ARTICLE pubs.acs.org/JPCC
Pulsed Laser Ablation Induced Fragmentation, Transformation, Internal Stress, Sn2+/H+ Cosignature, and Optical Property Change of SnO2 Powders in Water Hui-Di Lu,† Bo-Cheng Lin,† Shuei-Yuan Chen,‡ and Pouyan Shen*,† † ‡
Department of Materials and Optoelectronic Science, National Sun Yat-sen University, Kaohsiung 80424, Taiwan, ROC Department of Mechanical and Automation Engineering, I-Shou University, Kaohsiung 84001, Taiwan, ROC ABSTRACT: The rutile-type SnO2 powders originally ca. 2550 nm in size were successfully size minimized by pulsed laser ablation in liquid (PLAL) under 532 nm excitation and 400 mJ per pulse at a specified water depth (5 to 20 mm) for up to 20 min in a slender vial filled with deionized water. Transmission electron microscopic observations indicated the nanoparticles via PLAL fragmentation have a minimum diameter of ca. 5 nm with a predominant rutile-type structure and an occasional high-pressure stabilized α-PbO2-type structure with 1-D 2 and 4 commensurate superstructures in accordance with a (021) shuffling derivation from a fluorite-type parental phase. Miniature size of the nanoparticles was more effective at a lower water level and a longer time of laser excitation yet limited by a coalescence process. The combined effects of nanosize, dense phase, and internal compressive stress, as well as Sn2+/H+ cosignature according to X-ray diffraction and spectroscopic results account for a lower minimum band gap down to ca. 2 eV for potential optocatalytic applications of such SnO2 nanoparticles.
’ INTRODUCTION The study of tin oxide with a dual valency of Sn was mainly motivated by its applications as a solid state gas sensor material, oxidation catalyst, and transparent conductor.1 Extensive studies have revealed some critical factors, mainly, size,2 microstructure,3 and surface modification,1,4 that govern the sensor properties of SnO2. Decreasing the crystalline size of SnO2 was found to be very effective in improving its gas sensitivity.2 It is thus of great interest to find a simple and clean route to fabricate SnO2 with uniform nanosize and specific crystal structures having composition and/or defect microstructure modifications. The SnO2, in the form of powders or films for engineering applications, commonly has a rutile-type structure (cassiterite, space group P42/mnm) with a varied extent of nonstoichiometry when fabricated under ambient pressure by a variety of synthesis techniques such as coprecipitation,5 ion sputtering,6 microwave heating,7 surfactants mediating,8 and solgel process.9 However, sandwiched SnO2 nanobelts with a rutile core but a surprising orthorhombic outer layer (due to oxygen deficiency) have been fabricated using elevated temperature synthesis techniques.10 This result is distinct from that for bulk SnO2 where pressures in excess of 155 kbar are required to form the orthorhombic structure.1114 (Static compression coupled with laser heating indicated that cassiterite transforms into a α-PbO2-type (space group Pbcn)11,12 and then a fluorite-type like structure at a higher pressure.13,14) Laser ablation on bulk SnO2 has been used to form rutile-type nanoribbons.15 However, pulsed laser ablation (PLA) on a metallic r 2011 American Chemical Society
Sn plate in oxygen ambient was used to form rutile-type nanocondensates with planar defects due to a coalescence event16 and to condense minor SnO2 nanoparticles with a fluorite-type derived dense structure due to very rapid heating and cooling effects of the PLA process.17 Here, PLA on floating powders in liquid (PLAL), a simple physical chemical process commonly adopted for the fragmentation of noble metals1822 and copper oxide,23 was extended to oxide, i.e., rutile-type SnO2. We focused on (1) the internal stress, (2) the Sn2+/H+ cosignature, (3) the phase change into a highpressured stabilized type with shear-induced planar defects, and (4) the resultant band gap narrowing of the SnO2 nanoparticles subjected to PLAL fragmentation. Such knowledge is of importance to potential sensor and optocatalytic applications of Sn2+/H+ cosignified SnO2 nanoparticles and sheds light on the phase behavior of the SnOH system in natural dynamic settings.
’ EXPERIMENTAL SECTION A fixed amount (0.01 g) of cassiterite SnO2 powders (Alfa Aesar, 99.9%, faceted and ca. 25 to 50 nm in size, cf., Appendix 1) in a slender vial (8 mm in diameter and 32 mm in length) filled with deionized water (ca. 6 cc depending on accumulated ablation time) for a starting concentration of ca. 0.002 g/cm3 Received: August 8, 2011 Revised: November 10, 2011 Published: November 28, 2011 24577
dx.doi.org/10.1021/jp2090268 | J. Phys. Chem. C 2011, 115, 24577–24585
The Journal of Physical Chemistry C were subject to energetic Nd:YAG-laser (Lotis, 1064 nm, beam mode: TEM00) pulse irradiation. The laser beam was focused to a spot size of 0.03 mm2 on specified water depths (5, 10, 15, and 20 mm) below the water level under a laser pulse energy of 400 mJ/pulse using second harmonic excitation, i.e., 532 nm, for better laser penetration into the water. The Q-switch mode for a pulse duration of 16 ns was adopted to achieve a peak power density of 8.35 1010 W/cm2 (average power density 1.33 104 W/cm2) at 10 Hz. Water refill and solution shaking after every 5 min of PLAL fragmentation were employed to ensure a uniform colloidal solution upon energetic irradiation for an accumulated time of 5, 15, 20, and 30 min in order to see if the water level significantly affects laser absorption and shock wave/plume expansion distribution as of concern to the fragmentation of SnO2 particles. The resultant colloidal solutions remained white, yet warmed up to ca. 50 °C. The phase identity and lattice parameters of the SnO2 nanoparticles via PLAL fragmentation, as retrieved from the colloidal solutions by centrifugation and then collection on sodalime glass slide, were determined by X-ray diffraction (XRD, Siemens D5000, Cu Kα at 40 KV, 30 mA, and 3s for each 0.05° increment from 20 to 80 of the 2θ angle). The composition and crystal structures of the individual nanoparticles collected on Cu grids overlaid with a carbon-coated collodion film were characterized by transmission electron microscopy (TEM, JEOL 3010 at 200 kV) coupled with selected area electron diffraction (SAED) and point-count energy dispersive X-ray (EDX) analysis at a beam size of 5 nm. The UVvisible absorption of the colloidal solutions before and after specific PLAL fragmentation treatments was characterized by the instrument of U-3900H, Hitachi, with a resolution of 0.1 nm in the range of 190 to 900 nm. Such colloidal solutions were also dipped onto silica glass and then dried for an X-ray photoelectron spectroscopy (XPS, JEOL JPS-9010MX photoelectron spectrometer with a Mg KR X-ray source) study of Sn 3d and O 1s binding energies calibrated with a standard of C 1s at 284.2 eV and to determine the Sn4+/Sn2+ ratio of SnO2 nanoparticles. The Raman spectrum of the SnO2 nanoparticles centrifuged from the colloidal solutions was made using a HeNe laser (Horiba HR800) under 633 nm excitation having a spatial resolution of 1 μm. Such centrifuged SnO2 nanoparticles were also mixed with KBr for a Fourier transform infrared spectroscopy (FTIR, Bruker 66v/S, 64 scans in the range of 4004000 cm1 with 4 cm1 resolution) study of the OH signature, in particular hydrogenation, if any, within the lattice.
’ RESULTS X-ray Diffraction. XRD of the starting SnO2 powders and the SnO2 nanoparticles prepared by PLAL fragmentation for 5 min at a specified water depth ranging from 5 to 20 mm, as compiled in Figure 1, showed only diffractions of the predominant phase, i.e., cassiterite of the rutile-type structure. The d-spacings of cassiterite are significantly smaller after PLAL fragmentation regardless of the difference in water depths for excitation as manifested by the 220 diffraction magnified in Figure 1. Structure refinement of the d-spacings further indicates smaller lattice parameters a and c and a smaller c/a ratio for the cassiterite nanoparticles after PLAL fragmentation (Table 1). The water level in the PLAL process is expected to affect laser absorption and shock wave/plume expansion distribution to cause varied strain of the SnO2 nanoparticles. Such effect turned out to be
ARTICLE
Figure 1. XRD (CuKα) traces of (a) the starting SnO2 powder and the SnO2 nanoparticles via PLAL fragmentation at a specified water depth for (b) 5 mm, (c) 10 mm, (d) 15 mm, and (e) 20 mm to have smaller dspacings of the predominant rutile-type structure, cf., the magnified 220 diffraction peak inset.
Table 1. Refined d-Spacings ((0.0001 nm) and Lattice Parameters ((0.0001 nm) of Starting Rutile-type SnO2 Powders (standard) and the Same Type SnO2 Nanoparticles via PLAL Fragmentation at a Specified Water Depth of 5 to 20 mm for a Fixed Total Excitation Time of 5 min hkl
standard
5 mm
10 mm
15 mm
20 mm
110
0.3358
0.3349
0.3354
0.3349
0.3349
101
0.2652
0.2642
0.2647
0.2643
0.2644
200
0.2375
0.2368
0.2372
0.2368
0.2368
211
0.1769
0.1764
0.1766
0.1764
0.1764
220
0.1679
0.1675
0.1677
0.1675
0.1675
0.1592
0.1595
0.1592
0.1593
002 310 112
0.1502 0.1443
0.1498 0.1438
0.1500 0.1440
0.1498 0.1438
0.1498 0.1439
301
0.1419
0.1415
0.1417
0.1415
0.1415
202
0.1326
0.1321
0.1323
0.1322
0.1322
321
0.1218
0.1214
0.1216
0.1215
0.1215
a
0.4749
0.4737
0.4744
0.4737
0.4736
c
0.3197
0.3184
0.3189
0.3185
0.3186
c/a
0.6732
0.6722
0.6722
0.6724
0.6728
insignificant because the adopted depth of focus (118 mm, using a Gaussian beam under fundamental transverse TEM00 mode) is longer than the water levels. An additional XRD study of the sample subject to PLAL at a specified water depth of 15 mm for 5 min vs 30 min (Appendix 2) indicated that a longer laser excitation time did not cause further change of the cassiterite dspacings. By contrast, miniature size of the nanoparticles was found to be more effective at a longer time of laser excitation as well as a lower water level, yet limited by a coalescence process as revealed by the following TEM observations. Transmission Electron Microscopy. TEM bright field image (BFI) and SAED of the SnO2 powders subjected to various PLAL treatments in this study indicated that the predominant 24578
dx.doi.org/10.1021/jp2090268 |J. Phys. Chem. C 2011, 115, 24577–24585
The Journal of Physical Chemistry C
ARTICLE
Figure 2. TEM BFI (left) and SAED pattern (right) of the rutile-type SnO2 nanoparticles via PLAL fragmentation at (a) 10 mm and (b) 20 mm water depth for a fixed excitation time of 5 min. Note more significant fragmentation of the particles down to 5 nm in size for the case of a than for b.
rutile-type cassiterite particles were significantly reduced in size down to ca. 5 nm. In general, PLAL fragmentation was more effective at a shallower water depth at a given laser excitation time as shown in Figure 2 for the case of 10 vs 20 mm water depth at a fixed PLAL time of 5 min. Extending laser excitation time from 5 to 20 min at a fixed water depth also caused more complete fragmentation of the particles toward 5 nm in size as shown in Figure 3 and Appendix 3 for the cases of 5 mm and 15 mm water depths, respectively. In any case, the SnO2 nanoparticles via PLAL fragmentation tended to coalesce and agglomerate as a close packed manner. A lattice image coupled with 2-D forward and inverse Fourier transform further showed that the typical cassiterite nanoparticles via PLAL fragmentation have well-developed {110}, {101} (Figure 4), and (100) facets (Figure 5) and hence corrugated appearance, which could be affected also by the H+/Sn2+ cosignature as revealed by latter spectroscopic results. The SnO2 nanoparticles via PLAL fragmentation as 5 nm in size were occasionally found to have a high-pressure stabilized αPbO2-type structure. The lattice image coupled with 2-D forward and inverse Fourier transform (Figure 6) indicated such α-PbO2type SnO2 nanoparticles have well-developed (021) and (110) surfaces for parallel epitaxial coalescence into unity. The 1-D 2 and 4 commensurate superstructures in terms of (021) shuffling in one of the α-PbO2-type nanoparticles indicated it was derived from a fluorite-type parent phase as discussed later. Vibrational Spectroscopy. The starting cassiterite powders showed Raman bands at 474 cm1 (Eg), ∼500 cm1 (S2), ∼570 cm1 (S1), 634 cm1 (A1g), ∼700 cm1 (S3), and 775 cm1 (B2g) with the band assignments in parentheses from ref 24, where A1g, B2g, and Eg correspond to the classical vibration modes, while bands S1, S2, and S3 appear as a consequence of disorder activation. The SnO2 powders subjected to PLAL
Figure 3. TEM BFI of the rutile-type SnO2 nanoparticles via PLAL fragmentation for (a) 5 min and (b) 20 min at a fixed water depth of 5 mm. Note more significant fragmentation of the particles down to 5 nm in size for the case of b than for a.
fragmentation at a specified water depth of 15 mm for 5 to 30 min (Figure 7) showed basically the same Raman bands yet with different intensities as compiled in Table 2. The AS/AA1g ratio, i.e., the sum of the areas of bands S1 and S2 with respect to that of the A1g mode, increases with the increase of PLAL time and therefore the decrease of particle size. Note the nondegenerate mode A1g is about oxygen vibrating in the plane perpendicular to the c axis, while the doubly degenerated Eg mode vibrates in the direction of the c axis according to the study of uniaxial-stress dependence of the first-order Raman spectrum of TiO2 with a rutile-type isostructure.25 Thus, an increasing AS/A1g ratio indicates that oxygen vibration of the cassiterite nanoparticles via PLAL fragmentation experienced more constraint within the basal plane than along the c axis upon miniature size and accompanied increase of Sn4+ content. Note also that the classical vibration modes A1g and B2g have lower wave numbers, whereas the surface disorder modes S1 and S2 have higher wave numbers as the particle size decreases from ca. 100 to 4 nm (see Table 2 of ref 24). FTIR spectrum of the starting SnO2 powders and the representative sample after PLAL fragmentation under a specified water depth of 10 mm for 5 min showed the characteristic vibration modes and OH-signature of cassiterite (Figure 8). PLAL fragmentation caused an enhancement of the lattice vibration overtone at 24579
dx.doi.org/10.1021/jp2090268 |J. Phys. Chem. C 2011, 115, 24577–24585
The Journal of Physical Chemistry C
ARTICLE
Figure 4. TEM (a) lattice image of the representative rutile-type SnO2 nanoparticle via PLAL fragmentation at 15 mm water depth for 30 min to have a reduced size and corrugated appearance; (b and c) 2-D forward and inverse Fourier transform, respectively, of the nanoparticle in the square region of panel a showing well-developed {110} and {101} facet edge on in the [111] zone axis.
Figure 5. TEM (a) lattice image of the representative rutile-type SnO2 nanoparticle via PLAL fragmentation at 5 mm water depth for 20 min to have a reduced size and corrugated appearance; (b and c) 2-D forward and inverse Fourier transform, respectively, of the nanoparticle in the square region of panel a showing well-developed (100) beside the {110} facet edge on in the [001] zone axis.
1384 cm1 and the replacement of weak OH vibration mode at ∼3600 cm1 by the strong OH vibration mode at 3456 cm1, which can be attributed to extensive replacement of the terminal SnOH by the bridged SnOH, i.e., hydrogenation within the lattice, following the assignment of ref 26. XPS and UVVisible Absorption. Figures 9a and 9b showed the XPS binding energy of Sn and O, respectively, for the starting SnO2 powders and the SnO2 nanoparticles via PLAL fragmentation under a specified water depth of 10 mm for 5 min. The starting powders have characteristic Sn 3d5/2 (485.8 eV), Sn 3d3/2 (494.2 eV) (Figure 9a), and O 1s (530.4 eV) with the binding energy specified in parentheses. By contrast, the hydrogenated and Sn4+-enriched SnO2 nanocondensates via PLAL fragmentation have significantly higher binding energy for Sn 3d5/2 (486.0 eV), yet slightly higher binding energy for Sn 3d3/2 (494.4 eV) and O 1s (530.6 eV). The Sn 3d5/2 binding in fact has mixed valence states of +2 and +4, and the resolved peak height based on Lorentz fitting indicated that the 4+/2+ ratio increases from 1.2 for the starting powders (Figure 9c) to 3.7 for the nanoparticles via PLAL fragmentation (Figure 9d). The UVvisible absorption traces of the representative SnO2 nanoparticles via PLAL fragmentation at a specified water depth of 15 mm for 15 min (Figure 10a) and 20 min (Figure 10b) correspond to a minimum band gap of 2.88 and 1.99 eV, based on their intersection with the baseline at 542.78 and 624.48 nm, respectively.
PLAL can be explained by the Coulomb explosion model2731 or photothermal evaporation model,32 which are favored in fs and ns regimes, respectively, on the basis of the two-temperature model of electron temperature Te, lattice temperature Tl, and the temperature of the medium surrounding the particle.33 As summarized in ref 33, the Coulomb explosion model presumes ejection of quite a number of electrons to generate multiply ionized nanoparticles to undergo spontaneous fission because of the charge repulsion depending on the excitation wavelength of 532 and 355 nm that resonant to the intraband (surface plasmon band) and interband, respectively, as indicated by transient absorption spectra measurements,29 whereas the photothermal evaporation model is based on the classical thermodynamics for the observation of a size reduction of aqueous gold nanoparticles down to ca. 10 nm in diameter by exposure to a 532 nm ns pulsed-laser beam for melting and evaporation as indicated by the temperature rise estimated from the absorbed laser energy and the measurements of blackbody radiation from the particles.32 It is beyond the scope of this work to obtain transient absorption spectra of the aqueous SnO2 particles during PLAL fragmentation in order to prove the photoionization event involving single electron multiphoton absorption and single electron photoejection for fragmentation of the particles and enhanced reactivity with the solvent molecules. However, the lattice temperature of the floating SnO2 nanoparticles is expected to rise to the melting (Tm = 1630 °C) and boiling temperatures (Tb = 18001900 °C) in the present PLAL process in view of the same 532 nm excitation in the ns regime, yet at a 103 times higher fluence, given a higher pulse energy and a smaller beam size (400 mJ, 0.2 mm) than that (300 mJ, 7 mm) adopted in ref 32.
’ DISCUSSION Size Change Mechanisms via PLAL Fragmentation. Fragmentation of noble metal (mainly, Au and Ag) nanoparticles by
24580
dx.doi.org/10.1021/jp2090268 |J. Phys. Chem. C 2011, 115, 24577–24585
The Journal of Physical Chemistry C
ARTICLE
Table 2. Raman Shift Wavenumbers and Intensity Ratio of Specific Bands for the SnO2 Nanoparticles via PLAL Fragmentation for 5 to 30 min at a Fixed Water Depth of 15 mma total PLAL time
A1g
5 min
634.1
15 min
633.5
20 min 30 min
S1
S2
As/A1g
NA
506.5
0.047
NA
503.5
0.051
634.1
NA
500.7
0.067
633.8
569.1
NA
0.763
The unit of wave numbers is cm1; A1g is the sum of S1 and S2 (cf., text); and the abbreviation NA means not available. a
Figure 6. TEM (a) lattice image of the representative α-PbO2-type SnO2 nanoparticles via PLAL fragmentation at 15 mm water depth for 20 min; (b and c) 2-D forward and inverse Fourier transform, respectively, of the nanoparticles in the square region of panel a showing their parallel epitaxial coalescence over well-developed (021) and/or (110) surfaces into unity in the [112] zone axis. The left corner of panel c shows 1-D 2 and 4 commensurate superstructures (denoted as 2S and 4S, respectively) in terms of (021) shuffling indicating a fluorite-type parental phase (cf., text).
Figure 7. Raman shift of the SnO2 nanoparticles via PLAL fragmentation at a specified water depth of 15 mm for 5 to 30 min showing characteristic vibration modes of the rutile-type structure. Note the classical vibration modes A1g and B2g have lower wave numbers, whereas the surface disorder modes S1 and S2 have higher wave numbers compared to those of the original powders before PLAL fragmentation.
(The temperature of SnO2 nanoparticles during PLAL fragmentation can be calculated to fall in the temperature range of
Figure 8. Representative FTIR spectrum of the SnO2 powder before (dark) and after (red) PLAL fragmentation at a specified water depth of 10 mm for 5 min showing the characteristic vibration modes and OHsignature of the rutile-type structure.
30004000 °C, assuming 1% laser absorption efficiency by the nanoparticles following the equations in ref 32.) In the present 532 nm excitation, the photoionization and photothermal processes may both occur to account for the fragmentation of cassiterite nanoparticles and the nucleation of the α-PbO2-type SnO2, respectively. In any case, thermal shock by rapid cooling in water under the influence of the bubble effect would facilitate fragmentation/cleaving of the cassiterite nanoparticles and the formation of the α-PbO2-type SnO2. The SnO2 nanoparticles via PLAL fragmentation in water within the slender vial have a lower limit of size around 5 nm. It is not clear if such a miniature size limit has anything to do with much larger-sized cavitation bubbles associated with the particles for effective thermal shock.34 In any case, the radiant heating effect during PLAL fragmentation may cause significant Brownian motion and coalescence of the nanoparticles less than 5 nm in size. It should be noted that our additional PLAL fragmentation experiment of the same floating SnO2 powders in the vial with twice larger inner diameter did not efficiently produce nanoparticles smaller than 10 nm in size despite a prolonged excitation up to 1 h at a specified water depth of 15 mm. This indicates that space constraint of the colloidal solution may affect laser excitation and/or collision of the floating particles for an effective miniature size. In fact, a larger volume of irradiated solution would lower the absorbed laser energy32 and hence less fragmentation of the particles. 24581
dx.doi.org/10.1021/jp2090268 |J. Phys. Chem. C 2011, 115, 24577–24585
The Journal of Physical Chemistry C
ARTICLE
Figure 9. XPS of (a) Sn and (b) O of the starting SnO2 powder (black trace) and the SnO2 nanoparticles via PLAL fragmentation at a specified water depth of 10 mm for 5 min (red trace arrowed). The mixed valence states of +2 and +4 were revealed by Lorentz fittings of the Sn 3d5/2 doublets (c) for the starting powder and (d) the nanoparticles via PLAL fragmentation.
Defect Chemistry and Shape of Sn2+/H+-Codoped SnO2 Nanoparticles. Although electro-chemical measurements are re-
quired to understand the extent of departure from equilibrium, defect chemistry equations in terms of point defects are justified thermodynamically for the present (Sn2+,H+)-cosignified SnO2 particles. The defect chemistry for the substitution of Sn4+ with Sn2+ in coordination number (CN) 6 to form SnSn00 for the rutile-type can be assigned as the following equation in Kr€ogerVink notation:35 SnO s SnO Snsn 00 þ V o3 3 þ Oxo s2
ð1Þ
Alternatively, Sn4+ can enter the interstitial octahedral sites binding with oxygen vacancies36 via the following equation to affect the crystallographic shear and defect clustering schemes: Sni3 3 3 3 þ V o3 3 þ 3SnSn 00 þ 4Oxo 4SnO s SnO s2
ð2Þ
It is also likely to have a single-charge proton (or electron hole h 3 ), instead of VO.. in eq 1, to act as the charge-compensating defect for SnSn00 SnðOHÞ2 s SnO SnSn 00 þ 2h 3 þ 2Oxo s2
Figure 10. UVvisible absorption spectra of the SnO2 nanoparticles via PLAL fragmentation at a specified water depth of 15 mm for (a) 15 min and (b) 20 min.
ð3Þ
and hence a larger cell parameter of the present starting SnO2 powders prepared by a hydrothermal process. As the Sn4+/Sn2+ ratio increases by the PLAL fragmentation process, eq 2 would compete with eq 3 to have optimized internal hydrogen bonding and oxygen vacancies to account for the observed optical properties. In this regard, tin(II) oxy-hydroxide having a tetragonal symmetry, yet with 24582
dx.doi.org/10.1021/jp2090268 |J. Phys. Chem. C 2011, 115, 24577–24585
The Journal of Physical Chemistry C space group and cell parameters (P4/mnc; a = 0.793 nm and c = 0.913 nm) significantly different from that of the rutiletype (P4/mnm), has been prepared from a stannous perchlorate solution.37 Apparently, the SnO2 nanoparticles via the present dynamic PLAL fragmentation process are not necessarily in thermodynamic equilibrium and hence not hydrogenated enough to form such a tin(II) oxy-hydroxide. It is by no means clear if the defect chemistry eqs 13 proposed for the rutile-type SnO2 nanoparticles can be extended to high-pressure stabilized α-PbO2-type SnO2 and its parental fluorite-type related structure with Sn in CN 7 and/or 8. Theoretical calculation using the periodic quantum mechanical method with the density functional theory indicated the specific surface energy of the SnO2 crystal with rutile-type structure is (110) < (100) < (101) < (001) in the increasing order.38 The preferential growth of SnO2 nanorods along [112] was indeed reported to have well-developed (110) surface, although under the influence of the capping agent ethylenediamine.39 The rutile-type SnO2 nanoparticles via the present PLAL fragmentation process have well-developed (100) and {101} competing with {110} presumably due to the Sn2+/H+-cosignature and charge-compensating defects for the corrugated appearance of the nanoparticles. Internal Stress and High-Pressure Polymorphs of Sn2+/H+Codoped SnO2 Nanoparticles. The synthetic and/or natural rutile-type SnO2 has varied lattice parameters at ambient atmospheric conditions due to a valence change of Sn, tramp impurities, and charge-compensating defects. For example, the starting SnO2 powders with enriched Sn2+ as indicated by XPS results have larger lattice parameters (a = 0.4749 nm and c = 0.3197 nm; cf., Table 1) than the SnO2 powders used for the in situ XRD study of the compressed samples.40 The rutile-type SnO2 nanoparticles via PLAL fragmentation typically have the refined lattice parameters a = 0.4736 nm and c = 0.3186 nm (Table 1), which are 0.28% and 0.34% smaller than the starting powders. This indicates a significant internal compressive stress of the rutile-type SnO2 nanoparticles via PLAL fragmentation, aside from the effect of protonation and increasing Sn4+/Sn2+ ratio as indicated by the combined FTIR and XPS results. The bulk modulus and its pressure derivative are not available for H+-signified and/or Sn2+-doped SnO2 cassiterite to determine the internal stress level using BirchMurnaghan equation of state. The pressure-dependent Raman shift of rutile-type SnO2 codoped with Sn2+ and H+ is not available either for the stress level estimation of the present nanoparticles. Still, the coexistence of α-PbO2-type SnO2 nanoparticles indicates that the internal compressive stress could be up to ca. 16 GPa in view of static compression and laser heating results.8 As for the defect in the α-PbO2-type SnO2 nanoparticles via the present PLAL fragmentation, extended defects, i.e., 1-D commensurate (2 and 4) superstructures parallel to (021), were clearly identified by TEM. Such planar defects could have something to do with crystallographic shear (CS) due to the substitution of Sn4+ with Sn2+ in CN 6 to form SnSn00 and charge compensating oxygen vacancies for α-PbO2-type SnO2 for cassiterite with rutile-type structure following defect chemistry eq 1. In this connection, CS was known to occur for the nonstoichiometric TiO2x rutile with oxygen vacancy distribution along specific atomic planes,4145 Zr-doped TiO2 rutile,46 and ZrTiO4 with a α-PbO2-type structure.47 The α-PbO2-type SnO2 with (021) commensurate structures was likely derived from a parental fluorite-type (f) or its related monoclinic (m) structure by shearing (100)f,m along [010]f,m to
ARTICLE
become (021). In fact, such shearing to reach the epitaxial relationship (001)f,m//(100) and (100)f,m//(021) has been predicted for transition metal dioxides based on continuous topological distortions of three-dimensional anion nets48 and experimentally proven for the dense TiO2 nanocondensates fabricated by PLA on metallic Ti target in air with purged oxygen (see Figure 6 of ref 49). Minimum Band Gap of Sn2+/H+-Codoped SnO2. The Sn2+and H+-codoped SnO2 nanoparticles fabricated by the present PLAL fragmentation process has a minimum band gap narrowed down to ca. 2 eV, much lower than the reported band gap Eg = 3.6 eV for the undoped bulk material of cassiterite SnO2.50 A significant change of the band gap to a lower value for the present nanoparticles can be attributed to the following combined factors: (i) ultrafine particle size down to 5 nm, (ii) internal compressive stress of the rutile-type structure and the coexistence of minor high-pressure stabilized α-PbO2-type structure with extended defects, and (iii) enhancement of the bridged SnOH bonding and retention of Sn2+ despite the increase of the Sn4+/Sn2+ ratio via PLAL fragmentation. It is difficult, if not impossible, to evaluate the relative importance of the three factors of band gap narrowing. However, a miniature size would be the ultimate cause that affects the capillarity force and solute trapping of the particles,51 i.e., the (Sn2+,H+)-cosignature in this case. SnO2 is an optoelectrical n-type semiconductor with dual Sn valency and oxygen potential dependent surface state for gas sensing and catalysis applications as summarized in ref 1. In general, the surface state of SnO2 particles can be modified by size and additives to either increase the charge carrier concentration by donor atoms or to increase the gas sensitivity or the catalytic activity by metal additives.1 The extent of oxygen termination on the specific (hkl) surface also affects sensing and catalysis capability. For example, recent photoemission and density functional theory studies showed that water adsorbs dissociatively on the SnO2(101) surface in the presence of terminating oxygen atoms and molecularly if these surface oxygen atoms are removed.52 The different chemical surface responses of these two bulk terminations of SnO2 was also found to change the water induced band bending and consequently the conductivity of the gas sensing material.51 Regarding heterogeneous catalysis, SnO2 having dual Sn valency is an oxidation catalyst, which exhibits good activity toward CO/O2 and CO/NO reactions when doped or supported with suitable materials.1 Having miniature size, (Sn2+,H+)-cosignature, well-developed surfaces, (110), (101), and (100), internal stress, and hence a minimum band gap down to ca. 2 eV, the SnO2 nanoparticles via the present PLAL fragmentation process are thus expected to have potential sensing and catalytic applications.
’ CONCLUSIONS The Sn2+/H+-cosignified SnO2 nanoparticles with size reduced down to a lower limit of ca. 5 nm and minimum band gap narrowed down to ca. 2 eV for potential photoelectric/catalytic applications were fabricated by PLAL fragmentation of submicrometer sized cassiterite powders at a specified water depth and excitation time. The SnO2 nanoparticles via such a PLAL fragmentation process are mainly rutile-type structures and minor high-pressure stabilized α-PbO2-type structures having 1-D 2 and 4 commensurate superstructures parallel to the (021) plane, possibly due to shuffling of a fluorite-type related parental phase. The phase behavior of SnO2 nanoparticles via 24583
dx.doi.org/10.1021/jp2090268 |J. Phys. Chem. C 2011, 115, 24577–24585
The Journal of Physical Chemistry C PLAL fragmentation in water, in particular, valency change and the formation of dense phase with planar defects, may shed light on the natural aqueous shock occurrence of analogous dioxides with postrutile structures, such as SiO2 with the α-PbO2-like structure53 and the baddeleyite-type structure54 in the meteorite Shergotty, and the α-PbO2-type TiO2 in shocked gneisses from the Ries Meteorite Crater in Germany.55
ARTICLE
significant fragmentation of the particles down to 5 nm in size for the case of b than for a.
’ APPENDIX 1 TEM BFI of original cassiterite SnO2 powders, which are faceted and ca. 25 to 50 nm in size.
’ APPENDIX 2 XRD (CuKα) of the starting rutile-type SnO2 powders (bottom trace) and the SnO2 nanoparticles via PLAL fragmentation at a specified water depth of 15 mm for 5 min (middle trace) and 30 min (top trace) to have smaller d-spacings than the starting powders. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT We thank Miss S. Y. Shih for the help on XPS analysis and anonymous referees for constructive comments. This research was supported by the Center for Nanoscience and Nanotechnology at NSYSU and partly by the National Science Council, Taiwan, ROC. ’ REFERENCES
’ APPENDIX 3 TEM BFI of the SnO2 nanoparticles with a predominant rutile-type structure via PLAL fragmentation at a specified water depth of 15 mm for (a) 5 min and (b) 20 min. Note more
(1) Batzill, M.; Diebold, U. Prog. Surf. Sci. 2005, 79, 47. (2) Xu, C.; Tamaki, J.; Miura, N.; Yamazoe, N. Sens. Actuators, B 1991, 3, 147. (3) Sakai, G.; Baik, N. S.; Miura, N.; Yamazoe, N. Sens. Actuators, B 2001, 77, 116. (4) Epifani, M.; Arbiol, J.; Pellice, E.; Comini, E.; Sicilian, P.; Faglia, G.; Morante, J. R. Cryst. Growth Des. 2008, 8, 1774. (5) Mosquera, A.; Rodríguez-Paez, J. E.; Varela, J. A.; Bueno, P. R. J. Eur. Ceram. Soc. 2007, 27, 3893. (6) Ruske, M.; Brauer, G.; Pistner, J.; Pfafflin, U.; Szczyrbowski, J. Thin Solid Films 1999, 351, 146. 24584
dx.doi.org/10.1021/jp2090268 |J. Phys. Chem. C 2011, 115, 24577–24585
The Journal of Physical Chemistry C (7) Zhu, J. J.; Zhu, J. M.; Liao, X. H.; Fang, J. L.; Zhou, M. G.; Chen, H. Y. Mater. Lett. 2002, 53, 12. (8) Liang, C.; Shimizu, Y.; Sasaki, T.; Koshizaki, N. J. Phys. Chem. B 2003, 107, 9220. (9) Kurz, A.; Brakecha, K.; Puetz, J.; Aegerter, M. A. Thin Solid Films. 2006, 502, 212. (10) Dai, Z. R.; Gole, J. L.; Stout, J. D.; Wang, Z. L. J. Phys. Chem. B 2002, 106, 1274. (11) Liu, L. G. Phys. Earth Planet. Inter. 1974, 9, 338. (12) Suito, K.; Kawai, N.; Masuda, Y. Mater. Res. Bull. 1975, 10, 677. (13) Liu, L. G. Science 1978, 199, 422. (14) Endo, S.; Nitawaki, S.; Shige, T.; Akahama, Y.; Kikegawa, T.; Shimomura, O. High Pressure Res. 1990, 4, 408. (15) Hu, J.; Bando, Y.; Liu, Q.; Goldberg, D. Adv. Funct. Mater. 2003, 13, 493. (16) Tseng, W. J.; Shen, P.; Chen, S. Y. J. Solid State Chem. 2006, 179, 1237. (17) Tseng, W. J.; Shen, P.; Chen, S. Y. J. Phys. Chem. Solids 2009, 70, 334. (18) Yang, G. W. Prog. Mater. Sci. 2007, 52, 648. (19) Barcikowski, S.; Devasa, F.; Moldenhauer, K. J. Nanopart. Res. 2009, 11, 1883. (20) Amendola, V.; Meneghetti, M. Phys. Chem. Chem. Phys. 2009, 11, 3805. (21) Sakamoto, S.; Fujutsuka, M.; Majima, T. J. Photochem. Photobiol., C 2009, 10, 33. (22) Semaltianos, N. G. Crit. Rev. Solid State Mater. Sci. 2010, 35, 105. (23) Kwasaki, M.; Nishimura, N. EOS Conference on Laser Ablation and Nanoparticle Generation in Liquids, Ene_gelber_g, Switzerland, 2010. (24) Dieguez, A.; Romano-Rodríguez, A.; Vila, A.; Morante, J. R. J. Appl. Phys. 2001, 90, 1550. (25) Merle, P.; Pascual, J.; Camassel, J.; Mathieu, H. Phys. Rev. B 1980, 21, 1617. (26) Amalric-Popescu, D.; Bozon-Verduraz, F. Catal. Today 2001, 70, 139. (27) Kamat, P. V.; Fluminani, M.; Hartland, G. V. J. Phys. Chem. B 1998, 102, 3123. (28) Yamada, K.; Tokumoto, Y.; Nagata, T.; Mafune, F. J. Phys. Chem. B 2006, 110, 11751. (29) Yamada, K.; Miyajima, K.; Mafune, F. J. Phys. Chem. C 2007, 111, 11246. (30) Shoji, M.; Miyajima, K.; Mafune, F. J. Phys. Chem. C 2008, 112, 1929. (31) Muto, H.; Miyajima, K.; Mafune, F. J. Phys. Chem. C 2008, 112, 5810. (32) Takami, A.; Kurita, H.; Koda, S. J. Phys. Chem. B 1999, 103, 1226. (33) Werner, D.; Hashimoto, S. J. Phys. Chem. C 2011, 115, 5063. (34) Sajti, C. L.; Sattari, R.; Chichkov, B. N.; Barcikowski, S. J. Phys. Chem. C 2010, 114, 2421. (35) Kr€oger, F. A.; Vink, H. J. Solid State Phys. 1956, 3, 307. (36) Kílíc-, C.; Zunger, A. Phys. Rev. Lett. 2002, 88, 095501. (37) Howie, R. A.; Moser, W. Nature 1968, 219, 372. (38) Beltran, A.; Andres, J.; Sambrano, J. E.; Longo, E. J. Phys. Chem. A 2008, 112, 8943. (39) Cheng, G.; Wu, K.; Zhao, P.; Cheng, Y.; He, X.; Huang, K. Nanotechnology 2007, 18, 355604. (40) Haines, J.; Leg_ er, J. M. Phys. Rev. B 1997, 55, 11144. (41) Andersson, S. Acta Chem. Scand. 1960, 14, 1161. (42) Bursill, L. A.; Hyde, B. G.; Philp, D. K. Philos. Mag. 1970, 23, 1501. (43) Bursill, L. A.; Hyde, B. G. Acta Crystallogr., Sect. B: Struct. Sci. 1971, 27, 210. (44) Bursill, L. A.; Hyde, B. G. Prog. Solid State Chem. 1972, 7, 177. (45) Sørensen, O. T., Ed. Non-Stoichiometric Oxides; Academic Press: New York, 1981; pp 2831, 6465.
ARTICLE
(46) Yang, K. C.; Shen, P.; Gan, D. J. Solid State Chem. 2006, 179, 3478. (47) Yang, K. C.; Shen, P.; Gan, D. J. Eur. Ceram. Soc. 2008, 28, 1169. (48) Hyde, B. G.; Bursill, L. A.; O’Keeffe, M.; Andersson, S. Nature 1972, 237, 35. (49) Chen, S. Y.; Shen, P. Jpn. J. Appl. Phys. 2004, 43, 1519. (50) Fr€ohlich, D.; Kenklies, R.; Helbig, R. Phys. Rev. Lett. 1978, 41, 1750. (51) Porter., D. A.; Easterling, K. E.; Sherif, M. Y. Phase Transformations in Metals and Alloys, 3rd ed.; CRC Press: Boca Raton, FL, 2009. (52) Batzill, M.; Bergermayer, W.; Tanaka, I.; Diebold, U. Surf. Sci. 2006, 600, 29. (53) Sharp, T. G.; El Goresy, A.; Wopenka, B.; Chen, M. Science 1999, 284, 1511. (54) El Goresy, A.; Dubrovinsky, L.; Sharp, T. G.; Saxena, S. K.; Chen, M. Science 2000, 288, 1632. (55) El Goresy, A.; Gillet, P.; Chen, M.; Dubrovinsky, L.; Sharp, T.G. 10th Annual V.M. Goldschmidt Conference; Cambridge Publications, Ltd.: Cambridge, U.K., 2000; version 5, no. 2, p 379.
24585
dx.doi.org/10.1021/jp2090268 |J. Phys. Chem. C 2011, 115, 24577–24585
