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Comparative analysis of structural and photoelectrochemical properties of pure and Sb doped SnO2 functional electrode
Thin Solid Films 649 (2018) 225–231
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Comparative analysis of structural and photoelectrochemical properties of pure and Sb doped SnO2 functional electrode
T
⁎
H. Alia, R. Brahimib, , R. Outemzabeta, B. Bellalb, M. Trarib a b
Laboratory of Semiconductor Materials and Metallic Oxides, Faculty of Physics, USTHB, B.P.32, 16111 Algiers, Algeria Laboratory of Storage and Renewable Energy, Faculty of Chemistry, USTHB, B.P.32, 16111 Algiers, Algeria
A R T I C L E I N F O
A B S T R A C T
Keywords: SnO2 film APCVD Electrochemical impedance spectroscopy (EIS) Absorbance Chromate reduction
The purpose of this work was to analyze the electrochemical properties (C-E) and (I-E) of SnO2 thin films grown onto glass substrates by atmospheric pressure chemical vapor deposition (APCVD) and to correlate them with the microstructure i.e. texture and crystallography. The microstructure is one of the authentic monitor and the “memory” of materials processing that influences strongly the physical properties. The polycrystalline films present a good crystallinity; all peaks are assigned to the rutile phase and the films texture shows a dependence on the preparation conditions. The films deposited by APCVD under different conditions and Sb doping have been characterized by the Mott-Schottky plots and electrochemical impedance spectroscopy (EIS) both in the dark and under illumination. The photoelectrochemical results show the effectiveness of SnO2 films toward the chromate reduction. The structures like Sb doped SnO2-glass/chromate solution provide variable electrochemical properties as well as different absorbance under solar light which confirm the reduction of HCrO4− to trivalent state. We also show, through the EIS graphical representation of SnO2-Sb films-glass/chromate solution that the values of the electrical components relaxation system, conductivity and dielectric constants, are correlated with both the growth conditions and doping level.
1. Introduction SnO2 is widely used in optical and electronic applications because of its special properties. It is an insulator due to its stoichiometry but when doped with antimony (ATO), indium (ITO) or fluorine (FTO), its electrical conductivity is considerably improved [1,2]. It has been reported that both the nature and amount of impurities, improve the electrooptical properties. In addition, the oxygen deficiency in polycrystalline layers strongly influences the film conductivity [3,4]. On the other hand, the physical and chemical properties of metallic oxides depend on the growth conditions (temperature, type of substrate, surrounding atmosphere, etc.…). SnO2 is used as electrode in solar and fuel cells, liquid crystal devices, infrared reflectors, plasma display panels (PDPs) and transistors [5–8] to cite just few works. Among nanosized materials, SnO2 remains widely implemented in electrochemistry for the detection of oxidizing and reducing gases in chemical and gas sensors [9]. This is due to its large surface-to-volume ratio, size-morphology-tunable electronic and photocatalytic properties. In this form, SnO2 could be easily functionalized with selected chemical groups or combined with other materials to yield original nanohybrids, nano composites and hetero-structures
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with improved characteristics especially in the biosensors field [10,11]. The electrochemical impedance spectroscopy (EIS) is a powerful method for characterizing the electrical properties and allows the study of various phenomena at the solid/liquid interfaces [12] and industrial applications (corrosion, batteries) [1,13]. They are not only applied in quantitative chemical assays but also give the opportunity to work with non-electroactive species at low concentrations. In this regard, EIS is a convenient technique to correlate the electrical and structural characteristics of polycrystalline films in solution [12]. By analyzing the EIS spectra, it is possible to quantify the bulk and grain boundaries contributions of polycrystalline materials. Therefore, the frequency dependence of the electrical measurements can be related to the microstructure of the material using appropriate equivalent electrical circuits which fit the experimental responses. Because of the importance of the ac impedance spectroscopy for measuring the ionic conductivity, there have been some studies on the effect of the crystallinity on the impedance of thin layers [12]. Owing to the catalytic properties of SnO2 and its potential applications for the environmental protection, as well as its simplicity and low manufacturing cost, it appears as a model material for solving some pollution problems [14–16]. The choice of SnO2 is also motivated by its non-
Corresponding author. E-mail addresses: [email protected], [email protected] (R. Brahimi).
https://doi.org/10.1016/j.tsf.2018.01.044 Received 21 December 2016; Received in revised form 11 January 2018; Accepted 22 January 2018 0040-6090/ © 2018 Elsevier B.V. All rights reserved.
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toxicity, low cost and chemical stability over the whole pH range [17]. Even though the photo-catalysis is related to extrinsic characteristics of SnO2, an understanding of the photoelectrochemical (PEC) properties is required to predict the interfacial reactions. The physical and PEC characterizations are correlated to establish the energy band diagram, a prerequisite of the photocatalysis. In the present work, we report the physical and photoelectrochemical properties of SnO2-Sb (ATO) thin films deposited on glass substrates by atmospheric pressure chemical vapor deposition (APCVD). Furthermore, ATO provides variable electrochemical properties under illumination [18] and can be used for the reduction of chromate into less harmful forms under UV light. We have deposited SnO2 and Sb doped SnO2 supported on glass substrate at different temperatures in order to improve the physical properties of SnO2 and the thermal variation is clearly shown through the crystallinity. We have used the APCVD method for the deposition of Sb-SnO2 films on the glass substrate at different temperatures in order to improve the physical properties of SnO2 and the thermal variation is clearly shown through the crystallinity by EIS measurements and the crystallinity by XRD analysis. The photocatalytic activity on SnO2 showed less performance than on Sb-doped SnO2. But the effect of the substrate used in this work is less efficient than that used in our recent publication [16]. 2. Experimental details Pure SnO2 and antimony doped (Sb-SnO2) films deposited onto glass substrates in the temperatures range (375–443 °C) were prepared by APCVD technique using a horizontal reactor. A gas mixture (SnCl2, 2 H2O and SbCl3) and pure oxygen were used as precursors. The deposition time was fixed at 10 min under a compressed oxygen flow rate of 2 L min−1, oxygen was used as oxidant agent and carrier gas. The atomic ratio (Sb/Sn) was varied from 0.78 to 1.3% taking the hydrated SnCl4 mass constant and the mass of SbCl3 variable. With these conditions, the thickness of the films is around 2.2 μm. The as-prepared films have different colors varying from light yellow (0% Sb) to dark blue (1.3% Sb). The X-ray diffraction (XRD) analysis of the multi-dimensional structure was carried out with a Bragg geometry diffractometer (AXS BruckerD8 Advance). The X-ray radiation was directed with an angle of 0.5° to the sample surface and scanned with 0.05° step in the 2θ range (20–90°). For PEC measurements, the electrical contact of the working electrodes (1.5 cm2 surface area) was realized with silver paint and isolated with epoxy resin. The electrochemical measurements were carried on at ambient temperature in a three electrode cell; Pt-electrode and saturated calomel electrode (SCE) were used respectively as emergency and reference electrodes. The working solution was prepared by dissolving K2Cr2O7 (Merck, extra pure) in distilled water. The EIS measurements were performed at the open circuit potential (OCP) over the frequency range (10 mHz–100 kHz). After stabilization of OCP, the current-potential (I-E) characteristics were plotted in aerated solution using a PGZ301 potentiostat (Radiometer analytical). The variation of the interfacial capacitance was measured at 10 kHz. For the photocatalytic tests, pure and Sb doped films were immersed in a Pyrex reactor, containing 10 cm3 of HCrO4 solution (16 ppm, pH ~ 2.7) and exposed to solar light in a sunny day. The absorbance spectra of the chromate solutions were recorded with a UV–visible spectrophotometer (Specord ® 200 PLUS). All solutions were prepared from distilled water (resistivity ~ 0.8 Ω cm).
Fig. 1. a-Influence of deposition temperature and b-Antimony doping level on XRD patterns of SnO2 films deposited onto glass substrate.
Fig. 1a and b respectively. The prominent peaks at 2θ = 26.5° (d = 3.36 Å), 2θ = 33.7° (2.64 Å), 2θ = 37.9° (2.37 Å) and 2θ = 51.5° (d = 1.77 Å) correspond to (110), (101), (200) and (211) crystal lattice planes of SnO2 structure respectively. All other smaller peaks coincide with the corresponding peaks of the rutile phase given in the standard data file (JCPDS-ICDD 41-1445). In addition, the films exhibit a very good adherence on the glass substrates. 3.1.1. Effect of the deposition-crystallization temperature (T) on the structural properties The orientation is different depending on the substrate temperature, the (110) plane predominates at 375 °C while it changes at 443 °C to (101), (211) and (200) planes. The crystallinity of the films increases in a regular manner with raising temperature and similar results have been reported in the literature [19,20]. However, the XRD pattern recorded at 420 °C is abnormal; further, in the electrical properties, the measurement of the sheet resistance confirms this hypothesis. The increase (or appearance) and simultaneous disappearance of continuous lines of polycrystalline SnO2 phase with Miller indices (110), (101), (200) and (211) versus temperature is a result of an adjustment of crystallographic planes. The best crystallization is obtained at 443 °C and for the doping we have used this temperature for the films deposition.
3. Results and discussion 3.1. XRD characterizations
3.1.2. Effect of Sb doping on the structural properties The structural analysis made on SnO2-Sb thin films on glass substrates shows that they still crystallize in the tetragonal structure. With
The XRD spectra of SnO2 films deposited on glass substrates versus the deposition temperature (T) and Sb-doping level are represented in 226
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solid solution when heated in evacuated silica tubes. 3.3. Texture coefficient (Tc(hkl)) In a material, each grain possesses different crystallographic orientations toward its neighbors and the orientation in the material is random. The privileged orientation on the material surface is a preferential orientation or texture and this property is desired in thin films. The microstructure is quantified by defining the texture coefficient Tc(hkl) which can be calculated for each orientation from the following equation [11,23]:
Tc(hkl) =
I(hkl) /I0(hkl)
(
1 [∑N I(hkl) /I0(hkl) ] N
)
(1)
where I(hkl) is the measured intensity, I0(hkl) the relative intensity of the corresponding plane given in the JCPDS card and N the number of reflections. SnO2 is not statistically disordered and some directions predominate and form a crystallographic texture. The knowledge of the texture is important because it influences the physical properties and the performance of thin layers. A value equal to unity corresponds to a total random orientation while a greater value indicates the degree of texture. Furthermore, values between 0 and 1 result in a low intensity of XRD lines [24]. As shown in Fig. 3, the structure factor obtained for pure SnO2 is better since the doping can be considered as a perturbation of a system naturally balanced. Any system which deviates from its natural balance seeks a new equilibrium state and even if this state is reached it cannot be of the same quality. In some of our results (not yet published), the compact textured film is obtained at 443 °C. According to Tong et al.'s research [25], the surface roughness is related to the mobility and diffusion of the adatoms on the substrate surface. At low temperature (375 °C), both the atomic mobility and diffusion on the substrate surface are small. In this case, the adatoms cannot reach the lattice sites, thus leading to the formation of amorphous structure and large surface roughness. On the contrary, with increasing the growth temperature, the mobility and diffusion of the adatoms are increased, thus stabilizing the nanoparticles and preventing SnO2 agglomeration into large particles. The crystallized grains are formed and the orientation becomes perfect at 443 °C, corresponding to intense XRD peaks (Fig. 1a).
Fig. 2. Variation of crystalline size ag of SnO2 deposited onto glass substrate versus: aGrowth temperature, b-Antimony doping level.
increasing the Sb-content, one observes in the doped films the increase of the peak at 26.5° (110) and decrease of the peak at 37.9°, (200) (Fig. 1b) indicating a crystallographic distortion of the crystal lattice, in agreement with the literature [21,22]. There are no secondary impurities like Sb2O3 or Sb2O5 on the pattern and this means that no phases are present neither in the grains nor at the grain boundaries. This result implies that both Sb3+ and Sb5+ ions replace the octahedral site of Sn4+ in the SnO2 lattice and give a doping by substitution; Sb3+ is converted to Sb5+ during heating. Therefore, Sn4+ in the SnO2 crystal can be replaced by Sb5+ because of the closeness of both ionic radii and electronegativities of Sn4+ and Sb5+ which are 0.083 nm, 0.074 nm and 1.8, 1.9, respectively. The substitution occurs easily, resulting in a slight shift of the XRD peak position if we superpose Fig. 1a and b [21,22].
3.4. Raman spectroscopy measurement The films have been characterized by the Raman spectroscopy and
3.2. Crystallite size (ag) versus temperature and doping level The average crystallite size (ag) of the films was calculated by using the Scherrer relation. We have used the Origin-8 software package to extract the exact peak position and the full width at half maximum (FWHM) of the most intense XRD peak (Fig. 1a). The crystallite size ag (Fig. 2a) increases with raising the growth temperature and reaches a maximum (23.4 nm), which represents a weak value compared to the results reported elsewhere [22]. Fig. 2b shows the variation of the crystallite size of SnO2 for different Sb doping level. From this evolution, we can deduce that the size ag is slightly affected by the Sb doping. It is helpful to mention that powders SnO2 and Sb2O3 form a complete
Fig. 3. Effect of antimony doping level (Sb%) on texture coefficient (Tc(hkl)).
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Fig. 6. Intensity-Potential J(E) characteristic of pure SnO2 and SnO2–Sb(1.3%) deposited onto glass at 443 °C.
Fig. 4. Raman spectra of pure SnO2 and SnO2–Sb(1.3%) deposited onto glass at 443 °C.
electrochemical behavior of the film. The dark current (Jd), < 50 μA cm−2, indicates the delocalization of the lone pair cation Sb3+ (5s2) with a blocking contact with the electrolyte as should be expected for a semiconductor/electrolyte junction assimilated at a chemical diode. The peaks at ~0.5 V (O1) and 0.8 V (O2) are ascribed to the electrochemical couples Sn4+/2+ and Sb5+/3+ respectively. The peaks on the reverse scan are assigned to the reduction of Sb5+ (~−0.2 V, R2) and Sn4+ (−0.7 V, R1), the large difference between the oxidation and reduction peaks indicate irreversible redox processes (sluggish system). The increased current above 1.5 V is due to the water oxidation into oxygen while the decrease of the current below ~−1 V is assigned to the hydrogen evolution (H2O + 2 e− → H2 + 2 OH−). Owing to the low permittivity (ε) of SnO2, the double layer capacity (Cdl) is much larger than that of the semiconductor (CSC), resulting in a wide depletion width (nm range), in agreement with lightly doping and moderate conductivity (see Table 1). In addition, the current Jd is small and the reciprocal capacity (CSC−2) leads to an accurate determination of the flat band potential (Efb) from the capacitance measurements:
all spectra are shown in Fig. 4, The peaks indexed at 244, 450 and 705 cm−1 are assigned to EU(TO), A2U(TO) and A2U(LO) respectively in agreement with the literature [26,27]. 3.5. Growth grains modes As the texture and structural properties are dependent on the growth of grains, we give in what follows an overview of three modes of material growth (Fig. 5a (a, b, c)): (a) the land growth (Volmer–Weber model), (b) the layer growth (Frank–van der Merwe model) and (c) the multiple growth of layer and island growth (Stranski–Krastanov model). Fig. 5b shows textured surfaces with faceted and oriented grains. The transverse SEM image of Sb-SnO2 film given inside Fig. 5b reveals dense and columnar granular microstructure corresponding to the Volmer-Weber model. 3.6. Electrochemical analysis
C−2 =
3.6.1. Intensity-potential J-E and C-E characteristics The J-E characteristic of SnO2 deposited on glass at 443 °C (Fig. 6) is plotted in neutral medium (pH ~ 6) in order to elucidate the
2 (E − E fb) [eNDεε 0 ]
(2)
where εo is the dielectric constant of vacuum (8.85 × 10−12 F m−1) and Fig. 5. a-Overview of the three modes of material growth: a-The island growth (Volmer–Weber model), b-The layer growth (Frank–van der Merwe model) and c-The multiple growth of layer and then island growth (Stranski–Krastanov model). b-SEM images of: 1-pure SnO2 thin film deposited onto glass and b-2- SnO2-Sb (1.3%) deposited onto glass. (Inset-b-2- SnO2-Sb(1.3%) deposited onto silicium.
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Table 1 Electrochemical and electrical parameters of SnO2/glass. Deposit temperature (°C)
Rgb (MΩ)
Cgb (nF)
σ (Ω−1 cm−1)
τ (s)
ε (F m−1) × 1011
375 395 410 443
2.20 1.90 1.85 1.30
10 20 30 45
23.3 116.5 127 77.7
0.10 0.70 3.57 5
1.43 2.86 4.29 6.44
Fig. 7. Capacitance characteristic of pure SnO2 and SnO2-Sb(1.3%) deposited onto glass at 443 °C.
e the electron charge. The positive slope of the (C−2 - E) characteristic (Fig. 7) indicates n type conductivity. The potential (Efb = −0.76 V) is determined by extrapolating the straight line to the potential axis while the electrons density (ND = 2.73 × 1019 cm−3) is provided from the slope. The transition from the depletion zone to the accumulation region occurs at ~0 V. For a comparative purpose, we have also reported the plot of pure SnO2 deposited on glass. The energetic position of the conduction band with respect to vacuum is given by:
P = 4.75 + eE fb + Ea
(3)
The P value (3.89 eV) indicates that the conduction band (CB) derives from Sn4+: 5s2 orbital and corresponds to a potential of −0.81 V, negative enough to reduce chromate into Cr3+ (see Photocatalysis below).
Fig. 8. a-Nyquist representation of pure SnO2 (- ZImag versus ZReal) for different temperature of deposition in the dark. b-Nyquist representation of pure SnO2 and SnO2-Sb (1.3%) deposited onto glass at 443 °C in the dark and under illumination.
ZCPE =
3.6.2. EIS analysis In polycrystalline solids, the transport properties are affected by the microstructure, and the EIS analysis usually contains features that can be directly related to the morphology. As mentioned above, the EIS analysis permits the correlation electrical conductivity/microstructure of the films, and gives information on the interfacial reactions. The Nyquist representation of SnO2 (Fig. 8a) shows only one arc at high frequencies, attributed to the bulk resistance. The intercept of the semicircle with the real axis gives the bulk resistance. The large diameter of SnO2 film deposited at 375 °C agrees with the lower conductivity (Table 1). By contrast, SnO2 prepared at 443 °C exhibits a higher conductivity and larger crystallite size. In addition, the diameter decreases under UV light, confirming the semi conductivity of the material (Fig. 8b). We also report in Fig. 8b, the Nyquist spectrum of doped SnO2-Sb (1.3%)/glass deposited at 443 °C in the dark and under UV light. The center is positioned below the real axis, and such depression is assigned to different phenomena like the surface states within the gap region, the heterogeneity of the film surface or the potential field. In such a case, the phase impedance is defined by a constant phase element (CPE):
1 T(jꙍ) p
(4)
ZCPE (Ω cm2) is characterized by a deviation from pure capacity and is represented by two parameters T (Ω−1 cm−2sp) and (C = T ωmaxp-1), ω being the angular frequency, j the imaginary number (j2 = −1) and ωmax the characteristic frequency for which the impedance Zi reaches its maximum. The homogeneity factor (0 < p < 1) depends on the depression angle β:
p=1−
2β2 π
(5)
The p value is attributed to the roughness of the film as well as the surface states within the forbidden band. The absence of straight line at low frequencies indicates that the system is under kinetic control with no diffusion. The offset near the origin is due to the resistance (Rs) of the electrolytic solution. It is attributed to the molar conductivities (Λ∝) of the ionic electrolyte of K+ (50 Ω−1 cm2 mol−1) and HCrO4− (Ω−1 cm2 mol−1).
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orientations of the films which depend on both the Sb doping and the growth temperature. A slight increase in the particle size was observed with increasing the deposition temperature. It has also be seen that instead of introducing new phases, the Sb-doping shifts the peak positions slightly with a change of their relative intensities, indicating the incorporation of Sb in the Sn-sublattice. The decrease in the amplitude of the imaginary part with raising temperature is due to the presence of a space charge region while the decrease of the real part showed an enhanced conductivity. The impedance data indicated non-diffusion controlled process. Due to its hetero-valence, the Sb doping generates free electrons, responsible of the enhanced transport properties confirmed by the Nyquist representation and capacitance measurements. The photoelectrochemistry indicated through the energy band diagram, the chromate reduction under solar illumination. These coatings are promising as depollution technique under solar light. Acknowledgments Fig. 9. UV–visible absorbance spectra of Cr(VI) solution after 6-hour illumination in the presence of pure SnO2 and SnO2-Sb(1.3%) films immersed in.
The authors would like to thank Smaali Assia for her contribution in structural characterization. The work was supported financially by the faculties of Physics and Chemistry of University of Science and Technology USTHB (CNEPRU E00220140035) of Algiers, Algeria.
3.7. Photocatalysis The photocatalysis presents advantages over the solid state devices like the simplicity of the junction which forms spontaneously by simple contact. The solar energy is clean and inexhaustible and the photocatalytic tests are realized under direct sunlight. The photo-electrochemical characterizations provide the energy band diagram of the junction SnO2/Cr2O72− electrolyte; chromate is a hazardous compound and the effluents must be treated at the source [28]. Therefore, the photoreduction of chromate is evaluated under solar light and the further criterion for the spontaneous Cr2O72− reduction is that the free potential (Uf) is more anodic than the potential Efb which is the case for Sb-SnO2. The potential of Cr2O72−/Cr3+ couple is ~ 0.93 V, depending on both the concentrations of species in solution and pH [29]. Sb-SnO2 film (surface area 1 cm2) is immersed in 25 mL of Cr2O72− solution (16 mg L−1). The system presents higher activity as demonstrated by a large decrease in the absorbance (Fig. 9), which is directly related to the decrease of chromate concentration according to the Beer Lambert law. Concomitantly, the water oxidation (H2O + 2 h+ → O2 + 2H+) precludes the undesirable reactions owing to the wide gap and favours the separation of electron/hole (e−/h+) pairs [30] because the potential of the couple O2/H2O is located below the valence band (SnO2-Sb-VB). The quantum efficiency (η) of the photocatalytic process is given by:
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η = 6 × {number of converted Cr2O7−2 mol. sec−1/photons flux sec−1} (6) η value of 0.30% has been obtained, the number 6 is introduced because the chromate reaction requires six electrons (Cr2O72− + 14H+ + 6 e− → 2 Cr3+ + H2O). Despite its wide gap, SnO2 shows an appreciable Cr2O72− photo reduction under sunlight. In acidic medium (pH ~ 2.7), 39% of Cr2O72− is reduced over Sb-SnO2 (1.3%) against only 6% on pure SnO2. The temperature of the solution reached 27 °C at the end of the test. As perspectives, the physical properties among which the XPS analysis will be investigated on SnO2 deposited on silicon single crystal which was successfully tested for Rhodamine B oxidation. The preliminarily results were satisfactory and will be published very soon. 4. Conclusion Crystallized SnO2 and Sb-SnO2 films with the rutile structure were grown by atmospheric pressure chemical vapor deposition. SnO2 surface coatings fabricated onto glass substrates were highly oriented. All films are single-phase SnO2, with a difference between the preferred 230
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Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Comparative analysis of structural and photoelectrochemical properties of pure and Sb doped SnO2 functional electrode
T
⁎
H. Alia, R. Brahimib, , R. Outemzabeta, B. Bellalb, M. Trarib a b
Laboratory of Semiconductor Materials and Metallic Oxides, Faculty of Physics, USTHB, B.P.32, 16111 Algiers, Algeria Laboratory of Storage and Renewable Energy, Faculty of Chemistry, USTHB, B.P.32, 16111 Algiers, Algeria
A R T I C L E I N F O
A B S T R A C T
Keywords: SnO2 film APCVD Electrochemical impedance spectroscopy (EIS) Absorbance Chromate reduction
The purpose of this work was to analyze the electrochemical properties (C-E) and (I-E) of SnO2 thin films grown onto glass substrates by atmospheric pressure chemical vapor deposition (APCVD) and to correlate them with the microstructure i.e. texture and crystallography. The microstructure is one of the authentic monitor and the “memory” of materials processing that influences strongly the physical properties. The polycrystalline films present a good crystallinity; all peaks are assigned to the rutile phase and the films texture shows a dependence on the preparation conditions. The films deposited by APCVD under different conditions and Sb doping have been characterized by the Mott-Schottky plots and electrochemical impedance spectroscopy (EIS) both in the dark and under illumination. The photoelectrochemical results show the effectiveness of SnO2 films toward the chromate reduction. The structures like Sb doped SnO2-glass/chromate solution provide variable electrochemical properties as well as different absorbance under solar light which confirm the reduction of HCrO4− to trivalent state. We also show, through the EIS graphical representation of SnO2-Sb films-glass/chromate solution that the values of the electrical components relaxation system, conductivity and dielectric constants, are correlated with both the growth conditions and doping level.
1. Introduction SnO2 is widely used in optical and electronic applications because of its special properties. It is an insulator due to its stoichiometry but when doped with antimony (ATO), indium (ITO) or fluorine (FTO), its electrical conductivity is considerably improved [1,2]. It has been reported that both the nature and amount of impurities, improve the electrooptical properties. In addition, the oxygen deficiency in polycrystalline layers strongly influences the film conductivity [3,4]. On the other hand, the physical and chemical properties of metallic oxides depend on the growth conditions (temperature, type of substrate, surrounding atmosphere, etc.…). SnO2 is used as electrode in solar and fuel cells, liquid crystal devices, infrared reflectors, plasma display panels (PDPs) and transistors [5–8] to cite just few works. Among nanosized materials, SnO2 remains widely implemented in electrochemistry for the detection of oxidizing and reducing gases in chemical and gas sensors [9]. This is due to its large surface-to-volume ratio, size-morphology-tunable electronic and photocatalytic properties. In this form, SnO2 could be easily functionalized with selected chemical groups or combined with other materials to yield original nanohybrids, nano composites and hetero-structures
⁎
with improved characteristics especially in the biosensors field [10,11]. The electrochemical impedance spectroscopy (EIS) is a powerful method for characterizing the electrical properties and allows the study of various phenomena at the solid/liquid interfaces [12] and industrial applications (corrosion, batteries) [1,13]. They are not only applied in quantitative chemical assays but also give the opportunity to work with non-electroactive species at low concentrations. In this regard, EIS is a convenient technique to correlate the electrical and structural characteristics of polycrystalline films in solution [12]. By analyzing the EIS spectra, it is possible to quantify the bulk and grain boundaries contributions of polycrystalline materials. Therefore, the frequency dependence of the electrical measurements can be related to the microstructure of the material using appropriate equivalent electrical circuits which fit the experimental responses. Because of the importance of the ac impedance spectroscopy for measuring the ionic conductivity, there have been some studies on the effect of the crystallinity on the impedance of thin layers [12]. Owing to the catalytic properties of SnO2 and its potential applications for the environmental protection, as well as its simplicity and low manufacturing cost, it appears as a model material for solving some pollution problems [14–16]. The choice of SnO2 is also motivated by its non-
Corresponding author. E-mail addresses: [email protected], [email protected] (R. Brahimi).
https://doi.org/10.1016/j.tsf.2018.01.044 Received 21 December 2016; Received in revised form 11 January 2018; Accepted 22 January 2018 0040-6090/ © 2018 Elsevier B.V. All rights reserved.
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toxicity, low cost and chemical stability over the whole pH range [17]. Even though the photo-catalysis is related to extrinsic characteristics of SnO2, an understanding of the photoelectrochemical (PEC) properties is required to predict the interfacial reactions. The physical and PEC characterizations are correlated to establish the energy band diagram, a prerequisite of the photocatalysis. In the present work, we report the physical and photoelectrochemical properties of SnO2-Sb (ATO) thin films deposited on glass substrates by atmospheric pressure chemical vapor deposition (APCVD). Furthermore, ATO provides variable electrochemical properties under illumination [18] and can be used for the reduction of chromate into less harmful forms under UV light. We have deposited SnO2 and Sb doped SnO2 supported on glass substrate at different temperatures in order to improve the physical properties of SnO2 and the thermal variation is clearly shown through the crystallinity. We have used the APCVD method for the deposition of Sb-SnO2 films on the glass substrate at different temperatures in order to improve the physical properties of SnO2 and the thermal variation is clearly shown through the crystallinity by EIS measurements and the crystallinity by XRD analysis. The photocatalytic activity on SnO2 showed less performance than on Sb-doped SnO2. But the effect of the substrate used in this work is less efficient than that used in our recent publication [16]. 2. Experimental details Pure SnO2 and antimony doped (Sb-SnO2) films deposited onto glass substrates in the temperatures range (375–443 °C) were prepared by APCVD technique using a horizontal reactor. A gas mixture (SnCl2, 2 H2O and SbCl3) and pure oxygen were used as precursors. The deposition time was fixed at 10 min under a compressed oxygen flow rate of 2 L min−1, oxygen was used as oxidant agent and carrier gas. The atomic ratio (Sb/Sn) was varied from 0.78 to 1.3% taking the hydrated SnCl4 mass constant and the mass of SbCl3 variable. With these conditions, the thickness of the films is around 2.2 μm. The as-prepared films have different colors varying from light yellow (0% Sb) to dark blue (1.3% Sb). The X-ray diffraction (XRD) analysis of the multi-dimensional structure was carried out with a Bragg geometry diffractometer (AXS BruckerD8 Advance). The X-ray radiation was directed with an angle of 0.5° to the sample surface and scanned with 0.05° step in the 2θ range (20–90°). For PEC measurements, the electrical contact of the working electrodes (1.5 cm2 surface area) was realized with silver paint and isolated with epoxy resin. The electrochemical measurements were carried on at ambient temperature in a three electrode cell; Pt-electrode and saturated calomel electrode (SCE) were used respectively as emergency and reference electrodes. The working solution was prepared by dissolving K2Cr2O7 (Merck, extra pure) in distilled water. The EIS measurements were performed at the open circuit potential (OCP) over the frequency range (10 mHz–100 kHz). After stabilization of OCP, the current-potential (I-E) characteristics were plotted in aerated solution using a PGZ301 potentiostat (Radiometer analytical). The variation of the interfacial capacitance was measured at 10 kHz. For the photocatalytic tests, pure and Sb doped films were immersed in a Pyrex reactor, containing 10 cm3 of HCrO4 solution (16 ppm, pH ~ 2.7) and exposed to solar light in a sunny day. The absorbance spectra of the chromate solutions were recorded with a UV–visible spectrophotometer (Specord ® 200 PLUS). All solutions were prepared from distilled water (resistivity ~ 0.8 Ω cm).
Fig. 1. a-Influence of deposition temperature and b-Antimony doping level on XRD patterns of SnO2 films deposited onto glass substrate.
Fig. 1a and b respectively. The prominent peaks at 2θ = 26.5° (d = 3.36 Å), 2θ = 33.7° (2.64 Å), 2θ = 37.9° (2.37 Å) and 2θ = 51.5° (d = 1.77 Å) correspond to (110), (101), (200) and (211) crystal lattice planes of SnO2 structure respectively. All other smaller peaks coincide with the corresponding peaks of the rutile phase given in the standard data file (JCPDS-ICDD 41-1445). In addition, the films exhibit a very good adherence on the glass substrates. 3.1.1. Effect of the deposition-crystallization temperature (T) on the structural properties The orientation is different depending on the substrate temperature, the (110) plane predominates at 375 °C while it changes at 443 °C to (101), (211) and (200) planes. The crystallinity of the films increases in a regular manner with raising temperature and similar results have been reported in the literature [19,20]. However, the XRD pattern recorded at 420 °C is abnormal; further, in the electrical properties, the measurement of the sheet resistance confirms this hypothesis. The increase (or appearance) and simultaneous disappearance of continuous lines of polycrystalline SnO2 phase with Miller indices (110), (101), (200) and (211) versus temperature is a result of an adjustment of crystallographic planes. The best crystallization is obtained at 443 °C and for the doping we have used this temperature for the films deposition.
3. Results and discussion 3.1. XRD characterizations
3.1.2. Effect of Sb doping on the structural properties The structural analysis made on SnO2-Sb thin films on glass substrates shows that they still crystallize in the tetragonal structure. With
The XRD spectra of SnO2 films deposited on glass substrates versus the deposition temperature (T) and Sb-doping level are represented in 226
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solid solution when heated in evacuated silica tubes. 3.3. Texture coefficient (Tc(hkl)) In a material, each grain possesses different crystallographic orientations toward its neighbors and the orientation in the material is random. The privileged orientation on the material surface is a preferential orientation or texture and this property is desired in thin films. The microstructure is quantified by defining the texture coefficient Tc(hkl) which can be calculated for each orientation from the following equation [11,23]:
Tc(hkl) =
I(hkl) /I0(hkl)
(
1 [∑N I(hkl) /I0(hkl) ] N
)
(1)
where I(hkl) is the measured intensity, I0(hkl) the relative intensity of the corresponding plane given in the JCPDS card and N the number of reflections. SnO2 is not statistically disordered and some directions predominate and form a crystallographic texture. The knowledge of the texture is important because it influences the physical properties and the performance of thin layers. A value equal to unity corresponds to a total random orientation while a greater value indicates the degree of texture. Furthermore, values between 0 and 1 result in a low intensity of XRD lines [24]. As shown in Fig. 3, the structure factor obtained for pure SnO2 is better since the doping can be considered as a perturbation of a system naturally balanced. Any system which deviates from its natural balance seeks a new equilibrium state and even if this state is reached it cannot be of the same quality. In some of our results (not yet published), the compact textured film is obtained at 443 °C. According to Tong et al.'s research [25], the surface roughness is related to the mobility and diffusion of the adatoms on the substrate surface. At low temperature (375 °C), both the atomic mobility and diffusion on the substrate surface are small. In this case, the adatoms cannot reach the lattice sites, thus leading to the formation of amorphous structure and large surface roughness. On the contrary, with increasing the growth temperature, the mobility and diffusion of the adatoms are increased, thus stabilizing the nanoparticles and preventing SnO2 agglomeration into large particles. The crystallized grains are formed and the orientation becomes perfect at 443 °C, corresponding to intense XRD peaks (Fig. 1a).
Fig. 2. Variation of crystalline size ag of SnO2 deposited onto glass substrate versus: aGrowth temperature, b-Antimony doping level.
increasing the Sb-content, one observes in the doped films the increase of the peak at 26.5° (110) and decrease of the peak at 37.9°, (200) (Fig. 1b) indicating a crystallographic distortion of the crystal lattice, in agreement with the literature [21,22]. There are no secondary impurities like Sb2O3 or Sb2O5 on the pattern and this means that no phases are present neither in the grains nor at the grain boundaries. This result implies that both Sb3+ and Sb5+ ions replace the octahedral site of Sn4+ in the SnO2 lattice and give a doping by substitution; Sb3+ is converted to Sb5+ during heating. Therefore, Sn4+ in the SnO2 crystal can be replaced by Sb5+ because of the closeness of both ionic radii and electronegativities of Sn4+ and Sb5+ which are 0.083 nm, 0.074 nm and 1.8, 1.9, respectively. The substitution occurs easily, resulting in a slight shift of the XRD peak position if we superpose Fig. 1a and b [21,22].
3.4. Raman spectroscopy measurement The films have been characterized by the Raman spectroscopy and
3.2. Crystallite size (ag) versus temperature and doping level The average crystallite size (ag) of the films was calculated by using the Scherrer relation. We have used the Origin-8 software package to extract the exact peak position and the full width at half maximum (FWHM) of the most intense XRD peak (Fig. 1a). The crystallite size ag (Fig. 2a) increases with raising the growth temperature and reaches a maximum (23.4 nm), which represents a weak value compared to the results reported elsewhere [22]. Fig. 2b shows the variation of the crystallite size of SnO2 for different Sb doping level. From this evolution, we can deduce that the size ag is slightly affected by the Sb doping. It is helpful to mention that powders SnO2 and Sb2O3 form a complete
Fig. 3. Effect of antimony doping level (Sb%) on texture coefficient (Tc(hkl)).
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Fig. 6. Intensity-Potential J(E) characteristic of pure SnO2 and SnO2–Sb(1.3%) deposited onto glass at 443 °C.
Fig. 4. Raman spectra of pure SnO2 and SnO2–Sb(1.3%) deposited onto glass at 443 °C.
electrochemical behavior of the film. The dark current (Jd), < 50 μA cm−2, indicates the delocalization of the lone pair cation Sb3+ (5s2) with a blocking contact with the electrolyte as should be expected for a semiconductor/electrolyte junction assimilated at a chemical diode. The peaks at ~0.5 V (O1) and 0.8 V (O2) are ascribed to the electrochemical couples Sn4+/2+ and Sb5+/3+ respectively. The peaks on the reverse scan are assigned to the reduction of Sb5+ (~−0.2 V, R2) and Sn4+ (−0.7 V, R1), the large difference between the oxidation and reduction peaks indicate irreversible redox processes (sluggish system). The increased current above 1.5 V is due to the water oxidation into oxygen while the decrease of the current below ~−1 V is assigned to the hydrogen evolution (H2O + 2 e− → H2 + 2 OH−). Owing to the low permittivity (ε) of SnO2, the double layer capacity (Cdl) is much larger than that of the semiconductor (CSC), resulting in a wide depletion width (nm range), in agreement with lightly doping and moderate conductivity (see Table 1). In addition, the current Jd is small and the reciprocal capacity (CSC−2) leads to an accurate determination of the flat band potential (Efb) from the capacitance measurements:
all spectra are shown in Fig. 4, The peaks indexed at 244, 450 and 705 cm−1 are assigned to EU(TO), A2U(TO) and A2U(LO) respectively in agreement with the literature [26,27]. 3.5. Growth grains modes As the texture and structural properties are dependent on the growth of grains, we give in what follows an overview of three modes of material growth (Fig. 5a (a, b, c)): (a) the land growth (Volmer–Weber model), (b) the layer growth (Frank–van der Merwe model) and (c) the multiple growth of layer and island growth (Stranski–Krastanov model). Fig. 5b shows textured surfaces with faceted and oriented grains. The transverse SEM image of Sb-SnO2 film given inside Fig. 5b reveals dense and columnar granular microstructure corresponding to the Volmer-Weber model. 3.6. Electrochemical analysis
C−2 =
3.6.1. Intensity-potential J-E and C-E characteristics The J-E characteristic of SnO2 deposited on glass at 443 °C (Fig. 6) is plotted in neutral medium (pH ~ 6) in order to elucidate the
2 (E − E fb) [eNDεε 0 ]
(2)
where εo is the dielectric constant of vacuum (8.85 × 10−12 F m−1) and Fig. 5. a-Overview of the three modes of material growth: a-The island growth (Volmer–Weber model), b-The layer growth (Frank–van der Merwe model) and c-The multiple growth of layer and then island growth (Stranski–Krastanov model). b-SEM images of: 1-pure SnO2 thin film deposited onto glass and b-2- SnO2-Sb (1.3%) deposited onto glass. (Inset-b-2- SnO2-Sb(1.3%) deposited onto silicium.
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Table 1 Electrochemical and electrical parameters of SnO2/glass. Deposit temperature (°C)
Rgb (MΩ)
Cgb (nF)
σ (Ω−1 cm−1)
τ (s)
ε (F m−1) × 1011
375 395 410 443
2.20 1.90 1.85 1.30
10 20 30 45
23.3 116.5 127 77.7
0.10 0.70 3.57 5
1.43 2.86 4.29 6.44
Fig. 7. Capacitance characteristic of pure SnO2 and SnO2-Sb(1.3%) deposited onto glass at 443 °C.
e the electron charge. The positive slope of the (C−2 - E) characteristic (Fig. 7) indicates n type conductivity. The potential (Efb = −0.76 V) is determined by extrapolating the straight line to the potential axis while the electrons density (ND = 2.73 × 1019 cm−3) is provided from the slope. The transition from the depletion zone to the accumulation region occurs at ~0 V. For a comparative purpose, we have also reported the plot of pure SnO2 deposited on glass. The energetic position of the conduction band with respect to vacuum is given by:
P = 4.75 + eE fb + Ea
(3)
The P value (3.89 eV) indicates that the conduction band (CB) derives from Sn4+: 5s2 orbital and corresponds to a potential of −0.81 V, negative enough to reduce chromate into Cr3+ (see Photocatalysis below).
Fig. 8. a-Nyquist representation of pure SnO2 (- ZImag versus ZReal) for different temperature of deposition in the dark. b-Nyquist representation of pure SnO2 and SnO2-Sb (1.3%) deposited onto glass at 443 °C in the dark and under illumination.
ZCPE =
3.6.2. EIS analysis In polycrystalline solids, the transport properties are affected by the microstructure, and the EIS analysis usually contains features that can be directly related to the morphology. As mentioned above, the EIS analysis permits the correlation electrical conductivity/microstructure of the films, and gives information on the interfacial reactions. The Nyquist representation of SnO2 (Fig. 8a) shows only one arc at high frequencies, attributed to the bulk resistance. The intercept of the semicircle with the real axis gives the bulk resistance. The large diameter of SnO2 film deposited at 375 °C agrees with the lower conductivity (Table 1). By contrast, SnO2 prepared at 443 °C exhibits a higher conductivity and larger crystallite size. In addition, the diameter decreases under UV light, confirming the semi conductivity of the material (Fig. 8b). We also report in Fig. 8b, the Nyquist spectrum of doped SnO2-Sb (1.3%)/glass deposited at 443 °C in the dark and under UV light. The center is positioned below the real axis, and such depression is assigned to different phenomena like the surface states within the gap region, the heterogeneity of the film surface or the potential field. In such a case, the phase impedance is defined by a constant phase element (CPE):
1 T(jꙍ) p
(4)
ZCPE (Ω cm2) is characterized by a deviation from pure capacity and is represented by two parameters T (Ω−1 cm−2sp) and (C = T ωmaxp-1), ω being the angular frequency, j the imaginary number (j2 = −1) and ωmax the characteristic frequency for which the impedance Zi reaches its maximum. The homogeneity factor (0 < p < 1) depends on the depression angle β:
p=1−
2β2 π
(5)
The p value is attributed to the roughness of the film as well as the surface states within the forbidden band. The absence of straight line at low frequencies indicates that the system is under kinetic control with no diffusion. The offset near the origin is due to the resistance (Rs) of the electrolytic solution. It is attributed to the molar conductivities (Λ∝) of the ionic electrolyte of K+ (50 Ω−1 cm2 mol−1) and HCrO4− (Ω−1 cm2 mol−1).
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orientations of the films which depend on both the Sb doping and the growth temperature. A slight increase in the particle size was observed with increasing the deposition temperature. It has also be seen that instead of introducing new phases, the Sb-doping shifts the peak positions slightly with a change of their relative intensities, indicating the incorporation of Sb in the Sn-sublattice. The decrease in the amplitude of the imaginary part with raising temperature is due to the presence of a space charge region while the decrease of the real part showed an enhanced conductivity. The impedance data indicated non-diffusion controlled process. Due to its hetero-valence, the Sb doping generates free electrons, responsible of the enhanced transport properties confirmed by the Nyquist representation and capacitance measurements. The photoelectrochemistry indicated through the energy band diagram, the chromate reduction under solar illumination. These coatings are promising as depollution technique under solar light. Acknowledgments Fig. 9. UV–visible absorbance spectra of Cr(VI) solution after 6-hour illumination in the presence of pure SnO2 and SnO2-Sb(1.3%) films immersed in.
The authors would like to thank Smaali Assia for her contribution in structural characterization. The work was supported financially by the faculties of Physics and Chemistry of University of Science and Technology USTHB (CNEPRU E00220140035) of Algiers, Algeria.
3.7. Photocatalysis The photocatalysis presents advantages over the solid state devices like the simplicity of the junction which forms spontaneously by simple contact. The solar energy is clean and inexhaustible and the photocatalytic tests are realized under direct sunlight. The photo-electrochemical characterizations provide the energy band diagram of the junction SnO2/Cr2O72− electrolyte; chromate is a hazardous compound and the effluents must be treated at the source [28]. Therefore, the photoreduction of chromate is evaluated under solar light and the further criterion for the spontaneous Cr2O72− reduction is that the free potential (Uf) is more anodic than the potential Efb which is the case for Sb-SnO2. The potential of Cr2O72−/Cr3+ couple is ~ 0.93 V, depending on both the concentrations of species in solution and pH [29]. Sb-SnO2 film (surface area 1 cm2) is immersed in 25 mL of Cr2O72− solution (16 mg L−1). The system presents higher activity as demonstrated by a large decrease in the absorbance (Fig. 9), which is directly related to the decrease of chromate concentration according to the Beer Lambert law. Concomitantly, the water oxidation (H2O + 2 h+ → O2 + 2H+) precludes the undesirable reactions owing to the wide gap and favours the separation of electron/hole (e−/h+) pairs [30] because the potential of the couple O2/H2O is located below the valence band (SnO2-Sb-VB). The quantum efficiency (η) of the photocatalytic process is given by:
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η = 6 × {number of converted Cr2O7−2 mol. sec−1/photons flux sec−1} (6) η value of 0.30% has been obtained, the number 6 is introduced because the chromate reaction requires six electrons (Cr2O72− + 14H+ + 6 e− → 2 Cr3+ + H2O). Despite its wide gap, SnO2 shows an appreciable Cr2O72− photo reduction under sunlight. In acidic medium (pH ~ 2.7), 39% of Cr2O72− is reduced over Sb-SnO2 (1.3%) against only 6% on pure SnO2. The temperature of the solution reached 27 °C at the end of the test. As perspectives, the physical properties among which the XPS analysis will be investigated on SnO2 deposited on silicon single crystal which was successfully tested for Rhodamine B oxidation. The preliminarily results were satisfactory and will be published very soon. 4. Conclusion Crystallized SnO2 and Sb-SnO2 films with the rutile structure were grown by atmospheric pressure chemical vapor deposition. SnO2 surface coatings fabricated onto glass substrates were highly oriented. All films are single-phase SnO2, with a difference between the preferred 230
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