Oxygen vacancy density-dependent transformationfrom infrared to Raman active
vibration mode in SnO2 nanostructuresT. H. Li,1,2 L. Z. Liu,1 X. X. Li,1 X. L. Wu,1,* H. T. Chen,3 and Paul K. Chu4
1National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China2College of Electronic Engineering, Guangxi Normal University, Guilin 541004, China
3College of Physics Science and Technology, Yangzhou University, Yangzhou 225002, China4Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China
Received September 6, 2011; revised September 30, 2011; accepted September 30, 2011;posted October 3, 2011 (Doc. ID 154046); published November 1, 2011
Tin oxide (SnO2) has interesting fundamental physicalproperties and promising applications in new-generation,high-performance devices such as batteries, gas sensors,catalysts, and biosensors [1–4], and SnO2 nanostructureswith different morphologies, such as nanowires (nanor-ods), nanoribbons (nanobelts), and nanocrystals (NCs),have been fabricated [5,6]. Recently, a new Raman modeat ∼300 cm−1 has been identified from rutile SnO2 nano-wires [7]. Based on the presence of an IR active mode at297 cm−1 observed from bulk SnO2, the ∼300 cm−1 Ramanmode was suggested to stem from a transformation of anIR to Raman active mode because the k ¼ 0 selectionrule relaxes with increasing disorder or decreasingsize [8]. However, the mechanism causing this IR toRaman transformation is unclear. In addition, Ramanmodes at 216–289 cm−1 and a broad Raman band at310–350 cm−1 have been observed from rutile SnO2 na-norods [4,9,10] and SnO2 nanocrystalline powders [11],respectively. These modes have also been attributed tothe IR to Raman transformation, and the exact mechan-ism is, again, not well understood.Oxygen vacancies (OVs) exist in many transition metal
oxides and play a critical role in producing new physicalphenomena [12,13]. In this work, spherical, cubic, andcuboid nanorod SnO2 NCs are synthesized, and theirRaman and IR characteristics are studied. The Ramanpeak at ∼302 cm−1 is observed, and its frequency isaffected only slightly by the NC size, but the intensity in-creases obviously with decreasing NC size. Our densityfunctional theory (DFT) calculations reveal that thisRaman active mode arises from the dipole momentchange induced by OVs and the intensity increase stemsfrom increased phonon confinement.SnO2 NCs with different morphologies were fabricated
by a hydrothermal reaction involving SnCl4 · 5H2O andCOðNH2Þ2. In a typical process, 0:08 g of SnCl4 · 5H2Oand 0:8 g of COðNH2Þ2 were added to a 40ml closed cy-lindrical Teflon-lined stainless steel autoclave containing32ml of deionized water, and then 1:6ml of fuming HCl
was introduced, followed by ultrasonic treatment andheating to 90 °C for 8 (sample B) and 24 h (sample C)[14]. To produce sample A, 0:09 g of SnCl4 · 5H2O and0:8 g of COðNH2Þ2 were mixed for 24 h under the aboveconditions. After the reaction, the autoclave was cooledto room temperature naturally. The white precipitateswere centrifuged, rinsed thoroughly with water andethanol several times, and dried at 40 °C in an oven.The morphology of the specimens was examined byhigh-resolution transmission electron microscopy (HR-TEM, JEOL-2100). Raman scattering was performed ona T6400 triple Raman system with the 514:5 nm line ofan argon ion laser as the excitation source. Fourier-transform IR (FTIR) spectra were obtained on a Nicolet170SX, and x-ray photoelectron spectra were acquired ona Kratos AXIS spectrometer (Japan).
The HR-TEM images show that the NCs in sample Aare mostly spherical and have a mean diameter of about5:3 nm [Fig. 1(a)]. By adjusting the ratio of SnCl4 · 5H2O(0:08 g) to COðNH2Þ2 (0:8 g) (1∶10 in our experiments)and the reaction time, the slower nucleation rate retardsaggregation of SnO2 clusters, resulting in the formationof crystalline SnO2 nanocubes and cuboid nanorods[14]. Figure 1(b) shows some cubic NCs with a mean sidelength of 3:0nm (sample B). As the synthesis time is in-creased to 24 h, the cubic NCs become bigger and morphinto cuboid nanorods with a mean volume of 3:0nm ×3:0 nm × 8:4 nm [Fig. 1(c), sample C]. If we define avolume-mean size, the sizes of samples B, C, and A in-crease in that order. The NCs in the three HR-TEMimages reveal lattice fringes corresponding to theð110Þ and ð101Þ planes of rutile SnO2, and they are con-sistent with the selected-area electron diffraction(SAED) results [Fig. 1(d)].
Figure 2(a) shows the Raman spectra of samples A, B,and C, and five distinct modes can be observed. The S5peak at 629:5 cm−1 corresponds to the A1g mode of crys-talline SnO2. Its position and intensity depend on not onlythe NC size but also the type, density, and distribution of
OVs. The strongest S4 mode at 572:7 cm−1 originates fromthe surface in-plane OVs in SnO2 NCs, and its position isindependent of the NC morphology and size [15]. Thismode shows the same intensity from the three samplesdue to the same OV density (see below). Here, the 475 nmEg mode appears as a shoulder (dotted line). Its positionand intensity are affected slightly by the size andmorphology in addition to the OV content based onour previous calculation [15]. The S2 and S3 peaks arethe B1g and A1g modes, respectively, but their positionsshift from 358 and 440 cm−1 to 354.5 and 435:5 cm−1 com-pared to SnO2 powders. The shifts can be attributed toOVs in the NCs [16]. A new Raman peak (S1) at 302�1 cm−1 can also be observed from all the samples. Its po-sition has no distinct dependence on the NC morphologyand size, but the intensity increases with decreasing size.The S1 peak is not an intrinsic Raman mode because it
does not appear in bulk SnO2 according to symmetryanalysis.
Figure 3(a) presents the FTIR spectra of the three sam-ples. These spectra show vibration bands at 302.8 and553:4 cm−1 (Sn-O-Sn vibration) and have almost the sameline shape [8]. The frequency of the 302:8 cm−1 IR mode isconsistent with that of the observed Raman mode. Com-pared to the 320 cm−1 IR band (curve D) observed fromcommercial SnO2 powders (sample D) [17], the IR modedownshifts by about 17 cm−1. In SnO2 crystals, the fre-quencies of the IR modes depend strongly on long-rangeCoulombic force and dipole interaction [18], and changesin the shape, size, and filling factor alter the IR spectra[8]. Since the IR mode observed from the three sampleshas the same frequency, it can be inferred that the NCmorphology and size have no effects. In addition, sincethe spherical (A), cubic (B), and cuboid nanorod (C)NCs have the same rutile SnO2 crystalline structure, itis reasonable to ascribe the frequency shift to OVs. Itis known that OVs in SnO2 nanostructures displace thetin sublattice with respect to the oxygen sublattice, there-by producing a dipole moment and altering the IR vibra-tion frequencies. The atomic displacements also changethe phonon dispersion curve. In the Brillouin zone, therelaxation of the k ¼ 0 selection rule is progressive, withcrystal structure distortion thus enabling gradual trans-formation of the 302 cm−1 IR active mode into a Ramanactive one.
To confirm the existence of OVs, the Sn 3d XPS spectraobtained from samples A, B, and C are presented inFig. 3(b). The binding energy of the Sn 3d5=2 peak is487:1 eV, and the valence of Sn is thus 3.62 [19], whichis less than 4. This suggests nonstoichiometric SnO2−xwith a certain OV density (x ¼ 0:19). It is noted thatthe XPS measurement error is about 12%, and thus theOV density is 0:19� 0:02. It does not affect our conclu-sion. For comparison, the XPS spectrum [sample D inFig. 3(b)] is also acquired from commercial SnO2 pow-ders. A Raman peak in the range of 290–330 cm−1 cannotbe observed [inset in Fig. 2(a)], and x is calculated to be0.04, which is much smaller than 0.19. This indicates thata specific OV density is required in order to produce theIR to Raman transformation.
Fig. 1. HR-TEM images of SnO2 NCs with (a) spherical(sample A), (b) cubic (sample B), and (c) cuboid nanorod(sample C) morphologies; (d) the SAED pattern of sample C.
200 300 400 500 600 700
C
B
AC
A
(a)
S5
S4
S3
S2Inte
nsity
(a.
u.)
Raman Shift (cm-1)
S1
B
300 450 600
Sample D
Inte
nsit
y (a
.u.)
Raman Shift (cm-1)
200 250 300 350
(b)
Raman Shift (cm-1)
Fig. 2. (Color online) (a) Raman spectra acquired fromsamples A, B, and C. The inset shows the Raman spectrumof a commercial SnO2 powder (sample D). (b) CalculatedRaman spectra of the SnO2 NCs with different morphologiesand sizes for samples A, B, and C.
150 300 450 600 485 490 495
XPS
Int
ensi
ty (
a.u.
)
Tra
nsm
ittan
ce (
a.u.
)
Binding energy (eV)
(a)
S
D
B
C
A
Sn d3/2
(b)
Wavenumber (cm-1
)
Sn d5/2
D
B
C
A
Fig. 3. (Color online) (a) FTIR and (b) Sn 3d XPS spectra ofsamples A, B, and C and commercial SnO2 powders (sample D).
To further elucidate the mechanism, the DFT isadopted to calculate the Raman spectra and correspond-ing phonon dispersion curve in the presence of OVs [15].Since it is difficult to calculate the morphological effecton the Raman mode, a bulk crystalline SnO2 structurewith a certain OV density is adopted. The resultsshow that x ¼ 0:06 causes the appearance of a Ramanmode at ∼316:2 cm−1. With increasing OV density, thefrequency downshifts to 310:5 cm−1 for x ¼ 0:13,308:3 cm−1 for x ¼ 0:19, and 273:4 cm−1 for x ¼ 0:25.The theoretical derivation reveals clearly that the IR–Raman transformation can occur at different frequenciesas observed previously [4,9–11], because the amounts ofOVs can vary among different samples.To further investigate the relationship between the
Raman mode and NC morphology, Campbell andFauchet’s phonon confinement model is applied. Thefirst-order Raman spectrum IðωÞ can be described bythe following equation [20]:
IðωÞ ¼Z
d3qjCð0; qÞj2½ω − ωðqÞ�2 þ ðΓ=2Þ2 ; ð1Þ
where ωðqÞ represents the phonon dispersion (obtainedby DFT calculation), Γ is the natural linewidth, andCð0; qÞ is the Fourier coefficient describing phonon con-finement. With regard to the spherical and nanorod SnO2,the Fourier coefficients can be written as
jCSð0; qÞj2 ¼ exp½−ðq2D2Þ=ð4πÞ2� ð2Þand
jCð0; q1; q2Þj2 ¼ exp½−q21L21=16π2� × exp½−q22L2
2=16π2�
×
����1 − erf
�iq2L2ffiffiffiffiffi32
pπ
�����2; ð3Þ
where D is the spherical NC size and L1 and L2 are thediameter and length of the SnO2 nanorod. According tothe calculated Raman line shape in Fig. 2(b), as the NCsize decreases, the frequencies downshift slightly from303:2 cm−1 (A) to 301:7 cm−1 (B) and 303:1 cm−1 (C) whilethe intensity increases. They are in good agreement withthe aforementioned experimental results. This is under-standable, because phonon scattering can spread outover a much larger range by increasing the NC size,and meanwhile the frequencies will upshift. Hence, theintensity is enhanced significantly as the NC becomessmaller due to strong confinement in the phonon scatter-ing region.In summary, the Raman spectra acquired from
SnO2 NCs with different morphology and size show thatthe IR active mode at ∼302 cm−1 appears in the Ramanspectrum. Its frequency can be affected slightly by the
NC size, but its intensity obviously depends on the NCsize. Theoretical derivation based on the DFT revealsthat the IR to Raman mode transformation arises fromthe change in the dipole moment due to the presenceof a certain amount of OVs. Hence, this mode impartsdirect information pertaining to OVs in SnO2 nano-structures.
This work was jointly supported by a Project Fundedby the Priority Academic Program Development of Jiang-su Higher Education Institutions (PAPD) and NationalBasic Research Programs of China under GrantNos. 2011CB922102 and 60976063. Partial support wasalso received from the Hong Kong Research GrantsCouncil (RGC) General Research Fund (GRF) No. CityU112510.
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