Surface Of ZnO Nanostructures Research Articles

Thermal decomposition-assisted, aspect ratio controlled ZnO nanorods towards highly selective H2 gas detection

ZnO nanostructures with various aspect ratios have been synthesized for H2 gas detection applications. The thermal-decomposition method was employed at different annealing temperatures (350, 450, and 550 °C) and its impact on various shapes/sizes of ZnO nanostructures is demonstrated. Thermal decomposition performed at 350 °C exhibited a maximum (∼6.25) aspect ratio among them. Its capability of H2 sensing was also observed to be maximum by realizing ∼483% of sensor response at 180 °C under H2 gas concentration of 80 ppm. The sensor response is ∼3 times (∼177%) and ∼9 times (∼53%) higher at ZnO nanostructure synthesized at 350 °C than at 450 °C, and 550 °C, respectively. The higher sensor response has been attributed to the increased availability of active surface area for adsorption/desorption of gas molecules. ZnO@350 nanostructure showed significantly higher selectivity towards H2 gas than other target chemical inputs. We have also studied H2-induced metallization on the surface of ZnO nanostructures which plays an important role for improving the selectivity and sensor response. This study provides insight into the role of aspect-ratio-controlled shape/sized ZnO in improving H2 gas sensing.

Cu nanoparticles grafting on the surface of ZnO nanostructures to boost the porosity and surface area for effective removal of manganese ions from aqueous solutions

Designing highly adsorptive materials for wastewater treatment via facile approaches is still challenging. To boost the recovery of heavy metals from wastewater, surface and structure modification are considered a successful route. Herein, we report the design of ZnO nanoparticles by a simple thermal decomposition method followed by grafting Cu nanoparticles (Cu NPs) over the ZnO surface. Cu/ZnO was prepared with different Cu ratios, 0.01 and 1%. It was found that incorporating Cu into ZnO improved the porosity and surface area of ZnO. The adsorption ability of Cu/ZnO compared with bare ZnO was studied towards removing manganese ions from wastewater. The effects of several parameters, such as pH, temperature, contact time, and initial ion concentrations, were studied. The maximum removal of manganese was found at pH 2, 20 °C after 60 min in the presence of 1 g/L adsorbent. The role of Cu grafted on the surface of ZnO was discussed. The rates of adsorption were found to follow the pseudo-second-order model. The results showed better fitting to Freundlich isotherm. The thermodynamic study revealed that the sorption process is spontaneous, exothermic, and favorable at low temperatures. The free energy (ΔG°), enthalpy (ΔH°), and entropy (ΔS°) changes were calculated to predict the nature of adsorption.Graphical

Preparation of ZnO/Ag nanoflowers by hydrothermal assisted with galvanic effect and its surface enhanced Raman scattering activity

An effective approach is proposed for fabrication of ZnO/Ag nanocomposite for surface enhanced Raman scattering application by combining hydrothermal assisted with galvanic effect and sputtering process. The sputtering process is applied to distribute Ag on the surface of ZnO nanostructures. The morphology, structure, elemental composition and optical properties of the samples were studied. The most suitable sputtering condition was identified to achieve the highest Raman enhancement. The optimum sputtering time of 70 s provides strong Raman signal of methylene blue, and demonstrates the potential of using the ZnO/Ag nanoflowers as sensitive surface enhanced Raman substrates.

Graphene capping layer for improving the photovoltaic stability of ZnO nanorods solar cells under temperature variations

The present study evaluates the role of the graphene capping layer on the optoelectronic performance of ZnO/Si solar cells at different ambient temperatures. The n-ZnO buffer thin film was sputter-deposited on the p-Si substrates to form a p-n heterojunction. Next, ZnO nanostructures were chemically synthesized on top of the ZnO/Si substrates, and few-layer graphene was deposited on ZnO nanostructures using an electrophoretic deposition technique. The planar and cross-sectional electron microscopy analyses exhibited that the graphene capping layer entirely covered the surface of ZnO nanostructures, and their morphology remained intact after graphene transfer. Their photoluminescence (PL) properties before and after graphene deposition were analyzed at various temperatures to evaluate the graphene role in the optical behavior of ZnO nanostructures. The PL intensity was reduced for the graphene-containing sample because of the slight light absorption in each graphene layer. However, the stability of PL spectra at elevated temperatures was improved after graphene transfer. Next, a heterojunction solar cell was developed by depositing top and back contacts to study the photovoltaic performance of nanostructures at various temperatures. The current-voltage results, recorded under a solar light simulator, illustrated that the efficiency of the bare ZnO device was reduced at high temperatures because of the available interstitials and oxygen vacancies. After graphene transferring, this decay was moderated, which enhanced the photovoltaic stability of ZnO-based solar cells. Therefore, the graphene capping layer can act as a stabilizing thin film to enhance the photovoltaic durability of ZnO nanostructures under harsh conditions.

Improving natural sunlight photocatalytic efficiency of ZnO nanowires decorated by iron oxide cocatalyst via a simple drop method

In this study, iron oxide cocatalysts were deposited on the surface of ZnO nanostructures by a simple “drop, evaporation, calcination” method. Different deposition concentrations were studied, allowing to extend the photocatalyst absorption spectral range from UV to visible light, improving the photocatalytic efficiency under natural sunlight. An optimal deposition concentration of 0.06% molar percentage of iron against zinc led to improve by 11% the Methyl Orange (MO) photocatalytic degradation after 5 h under natural sunlight. No iron leaching was detected by ICP-MS analysis after the water purification by photocatalysis. By studying the lifetime stability of the optimal sample under a powerful UV light source, no losses of photocatalytic efficiency were recorded after several degradation cycles. Nevertheless, under this powerful light source, photocorrosion damages were observed with two antagonistic effects: (1) no photocorrosion effect for the ZnO regions covered by iron oxide and (2) ZnO photocorrosion and an acceleration of the dissolution of the ZnO nanostructures in uncovered regions. Thus, we assume the electron/hole delocalisation, which is responsible of the photocatalytic activity improvement, is also the reason justifying the ZnO photocorrosion acceleration of uncovered part. The photocorrosion phenomena was observed only under the powerful UV light source, not under natural sunlight.

Enzymes and phytochemicals from neem extract robustly tuned the photocatalytic activity of ZnO for the degradation of malachite green (MG) in aqueous media

The malachite green (MG) is very difficult to degrade in water; thus, it needs an efficient photocatalyst. In this study, neem extract was used to tune the surface and crystal properties of ZnO nanostructures for the photodegradation of MG. The biosynthesized ZnO samples were prepared by hydrothermal method in the presence of 5, 10 and 15 mL of neem extract. The structural characterization has shown nanoparticle like morphology of ZnO as revealed by scanning electron microscopy (SEM) and hexagonal phase was confirmed by powder X-ray diffraction (XRD) technique. The XRD analysis has shown a shift in the 2 theta towards lower angle for ZnO with increasing amount of neem extract. Also, the crystallite particle size of ZnO was decreased with increasing neem extract. The UV–visible spectroscopy has shown the decrease in the optical band gap of ZnO, and the lowest band gap is possessed by ZnO sample produced with 15 mL of neem extract. The ZnO sample obtained with 15 mL of neem extract has shown approximately 99% degradation efficiency for MG for 70 min in aqueous solution. The superior photocatalytic activity of ZnO sample with 15 mL of neem extract could be attributed from the decrease in charge recombination rate due to the decreased optical band gap and particle size.

PH-Dependent Formation of Oriented Zinc Oxide Nanostructures in the Presence of Tannic Acid.

To crucially comprehend the relaying factors behind the growth mechanism of ZnO nanostructures, the needs to understand the cause of preferences in the enhancement of desired physicochemical properties are essential. The particular oriented attachment (OA) is believed to become the cause of the classical growth pattern of ZnO nanostructures which is mainly controlled by the Ostwald ripening (OR) process. In the present work, the concerns over the systematic changes in size and the morphological surface of ZnO nanostructures upon exposure to tannic acid (TA) prepared by drop-wise method turns the particles to different surface adjustment state. Here, we assessed the TA capping ability and its tendency to influence the OA process of the ZnO nanostructures. The detailed process of the growth-based TA system via transmission electron microscopy (TEM), scanning electron microscopy (SEM), and FFT autocorrelation revealed the pH effect on their physical properties which proved the transition surface properties state of the particles from rough to smooth states due to oriented attachment. For pure ZnO nanostructures, the surface is almost smooth owing to the strong bonding particles which are then changed to coarsened surface structures upon the introduction of TA. Strong surface adsorption of Zn cations and phenol ligands mediated the agglomerated nanocrystals, surprisingly with smaller nanostructures dimension.

Water-Based Scalable Methods for Self-Cleaning Antibacterial ZnO-Nanostructured Surfaces.

Bacterial colonization poses significant health risks, such as infestation of surfaces in biomedical applications and clean water unavailability. If maintaining the surrounding water clean is a target, developing surfaces with strong bactericidal action, which is facilitated by bacterial access to the surface and mixing, can be a solution. On the other hand, if sustenance of a surface free of bacteria is the goal, developing surfaces with ultralow bacterial adhesion often suffices. Here we report a facile, scalable, and environmentally benign strategy that delivers customized surfaces for these challenges. For bactericidal action, nanostructures of inherently antibacterial ZnO, through simple immersion of zinc in hot water, are fabricated. The resulting nanostructured surface exhibits extreme bactericidal effectiveness (9250 cells cm–2 h–1) that eliminates bacteria in direct contact and also remotely through the action of reactive oxygen species. Remarkably, the remote bactericidal action is achieved without the need for any illumination, otherwise required in conventional approaches. As a result, ZnO nanostructures yield outstanding water disinfection of >99.98%, in the dark, by inactivating the bacteria within 3 h. Moreover, Zn2+ released to the aqueous medium from the nanostructured ZnO surface have a concentration of 0.73 ± 0.15 ppm, markedly below the legal limit for safe drinking water (5–6 ppm). The same nanostructures, when hydrophobized (through a water-based or fluorine-free spray process), exhibit strong bacterial repulsion, thus substantially reducing bacterial adhesion. Such environmentally benign and scalable methods showcase pathways toward inhibiting surface bacterial colonization.

Enhanced photocatalytic and antibacterial activity of ZnO/Ag nanostructure synthesized by Tamarindus indica pulp extract

In this paper, we report a one-step, sunlight-irradiated synthesis of ZnO nanostructures (NSs) covered with Ag nanoparticles (NPs) using Tamarindus indica pulp extract (TPE) and their applications to photocatalytic and antibacterial activity. TPE contains phytochemicals that act as reducing and stabilizing agents for the distribution of uniform Ag NPs of approximately 5 nm on the surface of ZnO NSs. The photocatalysis of the synthesized bare ZnO and ZnO/Ag (0.5 wt%) NSs was evaluated under UVA light and dye degradation results for methylene blue (MB) are 68% for ZnO and 94% for ZnO/Ag after 120 min. For methylene orange (MO), bare ZnO and ZnO/Ag NSs were degraded 40% and 81%, respectively, after 150 min. Moreover, the photocatalyst also showed good recycling stability even after five cycles. In addition to dye degradation, the ZnO/Ag NSs were able to show antibacterial activity against E. coli, P. aeruginosa, S. aureus, and B. subtilis at a concentration of 2 mg/ml. They showed higher activity against Gram-positive than gram-negative bacteria, which showed the influence of Ag metal on the enhanced photocatalytic and antibacterial activity of ZnO NSs. Our findings suggested that ZnO/Ag NSs can be synthesized by a facile and ecofriendly method using TPE and showed superior performance for UVA photocatalysis and antibacterial activity compared to bare ZnO.

Exciton Channel of Photoactivation for Redox Reactions on the Surface of 2D ZnO Nanostructures

The resonance excitation of excitons was used for the first time to activate photocatalytic redox reactions on ZnO. The sample activity was tested using the photoactivated oxygen isotope exchange (POIE), which can be used to predict both oxidation and reduction reactions. When the excitation energy is transferred to the surface by an electrically neutral exciton, then the main losses in transferring the photogenerated e–/h+ pairs due to recombination and to overcoming the surface potential barrier are strongly reduced. The radiative decay of an exciton on the ZnO surface is excluded by a 2D structure ZnO/ZnO1–x/O–, in which the exciton decays nonradiatively into a pair of long-lived (up to 8 × 103 s) electron and hole local states, on which in its turn a chemical reaction can take place. These centers decay at 450 K, emitting green luminescence. The exciton resonance excitation allows one to obtain the order-of-magnitude higher values of the efficiency of redox reaction compared to the case of interband excitation.

Controlled Surface Modification of ZnO Nanostructures with Amorphous TiO2 for Photoelectrochemical Water Splitting

AbstractThe utilization of solar radiation to trigger photoelectrochemical (PEC) water splitting has gained interest for sustainable energy production. In this study, attention is focused on the development of ZnO–TiO2 nanocomposite photoanodes. The target systems are obtained by growing porous arrays of highly crystalline, elongated ZnO nanostructures on indium tin oxide (ITO) by chemical vapor deposition. Subsequently, the obtained nanodeposits are functionalized with TiO2 via radio frequency‐sputtering for different process durations, and subjected to final annealing in air. Characterization results demonstrate the successful formation of high purity composite systems in which the surface of ZnO nanostructures is decorated by ultra‐small amounts of amorphous titania, whose content can be conveniently tailored as a function of deposition time. Photocurrent density measurements in sunlight‐triggered water splitting highlight a remarkable performance enhancement with respect to single‐phase zinc and titanium oxides, with up to a threefold photocurrent increase compared to bare ZnO. These results, mainly traced back to the formation of ZnO/TiO2 heterojunctions yielding an improved photocarrier separation, show that the target nanocomposites are attractive photoanodes for efficient PEC water splitting.

Rapid ultrasonic-microwave assisted synthesis of spindle-like Ag/ZnO nanostructures and their enhanced visible-light photocatalytic and antibacterial activities

Abstract Spindle-like Ag/ZnO nanostructure has been successfully synthesized via an ultrasonic-microwave combined assisted method. The as-prepared Ag/ZnO nanocomposites exhibited enhanced photodegradation efficiency for organic pollutants than pure ZnO and commercial TiO2 (P25) under visible light irradiation, which was probably due to the surface plasmon resonance (SPR) effect of Ag nanoparticles. The production of hydroxyl radicals ( OH) under visible light irradiation was greatly promoted via loading Ag nanoparticles on the surface of ZnO nanostructures, also resulting in the improved photocatalytic activity. Moreover, the spindle-like Ag/ZnO nanocomposites displayed excellent antibacterial activities for both Gram-positive and Gram-negative bacteria. The Ag/ZnO nanocomposites could penetrate and damage cell membrane, causing the leakage of cytosol and the inactivation of respiratory chain dehydrogenase. The synergistic effect of Ag and ZnO is also beneficial for the mass production of H2O2, leading to the enhanced antibacterial activities of nanocomposites.

Facile Fabrication of Multifunctional ZnO Urchins on Surfaces

Functional ZnO nanostructured surfaces are important in a wide range of applications. Here we report the simple fabrication of ZnO surface structures at near room temperature with morphology resembling that of sea urchins, with densely packed, μm-long, tapered nanoneedles radiating from the urchin center. The ZnO urchin structures were successfully formed on several different substrates with high surface density and coverage, including silicon (Si), glass, polydimethylsiloxane (PDMS), and copper (Cu) sheets, as well as Si seeded with ZnO nanocrystals. Time-resolved SEM revealed growth kinetics of the ZnO nanostructures on Si, capturing the emergence of “infant” urchins at the early growth stage and subsequent progressive increases in the urchin nanoneedle length and density, whilst the spiky nanoneedle morphology was retained throughout the growth. ε-Zn(OH)2 orthorhombic crystals were also observed alongside the urchins. The crystal structures of the nanostructures at different growth times were confirmed by synchrotron X-ray diffraction measurements. On seeded Si substrates, a two-stage growth mechanism was identified, with a primary growth step of vertically aligned ZnO nanoneedle arrays preceding the secondary growth of the urchins atop the nanoneedle array. The antibacterial, anti-reflective, and wetting functionality of the ZnO urchins—with spiky nanoneedles and at high surface density—on Si substrates was demonstrated. First, bacteria colonization was found to be suppressed on the surface after 24 h incubation in gram-negative Escherichia coli (E. coli) culture, in contrast to control substrates (bare Si and Si sputtered with a 20 nm ZnO thin film). Secondly, the ZnO urchin surface, exhibiting superhydrophilic property with a water contact angle ~ 0°, could be rendered superhydrophobic with a simple silanization step, characterized by an apparent water contact angle θ of 159° ± 1.4° and contact angle hysteresis ∆θ < 7°. The dynamic superhydrophobicity of the surface was demonstrated by the bouncing-off of a falling 10 μL water droplet, with a contact time of 15.3 milliseconds (ms), captured using a high-speed camera. Thirdly, it was shown that the presence of dense spiky ZnO nanoneedles and urchins on the seeded Si substrate exhibited a reflectance R < 1% over the wavelength range λ = 200–800 nm. The ZnO urchins with a unique morphology fabricated via a simple route at room temperature, and readily implementable on different substrates, may be further exploited for multifunctional surfaces and product formulations.

Preparation and characterization of ZnO/ZnS core/shell nanocomposites through a simple chemical method

In this paper, we reported the preparation of ZnO/ZnS core/shell nanocomposites by sulfidation of ZnO nanostructures via a simple hydrothermal method. The precursors of bare ZnO nanoparticles and ZnO nanorods were synthesized by a surfactant-assisted hydrothermal growth. The structural, morphological, and element compositional analysis of bare ZnO nanostructures and ZnO/ZnS core/shell nanocomposites were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, and transmission electron microscopy techniques. The XRD results indicated that the phase of bare ZnO nanoparticles and ZnO nanorods was wurtzite structure, and the phase of coated ZnS nanoparticles on the surface of bare ZnO nanostructures was sphalerite structure with the size of about 8 nm. Photoluminescence measurement was carried out, and the PL spectra of ZnO/ZnS core/shell nanocomposites revealed an enhanced UV emission and a passivated orange emission compared to that of bare ZnO nanostructures. In addition, the growth mechanism of ZnO/ZnS core/shell nanostructures through hydrothermal method was preliminarily discussed.

Plasmon mediated enhancement of visible light emission of Au-ZnO nanocomposites

We report on the enhancement of the visible luminescence of Au-ZnO nanocomposites synthesized via a simple photoreduction method. The attachment of Au nanoparticles on the surface of ZnO nanostructures has been observed using transmission electron microscopy. Raman analysis reveals an increase in disorderness via lattice defect formation due to Au attachment and thus results in the appearance of silent modes of ZnO. The photoluminescence spectra of ZnO is found to be dominated by defect related visible emission, which is enhanced with the inclusion of Au nanoparticles. At room temperature, the green emission for nanocomposites has been found to be enhanced by seven times as compared to the pristine ZnO. Temperature dependent photoluminescence of Au-ZnO nanocomposites has been investigated to elucidate the origin of green luminescence and the enhancement of intensity. The gradual dominance of the green emission is originated from the surface plasmon oscillation of Au followed by the electron capture at the singly ionized oxygen vacancies of ZnO. The observed enhancement of visible light emission of ZnO using Au nanoparticles can establish the platform to fabricate efficient ZnO based visible light emitting devices and display systems.

Tailoring surface and photocatalytic properties of ZnO and nitrogen-doped ZnO nanostructures using microwave-assisted facile hydrothermal synthesis

Microwave hydrothermal synthesis, using an experimental 23 factorial design, was used to produce tunable ZnO nano- and microstructures, and their potential as photocatalysts was explored. Photocatalytic reactions were conducted in a microreactor batch system under UV and visible light irradiation, while monitoring methylene blue degradation, as a model system. The variables considered in the microwave reactor to produce ZnO nano- or microstructures, were time, NaOH concentration and synthesis temperature. It was found that, specific surface area and volume/surface area ratio were affected as a consequence of the synthesis conditions. In the second stage, the samples were plasma treated in a nitrogen atmosphere, with the purpose of introducing nitrogen into the ZnO crystalline structure. The central idea is to induce changes in the material structure as well as in its optical absorption, to make the plasma-treated material useful as photocatalyst in the visible region of the electromagnetic spectrum. Pristine ZnO and nitrogen-doped ZnO compounds were characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), specific surface area (BET), XPS, and UV–Vis diffuse reflectance spectroscopy. The results show that the methodology presented in this work is effective in tailoring the specific surface area of the ZnO compounds and incorporation of nitrogen into their structure, factors which in turn, affect its photocatalytic behavior.

Photocatalytic efficiencies of Ni, Mn, Fe and Ag doped ZnO nanostructures synthesized by hydrothermal method: The synergistic/antagonistic effect between ZnO and metals

Abstract The ZnO and different noble and transition metals (Ni, Mn, Fe and Ag) doped ZnO (M/ZnO) nanostructures were synthesized by hydrothermal method. The ZnO and M/ZnO photocatalysts were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), fourier transform infrared (FTIR) and UV–vis spectrophotometry. The obtained XRD and FESEM results confirmed the well dispersion of Ni and Ag nanoparticles and highly agglomerated Mn and Fe nanoparticles on the surface of ZnO nanostructures. The optical band gap value was calculated as 3.24, 3.15, 3.10, 3.05 and 3.00 eV from UV–vis diffuse reflectance spectra of ZnO, Ag/ZnO, Ni/ZnO, Fe/ZnO and Mn/ZnO photocatalysts, respectively. The results of the photocatalytic degradation of tartrazine in aqueous solutions under the UV-light showed that Ni/ZnO exhibited higher photocatalytic activity than the other ones. Mn/ZnO demonstrated the lowest photocatalytic activity among the synthesized photocatalysts. Thus, Ni and Ag exhibited synergistic effect while Fe and Mn exhibited antagonistic effect on the ZnO photocatalytic activity. The maximum degradation rate of tartrazine was obtained to be 98.2% in the 60 min using Ni/ZnO. The plausible mechanisms were proposed and discussed for the enhanced or decreased photocatalytic activities resulting from different metals doped ZnO.

Improving sunlight-driven photocatalytic activity of ZnO nanostructures upon decoration with Fe(III) cocatalyst

In the study, the Fe(III) cocatalysts were grafted successfully on the surface of ZnO nanostructures to form Fe(III)/ZnO photocatalysts by a simple impregnation technique. It was found that the morphologies and crystal structures of ZnO remain unchanged after modification of the Fe(III) clusters. And the photocatalytic activity and the stability for the degradation of Rhodamine B were also evaluated. Under simulated sunlight illumination for 210min, an almost complete degradation was achieved over Fe(III)/ZnO nanocomposites. The mechanism of the enhancement is associated with a direct interfacial charge transfer process (IFCT). The Fe(III) clusters not only acted as electron sinks to enhance the separation of photoinduced electrons from holes, but also elevated the amount of the holes, leading to the production of more hydroxyl radical with high oxidizability.

Evaluation of gas-sensing properties of ZnO nanostructures electrochemically doped with Au nanophases.

A one-step electrochemical method based on sacrificial anode electrolysis (SAE) was used to deposit stabilized gold nanoparticles (Au NPs) directly on the surface of nanostructured ZnO powders, previously synthesized through a sol–gel process. The effect of thermal annealing temperatures (300 and 550 °C) on chemical, morphological, and structural properties of pristine and Au-doped ZnO nancomposites (Au@ZnO) was investigated. Transmission and scanning electron microscopy (TEM and SEM), as well as X-ray photoelectron spectroscopy (XPS), revealed the successful deposition of nanoscale gold on the surface of spherical and rod-like ZnO nanostructures, obtained after annealing at 300 and 550 °C, respectively. The pristine ZnO and Au@ZnO nanocomposites are proposed as active layer in chemiresistive gas sensors for low-cost processing. Gas-sensing measurements towards NO2 were collected at 300 °C, evaluating not only the Au-doping effect, but also the influence of the different ZnO nanostructures on the gas-sensing properties.

Point defects assisted NH3 gas sensing properties in ZnO nanostructures

Abstract In this report, the NH 3 gas sensing properties of ZnO nanostructures fabricated by radio frequency magnetron sputtering under various argon sputtering pressures have been investigated under various temperatures. The morphological transitions occur from vertical standing nanorods to inclined and tapered nanostructures with increasing the argon sputtering pressure. The dominant green emission at around 2.28 eV in the photoluminescence spectra signifies the presence of oxygen vacancies in the ZnO nanostructures which increases as a function of argon sputtering pressure. Despite low surface area, the nanostructures grown under higher argon sputtering pressure of 10 Pa exhibit excellent NH 3 gas response magnitude since it is exhibiting more oxygen vacancies as compared to other counterparts. For 25 ppm NH 3 gas at room temperature, a response time of 49 s and a fast recovery time of 19 s are attributed to the modification in the intermediate defect states induced by the oxygen vacancies through the adsorption and desorption of gas molecules on the surface of ZnO nanostructures.