•ZnO structures were topologically transformed from hydrozincite microspheres.•Hydrozincite with (0 0 1) facets synthesized by hydrothermal method with citrate.•Exposed (0 0 0 1) facets of ZnO0001 were derived from (0 0 1) facets of hydrozincite.Distinct from normal hierarchical mesoporous ZnO structures exposed with (2 −1 −1 0) facets derived from polycrystal hydrozincite precursors exposed with (0 1 0) facets, single-crystal hydrozincite precursors with exposed (0 0 1) facets were prepared by a hydrothermal method assisted with sodium citrate and then topologically transformed into hierarchical mesoporous ZnO microspheres with exposed (0 0 0 1) facets by calcination. To reveal the transformation process, a novel exposed facet dependent topological transformation mechanism was proposed as follows. Firstly, CO bonds and OH bonds in Zn5(CO3)2(OH)6 were broken with H2O and CO2 gases releasing; secondly, the remaining Zn atoms and O atoms rearranged their positions, leading to a transformation from a unit cell of monoclinic hydrozincite into eight unit cells of wurtzite ZnO based on their relationship of crystal lattice parameter. As a result, the (0 1 0) and (0 0 1) facets of hydrozincite can be transferred into the (2 −1 −1 0) and (0 0 0 1) facets of wurtzite ZnO, respectively. In this sense, the exposed facets of hydrozincite precursors determined the exposed facets of ZnO micro/nano structures. Therefore, the transformation mechanism can be also used to synthesize other metal oxide structures with special exposed facets.

•The responsivity of rPG sensor was several times higher than that of rGO sensor.•The detection limit of rPG sensor lowed to 500 ppb NO2.•High performance due to abundant edge-defective sites and unique porous structure.We report on a high-performance room-temperature NO2 sensor based on porous and multi-defective reduced porous graphene (rPG). The rPG sebsor exhibits much higher responsivity than that of sensor made from reduced graphene oxide (rGO) without artificial nanopores, especially under lower NO2 concentration. The limit of detection of rPG sensor is as much as 28% towards 500 ppb NO2. The remarkable surface reactivity stemmed from abundant defective sites, especially edge-defective sites of nanopores, is highly responsible for the excellent responsivity and sensitivity of rPG sensor. Moreover, the unique porous structure of rPG also contributes to the outstanding gas-sensing performance by providing open gas diffusion path, and thus significantly improving the utilization of surface active sites.

ZnO nanocrystals as catalysts have been widely employed in the catalytic thermal decomposition of ammonium perchlorate (AP). However, the catalytic mechanism is still controversial and the role of surface lattice oxygen is always ignored. Herein, a classical catalytic mechanism based on the surface lattice oxygen was proposed to reveal AP decomposition promoted by ZnO nanosheets. ZnO and ZnS nanosheets, both of which have the same wurtzite structure and the same (2−1−10) exposed facets, have been synthesized by the calcination of a ZnS(en)0.5 precursor in different conditions. ZnO nanosheets with a smaller surface area showed a better catalytic activity than ZnS nanosheets because the surface lattice oxygen of the ZnO nanosheets can react with NH3 (an intermediate of AP thermal decomposition) to generate oxygen vacancies that can subsequently be recovered, while the surface lattice sulfur of the ZnS nanosheets did not react with NH3. The generation and replenishment of oxygen vacancies on the (2−1−10) exposed facets of the ZnO nanosheets were confirmed by XPS and FTIR results, and thus revealed the origin of the efficient catalytic AP decomposition over (2−1−10) facets of ZnO nanosheets as surface lattice oxygen. Therefore, this work could provide a new insight into the catalytic mechanism of metal oxides to promote AP decomposition.

Correction for ‘Enhanced room temperature NO2 response of NiO–SnO2 nanocomposites induced by interface bonds at the p–n heterojunction’ by Jian Zhang et al., Phys. Chem. Chem. Phys., 2016, 18, 5386–5396.

Metal-oxide-semiconductor (MOS) based gas sensors have been considered a promising candidate for gas detection over the past few years. However, the sensing properties of MOS-based gas sensors also need to be further enhanced to satisfy the higher requirements for specific applications, such as medical diagnosis based on human breath, gas detection in harsh environments, etc. In these fields, excellent selectivity, low power consumption, a fast response/recovery rate, low humidity dependence and a low limit of detection concentration should be fulfilled simultaneously, which pose great challenges to the MOS-based gas sensors. Recently, in order to improve the sensing performances of MOS-based gas sensors, more and more researchers have carried out extensive research from theory to practice. For a similar purpose, on the basis of the whole fabrication process of gas sensors, this review gives a presentation of the important role of screening and the recent developments in high throughput screening. Subsequently, together with the sensing mechanism, the factors influencing the sensing properties of MOSs involved in material preparation processes were also discussed in detail. It was concluded that the sensing properties of MOSs not only depend on the morphological structure (particle size, morphology, pore size, etc.), but also rely on the defect structure and heterointerface structure (grain boundaries, heterointerfaces, defect concentrations, etc.). Therefore, the material-sensor integration was also introduced to maintain the structural stability in the sensor fabrication process, ensuring the sensing stability of MOS-based gas sensors. Finally, the perspectives of the MOS-based gas sensors in the aspects of fundamental research and the improvements in the sensing properties are pointed out.

•Reasons for room-temperature difficult recovery of WS2 sensors for NH3 detection.•The excellent recovery within 271.9 s was observed for single-layer WS2 sensor.•The recovery time of WS2 sensor has a anti-linear relation with number of layer.Tungsten disulfide (WS2), as a representative layered transition metal dichalcogenides (TMDs), is expected as a promising candidate for high-performance NH3 sensor at room temperature. Unfortunately, the common WS2 based NH3 sensors are difficult to recovery at room temperature, which severely limits its application. Hence, how to improve recovery has become an urgent problem to be solved. Herein, we prepare five types of WS2 nanosheets with different layer numbers from bulk to monolayer, and find that the recovery time of NH3 gas sensor is rapidly linear shorten as the number of layers decreasing. Through the first-principles calculation of the interaction between NH3 and WS2 substance, the different binding energy between ammonia and the surface (−0.179 eV) and interlayer (−0.356 eV) of layered WS2, as well as the different electron transfer way, should be responsible for the difficult recovery rate of various WS2 samples. Therefore, reducing the number of layer of WS2 is a promising approach to speed up recovery. Based on this conclusion, we successfully prepare a fast recoverable ammonia gas sensor based on single layer WS2, which exhibits exciting fast recovery within 271.9 s at room temperature without any condition. Moreover, our work also can act as a reference for other gas detection of TMDs based gas sensor to improve the gas performance at room-temperature.Download high-res image (192KB)Download full-size image

Recently, heterostructured nanomaterials have attracted great attention in gas sensing applications. However, the sensing mechanism of the enhanced sensitivity of heterostructured nanomaterials remains unclear, which is not conducive to further improvements in their sensing performances. In order to detail the fundamental studies on the gas sensing mechanism of heterostructured nanomaterials and improve the room temperature NO2 sensing properties of NiO-based nanomaterials, NiO–SnO2 heterojunction nanocomposites were fabricated. It was found that the sensitivity of the nanocomposites was largely enhanced compared to the bare NiO. On the basis of the intrinsic characteristics of the p–n heterojunction and the band structure of the NiO–SnO2 heterojunction, the largely enhanced room temperature NO2 response of the nanocomposites could be attributed to two factors. One was the significantly decreased initial conductance, and the increase in the equivalent hole concentration of the nanocomposites after exposure to NO2, associated with the effective electron transfer via the interface bonds at the heterojunction. Another was that the variation of contact potential in the nanocomposites, before and after exposure to NO2, exerted a drastic effect on the transducer function for gas sensing. According to the differentiation in the sensitivity of the nanocomposites with different molar ratios, the important role of interface bonds in gas sensing properties was further illustrated by the dependency of the sensitivity on the interface bond number and the interface resistance. Here, we hope that this work could give us a better understanding of the gas sensing mechanism of the p–n heterojunction, and provide a proper approach for heterojunction materials to further improve their sensing performances.

To improve the gas-sensing performance of metal-oxide-semiconductors, the effect of defects on gas-sensing properties has been widely investigated. Nevertheless, although the metal cation defect is the dominative acceptor defect in p-type semiconductors, its effect on the gas-sensing properties remains blank, which leads to a hindrance for further developing p-type semiconductor-based gas sensors. Accordingly, to eleborate the effect of metal cation defects on the sensing properties, mesoporous NiO nanosheets with different amounts of nickel vacancies were prepared by annealing at different temperatures. It was found that the amount of nickel vacancies increased with increasing the annealing temperature. Gas-sensing studies revealed that the NiO with a higher concentration of nickel vacancies exhibited higher sensitivity to NO2 at room temperature. With further increasing the annealing temperature to 600 °C, although the rapid decrease in the specific surface area of the NiO might limit the physisorption of NO2, the NiO could also present a better sensitivity to NO2 due to the abundant nickel vacancies with high activity. Furthermore, an in situ DRIFTS study demonstrated that the number of adsorbed nitrate and nitrite species on NiO surfaces increased with increasing the amount of nickel vacancies, indicating that the nickel vacancies acted as the dominative active sites participating in the gas–solid reaction and then determined the room-temperature sensing properties. According to the defect ionization equation, a hole conduction model was further proposed to decipher the dependency of sensing properties on the metal cation defects. We hope this work could make us better understand the roles of cation defect in the sensing properties, and it could also benefit the improvement of p-type semiconductor-based gas sensors.

Unique gas-sensing properties of semiconducting hybrids that are mainly related to the heterogeneous interfaces have been considerably reported. However, the effect of heterogeneous interfaces on the gas-sensing properties is still unclear, which hinders the development of semiconducting hybrids in gas-sensing applications. In this work, SnO2–SnS2 hybrids were synthesized by the oxidation of SnS2 at 300 °C with different times and exhibited high response to NH3 at room temperature. With the increasing oxidation time, the relative concentration of interfacial Sn bonds, O–Sn–S, among the total Sn species of the SnO2–SnS2 hybrids increased first and then decreased. Interestingly, it can be found that the response of SnO2–SnS2 hybrids to NH3 at room temperature exhibited a strong dependence on the interfacial bonds. With more chemical bonds at the interface, the lower interface state density and the higher charge density of SnO2 led to more chemisorbed oxygen, resulting in a high response to NH3. Our results revealed the real roles of the heterogeneous interface in gas-sensing properties of hybrids and the importance of the interfacial bonds, which offers guidance for the material design to develop hybrid-based sensors.Keywords: electron transfer; gas sensor; hybrid; interfacial bond; room temperature;

Lowering the working temperature without sacrificing other good gas-sensing properties is of particular interest to gas sensors for an excellent performance. In this work, La surface doped ZnO nanocrystals were successfully prepared by a facile thermal treatment with lanthanum nitrate (La(NO3)3) solution injected into ZnO thick films, which exhibited a remarkable decrease in the optimal working temperature for formaldehyde (HCHO) sensing properties. This was probably attributed to the formation of surface LaZn defects in the ZnO nanocrystals which was evidenced by XRD, XPS results and DFT calculations. The surface LaZn defects can introduce a shallower donor level than oxygen vacancies, and probably facilitate the charge transfer from oxygen species to ZnO for producing chemisorbed oxygen species more easily. This was in good agreement with the DFT results that the absorption energy of oxygen molecules on the surface of La doped ZnO was only −10.61 eV, much lower than that of pure ZnO. Moreover, the optimal working temperature of the La doped ZnO based sensor was significantly decreased from 350 to 250 °C without sacrificing the high and quick response to HCHO gas as the content of surface LaZn defects was increased gradually. Therefore, the behavior of the surface LaZn defects in the optimal working temperature revealed a HCHO response mechanism in ZnO, which can provide new insights into the enhanced HCHO sensing performance of gas sensors.

In recent years, there has been increasing interest in synthesis of reduced graphene oxide (rGO)–metal oxide semiconductor (MOS) nanocomposites for room temperature gas sensing applications. Generally, the sensitivity of a MOS can be obviously enhanced by the incorporation of rGO. However, a lack of knowledge regarding how rGO can enhance gas-sensing performances of MOSs impedes its sensing applications. Herein, in order to get an insight into the sensing mechanism of rGO–MOS nanocomposites and further improve the sensing performances of NiO-based sensors at room temperature, an rGO–NiO nanocomposite was synthesized. Through a comparison study on room temperature NO2 sensing of rGO–NiO and pristine NiO, an inverse gas-sensing behavior in different NO2 concentration ranges was observed and the sensitivity of rGO–NiO was enhanced obviously in the high concentration range (7–60 ppm). Significantly, the stimulating effect of rGO on the recovery rate was confirmed by the sensing characteristics of rGO–NiO that was advantageous for the development of NO2 sensors at room temperature. By comprehending the electronic interactions between the rGO–MOS nanocomposite and the target gas, this work may open up new possibilities for further improvement of graphene-based hybrid materials with even higher sensing performances.

Hierarchical porous ZnO hollow microspheres assembled from nanorods with exposed (001) facets on their external surface were prepared in one pot by a simple low-temperature wet chemical method without templates. The formation mechanism based on their chemical self-transformation was proposed. Importantly, these ZnO hollow microspheres exhibited better catalytic activity for the thermal decomposition of ammonium perchlorate (AP) than the dispersed ZnO nanorods by lowering its decomposition temperature from 409 °C to 308 °C and decreasing its activation energy from 150 ± 14 kJ mol−1 to 63 ± 7 kJ mol−1. This is attributed to their hierarchical porous structure with larger surface area and exposed (001) facets dominant on their external surface, which can facilitate the adsorption of HClO4 and NH3 gases from AP and the formation of active oxygen. The active oxygen will promote the oxidation reaction of NH3 more completely in AP decomposition, thus leading to a significant decrease in decomposition temperature and activation energy. Therefore, this work could provide a new insight into the thermal decomposition mechanism of AP catalyzed by hierarchical micro/nanostructures of metal oxides.

●The photocatalytic activity of rutile TiO2 nanorod array is significantly improved by photothermocatalytic method.●The oxidation rate of benzene first increases and then decreases as the temperature goes up.●The mineralization rate of benzene (the CO2 generation) keeps increasing as the temperature goes up.In this study, we successfully improved the photocatalytic mineralization of gaseous benzene by tuning up temperature from room temperature to 280 °C. It was discovered that the mineralization rate of gaseous benzene (CO2 generation) increased with temperature, while the oxidation rate first increased and then decreased. A mechanism is proposed in which the mineralization rate is promoted for more unoccupied reactive sites due to desorption of H2O at an elevated temperature. However, the oxidation rate is first promoted by thermo-energy and then decreased due to weakened ∙OH-initiated oxidation of benzene due to less ∙OH on the surface at the elevated temperature.

The discovery of gas sensing properties of single-crystalline nanostructures comparable or even better than their polycrystalline counterparts has triggered the attention of the gas sensor research community. To eleborate the sensing mechanisms of single-crystalline and polycrystalline nanostructures, the single-crystalline (SC) NiO hexagonal nanosheet and the nanoparticle self-assembled polycrystalline (PC) NiO nanosheet were prepared for room-temperature NO2 sensing studies. The gas sensing studies revealed that the SC NiO exhibited a remarkably higher response to NO2 at room temperaure than the PC NiO. The correlation between the structural features of NiO and the room temperature NO2 sensing performances was discussed in detail via the grain boundary scattering theory. It was found that the scattering potential played a vital role in the sensing process. Because the absence of grain boundary in the SC NiO reduced the NO2 chemisorption-induced carrier scattering at interfaces, the SC NiO performed the largely enhanced sensitivity. Here we hope that this work can help us to further understand the gas sensing mechanism and open up a new generation of gas sensors.

We describe a simple, cost-effective approach for surface modification of various substrates with SnO2 nanoparticles by controlled hydrolysis, which is not confined on specific chemical states of the surface. Specifically, these SnO2 nanoparticles were loaded onto the surfaces of carbon-based materials (graphene), ionic crystals (tin sulfide), polymer materials (silk fiber) and biological materials (yeast) with uniform distribution, despite the great differences in surface chemistries. Moreover, the formation mechanism of the SnO2 nanoparticles has been discussed, confirming that the process described can be easily implemented and adapted to other systems. These as-synthesized nanocomposites are expected to have wide applications in the fields of gas sensing, photocatalysis and biomaterials.

•Mg-doped anti-reflection single layer coatings were fabricated on glass substrates.•The maximum transmittance of the single layer coating was 98.5% at 0.04 mol% Mg2+.•An even porous structure with small pores was obtained at 0.04 mol% Mg2+.Mg-doped anti-reflection single-layer SiO2 coatings were fabricated on glass substrates by dip-coating method. The transmittance and refractive index of the coatings in the visible spectrum were significantly affected by the content of Mg2+ dopants and withdrawal speeds. A maximum transmittance of the Mg-doped anti-reflection coating was obtained when the content of Mg2+ dopants was 0.04 mol% and the withdrawal speed was 350 mm/min. Compared with bare glass substrate (89.7%), the maximum transmittance of the coating was greatly improved to 98.5% at 521 nm. An even porous structure with small pores (about 30 nm) was obtained and this structure can enhance transmittance of the coating in the visible spectrum.

•Sb doped ZnO nanocrystals was prepared by a vapor condensation method.•Sb doped ZnO nanocrystals exhibited an obvious shrinkage of crystal lattice.•The XRD results provide direct evidence for the model based on SbZn–2VZn complex.•This is in good agreement with the PL spectrum and red shift of absorption peak.There is a theoretical structure model for Sb doped ZnO in which Sb dopants occupy Zn sites to form a SbZn–2VZn complex and this model has been confirmed rarely by the structure characterizations. Here, Sb doped ZnO nanocrystals (NCs) were prepared by a vapor condensation method and exhibited a shrinkage of crystal lattice, providing a direct experimental evidence for this model. The shrinkage of crystal lattice was accurately characterized by XRD, and mainly attributed to the formation of SbZn–2VZn complex which agreed well with a new peak at ~3.10 eV in the PL spectrum and red shift of adsorption peak in the UV–vis spectrum compared with that of pure ZnO NCs.

The formation mechanism of SnO2 nanotubes (NTs) fabricated by generic electrospinning and calcining was revealed by systematically investigating the structural evolution of calcined fibers, product composition, and released volatile byproducts. The structural evolution of the fibers proceeded sequentially from dense fiber to wire-in-tube to nanotube. This remarkable structural evolution indicated a disparate thermal decomposition of poly(vinylpyrrolidone) (PVP) in the interior and the surface of the fibers. PVP on the surface of the outer fibers decomposed completely at a lower temperature (<340 °C), due to exposure to oxygen, and SnO2 crystallized and formed a shell on the fiber. Interior PVP of the fiber was prone to loss of side substituents due to the oxygen-deficient decomposition, leaving only the carbon main chain. The rest of the Sn crystallized when the pores formed resulting from the aggregation of SnO2 nanocrystals in the shell. The residual carbon chain did not decompose completely at temperatures less than 550 °C. We proposed a PVP-assisted Ostwald ripening mechanism for the formation of SnO2 NTs. This work directs the fabrication of diverse nanostructure metal oxide by generic electrospinning method.

ZnO micro/nanocrystals with different percentages of the exposed (0001) facets were synthesized by a facile chemical bath deposition method. Various characterizations were carried out to understand the relationship between particle shape, exposed (0001) facets, and catalytic activity of ZnO nanocrystals for the thermal decomposition of ammonium perchlorate (AP). An enhancement in the catalytic activity was observed for the ZnO micro/nanocrystals with a higher percentage of the exposed (0001) facets, in which the activation energy Ea of AP decomposition was lowered from 154.0 ± 13.9 kJ/mol to 90.8 ± 11.4 kJ/mol, 83.7 ± 15.1 kJ/mol, and 63.3 ± 3.7 kJ/mol for ZnO micro/nanocrystals with ca. 18.6%, 20.3%, and 39.3% of the exposed (0001) facets. Theoretically evidenced by density functional theory calculations, such highly exposed (0001) facets can be favorable for the adsorption and diffusion of perchloric acid, and also facilitate the formation of active oxygen which can lead to the oxidation reaction of ammonia more completely in the catalytic decomposition of AP.

Recently, graphene-based semiconductor photocatalysts have attracted more attention because of their enhanced photocatalytic activity caused by interfacial charge transfer (IFCT). However, the effect of a chemical bond is rarely involved for the IFCT. In this work, TiO2/graphene composites with a chemically bonded interface were prepared by a facile solvothermal method using tetrabutyl orthotitanate (TBOT) as the Ti source. The chemically bonded TiO2/graphene composites effectively enhanced their photocatalytic activity in photodegradation of formaldehyde in air, and the graphene content exhibited an obvious influence on the photocatalytic activity. The prepared composite with 2.5 wt % graphene (G2.5-TiO2) showed the highest photocatalytic activity, exceeding that of Degussa P25, as-prepared pure TiO2 nanoparticles, and the mechanically mixed TiO2/graphene (2.5 wt %) composite by a factor of 1.5, 2.6, and 2.3, respectively. The enhancement in the photocatalytic activity was attributed to the synergetic effect between graphene and TiO2 nanoparticles. Other than the graphene as an excellent electron acceptor and transporter, the enhanced photocatalytic activity was caused by IFCT through a C–Ti bond, which markedly decreased the recombination of electron–hole pairs and increased the number of holes participating in the photooxidation process, confirmed by XPS analysis, the gaseous phase transient photocurrent response, electrochemical impedance spectroscopy, and photoluminescence spectra. This work about effective IFCT through a chemically bonded interface can provide new insights for directing the design of new heterogeneous photocatalysts, which can be applied in environmental protection, water splitting, and photoelectrochemical conversion.Keywords: chemical bonding; C−Ti bond; gaseous phase photocurrent; interfacial charge transfer; photocatalytic activity

A BiOCl–Bi2WO6 heterojunction with a chemically bonded interface was synthesized via a facile one-step solvothermal method. A series of characterization techniques (XRD, XPS, TEM, SEM, EDS etc.) confirmed the existence of a BiOCl–Bi2WO6 interface. The heterojunction yielded a higher photodegradation rate of Rhodamine B under visible light irradiation compared to its individual components. Theoretical studies based on density functional theory calculations indicated that the enhanced photosensitized degradation activity could be attributed to the favorable band offsets across the BiI–O–BiII bonded interface, leading to efficient interfacial charge carrier transfer. Our results reveal the photosensitized mechanism of BiOCl–Bi2WO6 heterojunctions and demonstrate their practical use as visible-light-driven photocatalytic materials.

After gas sensors based on ZnO nanotetrapods (T-ZnO) were processed by sintering at different temperatures from 350 to 850 °C, a strong correlation was interestingly found among the sintering processing, material microstructure and gas-sensing properties. With increasing sintering temperature from 350 to 750 °C, the feet and cross of T-ZnO became gradually shorter and bigger, respectively. And subsequently tetrahedron-shaped ZnO nanoparticles were produced instead of T-ZnO at 850 °C. The morphological evolution was explained by a new physical model involving Thomson effect, leading to a decrease in the specific surface area. In addition, the contact between feet got better and then became poorer. Meanwhile, surface defects of T-ZnO were also altered: zinc interstitial (Zni••) was decreased in its amount while oxygen vacancy (VO×) showed an inverse trend as sintering temperature increased. Moreover, the best gas-sensing performance toward formaldehyde and methanol was obtained after sintering at 450 °C. This was mainly attributed to the synergetic effect between the best grain contact (meaning that more T-ZnO can make contributions to the sensor response) and more zinc interstitial as well as larger specific surface area (supplying more chemisorbed oxygen). Our work could offer important guidance for the process selection and material design to develop nanostructure-based sensors.

Distinct from SnO2 nanoparticulates whose gas-sensing properties depend deeply on grain size and specific surface area, hierarchical SnO2 mesoporous structures has been found to possess remarkable gas-sensing performance due to their large accessible surface area. Interestingly, our obtained hierarchical SnO2 mesoporous microfibers shows an increase in the response to formaldehyde (HCHO) gas with decreasing specific surface area and increasing pore size, characterized by TEM, BET and performance analysis. The pore-size-dependent gas-sensing properties are mainly attributed to the transport of detected gases inside SnO2 mesoporous microfibers. Smaller pores can not provide efficient gas transport to more active sites while larger pores can allow most detected gas molecules diffuse easily inside the deeper region of the mesoporous SnO2 microfibers and react with oxygen species adsorbed on the surface (these reactive surface can be called as effective surface), resulting in a large sensor response. Therefore, the pore size and its resultant effective surface area rather than specific surface area play a dominant role in the gas-sensing properties of mesoporous materials, which can provide us with new insight on the design of high-performance gas sensors in the future.

Metal oxide quasi-one-dimensional (quasi-1D) nanostructures have a very good gas-sensing performance due to their large surface area and porous structures with a less agglomerated configuration. However, the well-designed fragile nanostructures could be easily destroyed during the conventional fabrication process of gas sensors. Herein, we presented a novel materials-sensor integration fabrication strategy: on basis of screen printing (SP) technology and calcination, micro-injecting (MI) was introduced into the fabrication process of sensors, which was named as SPMIC, to obtain In2O3 nanowire-like network directly on the surface of coplanar sensors array by structure replication from sacrificial carbon nanotubes (CNTs). The obtained In2O3 nanowire-like network exhibited an excellent response (electrical resistance ratio Ra/Rg), about 63.5, for100 ppm formaldehyde at 300 °C, which was about 30 times larger than that of compact In2O3 nanoparticles film (non-network film). The enhanced gas-sensing properties were mainly attributed to the high surface-to-volume ratio and the nanoscopic structural properties of materials. Furthermore, the SPMIC could be employed not only in the preparation of other metal oxide nanowire-like network, but also in the fabrication of coplanar gas sensors arrays on the required sites with different materials.

Density functional theory calculations are performed to study the band offsets at the interface of two photocatalytic materials BiOCl:Bi2WO6. It is found that the W–O bonded interface shows the most stability. An intrinsic interface fails to enhance the charge-carrier separation due to the improper band alignment between these two materials. Sulfur (S) is proposed to replace the bulk oxygen (O) site and thus tune the band edges of BiOCl to enhance the photocatalytic performance of the heterojunction. Furthermore, the presence of S provides an extra charge to generate a clean interface with minimal gap states that also benefits carrier migration across the heterojunction.

To overcome the agglomeration of metal oxide (MOX) nanocatalysts mechanically mixed with ammonium perchlorate (AP) and other additives of rocket propellant, the shell–core nanocomposites of MOX/AP have been synthesized successfully by a facile liquid deposition method at room temperature. SEM analysis revealed that MOX (M = Zn, Co, Fe) nanoparticles were deposited on the surface of AP particles as either a continuous thin layer or small clusters. Owing to the existence of the shell of MOX nanocatalysts, ZnO/AP, Co3O4/AP and Fe2O3/AP nanocomposites showed excellent self-catalytic performances for AP thermal decomposition: lowering the decomposition temperature from 398 °C to 272 °C, 285 °C, 337 °C, and increasing the heat release from 584 J g−1 to 1137 J g−1, 1237 J g−1, 1010 J g−1, respectively. Moreover, their self-catalytic performances mainly relied on the content of MOX nanocatalysts, which was controlled by the concentration of metal salts in the precursor solution. In particular, ZnO/AP nanocomposites with the mass ratio of ZnO:AP = 4:100 exhibited the best self-catalytic performance in decreasing the activation energy from 154.0 kJ/mol to 96.5 kJ/mol. The MOX/AP (M = Zn, Co, Fe) shell–core nanocomposites could have a promising application in the rocket propellant for improving the thermal-catalytic decomposition performance of AP.Graphical abstractThe shell–core nanocomposites of MOX/AP were prepared by a facile liquid deposition method at room temperature and exhibited significant self-catalytic effects for the thermal decomposition of AP in lowering the decomposition temperature and increasing the heat release.Highlights► MOX/AP shell–core nanocomposites were synthesized by a facile liquid deposition method. ► MOX nanoparticles were homogeneously coated on the surface of AP. ► The nanocomposites have excellent self-catalytic performances for AP decomposition. ► ZnO/AP nanocomposites (mass ratio = 4:100) exhibit the best self-catalytic performance.

Defect graphene was reported by adding sugar through solvothermal method. The characterization results of XRD, IR, Raman, and XPS showed that the samples have tunable mount of oxygenated group, which plays a role as adsorption site to detecting humidity gas molecule. The sample from sucrose has the highest mount of functional oxygenated groups and shows the best humidity property.Highlights► Graphene with tunable defect was reported through solvothermal method. ► The defect was considered as oxygenated groups based on characterization results. ► The sample from sucrose has the most mount of functional oxygenated groups. ► The defect plays a role as adsorption site to detecting humidity gas molecule. ► The sample from sucrose shows the best humidity property.

The formaldehyde (HCHO) sensing properties of mesoporous WO3 sensors were investigated via being activated by the visible light at room temperature. It is found that the response of the mesoporous WO3 sensors to HCHO (100 ppm) reached to 1.3 × 10−7 (Ωs)−1 under white light (100 W/m2) and 9.5 × 10−8 (Ωs)−1 under blue light (100 W/m2), respectively, which are 4 times and 9 times as much as the response of the commercial WO3, mainly due to the high surface area and the existence of the mesopores. Moreover, the response of the WO3 sensors is linearly increased with the increase of the light intensity and the formaldehyde concentration, and high humidity significantly decreases the gas sensing properties of the mesoporous WO3 under visible light. Also, compared to the sensors made by commercial WO3 material, the mesoporous WO3 sensors exhibit high sensitivity to acetone and toluene. Enhancement of the gas sensing properties of the mesoporous WO3 is attributed to the higher surface interaction rate among photoelectron, photoinduced oxygen ion and target gas.

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 Ag), 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.

Novel C-doped WO3 microtubes (MTs) were successfully synthesized by a facile infiltration and calcination process using the cotton fibers as templates. The prepared MTs were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM), N2 adsorption and desorption measurements and ultraviolet–visible spectroscopy. XPS spectra show the carbon was doped into the lattice of the WO3 phase, resulting in a decrease of the band gap of the C-doped WO3 MTs from 2.45 eV to 2.12 eV. Moreover, the WO3 MTs were assembled by nanoparticles in size of ca. 40 nm and had larger specific surface area (21.3 m2/g) due to existence of meso/macro-pores inside them. At low operating temperature of 90 °C, the gas sensor based on the C-doped WO3 MTs had a detected limit of 50 ppb to the toluene gas (response of 2.0). The enhancement of toluene sensing performance of C-doped WO3 MTs was attributed to a larger surface area and higher porosity, which arises from its unique MTs. Furthermore, the band gap reduction and a new intragap band formation for C-doped WO3 MTs were proposed as the reason for the decrease in optimal operating temperature.

Zn-doped SnO2 nanorods clusters with controllable size (the length and the diameter of nanorods) are prepared by adjusting the Zn2+ concentration in the precursor solution using a facile hydrothermal process. The as-synthesized samples are characterized by powder X-ray diffraction, scanning electron microscopy, transmission electron microscopy (TEM), high-resolution TEM, and energy-dispersive X-ray spectroscopy (EDS). With an increase of the molar ratio of Zn2+/Sn4+ in the solution, the length of the Zn-doped SnO2 nanorods gradually increases, and the average length of the nanorods reaches the maximum of 239 nm at 0.133. However, when the molar ratio of Zn2+/Sn4+ is increased to 0.2, the length of the nanorods decreases accompanied with the generation of Zn2SnO4 phase. The Zn-doped SnO2 nanorods grow along the [1 1 2] direction and the possible growth mechanism is proposed. The gas-sensing properties of the obtained Zn-doped SnO2 samples are tested towards methanol gas. And, it is found the composition–morphology-performance relationship that the more the doping content of Zn, the longer the length of nanorods and the higher the response to methanol, which is significant for the preparation and the design of gas-sensing materials with the high performance.

Nanostructured ZnO network films have been fabricated on Al2O3 substrates by the combination of chemical bath deposition and thermal decomposition process. Layered basic zinc nitrate (LBZN) network films were deposited on the Al2O3 substrates with LBZN crystal seeds in methanol solution of zinc nitrate hexahydrate and hexamethylenetetramine. The LBZN precursor films were then transformed into nanostructured ZnO films by heating at 260 °C in air. During the thermal decomposition process abnormal exothermic heat effect was observed at 200–210 °C and CH3 groups were found in the as-deposited films. We propose that methanol molecules are integrated in the LBZN films forming LBZN–CH3OH complex and that the heat effect comes from the exothermic release of the methanol.

Bi2WO6 (BWO) nanostructures with QDS dispersed on single crystalline nanosheets were successfully prepared by a facile solvothermal method. The product possessed large surface area of 60 m2/g and exhibited excellent visible light absorption with a blue shift from 2.54 eV to 2.75 eV. The photocatalytic efficiency of the sample was six times that of nanoparticles assembled BWO nanostructures and three times that of nanoplates assembled BWO nanostructures. The photocatalytic mechanism for degradation of dyes over QDS modified BWO nanostructures was discussed, which revealed the important role of QDS in the generation, migration and consumption of the photogenerated electrons and holes.

Lowering the working temperature without sacrificing other good gas-sensing properties is of particular interest to gas sensors for an excellent performance. In this work, La surface doped ZnO nanocrystals were successfully prepared by a facile thermal treatment with lanthanum nitrate (La(NO3)3) solution injected into ZnO thick films, which exhibited a remarkable decrease in the optimal working temperature for formaldehyde (HCHO) sensing properties. This was probably attributed to the formation of surface LaZn defects in the ZnO nanocrystals which was evidenced by XRD, XPS results and DFT calculations. The surface LaZn defects can introduce a shallower donor level than oxygen vacancies, and probably facilitate the charge transfer from oxygen species to ZnO for producing chemisorbed oxygen species more easily. This was in good agreement with the DFT results that the absorption energy of oxygen molecules on the surface of La doped ZnO was only −10.61 eV, much lower than that of pure ZnO. Moreover, the optimal working temperature of the La doped ZnO based sensor was significantly decreased from 350 to 250 °C without sacrificing the high and quick response to HCHO gas as the content of surface LaZn defects was increased gradually. Therefore, the behavior of the surface LaZn defects in the optimal working temperature revealed a HCHO response mechanism in ZnO, which can provide new insights into the enhanced HCHO sensing performance of gas sensors.

Correction for ‘Enhanced room temperature NO2 response of NiO–SnO2 nanocomposites induced by interface bonds at the p–n heterojunction’ by Jian Zhang et al., Phys. Chem. Chem. Phys., 2016, 18, 5386–5396.

Density functional theory calculations are performed to study the band offsets at the interface of two photocatalytic materials BiOCl:Bi2WO6. It is found that the W–O bonded interface shows the most stability. An intrinsic interface fails to enhance the charge-carrier separation due to the improper band alignment between these two materials. Sulfur (S) is proposed to replace the bulk oxygen (O) site and thus tune the band edges of BiOCl to enhance the photocatalytic performance of the heterojunction. Furthermore, the presence of S provides an extra charge to generate a clean interface with minimal gap states that also benefits carrier migration across the heterojunction.

Recently, heterostructured nanomaterials have attracted great attention in gas sensing applications. However, the sensing mechanism of the enhanced sensitivity of heterostructured nanomaterials remains unclear, which is not conducive to further improvements in their sensing performances. In order to detail the fundamental studies on the gas sensing mechanism of heterostructured nanomaterials and improve the room temperature NO2 sensing properties of NiO-based nanomaterials, NiO–SnO2 heterojunction nanocomposites were fabricated. It was found that the sensitivity of the nanocomposites was largely enhanced compared to the bare NiO. On the basis of the intrinsic characteristics of the p–n heterojunction and the band structure of the NiO–SnO2 heterojunction, the largely enhanced room temperature NO2 response of the nanocomposites could be attributed to two factors. One was the significantly decreased initial conductance, and the increase in the equivalent hole concentration of the nanocomposites after exposure to NO2, associated with the effective electron transfer via the interface bonds at the heterojunction. Another was that the variation of contact potential in the nanocomposites, before and after exposure to NO2, exerted a drastic effect on the transducer function for gas sensing. According to the differentiation in the sensitivity of the nanocomposites with different molar ratios, the important role of interface bonds in gas sensing properties was further illustrated by the dependency of the sensitivity on the interface bond number and the interface resistance. Here, we hope that this work could give us a better understanding of the gas sensing mechanism of the p–n heterojunction, and provide a proper approach for heterojunction materials to further improve their sensing performances.

A BiOCl–Bi2WO6 heterojunction with a chemically bonded interface was synthesized via a facile one-step solvothermal method. A series of characterization techniques (XRD, XPS, TEM, SEM, EDS etc.) confirmed the existence of a BiOCl–Bi2WO6 interface. The heterojunction yielded a higher photodegradation rate of Rhodamine B under visible light irradiation compared to its individual components. Theoretical studies based on density functional theory calculations indicated that the enhanced photosensitized degradation activity could be attributed to the favorable band offsets across the BiI–O–BiII bonded interface, leading to efficient interfacial charge carrier transfer. Our results reveal the photosensitized mechanism of BiOCl–Bi2WO6 heterojunctions and demonstrate their practical use as visible-light-driven photocatalytic materials.

In recent years, there has been increasing interest in synthesis of reduced graphene oxide (rGO)–metal oxide semiconductor (MOS) nanocomposites for room temperature gas sensing applications. Generally, the sensitivity of a MOS can be obviously enhanced by the incorporation of rGO. However, a lack of knowledge regarding how rGO can enhance gas-sensing performances of MOSs impedes its sensing applications. Herein, in order to get an insight into the sensing mechanism of rGO–MOS nanocomposites and further improve the sensing performances of NiO-based sensors at room temperature, an rGO–NiO nanocomposite was synthesized. Through a comparison study on room temperature NO2 sensing of rGO–NiO and pristine NiO, an inverse gas-sensing behavior in different NO2 concentration ranges was observed and the sensitivity of rGO–NiO was enhanced obviously in the high concentration range (7–60 ppm). Significantly, the stimulating effect of rGO on the recovery rate was confirmed by the sensing characteristics of rGO–NiO that was advantageous for the development of NO2 sensors at room temperature. By comprehending the electronic interactions between the rGO–MOS nanocomposite and the target gas, this work may open up new possibilities for further improvement of graphene-based hybrid materials with even higher sensing performances.

Metal-oxide-semiconductor (MOS) based gas sensors have been considered a promising candidate for gas detection over the past few years. However, the sensing properties of MOS-based gas sensors also need to be further enhanced to satisfy the higher requirements for specific applications, such as medical diagnosis based on human breath, gas detection in harsh environments, etc. In these fields, excellent selectivity, low power consumption, a fast response/recovery rate, low humidity dependence and a low limit of detection concentration should be fulfilled simultaneously, which pose great challenges to the MOS-based gas sensors. Recently, in order to improve the sensing performances of MOS-based gas sensors, more and more researchers have carried out extensive research from theory to practice. For a similar purpose, on the basis of the whole fabrication process of gas sensors, this review gives a presentation of the important role of screening and the recent developments in high throughput screening. Subsequently, together with the sensing mechanism, the factors influencing the sensing properties of MOSs involved in material preparation processes were also discussed in detail. It was concluded that the sensing properties of MOSs not only depend on the morphological structure (particle size, morphology, pore size, etc.), but also rely on the defect structure and heterointerface structure (grain boundaries, heterointerfaces, defect concentrations, etc.). Therefore, the material-sensor integration was also introduced to maintain the structural stability in the sensor fabrication process, ensuring the sensing stability of MOS-based gas sensors. Finally, the perspectives of the MOS-based gas sensors in the aspects of fundamental research and the improvements in the sensing properties are pointed out.