Synthesis of tin oxide activated by DAN grafting and Mo nanoparticle insertion for optoelectronic properties improvement

Abstract

Tin oxide (SnO₂) was synthesized via a co-precipitation method and activated by 1,5 diaminonaphthalene (DAN) grafting and molybdenum nanoparticles (Mo-NPs) incorporation. The resulting SnO₂–DAN–Mo nanocomposites were characterized by X-ray diffraction, transmission electron microscopy, diffuse reflectance and FTIR spectrophotometry, photoluminescence spectroscopy and complex impedance spectroscopy measurements. The surface-grafting of DAN within mesopores was confirmed by Fourier-transform infrared spectroscopy. The XRD and TEM studies showed a dominant tetragonal structure. The dispersion of fine Mo-NPs on the surface of the matrices, produced slight structural compaction. The crystallite size decreased with the insertion of DAN and Mo-NPs. The photoluminescence study revealed the presence of oxygen vacancies and that the PL intensity strongly depends on DAN grafting and Mo-NPs insertion. In addition, both the incorporation of Mo-NPs and DAN grafting appear to be responsible for the changes in the conductance and relaxation phenomenon. The effects of surface groups of SnO₂–DAN–Mo and charge transfer were found to be almost proportional to the capacitance. The above properties make these nanocomposites efficient electrode materials for green energy storage.

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1. Introduction

Transparent conductive oxides have attracted considerable attention due to their potential applications in electrical, optical, electronic and electrochemical devices. Among them, the semiconductor tin oxide (SnO₂) presents a wide band gap of 3.8 eV ^1–3 and is a photoactive material,⁴ gas sensor^5,6 and also a potential attractive semiconductor host matrix for introducing transition metal ions.^7–13 Some authors, studying SnO₂ nanoparticles prepared by numerous methods, have put efforts to obtain the optimal solubility of Fe in the rutile structure. To avoid the segregation of metal-doped-SnO₂ impurity in phases, chemical vapor deposition, chemical vapor transport and sol–gel methods have been employed.^14–16 Co-precipitation appears to be a better approach that requires facile manipulation, less costs and simple equipments. It provides homogenous distribution of elements and high crystal quality. Different methods such as doping, increase in surface area, structure construction and catalyst addition have been used, aiming mainly to enhance the sensitivity of metal oxide gas sensors.

Transition metal-doped SnO₂ has received considerable attention recently as promising materials for properties and applications improvements. The incorporation of transition metals such as Mo into SnO₂ makes it a good candidate for sensor applications and ionic conductivity. In addition, some factors such as morphology and the presence of defects could largely affect the metallic nanoparticles site distribution as well as their specific magnetic interactions.^17,18 Knowledge of the local environment of the dopant is essential to understand the mechanisms giving rise to magnetic order in these compounds.¹⁹

The growing interest devoted to the activation of SnO₂ powder surfaces with organic chains has been stimulated by the continual tendency of re-aggregation due to the adhesion forces between the particles displaying high surface. A majority of the nanoparticle characteristics are driven by their dimensions and are altered dramatically upon aggregation. The organic chains grafted onto the materials are stabilized in particle length scales in the nanometer range. Moreover, the organic molecules grafted generate considerable free volume, which not only enhances the dispersion stability of the nanoparticles in organic and aqueous media, but can also prevent the segregation of the inorganic fillers due to the high affinity of the particle modifier for the organic medium.

SnO₂-based composites are expected to gather the physico-chemical properties of both the inorganic and organic moieties, and favor SnO₂ dispersion through interactions with the surrounding chelating sites. In other words, SnO₂ modified with organic compounds contain fewer particles compared to the unmodified counterpart. When loaded with metal nanoparticles, organically modified SnO₂ showed improved electrical properties.¹⁴ The electrical properties of SnO₂ has stimulated researchers to focus interest towards its use as a starting electrode material, and there exists an ample literature in this regard.

In the present study, an effective route was employed for the synthesis and functionalization of SnO₂ using 1,5-diaminonaphthalene (DAN) grafting and molybdenum nanoparticles (Mo-NPs) incorporation. Deeper insight in the effects of DAN grafting and Mo-NPs incorporation were achieved to assess the electrical properties of the surface. Molybdenum nanoparticles have been chosen for their interesting properties,^20,21 which opens new prospects for SnO₂–DAN–Mo as efficient materials for electrodes.²² The effects of DAN grafting and Mo-NPs insertion on the structural, electrical, and optical properties of SnO₂-NPs were investigated in detail.

2. Experimental

2.1. Material synthesis

Tin oxide (SnO₂) powders were synthesized by a co-precipitation technique. Appropriate amounts of stannic tetrachloride hydrated (SnCl₄·5H₂O) were first dissolved in 150 ml of deoxygenated distilled water. The salts were then precipitated at 343 K with 50 ml of NaOH to maintain a pH value and this temperature was maintained for 4 hours. The resulting precipitates were separated by centrifugation and washed with deionized water to remove the salt, which was tested using an aqueous AgNO₃ solution. The resulting products were dried overnight at 363 K to obtain the final pure SnO₂ powders.

1,5-Diaminonaphthalene was incorporated through impregnation in a 1 : 3 water–ethanol mixture as solvent at 353 K during 6 hours. The obtained product (SnO₂–DAN) was washed and filtrated, then dried at 323 K overnight. Mo-NP dispersion was achieved using (Mo(NO₃)₆) as a precursor in toluene (99.5%, d = 0.865 g ml⁻¹) in the presence of hydrazine as the reducing agent (Scheme 1). The resultant mixture turned black after stirring for 5 hours at room temperature, indicating the formation of molybdenum nanoparticles within SnO₂–DAN. SnO₂–DAN–Mo nanocomposite was dried at 353 K for 6 hours and then stored in a sealed enclosure containing dry and oxygen-free nitrogen.

Scheme 1 Schematic of the preparation of SnO₂–DAN–Mo nanocomposites.

2.2. Material characterization

The as-prepared samples were fully characterized by powder X-ray diffraction (XRD) (Siemens D5000 X-ray diffractometer and a Cu-Kα radiation at λ = 1.54056 Å), Fourier transform IR spectroscopy (using a KBr cell and a Fourier transform IR NICOLET IR200 equipment at wavenumbers ranging from 4000 to 400 cm⁻¹; each sample was scanned 40 times for spectrum integration, and the scanning resolution was 2 cm⁻¹). Transmission electron microscopy (TEM, JEM-200CX) was used to study the morphology and particle size of the powders. The samples for TEM were prepared through dispersion in EtOH in an ultrasonic bath for 15 min, and few drops of the resulting suspension containing the synthesized materials were put onto a TEM grid. The optical transmittance T(λ) of the samples were recorded using a UV-vis-IR spectrophotometer (Shimadzu UV 3100) provided with an integrating sphere (LISR 3200, 300–800 nm). In addition, the room temperature photoluminescence spectra were measured using a fluorescence spectrophotometer (Jobin Yvon FL3-21) with an Xe lamp (450 W) as the light source excited at 350 nm.

The electrical measurements of the real and imaginary components of impedance parameters (Z′ and Z′′) were made over a wide range of temperatures (273–333 K) and frequencies (5 Hz to 13 MHz) using a Hewlett Packard 4192 analyzer. The configuration for the electrical measurements was obtained using two-electrodes. The powder samples were compacted into pellets of 8 mm diameter and 1 mm thickness, using 3 tons per cm² uniaxial press. These electrodes were painted on the two extremities of the sample using silver paste (Scheme 2).

Scheme 2 Configuration of the pellet SnO₂/DAN/Mo samples for the electrical measurements.

3. Results and discussion

3.1. Interaction mechanism of SnO₂–DAN–Mo

Scheme 3 shows the mechanisms taking place during the process of tin oxide modification. Over the past decade, much research has been done on developing nanocomposites produced by the action of modified inorganic carriers with organic molecules. Such procedures make it possible to produce new classes of nanocomposite materials that combine properties of both inorganic particles and organic molecule matrices. The stabilization of metallic nanoparticles (M-NPs) still remains a major challenge because of their strong tendency to aggregate with loss of properties. M-NPs entrapment by organic macromolecules may favor stabilization through a combination of electrostatic and steric factors. However, the elasticity of most macromolecules cannot prevent M-NPs' mobility and aggregation. To date, a wide variety of inorganic supports have been tested for both their chemical and physical properties. This metal-organo-tin oxide has an edge structure bearing terminal hydroxyl groups that can generate bridges with organo amine via ethanol elimination upon heating.

Scheme 3 Idealized representations of SnO₂/DAN/Mo materials.

3.2. Morphological properties

TEM images of modified and unmodified SnO₂ materials are presented in Fig. 1. All samples were nearly spherical with an average diameter between 15 and 20 nm. This suggests that DAN–Mo can certainly prevent the growing of SnO₂ nanoparticles and the particle size is substantially decreased with the presence of DAN grafting and molybdenum nanoparticle incorporation. After treatment, a dark field image revealed that these particles are composed of crystallites, which explains the effects of DAN grafting that can be observed surrounding the particle surface.

Fig. 1 TEM image of SnO₂ (a), SnO₂–DAN (b) and SnO₂–DAN–Mo (c).

The surface morphology of the as-prepared materials was determined using Image-J software (Fig. 2). DAN grafting and Mo-NPs incorporation induced a slight modification in the SnO₂ surface morphology. The surface increased and became functionalized. This observation was already reported in previous work and is commonly associated with a reduced surface energy by DAN–Mo.²³

Fig. 2 Surface morphological plot of SnO₂ (a), SnO₂–DAN (b) and SnO₂–DAN–Mo (c).

3.3. Structural analysis

Fig. 3 shows the X-ray diffraction patterns of SnO₂, SnO₂–DAN and SnO₂–DAN–Mo. For SnO₂ and SnO₂–DAN materials, XRD reveals that the structures were polycrystalline in the tetragonal rutile form with P4₂/mnm space group (JCPDS card no. 41-1445).²⁴ No traces of any other impurity were observed, indicating that the materials were pure phases.

Fig. 3 Powder X-ray diffraction patterns of SnO₂ (1), SnO₂–DAN (2) and SnO₂–DAN–Mo (3).

However, after incorporation of Mo-NPs, the resulting materials displayed supplementary peaks at low angles corresponding to a molybdenum phase. This indicates that a higher dispersion of molybdenum (3%) leads to a localized and stabilized tin oxide lattice.

There was a clear shift of all peaks towards higher angles (Fig. 3 inside), which can be explained by slight framework compaction for SnO₂–DAN and SnO₂–DAN–Mo nanocomposites, as supported by the increase in intensity for all lines Table 1. These results must be related to the electrostatic attraction between Mo-NPs and the lattice oxygen atoms. Also, the results indicate a homogenous and ordered arrangement of the organic moiety around SnO₂-NPs.

Table 1 Variation of the parameters (a and c) and average crystallite size of SnO₂ (a), SnO₂–DAN (b) and SnO₂–DAN–Mo (C) nanocomposites^a

Samples	SnO2	SnO2–DAN	SnO2–DAN–Mo
a Where d(hkl) is calculated using the relation of Bragg.
d(110)	2.591	2.590	2.559
d(101)	2.417	2.332	2.389
d(200)	2.408	2.405	2.491
d(220)	1.597	1.648	1.595
d(002)	1.461	1.378	1.348
d(310)	1.380	1.320	1.267
d(112)	1.256	1.240	1.214
d(301)	1.184	1.170	1.112

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The average size of the particles was determined by the X-ray line broadening method using the Scherrer formula.²⁵ It was concluded that, the average grain size was around 10 nm and was found to decrease in the presence of DAN–Mo, which was in good agreement with the results obtained by TEM analysis. The calculated lattice parameters decreased monotonously from SnO₂ to SnO₂–DAN–Mo (Table 2). Therefore, it was reasonable to suggest that Mo-NPs are incorporated within the SnO₂–DAN lattice.

Table 2 Variation of d_(hkl) of SnO₂ (a), SnO₂–DAN (b) and SnO₂–DAN–Mo (C) nanocomposites^a

Samples	Average size, G (nm)	Parameters (Å)
		a = b	c
a Where G is calculated using the relation of Bragg for (110) plan.
SnO2	19.971	4.610	3.231
SnO2–DAN	15.050	4.509	3.219
SnO2–DAN–Mo	14.670	4.489	3.215

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3.4. FT-IR analysis

FT-IR spectroscopy is usually employed to evidence the successful grafting of DAN and other organic and inorganic species. Table 3 lists the FT-IR spectra of SnO₂, SnO₂–DAN and SnO–DAN–Mo nanocomposites. The typical absorption bands at 3365 cm⁻¹ and 1604 cm⁻¹ were attributed to the NH₂ stretching and OH bending vibration, respectively, taking into account that the NH₂ band overlaps the band of OH vibration.²⁶ The broad vibrationband at 3365 cm⁻¹ indicates the presence of hydrogen bonds, probably because SnO₂ retains certain amount of adsorbed water from the ambient atmosphere.²⁷ The characteristic peak at 2348 cm⁻¹ is probably due to the absorption of CO₂ from the ambient air atmosphere.²⁸ The weak peaks at 1049 cm⁻¹ indicates the occurrence of Sn–O and Mo–O bonds and their functional groups. In addition, the asymmetric vibration of O–Sn–O bridge functional groups of SnO₂ exhibits peaks around 514–468 cm⁻¹ and a lattice mode due to SnO₂ appears in the region 620–720 cm⁻¹.²⁹ The decrease in the intensity of the peaks with increasing Mo-NPs amount is due to the appearance of the Mo–Sn stretching.

Table 3 Transmittance FT-IR peaks and their assignments of SnO₂, SnO₂–DAN and SnO₂–DAN–Mo nanocomposites

Attributions	Wavenumber (cm^−1)
	SnO2	SnO2–DAN	SnO2–DAN–Mo
Hydrogen bonds involved in O–H oscillators	3335	3350	3365
O–H bending bonds associated with some residual water molecules	1604	1602	1590
The band of –NH2 overlaps with that of –OH vibration	—	1620	1621
Absorption bands of CO2	2348	2345	2341
The presence or absence of Sn–O	1049	1031	1025
Vibration of Mo–Sn	—	—	720
Vibration of Mo–O	—	—	635
Vibration of anti-symmetric O–Sn–O	520–490	514–479	514–468

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In addition, a slight shift in the position of the absorption band has been correlated to Mo nanoparticles dispersion. The additional peaks at 635 cm⁻¹ is related to the vibration of the Mo–O bond.

3.5. Band gap changes

The coordination and the bonding of the Mo-NPs' sites to the lattice are extremely important to explore the applications of the materials. Diffuse-reflectance UV-vis spectra are used for characterizing the coordination mode of Mo-NPs' centers in tin oxide.

Fig. 4 depicts the transmittance spectrum of SnO₂, SnO₂–DAN and SnO₂–DAN–Mo nanocomposites. It can be seen clearly that all spectra exhibited sharp transmittance peaks in the visible region, owing to the relatively large exciton binding energy. The slight difference observed in the peaks of SnO₂–DAN and SnO₂–DAN–Mo might be due to DAN grafting and the appreciable amount of dispersed molybdenum nanoparticles within SnO₂, may modify the optic parameter leading to improved transmittance in the visible spectrum range.³⁰

Fig. 4 Transmittance spectra of SnO₂, SnO₂–DAN and SnO₂–DAN–Mo nanocomposites.

The optical band gaps of the investigated samples were calculated using the Tauc relation (1)³¹:

(αhν)ⁿ = A(hν − E_g) (1)

where α is the linear absorption co-efficient, hν is the photon energy, A is a constant related to the material, E_g is the band gap and n is an exponent that can take a value of either 2 for a direct transition or 1/2 for an indirect transition. The plot of (αhν)² versus hν of the investigated samples is presented in Fig. 5.

Fig. 5 Plot of (αhν)² versus hν of SnO₂, SnO₂–DAN and SnO₂–DAN–Mo nanocomposites.

The band gap of activated SnO₂–DAN–Mo was estimated to be 3.52 eV, which is shifted by 0.17 eV from the bulk band gap of 3.69 eV for the starting materials (Table 4).

Table 4 Calculated band gaps of SnO₂, SnO₂–DAN and SnO₂–DAN–Mo nanocomposites

Samples	Eg (eV)
SnO2	3.69
SnO2–DAN	3.62
SnO2–DAN–Mo	3.52

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This shift can be explained based on the exchange interactions between the band electrons and the localized electrons of the Mo-NPs within the mesopores in the semiconducting host lattice.^32,33 The changes observed in the band gap of the modified samples might be correlated with the changes in the band structures caused by the disorder in the crystal structure due to DAN grafting and Mo-NPs insertion. To summarize, the DRS spectra evidenced the role of Mo-NPs dispersed in mesoporous SnO₂, which provides clear evidence of an improvement in the electrical properties and semiconductor characteristics.

3.6. Luminescence properties

The photoluminescence (PL) spectra of all samples at room temperature with excitation wavelength at 375 nm are shown in Fig. 6. PL spectra of all samples displayed a strong dominant emission at 380 nm due to the recombination of deep trapped charges and photo-generated electrons from the conduction band.³⁴

Fig. 6 Photoluminescence excitation spectra recorded at room temperature for SnO₂ (a), SnO₂–DAN (b) and SnO₂–DAN–Mo (c).

The intensity of the visible emission of SnO₂–DAN–Mo and SnO₂–DAN nanocomposites was lower than that of pure SnO₂. This result is due to the large incorporation of Mo-NPs into the SnO₂ lattice. The origin of visible luminescence can be attributed to the recombination of electrons trapped in singly charged ionized oxygen vacancies with photo generated holes. In fact, molybdenum nanoparticle loaded-SnO₂–DAN lattice has a lower oxygen vacancy concentration to ensure charge neutrality.³⁵ Therefore, the emission peak might correspond to the electron transition from the donor level, formed by oxygen vacancy, to the valence band.³⁶ There is a significant change in peak position that shifted to the higher wavelength values. The observed shift in UV emission peak was a result of the decreased optical band gap of the modified SnO₂ materials, which was in good agreement with the DRS results. Furthermore, this shift can be explained by the change in the effective refractive index of the SnO₂-NPs due to presence of DAN molecules and Mo-NPs.

3.7. Impedance spectrum analysis

Fig. 7 shows the complex impedance spectra (Z′′ vs. Z′) obtained by plotting the imaginary part with the corresponding real part for SnO₂, SnO₂–DAN and SnO₂–DAN–Mo at various temperatures. An analysis of all the samples shows that grafted DAN with Mo-NPs incorporation were found to induce visible improvement in the electrical properties of SnO₂, as supported by a much lower dispersion of the experimental value compared to the starting material. SnO₂–DAN–Mo displayed an arrangement in a semicircle at low Z′ values, due to the film impedance observed at low frequency. This corresponds to a “non-Debye” behavior, attributed to a polarization phenomenon. In addition, the ohmic and charge-transfer resistances of SnO₂–DAN–Mo nanoparticles are larger than those of pristine SnO₂. This suggests that the incorporation of the Mo-NPs in the SnO₂–DAN sample increases the charge-transfer resistance. A probable reason for the large electron-transfer resistance is the heterogeneity of the synthesized nanoparticles.

Fig. 7 Complex impedance spectra at various temperatures of SnO₂ (a), SnO₂–DAN (b) and SnO₂–DAN–Mo (c).

According to Anderson and Parks' protons conductivity model,^37,38 in the absence of molecular water, free protons can migrate 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 may also promote proton transfer.

At lower frequencies, electrode polarization effects transduced by constant-phase element (CPE) are evidenced. Fig. 8 shows the equivalent circuit associated to the SnO₂–DAN–Mo nanocomposites. The equivalent circuit involved a resistance R_g (grain resistance) associated in parallel with constant phase element impedance (CPE_g).

Fig. 8 Equivalent circuit model of SnO₂–DAN–Mo.

3.8. Relaxation phenomenon

The frequency and temperature dependent plots of the real Z′ and imaginary Z′′ part of impedance for SnO₂, SnO₂–DAN and SnO₂–DAN–Mo are shown in Fig. 9. For the imaginary part of SnO₂–DAN–Mo materials, the spectrum is characterized by the appearance of a peak at 5 × 10⁴ Hz, which is conventionally known as the “relaxation frequency”. It is clear that the peak height decreases, and the relaxation frequency shifts to higher frequencies when the temperature increases. Herein, the results confirm the semiconducting character of SnO₂–DAN–Mo and indicate a decrease in the resistive properties.³⁹

Fig. 9 Variation of the real (Z′) and imaginary (Z′′) part of the impedance as a function of the angular frequency for SnO₂ (a), SnO₂–DAN (b) and SnO₂–DAN–Mo (c).

However, regarding the real part of impedance, Z′, the magnitude was typically higher in the low-frequency region and then it decreased gradually with increasing frequency. The value of Z′ appears to merge in the high-frequency region irrespective of the temperature. This result confirms the presence of space charge in the materials, and suggests that it may be a factor responsible for the enhancement of conductance values.

3.9. Conductance measurements

Conductivity measurements were used widely to investigate the nature of defect centers in disordered systems because it is assumed that they are responsible for this type of conduction. Fig. 10 shows the change in conductance for several frequencies and temperatures. At low frequencies (<10⁴ Hz) the conductance is constant and it begins to increase for f > 10⁴ Hz. Herein, the DAN–Mo loading was found to induce increases in conductance up to a plateau with increasing frequency.

Fig. 10 Conductance as a function of temperature for SnO₂ (a), SnO₂–DAN (b) and SnO₂–DAN–Mo (c).

The conductivity achieved was 10⁻² s for SnO₂–DAN–Mo versus 4 × 10⁻³ s recorded for pure SnO₂. By referring to these values, it can be explained that proton conduction in these materials is suggested to take place according to the Grotthuss mechanism.⁴⁰ As shown, the maximum conductance value of SnO₂–DAN–Mo was obtained at about 10⁻⁴ S loading content. The occurrence of a maximum value indicates that the presence of an active effect induced by SnO₂–DAN–Mo, as mentioned above, can improve the conductance of the constant phase elements. On the other hand, the minimum conductance value (10⁻⁶ S) was explained by transportation hindrance of charge carriers resulting from SnO₂–DAN–Mo grain aggregation that may decrease the conductivity of the constant phase elements. Therefore, the pores of this activated material may be acting as the conducting pathway of the charge carrier.

This result suggests that the observed shift originates from a thermally activated process. Furthermore, this conductivity dependence on frequency was an indication of hopping conduction at higher frequency between localized states.⁴¹ The study of the temperature dependence on the angular frequency exponent can be used to validate if the hopping pattern is consistent with the correlated barrier hopping model.⁴²

3.10. SnO₂–DAN–Mo for high-performance of capacitance

In order to investigate the effects of different kinds of surface functional groups on the capacitance, Fig. 11 was designed to show the capacitance–frequency characteristics of pure and modified SnO₂ at several temperatures and fixed voltage. The capacitance of the materials increased with increasing frequency. The accumulation capacitance decreased with fluctuations for SnO₂ unlike SnO₂–DAN–Mo, which displayed a dependence between frequencies and capacitance. As the frequency increases, the capacitance increases and this trend is maintained up to a maximum value (3.4 × 10² F).

Fig. 11 Capacitance as a function of temperature for SnO₂ (a), SnO₂–DAN (b) and SnO₂–DAN–Mo (c).

In fact, at low frequencies, high capacitance depends on the ability of the charge carriers to follow the applied signal. A decrease in capacitance is observed at higher frequencies, and the charge at the interface may not follow the conductivity signal.^43,44

Regarding the detailed mechanism of the elevated capacitance, it was suggested that these surface functional groups enhance the charge transfer. This is possibly due to their high electron density from the Mo-NPs, implying capacitance enhancement as a result of the presence of surface functional groups. The presence of surface functional groups also enhanced the electrical double layer capacitance formed by charge separation on the interface between the pellet and the electrode. However, it still makes sense to explore the changing trend of capacitance, which was expected to correlate directly with the amount of functional groups. The capacitance of SnO₂ was slightly lower than that of SnO₂–DAN–Mo in spite of the similar composition of surface functional groups.⁴⁵

A negative electron charge was balanced by positive (hole) charges accumulated near the surface of the electrode-pellets. Switching the frequencies into a high value direction makes surface functional depleted from the holes. Thus, the total measured capacitance, similar to the accumulation state, approaches the capacitors.

This improved capacitance is attributed to the occurrence of surface functional groups that enhances the charge transfer and high electron density from DAN and Mo-NPs. The presence of surface functional groups is also supposed to enhance the electrical double layer capacitance formed by charge separation on the interface between the pellet and the electrode. Herein, the effect of the amount of functional groups on the capacitance still remains to be elucidated. The capacitance of SnO₂ was slightly lower than that of SnO₂–DAN–Mo in spite of their similar content of surface functional groups. On the other hand, at high frequency region, the capacitance displayed a marked decay, which indicates the ability of the charge carriers of the pellet interface at moderate frequencies to follow the conductivity.

Table 5 compares the capacitance values of the actual activated tin oxide samples to those obtained with other materials composites reported previously. It is worth mentioning that under similar experimental conditions, pronounced increases in the capacitance were achieved with SnO₂–DAN–Mo composites. This indicates the originality and efficacy of both DAN–Mo addition and the synthesis process.

Table 5 Comparison between the obtained capacitance values and published results

Samples	Capacitance (F)	References
SnO2 nanowires	76 × 10^−6	46
SnO2/CNFs	200	47
SnO2/Ni	310.4	48
SnO2/graphene	126	49
Hollow SnO2@C	25.8	50
SnO2–DAN–Mo	340	This works

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4. Conclusions

In summary, SnO₂–DAN–Mo nanocomposites were synthesized and activated by DAN grafting and incorporation of Mo-NPs. XRD results confirmed the tetragonal structure of both unmodified and modified SnO₂ nanocomposites. UV-visible diffuse reflectance spectroscopy revealed a shift of the band-gap energy, as a first precise indicator of the electrical properties improvement. PL studies showed strong enhancement in UV visible emissions from SnO₂–DAN–Mo due to the presence of DAN–Mo-NPs. Mo-NPs in the SnO₂–DAN sample increases the charge-transfer resistance. Deeper insight through impedance measurements revealed a semiconductor behavior due to the formation of effective proton-conducting in the materials and proton transfer between Mo-NPs within the lattice. Mo-NPs contribute to the capacitance of SnO₂–DAN mostly via the surface functional groups, which enhance the electronic double layer capacitance. These results open promising prospects for preparing electrode materials.

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