Impact of Nb(V) Substitution on the Structure and Optical and Photoelectrochemical Properties of the Cu₅(Ta_1–xNb_x)₁₁O₃₀ Solid Solution

Abstract

A family of solid solutions, Cu 5 (Ta 1−x Nb x ) 11 O 30 (0 ≤ x ≤ 0.4), was investigated as p-type semiconductors for their band gaps and energies and for their activity for the reduction of water to molecular hydrogen. Compositions from 0 to 40 mol % niobium were prepared in high purity by solid-state methods, accompanied by only very small increases in the lattice parameters of ∼0.05% and with the niobium and tantalum cations disordered over the same atomic sites. However, an increasing niobium content causes a significant decrease in the bandgap size from ∼2.58 to ∼2.05 eV owing to the decreasing conduction band energies. Linear-sweep voltammetry showed an increase in cathodic photocurrents with niobium content and applied negative potential of up to −0.6 mA/cm 2 (pH ∼7.3; AM 1.5 G light filter with an irradiation intensity of ∼100 mW/ cm 2 ). The cathodic photocurrents could be partially stabilized by heating the polycrystalline films in air at 550 °C for 1 h to produce surface nanoislands of CuO or using protecting layers of aluminum-doped zinc oxide and titania. Aqueous suspensions of the Cu 5 (Ta 1−x Nb x ) 11 O 30 powders were also found to be active for hydrogen production under visible-light irradiation in a 20% aqueous methanol solution with the highest apparent quantum yields for the 10% and 20% Nb-substituted samples. Electronic structure calculations show that the increased photocurrents and hydroen evolution activities of the solid solutions arise near the percolation threshold of the niobate/tantalate framework wherein the Nb cations establish an extended −O−Nb− O−Nb−O− diffusion pathway for the minority carriers. The latter also reveals a novel pathway for enhancing charge separation as a function of the niobium−oxide connectivity. Thus, these results illustrate the advantages of using solid solutions to achieve the smaller bandgap sizes and band energies that are needed for solar-driven photocatalytic reactions.

Capturing Metastable Oxide Semiconductors for Applications in Solar Energy Conversion

“…Shown in Figure A for NaCu­(Ta 1– x Nb x ) 4 O 11 , the bandgap undergoes a significant red-shift from ∼2.70 eV to ∼1.80 eV with increasing Nb content from 0% to 70%. For Cu 5 Ta 11– x Nb x O 30 , this gives a bandgap red shift from ∼2.58 eV to ∼1.97 eV . For both semiconductors, the red-shifting is caused by replacing Ta 5d 0 orbitals with Nb 4d 0 orbitals and lowering the conduction-band energy.…”

“…As described below, crystalline semiconductors containing new combinations of these M/M′ cations have been prepared in my laboratory and others and investigated for their photoelectrochemical properties. − ,− Many representatives within these systems are listed in Table in order of increasing band gaps. Notably, a predominant fraction of ∼80% of these semiconductors has been found to occur as thermodynamically unstable phases, as denoted in the right-hand column.…”

“…This strategy is based on requiring the cations to diffuse over longer distances to phase segregate into the simpler oxides, such as Nb 2 O 5 and Ta 2 O 5 . Many recent examples from my group have illustrated this concept, including the single metal-site (M) solid solutions of CuNb 1– x Ta x O 3 , Cu 5 Ta 11– x Nb x O 30 , and NaCu­(Ta 1– x Nb x ) 4 O 11 . ,, These compounds show that Nb­(V) and Ta­(V) will statistically disorder over the same crystallographic sites owing to their nearly identical Shannon radii. More complex examples are the double metal-site (both M and M′) solid solutions, including (Ba 1– x Sn x )­(Zr 1– y Ti y )­O 3 and (Na 2– x Sn x /2 )­(Ta 4– y Nb y )­O 11 , , with the cations separated by parentheses in the chemical formula.…”

“…The synthesis of solid solutions of many of the semiconductors in Table provides the capability of tuning their band gaps via changes in the Δ E red by substitutions for either M or M′. For example, the preparation of kinetically stabilized solid solutions Cu 5 Ta 11– x Nb x O 30 and NaCu­(Ta 1– x Nb x ) 4 O 11 results in smaller bandgaps with increasing Nb­(V) substitution for Ta­(V) cations. , As the Nb­(V)/Ta­(V) cations are statistically disordered, the only change is with respect to lower conduction band energies. Shown in Figure A for NaCu­(Ta 1– x Nb x ) 4 O 11 , the bandgap undergoes a significant red-shift from ∼2.70 eV to ∼1.80 eV with increasing Nb content from 0% to 70%.…”

“…Thermodynamically stable structures and compositions have been the foundation for establishing structure–property relationships and driving much technological progress. Recent research, however, has revealed growing numbers of crystalline semiconductors that occur near to or “over the edges” of stability, that is, existing as metastable solids that are thermodynamically unstable but that are synthesizable and kinetically trapped. − While compilations of thermodynamic data for solids extend back for many decades, the unknown vastness of metastable space has only begun to be revealed in recent experimental and computational studies. − Thermodynamic instability can result from nonoptimal structures and bonding that, concomitantly, yield physical properties that are both desirable and otherwise challenging to obtain. For example, concepts such as “redox balance” have been key to understanding the stability of oxides under oxidizing or reducing conditions in solid oxide fuel cells .…”