Article pubs.acs.org/JPCC
Thermodynamic Stability of SnO2 Nanoparticles: The Role of Interface Energies and Dopants Chi-Hsiu Chang,† Mingming Gong,‡ Sanchita Dey,† Feng Liu,‡ and Ricardo H. R. Castro*,† †
Department of Chemical Engineering and Materials Science & NEAT ORU, University of CaliforniaDavis, Davis, California 95616, United States ‡ State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an, Shaanxi 710072, P.R. China ABSTRACT: The stability of nanoparticles is strongly dependent on the thermodynamics of interfaces. Providing reliable data on surface and grain boundary energies is therefore of key importance for predicting and improving nanostability. In this work, we used a combination of high-temperature oxide melt drop solution calorimetry and water adsorption microcalorimetry to demonstrate the effect of a dopant (manganese) on both surface and grain boundary energies of SnO2, and discussed the impacts on the average particle size at a given temperature. The results show a significant decrease in the grain boundary energy with increasing manganese content and a concomitant moderate decrease in the surface energy, consistently with segregation enthalpy values acquired from an analytical fitting model. The results explain the measured increase in stability with increasing dopant content (smaller sizes) and suggest the grain boundary energy has a much more important role in defining particle stability than previously supposed.
1. INTRODUCTION The thermodynamic instability of nanoparticles can be simply attributed to their total surface free energy, which represents the driving force for coarsening, that is, the energy difference between the bulk (coarsened) and the nanosized phases. However, in a more complete energetic description of the system, because nanoparticles are rarely found isolated from each other as a result of partial coarsening/sintering (spontaneously occurring in synthesis or forced during processing), the energy associated with solid−solid interfaces (i.e., grain boundaries) should also be taken into account. Introducing this term enables not only a better estimation of the real stability of nanoparticles, but also enables a true prediction and understanding of the microstructural evolutions occurring in nanoparticles. Recently, significant effort has been placed in designing experiments capable of assessing both surface and grain boundary energies to provide this complete thermodynamic analysis of nanomaterials. The challenge is that these are very small thermodynamic quantities which require highly sensitive instrumentation for reliable assessment. Castro and Quach,1 and later Drazin and Castro,2 have proposed the usage of microcalorimetry of water adsorption to probe the surface energy of oxide nanoparticles in both hydrated and anhydrous states. This method has been proven effective for oxides1−5 and is based on a thermodynamic correlation between the heat of water adsorption on the surface and the surface energy itself. Values of grain boundary energies can also be attained using high precision calorimetric techniques. In this case, a combination of high temperature oxide melt solution © XXXX American Chemical Society
calorimetry and water adsorption microcalorimetry has been proven successful in different systems.3 In summary, the method is based on the fact that the heat of dissolution of nanoparticles includes contributions from both interfaces and the bulk of the particles. By appropriate thermochemical cycle design, one can effectively separate the energetic contribution from each, enabling accurate determination of grain boundary energies. Being able to measure interfacial energies enables testing previously proposed hypotheses on the role of dopants on the thermodynamic stability of nanocrystals.6 That is, when an ionic dopant is in added to an oxide system, the ion can go into solid solution and, depending on the bulk solubility limit, it can eventually segregate to interfaces. According to the Gibbs adsorption isotherm, a segregated compound at the surface should decrease the surface energy proportionally to the segregated amount.7−10 While this is well-known for liquid phases, it has been recently demonstrated to be true for oxide systems. 4,11,12 For instance, when doping CeO 2 with manganese,4 manganese ions form a surface excess and reduce the total surface energy. This leads to more stable nanoparticles as the total interfacial energy is decreased (driving force for growth is reduced). In a system with both surfaces and grain boundaries though, one may expect segregation to happen at both interfaces. In principle, the excess on each interface will depend on the Received: December 29, 2014 Revised: February 27, 2015
A
DOI: 10.1021/jp512969k J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
and ethylene glycol (Sigma-Aldrich, 99.8%) aqueous solution kept stirring at 90 °C; the molar ratio of the global metal cations, citric acid, and ethylene glycol was 1:4:16. To totally dissolve the solutes, 5 M nitric acid (Sigma-Aldrich, 70%) was slowly dosed until a transparent solution was obtained. The resulting solution was heated up to 140 °C for 30 min to achieve full polyesterification. The dark brown resin was heated up to 450 °C for 4 h for combustion of most polymeric chains and promotion of oxide nucleation. The resulted carbon-rich powder was ground on an agate mortar and then annealed at 500 °C for 15 h. The long-term annealing process helps to remove the residual carbon compound and to enhance particle nucleation and growth. As-synthesized powders were examined to define crystallographic phases, lattice parameters, and crystallite sizes by X-ray diffraction (XRD) analysis using an instrument from Bruker AXS Inc., Madison, WI (model D8 Advance; Cu Kα, radiation λ = 1.5418 Å). The operating parameters were 40 kV and 40 mA with 2 s/0.02° dwell time/step size, and the data were collected between 20° to 70° for 2θ. All SnO2 based patterns were fit by JDPDF# 41-1445. The lattice parameters were analyzed by JADE 6 software (Materials Data Inc.) and calibrated by mixing samples with lanthanum hexaboride (LaB6; JDPDF# 34-0427; lattice parameters at 22.5 °C are a = b = c = 0.41569162 ± 0.97 × 10−6 nm). Lattice parameters were additionally utilized to calculate theoretical densities (ρT) and lattice volume. The crystallite sizes were assessed by JADE 6 software based on full width at half maximum (FWHM) method, and reconfirmed by measuring grain diameters from transmission electron microscopy (TEM) images (JEOL JEM 2100F/Cs, 200 kV). At least 150 grains were counted by ImageJ software for each sample. To identify the composition of each Mn-doped SnO2 sample, a Cameca SX-100 electron microprobe was used. The operating parameters were 15 kV for an accelerating voltage, 10 nA for a beam current, and 4 μm for a beam size. Metallic Mn and Sn were utilized as standards. Backscattered electron images and X-ray dot maps were used to ensure no second phase presence. At least 10 randomly selected points of each sample were tested and used to calculate the average composition. To avoid the possible carbonate residual from this polymeric precursor method, Fourier transform infrared (FT-IR) spectroscopy (Magna 560 Nicolet instrument) was used to detect any contamination. No residual carbon was found on assynthesized nanoparticles. Drop Solution Calorimetry. The heat of dissolution of SnO2 nanoparticles doped with various Mn concentrations in lead borate was measured using a custom-built Calvet-type twin microcalorimeter.24,25 Each sample was lightly pressed as a 5 mg pellet and dropped into a 20 g of solvent kept at 800 °C in a platinum crucible. Details of this experimental setup can be found elsewhere.26 At least eight tests were performed in order to deliver a statistically significant drop solution enthalpy (ΔHDS). Because in nanosamples the large amount of surface area absorbs a significant amount of water, for consistency, all samples were placed in a room at 50% in humidity and 24 °C for a week to equilibrate water content. The water content (y) was assessed by thermogravimetric analysis (TGA; Netzsch STA 449 system, Netzsch GmbH, Selb, Germany) with 450 °C dwelling 5 h heating profile. The water content can easily convert to coverage (θ) if surface area is available. The coarsened samples were prepared from nanopowder calcination at 1100 °C for at least 12 h to obtain negligible interface area,
respective segregation enthalpy, which is related to the total energy decrease caused by the segregate. Hence, a “competition” for the dopant between the interfaces shall exist, which can ultimately affect the overall microstructure of the system. In this paper, we evaluate the relationship between dopant segregation, interfacial energy, and interfacial area (grain sizes) in tin dioxide nanoparticles doped with manganese to provide a more fundamental understanding of the nanostability dependences on interfacial energetics and composition. SnO2 is a widely utilized oxide in catalysts, gas sensors, and other technologies13−15 and has been doped by various dopants for engineering design purposes.6,16,17 When reporting on tin dioxide nanoparticles, many works suggest segregation of dopants to the interfaces as a result of low bulk solubility limit of SnO 2 , indicating possible interfacial energetic changes.11,18−20 For example, Gouvêa et al. observed that Mg segregating on SnO2 surfaces, which led to particle size decrease accompanying surface area to volume ratio increase; the change with developing excess energy suggested the surface energy has to reduce to avoid overall energy upsurge.11 In previous work by Chang et al., a reversed sintering behavior, called desintering, occurred while Mn dopants found segregated to surfaces at high temperatures redissolved into SnO2 bulk at lower ones.18 The fact evidently demonstrated that Mn distribution significantly linked microstructural evolutions to surface and grain boundary energies rather than kinetics. It is difficult to separate kinetic effects from thermodynamic when discussing the role of dopants in a particular system though because either in solid solution or segregated at interfaces dopants should affect both. The role of thermodynamics is typically harder to predict and, in many cases in the literature, it is considered less important than kinetics without satisfactory explanations.16,17 The goal of the present work is therefore to quantify the effects of manganese as a dopant on the thermodynamics of SnO2 nanoparticles, and identify direct dependences of microstructural evolution and interfacial energies independent of kinetic parameters. Manganese was selected as a testing dopant because it has been reported that it enables stabilization of SnO2 nanoparticles, but the nature of the stabilization is still unclear.17 The experiments were designed such that SnO 2 nanoparticles with different concentrations of manganese were synthesized and annealed at the same temperature. The crystallite’ sizes as well as the amounts of interfaces (both surfaces and grain boundaries) were quantified and correlated with the interfacial energies measured by calorimetry to establish the thermodynamic dependence. It was observed that, even at low temperatures, the growth of nanoparticles (crystallite size) is limited by the presence of both surfaces and grain boundaries, and not surfaces alone. Moreover, the interface with lower energy will preferentially be formed during annealing, and will be the one governing the overall crystal size and state of agglomeration.
2. EXPERIMENTAL PROCEDURES Synthesis and Characterization. Mn-doped SnO2 nanoparticles were synthesized by a polymeric precursor method.21,22 Tin(II) citrate was one of metal cation precursors and was prepared from citric acid (Sigma-Aldrich, 99%) and tin(II) chloride (Alfa Aesar, 99%) in molar ratio 1:2.23 The other metal cation precursor was manganese(II) carbonate (Alfa Aesar, 99.9%) which is also the source of dopants. Two precursors with appropriate ratio were added into well mixing citric acid B
DOI: 10.1021/jp512969k J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Table 1. Characteristics of Composition, Grain Size, Interface Area, Coverage, and Calorimetric Information Acquired from the Different Mn-Doped SnO2 Samplesa composition characteristics sample S0 S1 S2 S4 S8
xT (mol %)
θDS
sample S0 S1 S2 S4 S8
7.78 7.07 9.42 15.63 9.68
ρT (g/cm )
Mw (g/mol)
0 150.71 1.62 ± 0.38 149.68 ± 2.57 ± 0.35 149.07 ± 4.07 ± 0.42 148.11 ± 8.53 ± 0.44 145.27 ± coverage (H2O/nm2) θWA 10.13 9.39 9.94 8.23 9.21
0.24 0.22 0.27 0.28
7.02 7.00 6.97 6.94 6.86
± ± ± ±
0.01 0.01 0.01 0.01
ΔHhydrous,nano DS 460.55 460.45 473.87 518.13 501.00
interface area (m2/g)
grain size (nm) 3
± ± ± ± ±
2.26 1.84 2.45 2.80 3.08
GSXRD
GSTEM
ΔHanhydrous,nano DS 401.74 395.48 395.97 391.40 377.36
± ± ± ± ±
19.43 17.44 16.97 16.57 10.93 (J/g)
ΔHcoarsened DS
1.96 1.58 2.05 2.11 2.32
462.41 464.63 466.24 466.80 478.81
AI
GSBET
10.9 ± 0.12 10.21 ± 2.05 9.3 ± 0.07 9.58 ± 2.17 8.4 ± 0.03 8.78 ± 1.20 6.8 ± 0.05 6.43 ± 2.09 6.1 ± 0.03 6.09 ± 1.96 enthalpy of thermochemical cycle ± ± ± ± ±
2.29 2.18 3.02 2.06 2.15
± ± ± ± ±
0.15 0.05 0.03 0.11 0.06
78.37 92.20 102.43 127.08 143.39
± ± ± ± ±
AS 0.14 0.71 0.39 0.95 1.43
ΔHexs
γS
± ± ± ± ±
± ± ± ± ±
60.67 69.14 70.28 75.39 101.45
3.01 2.69 3.65 2.95 3.16
AGB
44.00 ± 0.33 49.16 ± 0.12 50.72 ± 0.07 52.17 ± 0.35 79.99 ± 0.39 interface energy 1.20 1.20 1.19 1.17 1.12
17.18 ± 0.18 21.51 ± 0.35 25.86 ± 0.20 37.46 ± 0.50 31.7 ± 0.73 (J/m2) γGB
0.02 0.02 0.02 0.02 0.02
3
0.70 0.48 0.39 0.38 0.37
± ± ± ± ±
0.083 0.13 0.14 0.08 0.11
a The subscript with θ presents the different purpose for energetics calculation. θDS was used to correct the heat of water desorption in the thermochemical cycle. θWA was the coverage where it reached waterlike state and used in eq 3.
synthesized Mn-doped SnO2 nanoparticles were measured by microprobe and are listed in Table 1. Based on them, the samples were denoted as S0, S1, S2, S4, and S8, as indicated in the table. Figure 1 shows XRD patterns for all samples
excess energy and water content. The compositions were consistent before and after coarsening which were also identified by microprobe. Surface areas (AS) of samples subjected to calorimetry were measured by Brunauer−Emmett−Teller (BET) method using a Micromeritics ASAP 2020 apparatus (Micromeritics, Norcross, GA). Grain boundary area (AGB) was then estimated depending on the correlation between total interface area (AI), surface area and grain size in diameter (GS) as follows: A GB =
AI =
AI − AS 2
6 ρT GS
(1)
(2)
Note that the estimation of AI is more precious if the grains are isolated and identical spheres in morphology.27 The constant 6 in eq 2 comes simply from geometrical analyses of the surface/ volume ratio of spherical particles. The heats of water desorption on the nanosamples were measured by the combination of a Micromeritics ASAP 2020 adsorption analyzer and a Setaram Sensys Evo (Setaram Instrumentation, Caluire, France). This combined instrument monitoring of heat differences and water coverage simultaneously was also applied on water adsorption microcalorimetry measurement, as described previously.1 Before BET and water adsorption microcalorimetry measurement, all tested samples were degassed under vacuum (
