MOF-based membrane encapsulated ZnO nanowires for enhanced ...
SUPPORTING INFORMATION
MOF-based membrane encapsulated ZnO nanowires for enhanced gas sensor selectivity Martin Drobek1±, Jae-Hun Kim2±, Mikhael Bechelany*1, Cyril Vallicari1, Anne Julbe1, Sang Sub Kim*2
1
Institut Européen des Membranes, UMR 5635, Université de Montpellier, ENSCM, CNRS, Place Eugène
Bataillon, F-34095 Montpellier cedex 5, France b
Department of Materials Science and Engineering, Inha University, Incheon 402-751, Republic of Korea
±
These 2 authors participated to this work equally
*corresponding authors: [email protected] , phone: +33467149167, Fax: +33467149119 [email protected] , phone: +82328607546, Fax: +82328625546
S-1
S-1. Experimental Section The protocol which has been elaborated for the preparation of optimized gas sensors composed of ZIF-8 membrane encapsulated ZnO nanowires results from a combination of authors’ expertise in the areas of both semiconductor metal oxides (SMOs)-based sensors and ZIF-8 membranes growth by solvothermal conversion of ZnO. The synthesis protocol has been optimized in order to ensure top-surface conversion of ZnO NWs and obtain a thin selective ZIF-8 layer encapsulating the ZnO nanowires.
1. Synthesis of ZnO nanowires ZnO NWs were grown on a deliberately-designed interdigital electrode (IDE) that had been fabricated on SiO2 (200 nm thick)-grown Si (100) substrates using a conventional lithography process. The IDE consisted of tri-layers; Au (3 nm)/Pt (100 nm)/Ti (50 nm) films were deposited sequentially by a sputtering method. The Au layer played the catalytic role of growing ZnO NWs, disappearing after fulfilling its catalytic role, and remaining as nanoparticles at the tip of nanowires.1 The Pt layer served as an electrical passage. The Ti layer was employed to enhance the adhesion between the Si substrate and Pt layer. The details of the IDE shape used in this work are as follows: the total number of electrode pads was 20; each electrode pad was 1.05 mm long and 20 µm wide; the gap between the neighboring electrode pads was 10 µm. The wafer with as-fabricated IDE was loaded in a horizontal-type tube furnace, in which an alumina boat containing Zn powders was placed. The wafer and the alumina boat was 2 cm apart. The temperature was maintained at 950 °C for 1 hour while flowing Ar and O2 through the tube furnace at rates of 800 and 50 standard cubic centimeter per minute (SCCM), respectively. According to the well-known vapor-growth behavior of oxide nanowires, ZnO NWs grown selectively on the Au catalytic layer were made to form three-dimensional interwoven mat. Such networking propensity was in an agreement with our previous studies dealing with the synthesis of SnO2 and CuO nanowires as reported elsewhere.2-5
2. Synthesis of ZnO@ZIF-8 composite nanowires The ZnO NWs were submitted to a solvothermal treatment in a closed pressure vessel (Teflon-lined stainless-steel autoclave - 45 ml) containing 2-methyl imidazole (2-mim) S-2
dissolved in methanol (10 wt%). The above reaction mixture was prepared under stirring for 10 minutes to obtain a clear solution. The autoclave was heated in a conventional oven at 100 °C for 24 h and cooled down to room temperature. The as-obtained ZnO@ZIF-8 composite nanowires (ZnO@ZIF-8 NWs) were then washed several times with methanol and dried at 70 °C for 2 h. The overall fabrication process of ZnO@ZIF-8 NWs sensors is schematically shown in Figure 1.
3. Characterizations The morphology, thickness and homogeneity of both virgin ZnO and composite ZnO@ZIF-8 NWs were studied using a high resolution scanning electron microscope (FESEM, Hitachi S4800) at 1.5 keV. The chemical composition of the material grown on the sensor surface has been determined by EDX analysis (Silicon Drift Detector (SDD), X-MaxN, Oxford Instrument). Finally, samples have been studied by transmission electron microscopy (HRTEM, FEI Tecnai F20) using different analyses including TEM imaging, EDS and EELS chemical analysis or STEM mapping. Chemical composition was studied by X-ray Photoelectron Spectroscopy (XPS) (ESCALAB 250 Thermal Electron ) with AlKα (1486.6 eV). Binding energies were calibrated by using the containment carbon (C1s = 284.4 eV). Structural characterization was performed using grazing incidence X-ray diffraction (GIXRD, Bruker D5000 with CuKα radiation). The N2 sorption-desorption isotherms were measured with Micromeritics ASAP 2010 equipment (outgassing conditions: 80°C-12h). TGA has been performed using a TA Instruments SDT 2960, under air atmosphere in the temperature range 25-800°C (heating rate 5°C/min).
4. Gas sensing measurements The gas sensing performance of the ZnO@ZIF-8 NWs was examined for C6H6, C7H8 and H2 gases. The gas concentrations were controlled by changing the mixing ratio of dry air and dry air–balanced analyte gas using mass flow controllers. The configuration and design of the gas sensing system used in this study were described in earlier reports.6-7 All sensing measurements were performed at 300 °C, which was chosen by preliminary experiments.8 The response (R) of the sensors to the tested reducing gases was evaluated by the following equation: Ra/Rg, where Ra and Rg were the resistances measured in the absence and presence of the analyte gas, respectively. S-3
Intensity (a. u.)
12000
8000
*
ZnO@ZIF-8
(011) +
4000
(112) (002)+ +
o
(002) + *
*
0
10
20
30
40
2 θ (°) Figure S1. XRD patterns of ZnO@ZIF-8 composite material (diffraction peaks of the support are marked with (o), those of ZnO are marked with (*) and those of ZIF-8 are marked with (+).
Table S1. Results of XPS analysis of ZnO@ZIF-8 composite materials Line
Peak BE (eV)
At. %
C1s
283.12
50.07
O1s
530
25.93
Zn2p3 1020.05
10.6
N1s
397.18
11.32
Si2p
102.21
2.08
S-4
Figure S2. EDX mapping measurements during SEM observations of the gas sensor surface
Table S2. EDX analysis of the ZnO@ZIF-8 composite sensor composition Element
Weight %
Atomic %
C
4.31
13.50
N
0.18
0.49
O
12.06
28.38
Si
12.49
16.74
Zn
70.97
40.89
S-5
Total:
100.00
100.00
Figure S3. TEM images of ZnO@ZIF-8 composite nanowires and the corresponding STEM/EDX line scans.
S-6
100 98
% Weight
96 94 92 90 88 86 0
100
200 300 Temperature (°C)
400
500
Figure S4. TGA (in air) of ZIF-8/ZnO composite material
Intensity (a.u)
40000
* ZnO
22500
*
*
600 °C
*
*
10000
500 °C 400 °C 300 °C 200 °C
2500
100 °C 30 °C
0 5
5
10
10
15
15
20
20
25
25
30
2theta (°) 30
2θ
35
35
40
40
45
45
50 2Theta (°)
50
Figure S5. In-situ X-ray thermodiffraction study (air, 10°C/min, plateau 30 min for each T°) of ZIF-8/ZnO composite material in the temperature range 30-600°C S-7
Figure S6. Evolution of normalized resistance curves of the composite ZnO@ZIF-8 NWs maintained at the operating temperature of 300 °C for various experiment durations (from 1h to 72h) and H2 concentrations in dry air (10, 30 and 50 ppm-grey areas).
S-8
Figure S7. Gas response measured at 250°C of the ZnO@ZIF-8 composite nanowires in contact with 10, 30 and 50 ppm H2.
REFERENCES 1. Yang, P.; Yan, H.; Mao, S.; Russo, R.; Johnson, J.; Saykally, R.; Morris, N.; Pham, J.; He, R.; Choi, H. J., Controlled Growth of Zno Nanowires and Their Optical Properties. Adv. Funct. Mater. 2002, 12, 323-331. 2. Park, J. Y.; Choi, S.-W.; Kim, S. S., Junction-Tuned Sno2 Nanowires and Their Sensing Properties. J. Phys. Chem. C 2011, 115, 12774-12781. 3. Choi, S.-W.; Katoch, A.; Sun, G.-J.; Wu, P.; Kim, S. S., No2-Sensing Performance of SnO2 Microrods by Functionalization of Ag Nanoparticles. J. Mater. Chem. C 2013, 1, 2834-2841. 4. Kim, J.-H.; Katoch, A.; Choi, S.-W.; Kim, S. S., Growth and Sensing Properties of Networked P-Cuo Nanowires. Sens. Actuators, B 2015, 212, 190-195. 5. Kim, J.-H.; Katoch, A.; Kim, S. S., Optimum Shell Thickness and Underlying Sensing Mechanism in P–N Cuo–ZnO Core–Shell Nanowires. Sens. Actuators, B 2016, 222, 249-256. 6. Choi, S.-W.; Katoch, A.; Kim, J.-H.; Kim, S. S., Prominent Reducing Gas-Sensing Performances of N-Sno2 Nanowires by Local Creation of P–N Heterojunctions by Functionalization with P-Cr2o3 Nanoparticles. ACS Appl. Mater. Interfaces 2014, 6, 17723-17729. 7. Katoch, A.; Sun, G.-J.; Choi, S.-W.; Hishita, S.; Kulish, V. V.; Wu, P.; Kim, S. S., AcceptorCompensated Charge Transport and Surface Chemical Reactions in Au-Implanted Sno2 Nanowires. Sci. Rep. 2014, 4, 4622. 8. Park, J. Y.; Choi, S.-W.; Kim, S. S., Tailoring the Number of Junctions Per Electrode Pair in Networked Zno Nanowire Sensors. J. Am. Ceram. Soc. 2011, 94, 3922-3926.
S-9
MOF-based membrane encapsulated ZnO nanowires for enhanced gas sensor selectivity Martin Drobek1±, Jae-Hun Kim2±, Mikhael Bechelany*1, Cyril Vallicari1, Anne Julbe1, Sang Sub Kim*2
1
Institut Européen des Membranes, UMR 5635, Université de Montpellier, ENSCM, CNRS, Place Eugène
Bataillon, F-34095 Montpellier cedex 5, France b
Department of Materials Science and Engineering, Inha University, Incheon 402-751, Republic of Korea
±
These 2 authors participated to this work equally
*corresponding authors: [email protected] , phone: +33467149167, Fax: +33467149119 [email protected] , phone: +82328607546, Fax: +82328625546
S-1
S-1. Experimental Section The protocol which has been elaborated for the preparation of optimized gas sensors composed of ZIF-8 membrane encapsulated ZnO nanowires results from a combination of authors’ expertise in the areas of both semiconductor metal oxides (SMOs)-based sensors and ZIF-8 membranes growth by solvothermal conversion of ZnO. The synthesis protocol has been optimized in order to ensure top-surface conversion of ZnO NWs and obtain a thin selective ZIF-8 layer encapsulating the ZnO nanowires.
1. Synthesis of ZnO nanowires ZnO NWs were grown on a deliberately-designed interdigital electrode (IDE) that had been fabricated on SiO2 (200 nm thick)-grown Si (100) substrates using a conventional lithography process. The IDE consisted of tri-layers; Au (3 nm)/Pt (100 nm)/Ti (50 nm) films were deposited sequentially by a sputtering method. The Au layer played the catalytic role of growing ZnO NWs, disappearing after fulfilling its catalytic role, and remaining as nanoparticles at the tip of nanowires.1 The Pt layer served as an electrical passage. The Ti layer was employed to enhance the adhesion between the Si substrate and Pt layer. The details of the IDE shape used in this work are as follows: the total number of electrode pads was 20; each electrode pad was 1.05 mm long and 20 µm wide; the gap between the neighboring electrode pads was 10 µm. The wafer with as-fabricated IDE was loaded in a horizontal-type tube furnace, in which an alumina boat containing Zn powders was placed. The wafer and the alumina boat was 2 cm apart. The temperature was maintained at 950 °C for 1 hour while flowing Ar and O2 through the tube furnace at rates of 800 and 50 standard cubic centimeter per minute (SCCM), respectively. According to the well-known vapor-growth behavior of oxide nanowires, ZnO NWs grown selectively on the Au catalytic layer were made to form three-dimensional interwoven mat. Such networking propensity was in an agreement with our previous studies dealing with the synthesis of SnO2 and CuO nanowires as reported elsewhere.2-5
2. Synthesis of ZnO@ZIF-8 composite nanowires The ZnO NWs were submitted to a solvothermal treatment in a closed pressure vessel (Teflon-lined stainless-steel autoclave - 45 ml) containing 2-methyl imidazole (2-mim) S-2
dissolved in methanol (10 wt%). The above reaction mixture was prepared under stirring for 10 minutes to obtain a clear solution. The autoclave was heated in a conventional oven at 100 °C for 24 h and cooled down to room temperature. The as-obtained ZnO@ZIF-8 composite nanowires (ZnO@ZIF-8 NWs) were then washed several times with methanol and dried at 70 °C for 2 h. The overall fabrication process of ZnO@ZIF-8 NWs sensors is schematically shown in Figure 1.
3. Characterizations The morphology, thickness and homogeneity of both virgin ZnO and composite ZnO@ZIF-8 NWs were studied using a high resolution scanning electron microscope (FESEM, Hitachi S4800) at 1.5 keV. The chemical composition of the material grown on the sensor surface has been determined by EDX analysis (Silicon Drift Detector (SDD), X-MaxN, Oxford Instrument). Finally, samples have been studied by transmission electron microscopy (HRTEM, FEI Tecnai F20) using different analyses including TEM imaging, EDS and EELS chemical analysis or STEM mapping. Chemical composition was studied by X-ray Photoelectron Spectroscopy (XPS) (ESCALAB 250 Thermal Electron ) with AlKα (1486.6 eV). Binding energies were calibrated by using the containment carbon (C1s = 284.4 eV). Structural characterization was performed using grazing incidence X-ray diffraction (GIXRD, Bruker D5000 with CuKα radiation). The N2 sorption-desorption isotherms were measured with Micromeritics ASAP 2010 equipment (outgassing conditions: 80°C-12h). TGA has been performed using a TA Instruments SDT 2960, under air atmosphere in the temperature range 25-800°C (heating rate 5°C/min).
4. Gas sensing measurements The gas sensing performance of the ZnO@ZIF-8 NWs was examined for C6H6, C7H8 and H2 gases. The gas concentrations were controlled by changing the mixing ratio of dry air and dry air–balanced analyte gas using mass flow controllers. The configuration and design of the gas sensing system used in this study were described in earlier reports.6-7 All sensing measurements were performed at 300 °C, which was chosen by preliminary experiments.8 The response (R) of the sensors to the tested reducing gases was evaluated by the following equation: Ra/Rg, where Ra and Rg were the resistances measured in the absence and presence of the analyte gas, respectively. S-3
Intensity (a. u.)
12000
8000
*
ZnO@ZIF-8
(011) +
4000
(112) (002)+ +
o
(002) + *
*
0
10
20
30
40
2 θ (°) Figure S1. XRD patterns of ZnO@ZIF-8 composite material (diffraction peaks of the support are marked with (o), those of ZnO are marked with (*) and those of ZIF-8 are marked with (+).
Table S1. Results of XPS analysis of ZnO@ZIF-8 composite materials Line
Peak BE (eV)
At. %
C1s
283.12
50.07
O1s
530
25.93
Zn2p3 1020.05
10.6
N1s
397.18
11.32
Si2p
102.21
2.08
S-4
Figure S2. EDX mapping measurements during SEM observations of the gas sensor surface
Table S2. EDX analysis of the ZnO@ZIF-8 composite sensor composition Element
Weight %
Atomic %
C
4.31
13.50
N
0.18
0.49
O
12.06
28.38
Si
12.49
16.74
Zn
70.97
40.89
S-5
Total:
100.00
100.00
Figure S3. TEM images of ZnO@ZIF-8 composite nanowires and the corresponding STEM/EDX line scans.
S-6
100 98
% Weight
96 94 92 90 88 86 0
100
200 300 Temperature (°C)
400
500
Figure S4. TGA (in air) of ZIF-8/ZnO composite material
Intensity (a.u)
40000
* ZnO
22500
*
*
600 °C
*
*
10000
500 °C 400 °C 300 °C 200 °C
2500
100 °C 30 °C
0 5
5
10
10
15
15
20
20
25
25
30
2theta (°) 30
2θ
35
35
40
40
45
45
50 2Theta (°)
50
Figure S5. In-situ X-ray thermodiffraction study (air, 10°C/min, plateau 30 min for each T°) of ZIF-8/ZnO composite material in the temperature range 30-600°C S-7
Figure S6. Evolution of normalized resistance curves of the composite ZnO@ZIF-8 NWs maintained at the operating temperature of 300 °C for various experiment durations (from 1h to 72h) and H2 concentrations in dry air (10, 30 and 50 ppm-grey areas).
S-8
Figure S7. Gas response measured at 250°C of the ZnO@ZIF-8 composite nanowires in contact with 10, 30 and 50 ppm H2.
REFERENCES 1. Yang, P.; Yan, H.; Mao, S.; Russo, R.; Johnson, J.; Saykally, R.; Morris, N.; Pham, J.; He, R.; Choi, H. J., Controlled Growth of Zno Nanowires and Their Optical Properties. Adv. Funct. Mater. 2002, 12, 323-331. 2. Park, J. Y.; Choi, S.-W.; Kim, S. S., Junction-Tuned Sno2 Nanowires and Their Sensing Properties. J. Phys. Chem. C 2011, 115, 12774-12781. 3. Choi, S.-W.; Katoch, A.; Sun, G.-J.; Wu, P.; Kim, S. S., No2-Sensing Performance of SnO2 Microrods by Functionalization of Ag Nanoparticles. J. Mater. Chem. C 2013, 1, 2834-2841. 4. Kim, J.-H.; Katoch, A.; Choi, S.-W.; Kim, S. S., Growth and Sensing Properties of Networked P-Cuo Nanowires. Sens. Actuators, B 2015, 212, 190-195. 5. Kim, J.-H.; Katoch, A.; Kim, S. S., Optimum Shell Thickness and Underlying Sensing Mechanism in P–N Cuo–ZnO Core–Shell Nanowires. Sens. Actuators, B 2016, 222, 249-256. 6. Choi, S.-W.; Katoch, A.; Kim, J.-H.; Kim, S. S., Prominent Reducing Gas-Sensing Performances of N-Sno2 Nanowires by Local Creation of P–N Heterojunctions by Functionalization with P-Cr2o3 Nanoparticles. ACS Appl. Mater. Interfaces 2014, 6, 17723-17729. 7. Katoch, A.; Sun, G.-J.; Choi, S.-W.; Hishita, S.; Kulish, V. V.; Wu, P.; Kim, S. S., AcceptorCompensated Charge Transport and Surface Chemical Reactions in Au-Implanted Sno2 Nanowires. Sci. Rep. 2014, 4, 4622. 8. Park, J. Y.; Choi, S.-W.; Kim, S. S., Tailoring the Number of Junctions Per Electrode Pair in Networked Zno Nanowire Sensors. J. Am. Ceram. Soc. 2011, 94, 3922-3926.
S-9
