Aiming to improve the photocatalytic properties of transition metal perovskites to be used as robust photoanodes, [LaFeO3]1-x/[SrTiO3]x nanocomposites (LFO1-x/STOx) are considered. This hybrid structure combines good semiconducting properties and an interesting intrinsic remanent polarization. All the studied samples were fabricated using a solid-state method followed by high-energy ball milling, and they were subsequently deposited by spray coating. The synthesized compounds were demonstrated to possess orthorhombic (Pnma) and cubic (Pm3¯m) structures for LFO and STO, respectively, with an average grain size of 55-70 nm. The LFO1-x/STOx nanocomposites appeared to exhibit high visible light absorption, corresponding to band gaps of 2.17-3.21 eV. Our findings show that LFO0.5/STO0.5 is the optimized heterostructure; it achieved a high photocurrent density of 11 µA/cm2 at 1.23 V bias vs. RHE and an applied bias photo-to-current efficiency of 4.1 × 10-3% at 0.76 V vs. RHE, as demonstrated by the photoelectrochemical measurements. These results underline the role of the two phases intermixing LFO and STO at the appropriate content to yield a high-performing photoanode ascribed to efficient charge separation and transfer. This suggests that LFO0.5/STO0.5 could be a potential candidate for the development of efficient photoanodes for hydrogen generation via photoelectrocatalytic water splitting.
We report on theoretical investigations of a methylammonium lead halide perovskite system loaded with iron oxide and aluminum zinc oxide (ZnO:Al/MAPbI3/Fe2O3) as a potential photocatalyst. When excited with visible light, this heterostructure is demonstrated to achieve a high hydrogen production yield via a z-scheme photocatalysis mechanism. The Fe2O3: MAPbI3 heterojunction plays the role of an electron donor, favoring the hydrogen evolution reaction (HER), and the ZnO:Al compound acts as a shield against ions, preventing the surface degradation of MAPbI3 during the reaction, hence improving the charge transfer in the electrolyte. Moreover, our findings indicate that the ZnO:Al/MAPbI3 heterostructure effectively enhances electrons/holes separation and reduces their recombination, which drastically improves the photocatalytic activity. Based on our calculations, our heterostructure yields a high hydrogen production rate, estimated to be 265.05 µmol/g and 362.99 µmol/g, respectively, for a neutral pH and an acidic pH of 5. These theoretical yield values are very promising and provide interesting inputs for the development of stable halide perovskites known for their superlative photocatalytic properties.
Zinc Oxide , Ferric Compounds , Zinc , Aluminum Oxide , Hydrogen
The potential use of down-sized BFO-xSTO systems (x ≤ 25%) as highly efficient photoanodes for photocatalytic water splitting is investigated. BFO-xSTO is prepared by a solid-state method and subsequently deposited by spray coating. The compounds possess rhombohedral symmetry for x ≤ 15% and phase coexistence for x > 15%, as demonstrated by Raman spectroscopy and transmission electron microscopy. Our findings revealed a drastic grain size decrease with increasing STO content, namely 260 nm for BFO to 50 nm for BFO with 25% STO. Moreover, BFO-xSTO, x > 10% exhibited high optical absorption (> 80%) in the full spectrum and interestingly a very promising band alignment with water redox potentials. Moreover, the photochemical measurements revealed a photocurrent density of â¼0.17 µA cm-2 achieved for x = 15% at 0 bias. Using DFT calculations, the substitution effects on the electronic, optical, and photocatalytic performances of the BFO system were investigated and quantified. Surprisingly, a high hydrogen yield (â¼191 µmol g-1) was achieved by BFO-12.5%STO compared to 1 µmol g-1 and 57 µmol g-1 for BFO and STO, respectively. This result highlights the beneficial effects of both the downsizing and substitution of BFO on the photocatalytic water splitting and hydrogen production performances of Bi1-x Sr x Fe1-x Ti x O3 systems.
Owing to their remarkable success in photocatalytic applications, multiferroic BiFeO3 and its derivatives have gained a highly promising position as electrode materials for future developments of efficient catalysts. In addition to their appropriate band gaps, these materials exhibit inherent intrinsic polarizations enabling efficient charge carrier separation and their high mobility without the need for additional co-catalysts. Here, we review the existing strategies for enhancing the photocatalytic performances of BiFeO3-based materials and we describe the physico-chemical properties at the origin of their exceptional photocatalytic behavior. A special focus is paid to the degradation of organic pollutants and water splitting, both driven through photocatalysis to unveil the correlation between BiFeO3 size, substitution, and doping on the one hand and the photocatalytic performances on the other hand. Finally, we provide practical recommendations for future developments of high-performing BiFeO3-based electrodes.
Tungsten disulfide nanosheets were successfully prepared by one-step chemical vapor deposition using tungsten oxide and thiourea in an inert gas environment. The size of the obtained nanosheets was subsequently reduced down to below 20 nm in width and 150 nm in length using high-energy ball milling, followed by 0.5 and 1 wt% graphene loading. The corresponding vibrational and structural characterizations are consistent with the fabrication of a pure WS2 structure for neat sampling and the presence of the graphene characteristic vibration modes in graphene@WS2 compounds. Additional morphological and crystal structures were examined and confirmed by high-resolution electron microscopy. Subsequently, the investigations of the optical properties evidenced the high optical absorption (98%) and lower band gap (1.75 eV) for the graphene@WS2 compared to the other samples, with good band-edge alignment to water-splitting reaction. In addition, the photoelectrochemical measurements revealed that the graphene@WS2 (1 wt%) exhibits an excellent photocurrent density (95 µA/cm2 at 1.23 V bias) compared with RHE and higher applied bias potential efficiency under standard simulated solar illumination AM1.5G. Precisely, graphene@WS2 (1 wt%) exhibits 3.3 times higher performance compared to pristine WS2 and higher charge transfer ability, as measured by electrical impedance spectroscopy, suggesting its potential use as an efficient photoanode for hydrogen evolution reaction.
Layered transition metals dichalcogenides such as MoS2 and WS2 have shown a tunable bandgap, making them highly desirable for optoelectronic applications. Here, we report on one-step chemical vapor deposited MoS2, WS2 and MoxW1-xS2 heterostructures incorporated into photoconductive devices to be examined and compared in view of their use as potential photodetectors. Vertically aligned MoS2 nanosheets and horizontally stacked WS2 layers, and their heterostructure form MoxW1-xS2, exhibit direct and indirect bandgap, respectively. To analyze these structures, various characterization methods were used to elucidate their properties including Raman spectroscopy, X-ray diffraction, X-ray photoelectron spectrometry and high-resolution transmission electron microscopy. While all the investigated samples show a photoresponse in a broad wavelength range between 400 nm and 700 nm, the vertical MoS2 nanosheets sample exhibits the highest performances at a low bias voltage of 5 V. Our findings demonstrate a responsivity and a specific detectivity of 47.4 mA W-1 and 1.4 × 1011 Jones, respectively, achieved by MoxW1-xS2. This study offers insights into the use of a facile elaboration technique for tuning the performance of MoxW1-xS2 heterostructure-based photodetectors.
In the surge of recent successes of 2D materials following the rise of graphene, molybdenum disulfide (2D-MoS2) has been attracting growing attention from both fundamental and applications viewpoints, owing to the combination of its unique nanoscale properties. For instance, the bandgap of 2D-MoS2, which changes from direct (in the bulk form) to indirect for ultrathin films (few layers), offers new prospects for various applications in optoelectronics. In this review, we present the latest scientific advances in the field of synthesis and characterization of 2D-MoS2 films while highlighting some of their applications in energy harvesting, gas sensing, and plasmonic devices. A survey of the physical and chemical processing routes of 2D-MoS2 is presented first, followed by a detailed description and listing of the most relevant characterization techniques used to study the MoS2 nanomaterial as well as theoretical simulations of its interesting optical properties. Finally, the challenges related to the synthesis of high quality and fairly controllable MoS2 thin films are discussed along with their integration into novel functional devices.
We demonstrate that under ambient and humidity-controlled conditions, operation of bimodal excitation single-scan electric force microscopy with no electrical feedback loop increases the spatial resolution of surface electrical property measurements down to the 5 nm limit. This technical improvement is featured on epitaxial graphene layers on SiC, which is used as a model sample. The experimental conditions developed to achieve such resolution are discussed and linked to the stable imaging achieved using the proposed method. The application of the herein reported method is achieved without the need to apply DC bias voltages, which benefits specimens that are highly sensitive to polarization. Besides, it allows the simultaneous parallel acquisition of surface electrical properties (such as contact potential difference) at the same scanning rate as in amplitude modulation atomic force microscopy (AFM) topography measurements. This makes it attractive for applications in high scanning speed AFM experiments in various fields for material screening and metrology of semiconductor systems.
Plasmonic nanostructures have played a key role in extending the activity of photocatalysts to the visible light spectrum, preventing the electron-hole combination and providing with hot electrons to the photocatalysts, a crucial step towards efficient broadband photocatalysis. One plasmonic photocatalyst, Au/TiO2, is of a particular interest because it combines chemical stability, suitable electronic structure, and photoactivity for a wide range of catalytic reactions such as water splitting. In this review, we describe key mechanisms involving plasmonics to enhance photocatalytic properties leading to efficient water splitting such as production and transport of hot electrons through advanced analytical techniques used to probe the photoactivity of plasmonics in engineered Au/TiO2 devices. This work also discusses the emerging strategies to better design plasmonic photocatalysts and understand the underlying mechanisms behind the enhanced photoactivity of plasmon-assisted catalysts.
In this study, the central composite design of response surface methodology was applied to optimize the ultrasonic synthesis of multiwalled carbon nanotube-titanium dioxide (MWNT-TiO2) composites. Twenty composites were prepared by adjusting three parameters (MWNT concentration in water, sonication to disperse/exfoliate MWNTs in water, and sonication to attach TiO2 onto MWNTs) at five levels. On the basis of the experimental design, semiempirical expressions were developed, analyzed, statistically assessed, and subsequently applied to predict the impact of the studied parameters on composite synthesis. The composite synthesis process was optimized to capture the experimental conditions favoring the highest productivity (i.e., MWNT-TiO2 formation or percent TiO2 attachment) utilizing minimal resources. The synthesis process optimization results showed that, to make a MWNT-TiO2 composite in 10 mL of water, 23.2 mg (â¼99% of 23.4 mg) of TiO2 can be attached to 2.6 mg of MWNTs. This process requires only 727 J sonication energy, of which 592 J is invested to exfoliate MWNTs (Sonication 1) and 135 J to attach TiO2 (Sonication 2) to MWNTs. Finally, the optimally synthesized composite was extensively characterized using SEM, surface area and porosity analysis, TGA, and ζ-potential analysis/DLS. Also, this composite was tested for stability under variable pH and solvent polarity. The approach developed in this study could be used to optimize the synthesis process of other similar composites.
Densely populated edge-terminated vertically aligned two-dimensional MoS2 nanosheets (NSs) with thicknesses ranging from 5 to 20 nm were directly synthesized on Mo films deposited on SiO2 by sulfurization. The quality of the obtained NSs was analyzed by scanning electron and transmission electron microscopy, and Raman and X-ray photoelectron spectroscopy. The as-grown NSs were then successfully transferred to the substrates using a wet chemical etching method. The transferred NSs sample showed excellent field-emission properties. A low turn-on field of 3.1 V/µm at a current density of 10 µA/cm2 was measured. The low turn-on field is attributed to the morphology of the NSs exhibiting vertically aligned sheets of MoS2 with sharp and exposed edges. Our findings show that the fabricated MoS2 NSs could have a great potential as robust high-performance electron-emitter material for various applications such as microelectronics and nanoelectronics, flat-panel displays and electron-microscopy emitter tips.
Cloud-seeding materials as a promising water-augmentation technology have drawn more attention recently. We designed and synthesized a type of core/shell NaCl/TiO2 (CSNT) particle with controlled particle size, which successfully adsorbed more water vapor (â¼295 times at low relative humidity, 20% RH) than that of pure NaCl, deliquesced at a lower environmental RH of 62-66% than the hygroscopic point (hg.p., 75% RH) of NaCl, and formed larger water droplets â¼6-10 times its original measured size area, whereas the pure NaCl still remained as a crystal at the same conditions. The enhanced performance was attributed to the synergistic effect of the hydrophilic TiO2 shell and hygroscopic NaCl core microstructure, which attracted a large amount of water vapor and turned it into a liquid faster. Moreover, the critical particle size of the CSNT particles (0.4-10 µm) as cloud-seeding materials was predicted via the classical Kelvin equation based on their surface hydrophilicity. Finally, the benefits of CSNT particles for cloud-seeding applications were determined visually through in situ observation under an environmental scanning electron microscope on the microscale and cloud chamber experiments on the macroscale, respectively. These excellent and consistent performances positively confirmed that CSNT particles could be promising cloud-seeding materials.
Photocatalytic nanostructures loaded with metallic nanoparticles are being considered as a potential candidate for designing efficient water splitting devices. Here, we aim to unveil the plasmonic behavior of a device made of Au-TiO2 nanostructures through in-depth investigations combining electron energy loss spectroscopy (EELS) and cathodoluminescence (CL). The experiments confirm the existence of Au bulk plasmon excitation, intrinsic interband transitions, and plasmon losses over a wide range of energies (0.6-2.4 eV). Depending on the size and the shape of the obtained nanostructures, such as fishing hook (FH), asymmetric nanorod (AR), and a/symmetric nanoparticles, in our devices, the dephasing times and the quality factors of the modes vary. Finite difference time domain simulations were then carried out on FH and AR structures. These simulations indicate good agreement between the electric field enhancement and the obtained plasmon excitation as observed in EELS. Moreover, the plasmonic activity obtained by CL and EELS was correlated with the photocurrent measurements recorded with the device, which confirmed that the localized plasmons in Au generate hot electrons and enhance the photoresponse of the device. This study confirms the functionality of the metal dielectric photocatalyst device over a wide range of wavelengths ranging from UV to near IR.
Mesoporous iron-oxide nanoparticles (mNPs) were prepared by using a modified nanocasting approach with mesoporous carbon as a hard template. mNPs were first loaded with doxorubicin (Dox), an anticancer drug, and then coated with the thermosensitive polymer Pluronic F108 to prevent the leakage of Dox molecules from the pores that would otherwise occur under physiological conditions. The Dox-loaded, Pluronic F108-coated system (Dox@F108-mNPs) was stable at room temperature and physiological pH and released its Dox cargo slowly under acidic conditions or in a sudden burst with magnetic heating. No significant toxicity was observed in vitro when Dox@F108-mNPs were incubated with noncancerous cells, a result consistent with the minimal internalization of the particles that occurs with normal cells. On the other hand, the drug-loaded particles significantly reduced the viability of cervical cancer cells (HeLa, IC50 =0.70â µm), wild-type ovarian cancer cells (A2780, IC50 =0.50â µm) and Dox-resistant ovarian cancer cells (A2780/AD, IC50 =0.53â µm). In addition, the treatment of HeLa cells with both Dox@F108-mNPs and subsequent alternating magnetic-field-induced hyperthermia was significantly more effective at reducing cell viability than either Dox or Dox@F108-mNP treatment alone. Thus, Dox@F108-mNPs constitute a novel soft/hard hybrid nanocarrier system that is highly stable under physiological conditions, temperature-responsive, and has chemo- and thermotherapeutic modes of action.
Doxorubicin/administration & dosage , Doxorubicin/chemistry , Drug Delivery Systems/methods , Ferric Compounds/chemistry , Magnetite Nanoparticles/chemistry , Ovarian Neoplasms/therapy , Antineoplastic Agents/administration & dosage , Antineoplastic Agents/chemistry , Antineoplastic Agents/pharmacology , Cell Survival/drug effects , Doxorubicin/pharmacology , Drug Stability , Female , HeLa Cells , Humans , Hydrogen-Ion Concentration , Hyperthermia, Induced , Porosity , Temperature
Plasmon assisted photoelectric hot electron collection in a metal-semiconductor junction can allow for sub-bandgap optical to electrical energy conversion. Here we report hot electron collection by wafer-scale Au/TiO2 metallic-semiconductor photonic crystals (MSPhC), with a broadband photoresponse below the bandgap of TiO2. Multiple absorption modes supported by the 2D nano-cavity structure of the MSPhC extend the photon-metal interaction time and fulfill a broadband light absorption. The surface plasmon absorption mode provides access to enhanced electric field oscillation and hot electron generation at the interface between Au and TiO2. A broadband sub-bandgap photoresponse centered at 590 nm was achieved due to surface plasmon absorption. Gold nanorods were deposited on the surface of MSPhC to study localized surface plasmon (LSP) mode absorption and subsequent injection to the TiO2 catalyst at different wavelengths. Applications of these results could lead to low-cost and robust photo-electrochemical applications such as more efficient solar water splitting.
Magnetic and fluorescent assemblies of iron-oxide nanoparticles (NPs) were constructed by threading a viologen-based ditopic ligand, DPV(2+), into the cavity of cucurbituril (CB[7]) macrocycles adsorbed on the surface of the NPs. Evidence for the formation of 1:2 inclusion complexes that involve DPV(2+) and two CB[7] macrocycles was first obtained in solution by (1)Hâ NMR and emission spectroscopy. DPV(2+) was found to induce self-assembly of nanoparticle arrays (DPV(2+)âCB[7]NPs) by bridging CB[7] molecules on different NPs. The resulting viologen-crosslinked iron-oxide nanoparticles exhibited increased saturation magnetization and emission properties. This facile supramolecular approach to NP self-assembly provides a platform for the synthesis of smart and innovative materials that can achieve a high degree of functionality and complexity and that are needed for a wide range of applications.
