The cocatalysts of noble metals are reported to play a significant role in improving the photocatalytic activity in water splitting and pollutant degradation reactions. The high price of noble metals limits their further application in industry. Herein, for the first time, we report that thiophene, rhodamine B (RhB) and methyl orange (MO) can be efficiently oxidized on BiVO4 co-loaded with Ni and CuO cocatalysts (denoted as Ni–CuO/BiVO4) under visible light irradiation with molecular oxygen as the oxidant. Moreover, 0.05 wt% Ni–0.5 wt% CuO/BiVO4 possesses high photocatalytic activity (over 94% conversion of thiophene), which is close to Pt–RuO2/BiVO4 (99% conversion of thiophene). XPS and ESR measurements showed that the activation of molecular oxygen and oxidation of pollutant molecules simultaneously take place on BiVO4 co-loaded with Ni/Cu and CuO/Cu2O cocatalysts. The considerable enhancement of photocatalytic activity can be attributed to the simultaneous presence of the reduction cocatalyst Ni/Cu and oxidation cocatalyst CuO/Cu2O, which are beneficial for the efficient separation and transfer of the photo-generated electrons and holes. Such visible-light-responsive semiconductor loaded with earth-abundant dual cocatalysts has great potential in both solar energy conversion and further industrial applications.

Ultrabroad-spectrum absorption and highly efficient generation of available charge carriers are two essential requirements for promising semiconductor-based photocatalysts, towards achieving the ultimate goal of solar-to-fuel conversion. Here, a fascinating nonmetal plasmonic Z-scheme photocatalyst with the W18O49/g-C3N4 heterostructure is reported, which can effectively harvest photon energies spanning from the UV to the nearinfrared region and simultaneously possesses improved charge-carrier dynamics to boost the generation of long-lived active electrons for the photocatalytic reduction of protons into H2. By combining with theoretical simulations, a unique synergistic photocatalysis effect between the semiconductive Z-scheme charge-carrier separation and metal-like localized-surface-plasmon-resonance-induced “hot electrons” injection process is demonstrated within this binary heterostructure.

•Photo-assisted self-optimizing of charge-carriers transport channel is realized in copper species/TiO2 electrospun nanofibers.•CuO/TiO2 nanofibers can be recrystallized to form Cu/Cu2O/CuO/TiO2 nanofibers during photocatalytic process.•Dynamics difference of excitons migration is responsible for the self-optimization of charge-carriers transport channel.•40-fold enhanced H2 generation was observed on the recrystallized nanofibers during photocatalytic decomposition of formic acid.Rational arrangement of nanosized semiconductor components in the multi-heterojunction photocatalyst can lead to the formation of a high-speed transport channel for charge-carriers separation and migration, which provides a promising way to achieve excellent photocatalytic efficiency for solar fuels generation. Herein, we develop a photo-assisted self-optimization strategy to re-build the charge-carriers transport channel in the copper species nanocrystals/TiO2 electrospun nanofibers with the organic hydrogen-carrier molecules as the photo-reactants. As a result of the dynamics difference of the excitons migration during the photocatalytic process, the binary CuO/TiO2 heterojunction nanofibers are recrystallized to form the quaternary Cu/Cu2O/CuO/TiO2 multi-heterojunction nanofibers that exhibits a higher rate constant (∼1.7×108S−1) for the interfacial electron-transfer than the former nanofibers (∼0.7×108S−1) due to the improved charge-carriers transport channel. In this way, a 40-fold enhanced H2 generation rate was observed on the recrystallized nanofibers photocatalyst during the photocatalytic decomposition of formic acid as compared to the pure TiO2 nanofibers. Our work presents an available paradigm to skillfully use a transient photo-physicochemical process to mildly engineer a high-quality charge-carriers transport channel in the hetero-nanophotocatalyst for realizing an optimal photocatalytic performance.Download high-res image (226KB)Download full-size image

In this paper, we report the temperature and rhodamine B (RhB)-concentration sensing behavior of Ag/ZnO/Er3+:YbMoO4 composite films based on the fluorescence intensity ratio (FIR) of two green upconversion (UC) emissions which are ascribed to the 2H11/2/4S3/2 → 4I15/2 transitions of Er3+. Through the strong and non-overlapping green UC emissions, the FIR of the two green emissions is closely related to temperature in the range of 300–650 K, which shows a high sensing accuracy and the maximum sensitivity of 0.01574 K−1. Due to the wavelength-dependent absorption of dye molecules, the FIR of the two green UC emissions exhibits an excellent exponential relationship with the RhB concentration in the range of 0–1000 ppm, which is ascribed to the radiative energy transfer (RET) from the composite film to RhB molecules. It is anticipated that the FIR technique based on the UC luminescence of rare-earth ions is a potential method for multifunctional application both in thermometers and biosensors.

•ZnO:Yb/Er nanocrystals clusters were synthesized by a microwave-assisted method.•FIR of green upconversion emissions in ZnO:Yb/Er was used for temperature sensing.•Impedance of ZnO:Yb/Er was utilized to perform humidity sensing.In this paper, we synthesized monodispersed ZnO:Yb/Er nanocrystal clusters (NCs) by a facile microwave-assisted approach. The fluorescence intensity ratio technique based on the two green upconversion emissions of ZnO:Yb/Er NCs was studied as a function of temperature with the maximum temperature sensitivity of 0.00323 K−1 in the range of 300–625 K. The ZnO:Yb/Er NCs also exhibited excellent impedance-dependent humidity sensing behavior with fast response and recovery characteristics time and dynamic response at a wide range of 11–95% relative humidity. It is anticipated that the present rare-earth doped ZnO NCs can furnish a simple, inexpensive and facile multifunctional application both in temperature and humidity sensing.

Aqueous solutions of zirconium oxychloride and aluminum nitrate were coprecipitated and crystallized to form a ZrO2–Al2O3 solid solution. The upconversion (UC) emission from different Er3+-doped samples was studied. An enhancement of the green UC emission by as much as 22 times was achieved by co-doping with Yb3+ and Mo6+ ions due to an energy transfer at a higher excited-state energy, which partly avoided the non-radiative decay processes at the lower energy levels of Er3+. The UC emission of the ZrO2–Al2O3 composite system series doped with different agents was enhanced. Excess oxygen vacancies are generated by forming ZrO2–Al2O3 solid solutions, which have an energy level close to the 4F7/2 level of the Er3+ ions. The defect state promoted the energy transfer process resulting in an eight-fold increased green UC emission in ZrO2–Al2O3 solid solutions. The solid solutions have a superior color chromaticity of x = 0.25 and y = 0.71 due to the evident enhancement in the green to red emission ratio in the 8ZrO2–2Al2O3 sample.

A Ln3+-doped (Yb3+, Tm3+ or Yb3+, Er3+ co-doped) NaYF4 nanoparticle/polystyrene hybrid fibrous membrane (HFM) was fabricated using an electrospinning technique. The HFM shows upconversion luminescence (UCL), flexibility, superhydrophobicity and processability. The UCL membrane can be used as a fluorescence sensor to detect bioinformation from a single water droplet (~10 μl). Based on the fluorescence resonance energy transfer, the detection limits of this sensor can reach 1 and 10 ppb for the biomolecule, avidin, and the dye molecule, Rhodamine B, respectively, which are superior to most of the fluorescence sensors reported in previous works. After the fluorescence detection, the target droplet was easily removed without residues on the UCL membrane surface due to its superhydrophobic property, which exhibits an excellent recyclability that cannot be achieved by traditional liquid-based detection systems.

One-dimensional SnO2/TiO2 heterostructures were successfully synthesized through the hydrothermal assembly of the single-crystalline SnO2 nanocubes onto the TiO2 electrospun nanofibers. The as-synthesized heterostructures with controllable coverage density of SnO2 nanocubes were then coated onto the ceramic-based interdigital electrodes to produce the humidity nanosensors for the investigation of their humidity sensing characteristics. The results showed that the optimal nanosensor with ∼20 at% SnO2-based heterostructure exhibited good humidity sensitivity, fast response–recovery behavior, low humidity hysteresis, and good reproducibility. In particular, the response and recovery times of this optimal nanosensor could reach ∼2.4 s and ∼30.2 s, respectively, which were considerably shorter than the corresponding values of TiO2 nanofiber-based humidity nanosensors. The improved sensitivity characteristics for the SnO2/TiO2 heterostructures can be attributed to the interfacial electron transfer between SnO2 nanocubes and TiO2 nanofibers, which leads to an appropriate height of the potential barrier on the surface of the heterostructures for water adsorption and desorption. Our proposed humidity sensing mechanism would provide opportunities to guide the design and fabrication of other high-performance humidity sensors based on semiconductor heterostructures.

We developed a novel kind of branched heterostructure by hydrothermal growth of ZnSnO3 nanostructures on TiO2 electrospun nanofibers, and demonstrated its enhanced ability to sense humidity through a sequential cactus-inspired tailoring of the ZnSnO3 nanostructures. Combining these results with first-principles calculations, it is deduced that the concentration of water molecules adsorbed on the ZnSnO3/TiO2 heterojunction surface can be increased by reducing the surface potential barrier. Meanwhile, the bioinspired ZnSnO3 nanoneedles, which form branches on the heterostructures, can further boost their adsorption abilities for water molecules via a water collection process. The adsorbed water molecules on the tips of the ZnSnO3 nanoneedles desorb easily in a low-humidity environment due to the small area of the tips (1.5–2.5 nm). Thus, the optimal ZnSnO3/TiO2 heterostructure exhibits response and recovery times of ∼2.5 s and ∼3 s, respectively. Its good sensitivity may enable it to detect tiny fluctuations in moisture and relative humidity that may surround any high-precision instrument.

The self-assembly of polymer composites of mixed carbon fillers including single-walled carbon nanotube (SWCNT) and carbon black nanoparticles (CB NPs) in diblock copolymer (BCP) template are investigated using hybrid particle-field molecular dynamics simulations in this work. Simulations show, in agreement with experiments, that composites of BCP template with SWCNT have lower percolation threshold than that of BCP template with CB NPs. Moreover, the ratio between SWCNT and CB NPs has a strong influence on the percolation threshold of composites. The results of percolation probability show that adding more SWCNT (compared with CB NPs) to the BCP template could decrease the percolation threshold. However, a synergistic effect of percolation of the mixed carbon fillers in BCP template has been found. In particular, a nonlinear relation following the Boltzman function has been found, and the lowest percolation threshold exists with the volume ratio 4:1 (SWCNT/CB NPs) compared with the volume ratios of 1:1, 2:1, and 8:1 (SWCNT/CB NPs). The mixed carbon fillers also affect the morphologies of the BCP template, and the calculated radius of gyration of BCP shows that, in a higher concentration of the mixed fillers, the stretching of BCP is stronger, which results in the deformation of BCP template.

The size-dependent temperature sensitivity is observed on the upconversion luminescence of NaYF4:Er,Yb microspheres with sizes between 0.7 and 2 μm that are prepared by a poly(acrylic acid)-assisted hydrothermal process. It is found that the fluorescence intensity ratio (FIR) of their green upconversion emissions (with peaks at 521 and 539 nm) is strongly size-dependent at temperatures between 223 and 403 K. As the size of the spheres increases from 0.7 to 1.6 μm, the maximum sensitivity decreases from 36.8 × 10−4 to 24.7 × 10−4 K−1. This effect is mainly attributed to the larger specific surface area of the smaller spheres where a relatively large number of Er(III) ions are located at the surface. This results in an increase in the efficiency of the 4S3/2 → 2H11/2 population process of the Er(III) ions due to stronger electron–phonon interactions with increasing T. Heating of the spheres by NIR light is also supposed to cause enhanced electron–phonon interactions in such particles.

•Electrospun Pt/TiO2 hybrid nanofibers exhibit high activities for visible-light-driven H2 evolution.•The Pt ions doping into TiO2 are responsible for the visible light absorption.•The hybrid nanofibers with 1.0 at% Pt showed the highest H2 evolution rate.•The H2 evolution rate is very dependent on the pH value of the sacrificial agent.One-dimensional (1D) Pt/TiO2 hybrid nanofibers (HNFs) with different concentrations of Pt were fabricated by a facile two-step synthesis route combining an electrospinning technique and calcination process. X-ray diffraction (XRD), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM) results showed that the Pt nanoparticles (NPs) with the size of 5–10 nm were well dispersed in the TiO2 nanofibers (NFs). Further investigations from the UV–Vis diffuse reflectance (DR) and X-ray photoelectron spectroscopy (XPS) analysis revealed that some Pt ions were incorporated into the TiO2 lattice as Pt4+ state, which contributed to the visible light absorption of TiO2 NFs. Meanwhile, the Pt2+ ions existing on the surface of Pt NPs resulted in the formation of Pt–O–Ti bond at Pt NPs/TiO2 NFs interfaces that might serve as an effective channel for improving the charge transfer. The as-electrospun Pt/TiO2 HNFs exhibited remarkable activities for photocatalytic H2 evolution under visible light irradiation in the presence of l-ascorbic acid as the sacrificial agent. In particular, the optimal HNFs containing 1.0 at% Pt showed the H2 evolution rate of 2.91 μmol h−1 and apparent quantum efficiency of 0.04% at 420 nm by using only 5 mg of photocatalysts. The higher photocatalytic activity could be ascribed to the appropriate amount of Pt ions doping and excellent electron-sink effect of Pt NPs co-catalysts.

We report on the synthesis and optical spectra of silver nanorice particles. Two strong absorption bands are resolved in the near UV and near-IR region, and the dark field scattering spectra are consistent with the absorption spectra. Finite-difference time-domain simulations reveal that the peak in the IR region can be attributed to the E field that is parallel to the long axis, while the peak in the UV can be attributed to the E field perpendicular to the short axis of the silver nanorice particles.

An innovative upconversion (UC) emissions route of Er3+ by Yb3+–Mn2+ dimer sensitizing in Er3+–Mn2+:Yb3Al5O12 (YbAG) nanocrystals is reported here, which resulted in the selective enhancement of green UC emission and suppression of red UC emission by a 976 nm laser diode excitation. By codoping of Mn2+, the green UC emission intensity increased about 260 times, while the red UC emission intensity decreased about 20 times than that of Er3+:YbAG nanocrystals. It indicates that the green enhancement and red suppression arise from the high excited state energy transfer with |2F7/2, 4T1g> (Yb3+–Mn2+ dimer) to the 4F7/2 (Er3+), which partly decreases the nonradiative processes happened in the lower levels of Er3+. The proposed sensitizing route here may constitute a promising step to realize high-efficient UC emissions of rare-earth ions doped oxides and significantly extend their scope of applications.Highlights► Er–Mn:Yb3Al5O12 nanocrystals exhibit excellent up-conversion emissions. ► Selective enhancement of green UC emission happened by Mn codoping. ► High excited state energy transfer caused intense green up-conversion emissions.

The extraordinary enhancement of green up-conversion emissions originated from the 2H11/2 → 4I15/2 and 4S3/2 → 4I15/2 transitions is obtained for the Er–Yb–Mo:Al2O3 with a 976 nm laser diode excitation. It indicates that such green enhancement arises from the high excited state energy transfer with |2F7/2, 3T2> state of the Yb3+–MoO42− dimer to the 4F7/2 level of Er3+. Fluorescence intensity ratio (FIR) technique based on the green up-conversion emissions of the Er–Yb–Mo:Al2O3 has been studied as a function of temperature. With an excitation power of 2 mW, the maximum of sensitivity and temperature revolution is approximately 0.0051 K−1 and 0.3 K in the range of 294–973 K, respectively. The Er–Yb–Mo:Al2O3 with intense green up-conversion emissions, a higher operating temperature and revolution indicates that it is a promising material for application in optical temperature sensing.

In this paper, we studied the pH dependent plasmon-driven surface-catalysis (PDSC) reactions of p,p′-dimercaptoazobenzene (DMAB) produced from para-aminothiophenol (PATP) and 4-nitrobenzenethiol (4NBT) both theoretically and experimentally. The surface enhanced Raman spectrum (SERS) of DMAB produced from PATP and 4NBT on Ag films in solutions with various pH values has been measured. The simulation and experimental results indicated that the pH dependence of PATP appeared in acidic environment and came from the amino group NH2. Furthermore, the ratio of intensity of Raman peak caused by PATP and DMAB indicated that this acidic sensor had higher pH sensitivity when it was excited by photons of higher energy.

Crystalline structures and infrared-to-visible upconversion luminescence spectra have been investigated in 1 mol% Er3+, 10 mol% Yb3+ and 0–20 mol% Li+ codoped TiO2 [1Er10Yb(0–20)Li:TiO2] nanocrystals. The crystalline structures of 1Er10Yb(0–20)Li:TiO2 were divided into three parts by the addition of Yb3+ and Li+. Both green and red upconversion emissions were observed from the 2H11/2/4S3/2 → 4I15/2 and 4F9/2 → 4I15/2 transitions of Er3+ in Er3+–Yb3+–Li+ codoped TiO2, respectively. The green and red upconversion emissions of 1Er:TiO2 were enhanced significantly by Yb3+ and Li+ codoping, in which the intensities of green and red emissions and the intensity ratio of green to red emissions (Igreen/Ired) were highly dependent on the crystalline structures. The significant enhanced upconversion emissions resulted from the energy migration between Er3+ and Yb3+ as well as the distortion of crystal field symmetry of Er3+ caused by the dissolving of Li+ at lower Li+ codoping concentration and the phase transformation at higher Li+ concentration. It is concluded that codoping with ions of smaller ionic radius like Li+ can efficiently improve the upconversion emissions of Er3+ or other rare-earth ions doped luminsecence materials.Research Highlights► Crystalline structures of Er:TiO2 are divided into three parts by Yb and Li codoping. ► Green and red UC emissions are strongly dependent on the crystalline structures. ► Change of Er symmetry and phase structure by Li codoping enhanced UC emissions.

The quasi-one dimensional (Q1D) Er3+–Yb3+ codoped single-crystal MoO3 ribbons with width range from 1 to 5 μm, and maximum length about 30 μm have been synthesized by the vapor transport method. The samples were characterized using X-ray diffraction, scanning electron microscopy, transmission electron microscope, and luminescence spectra. By a 975 nm laser diode (LD) as excitation source, the blue, green and red emission bands centered at about 408, 532, 553 and 657 nm were detected, which attributed to the 2H9/2 → 4I15/2, 2H11/2, 4S3/2 → 4I15/2 and 4F9/2 → 4I15/2 transitions of Er3+, respectively. The three-, and two-photon process was responsible for the blue, green and red up-conversion emissions mechanism for the Q1D Er3+–Yb3+ codoped single-crystal MoO3 ribbons, respectively. The results suggested that the Q1D Er3+–Yb3+ codoped single-crystal MoO3 ribbons will have potential applications in remote bio-imaging and surface enhanced Raman scattering.Research highlights► Er3+–Yb3+:MoO3 quasi-one dimensional ribbons synthesized by a vapor transport method. ► Blue, green and red emissions of Er3+–Yb3+:MoO3 Q1D ribbons were obtained. ► Er3+–Yb3+:MoO3 Q1D ribbons have potential applications in remote bio-imaging.

The Er–Mo:Yb2Ti2O7 nanocrystalline phosphor has been prepared by sol–gel method and used as an optical thermometry. By Mo codoping, the green upconversion (UC) emission intensity increased about 250 times than that of Er:Yb2Ti2O7 under a 976 nm laser diode excitation. It indicates that such green enhancement arises from the high excited state energy transfer (HESET) with the |2F7/2, 3T2> state of Yb3+–MoO42− dimer to the 4F7/2 level of Er3+. The fluorescence intensity ratio (FIR) of the two green UC emissions bands was studied as a function of temperature in a range of 290–610 K, and the maximum sensitivity and the temperature resolution were approximately 0.0074 K−1 and 0.1 K, respectively. It suggests that the Er–Mo:Yb2Ti2O7 nanophosphor with a higher green UC emissions efficiency is a promising prototype for applications in optical temperature sensing.

The Er3+-Li+ codoped TiO2 powders have been prepared by the non-aqueous sol–gel method. The green and red upconversion emissions centered at about 526, 550 and 663 nm were observed by the 2H11/2, 4S3/2 → 4I15/2 and 4F9/2 → 4I15/2 transitions of Er3+, respectively. Li+ codoping has opposite effect on the upconversion emissions intensities for Er3+-doped TiO2 at sintering temperatures of 1,073 and 1,273 K. At 1,073 K, the Er3+-doped TiO2 phase transition from anatase to rutile was accelerated with increasing Li+ codoping concentration, leading to the increase of crystal field symmetry of Er3+, thus the upconversion emissions intensities decreased. At 1,273 K, Li+ codoping had no effect on the phase structure of Er3+-doped TiO2 and only increased the Er–O bond length, it indicated that the upconversion emissions intensities greatly enhanced because of the decrease of crystal field symmetry of Er3+.

The 0.1 mol% Er3+ and 0–2 mol% Yb3+ codoped Al2O3 powders were prepared by the sol-gel method, and the phase structure, including only two crystalline types of doped Al2O3 phase, γ-(Al,Er,Yb)2O3 and θ-(Al,Er,Yb)2O3, was detected at the sintering temperature of 1000°C. The visible and near infrared emissions properties depended strongly on the Yb3+ codoping, and the corresponding maximal peak intensities centered at about 523, 545, 660 and 1533 nm were obtained respectively for the 0.1 mol% Er3+ and 0.5 mol% Yb3+ codoped Al2O3 powders, which were composed of θ-(Al,Er,Yb)2O3 and a small amount of γ-(Al,Er,Yb)2O3 phases. The two-photon absorption process was responsible for the visible up-conversion emissions, and the one-photon absorption process was involved in the near infrared emissions of the Er3+-Yb3+ codoped Al2O3 powders.

The Er3+–Yb3+ codoped Al2O3 has been prepared by the sol–gel method using the aluminium isopropoxide [Al(OC3H7)3]-derived Al2O3 sols with addition of the erbium nitrate [Er(NO3)3 · 5H2O] and ytterbium nitrate [Yb(NO3)3 · 5H2O]. The phase structure, including only two crystalline types of doped Al2O3 phases, θ and γ, was obtained for the 1 mol% Er3+ and 5 mol% Yb3+ codoped Al2O3 at the sintering temperature of 1,273 K. By a 978 nm semiconductor laser diodes excitation, the visible up-conversion emissions centered at about 523, 545, and 660 nm were obtained. The temperature dependence of the green up-conversion emissions was studied over a wide temperature range of 300–825 K, and the reasonable agreement between the calculated temperature by the fluorescence intensity ratio (FIR) theory and the measured temperature proved that Er3+–Yb3+ codoped Al2O3 plays an important role in the application of high temperature sensor.

•The intramolecular hydrogen bond N⋯HO is strengthened at S1 state, which provides a driving force to facilitate the proton transfer process.•The intramolecular proton transfer process is kinetically and thermodynamically favourable at S1 state, and it is forbidden at S0 state.•The double proton transfer mechanism is implemented by simultaneous double proton transfer, or successive single transfers.•The multiple fluorescence behavior of the fluorescence spectrum is predicted, which well explains the experimental observations.In this work, excited-state intramolecular single and double proton transfer mechanisms for 2,5-bis(benzoxazol-2-yl)thiophene-3,4-diol were investigated using time-dependent density functional theory (TDDFT) method for the first time. Our calculations showed that the intramolecular hydrogen bond N⋯HO is strengthened in the excited state, which provides a driving force to effectively facilitate the proton transfer process. The constructed potential energy surface of the excited state demonstrated that the double proton transfer reaction occurs more readily in both dynamics and thermodynamics aspects, and it is implemented by simultaneous double proton transfer, or successive single transfers. Furthermore, the assignments of multiple fluorescence bands in experiment were confirmed by fluorescence spectral simulation. The calculated emission spectra indicated that the experimental fluorescence maxima at 475 nm should be attributed to normal Stokes shifted emission; the large Stokes shifted fluorescence with peaks at 550 nm originates from the double proton transfer phototautomer emission, and the experimentally observed shoulder peak at ∼493 nm results from the excited state single proton transfer tautomer.The schematic energy diagrams for photo-induced tautomerization of bis-2,5-(2-benzoxazolyl)-hydroquinone.

Aqueous solutions of zirconium oxychloride and aluminum nitrate were coprecipitated and crystallized to form a ZrO2–Al2O3 solid solution. The upconversion (UC) emission from different Er3+-doped samples was studied. An enhancement of the green UC emission by as much as 22 times was achieved by co-doping with Yb3+ and Mo6+ ions due to an energy transfer at a higher excited-state energy, which partly avoided the non-radiative decay processes at the lower energy levels of Er3+. The UC emission of the ZrO2–Al2O3 composite system series doped with different agents was enhanced. Excess oxygen vacancies are generated by forming ZrO2–Al2O3 solid solutions, which have an energy level close to the 4F7/2 level of the Er3+ ions. The defect state promoted the energy transfer process resulting in an eight-fold increased green UC emission in ZrO2–Al2O3 solid solutions. The solid solutions have a superior color chromaticity of x = 0.25 and y = 0.71 due to the evident enhancement in the green to red emission ratio in the 8ZrO2–2Al2O3 sample.

The size-dependent temperature sensitivity is observed on the upconversion luminescence of NaYF4:Er,Yb microspheres with sizes between 0.7 and 2 μm that are prepared by a poly(acrylic acid)-assisted hydrothermal process. It is found that the fluorescence intensity ratio (FIR) of their green upconversion emissions (with peaks at 521 and 539 nm) is strongly size-dependent at temperatures between 223 and 403 K. As the size of the spheres increases from 0.7 to 1.6 μm, the maximum sensitivity decreases from 36.8 × 10−4 to 24.7 × 10−4 K−1. This effect is mainly attributed to the larger specific surface area of the smaller spheres where a relatively large number of Er(III) ions are located at the surface. This results in an increase in the efficiency of the 4S3/2 → 2H11/2 population process of the Er(III) ions due to stronger electron–phonon interactions with increasing T. Heating of the spheres by NIR light is also supposed to cause enhanced electron–phonon interactions in such particles.

One-dimensional SnO2/TiO2 heterostructures were successfully synthesized through the hydrothermal assembly of the single-crystalline SnO2 nanocubes onto the TiO2 electrospun nanofibers. The as-synthesized heterostructures with controllable coverage density of SnO2 nanocubes were then coated onto the ceramic-based interdigital electrodes to produce the humidity nanosensors for the investigation of their humidity sensing characteristics. The results showed that the optimal nanosensor with ∼20 at% SnO2-based heterostructure exhibited good humidity sensitivity, fast response–recovery behavior, low humidity hysteresis, and good reproducibility. In particular, the response and recovery times of this optimal nanosensor could reach ∼2.4 s and ∼30.2 s, respectively, which were considerably shorter than the corresponding values of TiO2 nanofiber-based humidity nanosensors. The improved sensitivity characteristics for the SnO2/TiO2 heterostructures can be attributed to the interfacial electron transfer between SnO2 nanocubes and TiO2 nanofibers, which leads to an appropriate height of the potential barrier on the surface of the heterostructures for water adsorption and desorption. Our proposed humidity sensing mechanism would provide opportunities to guide the design and fabrication of other high-performance humidity sensors based on semiconductor heterostructures.