Considering metal oxide nanoparticles as important technological materials, authors provide a comprehensive review of researches on metal oxide nanoparticles, their synthetic strategies, and techniques, nanoscale physicochemical properties, defining specific industrial applications in the various fields of applied nanotechnology. This work expansively reviews the recent developments of semiconducting metal oxide gas sensors for environmental gases including CO2, O2, O3, and NH3; highly toxic gases including CO, H2S, and NO2; combustible gases such as CH4, H2, and liquefied petroleum gas; and volatile organic compounds gases. The gas sensing properties of different metal oxides nanoparticles towards specific target gases have been individually discussed. Promising metal oxide nanoparticles for sensitive and selective detection of each gas have been identified. This review also categorizes metal oxides sensors by analyte gas and also summarizes the major techniques and synthesis strategies used in nanotechnology. Additionally, strategies, sensing mechanisms and related applications of semiconducting metal oxide materials are also discussed in detail. Related applications are innumerable trace to ultratrace-level gas sensors, batteries, magnetic storage media, various types of solar cells, metal oxide nanoparticles applications in catalysis, energy conversion, and antennas (including microstrip and patch-type optically transparent antennas), rectifiers, optoelectronic, and electronics.

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Metal oxide nanoparticles and their applications in nanotechnology

Over the past few years, very interestingly, two subclasses of perovskites — organic–inorganic halide perovskites and ferroelectric oxide perovskites, have simultaneously become the hotspots in the research field of photovoltaics. Organic–inorganic halide perovskites have launched a new era of low-cost, high-efficiency solar cells, due to their easy solution processability and superior optical and electrical properties for the photovoltaic effect. More recently, a so-called giant switchable photovoltaic effect has been demonstrated in organic–inorganic halide perovskites, thus promising a new memristive functionality. On the other hand, the recent renaissance of ferroelectric oxide perovskites for photovoltaics is caused by their fundamentally new photovoltaic mechanisms, which can produce a photovoltage far beyond the bandgap and may even lead to a boost of energy conversion efficiency. In addition, the combination of photovoltaic properties with the ferroic orders may create many novel functionalities for ferroelectric oxide perovskites. Toward the common goals of developing high-efficiency photovoltaics and novel opto-electronic functional devices, these two different subclasses of perovskites shall be brought together into a combined review. In this context, we review both organic–inorganic halide perovskites and ferroelectric oxide perovskites for photovoltaics, focusing on the material nature and the photovoltaic mechanisms. We also discuss their respective unresolved issues, along with useful suggestions for future research.

Perovskites for photovoltaics: a combined review of organic–inorganic halide perovskites and ferroelectric oxide perovskites

Intrinsic polarization of ferroelectrics (FE) helps separate photon-generated charge carriers thus enhances photovoltaic effects. However, traditional FE with transition-metal cations (M) of d0 electron in MO6 network typically has a band gap (Eg) exceeding 3.0 eV. Although a smaller Eg (2.6 eV) can be obtained in multiferroic BiFeO3, the value is still too high for optimal solar energy applications. Computational “materials genome” searches have predicted several exotic MO6 FE with Eg < 2.0 eV, all thus far unconfirmed because of synthesis difficulties. Here we report a new FE compound with MO4 tetrahedral network, KBiFe2O5, which features narrow Eg (1.6 eV), high Curie temperature (Tc ~ 780 K) and robust magnetic and photoelectric activities. The high photovoltage (8.8 V) and photocurrent density (15 μA/cm2) were obtained, which is comparable to the reported BiFeO3. This finding may open a new avenue to discovering and designing optimal FE compounds for solar energy applications.

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New high T c multiferroics KBiFe 2 O 5 with narrow band gap and promising photovoltaic effect

Ferroelectric materials have demonstrated novel photovoltaic effect to scavenge solar energy. However, most of the ferroelectric materials with wide bandgaps (2.7–4 eV) suffer from low power conversion efficiency of less than 0.5% due to absorbing only 8–20% of solar spectrum. Instead of harvesting solar energy, these ferroelectric materials can be well suited for photodetector applications, especially for sensing near-UV irradiations. Here, a ferroelectric BaTiO3 film-based photodetector is demonstrated that can be operated without using any external power source and a fast sensing of 405 nm light illumination is enabled. As compared with photovoltaic effect, both the responsivity and the specific detectivity of the photodetector can be dramatically enhanced by larger than 260% due to the light-induced photovoltaic–pyroelectric coupled effect. A self-powered photodetector array system can be utilized to achieve spatially resolved light intensity detection by recording the output voltage signals as a mapping figure.

Photovoltaic-Pyroelectric Coupled Effect Induced Electricity for Self-Powered Photodetector System.

Converting light energy to electrical energy in photovoltaic devices relies on the photogenerated electrons and holes separated by the built-in potential in semiconductors. Photo-excited electrons in metal electrodes are usually not considered in this process. Here, we report an enhanced photovoltaic effect in the ferroelectric lanthanum-modified lead zirconate titanate (PLZT) by using low work function metals as the electrodes. We believe that electrons in the metal with low work function could be photo-emitted into PLZT and form the dominant photocurrent in our devices. Under AM1.5 (100 mW/cm²) illumination, the short-circuit current and open-circuit voltage of Mg/PLZT/ITO are about 150 and 2 times of those of Pt/PLZT/ITO, respectively. The photovoltaic response of PLZT capacitor was expanded from ultraviolet to visible spectra, and it may have important impact on design and fabrication of high performance photovoltaic devices based on ferroelectric materials.

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Enlarging photovoltaic effect: combination of classic photoelectric and ferroelectric photovoltaic effects

A novel nanostructured ferroelectric photovoltaic material, consisting of the ferroelectric lead zirconate titanate (PZT) film and Ag(2) O semiconductor nanoparticles of comparatively narrow bandgap, has demonstrated a remarkable enhancement in the photovoltaic effects and the highest light-electricity conversion efficiency among those PZT-based photodiodes previously reported. This work sheds light on the design and enhanced performance of new optoelectronic and solar energy devices.

Enhancement of Photocurrent in Ferroelectric Films Via the Incorporation of Narrow Bandgap Nanoparticles

The investigation of the possibility of optical refrigeration (OR) on zinc-blende cadmium selenide/cadmium sulﬁde (CdSe/CdS) core/shell structure quantum dots (QDs) has been carried out. Quality samples were synthe-sized in our lab, and signiﬁcant energy up-conversion photoluminescence (UCPL) was observed in these samples, showing the potential of generating net cooling eﬀects. To better understand and predict the UCPL character-istics of the QDs, a semi-empirical model has been developed, showing good agreement with our experimental results. The model takes into account the corresponding quantum yield and cooling eﬃciency, predicting the possibility of realizing optical refrigeration on a CdSe QDs system. dispersed SET. Therefore, a net energy UC is, in principle, achievable by exciting the QDs at the tail of their SET. Based on experimental data, a semi-empirical model describing the PL processes was constructed. The simulated results show extraordinary agreement with the experimental data and further indicated that the broadening of zinc-blende CdSe QDs’ UCPL spectra is mainly due to the strong coupling between the photon induced exciton and the “bulk” acoustic phonon modes. They also suggest that surface passivation reduces excitons’ mobility in the surface band. Although the large cutoﬀ phonon energy cannot yet be explained, quantitative predictions can be produced by the model. With the model’s help, the possible cooling eﬃciency in CdSe QDs were calculated, showing the possibility of realizing OR in high quality zinc-blende CdSe QDs.

Muchuan Hua

Papers

Enhancement of Photocurrent in Ferroelectric Films Via the Incorporation of Narrow Bandgap Nanoparticles

Net energy up-conversion processes in CdSe/CdS (core/shell) quantum dots: A possible pathway towards optical cooling