Discoloration Effect and One-Step Synthesis of Hydrogen Tungsten and Molybdenum Bronze (H x MO3) using Liquid Metal at Room Temperature.

This paper presents a new route to one-step fabrication and in situ application of hydrogen tungsten and molybdenum bronze (H x MO3) at room temperature and triggers the interdisciplinary research of multifunctional materials between liquid metal and transition-metal oxides (TMOs). Gallium-based liquid metal (GBLM) enables the discoloration effect on TMOs in acid electrolytes at ambient temperature. The underlying mechanism behind this phenomenon can be ascribed to the redox effect at the interface of liquid metal and TMOs in acid electrolytes. Both the theoretical calculations and the experimental results demonstrate that the increasing intercalation of H+ ions into the lattice of WO3 raises the electron density at the Fermi level and charge carriers. H+ ion content in the obtained H x MO3 can be controlled in our approach to meet different requirements. Taking advantage of the one-step fabrication and room-temperature liquid phase nature of the liquid metal, H x MO3 is synthesized under ambient conditions in a very short time, which is inaccessible with conventional solution-processed mechanical alloying, or other methods. The H x MO3 obtained in this one-step approach enables convenient and simple applications for biomimetic camouflage, cost-effective energy storage, H+ ion sensor, and electronic switch.

Introduction

Transition-metal oxides (TMOs), like WO3 and MoO3, have been extensively studied because of their exceptional electronic properties. Owing to the feature of their charge injection and extraction, TMOs have great potential application prospect in organic electronic devices and energy storage.[1] Hydrogen bronzes (HBs) of nonstoichiometric materials are solid conductive mixed oxides where mobile H+ ions are incorporated into the crystal structure of TMOs.[2,3] They are generally in the formation of HMO3, where M is a transition metal such as element W or Mo. Thus, the hydrogen tungsten bronzes (HTBs) and molybdenum bronzes are appearing as the novel multifunctional materials with various particular properties and widespread applications, including near-infrared light photocatalysis,[4] solid oxide fuel cells,[5] organic photovoltaics,[6,7] energy storage,[8,9] organic solar cells,[10] electrochromic and photochromic,[11−13] and so forth.

HTB was initially reported by passing suspensions of tungstic oxide in hydrochloric acid through a specially constructed Jones reductor.[14] With the diffusion of H+ ions into WO3, blue color HWO3 of HTB (x ranging from 0.1 to 0.6) emerged.[15,16] Next, HTB was prepared via the reaction of Cu with monoclinic WO3 to form CuWO4, and then reduced in hydrogen flow, producing HWO3 and Cu.[17] In addition, mechanochemical[18] and mechanical alloying[19,20] synthesis routes have been successfully used for the HTB formation. More recently, pore-arrayed HTB is fabricated with polystyrene as template by electrodeposition.[21] A method for synthesizing low-temperature solution-processed (in the presence of ethanol at 80–140 °C for 10 min on a hotplate in air) was reported that the HMoO3 layers are prepared on an ITO substrate.[10] Both methods mentioned above are multistep and need prepreparation. The present work not only paves a new way to one-step fabrication and in situ applications of HWO3 coatings at room temperature but also triggers the interdisciplinary research of multifunctional materials between liquid metal and TMOs.

Gallium-based liquid metals (GBLMs) are ideal targets as reducing agents because they are nontoxic, remain liquid at room temperature, and present particular interface behaviors.[22−27] It was reported that CO2 was reduced to carbon species and copper oxide was reduced to copper phases on the surface of liquid metal at room temperature.[28,29] Moreover, a series of findings on biomimetic chromatic liquid-metal soft robots was disclosed recently.[30,31] Lightened by the above research studies, the discoloration effect of TMOs via liquid metal reduction was first expounded in the present work. To enlighten this, here HBs samples including HWO3 and HMoO3 were synthesized. HMO3 can be stably formed at ambient temperature because of the galvanic effect of liquid metal. In the public literature, there is no report on the preparation of HTB by the reduction of liquid metal. Therefore, we explored the discoloration process of HMO3 in a liquid metal-hydrochloric acid environment. We evaluated the absorbance of HMO3 in visible and near-infrared light, and the conductivity change with increasing H+ ion concentration. Moreover, the underlying mechanism in enhanced conductivity was elucidated combining experimental measurements as well as the first-principles calculations. This approach is expected to open up a new world of research studies in the important area of HB materials via the discoloration process.

Results and Discussion

Discoloration Process of WO3

The discoloration process of the WO3 coating on the tungsten substrate was obtained via the contact of GBLM and the surface of WO3 coating in HCl solutions (Movies S1–S4, Supporting Information), while the discoloration phenomenon did not appear in the deionized (DI) water (Movie S5, Supporting Information). This contacting process inspired the reaction where the H+ in acid solution was injected into the lattice of WO3. The reaction speed increased with the concentration of HCl solution and in turn discoloration speed of the WO3 coatings was increased, implying that the H+ ion injection in the lattice of WO3 had a proportion relationship with the H+ ion concentration of solution.[32] As displayed in the discoloration videos, a new route to produce HWO3 in various colors was demonstrated, which would expand their applications in biomimetic camouflage.

Figure a shows the variation in the color of the WO3 coatings. Color of the slabs gradually changed from pale green for pure WO3 coating to dusty blue for the treated WO3 coating beginning at 0.5 mol L–1 HCl solution, indicating that the W6+ ions or W5+ in WO3 may be reduced to a lower valence state of W atoms,[33−35] synthesizing HWO3.[4] Because HWO3 is deeply colored with an oscillator strength per W atom of near unity. Namely, some adsorbed H+ ions from the HCl solution entered into the lattice of WO3.[36]Figure b,c depicts that the thickness of the HWO3 coating on the tungsten substrate is about 20 μm. The X-ray diffraction (XRD) analysis in Figure d verified the above hypothesis that the semiconductivity of WO3 transformed into the metallic HWO3.

Figure 1

Discoloration process of WO3 coatings in different synthesis conditions and the corresponding characterization data. (a) Photographs of different HWO3 coatings on tungsten substrates. (i) Raw WO3, (ii) 0.1 mol L–1, (iii) 0.5 mol L–1, (iv) 1.0 mol L–1, (v) 2.0 mol L–1. (b) SEM image of [a(iv)]. (c) Cross-sectional SEM image of [a(iv)]. (d) XRD analysis of the corresponding samples of (a). (e) Corresponding surface resistivity of the corresponding samples of (a).

We also evaluated effective surface electronic resistivity ρs of the obtained coatings (Figure e). Dynamic process of the surface electronic resistance change is displayed (Movie S6, Supporting Information). ρs decreased with the increasing H+ ions, representing that the doped H+ ions in the HWO3 crystal structure introduced greater electrical conductivity. Evidently, HWO3 exhibits strong metallic features. The mechanism lying behind this phenomenon can be ascribed to the intercalation of H+ ions into the WO3 lattice, which may give rise to shallow donor energy levels and charge carriers are thus increased.[15] Five samples in Figure e follow linear voltage–current (U–I) relationship (constant slope k0) (Figure S1, Supporting Information), which reminds us of ideal resistors.

Figure a shows the variation in the color of the powders and the related suspensions in DI water. The discoloration process of the WO3 powders can be observed in Movie S7 (Supporting Information). With regard to the powders and its related intermixtures, color changes from yellow for pure WO3 to light blue for the treated WO3 powders in 0.1 mol L–1 HCl solution and dark blue in higher concentration of HCl solution. WO3 powders (Figure S2, Supporting Information) and HWO3 powders in Figure [a(iv)] (see Figure b) were consistent in morphology and average particle size, suggesting that the discoloration process does not change the microstructure in the nanometer scale. Optical properties of the WO3 and the prepared HWO3 were evaluated by using a UV–vis–NIR spectrophotometer in the range of 200–800 nm (see Figure c). Optical absorption of the WO3 powders is consistent with its indirect band gap absorption edge (2.53 eV). Meanwhile, absorbance magnitude and absorbance position of four HWO3 powders (ii to v) increase significantly from a wavelength of 405 to 479 nm.

Figure 2

Discoloration process of WO3 powders in different synthesis conditions and the corresponding characterization data. (a) Photographs of powders and corresponding suspension in DI water of raw WO3 and different HWO3 powders. (i) Raw WO3, (ii) 0.1, (iii) 0.5, (iv) 1.0, (v) 2.0 mol L–1. (b) SEM image of the HWO3 powders of (iv). (c) UV–vis–NIR transmittance spectra of the related powder in (a). (d) XRD analysis of the corresponding samples of (a).

As seen in Figure d, the H0.23WO3 appeared in the XRD spectra. Furthermore, the content of H+ ions in HWO3 increased with the enhancing concentration of H+ ions of solution. When the concentration of HCl solution exceeds 1.0 mol L–1, the reaction product became H0.53WO3. Compared to the WO3 coatings, the small size of the WO3 powders increased the incorporation content of H+ ions.[36] The crystal structure of the powders transformed from monoclinic to tetragonal and cubic with the incorporation of H+ ions. The underlying mechanism is that the injected H+ ions occupy the interstitial positions in the distorted perovskite lattice and are surrounded by nearly regular octahedrons of oxygen atoms with the latter displaced by the normal perovskite positions along ⟨110⟩, thus, the oxygen positions may differ slightly from the monoclinic WO3. H+ ions are significantly off center and are attached to oxygen atoms in hydroxyl groups.[14]

Besides the concentration of HCl solution, reaction time, and temperature are also vital parameters in the fabrication process. More verification experiments and analysis were conducted in this study (Figure S3, Supporting Information). With the increasing of preparation time and treated temperature, the product tetragonal H0.23WO3 developed into the cubic H0.53WO3 when the preparation time, synthesized temperature, and the concentration of HCl solution reached a certain value. The interactions of the above three factors decided the final crystal structure of HWO3.

To accurately analyze the HWO3’s surface chemical composition, X-ray photoelectron spectroscopy (XPS) measurements were performed and focused on the W 4f. WO3 powder features two peaks at 37.51 and 35.37 eV, related to the 4f5/2 and 4f7/2 core level components, respectively, with an approximately 2.14 eV spin–orbit splitting, which corresponds to a nearly stoichiometric (W6+) oxide composition (Figure S4, Supporting Information).

Figure shows the evolution of the W 4f core level peak upon the WO3 coatings in solutions with different concentrations of H+ ions of solution. As shown in Figure a, W 4f core-level spectrum was fitted with three spin–orbit doublets corresponding to the three different oxidation states of W atoms. The peaks of W 4f5/2 and W 4f7/2 appear at 37.95 and 35.81 eV, respectively, which can be attributed to the W6+ oxidation state. The second doublets of lower energy values at the peaks 34.41 and 36.54 eV arises owing to the emissions from W 4f7/2 and W 4f5/2 core levels, respectively, corresponding to W5+. Furthermore, two weak signals at around 33.25 eV (W4+ 4f7/2) and 35.54 eV (W5+ 4f5/2) are obtained.[37,38] The atomic ratio of W6+ to W5+ to W4+ was calculated to be 77.96:17.65:4.39, that is equal to W5.735+. Thus, the chemical formula can be written as H0.265WO3. Similar to the inference process for Figure a, the chemical formulas of substances from Figure b–d were defined as H0.285WO3, H0.548WO3, and H0.594WO3, successively. These data show that there is a strong concentration dependence of H+ ion activity in WO3 and that the enthalpy of reaction is also concentration dependent. The donor-state energy level is associated with W atoms and its variation and broadening with increasing x values of HWO3 are not yet fully elucidated. The H+ ions because of its small size, exists at several equivalent noncentral positions.

Figure 3

XPS W 4f spectrum of the synthesized HWO3 powders in different HCl solutions. (a) 0.1 mol L–1 HCl, (b) 0.5, (c) 1.0, (d) 2.0 mol L–1.

Discoloration Mechanism of WO3

GBLM features a self-limiting thin oxide layer under ambient conditions at the metal-air interface.[30] On the basis of thermodynamic considerations, gallium oxide existed as Ga2O3 on the surface of liquid metal, on which elements In and Sn exhibited simple substance.[26,30] An electrical double layer formed on the droplet surface gives rise to surface reduction properties.[39−41] Hydrochloric acid solution was introduced to remove Ga2O3 because of the reaction.[42] The reaction between HCl solution and Ga2O3 can be defined as eq , that is

As depicted in Figure a,b, determined by their material nature, electrode potential of the liquid eGaInSn becomes more negative than that of the WO3 with the increasing concentration of HCl solution. Therefore, WO3 coatings or WO3 powders in direct contact with the reducing liquid eGaInSn–acid interface is reduced to HTB and the discoloration phenomenon is thus observed. The redox reaction is described as eq .

Figure 4

Schematic of the discoloration process. (a) Mode of redox reaction formed between gallium and WO3 in HCl solution. (b) Electrode potential of HWO3 and eGaInSn in HCl solution with different concentrations.

Here, x represents the content of H+ ion in a HTB molecule.

First-Principles Calculations and Simulations

To further elucidate the underlying relationship between H+ concentration and sample conductivity, the first-principles calculations of the band structure and density of states (DOS) were performed. Pure WO3 is an indirect band gap semiconductor and the calculated band gap value is 1.32 eV (Figure S5, Supporting Information).

As depicted in Figure a,b, H+ ions fill into the positive hole of monoclinic WO3, which acts as a conductive function. The crystal structure changes from monoclinal to cubic with the increasing inserted H+ ions. The fact that H+ ion insertion into WO3 and diffusion throughout the lattice has low activation energy is because of the favorable H-bonding interactions available during the entire process, and the increasing intercalated H+ ions into WO3 increase electron density at the Fermi level and charge carriers.[15]Figure c–f display the electron DOS of HWO3 (x = 0.125, 0.25, 0.375, and 0.5). Clearly, when the H+ ions intercalate into the interstitial site, a local energy level resulting from H+ emerges at the Fermi level. With the improvement of H+ ion content in HWO3, the DOS at the Fermi level gradually increases, corresponding to increased free electron concentrations in HWO3, which is in accordance with experimental results.

Figure 5

(a) H+ ion insertion into the crystal structure of the WO3. (b) Orbital distribution of the HWO3 Fermi level. (c–f) DOS of different simulated HWO3. (c) H0.125WO3. (d) H0.25WO3. (e) H0.375WO3. (f) H0.5WO3.

In addition, the synthesized HMoO3 can also be color-controllable and stable in acid conditions (Figures S6 and S7, Supporting Information). MoO3 powders became dark with the H+ ion doping, which presents similar trends to the WO3, and hydrogen molybdenum bronzes were synthesized.[10,43] This can provide useful guidelines for the synthesis and application of hydrogen bronze structure materials in the future.

Conclusions

The particular discoloration phenomenon of TMOs in acid electrolyte can be ascribed to the redox effect at the interface of liquid metal/TMOs, which realizes one-step synthesis of hydrogen bronze structures at ambient. A new route to producing TMOs in a range of colors was inspired, which can expand their applications in biomimetic camouflage. Taking advantage of the one-step fabrication and room-temperature liquid phase nature of liquid metal, the HBs were synthesized under ambient conditions in a very short time, which is inaccessible with conventional solution-processed, mechanical alloying or other methods.

Both the theoretical calculations and the experimental results demonstrated that the increasing intercalation of H+ ions into the lattice of WO3 raises electron density at the Fermi level and charge carriers. Importantly, the H+ ion content of the obtained HMO3 can be controlled in our approach for meeting different requirements. The investigated one-step rapid-processed HMO3 under ambient enables convenient and simple applications for biomimetic camouflage, cost-effective energy storage, H+ ion sensor, and electronic switch.

Experimental Section

Materials Synthesis

The eutectic GaInSn alloy (eGaInSn), gallium (67% weight, 99.99% purity) indium (20.5% weight, 99.99% purity) and tin (12.5% weight, 99.99% purity), which was prepared in a furnace (KSL-1200X) at 200 °C for 2 h. The original WO3 and MoO3 powders were purchased from Beijing DK Nano Technology Co., Ltd (average size: 50 nm). 1.0 g oxide powders (WO3 or MoO3) and 5.0 g liquid eGaInSn were immersed in a centrifuge tube with 5.0 mL HCl solutions in different concentrations (0.1, 0.5, 1.0, and 2.0 mol L–1, respectively). The above intermixtures were vibrated on a platform vibrator for 1 min (IKA VORTEX GENIUS 3). HMO3 powders were separated, centrifuged, and washed by ethyl alcohol, dried in a furnace. WO3 coatings on the surface of tungsten substrate in Figure ai were oxidized in O2 atmosphere at 700 °C for 1 h.

Characterization

Synthesis and discoloration process were captured with a Canon camera (EOS 80D). Morphology of the HB powders and coatings were characterized using a scanning electron microscope (SEM) (S-4800, Hitachi, Ltd., Japan). XRD analysis was performed on a Rigaku Smart Lab 9 kW XRD system. XPS spectra were obtained using a Thermo Scientific ESCALAB 250Xi XPS system. The optical properties of the synthesized powders were evaluated by using a Cary 7000 UV–vis–NIR spectrophotometer. The electrode potential εSCE of liquid eGaInSn droplets (against a saturated calomel electrode, SCE) was measured using a dropping method (Figure S8, Supporting Information). Electrode potential acquisition instrument (Agilent 34970A, Keysight Technologies Co., Ltd., Beijing, China) was employed, which was connected to a computer. Linear U–I curves were obtained by Electrical Conductivity Measurement (HMS-3000).

Computational Methods

First-principles calculations are performed using density functional theory methods. Structural and elastic properties are calculated by using the plane-wave pseudo-potential method,[44] which is implemented through the CASTEP code.[45] The optimized norm-conserving pseudopotentials[46] are used to simulate ion–electron interactions for all constituent elements. A kinetic energy cutoff of 700 eV is chosen with Monkhorst–Pack k-point meshes (6 × 6 × 6) spanning less than 0.04 Å–3 in the Brillouin zone.[47] The cell parameters and the atomic positions in the unit cell of monoclinic WO3 are fully relaxed. The convergence threshold for SCF tolerance is set as 1.0 × 10–9 eV/atom. Optimized structure of WO3 (a = 7.413, b = 7.677 and c = 7.854 Å) is in good agreement with the experimental values (a = 7.306, b = 7.54 and c = 7.692 Å). For HWO3, a 2 × 2 × 1 supercell is adopted[48] and HW8O24, H2W8O24, H3W8O24, H4W8O24 models are used to model HWO3 with x equal to 0.125, 0.25, 0.375, and 0.5.