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Functional Inorganic Materials and Devices

Porous GaN Submicron-rods for Gas Sensor with High Sensitivity and Excellent Stability at High Temperature zhang mingxiang, Changhui Zhao, Huimin Gong, Gaoqiang Niu, and Fei Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b09769 • Publication Date (Web): 19 Aug 2019 Downloaded from pubs.acs.org on August 23, 2019

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ACS Applied Materials & Interfaces

Porous GaN Submicron-rods for Gas Sensor with High Sensitivity and Excellent Stability at High Temperature Mingxiang Zhang a, Changhui Zhao a, Huimin Gong a, Gaoqiang Niu a, Fei Wang a,b* a School

of Microelectronics, Southern University of Science and Technology, Shenzhen

518055, China b GaN

Device Engineering Technology Research Center of Guangdong, Southern

University of Science and Technology, Shenzhen 518055, China

ACS Paragon Plus Environment

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ABSTRACT

Highly porous GaN submicron-rods have been synthesized successfully by a facile hydrothermal method and heat treatment under controlled atmosphere. The morphology and size of the hydrothermal products are tailorable by adjusting the concentration of precursor solutions. Upon calcination in air, nanorod-assembled GaOOH submicron-rods are converted to bundle-like Ga2O3, and then converted to porous GaN submicron-rods under ammonia flow. Gas-sensing characterization demonstrates that the sensors based on porous GaN exhibit high sensitivity and fast response to ethanol vapor, as well as excellent stability and reliability at high temperature. The highly porous GaN submicronrods with large specific surface area, small grain size, and high length-to-diameter ratio show better response to ethanol. A possible sensing enhancement mechanism is also proposed. This study provides a promising route for the novel synthesis of GaN submicron-rods for high-performance gas sensors. KEYWORDS : hydrothermal, porous GaN submicron-rods, gas sensors, hightemperature, high sensitivity


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1. Introduction With the rapid development of industrialization and urbanization, gas sensors have attracted more and more attention because of their high sensitivity and extensive applications. And the applications include emission monitoring for industrial plants, indoor and outdoor environmental pollution detection, and breath analysis

1-2.

Over the past

decades, great efforts have been made to develop metal oxide semiconductor-based sensitive materials which exhibited excellent performance and good sensitivity to many gases 3-8. Recently, with the development of automation, mechanization and technology, the third generation semiconductor materials have received wide attention by scientists. GaN has been widely applied in research and in the design of light emitter device 9, high temperature piezoelectronics

10,

ultraviolet detectors

11-12,

high-temperature and high-

power electronic devices13-15, which depends on its wide direct-bandgap (~3.4 eV), high thermal conductivity, and high breakdown electric field, etc. During the past few years, GaN has also attracted increasing attention in the field of gas sensing under harsh environment (high temperature and corrosive ambient) thanks to its high mechanical, thermal and chemical stabilities. Zhong et al 16 developed H2 gas sensor


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through a monolithically integrated nano-sensor array on a GaN honeycomb nanonetwork, which combined molecular beam epitaxy method with micromachining techniques. The response/recovery time was shortened by a factor of 3 at 75 °C. Hsu et al 17 fabricated a Pd/AlGaN/GaN-based field effect transistor and studied the H2 sensing properties, and the sensor showed good performance under a low H2 concentration of 10-ppb. Hermawan et al. reported a GaN hydrogen gas sensor, which was pasted precisely on the comb-type electrode. The sensor exhibited excellent stability at high temperature (T > 400 °C) with rapid detection of hydrogen

18.

GaN-based high electron

mobility transistors (HEMT) have also been fabricated to detect trace amounts of different gases (e.g. H2, H2S, and NH3), which mainly use the effect of the two-dimensional electron gas 19-22. Furthermore, GaN schottky diodes have been reported with good gas sensitivity 23-25.

Above all, it can be seen that GaN could be a promising material to fabricate gas sensors with

high performance.

However, both the preparation of GaN material and the manufacturing of devices mentioned above demand complicated design and costly process. Among the various types of gas sensors, the resistive gas sensors still dominate the market because of low


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cost, high sensitivity, and simplicity of operation. The introduction of porous structure usually improves the sensing performance with considerably faster response and higher sensitivity than compact material for gas sensor applications. In this regard, a controllable hydrothermal reaction was adopted to prepare GaN submicron-rods. The gas sensors based on porous GaN submicron-rods show great potential for ethanol detection with high sensitivity and high-temperature stability.

2. Results and discussion 2.1. Characterization of the materials Figure 1 illustrates the preparation process of porous GaN submicron-rods. The GaOOH precursors were synthesized by a simple hydrothermal route. According to previous reports, the pH of the mixed solution plays a crucial role in shaping the morphology of GaOOH 26-27. In this work, the reaction pH is fixed at around 9, while the concentration of gallium salt increases by 7‒15 times in precursor solution. After the subsequent heat treatment in air and ammonia, the GaOOH was expected to transform to Ga2O3 and GaN, successively. To verify this transformation, XRD patterns of GaOOH and Ga2O3 are shown in Figure S1. All the strong diffraction peaks in Figure S1a can be indexed well to


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orthorhombic GaOOH (JCPDS No. 26-0674). In addition, the diffraction peaks in Figure S1b match well with the crystal structure of rhombohedra α-Ga2O3 (JCPDS No. 06-0503), indicating all the GaOOH precursors have been converted into α-Ga2O3 after calcination at 500 °C for 2 h in air.

Figure 1. Schematic diagram of formation process for porous GaN submicron-rods.

Figure 2 shows the typical SEM images of the GaOOH precursors and the corresponding Ga2O3 calcined in air. It can be seen that all the hydrothermal products are bundle-like GaOOH submicron-rods, and the GaOOH submicron-rods are assembled by numerous ultrathin nanorod subunits (Figure 2a‒d). Obviously, the adding amount of Ga(NO3)3·9H2O plays a key role in tailoring morphology during the nucleation and growth processes of GaOOH. The length-to-diameter ratio of GaOOH submicron-rods increases


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with the increasing amount of gallium salt, which reaches to its maximum value at 6.0 g (Figure 2c), and decreases with further addition of the gallium salt (8.0 g, Figure 2d). Especially, we have noticed that the length of GaOOH submicron-rods increases gradually with the increase of gallium concentration. After calcination in air at 500 °C, the GaOOH precursors were fully transformed to α-Ga2O3, which has been confirmed by XRD data (Figure S1b). Although the thermal oxidation treatment in air has a little effect on the size of Ga2O3 submicron-rods (Figure 2e‒h), a number of pores are produced on the surface of nanorod subunits based on the enlarged SEM images. Meanwhile, the cracks between adjacent nanorods become more pronounced, which can provide more channels for the diffusion of ammonia gas in the following nitridation process.


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Figure 2. SEM images of GaOOH precursors prepared with different amounts of Ga(NO3)3•9H2O: (a) 0.5 g, (b) 4.0 g, (c) 6.0 g and (d) 8.0 g, and the corresponding Ga2O3 calcined in air, (e) Ga2O3-1, (f) Ga2O3-2, (g) Ga2O3-3 and (h) Ga2O3-4. The insets are the enlarged SEM images for the respective samples.

Figure 3 shows the XRD patterns of four GaN products synthesized from the rapid nitridation process in ammonia atmosphere. Obviously, only the diffraction peaks of wurtzite GaN (JCPDS No. 50-0792) can be found in these curves. According to the Scherrer’s formula

28,

the mean grain sizes of GaN products calculated from the (101)

plane are about 15.2, 12.1, 11.4, and 13.0 nm, corresponding to GaN-1, GaN-2, GaN-3, and GaN-4, respectively. Moreover, no evidence of any other phases such as Ga2O3 is detected in Figure 3. It can be confirmed that the Ga2O3 intermediates have been completely converted to GaN phase.


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Figure 3. XRD patterns of GaN-based products.

The SEM images of the as-prepared GaN products with different adding amounts of Ga(NO3)3·9H2O (GaN-1, GaN-2, GaN-3, and GaN-4) are displayed in Figure 4. All the products maintain the basic architecture of submicron-rods. From the high magnification SEM images, the GaN products surface is rough and porous compared to the Ga2O3 shown in Figure 2. The nitridation process promotes conversion of the Ga2O3 nanorod subunits to numerous neck-connected GaN nanoparticles. It is worth noting that these polycrystalline porous GaN submicron-rods are quite different from nanoparticle aggregates. The average length-to-diameter ratio of GaN-3 shown in Figure 4c is about 4.90 (~0.41 μm in diameter), which is the highest ratio among the four GaN products. For


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GaN-1 (Figure 4a, ~0.51 μm in diameter), GaN-2 (Figure 4b, ~0.42 μm in diameter) and GaN-4 (Figure 4d, ~0.54 μm in diameter), the length-to-diameter ratios are about 1.51, 2.24, and 3.93, respectively.

Figure 4. SEM images of GaN products: (a) GaN-1, (b) GaN-2, (c) GaN-3, (d) GaN-4. The insets are the enlarged SEM images for the respective samples.

TEM observation was carried out to further clarify the microstructure of porous GaN-3 submicron-rods, as shown in Figure 5. The GaN-3 submicron-rods with uniform diameters


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exhibit relatively rough edges (Figure 5a and b). A magnified TEM image (Figure 5c) confirms that the GaN-3 rod has a highly porous architecture composed of neckconnected nanoparticles, which agrees well with the SEM results (Figure 4c). In addition, the (100) plane of wurtzite GaN with a d-spacing of 0.278 nm is observed in the lattice fringes (Figure 5d), revealing its high crystallinity.

Figure 5. (a, b) TEM and (c, d) HRTEM images of GaN-3 submicron-rods.


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Figure 6. N2 adsorption-desorption isotherms of GaN products. The inset shows the corresponding pore size distributions.

To further investigate the pore structure of the GaN products, N2 adsorption-desorption analyses have been carried out at 77 K, as illustrated in Figure 6. All the samples exhibit type-IV isotherms with hysteresis loops type H3, indicating the existence of mesoporous structure. The specific surface area of the GaN-3 is about 40.8 m2 g-1, which is much higher than the other GaN samples. The specific surface areas of GaN-1, GaN-2 and GaN-4 are 20.5 m2 g-1 (GaN-1), 28.1 m2 g-1 (GaN-2), and 19.4 m2 g-1 (GaN-4), respectively. The inset Figure 6 gives the corresponding pore size distributions, and the average pore diameters are about 16.5 nm (GaN-1), 10.6 nm (GaN-2), 11.5 nm (GaN-3)


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and 22.9 nm (GaN-4), respectively. In the case of GaN-3, most of the pores distribute in the range of 10‒50 nm, which is possibly ascribed to the incompact surface of porous GaN submicron-rods. 2.2. Gas sensing performance The GaN with porous structure was successfully applied to ceramic tube devices. As reported in our previous work

29,

the structure schematic diagram of the gas sensor and

the schematic diagram of the measurement chamber are shown in Figure S2 and Figure S3, respectively. Gas-sensing properties of the as-prepared porous GaN submicron-rods are shown in Figure 7. Figure 7a demonstrates the sensor response to 100 ppm ethanol at various operating temperatures (340‒440 °C). The response increases with the increase of operating temperature, reaching a maximum value, and then decreases with further increase of temperature. Among them, GaN-3 sensor shows the highest response value of 19.3 to 100 ppm ethanol at its optimum temperature (360 °C). By contrast, GaN1, GaN-2 and GaN-4 sensors achieve their highest responses at 380 °C, and the corresponding values are 7.5, 13.4 and 15.4, respectively. Consequently, the gas-sensing


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properties of GaN-based sensors were measured at their own optimum temperatures hereafter in this work.

Figure 7. Gas-sensing properties of GaN-based sensors: (a) response vs. temperature to 100 ppm ethanol; (b) responses of the sensors to various ethanol concentrations (5– 5000 ppm) at their own optimum temperatures; (c) transient response curves at different ethanol concentrations (5–200 ppm), the inset is the low-concentration range shown in (b); (d) response/recovery times of GaN-3 sensor to 100 ppm ethanol.


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Figure 8. (a) Long-term stability and (b) high-temperature stability of GaN-3 sensor to ethanol. Inset (a) shows the reproducibility to 100 ppm ethanol (10 cycles).

As can be seen from Figure 7b, the response value rises rapidly with the increasing concentration (5‒5000 ppm) of ethanol vapor. The sensor based on porous GaN-3 submicron-rods exhibits the best performance. It is clearly seen that the sensor does not saturate until 5000 ppm of ethanol, suggesting a wide detection range. Moreover, the inset Figure 7c shows nearly linear relationships within the ethanol concentration range from 5 to 200 ppm. It is worth noting that the relationship between sensor response is GaN-3 > GaN-4 > GaN-2 > GaN-1, when the ethanol concentration exceeds 50 ppm (50‒5000 ppm). The corresponding transient response curve in Figure 7c quickly goes to a plateau profile after injecting a certain concentration of ethanol, which may be attributed


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to the quick mass transport of target gases in the porous structures 30. For GaN-3 sensor, the responses are 2.1, 2.7, 4.6, 10.1, 19.3, and 38.5 to 5, 10, 20, 50, 100, and 200 ppm of ethanol, respectively. That is much higher than the response of many metallic oxides 31-34.

Simultaneously, the response of GaN has almost achieved the same level compared

with the ZnO at 200 ppm ethanol concentration

35.

What’s more, Luo et al detected the

ethanol sensing properties based porous GaN nanofibers which prepared by electrospinning

36.

The response is less than 10 for 100 ppm ethanol at optimal

temperature of 320 °C. It is clear that the response of GaN porous nanorods is higher than that of GaN nanofibers which may be attributed to high specific surface area of GaN porous nanorods. The ethanol gas response of the reported sensors is listed in Table 1 for comparison. Table 1. Ethanol sensing performance based on various metal oxide nanostructures reported before and the GaN submicron-rods presented in this work. Materials

Optimum temperature (°C)

Ethanol concentration (ppm)

Response at optimum temperature

Response time (s)

Recovery time (s)

Ref.

200

100

~ 12

-

-

32

260

100

~ 10

-

-

33

NiO NanorodFlowers In2O3 Nanoparticles


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50

-

11

24

Fe2O3 Porous Network

400

100

9.02

3

138

31

SnO2 Nanowires

400

100

10.5

2

136

37

ZnO Nanorod

320

200

~ 35

54

61

35

Porous GaN Nanofiber

320

100

8.5

8

5

36

GaN SubmicronRods

360

100

19.3

2

42

200

38.5

-

-

This work

Figure 7d shows the resistance change of GaN-3 sensor in the presence of 100 ppm ethanol. The response and recovery times are about 2 s and 42 s, respectively. The response/recovery time of GaN-1, GaN-2, and GaN-4 are shown in Figure S4. It can be clearly seen that the response characteristics are correlated with the diameter, morphology and the concentration of oxygen species 38. The sensor based on GaN-3 submicron-rods with the smallest diameter and largest specific surface area, exhibits fast response time as compared to other sensors. This can be ascribed to the enhanced effective surface area of porous structure because both the inner and outer surfaces of the GaN-3 are involved in the gas sensing reaction. Therefore, the porous GaN-3 can react with more gas


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molecules than GaN-1, GaN-2 and GaN-4, resulting in fast response. In order to evaluate the stability of the device, the GaN-3 sensor has been tested at 360 °C to 100 ppm ethanol vapor that continued for 7 weeks (Figure 8a). It can be seen that the GaN-3 sensor demonstrates excellent long-term stability, with a low fluctuation of about 10% during the test. As seen in the inset of Figure 8a, the GaN-3 sensor also exhibits rapid, reproducible, and reversible switching ability to 100 ppm ethanol during the 10-cycle test. To further study the high-temperature stability of the GaN-3 sensor, as shown in Figure 8b, the operating temperature is tuned from 380 °C to 440 °C. We can find that a stable and reliable sensor response can be achieved at the same concentration of ethanol, even at the highest operating temperature of 440 °C, which allows us to verify the robust nature of the GaN devices at high temperatures.


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Figure 9. Selectivity of GaN-based sensors to various gases. With respect to practicality, the selectivity of a gas sensor is also of great importance. Figure 9 shows the selectivity of the GaN-based sensors to various reducing gases at a fixed concentration of 100 ppm. All the sensors present the highest response to ethanol, and are almost insensitive to ammonia, benzene and toluene. A tiny difference of the selectivity could be observed among the four sensors (Figure S5), which might come from the surface morphology variation

39.

In general, the selectivity of the gas sensor means

the preferential chemiresistive sensing for a particular gas in the presence of another gas under same operating conditions

40.

However, It is usually very difficult to achieve an

absolutely selective material gas sensor which specifically sensitive to one compound, and most of the materials possess cross-sensitivity 41-42. It is still a challenge to improve the selectivity of gas sensors. Various methods have been reported, for instance, bulk/surface doping decreasing the grain size

44,

designing heterostructure

(nanowire

47,

nano-sheet

46,

nanoparticles

nano-network 48-49.

6

45,

43,

preparing special structures

and so on) and functionalizing noble metal

These methods have been well developed and applied for metal-oxide

materials, which could be promising to improve the gas selectivity for the GaN sensing.


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2.3. Gas-sensing mechanisms

Gas-sensing mechanisms of the GaN-based sensors to ethanol are illustrated in Figure 10. Similar to n-type metal oxide semiconductors 50, the GaN-based sensors show a rapid decrease in resistance toward ethanol vapor. Therefore, the sensor response can be explained by the reaction between the adsorbed oxygen species on the GaN surface and the ethanol molecules

51.

As shown in Figure 10a, oxygen molecules will adsorb on the

surface of GaN and diffused along the porous structure. Then, the adsorbed oxygen molecules subsequently capture electrons to form oxygen species (e.g. O2‒, O‒ and O2-) when exposing to air shown in Fig 10b. This leads to the generation of an electron depletion layer (EDL) on the near surface of GaN materials with decreasing carrier concentration, which increases the sensor resistance. When the ethanol vapor is injected into the chamber, as illustrated in Figure 10c, the oxygen species will react with ethanol molecules, and release trapped electrons back to the GaN, resulting in the narrow EDL width and a decrease of sensor resistance. The mainly reaction occurs on the surface of


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porous GaN submicron-rods at high temperatures (340‒440 °C) may be explained by the following Equation 19, 52-53:

O2 (gas)

O2 (ads),

O2 (ads) + e- (ads) O2- (ads) + eO- + e-

(1) O2- (ads) RT< T < 100 °C,

(2)

2O- (ads) 100 °C< T

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