Co₃O₄-ZnO P-N Heterostructure Nanomaterials Film and its Enhanced Photoelectric Response to Visible Lights at Near Room Temperature

Abstract

In this paper, Co₃O₄-ZnO nanomaterials with Co₃O₄ doping mass fractions of 0%, 2.13%, 4.13%, and 6.13% were prepared by sol-gel method. In order to explain and confirm the influence of the incorporation of Co₃O₄ on the surface morphology and gas sensitivity of ZnO at a relatively low gas concentration, additional studies such as XRD, XPS, SEM, EDS and UV-vis spectroscopy were performed. And its photoelectric response to 100 ppm acetone at near room temperature and visible light irradiation was studied. Due to the formation of P-N heterojunctions, the Co₃O₄-ZnO heterostructural nanoparticles has a highe response to low concentrations of acetone gas than undoped ZnO nanoparticles even at operating temperatures as low as 30ºC. The addition of Co₃O₄ improves the sensitivity and selectivity of ZnO thick films. The sensitivity of the 4.13wt% Co₃O₄-ZnO sample to 100 ppm acetone at a working temperature of 30ºC was 24.36. The light excitation effect was significantly enhanced. Under visible light irradiation, the sensitivity can reach 37.21. In addition, the Co₃O₄-ZnO P-N heterojunction model was combined with visible light excitation theory to further explore the mechanism of gas sensing reaction.

Keywords:
Zinc oxide; Acetone; Photoelectric response; Gas sensitivity; Heterojunction

1. Introduction

Metal oxide semiconductor materials have attracted wide attention from researchers as sensitive materials for detecting toxic and harmful gas ¹. Among these materials, zinc oxide is one of the earliest discovered and widely studied materials. which is a typical wide band gap n-type direct band gap semiconductor material with lower resistivity and higher light trapping properties ².Due to zinc oxide's wide band gap (3.37 eV at room temperature), it is necessary to increase the operating temperature to obtain better gas sensing characteristics, usually 250 to 400ºC. However, high temperatures not only lead to high power consumption, but also limit the detection of flammable and explosive gases, and it is also a challenge to the stability of the device ^3,4. Therefore, in order to lower the operating temperature of the sensor and increase the sensitivity, a large number of scientific papers on the use of zinc oxide as a gas sensor have been published. For example, many of precious metal elements, rare earth elements or metal compounds were doped into zinc oxide. The preparation of Tb-doped zinc oxide samples by Anita Hastir et al. significantly improved the sensitivity to ethanol and acetone ⁵. Qi Xu's Cu-doped ZnO nanorods prepared by the hydrothermal method have a response of 8 to 50 ppm TEA at a low temperature of 40ºC, and the response time and recovery time are 5 s and 25 s, respectively ⁶. A.P. Rambu et al. used magnetron sputtering deposition to obtain 3% (mass fraction) Co doped ZnO nanofibers ⁷. The best operating temperature for 600 ppm methane was 170ºC and the sensor was stable for a long time. Moreover, the formation of semiconductor heterostructures has been proven to be an effective way to improve gas-sensing properties, such as sensitivity and especially lower operating temperatures. For example, wang et al. synthesized CuO/ZnO nanorod hybrids have a sensitivity of up to 40 for 100 ppm H₂S at a working temperature of 100ºC ⁸. Xing Gao et al. reported that Co₃O₄-ZnO core-shell heterostructure NFs can detect 100 ppm of formaldehyde gas at 220ºC ⁹.

In addition, a large number of studies have found that UV radiation can greatly reduce the operating temperature of the sensor, leading to improved performance of ZnO-based photoelectric gas sensors. However, there are few articles on the application of visible light to ZnO gas sensors. Therefore, the improvement of the gas-sensitivity of zinc oxide sensors by visible light is one of the focuses of this experiment.

In this paper, different cobalt-doped precursors were prepared by simple sol-gel method, and different doped zinc oxide powder samples were produced by high temperature treatment. The effects of cobalt doping on the crystal structure and morphology of zinc oxide were investigated. The role of P-N heterojunction in sensor signal formation was analyzed, and the influence of visible light excitation on the gas sensitivity of CZO was studied. The mechanism of gas-sensing reaction was further explored.

2. Experimental

2.1 Co-doped ZnO composites and devices

A source of Zn(CH₃COO)₂·2H₂O, NaOH, CoCl₂.6H₂O as a raw material. Different ratios of zinc acetate and cobalt chloride were dissolved in a mixture of anhydrous ethanol and deionized water in equal proportions. An aqueous NaOH solution was prepared, and an aqueous NaOH solution was slowly added to a mixed solution of cobalt chloride and zinc acetate until the pH was equal to 7, and then the sol was obtained by magnetic stirring at room temperature. The stirring speed was 100r/min and the stirring time was 2h. The sol was aged at room temperature for 24 hours, the sample was placed in an 80ºCinfrared oven and dried to obtain gel. The gel was ground, washed and annealed to finally obtain Co₃O₄-ZnO powder. In the current work, we prepared four different doping amounts of ZnO powder, respectively 0 wt%, 2.13 wt%, 4.13 wt%, and 6.13 wt%.

To make a thick-film sensor, the above samples were each added with a proper amount of deionized water to each agate mortar, and then ground for about 10 minutes to form a paste. Dipping method was used to prepare different heat-dosed thick-film type gas sensors on the surface of Al₂O₃-based ceramic tube.

2.2 Characterization of ZnO films

The crystal structure of Co₃O₄-ZnO was analyzed by X-ray diffraction (XRD, Bruker AXS D8 FOCUS, Germany) with Cu-Kα1 radiation (λ =1.5406 Å) in the 2θ range of 20º-80º with 0.02º/s scanning step. The microstructures were analyzed by scanning electron microscopy (SEM, ZEISS Sigma500, Germany). The element surface distribution of Zinc oxide and cobalt were determined by Oxford X-MaxN150 EDS. Chemical binding analysis was conducted by an X-ray photoelectron spectrograph (XPS, ESCALAB MKK II) operated using Mg as the exciting source. The diffuse reflectance spectra of pure ZnO and 4.13wt% Co₃O₄-ZnO were measured by UV-vis spectrophotometer (Shimadzu, UV-2600, Japan) with a scanning range of 220-850 nm and a scanning speed of 0.5 nm /s.

2.3 Gas sensing properties testing

In this work, the sensitivity of the gas sensors was tested with the HW-30B gas sensor test system.The schematic of test circuit was shown in Fig. 1.

Figure 1
Schematic of gas sensing test circuit

In the above circuit, Vh represents the voltage of the heating circuit, which controls the working temperature.Vc is the circult voltage, Vout is the output voltage, and R1 is the load resistance that matches the component resistance. In this experiment, the gas detection is completed by taking a change in resistance when the sensor is in contact with the detected gas. Thesensitivity is defined as S.

(1) $\mathit{S}=\raisebox{1ex}{${\mathit{R}}_{\mathit{a}}$}\!\left/ \!\raisebox{-1ex}{${\mathit{R}}_{\mathit{g}}$}\right.$

R_a is the resistance of sensor in air; R_g is the resistance of sensor in target gas.

3. Result and Discussion

3.1 Structure and morphology

The crystal structure of Co₃O₄-ZnO was analyzed by X-ray diffraction (XRD). The XRD pattern of the different sample materials is shown in Fig. 2. All diffraction peaks can point to hexagonal (wurtzite) ZnO, which is well consistent with the JCPDS: #36-1451. In addition, the peak areas and intensities of the four samples vary with the different doping amount of Co₃O₄. The strong and sharp diffraction peak of 4.13wt% Co₃O₄-ZnO indicates that the product has high crystallinity¹⁰. The characteristic diffraction peak of the cobalt oxide crystal wasn't detected by XRD experiment ,and possible reason is that the content of Co₃O₄ in the sample is too low to be detected¹¹.

Figure 2
XRD patterns of Co₃O₄-ZnO with different doping concentration

Fig. 3a-d shows SEM images of different doping amounts of Co₃O₄-ZnO samples. It can be seen from the figure that the sample is a hexagonal prism structure, and the Co-doped ZnO material shows a rough and porous structure. Especially when the doping amount is 4.13wt%, the particles of the sample are more uniform and the surface is looser and more porous, and these changes in microscopic morphology caused by doping facilitate the adsorption and desorption of gases. It can be seen from Fig. 4 that the average crystal grain size of Co₃O₄-ZnO is significantly smaller than the average crystal grain size of pure ZnO, and the doping of Co can inhibit the agglomeration of crystal grains and limit the further growth of ZnO crystal grains.

Figure 3
SEM images of Co₃O₄-ZnO nanocomposites with different doping concentration a) ZnO b) 2.13wt% Co₃O₄-ZnO c) 4.13wt% Co₃O₄-ZnO d) 6.13wt% Co₃O₄-ZnO

Figure 4
The average crystal grain size of four samples in SEM images

X-ray photoelectron spectroscopy (XPS) was used to characterize and analyze the surface composition and structure of 4.13wt% Co₃O₄-ZnO sample. Fig. 5a shows the Zn 2p spectrum, there were two symmetric peaks in the spectra. One centered at 1020.9 eV corresponded to a Zn 2p_3/2 configuration and another one centered at 1044.0 eV was assigned to a Zn 2p_1/2 configuration ¹². The energy difference between these two peaks is 23.1 eV, which is consistent with the corresponding value of the standard ZnO nanoparticles ¹³, indicating that the Zn in the sample is in the +2 valence state. Fig. 5b shows the O1s spectrum of 4.13wt% Co₃O₄-ZnO sample. The peak centered at 529.9 eV should be assigned to surface lattice oxygen (Olat), the peak at 531.3eV could be generated by the oxygen species adsorbed on the ZnO surface ¹⁴. Fig. 5c shows the Co2p_3/2 spectrum of 4.13wt% Co₃O₄-ZnO sample. According to the literature ¹⁵, the sample decomposition Co 2p spectrum includes the contributions of Co³⁺ and Co^{2 +} and Co²⁺ satellites. Comparing the sample's Co2p_{3 / 2} binding energy with standard spectra ^15,16, we can designate the main peak near 779.9 eV as the Co^{3 +}_3/2 configuration, and the other main peak near 781.2 eV as Co^{2 +}_{3/ 2} configuration. The small peak at 786.5 eV is the Co^{2 +} rocking satellite peak. The surface area ratio of Co2p_{3 / 2} peaks of Co³⁺ and Co²⁺ is close to 2, corresponding to the formula Co^{2 +} (Co^{3 +})₂O₄¹⁷.

Figure 5
Zn 2p (a), O 1s (b) and Co 2p_2/3 (c) XP spectra of 4.13wt% Co₃O₄-ZnO sample

The results in Fig. 6 indicate the elemental spatial distribution of 4.13wt% Co₃O₄-ZnO nanocomposites. 4.13wt% Co₃O₄-ZnO nanocomposites have agglomerated cobalt-containing particles around their dispersed ZnO, which may be Co₃O₄. Co₃O₄ particles are fully exposed to ZnO and forms P-N heterojunction contact.

Figure 6
The EDS layered image pattern of 4.13wt% Co₃O₄-ZnO nanocomposites

3.2 Gas sensitive properties of Co₃O₄-ZnO P-N heterostructure nanocomposites

The sensitivity of ZnO with different doping concentrations was studied by using acetone as the gas to be tested at room temperature (30ºC). Fig. 7a shows the response and recovery curves of four doping concentrations pure ZnO, 2.13wt% Co₃O₄-ZnO, 4.13wt% Co₃O₄-ZnO, and 6.13wt% Co₃O₄-ZnO to 100 ppm acetone. The sensitivity are 6.5, 12.16, 24.36, and 14.22, respectively, indicating that doping Co₃O₄ can improve the sensitivity of ZnO sensor. Compared with undoped ZnO sensors (16 and 7 s, respectively), the response and recovery time of the 4.13wt% Co₃O₄-ZnO sample to 100 ppm acetone (Fig. 7b) were also only 4 seconds and 3 seconds, respectively. The results show that the addition of Co₃O₄ can reduce the response and recovery time of ZnO sensors. When Co₃O₄ is doped in ZnO, the band tail of the Co₃O₄ energy band enters the valence band of ZnO, which makes the band gap narrower and the electrons more prone to transition. Moreover, the addition of Co₃O₄ makes the surface of the sample more loose and porous, providing more active sites for the adsorption of gas molecules and oxygen molecules, thereby improving sensitivity.

Figure 7
Response and recovery curves for 100 ppm acetone at an operating temperature of 30ºC a) Samples of four different doping concentrations b) 4.13wt% Co₃O₄-ZnO sample

However, the sensitivity will decrease when the doping concentration is too high. The possible reason for sensitivity reduction is that ZnO has less contact with oxygen or target gas. The barrier height and resistivity will increase as the additional Co₃O₄ accumulates in the lattice gap or grain boundary of ZnO which hinders the contact¹⁸. Another possible reason will be discussed later.

Fig. 8 depicts the responses of pure ZnO sample and 4.13wt% Co₃O₄-ZnO sample to 100 ppm acetone at 30ºC as well as other gases such as toluene, ammonia, ethylene glycol, and methanol. And the gas sensing properties are shown in Table 1.

Figure 8
Selectivity of the 4.13wt% Co₃O₄-ZnO and pure ZnO sample to 100 ppm of various gases at 30ºC

Table 1
Gas sensing properties of 4.13wt% Co₃O₄-ZnO and puer ZnO to different gases (100 ppm)

Target gas	Gas response(S)		Response time/s		Recovery time/s
	4.13wt% Co3O4-ZnO	ZnO	4.13wt% Co3O4-ZnO	ZnO	4.13wt% Co3O4-ZnO	ZnO
Acetone	24.36	6.50	4	16	3	7
Toluene	11.58	8.85	4	14	2	8
Ammonia	2.87	0.96	6	22	6	26
Ethylene glycol	7.96	7.16	5	11	4	15
Methanol	4.34	2.45	7	19	2	15

It can be easily seen that the 4.13wt% Co₃O₄-ZnO sample always shows a higher response than the pure ZnO in all cases. Its response to acetone is 24.36. It is higher than other gases and shows good selectivity to acetone.

Fig. 9 shows the transient response and recovery curves of 4.13wt% Co₃O₄-ZnO sensor for different concentrations of acetone at an operating temperature of 30ºC. It can be seen from the figure that as the acetone concentration increases, the sensor response value also increases, and when the acetone concentration is as low as 10 ppm, there is also a significant response with a response value of 2.55.

Figure 9
Transient response and recovery curves of 4.13wt% Co₃O₄-ZnO sample for different concentrations of acetone

The long-term stability of ZnO and Co₃O₄-ZnO samples to 100 ppm acetone was tested at an operating temperature of 30ºC (Fig. 10). The results show that both sensors have good long-term stability. Compared to the previously reported sensing properties of Co₃O₄-ZnO materials (Table 2), low operating temperatures and relatively high sensitivity make this work very valuable.

Figure 10
Long-term stability of the ZnO and 4.13wt% Co₃O₄-ZnO to 100 ppm acetone at 30 ºC

Table 2
Sensing performances of Co₃O₄-ZnO gas sensors reported in literatures to VOCs.

Materials	Gas	Concentration /ppm	Temperature /ºC	Response(S)	References
mesoporous ZnO/Co3O4 microspheres	acetone	50	275	29	19
Pd@Co3O4-ZnO	ethanol	200	240	54	20
porous ZnO/Co3O4 nanocomposites	acetone	1	275	15.17	21
Co3O4/ZnO nanocomposites	formaldehyde	100	170	20	11
Co3O4-ZnO core-shell nanofibers	formaldehyde(acetone)	100	220	5.1(22)	9
Co3O4-ZnO P-N heterostructure nanomaterials	acetone	100	30	24.36	This work

3.3 Photoelectric gas sensing properties of Co₃O₄-ZnO P-N heterostructure nanocomposites

In order to study optical properties, diffuse reflectance spectra of pure ZnO and 4.13wt% Co₃O₄-ZnO were measured in the wavelength range of 220-850 nm, as shown in Fig. 11. It can be seen that the absorption of visible light by 4.13wt% Co₃O₄-ZnO is greater than that of pure ZnO.

Figure 11
Reflectance spectrum of pure ZnO and 4.13wt% Co₃O₄-ZnO

The energy gap was calculated by kubelka-Munk formula. (Give a graph:[ F(R)×E]^)1/2-E, F(R) is the kubelka-Munk function, F(R)=(1-R)²/2R, R is the reflectivity; E is the energy, E=1240/λ, the unit is eV; lengthen the part of line in the curve, intersecting with the X axis, the point is the value of energy gap.) As shown in Fig. 12, the band gap energies estimated from the intercept of the tangents to the plots are 3.13 eV and 2.11 eV for pure ZnO and 4.13wt% Co₃O₄-ZnO nanocomposites, respectively. It indicates that the incorporation of Co₃O₄ leads to a narrow band gap of the material. Since electronic transitions require the effect of thermal excitation, sensors made of materials with narrower band gaps have lower operating temperatures. In addition, reducing the width of the band gap can increase the utilization of visible light.

Figure 12
Energy band gap of pure ZnO and 4.13wt% Co₃O₄-ZnO

In order to investigate the influence of illumination conditions on the gas sensing properties of the components, Co₃O₄-ZnO samples with different doping amounts were irradiated with visible light with a wavelength greater than 420 nm, and their sensitivity to acetone (100 ppm) was tested. The result is shown in Fig. 13. Clearly, visible light illumination significantly increases the gas response of the gas sensing material, and the incorporation of Co₃O₄ results in a significant increase in photovoltaic gas sensing performance. In particular, when the doping amount of Co₃O₄ is 4.13wt%, the photoexcitation effect reaches the best with 37.21 gas response

Figure 13
Sensitivity of Co₃O₄-ZnO sample with and without light

Fig. 14 shows gas-sensing curves of pure ZnO samples prepared with and without visible light exposureat for 100 ppm acetone at an operating temperature of 30ºC. It can be seen from the figure that the response time of pure ZnO gas sensitive material under visible light irradiation is greatly reduced compared with pure ZnO gas sensitive material without visible light irradiation, but the sensitivity is not significantly improved. This is due to defects in ZnO, such as Zn vacancies, oxygen vacancies, Zn gaps, and oxygen gaps ^22,23.

When visible light is turned on, electrons will be excited by defects, and the electrons generated will reach the conduction band of ZnO, resulting in a decrease in response time. At the same time, the height of the grain boundary barrier will lower and the sensitivity will increase. However, since the amount of electrons excited by visible light associated with weak defects is limited, the sensitivity of pure ZnO under visible light irradiation is very small. When the acetone gas is exhausted, the response cannot immediately return to the initial value. This is because the recombination probability and rate of electrons and holes in pure ZnO are slow, resulting in an increase in recovery time.

Figure 14
Recovery curve of pure ZnO in response to 100 ppm acetone with and without visible light exposure

Fig. 15 compares the gas sensitivity curves for 4.13wt% Co₃O₄-ZnO with and without visible light illumination. Compared with no visible light irradiation, the gas response of 4.13wt% Co₃O₄-ZnO to the 100ppm acetone with visible light irradiation is significantly improved. The reason is that compared with pure ZnO, the band gap of 4.13wt% Co₃O₄-ZnO sample is narrower and the utilization of visible light is higher. In addition, the surface morphology of 4.13wt% Co₃O₄-ZnO is loose and porous, and the more pores, the more visible light the material captures ²⁴.

Figure 15
Recovery curve of 4.13wt% Co₃O₄-ZnO in response to 100 ppm acetone with and without visible light

4. Gas Sensing Mechanism

Co₃O₄ is p-type semiconductor withnarrow band gap energy of 1.6 eV and work function of 6.1 eV. As an n-type semiconductor, ZnO has wide band gap energy (3.37 eV) and work function of 5.2 eV at near room temperature. This can be seen from Fig. 16. When Co₃O₄ nanoparticles are adorned onto the surface of ZnO nanoparticles, due to Fermi level-mediated charge transfer^8,25,26, will lead to the for-mation of a P-N heterojunction at the interface between the p-Co₃O₄ nanoparticles and n-ZnO nanoparticles^27,28. The electrons in ZnO will be transferred to the Co₃O₄ nanoparticles, which significantly reduces the charge conduction channel and results in a higher resistance state than pure ZnO.

When the ZnO nanoparticles are heated by exposure to air, oxygen molecules are adsorbed on the surface of the ZnO nanoparticles. These chemisorbed oxygen trap electrons from the ZnO conduction band and form adsorbed oxygen ions, resulting in changes in carrier concentration and material resistance. Due to the low operating temperature of the sensor, oxygen ions mainly exist in the form of O₂^-. The adsorption process for n-type ZnO is as follows ^29,30:

(2) ${\mathit{O}}_2\left(g\right)\to {\mathit{O}}_2\left(\mathrm{ads}\right)$

(3) ${\mathit{O}}_2\left(\mathrm{ads}\right)+{\mathit{e}}^{-}\to {\mathit{O}}_2^{-}\left(\mathrm{ads}\right)$

For Co₃O₄, since it is a p-type semiconductor, the adsorption process is as follows⁹:

(4) ${\mathit{O}}_2\left(g\right)\to {\mathit{O}}_2^{-}\left(\mathrm{ads}\right)+{\mathit{h}}_{*}$

Obviously, the capture of electrons in ZnO leads to an increase in the barrier between ZnO nanoparticles, while the accumulation of holes in Co₃O₄ leads to a decrease in barrier. However, since Co₃O₄ is less in cobalt-doped ZnO, the conductivity of the ZnO and p-n heterojunction plays a major role. The resistance of pure ZnO and 4.13wt% Co₃O₄-ZnO in air at room temperature was 18384 ohms and 24689 ohms, respectively.

When the Co₃O₄-ZnO sensor is exposed to a certain concentration of acetone gas, the gas is oxidized by the negatively charged oxygen ions, and the electrons trapped by the O₂^- ion are automatically released back to the conduction band of ZnO, resulting in an increase in electron concentration in the conduction band and a significant decrease in the sensor resistance, the reaction occurred as follows:

(5) $\begin{array}{c}{\left({\mathit{CH}}_3\right)}_2\mathit{CO}\left(g\right)+4{\mathit{O}}_2^{-}\left(g\right)\to \hfill \\ 3{\mathit{CO}}_2\left(g\right)+3{\mathit{H}}_2\mathit{O}\left(l\right)+4{\mathit{e}}^{-}\hfill \end{array}$

In addition, the acetone gas also releases electrons into the p-type Co₃O₄ nanoparticles, recombines with their holes, thinning the interface depletion layer, and lowering the interface barrier height (Fig. 17a). Therefore, the resistance of the Co-doped zinc oxide based sensor is further reduced than that of the original ZnO sensor.

It is worth noting that after the acetone gas is introduced, part of the Co³⁺ is reduced to Co²⁺³¹. The reaction mechanism is as follows:

(6) $\begin{array}{c}2{\mathit{Co}}_3{\mathit{O}}_4+{\left({\mathit{CH}}_3\right)}_2\mathit{CO}\left(g\right)+3{\mathit{O}}_2\left(g\right)\to \hfill \\ 6\mathit{CoO}+3{\mathit{CO}}_2\left(g\right)+{\mathit{H}}_2\mathit{O}\left(l\right)\hfill \end{array}$

In the gas detection process, acetone molecules are directly oxidized on the surface of Co₃O₄ particles. This leads to the decrease in the quantity of acetone molecules which can react with the oxygen chemisorbed on ZnO surface, and consequently to the decrease in sensor signal of Co₃O₄-ZnO comparing with pure ZnO. However, the introduction of the Co₃O₄ impurity level greatly enhances the carrier concentration, and the reduction in the sensor signal will be offset by the rapid increase in carrier concentration. In addition, Co doping forms a new degeneracy band, resulting in a band gap narrowing effect, which makes it easier for electrons to excite from the valence band to the conduction band, adsorbing more surface oxygen species, leading to higher Ra.

With the increase of the amount of Co₃O₄ doping, Co₃O₄ will be transformed into the main conduction path. At this moment, the holes accumulation of Co₃O₄ in the air will not be ignored, which may cause the performance of the sensor to decrease. Therefore, the loading of Co₃O₄ nanoparticles should be strictly controlled to obtain better sensor response.

When visible light is irradiated on the surface of the sensor, the band gap of the Co₃O₄-ZnO sample is greatly reduced due to the addition of Co₃O₄³². Therefore, Co₃O₄-ZnO samples are more easily excited by visible light under visible light irradiation. High-energy photons are absorbed by the Co₃O₄-ZnO sample and produce electron-hole pairs. As the following equation shows:

(7) $\mathit{h}\mathit{\nu }\to {\mathit{h}}^{+}+{\mathit{e}}^{-}$

This process increases the carrier concentration and the conductivity of the nanocomposite. With the bandgap matching and influence of the heterojunction between Co₃O₄ and ZnO, the photogenerated electrons in the Co₃O₄-ZnO sample will migrate into ZnO, while the photogenerated holes will transfer to Co₃O₄, thus leading to the broadening of the Co₃O₄-ZnO interface depletion layer, and the resistance of the Co₃O₄-ZnO sensor is further increased. In addition, photogenerated electrons diffuse to the surface of ZnO and react with the oxygen molecules adsorbed on the surface. The reaction is as follows ³³:

(8) ${\mathit{O}}_2+{\mathit{e}}^{-}\left(\mathit{h}\mathit{\nu }\right)\to {\mathit{O}}_2^{-}\left(\mathit{h}\mathit{\nu }\right)$

This process effectively suppresses the recombination between photo-generated carriers and produces more photo-oxidizing ions than pure ZnO. When exposed to acetone gas, acetone reacts with adsorbed oxygen ions and photooxygen ions on the ZnO surface. The electrons generated by the reaction are reintroduced into the conduction band of ZnO.

(9) $\begin{array}{c}{\left({\mathit{CH}}_3\right)}_2\mathit{CO}\left(g\right)+4{\mathit{O}}_2^{-}\left(\mathit{h}\mathit{\nu }\right)\to \hfill \\ 3{\mathit{CO}}_2\left(g\right)+3{\mathit{H}}_2\mathit{O}\left(l\right)+4{\mathit{e}}^{-}\hfill \end{array}$

This can be seen from Fig. 17b. Therefore, this interfacial charge transfer can effectively promote the separation of electrons and holes, and further increase the number of electrons in ZnO, leading to more significant changes in resistance and improved sensitivity.

Figure 16
a) The energy band diagram of p-Co₃O₄ and n-ZnO b) the energy band diagram for P-N junction of Co₃O₄-ZnO nanocompoosites with a depletion layer at the interface

Figure 17
The scheme of P-N heterojunction models of both ZnO and Co₃O₄-ZnO nanocompoosites with and without visible light irradiation

5. Conclusion

In summary, the heterostructure Co₃O₄-ZnO thick film was successfully prepared by sol-gel method, and its gas sensitivity was studied. The doping of Co₃O₄ leads to an increase in the crystallinity of the Co₃O₄-ZnO sample and the formation of a p-n heterojunction, which leads to its more pronounced modulation of the resistance at the surface of the material. In the experiment, it is observed that Co₃O₄--doped ZnO thick-film sensors show higher sensitivity to acetone and better selectivity than undoped ZnO thick-film sensors at low temperatures. The sensitivity of 4.13wt% Co₃O₄-ZnO to 100 ppm acetone is 3.6 times higher than that of pure ZnO and the response recovery time is also reduced at room temperature 30ºC. In addition, optical excitation can enhance gas sensing characteristics and lower operating temperatures. The sensitivity of 4.13wt% Co₃O₄-ZnO under visible light was 1.52 times higher than that without visible light irradiation. Therefore, the doping of Co₃O₄ and the visible light illumination have a synergistic effect in improving the gas sensing characteristics of the sensor, further improving the sensitivity of the sensor and reducing the operating temperature of the element.

6. Acknowledgements

This work was supported by the National Natural Science Foundation of Tianjin (No. 17JCTPJC54500). The authors are also grateful to the Key Laboratory of Electronic Materials and Devices of Tianjin, China.

7. References

• ¹
  Schmidt-Mende L, MacManus-Driscoll JL. ZnO - nanostructures, defects, and devices. Materialstoday 2007;10(5):40-48.
• ²
  Barsan N, Koziej D, Weimar U. Metal oxide-based gas sensor research: How to? Sensors and Actuators B: Chemical 2007;121(1):18-35.
• ³
  Pisarev RV, Pavlov VV, Kalashnikova AM, Moskvin AS. Near-band gap electronic structure of the tetragonal rare-earth cuprates R₂CuO₄, and the bismuth cuprate Bi₂CuO₄ Physical Review B 2010;82(22):224502.
• ⁴
  Neri G, Bonavita A, Rizzo G, Galvagno S, Capone S, Siciliano P. Methanol gas sensing properties of CeO₂-Fe₂O₃ thin films. Sensors and Actuators B: Chemical 2006;114(2):687-695.
• ⁵
  Hastir A, Kohli N, Singh RC. Temperature dependent selective and sensitive terbium doped ZnO nanostructures. Sensors and Actuators B: Chemical 2016;231:110-119.
• ⁶
  Xu Q, Ju D, Zhang Z, Yuan S, Zhang J, Xu H, et al. Near room-temperature triethylamine sensor constructed with CuO/ZnO P-N heterostructural nanorods directly on flat electrode. Sensors and Actuators B: Chemical 2016;225:16-23.
• ⁷
  Rambu AP, Iftimie N, Nica V, Dobromir M, Tascu S. Efficient methane detection by Co doping of ZnO thin films. Superlattices & Microstructures 2015;78:61-70.
• ⁸
  Wang L, Kang Y, Wang Y, Zhu B, Zhang S, Huang W, et al. CuO nanoparticle decorated ZnO nanorod sensor for low-temperature H2S detection. Materials Science & Engineering: C 2012;32(7):2079-2085.
• ⁹
  Gao X, Li F, Wang R, Zhang T. A formaldehyde sensor: Significant role of p-n heterojunction in gas-sensitive core-shell nanofibers. Sensors and Actuators B: Chemical 2018;258:1230-1241.
• ¹⁰
  Peng C, Guo J, Yang W, Shi C, Liu M, Zheng M, et al. Synthesis of three-dimensional flower-like hierarchical ZnO nanostructure and its enhanced acetone gas sensing properties. Journal of Alloys and Compounds 2016;654:371-378.
• ¹¹
  Liu Y, Zhu G, Chen J, Xu H, Shen X, Yuan A. Co₃O₄/ZnO nanocomposites for gas-sensing applications. Applied Surface Science 2013;265:379-384.
• ¹²
  Choi SW, Katoch A, Sun GJ, Kim JH, Kim SH, Kim SS. Dual functional sensing mechanism in SnO₂-ZnO core-shell nanowires. ACS Applied Materials & Interfaces 2014;6(11):8281-8287.
• ¹³
  Moulder JF, Chastain J, King RC. Handbook of X-ray Photoelectron Spectroscopy: A Reference Book of Standard Spectra for Identification and Interpretation of XPS Data. Eden Prairie: Physical Electronics; 1995.
• ¹⁴
  Chen M, Wang X, Yu YH, Pei ZL, Bai XD, Sun C, et al. X-ray photoelectron spectroscopy and auger electron spectroscopy studies of Al-doped ZnO films. Applied Surface Science 2000;158(1-2):134-140.
• ¹⁵
  Tan BJ, Klabunde KJ, Sherwood PMA. XPS studies of solvated metal atom dispersed (SMAD) catalysts. Evidence for layered cobalt-manganese particles on alumina and silica. Journal of the American Chemical Society 1991;113(3):855-861.
• ¹⁶
  McIntyre NS, Cook MG. X-ray photoelectron studies on some oxides and hydroxides of cobalt, nickel, and copper. Analytical Chemistry 1975;47(13):2208-2213.
• ¹⁷
  Fu L, Liu Z, Liu Y, Han B, Hu P, Cao L, et al. Beaded Cobalt Oxide Nanoparticles along Carbon Nanotubes: Towards More Highly Integrated Electronic Devices. Advanced Materials 2005;17(2):217-221.
• ¹⁸
  Ng ZN, Chan KY, Low CY, Kamaruddin SA, Sahdan MZ. Al and Ga doped ZnO films prepared by a sol-gel spin coating technique. Ceramics International 2015;41(Suppl 1):S254-S258.
• ¹⁹
  Zhang L, Jing X, Liu J, Wang J, Sun Y. Facile synthesis of mesoporous ZnO/Co3O4 microspheres with enhanced gas-sensing for ethanol. Sensors and Actuators B: Chemical 2015;221:1492-1498.
• ²⁰
  Sun Y, Wang Z, Wang W, Li G, Li P, Lian K, et al. Electrospinning preparation of Pd@ Co₃O₄-ZnO composite nanofibers and their highly enhanced VOC sensing properties. Materials Research Bulletin 2019;109:255-264.
• ²¹
  Xiao J, Diao K, Zheng Z, Cui X. MOF-derived porous ZnO/Co₃O₄, nanocomposites for high performance acetone gas sensing. Journal of Materials Science: Materials in Electronics 2018;29(10):8535-8546.
• ²²
  Jeong D, Kim K, Park SI, Kim YH, Kim S, Kim SI. Characteristics of Ga and Ag-doped ZnO-based nanowires for an ethanol gas sensor prepared by hot-walled pulsed laser deposition. Research on Chemical Intermediates 2014;40(1):97-103.
• ²³
  Sarkar D, Khan GG, Singh AK, Mandal K. Enhanced Electrical, Optical, and Magnetic Properties in Multifunctional ZnO/a-Fe₂O₃ Semiconductor Nanoheterostructures by Heterojunction Engineering. Journal of Physical Chemistry C 2012;116(44):23540-23546.
• ²⁴
  Li Y, Li DL, Liu JC. Optical and gas sensing properties of mesoporous hollow ZnO microspheres fabricated via a solvothermal method. Chinese Chemical Letters 2015;26(3):304-308.
• ²⁵
  Miller DR, Akbar SA, Morris PA. Nanoscale metal oxide-based heterojunctions for gas sensing: A review. Sensors and Actuators B: Chemical 2014;204:250-272.
• ²⁶
  Xu Z, Duan G, Li Y, Liu G, Zhang H, Dai Z, et al. CuO-ZnO Micro/Nanoporous Array-Film-Based Chemosensors: New Sensing Properties to H₂S. Chemistry - A European Journal 2014;20(20):6040-6046.
• ²⁷
  Ge L, Jing X, Wang J, Wang J, Jamil S, Liu Q, et al. Trisodium citrate assisted synthesis of ZnO hollow spheres via a facile precipitation route and their application as gas sensor. Journal of Materials Chemistry 2011;21(29):10750-10754.
• ²⁸
  Lou Z, Li F, Deng J, Wang LL, Zhang T. Branch-like Hierarchical Heterostructure (a-Fe₂O₃/TiO₂): A Novel Sensing Material for Trimethylamine Gas Sensor. ACS Applied Materials & Interfaces 2013;5(23):12310-12316.
• ²⁹
  Mondal B, Basumatari B, Das J, Roychaudhury C, Saha H, Mukherjee N. ZnO-SnO₂, based composite type gas sensor for selective hydrogen sensing. Sensors and Actuators B: Chemical 2014;194:389-396.
• ³⁰
  Zeng Y, Zhang T, Yuan M, Kang M, Lu G, Wang R, et al. Growth and selective acetone detection based on ZnO nanorod arrays. Sensors and Actuators B: Chemical 2009;143(1):93-98.
• ³¹
  Rumyantesva MN, Vladimirova SA, Vorobyeva NA, Giebelhaus I, Mathur S, Chizhov AS, et al. p-CoOx/n-SnO₂ nanostructures: New highly selective materials for H₂S detection. Sensors and Actuators B: Chemical 2018;255(Pt 1):564-571.
• ³²
  Wang X, Zheng R, Liu Z, Ho HP, Xu J, Ringer SP. Structural, optical and magnetic properties of Co-doped ZnO nanorods with hidden secondary phases. Nanotechnology 2008;19(45):455702.
• ³³
  Panda SK, Jacob C. Preparation of transparent ZnO thin films and their application in UV sensor devices. Solid State Electronics 2012;73(14):44-50.

Publication Dates

• Publication in this collection
  11 Apr 2019
• Date of issue
  2019

History

• Received
  23 Oct 2018
• Reviewed
  04 Jan 2019
• Accepted
  09 Mar 2019