Structure and Photoluminescence Properties of Thermally Synthesized V2O5 and Al-Doped V2O5 Nanostructures

Al-free and Al-doped V2O5 nanostructures were synthesized by a thermal-chemical vapor deposition (CVD) process on Si(100) at 850 °C under 1.2 × 10−1 Torr via a vapor-solid (V-S) mechanism. X-ray diffraction (XRD), Raman, and high-resolution transmission electron microscopy (HRTEM) confirmed a typical orthorhombic V2O5 with the growth direction along [110]-direction of both nanostructures. Metallic Al, rather than Al3+-ion, was detected by X-ray photoelectron spectroscopy (XPS), affected the V2O5 crystallinity. The photoluminescence intensity of V2O5 nanostructure at 1.77 and 1.94 eV decreased with the increasing Al-dopant by about 61.6% and 59.9%, attributing to the metallic Al intercalated between the V2O5-layers and/or filled in the oxygen vacancies, which behaved as electron sinks. Thus the Al-doped V2O5 nanostructure shows the potential applications in smart windows and the electrodic material in a Li-ion battery.


Introduction
Vanadium is a multi-valent element with oxidation states of V 2+ , V 3+ , V 4+ , and V 5+ , and therefore has several phase-states, including vanadium monoxide (VO), vanadium sesquioxide (V 2 O 3 ), vanadium dioxide (VO 2 ), and vanadium pentoxide (V 2 O 5 ) [1,2]. Vanadium pentoxide (V 2 O 5 ) is the most stable phase; it has an orthorhombic structure and layered VO 5 structures of square pyramids that share corners and edges [3,4]. The layered structure results in the V3d split-off conduction band so that V 2 O 5 has direct and indirect band gaps at 2.3 and 1.9 eV [5]. Oxygen vacancies (V O ) are the most common defects in V 2 O 5 , and especially in the oxygen layer in the layered VO5 structure [6]. The conduction band that has split off and the V O -defects provide a flexible pathway for luminescence. Doping with elements and the formation of the core-shell nanostructures modify the photoluminescence properties [7,8]. Many works have reported upon methods of fabricating V 2 O 5 , which are: atomic layer chemical vapor deposition [9], the sol-gel process [10], thermal deposition [11], the hydrothermal process [12], and the thermal evaporation process [7,8]. The photoluminescence property of V 2 O 5 can be modified by the different dopants, such as Er [13], Co [14], Nd [15,16], Gd [16]; Al [10], and Ga [7]. The above doped V 2 O 5 materials possess potential applications in light emitters, photocatalysts, and cathodic materials for ionic batteries. Owing to the layered structures and tunable photoluminescence properties, V 2 O 5 has many applications, including electrochromic devices [17], solar cells [18,19], catalysts [20], solid-state batteries [21], gas sensors [22], chemical species sensors [23], smart windows [4], etc. The dopants as mentioned above were not metallic powders. Instead, the nitrate [10,13,15], sulfate [14], and oxide powders [16] were the dopants, which implied that the metallic ions were easy to form during the synthesized process, resulting in the subsequent influence in photoluminescence. However, the zero-valent metallic ion can also show the influence in the photoluminescence property. Compared to previous literature [7,10,[13][14][15][16], this work shows for the first time the metallic Al powders acting as dopants and doping V 2 O 5 nanostructures. This implies that the zero-valent and/or tri-valent Al ions can be observed, and their influence in the photoluminescence property of the V 2 O 5 nanostructures can be studied. Al-doped V 2 O 5 shows potential applications including as photodiode [24], in electrocatalysis [25], thin-film batteries [26], and as cathodic materials for lithium ion batteries [10].
In this work, metallic Al powder was directly used as a source of metallic Al dopant. Both Al-free and Al-doped V 2 O 5 nanostructures (NSs) were fabricated on Si(100) substrate using a thermal-chemical vapor deposition (CVD) process via a catalyst-free vapor-solid (V-S) mechanism at 850 • C under 1.2 × 10 −1 Torr. In addition, Ar gas was introduced during the whole synthesized process to lower the oxygen partial pressure and to prevent the formation of the Al 2 O 3 phase. The effects of Al doping on the crystal structures, binding energies, bonding vibration modes, and luminescence properties of V 2 O 5 NSs are systematically analyzed based on the results of X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and Raman, and photoluminescence spectra.

Fabrication of Al-Free and Al-Doped V 2 O 5 Nanostructures
Al-free V 2 O 5 NSs were deposited on the Si (100) substrates by the catalyst-free V-S mechanism using the thermal-CVD process at 850 • C in a quartz tube furnace. A 0.3 g mass of high-purity V 2 O 5 powder (Aldrich, Shanghai, China, 99.5%) was placed in an alumina crucible in the center of the quartz tube; about 10cm downstream of the crucible was placed the Si (100) substrate. The system pressure was pumped down to 4.0 × 10 −2 Torr and the temperature was increased to 850 • C at 25 • C/min in Ar 30 standard cubic centimeter per minute (sccm) V 2 O 5 powder was evaporated at 850 • C for 1.5 h, and its vapor was mixed with Ar (30 sccm) and O 2 (20 sccm) at 1.2 × 10 −1 Torr; V 2 O 5 NSs were grown on Si(100) substrate and then cooled to room-temperature at 30 sccm Ar. The final products were denoted as AV0 NSs. Al-doped V 2 O 5 NSs were synthesized in a similar manner. The starting materials were a mixture of 0.3 g powdered V 2 O 5 with various amounts of powdered Al (Aldrich, 99.9%)-1.5 mg (0.5 wt.%), 3 mg (1 wt.%), and 4.5 mg (1.5 wt.%)-that were co-evaporated at 850 • C to yield AV05, AV10, and AV15, respectively.
Before analyzing processes, all samples were preserved in a glass desiccator under the pressure of 1 Torr. The samples of the AV nanostructures being covered on the Si(100) substrates were directly analyzed using XRD, X-ray photoelectron spectroscopy (XPS), field-emission scanning electron microscopy (FESEM), Raman, and photoluminescence (PL). The AV nanostructures were detached from the Si(100) substrate by ultrasonication in the ethanol solution, followed by the HRTEM observation.

Characterizations
The crystal structures of the AV NSs were determined using a mass absorption coefficient glancing incident X-ray diffractometer with Cu kα radiation source (λ = 0.154 nm, 40 kV, 30 A, Bruker D2 PHASER) in the recording range from 20 to 60 • and high-resolution transmission electron microscope with the electron accelerating voltage of 200 kV (HRTEM, JEOL, JEM-3000F, Tokyo, Japan). The chemical binding energies were measured by X-ray photoelectron spectroscope using the Al kα radiation with the electron energy of 1486.6 eV (XPS, Perkin-Elmer model PHI1600 system, Waltham, MA, USA). The chemical bonding vibration modes were identified by Raman spectrometer (3D Nanometer Scale Raman PL microspectrometer, Tokyo Instruments, INC., Tokyo, Japan) with an excitation source of a semiconductor laser at the wavelength of 532 nm. Room-temperature PL spectra were recorded by a confocal Raman spectrometer (Alpha300, Witec, Ulm, Germany) with the source of semiconductor laser (λ = 532 nm, 0.5 mW). Figure 1a presents the XPS spectra of the V 2p and O 1s orbitals of AV0 and AV15 NSs. The peaks at 516.4 eV (V 2p 3/2 ) and 523.7 eV (V 2p 1/2 ) of the AV0 NSs with a splitting ∆-value of 7.3 eV are consistent with the characteristic of orthorhombic V 2 O 5 structures [27]. The AV15 NSs show a similar splitting ∆-value (7.4 eV), indicating that the Al dopants do not affect the V 2 O 5 orthorhombic structure. The O 1s peak at 529.3 eV is attributed to the oxygen ions (O L ) associated with the V-O bonds in the V 2 O 5 structures [28,29]; the peak at 530.7 eV (O C ) is attributed to the chemisorbed oxygen on the surface of the NSs [7]. The energy difference between V 2p 3/2 and O L is about 12.9 eV, indicating the formation of the V 2 O 5 phase [29]. The values of V 2p and O L in the AV15 NSs ( Figure 1a) are similar to those of the AV0 NSs. However, the binding energy of O C in AV15 NSs shifts upward to 532.2 eV relative to that of AV0 NSs (530.7 eV). The reduction potential of Al is as low as −0.677 V [30], indicating that Al more easily reacts with oxygen-containing species, such as oxygen and/or hydroxyl, and forms the amorphous phases of Al-O and/or Al-OH [31] at the surface of the NSs. Therefore, the O C position in AV15 NSs ends up with a blue-shift. Figure 1b shows that the energies of the Al 2p orbitals in AV05, AV10, and AV15 NSs have the same binding energy of 72.12 eV [32], showing that the Al dopants behave as a metal rather than the ions in the V 2 O 5 NSs. Table 1 presents the atomic percentage (at.%) of Al in the V 2 O 5 NSs, indicating that the at.% of Al dopant increases with Al contents.

X-ray Diffraction (XRD) Patterns
The powder XRD analysis of both Al-free and Al-doped V 2 O 5 NSs reveal the typical orthorhombic V 2 O 5 structure (joint committee on powder diffraction standard (JCPDS) 77-2418) with the planes (001) Based on the Bragg law and the plane-spacing equation for an orthorhombic structure, the lattice constants (a, b, and c), and the ratios c/a and c/b can be estimated; they are provided in Table 2   Therefore, the increase in c/a and c/b can be attributed to the filling by Al 0 of the sites of V O and/or intercalating between the two layered VO 5 structures in V 2 O 5 NSs. The Al dopant slightly increases the lattice distance between the two [VO 5 ]-layers in the a-c plane (inset in Figure 2b where φ is the angle between the (h 1 k 1 l 1 ) plane and the (h 2 k 2 l 2 ) plane; a (11.51 Å), b (3.559 Å), and c (4.371 Å) are the lattice constants of the orthorhombic V 2 O 5 structure Based on the above discussion, the growth direction of the AV NSs can thus be confirmed as being in the [110]-direction.

Raman Spectra
The V 2 O 5 structure is orthorhombic and has a P mnm symmetry; V 2 O 5 has typical Raman active peaks of AV0 and AV10 NSs which fitted by Gaussian deconvolution are observed at around 141.71, 195. 26 [3,[35][36][37], as shown Figure 4a and listed in Table 3. The crystal structure of V 2 O 5 NSs is layered V 2 O 5 s that is packed along the c-axis of the unit cell, and each V 2 O 5 layer comprises square pyramids of VO 5 that share edges and corners, as shown in Figure 4b [8,38]. One vanadium atom is connected to six oxygen atoms with five bond lengths, which are of 1.557, 1.779, 1.878, 2.017, and 2.791 Å, as shown schematically in Figure 4b [38]. The V-O1 bond has the strongest binding energy but the shortest bonding length (1.557 Å) of all of the V-O bonds; the V-O1 bond is a weak Van der Waals' bond between layered-VO 5 with a bonding length of 2.791 Å; along the b-axis, the V-O21 and V-O22 bonds have a length of 1.878 Å; along the a-axis, the V-O bonds are of two types: (1) The V-O2 1 bond has a length of 2.017 Å, and (2) The V-O3 bond has a length of 1.779 Å.  This orthorhombic V 2 O 5 structure has two bonding vibrations. They are: (1) stretching mode: the terminal oxygen of the V=O bond is the unshared oxygen (O1) and yields a peak at 993.36 cm −1 [39,40]; the doubly coordinated oxygen in the V-O-V bond is a cornersharing oxygen in one of the two adjacent pyramids yields a peak at 699.93 cm −1 [41]; the triply coordinated oxygen of the V-O bond is an edged-sharing oxygen in three adjacent pyramids yields a peak at 524.80 cm −1 [6,39]; and (2) bending mode: the bridging V-O-V bond at 480.45 cm −1 [39,41]; the terminal oxygen associated with the V=O bond yields a peak at 403.51 cm −1 [40]; the triply coordinated oxygen associated with the V-O bond yields a peak at 302.64 cm −1 [6]; the V=O terminal oxygen associated with the bond yields a peak at 282.04 cm −1 ; (V 2 O 2 ) n and (V 2 O 2 )' n bonds yield peaks at 194.26 and 141.71 cm −1 , respectively, corresponding to the chain translation and strongly associated with the layered structures [41]. Table 3 presents the Raman shifts of the AV0 and AV10 NSs. The deviations of the Raman shifts upon the addition of Al reveal a slight difference between AV0 and AV10 NSs, indicating that Al has no major effect on the V 2 O 5 binding structures, which is consistent with the XPS results.
The intensity of the Raman peaks (as shown in Figure 4a and Table 3) clearly decreases upon the addition of Al dopant. Jung et al. [42] reported a decrease in the Raman intensity from V 2 O 5 upon the insertion of Li into the Li x V 2 O 5 lattice owing to the formation of negative charge carriers by the reduction of V 5+ to V 4+ [43]; Park et al. determined that the decreasing Raman intensity is attributable to the extraction of Li from the Li x CoO 2 lattice because the oxidization state increases from Co 3+ to Co 4+ , generating positive charge carriers [44,45]. Both of these cases are attributable to the metallization of the host materials, so decreases in Raman intensity arise from the reduction of the optical skin depth of the Raman excitation light [42]. In this work, metallic Al is doped into the V 2 O 5 lattice, as revealed by XPS and XRD; the host materials, therefore, exhibit more metallic characters. Hence, the Raman intensity of the V 2 O 5 NSs clearly decreases with the addition of Al (Figure 4a). Figure 5a presents the PL spectra of the AV NSs, which reveal two prominent emissions at 1.77 (E1) and 1.94 eV (E2). V 2 O 5 has split-off conduction bands owing to its layered structure, suggesting that it has several energy bands from the top of the O2p valance band to the split-off V3d conduction band [11]. The V 2 O 5 crystal structure comprises from the [VO 5 ]-[VO 5 ] layers and oxygen vacancies (V O ) easily being formed between the layers [11]. Accordingly, the V O -related emission E1 occurs at 1.77 eV and E2 at 1.94 eV is attributed to the intrinsic emission band, which is split off from the V3d conduction band in the V 2 O 5 NSs [6,46]. Figure 5b presents variations of the intensities of E1 and E2 with the Al content. Such variations in luminescence intensities decay exponentially as Al content increases. The intensity of E1 decreases by about 61.6% and that of E2 decreases by about 59.9% as the Al content increases from 0 to 1.5 wt.%.   [14], gadolinium oxide (Gd 2 O 3 ) [16], neodymium oxide (Nd 2 O 3 ) [16], aluminum nitrate (Al(NO 3 ) 3 ) [24], and aluminium oxide (Al 2 O 3 ) [26]. Besides the Ga dopants, the PL intensities of V 2 O 5 decrease with the zero-valent Al the same as the results of the tri-valent elements. 3.6. Proposed Luminescence Mechanism Figure 6 presents a possible mechanism for E1 and E2 from the AV NSs. The conduction band (E C ), Fermi level (E F ), and valence band (E V ) of the V 2 O 5 structures in the Al-free V 2 O 5 NSs, are at −4.7, −5.45, and −7.00 eV [47], respectively, as shown in Figure  6a. Some of the excited electrons in the E1 level were generated by an incident light from the E v of the V 2 O 5 transiting to the E2 level, causing the intensity of E2 to exceed that of E1, as shown in Figure 5b. According to XPS and XRD results, the metallic Al exists in the V 2 O 5 NSs, which can be located between the layered V 2 O 5 structure and/or the oxygen vacancy. The Fermi level of the metallic Al is approximately 4.08-4.28 eV [48]. Hence, in the Al-doped V 2 O 5 NSs, as shown in Figure 6b, these excited electrons in the E2 level, which were excited by the incident radiation, can transit along three pathways. These pathways are (1) from E2 to E1, and then to the Fermi level of the metallic Al; (2) from E2 to the work-function level of the metallic Al; and (3) from E2 to E1. Along pathway 1, the excited electrons transit to the metallic Al, so the E2 intensity decreases; along pathway 2, the electrons that are generated upon the formation of the V O -defects transit to the metallic Al, leading the decreasing E1 intensity; along pathway 3, the excited electrons in the E2 level transit to the E1 level, so the intensity of E2 is lower than that of E1 for all of the AV NSs. Moreover, the concentration of V O is reduced upon the addition of Al dopant because the sites of the V O defects are filled by Al 0 . Then the intensity of E1 emission thus tends to decrease. These results suggest that Al as the dopant plays the following three roles in the AV NSs system: (1) as an electron sink, (2) filling the sites of the V O defects and reducing the V O concentration, and (3) intercalating the layered V 2 O 5 structure. Therefore, the intensity of E1 and E2 both decreases as the Al dopant concentration increases.

Conclusions
Both Al-free and Al-doped V 2 O 5 NSs were fabricated on the Si (100) substrate by a thermally activated CVD process at 850 • C via a V-S mechanism. XPS revealed the V-O binding energy of the V 2 O 5 phase and the presence of metallic Al rather than Al 3+ ions. XRD revealed the typical orthorhombic phase of the V 2 O 5 with c/a and c/b ratios that increase with the Al dopant concentration, suggesting that Al 0 (radius = 1.43 Å) preferentially fills the V O (radius = 1.4 Å) sites owing to their similar sizes. The FWHM has a similar value with a deviation of ±2.9% upon the addition of Al dopant, indicating that Al does not affect the V 2 O 5 crystal structure. HRTEM verified that the growth direction of both Al-free and Al-doped V 2 O 5 NSs is in the [110]-direction. Raman shifts revealed slight deviations upon the addition of Al, indicating that the Al content does not affect the binding structure of the V 2 O 5 . The photoluminescence results proved that the emission intensities at 1.77 and 1.94 eV decreased by factors of 61.6% and 59.9%, respectively, as the concentration of Al dopant increases from 0 to 1.5 wt.%. The decrease in the luminescence intensities was attributable to the following causes: (1) Al dopants reduced the V O -defect concentration and (2) Al acted as an electron sink. Therefore, the recombination rate of electrons and holes decreases, reducing the luminescence intensities at 1.77 and 1.94 eV, upon doping with Al. Al-doped V 2 O 5 NSs have potential applications in photocatalysts, solar cells, gas sensors, and electrodic materials.

Conflicts of Interest:
The authors declare no conflict of interest.