β-Ga2O3 nanowires and thin films for metal oxide semiconductor gas sensors: Sensing mechanisms and performance enhancement strategies

Journal of Materiomics xxx (xxxx) xxx

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b-Ga2O3 nanowires and thin films for metal oxide semiconductor gas sensors: Sensing mechanisms and performance enhancement strategies Adeel Afzal 1
degli Studi di Bari “Aldo Moro”, Via Orabona 4, 70126, Bari, Italy Dipartimento di Chimica, Universita

a r t i c l e i n f o

a b s t r a c t

Article history: Received 23 April 2019 Received in revised form 29 July 2019 Accepted 8 August 2019 Available online xxx

The reliable, selective, and fast detection of the inorganic and organic gases in indoor and outdoor air and industrial processes is a huge challenge for environmental sustainability, healthier life, and disease control and diagnosis. Metal oxides have been frequently explored as highly sensitive receptor elements in the electronic gas sensors since the 1960s. Gallium oxide (Ga2O3), often recognized as one of the widest-bandgap semiconductors, has shown tremendous potential as the inorganic gas receptor because of its extraordinary chemical and thermal stability, and excellent electronic properties. This article presents a comprehensive reference on the electrical properties, historical developments, detection mechanisms, and gas sensing performance of Ga2O3 nanowires and composite nanostructures. In particular, the relationships between composition, nanostructure, and gas sensing properties of galliumcontaining oxidic nanomaterials such as b-Ga2O3 nanowires, surface-modified Ga2O3, metal-doped Ga2O3 or Ga-doped metal oxides, and Ga2O3/metal oxide composite heterostructures are studied. The applications of Ga2O3 gas sensors are discussed with an emphasis on their practical limitations such as high-temperature operation, power consumption, and miniaturization issues. Finally, future research directions and potential developments are suggested. © 2019 The Chinese Ceramic Society. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

This article is dedicated to the memory of Prof. Klaus-Dieter Kohl (1945e2017). Keywords: b-Ga2O3 nanowires Gas sensors Nanomaterials Semiconductors Sensing mechanisms Thin films

Contents 1.

2. 3.

4. 5.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1. Electrical properties of b-Ga2O3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2. History of b-Ga2O3 gas sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . b-Ga2O3 gas sensing mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Gas sensing b-Ga2O3 nanowires and composite oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . b-Ga2O3 nanowires and thin films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. 3.2. Surface-modified b-Ga2O3 nanostructures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1. Noble metal-decorated b-Ga2O3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.2. Metal oxide-coated b-Ga2O3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Metal-doped b-Ga2O3 nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Ga-doped metal oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5. Composite Ga2O3/metal oxide heterostructures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Limitations and challenges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Funding sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conflicts of interest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00

E-mail address: [email protected]. Peer review under responsibility of The Chinese Ceramic Society. Present address: Department of Chemistry, College of Science, University of Hafr Al Batin, PO Box 1803, Hafr Al Batin, 39524, Saudi Arabia. 1

https://doi.org/10.1016/j.jmat.2019.08.003 2352-8478/© 2019 The Chinese Ceramic Society. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).

Please cite this article as: Afzal A, b-Ga2O3 nanowires and thin films for metal oxide semiconductor gas sensors: Sensing mechanisms and performance enhancement strategies, Journal of Materiomics, https://doi.org/10.1016/j.jmat.2019.08.003

2

A. Afzal / Journal of Materiomics xxx (xxxx) xxx

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

1. Introduction Over the past 30 years, the development of miniaturized electronic gas sensors that can detect very low concentrations (
100 ppm to 0.1 ppb level) of the toxic gases as well as oxygen in the indoor/outdoor environment and automotive/industrial processes have remained the focus of research [1e15]. These electronic devices often incorporate semiconducting metal oxides as the sensitive element due to their high sensitivity and low cost. Semiconducting metal oxides exhibit prompt changes in their electrical properties as a response to slight variations in the surrounding gaseous atmosphere. Thus, gas sensors based on metal oxide semiconductors can provide fast, inexpensive, sensitive, and often reliable detection of various gases if the sensitive material (i.e. metal oxide) is chosen carefully and possesses the essential structural and electronic properties. A comparison of metal oxides’ structural and electrical properties reveals that certain metal oxides, e.g. gallium oxide (Ga2O3), offer the unparalleled trade-offs such as excellent structural stability at very high temperatures, and they have tremendous potential for ideal gas sensing applications in harsh environments [16e18]. Herein, an overview of Ga2O3-based semiconductor gas sensors is presented. The semiconducting Ga2O3 has a wide bandgap (Eg ¼ 4.9) and a moderate free carrier concentration (1014e1018 cm1) [19e22]. Ga2O3 offers excellent chemical resistance and is thermally stable at high temperatures, i.e. above 1000  C [23,24]. It exists in several polymorphic forms, but all polymorphs are metastable and transform into the most stable monoclinic Ga2O3 or b-Ga2O3 at temperatures above 750  C [22,25]. b-Ga2O3 belongs to C2/m space group [26e28] and once formed, it is very much stable at all temperatures below the melting point (~1800  C) of Ga2O3 [29]. This article presents a comprehensive study of the advances in Ga2O3-based oxidic gas sensors. Firstly, we discuss the intrinsic electrical properties of the stable b-Ga2O3 and how they are influenced by gas adsorption/desorption, i.e. the detection mechanisms. Secondly, a brief history of Ga2O3-based semiconducting gas sensors and the evolution of b-Ga2O3 as the active sensing element for the detection of various gases at high temperatures are presented, especially acknowledging the remarkable contributions of Hans Meixner and Maximilian Fleischer. Thirdly, the strategies for improved performance of Ga-containing active materials for metal oxide gas sensors are reviewed and the gas sensing properties are studied with reference to the nature, composition, and microstructure of these materials. Subsequently, the limitations of Ga2O3-based gas sensors and possible future research directions are discussed.

temperatures. Harwig and coworkers measured DC/AC conductivity of b-Ga2O3 single crystals at 27e977  C [33,34]. They found bGa2O3 to exhibit ionic and electronic conductivity in the range of 27e627  C, while only the electronic conductivity above 627  C. The non-stoichiometric b-Ga2O3 is purely n-type semiconductor at high temperatures. Like most metal oxides, the intrinsic n-type electrical conductivity of b-Ga2O3 has been attributed to the oxygen vacancies for a long time [35e41]. Ueda et al. [35] prepared b-Ga2O3 single crystals under controlled atmosphere and observed their electrical conductivity as a function of oxygen flow rate, as shown in Fig. 1. Undoped b-Ga2O3 grown in pure O2 atmosphere was observed to be an insulator (s ¼ 109 U1 cm1), whereas the crystals grown in a mixture of N2 and O2 gases with a pressure ratio of 0.4: 0.6 demonstrated the maximum electrical conductivity (s ¼ 38 U1 cm1). Thus, a strong correlation between the increasing electrical conductivity and the decreasing O2 partial pressure during the processing (synthesis, growth, and-or annealing) of b-Ga2O3 led to the proposition that O2 vacancies were solely responsible for n-type semiconducting behavior of b-Ga2O3. Recently, however, Varley et al. [42] questioned this hypothesis that intrinsic n-type conductivity of b-Ga2O3 is due to oxygen deficiency. They used novel hybrid functionals to calculate the defect formation energy and charge-state transition for oxygen vacancies and unintended donor impurities [42]. They reported that the observed conductivity could not be attributed to the oxygen vacancies because of their deep donor characteristics. Instead, the observed conductivity of b-Ga2O3 could be due to the unintentional doping such as hydrogen impurities. Zacherle et al. [43] investigated the intrinsic point defects using density functional theory (DFT) and determined the free carrier concentration as a function of O2 partial pressure. They concluded that unintentional donor doping was inevitable from the growth ambient and it significantly contributed to the observed electrical conductivity of b-Ga2O3. The conclusive experimental and theoretical data, therefore, support the fact that the unintentional doping results in the intrinsic n-type electrical conductivity of b-Ga2O3 [44e46]. Intentional doping, on the other hand, is a well-known method to improve the electrical properties of oxide semiconductors. Several reports confirm the improvements in conductivity and free-

1.1. Electrical properties of b-Ga2O3 Perfectly stoichiometric, undoped b-Ga2O3 is an insulator [24,30], while it transforms into an n-type semiconductor if produced at the low partial pressure of oxygen. Cojocaru and coworkers first studied the electrical properties of b-Ga2O3 and found out that lattice oxygen deficiency in non-stoichiometric b-Ga2O3 was responsible for the high-temperature conductivity because of the donor-like doping phenomenon [31,32]. Later, many researchers studied the conductivity mechanism at different

Fig. 1. The electrical conductivity of b-Ga2O3 single crystals as a function of the oxygen flow rate. Reproduced with permission [35]. Copyright 1997, American Institute of Physics.

Please cite this article as: Afzal A, b-Ga2O3 nanowires and thin films for metal oxide semiconductor gas sensors: Sensing mechanisms and performance enhancement strategies, Journal of Materiomics, https://doi.org/10.1016/j.jmat.2019.08.003

A. Afzal / Journal of Materiomics xxx (xxxx) xxx

3

carrier concentration of b-Ga2O3 through intentional doping with Si, Sn, Zn, etc. [47e52]. Villora et al. [53] demonstrated that the electrical conductivity of b-Ga2O3 could be improved over three orders of magnitude by intentional Si-doping and that the associated free-carrier concentration (1016e1018 cm3) could correspond to the effective Si impurities. Hence, intentional donor doping not only increases the electrical properties of Ga2O3 but may also improve the performance and sensitivity of Ga2O3-based gas sensors. That is why both undoped and intentionally doped b-Ga2O3 have been employed in electronic gas sensors [54]. 1.2. History of b-Ga2O3 gas sensors

(1)

Where Debye length is a material property that can be calculated from Equation (2):


LD ¼

. εkT ne2

1 2 (2)

Where ε is the permittivity, k is the Boltzmann constant, T is the temperature, n is the electron density, and e is the electronic charge. When exposed to a reducing gas such as CO, the gas molecules react with the chemisorbed oxygen ions resulting in a change in charge career concentration. The chemical reactions between CO and oxygen species can be written as [74]: e 2COðgÞ þ Oe 2 ðadÞ/2CO2 ðgÞ þ e

(3)

COðgÞ þ Oe ðadÞ/CO2 ðgÞ þ ee

(4)

e 1 COðgÞ þ 2Oe ðadÞ/CO2e 3 ðadÞ/CO2 ðgÞ þ 2O2 ðgÞ þ 2e

(5)

=

b-Ga2O3 is an n-type semiconductor. Like other metal oxides, the response of semiconducting b-Ga2O3 gas sensors is usually based on the change in resistance due to change in charge career concentration as a consequence of the gas molecules’ interaction with the oxide surface. In principle, in the air (oxygen atmosphere), O2 molecules are chemisorbed on the semiconducting oxide surface, while trapping electrons from the conduction band and  2e furnishing surface oxygen ions (O 2 , O , and O ) [68e70]. Depending upon the operating temperature, different oxygen ions   may prevail, e.g. the chemisorbed O 2 ions exist below ~200 C, O ions in the range of 200e550  C, and O2 ions above ~550  C [71,72]. The capture of mobile electrons by chemisorbed oxygen species leads to the formation of a charge depletion region on the oxide surface, as shown in Fig. 2a. Width of this depleted region, (Wdep), is determined by the material properties such as Debye length (LD) and potential at the surface (Vsurface) that is given by Equation (1) [73]:

1 2
. Wdep ¼ LD 2eVsurface kT

=

2. b-Ga2O3 gas sensing mechanisms

Fig. 2. The conductivity models (the structural model and the band model) of gallium oxide presenting the charge depletion region and the energy barrier, respectively: (a) when the surface is exposed to clean air (i.e. oxygen-containing atmosphere), and (b) when it is exposed to a reducing gas.

=

Fleischer and Meixner [55] first reported the use of Ga2O3 thin films as high-temperature (850e1000  C) oxygen sensor. They demonstrated excellent response time (tres, in seconds) and good stability of the sputter-deposited Ga2O3 thin film sensor. In principle, the variation in thermodynamic equilibrium between bGa2O3 lattice and O2 gas content in the surrounding ambiance leads to prompt changes in n-type carriers concentration [56,57], and therefore, the conductivity of the device changes upon exposure to O2 gas. Later, they successfully tested the high-temperature stable Ga2O3 thin film sensor to monitor the automotive exhaust gases [58]. The high-temperature O2 sensors are a necessity for the automotive exhaust emission control, better fuel economy (i.e. by reduced fuel consumption), and to minimize the atmospheric pollution. At an early stage, b-Ga2O3 was primarily used for the hightemperature O2 gas sensors and the majority of academic research focused on improving and optimizing the oxygen gas sensing properties of b-Ga2O3 by varying deposition technique, source and/or substrate, and experimental conditions such as temperature, oxygen partial pressure, etc. [59e62]. Fleischer et al. [63e65] first applied b-Ga2O3 thin films for the detection of reducing gases such as H2, CO, CH4, ethanol, etc. The preliminary results indicated that b-Ga2O3 gas sensors could be used in oxidizing as well as reducing atmospheres. Ga2O3 gas sensors demonstrated stable electrical responses to 1% H2 in synthetic air and 5% H2 in nitrogen atmosphere at 600  C [63]. It was also suggested that Ga2O3 gas sensors made of polycrystalline Ga2O3 thin films could be tuned to sensing oxidizing or reducing gases by changing the operating temperature [66,67]. Further developments in b-Ga2O3 gas sensors including the more recent examples and strategies for enhanced sensitivity and gas response are discussed in section 3.

Consequently, the electrons are shifted back to the conduction band resulting in decreased width of the depletion region, as shown in Fig. 2b. Thus, the resistance of b-Ga2O3 gas sensor is reduced. In case of an oxidizing gas such as NO2, the electrons are removed from the conduction band resulting in an increase in the resistance of oxide gas sensor. The chemical interactions between the metal oxide surface, chemisorbed oxygens, and gas molecules can be written as [75]:

Please cite this article as: Afzal A, b-Ga2O3 nanowires and thin films for metal oxide semiconductor gas sensors: Sensing mechanisms and performance enhancement strategies, Journal of Materiomics, https://doi.org/10.1016/j.jmat.2019.08.003

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A. Afzal / Journal of Materiomics xxx (xxxx) xxx

NO2 ðgÞ þ ee ðadÞ/NOe 2

(6)

  e NO2 ðgÞ þ Oe 2 ðadÞ þ 2e /NO2 þ 2O

(7)

However, Hoefer et al. [68] have shown that the Debye length of

b-Ga2O3 is too big, i.e. extending over several mm at high temperature (~800  C), which means b-Ga2O3 may not exhibit high sensitivity toward NO2. Furthermore, Fleischer and Meixner [76] studied the conductivity mechanism of b-Ga2O3 at high temperatures through in situ Hall measurements and compared both singlecrystalline and polycrystalline b-Ga2O3. They observed that the Hall mobilities for both types of b-Ga2O3 were identical and were not influenced by the grain boundary effect. At high temperatures such as > 800  C, the conductivity changes and gas sensitivity were associated with the oxygen-defect equilibrium, as shown in Fig. 3 [77]. In many cases, metal (Pt) electrodes are in contact with b-Ga2O3 thin films. Pt/b-Ga2O3 contact interface can behave as Schottky diode because of the significant difference between the work functions of b-Ga2O3 (4.1 eV) [78] and Pt (5.65 eV) [79,80]. Therefore, the detection mechanism can be influenced and even dominated by Schottky behavior. For instance, Trinchi et al. [81e83] have fabricated Pt/b-Ga2O3/SiC-based Schottky diode gas sensors for H2. The gas sensing mechanism of these sensors is based on diffusion and decomposition of H2 molecules on the surface of the catalytic metal (Pt) resulting in the formation of an electrically polarized layer at the Pt/Ga2O3 interface [82]. Consequently, the currentvoltage (IeV) characteristics of the sensor are changed by the electric field of this polarized layer and the changes are translated as the sensor response. In summary, multiple detection mechanisms may simultaneously count for the actual gas response depending on the sensor's physical structure (such as type of metal electrodes, sensitive film thickness, porosity, and the interface), experimental conditions (such as the operating temperature, oxygen partial pressure,

Fig. 3. Gas sensing mechanism at high temperature showing the reaction between ntype semiconducting b-Ga2O3 and the surrounding gas atmosphere: (a) Surface defects: the reactions on the surface of b-Ga2O3 lead to the formation of surface oxygen defects, and (b) Bulk defects: the subsequent change in crystal defect equilibrium, i.e. an oxygen vacancy is created in the inner part (bulk) of the lattice when an oxygen atom leaves the crystal from the surface and the vacant position changes. Reproduced with permission [77]. Copyright 1999, American Vacuum Society.

thermal pre-treatment of oxide film, etc.), and the physicochemical nature of b-Ga2O3 (such as the microstructure, crystallinity, grain size, thickness or width of the depletion region, lattice oxygen storage capability, surface and-or bulk defects, and surface states). Thus, it is only possible to suggest the predominant gas sensing mechanism under any given set of conditions and is not possible to account for all the physicochemical processes or reactions taking place at the surface of b-Ga2O3 gas sensor. 3. Gas sensing b-Ga2O3 nanowires and composite oxides In this section, a review of the gas sensing properties of b-Ga2O3 and Ga-containing composite oxides, and the strategies to improve gas response and selectivity of these materials are presented. Table 1 presents a comparison of the gas sensing performance of bGa2O3 and Ga-containing composite oxides. 3.1. b-Ga2O3 nanowires and thin films The high-temperature b-Ga2O3 gas sensors are often used to determine the oxygen activity in different applications such as automobile exhaust, incinerator plants, and domestic heating appliances, but their applications are not limited to oxygen detection only. b-Ga2O3 can also selectively detect reducing gases such as H2, CO, CH4, and organic vapors at temperatures <700  C through surface redox reactions [84e87], also depicted in Fig. 2. At temperatures >800  C, however, b-Ga2O3 detects O2 gas through a predominant crystal defect mechanism [55,59,61], shown in Fig. 3. As stated earlier, the crystal defect equilibrium is achieved by the formation or annihilation of oxygen vacancies under variable O2 partial pressure. Therefore, the electrical conductivity is given by Equation (8):






s f Pm O2 exp

Ea= kT

(8)

Where s is the electrical conductivity, PO2 is the partial pressure of oxygen, Ea is the activation energy, k is the Boltzmann constant, and T is temperature. Baban et al. [88] tested b-Ga2O3 coated on interdigital Pt electrodes and Pt/Ga2O3/Pt sandwich-type configurations for hightemperature oxygen sensing. The sensors exhibit excellent gas response (~1.4), good stability at 20% O2, and fast tres of 14 and 27 s, respectively. In a similar work, Bartic et al. [89] achieved slightly better results with the gas response of 1.45 and tres of 10 s using polycrystalline b-Ga2O3 thin film sensors. Recently, b-Ga2O3 single crystals were grown by Czochralski method [90] and their response to O2 gas was investigated at different temperatures (700e1000  C) [91]. The maximum gas response of ~ 2.0 at 800  C and the shortest tres of ~5 s at 700  C were observed. Furthermore, these studies also confirmed that O2 sensitivity was greatly influenced by the surface redox reactions below 800  C, whereas the creation of oxygen vacancies and complex lattice defects governed the sensitivity above this temperature. The prospects of b-Ga2O3 gas sensors operating at lower working temperatures, yet effectively detecting oxygen and reducing gases are investigated by Liu et al. [92]. They developed b-Ga2O3 nanowire sensors using chemical thermal evaporation of gallium metal source. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of b-Ga2O3 nanowires (diameter: 50e150 nm) are shown in Fig. 4(a-b). These b-Ga2O3 nanowires exhibit reversible sensor response to different concentrations of O2 (Fig. 4c) and CO (Fig. 4d) gases at lower temperatures (100e300  C), and the sensitivity improves with the increasing gas concentration. Fig. 4e presents temperature dependence of the

Please cite this article as: Afzal A, b-Ga2O3 nanowires and thin films for metal oxide semiconductor gas sensors: Sensing mechanisms and performance enhancement strategies, Journal of Materiomics, https://doi.org/10.1016/j.jmat.2019.08.003

Material

Fabrication technique

Operating temperature Response a [Gas ( C) concentration]

b-Ga2O3 thin film b-Ga2O3 thin film b-Ga2O3 thin film b-Ga2O3 nanowires

(rf) magnetron sputtering (rf) magnetron sputtering Czochralski method Chemical thermal evaporation

1000 1000 700 300 200 300 30 100

500

b-Ga2O3 nanowires b-Ga2O3 thin film b-Ga2O3 nanowires b-Ga2O3/Pt nanowires

Vapor-liquid-solid method Spray pyrolysis Thermal evaporation Thermal evaporation and (dc) magnetron sputtering b-Ga2O3 nanorods Hydrothermal method b-Ga2O3/Pt nanorods Hydrothermal and chemical method b-Ga2O3/La0.8Sr0.2FeO3 nanorods Hydrothermal and magnetron sputtering Ce-doped Ga2O3 thin film Sol-gel method Sb-doped Ga2O3 thin film W-doped Ga2O3 thin film Zn-doped Ga2O3 thin film Ga-doped SnO2 thin film Spray pyrolysis

460 520 520 420 350

Ga-doped ZnO nanoparticles

Wet chemical method

250

Ga-doped ZnO thin films

Sol-gel method

130

Response time (tres, s)

Detection range

Other observations

Ref.

~ 1.4△ [100% O2] 1.45△ [100% O2] ~ 1.7△ [20% O2] ~ 4.7: [1% O2] ~ 4: [200 ppm CO] 5.3: [200 ppm H2] 332.5: [50 ppm NH3] 4.2: [10 ppm CO] 115.4: [10 ppm CO]

14e27 10 5 e e 48e52 40 440e560 540e660

1%e100% 1%e100% 1%e100% 0.5%e5% 50e500 ppm 40e200 ppm 0.5e100 ppm 10e100 ppm

Reproducible signal at 20% O2 e trec ¼ 29 s Selective CO sensor at 200  C

88 89 91 92

~ 2△ [20 ppm CO] ~ 40△ [20 ppm CO] ~ 45△ [20 ppm CO] 1.55△ [100 ppm O2] 1.27△ [100 ppm O2] 2.56△ [100 ppm O2] 2.93△ [100 ppm O2] 2.1: [133.3 Pa O2]

120e400 510e700 200e470 40 90 90 100 e

20e100 ppm

trec ¼ 500e880 s trec ¼ 180e200 s trec ¼ 230e380 s trec ¼ 30 s trec ¼ 65 s trec ¼ 80 s trec ¼ 70 s

56: [2 ppm NO2] 7: [1 ppm H2S] ~ 57%; [500 ppm H2]

e e 265e475

;

[1000 ppm H2]

18.8

Ga-doped ZnO/Pd nanorods

Wet chemical method

Room temperature

~ 91.2%

Ga-doped ZnO thin films Ga-doped SnO2 microflowers SnO2@Ga2O3 core-shell microribbons Ga2O3eZnO thin film

(rf) magnetron sputtering Hydrothermal method Chemical vapor deposition

300 230 25

~ 57%; [5 ppm H2S] 95.8△ [50 ppm HCHO] 3△ [75% RH]

50 3 28

Sol-gel method

450

3.47△ [100 ppm O2]

50

ZnO@Ga2O3 core-shell nanorods Thermal evaporation and atomic layer deposition Sol-gel method Ga2O3eTiO2 thin film WO3@Ga2O3 core-shell nanostructures Ga2O3eIn2O3 composite

Thermal evaporation Co-precipitation method

△

300

327.8

200

13.7△ [400 ppm CO] 4.3△ [10 ppm NO2] 5.2△ [200 ppm C2H5OH]

200 400 300

△

[100 ppm NO2]

100 e10000 ppm

trec ¼ 150e240 s 96 trec ¼ 18 s Low cross-sensitivity to organic vapors 97 trec ¼ 300e440 s 113 trec ¼ 400e610 s

0 3 at% Ga for best response e8.78  103 Pa 0.2e2 ppm 0.5 at% Ga for best response 500 e3000 ppm 0.5 e1000 ppm 1e10 ppm 0.1e100 ppm 5%e75%

0.3 at% Ga for best response

71

132

139 142 143

LOD: 0.2 ppm 3 wt% Ga for best response Selective 144 H2 sensor trec ¼ 175 s 3% Ga for best response 145 trec ¼ 39 s 3 wt% Ga for best response 146 trec ¼ 7 s 155 160

700

100 trec ¼ 80 s Ga: Zn (60: 40) for best response e10000 ppm 10e1200 ppm trec ¼ 270 s

30 108 20

25e40 ppm 0.5e10 ppm 40e200 ppm

trec ¼ ~ 80 s Ga: Ti (1: 1) for best response trec ¼ ~ 190 s Ga: Ti (1: 1) for best response trec ¼ ~ 120 s

164

1 vol% 300 ppm

trec ¼ 125 s 65 wt% Ga for best response trec ¼ 42 s 10 wt% Ga for best response

167

~ 38 [1 vol% CH4] 17 ~ 67△ [300 ppm C2H5OH] 13

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Please cite this article as: Afzal A, b-Ga2O3 nanowires and thin films for metal oxide semiconductor gas sensors: Sensing mechanisms and performance enhancement strategies, Journal of Materiomics, https://doi.org/10.1016/j.jmat.2019.08.003

Table 1 A comparison of the gas-sensing performance of b-Ga2O3 and Ga-containing composite oxides sensors.

161

165

a . In literature, sensor response (S) of the n-type semiconducting gas sensors is determined by the following relationships:△ Depending upon the oxidizing or reducing gas, sensitivity is reported as: [S¼Rg/Ra] or [S¼Ra/Rg], where Ra is the resistance in clean air or N2 atmosphere and Rg is the resistance in target gas atmosphere.: [S¼△R/Rg], where △R is the change in resistance upon exposure to target gas.; [S(%)¼(△R/Ro)100], where Ro is the baseline resistance in clean air or N2 atmosphere.

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Fig. 4. The micrographs of b-Ga2O3 nanowires: (a) SEM image, and (b) TEM image. Dynamic response of b-Ga2O3 nanowire sensor to: (c) O2 gas, and (d) CO gas. The sensor exhibits high sensitivity and fast response times. (e) The temperature dependent sensor response of b-Ga2O3 nanowires to O2 and CO gases. The nanowire sensor can be operated at low temperatures (100e200  C) with sufficient sensitivity toward CO and limited O2 interference. (f) The gas response of b-Ga2O3 nanowire sensor, when exposed to different reducing gases. The sensor shows considerable selectivity toward CO gas. Reproduced with permission [92]. Copyright 2008, Elsevier.

sensor's response with a peak O2 gas sensitivity at 300  C and a peak CO gas sensitivity at 200  C. The experiments also reveal high selectivity of b-Ga2O3 nanowires toward CO gas compared to other reducing gases, e.g. H2, NH3, H2S, as shown in Fig. 4f. On the other hand, reducing gases like H2 have been successfully detected by combining b-Ga2O3 and different device principles and geometries such as Schottky diodes in metal-reactive insulatorsemiconductor (MIS)-type configuration and transistors with bGa2O3 thin films and Pt gate electrodes [81e83,93e95]. These devices display stable response to H2 gas at different temperatures, which usually decreases with increasing temperature. The mechanism involves surface redox reactions of H2 with negatively charged oxygen species, as given below: e 2H2 ðgÞ þ Oe 2 ðadÞ/2H2 OðgÞ þ e

(9)

Yoon and coworkers [96] also fabricated H2 gas sensors via the high-temperature growth of single-crystalline b-Ga2O3 nanowires on Au-nanodot electrodes. The sensor exhibits good H2 gas sensitivity and tres of less than 1 min at 300  C. However, the recovery times (trec) are in the range of 3e4 min due to the low operating temperature. A room-temperature b-Ga2O3 thin film ammonia (NH3) sensor is also fabricated via spray pyrolysis [97]. The device exhibits promising results: high sensor response (332.5) to 50 ppm NH3 gas, low cross-sensitivity to organic vapors, and long-term stability, i.e. relatively unchanged response for 6 months. The detection mechanism involves the surface reactions with chemisorbed O 2 ions (the only oxygen species available in this temperature range [71,72]), as given below: e 4NH3 ðgÞ þ 3Oe 2 ðadÞ/2N2 ðgÞ þ 6H2 OðgÞ þ 3e

(10)

While investigating the effect of humidity on sensor performance, it is seen that increasing humidity slightly reduces the gas

response, because the chemisorption of H2O diminishes the number of active oxygen species [97,98]. More recent studies demonstrate that increasing relative humidity (RH) influences the conductivity of b-Ga2O3 nanowires [99,100]. It is due to the donortype effect of water vapors either by chemisorption in hydroxyl and molecular forms or by reaction with already chemisorbed oxygen species [101e103]. Hence, gas sensing in mixed gas-humid environments with significantly high RH can result in lowering the gas response of b-Ga2O3 nanowire sensors, especially at low working temperatures. 3.2. Surface-modified b-Ga2O3 nanostructures Improving the sensitivity and selectivity of b-Ga2O3 gas sensors is a great challenge. Over the years, several new approaches have been investigated in this regard such as surface modification of bGa2O3 nanostructures with catalytically active metal or metal oxides. The results of these studies are discussed in this section. 3.2.1. Noble metal-decorated b-Ga2O3 Noble metals such as palladium (Pd) and platinum (Pt) have often been used as catalytically active dispersions on metal oxide surfaces to enhance the performance of electronic gas sensors [104e109]. The gas sensing properties of the catalytically inactive Ga2O3 nanostructures are also improved by finely decorated noble metal catalysts [66,110]. For instance, the tres of CO gas sensor is drastically reduced at 600  C, while the sensitivity of H2 gas sensor is significantly improved at 300  C by Pt catalytic dispersion on Ga2O3 thin films [66]. Stegmeier et al. [111] fabricated a polycrystalline b-Ga2O3/Pt thick film gas sensor for the room-temperature operation and detection of volatile organic compounds and CO gas. Clearly, bGa2O3/Pt thick film sensor exhibits a higher response to all reducing

Please cite this article as: Afzal A, b-Ga2O3 nanowires and thin films for metal oxide semiconductor gas sensors: Sensing mechanisms and performance enhancement strategies, Journal of Materiomics, https://doi.org/10.1016/j.jmat.2019.08.003

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Fig. 5. (a) SEM image of the Pt-coated Ga2O3 nanowires. Inset, enlarged SEM image of a typical Pt-coated Ga2O3 nanowire. (b) Low-magnification TEM image of a typical Pt-coated Ga2O3 nanowire. (c) Local HRTEM image of the nanostructure at the interface region of a Ga2O3-core and a Pt-shell. (d) The electrical response of the pristine Ga2O3 nanowires sensor, and (e) the electrical response of Pt-functionalized Ga2O3 nanowires sensor to different concentrations of CO gas at 100  C. Reproduced with permission [113]. Copyright 2012, Elsevier.

gases compared to the pure b-Ga2O3 sensor. The room-temperature operation and improved sensor performance are achieved by brief thermal treatment at 175  C in the ambient atmosphere [111,112]. Thermal pre-treatment is believed to generate reactive oxygen species at the surface resulting in higher sensitivity. These results suggest that a thin layer of catalytically active noble metal can have substantial effects on the performance of b-Ga2O3 gas sensors. Kim et al. [113] developed Pt-coated b-Ga2O3 nanowires for CO gas sensing at 100  C. Fig. 5(a-c) show the microscopic images of pristine b-Ga2O3 and b-Ga2O3/Pt nanowires. Small Pt nanoparticles uniformly distributed on b-Ga2O3 nanowires significantly increase the sensor's electrical response toward CO gas. A comparison of the gas sensing properties of b-Ga2O3 and b-Ga2O3/Pt nanowires is also presented in Fig. 5d-e b-Ga2O3/Pt nanowires reveal 27.8-fold higher response toward 10 ppm CO gas compared to pure b-Ga2O3. The advantages of coating thin Pd or Pt dispersions on b-Ga2O3 nanowires are two-fold: (a) the sensitivity of the device is enhanced as demonstrated in these examples, and (b) the operating temperatures are significantly reduced. In fact, the reactive oxygen species (O2e, O, and O 2 ) are spontaneously chemisorbed to b-Ga2O3 nanowires at high temperatures, as stated earlier [89,91,114]. Such species facilitate the transfer of electrons between gaseous analyte and Ga2O3. At low temperature, however, the chemisorption of reactive species and electron transfer is hindered [113,115]. Thin film of catalytically active metal (Pd/Pt) allows the chemisorption of the active species at a lower temperature. Owing to their conductive nature, noble metal nanoparticles adsorb these reactive oxygen species and spill them over to b-Ga2O3 surface, i.e. also known as the spillover effect

[116e122]. Therefore, the greater adsorption of reactive species by noble metal nanoparticles and spillover effect are considered to improve the sensor performance.

3.2.2. Metal oxide-coated b-Ga2O3 Besides noble metals, certain metal oxides have also been investigated as the active oxidation catalysts for improving the electrical and sensing properties of b-Ga2O3 nanowires. The earliest attempts by Meixner and coworkers have utilized tantalum, tungsten, nickel, iridium, rhodium, ruthenium, and silicone oxides: Ta2O5, WO3, NiO, Ir2O3, Rh2O3, RuO2, and SiO2 [123e125]. Thin layers (thickness: 30e300 nm) of these oxides are either dispersed or sputter-coated onto a readily prepared Ga2O3 thick film (thickness: 2 mm) to optimize the electrical properties, gas sensitivity, selectivity, and operating temperature of Ga2O3 gas sensor. For instance, the fabrication of an amorphous SiO2 layer on the semiconducting Ga2O3 sensor is shown to remarkably affect the device's selectivity for H2 gas [124]. The SiO2 layer not only acts as a gas filter but significantly contributes to the signal transduction of Ga2O3 by manipulating the space charges within H2 gas sensor [126]. In a similar way, Schwebel et al. [60] constructed a selective oxygen sensor through surface modification of pure semiconducting Ga2O3 with catalytically active La2O3 or CeO2. They observed that pure Ga2O3 responded to oxygen as well as other reducing gases and organic vapors. However, a thin lanthanum oxide layer induces selectivity and the device reacts to the changes in O2 gas concentration only. Recently, perovskite-type La0.8Sr0.2FeO3 (LSFO) nanoparticles are used for the surface decoration of bGa2O3 nanorods [71]. Fig. 6(a-d) shows the TEM images of pristine

Please cite this article as: Afzal A, b-Ga2O3 nanowires and thin films for metal oxide semiconductor gas sensors: Sensing mechanisms and performance enhancement strategies, Journal of Materiomics, https://doi.org/10.1016/j.jmat.2019.08.003

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nanoparticles' sensitization effect. LSFO nanoparticles can produce a surface charge depletion layer, when in contact with Ga2O3, similar to the commonly perceived space-charge effect in pure bGa2O3 sensors [2,10,127]. Consequently, the thickness of the surface depletion layer is reduced upon exposure to a reducing gas such as CO; thus, increasing the device's conductivity. However, the electron transport mechanism is principally explained by the spilloverlike effect. CO molecules mainly interact with the already chemisorbed reactive oxygen species (as shown in equations (3-5)). The reactive oxygen anions are subsequently spread over to b-Ga2O3 from LSFO thin layer through the gas-LSFO-Ga2O3 interface resulting in rapid CO oxidation, high sensitivity, and tres [71]. 3.3. Metal-doped b-Ga2O3 nanomaterials

Fig. 6. (a) TEM image of a GaOOH nanorod grown at 150  C, and the inset is the corresponding electron diffraction pattern indicating the GaOOH nanorod growth plane is parallel to (111). (b) TEM image of a b-Ga2O3 nanorod annealed at 1000  C for 4 h, and the inset is the corresponding electron diffraction pattern indicating the growth plane is parallel to (001). (c) TEM image of post-annealing b-Ga2O3 coated with La0.8Sr0.2FeO3 (LSFO) 5 nm thin, and the inset is the electron diffraction pattern corresponding to the nanowire in c, which shows the preferred growth plane of b-Ga2O3 nanorod is (001). (d) TEM image of post-annealing b-Ga2O3 coated with Pt particles. (ef) CO gas sensing test results: (e) Currenttime characteristics of b-Ga2O3; b-Ga2O3/ LSFO 5 nm; b-Ga2O3/LSFO 10 nm; b-Ga2O3/Pt composite nanorod tested at 500  C with N2 as background atmosphere; (f) response time versus CO concentrations characteristics of b-Ga2O3; b-Ga2O3/LSFO; b-Ga2O3/Pt composite nanorod tested at 500  C. Reproduced with permission [71]. Copyright 2016, American Chemical Society.

b-Ga2O3, Ga2O3/LSFO, and Ga2O3/Pt functional nanorods. The

Ga2O3/LSFO nanorods exhibit high sensitivity to CO gas at 500  C. The overall gas sensing performance of Ga2O3/LSFO nanorods is analogous to Ga2O3/Pt sensors with slightly faster tres, while the functional (LSFO) layer thickness is small (~5e10 nm), as shown in Fig. 6e-f. Lin et al. [71] attribute the enhancement in sensitivity of Ga2O3/ LSFO sensor to the spillover-like mechanism. It is believed that different material properties may contribute to the LSFO

The dopants have a pronounced effect on the electrical properties of b-Ga2O3 [34]. SnO2 is an active material and highly regarded as one of the best gas sensing catalyst [128]. Sn-doping of b-Ga2O3 improves the electrical conductivity and the number of surface defect sites, which may significantly enhance the gas response of b-Ga2O3 nanowires and thin films [129]. Frank et al. [54,130] used SnO2 as a donor-type dopant for b-Ga2O3 thick film oxygen gas sensors. In a separate study, they also investigated the effects of ZrO2, TiO2, and MgO as donor-type and acceptor-type dopants on the electrical conductivity and gas sensing properties of n-type b-Ga2O3 thick films [131]. In the first experiments, it is observed that conductivity of doped b-Ga2O3 is increased, baseline resistance is lowered, and the surface-controlled sensitivity toward reducing gases is strongly influenced by the donor and acceptor dopants [130,131]. However, both donor-type (Zr4þ, Ti4þ, Sn4þ) and acceptor-type (Mg2þ) dopants do not influence the high-temperature, bulk-controlled O2 sensitivity [54,131]. Subsequent studies have proven otherwise, nonetheless. It is found that doping with an appropriate metal ion such as Sb, Ce, W, and Zn can actually improve the oxygen sensitivity of b-Ga2O3 thin films and can substantially decrease the working temperature of metal-doped-Ga2O3 oxygen gas sensors [132]. Zne and W-doped b-Ga2O3 thin film sensors exhibit good response kinetics (tres and trec: 70e100 s) and high sensitivity (S > 60%) toward oxygen gas at lower optimal working temperatures (420 for Zn-doped and 520 for W-doped b-Ga2O3). Fast responsive impedance-metric gas sensors have been fabricated by doping Ga2O3 with Au [133], and alkali metal ions [134]. Au-doped Ga2O3 sensor exhibits strong response dependency on CO gas concentration and fast sensor response and recovery times, i.e. both, tres and trec around 10 s [133]. Wang et al. [134] developed humidity sensors with alkali ions (Naþ and/or Kþ)doped Ga2O3 nanorods prepared from one-step thermal annealing of GaN powder. Fig. 7 present the sensor design, geometry and porous structure of Naþ and Kþ doped Ga2O3 nanorods (Ga2O3eNaeK) and their RH sensing properties. The sensors exhibit high sensitivity, fast tres (6 s) and trec (21 s), and good linearity in the range of 11e95% RH. A slight decrease in the humidity response is also observed with an increase in the environmental temperature. The temperature coefficient is determined to be 0.47% RH  C1 in the temperature range of 20e40  C [134]. 3.4. Ga-doped metal oxides The electrical properties of metal oxides such as SnO2 and ZnO can be optimized by intentional doping with Ga3þ, which can substitute Sn4þ and Zn2þ ions in the respective lattices resulting in a decrease or increase in the electrical conductivity [135e139]. This

Please cite this article as: Afzal A, b-Ga2O3 nanowires and thin films for metal oxide semiconductor gas sensors: Sensing mechanisms and performance enhancement strategies, Journal of Materiomics, https://doi.org/10.1016/j.jmat.2019.08.003

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Fig. 7. (a) TEM image of alkali metal ions-doped Ga2O3 nanorods. Inset shows the schematic design of sensor. (b) High resolution TEM images of alkali metal ions-doped Ga2O3 nanorods showing the porous structure. Inset shows the photo of sensing device. (c) The humidity sensing properties of various alkali metal ions-doped Ga2O3 nanorods sensors. (d) The sensing performance of Naþ and Kþ doped Ga2O3 nanorods at different temperatures. Reproduced with permission [134]. Copyright 2015, Elsevier.

process also leads to enhanced gas sensitivity [139e142]. For instance, the active SnO2 doping with 3 at% Ga improves its oxygen sensitivity up to 2.1 [139], while ZnO doping with 3 at% Ga improves the H2 gas response by approximately 2 orders of magnitude compared to pristine ZnO [143]. The latter also exhibits better sensitivity toward H2 compared to other reducing gases such as NH3 and CH4. Rashid et al. [144] fabricated a flexible H2 sensor on polyimide substrate by growing Ga doped ZnO nanorods followed by incorporation of Pd catalyst. At room temperature, the sensor shows excellent sensitivity (91%), low limit of detection (LOD: 0.2 ppm), and 4e5 folds increase in gas response compared to undoped ZnO. Furthermore, when exposed to different gases: O2, NO2, CO2, CO, or N2, the Ga-doped-ZnO/Pd sensor has a highly selective response to H2 gas that is attributed to the inclusion of catalytic Pd in the receptor. In a recent study, Girija et al. [145] employed Gadoped ZnO thin films in semiconducting H2S gas sensors. The sensors composed of 3% Ga-doped-ZnO thin films demonstrate the highest sensitivity and linear gas response in the range of 1e10 ppm H2S. Compared to pristine metal oxides (ZnO or bGa2O3), Ga-doped-oxides reveal promising results. The abovementioned investigations describe the surface redox phenomenon as the principal mechanism of detecting targeted reducing gases: H2, H2S, etc. Formaldehyde (HCHO) gas sensor is developed by Du et al. [146] using hydrothermally obtained undoped and Ga-doped SnO2 porous microflowers. Fig. 8 shows the SEM images of pure and Gadoped SnO2 microflowers and the corresponding sensor responses to different concentrations of HCHO gas at 230  C. The study has indicated 4.5 times higher gas response with 3 wt% Ga-doping. The higher sensitivity of Ga-doped SnO2 is attributed to the cationic substitution of high valence Sn4þ with low valence Ga3þ, which results in greater oxygen vacancies and crystal defects. Du et al.

[146] confirmed a significant increase in the oxygen vacancies from 41.2% to 49.7% due to Ga-doping. These vacancies act as active sites and play a pivotal role in the chemisorption of oxygen species (O2e and O); thus, enhancing the surface reactions with the target gas molecules [146,147]. The resulting Ga-doped SnO2 sensors also display good reproducibility, and faster tres and trec. 3.5. Composite Ga2O3/metal oxide heterostructures Recent progress in materials chemistry has seen new ways to blend metal oxides together into binary or ternary composites to take advantage of their individual properties. These composite heterostructures usually have superior electrical and gas sensing properties compared to pure oxides [148e152]. Furthermore, it is an attractive strategy to reduce operational temperature, enhance sensitivity and stability, and optimize selectivity toward certain gases. b-Ga2O3 is often combined with active oxides such as SnO2, TiO2, In2O3, and ZnO to form high-performance composite gas sensors [153e159]. A comparison of the gas sensing properties of Ga2O3eZnO thin films with various compositions (Ga: Zn at%) reveals significant differences. Ga2O3eZnO thin films with Ga: Zn (60: 40) exhibit the highest sensor response (3.47) to 100 ppm O2 and the best sensor kinetics (tres ¼ 50 s) [160]. Increasing the amount of Ga results in an increase in the base resistance as well as the optimal working temperature, while a decrease in the sensor response. Jin et al. [161] prepared the core-shell ZnO@Ga2O3 nanorods via thermal evaporation of GaN powder followed by atomic layer deposition and investigated their NO2 gas sensing properties. The core-shell ZnO@Ga2O3 nanorods sensors exhibit 354- and 1206-times superior response to 200 ppm NO2 compared to pure Ga2O3 and pure ZnO, respectively; which is huge. This is justified by the spacecharge model, i.e. a strong oxidizing gas like NO2 traps electrons

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Fig. 8. The SEM images of (a-b) pure SnO2, and (c-d) 3 wt% Ga-doped SnO2 porous microflowers. (e) Dynamic response curves, and (f) linear response of the doped and undoped SnO2 sensors with various concentrations of formaldehyde at the optimal temperature. Reproduced with permission [146]. Copyright 2018, Elsevier.

Fig. 9. (a) The SEM image of WO3@Ga2O3 core-shell nanostructures. (b) Low-magnification TEM image, (c) Local HRTEM image, and (d) corresponding SAED pattern of a typical WO3@Ga2O3 core-shell nanostructure. The dynamic sensor response of (e) the pristine Ga2O3 nanostructures, and (f) WO3@Ga2O3 core-shell nanostructures. Reproduced with permission [165]. Copyright 2015, Elsevier.

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from the ZnO shell thereby increasing the width of depletion region and resistance of the sensor. Furthermore, it is believed that the heterojunction barrier at the interface of core-shell structure also plays a significant role in electron transport and modulation, eventually combining the effects of Ga2O3-core and ZnO-shell to improve the sensitivity [161e163]. Interestingly, it has been noted earlier that b-Ga2O3 nanostructures do not respond to oxidizing gases other than oxygen [68], but combining b-Ga2O3 with other metal oxides disproves this hypothesis. Mohammadi et al. [164] also reported a sensitive TiO2eGa2O3 thin film sensor with a low detection limit (500 ppb) for NO2 and good sensing properties. The sensor is developed via the particulate sol-gel method with different compositions (Ti: Ga atomic ratios). TiO2eGa2O3 thin film sensor with Ti: Ga (1: 1) ratio exhibits good sensor response (4.3) to 10 ppm NO2 gas at 200  C, which is approximately 55% and 15% greater than pure TiO2 and TiO2eGa2O3 with Ti: Ga (1: 3) sensors, respectively. Fig. 9 presents the microscopic images and the dynamic ethanol sensing curves of pristine Ga2O3 and core-shell WO3@Ga2O3 nanostructures. The nanostructured core-shell WO3@Ga2O3 sensor is constructed by step-wise thermal evaporation of Ga2S3 and WO3 powders and yields excellent sensitivity toward 40e200 ppm ethanol compared to pristine Ga2O3 nanostructures [165]. A comparison of core-shell WO3@Ga2O3 sensor with pristine Ga2O3 reveals 160% higher sensor response and 2e3 min faster tres and trec for core-shell type heterostructures. As described above, the enhanced sensing performance is attributed to the space-charge effect, i.e. surface redox reaction of ethanol molecules with chemisorbed oxygen species decreases the width of depletion region and the resistance, while the Ga2O3-WO3 interface facilitates charge transport thereby reducing the potential barrier and improving the response [161e163,165,166]. Bagheri et al. [167] combined Ga2O3 and In2O3 in different ratios (0e75 wt% Ga2O3) via co-precipitation method to study the gas sensing properties of the resulting composite nanostructured sensors. It is observed that the sensor response and selectivity toward different reducing gases such as CO, CH4, and C2H5OH can be

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optimized by altering the amount of Ga2O3 in the composite, the post-synthesis calcination temperatures (500e850  C), and the sensor's operating temperatures. For instance, Ga2O3eIn2O3 composite sensor with 65 wt% Ga2O3 and calcined at 500  C presents high sensitivity and selectivity toward 1% CH4 at 400  C, while Ga2O3eIn2O3 composite sensor with 10 wt% Ga2O3 exhibits higher sensitivity and selectivity toward 300 ppm C2H5OH at 300  C. Thus, the selectivity of the semiconducting b-Ga2O3 sensors can be modulated by combining b-Ga2O3 with an appropriate active oxide and changing the processing/operational conditions [156,157,167]. 4. Limitations and challenges A review summary of b-Ga2O3 gas sensors is given in Table 2, which presents: (a) the advantages and drawbacks Ga2O3 sensors, (b) the strategies to improve their gas sensing performance and major challenges, and (c) widely studied applications of these sensors with relatively unexplored areas of research. b-Ga2O3 is very stable at high temperature and exhibits excellent sensitivity to oxygen often guided by lattice defects and n-type donor characteristics of b-Ga2O3 [88,91]. Hence, b-Ga2O3 nanostructured gas sensors are excellent for high-temperature applications, e.g. measuring oxygen in exhaust gases of incinerator plants, fossilfuel-vehicles, and indoor heating appliances for their controlled operation and restraining unnecessary emissions [77,168]. b-Ga2O3 nanostructures are also sensitive to certain reducing gases at a wide range of temperatures (100e1000  C). Both crystal defects and surface redox reactions of chemisorbed oxygen species with the target molecules are responsible for the detection of reducing gases depending on the operational temperature [77]. The high-temperature operation of stable b-Ga2O3 gas sensors is beneficial in many ways as it improves the response kinetics (tres and trec) of devices, reduces cross-sensitivity to water vapors, and prevents extra cleaning cycles. Hoefer et al. [68] compared commercially available FIGARO (SnO2-based) and STEINEL (Ga2O3based) gas sensors. They concluded that Ga2O3-based devices revealed lower product spread, smaller burn-in time, negligible

Table 2 A summary of b-Ga2O3 gas sensors with advantages and disadvantages: The strategies to enhance gas sensing properties of b-Ga2O3, the unmet challenges, already investigated and relatively unexplored applications of b-Ga2O3 gas sensors are presented. Advantages (þ)

Approaches for enhanced performance

þ High temperature stability. þ Wide working temperature range (room-temperature to 1000  C). þ Low product spread and burn-in time. þ Long-term stable ground resistance. þ High sensitivity to oxygen and many reducing gases such as H2, CO, CH4, C2H5OH, etc. þ Tunable selectivity. þ Stability in low O2 atmosphere. þ Very little humidity effect. þ Faster recovery times. þ Reproducibility. þ No cleaning cycles required at high temperature. Disadvantages (e) e Not responsive to some oxidizing gases such as NO2, CO2 etc. e High power consumption. e Miniaturization problems due to heating element integration. e Relatively higher cost.

⁃ The following approaches can further improve sensitivity, selectivity, ✓ b-Ga2O3 gas sensors highly suitable for highand response kinetics to reducing gases: temperature applications such as: a Constructing more versatile nanostructures with higher aspect ratio, a Measuring O2 activity in automotive exhaust surface availability, crystal defects, and-or oxygen storage capability gases. [71,146,161]. b Detecting O2 and CO in incinerator plants and b Intentional doping with a suitable metal [132]. household heating appliances. c Surface-modification with catalytically active metals [712 [113]]. ✓ Sensing reducing gases in indoor/outdoor d Composite formation with inherently sensitive oxides [160,164,167]. atmospheres. e Development of core-shell heterostructures [155,161,165]. f Working temperature optimization [156,157,167].

Challenges ⁃ Improving sensitivity toward oxidizing gases such as NO2. ⁃ Improving selectivity in gas mixtures. ⁃ Reducing operating temperature (
100  C). ⁃ Cost-effectiveness through miniaturization and low consumption.

Existing applications: Investigated (✓)

Potential applications: Unexplored (7) 7 Detecting olfactory gases and breath markers such as alcohols, phenols, ketones, NO, CO, NH3, (CH3)2S etc. power 7 Monitoring air for combustible and explosive gases such as natural gas, liquified petroleum gas, etc. 7 Measuring NO, NO2, CO2, and unburned hydrocarbons in exhaust gases. 7 Selective H2 sensor for machines and processes utilizing hydrogen energy.

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humidity influence, and better long-term stability over 5 months, while SnO2-based devices were more sensitive to different gases. However, a major drawback of the b-Ga2O3 gas sensors e i.e. highpower consumption e is also associated with their hightemperature operation, which demands the integration of a heating element thus, increasing the size and cost of the device compared to room temperature sensors. The reduction in the working temperature of b-Ga2O3 gas sensors without trading off their sensitivity, stability, and reproducibility is a great challenge. It has been noted that low-temperature operation influences the sensitivity of b-Ga2O3 toward reducing gases such as H2 in the humid environment because the oxide surface is covered with H2O molecules and hydroxyl (OH)e groups [169]. Nonetheless, the researchers have already presented a few promising examples of b-Ga2O3 gas sensors operating at room temperature and demonstrating high sensitivity and fast response kinetics (tres and trec) [97,155]. However, these devices have only been tested in a lab under controlled atmosphere. The low-temperature operation and high sensitivity can be achieved by suitable surface modification. For instance, the sensitivity of pristine b-Ga2O3 nanowires can be enhanced by a simple thermal ammonization treatment [170]. The surface-nitridated bGa2O3 nanowires show 3-times stronger electrical response to CO gas compared to pure b-Ga2O3. Thermal activation of b-Ga2O3 and b-Ga2O3/Pt sensors via intermittent heating at 175  C for short intervals is also proposed to improve their sensitivity toward reducing gases at low temperature [111,112]. However, such high sensitivity induced by the instant creation of reactive oxygen species on the sensor surface is ominously lost after a few weeks of operation [111]. The activation process may be repeated often to revive the sensor performance, which does not present a long-term solution. The selectivity of metal oxide gas sensors is not well-resolved. Like other metal oxides, b-Ga2O3 are not inherently selective toward a specific gas and often show significant cross-sensitivity to different gases or groups of gases. In certain cases, the selectivity toward different gases can be modulated by temperature [167,171]. The researchers have already taken up this challenge and some efforts to improve the selectivity and specificity of b-Ga2O3 nanowires and thin films have already been reported [85,92,144,172,173]. For instance, a thin layer of Pt metal on top of bGa2O3 sensor acts as a catalyst to induce greater sensitivity toward CO [113], while a thick SiO2 layer on top of b-Ga2O3 is reported to act as a filter thus, allowing only H2 to be detected [126]. Mazeina et al. [174,175] used acetic acid and pyruvic acid vapors to functionalize the surface of b-Ga2O3 nanowires and to improve their responses to specific groups of hydrocarbons. The results demonstrate sufficient selectivity toward triethylamine. However, there is still a long way to fully understand and optimize the selectivity of nanostructured b-Ga2O3 gas sensors. Another weakness of b-Ga2O3 sensors is their low sensitivity to certain oxidizing gases such as NO2, CO2, etc. [68]. Albeit some researchers have successfully demonstrated the detection of NO2 using b-Ga2O3-metal oxide composite gas sensors [161,164], this topic remains largely unexplored. A simple approach that combines b-Ga2O3 with active metals and-or metal oxides in the different ratios can prove its utility as an oxidizing gas sensor for a variety of applications, but there are not many examples of using b-Ga2O3 nanostructures for reducing gases. b-Ga2O3-based sensors have been commercialized for the past 20 years and are renowned for their excellent long-term stability and sensitivity to common O2, H2, CO, CH4, and ethanol, yet there is little concerted effort on the development of new b-Ga2O3-based materials for the detection of nitrogen oxides in e.g. exhaust gases. Furthermore, b-Ga2O3 gas sensors are not investigated for the point-of-care disease

diagnostics and due to their long-term stability and reliability, they can be applied for the detection of several olfactory gases and breath markers in future [176]. 5. Concluding remarks This article presents a comprehensive study of the gas sensing mechanisms, properties, and applications of nanostructured bGa2O3-based semiconductor gas sensors. b-Ga2O3 has emerged as an alternative of commonly used metal oxides, e.g. SnO2, for gas sensor measurements at high-temperature or under harsh conditions with low oxygen pressure due to its greater stability. Later, many researchers have focused on improving the gas sensing properties of b-Ga2O3 nanostructures. They have used multiple strategies: (a) surface-modification of b-Ga2O3 with catalytically active metals and-or metal oxides; (b) intentional doping of bGa2O3 nanostructures with donor- and-or acceptor-type metal ions; and (c) forming composite heterostructures with inherently more-sensitive metal oxides. These strategies promote the sensitivity, response times, selectivity of b-Ga2O3 sensor to targeted gases. The detection mechanisms of these sensors are discussed. A comparison of the gas sensor performance based on the nature and composition of b-Ga2O3 nanostructures and working temperatures is presented (see Table 1). An inclusive study of the advantages, disadvantages, challenges, investigated applications and unexplored areas of research for potential developments are also discussed (see Table 2). To summarize, b-Ga2O3 gas sensors have shown great potential in detecting oxygen and many reducing gases at different temperature ranges with excellent sensitivity, significant selectivity, and fast response kinetics. In future, the research developments may focus on the following: (a) synthesis of novel, more versatile b-Ga2O3 nanostructures (pristine, doped, or composite) with greater surface availability and oxygen storage capability; (b) room-temperature b-Ga2O3 sensors to avoid heater integration, thereby reducing power consumption, cost, and size (facilitating miniaturization); (c) improving b-Ga2O3 selectivity through suitable surface modifications, without temperature modulation; and (d) finding ways to enhance sensitivity to oxidizing gases and organic vapors, thereby extending the b-Ga2O3 sensor applications to e.g. point-of-care diagnosis. Funding sources This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. Conflicts of interest Author declares that there are no conflicts of interest. Acknowledgements I gratefully acknowledge the support and assistance provided by S. Irum in writing this manuscript. I dedicate this work to the memory of Dr. Claus-Dieter Kohl (1945e2017), my doctoral thesis examiner, Professor of Physics at Justus-Liebig-Universit€ at Gieben, Editor of the book entitled “Gas Sensing Fundamentals” (SpringerVerlag, 2014), and a brilliant scientist to work in the field of gas sensors. Rest in peace, Prof. C.-D. Kohl. References [1] Waitz T, Wagner T, Kohl C-D, Tiemann M. New mesoporous metal oxides as d e on A, Massiani P, Babonneau F, editors. Stud. Surf. Sci. gas sensors. In: Ge Catal. vol.174. Elsevier; 2008. p. 401e4. https://doi.org/10.1016/S0167-

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Adeel Afzal earned PhD in Chemistry from University of Vienna in 2007. He is a Collaborator (Researcher) at University of Bari and an Associate Professor of Chemistry at University of Hafr Al Batin. At Bari, he developed metal/ metal oxide composite nanoparticles for the hightemperature electronic gas sensors and catalytic applications. His research is focused on the design, synthesis, and characterization of imprinted and functional polymers, metal oxide nanostructures, and high-performance nanocomposites for chemical and gas sensors, industrial, and biomedical applications.

Please cite this article as: Afzal A, b-Ga2O3 nanowires and thin films for metal oxide semiconductor gas sensors: Sensing mechanisms and performance enhancement strategies, Journal of Materiomics, https://doi.org/10.1016/j.jmat.2019.08.003