Visible-Light-Driven Photochromism of Hexagonal Sodium Tungsten Bronze Nanorods

Abstract

Single-crystalline sodium tungsten bronze (Na-WO3) nanorods with typical diameters of 10–200 nm and lengths of several micrometers were prepared via hydrothermal synthesis. The as-prepared Na-WO3 nanorods crystallized in a hexagonal structure (space group P6/mmm) with unit cell parameters a = 7.3166(8) Å and c = 3.8990(8) Å and elongated along the ⟨001⟩ direction. Chemical analyses indicated a stoichiometry of Na0.18WO3.09·0.5H2O, revealing the existence of tunnel Na+ ions and water molecules in the structure, as confirmed also by the vibrational spectroscopic study. The as-prepared Na-WO3 nanorods exhibited a direct-allowed electronic transition with band-gap energy of about 2.5 eV, which allows a visible-light-driven photochromism related to photogenerated carriers and a proton–electron double injection process. The proposed photochromism was discussed in detail by means of Fourier transform infrared spectroscopy. The involved local structural evolutions such as water decomposition and ion intercalation during the photochromic process were identified.

Photoluminescence and Scintillation Modulation Upon UV/X‐ray‐Induced Photochromism in Europium Tungstate Phosphors

“…Although it is difficult to differ between direct and indirect transitions, since in both cases there are a linear dependence on h ν near the absorption edge, some trends could be pointed out: maintaining citric acid concentration, the bandgap values are not significantly changed when Eu III became the network forming metal ion instead of Y III . This suggests that the valence and conduction bands are mainly composed of oxygen 2p and tungsten 5d orbitals, respectively ,. The citric acid concentration increase leads to the decrease of bandgap values, since the increment of its concentration causes a larger reduction of W VI to W V ions .…”

“…According to some mechanisms proposed in the literature, electrons can be excited from valence to conduction band with the irradiation, creating electron‐hole pairs (e ‐ /h + ). The electrons can be trapped in W VI sites, reducing the ions to W V and the holes in its turn, may decompose water or other organic molecules adsorbed on particles surface . Since the REW 2 O 6 (OH) 3 structure contains the same layered WO 3 motif, as it can be seen in Figure (b), the above processes are described based on the tungsten trioxide, according the Equations (4‐: …”

“…This suggests that the valence and conduction bands are mainly composed of oxygen 2p and tungsten 5d orbitals, respectively. [17,47] The citric acid concentration increase leads to the decrease of bandgap values, since the increment of its concentration causes a larger reduction of W VI to W V ions. [48] That should lead to the formation of defects, creating novel levels into the bandgap and resulting in the observed E g decrease.…”

“…The electrons can be trapped in W VI sites, reducing the ions to W V and the holes in its turn, may decompose water or other organic molecules adsorbed on particles surface. [17] Since the REW 2 O 6 (OH) 3 structure contains the same layered WO 3 motif, as it can be seen in Figure 1(b), the above processes are described based on the tungsten trioxide, [55] according the Equations (4-6):…”

“…In this context, the development of inorganic photochromic compounds has received considerable attention due to their high thermal and chemical stability, controllable macroscopic shape molding and mechanical strength in comparison to the organic counterparts ,. Tungsten derivatives, like tungsten trioxide, tungsten bronzes, tungstates and polioxotungstates, for instance, are a well‐known class of inorganic photochromic materials. Tungstates, specifically, are also a kind of self‐activated luminescent material.…”