Suppression of Photoinduced Surface Oxidation of Vanadium Dioxide Nanostructures by Blocking Oxygen Adsorption.

Controlling the surface is necessary to adjust the essential properties and desired functions of nanomaterials and devices. For nanostructured multivalent vanadium oxides, unwanted surface oxidation occurs at ambient atmosphere generally and needs to be suppressed or avoided. We describe the suppressed surface oxidation of VO2 nanostructures through blocking oxygen adsorption. During an enhanced photoinduced surface oxidation process, the increased oxidation states of vanadium in VO2 nanostructures are suppressed by the use of an inert atmosphere or coating. Intermediate oxidation states are observed, and an ALD-TiO2 coating has a good antioxidant capacity for preventing the formation of oxygen-enriched components. Such oxidation suppression is beneficial to improving the stability of VO2 nanostructures. Controllable surface oxidation helps us to understand the physical essentials of surface chemical reactions and achieve better control of surface functions and performances on correlated vanadium oxide nanostructures.

Introduction

Surfaces are among essential and crucial issues in advanced materials, coatings, and devices. Some modified and decorated surfaces have been successfully exploited to improve the tolerance for surface corrosion[1,2] and catalysis capability[3,4] and even to create functional materials[5−7] with abnormal properties.[8,9] In general, surface engineering is critical to realize desired physical or chemical characteristics of nanomaterials. In the past decades, nanomaterials or low-dimensional structures have been widely investigated with unique chemical, optical, electrical, and magnetic characteristics.[10−12] Both the large surface area and high surface energy of nanomaterials provide an alternative way to adjust physical and chemical properties, which are beyond the capabilities of conventional bulk materials. Thus, the demand of surface compositions and structures governed at an atomic-layer scale is urgent because they can determine the construction of advanced nanomaterials and the extension of device performances. Various surface physical or chemical methods emerged and created more opportunities for the existing nanomaterials to meet the requirements of the real functions and devices. Meanwhile, a selected-area chemical nanoengineering of vanadium dioxide nanoparticles and nanowires has also been developed.[13]

Transition metal oxide nanostructures have attracted considerable attention with metal–insulator transition due to their capabilities of extending Moore’s law in the communities of fundamental physics and advanced electronics.[14−18] As one of prototypical vanadium oxides, VO2 undergoes a metal–insulator transition at approximately 68 °C and has shown superior performance in electronic and optical devices.[19,20] Due to the existence of various oxidization states of vanadium, VO2 is sensitive to oxygen and chemical reagents leading to the production of nonstoichiometric phases.[21−24] As we know, the oxygen components are easily adsorbed on the surface, and they can significantly change the valence state of vanadium on the surface of desired vanadium oxides.[19,21,25] Thus, a controllable surface is important for modulating the chemical characteristics and understanding the mechanism of the metal–insulator transition of VO2 nanostructures.[26−29] For example, the oxygen-induced high oxidation state of V on the surface affects metal–insulator transition.[30−33] Unfortunately, the adsorbed oxygen that is brought by moisture or ambient air is hardly eliminated from the surface. In addition, the large surface area makes the surface and phase purity of nanomaterials hard to control. Thus, it is necessary to understand the adsorption of oxygen on the surface and then make materials more stable. Some metal oxides (e.g., titanium dioxide) with good thermal and chemical stability have been demonstrated[34−36] and can be used to block the adsorption of oxygen or water on the surface of functional oxides.[37] Furthermore, atomic layer deposition (ALD) is among surface engineering techniques, and it is powerful for adjusting surface coating and passivation on the nanostructures used in optics, microelectronics, and catalysis.[38−40]

Here, we present the suppression of photoinduced surface oxidation by blocking oxygen adsorption on VO2 nanoparticles and nanowires. The ALD-TiO2 coating was used to investigate the prevention of VO2+ components forming on the nanostructure surface, which shows better antioxidant capacity, and it is much more stable and reliable than an inert gas. During an enhanced oxidation process, intermediate oxidation states were checked from low valence states to high valence states of V atoms and nonstoichiometric components. Such oxidation suppression will help to understand the physical essentials of surface chemical reactions and then achieve the control of surface functions on correlated vanadium oxide nanostructures.

Results and Discussion

Before artificial photoinduced oxidation processes, vanadium dioxide nanoparticles were prepared by the combination of atomic layer deposition and annealing processes. Figure a shows the initial VO2 nanoparticles with ∼400 nm in lateral size. In general, the VO2 nanoparticles are crystalline and randomly distributed via annealing an ∼70 nm-thick VO film (Figure S1) on the silica (∼280 nm)/silicon substrates according to a previous report.[13] In order to investigate the oxidation evolution of VO2 nanostructures, photoinduced oxidation is exploited as presented in Figure b. In these cases, a laser was used to irradiate VO2 nanoparticles in a line-scanning way sweep (Figure S2). Laser power values and irradiation time can be varied due to different specimens of nanoparticles. The chemical oxidations were triggered and observed once we artificially used a laser to irradiate VO2 nanoparticles at an ambient atmosphere. In Figure c, there are obviously different Raman spectra for the VO2 nanoparticles with and without a photoinduced oxidation process. In the case of the pristine VO2 nanoparticles (spectrum II), Raman shifts at 193 and 224 cm–1 were observed due to the characteristic V–V vibration of VO2. The peak of 520 cm–1 was from the Si substrate. However, in the case of the irradiated VO2 nanoparticles (spectrum I), Raman shifts at 103, 144, 283, and 700 cm–1 obviously appeared and are marked with dashed rectangles while the characteristic Raman peaks of VO2 remained. These emerged Raman shifts originated from the formation of V2O5 on VO2 nanoparticles during the photoinduced oxidation processes. Due to the photoinduced catalytic or thermal oxidation during the laser irradiation, the oxygen adsorbed on the surface of the VO2 nanostructure directly oxidized the V atoms from +4 to +5 valence.

Figure 1

(a) False-color SEM cross-sectional image of VO2 nanoparticles deposited on a SiO2/Si substrate. (b) Schematic diagram of photoinduced oxidation. (c) Raman spectra of VO2 nanoparticles before and after photoinduced oxidation.

As mentioned above, the photoinduced oxidation processes help us check the oxidation evolution of VO2 nanostructures. When an oxidation process was performed, we observed an obvious evolution as shown in Figure . No Raman variation of oxidation was found in Figure a because having a low-power value makes it difficult to trigger a photoinduced oxidation. Then, we noted the Raman signals of V6O13 that appeared once the oxidation occurred in a high-power case. In general, the oxygen adsorbed on the surface will oxidize parts of V atoms on the surface of nanoparticles under the assistance of a laser. The Raman shifts at ∼96, ∼139, and ∼685 cm–1 can be found and referred to V6O13 in Figure b.[41,42] We also found that the V6O13 is one of the intermediate oxidation states and not stable under laser irradiation. Here, the characteristic peak of VO2 was covered due to a high baseline in Raman spectra with a high laser power. The peak at 103 and 700 cm–1 for V2O5 is obvious, and the Raman shifts of VO2 retained after photoinduced oxidation as demonstrated in Figure c. This result indicates that the oxidation reaction involves two subsequent steps. First, the V6O13 intermediate phase formed and existed on the surface. As one of the intermediate oxidation states, the formed V6O13 is not stable under laser irradiation; thus, V6O13 can be easily and subsequently oxidized into V2O5 within a short time. We expect that the oxygen adsorbed on the surface plays a critical role in the photoinduced oxidation of VO2 nanoparticles. In an ambient environment, the adsorbed oxygen can oxidize V+4 into V+5 on the surface of the nanoparticles irradiated by a laser. In Figure , the schematic diagram on the right shows the whole oxidation process of VO2 nanoparticles.

Figure 2

Raman spectra and schematic diagrams during the photoinduced oxidation evolution. (a) Pristine VO2 nanoparticles. (b) VO2 nanoparticles irradiated with an enhanced laser power. (c) VO2 nanoparticles after irradiation.

To further confirm the roles of oxygen molecules adsorbed on the surface of nanoparticles, we utilized an inert nitrogen atmosphere to change the adsorption of oxygen during the photoinduced oxidation. Figure a shows different Raman shifts when the VO2 nanoparticles were irradiated in the air (spectrum I, Figure a) and nitrogen atmosphere (spectrum II, Figure a). By comparing with the pristine VO2 nanoparticles (spectrum III, Figure a), we did not observe the characteristic Raman shifts of V2O5 or any difference after irradiation in a nitrogen atmosphere. If exposed in air, however, the Raman shifts at ∼145 and ∼700 cm–1 are obvious in contrast to the cases of pristine nanoparticles and those in nitrogen. This result indicates that the nitrogen intervention can remove the adsorbed oxygen molecules on the surface and prevent oxygen molecules from reacting with VO2 under the irradiation of a laser. In Figure b, alternative Raman spectroscopy was further performed when we introduced nitrogen to protect the specimen and then retracted this protection during the photoinduced oxidation. From spectrum II′, we did not observe a Raman shift at ∼103 or ∼700 cm–1 belonging to V2O5. We found that, after removal of nitrogen, the nanoparticles were still stable and not easily oxidized in a very short time because the surface had not enough time to readsorb oxygen molecules. Subsequently, when the specimens were placed in air for a long time after removal of nitrogen, the oxygen molecules were readsorbed on the surface, which will lead to obvious surface oxidation during photoinduced oxidation. The characteristic Raman signals of V2O5 appear after irradiation as shown in spectrum II″ (Figure c). It means that the specimens can be reoxidized when being exposed to air for 30 min after removal of nitrogen. This depends on the fact that the oxygen molecules are easily adsorbed or readsorbed on the surface of the nanoparticles in air. Except for the continuous protection under inert gases, the photoinduced oxidation cannot be suppressed for a long time. Thus, blocking the oxygen adsorption on the surface is critical to suppress the surface oxidation of VO2 nanoparticles.

Figure 3

Raman spectra of VO2 nanoparticles irradiated in different atmospheres. (a) For the pristine VO2 nanoparticles and those irradiated in nitrogen and air. (b) For the irradiated nanoparticles without nitrogen protection (I′), and those irradiated after removal of nitrogen in a short time (II′). (c) For the reirradiated nanoparticles after several seconds and 30 min in air after removal of nitrogen protection.

The adsorbed oxygen on the surface determines the oxidation of VO2 nanoparticles as demonstrated above, and it is one of the center points for the suppression of surface oxidation to reliably block the oxygen adsorption. Therefore, we covered VO2 nanoparticles with ALD-TiO2 thin films to avoid the adsorption of oxygen on the surface. ALD-TiO2 thin films are powerful for protecting functional oxides by adjusting the amount of oxygen or water adsorbed on the surface.[37]Figure displays the suppression of photoinduced surface oxidation by the use of ALD-TiO2 thin films with different thicknesses. The thickness of the TiO2 layer was controlled by ALD, which can be reconfirmed by TEM and EDS spectra (e.g., Figures S3 and S4). During the photoinduced oxidation processes, Raman shifts of V2O5 were hardly detected in contrast to the case without the ALD-TiO2 thin-film coating. It is worth noting that the oxidation suppression and protective effect is extremely obvious although only a 5 nm-thick ALD-TiO2 thin film was coated on the VO2 nanoparticle. Moreover, The Raman shifts at ∼145 and ∼700 cm–1 were not observed, which implies that the 10 nm-thick ALD-TiO2 thin film (Figure S3) has completely suppressed the surface oxidation. In addition, with the increase of the coating thickness of titanium dioxide, the Raman peak at 145 cm–1 is gradually shifted to 140 cm–1, indicating that the thicker the titanium oxides, the better the protection (Figure S5). These results further suggest that ALD-TiO2 in nanoscale thickness can prevent the photoinduced oxidation of VO2 nanoparticles under strong laser irradiation by blocking the surface adsorption of oxygen.

Figure 4

Raman spectra and schematic outlines of VO2 nanoparticles protected by coating titanium oxide with different thicknesses after irradiation.

In order to further understand the role of the ALD-TiO2 thin film for blocking surface adsorption of oxygen on VO2 nanostructures, we subsequently explore the suppression of oxidation of VO2 nanowires coated with ALD-TiO2 thin films. According to the above oxidation processes in the cases of VO2 nanoparticles, we can also realize selected-area oxidation on a VO2 nanowire via a nonlithographic strategy for selected-area chemical nanoengineering of correlated VO2 nanostructures.[13] Once the VO2 nanowires (Figure S6) were irradiated by a laser, the characteristic Raman peaks of V2O5 such as 103 and 145 cm–1 appeared in Figure a. Similar to the cases of nanoparticles, the nanowire was partly oxidized during photoinduced surface oxidation.[13]Figure b shows that there was no significant difference in the Raman peak of VO2 nanowires before and after irradiation when the nanowire was coated with the 10 nm-thick ALD-TiO2 thin film. As shown in Figure a of the line-scanned Raman maps for VO2 nanowires, Raman shifts at 145 cm–1 obviously appeared in the case without the protection of the ALD-TiO2 thin film after irradiation.

Figure 5

Raman spectra and line-scanned Raman maps of the VO2 nanowire during the laser-irradiation processes: (a) no ALD-TiO2 thin film coated on VO2 nanowires and (b) a 10 nm-thick ALD-TiO2 thin film coated on VO2 nanowires.

However, no Raman peaks of V2O5 at 145 cm–1 were detected when the VO2 nanowire was coated with titanium oxide as shown in Figure b. All these results are consistent with the abovementioned in the cases of VO2 nanoparticles, which indicates that the photoinduced surface oxidation will be suppressed by the use of a surface coating for blocking oxygen adsorption on VO2 nanostructures. Moreover, we noted that the oxidation procedures were still restrained when the photo-oxidation processes were performed after several weeks.

Conclusions

We demonstrated the suppressed photoinduced oxidation through blocking the adsorption of oxygen on the surface of VO2 nanoparticles and nanowires. The adsorbed oxygen dominates the oxidation procedures, and the intermediate oxidation state has been observed. Similar with an inert atmosphere, the ALD-TiO2 thin films were used to isolate oxygen adsorbed on the surface for preventing the photoinduced surface oxidation on VO2 nanostructures. While the essential mechanism and function details need more exploration, we expected that such suppression of surface oxidation will help to improve the stability of vanadium dioxide nanostructures. Controlling surface oxidation is valuable for understanding the physical essentials of surface engineering, which enables us to obtain better surface functions and performances of vanadium oxide nanostructures.

Experimental Section

VO2 nanoparticles and nanowires were prepared on the substrates according to the method previously reported.[13] In brief, the SiO2/Si substrates were cleaned in isopropanol, acetone, ethanol, and deionized water to remove organic or other impurities and then dried under nitrogen (N2) gas. In the case of nanoparticles, vanadium(V) oxytriisopropoxide (VO(OC3H7)3, VTIP) and deionized water were used as precursors to obtain VO thin films on the substrate in an ALD reactor (Picosun R100, Finland). Subsequently, as-prepared VO thin films were annealed at 550 °C in argon for 4 h. In the case of nanowires, V2O5 powder was placed in a quartz boat in a horizontal quartz tube furnace upstream from the substrates. VO2 nanowires were obtained using a vapor transport method.

The ALD-TiO2 coating was deposited on the VO2 nanostructures in an ALD reactor. In brief, titanium isopropoxide (Ti[OCH(CH3)2]4,TTIP) and deionized water were used as precursors. For one cycle of ALD, TTIP was pulsed and released into the reaction chamber for 1.6 s followed by purging with nitrogen. Then, deionized water was pulsed for 0.1 s followed by purging with nitrogen. The thickness of the TiO2 thin film is controlled by the ALD cycles and checked by transmission electron microscopy (TEM). The cross-sectional TEM specimens were prepared by a focused ion beam (FIB) technique after the specimens were sputtered with a thin platinum layer as protection during the FIB process.

Raman measurements and photoinduced oxidation were conducted using Nanofinder 30 (TII Tokyo Instruments, Inc.) with an excitation laser wavelength of 532 nm and a spectral resolution of less than 1 cm–1. In order to acquire the observable Raman signals and spectrum with a high signal-to-noise ratio, the laser power was 1 mW with different exposure times. During the photo-oxidation processes, the laser power was up to 10 mW, and the exposure time was up to 5 s. The step was set to 100 nm for the Raman line scanning.