Resonant Two-Photon Oxidation in Vanadium Oxyhydrate Nanowires

Article pubs.acs.org/JPCC

Resonant Two-Photon Oxidation in Vanadium Oxyhydrate Nanowires above a Threshold Laser Intensity Ç . Ö zge Topal,† Susheng Tan,‡ Hongbing Lu,§ Nicholas Leventis,⊥ and A. Kaan Kalkan*,† †

School of Mechanical and Aerospace Engineering, Oklahoma State University, Stillwater, Oklahoma 74078, United States Department of Electrical and Computer Engineering & NanoScale Fabrication and Characterization Facility, Petersen Institute of NanoScience and Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, United States § Department of Mechanical Engineering, The University of Texas at Dallas, Richardson, Texas 75080, United States ⊥ Department of Chemistry, Missouri University of Science and Technology, Rolla, Missouri 65409, United States ‡

S Supporting Information *

ABSTRACT: The present work discloses the unusual photooxidation observed for V3O7·H2O nanowires under 514 nm excitation above a threshold intensity of 0.30 kW/cm2. We explicate this phenomenon by insitu Raman and photoluminescence spectroscopy at varying laser intensities as well as models for the transformation kinetics and energy band structure associated with H2OVO5 octahedron. The photooxidation is found to be triggered by two-photon cleavage of the H2O−V bond through excitation via nonbonding d-states. Subsequently, V3O7 spontaneously oxidizes to V2O5. However, the competing process of H2O’s rebonding is also realized. Hence, transformation to V2O5 occurs only if the H2O−V bond-cleavage rate exceeds a threshold, pushing the number of concomitantly broken bonds in the smallest structural unit to a critical number.

1. INTRODUCTION A transition metal has the versatility to form oxides (and oxide hydrates) of various stoichiometries and crystal structures, as it can exist in different oxidation states. Therefore, transition metal oxides (TMOs) can undergo oxidation/deoxidation under electron/ion bombardment,1 heat treatment,2 and electromagnetic radiation.3 This attribute of the TMOs is attractive for applications in catalysis, chemical sensing, and thermo- and photochromism.4−9 TMOs also have a unique energy band structure, which has rendered them as pigments in pottery, stained glass, and paintings for centuries. In addition to bonding and antibonding molecular orbitals associated with metal−oxygen bonds, TMOs accommodate nonbonding dorbitals, which may lie in energy in between the former two (i.e., depending on the coordination of the structural unit).10 Thereby, bonding-to-d transitions can be excited by visible photons in a number of TMOs with the potential to drive chemical reactions. Indeed, oxidation/deoxidation in TMOs under visible radiation was reported earlier.3,9 However, the common feature of these oxidation/deoxidation reactions is that they are all activated by the thermal energy generated by laser irradiation at intensity levels of 102−103 kW/cm2.3,9,11,12 The present work, on the other hand, reveals a photochemical (i.e., nonthermal) conversion in a vanadium metal oxide hydrate (oxyhydrate), namely the photooxidation of V3O7·H2O (V3O8H2) to V2O5 which is driven at a laser intensity as low as 0.30 kW/cm2. As measured by Raman spectroscopy, the laser-induced temperature rise during the transformation is as low as a few °C. We explore the © 2012 American Chemical Society

photooxidation using in-situ Raman and photoluminescence spectroscopy at varying laser intensities as well as models for the transformation kinetics and energy band structure on the basis of H2OVO5 octahedron. As different from other photochemical reactions, such as in the exposure of a photographic film or 3D photopolymerization by two-photon absorption (not resonant), the transformation investigated here occurs above a clear laser intensity threshold. For example, the V3O7·H2O nanowires are stable even at 95% of the threshold intensity for 1 h (i.e., the longest exposure tested). Intensity threshold or strong nonlinearity in photochemical transformations is of significant interest for laser writing of submicrometer features. Example applications are high density optical data storage and optical nanolithography. Essentially, the underlying principle in these applications is bypassing the diffraction limit of light by using only the central region of a diffraction-limited laser probe, where the intensity is at the highest. Hence, materials exhibiting photochemical transformations with intensity thresholds are ideal for selective excitation and photomodification in the beam center within a diameter significantly smaller than the wavelength.

2. EXPERIMENTAL SECTION Synthesis of Aerogels. V3O7·H2O aerogels were prepared by supercritical drying of wet vanadia gels obtained via Received: November 10, 2011 Revised: April 17, 2012 Published: April 17, 2012 10186

dx.doi.org/10.1021/jp2108494 | J. Phys. Chem. C 2012, 116, 10186−10192

The Journal of Physical Chemistry C

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camera saturation. A standard TEM grid (Ted Pella) was used for calibrating the size scale of the field of view acquired by the CCD camera. Then, the laser intensity was calculated as incident power divided by the laser spot size on the sample. The photooxidation threshold intensity, 0.30 kW/cm 2, corresponds to 0.6 mW incident power. For Raman acquisitions in an inert environment (e.g., oxygen-free and water-free), samples were enclosed in septum-sealed optical cells, which were subsequently purged with argon gas for 15 min prior to the acquisition. Photooxidation Kinetics by Photoluminescence. The V3O7·H2O to V2O5 transformation kinetics was monitored from the time variation of the photoluminescence (PL) intensity at 750 nm. At this wavelength, the PL intensity of V2O5 was measured to be 4.8 times that of V3O7·H2O. The fraction of transformed V2O5, X, was calculated using the level rule: I = XIV2O5 + (1 − X)IV3O7·H2O, where I is the measured PL intensity during transformation and IV2O5 and IV3O7·H2O are the PL counts for compositions of 100% V2O5 and 100% V3O7·H2O, respectively (spectra provided in Supporting Information). Here, all the counts were normalized by the incident laser power, as PL counts were observed to be linear with the laser power. IV3O7·H2O could only be acquired below the transformation threshold laser intensity. On the other hand, IV2O5 was obtained after complete transformation of V3O7·H2O to V2O5. The time rate of transformation at the beginning of the transformation, R = Ẋ (t ≅ 0), was derived from the two data points, X(t = 0) = 0; X(t = 2.5 s). The Renishaw RM 1000 micro-Raman system was also employed for the PL acquisitions. Again, a 514 nm Spectra-Physics 160 series Ar+ laser was used as the excitation source, and the laser spot size on the sample was set to 16 μm. All acquisitions were performed using a 150 lines/mm grating using a 20× objective lens (0.4 numerical aperture) in time series, acquiring a spectrum at every 2 s (1 s integration time + 1 s readout time per scan). The spectra were not corrected for the spectral response of the optics and the CCD detector. The as-recorded spectra show maxima at 750 nm (Supporting Information) which were used in the calculation of X and Ẋ (t ≅ 0).

modification of Dunn’s procedure as disclosed in the article of Leventis et al.13 In this process, a mixture of 5.58 mL of deionized water and 11.34 mL of acetone was added to 2.4 mL of vanadium(V) tripropoxide, VO(OCH2CH2CH3)3. In order to slow down the gelation process, the solutions were cooled in a CO2 ice/acetone bath (−78 °C) until ice appeared in water/ acetone mixture and vanadium(V) tripropoxide became more viscous. Prior to the mixing, water/acetone mixture was shaken vigorously until ice chunks disappeared. The water/acetone mixture was subsequently added into vanadium(V) tripropoxide at once. The obtained mixture (sol) was shaken for 10− 15 s and transferred to the polyethylene syringes (Becton Dickinson & Co., Luer-LokTM Tip, 5 mL) immediately while it was still cold and sealed with Parafilm. Samples were aged for 5 days. After aging, gels were removed from the syringes and placed into a jar filled with anhydrous acetone, approximately 4−5 times the total gel volume. The acetone was changed periodically, once every 24 h for four times. After the water inside the aerogels was fully exchanged with acetone, supercritical drying with CO2 was performed to remove the acetone. TEM and XRD. The structure of the vanadia aerogels was studied by a JEOL JEM-2100F transmission electron microscope, operated at 200 kV. The aerogel consists of entangled nanowires in the form of a “bird-nest” structure. A small piece was first cleaved and weighted. Subsequently, it was dissolved in a measured volume of deionized water to set the concentration to 3.4 g/L. The suspension was further diluted by 100 times in deionized water and spotted as 1 μL aliquots on carbon grids by a micropipet. The aerogel samples were also characterized using PanAnalytical XPert X-ray Powder diffractometer with a CCD detector using Cu Kα (0.152 nm) radiation. Band Gap Determination. To the best of the authors’ literature search, no information is available on the electronic and optical properties of V3O7·H2O. Accordingly, the optical absorption spectrum of V3O7·H2O was acquired to estimate the band gap of V3O7·H2O. Since, the sol−gel synthesized vanadia samples are opaque and they cannot be cleaved to thin films due their extreme fragileness, measurements were performed with samples dissolved in deionized water at a concentration of 0.11 g/L. Cary 300, a double beam spectrophotometer, was employed to measure the optical transmission using deionized water as the reference. Both the water reference and the vanadia sample were enclosed in Starna UV−vis quartz optical cells with an optical beam path length of 1 cm. The transmission coefficient (T) was converted to absorbance (A) as A = −log(T), from which the band gap was derived. Laser Exposures and Raman Scattering Acquisitions. A Renishaw RM 1000 micro-Raman system equipped with a CCD camera and Leica DMLM microscope was used for conducting and monitoring the photooxidation. Spectra were acquired at 1.6−1.8 cm−1 intervals using an 1800 lines/mm grating. A Spectra-Physics 160 series Ar+ laser of 514 nm was employed as the excitation source. The backscattered radiation was collected by a 20× objective lens with a numerical aperture of 0.4. The laser power incident on the sample was measured with an Edmund Optics hand-held laser power meter (silicon cell) with 0.01 μW power resolution and ±5% accuracy. The laser beam was defocused by 20% to disperse the irradiation over an area of 16 μm diameter. The laser spot size on the sample surface was measured using the microscope coupled to the Raman system with minimized interference and diffused reflectance as well as exposure conditions far below CCD

3. RESULTS AND DISCUSSION The X-ray diffraction (XRD) spectrum of the sol−gel synthesized vanadium oxide hydrate nanowires is seen in Figure 1a. The optimal fit for the diffraction peaks was found to be orthorhombic V3O7·H2O (trivanadium pentaoxide hydrate),14 in agreement with the previous work of Leventis et al.13 Their X-ray photoelectron spectroscopy findings revealed that V is present as vanadium(V) and vanadium(IV) at fractions of 67.61% and 32.39%, respectively. The optical absorbance spectrum of V3O7·H2O is provided in Figure 1b. The absorbance at above the bandgap energy is due to band to band transitions, and is governed by A = C(E − Eg)n, where C and n are positive constants, E is the photon energy, and Eg is the bandgap. Hence, the logarithm is expected to exhibit an asymptote at E = Eg. As shown in the inset of Figure 1b, the semilog plot reveals an asymptote at 2.2 eV, which is adopted as the bandgap. The transmission electron microscopy (TEM) image in Figure 1c shows V3O7·H2O aerogels consist of 9 ± 2 nm diameter and micrometers long nanowires. The electron interference fringes in Figure 1d are indicative of 10187



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Figure 1. (a) XRD spectrum of the aerogel sample acquired using Cu Kα (0.152 nm) radiation. The line spectrum is the V3O7·H2O reference (JCPDS-85-2401). (b) Optical absorbance of V3O7·H2O nanowires at concentration of 0.11 g/L in water (optical path length is 1 cm). The inset shows the derivation of band gap from the asymptote of the semilog plot. (c) TEM image of sol−gel synthesized V3O7·H2O showing 9 ± 2 nm diameter nanowires. The scale bar is 50 nm. (d) High-resolution TEM image of a single nanowire. The scale bar is 2 nm. The atomic planes indicated by lines are identified as (331) which are spaced by 0.218 nm.

Figure 2. Raman spectra of the vanadia aerogel samples acquired at 0.29 kW/cm2 laser (514 nm) intensity for 200 s unless otherwise stated. (a) V3O7·H2O. (b) V3O7·H2O under 2.9 kW/cm2 laser intensity that transformed to V2O5 during the acquisition. (c) V2O5 transformed from V3O7·H2O in (b). (d) V2O5 transformed from V3O7·H2O under 2.9 kW/cm2 laser intensity in Ar environment. (e) V2O5 transformed from V3O7·H2O after annealing at 350 °C for 10 min. Micrograph in the inset demonstrates a manual write process on V3O7·H2O by the Raman laser. Scale bar: 100 μm.

nanocrystals of varying crystal direction along the wire length as well as diameter. Figure 2a shows the Raman spectrum of as prepared V3O7·H2O acquired under 514 nm laser excitation of 0.29 kW/cm2 intensity. Figure 2b is the Raman spectrum collected with 10 times the laser intensity (2.9 kW/cm2). The higher laser intensity spectrum is dissimilar, indicative of a phase transformation or chemical reaction. After the material is fully transformed, a final Raman spectrum of the aerogel was collected at 0.29 kW/cm2 incident laser intensity that is provided in Figure 2c. This new phase is identified as crystalline V2O5 (Table 1)15 having a yellow-orange appearance, in contrast to the dark green of V3O7·H2O. The inset of Figure 2 demonstrates a laser-write process using this contrast. The transformation was also investigated in argon ambient. As seen in Figure 2d, a set of new peaks appeared at 167, 847, 880, 936, and 1032 cm−1 in addition to the V2O5 peaks observed in Figure 2c. Similar peaks were also reported by others for V2O5 nanotubes and thin films, which are Odeficient.18−20 The new peaks are attributed to emergence of V4+ sites in O-deficient V2O5 where the length and stiffness of V4+−O and V4+O bonds are different than those of V5+−O and V5+O accounting for frequency shifts of the vibrational modes.20 Subsequent to the transformation, the sample was also exposed to air by opening the vial cap, and Raman spectra were collected after 15, 30, 45, and 60 min exhibiting no change upon exposure to air. As detailed in the Supporting Information, the temperature at the transformation threshold laser intensity was measured by Raman spectroscopy. Figure 3 shows Stokes and anti-Stokes

Table 1. Raman Peak Assignments for V2O5 Raman peak [cm−1] 145 and 196 283 and 405 303 483 527 703 995 a

assignment and reference lattice vibrations due to the layered structurea,b VO bendinga bending vibrations of V−O groups and triply coordinated oxygensa bending in V−O−V groupsa V−O stretching of triply coordinated oxygen (associated with the edge-shared oxygen)a V−O stretching of doubly coordinated oxygen (associated with the corner-shared oxygen)a VO strecthinga

Reference 16. bReference 17.

Raman spectra of semitransformed vanadia aerogels. The temperature was measured as (27 ± 4 °C) by using the integrated intensities of the Stokes peaks at 405 and 995 cm−1 and the anti-Stokes peaks at −405 and −995 cm−1. The ambient temperature was 25 ± 1 °C. Similarly, Shebanova and Lazor12 have calculated the laser-induced temperature in Fe3O4 powder for 514 nm laser excitation by three different methods: (i) Stokes/anti-Stokes as in the present work; (ii) quasiharmonic approximation using the high-pressure phonon shift; (iii) thermodynamic Gruneisen parameter method. The agreement between the three methods is remarkable up to 150−200 °C. Fe3O4 is an efficient light absorber that is consistent with its low bandgap of 0.1 eV. Shebanova and Lazor 10188



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O’s in a VO6 octahedron belongs to a H2O molecule, where a coordinative bond between O of the H2O and V is established and both of the shared electrons originate from one of the lone pairs of the O. At the same time, the water molecule forms hydrogen bond(s) with the octahedra in the adjacent V3O8 layer. Similar to V3O7·H2O, divanadium pentoxide (V2O5) has a layered structure consisting of corrugated sheets, whose basic unit is VO5, sharing edges and corners.22 Unlike in V3O7·H2O, however, VO5 has optimized to a square pyramid in the absence of VO6 units. As we will disclose in the latter half of the present article, our findings suggest that H2O in V3O7·H2O is excluded by photocleavage of the H2O−V bond. Being metastable, V3O7 readily transforms to V2O5 through reaction with O2. Because of the structural similarity between the phases, the structural arrangement in the transformation is minimal once the H2O−V bonds are photocleaved, accounting for its nonthermal nature. We could not time-resolve the transformation kinetics by Raman spectroscopy. Time series Raman spectroscopy could only be conducted for a laser intensity just above the transformation threshold. (e.g., 0.31 kW/cm2). At this excitation intensity, the transformation is sufficiently slow for the acquisition of Raman spectra. If the laser intensity is increased by half from 0.31 to 0.45 kW/cm2, the transformation rate increases 7-fold, and the kinetics cannot be captured by Raman spectroscopy at reasonable signal-to-noise. However, an accurate investigation of the transformation rate as a function of laser intensity requires a sufficiently wide range of intensities. Therefore, we quantified the transformation kinetics by photoluminescence (PL) in the intensity range from 0.26 to 0.51 kW/cm2. Representative PL spectra, which were acquired at a laser intensity of 0.33 kW/cm2, are provided in the Supporting Information. Figure 4 plots X, the volume fraction

Figure 3. (a) Anti-Stokes Raman spectrum of semitransformed vanadia aerogel. (b) Stokes Raman spectrum of semitransformed vanadia aerogel. Raman peaks at ±285 and ±995 cm−1 were used for the temperature calculations which are well-defined peaks associated with the V2O5 phase.

show the laser-induced temperature can be as high as 400 °C in Fe3O4 for 40 mW laser power and laser spot size of ∼8 μm. On the other hand, measured temperature is only about 60 °C at 10 mW laser power or ∼20 kW/cm2 laser intensity. This laser intensity is more than 60 times that of the threshold reported by the manuscript under review. As a matter of fact, if the data of Shebanova and Lazor are extrapolated for the photooxidation threshold of 0.30 kW/cm2 in V3O7·H2O, no measurable temperature increase is predicted for ambient temperature of 25 °C. Therefore, our prediction of the laser-induced heating in V3O7·H2O being insignificant, is in agreement with the findings of Shebanova and Lazor. We also investigated thermal conversion of our V3O7·H2O nanowires to V2O5 by annealing them in a furnace for 10 min at 65, 100, 150, 200, 250, 300, and 350 °C. The transformation was found to occur at around 350 °C or above as inferred from color change from dark green to yellow as well as confirmed by Raman spectroscopy. Figure 2e shows the Raman spectrum of the aerogel after annealing at 350 °C for 10 min, confirming the V2O5 phase. The transformation of V3O7·H2O to V2O5 was also observed by Zakharova et al.2 for nanobelt samples at 350 °C. They have argued that the similarity in crystal structures of the two phases (i.e., V3O7·H2O and V2O5) minimize the structural rearrangements and facilitate the transformation. They explain the transformation as dehydration and oxidation steps in sequence: V3O7 ·H 2O → V3O7 + H 2O

(1)

4V3O7 + O2 → 6V2O5

(2)

Figure 4. Kinetics of V3O7·H2O to V2O5 transformation as monitored by PL. X is the fraction of V2O5. Inset shows the transformation rate computed at t ≅ 0 as a function of laser intensity. The fits are according to eq 4 with n = 2 and two different values of m: m = 4 (A = 0.016 s−1 and BRr = 0.10 MW2 cm−4); m = 5 (A = 0.037 s−1 and BRr = 0.14 MW2 cm−4). The data points are at 0.26, 0.33, 0.37, 0.41, 0.45, 0.49, and 0.51 kW/cm2.

of V2O5, as a function of time, t, as computed from PL intensity (explained in the Experimental Section). The kinetics curve is not “S-shaped” ruling out simultaneous nucleation and growth.23 Rather, the earlier regime is fit to X = 1 − e−Rt in Figure 4 that is characteristic of exponential decay of the V3O7·H2O phase as in a typical photochemical reaction. The fit exhibits discrepancy in the latter regime that we associate with the PL collected from below the surface, where the transformation rate, R, is lower due to laser intensity decay in the aerogel. The discrepancy is larger with higher incident laser

The formation of oxygen-deficient V2O5 in Ar ambient implies eq 2 or another O2-consuming reaction is also involved in the photooxidation of V3O7·H2O to V2O5. It is likely that the transformation being the subject of the present work also occurs via eqs 1 and 2, the only difference being the activation of eq 1 by photons rather than thermal energy. As described by Oka et al., the structure of V3O7·H2O consists of V3O8 layers, held together by hydrogen bonds. The building blocks for each layer are VO6 octahedra and VO5 trigonal bipyramids sharing edges and corners.2,21 One of the 10189



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transformation. For further investigation, we stopped the transformation halfway before its completion and subsequently acquired a spectrum below the transformation threshold intensity. Figure 6a shows the water band in the last spectrum before the transformation is halted.

intensity, as an increased range of thickness qualifies above the photooxidation threshold increasing the heterogeneity in signal. Our control volume for analysis is associated with the contribution 1 − e−Rt, which represents the “effective surface” region that essentially constitutes all of the signal at t ≅ 0. The inset of Figure 4 provides R as a function of the laser intensity. R was computed from the time derivative of X at t ≅ 0. A clear threshold laser intensity is seen for the transformation that is not typical. For example, the exposure in a photographic film as well as photobleaching rate in an organic dye is linear with the light intensity. Additionally, for two-photon excitation processes like 3D photopolymerization and high-resolution lithography, the transformation rate depends on the square of the light intensity. The remainder of the present paper elucidates the formation of the clear photooxidation threshold in V3O7·H2O. Figure 5 provides the time series Raman spectra of V3O7·H2O in the 900−1800 cm−1 range as it transforms to

Figure 6. (a) Water peak in Raman spectrum of vanadia aerogel as it transforms to V2O5 under 0.31 kW/cm2 laser intensity. The spectrum was captured at 471.5 s halfway before the completion of the transformation at an integration time of 15 s. The peak is fit by three Voigtians at 1550 (red), 1582 (green), and 1637 (blue) cm−1. The overall fit is represented by the bold black curve. (b) Water peak in Raman spectrum acquired after (a) at the same spot but at 0.19 kW/ cm2 for 300 s. The peak is deconvoluted into two Voigtians at 1550 (red) and 1582 (green) cm−1. The bold black curve represents the overall fit.

Deconvolution of the water band in Figure 6a reveals three peaks, two of which are the usual water peaks as stated above, whereas the 1550 cm−1 peak has not been reported elsewhere. We associate this peak with a different hydrogen-bonding configuration of water in V3O7·H2O. When we analyze the water band during the transformation at different spectra, the same three peaks are resolved. On the other hand, Figure 6b shows the subsequent spectrum after that in Figure 6a which was accumulated at the same location but with no transformation in progress (laser intensity below the photooxidation threshold). This time the water band is found to lack the 1640 cm−1 component, which is characteristic of bulk water. Because the 1640 cm−1 feature and the transformation are seen concomitantly, we consider the 1640 cm−1 band is contributed by O−H bending of water, which is photodetached from V in V3O7·H2O. Further, on the basis of its disappearance upon lowering of the laser intensity, we understand water photodetached from V rebonds with V. Because the water band, including the 1640 cm−1 feature (i.e., bulklike water), persists for more than 1000 s during the transformation as seen in Figure 5, the photodetached water’s diffusion out of vanadia is inferred to be limited. We owe the low mobility of photodetached water to its H-bonding with the adjacent V3O8 layer. This point also corroborates the nonthermal nature of the transformation. If the transformation were thermal, it would occur at 350 °C or above, and the H-bond cleavage and water’s diffusion out of the vanadia nanowires would be significantly faster. The dynamic competition between water’s photodetachment from V and its rebonding with V is thought to be responsible

Figure 5. Transformation under 0.31 kW/cm2 laser intensity captured at 7.5, 23.5, 39.5, 55.5, 71.5, 151.5, 231.5, 311.5, 391.5, 551.5, 711.5, 871.5, and 1191.5 s. Integration time is 15 s for each spectrum.

V2O5 under 0.31 kW/cm2 laser intensity. Here, the first spectrum is primarily characteristic of V3O7·H2O. Starting from the second spectrum, the emergence and growth of the peak at 995 cm−1 confirms transformation to V2O5 that saturates toward the end of the measurement. The spectra also reveal a band peaking at 1580 cm−1 which we assign to O−H bending of water.24 During transformation to V2O5, however, it slightly broadens toward higher wavenumbers. In liquid bulk water, the O−H bending mode is observed in terms of two convoluted features at 1640 and 1581 cm−1. The former, being the major peak, is assigned to partially H-bonded water, while the latter corresponds to fully H-bonded water.24 Accordingly, we associate the band peaking at 1580 cm−1 with water. As the control experiment, we also acquired the Raman spectra of V2O5 transformed from V3O7·H2O by furnace annealing as well as laser exposure. Both cases did not yield a measurable band at 1580 cm−1 (unless V2O5 is kept at 100% relative humidity during the acquisition). Indeed, as observed in Figure 5, the 1580 cm−1 band tends to vanish at the end of the 10190



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V bonds (Figure 7a), for which each O contributes one electron and V contributes all of its five valence electrons. The sixth Vd2sp3 orbital overlaps with one lone pair of water, sp3, forming the H2O−V bond (Figure 7a), for which both electrons are donated by the lone pair (i.e., coordinative bond). In reality, the electronic states in Figure 7b split to energy bands in the solid, but the molecular orbital model is still useful for estimating the energy band structure relevant to H2O−V bond. H2O−V bond cleavage may occur by either transition of an electron from H2O−V to (H2O−V)* (antibonding) or transition of both of the H2O−V electrons to Vt2g (nonbonding), setting the bond order to zero. The energy of the H2O−V to Vt2g transition is estimated as about 2.3 or 2.6 eV (Supporting Information), while a transition from H2O−V to (H2O−V)* is inferred to be about 4.6 or 4.9 eV. Given the excitation is at 514 nm (2.4 eV), H2O−V to (H2O−V)* can only be established via two-photon absorption. This process must either employ a virtual state between H2O−V and (H2O− V)* or Vt2g as the intermediate, the latter being more likely due to its being a resonant transition. The resonant case is illustrated in Figure 7b which is known as “1 + 1 absorption” or “resonant two-photon absorption” or “sequential two-photon absorption” in the literature. Hence, for either bond-cleavage mechanisms, H2O−V to (H2O−V)* (1 electron + 2 photons through Vt2g) or H2O−V to Vt2g (2 electrons + 2 photons), two photons must be involved, and n in eq 4 must be 2. Because the transition rate of a resonant two-photon process is several orders of magnitude higher than that of a nonresonant one (i.e., through a virtual state), it can occur at a lower laser intensity with no heat generation. Given n is 2, eq 4 yields a clear threshold and fits the experimental data (i.e., inset of Figure 4) only if m ≥ 4. Hence, it is inferred that the transformation occurs through the cleavage of ≥4 H2O−V bonds (all at the same time) at the smallest structural unit. Indeed, if we associate the smallest scale of the transformation with consumption of a single O2 molecule as suggested by eq 2, it follows from eq 1 that four H2O−V bonds must be cleaved for the smallest extent of the overall reaction (m = 4).

for the observed threshold laser intensity of the transformation. Under laser excitation, a H2O−V bond is repeatedly cleaved and re-established at rates of Rc and Rr, respectively, as illustrated in Figure 7a. We anticipate the freed H2O molecule

Figure 7. (a) Dynamic processes of photocleavage and re-establishment of the H2O−V bond in the H2OVO5 octahedron under laser excitation at rates of Rc and Rr, respectively. (b) Qualitative model for the formation of molecular orbitals in the H2OVO5 octahedron. A two-photon optical transition capable of breaking the H2O−V bond is illustrated.

rebonds with the same V, from which it is detached due to its limited diffusion in V3O7·H2O (i.e., unless transformation to V2O5 occurs). As discussed earlier, we attribute the low mobility of photodetached water to its H-bonding with the adjacent V3O8 layer. The probability of the H2O−V bond to be cleaved at any instant, Pc, is then given by Rc Pc = Rc + R r (3) In particular, Rc ∝ In, where I is the laser intensity and n is the number of photons involved in the cleavage of a H2O−V bond. If m is the number of H2O−V bonds that must be cleaved at the smallest V3O7·H2O structural unit (so that it transforms to V2O5), then the transformation rate, R, is proportional to Pcm. Combining these relations with eq 3, we obtain ⎞m ⎛ In R = A⎜ n ⎟ ⎝ I + BR r ⎠

4. CONCLUSION In conclusion, the present work reveals transformation of V3O7·H2O nanowires to V2O5 by resonant two-photon (514 nm) absorption. As substantiated by Raman spectroscopy, the transformation involves dehydration through photocleavage of the H2O−V bond and subsequent oxidation of V3O7 to V2O5. Most intriguingly, the transformation is observed above a clear threshold laser intensity of 0.30 kW/cm2 as a result of the following mechanisms: (i) dynamic competition between H2O−V bond cleavage and re-establishment, as verified by Raman spectroscopy and is consistent with transformation rate vs laser intensity; (ii) two-photon (514 nm) cleavage of the H2O−V bond by excitation via nonbonding d-states, as anticipated from molecular orbital energy scheme and consistent with transformation rate vs laser intensity; (iii) requirement for multiple number of cleaved H2O−V bonds at the smallest V3O7·H2O structural unit, so that it transforms to V2O5, as inferred from transformation rate vs laser intensity. The enabling role of the nanostructure is thought to be efficient O2 diffusion into the wires due to the short diameter as well as high surface to volume. A potential application of this unique photooxidation phenomenon is laser-writing of features smaller than the diffraction limit. Specific examples are high density optical data storage and optical nanolithography.

(4)

where A and B are constants. Equation 4 exhibits a threshold if n ≥ 2 and m ≥ 2 and essentially becomes a step function as n × m tends to infinity. Shown in the inset of Figure 4 is the fit of eq 4 to experimental data with n = 2 and m = 4 and 5. Hence, for the oxidation to occur, the bond-cleavage rate must exceed a threshold, pushing the number of concomitantly broken bonds in the smallest structural unit to a critical number, m. The fit is poor for n = 1, no matter how large m is picked, indicative of a multiphoton (n ≥ 2) process. The photocleavage of the H2O−V bond can be explained by transitions from bonding to nonbonding or antibonding orbitals resulting in zero bond order. Accordingly, we analyze the H2OVO5 octahedron (the structural unit, which accommodates water) in the framework of molecular orbital theory as illustrated in Figure 7b. Essentially, V establishes six bonds through its six d2sp3 hybrid orbitals. Five of these Vd2sp3 orbitals overlap with five O 2p, one-to-one, resulting in five O− 10191



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ASSOCIATED CONTENT

S Supporting Information *

Temperature measurement during transformation by Raman spectroscopy, transformation kinetics by photoluminescence, derivation of eq 4, and fourth-derivative analysis of optical absorption spectrum. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.K.K. and Ç .Ö .T. acknowledge funding by (i) US Air Force Office of Scientific Research (Grant #FA9550-10-C-0074) through a subcontract to Oklahoma State University by Techno-Sciences, Inc., Maryland; (ii) National Science Foundation (Award No. 0756791). We are indebted to Alejandro R. Navarro for the XRD characterization. We thank Gitogo Churu (supervised by H.L.) for providing his V3O7·H2O aerogel samples for comparison. We also thank Sean Maclaskey (supervised by A.K.K) for the optical absorption spectrum. Finally, we thank James Wicksted for his insightful discussions on the two-photon transitions.

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REFERENCES

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