High-Sensitivity Photoelectrochemical Ultraviolet Photodetector with Stable pH-Universal Adaptability Based on Whole Single-Crystal Integrated Self-Supporting 4H-SiC Nanoarrays.

Emerging photoelectrochemical (PEC) photodetectors (PDs) have notable advantages over conventional PDs and have attracted extensive attention. However, harsh liquid environments, such as those with high corrosivity and attenuation, substantially restrict their widespread application. Moreover, most PEC PDs are constructed by assembling numerous nanostructures on current collector substrates, which inevitably contain abundant interfaces and defects, thus greatly weakening the properties of PDs. To address these challenges, a high-performance pH-universal PEC ultraviolet (UV) PD based on a whole single-crystal integrated self-supporting 4H-SiC nanopore array photoelectrode is constructed, which is fabricated using a two-step anodic oxidation approach. The PD exhibits excellent photodetection behavior, with high responsivity (218.77 mA W-1 ), detectivity (6.64 × 1013  Jones), external quantum efficiency (72.47%), and rapid rise/decay times (17/48 ms) under 375 nm light illumination with a low intensity of 0.15 mW cm-2 and a bias voltage of 0.6 V, which is fall in the state-of-the-art of the wide-bandgap semiconductor-based PDs reported thus far. Furthermore, the SiC PEC PD exhibits excellent photoresponse and long-term operational stability in pH-universal liquid environments. The improved photodetection performance of the SiC PEC PD is primarily attributed to the synergistic effect of the nanopore array structure, integrated self-supporting configuration, and single-crystal structure of the whole photoelectrode.

Surroundings effects on the interfacial and temperature behaviors of NaOH/water bound to hydrophilic and hydrophobic nanosilicas.

HYPOTHESIS: It has been assumed that the temperature and interfacial behaviors of concentrated alkali solutions under confined space effects may depend on adsorbent surface structure, hydrophilicity/hydrophobicity, porosity of solids, and dispersion media properties causing kosmotropic or chaotropic effects onto hydrogen bond network (HBN) in bound water and NaOH solution. EXPERIMENTS: To analyze these effects, systems with NaOH/water (0.1 g/g/0.1 g/g) deposited onto compacted hydrophilic (A-300) and hydrophobic (AM1) nanosilicas were studied using 1H NMR spectroscopy (215-287 K). The materials were characterized using several experimental and theoretical methods. FINDINGS: It has been shown that bound water and water/NaOH represent various clusters and domains whose characteristics depend strongly on nanosilica hydrophilicity/hydrophobicity, dispersion media (air, CDCl3, DMSO, CDCl3/DMSO), subsequent or simultaneous deposition of NaOH and water, and temperature. Water amount (0.1 g/g) was selected too small to completely dissolve NaOH (0.1 g/g) under confined space effects and low temperatures. Chaotropic hydrophobic AM1 and CDCl3 enhance water clusterization and HBN disorder (weakly associated water, WAW appears) in contrast to kosmotropic hydrophilic A-300, NaOH, and DMSO reducing the clusterization and HBN disorder in bound water (WAW disappears). Several aspects related to the interfacial and temperature behaviors of water and co-adsorbates bound to the nanosilicas were elucidated.

Interfacial behavior of polar and nonpolar frozen/unfrozen liquids interacting with hydrophilic and hydrophobic nanosilicas alone and in blends.

HYPOTHESIS: Various nanosilica characteristics depend on hydrophobization strongly affecting interfacial phenomena. Is it possible to prepare hydrophilic samples with hydrophobic silica (AM1) alone and in blends with hydrophilic one (A-300)? It can be done with addition of a small amount of water to the powders which then are mechanically treated. EXPERIMENTS: Nanosilicas were characterized using adsorption, desorption, microscopic, spectroscopic, and quantum chemistry methods. 1H NMR spectroscopy and cryoporometry were applied to AM1 and AM1/A-300 blends wetted and mechanically treated. Wetted blends were studied with additions of n-decane and chloroform-d. FINDINGS: The powders wetted at h = 0.3-3.0 g of water per gram of dry solids have increased bulk density. Samples are in gel-like state at h = 4-5 g/g. Water interaction energy with nanoparticles nonmonotonically depends on h (maximal at h = 3 g/g). Upon mechanical treatment of wetted blends (h < 1.5 g/g), separated AM1 structures are absent. At greater h values, blend reorganization occurs to form AM1 aggregates covered by A-300 shells. Organics can displace water from mesovoids toward narrower pores inaccessible for larger molecules or into larger voids to reduce the contact area between immiscible liquids. Freezing point depression caused by confined space and dissolution effects is affected by the blend organization.

Nanostructured Polymethylsiloxane/Fumed Silica Blends.

Polymethylsiloxane (PMS) and fumed silica, alone and in a blended form (1:1 w/w), differently pretreated, hydrated, and treated again, were studied using TEM and SEM, nitrogen adsorption-desorption, 1H MAS and 29Si CP/MAS NMR spectroscopy, infrared spectroscopy, and methods of quantum chemistry. Analysis of the effects of adding water (0-0.5 g of water per gram of solids) to the blends while they are undergoing different mechanical treatment (stirring with weak (~1-2 kg/cm2) and strong (~20 kg/cm2) loading) show that both dry and wetted PMS (as a soft material) can be grafted onto a silica surface, even with weak mechanical loading, and enhanced mechanical loading leads to enhanced homogenization of the blends. The main evidence of this effect is strong nonadditive changes in the textural characteristics, which are 2-3 times smaller than additive those expected. All PMS/nanosilica blends, demonstrating a good distribution of nanosilica nanoparticles and their small aggregates in the polymer matrix (according to TEM and SEM images), are rather meso/microporous, with the main pore-size distribution peaks at R > 10 nm in radius and average values of 18-25 nm. The contributions of nanopores (R < 1 nm), mesopores (1 nm < R < 25 nm), and macropores (25 nm < R < 100 nm), which are of importance for studied medical sorbents and drug carriers, depend strongly on the types of the materials and treatments, as well the amounts of water added. The developed technique (based on small additions of water and controlled mechanical loading) allows one to significantly change the morphological and textural characteristics of fumed silica (hydrocompaction), PMS (drying-wetting-drying), and PMS/A-300 blends (wetting-drying under mechanical loading), which is of importance from a practical point of view.

Water Interactions with Hydrophobic versus Hydrophilic Nanosilica.

It is well-known that interaction of hydrophobic powders with water is weak, and upon mixing, they typically form separated phases. Preparation of hydrophobic nanosilica AM1 with a relatively large content of bound water with no formation of separated phases was the aim of this study. Unmodified nanosilica A-300 and initial AM1 (A-300 completely hydrophobized by dimethyldichlorosilane), compacted A-300 (cA-300), and compacted AM1 (cAM1) containing 50-58 wt % of bound water were studied using low-temperature 1H NMR spectroscopy, thermogravimetry, infrared spectroscopy, microscopy, small-angle X-ray scattering, nitrogen adsorption, and theoretical modeling. After mechanical activation (∼20 atm) upon stirring of AM1/water mixture at the degree of hydration h = 1.0 or 1.4 g of distilled water per gram of dry silica, all water is bound and the blend has the bulk density of 0.7 g/cm3. The temperature and interfacial behaviors of bound water depend strongly on a dispersion media type (air, chloroform, and chloroform with trifluoroacetic acid (4:1)) because the boundary area between immiscible water and chloroform should be minimal. Water and chloroform molecules are of different sizes affecting their distribution in pores (voids between silica nanoparticles in their aggregates) of different sizes. Structural, morphological, and textural characteristics of silicas, and environmental features affect not only the distribution of bound water, but also the amounts of strongly (frozen at T < 260 K) and weakly (frozen at 260 K < T < 273 K) bound and strongly (chemical shift δH = 4-6 ppm) and weakly (δH = 1-2 ppm) associated waters. Despite the changes in the characteristics of cAM1, it demonstrates a flotation effect. The developed system with cAM1/bound water could be of interest from a practical point of view due to controlled interactions with aqueous surroundings.

Interfacial behavior of n-decane bound to weakly hydrated silica gel and nanosilica over a broad temperature range.

The interfacial and temperature behavior of n-decane bound to weakly hydrated nanosilica A-400 (initial, heated, or compacted) or silica gel Si-60 was studied using low-temperature (1)H NMR spectroscopy applied to static samples that allowed us to observe signals only of mobile decane and unfrozen water molecules. For deeper insight into the phenomena studied, interactions of n-decane, 1-decanol, and water with a set of nanosilicas and silica gels were analyzed using DSC and thermoporometry. Both NMR and DSC results demonstrated that during heating of frozen samples at a heating rate of 5 K/min a portion of decane or decanol remained frozen at temperature higher than the freezing point of bulk liquid (Tf). For decane and decanol adsorbed onto silica gels Si-40, Si-60, and Si-100, the number, position, and intensity of freezing and melting peaks observed in the DSC thermograms over the 170-300 K range during cooling and heating of samples depended on the pore size distribution of silicas as well as on the amounts and type of adsorbates. The position of the main freezing peak of decane for all samples was close to Tf because the alkane amount was greater than the pore volume; i.e., a fraction of decane was bulk liquid. According to (1)H NMR data, a portion of decane, which was in a quasi-crystalline solid state characterized by fast molecular exchange (i.e., short transverse relaxation time) and not observed in the spectra, was greater than a portion of decane frozen at temperatures close to Tf during cooling that appears in the DSC endotherms of heated samples.