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The sensing mechanism of luminescent metal-organic framework [Zn(3-tzba)(2,2′-bipy)(H2O)] -3H2O for formaldehyde detection was explored by using density functional theory and time-dependent density functional theory methods. Our investigation found that luminescent metal-organic framework [Zn(3-tzba)(2,2′-bipy)(H2O)] • 3H2O is able to interact with formaldehyde through hydrogen bonding to the framework. The luminescent mechanism of the hydrogen-bonded complex is photo-induced electron transfer; while the luminescent mechanism of luminescent metal-organic framework [Zn(3-tzba)(2,2′-bipy)(H2O)]-3H2O is ligand-to-ligand charge transfer. The intermolecu-lar hydrogen bond was found to be stronger in the excited state than that in the ground state by analyzing the geometry nuclear magnetic resonance, binding energy and infrared spectrum in different electronic states. Calculated fluorescence radiative rate coefficient and internal conversion rate coefficient qualitatively indicated a reduced radiative process and an enhanced internal conversion process of the hydrogen-bonded complex. The hydrogen-bonded complex exhibits luminescence weakening or even quenching due to the enhancement of the intermolecular hydrogen bond in the excited state compare with luminescent metal-organic framework [Zn(3-tzba)(2,2′-bipy)(H2O)]-3H2O. The variable luminescence demonstrated the potential of luminescent metal-organic framework [Zn(3-tzba)(2,2′-bipy)(H2O)]-3H2O as luminescent sensor for formaldehyde detection.

Efficiently harvesting solar energy for photocatalysis remains a challenge because near infrared (NIR) light with low excitation energy cannot be directly utilized by general semiconductor photocatalysts, although it makes up 46% of the solar spectrum. Herein, a broad-spectrum photocatalyst with pollutant adsorption-enrichment capability to enable natural sunlight-driven photocatalysis was synthesized by decorating a small amount of silver orthophosphate (Ag3PO4) nanodots on mesoporous silica (mSiO2)-functionalized upconversion nanoparticles (UCNPs; NaYF4:Yb,Tm@NaLuF4). The UCNPs play the role of broadening the absorption spectrum by upconverting NIR light into visible light, which can be absorbed by the Ag3PO4 nanodots. The functionalized hydrophilic mSiO2 shell not only facilitates the decoration of Ag3PO4 nanodots but is also capable of adsorbing and enriching pollutants around the Ag3PO4 nanodots to enhance the photocatalytic activity. Upon natural sunlight irradiation, the as-prepared photocatalyst (UCNP@mSiO2-Ag3PO4) exhibited remarkable photocatalytic activity with degradation rate constants of 0.24 and 0.051 min−1 under stirring and static conditions, exceeding that of the pure Ag3PO4 sample by factors of 4.2 and 12.5, respectively. Thus, it can be anticipated that the design of photocatalysts with the synergy of broadband absorption and pollutant adsorption-enrichment will be a promising strategy to realize efficient sunlight-driven photocatalysis, even without extra mechanical stirring for future scaled-up water treatment.

Two twisted π-conjugated cruciform luminophores DMTCS-CC6 and DBTCS-CC6 characterized by rigid X-shaped geometry with two conjugated donor and acceptor intersecting at the central aromatic core have been rationally designed and successfully synthesized. Spectral analysis and theoretical calculations studies indicated that the two cross-conjugated compounds showed unique intramolecular charge-transfer (ICT) process from the donor (carbazole) to the acceptor (dicyanodistyrylbenzene) axis in the excited state. Especially, DMTCS-CC6 and DBTCS-CC6 exhibited efficient aggregation-induced emission (AIE) and high contrast mechanofluorochromic (MFC) properties. By simple grinding, the as-prepared powders of DMTCS-CC6 and DBTCS-CC6 emitting strong yellowish-green (540 nm) and yellow light (560 nm) under UV irradiation could be transferred into powders emitting yellow (582 nm) and red (609 nm), respectively, the red shifts of 42 and 49 nm were observed. Such mechanochromism was reversible upon the treatment of grinding and fuming with DCM. The XRD patterns suggested that the MFC behavior originated from the transition between the crystalline and amorphous states. More interestingly, DBTCS-CC6 exhibited more obvious AIE and more contrast MFC behavior relative to DMTCS-CC6. This is a result of the quite larger space hindrance induced by 3,5-bis(trifluoromethyl)phenyl with two trifluoromethyl groups bonded at meta positions could make DMTCS-CC6 exhibit more twisted spatial conformation, leading more efficient solid state fluorescence and better structural transformability.

SnS2 nanosheets with unique properties are excellent candidate materials for fabricating high-performance NO2 gas sensors. However, serious restacking and aggregation during sensor fabrication have greatly impacted the sensing response. In this study, flower-like hierarchical SnS2 was prepared by a simple microwave method and partially thermally oxidized to form hierarchical SnS2/SnO2 nanocomposites to further improve the sensing performance at low operating temperature. The fabricated SnS2/SnO2 sensor exhibited ultrahigh response (resistance ratio = 51.1) toward 1 ppm NO2 at 100 °C, roughly 10.2 times higher than that of pure SnS2 nanoflowers. The excellent and enhanced NO2 sensing performances of hierarchical SnS2/SnO2 nanocomposites were attributed to the novel hierarchical structure of SnS2 and the nanoheterojunction between SnS2 and the ultrafine SnO2 nanoparticles. The SnS2/SnO2 sensors also exhibited excellent selectivity and reliable repeatability. The simple fabrication of high performance sensing materials may facilitate the large-scale production of NO2 gas sensors.

Metal oxide/graphene nanocomposites are emerging as promising materials for developing room-temperature gas sensors. However, the unsatisfactory performances owing to the relatively low sensitivity, slow response, and recovery kinetics limit their applications. Herein, a highly sensitive and rapidly responding room-temperature NO2 gas sensor based on WO3 nanorods/sulfonated reduced graphene oxide (S-rGO) was prepared via a simple and cost-effective hydrothermal method. The optimal sensor response of the WO3/S-rGO sensor toward 20 ppm NO2 is 149% in 6 s, which is 4.7 times higher and 100 times faster than that of the corresponding WO3/rGO sensors. In addition, the sensor exhibits excellent reproducibility, selectivity, and extremely fast recovery kinetics. The mechanism of the WO3/S-rGO nanocomposite gas sensor is investigated in detail. In addition to the high transport capability of S-rGO as well as its excellent NO2 adsorption ability, the superior sensing performance of the S-rGO/WO3 sensor can be attributed to the favorable charge transfer occurring at the S-rGO/WO3 interfaces. We believe that the strategy of compositing a metal oxide with functionalized graphene provides a new insight for the future development of room-temperature gas sensors.

A theoretical investigation of hydroxide ion transport mechanism in quaternary ammonium functionalized polystyrene (QAPS) anion exchange membrane (AEM) was studied by density functional theory (DFT). The results showed that there were two steps for OH− transferring through QAPS-AEM. The first step was the movement of OH− in water channel, which was induced by frequently forming and breaking of hydrogen bonds (H-bonds) between H2O and OH−. The second step was that OH− transferred across the quaternary ammonium (QA) groups by following the rotation about C C single bond, which was the rate-determining step for OH− transferring in QAPS-AEM. We presented that the ionic groups on the side chain of polymer with smaller space steric should provide higher ion conductivity due to their lower rotation energy barriers.

Ever-increasing concerns over air quality and the newly emerged internet of things (IoT) for future environmental monitoring are stimulating the development of ultrasensitive room-temperature gas sensors, especially for nitrogen dioxide (NO2), one of the most harmful air pollution species released round-the-clock from power plants and vehicle exhausts. Herein, tin dioxide nanorods/ethylenediamine-modified reduced graphene oxide (SnO2/EDA-rGO) heterojunctions with selective adsorption and electronic structure modulation were engineered for highly sensitive and selective detection of NO2 at room temperature. The modified EDA groups not only enable selective adsorption to significantly enrich NO2 molecules around the interface but also realize a favorable modulation of SnO2/EDA-rGO electronic structure by increasing the Fermi level of rGO, through which the sensing performance of NO2 is synergistically enhanced. The response of the SnO2/EDA-rGO sensor toward 1 ppm NO2 reaches 282%, which exceeds the corresponding SnO2/rGO sensor by a factor of 2.8. It also exhibits a low detection limit down to 100 ppb, enhanced selectivity, and rapid response/recovery kinetics. This approach to designing a novel heterojunction with significantly enhanced chemical and electric effects may shed light on the future engineering of gas-sensing materials.

The rapid recombination and poor redox capability of photoinduced charge carriers seriously impede the photocatalytic activity. Herein, a direct Z-scheme photocatalyst was fabricated by selectively depositing Ag6Si2O7 nanoparticles (NPs) on the {0 4 0} facets of BiVO4. Owing to the facet-dependent band structure of BiVO4, the photoinduced electrons and holes can be pre-separated upon photoillumination and transfer to {0 4 0} and {1 1 0} facets of BiVO4, respectively. A rapid combination of the separated electrons at the {0 4 0} facets of BiVO4 with holes of the Ag6Si2O7 NPs then takes place triggering a direct Z-scheme photocatalysis. Such a charge pre-separation triggered by the faceted BiVO4, together with Z-scheme at the interface warrants not only effective photoinduced charge separation but also excellent redox capability owing to the increased redox potential. The photocatalytic rate constant of the optimum BiVO4{0 4 0}/Ag6Si2O7 heterostructure is 28.2 and 17.2 times higher than those of pristine BiVO4 and Ag6Si2O7, respectively. This study offers a new insight for constructing direct Z-scheme photocatalysts with high degradation efficiency based on facet-controlled crystals.

Graphene and its derivatives are attractive for electrocatalytical application in dye-sensitized solar cells because of their unique structures and electronic properties. By means of density functional theory calculations, the mechanism of triiodide reduction reaction on nitrogen-doped graphene (NDG) was studied in acetonitrile environment. The computations demonstrated that the rate-determining step was the ability of NDG to release electrons to active iodine atoms. According to the calculation, the optimal NDG was designed with nitrogen contents of 4.0 % graphite N and 3.0 % pyridinic N approximately. In order to precisely distinguish these two nitrogen species in the optimal NDG, we proposed the chemical shift of 15N NMR of nitrogen doped in graphene provided guidance for the experiments.

Carbon quantum dots (CQDs) had attracted much attention due to their unique structures and excellent properties. Their green preparation was one of the research frontiers. However, most of the CQDs were prepared by strong acid oxidation, the way of which was not friendly to the environment. In this study, CQDs were prepared by green ozone oxidation of lignite coal, which is abundant and inexpensive. The CQDs were well dispersed, the size distribution of the obtained CQDs centralized from 2 to 4 nm with the average diameter of about 2.8 nm. In addition, the as-prepared CQDs containing rich oxygen functional groups exhibited good water-solubility and optical properties with yield reached 35%. The CQDs showed a highly sensitive and selective quenching effect to Fe3+ with desirable anti-interference performance. Moreover, the fluorescence intensity of CQDs had a good linear response to the Fe3+ concentration ranging from 10 to 150 µmol/L with the detection limit of 0.26 µmol/L. This green and facile synthesis method had the prospect of large-scale preparation of CQDs.

It is demonstrated that silver nanoparticle (SNP) arrays fabricated by combining nanoimprint lithography and electrochemical deposition methods can be used as substrates for metal-enhanced fluorescence, which is widely used in optics, sensitive detection, and bioimaging. The method presented here is simple and efficient at controlling the nanoparticle density and interparticle distance within one array. Furthermore, it is found that the fluorescence intensity can be tuned by engineering the feature size of the SNP arrays. This is due to the different coupling efficiency between the emission of the fluorophores and surface plasmon resonance band of the metallic nanostructures.