Photoactive Basic Bismuth Nitrate/Nb2O5 Nanocomposites

Abstract

Basic bismuth nitrates (BBN) are generated from the incomplete hydrolysis of bismuth nitrate pentahydrate, it is a n-type semiconductor, and it was initially used as precursor of bismuth oxides and for medical applications. BBN also can be employed as photocatalysts, but usually exhibit low quantum efficiency due to the fast recombination of photogenerated charge carriers. In order to improve their photocatalytic performance, semiconductor composites have been proposed, being niobium pentoxide (Nb2O5) a promising candidate to be coupled with BBN. Hence, two different and promising photoactive nanocomposites (Nb-BBN) of BBN (Bi6O5(OH)3(NO3)5.3H2O and Bi2O2(OH)(NO3)) and Nb2O5 were obtained by the oxidant peroxide method followed by crystallization under mild hydrothermal conditions. Ammonium niobium oxalate and bismuth nitrate were the sources of Nb and Bi, respectively. Hydrogen peroxide was used as oxidant, and deionized water was used to prepare all solutions. Bismuth nitrate in contact with water forms immediately a highly hydrated basic bismuth nitrate precipitate (Bi6O5(OH)3(NO3)5.3H2O), named P-BBN. Hydrogen peroxide (10:1 molar ratio H2O2:M, M = Bi + Nb) and ammonium niobium oxalate (3:1 molar ratio Bi:Nb) were added into the suspension containing P-BBN, and the synthesis medium was poured in a teflon-lined hydrothermal reactor with autogenous pressure to promote the crystallization under two different procedures. The first nanocomposite (Nb-BBN120) was obtained using one single step: heating at 1 °C min-1 up to 120 ºC (maintained for 12 h), with a final pressure ca. 7 atm. The second nanocomposite (Nb-BBN230) was synthesized using three consecutive steps: 1) heating at 4 °C min-1 up to 150 ºC, with a final pressure ca. 10 atm; 2) heating at 2 °C min-1 up to 200 ºC, with a final pressure ca. 20 atm; and 3) heating at 1 °C min-1 up to 230 ºC (maintained for 12 h), with a final pressure ca. 31 atm. The photocatalytic performance was evaluated, under ultraviolet irradiation at room temperature, towards the discoloration of an aqueous solution of 10 mg L-1 rhodamine B (RhB) using 10 mL of a suspension containing 10 mg mL-1 photocatalyst. No direct photolysis and adsorption were observed. It was also evaluated the photocatalytic stability of the as-synthesized samples after four consecutives RhB discoloration 2-hour-cycles. XRD patterns revealed that the crystallographic structure of the samples Nb-BBN120 and Nb-BBN230 have a monoclinic Bi6O5(OH)3(NO3)5.3H2O (ICSD 00-2406) and orthorhombic Bi2O2(OH)(NO3) (ICSD 15-4359) phases, respectively. No significant peaks assigned to any Nb2O5 second phase are observed, probably due to its low content and high dispersion or the shielding effect of the strong diffraction signals from BBN. Although any XRD peak has been observed for Nb2O5, its presence in the nanocomposites was confirmed by XRF, ICP-OES, and XPS. The kinetics of RhB photodegradation shows that the hydrothermal treatment in presence of Nb2O5 is of paramount importance to enhance the BBN photoactivity. Although Nb-BBN120 and P-BBN have the same BBN phase, the first one has higher activity than the pristine material. Furthermore, despite of Nb-BBN230 shows the best photocatalytic activity, after performing three reuse cycles there was a loss of ~ 50% of its initial photoactivity, suggesting that the lamellar structure is a key factor on the photocatalyst stability. On the other hand, Nb-BBN120 or even the pristine P-BBN retained their initial photocatalytic activity after reuse. In summary, these novel BBN/Nb2O5 nanocomposites provide new insights into the synthesis of promising BBN photocatalysts for oxidation reactions.