SnO2 Nanoparticles-CeO2 Nanorods Enriched with Oxygen Vacancies for Bifunctional Sensing Performances toward Toxic CO Gas and Arsenate Ions.

In this paper, we present a novel, one-step synthesis of SnO2 nanoparticle-CeO2 nanorod sensing material using a surfactant-mediated hydrothermal method. The bifunctional utility of the synthesized sensing material toward room-temperature sensing of CO gas and low-concentration optosensing of arsenic has been thoroughly investigated. The CeO2-SnO2 nanohybrid was characterized using sophisticated analytical techniques such as transmission electron microscopy, X-ray diffraction analysis, energy-dispersive X-ray analysis, X-ray photoelectron spectroscopy, and so forth. The CeO2-SnO2 nanohybrid-based sensor exhibited a strong response toward CO gas at room temperature. Under a low concentration (3 ppm) of CO gas, the CeO2-SnO2 sensing material showed an excellent response time of 21.1 s for 90% of the response was achieved with a higher recovery time of 59.6 s. The nanohybrid sensor showed excellent low-concentration (1 ppm) sensing behavior which is ∼6.7 times higher than that of the pristine SnO2 sensors. The synergistically enhanced sensing properties of CeO2-SnO2 nanohybrid-based sensors were discussed from the viewpoint of the CeO2-SnO2 n-n heterojunction and the effect of oxygen vacancies. Furthermore, the SnO2-CeO2 nanoheterojunction showed luminescence centers and prolonged electron-hole recombination, thereby resulting in quenching of luminescence in the presence of arsenate ions. The photoluminescence of CeO2-SnO2 is sensitive to the arsenate ion concentration in water and can be used for sensing arsenate with a limit of detection of 4.5 ppb in a wide linear range of 0 to 100 ppb.

Introduction

The extensive industrialization and combustion of fossil fuels are posing a severe threat to the environment and public health.[1] Among various hazardous gases and volatile organic compounds released into the environment, carbon monoxide (CO) is very toxic to human health.[2] CO is a highly flammable, colorless, and odorless gas and when inhaled may result in dizziness, nausea, breathing difficulty, and even death.[3,4] As per the World Health Organization (WHO) regulations, the lethal exposure concentration and duration of CO gas are 9 ppm for 8 h and 26 ppm for 1 h, respectively.[5] Also, arsenic is an extensively dispersed and highly toxic heavy metal in the environment. Arsenic contamination of the groundwater has posed a global threat to public health and the environment.[6] The presence of arsenic in groundwater has been reported in many areas of the world, such as Argentina, Australia, Bangladesh, Cambodia, Canada, and many more.[7] In India, the northeastern states and some parts of West Bengal are affected by groundwater contamination of arsenic.[8] Therefore, real-time monitoring and alarming of the presence of CO in gas and arsenic in water are very crucial.

Recently, various metal oxide semiconductor-based nanomaterials have been explored in sensor technologies due to their high sensitivity, selectivity, and stability.[9−12] A greater surface-to-volume ratio, tunable surface defects, active sites, and good electrical and catalytic properties of nanostructured semiconductor-based sensors make them the promising material for electro-catalytic redox sensors and optosensing devices. A variety of semiconducting metal oxides like ZnO, CuO, TiO2, WO3, SnO2, and so forth have been reported in the literature as an efficient candidate for CO gas sensors.[13−17] However, semiconductor-based gas sensors have the disadvantage of having high operating temperatures. Gas sensing in elevated temperatures may result in high power consumption and can lead to the ignition of flammable gases.[18] For fluorescence-based sensing of aqueous pollutants, the operating concentration range is very crucial. Surface modification of semiconductor nanoparticles like ZnO with suitable fictionalization can alter the fluorescence properties, thereby facilitating arsenic sensing in water.[19] Therefore, to achieve low operating temperature in gas sensors and a suitable concentration range of operation for optosensors, various structural or morphological modifications such as doping, heterojunction formation, composite formation, and so forth have been introduced into semiconductor sensor materials.

Currently, great efforts have been made to design engineered semiconductor-based nanosensors for room-temperature sensing of CO gas. The gas sensing response of nanosensors is dependent on the interaction of the target gas molecules with the sensor surface, and thus, the heterojunction plays a critical role in sensor technologies.[20] In nanoheterojunctions, the interfacial interaction may result in the formation of the charge depletion layer to equalize the Fermi level of the materials.[21] The resultant charge flow can change the resistance of the material, thereby making the nanoheterojunction synergistically sensitive to target oxidizing or reducing gas.[22] Recently, n–n-junctions like ZnO–SnO2,[23] SnO2–CuO,[24] ZnO–CuO,[25] CeO2–TiO2,[26] and so forth have been extensively utilized for improved CO gas sensing performance.

Moreover, semiconductor nanoheterojunctions can also be explored as a fluorescent probe for real-time detection of arsenic in water.[27] Arsenate species commonly exist in water in the ionic form (H2AsO4– or HAsO42–) and can bring about changes in fluorescence properties of semiconductor sensing material upon adsorption.[19] The surface interaction of two crystalline solids in a nanoheterojunction can alter the electronic band structure of the nanohybrid, thereby significantly modifying the optical properties. Various nanohybrids such as CdTe quantum dots,[28] functionalized CeO2 nanowires,[29] MoS2 nanosheets,[30] ZnO quantum dots,[31] and so forth reported having suitable optical properties for an effective range of arsenic sensing operation.

Furthermore, oxygen vacancies dominate the physical and chemical properties of metal oxide nanosensors.[32,33] The electronic features of metal oxides can be largely dependent on the extent of oxygen vacancies, and thus, engineered oxygen vacancies in metal oxides can be an effective technique to regulate both gas sensing and optosensing performance of metal oxides.[34,35] The oxygen vacancies can act as electron donors and provide free electrons to improve conductivity and provide active sites for chemisorptions of oxygen/pollutant species, which can improve the sensitivity of the sensors.[36] Moreover, the oxygen vacancies can act as redox centers for aqueous pollutants leading to the change in emission properties in the presence of pollutants.[37]

Herein, we report on the facile fabrication of a SnO2 nanoparticle–CeO2 nanorod n–n heterojunction enriched with oxygen vacancies and its utility toward bifunctional sensing of CO gas at room temperature and low-concentration sensing of arsenate in aqueous solution.

Experimental Section

Materials and Methods

All the reagents were of the AR grade and used as received. Stannic chloride pentahydrate, ammonium cerium(IV) nitrate, sodium hydroxide, and distilled water were obtained from Sigma-Aldrich.

JEOL, 9JSM-100CX equipment was utilized for transmission electron microscopy (TEM). An energy-dispersive X-ray analysis (EDAX) spectrum was obtained using the JEOL model JSM-6390LV. Powder X-ray diffraction (XRD) patterns were obtained using a Philips X’PERT powder X-ray diffractometer. Brunauer–Emmett–Teller (BET) analysis and Barrett–Joyner–Halenda (BJH) measurements were conducted using a Quanta Chrome Nova 1000 gas adsorption analyzer. X-ray photoelectron spectroscopy (XPS) of the material was carried out by using a PHI 5000 Versa Probe II spectrometer. The absorbance spectra of the samples were recorded using a GENESYS 10S UV–visible spectrophotometer. Photoluminescence (PL) spectra were taken using an Ocean Optics QE Pro instrument.

Synthesis

In a typical synthesis, 3.5 g of stannic chloride pentahydrate was dissolved in 20 mL of distilled water. The solution was heated at ∼60 °C, and 1 M (10 mL) sodium hydroxide solution was added dropwise with constant stirring. In a separate beaker, 5.5 g of ammonium cerium nitrate was dissolved in 20 mL of distilled water, and 10 mL of 0.01 M cetyltrimethylammonium bromide was added to it. The solutions were mixed and sonicated for 20 min to get an even mixture. The mixture was then heated at ∼60 °C with constant magnetic stirring for 2 h. The resultant content was then transferred to a Teflon-lined autoclave and heated at 120 °C for 12 h to get the product. The obtained material was washed several times with ethanol and distilled water, centrifuged, and collected. The material was then subjected to calcination at ∼400 °C for 2 h to get the SnO2–CeO2 nanohybrid.

Fabrication of the Sensor and Measurement of the Sensing Response

The Ti/Pt interdigital electrode (IDE)-based gas sensor was fabricated using micro-electromechanical systems technology as presented in our previous studies.[21] The electrical measurements were carried out using a KEITHLEY 2450-SourceMeter at room temperature (∼27 °C) under constant applied voltage (1 V) while injecting different concentrations of target gases at a flow rate of 500 mL min–1. The gas response was recorded using eq .where Iair and Igas are the current values of the sensors in background gas and in the presence of CO or other test gases, respectively.

For optosensing measurements, the PL spectra were recorded with a 20 mg/L sensor concentration at an excitation wavelength of ∼285 nm. The arsenate ion concentration was varied in the range of 0–100 ppb. All measurements were carried out at pH ∼7 and at room temperature (∼27 °C).

Results and Discussion

Characterization

TEM and EDAX Studies

To investigate the morphology of the synthesized CeO2–SnO2 nanocomposite, electron microscopic techniques have been employed. The TEM micrograph (Figure a,b) revealed the existence of CeO2 nanorod arrays which were surface-decorated with quantum-sized SnO2 nanoparticles. The length of CeO2 nanorods was approximately 80–120 nm, and the diameter ranges between ∼10 and 15 nm. The surface-decorated SnO2 nanoparticles were of spherical shape having a diameter in the range of ∼2 ± 0.5 to 4 ± 0.5 nm. The high-resolution TEM (HRTEM) micrograph (Figure c) centered at the junction of two nanorods revealed two different types of lattice fringes. The labeled interplanar distances of 0.311 and 0.335 nm correspond well with lattice fringes of the (111) and (110) planes of cubic CeO2 and tetragonal SnO2, respectively. These results can be further confirmed by the selected area (electron) diffraction SAED pattern presented in Figure d. The concentric rings can be identified as the (111) plane of cubic CeO2 and (110), (220), and (311) planes of tetragonal SnO2 which were in well agreement with the XRD pattern of the nanocomposite (Figure a). Moreover, the bright and concentric rings of the SAED pattern revealed the high crystallinity and polycrystalline nature of the nanocomposite.

Figure 1

(a,b) TEM micrographs of the CeO2–SnO2 nanocomposite, (c) HRTEM micrograph, and (d) SAED pattern of the CeO2–SnO2 nanocomposite.

Figure 3

XRD pattern of pristine CeO2 and the CeO2–SnO2 nanocomposite.

EDAX of the CeO2–SnO2 nanocomposite (Figure ) exhibited signals corresponding to Sn, Ce, and O. The peaks at ∼3.5, ∼4.9, and ∼0.5 keV could be associated with L-series emissions of Sn and Ce and K-series emission of O, respectively. The atomic percentages of O, Sn, and Ce were found to be ∼80.2, 15.8, and 4.0%, respectively, which indicated a substantial physical integration among the individual components of the nanocomposite. Furthermore, the absence of impurity peaks in the EDAX spectrum confirmed the purity and composition of the nanocomposite.

Figure 2

EDAX spectrum of the CeO2–SnO2 nanocomposite.

X-ray Diffraction Analysis

The XRD measurements were performed to analyze the phase structure of the synthesized nanomaterials. Figure shows the XRD patterns of pristine CeO2 and the CeO2–SnO2 nanocomposite, in which all the characteristic peaks were assigned to cubic CeO2 and tetragonal SnO2.[38] The XRD spectrum of pristine SnO2 nanoparticles is given in Figure S1 (Supporting Information). The XRD pattern of pristine SnO2 showed peaks at 26.4, 34.2, 52.6, and 64.2° corresponding to (110), (101), (211), and (112) of tetragonal SnO2, respectively, with the lattice parameter a = 4.737 Å, c = 3.186 Å (JCPDS 88-0287). In the case of pristine CeO2, the distinct peaks centered at 28.2, 33.1, 47.4, and 56.2° can be assigned to the (111), (200), (220), and (311) planes of cubic CeO2, respectively, having the lattice parameter a = 5.412 Å (JCPDS 81-0792). Compared with pristine CeO2 and pristine SnO2, the XRD spectrum of CeO2–SnO2 showed several new peaks at 26.5, 33.8, 42.6, and 51.7° which can be indexed to the (110), (101), (210), and (211) planes of tetragonal SnO2, respectively. The overlapping sharp and strong peaks related to the (111) plane of CeO2 and the (110) plane of SnO2 signified the preferred direction of crystal growth, which correlates well with the HRTEM observations indicated in Figure c.

BET Analysis

Nitrogen sorption experiments have been employed to investigate the specific surface area and pore structures of the synthesized nanocomposite (Figure a,b). The CeO2–SnO2 nanocomposite exhibited a type IV isotherm with an H4 hysteresis loop (Figure a). This indicated narrow slit-like pores, particles with internal voids and irregular shapes, and broad size distribution.[39] Pristine SnO2 nanoparticles also showed a type IV isotherm; however, the hysteresis loop was found to be much narrow compared to the CeO2–SnO2 nanocomposite (Figure b). Furthermore, the BET specific surface area of the CeO2–SnO2 nanocomposite was estimated to be 28.44 m2/g, which is much higher compared to that of pristine SnO2 nanoparticles (3.06 m2/g). Moreover, the BJH pore diameter and pore volume of CeO2–SnO2 (3.36 nm and 0.041 cm3/g) were found to be comparable to those of pristine SnO2 (3.81 nm and 0.011 cm3/g). The high surface area and mesoporous nature of the material are likely to benefit the gas sensing performance.

Figure 4

(a) N2 adsorption–desorption isotherm of CeO2–SnO2 (inset: BJH pore size distribution) and (b) N2 adsorption–desorption isotherm of SnO2 (inset: BJH pore size distribution).

XPS Analysis

The compositional analysis of the synthesized nanocomposite was carried out using XPS, and the results demonstrated the surface elemental composition and chemical status of the CeO2–SnO2 nanohybrid. The XPS survey spectrum (Figure a) demonstrated peaks corresponding to Ce, Sn, O, and C, and no impurity could be found, which also confirmed the composition and high purity of the synthesized CeO2–SnO2 nanocomposite. The high-resolution XPS spectrum for Sn 3d (Figure b) showed two strong peaks at binding energies 486.8 and 495.2 eV, which can be indexed to Sn 3d5/2 and Sn 3d3/2, respectively, belonging to the Sn4+ oxidation state. The Ce 3d level consisted of two broad peaks ascribed to Ce 3d5/2 and Ce 3d3/2 (Figure c),which can further be deconvoluted into several peaks centered at 883.5, 885.9, and 888.3 and 899.2, 903.1, 907.4, 914.7, and 917.7 eV, respectively. It was reported that the peaks at 883.5, 888.3, 903.1, 914.7, and 917.7 eV are related to the Ce4+ oxidation state, and the peaks at 885.9, 899.2, and 907.4 eV are characteristics of the Ce3+ oxidation state.[38] This suggested that both Ce3+ and Ce4+ ions coexist in the CeO2–SnO2 sample, which may improve the surface reactivity of the composite. The high-resolution XPS spectrum of Ce 3d (Figure S2a Supporting Information) of pristine CeO2 nanoparticles exhibited the peaks at 882.54, 885.7, 898.6, 901.2, 904.1, 907.5, and 916.8 eV. The slight deviation of Ce 3d5/2 and Ce 3d3/2 peaks between pristine CeO2 and the CeO2–SnO2 nanocomposite indicated the surface chemical interaction in the binary nanocomposite. The deconvolution of the O 1s peak of the CeO2–SnO2 nanocomposite (Figure d) resulted in three distinct peaks centered at 530.4, 531.7, and 533.2 eV which can be ascribed to lattice oxygen (OL), oxygen vacancy regions (OV), and chemisorbed oxygen/water species (OC), respectively.[40] The lower peak intensity of lattice oxygen is attributed to the high reactivity and lability of the lattice oxygen in CeO2. The unstable lattice oxygen may get transferred to the surface of SnO2 in the CeO2–SnO2 nanohybrid and get adsorbed on it, thereby creating oxygen vacancies on the CeO2 surface. This justifies the abruptly increased intensity of the OV and OC peaks in the O 1s spectrum compared to OL. Furthermore, the electrons left behind by the labile lattice oxygen of the CeO2 surface can be captured by Ce4+ ions to get converted to Ce3+, which justifies the occurrence of Ce3+ in the Ce 3d spectrum.[40] Also, there occurs a slight shift of binding energy of the O 1s peak of CeO2–SnO2 and pristine SnO2 (Figure S2b Supporting Information), which indicated the surface interaction of SnO2 and CeO2 nanoparticles in the SnO2–CeO2 nanocomposite. Thus, the hybridization of CeO2 with SnO2 resulted in the generation of high oxygen vacancies which may enhance the ability to adsorb ionized oxygen species, thereby contributing to the high gas sensing performance of the CeO2–SnO2 nanohybrid.

Figure 5

(a) XPS survey spectrum and the high-resolution XPS spectrum of (b) Sn 3d, (c) Ce 3d, and (d) O 1s.

Sensing Characteristics of the CeO2–SnO2 Nanohybrid-Based Sensor

Room-Temperature CO Gas Sensing

The CO sensing characteristics of the CeO2–SnO2 nanocomposite were investigated using two different IDE-based sensors fabricated with pristine SnO2 and the CeO2–SnO2 nanocomposite. Figure a displays the sensor responses of pristine SnO2- and CeO2–SnO2-based sensors toward CO gas at different concentrations (1–5 ppm) at room temperature. In a cycle, the current values were found to be increasing with the influx of CO gas and gradually recovered to the initial values after discontinuing the flow of CO gas. This is due to the reducing nature of CO gas, which tends to inject electrons into the sensor material, thereby decreasing resistance and increasing current values. At a 1 ppm concentration of CO, the CeO2–SnO2-based sensor exhibited a relative response which is ∼6.7 times higher than that of the pristine SnO2-based sensor. Furthermore, with increasing CO gas concentrations from 1 to 5 ppm, the sensor responses were found to be increasing steadily. A plot of the logarithm of sensing responses as a function of the logarithm of the CO concentration for SnO2- and CeO2–SnO2-based sensors is presented in Figure b. The relationship was found to be almost linear with correlation coefficient (R2) values of 0.988 and 0.928 for SnO2 and CeO2–SnO2 sensors, respectively. The slopes of the response versus concentration plot were found to be 1.517 and 0.495 for SnO2- and CeO2–SnO2-based sensors, respectively. Although the CeO2–SnO2-based sensor exhibited a slightly lesser slope, the relative responses at low concentrations were much higher than those of the SnO2 sensor. This signified the superior CO sensing performance of the CeO2–SnO2-based sensor in a low concentration range compared to the pristine SnO2 sensor.

Figure 6

(a) Transient relative response of SnO2 and CeO2–SnO2 sensors to the 1, 3, and 5 ppm concentration of CO gas, (b) linear fitting of the logarithm of sensing responses as a function of the logarithm of the CO concentration for SnO2- and CeO2–SnO2-based sensors, (c) dynamic response–recovery curve of SnO2 and CeO2–SnO2 sensors at a 1 ppm CO concentration, and (d) repeatability testing of response–recovery curves at different concentrations.

The gas sensing performance of sensors can further be investigated by response and recovery time (i.e., the time required by the sensor to reach 90% of the final current value at the saturation state). At an intermediate concentration of CO gas (3 ppm), the CeO2–SnO2-based sensor exhibited a shorter response time (Tres) of 21.2 s and a delayed recovery time (Trec) of 59.6 s compared to the pristine SnO2 sensor (Tres = 30.6 s, Trec = 37.9 s) (Figure c). The rapid response and delayed recovery are the great characteristics of gas sensors, and the response/recovery times have been found comparable with various previously reported extraordinary CO sensors. Pt/SnO2 showed a response and recovery time of 15 and 14 s, respectively, at ∼250 °C for the 100 ppm concentration of CO gas.[41] Cu-OMS nanosensors exhibited CO sensing at room temperature at a 10 ppm concentration with a response and recovery times of 55 and 42 s, respectively,[42] whereas the Fe3O4–rGO nanocomposite sensor showed great CO sensing behavior with a response/recovery time of 21 s/8 s for a 10 ppm CO concentration at room temperature.[43] The response/recovery time obtained for the present nanosensor toward a 3 ppm CO gas concentration at room temperature of 21.1 s/59.6 s is very promising for practical application.

Furthermore, the repeatability of the sensing results of the CeO2–SnO2 sensor has been evaluated with six consecutive cycles of the response/recovery test with varying concentrations as shown in Figure d.[44] It was found that the sensor achieved comparable responses at a particular concentration and recovered completely to its initial baseline in each run. This indicated that the CeO2–SnO2-based sensor possesses good repeatability toward CO sensing.

The long-term stability of the CeO2–SnO2 sensor was investigated for 15 days and is displayed in Figure a. For the intermediate CO concentration (3 ppm), an average relative response of 21.5% with a standard deviation of 1.07% was recorded over six successive measurements at an interval of 3 days each. Furthermore, the CO gas selectivity of the CeO2–SnO2 sensor was examined by testing the sensor response to other interfering gases like NH3, CO2, ethanol, and water vapor at 3 ppm concentrations and is given in Figure b. Apparently, the CeO2–SnO2 sensor exhibited improved selectivity toward CO compared to other test gases at room temperature. This is possibly because the examined interfering gases require a much higher temperature for efficient detection.[38] Moreover, the influence of relative humidity on the CO sensing performance of the sensor was examined and found to be adventitious to the sensor as displayed in Figure c.[45] For 85% of relative humidity, a maximum increase of 16.5% in the overall response was observed. Under high relative humidity, water molecules tend to react with chemisorbed anionic oxygen species, thereby getting adsorbed over the sensor surface. These additive gas molecules can facilitate the release of more electrons back to the conduction channel (H2Ogas + Oads– → 2OHads + e–). As a result, the conductivity of the sensing material increases, and hence, the response toward the target CO gas also increases. Thus, these results showed that the CeO2–SnO2 sensor exhibits long time reliability, selectivity toward CO gas, and humidity resistance which are crucial for its practical utility.

Figure 7

(a) Long-term stability of CeO2–SnO2 sensor response, (b) selectivity histogram of the CeO2–SnO2 sensor, and (c) effect of relative humidity on the CO sensing performance.

Mechanism of Gas Sensing

The gas sensing mechanism of semiconductor oxides is mostly believed to be associated with interactions between the sensor surface, analyte gas, and adsorbed oxygen. Oxygen surface coverage of the sensing material plays a crucial role in optimum sensing performance. When the sensor surface is exposed to air, the space charge layer is generated over the sensor surface due to the chemisorptions of oxygen, which may accept electrons from the conduction band to form ionic oxygen species (O2–, O–, and O2–).[46] This may result in the growth of the electron depletion layer, thereby increasing the resistance of the material. The sensor surface coverage with oxygen species can be enhanced by increasing surface area, increasing active sites, and decreasing desorption. On exposure to the reducing gas CO, the chemisorbed oxygen species may interact with the reducing gas to release the trapped electrons back to the conduction band, thereby decreasing the resistance of the sensor (as evident from Figure a).[47] In this fashion, the target gas can continually replenish the sensor surface and maximize current modulation. The plausible mechanism is schematically represented as follows[47]

The choice of the heterojunction structure is very crucial in sensor technology as it dictates the resistance changes to the target gas. The target gas molecules get preferentially adsorbed at high-energy sites. The high-energy active site may include various imperfections like vacancy defects, atoms with low coordination, modulated interfaces, and surface grain boundaries.[48] The formation of nanoheterostructures not only allows complete carrier depletion of conduction channels but also modulates the surface reactivity due to the quantum confinement effect.[48] The prospect of fine-tuning the adsorption and surface activity of a nanoheterojunction by modulating various parameters provides control over sensor design and improves sensor selectivity.

The CO gas sensing performance of CeO2–SnO2 is found to be superior to that of the pristine SnO2 sensor, which may be attributed to the formation of a n–n heterojunction at the interface of the CeO2 nanorod–SnO2 quantum dot nanosensor. The 3D quantum confinement in SnO2 and 2D quantum confinement in CeO2 significantly alter the electronic structure of the CeO2–SnO2 nanoheterostructure. Also, the rod-like morphology of CeO2 can enhance the stability of the CeO2–SnO2 nanoheterojunction by effectively incorporating the SnO2 quantum dots.

Referring to the literature, the work function and energy band gap of SnO2 and CeO2 are W = 4.9 eV and Eg = 3.5 eV and W = 4.69 eV and Eg = 3.16 eV, respectively.[38] During the formation of the n–n heterojunction, the electrons tend to flow from CeO2 having a smaller work function to SnO2 until the Fermi levels get equalized.[49] Such electron migration may result in large electron density at the interface which may synergistically improve the oxygen adsorption at the sensor surface. This results in the bending of energy bands as shown in Figure .[50] The transfer of electrons from CeO2 to SnO2 along the n–n junction creates an accumulation layer. The subsequent oxygen adsorption on the sensor surface can further deplete the accumulation layer, thereby increasing the potential energy barrier at the interface and enhancing the response.[51,52] As a result, the reducing gas CO can extract more oxygen from the CeO2–SnO2 sensor surface and thus produce a greater sensor response compared to the pristine SnO2 sensor.

Figure 8

Energy band configurations of the SnO2, CeO2, and CeO2–SnO2 heterojunction before and after equilibrium.

Moreover, the separation of oppositely charged carriers can additionally enhance sensing performance. The formation of a nanoheterojunction between SnO2 nanoparticles and the CeO2 nanorod can significantly lower the recombination of electrons and holes compared to the pristine counterparts. The electric field generated across the depletion region at the interface can pull electrons in one direction and holes in the opposite direction.[52] The resulting charge separation increases carrier densities at the sensor surface, thereby enhancing sensing performance. Furthermore, the XPS results established the occurrence of significant oxygen vacancies in the CeO2–SnO2 nanohybrid. The oxygen vacancies can act as an electron donor which may result in further flow of electrons from CeO2 to the SnO2 surface, thereby increasing the conductivity.[33] The existence of a larger amount of oxygen vacancies in the CeO2–SnO2 sensor can induce stronger chemisorption of oxygen on the sensor surface leading to a quick response and slow recovery time of the gas sensor.

Low-Concentration Arsenate Sensing

The optical absorption and emission properties of the synthesized CeO2–SnO2 nanoheterojunction have been explored for the detection of arsenate in water. The UV–visible spectral analysis (Figure a) showed the superior optical absorption behavior of the CeO2–SnO2 nanohybrid compared to pristine SnO2 which is attributed to the formation of a n–n nanoheterojunction between SnO2 and CeO2. The CeO2–SnO2 nanohybrid exhibited an ultraviolet cutoff in the range 260–330 nm which is ascribed to the electronic excitation between valence and conduction bands of the nanoheterojunction. The radiative recombination of the photogenerated electrons and holes of the CeO2–SnO2 nanohybrid may result in a broad UV emission.[31] The PL spectra (Figure b) clearly showed an emission peak centered at ∼340 nm corresponding to an excitation wavelength of 285 nm. The addition of arsenate ions to the CeO2–SnO2 nanohybrid solution led to a drastic diminishment of both the excitation band and emission band of the CeO2–SnO2 nanohybrid. This quenching of the fluorescence intensity of the CeO2–SnO2 nanohybrid in the presence of arsenate ions can be explored for the detection of arsenate in water.[29]

Figure 9

(a) UV–vis spectra of pristine SnO2 and the SnO2–CeO2 nanohybrid, (b) PL spectra of the SnO2–CeO2 nanohybrid in the absence and presence of arsenate ions, (c) PL spectra of the SnO2–CeO2 nanohybrid (20 mg/L) in the presence of arsenate ions (0–100 ppb) at an excitation wavelength of 285 nm, and (d) linear regression analysis showing relative fluorescence intensity as a function of the concentration of arsenate ions.

To investigate the potential sensing of the CeO2–SnO2 nanohybrid toward As(V), a series of PL quenching-based titration experiments were performed with the addition of As(V) ions to the aqueous suspension of the CeO2–SnO2 nanohybrid. The fluorescence measurements were recorded in a low-arsenate concentration range of 0 to 100 ppb. The CeO2–SnO2 nanohybrid showed quenching of fluorescence intensity with increasing arsenate ion concentrations as presented in Figure c. Furthermore, there is a good linear correlation (R2 = 0.9923) between the relative fluorescence intensity and the concentration of arsenate ions in the range 0–100 ppb (Figure d).

The limit of detection (LOD) was calculated using the following equation: LOD = 3σ/m where σ is the standard deviation of the blank measurement and m is the slope of the intensity versus quencher concentration plot.[53] The LOD for the current fluorescence quenching assay is estimated to be approximately 4.5 ppb which is much lower than the maximum permissible limit of arsenic (10 ppb) in drinking water as recommended by the WHO.

Moreover, to ascertain the practical utility of the CeO2–SnO2 nanohybrid sensor, the selectivity toward arsenate ions in the presence of various interfering ions and with spiked real water samples has been investigated. The fluorescence responses of the CeO2–SnO2 nanohybrid in the presence of cations like Fe2+, Cu2+, Pb2+, Cr3+, Zn2+, Mn2+, Mg2+, Na+, and Al3+ at a concentration five times higher than the As5+ ion concentration have been recorded and presented in Figure a. As evident, the interfering ions do not cause any significant quenching of fluorescence intensity as compared to the arsenate ions that demonstrated the selectivity of the CeO2–SnO2 nanohybrid sensor. Additionally, the competitive experiments were carried out in real bore well water samples (collected from Hailakandi, Assam, India) spiked with arsenate ions, and no significant difference in sensing responses was observed (Figure b). Furthermore, the reliability of the CeO2–SnO2 nanohybrid sensor was tested by repeating the sensing experiment by three consecutive cycles under complex real environmental conditions. The sensor showed good recovery of the sensing response of up to ∼87% after the third cycle of operation. All in all, the CeO2–SnO2 nanohybrid sensing system showed good sensitivity, selectivity, and reliability toward arsenate ions, and the fluorescence quenching effects induced by As5+ ions are not significantly interfered by the coexisting ions and real environmental samples.

Figure 10

(a) Selectivity of the SnO2–CeO2 nanohybrid (20 mg/L) toward arsenate ions (100 ppb) in the presence of various interfering ions (500 ppb) and (b) reliability of the arsenate sensing performance of the SnO2–CeO2 nanohybrid in real water samples under multiple cycles of operation.

Mechanism of Arsenate Sensing

To elucidate the probable mechanism of fluorescence quenching of the CeO2–SnO2 nanohybrid in the presence of arsenate ions, the XPS measurements of the sensor after the sensing experiment have been recorded. The appearance of As 3d peaks at the binding energy region 42–48 eV revealed the interaction of arsenic species with the CeO2–SnO2 nanohybrid (Figure a). Furthermore, the comparison of the high-resolution O 1s spectra of the CeO2–SnO2 nanohybrid before (Figure d) and after (Figure b) arsenate addition revealed that the intensity of the deconvoluted peak centered at ∼532.75 eV decreased considerably in the presence of arsenate ions. The peak was attributed to the adsorbed water molecules, and a decrease in the peak height is indicative of substitution of surface-adsorbed water species by arsenate ions. Such surface adsorption of arsenate ions can facilitate the fluorescence quenching of the CeO2–SnO2 nanohybrid. The high-resolution XPS spectrum of As 3d (Figure a) revealed the existence of As3+ ions (binding energy ∼42.74 eV) along with the As5+ ions (binding energy ∼46.05 eV). This indicated a partial reduction of As5+ species during the interaction with the CeO2–SnO2 nanohybrid which confirmed the electron quenching ability of the arsenate ions.[54] The origin of PL in the CeO2–SnO2 semiconducting nanoheterojunction is probably the radiative recombination of the photogenerated electrons in the conduction band and holes in the valence band.[31] The surface adsorption of a large number of arsenate ions may pave an avenue for easy relaxation of electrons from the excited state of the nanoheterojunction to the ground state, thereby resulting in fluorescence quenching.

Figure 11

High-resolution XPS spectrum of (a) As 3d and (b) O 1s of the recovered SnO2–CeO2 nanohybrid after arsenate sensing.

Conclusions

In summary, a SnO2 nanoparticle–CeO2 nanorod hybrid has been successfully synthesized using a low-cost, surfactant-mediated hydrothermal technique. The bifunctional sensing capability of the synthesized nanohybrid has been evaluated toward room-temperature sensing of CO gas and low-concentration sensing of arsenate ions in water. The gas sensing results indicated that the CeO2–SnO2 nanohybrid exhibited synergistic CO sensing performances at room temperature. This improvement in gas sensing response can be attributed to the alteration of the electronic structure due to n–n heterojunction formation and generation of oxygen vacancies at the surface of the CeO2–SnO2 nanohybrid. Furthermore, the CeO2–SnO2 nanohybrid resulted in the quenching of the exciton emission at 340 nm in the presence of arsenate ions. In contrast to bare SnO2, CeO2–SnO2 could work as a fluorescent probe for arsenic sensing in the concentration ranges from 0 to 100 ppb in water at a low LOD of 4.5 ppb.