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Wastewater was sourced directly from an industrial anaerobic biodigestion system to assess its potential to serve as a growth medium and improve the digestibility of microalgae in a closed-loop bioenergy system. Cyanobacteria (Oscillatoria sp.) were cultured in the wastewater and further explored as a feedstock for biomethane production via methanogenesis. Promisingly high levels of nitrate (1180 mg L−1) and phosphate (61.5 mg L−1) were found in the nitrification-treated, NO3-rich wastewater stream. While cyanobacteria were not observed to grow directly in this NO3-rich wastewater stream, a modest dilution of the wastewater by a factor of two produced denser cultures than could be grown in standard f/2 media, which was made by enriching seawater. Growth in wastewater to improve the digestibility of the cyanobacteria resulted in significantly more methane from cultures in both the NO3-rich and the untreated, NH4-rich wastewater streams.

AgBiS2 nanocrystals are good candidates in optoelectronic applications due to their solution-processability, earth abundance and non-toxic properties. AgBiS2 thin film solar cells have demonstrated promising performance due to their high absorption coefficients and suitable bandgaps for light harvesting. However, their physical properties such as mobility, carrier concentration and photosensitivity, which are very important for photovoltaics, remain unreported. Herein, we develop an improved amine-based synthesis route and systematically investigate these properties by characterizing field effect transistors and photodetectors fabricated from AgBiS2 nanocrystals. We found that the amine-based synthesis improved their measurable semiconducting properties and their solar cell performance. The optimal thickness of a champion solar cell was found to increase to 65 nm, which achieved a power conversion efficiency of 4.3% based on a ZnO/AgBiS2/P3HT/Au device structure, compared with 35 nm of a champion solar cell with a power conversion efficiency of 3.9% reported previously. This modified approach could be applied in the preparation of other high quality nanocrystals that uses silver, where single valence Ag cations are of increasing importance in stable solution processed nanomaterials.

Solution-processable lead selenide (PbSe) colloidal quantum dots (QDs) are promising candidates for photovoltaics due to their efficient multiple exciton generation and carrier transport. However, despite these advantages, currently the best PbSe QD solar cells (QDSCs) still have short-circuit current densities (JSC) of about 25 mA cm−2. Here, we report the introduction a solution-processed trivalent antimony-doped tin oxide buffer layer at the interfaces in the device, which led to significant improvement in the JSC. Consistent with the optical simulations, the external quantum efficiency of the devices was improved in a region corresponding to the PbSe QD/buffer layer interfaces (400–600 nm), implying enhanced electron extraction. The improved performance is attributed to optimized gradient energy level alignment and shunt blocking at the interfaces. With this simple interfacial treatment, the JSC of the champion device was increased significantly to 26.7 mA cm−2, a more than 8% improvement compared to the control device. A further increase in the fill factor was also observed, leading to an over 11% improvement in the champion power conversion efficiency, from 7.1% to 7.9%. This work offers a simple method for interfacial engineering that led to PbSe QDSCs with efficiencies that are among the highest reported in the literature.

A major factor limiting the performance of nanostructured CuInS2 photovoltaic devices is the current density, pointing to poor charge carrier transport in CuInS2 nanoparticle films. In a typical CuInS2 core/shell structure synthesis, ZnS is typically chosen as a shell material for CuInS2 core passivation, which leads to a significant enhancement of the photoluminescence quantum yield from the CuInS2 nanoparticles. While typically a marker for excellent photovoltaic performance, in this case the increased photoluminescence likely signals increased charge carrier confinement and reduced transport through any thin films fabricated from the nanoparticles. Here we show that replacing the typical divalent Zn cation surface termination with a monovalent Ag cation leads to small improvements in charge carrier transport through nanostructured films. This surface termination intentionally introduces lower energy electronic states directly at the surface of the CuInS2 nanoparticles, reducing charge carrier confinement and thus increasing charge carrier mobility between nanoparticles. The study assessed appropriate Ag molar ratios to be used in synthesis, with an 8% Ag : Cu ratio found to be optimal. The passivation offered by Ag surface termination appears comparable to that from Zn with strong photoluminescence observed in both cases. Slight improvements in the performance of all-solid-state nanoparticle CuInS2 photovoltaic devices are obtained, with current densities in the Ag surface terminated case being increased by just under 10%. These findings outline a potential strategy for the synthesis of type II core–shell CuInS2 quantum dot thin film devices with improved charge transport.

A narrow size distribution of quantum dots (QDs) is needed for their application in photovoltaics but collection of such information is difficult. This paper demonstrates the application of Raman spectroscopy as a characterisation tool to extract the size distribution and crystalline fraction of Si QD samples fabricated through the sputter-anneal method. Measured Raman spectra of Si QD materials are de-convoluted into four components according to their origins and Raman scattering by Si QD cores is described by a modified one phonon confinement model, while other components are reproduced with Gaussian functions. Through fitting of Raman spectra, Si QD size distributions and Si crystalline fractions are obtained. The results are compared with the values extracted from PL modelling on a series of B doped Si QD samples. The good consistency between the values extracted by these two methods confirms the validity of the Raman model. The result confirms that Si crystallization has been suppressed by B doping as the average Si QD size and Si crystalline fraction are reduced with increased B doping level.

PbS QDs are studied as attractive candidates to be applied as hot carrier solar cell absorbers. The thermalization properties of PbS QDs are investigated with power and temperature dependent continuous wave photoluminescence (CWPL). Non-equilibrium hot carrier populations are generated by high energy laser excitation, the thermalization coefficient Q is estimated from the incident power dependent carrier temperature. A non-equilibrium hot carrier population 200 K above the lattice temperature is detected at mild illumination intensity. A higher energy carrier population and an increasing Q value are observed with rising lattice temperatures. State filling effects are proposed as a possible cause of the generation of the non-equilibrium hot carrier population and an enhanced electron–phonon coupling strength is suggested to account for the faster carrier cooling rate observed in closely packed Langmuir–Blodgett monolayer films. A thermalization coefficient, Q, as low as 6.55 W K−1 cm−2 was found for the drop cast sample and suggests that PbS QDs are good candidates for practical hot carrier absorbers.

PbS quantum dots have been synthesized using a PbBr2 precursor and the halide content has been examined. Slower reaction kinetics for quantum dots growth relative to the use of PbCl2 was observed for PbBr2, giving a possible route to increased control over quantum dot size with in situ passivation. Unambiguous determination of the surface conditions of nanomaterials is still a developing area of science, pushing the limits of current microscopy and analytical techniques. Contributions to a rigorous form of nanomaterial surface analysis are made here using X-ray photoelectron spectroscopy to analyse bonding in detail. Atomic resolution TEM is applied to produce energy dispersive X-ray spectroscopy maps with state of the art resolution. This analysis has been applied to air-stable halide terminated PbS nanoparticles, which is a nanomaterial of central importance for quantum confined solar cell applications. Chemical analysis from X-ray photoelectron spectroscopy is consistent with Br surface termination and high resolution energy dispersive X-ray spectroscopy (EDS) maps also show a positive spatial correlation for Br with quantum dot location. An observed excess Br content is attributed to the presence of bromine terminated PbS quantum dots nuclei in the final colloid.

Hematite (α-Fe2O3) is a very promising material for solar water splitting that requires a high anodic potential to initiate the oxygen evolution reaction (OER). In this work, we explore the correlation between the downshift in flat band potential of hematite, Vfb, and in onset potential of OER, Vonset, caused by prolonged annealing. We observed a cathodic shift (i.e., towards lower potentials) of 200 mV of Vonset on model photoanodes consisting of ultra-thin hematite films, upon increasing the oxidation time during fabrication and without any further modifications. Detailed physical characterization, electrochemical impedance spectroscopy, and Mott-Schottky analysis revealed a quantitative correlation between the cathodic shift of Vonset and a lowering of Vfb. We identified a reduction in concentration of grain boundaries with increasing oxidation time, as the mechanism behind the observed shift of the Vfb. The approach presented here can be seen as a complementary strategy to co-catalysts and other post-fabrication treatments to lower Vonset. Moreover, it is generically applicable to photoelectrodes used to carry out oxidation and reduction half-cell reactions.

Narrow size distribution of quantum dots (QDs) is needed for their application in photovoltaics but collection of such information is difficult. Many experiments have shown the photoluminescence (PL) spectrum of Si QDs will broaden and peak position is size dependent. However, there is still lack of quantitative analysis of such phenomenon. In this paper, a model is developed to fit the PL spectrum based on spontaneous emission and the size distribution of the QDs. With this model, we can quantitatively analyse the QD size and its distribution using the PL spectra only, saving the need of time consuming and destructive characterization methods such as transmission electron microscopy (TEM). The optical bandgap can be extracted naturally from this PL model. The size and distribution of the QD which are obtained by fitting the PL spectra are then confirmed by measurements using TEM and XRD.

Graphene oxide has been extensively investigated as an electron acceptor due to its exceptional electronic and optical properties. Here we report an unusual ultrafast electron transfer occurring in the nanocomposites of Au nanocluster (Au NC)–graphene oxide (GO) in which GO acts as an electron donor. An ultrafast electron transfer is corroborated from the excited states of graphene oxide into the highest occupied molecular orbital (HOMO) of Au NCs. It is found that the electron transfer rate is significantly higher in Au10–GO nanocomposites (4.17 × 1012 s−1) than that in Au25–GO (0.49 × 1012 s−1) due to a larger energy difference and smaller sized ligands. This finding suggests that graphene oxide–Au nanocluster nanocomposites can be very useful to construct novel nanostructures with enhanced visible light photovoltaic, photonic and photo-catalytic activities.

Carbon nanodots (CNDs) have emerged as fascinating materials with exceptional electronic and optical properties, and thus they offer many promising applications in photovoltaics and photocatalysis. In this paper we investigate electron transfer in nanocomposites of CNDs–graphene oxide (GO), –multi-walled carbon nanotubes (MWNTs) and –TiO2 nanoparticles without linker molecules, using steady state and time-resolved spectroscopy. Significant fluorescence quenching was observed in the CND–GO system, and it is attributed to the ultrafast electron transfer from CNDs to GO with a time constant of 400 fs. In comparison, carbon nanotubes result in static quenching of fluorescence in CNDs. No charge transfer was observed in both CND–MWNT and CND–TiO2 nanocomposites. This finding suggests that the CND–GO nanocomposite can be an excellent candidate for hot carrier solar cells due to the effective carrier extraction, broad spectral absorption, weak electron–phonon scattering, and thus a slow cooling rate for hot carriers.