NiO/ZnO p–n heterostructures and their gas sensing properties for reduced operating temperature

Abstract

NiO/ZnO p–n heterostructures were successfully synthesized by using a hydrothermal method followed by calcination. The morphology of the NiO/ZnO p–n heterostructures could be controlled by the amount of Ni concentration, with 10% Ni the optimum content. The structural features of the NiO/ZnO p–n heterostructures were characterized in detail by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). Additionally, direct current (DC) I–V curves of the NiO/ZnO p–n heterostructures showed diode-like behavior, which is indirect evidence demonstrating that p–n heterojunctions were formed between NiO and ZnO. The 10% NiO/ZnO heterostructures gas sensor exhibited a good gas response, fast response/recovery times and long-term stability to ethanol vapor even at 200 °C, the reduced operating temperature was much lower than for pure ZnO. The decline of the operating temperature was attributed to the formation of p–n heterojunctions. Meanwhile, a possible gas sensing mechanism is illustrated by the calculated energy band positions of the NiO/ZnO p–n heterostructures and alternating current (AC) impedance spectra.

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Introduction

Numerous kinds of gases are emitted from various sources into our living space, working space or outdoors. Many of them are hazardous to human beings and the environment, such as ammonia in industrial environments and formaldehyde in living environments. Besides, some gases are utilized for diagnosing the status of their sources, like ethanol and acetone in the breath and flavor components of food. Semiconductor gas sensors have been extensively investigated for practical applications, such as detection of gas leakage and environmental monitoring of gaseous pollutants.^1–4 Impedance-semiconductor gas sensors are typically operated at temperatures greater than 300 °C and usually through a change in resistance induced by the surface adsorption of oxygen and target gases at high temperatures.⁵ Such adsorbed oxygen plays a key role in several technological processes. For example, oxidation reactions are promoted by heterogeneous catalysts in gas sensors or in photocatalysis.^6–8 High-temperature operation also ensures the complete desorption of gaseous species on the surface of metal oxide following transduction. However, miniaturization of chemical gas sensors based on semiconducting oxides need to reduce power consumption to a degree that allows the employment of the gas sensors in miniaturized analytical devices. Semiconductor gas sensors maintaining at the high temperatures may have drawbacks, especially when high sensor power consumption is undesirable, such as when photovoltaic are the desired power source.^5,9 Additionally, the high operating temperature is an undesired factor for the long-term stability of the gas sensor. In this context, miniaturization, low-power and thus low temperature, long-term stability gas sensors would be advantageous.

Semiconductor gas sensors have been developed based on limited kinds of n-type metal oxides, such as SnO₂, In₂O₃, ZnO, Fe₂O₃ and WO₃. In particular, zinc oxide (ZnO), a typical n-type transparent oxide semiconductor with a wide band gap of 3.4 eV, has been recognized as an excellent material for the gas sensors, due to its high mobility of electrons, low cost and environmental friendliness.^10–14 However, the gas sensors based on pure ZnO exhibited poor gas response and selectivity and required high operating temperature of about 400 °C. It has been found that ZnO-based sensors could be improved in gas response and selectivity by the addition of noble metals such as Au, Ag, Pt and semiconductors like SnO₂, TiO₂, Co₃O₄.^15–18 Meanwhile, nickel oxide (NiO) as a p-type semiconducting material with a wide band gap of 3.7 eV, can easily form p–n heterojunctions with ZnO. The NiO/ZnO p–n heterojunction nanostructures have been extensively studied in light emitting diodes (LEDs),¹⁹ photodetector,^20,21 photocatalysts^22,23 and gas sensors.^24–26 Generally, the sensitivity and selectivity of the gas sensor can be substantially enhanced due to p–n heterojunction effects. The nano-coaxial p-Co₃O₄/n-TiO₂ heterojunction exhibited p-type response with excellent sensing performances.²⁷ Not only can the SnO₂@Co₃O₄ p–n heterostructures improve the gas response, but also can reduce the optimum operating temperature.²⁸ Predictably, the NiO/ZnO p–n heterostructures which were prepared by the hydrothermal synthesis should also be a good gas sensing material.

Herein, we report a facile hydrothermal approach to synthesize the Ni(OH)₂/ZnO precursor at 90 °C for 12 h. After heating the Ni(OH)₂/ZnO precursor at 350 °C for 2 h, the NiO/ZnO p–n heterostructures were obtained. The phase compositions and morphology of the NiO/ZnO p–n heterostructures were confirmed by different characterization methods. The NiO/ZnO p–n heterostructures sensor which was the non-thin film device was firstly observed the diode-like behavior by I–V characteristic, suggesting that p–n heterojunctions were prepared between NiO and ZnO. In the gas sensing measurement, the gas sensor of the NiO/ZnO p–n heterostructures exhibited good gas response to ethanol at reduced operating temperature of 200 °C, and revealed stable gas response toward ethanol vapor concentrations range from 10 to 1000 ppm. Additionally, the NiO/ZnO p–n heterostructures gas sensor displayed the long-term stability after 2 months and a quick response time of 6 s. And the possible gas sensing mechanism had been explained by the calculated energy band positions of the NiO/ZnO p–n heterostructures and AC impedance spectra.

Experimental

Materials and synthesis of the NiO/ZnO p–n heterostructures

Zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O), nickel acetate tetrahydrate (Ni(CH₃COO)₂·4H₂O), and sodium hydroxide (NaOH) were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). All the raw materials were analytical reagent grade and used as received without further purification.

In a typical procedure, Zn(NO₃)₂·6H₂O (0.54 g, 1.8 mmol), Ni(CH₃COO)₂·4H₂O (0.05 g, 0.2 mmol), and NaOH (0.16 g, 4 mmol) were successively added into 40 mL deionized water under continuous magnetic stirring. The resultant solution was heated in a sealed autoclave with a stainless steel shell at 90 °C for 12 h inside an oven. Subsequently, the light green precursor was collected after cooling naturally to room temperature, washed three times using deionized water and alcohol alternatively, and then dried at 60 °C for 8 h. Finally, NiO/ZnO p–n heterostructures were obtained by calcining the light green precursor at 350 °C for 2 h in air.²⁹ The sample with a molar ratio of NiO to ZnO of 1 : 9 was labeled as 10% NiO/ZnO. Furthermore, the samples of the 5% NiO/ZnO and 20% NiO/ZnO were synthesized with the same procedure. In comparison, pure ZnO and NiO nanostructures were prepared similarly.

Characterization

Phase compositions of the as-synthesized materials were examined by using X-ray diffraction (XRD; X'pert PRO MPD, Philips, Eindhoven, The Netherlands) with Cu Kα radiation (λ = 1.5406 Å) in the range of 20–80°. Product morphology and microstructure were observed by using scanning electron microscopy (SEM; JSM-6701F, JEOL, Tokyo, Japan) and transmission electron microscopy (TEM; Tecnai F30G², FEI, Hillsboro, OR, USA). Surface properties of the samples were analyzed with using X-ray photoelectron spectroscopy (XPS; VG ESCALA-B220i-XL, Thermo Scientific, Surrey, UK) using an Al Kα (hν = 1486.6 eV) source at a residual gas pressure below 10⁻⁸ Pa. All the binding energies were referenced to the C 1s peak at 284.7 eV of the surface adventitious carbon. DC current–voltage characteristics of the sensor were measured at room temperature by using a source meter (Model 2410, Keithley, Cleveland, USA) with the Cell I-V software (2410, CEAULIGHT, Beijing, China).

Gas sensing measurements

The gas-sensing measurements were performed on a gas response instrument (HW-30A, Hanwei Ltd., Zhengzhou, China) (Fig. S5a†). The volume of the gas-sensing test chamber is 15 L. Firstly, we kept gas sensors to stabilize their base-line during a testing when we closed a cover of the test chamber, and then the certain volume of the liquid reductive gas was injected into an evaporator with a syringe in 30 s. Subsequently, gas sensors were exposed in air when we opened a cover of the test chamber in 110 s to remove the reductive gas. Finally, the whole testing was stopped in 180 s. The whole testing was repeated when the base-line recovered. A loaded resistor is connected in series with a gas sensor in the measuring electric circuit (Fig. S5b†). The circuit voltage (V_c) is 5 V, and the output voltage (V_out) is the terminal voltage of the loaded resistor R_L. The heating voltage (V_h) which is added on the Ni–Cr alloy is adjusted for the working temperature of gas sensors. When an amount of tested gas is injected into a chamber, the sensor's resistance is changed. As a result, the output voltage of the loaded resistor is changed. The gas response (S) was defined as the ratio of R_air/R_gas, where R_air and R_gas are the resistance measured in air and reductive gas, respectively. And the gas response (S) also can be calculated with the output voltage of the loaded resistor (V_out) as following equation:

S = R_air/R_gas = (5 − V_air) × V_gas/(5 − V_gas) × V_air (1)

where V_air and V_gas are the average V_out of 0–30 s in air and 31–110 s in reductive gas, respectively. The response/recovery times were defined as the time to reach 90% of the final equilibrium value. Gas sensor devices were fabricated according to a previously published procedure.³⁰ An alumina tube with Au electrodes and platinum wires was used as the substrate. The sensors of the NiO/ZnO p–n heterostructures were prepared on alumina tube by the slurry spin coating. A Ni–Cr alloy crossed the alumina tube, which was applied as a heating resistor for controlling the operating temperature. Finally, the gas sensors were dried under infrared radiation light for several minutes and calcined at 400 °C for 2 h in air. For each composition, three sensors were made and tested three times at the same conditions to obtain an average value of sensitivity. Whilst, AC impedance spectroscopy of the sensor was measured by using an impedance analyzer (4294A, Agilent, CA, USA) with the gas sensing instrument (HW-30A, Hanwei Ltd., Zhengzhou, China) where the temperature of the sensor could be controlled. The measurement frequency range was from 100 Hz to 1 × 10⁷ Hz.

Results and discussion

Phase composition and morphology

XRD patterns of the as-prepared NiO/ZnO p–n heterostructures, ZnO and NiO are shown in Fig. 1. All diffraction peaks in Fig. 1a can be perfectly indexed as hexagonal structure of ZnO (JCPDS card: 36-1451) and cubic structure of NiO (JCPDS card: 78-0643). For the NiO/ZnO p–n heterostructures, XRD patterns consisted of all peaks of ZnO phase and NiO phase. As shown in Fig. 1b and c, two peaks at 36.3° and 62.5° of the NiO/ZnO p–n heterostructures can be fitted with the peak of ZnO phase and NiO phase, respectively. The intensity of (1 1 1) and (2 2 0) peaks of NiO is enhanced in the NiO/ZnO p–n heterostructures with the increase of Ni content. The (2 0 0) peak at 43.2° of NiO slightly shifted about 0.53°, the result can be seen in the inset of Fig. 1a. Moreover, no diffraction peaks from impurities can be detected. SEM images of ZnO and the 10% NiO/ZnO heterostructures are shown in Fig. 2. It can be seen that pure ZnO with sizes in the range of 2–5 μm are flower-like structures in Fig. 2a. As shown in Fig. 2b, the size of the 10% NiO/ZnO heterostructures decreases obviously, suggesting that the existence of Ni can affect nucleation and growth of ZnO. Moreover, response/recovery curves of ZnO, 5% NiO/ZnO, 10% NiO/ZnO, and 20% NiO/ZnO were measured at same experimental conditions, and the result is presented in Fig. S2a.† The 10% NiO/ZnO heterostructures reveal the biggest gas response to 100 ppm ethanol vapor at operating temperature of 200 °C (Fig. S2c†). We also have presented SEM images of the 5% NiO/ZnO and 20% NiO/ZnO heterostructures in Fig. S1a and b.† We can see a few nanoparticles of NiO are covered on the surface of the 5% NiO/ZnO heterostructures. Meanwhile, nanoparticles and nanosheets of NiO coexist on the surface of the 20% NiO/ZnO heterostructures. Additionally, XPS spectra of the 5% NiO/ZnO and 20% NiO/ZnO heterostructures demonstrate that the relative intensity of Ni 2p peak is increased (Fig. S1d†), indicating that NiO nanocrystals on the surface of ZnO are gradually increased by the addition of Ni content. For the 5% NiO/ZnO and 20% NiO/ZnO heterostructures, too less or too more NiO nanocrystals on the surface of ZnO are harmful for the gas response of materials. Based on the above discussion, we chose the 10% NiO/ZnO heterostructures to carry out subsequent characterizations and experiments.

Fig. 1 (a) XRD patterns of ZnO and NiO/ZnO p–n heterostructures and NiO. (b) The fitted peaks of ZnO (1 0 1) and NiO (1 1 1) and (c) the fitted peaks of ZnO (1 0 3) and NiO (2 2 0) in the NiO/ZnO p–n heterostructures. The inset shows the selected peak of (2 0 0) for the NiO/ZnO p–n heterostructures and NiO.

Fig. 2 (a) SEM image of ZnO and (b) SEM image of the 10% NiO/ZnO heterostructures (the insets of SEM images are high-magnification patterns).

The 10% NiO/ZnO heterostructures were further investigated by using TEM. The low-magnification TEM image of the 10% NiO/ZnO heterostructures can be seen in Fig. 3a. High-resolution transmission electron microscopy (HRTEM) image obtained within the red circle area is presented in Fig. 3b. The interplanar distance of ZnO (0 0 2) and NiO (1 1 1) in high-resolution TEM image is shown in Fig. 3b. In order to overcome the measurement error, we have calculated three interplanar distances of ZnO and NiO to obtain the average d-spacing. The average interplanar distance of 0.26(0) nm matches with the lattice spacing of the (0 0 2) planes in ZnO while NiO has a lattice spacing of 0.24(1) nm, corresponding to the interspacing of the (1 1 1) planes. Meanwhile, selected-area electro diffraction (SAED) pattern (Fig. 3c) shows that the diffraction rings can be indexed as the (1 0 0), (0 0 2) planes of hexagonal ZnO and the (2 0 0), (2 2 0) planes of cubic NiO, respectively. In order to confirm the interface between NiO and ZnO, the non-sputtered and Ar ion sputtered samples were analyzed by XPS. For the non-sputtered 10% NiO/ZnO heterostructures, the Ni 2p high resolution XPS spectrum can be deconvolved into five peaks (Fig. 3d), which are consistent with the result in Fig. 4d, and the position of Ni 2p peaks (2p_3/2 and 2p_1/2) has no shift in two samples. After sputtering (Fig. 3d), the relative intensity of Ni 2p peaks is decreased with adding the sputtering time, indicating that NiO nanocrystals on the surface of ZnO may be removed in the sputtered process. Thus, we can reasonable speculate that NiO nanocrystals are coated on the surface of ZnO in the 10% NiO/ZnO heterostructures.

Fig. 3 (a) TEM image and (b) HRTEM image and (c) the corresponding SAED pattern of the 10% NiO/ZnO heterostructures. (d) Survey XPS spectra of the 10% NiO/ZnO heterostructures at different conditions, (inset: Ni 2p high-resolution XPS of 10% NiO/ZnO heterostructures).

Fig. 4 (a) Survey XPS spectra of ZnO and the 10% NiO/ZnO heterostructures. (b) Zn 2p high-resolution XPS spectra. (c) O 1s high-resolution XPS spectra and corresponding Gaussian fittings. (d) Ni 2p high-resolution XPS of the 10% NiO/ZnO heterostructures and corresponding Gaussian fittings.

Fig. 4 shows XPS spectra of the 10% NiO/ZnO heterostructures and ZnO. All the peaks on the full-range spectra (Fig. 4a) can be assigned to Ni, Zn, O and C, and no peaks of other elements are observed. The C is attributed to the adventitious carbon based contaminant adsorbed on surface of the samples, while the C 1s binding energy peak at 284.7 eV is used as the reference for calibration. Accordingly, the Zn 2p high-resolution XPS spectra of ZnO and the 10% NiO/ZnO heterostructures can be observed in Fig. 4b. For pure ZnO, the peak at 1021.8 eV is attributed to Zn 2p_3/2 and the other peak at 1044.8 eV can be assigned to Zn 2p_1/2. Although the binding energy of Zn 2p (2p_3/2 and 2p_1/2) for the 10% NiO/ZnO heterostructures shifts 0.6 eV to higher binding energy, compared with the value of pure ZnO, the spin–orbit splitting between Zn 2p_3/2 and Zn 2p_1/2 is 23 eV, which is in agreement with the value of pure ZnO, indicating that Zn²⁺ has the same chemical state in ZnO and the 10% NiO/ZnO heterostructures.²² As shown in Fig. 4c, the peak at 529.9 eV for ZnO corresponds to the lattice oxygen and the peak at 531.4 eV belongs to the adsorbed oxygen or OH species on the surface of the sample. The O 1s scan peak of the 10% NiO/ZnO heterostructures at 529.1 eV is assigned to the lattice oxygen. The peak at 530.8 eV is attributed to the deficient oxygen and the peak at 532.3 eV belongs to the chemisorbed or dissociated oxygen or OH species. Note that because the different chemical environment of O in two samples, each characteristic peak has some shift in the 10% NiO/ZnO heterostructures.²⁴ Moreover, the Ni 2p high-resolution XPS spectrum of the 10% NiO/ZnO heterostructures can be deconvolved into five peaks in Fig. 4d. The binding energy Ni 2p peaks (2p_3/2 and 2p_1/2) for the 10% NiO/ZnO heterostructures are lower about 0.7 eV than that of pristine NiO. Consequently, the Zn 2p and Ni 2p peaks shift toward a higher and a lower binding energy in the 10% NiO/ZnO heterostructures, respectively. This can be attributed to the fact that the strong interaction was formed between NiO and ZnO.^22,24 In other words, the NiO/ZnO p–n heterostructures have been successfully synthesized.

DC I–V characteristics of ZnO and the 10% NiO/ZnO heterostructures were measured by using a source meter of Keithley 2410 and the results are presented in Fig. 5a. I–V curves show two different regions depending on the voltage at the forward voltage. Region I, V < 60 V, a linear dependence of the current on the voltage can be perfectly fitted with the experimental curve (Fig. 5b), which follows the ohmic law. At a higher voltage, V > 60 V, region II, the I–V curve follows a power law I ∼ V² (Fig. 5c), which is generally attributed to a space-charge-limited current (SCLC) conduction for single-carrier (electron) injection behavior observed in wide band gap semiconductors.^31,32 Correspondingly, the current of the 10% NiO/ZnO heterostructures also can be linearly fitted with the experimental curve in region III (Fig. 5d), and in region IV, the current is dramatically increased and can be exponentially fitted with the voltage (Fig. 5e), which is usually observed in wide band gap p–n diodes.³² Additionally, I–V curve of the 10% NiO/ZnO heterostructures is an asymmetrical curve, with rectification characteristic, which is similar to that reported for the p–n junction made in ZnO materials.^33,34 Therefore, the result further confirmed that the NiO/ZnO p–n heterostructures had been formed between NiO and ZnO.

Fig. 5 (a) I–V characteristics of ZnO and the 10% NiO/ZnO heterostructures. ZnO I–V curve show the experimental data (open symbols) and the theoretical fittings (red lines) in region I (b) and region II (c). I–V curve of the 10% NiO/ZnO heterostructures in region III (d) and region IV (e).

Gas sensing performance and mechanism

Operating temperature is one of the most important parameters of the metal oxide gas sensors. Fig. 6a represents the typical response curves of the 10% NiO/ZnO heterostructures sensor that was exposed to 100 ppm ethanol vapor at different temperatures.

Fig. 6 (a) Typical response curves of the 10% NiO/ZnO heterostructures sensor in 100 ppm ethanol vapor at different temperatures. (b) Gas response as a function of different operating temperatures for the ZnO and 10% NiO/ZnO heterostructures sensor in 100 ppm ethanol vapor.

It is well known that the resistance of semiconductor is notably dependent on temperature. The operating temperature dependence of gas response (R_air/R_gas) will take on two different characters (camelback or volcano frameworks) because of various oxygen species of chemical absorption on surface of the gas sensing materials.³⁵ As shown in Fig. 6b, the gas response of the 10% NiO/ZnO heterostructures sensor displays the camelback framework, which is consistent with the feature of the surface-controlled gas sensors.³⁶ The surface operating temperature of the gas sensor is approximately 200 °C when the heating voltage (V_h) is 3.8 V (Fig. S5a†), which had been confirmed by theoretical calculations and the measurement of the micro thermocouple probe in our previous works. Thus, we selected the heating voltage of 3.8 V, which means the operating temperature was 200 °C to process the subsequent experiments. Besides, the response and recovery curves of the gas sensor based on different materials at 200 °C and the ZnO sensor at different temperatures in 100 ppm ethanol vapor can be observed in Fig. S2.† The result indicates that the 10% NiO/ZnO heterostructures showed the highest gas response at 200 °C. It is noteworthy that the operating temperature of the 10% NiO/ZnO heterostructures sensor is much lower than that of the ZnO sensor. Table 1 lists the gas sensing properties of the NiO and ZnO-based gas sensors prepared by different methods.^37–40

Table 1 Comparison of gas sensing characteristics for different NiO and ZnO-based gas sensors

Materials	Target gas	Gas conc. (ppm)	T (°C)	S	Ref.
ZnO nanosheets	Ethanol	100	400	24	37
NiO hollow hemispheres	Ethanol	100	300	4	38
Mn3O4/ZnO nanobelts	Ethanol	100	400	30	39
5% NiO/SnO2 nanofibers	Ethanol	100	300	25	40
NiO/ZnO nanotubes	Ethanol	200	215	<5	24
NiO/ZnO heterostructure	Acetone	100	330	13	25
NiO/ZnO nanowires	Ethanol	5	450	29	26
10% NiO/ZnO nanostructures	Ethanol	100	200	16	This work

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We use the method of theoretically predicting from the absolute electronegativity (Pearson absolute electronegativity)⁴¹ to determine the energy band positions of the 10% NiO/ZnO p–n heterostructures in Scheme 1. The energy band potentials of a semiconductor at the point of zero charge can be calculated by the following empirical equation:^27,42

E_VB = X − E^e + 1/2E_g (2)

E_CB = X − E^e − 1/2E_g (3)

E₀ = −(X − E_CB) (4)

where X is the absolute electronegativity of the semiconductor, which is defined as the geometric mean of the absolute electronegativity of the constituent atoms, E^e is the energy of free electrons on the hydrogen scale (ca. 4.5 eV), E_VB and E_CB is the VB edge potential and CB edge potential, respectively. E_g is the band gap of the semiconductor and E₀ represents the vacuum level. The absolute electronegativity of NiO and ZnO by the above equation is shown in Table 2. The built-in electric field is formed at the interface of the 10% NiO/ZnO p–n heterostructures. Meanwhile, the equalization of Fermi level will bend the energy band positions in the depletion layer. The electrons can transfer from p-type NiO to n-type ZnO easily, but the migration of the holes will be blocked due to the potential barrier.²⁸ This is the reason why the 10% NiO/ZnO heterostructures show an n-type sensing property.

Scheme 1 Schematic illustration of the energy band positions between NiO and ZnO before and after contacting.

Table 2 Values of the theoretical calculation for the energy band positions of NiO and ZnO

	Constituent elements	Electron affinity (eV)	Ionization energy (eV)	Element electronegativity^a (eV)	Semiconductor electronegativity (X)	Eg (eV)
a The element electronegativity is the arithmetic mean of the electron affinity and the first ionization energy.
NiO	Ni	1.16	7.64	4.4	5.76	3.7
	O	1.46	13.6	7.53
ZnO	Zn	<0	9.4	4.45	5.79	3.4

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When the 10% NiO/ZnO p–n heterostructures are exposed in air, oxygen can be adsorbed on surface of the material, forming four kinds of oxygen species, which are O₂ (80 °C), O₂⁻ (150 °C), O⁻ (300–400 °C) and O²⁻ (550 °C).^10,43 Among these, O⁻ is the most reactive with reductive gases. When the 10% NiO/ZnO p–n heterostructures gas sensor is operated at 200 °C, O₂⁻ is the main oxygen species. Then, O₂⁻ captures one electron from the n-type ZnO to become the dissociative type oxygen species (O⁻) [O₂⁻(ad) + e⁻ → 2O⁻(ad)]. On contact with the ethanol vapor, it is consumed in the redox reaction and electrons are released back to ZnO, leading to a decrease in the electrical resistance.^20,44 Therefore, the formation of the 10% NiO/ZnO p–n heterostructures plays an important role to improve the gas response at the reduced operating temperature of 200 °C compared to the pure ZnO.

Fig. 7a illustrates the typical transient response curves of the 10% NiO/ZnO heterostructures gas sensor, when it was exposed to ethanol vapor with concentrations of 10, 50, 100, 200, 300, 500, 800 and 1000 ppm at operating temperature of 200 °C. The output voltage is increased with increasing concentration of the reductive gas, which means that the 10% NiO/ZnO heterostructures gas sensor reveals concentration dependence for ethanol vapor. As shown in the inset of Fig. 7a, the response and recovery times of the gas sensor in 100 ppm ethanol vapor are 6 and 22 s, respectively. The pattern of the gas response versus concentration of ethanol vapor is plotted in Fig. 7b and the gas response are 4, 11, 16, 22, 25, 30, 38 and 41 in 10, 50, 100, 200, 300, 500, 800 and 1000 ppm ethanol vapor, respectively. Generally, the gas response of the semiconducting oxide can be empirically represented as following formula:

S = 1 + A_g × (P_g)^β (5)

where P_g is the gas partial pressure, which is directly proportional to the gas concentration, A_g is a prefactor, and β is the response exponent on P_g, the exponent has an ideal value of either 0.5 or 1, which is derived from the surface interaction between chemisorbed oxygen and reductive gases to the n-type semiconductor.⁴⁵ The data can be fitted almost linearly, with a correlation coefficient R of 0.9933 in the inset of Fig. 7b. The value β towards ethanol vapor is 0.4931, which is closed to the ideal value of 0.5, indicating that the gas sensing mechanism of the 10% NiO/ZnO heterostructures is the surface reaction. Several volatile organic compounds (VOCs), including ethanol, methanol and acetone, can be well detected by using this sensor. Typical response curves and selectivity of the gas sensor based on the 10% NiO/ZnO heterostructures is showed in Fig. S3.† Stability of the gas sensor based on the 10% NiO/ZnO heterostructures is showed in Fig. S4.† The gas response was retained above 85% after 2 months, which means that the 10% NiO/ZnO heterostructures sensor has the long-term stability.

Fig. 7 (a) Typical transient response curves of the 10% NiO/ZnO heterostructures sensor in ethanol vapor at concentration range from 10 to 1000 ppm at 200 °C (inset: response and recovery times for the 10% NiO/ZnO heterostructures sensor). (b) Gas response of the 10% NiO/ZnO heterostructures sensor as a function of ethanol vapor concentration from 10 to 1000 ppm at 200 °C (inset: dilogarithm linear fitting of the gas response to the concentration of ethanol vapor).

In order to further demonstrate the gas sensing mechanism and the reason of reduced operating temperature, AC impedance spectra were performed and shown in Fig. 8. The complex impedance spectra of the ZnO and 10% NiO/ZnO heterostructures sensor in air can be seen in Fig. 8a and b, respectively. The resistance of the sample can be tentatively evaluated from the value of Z′ in the low frequency end of the semicircle, where Z′′ would supposedly reach the abscissa axis (Z′′ = 0).⁴⁶ The resistance of two samples revealed a monotonic decrease with increasing the operating temperature due to the higher mobility of charge carriers. The impedance data show the single Debye-like semicircle for ZnO and the 10% NiO/ZnO heterostructures at temperature range investigated. Thus, the electrical properties can be associated to a parallel resistance–capacitance (RC) equivalent circuit, indicating that only grain response is detected. Meanwhile, the imaginary parts of impedance (Z′′) and modulus (M′′) as a function of frequency are displayed in Fig. 8c. The plots of Z′′/M′′ show one single peak in both cases with maxima at similar frequencies, which means the sample is electrically homogeneous.^47,48 Additionally, the activation energy of samples for conduction process was extracted by the slope of the straight line in the logarithmic conductivity (log δ) against reciprocal temperature (1000/T) plot.^49,50 The Arrhenius plots of conductivity are plotted in Fig. 8d, the activation energy (E_a) of ZnO and 10% NiO/ZnO heterostructures is 0.99 eV and 0.94 eV, respectively. Therefore, the activation energy of the intrinsic conduction is further evidence to enhance the gas sensing properties at the reduced operating temperature of 200 °C for the 10% NiO/ZnO heterostructures. As shown in Fig. 8e, the resistance of the 10% NiO/ZnO heterostructures sensor is dramatically decreased in ethanol atmosphere at different operating temperatures because an amount of electrons are released back to the 10% NiO/ZnO heterostructures sensor in the redox reaction. It indicated that the 10% NiO/ZnO heterostructures sensor was n-type gas sensing mechanism, corresponding to the result of calculated energy bands. In AC impedance spectra, the resistance of the 10% NiO/ZnO heterostructures sensor can be obtained in air (Fig. 8b) and in ethanol (Fig. 8e), respectively. And the gas response (S = R_air/R_gas) of the 10% NiO/ZnO heterostructures sensor at different temperatures is shown in Fig. 8f. The tendency of the gas response changes similarly at different temperatures in comparison to the result of the gas response instrument in Fig. 6, which gives a further evidence to select the reduced operating temperature for the 10% NiO/ZnO heterostructures.

Fig. 8 (a) Nyquist plots of the ZnO sensor in air at different temperatures and (b) the 10% NiO/ZnO heterostructures sensor in air at investigated temperatures and (c) Z′′/M′′ spectroscopic plots. (d) The Arrhenius plots of conductivity for ZnO and the 10% NiO/ZnO heterostructures. (e) Impedance complex plane plots for the 10% NiO/ZnO heterostructures sensor in 100 ppm ethanol at different temperatures and (f) the relationship between the resistance and the gas response from panel (b) and (e).

Conclusions

The NiO/ZnO p–n heterostructures were synthesized after calcining the precursor by using a hydrothermal method at 90 °C for 12 h, which was confirmed by the results of XRD, SEM, TEM and XPS. According to DC I–V characteristic, the 10% NiO/ZnO heterostructures showed the diode-like behavior in the p–n heterojunctions. In the gas sensing measurement, the gas sensor of the 10% NiO/ZnO heterostructures showed high gas response, good sensor stability and quick response/recovery times even at the reduced operating temperature of 200 °C. Both the theoretical calculation of energy band positions and AC impedance spectra arrive at the same conclusion, the formation of the NiO/ZnO p–n heterostructures is an effective method to improve the gas sensing properties for the ZnO-based materials.

Acknowledgements

This work was supported by the National Natural Science Foundation (51172187), and 111 Program (B08040) of MOE, the Xi'an Science and Technology Foundation (CXY1510-2), the Fundamental Research Funds for the Central Universities (3102014JGY01004) of China.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra19520b

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