The Effects of Crystal Structure and Electronic Structure on Photocatalytic H₂ Evolution and CO₂ Reduction over Two Phases of Perovskite-Structured NaNbO₃

Abstract

Cubic and orthorhombic NaNbO 3 were fabricated to study the effects of crystal structure and electronic structure on the photocatalytic activities in detail. The samples were characterized by X-ray diffraction, field emission transmission electron microscopy, high-resolution transmission electron microscopy, UV−visible absorption spectroscopy, and X-ray photoelectron spectroscopy. The photocatalytic activities of the two phases of NaNbO 3 have been assessed by H 2 evolution from aqueous methanol solution and CO 2 photoreduction in gas phase. The photocatalytic H 2 evolution and CO 2 reduction activities over cubic NaNbO 3 were nearly twice of those over orthorhombic NaNbO 3 . The first-principles calculation reveals that the higher activity over cubic NaNbO 3 can be attributed to its unique electronic structure, which is beneficial for electron excitation and transfer. ■ INTRODUCTIONAs the fossil fuels have limitations in availability, a new source that can provide abundant and maintainable energy must be developed.1,2 For the past decades, photocatalysis has been developed as a candidate that can supply a renewable, unlimited, and environmentally friendly energy source to solve the energy crisis.3,4 The investigations on photocatalytic reaction mechanisms, 5−7 energy-band structure engineering (including the optimization of crystal structure and the modulation of band energy levels), 8−10 and morphology control 11−15 have been carried out to enhance the photocatalytic efficiency. Generally, to study the relationship between crystal structure and electronic structure is helpful to understand the process of photogenerated carrier excitation and transfer. Up to now, the relevant studies have been performed on the TiO 2 , CdS, BiVO 4 , and AgGaO 2 with different crystal structures. 16−19 Among intensively studied photocatalysts, the materials with perovskite and multilayered perovskite structures have received considerable attention.20−24 However, the report about the influence of crystallographic symmetry on photogenerated carrier excitation and transfer in the perovskite-structured photocatalysts is still limited. NaNbO 3 is nontoxic, highly stable, and with a typical perovskite structure and thus attracts extensive attention in the field of photocatalysis. Many investigations have proved that NaNbO 3 is a high-efficient photocatalyst for H 2 generation.25−29 Under the irradiation of high-pressure mercury lamp, H 2 O can be reduced into H 2 with quite high efficiency over NaNbO 3 nanoparticles.29 Nanofiber-structured NaNbO 3 was also verified to be useful to slit pure water and reduce CO 2 to CH 4 . 30 Moreover, NaNbO 3 normally belongs to the orthorhombic system at room temperature and exhibits an unusual complex sequence of temperature-, pressure-, and particle-size-driven phase transitions. 31−35 When the temperature ranges from room temperature to 1000 K, there are several other phases of NaNbO 3 existing, such as tetragonal and cubic structures. 36 All of them, cubic, tetragonal, and orthorhomb...

Electronic and Optical Properties of Sodium Niobate: A Density Functional Theory Study

“…e vertical black dashed lines correspond to the experimental ground state volumes. [34] Given are the lattice parameter a, equilibrium unit-cell volume V 0 , the bulk modulus B 0 and its pressure derivative B ′ 0 , and the direct E Table 2. e electronic band structure calculated using the PBEsol functional and based on the PBEsol ground state volume is shown in Figure 5(c).…”

A perspective on perovskite oxide semiconductor catalysts for gas phase photoreduction of carbon dioxide

“…This yield is relatively low compared with the conversion of water splitting. [14][15][16][45][46][47][48][49][50][51][52][53][54] The conversion efficiency and [ 45] ). [14] ).…”

“…3. [51] Li et al discussed the photocatalytic activities of CO 2 reduction over the cubic and orthorhombic NaNbO 3 in a gas phase system and found that the CO 2 reduction activity over the cubic NaNbO 3 was nearly twice that of the orthorhombic NaNbO 3 . These results might be due to the unique electron structure of cubic NaNbO 3 , which may support electron excitation and transfer.…”

“…These results might be due to the unique electron structure of cubic NaNbO 3 , which may support electron excitation and transfer. [51] In addition, they studied the influence of the preparation temperature on the activity of NaNbO 3 photocatalyst. [50] The cubic NaNbO 3 prepared at 500°C presented the best photocatalytic performances, and the CH 4 evolution for gas phase CO 2 photoreduction over NaNbO 3 loaded with 0.5 wt.% Pt under a 300 W Xe lamp (λ > 300 nm) could reach 12.6 µmol/m 2 /h.…”

“…(Copyright 2012, The Journal of Physical Chemistry. [51] ). Functional Oxides Prospective Article photocatalytic reduction of CO 2 into CH 4 over BaCeO 3 loaded with 0.3 wt% Ag cocatalyst was up to 0.55 µmol/h/g.…”