Evidence of Cr3+ and Cr4+ Coexistence in Chromium-Doped SnO2

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Evidence of Cr and Cr Coexistence in Chromium-Doped SnO Nanoparticles: A Structural and Magnetic Study 2

Juan Aquino, Fermin H. Aragon, Jose A.H. Coaquira, Xavier Gratens, Valmir Antonio Chitta, Ismael Jose Gonzalez, Waldemar Augusto de Almeida Macedo, and Paulo C. Morais J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b06054 • Publication Date (Web): 08 Sep 2017 Downloaded from http://pubs.acs.org on September 12, 2017

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The Journal of Physical Chemistry

Evidence of Cr3+ and Cr4+ Coexistence in Chromium-Doped SnO2 Nanoparticles: A Structural and Magnetic Study

Juan C. R. Aquino1, Fermin H. Aragón1*, José A. H. Coaquira1, Xavier Gratens2, Valmir A. Chitta2, Ismael Gonzales3, Waldemar A. A. Macedo3 and Paulo C. Morais1,4

1

Núcleo de Física Aplicada, Instituto de Física, Universidade de Brasília, Brasília DF 70910900, Brazil. 2 3

Instituto de Física, Universidade de São Paulo, 05315-970, São Paulo, Brazil.

Centro de Desenvolvimento da Tecnologia Nuclear, CDTN, 31270-901 Belo Horizonte, MG, Brazil. 4

Anhui University, School of Chemistry and Chemical Engineering, Hefei 230601, China

ABSTRACT The present study reports the successful synthesis of Sn1-xCrxO2 nanoparticles with doping-content (x) ranging from 0 to 0.20. Samples were synthesized by a polymer precursor method using SnCl2·H2O and Cr(NO3)3·5H2O as metal ion suppliers. In all samples X-ray diffraction data show one single phase formation (rutile-type), with crystalline size (crystal strain) decreasing (increasing) monotonically while increasing the x-content, which are assigned to substitutional solution of Cr- and Sn-ions in the crystalline structure. In addition to a weak magnetic ordering observed in a few samples (x = 0.01, 0.02, 0.03, and 0.05) paramagnetism is the main magnetic contribution in all synthesized samples, which is due to the presence of two chromium ions (Cr3+ and Cr4+). X-ray photo-electron spectroscopy measurements confirm the coexistence of Cr3+- and Cr4+-ions in excellent agreement with the monotonic decrease of the [Cr3+]/[Cr4+] versus doping-content (x), assessed from the fitting of the susceptibility versus temperature data using the Curie-Weiss law.

*Correspondig author: [email protected]

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1. INTRODUCTION 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nanostructured semiconductor oxides, such as tin dioxide (SnO2) based nanoparticles and thin films, had been extensively studying aiming to open up new frontiers in technology as well as to act as source of inspiration to uncover new phenomena. In this regard tin dioxide (SnO2) nanoparticles, with rutiletype crystalline structure (space group P42/mnm), had been successfully using nowadays as gas sensor for volatile inorganic and organic compounds, as for example in detecting H2S leakage of biogas1. Additionally, in recent years, SnO2 NPs had been gaining great interest as a biosensor platform, with wide applications in public health and biological sciences, as for instance in the detection of riboflavin (vitamin V2) using Crdoped SnO2 NPs2 and for detecting 1-nonanal gas (present in the breath of lung cancer patients) using SnO2 nanosheet/nanoparticles3 . In all cases, the key aspect in gas sensor application is the non-stoichiometric composition of the nanosized tin dioxide (SnO2-γ), the oxygen deficiency increasing as the crystallite size reduces, improving the efficiency of the sensor due to the increase of the chemically-active surface area. In this respect, physical (e.g. ball milling)4-5 as well as chemical6 preparation techniques were using to produce SnO2-based nanoparticles (NPs). Furthermore, the doping process was used to modulate the nanomaterial’s size and to introduce new properties into the system, as for the case of doping with incomplete d- or f-shells elements, bringing in magnetic properties into the SnO2 NPs, such as room temperature ferromagnetism (RTFM). The RTFM observed in metal-doped oxide semiconductors is a very interesting phenomenon, owing to introduce a new degree of freedom to the semiconducting matrix7. Actually, several works were reporting RTFM for low metal-doping levels8, even for undoped oxide semiconducting materials as suggested by Sundaresan et al.9, who stated that RTFM may be an universal characteristic of nanosized oxide semiconductors once oxygen vacancies play a critical role in the activation of the ferromagnetism10. Moreover, the chosen sample preparation method of nanosized oxide semiconductors markedly influenced its end magnetic properties, as reported by Lavanya et al.2 on Cr-doped SnO2 NPs (Sn1-xCrxO2) synthesized by a microwave irradiation method with Cr-content ranging from x=0 to x=0.05, while revealing RTFM only for the x=0.03 sample. Differently, RTFM was reporting in a broad range of doping content in Cr-doped SnO2 NPs synthesized by the co-precipitation and chemical vapor deposition methods11-12. In the present study, we report on results regarding the Cr-doping effects on the structural and magnetic properties of Sn12 ACS Paragon Plus Environment

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NPs prepared by a polymeric precursor method with doping content in the nominal range of x=0 to

x=0.20. While reporting on a comprehensive characterization of the as-prepared Sn1-xCrxO2 NPs employing a pool of standard experimental techniques emphasis is given to the relative content of two chromium-ion species (Cr3+ and Cr4+) as the doping level increases and its influence on the structural, morphological and magnetic properties to the end nanomaterial.

2. SAMPLES DESCRIPTION AND EXPERIMENTAL DETAILS Undoped and Cr-doped Sn1-xCrxO2 NPs were synthesizing by a polymer precursor method (Pechine's method) using SnCl2·H2O and Cr(NO3)3·5H2O as starting chemical reagents. Details of the synthesis process was reporting elsewhere13. The nominal Cr-content (x) of the samples was calculating based on the proportions of the starting material and assuming substitution of tin by chromium in the rutile type lattice of SnO2. We reported on samples prepared with nominal x = 0, 0.01, 0.02, 0.03, 0.05, 0.07, 0.10 and 0.20 whereas the analyses of the actual chemical composition had been carrying out by energydispersive x-ray spectroscopy (EDS) measurements. The obtained contents, x(EDS), were listing in Table 1, from which a good agreement (within the experimental error) between nominal x and x(EDS) values was observing. The results collected in Table I indicated that the values of x can be used in the data analyses as the actual Cr-content. The crystal structure of all specimens had been characterizing by X-ray powder diffraction (XRD) technique using a commercial Rigaku diffractometer equipped with Cu Kα radiation source. The XRD patterns were further analyzing by the Rietveld refinement method using the General Structure Analyses System (GSAS) software package with the graphical user EXPGUI interface14. The profile function used in the GSAS software to model the diffraction pattern was the Thompson-CoxHastings pseudo-Voigt function. Correction of the full width at half maximum (Гcorr) from instrumental contribution was including in the analysis as described in14 . The crystalline quality, lattice parameters, mean particle size and mean residual strain were determining. Additionally, NPs’ morphology, mean particle size and particle size dispersity had been assessing by transmission electron microscopy (TEM). X-ray photoelectron spectroscopy (XPS) analyses were performing using a SPECS surface analysis system 3 ACS Paragon Plus Environment


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equipped with the Phoibos 150 electron analyzer equipped with monochromatized Al Kα radiation (1486.6 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

eV) source operating at 350 W of output power. The C(1s) signal (284.6 eV) was employing as reference for the calibration of the binding energies of different elements in order to correct for the charge effect. The magnetic properties of the Cr-doped Sn1-xCrxO2 nanoparticles were investigating using a Cryogenics S600 superconducting quantum interference device (SQUID). The magnetization (M) was recorded as a function of the magnetic field (H) for up to 6.5 T, at temperatures T = 300 K and T = 1.7 K. The DC magnetic susceptibility of the paramagnetic phase (χ) as a function of the temperature (T) was measuring from 1.7 K to 300 K. For samples with only paramagnetic contribution, χ was determining by measuring the magnetization at low magnetic fields while using χ = M/H. For samples with both ferromagnetic and paramagnetic contributions, the paramagnetic component of χ was assessing by measuring M for a set of values of H (H > 10 kOe), for each temperature. Then, χ was determining from the linear fit of the M versus H data.

3. RESULTS AND DISCUSSION Figure 1 shows the XRD patterns of the as-produced Sn1-xCrxO2 NPs with x varying from 0 to 0.20. The XRD data analyses indicate the formation of the rutile-type phase for all samples and no evidence of additional crystalline or amorphous phase is observing. This finding shows that the solubility limit of the synthesis method is above x = 0.20 though the XRD peak linewidth (Г) increases with increasing x. This observation is attributing to the particle size reduction and/or to changes in the lattice strain extent. The mean crystalline particle size (XRD) and the mean residual strain contribution () were obtaining after performing the Rietveld refinements of the XRD patterns and using the modified Williamson-Hall relation for the pseudo-Voigt broadening given by15:

= 〈 〉

+

〈〉

, (1)

where K = 0.94 was using for spherically-shaped NPs and λ was the X-ray wavelength (1.5418 Å for Cu Kα) employed. Figure 2a shows typical Rietveld refinements of the undoped (x = 0.00), x = 0.10 and 0.20 Cr4 ACS Paragon Plus Environment

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doped Sn1-xCrxO2 NPs. XRD and values were obtaining from linear regression of versus 16!"# ⁄ plots, as represented in Figure 2b. The structural parameters obtained from this fit were listed in Table 1. The lattice parameters a and c determined from the Rietveld refinements were listing in Table 1. The obtained values of the lattice parameters were used to calculate the volume (V) of the unit cell as a function of the total Cr-content (x), as displayed in Figure 2c. We observed a linear decrease of V with increasing x, from 0 to x = 0.10. The reduction of V with increasing x is expecting once the ionic radius of Cr3+ (0.615 Å) or Cr4+ (0.55 Å) is smaller than the ionic radius of Sn4+ (0.69 Å)16. The linear variation of V versus x (up to x = 0.10) suggests a substitutional solution of tin- and chromium-ions in the rutile type structure as the main doping regime. Above x = 0.10, the occupation of interstitial sites and chromium enrichment at the NP’s surface seem to be determinant in the doping process. Furthermore, as shown in Figure 2d and 2e, the mean particle size decreases while the mean residual strain increases with increasing total Cr-content (x). The mean particle size reduction with increasing Cr-content had been attributing to the excess of dopant ions concentrated at the NP’s surface, leading to surface energy reduction17-19. Relevant results of TEM analyses for the Sn1-xCrxO2 NPs with x = 0.10 and x = 0.03 were presenting. For x = 0.10, the TEM image and corresponding particle size histogram are displayed in Figure. 3a and 3b, respectively. The histogram mounted using the Sturges’ protocol was well modeled by a log-normal distribution function (solid red line in Figure 3b). The mean particle size (TEM) was estimating by using: TEM=exp(σ2/2), where the mean value = 8.6 ± 0.1 nm and the polydispersity parameter σ = 0.14 ± 0.01 were using to obtain TEM = 8.7 ± 1.0 nm (x = 0.10), which is larger than the XRD = 5.6 nm value. The difference in the mean particle sizes, as assessed from XRD and TEM data, is attributing to the existence of an amorphous surface layer not probed by XRD measurements. HRTEM image obtained from the x = 0.03 Sn1-xCrxO2 NPs (Figure 3c) clearly shows the (110) planes of the rutile-type structure. The interplanar (110) spacing was estimating to be d ~ 3.3 Å. The selected area electron diffraction (SAED) pattern recorded for the x = 0.03 sample is shown in Figure 3d, from which the (110), (101) and (211) diffraction planes of the rutile type SnO2 structure were also determining. The oxidizing state of chromium-ions in the Sn1-xCrxO2 NPs was probing by high resolution X-ray photo-electron spectroscopy (XPS) measurements performed in the x = 0.03 and 0.07 NPs’ samples. Figures 5 ACS Paragon Plus Environment


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4a and 4b show the XPS spectra of Cr 2p1/2 and Cr 2p3/2 for the x = 0.03 and 0.07 Sn1-xCrxO2 NPs’ samples 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

around the Cr 2p core region. The XPS spectra of the x = 0.03 and 0.07 NPs’ samples were well fitting using two peaks centered at 585.9 eV and 576.2 eV ascribed respectively to Cr 2p1/2 and Cr 2p3/2 of Cr4+ and two peaks centered at 586.7 eV and 577.0 eV respectively due to Cr 2p1/2 and Cr 2p3/2 of Cr3+. The found positions of the Cr 2p peaks (Cr3+ and Cr4+) are in agreement with reports in the literature20-21, confirming the existence of two valence states of chromium-ions (Cr4+ and Cr3+) in the studied Sn1-xCrxO2 NPs. The ratio of Cr4+/Cr3+ contents was determining from the ratio of the peaks’ areas associated with Cr4+ and Cr3+, namely ~0.91 and ~1.89 for x = 0.03 and 0.07, respectively. These findings evidence the increase of the Cr4+ amount as the total Cr-doping increases, confirming the magnetic measurement results presented and discussed below. Furthermore, after a treatment with argon ion sputtering carried out in the x = 0.07 Sn1xCrxO2

NPs’ sample, the peaks associated with the Cr4+ were almost entirely suppressed, remaining only the

peaks associated to Cr3+, as shown in Figure 4c. This result strongly suggests that the Cr4+ is locating preferentially at NPs’ surface. On the other hand, taken x(Cr3+) as the actual Cr3+-content within the sample, the values found for the ratio x(Cr3+)/x in the x = 0.03 and 0.07 NPs’ samples were ~0.525 and ~0.347, respectively. These values were also plotting in Figure 7 (star symbols), showing excellent agreement with the values obtained from the magnetic measurements (solid dot symbols) presented and discussed below. Figure 5 shows the magnetization (M) as a function of the magnetic field (H) recorded at T = 300 K for all studied Sn1-xCrxO2 NPs’ samples. The undoped Sn1-xCrxO2 NPs were finding to be diamagnetic with susceptibility χd = -2.79×10-7 emu/g×Oe. For the Cr-doped Sn1-xCrxO2 NPs, the main component of the magnetization was paramagnetic (PM) and proportional to the total Cr-content, indicating that the observed PM component is associated to chromium-ions (Cr4+ and Cr3+), in agreement with the XPS results. In addition, for NPs’ samples with x = 0.01, 0.02, 0.03 and 0.05, a small FM contribution was detected, as shown in the inset of Figure 5. Both PM and FM contributions are intrinsic once the XDR measurements described above did not detect any Cr-related secondary phases, metallic tin- or chromium-clusters, or antiferromagnetic Cr2O3 and Cr3O4 clusters. In a previous work22, a study using Mössbauer spectroscopy has been done for Cr-doped SnO2 nanoparticles. In that work any evidence of magnetic splitting were determined. The absence of magnetic splitting, even for the lower Cr-content samples, is consistent with the 6 ACS Paragon Plus Environment

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low hyperfine magnetic field transferred to the 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

119

Sn probe nuclei from Cr ions showing any magnetic

ordering. The inverse of the paramagnetic susceptibility (1/χ) as a function of the temperature (T) for all Crdoped Sn1-xCrxO2 NPs was displaying in Figure. 6, with the plotted data correcting for the SnO2 diamagnetic component. For all Sn1-xCrxO2 NPs’ samples, the 1/χ exhibits a Curie-Weiss like behavior for T > 50-100 K. No cusp indicating any type of phase transition was observing in the entire temperature range. In the high temperature range (100 K - 300 K) the χ versus T data were fitting to the Curie-Weiss law, namely χ = C/(TθCW), from which the magnetic parameters C and θCW were extracting and listing in Table 2. However, the values found for θCW are negative, which indicates antiferromagnetic exchange coupling between chromium-ions. The actual chromium-ion contents (x(Cr3+)) and Cr4+ (x(Cr4+)) were determining from the values of the Curie constants (C), assuming the presence of a single chromium valence. For the Cr3+ and Cr4+ we had used g = 1.975 and S = 3/2 and g = 1.975 and S = 1, respectively 23. Our findings show that the values of x(Cr3+) underestimate the total Cr-content (x) in all samples, whereas the values of x(Cr4+) overestimate x (except for x = 0.20). These results indicate the existence of both valence states Cr4+ and Cr3+ in all Cr-doped samples. Actually, the contents x(Cr3+) and x(Cr4+) had been determining by using: x = x(Cr4+) + x(Cr3+)

(2)

and C = C(Cr4+) + C(Cr3+) . (3) Figure 7 shows the determined ratio x(Cr3+)/x as a function of the total Cr-content (x). For the x = 0.20 Sn1xCrxO2

NPs’ sample the Curie constant gives x(Cr4+) = 0.185 ± 0.005 with x(Cr3+) = 0, which is a very

reasonable result for the chromium-ions content in this sample. We observed from Figure 7 that for low xvalues (x = 0.01) the most populated species is Cr3+ (x(Cr3+) ~ 0.60) whereas for increasing x the x(Cr3+) value decreased while the x(Cr4+) value increased. This tendency was reinforcing by the x(Cr3+)/x ratio determined from the XPS measurements. Furthermore, the inset of Figure 7 shows θCW as a function of x. While assuming only one exchange constant, the x dependence of θCW should be linear. However, our 7 ACS Paragon Plus Environment


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findings presented in Figure 7 suggest the existence of different exchange constants for the Cr3+-Cr3+, Cr4+1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Cr4+ and Cr3+-Cr4+ exchange couplings. The θCW versus x shown in Figure 7 had been tentatively adjusting assuming only two exchange couplings (Cr3+-Cr3+ and Cr4+-Cr4+) using: 1 1 %&'()%*+, -./0 , (4) 2()%*3, -./

4 4 where %& and %&# are the reduced Curie-Weiss temperatures related to the two exchange constants for 4 Cr3+-Cr3+ and Cr4+-Cr4+, respectively. %&# was estimating using results of samples without Cr3+ (x = 0.20).

The fitting was performing with the contents of Cr3+ and Cr4+ determined before. In the whole range of x a 4 4 = -370 K and %&# = very good matching was obtaining (see solid red-line in the inset of Figure 7) for %&

-52 K. Worth mentioning that the exchange interaction constant (Ji) is related to the reduced Curie-Weiss temperature by 24. 61

./7 , (5) 5 = #89:92;

4 where Z is the coordination number of the neighboring pairs. According to Equation 5 the difference in %&

values is explaining by different exchange constants and/or coordination numbers of the coupling neighbors. For a rutile lattice, nearest neighboring pairs (J1) have Z = 2, next nearest-neighboring pairs (J2) have Z = 8 and third nearest-neighboring pairs (J3) have Z = 2

24

. Therefore, assuming J = J1 = J2 = J3 (Z = 12) the

calculation gives the following large exchange constants: J = -12 K and J = -3 K for Cr3+ and Cr4+, respectively. Below 100 K and for all Cr-doped Sn1-xCrxO2 NPs’ samples, the 1/χ versus T curve departed from the Curie-Weiss law in the form of a downturn. This feature is explaining by the existence of exchange interaction between magnetic ions in diluted magnetic semiconductor systems. Figure 8 shows the M versus H traces measured at T = 1.7 K for all Cr-doped Sn1-xCrxO2 NPs’ samples. The magnetization curves were well describing by the modified Brillouin function for total Cr-content up to x = 0.07. For higher total Crcontent, i.e. x = 0.10 and x = 0.20, we observed a small departure of the data from the Brillouin function. For all Cr-doped Sn1-xCrxO2 NPs’ samples the magnetization at 65 kOe (M65kOe) was closing to an apparent 8 ACS Paragon Plus Environment

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saturation which did not correspond to the expected full saturation. The values of M65kOe were finding to be 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

smaller than the calculated values for full saturation (M0). The difference between M65kOe and M0 increased with increasing x and reached about 50% of M0 for x = 0.20. The observed feature is due to the technical saturation phenomena and is consistent with the values of the exchange constants quoted above 25. As previously described (see Figure 5), magnetization data shows the coexistence of FM and PM phases for few samples. As for the FM contribution at room temperature, the saturation magnetization values MS(FM) were determining from a linear fit of the M versus H trace (with H > 10 kOe) and were listing in Table 2. The MS(FM) values were also displaying in Figure 9a as function of x. MS(FM) is found to be nearly temperature independent in the range 4 K - 300 K. For x = 0.03, the FM contribution has a Curie temperature of about 860 K, as shown in the inset of Figure 9a. The observed ferromagnetic order is intrinsic and is clearly dependent on the total Cr-content as shown in Figure 5. As shown in Figure 9a the values of MS(FM) regarding the FM contribution increased and then decreased with increasing x, while the maximum value of MS(FM) is obtained for x ~0.04. It is also observed that MS(FM) increased with increasing x(Cr3+) up to x(Cr3+) ~0.02 and then decreased with further increasing of x(Cr3+), as shown in Figure 9b. This finding regarding the Cr3+-content dependence of the FM contribution is similar to those obtained in previous work

26

. It was establishing that in undoped SnO2

nanostructures the observed FM is due to exchange interaction between localized electron spin moments resulting from oxygen vacancies at the nanostructures’ surface9,

27

. Nevertheless, for the as-produced

undoped SnO2 NPs no FM was detecting. On the other hand, given that Cr3+ substitutes for Sn4+ in the SnO2 lattice, the Cr-doping should generate oxygen vacancies. The double-charge of two Cr3+ added into the SnO2 lattice while doping has to be compensating in the form of one oxygen vacancy. Cr3+ located in a substitution site with oxygen vacancies in the near vicinity had been already detecting in bulk SnO2

24

.

Based on this result, the FM observed in the as-prepared samples are ascribing to Cr3+ while the associated oxygen vacancies are locating in the NPs’ core. A plausible explanation for the FM is given by the model proposed by Coey et al.28, suggesting that the long-range magnetic order is mediate by shallow donor



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electrons that form bound magnetic polarons, which can overlap to create a spin-split impurity band. This 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

model had been satisfactorily using to explain the FM in Gd- and Er-doped SnO2 NPs17, 29.

4. CONCLUSIONS Chromium-doped Sn1-xCrxO2 nanoparticles with tetragonal structure (rutile-type) have been successfully synthesizing using a variation of the Pechine’s polymer precursor method, which provides single-phased samples while using SnCl2·H2O and Cr(NO3)3·5H2O as the sources of metal ions. Our findings show that chromium-ions substitute tin-ions for nominal Cr-doping content (x) up to 0.20. We found monotonic reduction (increasing) of the nanoparticles’ size (strain) with increasing x-content. Paramagnetism ascribed to Cr3+- and Cr4+-ions dominates the magnetic behavior of all samples, although doping-dependent magnetic ordering emerges in samples with x = 0.01, 0.02, 0.03 and 0.05. X-ray photoelectron spectroscopy measurements confirm the presence of both Cr3+- and Cr4+-ions, although oxidation of the former into the latter is not yet explained. Evidence of a change in the doping regime, from bulk solubility (random distribution) at lower-doping to surface segregation at higher-doping, taking place around x = 0.03 may explain the presence of Cr3+-ions mainly in the nanoparticle’s core while promoting the formation of a nanoparticle surface rich in Cr4+-ions. Moreover, the oxidation of Cr3+-ions into Cr4+-ions is likely to be associated with oxygen vacancy enrichment of the nanoparticle’s core.

Supporting Information In order to exclude the existence of secondary phases we carried out room-temperature Raman spectroscopy measurements for all of the studied samples and no traces of secondary or amorphous phases (CrO2 or Cr2O3) were determined. Furthermore, to show the viability of the technique, Cr-doped SnO2 nanoparticulated samples with Cr content of 0.03, 0.10 and 0.20 Cr were subjected to thermal annealing in some temperatures in the range of 500-1050ºC. The results revealed the formation of a secondary chromium oxide phase only for samples with Cr content above 0.10 and annealing temperatures above 900 ºC, as seen


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it in the supplementary Figure S1. Meanwhile, XRD data analysis showed the presence of only the SnO2 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

phase.

ACKNOWLEDGMENTS: This work was financially supported by the Brazilian agencies CNPq, CAPES, FAP/DF, FAPEMIG, and FAPESP. The authors thank to Dr. S.W. da Silva for the Raman spectroscopy measurements and to LabMic (IF-UFG) for the high-resolutions TEM images.



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Page 15 of 23


Tables and table captions 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table 1. Structural and physical parameters assessed from Sn1-xCrxO2 nanoparticles. x is the total nominal content calculated based on the proportions of the starting material. x(EDS) is the core chemical content determined by energy dispersive x-ray spectrometry. The mean particle size , lattice parameters a and c and mean strain have been determined from Rietveld refinement analyses of the XRD patterns. S represents the quality of the refinement. x 0.000 0.010 0.020 0.030 0.050 0.070 0.100 0.200

x(EDS) 0.011±0.007 0.027±0.006 0.061±0.009 0.088±0.013 0.150 ±0.010

XRD ±1.0 (nm) 8.4 8.0 7.7 7.4 6.9 6.5 5.6 4.3

±0.01 (%) 0.25 0.27 0.31 0.28 0.33 0.33 0.36 0.41

a (Å) 4.736 4.733 4.731 4.732 4.725 4.727 4.722 4.721

c (Å) 3.188 3.185 3.183 3.185 3.181 3.181 3.180 3.179

S (%) 1.31 1.30 1.30 1.45 1.30 1.80 1.90 1.36

Table 2. Magnetic parameters assessed from Sn1-xCrxO2 nanoparticles. C is the Curie constant, θCW is the Curie-Weiss temperature determined from the susceptibility versus temperature data using the Curie-Weiss law. MS is the saturation magnetization value of the ferromagnetic contribution at room temperature. x 0.000 0.010 0.020 0.030 0.050 0.070 0.100 0.200 0.200

C (10-4 emu×K/g) 0 1.02 ± 0.15 1.86 ± 0.15 2.88 ± 0.10 4.60 ± 0.50 6.10 ± 0.30 7.70 ± 0.50 13.30 ± 0.80 12.90 ± 0.70

θCW (K) 0 -3.0 ± 1.0 -4.0 ± 1.5 -8.1 ± 2.0 -11.0 ± 3.0 -9.0 ± 3.0 -8.0 ± 3.0 -12.0 ± 2.0 -9.0 ± 3.0


MS (10-3 emu/g) 0 0.86 ± 0.04 1.65 ± 0.05 1.20 ± 0.20 1.97 ± 0.03 0.90 ± 0.07 0 0 0


Page 16 of 23

List of figure captions 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1. XRD patterns of Sn1-xCrxO2 nanoparticles with x content ranging from 0 to 0.20.

Figure 2. (a) Rietveld refinement of the undoped (x = 0.0), x = 0.10 and x = 0.20 Cr-doped Sn1-xCrxO2 nanoparticles. (b) Modified Williamson-Hall plot for Cr-doped Sn1-xCrxO2 nanoparticles according to Eq. (1). (c) Unit cell volume versus Cr-content. (d) Mean crystalline size versus total Cr-content. (e) Mean residual strain versus total Cr-content. The dashed lines are provided only as a guide to the eyes.

Figure 3. (a) Representative TEM image of the x = 0.10 Cr-doped Sn1-xCrxO2 nanoparticles. (b) Particle size histogram of the x = 0.10 Cr-doped Sn1-xCrxO2 nanoparticles. The solid red-line represented log-normal fitting. (c) High resolution TEM image and (d) selected area electron diffraction (SAED) pattern of the x = 0.03 Cr-doped Sn1-xCrxO2 nanoparticles. Figure 4. High resolution Cr 2p XPS core level spectrum of the Cr-doped Sn1-xCrxO2 nanoparticles with (a) x = 0.03, (b) x = 0.07 and (c) x = 0.07 after sputtering. Points represented the experimental data and solid redline is the best fitting using four peaks.

Figure 5. Magnetization as a function of magnetic field measured at T = 300 K for all Cr-doped Sn1-xCrxO2 nanoparticles. The inset shows an expanded view of the M-H curves at low magnetic fields for x = 0.01, 0.02, 0.03 and 0.05 Cr-doped Sn1-xCrxO2 nanoparticles. Figure 6. Inverse of the DC magnetic susceptibility (1/χ) of the paramagnetic phase as a function of temperature (T) for all Cr-doped Sn1-xCrxO2 nanoparticles. Solid lines represented fits of the 1/χ-T data to the Curie-Weiss law (see Table II for fitted parameters). Figure 7. Normalized Cr3+-content (x(Cr3+)/x) versus total Cr-content (x). The ion content was calculating based on x = x(Cr4+) + x(Cr3+). Stars symbols are values determined from XPS data. The dotted line is only a guide for the eyes. The inset shows the experimental values of the Curie-Weiss temperature. The solid redline was the fitting using the Curie-Weiss law as explained in the text.

Figure 8. Magnetization traces obtained at T = 1.7 K for all Cr-doped Sn1-xCrxO2 nanoparticles. Open circles represented the experimental data whereas solid lines were the fittings the M-H data using the modified Brillouin function.

Figure 9. (a) Saturation magnetization values of the FM contribution versus total Cr-content measured at T = 300 K. The inset shows the temperature dependence of the magnetization at high temperature for the x = 0.03 Cr-doped Sn1-xCrxO2 nanoparticles. (b) Cr3+-content versus total Cr-content.


Page 17 of 23


List of Figures

(321)

(202)

(301)

(310)

(022)

(220)

(211) (111)

(200)

(101)

(110)

Sn1-xCrxO2

20 % 10 % 7% 5% 3% 2% 1% 0%

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

20

30

40

50 60 2θ (degree) Fig. 1


70

80


71.5 x=0.20

(c)

2

Γ_corr Cos(θ) (x10 )

Sn1-xCrxO2

(a)

(b)

4

71.0 3

x=0.10

V(Å )

x= 0.20

x=0.07

3

70.5

x=0.03 x=0.00

70.0

2

x=0.10

40

Intensity

2

80

120

0.0

0.2

0.40

7

(%)

8

x=0.00

0.1

x

16 Sin (θ)/(Γ_corr Cos(θ))

DRX (nm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 23

(d)

6

0.35

(e)

0.30 5

20

30

40

50

60

2θ (degree)

70

80

0.0

0.1

x

0.2

Fig. 2


0.25 0.0

0.1

x

0.2

Page 19 of 23

Fig. 3

(c) x = 0.07 after sputtering Cr 2p1/2

(b) x = 0.07

Cr 2p3/2 3+

Cr

Sn1-xCrxO2

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4+

Cr

3+

Cr

(a) Sn1-xCrxO2 x = 0.03

3+

4+

Cr

591

588

585

582

579

Cr

576

573

Binding energy (eV)

Fig. 4


570


0.3

x = 0.20

0

0.10

-3

0.2

M (10 emu/g)

4

M (emu/g)

0.1

0.07

-4 -2

0

0.05 0.03 0.02 0.01

2

H (kOe) 0.0

x=0 -0.1

T = 300 K

-60

-40

-20

0

20

40

60

80

H (kOe) Fig. 5

x = 0.01

x = 0.02

12 x = 0.03

8 x= 0.05

-5

χ (10 g/emu)

x = 0.07

-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 23

4 x = 0.1 x = 0.2 0 0

50

100

150

200

T (K)

Fig. 6


250

300

Page 21 of 23

0 0.6

θ (K)

-5

-10

x (Cr )/x

0.4 3+

-15 0.00

0.05

0.10

0.15

0.20

x 0.2

Sn1-xCrxO2 0.0 0.00

0.05

0.10

0.15

0.20

x

Fig.7

7

T = 1.7 K

x = 0.20

6

0.10 5

M (emu/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.07 4

0.05 3

0.03 2

0.02

1

x = 0.01 0 0

10

20

30

40

50

H (kOe) Fig. 8


60

70

The Journal of Physical Chemistry 0.03

(b) 3+

x(Cr )

0.02

0.01

0.00 2.0

(a)

ln(M)

1.5

TC~860 K

-3

MS(FM.) (10 emu/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 23

1.0

x = 0.03 300 400 500 600 700 800 900

0.5

T (K)

0.0 0.00

0.05

0.10

0.15

x

Fig. 9


0.20

Page 23 of 23

0.6

Cr 2p3/2

x = 0.03 Cr 2p1/2

From XPS

x (Cr )/x

Intensity

0.4

3+

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.2

4+

3+

Cr

Cr

580

575

x = 0.07

−1

0.0

From χ -T

0.00 0.05 0.10 0.15 0.20

x

590

585

570

Binding energy (eV)

TOC Graphic


Evidence of Cr3+ and Cr4+ Coexistence in Chromium-Doped SnO2

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