Semiconducting metal oxide (SMO) has been extensively investigated over the past few decades for its use as a gas sensor. To fully understand the gas detection mechanism of MOS sensors, it is essential to analyze the material’s electronic structure when exposed to target gases under operating conditions. Herein, operando near-ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS) was employed to investigate the electronic structure of an n-type ZnO-based gas sensor under CO exposure. The present operando NAP-XPS study reveals that the decrease in resistance observed during CO exposure and dry and more realistic humid conditions is primarily due to surface oxidation states associated with the formation of oxygen vacancies. However, the sensor response to CO deteriorates significantly in humid conditions due to the formation of surface OH groups related to the filling of oxygen vacancies by water molecules. This is the first time NAP-XPS has been used to investigate a ZnO-based gas sensor under operando conditions in the presence of CO gas in both dry and humid air. This study provides insights into the relationship between ZnO surface properties and its promising performance as a CO gas sensor, establishing a solid foundation for the design of next-generation gas-sensing devices.

The increasing of pharmaceutical contaminants in aquatic environments has generated significant concern due to their persistence and potential toxicity. In this work, a series of metal-oxide-based carbon quantum dots (CQD/MFe2O4-ZnO, M = Fe2+, Co2+, Ni2+, Cu2+) was prepared using an eco-friendly method and characterized by various analytical techniques. The results show that iron oxide in the spinel phase and ZnO in the hexagonal wurtzite were observed along with carbon quantum dots (band energy ~ 1.87-2.10 eV). In the photoluminescence spectra, the intensity of peaks was decreased considerably as the electron-hole recombination effect decreased due to efficient charge transfer within the CQD/ferrite-ZnO heterojunction. For example, for CQD/Fe3O4-ZnO (391 nm, FWHM= 34.83 nm), close to the ZnO NBE, indicating preserved band-edge transitions with modified recombination dynamics because of strong interfacial coupling that facilitates an efficient charge transfer, suppressing the radiative recombination. Photocatalytic oxidation of naproxen under solar and visible lights was resulting to k= 0.0146 mM/s for CQD/Fe3O4-ZnO, k= 0.076 for CQD/CoFe2O4-ZnO, k= 0.075 for CQD/NiFe2O4-ZnO, k= 0.036 for CQD/CuFe2O4-ZnO, indicating the oxidation was 83.52% under solar light, that was further improved to 91.89% (k = 0.0214) when air bubbling was introduced into the reactor because high oxygen content in the reaction medium. Furthermore, the scavenging effect of free radicals (decreased to 48.38%) and cell imaging development of samples were also studied.

This manuscript investigates the effects of Mn addition on the microstructural, electronic, and surface properties of the WO3 compound synthesized via the modified polymeric precursor method. X-ray diffraction, Raman spectroscopy analyses indicated that Mn addition resulted in the formation of both monoclinic WO3 and MnWO4 phases. X-ray spectroscopies (XPS and XANES) revealed that the presence of a spurious phase did not alter the chemical environment of oxygen atoms. Regarding the light-assisted gas-sensing experiments, Mn addition improved the sensing performance of the compound towards sub-ppm NO2 levels, which was linked to the creation of additional adsorption sites resulting from the formation of the WO3/MnWO4 composite. These findings demonstrate that the WO3/MnWO4 composite is a promising material for application as NO2 gas sensors operating at room temperature.

Herein, we investigated the microstructural, electronic, and surface properties of hierarchical WO3 structures used as visible light-assisted NO2 gas sensors. For this purpose, we combined a variety of experimental techniques and theoretical simulations to understand the gas-sensing performance of the WO3 structures grown using distinct temperatures via aerosol-assisted chemical vapor deposition. X-ray diffraction and X-ray absorption spectroscopy analyses confirmed the formation of monoclinic phase tungsten trioxide, presenting a pronounced texture along the z-direction. Regarding the morphological features, scanning and transmission electron microscopies revealed well-oriented rod-like structures, exhibiting monocrystalline characteristics. X-ray photoelectron spectroscopy results pointed out that the choice of growth temperature did not influence the oxygen-totungsten ratio, exhibiting a surface close to stoichiometry. Room temperature gas-sensing measurements under blue illumination showed that the WO3 structures were able to detect sub-ppm NO2 gas levels, even in the presence of moisture. Also, the samples exhibited good selectivity towards reducing gases. Density functional theory simulations demonstrated the importance of crystal surfaces on the chemisorption processes, with the (110) and (010) surfaces benefiting the adsorption of NO2 molecules. The experimental and theoretical results revealed the relationship between the NO2 gas-sensing performance and the crystal shape of WO3 structures.

In this article, we investigate In2S3 thin films deposited via a two-step sulfurization process, focusing on the impact of substrate temperature during indium deposition for its use in NO2 detection. The films were prepared with indium deposited at 25 degrees C, 200 degrees C, and 300 degrees C prior to sulfurization. X-ray Diffraction (XRD) analysis shows that all films form /3-In2S3 in its tetragonal phase, with the film prepared by depositing indium at 300 degrees C exhibiting a peak corresponding to cubic phase also. Composition analysis reveals variations in the indium-tosulfur ratio among the films. Morphologically, the film prepared with depositing indium at 200 degrees C displays petal-like structures, enhancing surface area. Photoluminescence (PL) analysis indicates higher concentration of defect levels in this film compared to other samples. X-ray Photoelectron Spectroscopy (XPS) shows the presence of oxygen on the surface of the samples. This oxygen presence was notably surface specific as it was not found deeper within the sample. Film synthesized with depositing indium at 200 degrees C demonstrates promising application in sensing nitrogen dioxide (NO2) gas at temperature comparatively lower than the reported values. The fabricated sensor made without using any dopants or heterostructures detected 800 ppb of NO2 with a 12.25 % response. The sensor is also able to detect concentrations as low as 100 ppb.

p-type pristine InSe, pristine graphene, and the corresponding hybrid InSe-graphene gas sensor that is highly selective to NO2 have been developed. These materials are produced at an environmentally friendly temperature of 35 °C by the Liquid Phase Exfoliation (LPE) technique. Then their deposition was performed on alumina transducers for achieving chemoresistive gas sensors. X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), photoluminescence (PL), and Raman spectroscopy were used to analyze the materials. The multilayered crystalline structure is revealed by HRTEM. Studies on gas-sensing properties showed that the response of the hybrid InSe-graphene sensor to 1 ppm of NO2 is three times higher than the one of the pristine graphene sensor, whereas the pristine InSe sensor was not responsive. While under dry conditions, the response to NO2 (1 ppb) was 3.41%, under humid conditions (RH 50%), the responsiveness was significantly increased to 6.16% and to 14.42% for sensors operated at 150 and 250 °C, respectively.

This work presents a novel light-modulated gas sensor based on hybrid Indium Selenide (InSe)-graphene synthesized via the liquid phase exfoliation (LPE) technique. We investigated the effects of the operating temperature and light irradiation on the sensing layer performance. The morphology, composition and structural characteristics of the sensing layer are analyzed using different material characterization techniques including scanning and transmission electron microscopies, X-ray photoelectron spectroscopy, and Raman. The response to NO 2 of the InSe-graphene hybrid gas sensor while operated under dark conditions or excited at one of three different wavelengths (i.e., 375, 470, and 530 nm), which correspond to UV, blue and green light, respectively, is studied. Results show that UV light excitation of the film when operated either at room temperature or at 150 degrees C resulted in an enhanced NO2 response, with a limit of detection below 50 ppb, and an excellent selectivity against other gaseous like CO, CO2, C6H6 and H2. Remarkably, the hybrid nanomaterial is characterized by showing significantly faster response and recovery times than those often found in the literature.

ZnO nanowires (NWs) with wurzite structure and a very high [0001] preferred orientation were grown on a ZnO thin film, from a gas-solid process, in the absence of catalyst. ZnO NWs were characterized by FESEM, XRD, HRTEM, XPS, PL and Raman spectroscopy and used as photocatalysts for the photo-transformation of ethanol(aq) in the gas phase. The existence of different defects such as oxygen vacancies was evidenced. The photocatalytic process was followed by in-situ diffuse reflectance infrared spectroscopy (DRIFTS) coupled to on-line mass spectrometry (MS) analysis. The surface species determined during the irradiation (lambda = 365 nm) of ZnO NWs under ethanol/water vapor flow at room temperature are related with the hydrogen production and carboncontaining products evolved.

The reliable detection of ammonia at room temperature is crucial for not only maintaining environmental safety but also for reducing the risks of hazardous pollutants. In this study, the electrochemical modification of laser-induced graphene (LIG) with polyaniline (PANI) led to the development of a chemo-resistive nanocomposite (PANI@LIG) for detecting ammonia levels at room temperature. The composite is characterized by field emission scanning electron microscopy, Fourier transforms infrared, and Raman and X-ray photoelectron spectroscopy. This work marks the first utilization of PANI@LIG for gas sensing and introduces a simple but effective approach for fabricating low-cost wearable gas sensors with high sensitivity and flexibility.

The search for a highly selective ethanol sensor is still an open challenge. Metal oxides still face selectivity issues when it comes to gas sensing especially in case of ethanol. Keeping this in sight, in this work we explore the sensing properties of WO3 nanowires grown via the aerosol assisted chemical vapour deposition and decorated with CeO2 nanoparticles by drop casting. The nanocomposites were characterized by X-ray diffraction, field emission scanning electron microscopy, energy dispersive X-ray, photoluminescence, Raman and X-ray photoelectron spectroscopies. Upon exposure to different gas analytes, the electrical characterization showed that sensors responded well to ethanol both under dry and humid conditions (RH= 50%, 25 degrees C), though the response towards ethanol decreased in humid atmospheres. The long-term stability was studied, and a gas sensing mechanism is introduced and discussed.

ZnO nanoflowers (ZF) were successfully synthesized by a simple pH-controlled co-precipitation technique. Systematic investigations such as X-ray Diffraction (XRD), Field Emission Scanning Electron Microscopy (FESEM) and Transmission Electron Microscopy (TEM) were carried out to confirm the structure and morphology of the sample. It has been found that the petals of the nanoflowers are composed of nanorods. The higher Urbach tail energy calculated from the optical absorbance spectrum indicates the presence of structural defects in the sample. The findings confirmed that the photoluminescence (PL), photoconductivity and photocatalytic char-acteristics of ZnO are inextricably linked to oxygen defects. The presence of oxygen vacancy (Ov) and surface adsorbed oxygen in the sample is confirmed by X-ray Photoelectron Spectroscopy (XPS), PL, Fourier Transform Infrared Spectroscopy (FTIR) and Raman Spectroscopy. Also, it shows an improved photocatalytic efficiency with Rhodamine B (RhB) dye under UV (100%, 25 min) and visible (98%, 60 min) light irradiation. The higher Fo center dot rster Resonance Energy Transfer (FRET) between the ZF and RhB dye can have a significant role in the photocatalytic activity of ZnO. Also, the results confirm that the stronger the PL signal, the lesser the photocatalytic activity, because of the rapid recombination of photo-induced electrons and holes. The higher photocatalytic efficiency of ZF results from O-related defects such as Ov and surface adsorbed groups in the sample.

In this work, we studied new materials free of noble metals that are active in photocatalytic H2 generation from ethanol aqueous solutions (EtOHaq), which can be obtained from biomass. MoxC/g-C3N4 photocatalysts containing hexagonal (hcp) Mo2C and/or cubic (fcc) MoC nanoparticles on g-C3N4 nanosheets were prepared, characterized, and evaluated for photocatalytic hydrogen production from EtOHaq (25% v/v). Tailored MoxC/g-C3N4 nanocomposites with MoxC crystallite sizes in the 4-37 nm range were prepared by treatment with ultrasound of dispersions containing MoxC and g-C3N4 nanosheets, formerly synthesized. The characterization of the resulting nanocomposites, MoxC/g-C3N4, by different techniques, including photoelectrochemical measurements, allowed us to relate the photocatalytic performance of materials with the characteristics of the MoxC phase integrated onto g-C3N4. The samples containing smaller hcp Mo2C crystallites showed better photocatalytic performance. The most performant nanocomposite contained nanoparticles of both hcp Mo2C and fcc MoC and produced 27.9 mmol H2 g-1 Mo; this sample showed the lowest recombination of photogenerated charges, the highest photocurrent response, and the lowest electron transfer resistance, which can be related to the presence of MoC-Mo2C heterojunctions. Moreover, this material allows for easy reusability. This work provides new insights for future research on noble-metal-free g-C3N4-based photocatalysts.

The electrochemical glucose oxidation reaction (GOR) presents an opportunity to produce hydrogen and high-value chemical products. Herein, we investigate the effect of Sn in Ni nanoparticles for the GOR to formic acid (FA). Electrochemical results show that the maximum activity is related to the amount of Ni, as Ni sites are responsible for catalyzing the GOR via the NiOOH/Ni(OH)2 pair. However, the GOR kinetics increases with the amount of Sn, associated with an enhancement of the OH- supply to the catalyst surface for Ni(OH)2 reoxidation to NiOOH. NiSn nanoparticles supported on carbon nanotubes (NiSn/CNT) exhibit excellent current densities and direct GOR via C-C cleavage mechanism, obtaining FA with a Faradaic efficiency (FE) of 93 % at 1.45 V vs. reversible hydrogen electrode. GOR selectivity is further studied by varying the applied potential, glucose concentration, reaction time, and temperature. FE toward FA production decreases due to formic overoxidation to carbonates at low glucose concentrations and high applied potentials, while acetic and lactic acids are obtained with high selectivity at high glucose concentrations and 55 °C. Density functional theory calculations show that the SnO2 facilitates the adsorption of glucose on the surface of Ni and promotes the formation of the catalytic active Ni3+ species.

We describe the triboluminescence response of undoped (BaAl2Si2O8, h-BAS) and Eu-doped (h-BAS:Eu) barium hexacelsian powders and show that the triboluminescence behavior is dependent on the formation of barium vacancies. X-ray photoelectron spectroscopy of the h-BAS:Eu powders confirms the presence of Eu3+ and Eu2+ in the compound, leading to the formation of significant vacancy point defects in excess of those found in h-BAS as a result of the charge imbalance caused by the substitution of Eu3+ in Ba2+ sites. From electron paramagnetic resonance measurements and density functional theory (DFT) calculations, we demonstrate that the vacancy defects correspond to singly ionized barium vacancies. DFT-calculated thermodynamic transitions and electronic structure calculations reveal deep energy levels within the compound's energy band gap, with a strong emission at 3.33 eV correlated to an electron exchange between the conduction band minimum and a barium vacancy center. Time-resolved triboluminescence spectra show that the increased concentration of barium vacancies in h-BAS:Eu enhances the signal by about 75% compared to the signal from h-BAS. These results play an important role in the understanding of fundamental mechanisms behind the triboluminescence response of ceramic materials as well as the role of different types of defects in this process.

To overcome the limitations of ZnO as a photocatalyst, the present work reports a ternary nanocomposite (ZnO–TiO2/rGO) with a high photocatalytic activity under direct natural solar light irradiation. Reduced graphene oxide (rGO) was obtained after bio-reduction of GO using pomegranate peels. Techniques of FE-SEM, TEM, XRD, FTIR, UV–Vis DRS, Raman and PL were used for characterization purpose. The ternary nanocomposite exhibited a high photocatalytic activity towards the degradation of indigo carmine dye, resulting in an efficiency of 92% within 150 min under sunlight illumination. Accordingly, the hybridization of ZnO with TiO2 and rGO improves light absorption, promotes high separation of photogenerated charges, and solves the photocorrosion drawback of ZnO, leading to a better stability and reusability of the nanocomposite. Particularly, the prepared rGO allowed certain hydrophilicity and a better surface hydroxylation. In view of that, a comprehensive photocatalytic mechanism was proposed and discussed, referred to experiments showing the effect of holes and •OH scavengers. The findings revealed that the developed rGO hybridized with ZnO–TiO2 heterojunction can be a promising candidate for removing environmental contaminations using natural solar light.

This paper investigates the effect of decorating graphene with zinc oxide (ZnO) nanoparticles (NPs) for the detection of NO2. In this regard, two graphene sensors with different ZnO loadings of 5 wt.% and 20 wt.% were prepared, and their responses towards NO2 at room temperature and different conditions were compared. The experimental results demonstrate that the graphene loaded with 5 wt.% ZnO NPs (G95/5) shows better performance at detecting low concentrations of the target gas than the one loaded with 20 wt.% ZnO NPs (G80/20). Moreover, measurements under dry and humid conditions of the G95/5 sensor revealed that the material is very sensitive to ambient moisture, showing an almost eight-fold increase in NO2 sensitivity when the background changes from dry to 70% relative humidity. Regarding sensor selectivity, it presents a significant selectivity towards NO2 compared to other gas compounds.

ZnO films were fabricated by pulsed laser deposition using two different background atmospheres (argon/vacuum). The gas-sensing properties of these materials against reducing and oxidizing gases were examined. The microstructure and crystal symmetry of the deposited films were studied with X-ray diffraction (XRD), Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), Raman, and Photoluminescence (PL) spectroscopy. The XRD studies revealed that the ZnO films grown in an argon environment are highly textured in the c-axis with a hexagonal crystalline structure. The c-axis is perpendicular to the substrate plane orientation (002) compared to (100) plane orientation, which is developed in a vacuum environment. Usually, this orientation (100) is difficult to obtain. Raman scattering spectra for both types of ZnO films revealed the characteristic E2 (high) mode that is related to the vibration of oxygen atoms in wurtzite ZnO. Moreover, PL spectra showed that a high number of defects appear in both the vacuum and argon-grown ZnO films. XPS data indicated that the O1s peak consists of several components identified as lattice oxygen, oxygen close to defects, and chemisorbed species. Furthermore, gas-sensing properties were investigated for nitrogen dioxide (NO2) at different operating temperatures and concentrations. Although both types of ZnO films have shown a good response towards NO2 at ppb levels, the films prepared under vacuum conditions showed higher responses. This was attributed to differences in crystallinity, microstructure, and the type of defects present in these materials.

Novel arrays of Nb2O5-based ceramic nanostructures of various sizes (9-210 nm) and morphologies (dots, goblets, rods) aligned on substrates are fabricated via the anodizing of a thin Nb film through the initially formed porous anodic alumina (PAA) film in 1.5 M selenic acid (H2SeO4) - a new aqueous electrolyte generating extraordinarily thinner PAA pores than any other solutions. Accordingly, the nanostructures formed in the selenic acid are 1.3-fold thinner and better self-ordered than their counterparts formed from the same Al/Nb precursor bilayer in a reference oxalic-acid electrolyte. The nanostructures have a dual (core/shell) composition: the inner material (the core) is stoichiometric Nb2O5, whereas the outer layer (the shell) is a few nm-thick substoichiometric NbOx mixed with Al2O3. The composite-ceramic nanoarrays grow doped with selenium species such as selenate (SeO42-) and selenide (Se-2(-)) anions originating from the electrolyte and migrating inward under the high electric field. The incorporated Se species do not contribute to photoluminescence emission nor hinder the Raman signal from the nanoarrays, which makes them highly promising as Nb2O5-based SERS biosensing substrates. The planar PAA-inbuilt Se-doped Nb2O5-Al2O3 nanostructured ceramic film performs like a high-k low-loss low-leakage-current dielectric promising for on-chip integration. More potential applications of the Se-doped ceramic nanoarrays developed here include biomedical antibacterial coatings, advanced superhydrophobic surfaces, gas-sensing, and catalytic layers.

Nowadays, there is a rising urge to develop and implement cheaper, abundant, and environmentally friendly semiconducting materials into practical devices. In this regard, zinc oxide (ZnO) presents itself as a relevant and attractive prospect that fulfills the former requirements and stands out for its optoelectronic, photocatalytic, and sensing performance. Furthermore, obtaining ZnO nanostructures has reignited the research on the material. Despite the accurate control and reproducibility over the synthesis of nanoscale ZnO systems, the complete understanding of their physical properties at this scale remains. Especially for their application into scalable, practical devices. Thereby, the present thorough revision of recent results and investigations related to improving the photocatalytic and sensing performance of ZnO-based nanomaterials. The use of pristine, doped, and metallic ZnO composites for various emerging applications such as photocatalytic reactions, biosensors, and ozone chemosensors is presented. Furthermore, an analysis and description of the current challenges and prospectives are included.

The present work reports a ternary nanocomposite (ZnO-TiO2/rGO) synthesized through the sol-gel method. Graphene oxide (GO) was obtained using modified Hummer’s method and was subsequently bio-reduced using pomegranate peels to obtain reduced graphene oxide (rGO). A comprehensive characterization including FE-SEM, TEM, XRD, UV–Vis DRS, Raman, and photoluminescence spectroscopy was performed to investigate the morphology, crystallinity, and quality, optical and electrical properties of the active nanomaterials. The main purpose is to get a nanocomposite with a high photocatalytic activity under direct natural solar light irradiation. Thus, the ternary nanocomposite exhibited an outstanding photocatalytic activity towards the degradation of Indigo Carmine (IC) dye, resulting in an efficiency of 92 % within 150 min under sunlight illumination. Accordingly, the hybridization of ZnO with TiO2 and rGO improves light adsorption, promotes high separation of photogenerated charges, and solves the photocorrosion drawback of ZnO, leading to better stability and reusability of the nanocomposite. Finally, a comprehensive photocatalytic mechanism was proposed and discussed, referred to the band structure. These findings revealed that the developed rGO hybridized with ZnO-TiO2 heterojunction can be a promising candidate for removing environmental contaminations using natural solar light.