Compositing strategies to enhance the performance of chemiresistive CO2 gas sensors

Materials Science in Semiconductor Processing 107 (2020) 104820

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Compositing strategies to enhance the performance of chemiresistive CO2 gas sensors Yueqiang Lin, Zhuangjun Fan * School of Materials Science and Engineering, China University of Petroleum, Qingdao, 266580, Shandong, PR China

A R T I C L E I N F O

A B S T R A C T

Keywords: Chemiresistive CO2 sensors Doping Heterojunction Nanostructured materials Sensing performance

Reliable and inexpensive CO2 gas sensors hold huge market potential as they are widely used in areas such as food package sectors, indoor air quality testing and real time monitoring of man-made CO2 emissions in order to curb the global warming effectively. Nanostructured materials have some distinctive advantages, such as small grain size, controlled growth of morphology and heterojunction effect provided by compositing techniques, making them potential candidates for chemiresistive CO2 gas sensors. This article presents an overview of the most recent developments in chemiresistive CO2 gas sensors based on nanostructured semiconducting materials with a particular emphasis on semiconducting composite structures, which include the mixing composite structure, second-phase decorated structure and bi-layer film structure. The correlations between compositing structures and their CO2 sensing properties are also highlighted, followed by the discussion of challenges and outlooks of chemiresistive CO2 gas sensors.

1. Introduction As an important part of earth atmosphere, carbon dioxide (CO2) has a number of distinctive properties. Firstly, it is an essential ingredient of photosynthesis, in which CO2 is converted into glucose and O2, thus making human life possible. Secondly, owing to its unique physico­ chemical characteristics (e.g., inert and water-soluble feature, and high density (1.5 times heavier than that of air)), CO2 is widely used in food industry (such as food packaging and carbonated drinks), fire extin­ guishers, lasers and refrigerants [1,2]. However, CO2 is also considered as the most important contributor to the global warming, accounting for 76% of the increased greenhouse effect [3]. CO2 concentrations in the outdoor atmosphere have increased approximately 30% since pre-industrial times with a rate of about 1.5 ppm per year due to anthropogenic CO2 emissions [4] (Fig. 1A). June 2019 was character­ ized by the highest global land and ocean temperature departure from average since 1880 and the temperatures were 2.0 � C above average or higher across central and eastern Europe, northern Russia, northeastern Canada, and southern parts of South America [5]. Besides, in indoor environment, high CO2 levels may give rise to adverse health effects like headache, fatigue, eye symptoms, chronic asthma, bronchitis, sore throat, and respiratory tract symptoms [6]. According to the American Society of Heating, Refrigerating and Air Conditioning Engineers

(ASHRAE) Standard 62–1989, the CO2 concentration in occupied buildings should not exceed 1000 ppm [7]. In some fields like green­ house planting, there is a need for the detection of CO2 in low concen­ tration (usually below 300 ppm). While in some other fields such as modified atmosphere packaging for fruits and vegetables, high CO2 concentrations (up to 25%) need to be monitored. In order to better use of CO2 and curb globally warming effectively, dramatic efforts have been taken into developing CO2 sensors that can detect a wide range of CO2 contents with high response, rapid response/recovery speed and satisfying accuracy. Fig. 1B shows that the number of papers on CO2 sensors grows steadily from 2010. There are many types of CO2 gas sensors, such as gas chromatog­ raphy (GC) and mass spectrometers (MS) [8], Severinghaus electrode [9], optical [10,11], electrochemical [12–14], acoustic [15,16], work function [17–20], and capacitive based sensors [21–25]. However, compared with chemiresistive sensing devices, they suffer from many drawbacks, such as high maintenance cost (GC, MS and Severinghaus electrode) [8], complicated and short device life-time (optical and electrochemical) [26,27], and susceptible to interference gases (work-­ function based and acoustic) [28]. Chemiresistive devices, whose operation mechanism is based on the phenomenon that adsorption/de­ sorption of analyte gases can cause changes of electrical parameters, are drawing more and more research interests nowadays owing to their

* Corresponding author. E-mail address: [email protected] (Z. Fan). https://doi.org/10.1016/j.mssp.2019.104820 Received 18 September 2019; Received in revised form 30 October 2019; Accepted 31 October 2019 Available online 15 November 2019 1369-8001/© 2019 Elsevier Ltd. All rights reserved.

Y. Lin and Z. Fan

Materials Science in Semiconductor Processing 107 (2020) 104820

Table 1 The main advantages and disadvantages of nanostructured materials for chem­ iresistive CO2 sensors. Type

Advantages

Disadvantages

Metal oxides [29,30]

1) Low price and easy fabrication 2) Abundant morphologies with high area/volume ratio

1) High operating temperature and low stability 2) Post-treatments, such as annealing and calcination are often needed

Carbon nanotubes [50]

Graphene based [68, 69]

Polymers [70]

3) Easily modified with other nanostructured semiconductors 1) Low operating temperature 2) High carrier mobility 3) Good mechanical properties that hold potential usage for flexible sensors 1) High sensitivity due to super conductance and low crystal defect density 2) Lots of binding sites for gas adsorption and modification of RGO 3) Good mechanical properties that hold potential usage for flexible sensors 1) Low operating temperature and low fabrication cost 2) High sensitivity and easily modified with other phases

1) Relatively expensive 2) Poor selectivity for raw CNTs

1) Prone to conglomeration between layers 2) Low yield and high price

1) Low stability and long recovery time 2) Poor mechanical strength

Fig. 1. (A) Atmospheric CO2 concentration at Mauna Loa Observatory from July 1958 to July 2019 [5], (B) The number of CO2 gas sensor-related papers in Web of Science as of September 15, 2019.

small size, low cost, modest power consumption, long term stability and so on [29]. Nanostructured materials show great potential for chemir­ esistive gas sensors. First of all, the large surface-to-volume ratio of nanostructured materials provides much larger portion of surface atoms than buck atoms, contributing to more active sites for the analyte gas adsorption/reaction [30]. Secondly, recent studies reveal that crystal facets with high reactivity can be modulated to be exposed on the sur­ face of the nanostructured sensing layer [31,32]. Most importantly, when the grain size (D) of nanostructured material is comparable to the thickness of the electron depletion layer (2δ), the sensitivity of the gas sensor can be dramatically improved [33]. Metal oxide semiconductor (MOS) nanomaterials are among the first to be introduced as gas sensing materials due to high response, low cost and easy preparation [29]. Other types of nanostructured materials, such as carbon-based semi­ conductors [34,35] and conducting polymers [36] are also introduced into the family of sensing layer materials in recent years. Table 1 shows the main advantages and disadvantages of different types of nano­ structured materials used for sensing purposes. As is known, the appli­ cations of pristine nanostructured materials are limited because they typically suffer from drawbacks such as relatively low sensitivity, sus­ ceptible to RH and poor selectivity. Compositing, which is the emphasis of this paper, is thought to be an effective way to tackle the problems pristine nanostructured materials are facing. Nanostructured materials used for chemiresistive sensors are categorized and their working mechanisms are briefly summarized in Fig. 2. So far, significant efforts have been devoted to push chemiresistive CO2 gas sensors towards practical application, however, a comprehen­ sive review focused specifically on the recent development of chemir­ esistive CO2 gas sensors is still lacking. Therefore, the authors present here an overview of the past developments, the current progress of the

Fig. 2. Categories of nanostructured materials and their sensing mechanisms.

state-of-the-art chemiresistive CO2 sensors and their future perspectives. Several other excellent reviews on gas sensor materials [37–51], nano­ structures in gas sensing [29,52–56], p-type oxides in gas sensing [57, 58], gas sensing fundamentals [33,59–63], H2 sensors [28], NOx sensors [64] and CO2 sensors [65–67] may further aid in background under­ standing of the content presented here. The references selected here are far from enough and fully exhaustive studies on this field are still needed. However, this paper is meant to highlight some specific features in the field of chemiresistive CO2 gas sensors and to furnish a repre­ sentative scenario of the advances and progresses in CO2 monitoring domain. 2

Y. Lin and Z. Fan

Materials Science in Semiconductor Processing 107 (2020) 104820

In the present review, we first present the CO2 gas sensing mecha­ nism (section 2). Then we introduce research progresses of chemir­ esistive CO2 sensors based on pristine semiconductors (section 3). Afterwards we focus on compositing strategies to enhance the CO2 sensing properties in existing literatures by synergistic effects and their corresponding mechanisms (section 4). Finally, we conclude with cur­ rent challenges and future directions to explore (section 5). It is the hope of the authors that by compiling and comparing the information and results from relative studies, the readers will get a more coherent un­ derstanding of the CO2 sensing mechanisms and further advances in the CO2 detecting field will be facilitated.

versa for p-type semiconductors. Although lots of interesting results and theories behind their sensing activities have been revealed, there remain ample opportunities and challenges to probe the fundamental CO2 sensing mechanism. Herein, a brief introduction of the CO2 gas sensing mechanisms based on n-type semiconductor is given (here, CO2 is regarded as a reducing gas). Generally speaking, there are two basic theories regarding the sensing mechanisms: the ionosorption model [76,77] and the oxygen-vacancy model (reduction-reoxidation mechanism) [78]. Although neither the ionosorption nor the oxygen-vacancy model can be used to explain all the experimental observations, they do serve as crucial guidelines for understanding the relations between the nano­ material structures and their sensing performances.

2. Detection mechanism in chemiresistive CO2 sensors The simplest chemiresistive device can be constructed by printing metal electrodes on a substrate (cylindrical and planar), e.g., Al2O3 or Si wafer [71] (Fig. 3A). Besides, other types of chemiresistive sensor de­ vices including sensor arrays [72] (Fig. 3B), flexible sensor [73] (Fig. 3C) and UV-assisted sensor system [74] (Fig. 3D) are emerging in recent years. The structures of theses sensor devices well complement the traditional sensor devices and present a wide range of possibilities for future development of advanced sensor devices. Knowledge of sensing mechanisms is necessary to evaluate the wide variety of methods, which are capable of producing gas sensing films with different morphologies and structures. Semiconductors are typi­ cally classified into n-type semiconductors (in which electrons are the majority carriers) and the p-type (in which holes are the majority car­ riers) [75]. The resistance of n-type semiconductors is decreased upon contact with a reducing gas and increased with an oxidizing one, vice

2.1. Ionosorption model Compared with the oxygen-vacancy model, the ionosorption model is employed more often, where the process of gas detection is subdivided in a reception and a transduction sub-processes [76,77]. Initially, the atmospheric oxygen adsorbs on the surface of the semiconductor, extracting electrons from the conduction band (CB) of the semi­ conductor and molecular (O2 ) and atomic (O , O2 ) oxygen ions are formed (Eq. (1)) [76,79]. Depending on the ambient temperature, the formed oxygen ions are predominantly as O2 (below 420 K) or as O (between 420 and 670 K). Above 670 K, the parallel formation of O2 occurs, which is then directly incorporated into the lattice above 870 K [80]. As shown in Fig. 4, the delocalization of electrons results in the accumulation of oxygen ions on the surface of the sensing layer, which is

Fig. 3. Represents of chemiresistive gas sensor devices: (A) traditional prototype of gas sensors. Adapted from Ref. [71], copyright (2000), with permission from Elsevier. (B) Chemiresistive sensor arrays based on porphyrin functionalized single-walled carbon nanotubes. Adapted from Ref. [72], copyright (2015), with permission from American Chemical Society. (C) Graphene-oxide-based flexible sensor. Adapted from Ref. [73], copyright (2013), with permission from American Chemical Society. (D) UV assisted tellurium nanotube based chemiresistive sensor. Adapted from Ref. [74], copyright (2014), with permission from Elsevier. 3

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Materials Science in Semiconductor Processing 107 (2020) 104820

called depletion layer [81]. Band-bending is incurred in this process, in which the conduction band (CB) and the valence band (VB) bend up­ wards with respect to the Fermi level, creating potential barrier on the surface. The merging of two depletion layers between two grains leads to the formation of Schottky barrier, which is influenced by the material conductivity and thus can be modified by the surface reaction [82–84]. When CO2 is introduced (here, CO2 is regarded as a reducing gas), CO2 molecules react with oxygen ions pre-adsorbed on the surface of semi­ conductor, forming a meta-stable compound (CO3) with a very short life [85,86]. In this process electrons trapped by oxygen ions are released to the semiconductor, thereby increasing the electron concentrations in n-type semiconductors (Eq. (2)) [76,87]. Since the concentration of CO2 reacting with oxygen ions on the surface has a proportionable effect on the conductivity variation, the electrical parameter variations (i.e. cur­ rent, resistance) of CO2 sensors can be used to detect the concentration of CO2 gas. β O2ðgÞ þ αe ↔ OαβðadÞ 2

(1)

1 α CO2ðgÞ þ OαβðadÞ ↔ CO3ðadÞ þ e β β

(2)

channel, which is controlled by the concentration of oxygen ions on the surface of the sensing layer. When D � 2δ, the depletion layer occupies the whole grain and high sensitivity is obtained as the concentration of oxygen ions affects the whole semiconductor [33]. It should be noted that despite the wide use of this model, few spectroscopic evidences have been collected in situ to determine the oxygen ions’ contribution during gas detection [79]. 2.2. Oxygen-vacancy model Since this model has been reviewed in detail by Gurlo et al. [90], only a short summary is given here. Take the n-type SnO2, in which the ox­ ygen vacancies acting as electron donors, as an example. It is widely accepted that the conductivity of SnO2 was closely related to its non­ stoichiometry (SnO2-x, 0 < x < 2), which introduced oxygen vacancies (V⋅⋅O ) in the SnO2 bulk [91]. As illustrated in Fig. 5, in the absence of oxygen, V⋅⋅O dominates the sensing activities due to the nonstoichiometry of SnO2. When CO2 is introduced, alternate reduction and re-oxidation (Mars-van Krevelen mechanism) occur on the surface of SnO2, causing the variation of surface conductivity, which is characterized by the sensing behaviors. To be specific, CO2 reacts with V⋅⋅O , forming neutral oxygen vacancy (Vx0 ). Afterwards, the ionization of Vx0 will release electrons to the semiconductor, leading to the decrease of the sensor resistance. However, in the presence of oxygen, large amounts of V⋅⋅O will be transformed to lattice oxygen (Ox0 ). When CO2 is introduced, CO2 will react with the lattice oxygen (Ox0 ), leading to the formation of neutral oxygen vacancies (Vx0 ) and a meta-stable compound (CO3). Then the neutral oxygen vacancies go through the process of ionization, as mentioned above [90,92]. Although a number of works have been carried out to explain the gassensing performance using this theory [93,94], several problems remain

The depth of electron depletion layer is also known as Debye length (δ). Generally, the relationship between grain size (D) and δ has sig­ nificant impacts on the sensitivity of the materials [76,88,89]. As is known, the conductivity variation of semiconductor is controlled by the thickness of surface depletion layer. When D≫2δ, the sensitivity is relatively low since the depletion layer only accounts for a small part of the sensing materials and the potential barrier variations in the present of CO2 do not disturb the overall conductivity of the sensing layer very much. When D > 2δ, moderate sensitivity is expected as the conductivity variation is mainly influenced by the wideness of the conduction

Fig. 4. Schematic interaction reaction and energy level of n-type semiconducting materials (A) before and (B) after exposure to CO2. (C) The schematic diagram of the corresponding band structure. 4

Y. Lin and Z. Fan

Materials Science in Semiconductor Processing 107 (2020) 104820

XRD patterns for film S1 (40 nm) was reported to be larger than those of S2 (100 nm) and S3 (300 nm), indicating the smaller grain size of S1. Unfortunately, the low response of 1.01 toward 1000 ppm CO2 largely hinders the practical application of ZnO. A versatile way to improve the response of MOS is to increase the amount of active adsorption sites on its surface by morphology modification. In this regard, ZnO nanowires (NWs) [98] (Fig. 6B) and ZnO nanoparticles (NPs) based thin film [99] (Fig. 6C) were fabricated. Their gas sensing properties were greatly enhanced due to the fact that the large interior space of the super structures provided more active sites and thus much more space for the adsorption and reaction between CO2 gas and adsorbed oxygen ions. In addition to ZnO NWs, n-type CdO NWs also find important applications in CO2 detection. Krishnakumar et al. [100], for example, synthesized CdO NWs by a microwave-assisted synthesis method for sensing CO2. With a large surface-to-volume ratio and a Debye length comparable to the NW radius, CdO NWs possessed superior sensitivity to its thin film counterparts. SnO2 is also a promising material for CO2 gas sensing applications due to its low cost, high gas response and chemical stability. Hu et al. [101] fabricated SnO2 nanocrystalline powders for CO2 sensor (Fig. 6D). It was revealed that the response was much smaller at dry air (1.048@2000 ppm CO2) than that at 14% relative humidity (RH) (1.24@2000 ppm CO2) at 240 � C. The authors performed DFT calcula­ tion to reveal the sensing mechanism, verifying that in wet air the re­ action between CO2 and O of pre-adsorbed OH on the SnO2 (110) surface instigated the formation of carbonates and promoted the CO2 sensing performance. While in dry air, the interactions between CO2 and surface pre-adsorbed O2 /O were extremely weak, inhibiting the CO2 sensing performance. It should be noted that RH is a complex issue affecting CO2 sensing, which will be discussed in detail in section 5. Since the response of p-type MOS based gas sensor to a given gas is usually equal to the square root of that of an n-type MOS (with the identical morphological configurations) based gas sensor [57,58,102], n-type MOS based gas sensing materials are more widely used in the gas detection field. However, p-type MOS show fantastic response toward certain gases such as CO2 and H2S [57,58]. Many p-type oxides including CuO [103–105], rare earth oxides (Re2O3, ReOCl and Re2O2CO3. Where Re ¼ rare earth) [106,107], and perovskite oxides [108–113] have been widely studied for CO2 detection, among which rare earth oxides have drawn tremendous attention for monitoring CO2 owing to their outstanding catalytic properties and alkalinity. LaOCl is one of the most promising rare earth oxides for high sensitive and se­ lective CO2 gas sensors because of the favorable absorption of CO2 on the LaOCl surface through the formation of a carbonate on the lanthanum site [75]. Marsal et al. [106] studied the sensing characteristics of LaOCl NPs toward CO2 at a wide range of RH. The response toward 2000 ppm CO2 was 3.4 under dry air and RH had a positive effect on the response. Rare earth metal oxycarbonates such as neodymium dioxide carbonate (Nd2O2CO3) [107] and lanthanum dioxide carbonate (La2O2CO3) [114, 115] also show good sensing properties toward CO2 due to the formation of oxycarbonate phase. In addition to rare earth oxides, CuO with a narrow band gap of 1.3–2.1 eV has great merits for CO2 sensing appli­ cations as a result of high conductivity, nontoxicity and abundant availability of copper in nature. Abdelmounaïm et al. [103] reported nanostructured CuO porous film by spray pyrolysis method. The ob­ tained film using precursor with a concentration of 0.05 M led to a porous structure with larger active surface area, showing high perfor­ mance at room temperature (1.03@100 ppm CO2, response/recovery time of 10 s/6 s), and was very promising for practical applications. Perovskite oxides, commonly with the structure of ABO3 (A ¼ Rareearth element), are also widely used for CO2 detection [108–113]. Qin et al. [108] prepared LaFeO3 nanocrystalline powders by sol-gel method with subsequent annealing at 800 � C. At 300 � C, the response of the sensor to 2000 ppm CO2 was 2.19 with response/recovery time of 240 s/480 s. The possible CO2 sensing mechanisms for LaFeO3 sensor were investigated with first principles calculations, demonstrating that CO2 interacted with the pre-adsorbed oxygen ions on the surface of

Fig. 5. Schematic model representing oxygen-vacancy reaction in SnO2 in the absence (A) and in the presence (B) of oxygen in the atmosphere, respectively. Ox0 : lattice oxygen, Vx0 : neutral oxygen vacancy, V⋅O : singly ionized oxygen va­ cancy, V⋅⋅O : doubly ionized oxygen vacancy.

unaddressed. For example, the role of vacancy diffusion in the metal oxide bulk is strongly material and temperature dependent and requires further exploration. Furthermore, surface reduction-reoxidation mech­ anisms of metal oxides based gas sensors at their operating temperature ranges (250–450 � C) are relatively immatured. For example, the surface reduction-reoxidation kinetics of SnO2 based gas sensor at 250–450 � C are relatively slow compared with its measured short response times [33,95], suggesting that other mechanisms (e.g. chemisorption) could play a synergistic role in the gas sensing activities of such devices. 3. Chemiresistive CO2 sensors based on pristine semiconductors For simplicity, three most commonly used types of pristine CO2 gas sensing materials are presented in this section, namely metal oxide semiconductors (MOS), carbon-based semiconductors, and other semi­ conductors such as selenide (telluride), silicon-based semiconductors, and conducting polymers. Elaborate data for CO2 gas sensors based on pristine semiconductors are presented in Table 2. 3.1. Metal oxide semiconductors (MOS) The time that MOS were used as gas sensors can be dated back to 1960s [96,97]. Since then plenty of efforts have been devoted to the fabrication of MOS with fabulous morphologies and structures in order to improve their sensitivity, speed, selectivity and stability (known as “4S”). Among various MOS, n-type ZnO with a direct wide band gap of 3.3 eV at 300 K is most widely adopted for gas detection due to its high chemical sensitivity, ease of preparation with varied morphologies and non-toxicity. Saraswathi et al. [31] synthesized ZnO film by DC sput­ tering method for CO2 detection. By controlling the thickness of the deposited ZnO film, the grain size and crystalline orientation could be precisely modulated and the response was greatly enhanced with the decrease of grain size (Fig. 6A). Due to the formation of compressive stress during annealing, the obtained ZnO films were preferentially oriented, which led to the exposure of the unit cell toward (002) plane. With the increase of film thickness, the stress values were found to decrease due to film relaxation. The FWHM value of the (002) peak in 5

Y. Lin and Z. Fan

Materials Science in Semiconductor Processing 107 (2020) 104820

Table 2 A summary of CO2 gas sensors based on pristine semiconductors. CO2 character

Sensing material

Synthesis route

Morphology

Meas. Temp. (� C)/AT.

Response/ Concentration (ppm)

tres/trec (s)

Function of RH

Ref.

Oxidizing Oxidizing Reducing

ZnO ZnO ZnO

Thin film Nanowires Thin film

300/N2 200/air 350/air

1.01/1000 1.04/15 lit/min 2.86/400

20/20 8/40 75/108

– – Inhibited

[31] [145] [99]

Reducing

ZnO

Magnetron sputtering Sol-gel Chemical spray pyrolysis Hydrothermal

400/air

1.11/5000

–

–

[146]

Reducing

SnO2

Co-precipitation

Flower-like nanopowders Nanoparticles

240/wet air, 14%RH

1.24/2000

350/4

[101]

Reducing Reducing

SnO2 CdO

Mechanical milling Wet chemical

Nanoparticles Nanowires

400/air 250/dry air

1.14/1000 1.02/5000

Reducing

CdO

Coprecipitating

Nanopowders

250/dry air

1.03*/5000

–

[148]

Reducing Oxidizing

CeO2 CuO

Nanoparticles Porous film

400/N2 RT/vacuum

1.32*/800 1.04*/100

– –

[149] [103]

Reducing

YPO4

Nanobelts

400/air

–

[150]

La2O3 LaOCl Nd2O2CO3 La2O2CO3

Microrods Nanopowders Nanoparticles Nanorods

250/air 260/dry air 350/wet air, 50%RH 325/wet air, 50%RH

1.92/350 3.40/2000 4.00/1000 7.08/3000

Inhibited Promoted Promoted Promoted

[151] [106] [107] [115]

Reducing Reducing

La2O2CO3 LaFeO3

Hydrothermal Sol-gel

Ellipsoids Nanoparticles

320/dry air 300/dry air

2.25/2500 2.19/2000

– –

[114] [108]

Reducing

GdCoO3

Nanoparticles

400/dry air

1.10/75%

–

[111]

Reducing Reducing Reducing

Yb0.8Ca0.2FeO3 Sm0.9Ba0.1CoO3 La0.8Sr0.2FeO3

Solution polymerization Sol-gel Solution method Sol-gel

200/ 136 50/73 – – 900/ 1800 53/120 240/ 480 10/5.3

–

Reducing Reducing Reducing Reducing

Coprecipitating Pneumatic spray pyrolysis Surfactant-assisted colloidal Chemical bath Sol-gel Sol-gel Co-precipitation

– 200/ 300 200*/ 300* – 10/6

Maximum at 34% RH – –

Nanoparticles Nanoparticles Nanoparticles

260/wet air, 29%RH 410/air 380/dry air

2.01/5000 1.50/99.8% 1.25/2000

Promoted – –

[152] [110] [109]

Reducing Reducing

La0.875Ca0.125FeO3 In2Te3 (150 nm)

Sol-gel Flash evaporation

Nanoparticles Thin film

320/wet air, 38%RH RT/air

1.67/1000 1.12*/1000

Inhibited –

[153] [6]

Oxidizing

In2Te3

SHI irradiation

Thin film

RT/air

1.12/1000

–

[144]

Reducing

Porous Si

Thin film

–

[140]

Si NWs

RT/air (bia voltage ¼ 0.75V) RT/vacuum

1.90*/4mbar

Reducing

Electrochemical anodization Etching

24/31 202/— 660/ 300 – 0.05*/ — 15-20/ — 260/55

–

[141]

Reducing Reducing Oxidizing

CNT CNT CNT

CVD CVD CVD

RT/vacuum RT/vacuum RT/air

1.12/800mTorr 1.02/800 1.09/500

– – –

[117] [119] [118]

Oxidizing

CNT

CVD

RT/air

1.23/500

[118]

Mechanical cleavage Electrochemical exfoliation Airbrushing Hydrogen plasma

RT/air RT/air

1.26/100 1.04/200

– –

[120] [154]

Oxidizing Oxidizing

Graphene Few-layered graphene RGO RGO

385/ 412 8/10 11/—

–

Reducing Oxidizing

NTs NTs Horizontally aligned CNT array Random CNT network Nanosheets Nanosheets

104/ 125 – – 33/46

Nanosheets Nanosheets

RT/dry air RT/N2, 37% RH RT/air, 68% RH

1.02*/5000 3.45/1500 1.18/1500

– –

[155] [121]

Oxidizing

GO

Spray pyrolysis

Nanosheets

RT/N2

1.01*/30sccm

–

[156]

Oxidizing

Nanoporous GO

Spray pyrolysis

Nanoporous films

RT/dry air

1.37/60sccm

–

[157]

Oxidizing

Double-layer graphene

transferring

Nanosheets

RT/Ar, ~3% RH

~1/0.85 bar

– – 240/ 240 130*/ 150* 25/ 674.7 3-5/—

–

[125]

NWs

1.10*/2mbar

[147] [100]

Note: 1) Meas. Temp. ¼ Measure temperature, AT ¼ Background atmosphere, tres ¼ response time, trec ¼ recovery time. 2) S ¼ Rg/Ra when Rg > Ra and Ra/Rg when Rg < Ra. (Ra: resistance in carrier gas, Rg: resistance in CO2 atmosphere). 3) * Denotes a value not explicitly stated in the study, but approximated from a graphical plot. 4) “Function of RH” refers to the role of relative humidity in the CO2 sensing experiments, where “inhibited/promoted” means the presence of humidity inhibits/ promotes the sensing performance, and “—” denotes that the influence of humidity is not mentioned in the cited reference.

LaFeO3 (010) crystal face. However, the resistance of the ABO3 based sensor is very high, hampering its practical application. To solve this problem, alkaline-earth metal is commonly introduced as a partial substitution of rare earth element (M1-xNxBO3, where M ¼ Rare-earth element, N ¼ alkaline-earth metal). When the amount of alkaline-earth

metal doping is low, holes will be produced due to the ionization of ½NXM �, leading to an increase in conductivity. For instance, Hu et al. [109] developed a CO2 sensor using La0.8Sr0.2FeO3 via sol-gel route. When La3þ ions in LaFeO3 were replaced by a small amount of Sr2þ ions, holes were produced due to the ionization of ½SrxLa �, enhancing the 6

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Materials Science in Semiconductor Processing 107 (2020) 104820

Fig. 6. (A) Sensitivity as a function of operating temperature for the ZnO films S1 (40 nm), S2 (100 nm) and S3 (300 nm) toward 1000 ppm of CO2, with the insert chart showing the values of film stress, FWHM values in XRD patterns and particle sizes for films with different thicknesses. Adapted from Ref. [31], copyright (2014), with permission from Elsevier. (B) SEM micrograph of ZnO NW arrays. Adapted from Ref. [98], copyright (2016), with permission from Elsevier. (C) SEM micrograph of ZnO round-shaped nanoparticles. Adapted from Ref. [99], copyright (2018), with permission from Elsevier. (D) FE-SEM image of SnO2 powders. Adapted from Ref. [101], copyright (2016), with permission from Elsevier.

conductivity of the sensing layer. Similar cases can also be found in some other sensors based on SmCoO3 [110] and GdCoO3 [111]. In general, as summarized in Table 1, Sensors based on MOS are very advantageous because of their small size, simple construction, easy preparation and low cost. However, they suffer from narrow detection range, high power consumption because of high working temperatures and susceptible to RH and other interferences. Therefore, their perfor­ mance needs to be improved for practical application.

further improve the response of CNTs-based gas sensors, recently, hor­ izontally aligned CNTs (H-CNTs) and random CNTs (R-CNTs) were synthesized by Zhang et al. [118] using thermal CVD method and the obtained CNTs were functionalized by diazonium tetrafluoroborate (4-BBDT), as exhibited in Fig. 7A–B. It’s interesting to find that the H-CNTs based sensor had shorter response/recovery time of 33 s/46 s with however lower response of 1.09 toward 500 ppm CO2 gas compared with R-CNTs based sensor (response of 1.23, response/r­ ecovery time of 385 s/421 s) at RT. The faster response/recovery for the H-CNTs based sensors could be attributed to the absence of inter-tube junctions. The larger response value of R-CNTs based sensor might result from the bending of the CNTs, which could cause an overlap of the electron states in adjacent CNTs walls, leading to an increase in the accessible number of conduction channels. Thanks to their immense mechanical robustness, CNTs are also widely utilized in flexible gas sensors. Young et al. [119] reported the fabrication of flexible CNTs based CO2 gas sensors. It’s noteworthy that even under a curve radius of 0.5 cm, the sensors exhibited obvious CO2 response at RT and the sensing performance remained stable for a long period of time. Since the discovery of graphene by Novoselov and Geim in 2004 [122], tremendous efforts have been focused on graphene and its de­ rivatives viz. pristine graphene, graphene oxide (GO) and reduced gra­ phene oxide (RGO) due to their high mechanical strength, good thermal stability, ballistic conductivity, high carrier mobility at RT (200000 cm2V 1s 1), relatively low fabrication costs in large scale and large surface area (theoretical surface area of 2630 m2/g) [34,68,69]. Espe­ cially, graphene materials have relatively low Johnson noise because of their high conductance as well as low crystal defect density, making them capable of screening charge fluctuation better than one-dimensional CNTs [123]. A tiny change of the electrons amount

3.2. Carbon-based semiconductors Traditional MOS-based gas sensors normally have to be operated at high temperatures. In the search of new materials capable of operating at low temperatures, carbon-based semiconductors come into sight due to their high-quality crystal lattices, high carrier mobility, low noise and good mechanical properties [50]. Among all the carbon-based semi­ conductors, carbon nanotubes (CNTs) and graphene are most frequently used in the field of gas detection and both of them are elaborately elucidated below. CNTs are promising candidates for gas sensing owing to their large effective surface area with many available sites for adsorbing gas mol­ ecules, their hollow geometry and one-dimensional nanoscale morphology [35]. These advantages promote adsorption and reaction of target molecules with high efficiency and speed [35,116]. Huang et al. [117] reported a CNTs-based gas sensor synthesized by catalytic thermal chemical vapor deposition (CVD) and explored the influence of catalyst pretreatment on the sensor responses. It was revealed that catalyst pretreatment reduced the chemisorption of oxygenic contaminants and led to better graphitization of CNTs mats, enhancing the CO2 response (S ¼ 1.121@800 mTorr CO2) at room temperature (RT). In order to 7

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Materials Science in Semiconductor Processing 107 (2020) 104820

Fig. 7. SEM images of (A) R-CNTs and (B) H-CNTs with insert showing their sensor responses to 500 ppm CO2 before and after functionalization with 4-BBDT, respectively. Adapted from Ref. [118], copyright (2017), with permission from Elsevier. (C) Response of the graphene in the presence of 100 ppm CO2 at different temperatures, Adapted from Ref. [120], copyright (2011), with permission from Elsevier. (D) Sensor response of GO, rGO-P40 (hydrogen plasma reduced for 40 s fed with 50 sccm hydrogen flow rate) and rGO-F20 (hydrogen plasma reduced for 40 s fed with 20 sccm hydrogen) with CO2 concentration ranging from 0 to 1500 ppm at 37% RH, Adapted from Ref. [121], copyright (2013), with permission from Elsevier.

caused by gas adsorption can induce a noticeable variation in the conductance of graphene [124], making it possible of detecting even traces level of gas. The first graphene based gas sensor was reported in 2007 by Novoselov’s group [34], which was capable of detecting indi­ vidual gas molecules attached to or detached from the graphene surface. This study has opened up a new avenue to researchers for developing gas sensors based on graphene. Yang et al. [120] reported a graphene-based CO2 gas sensor by mechanical cleavage and micromachining. Due to the weak physical interaction of CO2 on the surface of graphene, which facilitated the adsorption and desorption of CO2 molecules in the detecting process, the sensor exhibited high response (S ¼ 1.26) toward 100 ppm CO2 gas with short response/recovery time of 8 s/10 s at room temperature. The room temperature sensing performances of the sensing device were comparable to those working at higher temperatures (40 � C, 60 � C), as illustrated in Fig. 7C. In order to further understand the gas sensing properties of graphene in the presence of humidity and CO2, Niklaus et al. [125] fabricated double-layer graphene device and investigated its resistance change when exposed to humidity. It was found that for relative humidity levels of less than ~3% RH, the resis­ tance of double-layer graphene was not significantly influenced by the humidity variation. Whereas for high RH levels, a decrease in chamber humidity caused a corresponding resistance increase in the graphene device. The CO2 sensing properties under low humidity (less than ~3% RH) was further explored, as shown in Table 2. RGO, containing many dangling oxygen atoms and defects as binding sites for gas analytes, is appreciated to be studied and used for gas sensors. Moreover, it can be produced on a large scale with a relatively low cost compared with graphene. Huang et al. [121] studied a RGO-based CO2 sensor developed by reducing GO via hydrogen plasma (Fig. 7D). Hydrogen plasma reduction created defects within the carbon basal plane, which can be related to the increase of SP2 crystallite sites

(adsorption active sites), thus contributing to the enhancement of the sensing performance. RGO based sensor with the hydrogen plasma reduction time of 40 s fed with 20 sccm hydrogen (RGO-F20) showed a response of 3.45 toward 1500 ppm CO2 in N2 (37% RH) at RT. Although graphene based nanomaterials have been continuously applied in various areas, its strong π-π interaction, hydrophobic interactions, and van der Waals forces between graphene sheets make it readily aggregate or restack, resulting in the decrease of surface area and carriers diffusion rate [126]. By contrast, 3D porous graphene hydrogel network (3DGH) has more advantages owing to its increased surface areas and reactive sites, and multidimensional electron transport pathways, which makes it an attractive alternative for gas sensing [127,128]. Sahiner et al. [129] prepared 3D super porous graphene aerogel (GA) by chemical reduction of GO with L-Ascorbic, which showed great CO2 detection potential for its super high conductivity and large surface area with high porosity. Its CO2 response could be tremendously improved (4000 times higher) after compositing with PANi polymer. Despite that graphene based materials have evoked upheaval for the last few years, there are many bottlenecks with graphene that need keen attentions to ensure the transformation of graphene from laboratory level development to commercial viability. For example, the selective detection of one specific gas in a gas mixture requires to be improved. The high fabrication cost and complicated fabrication techniques for high-quality graphene also need to be addressed. Albeit immeasurable potential, the lack of bandgap of graphene materials limits their use in the fields of high-performance gas sensors [130]. Other 2D layered nanomaterials such as layered metal disulfides and diselenides have also aroused huge interests for fabricating low temperature or RT gas sensors. Typical layered metal disulfides and diselenides based gas sensing materials include MoS2 [131,132], MoSe2 [133], WS2 [134], WSe2 [135], and SnS2 [136,137], which can be 8

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Materials Science in Semiconductor Processing 107 (2020) 104820

synthesized by mechanical exfoliation, chemical vapor deposition, epitaxial growth and liquid exfoliation. Regrettably, layered metal disulfides are, to the best of our knowledge, most widely used for the detection of nitride gases (NO2, NH3) due to large surface binding en­ ergies toward these gases [138]. Up to now, work has not been focused on CO2 detection using layered metal disulfides or diselenides. Although the CO2 sensing properties based on layered metal disulfides (dis­ elenides) remain yet to be seen, there will be no slow-down in scientific efforts to continuously improve the synthesis technology of layered metal disulfides (diselenides) and to explore advanced layered metal disulfides (diselenides) in the field of CO2 sensors.

catalytic effect can be classified as chemical sensitization and electronic sensitization, depending on whether the promoter changes the work function of the support material [160]. Chemical sensitization (Au, Pt, etc.) takes place via a “spill-over effect”. The adsorbed oxygen molecules on the surface of noble metal dissociate into chemisorbed monatomic oxygen and “spill-over” onto the base material surface. Electronic sensitization (Pd, Ag, etc.) results in a direct electronic interaction on the interface between the promoter and the host material. (4) Geometrical effects such as grain refinement and increased porosity play an impor­ tant role in surface area enhancement, thus improving gas accessibility [161]. In many cases, the improvement in the sensing performance of the composite is due to a synergistic effect of these factors. Among these four factors, the heterojunction interface is the most important consid­ eration when we design a semiconducting composite. Of all the in­ terfaces (p-n, n-n, p-p), the most common interface used to modulate gas sensing properties is the p-n junction (n-type backbone) [53,162,163]. In a p-n heterojunction, electrons transfer from the n-type to p-type with the holes flowing in the opposite direction until the system reaches equilibrium at the Fermi level. The intrinsic Fermi level of a n-type is usually higher than that of a p-type, thus leading to the formation of thick space charge layers (electron depletion layer in n-type and hole depletion layer in p-type) associated with the p-n junction, which significantly narrows the electrical transport channels and results in an increased resistance. When CO2 is introduced (here, CO2 is regarded as a reducing gas), the electrical transport channels widen due to the dona­ tion of electrons by reducing gas and a large decrease in the resistance occurs. As a result, the gas-sensing performance can be significantly improved attributing to the initial high Ra and largely decreased Rgas.

3.3. Other semiconductors Silicon-based materials such as porous silicon (PSi) and silicon nanowire arrays (SiNWs) are also potential candidates for gas sensing devices due to their ease of fabrication, low cost and large surface-tovolume ratio [139]. For example, Zouadi et al. [140] developed a CO2 gas sensor based on Al/CNx/PSi/Si structure by electrochemical anod­ ization. Here, CNx film was used to stabilize the PSi layer. It was found that the response value was about 1.9 with response/recovery time of 260 s/55 s toward a CO2 gas pressure of 4 mbar. Meanwhile, Naama et al. [141] reported the preparation of SiNWs by one-step chemical etching, which was used to assemble Al/SiNWs/p-Si/Al structure for CO2 gas detection. Its response was 1.10 with response/recovery time of 104 s/125 s toward 2 mbar CO2. However, Si is a chemically active material and has unstable surface properties due to oxidation and interaction with surrounding atmosphere, especially at higher temper­ atures. The operation temperature of Si-based devices is also limited due to the small band gap of silicon, which means that silicon-based devices have strong limitations for operation at high temperatures and in cor­ rosive atmospheres [142]. II–VI-based gas sensors (such as CdSe [143], In2Te3 [144]) and conducting polymer (CP) semiconductors like emer­ aldine base polyaniline [36] have also been investigated for CO2 detection. Nevertheless, II–VI-based gas sensors do not have the required stability and therefore these devices are not very promising for the sensor market [142]. CPs lead us to a new stage of gases detection owing to scalability, RT operations, facile property adjustment and high sensitivity. However, their long-term stability and recovery speed are not satisfactory [142].

4.1. Mixed composite structure Generally, mixed composite structure is formed by a simple mixture of two or more constituents, identified by a dash ( ) between the names of the compounds in this review. Mixed composite structures are commonly prepared during the process of synthesis or deposition of initial materials. Besides, they can also be formed by simply mixing the already-synthesized materials in certain proportions. It is believed that mixing two semiconductors in varied ratios could have analogous or at times substantially pronounced influences on their electrical and sensing performances. Based on their inherent characteristics, the mixing ratio of the two phases and synthesis methods, different mechanisms may account for their improved sensing performances.

4. Composite semiconductors and mechanisms for enhanced sensing performance

4.1.1. Interface effect of mixed composite structure Since the composite of perovskite-type BaTiO3 and PbO was reported to be used for CO2 sensing by Takita et al. [21] in 1990, more CO2 sensors based on perovskite complex structures such as BaTiO3–CuO [22–25,164–169] and LaFeO3–SnO2 [170] have been introduced. For example, Herr� an et al. [25] reported that the chemiresistive CO2 sensor based on BaTiO3–CuO thin film by RF magnetron sputtering from a BaTiO3–CuO equimolar target had a high response of about 1.09 toward 5000 ppm CO2 at 300 � C. The effect of mixing ratio on gas sensing properties was recently investigated by Xie et al. [170]. In his work, LaFeO3–SnO2 nanopowders were formed by mixing the as-synthesized LaFeO3 and SnO2 nanopowders. It was reported that LaFeO3–SnO2 in an equimolar amount (50Sn–La) showed the best response of 2.72 and shortest response time of 20 s toward 4000 ppm CO2 at 250 � C (Fig. 9A). The enhanced gas sensing performance of 50Sn–La was attributed to the formation of p-n heterojunction between p-type LaFeO3 and n-type SnO2, inducing the formation of built-in electric field between their interface. As a result, the energy band bended in the accumulation layer of LaFeO3 and the depletion layer of SnO2 in the interface region (Fig. 9B). Low molar ratio of SnO2 (e.g., 0, 20%) decreased the as-formed p-n heterojunctions amount whereas high molar ratio of SnO2 (e.g., 80%) decreased the amount of hydroxyl-related species, both of which deteriorated the sensor performance.

Since CO2 is a chemically stable gas, detection of CO2 using pristine chemiresistors is relatively difficult. A large number of CO2 gas sensors based on composite semiconductors therefore come into sight. Table 3 presents exhaustive CO2 sensor performance data using composite semiconductors as sensing materials. As we know, the combination types of composite semiconductors can dramatically affect the morphologies and thus the gas sensing perfor­ mances of the composite-based gas sensors. Three different structurearchitecture types namely mixed composite structure, second-phase decorated structure, and bi-layer/multi-layer film structure are most frequently implemented to enhance CO2 gas sensing properties (Fig. 8). The improvements in sensing performances of composite have been attributed to many factors such as interface effect, doping effect, cata­ lytic effect and geometrical effects. (1) Interface effect. The intimate electrical contact at the interface between two dissimilar semi­ conducting materials will lead to the equilibrium of Fermi levels across the interface to the same energy, usually resulting in charge transfer and the formation of a charge depletion layer. This is the basis for sensor performance enhancement [158]. (2) Doping effect. Incorporation of one material into a host material will result in lattice mismatch or conductivity change of host material and may provide more adsorption sites for both oxygen and the analyte gas [159]. (3) Catalytic effect. The 9

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Materials Science in Semiconductor Processing 107 (2020) 104820

Table 3 A summary of CO2 gas sensors based on composite semiconductors. CO2 character

Sensing material

Synthesis route

Morphology

Meas.temp. (� C)/AT.

Response/ Concentration (ppm)

tres/trec (s)

Function of RH

Ref.

Reducing

2at%LaOCl–SnO2

Porous film

425/synthetic air

1.38/2000

–

–

[173]

Reducing Reducing

2.2mol%La2O3–SnO2 50at%La–ZnO

Electrostatic spray pyrolysis Mechanical milling Hydrothermal

400/air 400/air

1.52/1000 1.65/5000

– 90/38

– –

[147] [146]

Reducing Reducing

8at% LaOCl–SnO2 CuO–BaTiO3

Electrospinning Magnetron sputtering

Nanoparticles Flower-like Nanopowders Nanofibers Thin film

300/air 300/wet air, 40% RH

3.70/1000 1.09/5000

– –

[91] [25]

Reducing

CuO–BaTiO3

Magnetron sputtering

Thin film

3.30/5000

–

[203]

Reducing

CuO–BaTiO3

Magnetron sputtering

Thin film

RT/wet air, 40% RH, photoactivated 250/air

–

[204]

Reducing

LaFeO3–SnO2

Mixing

Porous film

250/dry air

2.72/4000

–

[170]

Reducing

ZnO2–CuO

Mixing

Thick film

300/wet air, 40% RH

1.33/4000

24/92 >120/ >80 300/ 300 >90/ >120 ＜20/ — –

[175]

Reducing Reducing

5at%Ca–ZnO 5 wt%Sn–CdO

Sol-gel Coprecipitating

Nanoparticles Nanopowders

450/synthetic dry air 250/dry air

2.13/5000 1.18*/5000

Maximum at 30%RH – –

Oxidizing Oxidizing Reducing Oxidizing Reducing

0.4SnO2-0.6WO3 Cr–TiO2 (3/3) Gd-CeO2 CuO-CuxFe3-xO4 2at%BaCO3–Co3O4

Mixing Magnetron sputtering Coprecipitating RF sputtering Grounding

Nanoparticles Thin film Nanoparticles Thin film Nanoparticles

RT/air 55/dry air 250/nitrogen 250/dry air 150/air, 20%RH

1.06*/300 1.09*/10000 1.82*/800 1.90/5000 1.04*/1000

– – – – Inhibited

[172] [174] [149] [104] [205]

Reducing Reducing

Sol-gel Chemical oxidative polymerization Impregnation Drop-coating Impregnation Drop-coating Drop-coating

Nanocomposites Porous film

RT/air RT/air

1.23/40 psi 1.56*/700

Promoted –

[206] [207]

Reducing Reducing Reducing Reducing Reducing

ppy@Fe2O3 ppy@FeCl3 (Py/ FeCl3 ¼ 4.29) SnO2 @ La2O3 SnO2@La2O3 (0.01 M) SnO2@LaOCl SnO2@LaOCl ZnO@LaOCl

Powder Nanopowders Nanopowders Nanowires Nanowires

400/dry air 400/air 350/air 400/dry air 400/air

1.79/2080 1.38/2000 1.02/2000 6.80/4000 3.50/2000

– Inhibited Promoted – –

[208] [190] [75] [192] [209]

Reducing

SnO2@4at%La

Impregnation

Nanoparticles

250/N2

1.42/500

–

[191]

Reducing

BaTiO3–CuO@1 mol% Ag BaTiO3–CuO@Ag

Doping

Powders

430/air

1.59/5000

–

[204]

Magnetron sputtering

Thin film

250/wet air, 40%RH

1.28/5000

–

[210]

CuO–[email protected]% Ag CuO–BaTiO3 @ 1%Ag CuO@BaTiO3

Magnetron sputtering

Thin film

300/wet air, 40% RH

1.15/5000

900/ 600 120/80

–

[167]

Magnetron sputtering Co-precipitating

250/air 120/dry air

3.00*/1500 1.24/700

90/120 5/18

– –

[204] [181]

Mixing

120/dry air

1.40/700

3/8

–

[181]

Reducing

CuO@1 wt% Ag–BaTiO3 ZnO@1 wt%Ag–CuO

320/air

1.34/1000

76/265

–

[211]

Reducing

ZnO@CuO

Mixing

320/air

1.28/1000

82/286

–

[211]

Reducing Reducing

In2O3@6at%CaO La2O3@Pd

Impregnation Dipping

Thin film Spheres decorated leaves Spheres decorated leaves Spheres decorated with leaves Spheres decorated with leaves Mesoporous Porous film

230/dry air 250/air

1.80*/2000 1.39/500

Promoted –

[182] [194]

Reducing Reducing

La2O3@Pd Si NWs@Au

Dipping Immersion

Thin film NWs

2.78/400 1.26*/0.5 mbar

Inhibited –

[212] [141]

Reducing

[email protected] wt% Sb2O3 PILs@La2O2CO3 SnO2@ZIF-67 SWCNT@PIL PCF@N-doped carbon GA/PANi TiO2/Al2O3 Pd/TiO2 RGO/PEI

In-situ chemical route

Thin film

250/air RT/vacuum, bia voltage ¼ 1.4V RT/dry air

1.22*/50

– 105/ 145 80/50 303/ 311 16/22

–

[183]

Drop-casting Mixing Grinding Deposition Chemical reduction ALD Thermal evaporation Airbrushing

Thin films Core-shell NT Porous NFs 3D aerogel Thin films Thin films Thin films

RT/air, 50%RH 205/Argon RT/N2 RT/N2 RT/air RT/air, UV light RT/N2 RT/air

1.12*/2400 1.165/5000 1.02/10 1.03/20000 4000/100% 1.44/5 3.91/12.5% 1.01*/3667

300/— 220/25 �60/— – – – – 14/14

– – Tolerant Tolerant – – – –

[186] [188] [184] [189] [129] [199] [201] [200]

Reducing Reducing Reducing Reducing Reducing

Reducing Oxidizing Oxidizing Oxidizing Reducing Oxidizing Reducing Reducing

Impregnation

1.80*/1000

– 110*/ 140* 127/42 – – 9.5h/— 192/ 215 – 210/ 1560 24/120 – – 15/19 15*/ 17* 20*/ 75* –

[171] [148]

Note: 1) 2) 3) 4)

Meas. Temp. ¼ Measure temperature, AT ¼ Background atmosphere, tres ¼ response time, trec ¼ recovery time. S ¼ Rg/Ra when Rg > Ra and Ra/Rg when Rg < Ra. (Ra: resistance in carrier gas, Rg: resistance in CO2 atmosphere). * Denotes a value not explicitly stated in the study, but approximated from a graphical plot. “Function of RH” refers to the role of relative humidity in the CO2 sensing experiments, where “inhibited/promoted” means the presence of humidity inhibits/promotes the sensing performance, and “—” denotes that the influence of humidity is not mentioned in the cited reference.

10

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Materials Science in Semiconductor Processing 107 (2020) 104820

Fig. 8. Schematic of the three composite combination types according to their morphology.

Fig. 9. (A) The response curves with error bar of four samples at 250 � C toward 4000 ppm CO2. (B) Energy band structure of LaFeO3 and SnO2 before contact (a) and the band bending upon the formation of p-n heterojunction (b). Adapted from Ref. [170]. Copyright (2016), with permission from Elsevier.

However, in some cases, there is no electron transfer between the two phases of the composite. One phase is very reactive to the target gas and acts as an “antenna” material, while the other phase serves as the conducting channel. Neri et al. [171] prepared ZnO nanopowders with different concentrations of Ca loading by mixing their precursors and subsequently annealing treatment. Different from the work of Xie et al. [170] that there existed an optimal ratio between the two phases, the sensor response showed a monotonic increase upon increasing Ca con­ tent from 0% to 5 at% and the 5 at% Ca-loaded ZnO sensors showed the highest response (2.13–5000 ppm CO2) compared with those with lower doping concentrations. Instead of forming heterojunction between two phases, the improved response of the Ca-loaded ZnO sensor was attrib­ uted to the increased CO2 adsorption in the presence of Ca loading, which was evidenced by FT-IR analysis. Instead of p-n heterojunction, the formation of n-n heterojunction can also promote the CO2 gas sensing performance. Dhannasare et al. [172] fabricated nanosized SnO2-WO3 polycrystalline by directly mixing SnO2 and WO3 powders and then heated at 950 � C. The composite based sensor showed linear variation of response with CO2 gas up to 1100 ppm and the 0.4SnO2-0.6WO3 (atom ratio of Sn and W is 40:60) based sensor showed the highest response of about 1.06 toward 300 ppm CO2 at RT while pure SnO2 showed poor response. The enhanced sensing perfor­ mance was attributed to the combined effect of n-n heterojunction be­ tween SnO2-WO3 which increased defects in the structure of SnO2, and the enlarged surface area by WO3 loading.

In an early work, La–SnO2 nanopowders were synthesized by Oh et al. [147] via mechanical milling of commercial SnO2 and La2O3 powders. The sensor response of 2.2 mol% La2O3–SnO2 (1.52@1000 ppm CO2) was significantly higher than that of Diagne’s work (1.38@2000 ppm CO2) [173] due to the decreased grain size by me­ chanical milling. Recently LaOCl-doped SnO2 nanofibers (NFs) were prepared by Xue et al. [91] via mixing the precursor solution of SnCl2 and LaCl3 followed by electrospinning for CO2 monitoring. Owing to the porous 1D NFs structures (Fig. 10A), the sensor based on 8 at% LaOCl–SnO2 NFs exhibited exceptional response (3.7) toward 1000 ppm CO2 at 300 � C with short response/recovery time of 24 s/92 s. The gas sensing mechanism was demonstrated by oxygen-vacancy model, which was similar with that elaborated in section 2. Lee et al. [146] synthesized La-loaded porous flower-like ZnO nanopowders consisted of spherical-shape NPs by mixing their precursor solutions/hydrothermal treatment (Fig. 10B–D). The results indicated that 50 at% La-loaded ZnO based sensor showed significantly higher response of 1.65 and more stable base resistance behavior at 400 � C than those with other con­ centrations of La doping (0, 1 at%, 4 at%, 10 at%). La doping led to the increase of active reaction site in the composite and facilitated the spillover effect. Besides rare-earth elements doping, Sn-doped CdO nanostructures were synthesized by Krishnakumar et al. [148] via adding Sn precursor to Cd precursor. It was confirmed that 5 wt% Sn–CdO promoted the CO2 sensing properties in terms of higher response (7 times-fold) and shorter response/recovery time compared with undoped CdO nanostructures. Sn4þ doping increased the resistance of CdO complex, contributing to more surface oxygens at the surface of the material. What’s more, Mardare et al. [174] prepared Cr-doped TiO2 thin films by RF reactive co-sputtering. There was an appreciable in­ crease in the sensor response of TiO2:Cr 3/3 (about nine times higher compared with the undoped TiO2 film) toward 10% CO2 at 55 � C. The substitution of Ti4þ by chromium assisted the phase transformation from

4.1.2. Doping effect of mixed composite structure Besides interface effect, doping is also a well-recognized strategy to promote and optimize sensing characteristics of the mixed composite structure. Incorporation of one cation into the lattice of a different metal oxide will result in lattice mismatch, thus tailoring the structure, morphology and electronic properties of the mixed composite. 11

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Materials Science in Semiconductor Processing 107 (2020) 104820

Fig. 10. (A) SEM (the insert shows the corresponding EDS spectra) micrograph of 8 at% La–SnO2. Adapted from Ref. [91]. Copyright (2016), with permission from Elsevier. EDX mapping spectra (B), low-magnification (C) and high-magnification (D) SEM image of 50 at% La-loaded ZnO. Adapted from Ref. [146], Copyright (2016), with permission from Elsevier.

anatase to rutile, increasing the number of oxygen vacancies and decreasing the crystallites size, both of which contributed to the enhancement of CO2 sensing activities.

4.2. Second-phase decorated structure Nanocomposite can also be prepared by incorporation of a second phase into an already-prepared nanomaterial matrix using methods such as co-condensation, wet impregnation, ion exchange, and covalent bonding [176–179]. This kind of structure is often identified by an “@” sign between compounds (e.g. A@B), where A represents a base material with the following B added over it [29,180]. It should be noted that all the nomenclatures of second-phase decorated structure composites referred in this paper are normalized into the name pattern ‘A@B’.

4.1.3. Geometrical effect of mixed composite structure Geometrical effect is another factor contributing to the sensor per­ formance enhancement in mixed composite structures. Mixing in­ troduces electronic effects and simultaneously affects the microstructure and growth kinetics. Purely geometric consideration involves grain size reduction and surface roughness increase, which induces surface area enhancement, thus creating more active sites for gas adsorption/ desorption. Furthermore, grain size reduction allows the entire nano­ particle to exist within the depletion zone, thus enhancing sensor response [33]. Beyond grain size reduction, geometric effects also include the modification of porosity to control the gas diffusion rates and modulating to expose active crystallographic facets on the surface of the sensing layer. Yurchenko et al. [175] prepared CuO film by drop-coating and ZnO2 was added as the combustion promotor. The addition of ZnO2 not only decreased the annealing temperature of CuO but also prevented the CuO-NPs from agglomeration to bigger particles, contributing to higher sensitivity. The same phenomenon was also re­ ported by Al-Kelesh et al. [149] in Gd-doped CeO2, where the Gd-doped CeO2 showed lower working temperature and better sensitivity due to the decrease of particle size by doping. In addition, the creation of a rough surface in mixed composite structures can also contribute to more surface area and enhance CO2 sensing properties, as reported by Dhannasare et al. for SnO2-WO3 based sensor [172]. Recently the effect of exposed crystallographic facets on sensor response has been exten­ sively investigated. Crystallographic facets with lower packing density have higher energy which can facilitate surface reactivity [101,161]. For example, LaFeO3 (010) could adsorb more oxygen, facilitating the re­ action between CO2 and oxygen ions [108]. Unfortunately, these re­ searches mainly focus on pristine semiconductors for CO2 sensing, the influence of exposed crystallographic facets on the gas sensing perfor­ mance of composite structures needs to be further explored.

4.2.1. Interface effect of second-phase decorated structure Recently, Sunkara et al. [181] synthesized CuO microleaves deco­ rated with BaTiO3 spheroids and Ag nanoparticles (CuO@Ag–BaTiO3). CuO@BaTiO3 was initially synthesized by mixing BaTiO3 powder with Cu precursor. Then CuO@Ag–BaTiO3 was prepared by incorporating the as-synthesized CuO@BaTiO3 nanocomposite with silver. The results revealed that 1:1 mol ratio of BaTiO3 and CuO was the most suitable composition (Fig. 11A) since the formation of equal p-n junction units facilitated the dissociation of molecule oxygen into atomic form and then oxygen ions (O and O2 ) on the surface, creating electron-depletion zone. In the subsequent reaction between CO2 and oxygen ions, chemisorbed oxygen ions were depleted, causing changes of electron-depletion layers, which in turn changed the resistance of the sensing materials and the energy barrier decreased at the grain bound­ ary of BaTiO3 and CuO (Fig. 11C). Ag additive served as catalyst that promoted CO2 adsorption by catalyzing the carbonation process of CuO, which not only decreased the optimum operating temperature from 140 to 120 � C but also enhanced the gas sensing performances (Fig. 11B). Just like the situation discussed in the mixed structure (section 4.1.1), in the second-phase decorated structure there also exist cases that no electron transfer occurs between two phases with one phase serving as an ‘antenna’ material. For example, mesoporous In2O3@CaO was synthesized by Pellicer et al. [182] through hard-template route fol­ lowed by adding Ca precursor to the as-synthesized In2O3. The improved sensitivity of CaO loaded In2O3 (1.8 toward 2000 ppm CO2) was attributed to the improved CO2 adsorption (CaO can easily react with 12

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Fig. 11. (A) Sensor response as a function of operating temperature toward 1000 ppm CO2 gas, with CuO loading in different mole ratios. (B) Sensor response of CB1:1 nanocomposite vs operating temperature for various weight percentages of Ag toward 1000 ppm CO2 gas. (C) Schematic illustration of energy band structures of p-CuO and n-BaTiO3 in the presence of CO2. Reprinted from Ref. [181]. Copyright (2017), with permission from ACS.

CO2 to form bicarbonates) by Ca loading and the high surface-to-volume ratio provided by the mesostructure composite. Recent studies have found that the combination of graphene with MO (graphene@MO), the combinations of polymer with MO (poly­ mer@MO) or polymer (polymer@polymer) are also promising strategies to improve the sensitivity, enhance the selectivity and lower the oper­ ating temperature of gas sensors. Waghuley et al. [183] prepared Sb2O3 quantum dots (QDs) anchored graphene by impregnating graphene with Sb precursor and subsequently heat-treatment in N2. With the increase of graphene composition, more graphene sheets were damaged by Sb2O3 QDs, thus generating much more defects characterized by intensities ratio of the ultraviolet (IUV) to visible deep levels (IDL). The increased defects density (mainly oxygen vacancies) worked as active adsorption sites for atmospheric oxygen, thus enhancing the CO2 sensing perfor­ mance (1.22@50 ppm CO2 with response/recovery time of 16 s/22 s at RT for 1.6 wt% graphene@Sb2O3) (Fig. 12A and B). Poly (ionic liquid)

wrapped SWNTs (SWNTs@PIL) were fabricated by grinding SWNTs powder in concentrated PIL solution by Jin et al. [184]. The SWNTs@PIL based sensor exhibited superior selectivity toward CO, H2, CH4, ethanol and was not susceptible to RH. It was believed that the unique Lewis acid-base interaction between [BF4 ] and CO2 accounted for the exclusive CO2 response of SWNTs@PIL (Fig. 12C and D) [185]. What’s more, by compositing tetraalkylammonium based Poly (ionic liquid) with La2O2CO3 nanoparticles (PIL@La2O2CO3), Koziej et al. [186] designed composite sensing films with good conductivity due to the improved mobility of [PF6]-. When the concentration of La2O2CO3 reached 60–80 wt%, the interface phenomenon dominated the sensing behavior with the formation of surface conductivity channels [187] (Fig. 12E and F). It should be noted that if the decorated second-phase fully covers the host material, then the core@shell structure forms. The most prominent advantage of core@shell structure is maximizing the interfacial contact 13

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Fig. 12. (A) FE-SEM image of 1.6 wt% graphe­ ne@Sb2O3 QDs composite and (B) variation of den­ sity (IUV/IDL) ratio and CO2 sensing response with the increasing wt% of graphene. Reprinted from Ref. [183]. Copyright (2014), with permission from Elsevier. (C) TEM image of an individual SWNT partially wrapped by PIL and (D) the sensitivity of SWNTs@PIL toward CO2, CO2 (42% RH), CO, H2, CH4, ethanol, O2, and water vapor (42% RH). Reprinted from Ref. [184], Copyright (2012), with permission from RSC. (E) SEM image of film composed of 70 wt% of La2O2CO3 and 30 wt% of P [VBTMA][PF6] and maps of elemental distribution of lanthanum (green) and phosphorus (red). (F) Sche­ matic drawing illustrating the formation of the con­ ductivity channels at the interface between La2O2CO3 and P[VBTMA][PF6]. Reprinted from Ref. [186], Copyright (2015), with permission from Wiley.

areas between the core and the shell materials. In recent time, Kalidindi et al. [188] synthesized SnO2@ZIF-67 core-shell like architecture by assembling ZIF-67 over SnO2 for CO2 detection. It was revealed that the electronic structure of SnO2 changed at the interface, contributing to more CO2 uptakes (an additional 23.5% compared with that of pristine SnO2) and facile formation of CO23 , which enhanced CO2 sensing properties (up to 12-fold for 50% CO2 compared with that of pristine SnO2). N-doped carbon/carbon heterojunction was created between N-doped porous carbon layer and porous carbon fibers (PCFs) using a poly(ionic liquid) (PIL) as a “soft” activation agent [189], as observed in Fig. 13A–D. The as-synthesized PCF@N-doped carbon captured CO2 in amounts as much as 30% of its own mass and exhibited fast recovery and humidity-tolerant properties, due to the fact that N-doped carbon/­ carbon heterojunction promoted the interaction between PCF and CO2 as the N-doped carbon layers took electrons from the underlying carbon. The proposed core-shell PCF@N-doped carbon shows great potential as a simple real-time breath analysis device, as shown in Fig. 13E–F.

4.2.2. Doping effect of second-phase decorated structure Generally, the doping effect mainly exists in the mixed composite structure, which has already been discussed in detail in section 4.1.2. However, in some cases this effect does also influence the gas sensing performance of second-phase decorated structure and a brief summary is given here. Second-phase elements, such as rare earth elements are commonly doped by methods of drop-coating, thermal evaporation, and impregnation. SnO2@La2O3 [190], SnO2@LaOCl [75] and SnO2@La [191] were reported for CO2 detection, respectively. It’s widely believed that doping creates defects (such as oxygen vacancies) at the surface of the host material, thus increasing the active sites for the CO2 adsorption [191]. It should be mentioned that in the doped second-phase structure, there may also exist other promotion effects that enhance the sensing performance. For example, in the work of Van Hieu et al. [192], LaOCl functionalized SnO2 NWs based sensor was synthesized for CO2 detec­ tion and the super sensing behavior (S ¼ 6.8 toward 4000 ppm CO2 at 400 � C) was attributed to the combined effects of p-n junction (interface effect) and the favorable catalytic effect of LaOCl to CO2 gas.

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Fig. 13. (A) Schematic process of NPCF (PCF@N-doped carbon) preparation. (B–D) EDX maps of NPCF-10. (E) Kinetic responses of the PCF and NPCF-10 based sensors toward 2.0% CO2. (F) Kinetic response of NPCF-10-derived device to exhaled human breath subject while sitting or jogging. Reproduced with permission, Ref. [189]. Copyright (2017), with permission from Wiley.

4.2.3. Catalytic effect of second-phase decorated structure Noble metals, e.g., Pd, Pt, Ag and Au are the foremost choices for modifying the base material owing to their privileged catalytic proper­ ties and distinctive selectivity toward certain gases [30,193]. Catalytic metals will activate the base material by chemical or electrical sensiti­ zation. The chemical sensitization occurs via the ‘spillover effect’, where the metal promoters (Au, Pt, etc.) activate the test gas molecules to easier its oxidation or decomposition or dissociate oxygen molecules into oxygen atoms thus increasing the concentration of adsorbed oxygen ions on the surface of the base material. On the other hand, the elec­ tronic sensitization takes place with a direct exchange of electrons be­ tween the metal promoters (such as Pd, Ag, etc.) and the base materials. To be specific, in the process of electronic sensitization, noble metal promoters tend to form stable oxides (PdO, Ag2O, etc.) in air thus inducing the formation of an electron-depletion layer at the interface between metal oxides and the base materials. When CO2 is introduced, the oxide form of metal promoters will be reduced, causing the changes of the electron-depletion layer. The schematic mechanism of chemical sensitization and electronic sensitization is illustrated in Fig. 14A. It should be pointed out that in some metal modified semiconductors, chemical sensitization and electronic sensitization can work coherently for CO2 detection. For example, Lokhande et al. [194] synthesized La2O3 thin films decorated with Pd NPs using successive ionic layer adsorption and reaction method. La2O3@Pd not only exhibited higher response of

1.39 compared with 1.149 of pristine La2O3, but also showed short response/recovery of 105 s/145 s toward 500 ppm CO2 at 250 � C (Fig. 14B and C). The catalytic effect of Pd is shown in two ways. Firstly, it promotes the dissociation of oxygen molecules into more active oxy­ gen atoms, which capture electrons from the conduction band of La2O3, resulting in the formation of electron-depletion layer. Secondly, it’s believed that Pd atoms interact with oxygen and form weakly bonded species (Pd:O) in air, which is easily separated at low temperature and the active oxygen atoms are produced. The created oxygen atoms will capture electrons from La2O3 and the number of oxygen species in­ creases, leading to high response. Naama et al. [141] prepared Pt and Au modified Si NW arrays by simple immersion in HF aqueous solutions containing platinum and gold salts, respectively. I–V characteristics showed that unmodified and Pt modified structures behaved as an Ohmic contact while Au modified structure exhibited rectification properties due to the formation of Schottky structure between Au and Si NW arrays, which is different from the mainstream view of the chemical sensitization effect of Au [195,196]. It was also found that Schottky structure was more sensitive to CO2 gas than an Ohmic structure. This could be attributed to the fact that for a Schottky structure, when exposed to CO2, the height barrier increased as a result of reduced work function of Au NPs and a large decrease of forward current was expected.

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Fig. 14. (A) Schematic mechanism of chemical sensitization and electronic sensitization. (B) SEM image of La2O3@Pd thin film and (C) the response of La2O3 and La2O3@Pd thin films toward various CO2 concentrations. Adapted from Ref. [194]. Copyright (2017), with permission from Elsevier.

4.3. Bi-layer and multi-layer film structure

CO2 concentrations between 2.5% and 12.5% at RT with response of 3.97 toward 12.5% CO2.

Bi-layer/multi-layer film structure is characterized by its welldefined dimensions and contact areas. Generally, in a bi-layer struc­ ture of A/B, A is the top layer and B is the bottom layer. This type of structure can be fabricated by methods of sol-gel [197], sputtering [198], atomic layer deposition (ALD) [199], airbrushing [200], etc. The unique A/B structure makes it much easier to form well-defined heter­ ojunction at the interface, which may promote the charge diffusion across the interface thus making high sensitivities possible. However, this structure may suffer from drawbacks such as low surface area and low gas-accessibility to the heterojunction [29], where gas molecules have to get through the top material to reach. In order to minimize these problems, Karaduman et al. [199] introduced UV irradiation to TiO2/Al2O3 sensor. The UV induced electron-hole pairs increased the number of oxygen ions participating in the reactions with CO2. The higher density of electron trap contributed to a wider depletion layer in air, thus more CO2 molecules could participate in the reaction with oxygen ions and the sensitivity could be greatly enhanced. Meanwhile, as UV light could affect the energetic state of target gas adsorbed on the semiconductor surface and alter the chemisorption/desorption pro­ cesses, the faster reaction rate of TiO2/Al2O3 sensor under UV light was also achieved. Kim et al. [201] added Pd thin film on TiO2/SiO2 bi-layer structure to form periodic interfaces (Pd/TiO2/SiO2) by thermal evap­ oration. The formed Pd/TiO2 served as catalyst that promoted the dissociation of CO2 into ionic species (CO , O2 , etc.), which could diffuse on the surface of Pd–TiO2 and infiltrate into the TiO2 layer. These ions changed the work function of Pd and decreased the resistance. The catalytic effect of Pd/TiO2 shown in this unique structure improved the sensitivity and lowered the working temperature of the fabricated sen­ sors to RT as compared with previous reports [202], which could detect

5. Challenges and future outlooks As shown in previous sections, the nanostructured materials could provide an improvement of gas sensors in terms of sensitivity, speed, selectivity and stability. However, there are some challenges that must be overcome before putting them into practical use. Since typical drawbacks and challenges in chemiresistive gas sensors have already been highlighted in previously-mentioned reviews [29,213], here we only focus on challenges specific to nanostructured CO2 sensing mate­ rials and they are discussed briefly in the following aspects: (1) The influence of ambient humidity. Environmental humidity is an important consideration for practical use of chemiresistive gas sensors and water may be present onto a sensor surface as a molecule, hydroxyl radical, or hydroxyl in multiple oxidation states depending not only on the working temperature but also on the intrinsic characteristics of sensor materials [214]. However, the influence of humidity on CO2 gas sensors is still in hot debate. In some papars, humidity inhibits the sensor performance [190, 205] since the reactions between water molecules and surface oxygen ions consume large numbers of active oxygen ions. However, in some instances humidity acts as an enhancer to the sensor response. For example, in the work of Marsal et al. [106], RH (30%, 50%, 70%) had a positive effect on the LaOCl based sensor due to the formation of La2(CO3)x(OH)2(3-x), which was proved to be more favorable for CO2 adsorption than the simple carbonates, leading to a higher sensor response. Similar promo­ tion behavior in humid atmosphere was also described in other 16

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works based on LaOCl–SnO2 [75], CaO@In2O3 [182] and Nd2O2CO3 [107]. Interestingly, in Hu’s work regarding SnO2 based sensors [101], the influence of humidity depended on CO2 concentrations. With an increase of RH from 14% to 66%, the sensing response to low concentrations of CO2 (2000–4000 ppm) initially increased, reached a peak volume at about 34% RH, and then decreased. At higher CO2 concentrations (6000–20, 000 ppm), the sensing response of SnO2 dropped with an increase of humidity. The effect of RH on CO2 sensing performance of chemiresistive sensors are summarized in Tables 2 and 3. How­ ever, the influence of RH is so intricated that further in­ vestigations need to be performed to clarify the effect of water vapor on CO2 gas sensing performances. (2) The effect of oxygen concentration in carried gas. According to ionosorption model, surface oxygens are needed for the sensing behaviors of chemiresistive gas sensors. So in most cases, the sensors are employed in air ambient and the presence of oxygen in carrier gas facilities the gas-solid interaction [109,155]. However, in some reports, things are different. For example, in the work of Nd2O2CO3 based CO2 sensor reported by Haensch et al. [215], the sensitivity didn’t change too much when carrier gas was switched from air to almost pure nitrogen, indicating that oxygen is not essential for the sensing. Similarly, for ZnO [31], CuO [103], CNTs [117,119], 4at.% La–SnO2 [191], Pd/TiO2 [201], SWCNT@PIL [184] and N-doped microporous carbon fi­ bers [216] based sensors, ambient oxygen has little influence on the sensing performance. The function of oxygen in chemir­ esistive CO2 gas sensors remains open for further investigation. (3) Property of CO2 (reducing gas or oxidizing gas?). Although in most literatures the behavior of CO2 is reported to be a reducing gas, some literatures report it as an oxidizing gas [103,154,184, 216]. In the work of Saraswathi et al. [31], it was reported that when the ZnO thin film was exposed to CO2 gas, the adsorbed O ions reacted with the CO2 gas and extracted the electrons from the conduction band of the n-type ZnO thin film, leading to an increase of film resistance. It can be seen that in his research CO2 behaves as an oxidizing gas. However, in another work about ZnO sensing by Hunge et al. [99], the resistance of n-type ZnO thin film decreased when exposed to CO2, which was attributed to the fact that the reaction between CO2 molecules and the pre-adsorbed O species released electrons initially trapped by oxygen anions back into the ZnO solid. Thus it can be concluded that even with the same kind of sensing material (ZnO), the gas sensing mechanisms can be totally different. This situation is not rare, similar contradictory conclusions can also be found in CO2 sensors based on In2Te3 [6,144] and graphene [120,154]. The characteristics of CO2 in the reported literatures are summarized in Tables 2 and 3. (4) Selectivity. In practical applications such as environmental monitoring, oxygen and sulfur containing compounds (NOX compounds, volatile organic compounds, etc.) can be simulta­ neously present in gas sensing atmosphere. For this reason, studies on selectivity are crucial to fulfill the real-word applica­ tions of gas sensors. Although there exist some literatures high­ lighting the selectivity of CO2 sensors [184,212], it should be mentioned that these devices usually work at atmosphere that only one type of gas exists at a time and they may not work efficiently in the presence of various gases at a time. Moreover, because the detection mechanism of almost all resistive gas sen­ sors is closely related to the changes in resistance upon adsorp­ tion/desorption of gas molecules, it is difficult to discriminate a group of gases which produce similar resistance changing trends toward CO2. With little information available for gas sensing materials that are intrinsically selective toward CO2 gas, it has rather been a trial-and-error approach for scientists to develop CO2 sensors with very low cross-sensitivity.

As for the future research on CO2 gas sensors based on nano­ structured materials, we believe efforts should be devoted to the following aspects: (1) Since CO2 sensing mechanisms of nanostructured materials-based sensors are still ambiguous, more effective in situ spectrometric analysis should be carried out to understand the theories behind the effect of RH and ambient oxygen on the sensing properties of CO2 gas. For instance, according to the ionosorption model (Section 2.1), oxygen adsorbs on the surface of the semi­ conductor, extracting electrons from the CB and forming molec­ ular (O2 ) and atomic (O , O2 ) oxygen ions. This process is suggested to occur under real sensor working conditions, namely, at 150–400 � C under air atmosphere. However, there is not yet any convincing spectroscopic evidence for the existence of O2 and O . Hence, any in situ spectroscopic evidence, either for or against this mechanism, will greatly advance the basic under­ standing of gas sensing. (2) Improving the selectivity of CO2 gas sensor is one of the priority things in order to put it into commercial use. Nowadays, selec­ tivity of chemiresistive gas sensors is commonly achieved by the fact that different gases usually have different optimum working temperatures. More strategies need to be developed, such as creation of electronic “nose” on the base of an array of sensors [75], use of surface membranes (filters), addition of suitable el­ ements in gas sensing matrix which selectively catalyzes the particular redox reaction of our choice [212], etc. For electronic “nose” (e-nose) technique comprising an array of electronic chemical sensors with partial specificity and an appropriate pattern recognition system, each sensor in the array responds more selectively to a certain chemical parameter and the array is able to image in “chemical” space. Consequently, the e-nose is capable of recognizing simple or complex odors, maximizing the selectivity [75]. The use of membranes incorporated into the measuring unit or directly into the sensor construction is also an effective method to improve sensor selectivity, which can selec­ tively remove interferences or transform them into an inactive phase. For example, it is proved that thick Al2O3 layers can be used as filters for benzene in toluene sensors [217]; Teflon is helpful in stopping H2O reaching the sensor [218]; Zn-based zeolite imidazole framework (ZIF-8) filter with various micro­ pores can act as a molecular sieve to reduce the penetration of large molecules such as O2 and N2, while H2 molecules pass through easily [219], to just name a few. Additionally, the modification of palladium on the surface of MOS films is widely used to greatly enhance the response toward H2 via electronic sensitization effect, thus increasing H2 selectivity [220]. (3) The influence of the composition of composite on gas sensing properties is too multifactorial. As a result, at present the search for optimal composite for gas sensors is a trial and error test of all possible compositions. It is without doubt that more efforts in basic studies are of critical importance for better understanding of the nature of the interaction of CO2 with various composites, and the mechanisms of conductivity response in composite-based sensors. For example, some precise calculations and computa­ tional modeling of the interactions between CO2 molecules and semiconductor composites, and the subsequent changes in elec­ tronic band structure of gas-adsorbed semiconductor composites are required. Understanding the relationship between in­ teractions and the performances of composite-based sensors is also of crucial importance to design new materials for CO2 sensors. Acknowledgements This work was supported by the National Natural Science Foundation 17

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of China (NSFC) (Grant No. 51972342).

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