Structural and Surface Study of Praseodymium-Doped SnO2

Article pubs.acs.org/JPCC

Structural and Surface Study of Praseodymium-Doped SnO2 Nanoparticles Prepared by the Polymeric Precursor Method Fermin H. Aragón,*,† Ismael Gonzalez,† José. A. H. Coaquira,‡ Pilar Hidalgo,§ Hermi F. Brito,∥ José. D. Ardisson,† Waldemar A. A. Macedo,† and Paulo C. Morais‡,⊥ †

Laboratório de Física Aplicada, Centro de Desenvolvimento da Tecnologia Nuclear, Belo Horizonte MG 31270-901, Brazil Núcleo de Física Aplicada, Instituto de Física, Universidade de Brasília, Brasília DF 70910-900, Brazil § Faculdade Gama-FGA, Setor Central Gama, Universidade de Brasília, Brasília DF 72405-610, Brazil ∥ Instituto de Química, Universidade de São Paulo, São Paulo SP 05508-000, Brazil ⊥ School of Automation, Huazhong University of Science and Technology, Wuhan 430074, China ‡

ABSTRACT: In the present study, we report on the successful synthesis of Pr-doped SnO2 (SnO2:Pr) nanoparticles using a polymeric precursor method while setting the Pr-content in the range from 0 to 10.0 mol %. The as-prepared samples were characterized in regard to their structural, morphological and surface properties. X-ray diffraction (XRD) patterns recorded from all samples revealed the tetragonal rutile-type structure with a systematic average size reduction (in the range from 11 to 4 nm) while enhancing the residual strain (in the range of 0.186 to 0.480%) as the Pr-content was increased. From the Rietveld refinement analysis we found that the lattice parameters (a, c, u, and V) showed a linear behavior, indicating a solid solution regimen for the Pr-doping. Transmission electron micrographs provided mean particle sizes of 8.7 ± 0.5 nm, for 2.5 mol % Pr-content, and 5.2 ± 0.5 nm,for 10.0 mol % Prcontent, which are in very good agreement with values obtained from the XRD data analysis: 7.4 ± 1.0 nm and 4.0 ± 1.0 nm, respectively. From X-ray photoelectron spectroscopy (XPS) measurements [O]/[Sn] = 1.44 ratio has been estimated at the surface of the undoped SnO2 nanoparticles, which is below the expected value for bulk compound ([O]/[Sn] = 2), suggesting that the system is strongly nonstoichiometric at the nanoparticle surface. Actually, we found the [O]/[Sn] ratio value increasing monotonically as the Pr-content was increased, which was interpreted as due to the elimination of the surface chemisorbed oxygen and/or oxygen-related vacancies. Moreover, a redshift of the Sn(3d) XPS peaks has been determined as the Pr-content was increased, evidencing the change of the oxidation state of tin ions from Sn4+ to Sn2+. Our analyzes of the Pr(3d) XPS peaks indicated the preference of the Pr-ions for the Pr3+ oxidation state, although small amounts of the Pr4+-ions could not be completely ruled out, particularly for the lower Pr-doping samples.

1. INTRODUCTION In recent years, the growing interest in nanomaterials based on metal oxide (SnO2, CeO2, Al2O3, and WO3 among others) nanoparticles, nanotubes, nanospheres, nanodisks, nanolamellae, and nanofibers1,2 is mainly due to the diversity of applications in different areas.3 More specifically, these nanomaterials are attractive candidates for gas sensors due to their high surface area. In this regard, tin dioxide (SnO2) compound is one of the n-type semiconductors commonly used and its natural nonstoichiometry, low cost and high chemical stability are features to be emphasized.4 Moreover, the surface of the SnO2 nanoparticles exhibits good absorption properties and reactivity due to the presence of free electrons in the conduction band plus the presence of surface and bulk oxygen vacancies and active chemisorbed oxygen.5 A main drawback of the SnO2-based chemical sensors is the low chemical selectivity, which does not allow one to separate the contribution made by a particular type of molecule within © XXXX American Chemical Society

the gas phase to the total electric signal. In order to improve the selectivity for specific gases, oxides nanoparticles are doped with different metal ions.6,7 However, the doping approach induces strong effects on the structural,8 electronic,9 catalytic, and magnetic7,10,11 properties. Regarding the magnetic properties many oxides such as CeO2, ZnO, SnO2, etc.12−15 exhibit room temperature ferromagnetism (RTFM) when doped with a small amount of metals. The most used ions to produce the diluted magnetic oxides (DMO) are the transition metals (TM), meanwhile few reports are found regarding rare earth (RE) ions. The introduction of the RE ions can provide desirable optoelectronic properties for the host matrix, which can be used for the development of new devices such as lasers, LEDs, and optical amplifiers. Received: January 24, 2015 Revised: March 28, 2015

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DOI: 10.1021/acs.jpcc.5b00761 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C On the other hand, divalent earth rare ions such as Ce3+,4+, Pr , and Tb3+,4+ are interesting from the standpoint of DMO, due to the possibility of doping with mixed valence state of RE ions, which can favor a charge-transfer mechanism. Therefore, the systematic study of the structural and surface properties of those systems can contribute to the better understanding of the DMO. In this respect, Paunovic et al.7 show that the suppression of the ferromagnetism observed in Pr-doped CeO2 nanocrystals can be well explained in terms of different dopant valence state and the presence of Pr3+-ions on the surface. On the other hand, the entrance of a multivalent ions as Pr(3+,4+) substituting Sn4+ ions as solid solution in the Pr-doped SnO2 system can generate oxygen vacancies due to the charge imbalance, providing free electrons to the conduction band, which can be strongly dependent on the dopant content, however if this ions will located on the surface layer there may be a annihilation of defects as oxygen vacancies. As exposed above, in the present work we reports the successful Pr-doping of SnO2 nanoparticles with Pr-content up to 10.0 mol % and synthesized via a polymer precursor method. The structural, morphological and surface properties of the asproduced undoped SnO2 and Pr-doped SnO2 nanoparticles were assessed by X-ray diffraction (XRD) measurements, transmission electron microscopy (TEM), high-resolution TEM (HRTEM), Energy-dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS), respectively.

were carried out. Figure 1a shows the EDS spectrum of the 10 mol % Pr-doped sample. The peaks located at around 0.9, 5.0,

3+,4+

Figure 1. (a) EDS spectrum obtained for the 10.0 mol % SnO2:Pr nanoparticles. (b) XRD patterns of the SnO 2 and SnO2:Pr nanoparticles. (c) Rietveld refinement of a XRD pattern for the 5.0 mol % SnO2:Pr sample. The experimental and the calculated data are represented by symbols and solid lines, respectively. The green solid line represents the difference between the experimental and the calculated data.

2. EXPERIMENTAL DETAILS Undoped and the Pr-doped SnO2 nanoparticles were produced following Pechini’s method,16 in which the undoped sample was obtained from the SnCl2·2H2O precursor whereas the Prdoped samples used Pr(NO3)3·6H2O as the source for the Prions. Following the mentioned protocol the SnO2:Pr nanoparticles with Pr-content in the range from 0.0 to 10.0 mol % have been obtained. Details of the sample preparation were described elsewhere in the literature.6 The crystalline quality, structural parameters, crystallite size, and residual strain were obtained from the XRD technique using a commercial diffractometer (Bruker, D8 Advanced) equipped with Cu Kα radiation. The particle size distribution was assessed from the TEM micrographs by counting 1368 (for 10.0 mol % Prdoped) and 974 (for 2.5 mol % Pr-doped) particles and using the Sturges approach.17 In order to confirm the nominal doping concentration elemental analysis was carried out using the EDS option of a scanning electron microscope (FESEM, model SIGMA VP). The XPS analysis was performed on a SPECS surface analysis system equipped with the Phoibos 150 electron analyzer. The monochromatized Aluminum radiation (1486.6 eV) with the output power set at 400W was used for the XPS analyses of all samples. The C(1s) signal (284.6 eV) was employed as the reference for the calibration of the binding energies (BE) of different elements in order to correct for the charge effect. The CasaXPS software was used to analyze all XPS data. The surface atomic concentration in the samples was estimated using the instrument sensitivity factors to scale for the calculated photoelectron peak areas.

5.5, and 5.9 keV are assigned to the characteristic X-ray emissions of Pr-ions. In order to quantify the Pr-content several measurements have been performed by considering different regions of the samples. Within the experimental uncertainties, results shown in Table 1 confirm the nominal values of Prcontent in the SnO2:Pr samples. The XRD data analysis indicated the formation of pure tetragonal rutile-type SnO2 phase (JCPDS card, 41-1445; space group, P42/mnm) in all samples, with no evidence of any other crystalline or amorphous phase, as can be seen in Figure 1b. Likewise, we found that the line width (full width at half-maximum) of the XRD peaks tends to increase as the nominal Pr-content increases. This finding can be correlated with the decrease of the particle size and/or to changes in the residual strain extent. In order to obtain further information the XRD patterns were analyzed using the Rietveld refinement method using the Thompson-Cox-Hastings pseudo-Voigt function (TCH-pV) of the DBWS program, given by18 TCH − pV = ηL + (1 − η)G, where L and G represent the Lorentzian (L) and the Gaussian (G) functions whereas η is a mixing parameter. A typical Rietveld refinement pattern is shown in Figure 1c for the 5.0 mol % Pr-doped sample. The line broadening related to the instrumental contribution was corrected by subtracting the line width of a standard sample (SiO2 single crystal) from the line width of the studied samples. The deconvolution of the TCHpV function provides the line widths of the Gaussian and the Lorentzian components given by HG = (U tan2 θ + V tan θ + W + Z/cos2θ)1/2 and HL = X tan θ + Y/cos θ, where U, V, W, Z, X, and Y are the refined parameters and can be used to determine the mean crystallite size (D) and the residual strain (ε). Details regarding the calculation of D and ε can be found in the literature.19 The list of the structural parameters obtained from the Rietveld refinements is collected in Table 1. Figure 2a shows the expected decrease of the mean crystallite size (D) as the nominal Pr-content increases, which is a behavior already

3. RESULTS AND DISCUSSION In order to determine experimentally the praseodymium content in the SnO2:Pr nanoparticles EDS measurements B


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The Journal of Physical Chemistry C

Table 1. List of Parameters Obtained from the XRD data Rietveld Refinement of the SnO2 and SnO2:Pr Nanoparticlesa Pr (x, mol %) (nominal) 0 1.0 2.5 5.0 7.5 10.0 a

Pr (x, mol %) (EDS) − 0.9 2.3 4.5 6.9 8.9

± ± ± ± ±

0.1 0.1 0.2 0.3 0.5

mean size (D) (nm)

strain (ε) (%)

a (Å)

c (Å)

c/a (−)

u (−)

V (Å3)

S (−)

± ± ± ± ± ±

0.1864 0.1787 0.2996 0.3724 0.4715 0.4796

4.7334 4.7361 4.7370 4.7397 4.7413 4.7447

3.1842 3.1863 3.1878 3.1919 3.1941 3.1999

0.6727 0.6728 0.6730 0.6734 0.6737 0.6744

0.3004 0.2990 0.2980 0.2959 0.2933 0.2890

71.34 71.47 71.53 71.71 71.80 72.04

1.56 1.48 1.94 1.41 1.36 1.23

11.0 9.1 7.4 5.2 4.9 4.0

1.0 1.0 1.0 1.0 1.0 1.0

S (R-wp/R-expected) values represent the goodness of the fit.

parts b and c. Whereas the c/a ratio shows an increasing tendency as the nominal Pr-content increases the internal parameter of the rutile structure (u) tends to decrease. These two opposite trends provide key structural changes to the intrinsically flattened octahedron of oxygen ions surrounding the tin ions. The almost linear increase of the c/a ratio by increasing of the nominal Pr-content indicates an anisotropic expansion of the unit cell along the c-axis induced by the Prdoping. Parts a and b of Figure 3 show the TEM micrographs of the 10.0 and 2.5 mol % SnO2:Pr along with their corresponding particle size histograms, which are well modeled by a lognormal distribution function. The mean particle size can be estimated by using the relation: ⟨D⟩ = D0 exp(σ2/2), where D0 is the median value and σ is the polydispersion parameter. The values found for ⟨D⟩ from the fits (see the red solid line in the insets of Figure 3, parts a and b, were 5.2 ± 0.5 and 8.7 ± 0.5 nm for the 10.0 and 2.5 mol % SnO2:Pr nanoparticles, respectively. The particle size determined from the TEM data are in very good agreement with the mean crystalline sizes values determined from the XRD data analysis (see Table 1). As presented in Figure 3c the HRTEM image obtained for the

Figure 2. (a) Mean particle size (D) as a function of the Pr-content. (b and c) Pr-content dependence of the lattice parameters (a and c). The dashed lines are drawn only to guide the eyes.

reported in the literature regarding the TM- and RE-doping of SnO2 nanoparticles.6,8,20 The entry of the Pr-ion originates distortions in the crystal lattice evidenced by the increase of the lattice strain (ε). The unit cell volume becomes larger as the nominal Pr-content increases. The same tendency is observed for both lattice constants (a and c), as can be seen in Figure 2,

Figure 3. TEM images of the SnO2:Pr nanoparticles doped with (a) 10.0 and (b) 2.5 mol % with their corresponding particle size histograms (the red solid lines represent the log-normal function). (c) HRTEM image of 2.5 mol % SnO2:Pr sample. (d) Selected area electron diffraction (SAED) pattern of the 2.5 mol % SnO2:Pr nanoparticles. C


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The Journal of Physical Chemistry C 2.5 mol % SnO2:Pr sample shows two interplanar distances (d). By using the relation: 1/d2 = (h2 + k2)/a2 + l2/c2, valid for a tetragonal structure, where h, k, l are the Miller indices and a and c are the lattice constants, the d1 = 0.34 nm and d2 = 0.26 nm distances (see Figure 3c) have been assigned to the (110) and (101) diffraction planes of the rutile type structure, respectively. It is worth mentioning that these diffraction planes are commonly found in nanosized SnO2 systems.21 The selected area electron diffraction (SAED) pattern recorded from a set of nanoparticles is shown in Figure 3d, revealing three bright rings corresponding to d-values of 0.337, 0.266, and 0.177 nm expected for the (110), (101), and (211) diffraction planes of the rutile type SnO2 structure. The XPS spectrum of the undoped SnO2 nanoparticles presented in Figure 4 shows peaks originated from Sn and O

Figure 5. Sn 3d XPS spectra of the SnO2:Pr nanoparticles. The inset shows the experimental evolution of the Sn(3d5/2) peak with the nominal Pr-content.

Figure 4. XPS spectrum of the SnO2 nanoparticles, showing the Sn-, O-, and C-related peaks.

plus a weak feature C(1s) located at 284.6 eV due to carbon contamination. The energy difference between the Sn(3d5/2) and the O(1s) XPS peaks was found to be 43.89 (1) eV. This difference can be used as the index of the Sn oxidation state. Thus, for the stoichiometric SnO2 the expected index would be 43.90 eV, which is 0.08 eV lower than the index for the stoichiometric SnO.22 This finding confirms that the Sn4+ species are the dominant one in the SnO2 nanoparticles. After the praseodymium doping XPS peaks corresponding to Pr(3d) emerge, evidencing the successful doping. In order to show the chemical composition evolution of the as-synthesized samples due to the Pr-doping we recorded high resolution XPS spectra for both the SnO2 and SnO2:Pr nanoparticles in the typical Sn, Pr, and O binding energy ranges. Figure 5 shows the Sn(3d5/2) and the Sn(3d3/2) XPS peaks which were curve-fitted using Gaussian-like functions, providing for the SnO2 nanoparticles peak positions at 486.6 and 495.1 eV, respectively. The Sn(3d5/2) XPS peak position observed in Figure 5 confirms that the oxidation state of the tin ions in the SnO2 nanoparticle is 4+. Actually, the binding energies for the Sn(3d5/2) associated with the Sn2+ and Sn4+ are expected at 485.9 and 486.6 eV, respectively.23 For the Pr-doped samples we found a systematic shift of the Sn(3d5/2) and Sn(3d5/2) peaks to lower energy values as the nominal Pr-content increases (see the inset of Figure 5 for the Sn(3d5/2) peak). This finding is clear evidence that the oxidation state of the tin ions changes from Sn4+ to Sn2+ as the Pr-content increases. Figure 6 shows the XPS spectra of the 2.5, 5.0, and 10.0 mol % SnO2:Pr nanoparticles in the binding energy region of Pr(3d) ions. As indicated in Figure. 6 the two XPS peaks located at around 933.6 and 954.0 eV have been assigned to the Pr(3d5/2) and Pr(3d3/2) levels, respectively. The difference of

Figure 6. Pr 3d XPS spectra of the SnO2:Pr nanoparticles for 2.5, 5.0, and 10.0 mol % Pr-doping.

20.4 eV is commonly reported in the literature for the spin− orbit splitting of the 3d5/2 and 3d3/2 levels.24 The presence of both peaks, namely Pr(3d5/2) and Pr(3d3/2) does not permit to exclude one of the two possible oxidation states of Pr-ions (Pr3+ and Pr4+) in the as-synthesized samples, since the 3d5/2 and 3d3/2 peaks have been observed in both Pr2O3 and PrO2 compounds.24 Furthermore, a shoulder at around 967 eV was also observed for samples with lower Pr-content (see the peak denoted by two asterisks in Figure 6). A satellite peak centered at 967 eV in the XPS spectrum of PrO2 has been reported as being exclusively related to the tetravalent state of Pr in the PrO2 compound.24,25 Unfortunately, this XPS satellite peak is overlapped with the oxygen Auger peak (OKLL), making its exact determination difficult. On the other hand, as can be seen in the inset of Figure 6, the position of the binding energy of the 3d5/2 peak is located between 933.2 and 933.9 eV. According to Molder et al. this peak position is expected for the Pr2O3 compound and it is quite different from the peak position expected for the PrO2 compound whose 3d5/2 XPS peak should appear between 935 and 936 eV.26 This analysis indicates that most of the praseodymium ions in the as-synthesized samples are in the Pr3+ oxidation state, in agreement with the preferentially trivalent state like most of the RE-ions.27 However, small amounts of the Pr4+ ion cannot be completely ruled out, especially in the lower Pr-content region, where the binding energy is shifting toward higher values as the Pr-content is D


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Table 2. [Pr]/[Sn] and [O]/[Sn+Pr] ratio determined for the SnO2 and SnO2:Pr nanoparticles form XPS measurements 4+

2+

4+

2+

nominal Pr-content (x mol %)

[Pr]/[Sn]

[O]/[Sn + Pr]

[OSn ]/[OSn ]

[OChem, OVac]/[OSn + OSn ]

0 1.0 2.5 5.0 7.5 10.0 10.0 sputtering

− 0.012 0.029 0.046 0.065 0.070 0.059

1.44 1.45 1.53 1.52 1.55 1.61 1.19

4.04 4.07 1.13 1.06 0.62 0.73 0.64

0.44 0.48 0.47 0.41 0.27 0.13 0.06

reduced (see the inset of Figure 6). Finally, the 3d5/2 and 3d3/2 XPS components also show satellites at the lower binding energy end (represent by the asterisks in Figure 6) attributed to the well-screened 4f 3 final state.28 In order to estimate the relative contents of Pr and Sn on the nanoparticle’s surface the total areas of the Pr(3d) and Sn(3d) peaks were divided and corrected to the atomic sensitivity factor taken from the used software (see Table 2). As can be seen from data collected in Table 2 the [Pr]/[Sn] ratio shows a monotonic increase as the nominal Pr-content increases. Moreover, after the sputtering treatment on the 10.0 mol % Pr-doped sample we found a reduction of the [Pr]/[Sn] ratio of about 16%, which shows that the Pr-ions are segregated on the nanoparticle’s surface. This results is in agreement with reports found in the literature and it can explain the observed crystallite size reduction as the Pr-content increases (see Table 1).16 On the other hand, a [O]/[Sn] ratio of 1.44 has been determined for the undoped SnO2 nanoparticles, which is well below the expected value for the bulk system.2 Nevertheless, [O]/[Sn] ratio values below 2 have been already reported in the literature, which increases while exposing the sample to molecular oxygen O2.23,29,30 The low [O]/[Sn] ratio found in the undoped SnO2 nanoparticles confirms the nonstoichiometric character at the nanoparticle’s surface. For the SnO2:Pr nanoparticles the [O]/[Sn+Pr] ratio increase as the praseodymium content increases (see data in Table 2). It is worth mentioning that the oxygen concentration ([O]) is given by the sum of the structural oxygen (OStru), the chemisorbed oxygen (OChem), and the oxygen-related vacancies (OVac). Within this viewpoint, the increase of the [O]/[Sn + Pr] ratio shows that the Pr-doping takes place while removing oxygen-related vacancy and/or chemisorbed oxygen. Figure 7a and 7b show the O(1s) region of the XPS spectra for both the SnO2 and the 10.0 mol % SnO2:Pr nanoparticles. The spectrum for the 10.0 mol % SnO2:Pr sample submitted to the sputtering process is also shown. After a simple visual inspection one can observe that the O(1s) feature is wide and asymmetric, which can be deconvoluted into three well-defined Gaussian-like peaks, revealing the presence of three types of oxygen-related species. In agreement with the literature the first two components should correspond to the structural oxygen (OStru) with chemical states Sn2+ (SnO) and Sn4+ (SnO2) located at 529.8 and 530.5 eV, respectively.29 The origin of the third component located at around 531.4 eV is controversial and it has been assigned either to the chemisorbed oxygenrelated species, such as the hydroxyl group (OH−) or other radicals (CO, CO2) at the sample’s surface,31−33 or it can be associated with the presence of oxygen vacancies (OVac).34,35 We believe that any of these sources cannot be excluded since the doping with Pr-ions substituting Sn4+-ions must develop OVac to compensate charge.

Figure 7. O (1s) XPS spectra of the (a) undoped SnO2 nanoparticles, (b) 10.0 mol % SnO2:Pr nanoparticles before sputtering, and (c) 10.0 mol % SnO2:Pr nanoparticles after sputtering. The black symbols are the experimental data whereas the red solid lines are the best fit considering three components (brown, blue, and green solid lines).

Additionally, the OChem is present in small amount as indicated by the presence of the XPS feature around 288 eV in the C(1s) region, which is commonly attributed to the COx species (see Figure 4) and therefore confirming the presence of this type of oxygen species in the O(1s) region. After the sputtering treatment the XPS spectrum of the 10.0 mol % SnO2:Pr sample revealed almost a complete reduction of the carbon signal (centered at ∼288 eV) while reducing the XPS peak located at around 531.4 eV (the third component to O(1s) peak) in about 46%. The remaining XPS signal (see Figure 7c) was attributed to the oxygen-related vacancies (OVac).34,35 This result can be interpreted as the elimination of the chemisorbed oxygen and/or oxygen-related vacancy. 4+ 2+ On the other hand, the [OSn ]/[OSn ] ratio decreases as the Pr-content increases as can observed in Table 2, thus supporting the assumption that the oxidation state of tin ions gradually changes from Sn4+ to Sn2+, in agreement with the results obtained from the Sn(3d5/2) binding energy. As 4+ 2+ observed in Table 2 the [OChem, OVac]/[OSn + OSn ] ratio shows a slight increase while going from the undoped to the 1.0 mol % Pr-doped SnO2 nanoparticle, decreasing afterward as the Pr-content increases up to 10.0 mol %. This behavior evidence that the surface segregation of the Pr-ions leads to the reduction of OChem and/or OVac.

4. CONCLUSIONS SnO2:Pr nanoparticles with rutile-type structure have been successfully synthesized by the polymeric precursor method. The estimated crystallite size monotonically decreases (below E


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10 nm) upon the increase of the Pr-doping. From the XRD analysis we determined that the Pr-ions enter the rutile structure mainly substituting tin ions. The XPS data analysis reveals the presence of mainly Pr3+-ions and a continuous change of the oxidation state of tin ions from the Sn4+ to the Sn2+as the Pr-content is increased. Evidence of a surface segregation of Pr-ions has been determined for the 10.0 mol % SnO2:Pr sample. The surface enrichment seems to become enhanced as the Pr-content is increased and leads to the reduction of the chemisorbed oxygen and/or oxygen-related vacancies at the nanoparticle’s surface.

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AUTHOR INFORMATION

Corresponding Author

*(F.H.A.) Telephone +55 31 30693210. E-mail: ff[email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Brazilian agencies CNPq, CAPES, and FAPEMIG.

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REFERENCES

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