Hydrogenating VO2 with protons in acid solution

Hydrogenation is an effective way to tune material property1-5. Traditional techniques for doping hydrogen atoms into solid materials are very costly due to the need for noble metal catalysis and high-temperature/pressure annealing treatment or even high energy proton implantation in vacuum condition5-8. Acid solution contains plenty of freely-wandering protons, but it is difficult to act as a proton source for doping, since the protons always cause corrosions by destroying solid lattices before residing into them. Here we achieve a facile way to hydrogenate monoclinic vanadium dioxide (VO2) with protons in acid solution by attaching suitable metal to it. Considering the Schottky contact at the metal/VO2 interface, electrons flow from metal to VO2 due to workfunction difference and simultaneously attract free protons in acid solution to penetrate, forming the hydrogens dopants inside VO2 lattice. This metal-acid treatment constitutes an electron-proton co-doping strategy, which not only protects the VO2 lattice from corrosion, but also causes pronounced insulator-to-metal transitions. In addition, the metal-acid induced hydrogen doping behavior shows a ripple effect, and it can spread contagiously up to wafer-size area (>2 inch) even triggered by a tiny metal particle attachment (~1.0mm). This will stimulate a new way of simple and cost-effective atomic doping technique for some other oxide materials.

visible-light driving photocatalysis ability to TiO2 5 , and modulate metal-insulator transitions (MIT) in strongly correlated oxides such as SmNiO3 and other correlated oxides 1,3,6 . Recent experiments 2,8 also observed that H-incorporations in M-VO2 resulted in a very stable metallic phase at room temperature, giving excellent thermoelectric performance 25 . Interestingly, further injecting H into the lightly doped M-VO2 created another insulating state at the heavily H-doping situation 3 , making it a promising technique of controlling MIT in a reversible and consecutive way. However, for all of these hydrogenation treatments, the H-intercalations into crystal lattice are very costly because of the needs of noble metal (Au, Pt, Pd) catalysts or highpressure/temperature annealing process. That is, the creation of single-atom based hydrogens and their injection into solid cost a large amount of energies.
In this work, we propose a facile route to hydrogenate M-VO2 with protons in acid solution. It is known that some metallic oxides including VO2 are easily dissolved in acid. Protons (H + ) with positive charges attack oxygen atoms in oxide, and soon break the crystal lattice by dragging oxygen into solution, through a traditional reaction of VO2+4H +→ V 4+ +2H2O. As shown in Fig. 1a, a 30 nm M-VO2/Al2O3 (0001) epitaxial film grown by molecular beam epitaxy method 26 ( Supplementary Information Fig. S1) , was held by a plastic tweezers and put into a 2% H2SO4 acid solution. As expected, the yellowy VO2 epitaxial film completely disappeared after 3 hours. In contrast, when we used a steel tweezers as shown in Fig. 1b, the same VO2 film suddenly obtained excellent anti-corrosion ability, as it was nearly intact by 3 hours in the acid solution.
Obviously, the magic trick is ascribed to the metal attachment of the steel tweezers.
Scanning electron microscope (SEM) images in Fig. 1c show that the thickness of VO2 film remained unchanged and the surface maintained almost the same grain-like morphologies even after 20 hours in acid solution. More convincingly, the trace element analysis in Fig. 1d found that the V 4+ cations concentration in solution increased from 0.11μg/ml to 1.82 μg/ml after immersing a VO2 film without metal-attachement in acid from 30 minutes to 20 hours, while that of metal-acid treated sample kept very low V 4+ concentration at 0.03~0.06 μg/ml in 20 hours. These results demonstrated excellent anti-corrosion ability of VO2 lattice in acid if metal was attached.  Supplementary Information Fig. S3).
The anti-corrosion ability of metal-acid treated VO2 should be ascribed to hydrogenation, since conventional hydrogenated VO2 assisted by Au or Pd catalyst is also very stable in acid solution ( Supplementary Information Fig. S2). For the case of metal-acid treated VO2 film, protons in acid solution reside into VO2 crystal with the help of attached metal to form O-H bonds instead of destroying the VO2 lattice, which then prohibit the attack of H + to oxygen atoms. The X-ray diffraction (XRD) spectra in Fig. 1e show the dynamics shifts of (020) diffraction peak from 39.8 o to 36.7 o after the metal-acid treatment, which agree with XRD curves of lightly and heavily hydrogenated VO2 through conventional noble-metal catalysis at high temperature ( Supplementary Information Fig. S3). These suggest slightly expanded cell volumes due to H-incorporation. Fig. 1f presented the XPS measurement results, showing the conversion from V 4+ to V 3+ state due to H intercalation. The variations of O1s peak at ~531.6 eV for the O-H species further confirmed H-incorporation in VO2 after metalacid treatment. Furthermore, the XANES spectra in Fig. 1g show the V L-edge curves shifting continuously to lower energy, indicating the polarized charge in V atoms and the evolution of valence state from V 4+ to V (4δ )+ or even to V 3+ state. The polarized charge effect was also inferred from the O K-edge signal. After metal-acid treatment, the relative intensity ratio of the t2g and eg peaks decreased substantially, reflecting the variation of electron occupancy due to electron doping. All of these spectroscopic variations induced by metal-acid treatment with 1.5 and 10 hours, agree well with corresponding measurements on lightly and heavily hydrogenated VO2 through conventional catalysis techniques (Fig. S3), respectively. These together with above corrosion-protection tests, demonstrated that such metal-acid treatment indeed created H-doping in the VO2 film. 6 To further explore the effect of metal attachment, a tiny Cu particle (~1.0mm in diameter) was attached to the center of one 2-inch M-VO2/Al2O3(0001) epitaxial film, which were immersed into 2% H2SO4 solution. It was observed in Fig. 2a that, the bare VO2 film with yellowy color was dissolved within 1.5~3 hours, indicating the complete corrosion of VO2 layer. In sharp contrast, although the ~1.0mm copper particle was contacted to the VO2 film with a very small area, it protected the whole 2-inch wafer from acid corrosion. In addition, even if we took away the Cu particle after the treatment, the film was still stable in acid solution ( Supplementary Information Fig. S4).
It is known that hydrogenation for VO2 can induce MIT at room temperature 2, 8 , i. e. hydrogenation converted the insulated M-VO2 to be metallic ( Supplementary   Information Fig. S5). Starting from the original insulating M-VO2 film (Fig. 2b), the above Cu-acid treatment lowered down its surface resistance for ~3 orders of magnitude ( Fig. 2c). Applying heat the sample in air at 120 o C to remove the intercalated hydrogens within half an hour, the film was recovered back to the insulated phase (Fig. 2d), which is consistent with the results of hydrogenated samples through conventional catalysis ( Supplementary Information Fig. S6). From the resistance distribution map in Fig. 2b, one should note again that the ~1.0mm copper particle accomplished MIT for the whole 2-inch VO2 wafer, including the center copper-covered area where copper was gradually eliminated by acid (Fig. 2a). These are of great advantages for achieving complete and clean H-doping materials, as the conventional catalysis-based technique is suffered to the limited hydrogenation area covered by catalysts (Fig. S5b), as well as the difficulty to remove metal catalysts after hydrogenation. We thus move forward to examine the underlying mechanism of the metal-acid 8 induced VO2 hydrogenation. In Fig. 3a, it is found that active metals including Al, Cu, Ag, Zn or Fe (the pictures of Zn and Fe were not shown here) could all induce hydrogenation and thereby protect VO2 from corrosion in acid. In sharp contrast, the noble and relatively stable metal of Au and Pt could not. Here metals and VO2 actually formed the typical metal-semiconductor interfaces, forming the so-called Schottky Contacts. Theoretical investigations at the first-principle level were performed to compute the workfunctions of metals and M-VO2 ( Supplementary Information Fig. S7), and listed together with reported experimental values 27 in Fig. 3b. Due to the workfunction differences, metals with higher electric Fermi level (EF) would donate electrons to the interfaced semiconductor with lower EF (Fig. 3c). As expected, Al, Cu, Ag, Zn have lower workfunction than M-VO2 (Fig. 3b), so that one (1×1) VO2 unit could extract 0.47~2.50 efrom metals (Fig. 3d, Supplementary Information Fig. S8 and Table. S1). On the other hand, Au and Pt metal-attachments with higher workfunction than VO2 induced nearly no extra electrons in the interface M-VO2. for metals, M-VO 2 , and lightly hydrogenated H 0.5 VO 2 , with the order of Pt > Au > VO 2 > Cu > Ag > H 0.5 VO 2 > Zn > Al. XRD measurements in Fig. 1e identified our VO 2 sample with the (020) facet.
Here we focused on three facets of (020), (011), and (100), among which the latter two are the most stable surfaces for M-VO 2 . See calculation details in Supplementary Information Fig. S7 Consequently, the extra electrons in M-VO2 doped from active metals would drive surrounding acid protons to penetrate. By examining six VO2 surface sites for a proton to adsorb (Inset graph in Fig. 4a and Supplementary Information Fig. S9), we found that more doped electrons led to higher adsorption energies for all sites (Fig. 4a). For instance, on site 1, the proton adsorption energy of 3.68 eV in neutral circumstance was increased to 5.04 eV for a VO2 unit with 4 echarge. The diffusion of surface protons into the VO2 crystal could also be promoted by doped electrons ( Supplementary   Information Fig. S10). Therefore, driving by the electrostatic attraction force, the surrounding protons could penetrate into VO2 to meet electrons, resulting in neutral H intercalation. The incorporation of H in the VO2 crystal then prohibited further attack/adsorption of protons to oxygen, and increased the formation energy required for oxygen vacancy defect ( Supplementary Information Fig. S11), leading to anti-corrosion ability in acid solutions. This thus constitutes an electron-proton co-doping strategy, which creates stable neutral H-doping in VO2.
The H-doping then changed the VO2 electronic structures. For a VO2 unit with small H-doping concentration of H0.25VO2 (Fig. 4b), the evolutions of electronic structures were reflected by the computed partial density of state (PDOS) of the V-3d orbitals in Fig. 4b. The formation of H-O bonds causes electrons transferring from H to O atoms (Supplementary Information Table S2), which in turn promoted the electron 10 occupancy of V-3d orbitals (Table S2). Such effects were reflected by the up-shifting of Fermi level from the pure VO2 to H0.25VO2 (Fig. 4b). Originally, VO2 exhibited a typical insulating state, with wide energy gap consisting of fully-occupied valence band and empty conduction band. The H-doping then made the conduction band edge states partially occupied, causing the MIT to H0.25VO2.
Furthermore, theoretical simulations explained the contagious hydrogenation process which enabled a ~1.0mm metal particle to convert a 2-inch semiconductor wafer. The work functions of the lightly hydrogenated H0.25VO2 with three facets of It should be noticed that this metal-acid treatment induced hydrogenation in VO2 crystal completely lies on the synergetic electron-proton co-doping route. The tiny Cu particle induced hydrogenation behavior in large-area VO2 film with a ripple effect will be terminated at the liquid level if the VO2 film is partially immersed in acid solution ( Supplementary Information Fig. S13).   Supplementary Information Fig. S14). In Fig. 3b, we noticed that Al and Zn actually hold even lower workfunctions than those of lightly hydrogenated H0.25VO2. Therefore, the Al and Zn metal particles would continue to donate much electrons to H0.25VO2, which in turn continued to drive protons penetration until the H-incorporation became saturated. Such transformation behavior of VO2 from the initial insulating phase to the metallic phase and later to insulator, is consistent with very recent findings of the consecutive insulator-metal-insulator transitions induced by increasing H-doping concentration 3 . Amazingly, this metal-acid treatment could be extended to a metal-ions strategy for other elements doping into solids. Replacing the acid solution by Li+ involved polymeric solution, metallic Lidoped VO2 films were obtained ( Supplementary Information Fig S15). This demonstrated the universality of the metal-acid induced doping strategy.
In summary, we have proposed a novel electron-proton co-doping strategy to accomplish contagious hydrogenation to VO2 crystal with protons in acid solution.
Using non-noble metal (Cu, Ag, Al, Zn, Fe) attachment and diluted acid solution as the electron and proton sources, we achieved the hydrogenation treatment under ambient conditions by injecting both electrons and protons into VO2 film. The resulted H-doping modulated VO2 3d-orbital electron occupancy and drive consecutive phase transitions between metallic and insulator states. The whole process is extremely efficient as the repeated "electron flowing-proton penetration-phase transition-electron further flowing" cycle is contagious and expanding quickly, so that a tiny metal attachment (~1mm) would convert a very large area of semiconductor (>2 inch). Utilizing the uniformlydistributed protons with controllable concentration in acid, and the well-developed techniques for preparing metal-semiconductor hybrid, this co-doping method would not only help to achieve a peaceable way for hydrogen or Li storage/doping and selective material corrosions, but also stimulate a new way of thinking to develop simple and cost-effective atomic doping technique.
This work was partially supported by the National Basic Research Program of