ZnTe-coated ZnO nanorods

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ZnTe-coated ZnO nanorods: Hydrogen sul ﬁ de nano-sensor purely controlled by pn junction

Nguyen Minh Hieu

^a

, Do Van Lam

^b,c

, Truong Thi Hien

^a

, Nguyen Duc Chinh

^a

, Nguyen Duc Quang

^a

,

Nguyen Manh Hung

^a,d

, Cao Van Phuoc

^a

, Seung-Mo Lee

^b,c

, Jong-Ryul Jeong

^a

, Chunjoong Kim

^a,

⁎ , Dojin Kim

^a,

⁎

aDepartment of Materials Science and Engineering, Chungnam National University, Daejeon 34134, Republic of Korea

bDepartment of Nanomechanics, Korea Institute of Machinery and Materials, Daejeon 34103, Republic of Korea

cNano Mechatronics, Korea University of Science and Technology, Daejeon 34103, Republic of Korea

dDepartment of Materials Science and Engineering, Le Quy Don Technical University, Hanoi 100000, Viet Nam

H I G H L I G H T S

• Hydrothermal processing for ZnTe coated ZnO-nanorod structures synthesis.

•A nano-composite gas sensor whose response is purely controlled by pn junction.

• Nano-sensor principle showing the maximum response at the depletion depth scale.

•Supporting the depletion model for the conduction type gas sensor device.

G R A P H I C A L A B S T R A C T

a b s t r a c t a r t i c l e i n f o

Article history:

Received 2 January 2020

Received in revised form 3 March 2020 Accepted 4 March 2020

Available online 5 March 2020 Keywords:

ZnO gas sensor H2S gas sensor p-n junction Nanosensor

In this study, the double hydrothermal method is proposed as a facile approach to the synthesis of ZnTe/ZnO core–shell nanorods. The coating thickness of the p-type ZnTe is varied to adjust the junction depth in the n- type ZnO nanorods, and the conductance measurements reveal the change in the conduction path in the heterojunction structures. Structural and chemical investigations conducted using X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy conﬁrm the hetero-nanostructure formation of ZnTe/ZnO. The role of ZnTe in H2S-gas sensing by the ZnO nanorods is discussed. The enhanced sensing performance observed with a thin ZnTe coating conﬁrms the importance of the base resistance of the nano- transducer in achieving high response characteristics. The composite structure also demonstrates a superior sensing performance of good repeatability, stability, linearity, and gas selectivity at temperatures greater than 200 °C.

creativecommons.org/licenses/by-nc-nd/4.0/).Data availability statement:

All the data included in this article are the raw and/or processed data.

⁎ Corresponding authors.

E-mail addresses:[email protected](C. Kim),[email protected](D. Kim).

https://doi.org/10.1016/j.matdes.2020.108628

Contents lists available atScienceDirect

Materials and Design

j o u r n a l h o m e p a g e :w w w . e l s e v i e r . c o m / l o c a t e / m a t d e s

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1. Introduction

H₂S, which is commonly found in nature and industrial environ- ments, is one of the most toxic gases to the human respiratory system [1–6]. Long-term exposure to H2S can cause fatigue, loss of appetite, headaches, irritability, and poor memory; the H₂S threshold limit is 10 ppm for 8 h as speciﬁed by the Association Advancing Occupational and Environmental Health [7]. The detection of H2S in exhaled breath can thus be used for the diagnosis of diseases such as lung disease [8].

Furthermore, a sensitive and selective H2S sensor is required for ensur- ing safety in the chemical and food industries as well as for environmental safety [9]. Therefore, it is necessary to study and develop a highly sensitive gas sensor that can be used to precisely monitor H₂S gas concentrations at lower than the ppm level.

Metal-oxide-based semiconducting gas sensors have attracted im- mense interest owing to their small size, easy and low-cost fabrication, and good sensitivity [10–19]. There have been several studies on H2S- sensing metal oxides, which include WO3[20–22], SnO2[23,24], ZnO [25–27], Co₃O₄[28], CuO [29,30], and Fe₂O₃[31,32]. In particular, hydrothermally synthesized ZnO nanorods showed well-deﬁned surfaces of high crystallinity, and various strategies have been employed to achieve a high gas-sensing performance, such as doping [33], catalytic functionalization [34], the use of composites [35], and heterojunction formation [36]. However, in many cases, we found that the effects are multiply cooperative, and each mechanism or effect could not be separated [37–39].

ZnTe is a direct-bandgap p-type conducting semiconductor with a bandgap energy in the approximate range of 2.1 to 2.26 eV [40]. While it is relatively immune to oxygen adsorption–desorption reactions, its contact with ZnO can result in the formation of a pn heterojunction, which can be used to control the conductance of the ZnO transducer.

However, a thick deposition of ZnTe can prevent the occurrence of contact between the ZnO receptor and analyte gas molecules. In this study, we propose the use of a double hydrothermal method for constructing high-performance one-dimensional p-ZnTe/n-ZnO core–shell nanorods for H2S-gas detection. In the design of the sensor structure, ZnO nanorods were hydrothermally grown to form the receptor and transducer of the sensor body, ZnTe layers were formed on the ZnO nanorods surface through another hydrothermal process via an ion-exchange reaction. The ZnTe played roles of controlling the receptor area for H2S sensing and the transducer resistance via pn junction formation. There- fore, it is basically a ZnO nanostructured sensor controlled by ZnTe at- tachment. As such, we conﬁrm the depletion model of the nanostructured conduction type gas sensors. The structural investigations of the ZnTe/ZnO heterojunctions were correlated with the H2S- gas sensing performance, and the relevant mechanisms were discussed.

2. Experimental

2.1. Synthesis of p-ZnTe/n-ZnO core–shell nanorods

One-dimensional p-ZnTe/n-ZnO core–shell nanorod arrays were synthesized using a double hydrothermal method, which is schematically illustrated inFig. 1. A Zn thinﬁlm was sputter-deposited onto Al₂O₃substrates, which have two Au-electrode line patterns at a dis- tance of 1 mm from each other; this was followed by oxidation at

500 °C to form ZnO seed layers. The solution for the ZnO hydrothermal growth was prepared by mixing 2.4 g Zn(NO3)2·6H2O dissolved in 100 ml of distilled water with 1.12 g hexamethylenetetramine [(CH2)

6N4] (HTMA) dissolved in another 100 ml of distilled water. The solution was stirred continuously (at 300 rpm for 15 min) at room temperature (25 °C) until a clean and transparent solution was obtained, and was thenﬁlled into a plastic syringe. The substrates were submerged in the solution and inserted into a furnace at 90 °C for 4 h. The prepared samples were subjected to an oxidation annealing process at 500 °C to conﬁrm the crystalline ZnO structure.

In addition, 0.05 g Na2TeO3was dissolved in 100 ml of distilled water and stirred continuously (at 300 rpm for 15 min), while 0.125 g NaBH4

was dissolved in 20 ml of distilled water and stirred continuously. Pipets were used to drop different amounts of the NaBH4solution (1 ml, 2 ml, and 5 ml) into a 100 ml Na2TeO3solution to obtain solutions of various Te contents. The slow dropping of the NaBH₄solution into the Na₂TeO₃ solution caused the solution color to gradually change to brown andfi- nally to red. The solution was then transferred to a Teflon-lined stainless autoclave. The substrates of the ZnO nanorod array were dipped into the autoclave and maintained at 95 °C for 2 h to realize the hydrothermal growth of Te over the ZnO nanorods. The Te-coated ZnO nanorod- array samples dipped in solutions of various Te contents were labeled as ZnO-T1, ZnO-T2, and ZnO-T5, corresponding to the 1 ml, 2 ml, and 5 ml NaBH4solutions, respectively. The samples obtained werefiltered and washed several times using deionized water and blow-dried at 80 °C. Finally, the samples were calcined at 350 °C in an argon environment, which resulted in the conversion of Te into ZnTe owing to the Zn obtained from the underlying ZnO. Thus, a series of p-ZnTe/n-ZnO core– shell heterojunction nanorod arrays was obtained. Detailed information regarding the experimental process is presented in Fig. S1.

2.2. Characterization

The surface morphology of the synthesized nanorod arrays was examined usingﬁeld-emission scanning electron microscopy (FE-SEM, JSM 700F; JEOL). High-resolution transmission electron microscopy (JEM-ARM200F, JEOL) was used to examine the crystallinity and junction interfaces. Energy dispersive spectroscopy (EDS) equipped with SEM and TEM were used to examine the elemental distribution. The structure and chemical composition were studied using X-ray diffraction (XRD, Empyrean, PANalytical) with Cu Kαradiation, and Raman spectrometry (Horiba JobinYvon, LabRAM HF-800). An X-ray photoelectron spectroscopy (XPS) measurement was also performed using a Multi lab 2000 instrument with a Mg/Al twin anode source having a maximum power of Mg (300 W) and Al (400 W).

2.3. Gas-sensing property measurements

The gas-sensing properties were measured using a pico-ammeter/

voltage source (Keithley 6487) in a custom-made measurement system [28]. Fig. S2 presents a schematic of the sensor measurement system, which is equipped with a gas-ﬂow vacuum chamber, mass-ﬂow control system, substrate heater, and measuring instruments in interface with a computer. The analyte gases of H2, NH3, H2S, and CH4used were sup- plied by gas cylinders of 1000 ppm diluted in nitrogen. The gases were further diluted in dry air by varying the analyte gas concentration at a

Fig. 1.Schematic of the fabrication of p-ZnTe/n-ZnO core–shell nanorod arrays on the Au-electrode-patterned alumina substrate.

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constant dry-airﬂow of 100 sccm when fed into the test chamber. The gas concentrations were determined using C(ppm) = Cstd(ppm) × f/

(f + F), where f and F are theflow rates of the analyte gas and carrier gas, respectively. Cstd(ppm) is the concentration of the analyte gas in the gas cylinder (1000 ppm). Dry air was used as the carrier gas, and the gasflow rate was controlled by a mass-flow controller. The resistances were measured by applying a 1 V bias between the two Au elec- trodes while blowing dry air (Ro) or analyte gases (Rg) into the chamber.

The response of the n-type doped gas sensor such as ZnO nanorods is deﬁned by S = Ro/Rgfor reducing gases such as H2S, while it is expressed as S =Rg/Rofor oxidizing gases.

3. Results and discussion

3.1. Morphological and structural properties

Fig. 2a–d illustrate the representative SEM images of the fabricated ZnO nanorod-array sample (ZnO-P) and ZnTe/ZnO core–shell nanorod-array samples (ZnO-T1, ZnO-T2, and ZnO-T5). The ZnO nanorods have a hexagonal cross-section of diameter approximately 40 nm (Fig. 2a). ZnO-T1 and ZnO-T2 exhibited roughening surfaces with an increased diameter owing to ZnTe formation on the ZnO nanorods (Fig. 2b and c). The thicker coating of Te in ZnO-T5 was found toﬁll the gaps among the nanorods in addition to the ZnTe formation (Fig. 2d).

XRD patterns of the samples are shown inFig. 2e. ZnO-P revealed three main peaks at 34.36°, 47.5°, and 62.8° corresponding to the diffraction at the (002), (012), and (013) planes of the hexagonal ZnO structure. In the case of ZnO-T1, additional diffraction peaks were observed at 25.6°, 41.8°, 51.8°, and 60.6°, which were ascribed to the (111), (022), (222), and (004) planes of cubic ZnTe, respectively. This is in good agreement with the standard reference code 98–004-1984 of the HighScore Plus software (PANalytical). However, ZnTe formation occurred in a limited range at the Te/ZnO interface, as illustrated by the comparison between ZnO-T2 and ZnO-T5. The (021) peak for Te is only dominantly observed in the case of the ZnO-T2 sample, while many other peaks for Te are visible in the ZnO-T5 sample as the Te coating became thicker. Therefore, ZnO-T1, which had a thinner Te coating, revealed only ZnTe layer formation on the ZnO nanorods (or ZnTe/ZnO pn junction), while ZnO-T2 and ZnO-T5 showed a double-junction formation of Te/ZnTe/ZnO.

A detailed investigation of the main diffraction peak (002) of ZnO showed that the full width at half maximum (FWHM) of the peak increased with the ZnTe formation, as shown in Fig. S3. This indicates that the accumulation of ZnTe formed via ion exchange degraded the crystallinity of the underlying ZnO nanorods. The degradation of the ZnO nanorods was also conﬁrmed by the increasing relative peak

intensities for the (010), (011), (110), and (112) planes of ZnO with respect to the (002) peak intensity, as shown inFig. 2e.

The chemical compositions of the samples were further conﬁrmed by an XPS investigation, as shown inFig. 3.Fig. 3a presents a comparison of the binding energies of Zn 2p for the ZnO-P and ZnO-T1 samples. The negative shift of 0.2 eV of the Zn 2p peak in ZnO–T1 with respect to that in ZnO-P was due to the formation of ZnTe. It can be observed that the peak for ZnO–T1 is deconvoluted into two peaks: one for ZnO and the other for ZnTe.Fig. 3b presents a comparison of the Te 3d peaks of the ZnO–T1 and ZnO–T5 samples having different Te thicknesses. The Te 3d peaks centered at 572.91 and 583.31 eV in ZnO–T1 were assigned to Te²⁻3d5/2and Te²⁻3d3/2from the ZnTe layer formed over the ZnO nanorods. Furthermore, in ZnO–T5, the Te 3d peaks could be deconvoluted into two peaks: one from ZnTe (Te²⁻) and the other from the Te metal. The two peaks at 573.51 eV and 583.98 eV were assigned to Te metal 3d^5/2and Te metal 3d^3/2, respectively. This analysis again clearly conﬁrms the existence of Te metal, which was not con- verted into ZnTe in ZnO–T5. Fig. S4 shows the EDS measurements and also clearly proves that the Te content increases as more drops of the NaBH4solution are added into the solution of Na2TeO3.

The TEM images and corresponding elemental mapping presented inFig. 4further conﬁrm the crystalline nature and distribution of the elements in ZnO and ZnTe. It can be observed that the shiny surface of the ZnO nanorods became undulating after ZnTe formation on the surface, as shown inFig. 4a and b. This observation indicates that the ZnO-to- ZnTe conversion did not occur uniformly over the ZnO nanorod surface.

Nevertheless, the ZnTe layer appeared to cover the entire surface of the ZnO nanorod with a thickness of approximately 10 nm in the case of the thinnest Te coating or the ZnO-T1 sample (Fig. 4a). The interplanar dis- tances of 0.300 nm and 0.255 nm correspond to the (002) lattice plane of ZnTe and (104) lattice plane of ZnO, respectively, which conﬁrms the ZnTe formation (Fig. 4b). While the elemental mapping inFig. 4c–f reveals a uniform distribution of the elements, the relatively spotty image of Te indicates the local distribution of ZnTe. The TEM analyses of the ZnO-T5 sample were shown in Fig. S5, which reveals not only the ZnTe layer but also the Te metal. All the TEM observations coincide with the XRD and XPS measurement results.

3.2. Gas-sensing properties

Fig. 5a illustrates the temperature-dependent resistance of the samples measured in dry air. For all the samples, the decrease in resistance with temperature indicates the semiconducting property of the materials. A remarkable observation is the resistances of the structures at room temperature, which were 17 kΩ, 12 MΩ, 50 kΩ, and 16 kΩfor ZnO-P, ZnO-T1, ZnO-T2, and ZnO-T5, respectively. The dramatic resistance increase in the case of ZnO-T1 is due to the ZnTe/ZnO pn junction

Fig. 2.Plan-view and side-view SEM images of (a) ZnO-P, (b) ZnO–T1, (c) ZnO–T2, and (d) ZnO–T5. (e) XRD patterns of the samples with ZnO peaks (black), ZnTe peaks (red), and Te peaks (pink).

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formation, which results in the formation of depletion layers. Most probably, the thin p-type ZnTe layer was completely depleted of holes, and the electron-depleted space-charge region is also formed on the ZnO-nanorod surfaces, which results in a resistance increase. Mean- while, the thickening of the ZnTe and/or Te layer in ZnO-T2 and ZnO- T5 resulted in a decrease in the resistance owing to the addition of un- depleted conducting layers above the depletion junction. All the electri- cal measurements also coincide with the structural characterization results obtained thus far.

The resultant schematics of the surface structures of the core–shell structures are presented inFig. 5b–e. The relatively low resistance of ZnO-P is the result of a thin surface-depletion layer with a central conduction region in the nanorod (Fig. 5b). When a thin Te layer is coated and calcined, two things occur: one is the conversion of the surface ZnO into ZnTe accompanying a thinning of the ZnO nanorod, and the other is ZnTe/ZnO pn-junction formation with depletion regions on both sides. The thin p-type ZnTe shown inFig. 4a may be almost depleted owing to the contact with the n-type ZnO. The reduction in conduction of ZnO-T1 by more than two orders of magnitude, as shown in Fig. 5a, corresponds to the greatly reduced conduction region in the ZnO nanorod shown inFig. 5c. The thicker deposition of Te in ZnO-T2 may or may not extend the depletion region in the ZnO and ZnTe layers, but an increased conduction in the structure due to an added p-type conduction path through ZnTe (and Te) is clearly observed. The XRD examina- tion (Fig. 2e) showed that a small amount of unreacted Te remained on top of the ZnTe layer in the case of ZnO-T2, as schematically illustrated inFig. 5d. An even thicker Te coating would further increase the rem- nant Te-layer thickness, thus resulting in a further lowered resistance (Fig. 5e).

Fig. 6shows the response–recovery curves of the sensors toward 100 ppm H2S measured at various temperatures up to 350 °C, which is the maximum temperature that the measurement system can reach.

The resistance variation of the sensors is shown in Fig. S6. As the sensing

performance was observed at high temperatures of greater than 200 °C, the chemical routes for H2S-gas sensing are governed by the interplay of the following adsorption/desorption and combustion reactions [41].

O2ð Þ þg 2e⁻→2O⁻_ad ð1Þ

H2S gð Þ þ3O⁻_ad→H2O gð Þ þSO2ð Þ þg 3e⁻ ð2Þ In the response cycle, Eqs.(1) and (2)occur concomitantly, and in the recovery cycle, only Eq.(1)occurs. The observations ofFig. 6can thus be summarized as follows.

i) The highest response of ZnO to H2S was ~17 at 350 °C. The response level increased to ~70 in the case of the thin ZnTe coating in ZnO-T1, but decreased in the case of a thicker ZnTe coating.

ii) As the Te-coating thickness increased, the temperature for the highest response increased.

iii) The response rate was high, i.e., less than 1 min at temperatures greater than 300 °C, in the case of pure ZnO, but kept decreasing as the Te coating became thicker, and exhibited degrading response kinetics.

In addition to the response level changes as a function of the structure and temperature inFig. 6, we further analyzed observation iii) by plotting the normalized response–recovery curves of the sensors at 350 °C inFig. 7to reveal the deteriorating sensing kinetics in the response and recovery cycles with the increase in the ZnTe (and Te) thickness.

It should be noted that there were no reports on the use of ZnTe for H2S sensing, and we actually observed that pure Te exhibited no sensing signal toward H2S gas. These facts led to observations i) and ii), because H2S reacts only with the O^–adsorbed on the ZnO surface while ZnTe

a

1014 1017 1020 1023 1026 1029 ZnO

ZnO-P ZnO - T1

ZnTe

ZnO 0.2 eV

Zn 2p

570 575 580 585 590 595

Te2-

Te 3d ZnO - T1 Te2-

Te metal Te2-

Te metal

ZnO - T5

b

Fig. 3.XPS proﬁles for (a) Zn 2p from ZnO-P and ZnO–T1 and (b) Te 3d from ZnO–T1 and ZnO–T5.

Fig. 4.(a, b) TEM images and (c–f) elemental mapping of ZnO-T1.

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(and Te) did not show detectable oxygen ionosorption on the surface.

The latter result was derived from the inert calcination environment of argon. Therefore, it is necessary to distinguish the enhanced sensing performance in the case of thin ZnTe (ZnO-T1) and the degrading performance in the case of thicker ZnTe (ZnO-T2 and ZnO-T5).

A clear decrease in the response and recovery rates can be observed in the case of the thicker ZnTe in contrast to the thin ZnTe sample of ZnO-T1. The decreasing response/recovery kinetics as well as the degrading response levels with the thick ZnTe layer can be explained by the role of the layer as a diffusion barrier for the H2S (for response) and oxygen gases (for recovery), which must diffuse in and out through the ZnTe layer and/or the ZnTe/ZnO interface to reach and escape from the ZnO receptor surface. As shown inFig. 5c, for ZnO-T1, the ZnTe coating covers a part of the ZnO surface and reduces the area for the combustion reaction of Eq. (2) and delay the interaction with the impinging H2S gas molecules at the ZnTe/ZnO interface, thus resulting

in fewer H₂S molecules interacting with O^–at the ZnO surface/interface.

Both reduced surface area and diffusion effect will reduce the sensing signal via a reduced charge exchange reaction (or smallerΔR=Ro– Rg). The resistance of ZnO-T5 sensor decreased approaching that of ZnO-P owing to the thick ZnTe (and Te) layer (Fig. 5a), but its response value is more thanﬁve times lower than that of ZnO-P because of a less gas reaction rate due to the blocking effect of ZnTe (and Te) toward impinging gas molecules.

In addition to the change in the gas reaction rate on the receptor surface, the contact of ZnTe with ZnO forms pn junctions and increases the resistance of the ZnO nanorod structure (R_o), as shown inFig. 5a. The thin p-ZnTe cluster may be totally depleted of holes, while the electron depletion depth in n-ZnO has increased resulting in the three orders of magnitude increased sensor resistance. This condition is illustrated in Fig. 5c, which reveals the reduced neutral region depth in ZnO corresponding to an increased transducer resistance (orRo) in the case of RT 100 150 200 250 300 350

10² 10³ 10⁴ 10⁵ 10⁶ 10⁷

Resistance ( )

Temperature

(

o C

)

ZnO-P ZnO-T1 ZnO-T2 ZnO-T5

(a)

(b) ZnO-P (c) ZnO-T1

substrate ZnO

ZnO neutral ZnO

depleted ZnTe depleted

Te ZnTe

Fig. 5.(a) Resistance versus temperature of the sensor structures. (b–e) Schematic of junction structures of ZnO-P, ZnO-T1, ZnO-T2, and ZnO-T5 core–shell structures.

Fig. 6.Response curves to 100 ppm H2S of the sensors (a) ZnO-P, (b) ZnO-T1, (c) ZnO-T2, and (d) ZnO-T5.

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ZnO-T1. Since the sensor response is the ratio of the two resistances (S =Ro/Rg), the response is determined by both the gas reaction rate on the receptor surface and the pn junction formation. WhenRois relatively high, the modulation of the depletion depth can be far high for a given surface reaction frequency as exhibited by the highly enhanced response shown inFig. 6b. We have repeatedly observed that such high sensing signal in nano structures could be obtained with high resistances of mega ohm ranges, in which the sensor dimension may approach the depletion depth scale [37,42–45]. Therefore, a pn junction can be a route to realization of high performance chemoresistive gas sensors via controlling the conduction of nano structures.

The optimum temperature (which is deﬁned herein as the temperature for the maximum response level) has shifted from 350 °C for ZnO-P to 200 °C for ZnO-T1, as shown inFig. 6a and b. The reverse shift of the optimum temperature to higher temperatures of 250 °C and 300 °C for ZnO-T2 and ZnO-T5, respectively, with a continued decrease in the response level (Fig. 6c and d) was also observed. Recall that the electron

concentration, the depletion depth, and consequently, the transducer resistance is a function of temperature. These apparent shifts of the optimum temperature at different ZnTe thicknesses is again an artefact caused by the combined effect of the combustion reaction frequency controlled by the ZnTe thickness and the resistance of the sensor determined by the pn junction depletion depth in ZnO.

In conclusion, the role of ZnTe (and Te) is only to control the resistance of the ZnO receptor and transducer by forming a pn junction with the underlying ZnO. ZnTe played neither of a role of a receptor nor that of a catalyst to aid in the adsorption and/or interaction of H2S on ZnO. In addition, the un-depleted ZnTe and Te acted as a conducting transducer to give loweredRoresulting in the degraded response levels.

For the thick ZnTe (and even with unreacted Te above the ZnTe layer;

Fig. 5d and e), ZnTe further hindered gas diffusion into the underlying ZnO resulting in the delayed response (Fig. 7). The incomplete recovery with thicker ZnTe (Fig. 7) can be also explained similarly by the delayed diffusion of oxygen into the ZnTe/ZnO interface. The negligible change in the response/recovery rates of ZnO-T1 with respect to ZnO-P suggests indicates that the thin ZnTe coverage did not effectively hinder the gas diffusion to and from the ZnO surface.

Furthermore, we must mention that the conventional Schottky barrier model to explain the conductive gas sensing behaviors may not explain the results inFig. 6such as the apparent optimum temperature shifting with ZnTe thickness. Since ZnTe was formed later on the surface of the ZnO nanorods, the currentﬂows directly from a ZnO nanorod to another ZnO nanorod (see the morphology ofFig. 2), and ZnTe only con- trols the resistance of the ZnO nanorods. Unless ZnTe is formed at the ZnO-ZnO interface, the Schottky barrier at the ZnO-ZnO interface will not be affected by the ZnTe formation. Since all the Schottky barrier heights are the same for a given temperature and an interface area re- gardless of the ZnTe coating, the changing optimum temperature among the samples cannot be explained by the Schottky barrier model, but by the depletion model at least qualitatively.

The repeatability, concentration dependence, and selectivity of the sensors were examined at 200 °C and are summarized inFig. 8. The

5 10 15 20

ZnO-P ZnO-T1 ZnO-T2 ZnO-T5

@100ppm H2S/350^oC

Fig. 7.Normalized response–recovery curves to 100 ppm H2S measured at 350 °C for comparing the response and recovery kinetics of the sensors. The arrows indicate the delayed kinetics for the thicker ZnTe layers.

Fig. 8.(a) Dynamic resistance change in the ZnO-T1 sensor to 100 ppm H2S at 200 °C. (b) Responses of the ZnO-T1 sensor measured while varying the H2S concentration. (c) H2S- concentration-dependent response levels measured while varying the H2S concentrations for ZnO-P, ZnO-T1, and ZnO-T2. (d) Gas selectivity measured at 200 °C upon exposure to various interfering gases: 100 ppm H2S, CH4, H2, and NH3; 2000 ppm acetone and methanol; and 7000 ppm CO.

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measured repeatability for a 100 ppm concentration was excellent (Fig. 8a). The concentration-dependent response–recovery curves of ZnO-T1 measured at 200 °C in the range of 0.25 to 200 ppm are presented inFig. 8b, wherein it can be observed that a sub-ppm level H2S concentration could be detected by the ZnO-T1 sensor. These responses are summarized inFig. 8c, which also includes the measurement results obtained for ZnO-P and ZnO-T2.Fig. 8d reveals the gas-sensing response of the sensors for 100 ppm of H2S. These responses were compared with the responses measured at 200 °C for 100 ppm of CH4, H2, and NH3; 2000 ppm of acetone and ethanol; and 7000 ppm of CO. The high selectivity for H2S gas can be easily conﬁrmed by the comparison plot.

The performance of the ZnTe/ZnO pn heterojunction sensor for H2S detection is compared with the previously reported ZnO-based H2S chemoresistive gas sensors listed inTable 1. Our sensor structure com- prises a ZnO nanorod with a sensor response that was maximized only via conductance control through the pn-junction formation. The sensor structure was fabricated via a facile and cost-effective approach, and its performance was similar to that of other ZnO-based sensors at a low operation temperature. It can detect hundreds of ppb-level H₂S concentrations at temperatures greater than 200 °C.

4. Conclusions

ZnTe-coated ZnO-nanorod structures were fabricated via the facile and cost-effective double-hydrothermal-growth method. The deposition of Te followed by heat treatment in Ar ambient resulted in the conversion of Te into ZnTe, which covered ZnO to form a pn junction. The thick Te deposition produced a thicker ZnTe layer and unreacted Te layer, which only hindered the diffusion of gas molecules into and out of the ZnO receptor surface without intervening in the chemical reactions occurring on the receptor; this resulted in deteriorated sensor sig- nals and kinetics. However, a thin ZnTe layer can control the ZnO nanorod sensor resistance via pn junction formation and contribute to an enhanced sensing response without degrading the sensing kinetics.

We thus demonstrated a pn junction formation can be an effective route for improving the sensor performance by adjusting the sensor conductance. We further conﬁrmed the principle of the conduction type gas sensor particularly of nanostructured morphology, where the physicochemical gas reaction rate on the receptor surfaceandthe elec- trical transducer conductance determine the sensing levels. In addition, we conﬁrmed that the depletion model can better explain the sensing results of nanostructures than the Schottky barrier model.

Authors contribution

NMH conceived of the sensor structures, performed the most of the experiments, measurement, and analyses. He also wrote the manuscript. CVP and JRJ drew some of the plots with discussion, TTH, NDC, NMH and NDQ carried out materials preparation and SEM examinations. DVL and SML supported XRD and FWHM analyses with high qual- ity examinations. DK supervised the project and revised the manuscript via discussion with CJK.

Declaration of competing interest

We have no competing interests to declare regarding the work reported in this paper.

Acknowledgments

This work was supported by the National Research Lab (NRF- 2018R1A2A1A05023126) of the National Research Foundation of Korea.

Appendix A. Supplementary data

Supplementary data to this article can be found online athttps://doi.

org/10.1016/j.matdes.2020.108628.

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Table 1

H2S-sensing behaviors of ZnO-based chemoresistive gas sensors.

Materials Concentration

(ppm)

Response Optimum temperature (°C)

Reference

CuO/ZnO 100 62 250 [46]

CuO/ZnO 20 0.42 50 [47]

CuO/ZnO 5 0.65 250 [48]

ZnO tetrapods 5 100 300 [49]

ZnO nanorod bundle

50 30 500 [41]

ZnTe/ZnO nanorod 100 ~70 200 This work

(8)

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