Amorphous and Crystalline Sodium Tantalate Composites for Photocatalytic Water Splitting

Abstract

A facile hydrothermal synthesis protocol for the fabrication of sodium tantalates for photocatalytic water splitting is presented. Mixtures of tantalum and sodium ethoxide precursors were dispersed in ethanol, and ammonium hydroxide solution was used as mineralizer. By adjusting the amount of mineralizer, a variety of sodium tantalates with various morphologies, textural parameters, band gaps, crystal phases, and degrees of crystallinity were fabricated. The reaction was carefully monitored with a pressure sensor inside the autoclave reactor, and the obtained samples were characterized using X-ray diffraction, transmission electron microscopy, N2-physisorption, and ultraviolet-visible light spectroscopy. Among the series, the amorphous sample and the composite sample that consists of amorphous and crystalline phases showed superior activity toward photocatalytic hydrogen production than highly crystalline samples. Particularly, an amorphous sodium tantalate with a small fraction of crystalline nanoparticles with perovskite structure was found to be the most active sample, reaching a hydrogen rate of 3.6 mmol h(-1) from water/methanol without the use of any cocatalyst. Despite its amorphous nature, this photocatalyst gave an apparent photocatalyst activity of 1200 μmol g(-1) L(-1) h(-1) W(1-), which is 4.5-fold higher than highly crystalline NaTaO3. In addition, the most active sample gave promising activity for overall water splitting with a hydrogen production rate of 94 μmol h(-1), which is superior to highly crystalline NaTaO3 prepared by conventional solid-solid state route.

Activated‐Carbon‐Templated Crystalline Tantalates for Photocatalytic Water Splitting

“…Ac ubic morphologyi st ypical for crystalline NaTaO 3 and has been reported for nanoparticles and microparticles of NaTaO 3 . [16,19] Via the TEM analysis, the estimated nanoparticle sizes of AC-NaTaO 3 are around 5-19 nm, with an average particle size of 11 nm, which is in good agreement with the calculated crystallite size determinedb yX RD analysis (9 nm). This result shows that AC-NaTaO 3 has al arger nanoparticle size than AC-Ta 2 O 5 ,w hich is consistentw ith the analyzed surface area that was lower for AC-NaTaO 3 .T he atomic planeso ft he particles observed via HR-TEM displayahighly crystalline material, and the atomic planes of the individual particlesa re in different directions (Figure 3f), as it was observed for AC-Ta 2 O 5 .…”

“…The sample has been prepared via hydrothermal synthesis, and its activity towards photocatalytic water splitting hasb een determined using the same reactor set-up as for the hard-templated samples. [19] Althought he surface area of the amorphous sample is substantially higher than the one of the hard-templated AC-NaTaO 3 (75 m 2 g À1 ), the amorphouss ample produces al ower hydrogen production rate of 2.4 mmol h À1 ,while AC-NaTaO 3 exhibits ahydrogen production rate of 5.7 mmol h À1 .T he higher activity of AC-NaTaO 3 is related to the highly crystalline structure which compensates the lower surfacea rea. By the comparison of hydrogen production rates of the hard-templated samples with the amorphous-nanostructure samples, whosep hotocatalytic activity have been determined in the same photoreactor,w e were able to demonstrate the positive features of photocatalysts prepared via hard templating.T he high surfacea rea and crystalline structure generate ah igh number of active sites on the surface and allow fast chargecarrier diffusion.…”

“…These mesoporous domains consist of a network of linked NaTaO 3 nanoparticles, which exhibit a more cubical shape than the Ta 2 O 5 nanoparticles (Figure e). A cubic morphology is typical for crystalline NaTaO 3 and has been reported for nanoparticles and microparticles of NaTaO 3 . Via the TEM analysis, the estimated nanoparticle sizes of AC‐NaTaO 3 are around 5–19 nm, with an average particle size of 11 nm, which is in good agreement with the calculated crystallite size determined by XRD analysis (9 nm).…”

“…A similar comparison was made to AC‐NaTaO 3 using a previously reported amorphous sodium tantalate with a high surface area (200 m 2 g −1 ). The sample has been prepared via hydrothermal synthesis, and its activity towards photocatalytic water splitting has been determined using the same reactor set‐up as for the hard‐templated samples . Although the surface area of the amorphous sample is substantially higher than the one of the hard‐templated AC‐NaTaO 3 (75 m 2 g −1 ), the amorphous sample produces a lower hydrogen production rate of 2.4 mmol h −1 , while AC‐NaTaO 3 exhibits a hydrogen production rate of 5.7 mmol h −1 .…”

“…Furthermore, to gain an insight into the advantages of the porous network structure of hard‐templated photocatalysts, the photocatalytic performance of AC‐NaTaO 3 was compared with a crystalline NaTaO 3 nanoparticle sample, containing a small amount of amorphous structure, with a comparable surface area to AC‐NaTaO 3 . The nanoparticle sample has been obtained via hydrothermal synthesis and exhibits crystalline NaTaO 3 particles with a surface area of 83 m 2 g −1 and an average particle size of 69 nm . The APA for the nanoparticle sample has been determined to be 1000 μmol g −1 L −1 h −1 W −1 , showing a hydrogen production rate of 3.0 mmol h −1 .…”