ultimately enhancing light absorption and scattering. The final publication centres on substituting the noble and costly platinum catalyst with a more abundant MoSx catalyst. While prior research explored the amorphous variant of this catalyst, it was hypothesised that MoSx clusters, such as [Mo3S4]4+ and [Mo3S13]2–, would offer enhanced selectivity and reactivity due to the catalyst's increased reaction sites. Overall, this thesis has successfully addressed several factors hindering the performance of Sb2Se3, leading to an enhancement in both photovoltage and photocurrent. This study serves as a robust foundation that can be further refined to increase the practical potential of Sb2Se3 for large-scale water-splitting applications

Thiol-Amine-Based Solution Processing of Cu2S Thin Films for Photoelectrochemical Water Splitting

Cu2S is a promising solar energy conversion material owing to its good optical properties, elemental earth abundance, and low cost. However, simple and cheap methods to prepare phase-pure and photo-active Cu2S thin films are lacking. This study concerns the development of a cost-effective and high-throughput method that consists of dissolving high-purity commercial Cu2S powder in a thiol-amine solvent mixture followed by spin coating and low-temperature annealing to obtain phase-pure crystalline low chalcocite Cu2S thin films. After coupling with a CdS buffer layer, a TiO2 protective layer and a RuOx hydrogen evolution catalyst, the champion Cu2S photocathode gives a photocurrent density of 2.5 mA cm−2 at −0.3 V vs. reversible hydrogen electrode (VRHE), an onset potential of 0.42 VRHE, and high stability over 12 h in pH 7 buffer solution under AM1.5 G simulated sunlight illumination (100 mW cm−2). This is the first thiol-amine-based ink deposition strategy to prepare phase-pure Cu2S thin films achieving decent photoelectrochemical performance, which will facilitate its future scalable application for solar-driven hydrogen fuel production