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Here, we report the use of Li2Mn(SO4)2 as a potential energy storage material and describe its route of synthesis and structural characterization over one electrochemical cycle. Li2Mn(SO4)2 is synthesized by ball milling of MnSO4·H2O and Li2SO4·H2O and characterized using a suite of techniques, in particular, ex situ X-ray diffraction, X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy on the Mn and S K-edges to investigate the electronic and local geometry around the absorbing atoms. The prepared Li2Mn(SO4)2 electrodes undergo electrochemical cycles to different potential points on the charge–discharge curve and are then extracted from the cells at these points for ex situ structural analysis. Analysis of X-ray absorption spectroscopy (both near and fine structure part of the data) data suggests that there are minimal changes to the oxidation state of Mn and S ions during charge–discharge cycles. However, X-ray photoelectron spectroscopy analysis suggests that there are changes in the oxidation state of Mn, which appears to be different from the conclusion drawn from X-ray absorption spectroscopy. This difference in results during cycling can thus be attributed to electrochemical reactions being dominant at the surface of the Li2Mn(SO4)2 particles rather than in the bulk.

The power conversion efficiency (PCE) of hole conductor free carbon-based perovskite solar cells (PSCs) is restricted by the poor charge extraction and recombination losses at the carbon-perovskite interface. For the first time we successfully demonstrated incorporation of thin layer of copper doped nickel oxide (Cu:NiOx) nanoparticles in carbon-based PSCs, which helps in improving the performance of these solar devices. Cu:NiOx nanoparticles have been synthesized by a facile chemical method, and processed into a paste for screen printing. Extensive X-ray Absorption Spectroscopy (XAS) analysis elucidates the co-ordination of Cu in a NiOx matrix and indicates the presence of around 5.4% Cu in the sample. We fabricated a monolithic perovskite module on a 100 cm2 glass substrate (active area of 70 cm2) with a thin Cu:NiOx layer (80 nm), where the champion device shows an appreciated power conversion efficiency of 12.1% under an AM 1.5G illumination. To the best of our knowledge, this is the highest reported efficiency for such a large area perovskite solar device. I-V scans show that the introduction of Cu:NiOx mesoporous scaffold increases the photocurrent, and yields fill factor (FF) values exceeding 57% due to the better interface and increased hole extraction efficiency. Electrochemical Impedance Spectroscopy (EIS) results reinforce the above results by showing the reduction in recombination resistance (Rrec) of the PSCs that incorporates Cu:NiOx interlayer. The perovskite solar modules with a Cu:NiOx layer are stable for more than 4500 h in an ambient environment (25 °C and 65% RH), with PCE degradation of less than 5% of the initial value. NRF (Natl Research Foundation, S’pore) Accepted version

Despite extensive reports on highly efficient perovskite light-emitting diodes, rules governing the design of suitable two-dimensional (2D) perovskite templating cation to facilitate formation of optimal emitter landscape for energy cascade remain largely elusive. With factors such as structure, size, functionalization, and charge capable of influencing the distribution of multidimensional perovskite phases, the importance of 2D templating cation design in determining film optoelectronic properties is indisputable. However, typical mono-functionalized 2D templating cations often result in larger lead halide octahedral spacing, which impedes effective charge transport. This has fueled investigation into the use of multiple cations for optimal domain distribution and improved charge transfer kinetics to the emitting species. In this study, we attempt to impart enhanced charge transfer characteristics to the resultant multidimensional perovskite by employing two bi-functionalized aromatic cations, namely, pyridinium ethyl ammonium and imidazolium ethyl ammonium, reminiscent of mono-functionalized phenyl ethyl ammonium, a widely used 2D perovskite templating cation. Although it is proposed that greater intermolecular bonding would enhance charge transfer rates, the simultaneous increase in lead halide octahedral distortion results in quenching of their corresponding 2D and multidimensional perovskite luminescence properties, correlated with increased defect density within the material. This manifests in the form of shorter PL decay lifetimes, lower PLQY, and device performance arising from inferior energy funneling. This study highlights the importance of designing 2D perovskite templating cations offering better transport and reduced octahedral distortion for the development of energy cascade-efficient, multidimensional perovskites. Ministry of Education (MOE) National Research Foundation (NRF) Accepted version This research was primarily supported by the National Research Foundation under its Competitive Research Programme (CRP Award No. NRF-CRP14-2014-03) and the Ministry of Education under MOE2018-T2-2-083. The photophysical measurements are supported by National Research Foundation Investigatorship (NRF-NRFI-2018-04) and by the Ministry of Education under MOE Tier 2 grant MOET2EP50120- 0004.

In the present study, we describe a new approach to the synthesis of a Bi-containing apatite, which allows for a reduced reaction time and temperature. Using in-situ variable temperature X-ray diffraction measurements, we have refined the crystal structure of the material, which shows that its composition may be described as [Ca3.88±0.01Bi0.12±0.01]F[Ca4.42±0.01Bi1.58±0.01]T(PO4)6O1.85±0.01. The compound displays isotropic thermal expansion in the temperature range (173–373) K. A thermodynamic investigation of the sample was undertaken, which revealed the low-temperature heat capacity, C p ,m o , which was determined using adiabatic calorimetry from T = 6.4 K to 305.0 K. Smoothed C p ,m o ( T ) values between 6.5 K and 305.0 K are presented, along with the functions [ S m o ( T ) - S m o ( 6.5 ) ] , [ H m o ( T ) - H m o ( 6.5 ) ] , and [ Φ m o ( T ) - Φ m o ( 6.5 ) ] . Possible causes of the abnormal increase on the heat capacity curve in the low-temperature region are discussed.

A novel high-voltage solid solution of Li2CoxMn1–x(SO4)2 (x = 0–1) was synthesized using a simple solid-state reaction. Single-phase monoclinic crystal structure was identified, and there was no evidence for the presence of an orthorhombic polymorph at a sample preparation temperature of 450 °C. Interestingly, the bimetallic sulfates were found to be thermally stable up to 550 °C, and it was validated through various characterization techniques. Beyond 550 °C, the materials start to decompose and form Li2SO4 and α-CoSO4 in the case of Li2Co(SO4)2, while Li2Mn(SO4)2 decomposes to form Li2Mn2(SO4)3. In addition, an important property called sensitivity toward moisture was also characterized, which plays a critical role in the handling of the sulfate-based cathode during the cell fabrication. Thus, the reaction mechanism involved during the absorption of the atmospheric moisture was also investigated carefully. Finally, for the first time electrochemical activity of both Co and Mn systems was discovered wher...

To control the process of cation exchange (CE) in a multielemental system, a detailed understanding of structural changes at the microscopic level is imperative. However, the synthesis of a multielemental system has so far relied on the CE phenomenon of a binary system, which does not necessarily extend to the higher-order systems. Here, direct experimental evidence supported by theoretical calculations reveals a growth model of binary Cu–S to ternary Cu–Sn–S to quaternary Cu–Zn–Sn–S, which shows that cations preferentially diffuse along a specific lattice plane with the preservation of sulfuric anionic framework. In addition, we also discover that, unlike the commonly accepted structure (P63mc), the metastable crystal structure of Cu–Zn–Sn–S phase possesses fixed Sn occupancy sites. By revealing the preferential nature of cations diffusion and growth mechanism, our work provides insight into controlling the stoichiometry and phase purity of novel multielemental materials. Ministry of Education (MOE) National Research Foundation (NRF) Accepted version We acknowledge financial support from National Research Foundation (NRF), Singapore, through the Singapore-Berkeley Research Initiative for Sustainable Energy (SinBeRISE) and Nanomaterials for Energy and Water Management (SHARE NEW) CREATE program. L.H.W. thanks the funding support from Singapore Ministry of Education, Tier 2 (2016-T2-1-030). S.L. acknowledges the funding support from Singapore Ministry of Education Tier 1 (107/15). H.Z. thanks the funding support from U.S. DOE BES Materials Sciences and Engineering Division Under Contract No. KC22ZH. X.Y.L. thanks the funding support from Singapore Ministry of Education, Tier 1 (RG21/16) and Tier 2 (MOE2016-T2-1- 043) grants.. The work at the Molecular Foundry was supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. We thank Fiona Doyle for lending us her synthetic laboratory in University of California Berkeley (UCB), Song Chengyu and Karen Bustilo for their help and assistance on TEM, and Matthew P. Sherburne for nanoparticle growth discussion.

Altering cation and anion ratios in perovskites has been an excellent avenue in tuning the perovskite properties and enhancing the performance. Recently, MA/FA/Cs triple cation mixed halide perovskites have demonstrated efficiencies reaching up to 22 %. Similar to the widely explored MAPbI3, excess PbI2 is added in these perovskite films to enhance the performance. Previous reports demonstrate that the excess PbI2 is beneficial for the performance. However, not much work has been conducted about its impact on stability. Triple cation perovskites (TCP) deploy excess PbI2 up to 8 %. Thus, it is imperative to analyze the role of excess PbI2 in the degradation kinetics. In this paper, we have varied the amount of PbI2 in the triple cation perovskite films and monitored the degradation kinetics by X-ray diffraction (XRD) and optical absorption spectroscopy. We found that the inclusion of excess PbI2 adversely affects the stability of the material. Faster degradation kinetics is observed for higher PbI2 samples. However, excess PbI2 samples showed superior properties such as enhanced grain sizes and better optical absorption. Thus, careful management of the PbI2 quantity is required to obtain better stability and alternative pathways should be explored to achieve better device performance rather than adding excess PbI2. NRF (Natl Research Foundation, S’pore) Accepted version

With the objective of moving towards the commercialisation of Fe-based high voltage cathode Li2Fe(SO4)2, the effect of fluorinated binder (polyvinylidene fluoride – PVDF) and non-fluorinated binder (polyacrylonitrile – PAN) was evaluated. First, the redox mechanism involved in the cycling of Li2Fe(SO4)2 cathode in the presence of these two binders was investigated through ex situ X-ray Absorption Spectroscopy (XAS) studies which can directly determine oxidation state changes of the Fe ion during the charge–discharge process. Subsequently, the effect of the binders on the cycling and rate performance was evaluated. In the present work, consistent cycling performance with discharge capacity of 60 mA h g−1 was recorded at low current rate of C/15. However, it was noted that at high charging rate the cell with binder retained only 29% of the discharge capacity in comparison to the cell without binder. This was attributed to the high charge transfer resistance of binders through cyclic voltammogram and electrochemical impedance spectroscopy studies. This in turn suggests the requirement of high conductivity binders for such sulphate based electrodes.

In this work, we present the first report on thin film solar cells that employ Cu 2 MnSnS 4 (CMTS) and Cu 2 MnSn(S, Se) 4 (CMTSSe) as the absorber. CMTS and CMTSSe thin films are fabricated using a low cost spray pyrolysis technique in ambient atmosphere using water as a solvent. The crystal structure of the materials are similar to the established kesterite type photovoltaic materials such as Cu 2 ZnSn(S, Se) 4 (CZTSSe). The bandgap of these materials is between 1.4−1.7 eV, which is ideal for optimum solar absorption and can be tuned by varying the ratio of the elemental constituents. The photovoltaic device with a structure of Mo/CMTS/CdS/TCO/top electrode is fabricated as a proof-of-concept and yields a power conversion efficiency of ~0.73% for the best optimized CMTS device with Na doping. Through analysis of the device characteristics, we identify a key problem of very high carrier density in the CMTS/CMTSSe absorber that leads to short collection length, low V oc , low quantum efficiency at long wavelength and high shunt conductance that quench the fill factor. We also discuss several routes to improve the device performance.

Copper-doped hydroxyapatite (HA) of nominal composition Ca10(PO4)6[Cux(OH)2-2xOx] (0.0 &le, x &le, 0.8) was prepared by solid-state and wet chemical processing to explore the impact of the synthesis route and mode of crystal chemical incorporation of copper on the antibacterial efficacy against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) strains. Apatites prepared by solid-state reaction showed unit cell volume dilation from 527.17 &Aring, 3 for copper-free HA to 533.31 &Aring, 3 for material of the putative composition Ca10(PO4)6[Cu0.8(OH)0.4O0.8] consistent with Cu+ insertion into the [001] hydroxyapatite channel. This was less pronounced (528.30 &Aring, 3 to 529.3 &Aring, 3) in the corresponding wet chemical synthesised products, suggesting less complete Cu tunnel incorporation and partial tenancy of Cu in place of calcium. X-ray absorption spectroscopy suggests fast quenching is necessary to prevent oxidation of Cu+ to Cu2+. Raman spectroscopy revealed an absorption band at 630 cm&minus, 1 characteristic of symmetric O-Cu+-O units tenanted in the apatite channel while solid-state 31P magic-angle-spinning nuclear magnetic resonance (MAS NMR) supported a vacancy-Cu+ substitution model within the apatite channel. The copper doping strategy increases antibacterial efficiency by 25% to 55% compared to undoped HA, with the finer particle sizes and greater specific surface areas of the wet chemical material demonstrating superior efficacy.

Here, we report the use of Li2Mn(SO4)2 as a potential energy storage material and describe its route of synthesis and structural characterization over one electrochemical cycle. Li2Mn(SO4)2 is synthesized by ball milling of MnSO4·H2O and Li2SO4·H2O and characterized using a suite of techniques, in particular, ex situ X-ray diffraction, X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy on the Mn and S K-edges to investigate the electronic and local geometry around the absorbing atoms. The prepared Li2Mn(SO4)2 electrodes undergo electrochemical cycles to different potential points on the charge–discharge curve and are then extracted from the cells at these points for ex situ structural analysis. Analysis of X-ray absorption spectroscopy (both near and fine structure part of the data) data suggests that there are minimal changes to the oxidation state of Mn and S ions during charge–discharge cycles. However, X-ray photoelectron spectroscopy analysis suggests that there are changes in the oxidation state of Mn, which appears to be different from the conclusion drawn from X-ray absorption spectroscopy. This difference in results during cycling can thus be attributed to electrochemical reactions being dominant at the surface of the Li2Mn(SO4)2 particles rather than in the bulk.

CeNbO4.25 is reported to exhibit fast oxygen ion diffusion at moderate temperatures, making this the prototype of a new class of ion conductor with applications in a range of energy generation and storage devices. To date, the mechanism by which this ion transport is achieved has remained obscure, in part due to the long-range commensurately modulated structural motif. Here we show that CeNbO4.25 forms with a unit cell ∼12 times larger than the stoichiometric tetragonal parent phase of CeNbO4 as a result of the helical ordering of Ce(3+) and Ce(4+) ions along z. Interstitial oxygen ion incorporation leads to a cooperative displacement of the surrounding oxygen species, creating interlayer "NbO6" connectivity by extending the oxygen coordination number to 7 and 8. Molecular dynamic simulations suggest that fast ion migration occurs predominantly within the xz plane. It is concluded that the oxide ion diffuses anisotropically, with the major migration mechanism being intralayer; however, when obstructed, oxygen can readily move to an adjacent layer along y via alternate lower energy barrier pathways.

Despite their extremely good performance in solar cells with efficiencies approaching 20% and the emerging application for light-emitting devices, organic-inorganic lead halide perovskites suffer from high content of toxic, polluting, and bioaccumulative Pb, which may eventually hamper their commercialization. Here, we present the synthesis of two-dimensional (2D) Cu-based hybrid perovskites and study their optoelectronic properties to investigate their potential application in solar cells and light-emitting devices, providing a new environmental-friendly alternative to Pb. The series (CH3NH3)2CuCl(x)Br(4-x) was studied in detail, with the role of Cl found to be essential for stabilization. By exploiting the additional Cu d-d transitions and appropriately tuning the Br/Cl ratio, which affects ligand-to-metal charge transfer transitions, the optical absorption in this series of compounds can be extended to the near-infrared for optimal spectral overlap with the solar irradiance. In situ formation of Cu(+) ions was found to be responsible for the green photoluminescence of this material set. Processing conditions for integrating Cu-based perovskites into photovoltaic device architectures, as well as the factors currently limiting photovoltaic performance, are discussed: among them, we identified the combination of low absorption coefficient and heavy mass of the holes as main limitations for the solar cell efficiency. To the best of our knowledge, this is the first demonstration of the potential of 2D copper perovskite as light harvesters and lays the foundation for further development of perovskite based on transition metals as alternative lead-free materials. Appropriate molecular design will be necessary to improve the material's properties and solar cell performance filling the gap with the state-of-the-art Pb-based perovskite devices.

Thorium silicate apatite with the formula [Ca3.84Th0.16]F[Ca2.79Th3.21]T(SiO4)6O2 · x(H) was synthesized by solid-state reaction, and its structure refined in P63/m from powder X-ray diffraction (XRD) data using the Rietveld method (a = 9.50172(9) Å, c = 6.98302(8) Å, V = 545.98(1) Å(3); R-Bragg = 2.102%). It was found that thorium partitions strongly to the tunnel (T) 6h position rather than the framework (F) 4f site. Fourier transform infrared spectroscopy revealed only SiO4 tetrahedron, with SiO5 and SiO6 groups, sometimes observed in siliceous apatites absent, at least to the limit of detection of this technique. Thermal expansion of the thorium apatite determined by high-temperature XRD from 298-1173 K found Δa (0.87%) dilation to exceed Δc (0.73%) with increasing temperature consistent with other silicate apatites.

Carbon based perovskite solar cells (PSCs) are fabricated through easily scalable screen printing techniques, using abundant and cheap carbon to replace the hole transport material (HTM) and the gold electrode further reduces costs, and carbon acts as a moisture repellent that helps in maintaining the stability of the underlying perovskite active layer. An inorganic interlayer of spinel cobaltite oxides (Co

Pressure-induced crystallographic transitions and optical behavior of MAPbI3 (MA=methylammonium) were investigated using in-situ synchrotron X-ray diffraction and laser-excited photoluminescence spectroscopy. We observed that the tetragonal phase that presents under ambient pressure transformed to a ReO3-type cubic phase at 0.3 GPa, which further converted into a putative orthorhombic structure at 2.7 GPa. The sample was finally separated into crystalline and amorphous fractions beyond 4.7 GPa. During the decompression, the phase-mixed material restored the original structure in two distinct pathways depending on the peak pressures. Being monitored using a laser-excited photoluminescence technique under each applied pressure, it was determined that the bandgap reduced with an increase of the pressure till 0.3 GPa and then enlarged with an increase of the pressure up to 2.7 GPa. This work lays the foundation for understanding pressure-induced phase transitions and bandgap tuning of MAPbI3, enriching potentially the toolkit for engineering perovskites related photovoltaic devices. National Research Foundation (NRF) Accepted version This work was partially supported by NRF-CRP14-2014-03 and Custom Electronics, Inc. CHESS was supported by the NSF award DMR-1332208. Bandgap calculations were contributed by Hai Xiao, Jason Crowley and William A. Goddard III from Materials and Process Simulation Center (MSC) and Joint Center for Artificial Photosynthesis (JCAP), California Institute of Technology. S.J. acknowledges the support from Binghamton University.

Bismuth-based ternary halides have recently gained a lot of attention as lead-free perovskite materials. However, photovoltaic performances of these devices remain poor, mostly due to their low-dimensional crystal structure and large bandgap. Here, a dynamic hot casting technique to fabricate silver bismuth iodide-based perovskite solar cells under an ambient atmosphere with power conversion efficiencies above 2.5% is demonstrated. Silver bismuth iodides are 3D analogs of complex ternary bismuth halides with a suitable bandgap for a single junction solar cell. As far as it is known, these results represent the highest efficiency for solution processed air-stable lead-free perovskite solar cells. The enhanced solar cell performance via this dynamic hot casting technique can be attributed to long carrier lifetimes, micrometer-sized crystalline grains, and pinhole free thin-film formation with uniform morphology. This work provides a new direction for fabrication of solution-processed lead-free perovskite solar cells with a rapid fabrication strategy irrespective of the processing environment. NRF (Natl Research Foundation, S’pore) MOE (Min. of Education, S’pore) Accepted version

Catalyst deactivation is a persistent problem not only for the scientific community but also in industry. Isolated single-site heterogeneous catalysts have shown great promise to overcome these problems. Here, a versatile anchoring strategy for molecular complex immobilization on a broad range of semiconducting or insulating metal oxide (e.g. titanium dioxide, mesoporous silica, cerium oxide, and tungsten oxide) nanoparticles to synthesize isolated single-site catalysts has been studied systematically. An oxidatively stable anchoring group, maleimide, is shown to form covalent linkages with surface hydroxyl functionalities of metal oxide nanoparticles by photoclick chemistry. The nanocomposites have been thoroughly characterized by techniques including UV-visible diffuse reflectance spectroscopy (UV-DRS), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), infrared spectroscopy (IR), and X-ray absorption spectroscopy (XAS). The IR spectroscopic studies confirm the covalent linkages between the maleimide group and surface hydroxyl functionalities of the oxide nanoparticles. The hybrid nanomaterials function as highly efficient catalysts for essentially quantitative oxidations of terminal and internal alkenes, and show molecular catalyst product selectivities even in more eco-friendly solvents. XAS studies verify the robustness of the catalysts after several catalytic cycles. We have applied the photoclick anchoring methodology to precisely control the deposition of a luminescent variant of our catalyst on the metal oxide nanoparticles. Overall, we demonstrate a general approach to use irradiation to anchor molecular complexes on oxide nanoparticles to create recyclable, hybrid, single-site catalysts that function with high selectivity in a broad range of solvents. We have achieved a facile, spatially and temporally controllable photoclick method that can potentially be extended to other ligands, catalysts, functional molecules, and surfaces. MOE (Min. of Education, S’pore) ASTAR (Agency for Sci., Tech. and Research, S’pore) Accepted version

The 1H and 13C NMR spectra in methylammonium lead halide perovskites, CH3NH3PbX3 (X = I, Br and Cl) show that the CH3NH3+ units undergo dynamic reorientation, as the organic component tumbles in the perovskite cage. In addition, the differences in the anomalously long relaxation times of the protons associated with the CH3 and not the NH3 groups indicate that only the amine end of the CH3NH3+ group is interacting with the inorganic network. Using this information, we have refined some single crystal X-ray and neutron diffraction data to probe their unusual structures in more detail. Furthermore, impedance spectroscopy has been used to monitor the high-temperature phase transition of CH3NH3PbI3, which confirms a significant increase in conductivity, when it is in its high temperature and higher symmetry structural regime. The optical band-gaps of each halide perovskite were determined using UV-visible spectroscopy and are consistent with previous reports. NRF (Natl Research Foundation, S’pore) Accepted version

Electrolytes with oxide ion conductivities higher than 10−2 S cm−1 at moderate temperatures (∼500–900 °C) offer the possibility for solid oxide fuel cells to operate with less maintenance. This study of [A1+xB1−x]2[Ga]2[Ga2O7+x/2]2 (0 ≤ x ≤ 0.5) (A = La, Nd; B = Ca, Sr) layered-melilite found that in large single crystals intralayer oxide ion conduction is dominant. This anisotropic behavior arises by relaxation about the interstitial oxygen through changes in the interlayer A and Ga coordination, and at 850 °C conductivities are ∼0.008 S cm−1 along the c direction and ∼0.036 S cm−1 perpendicular to the c axis. It is found that the ionic conductivity can be optimized by increasing the number of interstitial oxygen and reducing the size of interlayer cations. ASTAR (Agency for Sci., Tech. and Research, S’pore) MOE (Min. of Education, S’pore) Accepted version

Altering cation and anion ratios in perovskites has proven an excellent means of tuning the perovskite properties and enhancing the performance. Recently, methylammonium/formamidinium/cesium triple-cation mixed-halide perovskites have demonstrated efficiencies up to 22 %. Similar to the widely explored methylammonium lead halide, excess PbI

In this work we have studied the feasibility of highly symmetric guanidinium (GA; CH6N3+) cation in perovskite based solar cells. It is an alternative to the methyl ammonium (MA; CH3NH3+) or formamidinium (FA; CH(NH2)2+) ions commonly utilized in the perovskite solar cells, may bring additional advantages due to its triad rotational symmetry and zero dipole moment (μ). We noticed that due to steric factors, GA preferably forms two-dimensional (2D) layer, where GA2PbI4 is favored monoclinic structure (E g = 2.5 eV) and the obtained device efficiency is η = 0.45%. This study provides a guideline for designing guanidinium based perovskite absorbers for solar cell devices. In addition, through further compositional engineering with mixed organic cations using GA may enhance the device performance. NRF (Natl Research Foundation, S’pore) ASTAR (Agency for Sci., Tech. and Research, S’pore) MOE (Min. of Education, S’pore) Accepted version

Carbon based perovskite solar cells (PSCs) are fabricated through easily scalable screen printing techniques, using abundant and cheap carbon to replace the hole transport material (HTM) and the gold electrode further reduces costs, and carbon acts as a moisture repellent that helps in maintaining the stability of the underlying perovskite active layer. An inorganic interlayer of spinel cobaltite oxides (Co3O4) can greatly enhance the carbon based PSC performance by suppressing charge recombination and extracting holes efficiently. The main focus of this research work is to investigate the effectiveness of Co3O4 spinel oxide as the hole transporting interlayer for carbon based perovskite solar cells (PSCs). In these types of PSCs, the power conversion efficiency (PCE) is restricted by the charge carrier transport and recombination processes at the carbon–perovskite interface. The spinel Co3O4 nanoparticles are synthesized using the chemical precipitation method, and characterized by X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and UV-Vis spectroscopy. A screen printed thin layer of p-type inorganic spinel Co3O4 in carbon PSCs provides a better-energy level matching, superior efficiency, and stability. Compared to standard carbon PSCs (PCE of 11.25%) an improved PCE of 13.27% with long-term stability, up to 2500 hours under ambient conditions, is achieved. Finally, the fabrication of a monolithic perovskite module is demonstrated, having an active area of 70 cm2 and showing a power conversion efficiency of >11% with virtually no hysteresis. This indicates that Co3O4 is a promising interlayer for efficient and stable large area carbon PSCs. NRF (Natl Research Foundation, S’pore) Accepted version

Lead-containing (Pb-B-X)-2Hapatites encompass a number of [AF]4[AT]6[(BO4)6]X2compounds used for waste stabilization, environmental catalysis and ion conduction, but the influence of the stereochemically active lone-pair electrons of Pb2+on crystal chemistry and functionality is poorly understood. This article presents a compilation of existing structural data for Pb apatites that demonstrate paired electrons of Pb2+at both theAFandATresults in substantial adjustments to the PbFO6metaprism twist angle, φ. New structure refinements are presented for several natural varieties as a function of temperature by single-crystal X-ray diffraction (XRD) of vanadinite-2H(ideally Pb10(VO4)6Cl2), pyromorphite-2H(Pb10(PO4)6Cl2), mimetite-2H/M(Pb10(As5+O4)6Cl2) and finnemanite-2H(Pb10(As3+O3)6Cl2). A supercell for mimetite is confirmed using synchrotron single-crystal XRD. It is suggested the superstructure is necessary to accommodate displacement of the stereochemically active 6s2lone-pair electrons on the Pb2+that occupy a volume similar to an O2−anion. We propose that depending on the temperature and concentration of minor substitutional ions, the mimetite superstructure is a structural adaptation common to all Pb-containing apatites and by extension apatite electrolytes, where oxide ion interstitials are found at similar positions to the lonepair electrons. It is also shown that plumbous apatite framework flexes substantially through adjustments of the PbFO6metaprism twist-angles (φ) as the temperature changes. Finally, crystalchemical [100] zoning observed at submicron scales will probably impact on the treatment of diffraction data and may account for certain inconsistencies in reported structures.

Organic-inorganic lead halide perovskites have been widely used as absorbers on mesoporous TiO2 films as well as thin films in planar heterojunction solar cells, yielding very high photovoltaic conversion efficiencies. Both the addition of chloride and sequential deposition methods were successfully employed to enhance the photovoltaic performance. Here, both approaches are combined in a sequential method by spincoating PbCl2 + PbI2 on a mesoporous TiO2 film followed by the perovskite transformation. The role of Cl in determining the optical, electrical, structural and morphological properties is correlated with the photovoltaic performance. The highest photovoltaic efficiency of 14.15% with the V(oc), FF and J(sc) being 1.09 V, 0.65 and 19.91 mA cm(-2) respectively was achieved with 10 mol% of PbCl2 addition due to an increase of the film conductivity induced by a better perovskite morphology. This is linked to an improvement of the hysteresis and reproducibility of the solar cells.

Solid-state, solution processed solar-cells based on organic–inorganic methyl ammonium lead halide absorbers have achieved efficiencies in excess of 15%, which has superseded liquid dye sensitized cells, as well as various thin film-based photovoltaics. This report introduces a new metal-halide perovskite, based on the formamidinium cation (HC(NH2)2+), that displays a favorable band gap (1.47 eV) and represents a broader absorption compared to previously reported absorbers that contained the methylammonium cation (CH3NH3+). The high open-circuit voltage (Voc = 0.97 V) and promising fill-factor (FF = 68.7%) yield an efficiency of 4.3%, which make this material an excellent candidate for this new class of perovskite solar cell. This report also investigates the formation of a black trigonal (P3m1) perovskite polymorph and a yellow hexagonal nonperovskite (P63mc) polymorph. Further solar cell development would entail the stabilization of the black trigonal (P3m1) perovskite polymorph over the yellow hexagona...

Neodymium silicate apatites are promising intermediate temperature (500°C–700°C) electrolytes for solid oxide fuel cells. The introduction of Al promotes isotropic percolation of O2−, and at low levels (0.83–2.0 wt% Al) enhances bulk conductivity. To better understand the effect of Al-doping on intrinsic conductivity, and the impact of grain boundaries on the transport, dense Nd9.33+x/3AlxSi6−xO26 (0 ≤ x ≤ 2) pellets were prepared by spark plasma sintering. Phase purity of the products was established by powder X-ray diffraction and the microstructure examined by scanning electron microscopy. The ionic conductivity measured by AC impedance spectroscopy for the spark plasma sintered ceramics were compared with transport in single crystals of similar composition. Intermediate Al-doping (0.5 ≤ x ≤ 1.5) delivered superior overall conductivity for both the polycrystalline and single crystal specimens. ASTAR (Agency for Sci., Tech. and Research, S’pore) Accepted version

The paradigm that functional materials are adequately described as three-dimensional crystal structures is not universally tenable. Gallate melilites are efficient oxide ion conductors at intermediate temperatures (∼750 °C) with non-rational crystallographic modulations presumed to play a key role in significantly enhancing oxygen mobility. Lattice distortions associated with incommensuration are usually extrapolated from diffraction analysis of volumes greatly exceeding the scale of modulation. Therefore, opportunities for making direct nanometric measurements are exceptionally valuable for correlating structure with function. In [Ca Ln ] 2 [Ga] 2 [Ga 2 O 7 ] 2 ( Ln =Nd, La) melilites, atomic displacive and compositional modulation waves can be imaged by high angle annular dark field and bright field scanning transmission electron microscopy with contrast quantified through electron scattering simulation. Here, we present atomic scale observations of (3+2)-dimensional modulations in gallate melilites which expands our understanding of the ion conduction mechanism and provides guidance for enhancing the performance of solid oxide fuel cells through crystal chemical tailoring.

To better understand the oxide ion conduction mechanism of rare earth silicate apatites as intermediate temperature electrolytes for solid oxide fuel cells (SOFC), the effect of lower valent metal doping on the performance of Nd(28+x)/3AlxSi6-xO26 (0 ≤ x ≤ 2) single crystals has been examined. The measurement of ionic conductivity via AC impedance spectroscopy showed that the conductivities were anisotropic and superior along the c direction. An interesting aspect from the impedance studies was the identification of a second semicircle with capacitance similar to that of a grain boundary component, despite the fact that polarized optical microscopy and electron backscattered diffraction showed that the single crystals consisted of a single grain. This semicircle disappeared after long-term (up to 3 months) annealing of the single crystals at 950 °C, also leading to a reduction in the bulk conductivity. In order to explain these observations, single-crystal X-ray diffraction studies were performed both before and after annealing. These studies found the undoped crystal conformed to P63/m, but with the O(3) oxygen positions, that participate in conduction, split nonstatistically across two sites with a shortened Si–O(3) bond. Consequently, the bond valence sum (BVS) of the Si (4.20) is larger than the formal valence. Fourier difference maps of the Al-doped crystals contain regions of excess scattering, suggesting the possible lowering of symmetry or creation of superstructures. After long-term annealing, the single crystal structure determinations were of higher quality and the experimental and nominal compositions were in better agreement. From these observations, we propose that in the as-prepared single crystals there are regions of high and low interstitial content (e.g., Nd9.67Si6O26.5 and Nd9.33Si6O26), and the second semicircle relates to the interface between such regions. On annealing, Nd redistribution and homogenization removes these interfaces and also reduces the number of interstitial oxide ions, hence eliminating this second semicircle while reducing the bulk conductivity. The results therefore show for the first time that the conductivity of apatite materials containing cation vacancies is affected by the thermal history. ASTAR (Agency for Sci., Tech. and Research, S’pore) MOE (Min. of Education, S’pore) Accepted version

The hybrid organic–inorganic perovskite (CH3NH3)PbI3 may find application in next generation solid-state sensitised solar cells. Although this material and related perovskites were discovered many decades ago, questions remain concerning their diverse structural chemistry and unusual properties. The article presents a review of previous work and provides a detailed description of the preparation, structural characterisation and physical characteristics of (CH3NH3)PbI3. The phase changes exhibited by (CH3NH3)PbI3 have been probed using variable temperature powder and single crystal X-ray diffraction, combined with differential scanning calorimetry, thermogravimetric analysis and phase contrast transmission electron microscopy. The optical band gap for (CH3NH3)PbI3 determined by UV-Visible spectroscopy was compared to values obtained from density-of-state simulation of the electronic band structure. Accepted version

We report the pressure-induced crystallographic transitions and optical behavior of MAPbI3 (MA=methylammonium) using in situ synchrotron X-ray diffraction and laser-excited photoluminescence spectroscopy, supported by density functional theory (DFT) calculations using the hybrid functional B3PW91 with spin-orbit coupling. The tetragonal polymorph determined at ambient pressure transforms to a ReO3-type cubic phase at 0.3 GPa. Upon continuous compression to 2.7 GPa this cubic polymorph converts into a putative orthorhombic structure. Beyond 4.7 GPa it separates into crystalline and amorphous fractions. During decompression, this phase-mixed material undergoes distinct restoration pathways depending on the peak pressure. In situ pressure photoluminescence investigation suggests a reduction in band gap with increasing pressure up to ≈0.3 GPa and then an increase in band gap up to a pressure of 2.7 GPa, in excellent agreement with our DFT calculation prediction. This work lays the foundation for understanding the pressure-dependent phase transition of MAPbI3 and potentially enriches the toolkit for engineering perovskite polymorphs with exceptional optical properties. NRF (Natl Research Foundation, S’pore) Accepted version

Although being considered as one of the most promising cathode materials for Lithium-ion batteries (LIBs), LiNi1/3Co1/3Mn1/3O2 (NCM) is currently limited by its poor rate performance and cycle stability resulting from the thermodynamically favorable Li+/Ni2+ cation mixing which depresses the Li+ mobility. In this study, we developed a two-step method using fluffy MnO2 as template to prepare hierarchical porous nano-/microsphere NCM (PNM-NCM). Specifically, PNM-NCM microspheres achieves a high reversible specific capacity of 207.7 mAh g−1 at 0.1 C with excellent rate capability (163.6 and 148.9 mAh g−1 at 1 C and 2 C), and the reversible capacity retention can be well-maintained as high as 90.3% after 50 cycles. This excellent electrochemical performance is attributed to unique hierarchical porous nano-/microsphere structure which can increase the contact area with electrolyte, shorten Li+ diffusion path and thus improve the Li+ mobility. Moreover, as revealed by XRD Rietveld refinement analysis, a negligible cation mixing (1.9%) and high crystallinity with a well-formed layered structure also contribute to the enhanced C-rates performance and cycle stability. On the basis of our study, an effective strategy can be established to reveal the fundamental relationship between the structure/chemistry of these materials and their properties.

et al., Rare earth silicate apatites are one-dimensional channel structures that show potential as electrolytes for solid oxide fuel cells (SOFC) due to their high ionic conductivity at intermediate temperatures (500-700 °C). This advantageous property can be attributed to the presence of both interstitial oxygen and cation vacancies, that create diffusion paths which computational studies suggest are less tortuous and have lower activation energies for migration than in stoichiometric compounds. In this work, neutron diffraction of NdAlSiO (0 ≤ x ≤ 1.5) single crystals identified the locations of oxygen interstitials, and allowed the deduction of a dual-path conduction mechanism that is a natural extension of the single-path sinusoidal channel trajectory arrived at through computation. This discovery provides the most thorough understanding of the O transport mechanism along the channels to date, clarifies the mode of interchannel motion, and presents a complete picture of O percolation through apatite. Previously reported crystallographic and conductivity measurements are re-examined in the light of these new findings., We are pleased to acknowledge the Agency for Science, Technology and Research (A*STAR) PSF grant 082 101 0021 “Optimization of Oxygen Sublattices in Solid Oxide Fuel Cell Apatite Electrolytes” for funding the work and the Ministry of Education (MOE) Tier 2 grant T208B1212 for enabling the purchase of a single crystal X-ray diffractometer.

Improved photoelectrochemical activity for tetragonal BiV 0.8 Mo 0.2 O 4 fabricated by hydrothermal synthesis is reported in the present study. The enhanced water oxidation efficiency is attributed to the formation of cation vacancies (Bi 5+ )/oxygen interstitials due to high amount of Mo doping. Efficient charge transport and electron hole separation for water oxidation using BiV 0.8 Mo 0.2 O 4 photoanode was supported by electrochemical impedance investigations and open circuit photovoltage measurements. The present study gives a significant insight into the role of nonstoichiometry related efficient water oxidation using tetragonal BiVO 4 .

A gem-grade apatite from Brazil of general composition (Ca,Na) 10 [(P,Si,S)O 4 ] 6 (F,Cl,OH) 2 has been studied using single-crystal X-ray and neutron diffraction together with synchrotron powder X-ray diffraction. Earlier electron microscopy studies had shown the nominally single-phase apatite contains an abundant fluorapatite (F - Ap) host, together with chloro-hydroxylapatites (Cl/OH - Ap) guest phases that encapsulate hydroxylellestadite (OH - El) nanocrystals. While the latter features appear as small (200–400 nm) chemically distinct regions by transmission electron microscopy, and can be identified as separate phases by synchrotron powder X-ray diffraction, these could not be detected by single-crystal X-ray and neutron analysis. The observations using neutron, X-ray and electron probes are however consistent and complementary. After refinement in the space group P 6 3 / m the tunnel anions F − are fixed at z = ¼ along , while the anions Cl − and OH − are disordered, with the suggestion that O-H···O-H··· hydrogen-bonded chains form in localized regions, such that no net poling results. The major cations are located in the 4 f A F O 6 metaprism (Ca+Na), 6 h A T O 6 X tunnel site (Ca only), and 6 h B O 4 tetrahedron (P+Si+S). The structural intricacy of this gem stone provides further evidence that apatite microstructures display a nano-phase separation that is generally unrecognized, with the implication that such complexity may impact upon the functionality of technological analogues.

The crystal chemistry and oxygen conductivity of vanadium-doped apatites of nominal composition [La8AE2][Ge6−xVx]O26+x/2 (AE = Ca, Sr, Ba; 0 ≤ x ≤ 1.5) were studied to establish their potential as intermediate temperature solid oxide fuel cell (SOFC) electrolytes. Single-phase products obtained for x ≤ 0.5 were found, using a combination of powder synchrotron X-ray and neutron diffraction, to be P63/m apatites. The ionic conductivities extracted by complex impedance spectroscopy showed that small vanadium amendments enhanced oxygen mobility at intermediate temperatures (500–700 °C) by more than one order of magnitude, as the incorporation of V5+ through displacement of Ge4+ is charge balanced with interstitial O2− that improves ionic conduction. The most promising composition was La7.88Ca2Ge5.35V0.65O26.15 that delivered σ = 3.44 × 10−4 S cm−1 at 500 °C. The superstoichiometric oxygen was delocalised, without fixed X-ray or neutron scattering centres. Crystal chemistry systematics demonstrate that the Ca-apatite was superior because the relatively small framework expanded the tunnel through reduced (La/AE)O6 metaprism twisting (φ), ensuring the P63/m symmetry was adopted, which favours the passage of O2− with lower activation energy. Accepted version

[ A I ] 4 [ A II ] 6 ( B O 4 ) 6 X 2 apatites can flexibly accommodate numerous cationic, metalloid and anionic substitutions. Using a combination of new refinements and published structures, this paper reviews correlations between substituent type and framework adaptation through adjustment of the A I O 6 metaprism twist angle, φ . These systematics are illustrated through powder neutron diffraction refinement of the crystal chemistry of A 10 (PO 4 ) 6 F 2 ( A =Ca, Sr) fluorapatites. Variations in φ reflect changes in the relative size of the A I 4 ( B O 4 ) 6 framework and A II 6 X 2 tunnel content and can be used to quantitatively assess the reliability of A I / A II cation partitioning coefficients determined by Rietveld analysis. In the simplest cases of bi-ionic substitution, the metaprism twist systematics conform to three principle trends (i) For A -type divalent substitution, the larger A 2 + species preferentially enters the channel before partitioning to the framework. This leads to parabolic modification in φ across the compositional series; (ii) For B -type pentavalent compounds, the φ variation will be linear in accord with the relative B 5+ ionic size; and (iii) For X -type substitution of halide anions, φ will be reduced as the average size increases. Departures from these trends may indicate polymorphism, compositional anomalies, A I /A II order disequilibrium, or poor structure refinement, and may be extended to chemically complex apatites with simultaneous substitutions over the A , B and X sites.

Lanthanoid silicates (Ln9.33Si6O26) adopt the hexagonal apatite structure and show potential as solid oxide fuel cell (SOFC) electrolytes due to their high oxide ion conductivity at intermediate temperatures (500–700 °C). Ions migrate preferentially along the crystallographic c-axis, and can be improved by introducing lower valent elements at the silicon sites. To better understand this phenomenon, single crystals of aluminium-doped neodymium silicate Nd9.33+x/3AlxSi6−xO26 (0≤x≤1) were synthesised using the floating-zone method in an inert environment at a growth rate of 5 mm/L. The products with x

Calcium (La10 − xCaxGe6O27 − x/2, 0 ≤ x ≤ 2.57) and barium (La10 − xBaxGe6O27 − x/2, 0 ≤ nominal x ≤ 3) doped lanthanum germanate apatites were synthesized by high temperature solid-state sintering. The lattice parameters obtained from Rietveld refinement of powder X-ray diffraction data showed that pseudomorphic transformations from 2A to 2H occurred as the alkaline earth was introduced. The likely appearance of an intervening 2M pseudomorph was not confirmed directly in this study; however, compositionally dependent inflections in cell constants and abrupt changes in oxygen mobility as a function of temperature support its existence, particularly for low alkaline earth contents. Oxygen excess apatites La9AEGe6O26.5 (AE = Ca, Sr, Ba) prove most promising as solid oxide fuel cell (SOFC) electrolytes at intermediate temperatures, by delivering the optimal compromise between higher mobile oxygen concentration and lowering of activation energy. In contrast, stoichiometric La8AE2Ge6O26 showed inferior conductivity, because although the activation energy was reduced, the proportion of extrastoichiometric oxygen was negligible. Atomistic modeling suggests that interstitial oxygen is located near (0, 1/2, 1/2) and causes the O3 position to be split statistically into the (0.050, 0.278, 0.587) and (0.084, 0.335, 0.565) sites. These co-existing locations may facilitate inter-tunnel oxygen transport.

Certain complex structures are logically regarded as intergrowths of chemically or topologically discrete modules. When the proportions of these components vary systematically a polysomatic series is created, whose construction provides a basis for understanding defects, symmetry alternation and trends in physical properties. Here, we describe the polysomatic familyA5NB3NO9N + 6XNδ(2 ≤N≤ ∞) that is built by condensingNapatite modules (A5B3O18Xδ) in configurations to createBnO3n + 1(1 ≤n≤ ∞) tetrahedral chains. Hydroxyapatite [Ca10(PO4)6(OH)2] typifies a widely studied polysome whereN= 2 and the tetrahedra are isolated inA10(BO4)6X2compounds, butN= 3A15(B2O7)3(BO4)3X3(ganomalite) andN= 4A20(B2O7)6X4(nasonite) are also known, with theXsite untenanted or partially occupied as required for charge balance. The apatite modules, while topologically identical, are often compositionally or symmetrically distinct, and an infinite number of polysomes is feasible, generally with the restriction being that anA:B= 5:3 cation ratio be maintained. The end-members are theN= 2 polysome with all tetrahedra separated, andN= ∞, in which the hypothetical compoundA5B3O9Xcontains infinite, corner-connected tetrahedral strings. The principal characteristics of a polysome are summarized using the nomenclatureapatite-(A B X)-NS, whereA/B/Xare the most abundant species in these sites,Nis the number of modules in the crystallographic repeat, andSis the symmetry symbol (usuallyH,T,MorA). This article examines the state-of-the-art in polysomatic apatite synthesis and crystallochemical design. It also presents X-ray and neutron powder diffraction investigations for several polysome chemical series and examines the prevalence of stacking disorder by electron microscopy. These insights into the structure-building principles of apatite polysomes will guide their development as functional materials.

The crystal chemistries of synthetic mimetite, Pb10(As5+O4)6(Cl2 − x O x/2), a neutral apatite, and finnemanite, Pb10(As3+O3)6Cl2, a reduced apatite, were characterized using a combination of X-ray powder diffraction, neutron diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy. Both phases conform to hexagonal P63/m symmetry; however, the temperature-driven transformation of clinomimetite to mimetite described earlier was not confirmed. The average mimetite structure is best described through the introduction of partially occupied oxygen sites. A better understanding of the mixed arsenic speciation in apatites can guide the formulation of waste form ceramics and improve models of long-term durability after landfill disposal.

Extraction of reliable bond distances and angles for Ca10(V x P1 − x O4)6F2 apatites using standard Rietveld refinement with Cu Kα X-ray powder data was significantly impaired by large imprecision for the O-atom coordinates. An initial attempt to apply crystal-chemical Rietveld refinements to the same compounds was partly successful, and exposed the problematic determination of two oxygen–metal–oxygen angles. Ab initio modeling with VASP in space groups P63/m, P21/m and Pm showed that both these angular parameters exhibited a linear dependence with the vanadium content. Stable crystal-chemical Rietveld refinements in agreement with quantum results were obtained by fixing these angles at the values from ab initio simulations. Residuals were comparable with the less precise standard refinements. The larger vanadium ion is accommodated primarily by uniform expansion and rotation of BO4 tetrahedra combined with a rotation of the Ca–Ca–Ca triangular units. It is proposed that the reduction of symmetry for the vanadium end-member is necessary to avoid considerable departures from formal valences at the A II and B sites in P63/m. The complementarity of quantum methods and structural analysis by powder diffraction in cases with problematic least-squares extraction of the crystal chemistry is discussed.

Lanthanoid silicate apatite solid electrolytes contain one-dimensional channels. These materials display substantial oxygen mobility at temperatures lower than conventional zirconia-based ionic conductors because interstitial oxygen displacements, mediated by Ln cation vacancies, have a lower activation energy. For these nonstoichiometric apatites, crystal structure solutions derived from X-ray and neutron powder diffraction yield the average atomic arrangement, but these techniques also average over local lattice disorders. Large apatite single crystals permit the evaluation of oxygen migration anisotropy using impedance spectroscopy and the correlation of this behavior to atomic scale domain formation or defect cluster aggregation if present. Aberration-corrected scanning transmission electron microscopy, in both high angle annular dark field (HAADF) and bright field (BF) modalities, was applied to characterize the local atomic structure of Nd9.33Si6O26 and Nd8Sr2Si6O26 apatite electrolytes. Quantitative image analysis found the distribution of metal vacancies and dopant metal in apatites to be remarkably homogeneous at the unit cell scale. This is distinct from other oxide electrolytes including fluorites, perovskites, and melilites, where domain and superstructure formation are a consequence of interstitial oxygen incorporation and prescribe the mode of ionic transport. In the present case, the unexpectedly high perfection of silicate apatites arises from the flexible topological response of one-dimensional channels penetrating the structure, which, in turn, allows robust chemical tailoring of these electrolytes. ASTAR (Agency for Sci., Tech. and Research, S’pore) MOE (Min. of Education, S’pore) Accepted version

Computational screening based on density-functional-theory calculations reveals Ge as a candidate element for replacing Pb in halide perovskite compounds suitable for light harvesting. Experimentally, three AGeI3 (A = Cs, CH3NH3 or HC(NH2)2) halide perovskite materials have been synthesized. These compounds are stable up to 150 °C, and have bandgaps correlated with the A-site cation size. CsGeI3-based solar cells display higher photocurrents, of about 6 mA cm−2, but are limited by poor film forming abilities and oxidising tendencies. The present results demonstrate the utility of combining computational screening and experimental efforts to develop lead-free halide perovskite compounds for photovoltaic applications. NRF (Natl Research Foundation, S’pore) Accepted version

A single-crystal structure determination of Nd8Sr2Si6O26 apatite, a prototype intermediate-temperature electrolyte for solid oxide fuel cells grown by the floating-zone method, was completed using the combination of Laue neutron diffraction and Raman spectroscopy. While neutron diffraction was in good agreement with P6₃/m symmetry, the possibility of P6₃ could not be convincingly excluded. This ambiguity was removed by the collection of orientation-dependent Raman spectra that could only be consistent with P6₃/m. The composition of Nd8Sr2Si6O26 was independently verified by powder X-ray diffraction in combination with electron probe microanalysis, with the latter confirming a homogeneous distribution of Sr and the absence of chemical zonation commonly observed in apatites. This comprehensive crystallochemical description of Nd8Sr2Si6O26 provides a baseline to quantify the efficacy of cation vacancies, oxygen superstoichiometry, and symmetry modification for promoting oxygen-ion mobility.

Mixed Si/Ge-based single phase apatites ([AI4][AII6] [(BO4)6]O2) of the formula La9.33□0.67Si6-xGex O26 (□: vacancy; 0 ≤ x ≤ 6) are prepared by mild hydrothermal synthesis from La2O3/(SiO2 + GeO2)/Na2O/H2O gels (autoclave, 180—240 °C).

Single-phase Cu2ZnSnS4 (CZTS) is an essential prerequisite toward a high-efficiency thin-film solar cell device. Herein, the selective phase formation of single-phase CZTS nanoparticles by ligand control is reported. Surface-enhanced Raman scattering (SERS) spectroscopy is demonstrated for the first time as a characterization tool for nanoparticles to differentiate the mixed compositional phase (e.g., CZTS, CTS, and ZnS), which cannot be distinguished by X-ray diffraction. Due to the superior selectivity and sensitivity of SERS, the growth mechanism of CZTS nanoparticle formation by hot injection is revealed to involve three growth steps. First, it starts with nucleation of Cu(2-x)S nanoparticles, followed by diffusion of Sn(4+) into Cu(2-x)S nanoparticles to form the Cu3SnS4 (CTS) phase and diffusion of Zn(2+) into CTS nanoparticles to form the CZTS phase. In addition, it is revealed that single-phase CZTS nanoparticles can be obtained via balancing the rate of CTS phase formation and diffusion of Zn(2+) into the CTS phase. We demonstrate that this balance can be achieved by 1 mL of thiol with Cu(OAc)2, Sn(OAc)4, and Zn(acac)2 metal salts to synthesize the CZTS phase without the presence of a detectable binary/ternary phase with SERS.

Single-phase Cu2ZnSnS4 (CZTS) is an essential prerequisite toward a high-efficiency thin-film solar cell device. Herein, the selective phase formation of single-phase CZTS nanoparticles by ligand control is reported. Surface-enhanced Raman scattering (SERS) spectroscopy is demonstrated for the first time as a characterization tool for nanoparticles to differentiate the mixed compositional phase (e.g., CZTS, CTS, and ZnS), which cannot be distinguished by X-ray diffraction. Due to the superior selectivity and sensitivity of SERS, the growth mechanism of CZTS nanoparticle formation by hot injection is revealed to involve three growth steps. First, it starts with nucleation of Cu2–xS nanoparticles, followed by diffusion of Sn4+ into Cu2–xS nanoparticles to form the Cu3SnS4 (CTS) phase and diffusion of Zn2+ into CTS nanoparticles to form the CZTS phase. In addition, it is revealed that single-phase CZTS nanoparticles can be obtained via balancing the rate of CTS phase formation and diffusion of Zn2+ into the CTS phase. We demonstrate that this balance can be achieved by 1 mL of thiol with Cu(OAc)2, Sn(OAc)4, and Zn(acac)2 metal salts to synthesize the CZTS phase without the presence of a detectable binary/ternary phase with SERS. NRF (Natl Research Foundation, S’pore) Accepted version

et al., A single-crystal structure determination of Nd8Sr 2Si6O26 apatite, a prototype intermediate-temperature electrolyte for solid oxide fuel cells grown by the floating-zone method, was completed using the combination of Laue neutron diffraction and Raman spectroscopy. While neutron diffraction was in good agreement with P63/m symmetry, the possibility of P63 could not be convincingly excluded. This ambiguity was removed by the collection of orientation-dependent Raman spectra that could only be consistent with P63/m. The composition of Nd8Sr2Si 6O26 was independently verified by powder X-ray diffraction in combination with electron probe microanalysis, with the latter confirming a homogeneous distribution of Sr and the absence of chemical zonation commonly observed in apatites. This comprehensive crystallochemical description of Nd8Sr2Si6O26 provides a baseline to quantify the efficacy of cation vacancies, oxygen superstoichiometry, and symmetry modification for promoting oxygen-ion mobility. © 2014 American Chemical Society., This work was supported by A*STAR (Agency for Science, Technology and Research) SERC Grant 082 101 0021 (“Optimization of Apatite Anion Sublattices in Solid Oxide Fuel Cell Electrolytes”) and by Spanish Government Grant MAT2010-19837-C06-06. A.O. acknowledges financial support provided by the Spanish Ministerio de Ciencia e Innovación through a Juan de la Cierva contract.

Crystals of CeNbO4 are prepared in a floating zone mirror furnace under air or Ar atmospheres using stoichiometric amounts of CeO2 and NbO2 (calcined at 1000 °C, 12 h).