•the cobweb-like WO3 was successfully synthesized.•Such cobweb-like WO3 realizes an enhanced sensitivity.•This method may enlighten the synthesis of other oxides.Owing to the slenderness of the single spider silk and the fluffy arrangement of numerous monomers, the cobweb has the sufficient contact with the water vapor in air and can form dewdrops on it. Enlightened by this, WO3 nanowires with a great length-diameter ratio and a non-compact assembly are inferred to possess an excellent gas-sensing property. As a proof-of-concept, the cobweb-like WO3 was firstly synthesized through a facile solvothermal method using the water-ethanol mixed solution. The subsequent gas-sensing measurements testified this kind of cobweb-like WO3 indeed displays an enhanced sensitivity towards the ethanol at a relatively low temperature.Enlightened by the naturally occurring phenomenon that spider silks could capture the water vapor in air, the cobweb-like WO3 with an enhanced gas-sensing property was successfully synthesized.

•Controllable synthesis of two novel NiO nanoflowers.•The growth mechanisms are elaborated.•The novel morphology evolution of two nanoflowers is presented.•A plausible gas-sensing mechanism is proposed.Novel sparse-type and close-type NiO nanoflowers have been controllably synthesized via hydrothermal process assisted with CTAB and SDS, respectively. The sensing performance of the close-type NiO was superior to the other, which benefits from the following factors: In the close-type nanoflower, (1) the architecture is more porous and closer; (2) more nanojunctions between adjacent nanosheets are formed; (3) abundant micro reaction rooms are assembled by nanosheets; (4) the thickness of nanosheets are much smaller than twice Debye length.In this letter, we report the synthesis of novel sparse-type and close-type NiO nanoflowers and investigate their gas-sensing properties.

•Hierarchical porous and conventional smooth WO3·H2O micro-spheres were successfully fabricated.•Porous spherical architectures exhibited the superior gas-sensing properties.•Porous spherical structure is a promising strategy to improve the gas-sensing behaviors.In this work, hierarchical WO3·H2O porous microspheres and conventional smooth WO3·H2O microspheres were successfully fabricated via a simple hydrothermal process. The products were characterized by X-ray diffraction and scanning electron microscope. Meanwhile, the possible evolution processes of the two products were discussed based on time-dependent experiments. Remarkably, hierarchical porous spherical architectures which possess abundant micro reaction rooms, exhibited improved gas-sensing performance compared to smooth sphere-like structures. It is believed that this unique nanostructure has great potential to enhance the gas-sensing properties of tungsten oxides and other semiconductor materials.Hierarchical WO3·H2O porous spherical structure performed an outstanding gas sensing performance to the ethanol gas compared to the conventional smooth WO3·H2O microsphere, indicating this special nanostructure holds great potential to enhance the properties of gas-sensing materials.

•A novel coral rock-like ZnO with porous structure was successfully synthesized.•The gas sensor based on the coral rock-like ZnO processed super gas response.•The porous nature may play a vital role in the surface gas reaction.In present work, a novel coral rock-like ZnO with porous structure was successfully synthesized via a solvothermal method and subsequent calcination. We were surprise to find that such ZnO nanostructure exhibited super high gas response to ethanol, which was attributed to the novel coral rock-like structure with porous nature, leading to effective gas diffusion and sufficient surface chemical reaction during sensing measurement.A coral rock-like ZnO with porous structure was successfully synthesized via a solvothermal method and subsequent calcination. The sensor based on the coral rock-like ZnO presented super high response to ethanol. The porous nature played a vital role in the surface chemical reaction during sensing tests.Download high-res image (119KB)Download full-size image

•The array structure which is inspired by naturally-occurring root hairs.•WO3 nanorod arrays were synthesized through a substrate-free hydrothermal way.•WO3 nanorod arrays realize an improved sensitivity toward NH3.Nanorod arrays possess a large surface area due to the orderly arrangement of monomers, and thus are considered as an ideal microstructure to be utilized in the interface-related process such as the gas-sensing reaction. But their adoption is plagued by the complicated synthesis route in which the substrate is usually necessary. Here we propose a substrate-free fabrication of WO3 nanorod arrays through the facile hydrothermal method. Subsequent measurements testify the superior NH3-sensing property of WO3 nanorod arrays. The corresponding response is as high as 8.3 at a concentration of 50 ppm and a temperature of 200 °C, surpassing other WO3-based NH3 sensors reported.WO3 nanorod arrays were successfully synthesized through a substrate-free hydrothermal method and confirmed an enhanced sensitivity toward NH3 at a relatively low temperature and concentration.Download high-res image (180KB)Download full-size image

•NiO nanoflowers with various morphologies exhibit excellent gas response.•A growth mechanism for NiO nanoflower has been established.•Surfactant plays a critical role in the formation of NiO nanostructure.The NiO nanoflowers were prepared by a facile surfactant assisted hydrothermal method using Ni(NO3)2–6H2O or NiCl2–6H2O as precursor compound. The microstructure of the samples was characterized by SEM and XRD. The gas sensing properties of the NiO nanoflowers toward ethanol was also investigated. The results show that surfactant plays a key role in the synthesis of flower-like NiO. The NiO nanoflowers show excellent sensing performances to ethanol gas. This morphology holds substantial promise for applying NiO as a potential gas sensing material for future sensor application.The gas sensing performances of nanoflowers with various morphologies and its sensing mechanism were presented in current work.

•A distinct growth manner of Cu2O polyhedron was observed, which was referred to All-direction Pattern.•An interesting solution color changes during polyhedral Cu2O formation were carefully traced.•Extensive observations were made to character the sample by SEM, TEM, and XRD.•Discuss the formation mechanism of the polyhedron in terms of atomic scale and micro/nano scope.•The Cu2O polyhedron was applied to gas-sensing.Previously, the formation of polyhedral Cu2O crystal is usually proposed like that the seed crystals grow outward following the relative growth rates (R values) along different directions (Inside-out Pattern) or that the surrounding building blocks aggregate toward the center followed by ripening (Outside-in Pattern). However, in this study, simply mixing aqueous Cu(CH3COO)2, NaOH, and D-(+)-glucose in the order listed at 70 °C for a period, a distinct growth manner of Cu2O polyhedron was observed based on carefully recording the solution color changes and the elaborate time-dependent SEM/TEM observations, which was referred to All-direction Pattern. More specifically, the initially formed Cu2O nanoshuttles first self-aggregated into porous braided structure with well-defined cubic framework and then condensed into solid cube from all direction via consuming the surrounding fragments everywhere possible, rather than in gradient densification from inside-out or outside-in. After that, the solid cube fell into the common Outside-in Pattern, i.e., adsorbing the surroundings followed by ripening. Although many complex polyhedral submicrostructures have been already reported, to our best knowledge, Cu2O crystal that grows in such manner is barely observed. It is believed that such interesting growth mechanism of polyhedral crystals could be general. What's more, the 30 min-Cu2O powder was made into a gas sensor towards ethanol, the performance of which has reached the level of application.An interesting growth manner of polyhedral Cu2O crystal was observed, termed Outside-in Pattern, which was evolved from braided cubic framework.Download high-res image (182KB)Download full-size image

The pristine and Cr-doped ZnO nanorods were successfully synthesized via a facile hydrothermal route. We found that the ethanol gas response of ZnO was improved significantly by Cr doping. In particular, the enhanced gas-sensing mechanism was investigated by first-principles calculations upon proposed surface adsorption models. The calculated results revealed that the Cr-doped ZnO (0 0 0 1) surface enabled transfer larger electrons and adsorb more oxygen molecules than that of undoped one, thus holding the potential for further enhancement in gas response of ZnO-based sensors.

Nanomaterials with three dimensional architectures frequently exhibit novel functional properties. In current work, a novel rutile TiO2 3D nanoflowers with the powder partly agglomerating have been successfully synthesized via a facile hydrothermal route in the saturated sodium chloride solution. In addition, the effect of the solvent in hydrothermal process was preliminarily discussed on the basis of comparative experiments. Such an unexpected morphology provides a non-trivial behavior driven by the properties of partly agglomerating TiO2 nanoflowers in gas sensing.

We report a preparation of titanium dioxide regular nano-polyhedron with reactive (001) facets by simple yet efficient hydrothermal synthesis process. It has been found that the (001) surfaces are terminated with F atoms, and the F-terminated surfaces, which reduce the surface energy of anatase titanium dioxide, provide a stable condition for the existence of (001) facets. Ni doping, additionally, restrict the growth of particle, making the morphology of anatase titanium dioxide regular plus uniform. Besides, the as prepared titanium dioxide with high reactive (001) facets exhibit far better gas sensing performance over ethanol than those mainly having exposed (101) facets. Such a facile method and interpretation mechanisms could be applicable to many other nanomaterials for a wide range of study.

Novel gas sensors with high sensing properties, simultaneously operating at room temperature are considerably more attractive owing to their low power consumption, high security and long-term stability. Till date, zinc oxide (ZnO) as semiconducting metal oxide is considered as the promising resistive-type gas sensing material, but elevated operating temperature becomes the bottleneck of its extensive applications in the field of real-time gas monitoring, especially in flammable and explosive gas atmosphere. In this respect, worldwide efforts have been devoted to reducing the operating temperature by means of multiple methods In this communication, room-temperature gas sensing properties of ZnO based gas sensors are comprehensively reviewed. Much more attention is particularly paid to the effective strategies that create room-temperature gas sensing of ZnO based gas sensors, mainly including surface modification, additive doping and light activation. Finally, some perspectives for future investigation on room-temperature gas-sensing materials are discussed as well.

•The 3D flower-like ZnO assembled by 1D nanorods was successfully synthesized.•Such flower-like ZnO realizes an enhanced ethanol sensing properties under UV illumination.•The strategy for the joint role may provide a way to further enhance the gas sensing properties.Nanorod-assembled flower-like ZnO was successfully synthesized using a simple hydrothermal method. The measurement results demonstrated that the ethanol sensing properties of the as-prepared ZnO nanostructure were improved once under UV illumination, with higher response and faster response-recovery speed as compared to that in dark condition. The sensing enhancement could be ascribed to the co-effect of the 3D ZnO nanostructure and UV irradiation. Specifically, the 3D framework constructed from 1D nanorods provided plenty of reaction sites and effective gas diffusion pathways. In addition, owing to photo-induced electron-hole pairs under UV irradiation, more energetically free carriers interacted with gas molecules, which accounted for the improvement in ethanol sensing behaviours.The 3D flower-like ZnO assembled by 1D nanorods was successfully synthesized hydrothermally, which exhibited enhanced ethanol sensing properties once under UV illumination as compared to that in dark conditionDownload high-res image (256KB)Download full-size image

•Hierarchical net-like NiO architectures were fabricated via a simple hydrothermal process.•Multiple continuous networked NiO structures exhibited enhanced gas-sensing properties.•Multiple continuous net-like structure is a possible strategy to enhance the gas-sensing performance.Multiply networked NiO nanostructures were synthesized via a simple hydrothermal process, also, discontinuous net-like NiO structures were prepared as a comparison. The morphologies of two samples were analyzed by SEM and TEM observation. Moreover, the gas sensing results indicated that the multiple networked NiO architectures assembled by nanowires which possess continuous structure exhibit an enhanced gas-sensing performance compared to the other one, which is likely attributed to efficient transmission of carrier and abundant open diffusion channels provided by this novel structure.Hierarchical continuous networked NiO architectures assembled by nanowires displayed an outstanding gas sensing performance to the target gas than that of discontinuous net-like structure, indicating this unique structure might be a promising strategy to enhance the properties of gas-sensing materials.Download high-res image (158KB)Download full-size image

We report on the synthesis of pristine and Cu-doped SnO2 spheres using a facile hydrothermal method and investigate their microstructures and gas-sensing response. We focus on how Cu doping can have an impact on gas-sensing behavior of SnO2-based sensors toward H2S. We find that Cu doping can enhance significantly the gas response of SnO2 toward H2S, the origin of which can be clarified with a proposed adsorption model. First-principles calculations reveal that adsorption energy of H2S on Cu-doped surface is lower than that on undoped one and the interaction between adsorbed H2S and Cu-doped surface is stronger than that between adsorbed H2S and pure surface, which consequently improves gas-sensing performances of SnO2 toward H2S. Such a combined experimental and calculational study offers an explanation on how Cu doping affects gas-sensing performances of SnO2.

•Novel hollow Cu2O polyhedrons were fabricated without template.•Extensively observations were made to character the sample.•Proposal the formation mechanism of the hollow polyhedron.Previously, hollow Cu2O configurations have been fabricated with the use of template or etchant. However, in this study, simply mixing aqueous sodium dodecyl sulfate (SDS), CuCl2, NaOH, and NH2OH·HCl in the order listed, Cu2O polyhedron with empty interior arise, which is rarely reported. Observations made on the novel particles confirm the hollow polyhedral feature and reveal that the uniform particles exhibit a narrow size distribution at around 300–400 nm in diameter and ca. 50 nm in wall thickness. It was recorded that, upon the addition of reductant (NH2OH·HCl), the solution color changed orderly from light blue to green, yellow, and then orange within 1 h, while the basic mixture solution turned to acid immediately. We have reason to attribute the formation of hollow polyhedron to the surfactant role of SDS and the co-etching effect of oxidative etching and acidic etching, acting on dissolving Cu2O nanocrystals via oxidation with oxygen and reaction with HCl (from NH2OH·HCl).In this letter, hollow Cu2O polyhedrons were successfully fabricated in a facile wet chemical system at room temperature without template or etchant.

•Flower-like WO3·H2O hierarchical architectures assembled by nanosheets were fabricated.•The formation processes are proposed based on comparative studies.•Closely WO3·H2O nanoflowers exhibited the superior gas sensing performance.•Semi-closed structure is a possible strategy to enhance the gas-sensing properties.Sparsely and closely flower-like WO3·H2O hierarchical architectures were successfully synthesized via a one-step hydrothermal process. The obtained samples are both WO3·H2O nanoflowers assembled by ultrathin nanosheets with the thickness less than electron deletion layer. And the possible formation processed was proposed based on the comparative studies. Furthermore, gas sensing experiments demonstrated that the closely WO3·H2O nanoflowers which possess numerous semi-closed spaces have a superior gas-sensing performance compared with the sparsely one. We believe this unique structure will critically enhance the gas sensing properties of WO3·H2O nanomaterials.The closely WO3·H2O nanoflowers displayed an outstanding gas sensing performance to the target gas than that of sparsely one, rendering this special structure might a possible strategy to enhance the gas-sensing properties of tungsten trioxide nanomaterials.

•Controllable synthesis of NiO flower-like microspheres with abundant nanoparticles adhering to the petals.•The NiO sensors exhibit excellent gas sensing performances.•A distinctive growth mechanism is elaborated.•The novel morphology evolution of the NiO flower-like microsphere is presented.Nanomaterials with three-dimensional architectures, frequently exhibiting novel functional properties, have attracted considerable attention of the researchers. In current work, we have successfully synthesized the NiO flower-like microspheres with abundant nanoparticles adhering to the petals via a facile hydrothermal method. Interestingly, it is found that the excellent gas sensing properties of the samples towards ethanol are likely attributed to the well-aligned porous architectures as well as great amounts of nanoparticles adhering to the petals, considered as active sites. Meanwhile, we also elaborated a distinctive growth mechanism of such a unique architecture and a plausible gas sensing mechanism, respectively.In this letter, we report the synthesis of NiO flower-like microspheres with abundant nanoparticles adhering to the petals and investigate their gas-sensing properties.

•Controllable synthesis of flake-flower NiO hierarchical architectures.•NiO sensors exhibit excellent gas sensing performances.•A feasible growth mechanism has been proposal.•The novel morphology evolution of the NiO nanoflowers is presented.Nanomaterials with hierarchical architectures consisting of two-dimensional nanostructures have attracted considerable attention for their potential applications in recent years. In current work, flake-flower NiO architectures self-assembled by aggregative nanosheets have been successfully synthesized via the hydrothermal route. It is notable that the NiO sensors exhibit excellent gas sensing performances owing to the open and well-defined structure as well as abundant micro reaction rooms built by two-dimensional ultrathin nanosheets, which are thinner than the thickness of electron depletion layer. Moreover, a possible growth mechanism for the interactions between PVP and Ni2+ ions in the synthesis of the precursors Ni(OH)2 and the novel morphology evolution of the NiO nanoflowers are proposed in detail.

•Junction-rich 2D SnO2 network assembled by nanorods were controllably synthesized.•The two type 2D SnO2 networks were applied to ethanol detection.•Discuss the gas-sensing mechanism in terms of potential barrier modulation and electronic transport.Previous reports demonstrate that sensors based on multiple nanowires exhibit improved gas-sensing performance compared to a single nanowire based sensor due to the potential barrier modulation at nanojunctions, however, little attention has been paid to the effect between junction and electronic transport. Herein, junction-rich 2D SnO2 network assembled by nanorods and transfer-favored network assembled by nanowires were controllably synthesized, which were applied to ethanol detection. In comparison, sensor based on the transfer-favored 2D network exhibited significantly superior performance than that of the junction-rich one, not only in terms of the sensitivity but also the response/recovery time, which seems inclined to reveal that effective electronic transport contributes more in the gas-sensing process than that of potential barrier modulation at interface.

•Blooming, semi-blooming and porous semi-blooming SnO2 nanoflowers were prepared via facile hydrothermal conditions.•The three nanosheet-assembled hierarchical SnO2 nanostructures were applied to test the ethanol gas-sensing performances.•It is found that the sufficient amount of gas diffusion matters for the gas sensing rather than the fast gas diffusion speed.•Gas-sensing mechanism was discussed in detail.•The mesoporous semi-blooming nanoflowers based sensor was applied to monitor the existence of beer by using a simple integrated device.The manner how nano building blocks assemble into hierarchical architectures exerts a tremendous influence on gas-sensing performance of the metal oxides. Here, we focus on tuning the 2D SnO2 nanosheets into 3D hierarchical nanoflowers by manipulating the presence of NaOH, and investigate their gas-sensing functionalities. We find that the blooming SnO2 nanoflowers assembled by ultrathin nanosheets (∼50 nm) are shrunk into semi-blooming state, and that the semi-blooming nanoflowers based sensor shows enhanced gas-sensing performance towards the ethanol, which is attributed mainly to the confined effect due to numerous nano or micro reaction rooms by keeping oxygen and ethanol molecules to complete gas-sensing reactions. While the semi-blooming nanoflowers turn into ordered mesoporous via thermally removing the periodically arranged polyvinyl pyrrolidone micelles, their gas-sensing performance is found to be improved dramatically, indicating that sufficient amount of gas diffusion is crucial to gas-sensing properties rather than the fast gas diffusion speed. As a final verification, we fabricate the sensors using the mesoporous semi-blooming SnO2 nanoflowers and successfully monitor the existence of beer by a simple integrated device, making it a promising candidate in detecting drunk driving.In this paper, blooming, semi-blooming and porous semi-blooming SnO2 nanoflowers were applied to test the gas-sensing performance towards ethanol.

•Synthesis of different WO3-H2O nanostructures from 0D to 3D.•Formation mechanisms is proposed based on comparative studies.•3D hierarchical architecture showed excellent sensing properties.In this paper, WO3·H2O with different nanostructures from 0D to 3D were successfully synthesized via a simple yet cost-effective hydrothermal method with the assistance of surfactants. The structures and morphologies of products were investigated by XRD and SEM. Besides, we systematically explained the evolution process and formation mechanisms of different WO3·H2O morphologies. It is noted that both the kinds and amounts of surfactants strongly affect the formation of WO3·H2O crystals, as reflected in the tailoring of WO3·H2O morphologies. Furthermore, the gas sensing performance of the as-prepared samples towards methanol was also investigated. 3D flower-like hierarchical architecture displayed outstanding response to target gas among the four samples. We hoped our results could be of great benefit to further investigations of synthesizing different dimensional WO3·H2O nanostructures and their gas sensing applications.In this work, we prepare different WO3·H2O nanostructures from 0D to 3D via a simple hydrothermal process, and report their growth formation mechanism and gas sensing properties.

•Urchinlike hex-WO3 microstructures assembled by numerous nanorods were fabricated via hydrothermal process.•A growth mechanism is proposed based on comparative studies.•Urchinlike hex-WO3 microstructures showed the excellent gas sensing properties.Novel urchinlike hexagonal WO3 microspheres have been fabricated by a facile hydrothermal method in the presence of K2SO4. Morphology analysis reveals that the as-synthesized WO3 microstructures are assembled by numerous one-dimensional nanorods. Based on comparative studies, a possible formation mechanism is proposed in detail. What’s more, gas-sensing measurement indicates that the well-defined urchinelike WO3 microspheres exhibit excellent gas sensing properties.In this work, we prepare urchinlike hex-WO3 microspheres via a simple hydrothermal process, and report their growth formation mechanism and gas sensing properties.

•A unique nanoflower assembled from porous nanosheets was syntheized successfully via a one-step hydrothermal method without calcination.•The unique nanostructure show an great enhanced gas-sensing performance.•A possible growth mechanism for the unique nanostructure is proposed.A unique nanoflower assembled from porous nanosheets was synthesized successfully via a novel one-step hydrothermal method for the first time. Namely, there was no calcination conducted on the whole preparation process, which was different from what the previous researches did. The unique architecture was characterized in detail using X-ray diffractometer, field-emission electron microscope. A possible growth mechanism was further proposed. We find that the obtained material shows a very excellent gas-sensing performance due to unique architecture with lots of pores on the petals.

•MoO3 nanostructures were successfully synthesized through a hydrothermal method.•Different nanostructures were obtained by altering the added order of CTAB and HNO3.•This research is important for the preparations of other oxide materials.Molybdenum trioxides (MoO3) with flower-like hierarchical structure and distinct nanobelts have been synthesized via a hydrothermal method. A series of comparative experiments show that cetyltrimethyl ammonium bromide (CTAB) and HNO3 have evident impact on forming the morphologies of MoO3. Interestingly, the add order of CTAB and HNO3 can be also a determinant factor in the assemble process. This research is of great significance in the preparations, characterizations and applications of various 1D and 3D semiconductor metal oxides. The possible growth mechanism is also proposed.College of Materials Science and Engineering, Chongqing University, Chongqing, ChinaIn this work, we prepare hierarchical MoO3 flower-like hierarchical structure and distinct nanobelts via a simple hydrothermal process, and report systematically the mechanism of formation for these structures.

•Novel needle-flower NiO architectures have been successfully synthesized.•The amount of sodium oxalate play a key role on formation of NiO 3D architectures.•Such a synthetic way may open up an avenue to enhance the gas sensing performance.Self-assembly of one-dimensional nanoscale building blocks into functional three-dimensional super-structures has attracted vast interests. In current work, novel NiO flower-like architectures have been successfully synthesized via a facile hydrothermal method and subsequent calcination. Notably, it has been observed that the sodium oxalate plays a vital role on the aggregation of needles and a novel growth mechanism of needle-flower NiO has been proposed in detail based on the experimental results. More importantly, it is noted that the gas sensing properties of the flower-like architectures are superior to needle-like structures on the basis of the investigation. Such a synthetic way may open up an avenue to tailor the morphologies of some other metal oxides and enhance their gas sensing performance.

•NiO nanobelts with uniform size and well-defined architectures were synthesized via a facile hydrothermal method.•A novel formation mechanism of NiO nanobelts was proposed.•The adjunction of sodium oxalate plays a key role on formation of NiO 1D architectures.•Morphologies of NiO nanobelts can be tailored by controlling the dose of sodium oxalate.Nanomaterials with low-dimensional architectures frequently exhibit novel functional properties. In current work, NiO nanobelts with well-defined morphologies and uniform size have been successfully synthesized via a facile hydrothermal method. Furthermore, a novel growth mechanism of NiO nanobelts has been proposed in detail. Surprisingly, it is worth mentioning that the sodium oxalate is of great benefit to the formation of NiO one-dimensional nanostructures and plays a vital role on tailoring morphologies of nanobelts on the basis of further comparative experiments. Such a synthetic way may open up an avenue to prepare some other oxides.In this work, we have successfully synthesized NiO nanobelts via a facile hydrothermal route and investigated the effect of sodium oxalate.

•NiO flower-like architectures fabricated by 1D nanobundle have been synthesized.•The hierarchical architectures can be tailored by varying the amount of ethylene glycol.•Such an unexpected morphology may provide a non-trivial behavior.Monodisperse NiO hierarchical nanoflowers fabricated by nanobundles have been successfully synthesized under hydrothermal conditions followed by a calcination treatment. It is amazingly observed that the bundle-like nanoflowers possessed the emanative needle-like ends on the nanoscale, which was different from the previous 3D hierarchical architectures only fabricated by nanosheets, entire solid rod-like and emanative needle-like structures. Furthermore, based on the further experiments, the amount of ethylene glycol (EG, 99.5 wt%) play a key role on the formation of emanative needle-like ends and constructions of the 3D hierarchical flower-like architectures, and the relative formation mechanism was primarily discussed. Such an unexpected morphology may provide a non-trivial behavior driven by the properties of emanative needle-like ends and electron transformation continuity.

•Mondispersed hexagonal WO3 nanowires were novely controlled prepared by hydrothermal method.•Detail XRD, SEM and TEM characterizations and crystal structure analyses were carried out.•An interesting growth mechanism for the morphology evolvement and the growth mechanism were given.Monodisperse hexagonal tungsten oxide (h-WO3) nanowires were novelly prepared on a large scale by the hydrothermal method with the assistance of K2SO4 and Na2SO4. The morphologies and structures of the nanowires were characterized by X-ray diffraction (XRD), focused ion beam scanning electron microscopy (FIB-SEM) and high-resolution transmission electron microscopy (TEM). These nanowires with a diameter of 80 nm and high crystallinity are especially bundles-like structure, which are growth from primary particles with same growth direction of [001] and the exposure to 〈200〉 facets. These primary nanoparticles are believed to have a crystal structure of hexagonal prism morphology. Based on the TEM characterizations and crystal structure analyses, the morphology evolvement and is given and the growth mechanism is discussed. This research is potentially applied to controlled synthesis 1D semiconductor oxides through hydrothermal route.

•A novel hierarchical SnO2 flower-like nanostructure was reported.•The SnO2 sheet-flower sensor shows a significantly enhanced gas response.•A possible growth mechanism for the flower-like architectures was proposed.Nanomaterials with three-dimensional (3D) hierarchical architectures often exhibit good functional properties. In current work, flower-like SnO2 architectures self-assembled by aggregative nanosheets have been successfully synthesized via a simple yet facile hydrothermal method. Interestingly, it is found that the obtained sample shows excellent gas sensing performances towards ethanol and this is owing to that nano-petal can provide many quick passages to absorb and desorb gas. Moreover, a possible growth mechanism of this novel structure has been proposed. Such an unexpected morphology holds substantial promise for rendering SnO2 as an excellent gas sensing material.In this work, we prepare hierarchical SnO2 flower-like architectures via a simple hydrothermal process, and report systematically the gas sensing performances and the mechanism of formation for this structure.

•O molecule sitting atop the Ti5C atom is the most possible adsorption mode.•The O vacancies of (101) face are mainly formed in the inner layer.•(101) Surface containing O vacancies has the same gas adsorption behavior as the perfect (101) face.The structural, adsorptive and electronic properties of oxygen adsorption on TiO2 (101) surface are investigated by first-principles calculation. The results show that adsorbed oxygen prefers to locate at Ti5C side, the adsorption energy is relatively low. The electronic structure of surface has no significant changes after oxygen adsorption, indicating weak interaction between adsorbed oxygen and surface of TiO2. The defective (101) face is mainly formed by oxygen vacancy in the inner layer, while the oxygen vacancy in the subsurface affect the adsorption performance slightly. The adsorption site is mainly at Ti5C for case of defective (101) face. That means the anatase TiO2 (101) face containing oxygen vacancy has the same gas adsorption mechanism as the perfect (101) face. This simulation mechanism may provide the instruction to further explore the TiO2-based thin film sensor.The main objective of this study is to understand the sensing origin of anatase TiO2 and provide an insight into why gas sensitivity of anatase TiO2 is not so excellent.

•Monodisperse h-WO3 nanowires with a diameter of 80 nm were hydrothermally synthesized.•These h-WO3 nanowires grow along the direction of [001] and with < 200 > facets exposed.•Sensor based on h-WO3 nanowires show excellent gas sensing properties to toxic gases.The monodisperse hexagonal WO3 (h-WO3) nanowires were synthesized using hydrothermal treatment through the acidification of Na2WO4 · 2H2O by addition of K2SO4 and Na2SO4. The obtained products were characterized using X-ray powder diffraction, field emission scanning electron microscopy and transmission electron microscopy. It showed a high crystallinity and good dispersity of nanowire structure with the exposure of < 200 > crystal facets. Based on h-WO3 products, thin film sensors were prepared. The gas-sensing properties to various concentrations (10, 20, 50, 100, and 200 ppm) of ethanol and formaldehyde were investigated. The h-WO3 nanowires exhibited high responses to both ethanol and formaldehyde gas. The sensor exhibited remarkably good response and fast response/recovery time, which were as short as 6–8 s. A possible reason for the influence of oxide structure on the sensing properties of thin film sensors is proposed. This work shows great potential for the preparation of 1D h-WO3 nanostructures and their application in the detection of toxic gases.

•Sub-micron porous WO3 spheres with uniform size were firstly prepared from WCl6.•These WO3 spheres are critically determined by C templates and WO3 H2O/C precursor.•This porous sphere has a large porosity and exhibits a high gas sensitivity to alcohol.•This research is significant in the synthesis and applications of functional nanomaterials.In this paper, sub-micron porous WO3 spheres with uniform size were firstly prepared by the hydrolysis of WCl6 using carbon microspheres as templates in the hydrothermal system and the following heating treatment. The carbon template, precursor, and the final products were characterized by XRD, SEM, TEM and BET analysis. The as-prepared sub-micron porous WO3 spheres had a large diameter of about 200 nm, which is critically determined by the size of carbon templates and WO3·H2O/C precursor. In addition, these porous spheres have a large porosity and exhibit high gas sensitivity to alcohol in different concentrations. The facile preparation method and the improved properties derived from the porous spheres structures are significant in the synthesis and future applications of functional nanomaterials.Sub-micron porous WO3 spheres with uniform size were firstly prepared by the hydrolysis of WCl6. These WO3 spheres are critically determined by the size of carbon templates and WO3 H2O/C precursor. In addition, this porous sphere has a large porosity and exhibits high gas sensitivity to alcohol in different concentrations.

•Variety low-dimensional nano architectures were reported in the field of WO3 via hydrothermal process.•A growth mechanism is proposed based on comparative studies.•Nanosheets-like WO3 hierarchical architectures showed the excellent propertiesWO3H2O with different morphologies were synthesized through hydrothermal method by adding different surfactants. The effect of surfactants on tailing morphology was investigated through XRD and SEM analyses in detail and the possible formation mechanism was discussed. Furthermore, the gas-sensing performances of obtained samples to ethanol were investigated. The results revealed that the WO3H2O with the hierarchical architectures assembled by nanosheets shows the highest response to ethanol among four kinds of the nanostructures WO3. These results on the preparation, mechanism and gas sensing properties of WO3H2O nanostructures hold great potential for the syntheses of oxide materials with novel nanostructures and their applications.In this work, we prepare various kinds of WO3 low-dimensional architectures via a simple hydrothermal process, and report their growth formation mechanism and gas sensing properties.

We successfully synthesized hierarchical SnO2 flower-like architectures assembled with nanorods via a facile hydrothermal route under mild condition. The structures and morphologies of the obtained hierarchical architectures were characterized by means of powder X-ray diffraction, scanning electron microscopy and transmission electron microscopy. By varying hydrothermal reaction time, we obtained the sphere-like and flower-like SnO2. The evolution that the morphology changing from sphere-like to flower-like was systematically investigated. Moreover, the mechanism of formation for this structure was proposed in detail.

CuO nano-particles, nano-rods, nano-sheets and nano-flowers were synthesized by the hydrothermal routes with copper salts under different additions. The obtained samples were characterized by X-ray diffraction and scanning electron microscopy. We investigated the effects of precursors on the formation of CuO with different morphologies and proposed their possible formation mechanisms. In addition, the obtained CuO nano-flowers are found to show better sensing performances than the other three low-dimensional CuO nanostructures. Our results also demonstrate that gas sensing properties of nanocrystals can be significantly improved by tailoring shape and morphology of the nanocrystals. The CuO nano-flowers may hold substantial promise in low-dimensional gas-sensing applications.

Morphology plays an important role in the properties of nanomaterials. We successfully synthesized three new SnO2 morphologies, nanocorals, nanofragments and nanograss, via a simple hydrothermal method with surfactant additives. The final products were characterized by field-emission scanning electron microscopy and X-ray diffraction. We found that the surfactants played a critical role in the synthesis of different SnO2 nanostructures. In particular, when poly(ethylene glycol) was added as a surfactant, a unique grass-like structure was obtained. The mechanism of formation for this novel structure is discussed. The gas-sensing performance of all the products was investigated. The results show that the nanograss morphology had a superior gas response to formaldehyde than the other morphologies.

•Unique nanoflowers were firstly reported in the field of SnO2 nanostructures by hydrothermal process.•A novel growth mechanism is proposed based on comparative studies.•SnO2 nanoflowers showed the excellent properties toward the ethanol.•Enhanced key gas sensing property meet basic needs for practical applications.Self-assembly of one-dimensional nanoscale building blocks into functional 2D or 3D complex superstructures has stimulated a great deal of interest. In current work, using the hydrothermal method and reagent of hexamethylenetetramine (HMT), we synthesize the SnO2 3D hierarchical nanostructures with an average diameter of 200–400 nm, which exhibit flower-like architectures assembled by numerous one-dimensional tetragonal prism nanorods. Further comparative studies demonstrate that the HMT provides nucleation sites for the assembling of the nanorods, which plays a crucial role in producing such unique flower-like architectures. Meantime, a novel growth mechanism is proposed in detail. In property, the prepared SnO2 nanoflowers show excellent gas-sensing performances to ethanol of 50 ppm at an optimal temperature as low as 250 °C. Such unique architectures may open up an avenue to further enhance the gas-sensing performances of SnO2 nanostructures for future sensor application.In this work, we prepare SnO2 nanoflowers through a simple HMT-assisted hydrothermal process, and report systematically their gas-sensing properties.

•Monodisperse WO3·H2O square nanoplates can be obtained under hydrothermal conditions.•Discussions on the formation mechanism of the WO3·H2O nanoplates was less reported.•It is proved that malic acid play an important role in the formation of square nanoplates.•This research is important for the preparations and applications of other metal oxides materials.Monodisperse WO3·H2O square nanoplates were obtained under hydrothermal conditions with the assistance of malic acid. The morphologies and structures of the plates are characterized by the analyses of XRD, SEM and TEM. In addition, the growth mechanism of the plate-like tungsten oxides has been primarily discussed and the schematic model and crystal structure have been given out. These WO3·H2O nanoplates are revealed as orthorhombic crystal structure with exposure (010) facet. It is proved that malic acid will preferable absorb on the (010) facets during the crystal growth, leading to the growth along the crystal direction [100] and [001], and further determining the formation of plate-like WO3·H2O. This research is of great significance in the preparations, characterizations and applications of various 2D semiconductor metal oxides.Monodisperse WO3·H2O square nanoplates were obtained under hydrothermal conditions. We give a primarily discussion on the formation mechanism of the prepared WO3·H2O nanoplates with the existence of malic acid.

•The synthesized SnO2 sample presents unique flower-like hierarchical architectures which were much less reported.•The role of additives on the morphology of the products was investigated in detail.•A novel growth mechanism is proposed based on comparative studies.Materials with hierarchical architectures often exhibit novel functional properties. In this paper, the hierarchical SnO2 flower-like architecture, consisting of numerous aggregative nanosheets was successfully synthesized via a facile hydrothermal method and subsequent calcination. The structures and morphologies of this hierarchical architecture were characterized in detail by means of powder X-ray diffraction (XRD) and field-emission scanning electron microscopy (FE-SEM). Further comparative experiments show that the sodium citrate was benefit to promote the growth of SnO2 nanosheets and accelerate the assembling of nanosheets into flower-like architecture, the PEG provided nucleation sites and made the nanoflowers grow uniformly and separated. Moreover, a possible growth mechanism of the flower-like architectures was proposed. The non-trivial behavior of flower-like SnO2 may be driven by the properties of grain boundaries.We successfully synthesized hierarchical assembled flower-like SnO2 with the reagents of sodium citrate and PEG via a simple hydrothermal technique and subsequent calcination.

•The synthesized NiO sample presents perfect regular hexagonal nanosheet which was much less reported.•The concentration of ammonia solution and the additive of EG were controlled in the synthesis processing.•It was the first to illustrate the growth mechanism for this hexagonal structure by the crystal structure theory.•The hexagonal NiO nanosheets were promising sensing materials for alcohol among various gases.Uniform hexagonal NiO nanosheets were perfectly prepared by a controlled hydrothermal method followed by calcination. The as-prepared products were characterized by X-ray powder diffraction (XRD), which illustrated the precursor Ni(OH)2 and the final product NiO. The morphologies of NiO were characterized in detail by field-emission scanning electron microscopy (FE-SEM) and high-resolution transmission electron microscopy (HRTEM). The SAED pattern is detected from a sampling area covering a collection of constituent nanoparticles. Based on the experimental results and the crystal structure theory, the chemical reaction processes and the growth mechanism for hexagonal structure were proposed. In addition, the gas-sensing performances of uniform hexagonal NiO were investigated toward 50 ppm of several reductive gases including ethanol, CO, H2S, CH4 and NH3 at different working temperatures.Graphical abstract(a) The FE-SEM image shows the as-synthesized NiO product presents uniform hexagonal nanosheets, and (b) the HRTEM image gives a special feature of a perfect hexagonal nanosheet.

•The synthesized NiO sample presents chrysanthemum-like morphology which was much less reported.•Doping the WO3·0.33H2O into the NiO to form a special p–n heterojunction was never reported before.•The composite exhibited outstanding ethanol-sensing performance.•Schematic model for the doped sensor was built to make the response process clear.Tungsten doped chrysanthemum-like NiO composite was successfully prepared via the hydrothermal process, then the calcination after mixing. Three relative samples (WO3·0.33H2O–NiO, NiO, WO3·0.33H2O) were tested and confirmed by X-ray diffraction. At the meantime, the Energy Dispersive Spectrometer (EDS) was used to prove that the dopant, tungsten, had been doped into the NiO. The morphologies of NiO and WO3·0.33H2O–NiO were characterized by field-emission scanning electron microscopy (FE-SEM) and high-resolution transmission electron microscopy (HRTEM). Significantly, the comparation of ethanol-sensing performances among the above three samples was disscussed in detail revealing that the doping of tungsten enhanced the gas-sensing property extremely. In addition, the doping model and gas-sensing mechanism were proposed to elaborate the high improvement of the sensor response to the ethanol.

•NiO nanowires with large aspect ratio were prepared by the hydrothermal method.•The first use of EG and PEG as surfactants is presented in the current manuscript.•The paper presents a new mechanism for the formation of NiO nanowires.•The as-prepared NiO with the morphology of nanowires has excellent gas response.NiO nanowires with high aspect ratio and dispersive distribution have been synthesized by a hydrothermal reaction of NiCl2 with Na2C2O4 and H2O in the simultaneous presence of ethylene glycol (EG) and polyethylene glycol (PEG). Then the products were obtained by the subsequent annealing at 400 °C in air. The effect of –OH from EG and –O– from PEG in the formation of nanowires was discussed. And the gas sensing properties of the as-prepared NiO nanowires toward ethanol were investigated. A novel formation mechanism of nanowires was presented and the NiO nanowires were proved to have an excellent gas sensing performance.Graphical abstractNiO nanowires with high aspect ratio and excellent gas sensing properties have been synthesized by a hydrothermal reaction and the formation mechanism was discussed in detail.Figure optionsDownload full-size imageDownload as PowerPoint slide

New morphologies often play an important role in deciding the properties of nanomaterials. In current study, we synthesize various types of low dimensional SnO2 nanostructure materials using a simple hydrothermal process. We find that the SnO2 nanorods, nanowires and nanosheets show higher gas response as well as lower operating temperature as compared to that of nanospheres. In addition, the obtained nanosheets SnO2 are found to show best sensing performances owing to the largest surface area and porous structure providing many quick passages to absorb and desorb gas, suggesting that the gas sensing properties of nanocrystals can be significantly improved by tailoring the shape and morphology of nanocrystals. Such an unexpected morphology holds substantial promise for rendering low dimensional nano-SnO2 as a potential gas sensing material for future sensor application.Graphical abstractIn this work, we prepare variety low dimensional nanostructure of SnO2 through a simple hydrothermal process, and report systematically their gas sensing functionality.Highlights► SnO2 nanosheets exhibit best sensing performance toward ethanol. ► Property improvement is independent of operating temperature and gas concentration. ► Enhanced key gas sensing property meet basic needs for practical applications. ► Sensing properties of nanocrystals can be improved by tailoring their shape and structure.

The structural, adsorptive and electronic properties of H2 adsorption on SnO2–F (1 1 0) surface are investigated by first-principles calculation. The results show that the F-doped (1 1 0) surface is more reducible than that of undoped SnO2 surfaces, which is mainly attributed to formation of the surface states and larger charges transfer between H2 molecule and the F-doped (1 1 0) surface. This simulation mechanism may provide the instruction to further explore the SnO2-based sensing materials.Graphical abstractA first principles calculation study on the gas sensing performances of SnO2–F surface and its sensing mechanism were presented in current work.Highlights► H2 molecule sitting atop the Obright atom is the most possible adsorption mode. ► The SnO2–F (1 1 0) surface hold more benefit for H2 adsorption than that of SnO2. ► F-doped SnO2 enable exhibit distinct sensing performance.

In this work, 2D SnO2 nanosheets were synthesized via a polyvinylpyrrolidone (PVP)-assisted hydrothermal method. X-ray diffraction and scanning electron microscopy were used to examine the chemical composition and nanostructures. It was found that the concentration of PVP played a critical role in governing the assembly process of nanostructures. Further, their gas-sensing performances were investigated comprehensively. The nanosheet-III structures were found to show the most superior gas-sensing properties due to their largest specific surface and fashionable assembly.

•The flower-sheet SnO2 sensor displayed particular response to the target gas.•Property improvement is independent of operating temperature and gas concentration.•Enhanced key gas sensing property meets basic needs for practical applications.•Sensing properties of nanocrystals can be improved by tailoring their shape and structure.We successfully synthesized variant hierarchical assembled flower-like SnO2 via a simple hydrothermal technique and subsequent calcination. The structures and morphologies of the 3D nanostructures were investigated by means of powder X-ray diffraction (XRD) and field-emission scanning electron microscopy (FE-SEM). The formation mechanism of these materials were proposed in detail. The gas-sensing performances of the as-prepared SnO2 were investigated towards ethanol. It is noted that the flower-sheet SnO2 sensor displayed particular response to the target gas, rendering SnO2 as a potential gas-sensing material for a broad range of future sensor applications.The flower-sheet SnO2 sensor displayed particular response to the target gas, rendering SnO2 as a potential gas-sensing material for a broad range of future sensor applications.

We report synthesis of hierarchical nanostructures of SnO2 nanosphere functionalized TiO2 nanobelts as a novel sensing material by a simple hydrothermal technique. A systematic comparison study reveals an enhanced gas sensing performance for the sensor made of SnO2 and TiO2 toward volatile organic compounds of several species over that of the commonly applied undecorated TiO2 nanobelts. The improved gas sensing properties are attributed to the pronounced electron transfer between the hierarchical nanostructures and the absorbed oxygen species as well as to the heterojunctions of the SnO2 nanospheres to the TiO2 nanobelts which provide additional reaction rooms. The results represent an advance of hierarchical nanostructures in further enhancing the functionality of gas sensors, and this facile method could be applicable to many sensing materials.

Zinc tin oxide (ZnSnO3) materials, native and doped with Ti nanocomposites, were prepared by coprecipitation. In particular, we focus on the effect of Ti dopant on the ethanol sensing behavior of the ZnSnO3-based sensor. Results show that adding Ti ions evidently enhanced gas response of ZnSnO3. A gas adsorption model based on first principles was proposed to address the gas sensing mechanism. This study may open up an avenue for exploring other ABO3-type sensing materials.

In this work we report microstructure and gas sensing property of the pristine SnO2 and (Ti,Sn)O2 solid solutions prepared via the hydrothermal method. Of all the examined volatile organic compound gases, the (Ti,Sn)O2 solid solution exhibits somewhat lower gas response as compared to that of pristine SnO2. Further, we perform a first principles calculation to establish a surface model for solid solution and estimate its atomic configuration. The tightness of Ti6C(Sn6C)O2C bond and high oxygen vacancy formation energy for solid solution suggest that less amount of pre-adsorbed oxygen was formed on the surface, ultimately resulting in sensing performance weakening.Graphical abstractA first principles calculation and experimental study on the gas sensing performances of Ti0.5Sn0.5O2 and its sensing mechanism were presented in current work.Highlights► (Ti,Sn)O2 solid solution exhibits somewhat lower gas response than pristine SnO2. ► Surface model for solid solution has been established. ► Solid solution has relatively high oxygen vacancy formation energy. ► The tight bonding for Ti6C(Sn6C)O2C leads to less adsorbed oxygen formed on the surface.

A type of titanium precursor, H-exchanged titanate nanobelts, was used to prepare nanosized anatase titanium dioxide (TiO2) with various morphologies by hydrothermal method. Nanorods, nanobelts, nano-polyhedrons and nanoparticles were successfully synthesized. We found that CTAB and EDTA-4Na+ play critical roles in synthesizing the nanorods and nano-polyhedrons. All the samples exhibit rapid response and recovery time to ethanol, but Nanorods, nanobelts and nano-polyhedrons show lager response than nanoparticles.

Unique assembled sphere-like WO3 architectures were successfully synthesized through a facile hydrothermal method in the presence of malic acid followed by subsequent heat treatment. We found that malic acid played a significant role in governing morphologies of WO3·xH2O precursors during hydrothermal process. A possible formation mechanism was also proposed in detail. Experimental results showed that the optimized hydrothermal precursor could be dehydrated to mixed composition of hexagonal and monoclinic WO3 with the unique sphere-like porous architecture after being annealed at 400 °C for 2.5 h. Besides, gas-sensing measurement indicated that the well-defined 3D assembled sphere-like architectures exhibited the highest sensor response to ethanol at the optimal temperature of 250 °C among the samples.Graphical abstractWe have prepared unique three-dimensional assembled sphere-like WO3 architectures successfully via a facile hydrothermal method followed by a subsequent heat treatment. The well-defined porous structures exhibit novel gas-sensing properties to ethanol.Highlights► Unique assembled sphere-like WO3 architectures are successfully obtained. ► Malic acid is introduced as an assembling and structure-directing agent to controllable synthesis of WO3 architectures. ► The well-defined sphere-like WO3 architectures exhibit excellent gas-sensing properties to ethanol. ► The malic acid presented in current experiment may provide inspiration for controllably synthesizing other metals oxide.

We report a successful synthesis of ZnO nanoparticles, nanosheets and nanoflowers via a simple hydrothermal process, and investigate comprehensively their gas-sensing performances. Of all the nanostructures, nanoflowers are found to show the most superior gas-sensing properties, e.g., highest gas response, shortest response and recovery time, excellent selectivity, and good repeatability and stability, which are attributed to their unique three-dimensional hierarchical structures with the largest specific surface area arising from remarkable amount of petals and pores. Further, the sodium citrate is found to be the key to producing such unique flower-like architecture, which can be understood upon the nucleation and self-assembly of building blocks of ZnO. Such development of the hierarchical architectures may open up an avenue to further enhance the gas-sensing performances of ZnO nanostructures for the on-site detection of the gases of interest.

Using a simple hydrothermal method, the pristine and Nb doped TiO2 is prepared, and their microstructures and gas-sensing responses to the harmful volatile organic compounds are investigated with a special focus on the impact of Nb additive. We find that the gas response of TiO2 is enhanced significantly by doping Nb, which is understood in theory upon proposed adsorption models. Combining experimental measurements with first-principles calculations, the working mechanism underlying such improvement in gas-sensing functions by the Nb additive is discussed.

•The porous WO3 monomer and the one-dimensional NiO monomer were synthesized firstly.•Such nature-inspired WO3-based nanocomposite shows an enhanced sensitivity to the ethanol.•A possible growth mechanism for the such architectures was proposed.Composed of the heart, cardiac valves and blood vessels, the blood circulation system can manage the aggregation and the movement of the blood regularly and efficiently. Due to the analogical fluidity of the charge carrier and the blood, such system may act as the inspiration to design the advanced gas-sensing material. As a proof-of-concept, firstly, the porous WO3 monomer was synthesized and recombined with the one-dimensional NiO monomer through the P-N junction to construct a similar structure. Subsequently, a series of gas-sensing tests towards the ethanol were measured. The obtained result indicates this nature-inspired nanocomposite indeed shows an enhanced gas-sensing property towards the ethanol, testifying the rationality of our design.Simulating the blood circulation system, a similar WO3-based nanocomposite with an enhanced gas-sensing property was constructed on the basis of the porous WO3 monomer and the one-dimensional NiO monomer.Download high-res image (325KB)Download full-size image

We report a successful synthesis of ZnO nanoparticles, nanosheets and nanoflowers via a simple hydrothermal process, and investigate comprehensively their gas-sensing performances. Of all the nanostructures, nanoflowers are found to show the most superior gas-sensing properties, e.g., highest gas response, shortest response and recovery time, excellent selectivity, and good repeatability and stability, which are attributed to their unique three-dimensional hierarchical structures with the largest specific surface area arising from remarkable amount of petals and pores. Further, the sodium citrate is found to be the key to producing such unique flower-like architecture, which can be understood upon the nucleation and self-assembly of building blocks of ZnO. Such development of the hierarchical architectures may open up an avenue to further enhance the gas-sensing performances of ZnO nanostructures for the on-site detection of the gases of interest.

•Nanoneedle assembled NiO nanoflowers are synthesized via hydrothermal method.•Growth mechanisms for the morphologies evolution were proposed detailly.•Nanoneedle-assembled NiO exhibit quicker gas response, recovery than nanosheet ones.Nanoneedle-assembled hierarchical and nanosheet-assembled hierarchical NiO nanoflowers were successfully synthesized via a facile hydrothermal method and subsequent calcination. And we proposed their growth mechanisms and the influences on gas-sensing properties. The results indicated that the gas sensing properties of NiO nanocrystals can be dramatically enhanced by tailoring the nanoscale building blocks. In addition, it was surprisingly noticed that the nanoneedle-assembled hierarchical NiO structures exhibited quicker gas response and recovery than the nanosheet-assembled ones. While nanosheet-assembled hierarchical NiO nanoflowers demonstrated higher gas response towards ethanol.In the study, nanoneedle-assembled hierarchical and nanosheet-assembled hierarchical NiO nanoflowers were synthesized via hydrothermal method and subsequent calcination. And we explored about their growth mechanisms and the influences on gas-sensing properties. The results indicated that the gas sensing properties of NiO nanocrystals can be dramatically enhanced by tailoring the nanoscale building blocks.

We report synthesis of hierarchical nanostructures of SnO2 nanosphere functionalized TiO2 nanobelts as a novel sensing material by a simple hydrothermal technique. A systematic comparison study reveals an enhanced gas sensing performance for the sensor made of SnO2 and TiO2 toward volatile organic compounds of several species over that of the commonly applied undecorated TiO2 nanobelts. The improved gas sensing properties are attributed to the pronounced electron transfer between the hierarchical nanostructures and the absorbed oxygen species as well as to the heterojunctions of the SnO2 nanospheres to the TiO2 nanobelts which provide additional reaction rooms. The results represent an advance of hierarchical nanostructures in further enhancing the functionality of gas sensors, and this facile method could be applicable to many sensing materials.