Suppressed phase separation of mixed-halide perovskites confined in endotaxial matrices.

The functionality and performance of a semiconductor is determined by its bandgap. Alloying, as for instance in InxGa1-xN, has been a mainstream strategy for tuning the bandgap. Keeping the semiconductor alloys in the miscibility gap (being homogeneous), however, is non-trivial. This challenge is now being extended to halide perovskites - an emerging class of photovoltaic materials. While the bandgap can be conveniently tuned by mixing different halogen ions, as in CsPb(BrxI1-x)3, the so-called mixed-halide perovskites suffer from severe phase separation under illumination. Here, we discover that such phase separation can be highly suppressed by embedding nanocrystals of mixed-halide perovskites in an endotaxial matrix. The tuned bandgap remains remarkably stable under extremely intensive illumination. The agreement between the experiments and a nucleation model suggests that the size of the nanocrystals and the host-guest interfaces are critical for the photo-stability. The stabilized bandgap will be essential for the development of perovskite-based optoelectronics, such as tandem solar cells and full-color LEDs.

Highly Efficient Spectrally Stable Red Perovskite Light-Emitting Diodes.

Perovskite light-emitting diodes (LEDs) have recently attracted great research interest for their narrow emissions and solution processability. Remarkable progress has been achieved in green perovskite LEDs in recent years, but not blue or red ones. Here, highly efficient and spectrally stable red perovskite LEDs with quasi-2D perovskite/poly(ethylene oxide) (PEO) composite thin films as the light-emitting layer are reported. By controlling the molar ratios of organic salt (benzylammonium iodide) to inorganic salts (cesium iodide and lead iodide), luminescent quasi-2D perovskite thin films are obtained with tunable emission colors from red to deep red. The perovskite/polymer composite approach enables quasi-2D perovskite/PEO composite thin films to possess much higher photoluminescence quantum efficiencies and smoothness than their neat quasi-2D perovskite counterparts. Electrically driven LEDs with emissions peaked at 638, 664, 680, and 690 nm have been fabricated to exhibit high brightness and external quantum efficiencies (EQEs). For instance, the perovskite LED with an emission peaked at 680 nm exhibits a brightness of 1392 cd m-2 and an EQE of 6.23%. Moreover, exceptional electroluminescence spectral stability under continuous device operation has been achieved for these red perovskite LEDs.

Composite Perovskites of Cesium Lead Bromide for Optimized Photoluminescence.

The halide perovskite CsPbBr3 has shown its promise for green light-emitting diodes. The optimal conditions of photoluminescence and the underlying photophysics, however, remain controversial. To address the inconsistency seen in the previous reports and to offer high-quality luminescent materials that can be readily integrated into functional devices with layered architecture, we created thin films of CsPbBr3/Cs4PbBr6 composites based on a dual-source vapor-deposition method. With the capability of tuning the material composition in a broad range, CsPbBr3 is identified as the only light emitter in the composites. Interestingly, the presence of the photoluminescence-inactive Cs4PbBr6 can significantly enhance the light emitting efficiency of the composites. The unique negative thermal quenching observed near the liquid nitrogen temperature indicates that a type of shallow state generated at the CsPbBr3/Cs4PbBr6 interfaces is responsible for the enhancement of photoluminescence.

Dynamic Electronic Junctions in Organic-Inorganic Hybrid Perovskites.

Organic-inorganic hybrid perovskites have shown great potential as building blocks for low-cost optoelectronics for their exceptional optical and electrical properties. Despite the remarkable progress in device demonstration, fundamental understanding of the physical processes in halide perovskites remains limited, especially the unusual electronic behaviors such as the current-voltage hysteresis and the switchable photovoltaic effect. These phenomena are of particular interests for being closely related to device functionalities and performance. In this work, a microscopic picture of electric fields in halide perovskite thin films was obtained using scanning laser microscopy. Unlike conventional semiconductors, distribution of the built-in electric fields in the halide perovskite evolves dynamically under the stimulation of external biases. The observations can be well explained using a model based on field-assisted ion migration, indicating that the mechanism responsible for the evolving charge transport observed in this material is not purely electronic. The anomalous dynamic responses to the applied bias are found to be effectively suppressed by operating the devices at reduced temperature or processing the materials at elevated temperature, which provide potential strategies for designing and creating halide perovskites with more stable charge transport properties in the development of viable perovskite-based optoelectronics.

Morphology and Doping Engineering of Sn-Doped Hematite Nanowire Photoanodes.

High-temperature activation has been commonly used to boost the photoelectrochemical (PEC) performance of hematite nanowires for water oxidation, by inducing Sn diffusion from fluorine-doped tin oxide (FTO) substrate into hematite. Yet, hematite nanowires thermally annealed at high temperature suffer from two major drawbacks that negatively affect their performance. First, the structural deformation reduces light absorption capability of nanowire. Second, this "passive" doping method leads to nonuniform distribution of Sn dopant in nanowire and limits the Sn doping concentration. Both factors impair the electrochemical properties of hematite nanowire. Here we demonstrate a silica encapsulation method that is able to simultaneously retain the hematite nanowire morphology even after high-temperature calcination at 800 °C and improve the concentration and uniformity of dopant distribution along the nanowire growth axis. The capability of retaining nanowire morphology allows tuning the nanowire length for optimal light absorption. Uniform distribution of Sn doping enhances the donor density and charge transport of hematite nanowire. The morphology and doping engineered hematite nanowire photoanode decorated with a cobalt oxide-based oxygen evolution reaction (OER) catalyst achieves an outstanding photocurrent density of 2.2 mA cm-2 at 0.23 V vs Ag/AgCl. This work provides important insights on how the morphology and doping uniformity of hematite photoanodes affect their PEC performance.

Enhanced Optical and Electrical Properties of Polymer-Assisted All-Inorganic Perovskites for Light-Emitting Diodes.

Highly bright light-emitting diodes based on solution-processed all-inorganic perovskite thin film are demonstrated. The cesium lead bromide (CsPbBr3 ) created using a new poly(ethylene oxide)-additive spin-coating method exhibits photoluminescence quantum yield up to 60% and excellent uniformity of electrical current distribution. Using the smooth CsPbBr3 films as emitting layers, green perovskite-based light-emitting diodes (PeLEDs) exhibit electroluminescent brightness and efficiency above 53 000 cd m-2 and 4%: a new benchmark of device performance for all-inorganic PeLEDs.

Acid Treatment Enables Suppression of Electron-Hole Recombination in Hematite for Photoelectrochemical Water Splitting.

We report a strategy for efficient suppression of electron-hole recombination in hematite photoanodes. Acid-treated hematite showed a substantially enhanced photocurrent density compared to untreated samples. Electrochemical impedance spectroscopy studies revealed that the enhanced photocurrent is partly due to improved efficiency of charge separation. Transient absorption spectroscopic studies coupled to electrochemical measurements indicate that, in addition to improved bulk electrochemical properties, acid-treated hematite has significantly decreased surface electron-hole recombination losses owing to a greater yield of the trapped photoelectrons being extracted to the external circuit.

Bright Light-Emitting Diodes Based on Organometal Halide Perovskite Nanoplatelets.

Bright light-emitting diodes based on solution-processable organometal halide perovskite nanoplatelets are demonstrated. The nanoplatelets created using a facile one-pot synthesis exhibit narrow-band emissions at 529 nm and quantum yield up to 85%. Using these nanoparticles as emitters, efficient electroluminescence is achieved with a brightness of 10 590 cd m(-2) . These ligand-capped nanoplatelets appear to be quite stable in moisture, allowing out-of-glovebox device fabrication.

Fully Printed Halide Perovskite Light-Emitting Diodes with Silver Nanowire Electrodes.

Printed organometal halide perovskite light-emitting diodes (LEDs) are reported that have indium tin oxide (ITO) or carbon nanotubes (CNTs) as the transparent anode, a printed composite film consisting of methylammonium lead tribromide (Br-Pero) and poly(ethylene oxide) (PEO) as the emissive layer, and printed silver nanowires as the cathode. The fabrication can be carried out in ambient air without humidity control. The devices on ITO/glass have a low turn-on voltage of 2.6 V, a maximum luminance intensity of 21014 cd m(-2), and a maximum external quantum efficiency (EQE) of 1.1%, surpassing previous reported perovskite LEDs. The devices on CNTs/polymer were able to be strained to 5 mm radius of curvature without affecting device properties.

Photohole Induced Corrosion of Titanium Dioxide: Mechanism and Solutions.

Titanium dioxide (TiO2) has been extensively investigated as photoanode for water oxidation, as it is believed to be one of the most stable photoanode materials. Yet, we surprisingly found that TiO2 photoanodes (rutile nanowire, anatase nanotube, and P25 nanoparticle film) suffered from substantial photocurrent decay in neutral (Na2SO4) as well as basic (KOH) electrolyte solution. Photoelectrochemical measurements togehter with electron microscopy studies performed on rutile TiO2 nanowire photoanode show that the photocurrent decay is due to photohole induced corrosion, which competes with water oxidation reaction. Further studies reveal that photocurrent decay profile in neutral and basic solutions are fundamentally different. Notably, the structural reconstruction of nanowire surface occurs simultaneously with the corrosion of TiO2 in KOH solution resulting in the formation of an amorphous layer of titanium hydroxide, which slows down the photocorrosion. Based on this discovery, we demonstrate that the photoelectrochemical stability of TiO2 photoanode can be significantly improved by intentionally coating an amorphous layer of titanium hydroxide on the nanowire surface. The pretreated TiO2 photaonode exhibits an excellent photocurrent retention rate of 97% after testing in KOH solution for 72 h, while in comparison the untreated sample lost 10-20% of photocurrent in 12 h under the same measurement conditions. This work provides new insights in understanding of the photoelectrochemical stability of bare TiO2 photoanodes.

Oxygen deficient α-Fe2O3 photoelectrodes: a balance between enhanced electrical properties and trap-mediated losses.

Intrinsic doping of hematite through the inclusion of oxygen vacancies (VO) is being increasingly explored as a simple, low temperature route to preparing active water splitting α-Fe2O3-x photoelectrodes. Whilst it is widely accepted that the introduction of VO leads to improved conductivities, little else is verified regarding the actual mechanism of enhancement. Here we employ transient absorption (TA) spectroscopy to build a comprehensive kinetic model for water oxidation on α-Fe2O3-x . In contrast to previous suggestions, the primary effect of introducing VO is to block very slow (ms) surface hole - bulk electron recombination pathways. In light of our mechanistic research we are also able to identify and address a cause of the high photocurrent onset potential, a common issue with this class of electrodes. Atomic layer deposition (ALD) of Al2O3 is found to be particularly effective with α-Fe2O3-x , leading to the photocurrent onset potential shifting by ca. 200 mV. Significantly TA measurements on these ALD passivated electrodes also provide important insights into the role of passivating layers, that are relevant to the wider development of α-Fe2O3 photoelectrodes.

A new benchmark capacitance for supercapacitor anodes by mixed-valence sulfur-doped V6O(13-x).

A new pseudocapacitor anode, sulfur-doped V6O(13-x), is reported. It achieves a benchmark capacitance of 1353 F/g (0.72 F/cm(2)) at a current density of 1.9 A/g (1 mA/cm(2)) in 5 M LiCl solution. The charges are stored chemically in the electrode via reversible redox reactions that involve multiple oxidation states of vanadium (V(3+), V(4+) and V(5+)).

Photoenhanced electrochemical interaction between Shewanella and a hematite nanowire photoanode.

Here we report the investigation of interplay between light, a hematite nanowire-arrayed photoelectrode, and Shewanella oneidensis MR-1 in a solar-assisted microbial photoelectrochemical system (solar MPS). Whole cell electrochemistry and microbial fuel cell (MFC) characterization of Shewanella oneidensis strain MR-1 showed that these cells cultured under (semi)anaerobic conditions expressed substantial c-type cytochrome outer membrane proteins, exhibited well-defined redox peaks, and generated bioelectricity in a MFC device. Cyclic voltammogram studies of hematite nanowire electrodes revealed active electron transfer at the hematite/cell interface. Notably, under a positive bias and light illumination, the hematite electrode immersed in a live cell culture was able to produce 150% more photocurrent than that in the abiotic control of medium or dead culture, suggesting a photoenhanced electrochemical interaction between hematite and Shewanella. The enhanced photocurrent was attributed to the additional redox species associated with MR-1 cells that are more thermodynamically favorable to be oxidized than water. Long-term operation of the hematite solar MPS with light on/off cycles showed stable current generation up to 2 weeks. Fluorescent optical microscope and scanning electron microscope imaging revealed that the top of the hematite nanowire arrays were covered by a biofilm, and iron determination colorimetric assay revealed 11% iron loss after a 10-day operation. To our knowledge, this is the first report on interfacing a photoanode directly with electricigens in a MFC system. Such a system could open up new possibilities in solar-microbial device that can harvest solar energy and recycle biomass simultaneously to treat wastewater, produce electricity, and chemical fuels in a self-sustained manner.

Solid-state supercapacitor based on activated carbon cloths exhibits excellent rate capability.

Activated carbon cloth is used as an electrode, achieving an excellent areal capacitance of 88 mF/cm(2) (8.8 mF/g) without the use of any other capacitive materials. Significantly, when it is incorporated as part of a symmetric solid-state supercapacitor device, a remarkable charge/discharge rate capability is observed; 50% of the capacitance is retained when the charging rate increases from 10 to 10,000 mV/s.

Low-temperature activation of hematite nanowires for photoelectrochemical water oxidation.

Hematite (α-Fe2 O3 ) nanostructures have been extensively studied as photoanode materials for photoelectrochemical (PEC) water oxidation. However, the photoactivity of pristine hematite nanostructures is fairly low and typically requires thermal activation at temperature of 650 °C or above. Here, we report a new method for enhancing the photocurrent of hematite nanowires at a substantially lower temperature of 350 °C by means of a two-step annealing process (activation process). Hydrothermally grown ß-FeOOH nanowires were first annealed in a pure N2 environment at 350 °C to form magnetite, followed by partial oxidation in air to convert magnetite to hematite. During this process, Fe(2+) sites (oxygen vacancies) were intentionally created to increase the donor density and therefore the electrical conductivity of hematite. The oxygen-deficient hematite nanowire photoanode created at low temperature (350 °C) show considerably enhanced photoactivity compared to pristine hematite sample that prepared by thermal annealing of ß-FeOOH nanowires at 550 °C in air. Moreover, this low-temperature annealing method can be coupled with an element doping method to further increase the photoactivity of hematite nanowire. Sn-doped hematite nanowires prepared by the same low-temperature annealing method show at least three fold enhanced photocurrent compared to the undoped sample. Significantly, the highest temperature in the entire annealing process was 350 °C, which is the lowest activation temperature ever reported for hematite nanowire photoanodes.

High power density microbial fuel cell with flexible 3D graphene-nickel foam as anode.

The structure and electrical conductivity of anode play a significant role in the power generation of microbial fuel cells (MFCs). In this study, we developed a three-dimensional (3D) reduced graphene oxide-nickel (denoted as rGO-Ni) foam as an anode for MFC through controlled deposition of rGO sheets onto the nickel foam substrate. The loading amount of rGO sheets and electrode surface area can be controlled by the number of rGO loading cycles. 3D rGO-Ni foam anode provides not only a large accessible surface area for microbial colonization and electron mediators, but also a uniform macro-porous scaffold for effective mass diffusion of the culture medium. Significantly, at a steady state of the power generation, the MFC device with flexible rGO-Ni electrodes produced an optimal volumetric power density of 661 W m(-3) calculated based on the volume of anode material, or 27 W m(-3) based on the volume of the anode chamber. These values are substantially higher than that of plain nickel foam, and other conventional carbon based electrodes (e.g., carbon cloth, carbon felt, and carbon paper) measured in the same conditions. To our knowledge, this is the highest volumetric power density reported for mL-scale MFC device with a pure strain of Shewanella oneidensis MR-1. We also demonstrated that the MFC device can be operated effectively in a batch-mode at least for a week. These new 3D rGO-Ni electrodes show great promise for improving the power generation of MFC devices.

Au nanostructure-decorated TiO2 nanowires exhibiting photoactivity across entire UV-visible region for photoelectrochemical water splitting.

Here we demonstrate that the photoactivity of Au-decorated TiO2 electrodes for photoelectrochemical water oxidation can be effectively enhanced in the entire UV-visible region from 300 to 800 nm by manipulating the shape of the decorated Au nanostructures. The samples were prepared by carefully depositing Au nanoparticles (NPs), Au nanorods (NRs), and a mixture of Au NPs and NRs on the surface of TiO2 nanowire arrays. As compared with bare TiO2, Au NP-decorated TiO2 nanowire electrodes exhibited significantly enhanced photoactivity in both the UV and visible regions. For Au NR-decorated TiO2 electrodes, the photoactivity enhancement was, however, observed in the visible region only, with the largest photocurrent generation achieved at 710 nm. Significantly, TiO2 nanowires deposited with a mixture of Au NPs and NRs showed enhanced photoactivity in the entire UV-visible region. Monochromatic incident photon-to-electron conversion efficiency measurements indicated that excitation of surface plasmon resonance of Au is responsible for the enhanced photoactivity of Au nanostructure-decorated TiO2 nanowires. Photovoltage experiment showed that the enhanced photoactivity of Au NP-decorated TiO2 in the UV region was attributable to the effective surface passivation of Au NPs. Furthermore, 3D finite-difference time domain simulation was performed to investigate the electrical field amplification at the interface between Au nanostructures and TiO2 upon SPR excitation. The results suggested that the enhanced photoactivity of Au NP-decorated TiO2 in the UV region was partially due to the increased optical absorption of TiO2 associated with SPR electrical field amplification. The current study could provide a new paradigm for designing plasmonic metal/semiconductor composite systems to effectively harvest the entire UV-visible light for solar fuel production.

High energy density asymmetric supercapacitors with a nickel oxide nanoflake cathode and a 3D reduced graphene oxide anode.

Here we demonstrate a high energy density asymmetric supercapacitor with nickel oxide nanoflake arrays as the cathode and reduced graphene oxide as the anode. Nickel oxide nanoflake arrays were synthesized on a flexible carbon cloth substrate using a seed-mediated hydrothermal method. The reduced graphene oxide sheets were deposited on three-dimensional (3D) nickel foam by hydrothermal treatment of nickel foam in graphene oxide solution. The nanostructured electrodes provide a large effective surface area. The asymmetric supercapacitor device operates with a voltage of 1.7 V and achieved a remarkable areal capacitance of 248 mF cm(-2) (specific capacitance of 50 F g(-1)) at a charge/discharge current density of 1 mA cm(-2) and a maximum energy density of 39.9 W h kg(-1) (based on the total mass of active materials of 5.0 mg). Furthermore, the device showed an excellent charge/discharge cycling performance in 1.0 M KOH electrolyte at a current density of 5 mA cm(-2), with a capacitance retention of 95% after 3000 cycles.

A mechanistic study into the catalytic effect of Ni(OH)2 on hematite for photoelectrochemical water oxidation.

We report a mechanistic study of the catalytic effect of Ni(OH)2 on hematite nanowires for photoelectrochemical water oxidation. Ni compounds have been shown to be good catalysts for electrochemical and photoelectrochemical water oxidation. While we also observed improved photocurrents for Ni-catalyst decorated hematite photoanodes, we found that the photocurrents decay rapidly, indicating the photocurrents were not stable. Importantly, we revealed that the enhanced photocurrent was due to water oxidation as well as the photo-induced charging effect. In addition to oxidizing water, the photoexcited holes generated in hematite efficiently oxidize Ni(2+) to Ni(3+) (0.35 V vs. Ag/AgCl). The instability of photocurrent was due to the depletion of Ni(2+). We proposed that the catalytic mechanism of the Ni(II) catalyst for water oxidation is a two-step process that involves the fast initial oxidation of Ni(2+) to Ni(3+), and followed by the slow oxidation of Ni(3+) to Ni(4+), which is believed to be the active catalytic species for water oxidation. The catalytic effect of the Ni(II) catalyst was limited by the slow formation of Ni(4+). Finally, we elucidated the real catalytic performance of Ni(OH)2 on hematite for photoelectrochemical water oxidation by suppressing the photo-induced charging effect. This work could provide important insights for future studies on Ni based catalyst modified photoelectrodes for water oxidation.

Growth of gallium nitride and indium nitride nanowires on conductive and flexible carbon cloth substrates.

We report a general strategy for synthesis of gallium nitride (GaN) and indium nitride (InN) nanowires on conductive and flexible carbon cloth substrates. GaN and InN nanowires were prepared via a nanocluster-mediated growth method using a home built chemical vapor deposition (CVD) system with Ga and In metals as group III precursors and ammonia as a group V precursor. Electron microscopy studies reveal that the group III-nitride nanowires are single crystalline wurtzite structures. The morphology, density and growth mechanism of these nanowires are determined by the growth temperature. Importantly, a photoelectrode fabricated by contacting the GaN nanowires through a carbon cloth substrate shows pronounced photoactivity for photoelectrochemical water oxidation. The ability to synthesize group III-nitride nanowires on conductive and flexible substrates should open up new opportunities for nanoscale photonic, electronic and electrochemical devices.