Synthesis and characterization of SnO2-HMD-Fe materials with improved electric properties and affinity towards hydrogen

Author’s Accepted Manuscript Synthesis and characterization of SnO2-HMD-Fe materials with improved electric properties and affinity towards hydrogen N. Bouazizi, R. Bargougui, T. Boudharaa, M. Khelil, A. Benghnia, L. Labiadh, R. Ben slama, B. Chaouachi, S. Ammar, A. Azzouz www.elsevier.com/locate/ceri

PII: DOI: Reference:

S0272-8842(16)30110-9 http://dx.doi.org/10.1016/j.ceramint.2016.02.167 CERI12358

To appear in: Ceramics International Received date: 24 January 2016 Revised date: 27 February 2016 Accepted date: 27 February 2016 Cite this article as: N. Bouazizi, R. Bargougui, T. Boudharaa, M. Khelil, A. Benghnia, L. Labiadh, R. Ben slama, B. Chaouachi, S. Ammar and A. Azzouz, Synthesis and characterization of SnO2-HMD-Fe materials with improved electric properties and affinity towards hydrogen, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2016.02.167 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Synthesis and characterization of SnO2-HMD-Fe materials with improved electric properties and affinity towards hydrogen N. Bouazizi1, 2, R. Bargougui3, T. Boudharaa2, M. Khelil4, A. Benghnia2, L. Labiadh3, R. Ben slama2, B. Chaouachi2, S. Ammar3, A. Azzouz1* 1

Nanoqam, Department of Chemistry, University of Quebec at Montreal, QC, Canada H3C 3P8.

2

Research unit: Environment, Catalyzes and Process Analysis, ENI,G University of Gabes, Tunisia. 3

Department of chemistry, Faculty of Sciences, University of Gabes, Tunisia.

4

Laboratory of Materials Physics and Nanomaterials Applied at Environment (LaPhyMNE), FSG University of Gabes, Tunisia.

*Corresponding author: Pr. Abdelkrim AZZOUZ; E-mail : [email protected]; Tel: 1 514 987 3000 Ext. 4119, Fax: 1 514 987 4054

Abstract Tin oxide (SnO2) was functionalized by hexamethylenediamine (HMD) grafting and incorporation of iron nanoparticles (Fe-NPs) and, then, fully characterized by X-ray diffractometry, transmission electron microscopy, diffuse reflectance spectrometry and FTIR spectrophotometery,

photoluminescence

spectroscopy

and

complexes

impedance

spectroscopy measurements. The covalent surface-grafting of HMD within mesopores was confirmed by the results from Fourier-transformed infrared spectroscopy. XRD and TEM study showed a dominant tetragonal structure.

Fe-NPs were finely dispersed inside

mesopores, producing a slight structure compaction. The crystallite size decreased in the presence of HMD and Fe-NPs. Photoluminescence (PL) insights revealed the presence of oxygen vacancies and the PL intensity was found to strongly depend on HMD grafting and Fe-NPs insertion. The incorporation of both Fe-NPs and HMD grafting appear to be responsible for electrical properties improvements. This material displayed an unprecedented 1

surface affinity factor towards hydrogen of 8.5 mol.m-2 at ambient temperature and pressure. This was attributed to the contribution of both physical and chemical hydrogen adsorption, and to the presence of fine Fe-NPs. These properties open promising prospects for green energy storage.

Keywords: SnO2; Electrical properties; Hexamethylenediamine; Iron nanoparticles; Hydrogen sensors.

1. Introduction Metals are known to display electric features, redox behavior and affinity towards hydrogen. Metal (0) dispersion in the form of fine particles is expected to induce such properties, resulting in interesting materials that can act as semi-conductors [1] or hydrogen adsorbent [2] and sensors [3]. For instance, mesoporous nanosheets decorated with palladium nanoparticles exhibited sensing properties for hydrogen for diverse purposes, including H2monitoring in radioactive environments [4-7]. In this regard, transparent conductive oxides have attracted considerable attention due to their potential applications in electrical, optical, electronic and electrochemical devices. Among them, tin oxide (SnO2) is a semiconductor with a wide band gap (Eg = 3.8 eV) and photoactive material [8], gas sensor [9,10] and also a potential attractive semiconductor matrix for hosting transition metal ions [11,12]. SnO2 nanoparticles can be prepared by numerous methods [13-15]. The sol-gel method requires less sophisticated and cost-effective procedure for producing highly performant materials. Doping with metal is expected to improve the sensitivity of the metal oxide and its surface area. Transition metal doped SnO2 received considerable attention, being regarded as promising materials for their properties and applications. The incorporation of Fe into SnO2

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may be a judicious route for inducing ionic conductivity and envisaging sensors applications. Fe-doped SnO2 was already found to display higher gas sensitivities than its unmodified counterpart [16]. In addition, some factors such as the structure morphology and the presence of defects could markedly influence the iron site distribution as well as their specific magnetic interactions [17]. Knowledge regarding the local environment of the dopant is an essential requirement to understand how the magnetic order arises in such matrices [17,18]. High dopant dispersion often requires an improved diffusion of the metal source, fine oxide powders are suitable for such a purpose. However, SnO2 in powder form, like any other metal oxide, has a strong tendency to re-aggregate into bulky clusters due to strong adhesion forces. An improvement in this regard can be achieved by grafting organic moieties on the external surface of the oxide particles. This should also generate porosity within the entanglement of the organic chains and thereby contributing to a higher dispersion and stabilization of the dopant on the external surface. In the present work, an effective route for the synthesis of such a material involved SnO2 functionalization by hexamethylenediamine (HMD) followed by the incorporation of iron nanoparticles (Fe-NPs). The interest devoted to iron nanoparticles is justified by their specific sensing properties [19,20]. These properties of SnO2-HMD-Fe allow envisaging potential applications as efficient material for electrodes [21]. Attempts were further made to assess not only the electrical properties of the surface but also the affinity towards hydrogen. Both properties converge towards the preparation of materials that can act as semi-conductors or hydrogen sensors of both simultaneously. To demonstrate this strategy, our approach involved a specific succession of characterization technique and adsorption test. Photoluminescence measurements provide valuable data on the band gap energy and the presence of electron acceptors and donors. To the best of our knowledge, the possible application of SnO2-HMD-

3

Fe as potential hydrogen sensor or adsorbent has not been tackled so far. The hydrogen sensing properties were investigated with respect to their response at ambient conditions.

2. Experimental 2. 1. Material synthesis The SnO2 powders were synthesized by co-precipitation of appropriate amounts of stannic tetrachloride hydrated (SnCl4.5H2O) previously dissolved in 150 ml of deoxygenated distilled water in the presence of 50 ml of NaOH at 70 °C for 4 hours. The resulting precipitates were separated by centrifugation a repeatedly washed with deionized water. Total removal of chloride was monitored though periodical tests with an aqueous AgNO3 solution. The final pure SnO2 powders were dried at 90 °C overnight and then impregnated with hexamethylenediamine in a water-ethanol solvent mixture (25:75 Vol.) at 80°C for 6 hours. The obtained SnO2-HMD material was repeatedly washed and filtrated, then dried at 323 K overnight. Fe-NPs dispersion was achieved using (Fe(NO3)3. 9H2O) as precursor in toluene (99.5 %, d = 0.865 g.mL-1) in the presence of hydrazine as the reducing agent. The mixture turned black after 5 hours stirring at room temperature, indicating the formation of Fe-NPs within SnO2-HMD. The final SnO2-HMD-Fe materials were dried at 80°C for 6 hours and then stored in sealed enclosure containing dry and O2-free nitrogen (Scheme 1).

2. 2. Material characterization The as-prepared samples were previously characterized through powder X-ray diffraction (XRD) (Siemens D5000 X-ray diffractometer and a Cu-Kα radiation at λ = 1.54056 A˚) in the 2θ ranges of 0.6–10°, Fourier transform IR spectroscopy (KBr cell, Fourier FTIR equipment and infrared spectroscopy (NICOLET IR200) and transmission electron microscopy (TEM, JEM-200CX). The samples for TEM were dispersed in EtOH under ultrasound exposure and

4

few drops of the resulting suspension were deposited onto the TEM grid. In a second step, the band gap was assessed using diffuse-reflectance UV–vis spectra (DRS) recorded on a Sinco S-4100 spectrometer, and photoluminescence spectra were recorded at room temperature using a fluorescence spectrophotometer (Jobin Yvon FL3-21) with an Xe lamp (450 W) at 325 nm. N2 adsorption-desorption measurements were performed at 77 K on a Micromeritics ASAP 2020 instrument. The samples were all outgassed at 80 °C for 5 h before analysis. Electrical measurements were performed using a two electrode configurations. The samples were compacted into pellets of 8 mm diameter and 1 mm thickness under a 3 tons.cm-2 uniaxial pressure. Electrical impedances, i.e. the variations of both real (Z') and imaginary (Z'') parts of the complex impedance (Z* = Z' - i Z''), were assessed in the frequency range 40 Hz-100 kHz with a TEGAM 3550 ALF automatic bridge at room temperature. Hydrogen adsorption tests were conducted by contacting, at room temperature and normal pressure, 0.10 g of adsorbent, previously dried overnight at ambient temperature and pressure, with 10 mL of pure dry hydrogen within a 20 mL sealed enclosure containing dry air-free hydrogen. The amount of adsorbed hydrogen was assessed through triplicate measurements of the volume change within a capillary glass tube having a 0.25 mm internal diameter containing a colored drop of oil serving as mobile hydraulic cap.

3. Results and discussion 3.1. Morphology and structure TEM images show that all samples appear as pseudo-spheres with an average diameter ranging between 7 and 10 nm (Figure 1). Contrast improvement revealed dark cores corresponding to SnO2 and Fe-NPs covered light shells of HMD. X-ray line broadening method using the Scheerer formula [22] revealed a substantial decrease in the average particle 5

size from 10.710 nm (SnO2) to 8.361 nm (SnO2-HMD) and 7.845 nm (SnO2-HMD-Fe), as supported by calculation using Image-J software (Table 1). HMD grafting and Fe-NPs incorporation induced a slight modification on the nature of SnO2 surface morphology. This effect was already reported in previous work and is commonly associated to a reduced surface energy by HMD-Fe [23]. Figure 1 Table 1 SnO2-HMD-Fe displayed a higher specific surface area (SSA) of 82 g.m-2for as compared to SnO2-HMD (60 g.m-2) and SnO2 (41 g.m-2), but lower average pore size of 5.03 nm versus 6.55 and 6.94 nm, respectively. The SSA increase and porosity decay are both attributed to the presence of Fe-NPs between the organic moiety chains. In other words, the contribution of the external surface of Fe-NPs improves the SSA, but their volumes reduce the pore volume. According to XRD measurements, the structure agrees with a tetragonal rutile structure with P42/mnm space group (JCPDS card No. 41–1445) [24]. All prepared materials showed appreciable crystallinity, but slight decreases in intensity for most XRD peak intensity of SnO2 indicate a slight decay in crystallinity, more particularly after HMD incorporation (Figure 2). This accounts for a quite uniform dispersion of the organic moiety, and the formation of organic clusters, if any, should be negligible. Figure 2 The slight shift of all lines towards higher values of 2-theta reflect increases in the reticular distances of the corresponding plane families (Table. 1). The calculated lattice parameters were found to decrease with HMD grafting and then Fe-NP insertion. This indicates changes in the lattice parameters of SnO2 that suggest a slight structure compaction, presumably as a result of the appearance of strong interaction between the lattice oxygen and Fe-NPs within the SnO2 structure.

6

SnO2 was identified by the anti-symmetric vibration of the O–Sn–O groups around 485 cm-1 and a lattice mode in the region 620–710 cm-1 [25]. HMD incorporation was reflected by the appearance of typical IR absorption bands at 3434 cm-1 and 1630 cm-1. These bands were attributed to the NH2 stretching, which overlaps that of the OH vibration [26] and OH bending respectively. The peak at 2340 cm-1 is probably due to the absorption of CO2 from the ambient air [27]. The shift of this band towards lower wavenumbers after SnO2 modifications indicates a strengthening of the vibration energy, presumably due to a decay in the affinity towards CO2 as result of SnO2 modifications. This is agreement with similar shift of the broad vibration band at 3360 cm-1, suggesting a similar strengthening of the vibration energy of the OH groups belonging to SnO2 and of retained moisture [28]. The most probable explanation resides in a decrease in the electron density on the oxygen atom. Here, the incorporation of both HMD and Fe-NPs is expected to attenuate the hydrophilic character and basicity leading to dehydration and CO2 release, given their well-known affinity towards hydroxyl groups [2932]. The decrease in peak intensity for most IR bands is an additional evidence in this regard.

3.2. Band gap changes UV-Vis DRS measurements (Figure 3) showed that, as a common feature, all samples exhibit a sharp absorption band in the range 200–420 nm and an absorption edge between 300 and 370 nm, owing to the relatively large exciton binding energy. The slight difference observed in the absorbance of SnO2-HMD and SnO2-HMD-Fe might be due to the appreciable amount of dispersed iron nanoparticles [33]. The optical band gaps of the investigated samples were calculated using Tauc relation (1) [34]: (αhυ)n = A(hυ-Eg)

(1)

where α is the linear absorption coefficient, hυ is the photon energy, A is a constant related to the material, Eg is band gap, n is an exponent that can take a value of either 2 for direct

7

transition or ½ for an indirect transition. The plot of (αhυ)2 versus hυ of the investigated samples (Figure 4) gave band gap values of 3.78 eV for SnO2-HMD-Fe and of 3.96 eV for the starting material. This accounts for a shift of 18 eV after SnO2 modifications. Figure 3 Figure 4 This shift can be explained by the occurrence of exchange interactions between the band electrons and the localized electrons of the Fe-NPs within the mesopores of the semiconducting host lattice [35,36]. In other words, this corresponds to a decrease in the band gap attributed to metal incorporation and an increase in the electron density around Fe-NPs, as previously reported [1]. This provides clear evidence of an improvement of the electrical properties and semiconductor character of SnO2.

3.3. Luminescence properties The photoluminescence (PL) spectra of all samples (Figure 5) displayed a strong dominant emission at 427 nm due to the recombination of deep trapped charges and photo-generated electrons from the conduction band [37]. The intensity of the visible emission of SnO2-HMDFe and SnO2-HMD was lower than that of pure SnO2. A part of this phenomenon is due to the incorporation of Fe-NPs into the SnO2 lattice. The visible luminescence can be attributed to electron transition from the donor level formed by oxygen vacancy to the valence band [38]. Fe-NPs incorporated into the SnO2-HMD lattice are assumed to reduce the oxygen vacancy concentration to ensure charge neutrality [39]. Figure 5 The shift of the UV emission peak is a result of the decrease of the optical band gap SnO2 modifications, in agreement with the DRS data. Additionally, this shift can also be explained by change in the refraction index of SnO2 due to presence of HMD molecules and Fe-NPs.

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3. 4. Impedance spectrum analysis Plotting the imaginary part with corresponding real part of the complex impedance for all samples at room temperature (Figures 6-8) revealed a significant improvement of the electrical properties of SnO2, as supported by the progressive decrease of the dispersion of the experimental values after HMD grafting and then Fe-NP incorporation. SnO2-HMD-Fe displayed a regular semicircle-shaped curve at low Z' values. This was explained in terms of the appearance of a film impedance at low frequency, and corresponds to a “non-Debye” behavior arising from a polarization process analogous to that involved in a circuit of parallel resistors and capacitors [40]. Figure 6 Figure 7 Figure 8 Here, the insertion of Fe-NPs in SnO2-HMD is supposed to induce an increase in the charge transfer resistance, presumably due to the surface heterogeneity, which should promote proton mobility and conductivity. The straight line observed at higher Z’ values corresponds to a Warburg impedance that arises from the diffusion of electro-active species [41]. According to Anderson and Parks’ proton conductivity model [42], in the absence of molecular water, free protons can bounce from site to site on the solid surface. In the presence of traces of water molecules, H-bridges with Sn–OH groups and lattice oxygen atoms is expected to favor easier proton transfer.

3. 5. Hydrogen storage The presence of HMD and Fe-NPs was also found to induce affinity towards hydrogen at room temperature, a supported by the higher amounts of adsorbed hydrogen observed after

9

each modification step (Figure 9). The hydrogen retention capacity appears to increase in the following sequence SnO2  SnO2-HMD  SnO2–HMD-Fe. The weak amount of hydrogen retained by the starting SnO2 material was found to vary significantly according to slight fluctuations of the moisture content, and must be due to van der Waals interaction between hydrogen and the solid surface. The increase in time of the amount of retained hydrogen indicates the occurrence diffusion hindrance. The hydrogen adsorbed on SnO2-HMD can be totally released upon slight heating up to 40oC or merely upon forced convection upon strong dry nitrogen stream (20-25 mL.min-1). Most of this hydrogen must be only entrapped by the matrice, which appears to act as a sponge. Figure 9 This suggests a purely physical adsorption and probably hydrogen condensation within HMD branch entanglement, in agreement with previous data [2,43]. Fe-NP insertion induced a much higher improvement of the hydrogen retention capacity, which was attained after longer impregnation time of ca. 50-60 min. In this case, only less than 20% of the captured hydrogen on SnO2-HMD-Fe can be desorbed upon heating to 40oC or upon forced convection, while 60-70% were released at 120oC. The release of the remaining hydrogen requires higher temperatures beyond the stability limit established through thermogravimetry (160oC). The maximum retention capacity of 0.7 mmol.g-1 registered after 60 min of saturation with dry hydrogen accounts for a hydrogen uptake of 0.14 wt.% at room temperature for a specific surface area of 82 m2.g-1, i.e. a surface affinity factor of 8.5 mol.m-2. Values in the same magnitude order were already reported [2]. They were explained in terms of simultaneous contributions of physical and chemical adsorption of hydrogen, with multilayer physical condensation around metal nanoparticles in compacted entanglements of grafted organic moieties. Investigations are still in progress in this regard.

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4. Conclusions These results allow concluding that HMD grafting and Fe-NPs incorporation on SnO2 played a key role in genesis of electrical properties and affinity towards hydrogen. A judicious approach involving different characterization techniques aimed to demonstrate the strong correlation between the presence de Fe-NP and the newly induced properties. The shift of the band-gap energy is a precise indicator of the electrical properties improvement. Deeper insight

through

impedance

measurements

showed

a

semiconductor

behavior.

Photoluminescence (PL) measurements revealed the presence of oxygen vacancies and the PL intensity seems to strongly depend on HMD grafting and Fe-NPs insertion. This material showed unprecedented surface affinity factor towards hydrogen of 8.5 mol.m-2 at ambient temperature and pressure. This was attributed to the presence of fine and highly dispersed FeNPs. These properties make this material to be regarded as an interesting candidate as semiconductor or hydrogen sensor. This opens promising prospects for green energy storage.

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adsorbed water, J. Phys. Chem. 72 (1968) 3662–3668.

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Figures captions Scheme. 1. Synthetic strategy towards SnO2-HMD-Fe for hydrogen storage. Fig. 1. TEM image of a) SnO2 and b) SnO2 -HMD-Fe. Fig. 2. Powder XRD spectra of SnO2, SnO2-HMD and SnO2-HMD-Fe. Fig. 3. UV–Vis diffuse reflectance spectra of SnO2, SnO2-HMD and SnO2-HMD-Fe. Fig. 4. Plot of (αhν)2 versus hν of SnO2, SnO2-HMD and SnO2-HMD-Fe nanoparticles. Fig. 5. Photoluminescence excitation spectra recorded at room temperature of SnO2, SnO2HMD and SnO2-HMD-Fe. The photoluminescence (PL) spectra of all samples were recorded at room temperature and with excitation wavelength at 425 nm. Fig. 6. Complex impedance spectra of SnO2 nanoparticles at room temperature. Fig. 7. Complex impedance spectra of SnO2-HMD at room temperature. Fig. 8. Complex impedance spectra of SnO2-HMD-Fe nanoparticles at room temperature. Fig. 9. Evolution in time of the hydrogen retention capacity. These experiment were conducted by contacting at room temperature and normal pressure 0.5 g of adsorbent, previously dried overnight at room temperature with 10 mL of pure dry hydrogen within a 20 mL sealed enclosure containing dry air.

Figures captions

17

Scheme. 1: Synthetic strategy towards SnO2-HMD-Fe for hydrogen capture.

18

Fig. 1: TEM image of a) SnO2 and b) SnO2 -HMD-Fe.

19

Fig. 2: Powder XRD spectra of a. SnO2, b. SnO2-HMD and c. SnO2-HMD-Fe.

4

SnO2-HMD-Fe SnO2-HMD SnO2

Absorbance (a.u)

3

2

1

0 200

300

400

500

600

700

800

Wavelenght (nm)

20

Fig. 3: UV–Vis diffuse reflectance spectra of SnO2, SnO2-HMD and SnO2-HMD-Fe.

80

SnO2-HMD-Fe SnO2-HMD SnO2

2

(h ) (a.u)

60

40

20

0 2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

Energy (eV)

Fig. 4: Plot of (αhν)2 versus hν of SnO2, SnO2-HMD and SnO2-HMD-Fe nanoparticles.

21

Fig. 5. Photoluminescence excitation spectra recorded at room temperature of SnO2, SnO2HMD and SnO2-HMD-Fe. The photoluminescence (PL) spectra of all samples were recorded at room temperature and with excitation wavelength at 425 nm.

Fig. 6. Complex impedance spectra of SnO2 nanoparticles at room temperature.

22

5x10

6

SnO2-HMD 6

3x10

6

2x10

6

1x10

6

Z' ()

4x10

0 0.00

2.50x10

6

5.00x10

6

7.50x10

6

1.00x10

7

Z" ()

Fig. 7. Complex impedance spectra of SnO2-HMD at room temperature.

23

6.0x10

3

4.0x10

3

Z' ()

SnO2-HMD-Fe

2.0x10

3

0.0 0.0

5.0x10

3

1.0x10

4

1.5x10

4

2.0x10

4

Z" ()

Hydrogen retention capacity (mmol. g-1)

Fig. 8. Complex impedance spectra of SnO2-HMD-Fe nanoparticles at room temperature.

0.7

SnO2 SnO2-HMD

0.6

SnO2-HMD-Fe

0.5 0.4 0.3 0.2 0.1 0.0 0

20

40

60

80

Contact time (mn)

24

Fig. 9. Evolution in time of the hydrogen retention capacity. These experiment were conducted by contacting at room temperature and normal pressure 0.5 g of adsorbent, previously dried overnight at room temperature with 10 mL of pure dry hydrogen within a 20 mL sealed enclosure containing dry air.

Scheme 1. Synthetic strategy towards SnO2-HMD-Fe for hydrogen capture.

25

Table. 1. Main features of SnO2 and its modified counterparts Analysis method

Features

Average particle size (nm)a Specific surface SBET (m2/g)b area DBET (nm)c Cell parameters, a=b Cell parameter, c

SnO2 10.710 41 6.94 4.699 3.211

SnO2HMD 8.361 60 6.55 4.678 3.209

SnO2HMD-Fe 7.845 82 5.03 4.679 3.210

d(110) 2.650 2.615 2.520 d(101) 2.460 2.424 2.390 X-ray d(200) 2.444 2.460 2.478 diffraction d(220) 1.681 1.650 1.600 d(002) 1.450 1.401 1.352 d(310) 1.365 1.327 1.279 d(112) 1.268 1.220 1.200 d(301) 1.180 1.155 1.105 H-bonds involved in O–H oscillators 3360 3348 3350 O–H bending bond associated with moisture 1620 1610 1587 Infrared NH2 band overlaps that of –OH vibration -1630 1633 absorption Absorption bands of CO2 2340 2320 2317 bands The presence or absence of Sn–O 1045 1020 1033 Vibration of anti-symmetric O–Sn–O 520-480 509-480 514-480 Eg (eV) 3.96 3.85 3.78 a Assessed through ImageJ and X-ray line broadening method using the Scheerer formula and the Bragg’s model for (110) plan. b Surface area determined by applying Brunauer-Emmett-Teller (BET) equation. c Pore size is calculated from the Barrett-Joyner-Halenda (BJH) equation using the desorption isotherm.

26