Ammonium phosphotungstate nanoparticle/ultrathin g-C₃N₄ composite photocatalysts with Z-scheme heterojunctions: preparation and application to organic contaminant degradation

Abstract

Owing to their advantageous physical, chemical, electronic, and optical properties, two-dimensional (2D) materials are widely used as highly active visible-light photocatalysts. In this study, 3D/2D composite photocatalysts were prepared from ammonium phosphotungstate nanoparticles and 2D ultrathin graphitic carbon nitride by an impregnation method and were found to efficiently promote the photodegradation of rhodamine B and tetracycline in water under visible light irradiation. Photocatalytic degradation was maximum at a PW12NH4 loading of 20 wt% (UCN20), with photodegradation efficiencies of 93.40% and 69.6% for RhB and TC, respectively. The photocatalytic degradation rate using UCN20 was 5.8-fold higher for RhB and 2.2-fold higher for TC compared to those with UCN. This high performance is ascribed to the large specific surface areas and pore volumes of the composites and the reduced probability of photogenerated carrier recombination therein. The related degradation mechanism is believed to involve a Z-scheme charge transfer mode, which enhances the redox capability of composite photocatalysts and renders them suitable for the visible light-driven remediation of wastewater containing organic contaminants.

Mechanochemical synthesis of ionic solids based on phosphotungstic acid and triethylamine for photocatalytic dye degradation

“…This is because of the existence of high percentage of phosphotungstate anions produced upon proton transfer from PTA to triethylamine and band gap that allows efficient use of sunlight. The degradation mechanism may also be similar to that reported previously for ammonium phosphotungstate [15]. It may involve the following steps [13,21].…”

“…The representative peaks of Keggin structure of phosphotungstic acid appear in the region 1200-700 cm −1 [33]. The peaks at frequencies 1097, 987, 892, and 809 cm −1 are due to ν as (P-O), β(W = O), and ν as (W-O-W) [15]. The major peaks from triethylamine are due to symmetric and antisymmetric stretching vibrations of CH 2 groups that appear in the range 3000-2800 cm −1 , and C-N stretching vibrations having frequencies 1489 and 1378 cm −1 in pure TEA [34,35].…”

“…The 2θ values are slightly diverted in TTA8 where mole fraction of triethylamine is four times that of phosphotungstic acid. To ascertain the salt formation, the data was compared with standard data on ammonium phosphotungstate (JCPDS 50-0305) where sharp peaks representative of Keggin structure were noticed between 10°−36°due to reflections from (110), ( 220), (222), and (332) planes [15]. The magnitude of peak shift reflects a change in distance between crystal planes.…”

“…The absorption edge of PTA (i.e., 377 nm) is shifted to 397 nm in ammonium phosphotungstate. A further reduction in bandgap occurs to 2.75 eV upon composite formation with ultrathin g-C 3 N 4 [15]. The ammonium hydrates of PTA show nano scale heterogeneities in structural features [24].…”

“…The stability, facile synthesis, and possibility of tunning their bandgap reflect the potential of phosphotungstates as photocatalysts for degradation of dyes [11][12][13]. For instance, the thermal stability of PTA is enhanced and band gap narrowed upon conversion to ammonium salt [14,15]. However, it is arduous to recycle these remarkable catalysts owing to their high solubility in polar organic solvents and water that decreases their utility as environmentally benign products.…”