Novel Materials for Urban Farming.

Scarcity of natural resources, shifting demographics, climate change, and increasing waste are four major challenges in the quest to feed the exploding world population. These challenges serve as the impetus to harness novel technologies to improve agriculture, productivity, and sustainability. Urban farming has several advantages over conventional farming: higher productivity, improved sustainability, and the ability to provide fresh food all year round. Novel materials are key to accelerating the evolution of urban farming - with their ability to facilitate controlled release of nutrients and pesticides, improved seed health, substrates with better water retention capability, more efficient recycling of agricultural waste, and precise plant health monitoring. Materials science enables environmental sustainability and higher harvest yields in urban farms. Here, Singapore is used as an example of a land-scarce city where urban farming may be the solution for future food production. Potential research directions and challenges in urban farming are highlighted, and how material optimization and innovation drive the development of urban farming to meet national and global food demands is briefly discussed. This review serves as a guide for researchers and a reference for stakeholders of urban farms, policy makers, and other interested parties.

Pressure-Responsive Two-Dimensional Metal-Organic Framework Composite Membranes for CO2 Separation.

The regulation of permeance and selectivity in membrane systems may allow effective relief of conventional energy-intensive separations. Here, pressure-responsive ultrathin membranes (≈100 nm) fabricated by compositing flexible two-dimensional metal-organic framework nanosheets (MONs) with graphene oxide nanosheets for CO2 separation are reported. By controlling the gas permeation direction to leverage the pressure-responsive phase transition of the MONs, CO2 -induced gate opening and closing behaviors are observed in the resultant membranes, which are accompanied with the sharp increase of CO2 permeance (from 173.8 to 1144 gas permeation units) as well as CO2 /N2 and CO2 /CH4 selectivities (from 4.1 to 22.8 and from 4 to 19.6, respectively). The flexible behaviors and separation mechanism are further elucidated by molecular dynamics simulations. This work establishes the relevance of structural transformation-based framework dynamics chemistry in smart membrane systems.

Undesired Bulk Oxidation of LiMn2 O4 Increases Overpotential of Electrocatalytic Water Oxidation in Lithium Hydroxide Electrolytes.

Chemical and structural changes preceding electrocatalysis obfuscate the nature of the active state of electrocatalysts for the oxygen evolution reaction (OER), which calls for model systems to gain systematic insight. We investigated the effect of bulk oxidation on the overpotential of ink-casted LiMn2 O4 electrodes by a rotating ring-disk electrode (RRDE) setup and X-ray absorption spectroscopy (XAS) at the K shell core level of manganese ions (Mn-K edge). The cyclic voltammogram of the RRDE disk shows pronounced redox peaks in lithium hydroxide electrolytes with pH between 12 and 13.5, which we assign to bulk manganese redox based on XAS. The onset of the OER is pH-dependent on the scale of the reversible hydrogen electrode (RHE) with a Nernst slope of -40(4) mV/pH at -5 µA monitored at the RRDE ring. To connect this trend to catalyst changes, we develop a simple model for delithiation of LiMn2 O4 in LiOH electrolytes, which gives the same Nernst slope of delithiation as our experimental data, i. e., 116(25) mV/pH. From this data, we construct an ERHE -pH diagram that illustrates robustness of LiMn2 O4 against oxidation above pH 13.5 as also verified by XAS. We conclude that manganese oxidation is the origin of the increase of the OER overpotential at pH lower than 14 and also of the pH dependence on the RHE scale. Our work highlights that vulnerability to transition metal redox may lead to increased overpotentials, which is important for the design of stable electrocatalysts.

Uncovering The Role of Oxygen in Ni-Fe(OxHy) Electrocatalysts using In situ Soft X-ray Absorption Spectroscopy during the Oxygen Evolution Reaction.

In-situ X-ray absorption spectroscopy (XAS) at the oxygen K-edge was used to investigate the role of oxygen during the oxygen evolution reaction (OER) in an electrodeposited Ni-Fe(OxHy) electrocatalyst in alkaline pH. We show the rise of a pre-peak feature at 529 eV in the O K-edge spectra, correlated to the appearance of a shoulder at the Ni L3-edge and formation of oxidized Ni3+/4+-O. Then, for the first time, we track the spectral changes in a dynamic fashion in both the soft and hard X-ray regimes during cyclic voltammetry (in situ CV-XAS) to obtain a fine-tuned resolution of the potential-related changes. The pre-peak feature at the O K-edge likely signifies formation of an electron deficient oxygen site. The electrophilic oxygen species appears and disappears reversibly in correlation with the Ni2+ ↔ Ni3+/4+ process, and persists during OER catalysis as long the metal is oxidized. Our study provides new insight into OER electrocatalysis: Before onset of the O-O bond formation step, the catalytic oxyhydroxide has accumulated electron deficiencies by both, oxidation of transition metal ions and formation of partially oxidized oxygen sites.

Single-Atom Au/NiFe Layered Double Hydroxide Electrocatalyst: Probing the Origin of Activity for Oxygen Evolution Reaction.

A fundamental understanding of the origin of oxygen evolution reaction (OER) activity of transition-metal-based electrocatalysts, especially for single precious metal atoms supported on layered double hydroxides (LDHs), is highly required for the design of efficient electrocatalysts toward further energy conversion technologies. Here, we aim toward single-atom Au supported on NiFe LDH (sAu/NiFe LDH) to clarify the activity origin of LDHs system and a 6-fold OER activity enhancement by 0.4 wt % sAu decoration. Combining with theoretical calculations, the active behavior of NiFe LDH results from the in situ generated NiFe oxyhydroxide from LDH during the OER process. With the presence of sAu, sAu/NiFe LDH possesses an overpotential of 0.21 V in contrast to the calculated result (0.18 V). We ascribe the excellent OER activity of sAu/NiFe LDH to the charge redistribution of active Fe as well as its surrounding atoms causing by the neighboring sAu on NiFe oxyhydroxide stabilized by interfacial CO32- and H2O interfacing with LDH.

In situ XAS study of CoBi modified hematite photoanodes.

Solar water splitting is a potentially scalable method to store solar energy in the form of renewable hydrogen gas. In this study, we demonstrate that the photoelectrochemical (PEC) performance of hematite photoanodes can be improved by modification with the oxygen evolution catalyst CoBi. The current density at 1.23 V of the pristine hematite under one sun is 0.88 mA cm-2 and it increases to 1.12 mA cm-2 after CoBi modification (∼27% improvement). The presence of a CoBi cocatalayst layer is proposed to improve the oxygen evolution reaction (OER) kinetics and also to prevent electron-hole recombination at the surface via passivating surface defects as well as suppressing the tunneling of electrons from the hematite core, thus improving the photocurrents and resulting in a negative shift of photocurrent onset potentials. These effects of CoBi modification are supported by experimental data obtained by performing electrochemical impedance spectroscopy (EIS), PEC and incident photon-to-current efficiency (IPCE) measurements. To investigate the electronic structure of the CoBi cocatalyst deposited on hematite, XPS and in situ X-ray absorption spectroscopy (XAS) are employed. Co K-edge spectra at different potentials and light conditions are recorded. This makes the present work different from most of the previous studies. Using a quantitative analysis method, information on the mean oxidation state of Co in the CoBi film under applied potential and illumination is revealed. We also compare different methods for determining the oxidation state from the edge position and find that the integral method and half height methods are most suitable. In summary, the present work underlines the improvement of the semiconductor/cocatalyst interface of oxygen evolving photoanodes and strengthens the importance of in situ XAS spectroscopy when studying catalysts. This study is the first report so far combining the studies of the PEC performance of a CoBi modified hematite nanorod array photoanode and in situ XAS at the Co K-edge.

One-Pot Synthesis of Nickel-Modified Carbon Nitride Layers Toward Efficient Photoelectrochemical Cells.

A new method to significantly enhance the photoelectrochemical properties of phenyl-modified carbon nitride layers via the insertion of nickel ions into carbon nitride layers is reported. The nickel ions are embedded within the carbon nitride layers by manipulating the interaction of Ni ions and molten organic molecules at elevated temperature prior to their condensation. A detailed analysis of the chemical and photophysical properties suggests that the nickel ions dissolve in the molten molecules, leading to the homogeneous distribution of nickel atoms within the carbon nitride layers. We found that the nickel atoms can alter the growth mechanism of carbon nitride layers, resulting in extended light absorption, charge transfer properties, and the total photoelectrochemical performance. For the most photoactive electrode, the Ni ions have an oxidation state of 2.8, as confirmed by soft X-ray absorption spectroscopy. Furthermore, important parameters such as absorption coefficient, exciton lifetime, and diffusion length were studied in depth, providing substantial progress in our understanding of the photoelectrochemical properties of carbon nitride films. This work opens new opportunities for the growth of carbon nitride layers and similar materials on different surfaces and provides important progress in our understanding of the photophysical and photoelectrochemical properties of carbon nitride layers toward their implantation in photoelectronic and other devices.

C=C π Bond Modified Graphitic Carbon Nitride Films for Enhanced Photoelectrochemical Cell Performance.

Applications of graphitic carbon nitride (g-CN) in photoelectrochemical and optoelectronic devices are still hindered due to the difficulties in synthesis of g-CN films with tunable chemical, physical and catalytic properties. Herein we present a general method to alter the electronic and photoelectrochemical properties of g-CN films by annealing. We found that N atoms can be removed from the g-CN networks after annealing treatment. Assisted by theoretical calculations, we confirm that upon appropriate N removal, the adjacent C atoms will form new C=C π bonds. Detailed calculations demonstrate that the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are located at the structure unit with C=C π bonds and the electrons are more delocalized. Valence band X-ray photoelectron spectroscopy spectra together with optical absorption spectra unveil that the structure changes result in the alteration of the g-CN energy levels and position of band edges. Our results show that the photocurrent density of the annealed g-CN film is doubled compared with the pristine one, thanks to the better charge separation and transport within the film induced by the new C=C π bonds. An ultrathin TiO2 film (2.2 nm) is further deposited on the g-CN film as stabilizer and the photocurrent density is kept at 0.05 mA cm-2 at 1.23 V versus reversible hydrogen electrode after two-cycle stability assessment. This work enables the applications of g-CN films in many electronic and optoelectronic devices.

Correlating Oxygen Evolution Catalysts Activity and Electronic Structure by a High-Throughput Investigation of Ni1-y-zFeyCrzOx.

High-throughput characterization by soft X-ray absorption spectroscopy (XAS) and electrochemical characterization is used to establish a correlation between electronic structure and catalytic activity of oxygen evolution reaction (OER) catalysts. As a model system a quasi-ternary materials library of Ni1-y-zFeyCrzOx was synthesized by combinatorial reactive magnetron sputtering, characterized by XAS, and an automated scanning droplet cell. The presence of Cr was found to increase the OER activity in the investigated compositional range. The electronic structure of NiII and CrIII remains unchanged over the investigated composition spread. At the Fe L-edge a linear combination of two spectra was observed. These spectra were assigned to FeIII in Oh symmetry and FeIII in Td symmetry. The ratio of FeIII Oh to FeIII Td increases with the amount of Cr and a correlation between the presence of the FeIII Oh and a high OER activity is found.

A soft XAS transmission cell for operando studies.

A new cell for operando soft X-ray absorption spectroscopy in transmission mode is presented. Developed for investigations on solar water-splitting catalysts, the cell allows the study of solid films in direct contact with electrolyte solution while applying voltage and visible light. The design is optimized for fast sample exchange and the simultaneous measurement of fluorescence and transmission signal. The capability of the cell is presented on a manganese oxide (MnOx) film, where electronic structure changes are monitored during forward and backward potential changes. Detailed information about the varying contributions of several Mn oxidation states during this process was revealed.

Effect of Zinc Incorporation on the Performance of Red Light Emitting InP Core Nanocrystals.

This report presents a systematic study on the effect of zinc (Zn) carboxylate precursor on the structural and optical properties of red light emitting InP nanocrystals (NCs). NC cores were assessed using X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), energy-dispersive X-ray spectroscopy (EDX), and high-resolution transmission electron microscopy (HRTEM). When moderate Zn:In ratios in the reaction pot were used, the incorporation of Zn in InP was insufficient to change the crystal structure or band gap of the NCs, but photoluminescence quantum yield (PLQY) increased dramatically compared with pure InP NCs. Zn was found to incorporate mostly in the phosphate layer on the NCs. PL, PLQY, and time-resolved PL (TRPL) show that Zn carboxylates added to the precursors during NC cores facilitate the synthesis of high-quality InP NCs by suppressing nonradiative and sub-band-gap recombination, and the effect is visible also after a ZnS shell is grown on the cores.

The role of ion exchange in the passivation of In(Zn)P nanocrystals with ZnS.

We have investigated the chemical state of In(Zn)P/ZnS core/shell nanocrystals (NCs) for color conversion applications using hard X-ray absorption spectroscopy (XAS) and photoluminescence excitation (PLE). Analyses of the edge energies as well as the X-ray absorption fine structure (XAFS) reveal that the Zn(2+) ions from ZnS remain in the shell while the S(2-) ions penetrate into the core at an early stage of the ZnS deposition. It is further demonstrated that for short growth times, the ZnS shell coverage on the core was incomplete, whereas the coverage improved gradually as the shell deposition time increased. Together with evidence from PLE spectra, where there is a strong indication of the presence of P vacancies, this suggests that the core-shell interface in the In(Zn)P/ZnS NCs are subject to substantial atomic exchanges and detailed models for the shell structure beyond simple layer coverage are needed. This substantial atomic exchange is very likely to be the reason for the improved photoluminescence behavior of the core-shell particles compare to In(Zn)P-only NCs as S can passivate the NCs surfaces.

Understanding the role of single molecular ZnS precursors in the synthesis of In(Zn)P/ZnS nanocrystals.

Environmentally friendly nanocrystals (NCs) such as InP are in demand for various applications, such as biomedical labeling, solar cells, sensors, and light-emitting diodes (LEDs). To fulfill their potential applications, the synthesis of such high-quality "green" InP NCs required further improvement so as to achieve better stability, higher brightness NCs, and also to have a more robust synthesis route. The present study addresses our efforts on the synthesis of high-quality In(Zn)P/ZnS core-shell NCs using an air- and moisture-stable ZnS single molecular precursor (SMP) and In(Zn)P cores. The SMP method has recently emerged as a promising route for the surface overcoating of NCs due to its simplicity, high reproducibility, low reaction temperature, and flexibility in controlling the reaction. The synthesis involved heating the In(Zn)P core solution and Zn(S2CNR2) (where R = methyl, ethyl, butyl, or benzyl and referred to as ZDMT, ZDET, ZDBT, or ZDBzT, respectively) in oleylamine (OLA) to 90-250 °C for 0.5-2.5 h. In this work, we systematically studied the influence of different SMP end groups, the complex formation and stability between the SMP and oleylamine (OLA), the reaction temperature, and the amount of SMP on the synthesis of high-quality In(Zn)P/ZnS NCs. We found that thiocarbamate end groups are an important factor contributing to the low-temperature growth of high-quality In(Zn)P/ZnS NCs, as the end groups affect the polarity of the molecules and result in a different steric arrangement. We found that use of SMP with bulky end groups (ZDBzT) results in nanocrystals with higher photoluminescence quantum yield (PL QY) and better dispersibility than those synthesized with SMPs with the shorter alkyl chain groups (ZDMT, ZDET, or ZDBT). At the optimal conditions, the PL QY of red emission In(Zn)P/ZnS NCs is 55 ± 4%, which is one of the highest values reported. On the basis of structural (XAS, XPS, XRD, TEM) and optical characterization, we propose a mechanism for the growth of a ZnS shell on an In(Zn)P core.

Phase-selective synthesis of Cu2ZnSnS4 nanocrystals using different sulfur precursors.

Cu2ZnSnS4 (CZTS) holds great promises as an absorber material for sustainable and low cost thin film solar cells. Kesterite and wurtzite are two common phases of CZTS. Until now, the synthesis and the growth of both phases are not clearly understood. In this work, kesterite CZTS nanoparticles, wurtzite CZTS nanoparticles as well as CZTS particles with a mixture of both structures were prepared by using elemental sulfur, 1-dodecanethiol, and thioacetamide, respectively. Time dependent studies were conducted and the reaction rate of sulfur source was found to be the key factor in determining the phase formation. Elemental sulfur reacts with oleylamine to produce highly reactive small molecule H2S, which leads to the formation of kesterite phase. The reaction pathways of the long alkane chain 1-dodecanethiol yield the formation of wurtzite phase via a binary phase. Thioacetamide yields a mixture of kesterite and wurtzite phase in the final product. The optical and electrical properties of kesterite and wurtzite CZTS were also evaluated.

Enhancing the photocatalytic efficiency of TiO2 nanopowders for H2 production by using non-noble transition metal co-catalysts.

Co and Ni-nanoclusters are attractive alternatives to Pt catalysts for hydrogen generation. These earth abundant elements when loaded onto the TiO(2) nanopowders surface act as efficient co-catalysts. Co, Ni-decorated TiO(2) photocatalysts display only three (3) times lower catalytic activities for H(2) evolution under UV illumination compared with Pt-decorated TiO(2) photocatalysts.

Surface treatment of hematite photoanodes with zinc acetate for water oxidation.

A simple and inexpensive method to form a hematite photoanode for efficient water oxidation is reported. A very thin ZnO overlayer was deposited on top of a thin film of hematite and found, compared with non-treated hematite, to increase the photocurrent and reduce the onset potential for generating oxygen from water. After 3 cycles of ZnAc treatment, the photocurrent increased more than 40% to 1.08 mA cm(-2) at 0.23 V vs. Ag/AgCl and onset potential for water oxidation shifted by -170 mV. It is proposed that the ZnO overlayer changes the flat band potential of hematite and reduces the surface defects.

Influences of graphene oxide support on the electrochemical performances of graphene oxide-MnO2 nanocomposites.

MnO2 supported on graphene oxide (GO) made from different graphite materials has been synthesized and further investigated as electrode materials for supercapacitors. The structure and morphology of MnO2-GO nanocomposites are characterized by X-ray diffraction, X-ray photoemission spectroscopy, scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, and Nitrogen adsorption-desorption. As demonstrated, the GO fabricated from commercial expanded graphite (denoted as GO(1)) possesses more functional groups and larger interplane gap compared to the GO from commercial graphite powder (denoted as GO(2)). The surface area and functionalities of GO have significant effects on the morphology and electrochemical activity of MnO2, which lead to the fact that the loading amount of MnO2 on GO(1) is much higher than that on GO(2). Elemental analysis performed via inductively coupled plasma optical emission spectroscopy confirmed higher amounts of MnO2 loading on GO(1). As the electrode of supercapacitor, MnO2-GO(1) nanocomposites show larger capacitance (307.7 F g-1) and better electrochemical activity than MnO2-GO(2) possibly due to the high loading, good uniformity, and homogeneous distribution of MnO2 on GO(1) support.

Stability studies of CdSe nanocrystals in an aqueous environment.

In this paper, CdSe nanocrystal dissolution in an aqueous solution was studied. It was found that light is a key factor affecting the dissolution of nanocrystals. In the presence of light, the electrons generated from CdSe nanocrystals reduce water to hydrogen and hydroxide ions (OH-) while photo-generated holes oxidize CdSe to Cd2+ and elemental Se. The dissolution was accelerated in an acidic medium while moderate alkalinity (pH=10.3) can slow down the dissolution possibly due to precipitation of nanocrystals. This study has strong implications for the use of these crystals in aqueous environments (bioimaging and dye-sensitized solar cells).

Understanding the effect of surface chemistry on charge generation and transport in poly (3-hexylthiophene)/CdSe hybrid solar cells.

For hybrid solar cells, interfacial chemistry is one of the most critical factors for good device performance. We have demonstrated that the size of the surface ligands and the dispersion of nanoparticles in the solvent and in the polymer are important criteria in obtaining optimized device performance. The size of the ligands will affect the charge transport at the particle/particle and particle/polymer interfaces and the chemical structures of the ligands will determine their compatibility with the solvent and polymer. Hence other than pyridine, 2-thiophenemethylamine also showed good potential as ligand replacement for poly(3-hexylthiophene)/CdSe hybrid solar cells. With the right ligand combination, we have shown that the power conversion efficiency improved by a factor of 6 after ligand exchange.

Packing behaviors of structurally different polyhedral oligomeric silsesquioxane-imidazolium surfactants in clay.

This paper reports the packing structures of two types of polyhedral oligomeric silsesquioxane (POSS)-imidazolium surfactants with different molecular rigidity in the intergalleries of montmorillonite clay. Wide-angle X-ray scattering (WAXS) and molecular modeling studies suggest that the POSS-imidazolium cations have a bilayer packing structure in clay with the long axes of the molecules largely tilted with respect to the basal plane. Direct evidence for the bilayer structure is provided by transmission electron microscopy (TEM). WAXS and differential scanning calorimetry (DSC) results indicate that the relatively flexible POSS-imidazolium cation is able to form a two-dimensionally ordered structure in the clay intergalleries, while the relatively rigid one exhibits a disordered structure in clay. Furthermore, with reducing surfactant loading, the clay modified with the rigid surfactant exhibits increased interlayer d-spacings probably because the surfactant is able to take more extended conformation at lower loadings. With its low organic content and disordered surfactant packing structure, the clay modified with the rigid surfactant also exhibits excellent thermal and thermo-oxidative stabilities.