Xinmin Cui

A broadband antireflective coating based on a double-layer system containing mesoporous silica and nanoporous silica

A double-layer broadband antireflective (AR) coating with excellent transmittance was successfully fabricated using two kinds of silica coatings. The excellent optical performance in a wide wavelength range came from a reasonable refractive index gradient from air to the substrate produced by the mesopores in the bottom-layer silica coating and the particle-packed pores in the top-layer silica coating. The maximum transmittance of the broadband AR coating was approximated to 100.0% at the peak value and above 99.6% in the visible region from 400 to 800 nm. Meanwhile, the average transmittance of the coating was more than 99.0% over the range of 360 to 920 nm. In addition, the double-layer broadband silica AR coating showed strong mechanical performance and good environmental stability. This work provides an alternative way to prepare a broadband AR coating for some applications in energy harvesting and optical devices.

Tri-wavelength broadband antireflective coating built from refractive index controlled MgF2 films

Regulation of the refractive index over a wide range is very important in the realization of tri-wavelength antireflective (AR) coating in high power laser systems, but many regulation approaches are too complex or violent to satisfy the practical requirements. Here, a simple, template-free sol–gel route was proposed to regulate the refractive index of MgF2 film by heat treating MgF2 sol and hence to control the self-assembly process of colloidal MgF2 nanocrystals. In this self-assembly process, the originally packed nanocrystals gradually evolved into bigger hollow vesicles, which reduced the refractive index of the MgF2 film from 1.38 to 1.2. When the refractive indices of the bottom and top layers were set as 1.34 and 1.2, the tri-wavelength broadband AR coating was finally realized on a quartz substrate, with the transmittance of 99.54%, 98.65% and 98.58% at 351 nm, 527 nm and 1053 nm, respectively.

Broadband antireflective double-layer mesoporous silica coating with strong abrasion-resistance for solar cell glass

To enhance the efficiency of solar cells, a broadband double-layer antireflective (AR) coating with excellent transmittance and abrasion-resistance, was successfully fabricated using two layer mesoporous silica coatings. Both layers were prepared via a solvent evaporation self-assembly method in which the top- and bottom-layer mesoporous silica coatings used Pluronic F127 and cetyltrimethylammonium bromide (CTAB) as templates, respectively. The grazing incidence small angle X-ray scattering (GISAXS) and the transmission electron microscope (TEM) results indicated that the mesopores in the double-layer AR coating belonged to a Fmmm orthorhombic symmetry structure of the SBA-16 in the top layer and a P6/mmc 3D-hexagonal structure of the MCM-41 in the bottom layer. The solar-weighted average transmittance (TPV) of the broadband AR coating is approximately 99.10% on quartz, 98.62% on borosilicate glass, and 98.55% on K9 glass in the solar spectrum range of 300–2400 nm. By introducing broadband AR coating, the overall power conversion efficiency (η) of the solar cell showed an increase of 1.23% for quartz, 1.31% for borosilicate glass, and 1.37% for K9 glass. Meanwhile, the double-layer AR coating had excellent mechanical stability; the TPV value of coating after abrasion by CS-10F wearaser only decreased 0.16% on quartz, 0.29% on borosilicate glass and K9 glass. The pencil hardness of the double-layer AR coating was found to be 6H.

Thermochromic VO2 films from ammonium citrato-oxovanadate(IV) with excellent optical and phase transition properties

Thermochromic VO2 film is a potential material for energy-saving windows in future buildings. Considering the difficulty in controlling the valence state and polymorphism during the formation of VO2 (M) film, we propose a facile and safe solution method of fabricating monoclinic (M) VO2 film directly from the newly synthesized ammonium citrato-oxovanadate(IV) compound [(NH4)4[V2O2(C6H4O7)2]·2H2O, denoted as CA-V(IV)], as a vanadium(IV) precursor to stabilize vanadium in the 4+ valence state without utilizing V2O5 as an intermediate or complex post-treatment in vacuum. This new ambient-stable compound CA-V(IV) contains centrosymmetric dinuclear complex anions [{VO(C6H4O7)}2]4−, in which the carboxylate groups are coordinated to the V4+ ions in a monodentate fashion and each vanadium atom exhibits distorted octahedral geometry. With a perfect monoclinic phase, the VO2 films possessed excellent thermochromic and visible transmissive properties. Accompanying a change in film thickness from 119 nm to 41 nm, the integral visible transmittance Tvis at 25 °C of VO2 films ranged from 38.4% to 70.0%. The maximum visible transmittance (Tmax) reached 77.2% for VO2 film with a thickness of 41 nm, which shows that the phase transition of VO2 did not greatly affect the visible transmittance of the film. For the VO2 film with a thickness of 41 nm, of which the visible transmittance modulation (ΔTvis = 2.1%) was the largest among the three samples, the solar energy modulation (ΔTsol) reached 11.3%. Moreover, the transition temperature of our VO2 films was far below that of bulk VO2 (68 °C), and the best transition temperature was as low as 50.8 °C.

Nonspherical hollow α-Fe2O3 structures synthesized by stepwise effect of fluoride and phosphate anions

Despite the significant progress in making hollow structures, it is still a challenge to synthesize some specialized hollow structures. In the present work, we obtained a new hollow hematite structure, tube-in-dodecahedron, by using the stepwise influences of fluoride and phosphate anions. Based on condition-dependent experiments, we proposed a “nucleation–aggregation–recrystallition and etching” mechanism, which also directed us to synthesize a series of hematite hollow structures, including hollow dodecahedron and hollow ellipsoid. The concentration of phosphate was found to play a decisive role in the control of these hollow structures. 0.08 mM is the critical point for keeping the top facets of dodecahedral hematite particles while 0.2 mM is the upper limit for keeping the lateral facets. The magnetic properties of these synthesized hollow hematite structures were found to be closely associated with the structures. The synthesized tube-in-dodecahedral hematite particles exhibited excellent photocatalytic reactivity toward organic dyes.

A hydrophobic and abrasion-resistant MgF2 coating with an ultralow refractive index for double-layer broadband antireflective coatings

An ultralow-index top layer is a prerequisite to preparing high-performance broadband AR coatings. When coupled with hydrophobicity and the abrasion-resistance required in practical applications, the challenge becomes even greater. In this work, a MgF2 AR coating was studied because of the low refractive index of MgF2 and its easily realized strong adhesion through low-temperature heat treatment. In order to obtain an ultralow refractive index and endow the coating with hydrophobicity, we designed several experimental routes and finally adopted MTES/TEOS co-precursors to direct MgF2 particles to form a honeycomb-like network structure without a template. A final refractive index of 1.15 and good hydrophobicity, with a water contact angle of 122°, were obtained using the MgF2–SiO2(CH3) coating. These superior properties were attributed to the incorporation of methyl groups, which not only endowed the coating with hydrophobicity, but also changed the original linear assembly of MgF2 particles to a circular assembly. Using this hydrophobic ultralow-index coating as a top layer, a high-performance double-layer AR coating was fabricated with a high average transmittance of 99.43% in the wavelength range of 400–1000 nm, good abrasion-resistance, and damp heat resistance after a low-temperature heat treatment of 250 °C. This MgF2 double-layer AR coating may be used in display devices or lenses.

Transparent and Dense Ladder-Like Alkylene-Bridged Polymethylsiloxane Coating with Enhanced Water Vapor Barrier Property

Organic-inorganic hybrid composites have been well-studied as water vapor barrier materials for their long diffusion length of water vapor in coatings which can be realized by improving the aspect ratio of inorganic components and regularity of nanostructure in coatings. In this paper, dense organic-inorganic hybrid coating based on ladder-like alkylene-bridged polymethylsiloxane (ABPMS) was successfully fabricated through the hydrosilylation reaction between polymethylhydrosiloxane and diene (1,5-hexadiene or 1,7-octadiene) in toluene under Pt/C catalysis. Its ladder-like structure was verified by 29Si magic angle spinning (MAS) NMR, 13C MAS NMR, and in-plane and out-of-plane glance-incident X-ray diffraction (GIXRD) techniques. Its corresponding coating showed excellent water vapor barrier ability for a typical water-soluble crystal, potassium dihydrogen phosphate (KDP). When treated in 50% relative humidity (RH) condition at 25 °C for 8 months, the ABPMS coating with 100 nm thickness displayed a very low transmittance loss of 1.6% compared with the high transmittance loss of 10% for uncoated KDP. Moreover, the ABPMS coating showed good ultraviolet radiation resistance, thermal stability, low mechanical property, and excellent compatibility with hydrophobic antireflective (AR) coatings.

New Water Vapor Barrier Film Based on Lamellar Aliphatic-Monoamine-Bridged Polysilsesquioxane

Siloxane-based hybrid lamellar materials with ordered nanostructure units paralleling to the substrate have been widely used for water vapor barrier. However, it is very difficult to control the orientation of the lamellar units at molecular level. In this Research Article, a new lamellar bridged polysilsesquioxane (BPSQ) film, whose voids between lamellae were filled by pendant alkyl chains in the organic bridge, was prepared via the stoichiometric reaction between 3-glycidoxypropyltrimethoxysilane and aliphatic monoamine at 60 °C without catalyst. Experimental evidence obtained from FT-IR, MS, NMR, and GIXRD techniques suggested that the as-prepared BPSQ films were constructed by lamellar units with disordered orientation. Nonetheless, they possessed satisfactory water vapor barrier performance for potassium dihydrogen phosphate (KDP) and deuterated potassium dihydrogen phosphate (DKDP) optical crystals, and the water vapor transmission rate through BPSQ film with thickness of 25 μm was as low as 20.3 g·m(-2)·d(-1). Those results proved that filling the voids between molecular lamellae with alkyl chains greatly weakened the effect of lamellar unit orientation on the vapor barrier property of BPSQ film.