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Nanostructured palladium modified graphitic carbon nitride – High performance room temperature hydrogen sensor
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Nanostructured palladium modified graphitic carbon nitride e High performance room temperature hydrogen sensor Raghu S. a,b, Santhosh P.N. b, Ramaprabhu S. a,* a
Alternative Energy and Nanotechnology and Laboratory, Department of Physics, Indian Institute of Technology Madras, Chennai, 600036, India b Low Temperature Physics Laboratory, Department of Physics, Indian Institute of Technology Madras, Chennai, 600036, India
article info
abstract
Article history:
Graphitic carbon nitride has attracted huge interest due to its due to low cost and easy
Received 27 May 2016
synthesis. We report for the first time graphitic carbon nitride composite as room tem-
Received in revised form
perature hydrogen sensing material. Nano structured palladium nanoparticle dispersed
31 August 2016
graphitic carbon nitride (Pd/g-C3N4) composite was investigated as room temperature
Accepted 1 September 2016
resistive gas sensor. Graphitic carbon nitride (g-C3N4) was prepared by pyrolysis of mel-
Available online xxx
amine. Palladium nanoparticles were dispersed on g-C3N4 by polyol reduction method. Hydrogen sensing properties of Pd/g-C3N4 were studied within near flammability range
Keywords:
from 1 to 4 vol%. The performance was evaluated with variation in temperature from 30 to
Graphitic carbon nitride
80 C. For 4% H2 at room temperature, 99.8% sensitivity is achieved with response time of
Palladium nanoparticles
88 s while for the same concentration of H2 at 80 C, the sensor shows 97.4% sensitivity.
Hydrogen sensor
The results show prospect of reliable sensing material.
Room temperature hydrogen sensor
© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen (H2) is promising environmental friendly zero emission energy carrier and is expected to replace the usage of many conventional energy carriers to a possible extent which release pollutants. H2 is used in wide range of applications such as fuel cells, semiconductor industry, mines, petroleum, nuclear power plants, and chemical industries [1]. Combustion of hydrogen (142 kJ/mol of H2) can produce three times the energy produced from gasoline, diesel and petroleum gas of the same weight. Considering this H2 is the choice above the conventional sources, but containing H2 and transporting it
pose more safety challenges. It is highly inflammable gas with wide flammability range from 4% to 75% in air and very low ignition energy of 0.017 mJ [2]. Therefore, sensors are necessary for sensing hydrogen and concentration monitoring. Various materials like Pd nanowire, Pd thin films, porous silicon, carbon nanotubes, WO3, TiO2, TeO2, In2O3, RuO2, MoO2, ZnO were explored as hydrogen sensors [3e12]. Most of the sensors available commercially are made of semiconducting metal oxides which operate at higher temperatures [4,7,13e15]. For concentration monitoring nearer to the flammability limit these may cause ignition. Most of the carbon based materials like graphene, carbon nanotubes composites though are operational at room temperature but suffer low
* Corresponding author. E-mail address: [email protected] (R. S.). http://dx.doi.org/10.1016/j.ijhydene.2016.09.002 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: S. R, et al., Nanostructured palladium modified graphitic carbon nitride e High performance room temperature hydrogen sensor, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.002
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sensitivity [6,16e18]. Sensors with high sensitivity at room temperature at these concentrations are desirable. There is a need to find new alternative to graphene and carbon nanotube like materials. Graphitic carbon nitride (g-C3N4) possesses the desirable properties for a sensing material. It is indirect band gap semiconducting material (band gap of 2.7 eV) consisting of carbon and nitrogen atoms arranged in graphite like layered structure [19e21]. It is highly stable and non-toxic. It can be synthesized easily at large scale and can with stand harsh conditions [22]. Many ways of synthesis are reported for synthesis of g-C3N4 with difference precursors like cyanamide, dicyanamide, melamine and urea [23e27]. In the present work, g-C3N4 was investigated as a room temperature resistive hydrogen sensor. By modifying g-C3N4 matrix by material that has strong affinity toward hydrogen its gas sensing properties can be explored. Palladium is well known for its activity towards hydrogen [28e33]. g-C3N4 was modified by dispersing palladium nanoparticles on its surface by polyol reduction technique. Sensor film was fabricated by simple and cost effective drop casting technique on an insulating substrate (alumina).
Experimental section Materials Melamine was purchased from Himedia Laboratories Pvt Ltd, Mumbai, India. Ethylene glycol, sodium hydroxide pellets were purchased from Merck, Mumbai, India Pvt Ltd. Palladium chloride was purchased from Sigma Aldrich chemicals Pvt Ltd. Ultra-pure deionized water was from water purification system Milli-Q Advantage A10 (Merck Millipore, USA).
Material synthesis Graphitic carbon nitride was prepared by direct pyrolysis of melamine [34,35]. 4 g of melamine was placed in an alumina boat and loaded inside a quartz tube in a tubular furnace. Two-end coupling were fitted on both ends and has provision for gas inlet and outlet. Initially it was purged with nitrogen for 20 min and the temperature was raised to 550 C and maintained for 4 h. Further it was allowed to cool down naturally to room temperature. Nitrogen was allowed throughout the experiment with flow rate of 0.16 SLPM. A condensed pale yellow colored powder of 645 mg was obtained which is g-C3N4. It was further ground to fine powder using mortar and pestle. Palladium nanoparticles were incorporated into the g-C3N4 matrix by modified polyol reduction. 80 mg of g-C3N4 is dispersed in 100 mL of a solution of ethylene glycol and DI water in the ratio of 3:1. This mixture is ultrasonicated for 20 min and kept for strong stirring. 3.333 mL of 1 wt% of PdCl2 solution was added drop wise for obtaining 20 wt% of palladium loading. Further, 2.5 M NaOH solution was added slowly to maintain pH of 10. After vigorous stirring for 12 h, the mixture is refluxed for 6 h at 130 C to ensure complete reduction of the metal salt. This sample was washed with copious amount of water. Further filtered, dried and labeled as Pd/g-C3N4.
Film fabrication Pd/g-C3N4 of 15 mg and 40 mL of 1 wt% Nafion solution was added to 0.5 mL of ethanol and ultrasonicated for 15 min and slurry was obtained. This slurry was drop casted on an alumina substrate with two gold-coated electrodes to make a thick film. This film was mounted inside the sample chamber and electrical contacts were given using silver epoxy paste. Schematic of the sensor test station is shown in Fig. 1. Sample chamber was then closed and purged with argon gas for 15 min. In presence of argon the film was degassed by heating it to 150 C. A 100 MU resistance is connected in parallel with the two electrodes outside the sample chamber externally. Keithley 2601 A Source meter was used to provide constant current and Keithley 2182 nanovoltmeter for measuring the voltage. Nanovoltmeter was interfaced to a computer through Lab-view program for data collection.
Results and discussion X-ray photoelectron spectroscopy was carried out by Specs X-ray photoelectron spectrometer, X-ray source being Mg Ka and analyzer PHOIBOS 100MCD. FTIR analysis was carried out in Agilent carry 630 diamond ATR by placing the powder sample directly without any dilution. For X-ray diffraction measurements PANalytical X'Pert pro X-ray diffractometer was used with Cu Ka radiation as source with nickel filter. Measurements were carried out from 5 to 90 with step size 0.02 . Thermogravimetric analysis was performed using TA instruments SDT Q 600. For morphological studies FEI quanta 3D transmission electron microscope FEI tecnai G2 was used. The powder X-ray diffraction patterns of g-C3N4 and Pd/gC3N4 are shown in Fig. 2. Peak at 27.6 is due to the stacking of tri-s-triazine rings which are graphitic like planar structures [36] and in addition to it a weak (100) peak at 13.3 are present. It arises from in-plane structural motif [37e39]. In Fig. 2(b) the characteristic peaks of palladium in FCC structure such as (111), (200), (220) and (311) are indexed. This shows that palladium nanoparticles are decorated on g-C3N4. Palladium average crystallite size was calculated using Scherrer's
Fig. 1 e Block diagram of sensor test station setup.
Please cite this article in press as: S. R, et al., Nanostructured palladium modified graphitic carbon nitride e High performance room temperature hydrogen sensor, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.002
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Fig. 2 e Powder X-ray diffraction of (a) melamine and (b) gC3N4.
formula and it was found to be 6.4 nm. Also the broadening of 27.6 peak suggests absence of long-range order in g-C3N4. X-ray photoelectron spectrum of Pd/g-C3N4 is shown in Fig. 3. A pallet of the material prepared to XPS so that the carbon tape used to stick sample would not contribute to carbon peaks emerging from the sample. In survey spectrum shown in Fig. 3(a), along with carbon, nitrogen and palladium small quantity of adventitious oxygen is present. The carbon and nitrogen atomic percentage in gC3N4 were found to be 45% and 55% respectively. In high resolution XPS spectrum of C 1s (Fig. 3(b)) peak at 284.6 eV is attributed to aromatic ring
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consisting of sp2 hybridized CeC [22,40]. Peak at 288 eV is due to carbon present in aromatic ring as N]CeN [41,42]. Small peak at 293.8 eV is due to charging effect. In deconvoluted high resolution XPS spectrum of N 1s (Fig. 3(c)) peak at 398.5 eV corresponds to nitrogen present in aromatic ring bonded as C]NeC [43]. Peak at 400.1 eV is attributed to nitrogen bonded with three carbon atoms. Small peak at 401.2 eV is due to Ce NeH, this shows that even after pyrolysis of the melamine some amount of hydrogen is still present. Fig. 3(d) shows deconvoluted high resolution XPS spectrum of palladium; Pd0 peaks of 3d5/2 and 3d3/2 at 335.2 eV and 340.5 eV respectively and Pdþ2 peaks of 3d5/2 and 3d3/2 at 337.1 eV and 342.6 eV respectively shows that palladium is present in 0 and þ2 oxidation states. The FTIR-ATR spectra of Melamine and g-C3N4 are shown in Fig. 4(a, b). Several peaks in the range 1650-1120 cm1 are due to the CeN and C]N stretching vibrations in tri-s-triazine rings. Sharp peak at 801 cm1 is ascribed to breathing mode of tri-s-triazine rings. Broad absorption bands present in the region from 3350 to 3000 cm1 are attributed to NH stretching vibrational modes [34,44,45]. This is in agreement with XPS again that the hydrogen is not completely removed. To find amount of metal nanoparticle loading on g-C3N4 thermogravimetric analysis was performed in zero-air atmosphere with heating rate 20 C/min and is shown in Fig. 5. g-C3N4 starts to decompose at around 400 C. After complete decomposition of g-C3N4 19 wt% of residue was left. In case of bare gC3N4 there no residue remained showing complete
Fig. 3 e (a) X-ray photoelectron spectroscopy survey spectrum, deconvoluted high resolution spectrum of (b) C 1s, (c) N 1s and (d) Pd 3d. Please cite this article in press as: S. R, et al., Nanostructured palladium modified graphitic carbon nitride e High performance room temperature hydrogen sensor, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.002
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test station. After degassing in argon atmosphere at temperature 150 C the film was allowed to cool to room temperature. Zero air was allowed for 30 min, then 4 vol% of H2 was allowed into the sample cell. At this instance resistance gradually decreased and stabilized at a value proportional to the concentration. Then hydrogen was stopped, the resistance of the film gradually increased and stabilized at the initial value. For repeated cycles we could observe repeatable values. Sensitivity of the sensor is defined as, Sensitivity % ¼
Fig. 4 e FTIR ATR spectra of (a) Melamine and (b) g-C3N4.
Fig. 5 e Thermogravimetric analysis plot of Pd/g-C3N4 and g-C3N4.
decomposition. It can be confirmed the 19 wt% residue left is the palladium as the metal containing material were used except the palladium precursor which was added in a proportion to make 20 wt% palladium loading. Presence of Palladium leads to catalytic decomposition due to which decomposition in Pd/g-C3N4 begins at lower temperature than bare g-C3N4. The morphology of Pd/g-C3N4 is shown in Fig. 6(a, b and c). Though g-C3N4 present in irregular shapes, stacked layered structure of g-C3N4 is evident from the transmission electron micrograph as g-C3N4 is formed from polymeric condensation of melamine. The Pd NPs are uniformly decorated on the surface of planar structures of gC3N4. Selected area electron diffraction is indexed and shown in Fig. 6(d). Sensor response towards hydrogen was studied at various temperatures (30 C, 40 C, 60 C and 80 C) in the home made
R0 Rg
R0 100
R0 and Rg represent resistance of the film in 100% air and in presence of the gas respectively. Response time is defined as time taken to 90% of the total change in sensor response parameter. Recovery time is defined as the time taken to reach 10% of the total change in base resistance. Sensor response of the bare material gC3N4 film for 4% H2 at 30 C is shown in Fig. 7. No change was observed even after prolonged exposure. This implies that bare gC3N4 as such has no interaction with H2 to induce change in resistance. Sensor response of the film for 4% H2 at 30 C is shown in Fig. 8 which shows good repeatability and high sensitivity of 99.8% with response time of 1 min 28 s. The recovery time was found to be 11 min. Further measurements were carried out from (4-1) % at temperatures 30 C, 40 C, 60 C and 80 C and are shown in Fig. 9. Up to 80 C the sensor shows very high performance near the flammability range of hydrogen. The sensor response within the concentration range of 4-1 vol% of H2 is tabulated in Table 1. For concentration below 1 vol% no change in the resistance was observed. Therefore, the lowest limit detectable by our configuration was 1 vol% H2 in air. Fig. 10 (a) shows the sensitivity with respect to H2 concentration at different temperatures. At lower concentration the sensitivity decreases rapidly with increase in temperature but, at higher concentration only a slight change was observed. Fig. 10(b) shows the response time vs concentration of H2 in air. It indicates that the response time increase with decrease in the concentration of H2. When H2 molecules move near the proximity of Pd NP surface, due to mutual interaction between outer shell electrons of hydrogen and palladium, dipole moment is induced which leads to Van der Waal's forces between them [46]. These Van der Waal forces lead to physisorption, hydrogen molecules form a layer over the Pd surface and are dissociated into atoms forming Pd/H solution [33]. This is represented in Fig. 11. As a result, the work function of Pd decreases and transfer of electrons takes place from Pd to gC3N4. This enhances the charge carrier concentration resulting in decrement in resistance [47]. The variation in the resistance is proportional to the concentration of H2. Oxygen present in air reacts with hydrogen on Pd surface forming water. As hydrogen atoms are removed from the surface hydrogen atoms in bulk diffuse to the surface. And more hydrogen is desorbed. When H2 is allowed, adsorption of H2 and desorption due to oxygen both the events occur simultaneously. Former event leads when there is rise in H2
Please cite this article in press as: S. R, et al., Nanostructured palladium modified graphitic carbon nitride e High performance room temperature hydrogen sensor, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.002
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Fig. 6 e (a), (b) Transmission electron micrographs (TEM), (c) High resolution TEM and (d) Selected area electron diffraction pattern of Pd/g-C3N4.
Fig. 7 e Sensor response of bare gC3N4 at 30 C towards 4% hydrogen in zero air.
Fig. 8 e Sensor response at 30 C towards 4% hydrogen in zero air.
Please cite this article in press as: S. R, et al., Nanostructured palladium modified graphitic carbon nitride e High performance room temperature hydrogen sensor, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.002
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Fig. 9 e Sensor response for 4 to 1% H2 at (a) 30 C, (b) 40 C, (c) 60 C and (d) 80 C.
Table 1 e Sensitivity of the sensor at various concentration of H2 at different temperatures. H2 concentration (vol%)
1 2 3 4
Sensitivity (%) 30 C
40 C
60 C
80 C
85 98.1 99.5 99.8
84.2 97.5 98.7 99
48.6 89.1 95.6 98.1
47.6 79.9 92.7 97.4
concentration, as more H2 adsorbed the resistance decreases. At some point both events attain equilibrium and the resistance stabilizes. As soon as H2 is stopped the later event leads and H2 is desorbed, as a result the resistance gradually increases and reaches initial value. In this way the sensor goes to its initial standby state. The material shows the capability to operate up to 80 C. Though the films show high resistance a shunt resistance can be used to scale down the resistance so that operational voltage will be practical.
Fig. 10 e (a) Sensitivity vs concentration and (b) response time vs concentration of hydrogen at various temperatures. Please cite this article in press as: S. R, et al., Nanostructured palladium modified graphitic carbon nitride e High performance room temperature hydrogen sensor, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.002
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Fig. 11 e Schematic shows hydrogen physisorption and chemisorption over palladium nanoparticles.
These results are encouraging to further explore g-C3N4 composites in gas sensors application.
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Conclusions [11]
Graphitic carbon nitride was prepared by simple pyrolysis of melamine. Uniform dispersion of palladium nanoparticles on graphitic carbon nitride was achieved by polyol reduction technique. Palladium nanoparticles modified graphitic carbon nitride was demonstrated as high performance resistive hydrogen sensor. At 30 C for 4% hydrogen, Pd/g-C3N4 shows high sensitivity of 99.8% with a response time of 88 s. Resistance changes from 12 GU to 19 MU for 4% hydrogen at room temperature. Compared to regular room temperature sensors, Pd/g-C3N4 shows very high sensitivity even at higher temperatures up to 80 C although there is feeble decline in the sensitivity. Pd/g-C3N4 can thus be a promising material for hydrogen sensing and also for H2 concentration monitoring.
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Acknowledgement [15]
Authors are thankful to Indian Institute of Technology Madras and Government of India for supporting this work.
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Please cite this article in press as: S. R, et al., Nanostructured palladium modified graphitic carbon nitride e High performance room temperature hydrogen sensor, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.002
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Nanostructured palladium modified graphitic carbon nitride e High performance room temperature hydrogen sensor Raghu S. a,b, Santhosh P.N. b, Ramaprabhu S. a,* a
Alternative Energy and Nanotechnology and Laboratory, Department of Physics, Indian Institute of Technology Madras, Chennai, 600036, India b Low Temperature Physics Laboratory, Department of Physics, Indian Institute of Technology Madras, Chennai, 600036, India
article info
abstract
Article history:
Graphitic carbon nitride has attracted huge interest due to its due to low cost and easy
Received 27 May 2016
synthesis. We report for the first time graphitic carbon nitride composite as room tem-
Received in revised form
perature hydrogen sensing material. Nano structured palladium nanoparticle dispersed
31 August 2016
graphitic carbon nitride (Pd/g-C3N4) composite was investigated as room temperature
Accepted 1 September 2016
resistive gas sensor. Graphitic carbon nitride (g-C3N4) was prepared by pyrolysis of mel-
Available online xxx
amine. Palladium nanoparticles were dispersed on g-C3N4 by polyol reduction method. Hydrogen sensing properties of Pd/g-C3N4 were studied within near flammability range
Keywords:
from 1 to 4 vol%. The performance was evaluated with variation in temperature from 30 to
Graphitic carbon nitride
80 C. For 4% H2 at room temperature, 99.8% sensitivity is achieved with response time of
Palladium nanoparticles
88 s while for the same concentration of H2 at 80 C, the sensor shows 97.4% sensitivity.
Hydrogen sensor
The results show prospect of reliable sensing material.
Room temperature hydrogen sensor
© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen (H2) is promising environmental friendly zero emission energy carrier and is expected to replace the usage of many conventional energy carriers to a possible extent which release pollutants. H2 is used in wide range of applications such as fuel cells, semiconductor industry, mines, petroleum, nuclear power plants, and chemical industries [1]. Combustion of hydrogen (142 kJ/mol of H2) can produce three times the energy produced from gasoline, diesel and petroleum gas of the same weight. Considering this H2 is the choice above the conventional sources, but containing H2 and transporting it
pose more safety challenges. It is highly inflammable gas with wide flammability range from 4% to 75% in air and very low ignition energy of 0.017 mJ [2]. Therefore, sensors are necessary for sensing hydrogen and concentration monitoring. Various materials like Pd nanowire, Pd thin films, porous silicon, carbon nanotubes, WO3, TiO2, TeO2, In2O3, RuO2, MoO2, ZnO were explored as hydrogen sensors [3e12]. Most of the sensors available commercially are made of semiconducting metal oxides which operate at higher temperatures [4,7,13e15]. For concentration monitoring nearer to the flammability limit these may cause ignition. Most of the carbon based materials like graphene, carbon nanotubes composites though are operational at room temperature but suffer low
* Corresponding author. E-mail address: [email protected] (R. S.). http://dx.doi.org/10.1016/j.ijhydene.2016.09.002 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: S. R, et al., Nanostructured palladium modified graphitic carbon nitride e High performance room temperature hydrogen sensor, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.002
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sensitivity [6,16e18]. Sensors with high sensitivity at room temperature at these concentrations are desirable. There is a need to find new alternative to graphene and carbon nanotube like materials. Graphitic carbon nitride (g-C3N4) possesses the desirable properties for a sensing material. It is indirect band gap semiconducting material (band gap of 2.7 eV) consisting of carbon and nitrogen atoms arranged in graphite like layered structure [19e21]. It is highly stable and non-toxic. It can be synthesized easily at large scale and can with stand harsh conditions [22]. Many ways of synthesis are reported for synthesis of g-C3N4 with difference precursors like cyanamide, dicyanamide, melamine and urea [23e27]. In the present work, g-C3N4 was investigated as a room temperature resistive hydrogen sensor. By modifying g-C3N4 matrix by material that has strong affinity toward hydrogen its gas sensing properties can be explored. Palladium is well known for its activity towards hydrogen [28e33]. g-C3N4 was modified by dispersing palladium nanoparticles on its surface by polyol reduction technique. Sensor film was fabricated by simple and cost effective drop casting technique on an insulating substrate (alumina).
Experimental section Materials Melamine was purchased from Himedia Laboratories Pvt Ltd, Mumbai, India. Ethylene glycol, sodium hydroxide pellets were purchased from Merck, Mumbai, India Pvt Ltd. Palladium chloride was purchased from Sigma Aldrich chemicals Pvt Ltd. Ultra-pure deionized water was from water purification system Milli-Q Advantage A10 (Merck Millipore, USA).
Material synthesis Graphitic carbon nitride was prepared by direct pyrolysis of melamine [34,35]. 4 g of melamine was placed in an alumina boat and loaded inside a quartz tube in a tubular furnace. Two-end coupling were fitted on both ends and has provision for gas inlet and outlet. Initially it was purged with nitrogen for 20 min and the temperature was raised to 550 C and maintained for 4 h. Further it was allowed to cool down naturally to room temperature. Nitrogen was allowed throughout the experiment with flow rate of 0.16 SLPM. A condensed pale yellow colored powder of 645 mg was obtained which is g-C3N4. It was further ground to fine powder using mortar and pestle. Palladium nanoparticles were incorporated into the g-C3N4 matrix by modified polyol reduction. 80 mg of g-C3N4 is dispersed in 100 mL of a solution of ethylene glycol and DI water in the ratio of 3:1. This mixture is ultrasonicated for 20 min and kept for strong stirring. 3.333 mL of 1 wt% of PdCl2 solution was added drop wise for obtaining 20 wt% of palladium loading. Further, 2.5 M NaOH solution was added slowly to maintain pH of 10. After vigorous stirring for 12 h, the mixture is refluxed for 6 h at 130 C to ensure complete reduction of the metal salt. This sample was washed with copious amount of water. Further filtered, dried and labeled as Pd/g-C3N4.
Film fabrication Pd/g-C3N4 of 15 mg and 40 mL of 1 wt% Nafion solution was added to 0.5 mL of ethanol and ultrasonicated for 15 min and slurry was obtained. This slurry was drop casted on an alumina substrate with two gold-coated electrodes to make a thick film. This film was mounted inside the sample chamber and electrical contacts were given using silver epoxy paste. Schematic of the sensor test station is shown in Fig. 1. Sample chamber was then closed and purged with argon gas for 15 min. In presence of argon the film was degassed by heating it to 150 C. A 100 MU resistance is connected in parallel with the two electrodes outside the sample chamber externally. Keithley 2601 A Source meter was used to provide constant current and Keithley 2182 nanovoltmeter for measuring the voltage. Nanovoltmeter was interfaced to a computer through Lab-view program for data collection.
Results and discussion X-ray photoelectron spectroscopy was carried out by Specs X-ray photoelectron spectrometer, X-ray source being Mg Ka and analyzer PHOIBOS 100MCD. FTIR analysis was carried out in Agilent carry 630 diamond ATR by placing the powder sample directly without any dilution. For X-ray diffraction measurements PANalytical X'Pert pro X-ray diffractometer was used with Cu Ka radiation as source with nickel filter. Measurements were carried out from 5 to 90 with step size 0.02 . Thermogravimetric analysis was performed using TA instruments SDT Q 600. For morphological studies FEI quanta 3D transmission electron microscope FEI tecnai G2 was used. The powder X-ray diffraction patterns of g-C3N4 and Pd/gC3N4 are shown in Fig. 2. Peak at 27.6 is due to the stacking of tri-s-triazine rings which are graphitic like planar structures [36] and in addition to it a weak (100) peak at 13.3 are present. It arises from in-plane structural motif [37e39]. In Fig. 2(b) the characteristic peaks of palladium in FCC structure such as (111), (200), (220) and (311) are indexed. This shows that palladium nanoparticles are decorated on g-C3N4. Palladium average crystallite size was calculated using Scherrer's
Fig. 1 e Block diagram of sensor test station setup.
Please cite this article in press as: S. R, et al., Nanostructured palladium modified graphitic carbon nitride e High performance room temperature hydrogen sensor, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.002
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Fig. 2 e Powder X-ray diffraction of (a) melamine and (b) gC3N4.
formula and it was found to be 6.4 nm. Also the broadening of 27.6 peak suggests absence of long-range order in g-C3N4. X-ray photoelectron spectrum of Pd/g-C3N4 is shown in Fig. 3. A pallet of the material prepared to XPS so that the carbon tape used to stick sample would not contribute to carbon peaks emerging from the sample. In survey spectrum shown in Fig. 3(a), along with carbon, nitrogen and palladium small quantity of adventitious oxygen is present. The carbon and nitrogen atomic percentage in gC3N4 were found to be 45% and 55% respectively. In high resolution XPS spectrum of C 1s (Fig. 3(b)) peak at 284.6 eV is attributed to aromatic ring
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consisting of sp2 hybridized CeC [22,40]. Peak at 288 eV is due to carbon present in aromatic ring as N]CeN [41,42]. Small peak at 293.8 eV is due to charging effect. In deconvoluted high resolution XPS spectrum of N 1s (Fig. 3(c)) peak at 398.5 eV corresponds to nitrogen present in aromatic ring bonded as C]NeC [43]. Peak at 400.1 eV is attributed to nitrogen bonded with three carbon atoms. Small peak at 401.2 eV is due to Ce NeH, this shows that even after pyrolysis of the melamine some amount of hydrogen is still present. Fig. 3(d) shows deconvoluted high resolution XPS spectrum of palladium; Pd0 peaks of 3d5/2 and 3d3/2 at 335.2 eV and 340.5 eV respectively and Pdþ2 peaks of 3d5/2 and 3d3/2 at 337.1 eV and 342.6 eV respectively shows that palladium is present in 0 and þ2 oxidation states. The FTIR-ATR spectra of Melamine and g-C3N4 are shown in Fig. 4(a, b). Several peaks in the range 1650-1120 cm1 are due to the CeN and C]N stretching vibrations in tri-s-triazine rings. Sharp peak at 801 cm1 is ascribed to breathing mode of tri-s-triazine rings. Broad absorption bands present in the region from 3350 to 3000 cm1 are attributed to NH stretching vibrational modes [34,44,45]. This is in agreement with XPS again that the hydrogen is not completely removed. To find amount of metal nanoparticle loading on g-C3N4 thermogravimetric analysis was performed in zero-air atmosphere with heating rate 20 C/min and is shown in Fig. 5. g-C3N4 starts to decompose at around 400 C. After complete decomposition of g-C3N4 19 wt% of residue was left. In case of bare gC3N4 there no residue remained showing complete
Fig. 3 e (a) X-ray photoelectron spectroscopy survey spectrum, deconvoluted high resolution spectrum of (b) C 1s, (c) N 1s and (d) Pd 3d. Please cite this article in press as: S. R, et al., Nanostructured palladium modified graphitic carbon nitride e High performance room temperature hydrogen sensor, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.002
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test station. After degassing in argon atmosphere at temperature 150 C the film was allowed to cool to room temperature. Zero air was allowed for 30 min, then 4 vol% of H2 was allowed into the sample cell. At this instance resistance gradually decreased and stabilized at a value proportional to the concentration. Then hydrogen was stopped, the resistance of the film gradually increased and stabilized at the initial value. For repeated cycles we could observe repeatable values. Sensitivity of the sensor is defined as, Sensitivity % ¼
Fig. 4 e FTIR ATR spectra of (a) Melamine and (b) g-C3N4.
Fig. 5 e Thermogravimetric analysis plot of Pd/g-C3N4 and g-C3N4.
decomposition. It can be confirmed the 19 wt% residue left is the palladium as the metal containing material were used except the palladium precursor which was added in a proportion to make 20 wt% palladium loading. Presence of Palladium leads to catalytic decomposition due to which decomposition in Pd/g-C3N4 begins at lower temperature than bare g-C3N4. The morphology of Pd/g-C3N4 is shown in Fig. 6(a, b and c). Though g-C3N4 present in irregular shapes, stacked layered structure of g-C3N4 is evident from the transmission electron micrograph as g-C3N4 is formed from polymeric condensation of melamine. The Pd NPs are uniformly decorated on the surface of planar structures of gC3N4. Selected area electron diffraction is indexed and shown in Fig. 6(d). Sensor response towards hydrogen was studied at various temperatures (30 C, 40 C, 60 C and 80 C) in the home made
R0 Rg
R0 100
R0 and Rg represent resistance of the film in 100% air and in presence of the gas respectively. Response time is defined as time taken to 90% of the total change in sensor response parameter. Recovery time is defined as the time taken to reach 10% of the total change in base resistance. Sensor response of the bare material gC3N4 film for 4% H2 at 30 C is shown in Fig. 7. No change was observed even after prolonged exposure. This implies that bare gC3N4 as such has no interaction with H2 to induce change in resistance. Sensor response of the film for 4% H2 at 30 C is shown in Fig. 8 which shows good repeatability and high sensitivity of 99.8% with response time of 1 min 28 s. The recovery time was found to be 11 min. Further measurements were carried out from (4-1) % at temperatures 30 C, 40 C, 60 C and 80 C and are shown in Fig. 9. Up to 80 C the sensor shows very high performance near the flammability range of hydrogen. The sensor response within the concentration range of 4-1 vol% of H2 is tabulated in Table 1. For concentration below 1 vol% no change in the resistance was observed. Therefore, the lowest limit detectable by our configuration was 1 vol% H2 in air. Fig. 10 (a) shows the sensitivity with respect to H2 concentration at different temperatures. At lower concentration the sensitivity decreases rapidly with increase in temperature but, at higher concentration only a slight change was observed. Fig. 10(b) shows the response time vs concentration of H2 in air. It indicates that the response time increase with decrease in the concentration of H2. When H2 molecules move near the proximity of Pd NP surface, due to mutual interaction between outer shell electrons of hydrogen and palladium, dipole moment is induced which leads to Van der Waal's forces between them [46]. These Van der Waal forces lead to physisorption, hydrogen molecules form a layer over the Pd surface and are dissociated into atoms forming Pd/H solution [33]. This is represented in Fig. 11. As a result, the work function of Pd decreases and transfer of electrons takes place from Pd to gC3N4. This enhances the charge carrier concentration resulting in decrement in resistance [47]. The variation in the resistance is proportional to the concentration of H2. Oxygen present in air reacts with hydrogen on Pd surface forming water. As hydrogen atoms are removed from the surface hydrogen atoms in bulk diffuse to the surface. And more hydrogen is desorbed. When H2 is allowed, adsorption of H2 and desorption due to oxygen both the events occur simultaneously. Former event leads when there is rise in H2
Please cite this article in press as: S. R, et al., Nanostructured palladium modified graphitic carbon nitride e High performance room temperature hydrogen sensor, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.002
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Fig. 6 e (a), (b) Transmission electron micrographs (TEM), (c) High resolution TEM and (d) Selected area electron diffraction pattern of Pd/g-C3N4.
Fig. 7 e Sensor response of bare gC3N4 at 30 C towards 4% hydrogen in zero air.
Fig. 8 e Sensor response at 30 C towards 4% hydrogen in zero air.
Please cite this article in press as: S. R, et al., Nanostructured palladium modified graphitic carbon nitride e High performance room temperature hydrogen sensor, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.002
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Fig. 9 e Sensor response for 4 to 1% H2 at (a) 30 C, (b) 40 C, (c) 60 C and (d) 80 C.
Table 1 e Sensitivity of the sensor at various concentration of H2 at different temperatures. H2 concentration (vol%)
1 2 3 4
Sensitivity (%) 30 C
40 C
60 C
80 C
85 98.1 99.5 99.8
84.2 97.5 98.7 99
48.6 89.1 95.6 98.1
47.6 79.9 92.7 97.4
concentration, as more H2 adsorbed the resistance decreases. At some point both events attain equilibrium and the resistance stabilizes. As soon as H2 is stopped the later event leads and H2 is desorbed, as a result the resistance gradually increases and reaches initial value. In this way the sensor goes to its initial standby state. The material shows the capability to operate up to 80 C. Though the films show high resistance a shunt resistance can be used to scale down the resistance so that operational voltage will be practical.
Fig. 10 e (a) Sensitivity vs concentration and (b) response time vs concentration of hydrogen at various temperatures. Please cite this article in press as: S. R, et al., Nanostructured palladium modified graphitic carbon nitride e High performance room temperature hydrogen sensor, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.002
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[6]
[7]
[8]
Fig. 11 e Schematic shows hydrogen physisorption and chemisorption over palladium nanoparticles.
These results are encouraging to further explore g-C3N4 composites in gas sensors application.
[9]
[10]
Conclusions [11]
Graphitic carbon nitride was prepared by simple pyrolysis of melamine. Uniform dispersion of palladium nanoparticles on graphitic carbon nitride was achieved by polyol reduction technique. Palladium nanoparticles modified graphitic carbon nitride was demonstrated as high performance resistive hydrogen sensor. At 30 C for 4% hydrogen, Pd/g-C3N4 shows high sensitivity of 99.8% with a response time of 88 s. Resistance changes from 12 GU to 19 MU for 4% hydrogen at room temperature. Compared to regular room temperature sensors, Pd/g-C3N4 shows very high sensitivity even at higher temperatures up to 80 C although there is feeble decline in the sensitivity. Pd/g-C3N4 can thus be a promising material for hydrogen sensing and also for H2 concentration monitoring.
[12]
[13]
[14]
Acknowledgement [15]
Authors are thankful to Indian Institute of Technology Madras and Government of India for supporting this work.
[16]
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