and n-Doped SnO2 Nanocrystalline Films - American Chemical Society

Article pubs.acs.org/JPCC

Defect Chemistry of the Metal Cation Defects in the p- and n‑Doped SnO2 Nanocrystalline Films Guozhu Zhang, Changsheng Xie,* Shunping Zhang, Shasha Zhang, and Ya Xiong State Key Laboratory of Materials Processing and Die & Mould Technology, Nanomaterials and Smart Sensors Research Laboratory (NSSRL), Department of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, People’s Republic of China S Supporting Information *

ABSTRACT: Cationic interstitial and substitutional defects, which serve as a key role in shaping the material’s performance, are considered as two kinds of important defect structures in the doped SnO2. To give a clear characterization of such metal cation defects, temperature-dependent electrical conduction measurement by the high throughput screening platform of gas-sensing materials is carried out, for the first time, to perform the defect structure studies of the p-type (Li+, Cd2+, Al3+), isovalent (Ti4+), and n-type (Nb5+, W6+) doped SnO2 nanocrystalline films in the oxygen-free atmosphere. The temperature-dependent measurements indicate that subtle induced impurities are capable of evidently modifying the electrical conduction mechanism of the SnO2. In terms of the smallpolaron hopping mechanism, an improved defect chemical model is proposed in which the properties of the metal cation defects are explicitly depicted. Values for the ionization energy (ΔED) of the metal cation defects and electron hopping energy (EH) in the doped SnO2 are extracted by fitting the experimental data to the defect model. These data that reflect the nature of the metal cation defects and their effects on the electronic structure of the SnO2 are first introduced here, and the validity of these data are confirmed. What’s more, the ΔED calculated here is of critical importance for understanding the defect structure of the metal dopants in the SnO2.

1. INTRODUCTION Research in tin oxide (SnO2) is gaining dramatic interest among the wide-band-gap semiconductor community due to its outstanding electrical conduction and unique photoelectric properties.1,2 Furthermore, in order to tailor the base material, doping of SnO2 with metal ions, which, in turn, enhances the device performance, has become a convenient and effective way in the material science.3,4 This makes them excellent candidates for large-scale applications in gas sensors,5−8 solar cells,9,10 lithium-ion batteries,11,12 and UV photodetectors.13−15 To assist the design of SnO2-based nanomaterials for widespread usage, it would be helpful to understand the role of the dopants in the SnO2 and, eventually, formulate design rules for the metal oxide semiconductors (MOSs) of this kind. It is known to all that SnO2 is a defect-rich MOS,1,16 and the doped metal ions often result in two kinds of point defects in the SnO2 lattice, i.e., the substitutional and interstitial point defects,17−20 as shown in Figure 1. Incorporating such defects may destroy the original electronic structure and lead to the rearrangement of the internal defect equilibrium.21,22 For the ptype doped SnO2, the metal ions substituting the Sn4+ are always accompanied by the introducing of excess holes, mainly as oxygen vacancies,6,23−25 whereas the n-type doped SnO2 are in favor of creating more electrons.26−29 Similarly to the n-type doping, the interstitial defects are generally characterized as donors and the inducing of such defects often results in the © 2014 American Chemical Society

Figure 1. Two possible extrinsic point defects (substitutional and interstitial) formed in the lattice of rutile SnO2.

creation of an excess of electrons in the SnO2.6,19,20 Because of the high activity as the oxygen vacancy and the great contribution of the electrons, both of them are believed to be responsible for the material’s performance.28,30,31 In SnO2based gas sensors, for instance, various aliovalent metal Received: March 27, 2014 Revised: June 30, 2014 Published: July 5, 2014 18097

dx.doi.org/10.1021/jp503059e | J. Phys. Chem. C 2014, 118, 18097−18109

The Journal of Physical Chemistry C

Article

Figure 2. Scheme of high throughput screening platform of gas-sensing materials (HTSP-GM) integrated with a temperature-programmed system.

impurities, such as Zn,5 Cd,6,32 and Al,33 were chosen to dope the SnO2 and suggested that, with the presence of the substitutional defects MSn (M = Zn, Cd, and Al), more oxygen vacancies were formed, which resulted in the sensing performance enhancement.5,6,32,33 In the case of the electronic and optoelectronic applications, to achieve high n-type conductivity devices, Sb34 and Ta35 were used as donor dopants to introduce in the SnO2 and emphasis was put on functions of the derived electrons or VSn ′′′′. Despite that great efforts are devoted to the point defect studies on the metal species doped SnO2, the recent studies have a major drawback of putting all the interests on the derivative defects (anionic defects) instead of the metal cation defects themselves. In the field of catalysis, it has been demonstrated that the doped metal species are vitally important in the SnO2-based catalysts and draw predominant attention.36−38 Take the Mo/SnO2 catalyst as an example, it was concluded that the formation of the Mo = O on the SnO2 surface was considered as the active sites to promote the oxidation of the ethanol.36 The defect structures of these metal dopants in the SnO2, however, are lacking considerations. In analogy to the oxygen vacancy, the metal cation defects often are characterized as donor or acceptor, which also can ionize out electrons and holes,21 and this endows them with the ability to serve as the activity center. Moreover, that the defect structure depends on the valence of the doping metal ions is not taken into account in previous studies. In this way, a deeper understanding of the metal cation defects in SnO2 is urgently needed. Based on the small-polaron hopping (SPH) conduction model,39,40 electrical conduction studies in terms of the defect chemistry might help us to address this issue. It is generally accepted that the introduced metal cation defects are able to form different defect levels in the band gap.41,42 Considering the defect ionization mechanism, thermal activation is a good pathway to give a clear characterization of these defects. Thus, studying the temperature-dependent electrical conductivity of the doped SnO2 in an oxygen-free atmosphere, for eliminating the influence of the ambient, can provide meaningful information about the defect ionization, defect equilibrium, and electron conduction mechanisms. Thereby, monitoring the temperature-dependent changes in conductivity of the doped SnO2, ideally real-time and rapid, has become of great importance. Fortunately, our research group recently proposed a method that enabled us to examine the temperature-dependent conductivity of the MOS films in real-

time by utilizing the high throughput screening platform of the gas-sensing materials (HTSP-GM) integrated with the temperature-programmed system.43 This platform, therefore, can be used to study the temperature-dependent conductivities of the different metal species doped SnO2 nanocrystalline films simultaneously, subsequently offering a clear insight into incorporating metal ion defects in the electron conduction process. In terms of the electrical conduction studies, we herein report an investigation of the defect chemistry in a series of metal ions doped SnO2 nanocrystalline films, p-type (Li+, Cd2+, Al3+), identical valence (Ti4+), and n-type (Nb5+, W6+). Considering the relations between the impurity defects and electrical transport properties, we will intensively present a clear depiction of the metal cation defect from formation and ionization to their influence on electrical conduction. In the following, an improved electrical conduction model that focuses on ionization of these defects is proposed here. By understanding the electrical conduction mechanism in such systems, we attempt to quantitatively acquire the ionization energy of the metal cation defects and the electron hopping energy in the doped SnO2, and this may be considered as a first step in such MOS studies.

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. Doped and undoped SnO2 nanomaterials were prepared by a hydrothermal method. The metal chlorides was chosen as doping sources in this work. All of these reagents (LiCl·H2O, CdCl2·2.5H2O, AlCl3·6H2O, TiCl4, NbCl5, and WCl6) were AR. Typically, 3.5 g (10 mmol) of SnCl4·5H2O was resolved in 100 mL of distilled water under ultrasonic, and then the transparent solution was hydrolyzed with diluted ammonia hydroxide solution under vigorous stirring. During the addition of ammonia hydroxide, the tin oxide was settled out as a white precipitate. Afterward, the fluffy precipitate was carefully washed with distilled water to remove the chloride ion. Finally, the tin oxide sol solution (pH = 10.5) was obtained by peptizing the tin oxide gel precipitate with diluted ammonia hydroxide solution. After ultrasonic dispersing for several minutes, the formed microemulsion was mixed with the solution of metal chloride (a dopant concentration was given as 1 mol %), and then they were transferred to an 80 mL Teflon-lined stainless steel autoclave, at 180 °C, for 3 h. In the end, a yellowish transparent xerogel was obtained after the viscous gel was dried at 80 °C for 24 h. 18098



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Figure 3. (a) XRD patterns of doped and undoped SnO2 after heat treatment at 600 °C, 2 h. Structural Rietveld analysis: (b) evolution of rutile lattice parameters and (c) evolution of cell volume of rutile.

temperature range was 300−700 K, and the heating rate β was set at 10 K/min.44

2.2. Film Preparation. As the nanopowders of the doped and undoped SnO2 had been prepared, pastes were obtained by mixing oxide powders with organic solvent in an agate mortar. Then, a screen printing technique was used for preparing a thick film on a 36-matrices flat-type material chip. Each film was deposited in a diameter of 4 mm and a thickness of 8 μm. The specific procedures of the paste making and material chip fabricating were according to our previous studies.43 Afterward, the SnO2-based 36-matrices material chip was transferred into the furnace and sintered at 600 °C for 2 h in air. Finally, the asprepared chip was aged at 350 °C in the platform for 12 h to enhance its stability. 2.3. Characterization. The deposited films were then characterized by performing structural measurements. X-ray diffraction (XRD) patterns of the doped and undoped SnO2 films were recorded on a Philips X’Pert diffractometer from 2θ = 10 to 80° using Cu−Kα1 radiation (λ = 1.5406 Å). X-ray photoelectron spectroscopy (XPS) measurements were carried out with a Kratos AXIS-ULTRA DLD-600W spectrometer employing Al−Kα radiation. The measurement spectra were further corrected based on adventitious C 1s at 285.0 eV using the manufacturer’s standard software. 2.4. Electrical Conductivity Testing. The temperaturedependent conductivity measurements were carried out under high-purity nitrogen (99.999%) in the high throughput screening platform of the gas-sensing materials (HTSP-GM), which has been reported in our previous work.43 The measurement platform consisted of five components, including the computer, the data control module, gas flow rate control module, temperature control module, and the test chamber, as shown in Figure 2. The temperature controller used here was an SDC35 controller (Azbil Co., Tokyo, Japan), of which the working temperature could be controlled in a range of room temperature to 500 °C with a sensitivity better than ±0.1 °C. As a temperature testing and feedback part, a Pt resistor was printed on the material chip. The testing resistance range of the platform was 10−108 Ω within an error of 5%. The performed

3. RESULTS AND DISCUSSION 3.1. Sample Analysis. XRD patterns of the doped and undoped SnO2 are shown in Figure 3a. It was found that the incorporation of the metal elements did not produce any significant change in the crystalline phase present. From Figure 3a, the as-synthesized nanomaterials were indexed to the rutile SnO2 (P42/mnm, JCPDS 77-0447). The structure of doped SnO2 was characterized by Rietveld analysis of the XRD diffraction patterns. From the analysis, the lattice parameters of the tetragonal cell were obtained. In Figure 3b,c, it can be observed that, upon incorporation of the metal elements, a clear distortion of the lattice was achieved, which indicated that the substitution or insertion of the elements may occur. The radius and the final doped concentration difference of the metal ions were ascribed to the difference of the distortion degree of the SnO2 lattice. Moreover, on the basis of the XRD results, the doped and undoped SnO2 nanopowders had an average diameter of about 10−15 nm, which indicated that the dopants have no influence on the morphology structure of the nanocrystalline SnO2. The XPS measurements have been performed at room temperature and are shown in Figure 4. On the basis of the global XPS spectra in Figure 4a, the high-resolution scans of the O 1s, Sn 3d, Li 1s, Cd 3d, Al 2p, Ti 2p, Nb 3d, and W 4f regions were conducted in Figure 4b,c. Binding energies (EB) of the component peaks obtained by peak fitting are given in Table 1. From the obtained data, it can be confirmed that the introduced metal elements are mainly in the state of Cd2+, Al3+, Ti4+, Nb5+, and W6+, except for lithium. The Gaussian deconvolution to the Li 1s spectra revealed that there were two components in the Li-doped SnO2 sample, as shown in Figure 4d. The Li 1s peak located at 63.0 eV was assigned to the Li+ and the peak at 59.3 eV was related to Li interstitial (Lii) that corresponded to the valence state of incomplete 18099



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Figure 4. XPS spectra of the (a) doped and undoped SnO2, (b) O 1s, (c) Sn 3d, (d) Li 1s, (e) Cd 3d, (f) Al 2p, (g) Ti 2p, (h) Nb 3d, and (i) W 4f.

Table 1. Binding Energies (EB) of the XPS Component Peaks Obtained by Peak Fitting for the Doped and Undoped SnO2 undoped species O Sn Li 1s Cd Al 2p Ti Nb W

1sA 1sB 3d3/2 3d5/2

Li-SnO2

Cd-SnO2

Al-SnO2

Ti-SnO2

Nb-SnO2

W-SnO2

EB (eV)

EB (eV)

EB (eV)

EB (eV)

EB (eV)

EB (eV)

EB (eV)

530.88 532.05 495.39 486.99

530.79 532.05 495.31 486.91 59.3, 60.0

530.69 532.05 495.28 486.78

530.86 532.05 495.38 487.03

530.87 532.05 495.48 487.08

530.91 532.05 495.50 487.00

530.93 532.05 495.50 487.05

3d3/2 3d5/2

412.4 405.6 74.5

2p1/2 2p3/2 3d3/2 3d5/2 4f5/2 4f7/2

465.3 459.4 210.3 207.6 38.0 35.8

oxidation.45 In addition, the two peaks were resolved in terms of the pristine O 1s peak, a major component peak (O 1sA) at EB = 530.88 eV, and a minor component peak (O 1sB) at EB = 530.05 eV. The former was attributed to the Sn−O bond in the SnO2, and the latter was ascribed to the surface loosely bound

oxygen species that appeared. A little shift of the O 1sA peak toward low binding energy was observed for the p-type doped SnO2 and an opposite shift appeared in the n-type doped SnO2, which indicated that the M−O (M = Li, Cd, Al, Ti, Nb, and W) bonds were formed in the SnO2. The shift of the Sn 3d peaks, 18100



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defect ionization stage, which directly corresponds to the point defect, was the emphasis of this work. Moreover, the defect ionization stage also can be fractionized into two regions: a conductivity slowly (I) and a rapidly (II) increase stage. Similarly, this change regulation emerged in the doped SnO2. To clearly reveal the nature of the incorporating defect in the doped SnO2, details from the defect formation and ionization to their influences on the electrical conduction were explored as follows. 3.2.1. Point Defects in the Doped SnO2. In recent work by the authors, the incorporating metal oxide (MnOm) often thermally results in two types of point defects in the SnO2 lattice, the substitutional (MSn) and interstitial (Mi) defect.18 In this way, if the valence (r) of the incorporating metal ions is lower than Sn4+ (r < 4), then acceptor-like defects are formed by the substitution of Sn4+ by metal elements in the SnO2. On the contrary, donor-like defects are formed in the higher valence (r > 4) metal ions doped SnO2. At the same time, donor-like interstitial defects (Mi) can be formed in the SnO2. The universal formation formulas of these point defects are described below in Table 2 in the Kröger−Vink notation. According to the universal formulas, the point defects in the ptype (Li, Cd, Al), isovalent (Ti), and n-type (Nb, W) doped SnO2 are shown in Table 3. 3.2.2. Electron Transport Properties. It is generally assumed that a thermally activated small-polaron hopping mechanism can be well used to depict the electron transport property in such defect-rich MOSs.39,40 On the basis of the previous studies of the transport properties of the undoped SnO2,44,51 the motion of the electrons in such a disorder system can be divided into three steps, which include the ionization of the impurities, the hopping of the electrons from the “well”, and the motion of the ionized electrons. The specific process in the undoped SnO2 is shown in Figure 6. Taking the SPH model into account for the tin oxide, the electron conduction proceeds in the nonadiabatic regime at high temperature (T > 300 K) due to T > θD/2 (θD is the Debye temperature (500 K for SnO2)). The temperature-dependent conductivity (σ) and mobility (μe) follow the expressions

seen in Figure 4c, also demonstrated the incorporation of the metal ions into the SnO2 lattice. 3.2. Electrical Conduction Study. Temperature-dependent electrical conductivity plots of the doped and undoped SnO2 nanocrystalline films are shown in Figure 5a. A

Figure 5. (a) Temperature-dependent conductivity of the doped and undoped SnO2 nanocrystalline films measured at 10 K/min. (b) The conductivity property of the undoped SnO2 in the measurement temperature range (300−700 K).

conductivity decrease emerged for the p-type doped SnO2 and an increase for the n-type doped SnO2, which indicated that acceptor and donor defects formed in the SnO2 lattice, respectively. For the isovalent doping, since the Ti4+ can be reduced into Ti3+,46,47 acceptor-like defects (TiSn ′ ) might stably form in the SnO2, which resulted in the conductivity decrease as compared with the undoped SnO2. These formed defects, whether they are in the bulk or in the surface, may play a determined role in the electrical properties of the SnO2.48 In addition, temperature-dependent conductivity showed a typical sigmoid behavior that was corresponding to the related study.49,50 Take the undoped SnO2 as an example, seen clearly in Figure 5b, the electrical behavior can be distributed to three temperature regions: (A) defect ionization, (B) electron−lattice scattering, and (C) intrinsic ionization (VB-CB) region. The

σ = eneμe

(1)

μe = (μ0 /T 3/2) exp( −EH /kT )

(2)

where ne is the electron concentration, μe is the electron mobility, EH is the charge hopping energy, and μ0 indicates the pre-exponential factor. 3.3. Undoped SnO2. As an n-type semiconductor, it is generally accepted that the electrical property in the undoped SnO2 is correlated with its nonstoichiometry. The oxygen vacancy (V×O) and interstitial tin (Sni) are the primary donor defects in the bulk. With regard to the oxygen vacancy defect, a shallow donor level for V•O/V×O (singly ionized oxygen vacancy

Table 2. Point Defects in the p- and n-Type Doped SnO2a doping type

a

substitution defect

p-type (r < 4)

(4 − 2m / n) ′ + mOO× + (2n − m)VO•• MnOm ⎯⎯⎯⎯→ nMSn

isovalent (r = 4)

× MO2 ⎯⎯⎯⎯→ MSn + 2OO×

n-type (r > 4)

(2m / n − 4) • MnOm ⎯⎯⎯⎯→ nMSn + mOO× + 2(m − 2n)e′

SnO2

interstitial defect SnO2

MnOm ⎯⎯⎯⎯→ nMi(2m / n) • + mOO× + 2me′ SnO2

SnO2

MO2 ⎯⎯⎯⎯→ Mi•••• + 2OO× + 4e′

SnO2

SnO2

•

MnOm ⎯⎯⎯⎯→ nMi(2m / n) + mOO× + 2me′

r is the valence of the doped metal ions. 18101



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Table 3. Point Defects in the p-Type (Li, Cd, Al), Isovalent (Ti), and n-Type (Nb, W) Doped SnO2 impurity

substitution defect

interstitial defect

Li

‴+ Li 2O ⎯⎯⎯⎯→ 2LiSn

Cd

″ + OO× + VO•• CdO ⎯⎯⎯⎯→ CdSn

Al

′ + 3OO× + VO•• Al 2O3 ⎯⎯⎯⎯→ 2AlSn

Ti

× TiO2 ⎯⎯⎯⎯→ TiSn + 2OO×

Nb

• Nb2O5 ⎯⎯⎯⎯→ 2NbSn + 5OO× + 2e′

W

•• WO3 ⎯⎯⎯⎯→W Sn + 3OO× + 2e′

SnO2

OO×

+

3VO••

SnO2

SnO2

SnO2

Li 2O ⎯⎯⎯⎯→ 2Lii• + OO× + 2e′ SnO2

CdO ⎯⎯⎯⎯→ Cdi•• + OO× + 2e′ SnO2

Al 2O3 ⎯⎯⎯⎯→ 2Ali••• + 3OO× + 6e′ SnO2

SnO2

TiO2 ⎯⎯⎯⎯→ Tii•••• + 2OO× + 4e′

SnO2

SnO2

Nb2O5 ⎯⎯⎯⎯→ 2Nbi••••• + 5OO× + 10e′ SnO2

SnO2

WO3 ⎯⎯⎯⎯→ W i•••••• + 3OO× + 6e′

the DIOV can produce 2 times the electrons than the SIOV, the conductivity slowly (I) and rapidly (II) increase stages in the defect ionization region can be ascribed to these two kinds of oxygen vacancies; i.e., the ionization of the SIOV dominates the conductivity change in stage I and the DIOV in stage II. Therefore, the conducitivity (σ) in these two stages is expressed based on our previous studies44 σI = (B1′/T 3/2) exp[−(EH1 + ΔED1/2)/kT ]

(3)

σII = (B2′ /T 3/2) exp[−(EH 2 + ΔED2 /3)/kT ]

(4)

where B1′ = eμ0(K10 ′ ND1)1/2 and B2′ = eμ0(2K20 ′ ND2)1/3. ND1, ND2, EH1, EH2, ΔED1, and ΔED2 represent the concentration, the electron hopping energy, and the ionization energy of the SIOV and DIOV, respectively. For the ionization energy, it has been identified that ΔED1 = 0.03 eV and ΔED2 = 0.15 eV.53 On the basis of the above eqs 3 and 4, the EH1 and EH2 are obtained after exponent fitting (σT3/2 − 1000/T) these two stages, with EH1 = 0.204 eV and EH2 = 0.306 eV. The fitting results in stage I and stage II are shown in Figure 7. However, the ionization has become complexed once the impurities are introduced: for p-type doped SnO2, due to the self-compensation effect, some relevant defect associates 54,55 (nM(4−2m/n) ′ − (2n − m)V•• The electrons Sn O ) are formed. located in the associate can be transferred to the oxygen vacancy (V•• O ), then promoting the ionization of these associate defects, which have a lower ionization energy as compared with the acceptor-like defect (M(4−2m/n) ′). For the n-type doped Sn SnO2, newly created donor-like defects (M(2m/n−4)• ) are likely Sn to participate in the ionization. All of these defects play a

Figure 6. Model representing the electron hopping conduction in the nonstoichiometric SnO2, involving the energy band expression of the hopping conduction process in the defects. ED is the donor level, and EH denotes the polaron hopping energy. • (SIOV)) and a deep donor level for V•• O /VO (doubly ionized oxygen vacancy (DIOV)) have been identified 0.03 and 0.15 eV below the bottom of the conduction band (CB), respectively.16,52,53 However, the interstitial tin donor level is formed in the conduction band that has been fully ionized.1 Thus, in the defect ionization region of the undoped SnO2, we can make an assertion that the electron concentration (ne) is controlled by the ionization of the oxygen vacancy (V×O). Moreover, since

Figure 7. Exponential fitting of the temperature-dependent conductivity of the undoped SnO2 nanocrystalline films measured at 10 K/min. (a) In the defect ionization stage I. (b) In the defect ionization stage II. (correlation coefficient r2 > 0.999). 18102



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significant role in shaping the electronic structure and performance of the SnO2. Although the impurities are introduced, the temperaturedependent conductivity in the defect ionization region can also be divided into two stages: a slowly (I) and a rapidly (II) increase stage of the conductivity in the lower and higher temperature ranges, respectively, as seen in Figure 5a. Both of these two stages present a higher increase rate than the undoped SnO2 nanocrystalline films. Considering that the concentration of the introduced defects in the doped SnO2 is lower than that of the intrinsic defects (the SIOV and DIOV), the conductivity of these two stages is still mainly determined by the SIOV and DIOV. In addition, alteration of the hopping energy (EH) may appear in the doped SnO2, since the lattice structure of the SnO2 is subjected to distortion from the incorporating impurities. To accurately model the electrical conduction properties of the p- and n-type doped SnO2, Li- and W-doped SnO2 are selected as examples in the following discussions. 3.4. p-Type (Li) Doped SnO2. The defects taken into consideration for the Li-doped SnO2 are acceptor-like species • •• Li‴ Sn, donor-like species Lii , (Li‴ Sn − 3VO /2, and the intrinsic • × •• • defects VO/VO (SIOV) and VO /VO (DIOV). Thus, here, the ionization reaction of these defects can be written in the form of VO× ⇔ VO• + e′

(3-1)

VO× ⇔ VO•• + 2e′

(3-2)

Lii× ⇔ Lii• + e′

(3-3)

× ‴ − 3VO••/2) ⇔ (LiSn (LiSn − 3VO••/2) + 3e′

(3-4)

× ‴ + 3h• LiSn ⇔ LiSn

(3-5)

(5)

′ exp( −ΔED2 /kT ) [VO••]n22 /[VO×]2 = K 2′(T ) = K 20

(6)

′ exp( −ΔED3/kT ) [Lii•]n3/[Lii×] = K3′(T ) = K30

(7)

(10)

′ )1/3 exp(−ΔED2 /3kT ) n2 = (2NDLi2K 20

(11)

′ )1/2 exp( −ΔED3/2kT ) n3 = (NDLi3K30

(12)

′ )1/4 exp(−ΔED4 /4kT ) n4 = (4NDLi4K40

(13)

″ )1/4 exp(−ΔED′ 4 /4kT ) h1 = (3NALi4K40

(14)

As the defect ionization reactions are identified, the ownership of the ionization temperature ranges that belong to these defects should be characterized according to their ionization properties. The interstitial Li often results in a shallow donor level following the conduction band and the substitutional Li is apt to form a shallow acceptor level above the valence band.55 An assumption is made here that the interstitial Li is in favor of ionizing in the low temperature range and the substitutional Li is engaged to ionize in the high temperature range. Thus, the contribution of the conductivity altering in the ionization stage I belong to the Li×i , whereas, in the ionization stage II, (LiSn ‴ − 3V•• O /2) is responsible for the conductivity variation. Taken as a whole, the temperaturedependent conductivity of the Li-doped SnO2 in the defect ionization region can be concluded. In the ionization stage of SIOV σI = eμeLi (n1 + n3) 1 σII = eμeLi2 (n2 + n4) + eμh h1

× ‴ ]h13/[LiSn ″ exp( −ΔED′ 4 /kT ) [LiSn ] = K4″(T ) = K40

(16)

where μh is the hole mobility. However, for the n-type semiconductor, the contribution of the hole to the conductivity can be ignored due to n ≫ h, and the second term in the right of eq 16 is neglected in the following derivation. Then, combining eq 2 and eqs 10−16, the conductivities in these two stages are given as σI =

⎤ ⎡⎛ eμ0 ⎧ Li ΔED1 ⎞ ⎨(ND1K10 ′ )1/2 exp⎢⎜ −EHLi1 − ⎟ /kT ⎥ 3/2 ⎦ ⎣⎝ 2 ⎠ T ⎩ ⎡⎛ ⎤⎫ ΔED3 ⎞ ′ )1/2 exp⎢⎜ −EHLi1 − ⎟ /kT ⎥⎬ + (NDLi3K30 ⎣⎝ ⎦⎭ 2 ⎠

× ‴ − 3VO••/2)] [(LiSn − 3VO••/2)]n43/[(LiSn

′ exp( −ΔED4 /kT ) = K4′(T ) = K40

(15)

In the ionization stage of DIOV

The dilute concentrations have the mass action equilibrium formulas ′ exp( −ΔED1/kT ) [VO•]n1/[VO×]1 = K1′(T ) = K10

′ )1/2 exp( −ΔED1/2kT ) n1 = (NDLi1K10

(8)

σII = (9)

⎤ ⎡⎛ eμ0 ⎧ ΔED2 ⎞ ⎨(2NDLi2K 20 ′ )1/2 exp⎢⎜ − EHLi2 − ⎟ /kT ⎥ 3/2 ⎝ ⎠ ⎦ ⎣ 3 T ⎩ ⎡⎛ ⎤⎫ ΔED4 ⎞ ′ )1/4 exp⎢⎜ −EH 2 − ⎟ /kT ⎥⎬ + (4NDLi4K40 ⎣⎝ ⎦⎭ 4 ⎠

where n and h are the concentrations of electrons and holes. • × •• [V•O], [V•• ‴ ] are the O ], [Lii ], [(LiSn − 3VO /2)], and [LiSn concentrations of the ionized defects. [V×O]1, [V×O]2, [Li×i ], [(Li‴ Sn × − 3V•• O /2)], and [LiSn] are the concentrations of the relevant defects, which can be seen as constants ND/NA. ΔED3, ΔED4, and ΔED4 ′ represent the ionization energy of the Li•i , (LiSn ‴ − 3V•• /2) and Li‴ O Sn, and ΔED3 = EC − ED3, ΔED4 = EC − ED4, and ΔE′D4 = E′D4 − EV. K′10, K′20, K′30, K′40, and K″40 are preexponentials of the reactions. In the ionization process, the conduction electron/hole comes from the ionization of the defects and the relations between the ionized defects and the electron/hole are given as n = D+, h = A−. Therefore, based on the above mass reaction equations, the electron/hole concentration can be expressed as

(17)

(18)

The temperature-dependent conductivity of the p-type doped (Li) SnO2 in the defect ionization region is modeled in the above. On the basis of the measurements, the ionization •• energy of metal cation defects (Li•i , (Li‴ Sn − 3VO /2)) and the electron hopping energy in the Li-doped SnO2 can be determined through exponential fitting. 3.5. n-Type (W) Doped SnO2. For the W-doped SnO2, the •••••• mainly created defects are donor-like species W•• , Sn and Wi • × •• • and the intrinsic defects VO/VO (SIOV) and VO /VO (DIOV). Since the W•••••• has been fully ionized at room temperature i (300 K) according to the XPS result, thus here, the ionization 18103


Al

Cd

Li

+ e′

18104

II

−

3VO••/2) + 3e′

⇔

× ′ + h• AlSn ⇔ AlSn

′ − (AlSn

VO••/2)

VO× ⇔ VO•• + 2e′

VO× ⇔ VO• + e′

× (AlSn

−

VO••/2) + e′

−

′ VO••/2)]n/[(AlSn −

VO••/2)]

′ exp(−ΔED6 /kT ) = K 6′(T ) = K 60

× ′ ]h/[AlSn ″ exp(−ΔED′ 6 /kT ) [AlSn ] = K 6″(T ) = K 60

× [(AlSn

′ exp(−ΔED2 /kT ) [VO••]n2 /[VO×] = K 2′(T ) = K 20

′ exp(−ΔED1/kT ) [VO•]n/[VO×] = K1′(T ) = K10

× ″ ]h2 /[CdSn ″ exp(−ΔED′ 5/kT ) [CdSn ] = K5″(T ) = K50

″ + 2h ⇔ CdSn

•

× ″ − VO••)] = K5′(T ) = K50 ′ exp( −ΔED5/kT ) − VO••)]n2 /[(CdSn [(CdSn

′ exp(−ΔED2 /kT ) = K 2′(T ) = K 20

× CdSn

+ 2e′

[VO••]n2 /VO×


× ‴ ]h3/[LiSn ″ exp(−ΔED′ 4 /kT ) [LiSn ] = K4″(T ) = K40

‴ − 3VO••/2)] = K4′(T ) = K40 ′ exp( −ΔED4 /kT ) [(LiSn − 3VO••/2)]n3/[(LiSn


′ exp(−ΔED3/kT ) = K3′(T ) = K30

× ″ − VO••) ⇔ (CdSn (CdSn − VO••) + 2e′

⇔

VO••

VO×

II

I

⇔ •

‴ + 3h ⇔ LiSn

VO× ⇔ VO• + e′

× LiSn

‴− (LiSn

3VO••/2)

VO× ⇔ VO•• + 2e′

⇔

[Lii•]n/[Lii×]

Lii×

Lii•


× (LiSn

′ exp(−ΔED1/kT ) = K1′(T ) = K10

mass action law

′ exp(−ΔED2 /kT ) [VO••]n2 /VO× = K 2′(T ) = K 20

[VO•]n/[VO×]

VO× ⇔ VO• + e′

I

II

I

VO× ⇔ VO•• + 2e′

+ e′

II

⇔

I

ionization reaction

undoped

VO•

VO×

ionization stage

impurity

σII =

σI =

σII =

σI =

σII =

σI =

σII =

σI =

conductivity

⎤ ⎡⎛ ΔED1 ⎞ ′ )1/2 exp⎢⎜− EHCd1 − ⎟/kT ⎥ (NDCd1K10 ⎝ ⎠ ⎦ ⎣ 2

⎤ ⎡⎛ ΔED1 ⎞ ′ )1/2 exp⎢⎜− EHAl1 − ⎟/kT ⎥ (NDAl1K10 ⎦ ⎣⎝ 2 ⎠

⎤⎫ ⎡⎛ ΔED6 ⎞ ⎟/kT ⎥⎬ + (NDAl6K ′60 )1/2 exp⎢⎜− EHAl2 − ⎦⎭ ⎣⎝ 2 ⎠

⎡⎛ ⎤ eμ0 ⎧ ΔED2 ⎞ ⎨(2NDAl2K ′20 )1/3 exp⎢⎜− EHAl2 − ⎟/kT ⎥ ⎣⎝ ⎦ 3 ⎠ T 3/2 ⎩

T 3/2

eμ0

⎡⎛ ⎤⎫ ΔED5 ⎞ ′ )1/3 exp⎢⎜− EHCd2 − ⎟/kT ⎥⎬ + (2NDCd5K50 ⎝ ⎠ ⎦⎭ ⎣ 3

⎡⎛ ⎤ eμ0 ⎧ ΔED2 ⎞ ⎨(2NDCd2K 20 ′ )1/3 exp⎢⎜− EHCd2 − ⎟/kT ⎥ ⎣⎝ ⎦ 3 ⎠ T 3/2 ⎩

T

3/2

eμ0

⎡⎛ ⎤⎫ ΔED4 ⎞ ′ )1/4 exp⎢⎜−EHLi2 − ⎟/kT ⎥⎬ + (3NDLi4K40 ⎣⎝ ⎦⎭ 4 ⎠

⎤ ⎡⎛ eμ0 ⎧ ΔED2 ⎞ ⎨(2NDLi2K 20 ′ )1/3 exp⎢⎜− EHLi2 − ⎟/kT ⎥ 3/2 ⎝ ⎠ ⎦ ⎣ 3 T ⎩

⎡⎛ ⎤⎫ ΔED3 ⎞ ′ )1/2 exp⎢⎜− EHLi1 − ⎟/kT ⎥⎬ + (NDLi3K30 ⎣⎝ ⎦⎭ 2 ⎠

⎤ ⎡⎛ eμ0 ⎧ Li ΔED1 ⎞ ⎨(ND1K10 ′ )1/2 exp⎢⎜− EHLi1 − ⎟/kT ⎥ ⎦ ⎣⎝ 2 ⎠ T 3/2 ⎩

⎤ ⎡ ⎛ ΔED2 ⎞ ′ ND2)1/3 exp⎢− ⎜EH 2 + ⎟/kT ⎥ (K 20 ⎦ ⎣ ⎝ 3 ⎠

⎤ ⎡ ⎛ ΔED1 ⎞ ′ ND1)1/2 exp⎢− ⎜EH1 + ⎟/kT ⎥ (K10 ⎦ ⎣ ⎝ 2 ⎠

T 3/2

eμ0

T 3/2

eμ0

Table 4a. Key Defect Ionization Equilibrium Reactions, Relevant Mass Action Equations, and Conductivities of the Undoped and Doped SnO2

The Journal of Physical Chemistry C Article


a

18105

II

I

II

I

× ′ ]h/[TiSn [TiSn ]

−

−

VO••/2)]

+ e′

+ 2e′

4+


′ exp(−ΔED9 /kT ) = K 9′(T ) = K 90

can be reduced into Ti , an acceptor-like defect (TiSn ′ ) may be formed in the SnO2.

VO× ⇔ VO•• + 2e′

⇔

× 2 •• [WSn ]n /[W Sn ]

× WSn

•• W Sn



′ exp(−ΔED8/kT ) = K8′(T ) = K80

VO× ⇔ VO• + e′

VO× ⇔ VO•• + 2e′

⇔

• NbSn

43,44

′ exp(−ΔED7 /kT ) = K 7′(T ) = K 70

″ exp(−ΔED′ 7 /kT ) = K 7″(T ) = K 70

′ VO••/2)]n/[(TiSn

× • [NbSn ]n/[NbSn ]

3+

+ e′

× [(TiSn


−

VO••/2)

× NbSn

′ +h ⇔ TiSn

⇔ •

× (TiSn

′ exp(−ΔED1/kT ) = K1′(T ) = K10

mass action law


[VO•]n/[VO×]

VO× ⇔ VO• + e′

× TiSn

′ − (TiSn

VO••/2)

VO× ⇔ VO•• + 2e′

+ e′

II

⇔

ionization reaction

I

VO•

VO×

ionization stage

For titanium-doped SnO2, since the Ti

W

Nb

Ti

a

impurity

σII =

σI =

σII =

σI =

σII =

σI =

⎤ ⎡⎛ ΔED1 ⎞ ′ )1/2 exp⎢⎜− EHTi1 − ⎟/kT ⎥ (NDTi1K10 ⎦ ⎣⎝ 2 ⎠

conductivity

⎤ ⎡⎛ ΔED2 ⎞ ′ )1/3 exp⎢⎜− EHNb2 − ⎟/kT ⎥ (2NDNb2 K 20 ⎣⎝ ⎦ 3 ⎠

T

3/2

eμ0

⎤ ⎡⎛ ΔED2 ⎞ ′ )1/3 exp⎢⎜− EHW2 − ⎟/kT ⎥ (2NDW2K 20 ⎦ ⎣⎝ 3 ⎠

⎪

⎤⎫ ⎡⎛ ΔED9 ⎞ ′ )1/3 exp⎢⎜− EHW1 − ⎟/kT ⎥⎬ + (2NDW9K 90 ⎝ ⎠ 3 ⎣ ⎦⎭

⎪

eμ0 ⎧ W ⎛ ΔED1 ⎞ ⎨(ND1K10 ′ )1/2 exp⎜− EHW1 − ⎟/kT ⎝ 2 ⎠ T 3/2 ⎩

T

3/2

eμ0

⎡⎛ ⎤⎫ ΔED8 ⎞ ′ )1/2 exp⎢⎜− EHNb1 − ⎟/kT ⎥⎬ + (NDNb8 K80 ⎣⎝ ⎦⎭ 2 ⎠

⎤ ⎡⎛ eμ0 ⎧ Nb ΔED1 ⎞ ′ )1/2 exp⎢⎜− EHNb1 − ⎨(ND1 K10 ⎟/kT ⎥ 3/2 ⎝ ⎠ ⎦ ⎣ 2 T ⎩

⎤⎫ ⎡⎛ ΔED7 ⎞ ′ )1/2 exp⎢⎜− EHTi2 − ⎟/kT ⎥⎬ + (NDTi7K 70 ⎦⎭ ⎣⎝ 2 ⎠

⎡⎛ ⎤ eμ0 ⎧ ΔED2 ⎞ ⎨(2NDTi2K 20 ′ )1/3 exp⎢⎜− EHTi2 − ⎟/kT ⎥ ⎣⎝ ⎦ 3 ⎠ T 3/2 ⎩

T 3/2

eμ0

Table 4b. Key Defect Ionization Equilibrium Reactions, Relevant Mass Action Equations, and Conductivities of the Undoped and Doped SnO2

The Journal of Physical Chemistry C Article



Article

properties of the undoped and doped (Li, Cd, Al, Ti, Nb, and W) SnO2 are summarized in Tables 4a and 4b. In order to determine the ionization energy ΔED of the introduced defects, the electrical conductivity model conducted above (Tables 4a and 4b) is applied to the investigation of the measurement results. To clearly determine the temperature ranges of the stage I and stage II, the fitting range division and the specific fitting results are displayed in the Supporting Information. For the value of the hopping energy (EM H ) of the doped SnO2, since the ΔED1 and ΔED2 are known for the SnO2, the hopping energy EM H1 (M = Li, Cd, Al, and Ti) in stage I and the EM H2 (M = Nb and W) in stage II can be obtained after exponential fitting the plots in these temperature ranges in terms of the expressions of the temperature-dependent conductivity; specifically, simulated results are shown in Figure S1 (a-1, b-1, c-1, and d-1) and Figure S2 (a-2 and b-2) (Supporting Information). In the same time, the hopping energy can be calculated using the Mott’s small-polaron hopping model with the relation by56

reaction of these defects above 300 K can be written in the form of VO× ⇔ VO• + e′

(3-1)

VO× ⇔ VO•• + 2e′

(3-2)

× •• WSn ⇔ W Sn + 2e′

(3-6)

The dilute concentrations have the mass action equilibrium formulas ′ exp( −ΔED1/kT ) [VO•]n1/[VO×]1 = K1′(T ) = K10

(19)

′ exp( −ΔED2 /kT ) [VO••]n22 /[VO×]2 = K 2′(T ) = K 20

(20)

•• 2 × ′ exp( −ΔED9 /kT ) [W Sn ]n9 /[WSn ] = K 9′(T ) = K 90

(21)

Taking the same as the Li-doped SnO2, the electron concentrations in each reaction are ′ )1/2 exp( −ΔED1/2kT ) n1 = (NDW1K10 n2 =

′ )1/3 (2NDW2K 20

(22)

exp(−ΔED2 /3kT )

(23)

′ )1/3 exp(−ΔED9 /3kT ) n9 = (2NDW9K 90

(24)

⎛ e 2 ⎞⎛ 1 ⎞ ⎟⎟⎜⎜ − 1 ⎟⎟ EH = ⎜⎜ R⎠ ⎝ 4εp ⎠⎝ rp

rp =

As for the n-type doping, due to that the interstitial defects have been fully ionized at room temperature, a consensus is reached that the substitutional defect is able to form a shallow donor level. An assumption is made here that substitutional defects (W×Sn) are chosen to ionize in the low temperature region. Therefore, the temperature-dependent conductivity of the W-doped SnO2 in the defect ionization region can be concluded. In the ionization stage of SIOV

σI = eμeW (n1 + n9) 1

σII =

(25)

εpM εp

(26)

⎡⎛ ⎤ eμ0 ⎧ W ΔED1 ⎞ ⎨(ND1K10 ′ )1/2 exp⎢⎜ −EHW1 − ⎟ /kT ⎥ 3/2 ⎣⎝ ⎦ 2 ⎠ T ⎩ ⎡⎛ ⎤⎫ ΔED9 ⎞ ′ )1/3 exp⎢⎜ −EHW2 − + (2NDW9K 90 ⎟ /kT ⎥⎬ 3 ⎠ ⎣⎝ ⎦⎭

σII =

eμ0 T

3/2

=

EH1 EHM1

=

EH 2 EHM2

(M = Li, Cd, Al, Ti, Nb, and W) (31)

Since the EH1 and EH2 are calculated based on the undoped SnO2 nanocrystalline film, and the EM H1 (M = Li, Cd, Al, and Ti) and the EM (M = Nb and W) are extracted from the above H2 fitting, respectively, the hopping energy EM H2 (M = Li, Cd, Al, and Ti) in stage II and the EM H1 (M = Nb and W) in stage I can be calculated according to eq 31. As the hopping energies EH are known, the ionization energy ΔED of the introduced defects can be clearly calculated through exponential fitting the temperature-dependent conductivity plots in Figure S1 (a-2, b-2, c-2, and d-2) and Figure S2 (a-1 and b-1) (Supporting Information). The specific values of the acquired are summarized in Table 5. It is found that the hopping energies of the doped SnO2 are higher as compared to those of the undoped SnO2. The difference can be assigned to the change of the dielectric constant that stemmed from the introduced defects in the SnO2.57 These data are found to be consistent with the relevant research results.56,58 As for the value of the ionization energy of the introduced defects, an apparent difference arises on the p-type, isovalent, and n-type doped SnO2, which may correlate with the different level positions of the formed defects. The ionization energy (ΔED) calculated here is of critical importance for understanding the defect structure of the metal dopants in the SnO2. Because there are no previously reported experimental values of the ionization energy of such defect associates and donor-like

Then, combing eq 2 and eqs 22−26, the conductivities in these two stages are given as σI =

(30)

where rp is polaron radius, R is the average distance between the adjacent defects, and εp is the effective dielectric constant. Because the introduced impurity defects are relatively seldom, a dielectric constant difference may occur for the doped SnO2 and an assumption is made here that R does not evidently change. Then, the change of the effective dielectric constant for the doped SnO2 can be derived based on the above two equations

In the ionization stage of DIOV

eμeW n 2 2

⎛ π ⎞1/3⎛ R ⎞ ⎜ ⎟ ⎜ ⎟ ⎝6⎠ ⎝2⎠

(29)

(27)

⎡⎛ ⎤ ΔED2 ⎞ ′ )1/3 exp⎢⎜ −EHW2 − ⎟ /kT ⎥ (2NDW2K 20 ⎣⎝ ⎦ 3 ⎠ (28)

The temperature-dependent conductivity of the n-type doped (W) SnO2 in the defect ionization region is obtained. It can be seen that the temperature-dependent conductivity shows an exponent relation between σT3 and 1/T. Therefore, according to the experimental measurements, the ionization energy of the introduced impurity defects can be obtained through exponential fitting the temperature-dependent conductivity in the defect ionization region, since EH1, EH2, ΔED1, and ΔED2 are quantitatively calculated from the undoped SnO2. Similarly, on the basis of the above discussions, a systematic study of the defect chemistry and electrical conductivity 18106



Article

Table 5. Electron Hopping Energy EH and the Defect Ionization Energy ΔED in the Doped SnO2 species Li-SnO2 Cd-SnO2 Al-SnO2 Ti-SnO2 Nb-SnO2 W-SnO2

hopping energy EH (eV) ELi H1 ELi H2 ECd H1 ECd H2 EAl H1 EAl H2 ETi H1 ETi H2 ENb H1 ENb H2 EW H1 EW H2

0.242 0.363 0.419 0.628 0.194 0.290 0.294 0.441 0.291 0.437 0.245 0.367

All of these results indicated that the temperatureprogrammed technique combined with conductivity measurement can provide the information about the metal cation defects and electrical conductivity changes that occur in the doped SnO2 in detail. Furthermore, the obtained ionization energy (ΔED) is able to give a clear understanding of the defect structure of the metal dopants in the SnO2.

ionization energy ΔED (eV) ΔED(Li•i ) ΔED((LiSn ‴ − 3V•• O /2)) •• ΔED((Cd‴ Sn − VO ))

0.068 1.91 3.04

ΔED((AlSn ′ − V•• O /2))

0.708

ΔED((Ti′Sn − V•• O /2))

2.23

ΔED(Nb•Sn)

0.170

ΔED(W•• Sn )

0.363

■

ASSOCIATED CONTENT

S Supporting Information *

Exponential fitting of the temperature-dependent conductivity of the p-type (Li, Cd, and Al), isovalent (Ti), and n-type (Nb, W) doped SnO2 nanocrystalline films measured at 10 K/min. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

defects for the doped SnO2, a direct comparison may not be able to be given here. However, these values were identified corresponding to the definition of the acceptor- and donor-like defects,21 and the validity of the calculated ionization energy will be further confirmed in our following research.

*Tel: +86-27-8755-6544. Fax: +86-27-8754-3778. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

4. CONCLUSIONS In terms of an improved defect chemical model proposed here, an investigation of the defect chemistry in a series of metal ions, p-type (Li+, Cd2+, Al3+), isovalent (Ti4+), and n-type (Nb5+, W6+), doped SnO2 nanocrystalline films was performed through the temperature-programmed system integrated with the high throughput screening platform of gas-sensing materials (HTSPGM). With this platform, the acquired temperature-dependent conductivity plots in the defect ionization region can be used to characterize the defect structure of the metal cation defects and the electrical conduction mechanism in the doped SnO2. The main conclusions drawn from the present work are summarized as follows. A decrease of the conductivity that emerged in the p-type doped SnO2 indicates that more oxygen vacancies are formed, and owing to the charge compensation effect, defect associates (nM(4−2m/n) ′ − (2n−m)V•• Sn O ) (M = Li, Cd, Al) are presented in the p-type doped SnO2. As a result, the electron ionization rate and quantity are improved and have been identified by the temperature-dependent conductivity plots. For the isovalent (Ti) doping, since the Ti4+ can be reduced into Ti3+ (Ti4+ + e′ ↔ Ti3+), an acceptor-like defect (TiSn ′ ) may be stably formed in the SnO2, which resulted in the conductivity decrease as compared with the undoped SnO2. As for n-type doping SnO2, accompanied by the formation of donor-like defects M(2m/n−4)• (M = Nb, W), more electrons are Sn produced, which results in an increase of the conductivity. Furthermore, the donor-like defects that also can ionize out electrons have a deep influence on the defect ionization region. On the basis of the ionization nature, the ionization energy (ΔED) of these metal cation defects and the electron hopping energy (EH) are extracted from the plots of the SIOV and DIOV ionization stages. The obtained values of EH (∼0.2−0.6 eV) are consistent with the relevant studies, which indicates that incorporating the impurity defects leads to a change of the lattice electronic structure of the SnO2. Moreover, it is found that the ΔED of these metal cation defects are first determined in this work, but these values are confirmed corresponding to the theoretical studies.

ACKNOWLEDGMENTS This work was supported by the Nature Science Foundation of China (Nos. 51204072 and 50927201), the National Basic Research Program of China (Grant Nos. 2009CB939705 and 2009CB939702). The authors are also grateful to Analytical and Testing Center of Huazhong University of Science and Technology.

■

REFERENCES

(1) Kılıç, Ç .; Zunger, A. Origins of Coexistence of Conductivity and Transparency in SnO2. Phys. Rev. Lett. 2002, 88, 095501. (2) Batzill, M.; Diebold, U. The Surface and Materials Science of Tin Oxide. Prog. Surf. Sci. 2005, 79, 47−154. (3) Franke, M. E.; Koplin, T. J.; Simon, U. Metal and Metal Oxide Nanoparticles in Chemiresistors: Does the Nanoscale Matter? Small 2006, 2, 36−50. (4) Wang, H.; Rogach, A. L. Hierarchical SnO2 Nanostructures: Recent Advances in Design, Synthesis, and Applications. Chem. Mater. 2014, 26, 123−133. (5) Ding, X.; Zeng, D.; Xie, C. Controlled Growth of SnO2 Nanorods Clusters via Zn Doping and Its Influence on Gas-Sensing Properties. Sens. Actuators, B 2010, 149, 336−344. (6) Tianshu, Z.; Hing, P.; Li, Y.; Jiancheng, Z. Selective Detection of Ethanol Vapor and Hydrogen Using Cd-Doped SnO2-Based Sensors. Sens. Actuators, B 1999, 60, 208−215. (7) Bontempi, E.; Zampiceni, E.; Sberveglieri, G.; Depero, L. E. Structural and Morphological Study of W-Sn-O Thin Films Deposited by Rheotaxial Growth and Thermal Oxidation. Chem. Mater. 2002, 14, 3422−3426. (8) Fiz, R.; Hernandez-Ramirez, F.; Fischer, T.; Lopez-Conesa, L.; Estrade, S.; Peiro, F.; Mathur, S. Synthesis, Characterization, and Humidity Detection Properties of Nb2O5 Nanorods and SnO2/Nb2O5 Heterostructures. J. Phys. Chem. C 2013, 117, 10086−10094. (9) Dou, X.; Sabba, D.; Mathews, N.; Wong, L. H.; Lam, Y. M.; Mhaisalkar, S. Hydrothermal Synthesis of High Electron Mobility ZnDoped SnO2 Nanoflowers as Photoanode Material for Efficient DyeSensitized Solar Cells. Chem. Mater. 2011, 23, 3938−3945. (10) Li, Z.; Zhou, Y.; Yu, T.; Liu, J.; Zou, Z. Unique Zn-Doped SnO2 Nano-Echinus with Excellent Electron Transport and Light Harvesting Properties as Photoanode Materials for High Performance DyeSensitized Solar Cell. CrystEngComm 2012, 14, 6462−6468. 18107



Article

(32) Zhang, X.; Yang, H. Structural Characterization and Gas Sensing Property of Cd-Doped SnO2 Nanocrystallites Synthesized by Mechanochemical Reaction. Sens. Actuators, B 2012, 173, 127−132. (33) Xu, X.; Sun, J.; Zhang, H.; Wang, Z.; Dong, B.; Jiang, T.; Wang, W.; Li, Z.; Wang, C. Effects of Al Doping On SnO2 Nanofibers in Hydrogen Sensor. Sens. Actuators, B 2011, 160, 858−863. (34) Kim, Y.; Chen, H. Electrical Properties of Ta-Doped SnO2 Thin Films Prepared by the Metal-Organic Chemical-Vapor Deposition Method. Appl. Phys. Lett. 2001, 78, 350−352. (35) Dhage, S. R.; Ravi, V. Influence of Various Donors on Nonlinear I-V Characteristics of Tin Dioxide Ceramics. Appl. Phys. Lett. 2003, 83, 4539−4541. (36) Gonçalves, F.; Medeiros, P. R.; Eon, J. G.; Appel, L. G. Active Sites for Ethanol Oxidation Over SnO2-Supported Molybdenum Oxides. Appl. Catal., A 2000, 193, 195−202. (37) Sá, J.; Montero, J.; Duncan, E.; Anderson, J. A. Bi Modified Pd/ SnO2 Catalysts for Water Denitration. Appl.Catal., B 2007, 73, 98− 105. (38) Dong, L.; Zhang, L.; Sun, C.; Yu, W.; Zhu, J.; Liu, L.; Liu, B.; Hu, Y.; Gao, F.; Dong, L.; Chen, Y. Study of the Properties of CuO/ VOx/Ti0.5Sn0.5O2 Catalysts and Their Activities in NO + CO Reaction. ACS Catal. 2011, 1, 468−480. (39) Austin, I. G.; Mott, N. F. Polarons in Crystalline and Noncrystalline Materials. Adv. Phys. 1969, 18, 41−102. (40) Tuller, H. L.; Nowick, A. S. Small Polaron Electron Transport in Reduced CeO2 Single Crystals. J. Phys. Chem. Solids 1977, 38, 859− 867. (41) Von Wenckstern, H.; Schmidt, H.; Brandt, M.; Lajn, A.; Pickenhain, R.; Lorenz, M.; Grundmann, M.; Hofmann, D. M.; Polity, A.; Meyer, B. K. Anionic and Cationic Substitution in ZnO. Prog. Solid State Chem. 2009, 37, 153−172. (42) Van Vechten, J. A.; Thurmond, C. D. Entropy of Ionization and Temperature Variation of Ionization Levels of Defects in Semiconductors. Phys. Rev. B 1976, 14, 3539. (43) Zhang, G.; Zhang, S.; Yang, L.; Zou, Z.; Zeng, D.; Xie, C. La2O3Sensitized SnO2 Nanocrystalline Porous Film Gas Sensors and Sensing Mechanism toward Formaldehyde. Sens. Actuators, B 2013, 188, 137− 146. (44) Zhang, G.; Xie, C.; Zhang, S.; Yang, L.; Xiong, Y.; Zeng, D. Temperature- and Atmosphere-Dependent Defect Chemistry Model of SnO2 Nanocrystalline Film. J. Am. Ceram. Soc. 2014, 97, 2091− 2098. (45) Lu, J. G.; Zhang, Y. Z.; Ye, Z. Z.; Zeng, Y. J.; He, H. P.; Zhu, L. P.; Huang, J. Y.; Wang, L.; Yuan, J.; Zhao, B. H. Control of P- and NType Conductivities in Li-Doped ZnO Thin Films. Appl. Phys. Lett. 2006, 89, 112113. (46) Di Valentin, C.; Pacchioni, G.; Selloni, A. Reduced and N-Type Doped TiO2: Nature of Ti3+ Species. J. Phys. Chem. C 2009, 113, 20543−20552. (47) Zuo, F.; Wang, L.; Wu, T.; Zhang, Z.; Borchardt, D.; Feng, P. Self-Doped Ti3+ Enhanced Photocatalyst for Hydrogen Production under Visible Light. J. Am. Chem. Soc. 2010, 132, 11856−11857. (48) Luo, L.; Yang, X.; Liang, F.; Xu, H.; Zhao, Y.; Xie, X.; Zhang, W.; Lee, S. Surface Defects-Induced p-Type Conduction of Silicon Nanowires. J. Phys. Chem. C 2011, 115, 18453−18458. (49) Willett, M. J.; Burganos, V. N.; Tsakiroglou, C. D.; Payatakes, A. C. Gas Sensing and Structural Properties of Variously Pretreated Nanopowder Tin (IV) Oxide Samples. Sens. Actuators, B 1998, 53, 76−90. (50) McAleer, J. F.; Moseley, P. T.; Norris, J. O.; Williams, D. E. Tin Dioxide Gas Sensors. Part 1.Aspects of the Surface Chemistry Revealed by Electrical Conductance Variations. J. Chem. Soc., Faraday Trans. 1 1987, 83, 1323−1346. (51) Barsan, N.; Weimar, U. Conduction Model of Metal Oxide Gas Sensors. J. Electroceram. 2001, 7, 143−167. (52) Samson, S.; Fonstad, C. G. Defect Structure and Electronic Donor Levels in Stannic Oxide Crystals. J. Appl. Phys. 1973, 44, 4618− 4621.

(11) Wang, Y.; Chen, T. Nonaqueous and Template-Free Synthesis of Sb Doped SnO2 Microspheres and Their Application to LithiumIon Battery Anode. Electrochim. Acta 2009, 54, 3510−3515. (12) Alcántara, R.; Fernández-Madrigal, F. J.; Pérez-Vicente, C.; Tirado, J. L.; Jumas, J. C.; Olivier-Fourcade, J. Preparation, Sintering, and Electrochemical Properties of Tin Dioxide and Al-Doped Tin Dioxides Obtained from Citrate Precursors. Chem. Mater. 2000, 12, 3044−3051. (13) Luo, L.; Liang, F.; Jie, J. Sn-Catalyzed Synthesis of SnO2 Nanowires and their Optoelectronic Characteristics. Nanotechnology 2011, 22, 485701. (14) Kim, D.; Kim, Y.-K.; Park, S. C.; Ha, J. S.; Huh, J.; Na, J.; Kim, G.-T. Photoconductance of Aligned SnO2 Nanowire Field Effect Transistors. Appl. Phys. Lett. 2009, 95, 043107. (15) Wan, Q.; Dattoli, E.; Lu, W. Doping-Dependent Electrical Characteristics of SnO2 Nanowires. Small 2008, 4, 451−454. (16) Godinho, K. G.; Walsh, A.; Watson, G. W. Energetic and Electronic Structure Analysis of Intrinsic Defects in SnO2. J. Phys. Chem. C 2008, 113, 439−448. (17) Archer, P. I.; Radovanovic, P. V.; Heald, S. M.; Gamelin, D. R. Low-Temperature Activation and Deactivation of High-CurieTemperature Ferromagnetism in a New Diluted Magnetic Semiconductor: Ni2+-Doped SnO2. J. Am. Chem. Soc. 2005, 127, 14479− 14487. (18) Poupon, L.; Iacconi, P.; Pijolat, C. Physical and Quasi-Chemical Study of Point Defects in Aluminium or Niobium-Doped Polycrystalline Tin Dioxide. J. Eur. Ceram. Soc. 1999, 19, 747−751. (19) Wu, P.; Zhou, B.; Zhou, W. Room-Temperature Ferromagnetism in Epitaxial Mg-Doped SnO2 Thin Films. Appl. Phys. Lett. 2012, 100, 182405. (20) Van Komen, C.; Thurber, A.; Reddy, K. M.; Hays, J.; Punnoose, A. Structure-Magnetic Property Relationship in Transition Metal (M = V, Cr, Mn, Fe, Co, Ni) Doped SnO2 Nanoparticles. J. Appl. Phys. 2008, 103, 07D141. (21) Sasaki, K.; Maier, J. Low-Temperature Defect Chemistry of Oxides. I. General Aspects and Numerical Calculations. J. Appl. Phys. 1999, 86, 5422−5433. (22) Tuller, H. L.; Bishop, S. R. Point Defects in Oxides: Tailoring Materials through Defect Engineering. Annu. Rev. Mater. Res. 2011, 41, 369−398. (23) Kar, A.; Patra, A. Optical and Electrical Properties of Eu3+Doped SnO2 Nanocrystals. J. Phys. Chem. C 2009, 113, 4375−4380. (24) Wei, W.; Dai, Y.; Huang, B. Role of Cu Doping in SnO2 Sensing Properties toward H2S. J. Phys. Chem. C 2011, 115, 18597−18602. (25) Liu, X.; Iqbal, J.; Wu, Z.; He, B.; Yu, R. Structure and RoomTemperature Ferromagnetism of Zn-Doped SnO2 Nanorods Prepared by Solvothermal Method. J. Phys. Chem. C 2010, 114, 4790−4796. (26) Müller, V.; Rasp, M.; Šefanić, G.; Ba, J.; Günther, S.; Rathousky, J.; Niederberger, M.; Fattakhova-Rohlfing, D. Highly Conducting Nanosized Monodispersed Antimony-Doped Tin Oxide Particles Synthesized via Nonaqueous Sol-Gel Procedure. Chem. Mater. 2009, 21, 5229−5236. (27) Huang, Y.; Li, G.; Feng, J.; Zhang, Q. Investigation on Structural, Electrical and Optical Properties of Tungsten-Doped Tin Oxide Thin Films. Thin Solid Films 2010, 518, 1892−1896. (28) Wang, Y.; Brezesinski, T.; Antonietti, M.; Smarsly, B. Ordered Mesoporous Sb-, Nb-, and Ta-Doped SnO2 Thin Films with Adjustable Doping Levels and High Electrical Conductivity. ACS Nano 2009, 3, 1373−1378. (29) Nütz, T.; Haase, M. Wet-Chemical Synthesis of Doped Nanoparticles: Optical Properties of Oxygen-Deficient and Antimony-Doped Colloidal SnO2. J. Phys. Chem. B 2000, 104, 8430−8437. (30) Gurlo, A.; Riedel, R. In Situ and Operando Spectroscopy for Assessing Mechanisms of Gas Sensing. Angew. Chem., Int. Ed. 2007, 46, 3826−3848. (31) Schaub, R.; Thostrup, P.; Lopez, N.; Lægsgaard, E.; Stensgaard, I.; Nørskov, J. K.; Besenbacher, F. Oxygen Vacancies as Active Sites for Water Dissociation on Rutile TiO2 (110). Phys. Rev. Lett. 2001, 87, 266104. 18108



Article

(53) Jarzebski, Z. M.; Marton, J. P. Physical Properties of SnO2 Materials II. Electrical Properties. J. Electrochem. Soc. 1976, 123, 299C−310C. (54) Tsur, Y.; Riess, I. Self-Compensation in Semiconductors. Phys. Rev. B 1999, 60, 8138. (55) Wardle, M. G.; Goss, J. P.; Briddon, P. R. Theory of Li in ZnO: A Limitation for Li-Based p-Type Doping. Phys. Rev. B 2005, 71, 155205. (56) Vemuri, R. S.; Bharathi, K. K.; Gullapalli, S. K.; Ramana, C. V. Effect of Structure and Size on the Electrical Properties of Nanocrystalline WO3 Films. ACS Appl. Mater. Interfaces 2010, 2, 2623−2628. (57) Fu, D.; Taniguchi, H.; Taniyama, T.; Itoh, M.; Koshihara, S. Origin of Giant Dielectric Response in Nonferroelectric CaCu3Ti4O12: Inhomogeneous Conduction Nature Probed by Atomic Force Microscopy. Chem. Mater. 2008, 20, 1694−1698. (58) Chiang, Y. M.; Lavik, E. B.; Kosacki, I.; Tuller, H. L.; Ying, J. Y. Defect and Transport Properties of Nanocrystalline CeO2‑x. Appl. Phys. Lett. 1996, 69, 185−187.

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