Catalytic Applications of Non‐Centrosymmetric Oxide Nanomaterials

Abstract

Noble‐metal‐free catalytic nanoparticles hold the promise being abundant, low‐cost materials having a small environmental footprint and excellent performance, albeit inferior to that of noble metal counterparts. Several materials have a long‐standing history of success in photocatalysis, in particular titanium dioxide, and in recent years more complex oxides and added functionality have emerged with enhanced performance. We will discuss different approaches related to the use of non‐centrosymmetric and polar oxide nanoparticles and how the bulk photovoltaic effect, piezoelectricity, and pyroelectricity add to photocatalysis and tribocatalysis. We pay special attention to discriminate between the role of free versus that of bound charges within the catalyst, which is crucial to disentangle the different contributions to the catalytic reaction for the benefit of the overall enhanced catalytic performance in e.g. wastewater treatment and ultimately water‐splitting.

Charge Separation in BaTiO3 Nanocrystals: Spontaneous Polarization Versus Point Defect Chemistry

“…This trend breaks down as soon as the nanoparticles are small enough to be cubic, leading to reduced photoactivity. [ 26–28 ] Our results do show that the smallest BTO (600) nanoparticles (13 nm in size) have the highest photoactivity. However, BTO (600) is predominantly cubic according to our XRD and Raman measurements (Figure 1c–e).…”

“…Thus, our results show that particle size plays a minor role in the charge separation process, while the biggest contribution originates from the presence of different recombination centers. [ 26–28 ]…”

“…Thus, our results show that particle size plays a minor role in the charge separation process, while the biggest contribution originates from the presence of different recombination centers. [26][27][28] To rationalize the trends in charge separation, one needs to consider the structural and spectroscopic changes the nanoparticle powders undergo during annealing from 600 °C (BTO (600)) to 900 °C (BTO (900)). [10] First of all, the BTO (700), BTO (800), and BTO (900) powders show unambiguous XRD and Raman evidence for tetragonal distortion of the lattice (Figure 1c,e), and, thus, of ferroelectric spontaneous polarization, which should enhance charge separation.…”

Tribocatalytic Degradation of Organic Pollutants Using Fe2O3 Nanoparticles

“…9,10 Therefore, advanced oxide techniques, such as photocatalysis, piezocatalysis, and tribocatalysis, have received increasing attention because these techniques can transform solar energy or mechanical energy into chemical energy using naturally existing clean energy. 11,12 The photocatalytic process is used to produce holes and electrons within semiconductors under light irradiation; then, these photogenerated holes and electrons are employed in catalytic reactions. 13 However, photocatalysts should have proper optical band gaps to respond to visible light because there is ∼48% visible light in the solar light.…”

“…Traditional wastewater purification methods, including the physical adsorption method, membrane filtration, the biological method, and so on, cannot solve the problem completely. − The physical adsorption method just transfers pollutants from one system to another, requiring additional treatment. − The membrane filtration method needs high costs and high energy consumption . The biological method demands suitable pH value, salt content, and temperature of wastewater and is time-consuming. , Therefore, advanced oxide techniques, such as photocatalysis, piezocatalysis, and tribocatalysis, have received increasing attention because these techniques can transform solar energy or mechanical energy into chemical energy using naturally existing clean energy. , The photocatalytic process is used to produce holes and electrons within semiconductors under light irradiation; then, these photogenerated holes and electrons are employed in catalytic reactions . However, photocatalysts should have proper optical band gaps to respond to visible light because there is ∼48% visible light in the solar light.…”