VO₂/ZnO bilayer films with enhanced thermochromic property and durability for smart windows

VO_{2} films are widely considered as one of the most suitable material to act as smart windows. Although this system is able to function, the durability of the film has been an issue as the surface of the films may oxidize by converting V^{4+} to V^{5+}. To overcome this problem, attempt is made to coat the VO_{2} film with ZnO, which can assist by creating a resistance layer to prevent further oxidation of VO_{2}. Here, VO_{2}/ZnO bilayer film was prepared by a facile method comprised of spin-coating and dip-coating process and shows excellent durability, and in particular. the solar modulation efficiency (△T_{sol}) maintaining ca 89.9% (from 17.8% to 16.0%) after 300 days in a humid environment, however, the △T_{sol} of pure VO_{2} film is decreased from 11.8% to 4.1%. Also, the VO2/ZnO bilayer exhibits an enhanced thermochromic property of visible transmittance (T_{lum} = 55.7 ± 2.1%) and △T_{sol} (17.1 ± 1.4%) which is 1.49 times higher than that of pure VO2 film (△T_{sol} = 11.5 ± 0.4%). The enhancement in the thermochromic performance and durability is probably attributed to the anti-reflection and protection of ZnO layer. Therefore, this work can provide an effective way to optimize thermochromic property for practical application of VO_{2}-based smart windows

Facile Solution Process of VO2 Film with Mesh Morphology for Enhanced Thermochromic Performance

The fabrication and applications of VO2 film continue to be of considerable interest due to their good thermochromic performance for smart windows. However, low visible transmittance (Tlum) and solar modulation efficiency (∆Tsol) impede the application of VO2 film, and they are difficult to improve simultaneously. Here, a facile zinc solution process was employed to control the surface structure of dense VO2 film and the processed VO2 film showed enhanced visible transmittance and solar modulation efficiency, which were increased by 7.5% and 9.5%, respectively, compared with unprocessed VO2 film. This process facilitated the growth of layered basic zinc acetate (LBZA) nanosheets to form mesh morphology on the surface of VO2 film, where LBZA nanosheets enhance the visible transmittance as an anti-reflection film. The mesh morphology also strengthened the solar modulation efficiency with small caves between nanosheets by multiplying the times of reflection. By increasing the zinc concentration from 0.05 mol/L to 0.20 mol/L, there were more LBZA nanosheets on the surface of the VO2 film, leading to an increase in the solar/near-infrared modulation efficiency. Therefore, this work revealed the relationship between the solution process, surface structure, and optical properties, and thus can provide a new method to prepare VO2 composite film with desirable performance for applications in smart windows

Solution-Processed Gas Sensors Based on ZnO Nanorods Array with an Exposed (0001) Facet for Enhanced Gas-Sensing Properties

In this paper, thin film gas sensors made from 8-nm-diameter and exposed-{10-10}-facet ZnO nanorods self-aligning along the ceramic tube are fabricated by a simple dip-coating method. On the sensor surface, we successfully synthesize a ZnO nanorods array exposed with (0001) plane in situ by a facile solution-processing technique. Compared with the unprocessed sensor (i.e., dip-coated ZnO film based sensor), the main advantages of the solution-processed sensor are a high sensitivity (3-fold prefactor <i>A</i>_g), fast response (less than 10 s), and low detection limit (1 ppm) to benzene and ethanol. The enhancement in the gas-sensing performance suggests that the effect of exposed facet is dominant rather than the size effect, and the order of gas-sensing properties of ZnO crystal face is (0001) > {10-10}. On the basis of these results, it is found that the surface structure at the atomic level is a key factor in improving the oxygen adsorption and, consequently, the gas-sensing performance of a ZnO nanorods array based gas sensor

Enhanced Luminescence of Dye-Decorated ZIF-8 Composite Films via Controllable D-A Interactions for White Light Emission

Metal-organic frameworks (MOFs) constructed by metal ions/clusters and organic linkers are used to encapsulate fluorescent guest species with aggregation-caused quenching (ACQ) effects to enhance fluorescence properties due to their porous structures and high specific surface areas. However, there would be a problem of matching between MOF pores and guest molecules' sizes. In this paper, amorphous ZIF-8 was modified by carboxyl functional groups (H3BTC-ZIF-8) via introducing the 1,2,4-benzenetricarbonic acid (H3BTC) ligand into the ZIF-8 sol system. Moreover, H3BTC-ZIF-8 was used for the loading of organic fluorescent dyes rhodamine 6G (R6G) and coumarin 151 (C151) to prepare R6G/C151/H3BTC-ZIF-8 composite films. A white-light-emitting composite film (R6G/C151/H3BTC-ZIF-8) with CIE coordinates of (0.323, 0.347) was successfully prepared by compounding fluorescent dyes (R6G and C151) with H3BTC-modified ZIF-8, whose photoluminescence quantum yield (PLQY) can reach 64.0%. It was higher than the PLQY of the composite films prepared by crystalline ZIF-8 (40.2%) or amorphous ZIF-8 without H3BTC (48.0%) compounded with the same concentrations of dyes. The fluorescence enhancement was probably attributed to an increased amount of active sites of H3BTC-modified ZIF-8 interacting with dyes C151 and R6G. This can form hydrogen bonds between H3BTC-ZIF-8 and C151, and weak electron donor-acceptor (D-A) interactions between H3BTC-ZIF-8 and R6G molecules, respectively, thus enhancing the interactions between dyes and ZIF-8 and reducing the ACQ effect existing between dye molecules. Therefore, this strategy could provide an important guidance to develop white-light-emissive materials

Modulation of Structure and Optical Property of Nitrogen-Incorporated VO2 (M1) Thin Films by Polyvinyl Pyrrolidone

VO2, as a promising material for smart windows, has attracted much attention, and researchers have been continuously striving to optimize the performance of VO2-based materials. Herein, nitrogen-incorporated VO2 (M1) thin films, using a polyvinylpyrrolidone (PVP)-assisted sol&ndash;gel method followed by heat treatment in NH3 atmosphere, were synthesized, which exhibited a good solar modulation efficiency (&Delta;Tsol) of 4.99% and modulation efficiency of 37.6% at 2000 nm (&Delta;T2000 nm), while their visible integrated transmittance (Tlum) ranged from 52.19% to 56.79% after the phase transition. The crystallization, microstructure, and thickness of the film could be regulated by varying PVP concentrations. XPS results showed that, in addition to the NH3 atmosphere-N doped into VO2 lattice, the pyrrolidone-N introduced N-containing groups with N&ndash;N, N&ndash;O, or N&ndash;H bonds into the vicinity of the surface or void of the film in the form of molecular adsorption or atom (N, O, and H) filling. According to the Tauc plot, the estimated bandgap of N-incorporated VO2 thin films related to metal-to-insulator transition (Eg1) was 0.16&ndash;0.26 eV, while that associated with the visible transparency (Eg2) was 1.31&ndash;1.45 eV. The calculated Eg1 and Eg2 from the first-principles theory were 0.1&ndash;0.5 eV and 1.4&ndash;1.6 eV, respectively. The Tauc plot estimation and theoretical calculations suggested that the combined effect of N-doping and N-adsorption with the extra atom (H, N, and O) decreased the critical temperature (&tau;c) due to the reduction in Eg1

Facile Synthesis of Island-like ZrO2-VO2 Composite Films with Enhanced Thermochromic Performance for Smart Windows

VO2-based film, as a very promising thermochromic material for smart windows, has attracted extensive attention but has not been widely applied because it is difficult to simultaneously improve in terms of both solar-modulation efficiency (&Delta;Tsol) and visible transmittance (Tlum) when made using the magnetron-sputtering method, and it has poor durability when made using the wet chemical method. Herein, island-like ZrO2-VO2 composite films with improved thermochromic performance (&Delta;Tsol: 12.6%, Tlum: 45.0%) were created using a simple approach combining a dual magnetron-sputtering and acid-solution procedure. The film&rsquo;s &Delta;Tsol and Tlum values were increased initially and subsequently declined as the sputtering power of the ZrO2 target was raised from 30 W to 120 W. &Delta;Tsol achieved its maximum of 12.6% at 60 W, and Tlum reached its maximum of 51.1% at 90 W. This is likely the result of the interaction of two opposing effects: Some VO2 nanocrystals in the composite film were isolated by a few ZrO2 grains, and some pores could utilize their surface-plasmon-resonance effect at high temperature to absorb some near-infrared light for an enhanced &Delta;Tsol and Tlum. More ZrO2 grains means fewer VO2 grains in the composite film and increased film thickness, which also results in a decrease in &Delta;Tsol and Tlum. As a result, this work may offer a facile strategy to prepare VO2-based films with high thermochromic performance and promote their application in smart windows

Divalent Folate Modification on PEG: An Effective Strategy for Improving the Cellular Uptake and Targetability of PEGylated Polyamidoamine–Polyethylenimine Copolymer

The stability and targeting ability of nanocarrier gene delivery systems are necessary conditions to ensure the good therapeutic effect and low nonspecific toxicity of cancer treatment. Poly­(ethylene glycol) (PEG) has been widely applied for improving stability and as a spacer for linking ligands and nanocarriers to improve targetability. However, the cellular uptake and endosomal escape capacity of nanocarriers has been seriously harmed due to the introduction of PEG. In the present study, we synthesized a new gene delivery vector by coupling divalent folate-PEG (PEG_3.4k-FA₂) onto polyamidoamine–polyethylenimine (PME) copolymer (PME–(PEG_3.4k-FA₂)_1.72). Both PEG and monovalent folate-PEG (PEG_3.4k-FA₁) modified PME were prepared as control polymers, which were named as PME–(PEG_3.5k)_1.69 and PME–(PEG_3.4k-FA₁)_1.66, respectively. PME–(PEG_3.4k-FA₂)_1.72 exhibited strong DNA condensation capacity like parent polymer PME which was not significantly influenced by PEG. PME–(PEG_3.4k-FA₂)_1.72/DNA complexes at N/P = 10 had a diameter ∼143 nm and zeta potential ∼13 mV and showed the lowest cytotoxicity and hemolysis and the highest transfection efficiency among all tested polymers. In folate receptor positive (FR-positive) cells, the cellular uptake and transfection efficiency were increased with the increase in the number of folates coupled on PEG; the order was PME–(PEG_3.4k-FA₂)_1.72 > PME–(PEG_3.4k-FA₁)_1.66 > PME–(PEG_3.5k)_1.69. Folate competition assays showed that PME–(PEG_3.4k-FA₂)_1.72 complexes had stronger targeting ability than PME–(PEG_3.5k)_1.69 and PME–(PEG_3.4k-FA₁)_1.66 complexes due to their higher folate density per PEG molecule. Cellular uptake mechanism study showed that the folate density on PEG could change the endocytosis pathway of PME–(PEG_3.5k)_1.69 from clathrin-mediated endocytosis to caveolae-mediated endocytosis, leading to less lysosomal degradation. Distribution and uptake in 3D multicellular spheroid assays showed that divalent folate could offer PME–(PEG_3.4k-FA₂)_1.72 complexes stronger penetrating ability and higher cellular uptake. With these advantages, PME–(PEG_3.4k-FA₂)_1.72 may be a promising nonviral vector candidate for efficient gene delivery. This study also indicates that divalent folate modification on PEG can serve as an efficient strategy to improve the cellular uptake and targeting ability of PEGylated cationic polymers for gene delivery

Aluminium and zinc co-doped CuInS2 QDs for enhanced trion modulation in monolayer WS2 toward improved electrical properties

Considering the significant influence of trions on the optical and electronic properties of two-dimensional transition metal dichalcogenides, the precise tuning of trions in a large range is important for optoelectronic or trion-related applications. Herein, a detailed comparison of eco-friendly CuInS2 (CIS) quantum dots (QDs) and (Al, Zn) co-doped CIS QDs on tuning the trion ratio and electrical properties in a mechanically-exfoliated monolayer (ML) WS2 is presented. With a similar QD thickness, the trion modulation ability of CIS QDs is largely enhanced after co-doping due to existing extra donor states. In particular, the trion ratio of ML WS2 can be precisely tuned from 0.05 to 0.7 by varying the QD species and QD thickness, while only a small range (0.05–0.15) of the trion ratio in ML WS2 is achieved under gate voltage. Moreover, the electron mobility and electron concentration of WS2-based field-effect transistors (FETs) are significantly improved after QD modification, exhibiting potential applications in FETs and photodetectors.</p