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SN Applied Sciences

, 1:719 | Cite as

High-sensitivity detection of H2S by In2O3/C composite prepared by inert-ambient sealed-tube pyrolysis

  • M. Srinidhi RaghavanEmail author
  • S. A. Shivashankar
Research Article
Part of the following topical collections:
  1. 1. Chemistry (general)

Abstract

Indium acetylacetonate, a β-diketonate complex, was pyrolyzed at 700 °C in inert ambient in a sealed quartz tube, to yield a powder composite of nanocrystalline In2O3 and elemental carbon (In2O3/C), as deduced from characterization by powder X-ray diffraction and Raman spectroscopy. Scanning electron microscopy shows that the metal oxide is embedded in micrometer-sized spherical structures, composed largely of carbon. The spherical entities are likely formed when the metal complex melts, decomposes, and vaporizes during pyrolysis, with the vapour condensing into spherical “droplets” as the sealed tube cools gradually to room temperature. The In2O3/C composite was tested (in pellet form) as a gas sensor, specifically the conductometric sensing of H2S. At 5 ppm of H2S, the composite shows a high response of 225% at 250 °C, with the response and recovery being swift (~ 5 s and ~ 15 s, respectively). At 250 °C, the detection limit is found to be of 500 ppb of H2S, with selectivity over NH3, NO2, CH4, and SO2 being considerable. The In2O3/C sensor also displays good cyclability. When carbon in the composite is removed by annealing it in air (550 °C, 60 min), the resulting In2O3 powder (in pellet form) shows a much poorer response to H2S at 250 °C (25% to 5 ppm), illustrating that the elemental carbon in In2O3/C enhances sensitivity to H2S. The advantages of a simple fabrication process and low power consumption make the carbonaceous composite sensor potentially useful.

Keywords

Metal oxide/carbon composite In2O3/C Sealed tube pyrolysis H2S sensor Low ppm detection High selectivity 

1 Introduction

Hydrogen sulfide (H2S) is a very toxic gas with a characteristic foul odour. It is a strong neurotoxin, capable of causing severe damage to humans even at low concentration [1, 2]. It is also a naturally occurring gas in mines and is often present in industrial processing areas [3]. H2S is flammable and highly corrosive and is easily absorbed by the human body at relatively low concentrations [4]. Therefore, the detection of hydrogen sulfide with high sensitivity is attracting significant effort worldwide [5]. Various H2S-sensing elements and methods have been studied, such as optical sensors [6, 7], electrochemical sensors [8, 9], and metal-oxide conductometric sensors [10, 11, 12, 13]. Sensors reported to date are usually expensive, show poor stability and unreliable long-term performance [14].

Metal oxide gas sensors are frequently considered attractive for the fabrication of portable devices, owing to their high sensitivity towards target gases, simple device structure, and low power consumption [15]. However, a few major challenges need to be addressed to make metal oxide-based gas sensors more useful. In general, metal oxide sensors react not only with the analyte (target) gas but also with the surrounding atmosphere [16]. Consequently, in conductometric sensors, response and recovery times, and resistance of the sensor in the ambient, deteriorate significantly over time due to presence of water vapor and other trace gases in the ambient. Selectivity of the gas sensor, which refers to the ability of a sensor to respond selectively to a specific analyte, must be high and is essential for reliable analysis.

Indium oxide, In2O3, is among the binary oxides studied for sensing H2S. The strong chemical interaction between In2O3 and H2S and the conversion of In2O3 into In2S3 upon exposure to H2S is considered the basis for sensing. Recent investigations have shown that In2O3 is a promising candidate for gas-sensor applications. Though indium oxide (In2O3) has been studied as sensor material for NO2, CO, and O3 [17, 18, 19, 20], there are only a few published investigations of In2O3 and In2O3-based materials as the conductometric sensor for H2S. Two others depend on transistors fabricated with In2O3 as the sensor [21, 22]. Tu et al. [23] have reported detecting 2 ppm H2S at 150 °C with Pt-doped In2O3, whereas Kapse et al. [24] have shown detection of 50 ppm H2S at 100 °C by Pd/Au-impregnated, 5% La-doped In2O3 nanoparticles. Xu et al. [10] have reported sensing 50 ppm at 270 °C by nanocrystalline In2O3 prepared by hydrothermal synthesis. Bari et al. have reported detecting 500 ppm H2S at 25–150 °C by spray-pyrolysed films of nano-In2O3 [25, 26, 27, 28, 29, 30]. To date, there is no report in the literature of conductometric sensing of H2S at sub-ppm concentrations by an In2O3-based sensor.

We report on the synthesis of a novel carbonaceous composite of In2O3 nanocrystals with high surface-to-volume ratio by the simple, one-step process of sealed-tube pyrolysis (STP) of the acetylacetonate complex of indium in an inert ambient. We demonstrate the ability of the carbonaceous composite to sense even sub-ppm concentrations of H2S at a modest sensing temperature, and the high selectivity of the sensor over several other hazardous gases like NH3, NO2, and SO2. We show that, when the carbon in the composite is removed, the oxide alone has a much lower sensitivity to H2S.

2 Experimental

2.1 Preparation of indium acetylacetonate, In(acac)3

The metal complex, denoted by In(acac)3, was prepared using indium nitrate. All reagents used were of analytical grade. A calculated amount of the nitrate dissolved in water and 10% of methanol was stirred for 20 min to achieve homogeneity. Next, 2.4 ml of acetylacetone (20 mmol) was added to 2.5 ml of methanol and mixed well, following which this acetylacetonate solution was added to the metal nitrate solution and stirred until the solution became clear. The reaction mixture was neutralized by 30% ammonia solution added dropwise until a pH of 7 was attained. The resulting pearl-white solution was stirred for 1 h at room temperature. The solid precipitate thus obtained was separated through vacuum filtration, dried, and crystallized at 4 °C using a mixture of 10% methanol in acetone as the solvent. This resulted in the formation of the desired complex as a white powder. The average yield of the reaction was 92%. The formation of the complex was verified by various means, such as powder XRD and FTIR spectroscopy (ESI).

Pyrolysis of the indium complex, In(acac)3, was carried out in a quartz tube 1.5 cm in diameter and ~ 15 cm in length. The middle part of each tube was thinned to form a small neck, which could be easily cut and sealed. The tube was evacuated and backfilled with ultra-high purity (UHP) nitrogen. About 0.5 g of In(acac)3 powder was introduced into the tube, followed by evacuation and backfilling with UHP nitrogen. The tube was cut in the middle and sealed so that the indium complex could be pyrolyzed in the inert ambient of the sealed tube.

The sealed-tube was heated to 700 °C and held at that temperature for 24 h, after which the tube was allowed to cool naturally to room temperature. After cooling to room temperature, the tube appeared completely black. (The seal had to be broken with care, as high pressure had developed inside the tube because of decomposition taking place at elevated temperatures, forming different vaporous species.) The inner wall of the tube was found to be coated with a black reaction product. So, the inner wall of the broken tube was scraped carefully to collect the powdered product of the STP process.

2.2 Characterization

The crystallized powder of the metal acetylacetone complex was characterized Fourier transform infrared (FTIR, Perkin Elmer Model BX) spectroscopy to verify the presence of metal–oxygen bonds, viz., In–O bonds (ESI). Powder XRD data on the STP sample were collected using Cu-Kα radiation (Panalytical, Model MPD). The morphology of the STP product was examined by field emission scanning electron microscopy (FESEM, Zeiss Ultra 55). Transmission electron microscopy (TEM, TECNAI F-30) was used to analyze the finer details of the morphology and the crystallinity of the powder product of the STP process. Raman spectra were obtained for the STP product (514 nm argon ion laser, Horiba Jobin–Yvon LabRAM HR 100 spectrometer). Further, H2S sensing by the carbonaceous product, taken in pellet form, was carried out at various ppm concentrations of the H2S diluted with nitrogen. During sensing, gas flow and H2S concentration were regulated with electronic mass flow controllers (Alicat). A source-measure unit (Keithley Model 2450) was used for recording the resistance of the sample in the presence and absence of H2S.

3 Results and discussion

Indium acetylacetonate in powder form could be obtained at high yield. It was found by X-ray powder diffraction to be crystalline in the as-prepared and recrystallised forms. The FTIR spectra [SI] show the presence of In–O bond in the complex, providing preliminary evidence that the metal complex had been formed.

The XRD pattern of the black powder product of STP is shown in Fig. 1. The material is clearly well crystallized, and the peaks can be indexed to indium oxide, In2O3, of the cubic bixbyite structure (JCPDS card number-06-0416). The average crystallite size is estimated to be 50 nm (Scherrer formula). In addition to the peaks due to In2O3, two peaks are observed and are labeled in blue. These two peaks, centred around 2θ = 43° and 54°, may be indexed respectively to the (101) and (004) planes of the graphite (elemental carbon) in the powder composite [31].
Fig. 1

XRD of powder from the STP of In(acac)3 at 700 °C, 24 h

The Raman spectrum of the powder product of STP shows very clearly the presence of elemental carbon (Fig. 2), confirming the preliminary inference based on its black colour. The first-order Raman spectrum consists of an envelope of peaks centered at G (1596 cm−1) and D (1352 cm−1), typical of amorphous graphitic carbon. The second-order feature D′ is present at 2489 cm−1. Thus, the STP process results in carbon with structural disorder, as confirmed by Raman-active vibrational modes that are forbidden in the case of graphite [32]. Based on the XRD and Raman data, therefore, the STP product may be designated In2O3/C, denoting a carbonaceous composite.
Fig. 2

Raman spectrum of the STP product of In(acac)3 (700 °C, 24 h), evidencing the presence of elemental carbon

XPS spectra (Krotos Axis instrument, Al-Kα radiation) collected on the STP powder sample exhibit characteristic peaks for indium and oxygen (Figs. 3a, b and 4), in good agreement with data in the literature for In2O3 [31]. In order to acquire a more accurate chemical composition of the as–prepared In2O3/C, the XPS spectra were obtained. The XPS spectra of the samples confirm the presence of In, O, and C atoms in the STP product. The O1s XPS spectrum of In2O3/C samples may be deconvoluted into two peaks, with the binding energies at ~ 530.3 eV and 531.9 eV assignable to oxygen and the adsorbed oxygen species, respectively [31]. The main O1s XPS peak at 529.4 eV corresponds to lattice oxygen in crystalline In2O3. The binding energies corresponding to In3d5/2 and In3d3/2 are at 444.1 and 452.7 eV, respectively [32].
Fig. 3

a, b XPS spectrum of the STP product, In2O3/C, confirms the formation of In2O3 as shown by the In and O spectra

Fig. 4

XPS spectrum of the STP product, confirming the presence of elemental carbon

Figure 4 shows the C1s spectrum which may be deconvoluted into peaks at 284.6, 285.2, and 286.4 eV, which are associated with carbon in the states of C–H, O=C–H, and C=O, respectively. Quantitative analysis of the XPS data spectrum shows that the percentage of sp3 carbon (81.3%) is higher than that of sp2 carbon (19.6%), evidencing the presence of the carbon.

The SEM micrograph of the STP product, In2O3/C, is shown in Fig. 5a. The sample consists of numerous highly spherical entities ranging in diameter from ~ 1 to ~ 6 μm, together with non-spherical entities several microns in diameter, often agglomerated with the spheres (Fig. 5a, b). Elemental analysis shows that the spheres are made of carbon. TEM analysis of the STP products shows (Fig. 6a, b) nanocrystalline In2O3 present in a carbon-rich matrix. Individual units of unique morphology could be observed (Fig. 6a), which comprise In2O3 crystallites embedded in amorphous carbon. These data corroborate the Raman analysis, and confirm that inert-ambient STP of In(acac)3 results in a composite of carbon and crystalline In2O3.
Fig. 5

a, b Morphology of In2O3/C obtained by STP (700 °C, 24 h)

Fig. 6

a, b TEM micrograph and SAED pattern of the STP product (700 °C, 24 h); the SAED pattern is that of nanocrystalline In2O3, which is embedded in carbon; some nanocrystals of In2O3 feature apparently “exposed” surface, as seen in a

3.1 Possible mechanism of formation of the carbonaceous oxide

The presence of highly spherical, micron-sized entities (Fig. 5a, b) is indicative of the solidification of liquid droplets upon cooling. This can be understood by consulting the thermal analysis data for In(acac)3, which shows complete vapourisation of the complex at about 250 °C. During pyrolysis at an elevated temperature, the vapour fills the sealed tube, creating an elevated pressure in the tube as the temperature is raised during the STP process. At an unknown temperature above ~ 300 °C, the metal complex decomposes into various entities, presumably including carbonaceous ones.

Some of the vapours condense into spherical solid entities as the tube is cooled to room temperature. Decomposition of the complex also leads to the formation of oxide moieties, which form the “core” of the molecular structure of acetylacetonate complexes of metals. Given that the cooling is fairly rapid, the oxide crystallites formed are fairly small and are embedded in a carbon-rich matrix, thereby forming the observed carbonaceous oxide composite. The process is analogous to the formation of a similar composite during the inert-ambient STP of Fe(acac)3 [33, 34, 35].

3.2 H2S-sensing studies

To examine the potential for the In2O3/C composite to serve as a gas sensor, suitable structures based on In2O3/C composites were fabricated, and their ability to sense H2S was investigated. The carbonaceous indium oxide powder (formed by STP) was cold-pressed into pellets, to which electrical contacts were made with silver paste. The resistivity of the sensors was measured using the four-probe technique. The In2O3/C pellets fabricated for use as sensors showed typical semiconducting behavior, where their resistivity decreased with increasing temperature (ESI). The resistance of the sensor elements was measured as a function of temperature in the presence and in the absence of the analyte, H2S. Specifically, the sensor (pellet) was placed in a probe station provided with a heater and flow lines for supplying H2S at the desired concentration in UHP nitrogen. Electronic mass flow controllers enabled the tuning of the analyte concentration precisely. The temperature of the sensor could be set precisely using a PID controller. In2O3 is a p-type semiconducting oxide, in which the formation of a hole accumulation layer (HAL) is the operative mechanism for gas detection. H2S is a reducing gas, which injects electrons into the surface of the hole-rich surface of the sensing oxide, altering the resistance of the sensor element. To quantify the sensitivity of In2O3/C to H2S, synthetic air (80% nitrogen and 20% oxygen) was used as the reference gas.

The response to the analyte H2S is calculated using the equation:
$$Response = \left( {I_{g} - I_{a} } \right)/I_{g} \times 100\%$$
(1)
where \(I_{a}\) = current in In2O3/C exposed to synthetic air and \(I_{g}\) = current in In2O3/C in the presence of H2S, at the same temperature. The response of the In2O3/C sensor H2S is presented in Fig. 7. It is seen that not only does the sensor provide a rapid and significant response at a concentration as low as 500 ppb but that the response is effectively reproducible when the sensor is cycled thrice through a series of increasing H2S concentrations. The apparent reduction in sensitivity from cycle 1 to cycle 3 is believed to be an artifact of the long interval (2 weeks) between successive measurements (due to limited availability of the measurement setup). The relevant statistics of the linear regression analysis of the data are given in the ESI.
Fig. 7

Typical response of a In2O3/C pellet sensor to H2S at 250 °C. b Repeated response of In2O3/C sensor to various concentrations of H2S

The present sensor based on In2O3/C composites shows a significantly greater response to H2S than sensors based on In2O3 reported in the literature, as seen from Table 1 below.
Table 1

summarizes and compares published results on H2S sensing by various metal oxide nanostructure-based sensors; Ra and Rg are the resistance of the sensor measured, respectively, in synthetic air and in the presence of H2S gas at the respective optimum temperature

Material and morphology

Synthesis method

Concentration of H2S (ppm)

Sensor response

Optimum temperature (°C)

Response time (s)

References

Pt-doped mesoporous indium oxide

In situ Nano casting

2–300

NA

150

1500

[23]

La-doped In2O3

Hydrothermal decomposition

10–50

Ra–Rg = 0.88%

100–150

NA

[24]

In2O3–ZnO

spray pyrolysis technique

1000

NA

250

NA

[38]

In2O3–SnO2 nanorods

Co-precipitation method

10

Ra/Rg = 4.98%

300

NA

[39]

Nanocrystalline In2O3 sensors

Carbothermal method

10

Ra/Rg = 0.82%

RT

240

[40]

Flower like In2O3

Hydrothermal process

NA

Ra/Rg = 98%

400

NA

[41]

In2O3/C composite

Sealed tube pyrolysis

0.5

Ig–Ia/Ig = 225%

250

4

Present work

Furthermore, the sensor based on In2O3/C displays a strong, steady response to H2S rather than to other gases. Specifically, the response to H2S is much stronger than to other reducing gases, showing that the carbonaceous composite is selective to H2S. It can be seen from Fig. 7 that the sensor response to H2S increases significantly when operating temperature is raised to 250 °C.

As the sensitivity of gas sensors is influenced by temperature, the experiments were carried from 100 to 250 °C to optimize the operating temperature of the material.

It is found that the response of the sensor to H2S is strongest at 250 °C. The detailed responses of H2S sensor at elevated temperatures in the range 50–300 °C is listed in Table 2.
Table 2

Response of the In2O3/C sensor to H2S at temperatures in the range 50–300 °C

S. no.

Temperature (°C)

Concentration (ppm)

Response (%)

Response time (s)

1

50

5

Nil

Nil

2

100

5

30

2000

3

150

5

47

1200

4

200

5

110

750

5

225

5

146

600

6

250

5

225

300

7

275

5

210

560

8

300

5

170

900

The response time of a sensor is defined as the time taken to reach 90% of the full (eventual) response of the sensor after it is exposed to the target gas, and the recovery time defined as the time taken for the response to decrease to 10% of the full response, after the flow of the target gas is terminated. It is to be noted that the response of the sensor is fast and nearly instantaneous when the analyte H2S gas is introduced, reaching a steady state rapidly, within a few seconds.

As shown in Fig. 7a, the In2O3/C composite sensor exhibits quite linear response from 500 ppb to 5 ppm of H2S concentration. At 250 °C, the response and recovery times are 5 s and 15 s, respectively. The detection limit of the sensor was determined to be 500 ppb on the basis of reproducibility of sensing at 250 °C. Although H2S could be detected at 200 ppb, the signal-to-noise ratio was not high enough.

At operating temperatures in the range 100–150 °C, the sensor exhibits lower sensitivity and slower recovery than at the more favorable operating temperatures in the range 225–300 °C, with 250 °C as the optimum (Table 2). This is due to the relatively slow kinetics of the sensing reactions at lower temperatures.

We believe that the unique spherical morphology provides “great numbers of carbon atoms on the surface”, facilitating the availability of many active sites for the adsorption and diffusion of H2S molecules. As such, chemisorbed oxygen can participate in the oxidation–reduction reaction happening on the surface of the sensor, causing a large change in sensor resistance. It is clear that the observed high sensitivity to H2S is closely associated with the unique morphology of In2O3/C.

To ascertain whether the carbon in the In2O3/C composite plays a role in the sensing of H2S, the composite powder material was annealed in air at 550 °C for 60 min to remove the carbon in the composite, yielding a powder of In2O3. The oxide powder was made into a pellet and examined for its ability to sense H2S gas, using the same protocols as described above.

The results of the sensing measurement on In2O3 at 250 °C are shown in Fig. 8a. Comparing the sensing responses data in Figs. 7a and 8a, it is evident that, though In2O3 without carbon does sense H2S, the sensitivity is significantly less than that of In2O3/C (at 250 °C), and that the response and recovery times are poorer. The comparison of the results confirms that the carbonaceous composite of In2O3 is an excellent sensor for H2S. The carbon matrix plays a role in increasing sensitivity to H2S significantly. Figure 9 compares the sensitivity of indium oxide to H2S with and without carbon.
Fig. 8

Response of a In2O3 pellet sensors to 5 ppm H2S at 250 °C. b Response of In2O3 sensor to different concentrations of H2S

Fig. 9

Comparison of responses of the In2O3/C sensor and the In2O3 sensor to various concentrations of H2S at 250 °C

It is also seen that the rise time upon exposure to the analyte is noticeably longer than it is for the composite. The results imply, of course, that the oxide/carbon composite is more sensitive to the reducing gas than the oxide alone. Further investigation is needed to understand the role of carbon in enhancing sensitivity to H2S.

3.3 Gas sensing mechanism

According to a generally accepted sensing mechanism, a reducing gas such as H2S, upon interacting with the surface of the metal oxide through surface adsorbed oxygen, disturbs the carrier concentration, altering the resistance of the material.

Here the majority charge carriers are holes. When a p-type semiconductor material interacts with a reducing gas, it liberates electrons. These electrons combine with holes present in the material and neutralize it resulting an increase in resistance. A reverse effect is observed in the presence of oxidizing gas [36].

It is proposed that the sensing of H2S gas sensing by In2O3/C composites is related to surface-adsorbed oxygen species. When the sensors fabricated with the carbon-indium oxide composite are exposed to air, oxygen molecules readily easily adsorb on the surface of these composite and capture free electrons, giving rise to chemisorbed oxygen molecular ions (O2). These chemisorbed oxygen species react with H2S molecules on the surface, resulting in sensing action.

In overall reducing reaction between H2S gas and chemisorbed oxygen spices may be represented by the following equations [36, 37]
$${\text{O}}_{2} + 2{\text{e}}^{ - } \to 2{\text{O}}_{2}^{ - }$$
(2)
$$2{\text{H}}_{2} {\text{S}} + 3{\text{O}}_{2}^{ - } \to 2{\text{H}}_{2} {\text{O}} + 2{\text{SO}}_{2} + 3{\text{e}}^{ - }$$
(3)

The above equation shows that the concentration of electrons increases as the H2S gas interacts with the adsorbed oxygen molecules. These electrons neutralize the holes present in In2O3/C, thus reducing the majority charge carriers. This would result in increase of the resistance of the composite material, accounting for the sensitivity of the material to low concentrations to H2S.

3.4 Selectivity

Metal-oxide gas sensors are generally sensitive to more than one target gas and normally show cross-sensitivity. Thus, selectivity is an important parameter which can be assessed by measuring cross-sensitivity. Figure 10 shows the sensitivity of In2O3/C to H2S relative to other reducing gases at the operating temperature of 250 °C, demonstrating significant selective response of the sensor to H2S.
Fig. 10

Selectivity of In2O3/C towards H2S vis-à-vis other gases (at 250 °C)

4 Conclusion

Inert-ambient sealed-tube pyrolysis is a simple route to the synthesis of carbonaceous composites of metal oxides from the corresponding metal acetylacetonates (acac). Using In(acac)3, a composite of crystalline In2O3 and elemental carbon is obtained by pyrolyzing the complex at 700 °C. A conductometric sensor made of a pellet of the In2O3/C composite is found to sense H2S at low concentrations as low as 500 ppb, at modest temperatures. The observed sensing action is better than that reported in the literature for conductometric sensors made of In2O3 nanoparticles and thin films when exposed to similar concentrations of H2S. Measurements show that, when carbon in the composite is removed, sensing action is diminished significantly. Thus, the In2O3/C composite obtained by sealed-tube pyrolysis is a good candidate for sensing H2S at modest temperatures and in low concentrations.

Notes

Acknowledgements

The authors thank the Ministry of Electronics and Information Technology (MeitY, Govt. of India) for funding support and Prof. Navakanta Bhat for permitting the use of the gas-sensing laboratory. They also acknowledge the technical support provided by the Micro and Nano Characterization Facility (MNCF) at the Centre for Nano Science and Engineering (CeNSE).

Compliance with ethical standards

Conflict of interest

The authors declare that they have no conflict of interest.

Supplementary material

42452_2019_760_MOESM1_ESM.docx (676 kb)
Supplementary material 1 (DOCX 675 kb)

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Copyright information

© Springer Nature Switzerland AG 2019

Authors and Affiliations

  1. 1.Centre for Nano Science and EngineeringIndian Institute of ScienceBengaluruIndia
  2. 2.Department of ChemistryBMS College of EngineeringBasavangudi, BengaluruIndia

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