NO2 sensing with CdS nanowires at room temperature under green light illumination

Detection of ppb-level NO2 gas under atmosphere is urgent to meet the requirements of the rapidly developing internet of things. Compared with traditional sensing methods, light illumination has been considered as a key approach for excellent gas sensor performance under moderate conditions. Herein, we developed a green-light-assisted gas sensor based on cadmium sulfide nanowires (CdS NWs) that has good NO2 sensing capability at ambient temperature. The response values of NO2 are 236% and 11% to 10 ppm and 12.5 ppb, respectively. Furthermore, the CdS NWs sensor has a high selectivity for NO2 over a variety of interference gases, as well as good stability. The cleaning light activation and the sulfur vacancy-trapped charge behavior of CdS NWs are observed, which suggest a light-assisted sensing mechanism. These results suggest that light-induced charge separation behavior might significantly improve gas-sensing characteristics.


Introduction
With the continuous advancement of 'internet of things' (IoT) technology, the function of gas sensors in a variety of applications, such as air-quality control [1], industrial production [2], non-invasive medical diagnosis [3], and food preservation [4], has received much attention.Nitrogen dioxide (NO 2 ) is 3 These authors contributed equally to this work.* Authors to whom any correspondence should be addressed.
Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence.Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. a poisonous gas produced by the burning of fossil fuels and the emission of automotive exhaust, causing severe air pollution and posing a health risk to humans [5][6][7].According to the US Environmental Protection Agency (EPA), long-term exposure to even ppm-level NO 2 can harm the human respiratory system and brain system, leading to significant lung disorders (such as pneumonia or lung cancer) [8,9].Furthermore, reliable measurement of NO (20-50 ppb), which may readily convert to NO 2 , can lead to an early asthma diagnosis [10][11][12].Therefore, the selective detection of trace NO 2 is of great significance to air-quality monitoring and non-invasive medical diagnosis [13].
Gas sensors should have great sensitivity and selectivity to low-concentration gases at room temperature, in order to meet above requirements [14].However, due to its high adsorption Air quality monitoring is of great significance to human health and environmental protection.The high cost and low spatial coverage of current environmental monitoring methods limit their practicability for individual applications.Gas sensor technology is a convenient monitoring method which has been proven by the market.However, it still faces many challenges, such as high power consumption, poor selectivity, and unclear sensitive mechanism.Therefore, the development of new sensitive materials, the exploration of sensitization methods, and the clarification of the sensing mechanism will contribute towards addressing the limitations of gas sensors.In addition, with the development of microfabrication technology, advanced gas sensors can be integrated into low-cost devices, or even consumer electronic products such as smartphones and wearable devices.In recent years, the rapid development of the internet of things has promoted gas sensors to play an important role in smart homes and smart cities, but this also places stricter requirements on the performance of gas sensors.Therefore, exploring new materials, new processes, new theories and new technologies are the main ways for the development of future gas sensors.energy [15,16], the response/recovery of NO 2 at room temperature is a sluggish process, making it problematic for real-time monitoring.To date, enormous efforts have been made to lower the working temperature of gas sensors, including development of new materials [13,17,18], morphological control [19][20][21], noble metal modification [7,22,23], and so on.Among them, the photoactivation method can be considered as a promising way to enhance the sensing performance by increasing the carrier concentration [24,25].Although ultraviolet (UV) radiation had realized the outstanding sensitivity and rapid response/recovery via cleaning effect [26][27][28][29], the negative influence cannot be ignored.UV light can cause damage to human eyes or skin because of high photo energy [30].In addition, UV light will chemically damage the sensing material by ozone produced in an oxygen-enriched condition [31].Besides, the low content UV light of sunlight make it necessary for gas sensor to equip an addition UV light source, which present extra challenge to device design [32].By contrast, photoactivation via visible light has no harm to human health and sensing materials.Meanwhile, visible light is more desirable in energy harvest and energy utilization when there is sunlight or indoors lamp, which is more practical for IoT application.
Generally, gas-sensing materials with narrow band gap is desired to achieve the visible light activation.Cadmium sulfide (CdS), one of classic metal chalcogenides, by virtue of its suitable band gap (ca.2.4 eV) for visible light adsorption and a relatively facile synthetic method, has been an ideal candidate in gas sensing [33][34][35].However, most of researches underestimate or even conceal the intrinsic performance of CdS for gas sensor.For example, commercial ZnO coated with CdS can enhance the sensing performance for formaldehyde induced by visible-light at room temperature, but the wide bandgap of ZnO (3.4 eV) leads to a low utilization of long wavelength light [36].Guo et al synthesized CdS/Co 3 O 4 heterojunction material, which showed an improvement to 50 ppm acetone by 25% under green light at room temperature [37].The sensing limit was still not satisfactory.Exposing high-energy facet of CdS via in-situ synthesis on interdigitated electrodes can provide more active sites for gas adsorption and possess a light-trapping effect, exhibiting a remarkable sensing performance towards NO 2 [38].However, the extra high working consumption and complicated synthesis method limit the scope of further practical applications.Therefore, we developed a facile strategy for efficient detection of NO 2 gas, which is of great importance in gas sensing.
In this work, we synthesized CdS nanowires (NWs) by a simple solvothermal method and investigated the sensing performance towards NO 2 at room temperature under illumination with different light emitting diodes (LED).The prepared CdS NWs sensor displayed the best performance with the gas response values of 236% towards 10 ppm NO 2 and 11% towards 12.5 ppb NO 2 under green LED.In addition, the sensor exhibited significantly higher selectivity to NO 2 among typical interfering volatile organic compounds (VOCs) gases, such as NH 3 , CO, H 2 S, triethylamine, toluene, ethanol and acetone.Moreover, the sensor showed a fast response/ recovery process.All the results indicated the promising application of CdS NWs in air-quality monitoring and noninvasive medical diagnosis.

Preparation of CdS NWs
A desired mount of Cd(NO 3 ) 2 • 4H 2 O (1 mmol) and CS(NH 2 ) 2 (1 mmol) were firstly added into 40 ml of ethylenediamine with vigorously stirring for 1 h.Afterwards, the mixture system was transferred to a 100 ml Teflon autoclave and kept at 220 • C for 24 h.Then, the as-prepared CdS NWs were collected, and washed with deionized water and ethanol several times for further applications.

Characterization
X-ray diffraction (XRD) patterns were performed by Rigaku D/max-2500 diffractometer with Cu Kα radiation (λ = 1.5418Å) at 40 kV and 200 mA.The surface structure and morphology of the sample was characterized using scanning electron microscopy (SEM, JSM-6701F) and transmission electron microscopy (TEM, JEOL JEM-2100F).Energy dispersive x-ray spectroscopy (EDS) mappings of the materials were obtained by scanning TEM (STEM) equipped with an EDS detector (JEOL 2100F).X-ray photoelectron spectroscopy (XPS) measurement was determined on the VG Scientific ESCALab220i-XL spectrometer using Al Kα radiation.The photoluminescence (PL) spectra were recorded with a Horiba Jobin Yvon LabRam HR800 at room temperature.Raman spectra were performed on LabRAM HR Evolution Raman microscope.UV-vis absorption spectra were acquired by Shimadzu UV-2550 spectrophotometer with BaSO 4 as a reflectance standard.

Electrochemical measurements
Electrochemical tests were carried out with a standard three-electrode system on a CHI660 electrochemical workstation.To prepare sample inks, 2 mg of CdS nanosheets were dispersed in a mixture solution containing 1 ml isopropanol (IPA) and 10 µl Nafion though ultrasonication.Then, 10 µl of the sample ink was drip-coated on a glassy carbon electrode and dried at room temperature.The photocurrent response was measured in 0.5 M Na 2 SO 4 solution with a Pt net and Ag/AgCl used as the counter electrode and reference electrode, respectively.Mott-Schottky (MS) plots were obtained by three sweeps in a voltage range from −1.0 to 1.0 V at the selected frequencies of 0.5, 1.0, and 1.5 kHz.

Sensor fabrication
The gas sensor based on CdS NWs was fabricated via a drop-coating process.The CdS NWs powders were dispersed in ethanol by ultrasonication for 2 h to prepare corresponding 10 mg ml −1 dispersion firstly, followed by drop coating on Pt interdigital electrodes (IEs) in a circular region (6 mm × 6 mm) to form a uniform film.The Pt IEs was pre-coated on ceramic substrates.The sensors for test were obtained by heat treatment at 200 • C for 5 h in air.The substrates were washed with water, ethanol and acetone for three times before use.

Performance testing
The gas sensors were evaluated by a commercial gas-sensing performance testing device (SD-101, Huachuang Ruike Tech.Co. Ltd, Wuhan, China) at room temperature.The light source uses the LED light that comes with the instrument.The power of UV light, blue light, and green light is 0.06 W, and the power of red light is 0.04 W.
Nitrogen dioxide (NO 2 ), carbon monoxide (CO), ammonia (NH 3 ), hydrogen sulfide (H 2 S) and toluene (balanced by nitrogen with 99.99% purity) were adjusted by mass flow controller (PQ-8020, Huachuang Ruike Tech.Co., Ltd, Wuhan, China).The gas concentration is calculated by formula: c = MFC3+MFC4 MFC1+MFC2+MFC3+MFC4 c 0 , where c (ppm) is the concentration of target gases, c 0 (ppm) is the concentration of source gases, MFC1 and MFC2 are the flow values of dry synthetic air, and MFC3 and MFC4 are the flow values of source gases.Triethylamine, ethanol and acetone were obtained via static gas distribution system, by heating liquid on a heating platform.The gas concentration is calculated by formula: c = 22.4×ρ×V1  V2×M , where c (ppm) is concentration of target gas, ρ (g l −1 ) is the liquid density of target gas, V 1 (µl) is liquid volume of target gas, V 2 (l) is volume of chamber and M (g mol −1 ) is molecular weight of target gas.The response values were calculated according to following formula: S = (|Ra − Rg|)/Ra × 100%, where Rg is the sensor resistance in target gas and Ra is the sensor resistance in the air.The response and recovery time are determined by the time required for sensor to achieve 90% change of the total resistance in the process of adsorption and desorption.All the tests of gas-sensitive properties were performed at room temperature (∼25 • C).

Results and discussion
As shown in figure 1(a), CdS NWs with abundance S-vacancy can be easily prepared through the hydrothermal route in ethylenediamine with cadmium nitrate and thiourea as precursors according to our previous work [39].Electron spin resonance (ESR) spectroscopy (figure 1(b)) indicates the abundant S vacancies in CdS NWs as a peak at ∼351 mT with significantly higher intensity is observed [40].XRD analysis was performed to investigate the crystalline phase of the obtained sample, which can be assigned to hexagonalphase of CdS (PDF#41-1049) (supplementary figure S1 (available online at stacks.iop.org/MF/1/025303/mmedia)). TEM and SEM images display that the synthesized CdS sample is composed of one-dimensional NWs in a diameter of 60-100 nm (figures 2(c) and S2).The lattice fringe spacing in high-resolution TEM (HRTEM) image and the corresponding fast Fourier transform (FFT) pattern reveal the singlecrystalline nature of these NWs (figures 1(d) and (e)).The measured average distance of lattice fringe from figure 1(d) (marked by blue line) is 0.334 nm (figure 1(f)), which matches well with the d-spacing of (002) crystallographic planes of hexagonal CdS, consistent with the result of XRD measurement (figure S1).Therefore, it suggests that the CdS NWs are grown along the [001] direction (figure 1(g)).Moreover, the EDS mapping analysis of Cd NWs displays the homogeneous distribution of elemental S and Cd (figure 1(h)).
XPS measurement was carried out to identify the chemical composition and valence state of CdS NWs. Figure 2(a) shows a high resolution XPS spectrum of Cd 3d, which exhibits two peaks at 412.0 and 405.1 eV, corresponding to the Cd 3d 3/2 and Cd 3d 5/2 , respectively.Besides, the typical characteristic peaks at 162.8 and 161.7 eV are in good consistency with the binding energies of S 2p 1/2 and S 2p 3/2 , respectively, indicating the existence of S 2− (figure 2(b), table S1).In the Raman spectrum (figure S3), the CdS NWs sample clearly presents two peaks at 300 and 601 cm −1 , which can be assigned to first-order longitudinal optical mode (1LO) and the second-order longitudinal optical mode (2LO), respectively [41].
To reveal the photoelectric properties, transient photocurrent measurement was first applied to examine the charge separation behavior over CdS NWs.As shown in figure 2(c), it exhibits the high photocurrent density (∼0.35 mA cm −2 ), suggesting fast charge separation during the light irradiation in CdS NWs.PL spectra for the CdS NWs was then taken at room temperature.As shown in figure 2(d), the peak maximum at 512 nm demonstrates the existence of photo-excited charges, which is consistent with room-temperature bandedge emission from CdS [42].To evaluate the band gaps, the Tauc plots converted from the absorption spectra via Kubelka-Munk function were investigated.The direct bandgap of CdS NWs was estimated at ∼2.38 eV (figure 2(e)).In addition, we  carried out MS measurements to confirm the exact conduction band (CB) potential of CdS NWs, which was −0.6 eV vs reversible hydrogen electrode (figure 2(f)).Therefore, we speculate CdS NWs can be designated as the photo-harvester in sensing because of the excellent photo-response capability.

Gas-sensing property
To investigate the effect of illumination for CdS NWs, the sensing transients to 10 ppm NO 2 were evaluated under different light sources as shown in figure 3(a).According to the result, the resistance of the sensor (>1 GΩ) is beyond the detection limit of a gas-sensing device in the dark and decrease as the light wavelengths decrease after illumination.Unfortunately, although red LED enhanced the conductivity, the resistance was still too high to calculate the response value.The responses under light illumination suggest a typical gas-sensing characteristic of n-type semiconductors.When CdS NWs contacted NO 2 , the adsorbed NO 2 molecular will capture electron from the CB of CdS NWs, making electron concentration lower and resulting a higher resistance.The sensor has the fastest response/recovery under UV light, but the response value is fairly low.By comparison, the sensing properties under green light and blue light show a fast response/recovery process, and the gas sensitivity assisted by green light is higher than by others.Thus, we choose green light as the optimum light source, which is consistent to the measured optical bandgap (2.38 eV) that corresponds to green light wavelength.
We then explored the gas-sensing performance of the sensor based on CdS NWs under the activation of green light.Firstly, good selectivity to target gases is significantly important for practical applications.Therefore, CO, NH 3 , H 2 S, triethylamine, toluene, ethanol and acetone were selected as interfering gases to investigate the selectivity of the sensing materials by green LED activation (figure S4).The responses to 10 ppm NO 2 and interfering gases of CdS NWs at room temperature are depicted in figure 3(b).It can be seen that CdS NWs exhibited particular high response for NO 2 with S NO2 of 236%.Selectivity factor can be defined as the ratios between the responses to NO 2 and interfering gases (S NO2 /S interfering gases ) which is plotted in figure 3(c), suggesting significantly higher selectivity to NO 2 among interfering gases.
The dynamic response curve of gas sensor based on CdS NWs to different NO 2 concentration ranging from 1.25 to 12.5 ppm are show in figure S5(a).It could be found that the corresponding responses had a significant enhancement with the increasing concentration of NO 2 , and it exhibited a good linear relationship with gas concentration (figure S5(b)).The detection limit of the CdS NWs for ppb level gas was further explored as figure 3(d), displaying the transient resistance variation curve of low concentration NO 2 from 12.5 to 200 ppb, the response to 12.5 ppb NO 2 is 11% and the sensor also showed a linear relationship between response and gas concentration (figure 3(e)).The response and recovery time to 10 ppm NO 2 are calculated in figure S6, indicating that the illumination is very conducive to expediting response and recovery speed of sensors.In order to assess the response repeatability and long-term stability, the response to 200 ppb NO 2 under green LED light activation at room temperature was continuously tested for ten cycles (figure 3(f)).In addition, the sensor was tested for 30 times within one month, as shown in figure 3(g).The response values varied in small ranges with a slight decrease, which can be ascribed to the effect of humidity in the environment for CdS sensor, suggesting a relatively satisfactory stability.The comparisons of NO 2 sensing performances of CdS-based and visible-light-assisted gas sensors were shown in table S2.
Therefore, take the suggestion of the US EPA and World Health Organization into consideration, all above results indicate the CdS NWs based gas sensor with ppm and ppb-level NO 2 detection can be used various applications such as airquality monitoring and non-invasive medical diagnosis.

Gas-sensing mechanisms
For semiconductor gas sensors, the traditional sensing mechanism is based on space-charge-layer model that the resistance is determined by the change of carrier concentration.CdS NWs demonstrate the characteristic of n-type semiconductors, indicating the carrier is electron.Surface adsorbed oxygen species, such as O 2 − , O − and O 2− , will form when free electrons in CB are captured by oxygen molecules, produce electron depletion layer (EDL) on the surface of CdS NWs.For NO 2 sensing, the adsorbed NO 2 molecules will compete with oxygen molecules and withdraw more e − in the CB of CdS due to its strong electrophilicity, which will further increase the thickness of EDL and create a response.
Therefore, the light-assisted gas-sensing mechanism must consider the gas adsorption/desorption kinetics.We talk about the desorption of adsorbed oxygen first according to two main means [43] (figure 4(a)).One case is that the stable chemisorbed O 2 − ion interact with the photogenerated holes, result in the oxygen desorb by releasing electron to CdS.And the photogenerated electrons will react with other oxygen molecules to form a kind of photoinduced oxygen ions, which are weakly bound to the surface of CdS NWs.Another case [44] is that the surface oxygen remove by reacting directly with photon and release bound electrons to CdS.These can be called the cleaning effect [45]: To confirm the practicality of this mode on our work, we investigated the photoresponse curves under different light via gas-sensing test system (figure S7).It is clear that the resistance of gas sensor decreased under the illumination even by red LED, which cannot conduct band to band excitation due to the inadequate photon energy (about 1.65 eV).In addition, although the resistance rapidly drops to a low value till equilibrium under the illumination by green LED, it can be recovered to the initial state once removing the light source.All the results indicate that the resistance change of sensor caused by not only electronic transitions but also the adsorption and desorption process of oxygen molecules.
Figure 4(b) studied the existence of activated oxygen ions (O 2 -), the sensor is exposed to dry air in lamp light at first, chemisorbed O 2 − ions form on the surface and produce an EDL.However, when dry air is switched to N 2 , the resistance is almost constant.And when the sensor is exposed to dry air again, no significant change of resistance was observed.Above results suggested that the adsorbed O 2 − ions are stable and hard to escapade in N 2 under lamp light at room temperature.It is interesting when the sensor is placed in dry air and followed in N 2 under green LED with 0.24 W cm −2 .Firstly, the base resistance in dry air decreases sharply due to electronic transition and the desorption of O 2 − .Secondly, the resistance is further decreased when the air is turned to N 2 by the desorption of activated O 2 − , which is weakly bound to the surface of CdS NWs.Therefore, activated oxygen ions (O 2 − ) are present in CdS NWs.
For CdS NWs, abundant sulfur vacancies can act as the adsorption sites for oxygen molecules and form a thick EDL like oxygen vacancies [46,47], resulting in the ultrahigh baseline resistance.However, since the adsorbed oxygen species are very stable at room temperature and CdS is in an almost completely depleted state, it makes the adsorption reaction of NO 2 difficult, thus difficult to achieve response.However, things get different under light activation, the lightassisted sensing mechanism is illustrated in figure 4(c).Benefit from the unique charge separation behavior of CdS NWs, that the photoinduced holes are mainly located at CdS NW stems, whereas the photoinduced electrons tend to localize at the CdS NWs tips [48].Therefore, the adsorbed oxygen at the stems of the highly exposed CdS NWs can be easily removed by recombining with holes to form oxygen molecules and release bound electrons into the sulfur vacancies, reducing the thickness of the EDL, resulting in a sharp drop in resistance over three orders of magnitude.At the same time, the desorption of oxygen molecules leaves abundant adsorption sites for gas sensing.For NO 2 gas sensing, oxidizing NO 2 molecules will mainly accumulate at the tip of CdS, while reducing gas will be mainly distributed at the neck of CdS during the response.This sensing process can be divided into two parts as follows: A large amount of adsorbed oxygen species are either desorbed or becomes weakly adsorbed, which not only greatly improves the responsiveness to NO 2 , but also promotes the kinetic process of the response and reduces the response time.Besides, the slower recovery process compared to the response process can be attributed to the fact that the tips mainly concentrate photogenerated electrons, so the process similar to the recombination of oxygen molecules in the stems with photogenerated holes does not occur.

Conclusions
In summary, we successfully used CdS NWs sensor for low concentration NO 2 detection at room temperature and proposed a feasible sensing mechanism under green light illumination.The prepared CdS NWs sensor displayed an excellent performance, e.g.high sensitivity (11% to 12.5 ppb NO 2 ), remarkable selectivity among several interferants, fast response/recovery speed, good repeatability and long-term stability.The gas-sensing mechanism of the CdS NWs sensor activated by the green light illumination is closely related to two aspects.On one hand, the photo-excited electron-hole pairs yield a clean effect by removing stable chemisorbed O 2 − ion, leaving vast available adsorption sites for NO 2 .On the other hand, because of the unique charge separation behavior of CdS NWs, NO 2 can easily capture the abundant photogenerated electrons at the tips with little affect by the photogenerated holes in the stems.This work suggests that CdS NWs is an effective sensing material for building a high-performance NO 2 gas sensor at room temperature, and will be helpful for designing sensors for air-quality monitoring and non-invasive medical diagnosis.

Figure 1 .
Figure 1.(a) Scheme illustration of the synthetic S-vacancy CdS NWs via hydrothermal method.(b) ESR spectra of CdS NWs.(c) TEM and (d) HRTEM images of CdS NWs.(e) FFT patterns derived from the red square in (d).(f) The section profile along the blue line in (d).(g) Schematic illustration of the [001] direction of hexagonal CdS crystal structure.(h) High angle annular dark field-STEM (HAADF-STEM) image with corresponding EDS elemental mapping images of CdS NWs.

Figure 2 .
Figure 2. High-resolution XPS spectra of (a) Cd 3d and (b) S 2p.(c) Transient photocurrent response on CdS NWs in 0.5 M Na 2 SO 4 solution.(d) Room-temperature PL spectra.The PL excitation wavelength was 325 nm.(e) UV-vis absorption spectra and (inset) corresponding band gap energies of CdS NWs.(f) MS plots of CdS NWs collected at various frequencies.

Figure 3 .
Figure 3. (a) CdS NWs sensor's response to 10 ppm NO 2 at room temperature under different light.(b) and (c) Gas responses to 10 ppm various gases and the ratio of the responses to 10 ppm NO 2 and 10 ppm interfering gases under green LED of sensor based on CdS NWs.(d) and (e) Dynamic sensing transients to different NO 2 concentration and the function of gas response and NO 2 concentration.Error bars were derived from several independent tests.(f) Successive sensing performance in ten cycles.(g) Gas responses of CdS NWs for 200 ppb NO 2 against several times within 1 month.

Figure 4 .
Figure 4. (a) Schematic diagram of the CdS band structure under light activation.(b) Effect of gas atmosphere on the gas sensor based on CdS NWs in lamp light and in green light.(c) Schematic illustration of the NO 2 sensing mechanism of the light-assisted CdS NWs based gas sensor.