Novel and green synthesis of BiVO4 and GO/BiVO4 photocatalysts for efficient dyes degradation under blue LED illumination
Introduction
The nanoscience of photocatalytic semiconductors has attracted great attention due to the effective utilization of solar energy in water-splitting to produce hydrogen, carbon dioxide conversion to fuels, photoreduction of heavy metals to less toxic oxidation states, and mineralization of organic pollutants by photodegradation [[1], [2], [3], [4]]. Using photocatalytic compounds can produce fuel and remediate polluted water from green sources and technologies, attending to global warming and environmental pollution concerns.
Bi-based semiconductors (Bi2O3, Bi2O2CO3, Bi2WO6, BiVO4, BiOI and others) have gained significant interest due to their high photocatalytic activity, related to their wide energy gap (Eg) values, varying from 1.8 to 3.3 eV. Moreover, they exhibit superior visible light absorption, attributed to the hybridized valence band by O 2p and Bi 6s, narrowing the Eg Ref. [5]. BiVO4 exhibits outstanding physicochemical properties such as ionic conductivity, visible light photoactivity, chemical stability, high resistance to photocorrosion and non-toxicity. Synthetic BiVO4 crystallizes into three different polymorphic structures (tetragonal, and monoclinic scheelite and tetragonal zircon-type), among which the thermodynamically stable monoclinic scheelite shows high photocatalytic performance [6]. Monoclinic scheelite polymorph of BiVO4 (m-BiVO4) is an n-type semiconductor visible-light-driven (Eg = 2.4 eV) that has become a promising catalyst for pollutants degradation [7]. Besides, BiVO4 can serve as an efficient photoanode for water oxidation under solar exposure [8]. Nevertheless, BiVO4 presents a high recombination rate of the photogenerated electron-hole pairs (e- - h+), diminishing the degradation efficiency. To overcome this drawback, modifying the BiVO4 texture has focused on the addition of noble metals (acting as a dopant in the host structure or plasmonic particle deposited on the semiconductor surface) or designing BiVO4-based heterojunctions [9]. On the other hand, graphene oxide (GO) possesses outstanding photoelectric activity, efficient carrier migration rate, and high chemical stability. A coupling of GO with a semiconductor allows enhancing the degradation rate of organic pollutants due to the efficient separation and migration of the photogenerated e- – h+ pair, playing an essential role as a receiver and migration bridge electrons [[10], [11], [12], [13]].
BiVO4 photocatalytic and photophysical properties are strongly dependent on the synthesis procedure. Many techniques have been applied for the BiVO4 synthesis, such as hydrothermal, co-precipitation, sol-gel, flame spray, ultrasonic- and microwave-assisted routes, and so forth [9,14]. Scientific investigations have been mainly focused on the crystallinity, particle size, morphology characteristics of BiVO4, the viability of obtaining pure phases (especially m-BiVO4), and their effect on photocatalytic activity. Currently, great emphasis is placed on designing and implementing green synthesis methods [4,7,[15], [16], [17]]. Among these, metathesis and molten salts represent straightforward, cost-effective, high-yield, and green methods and have been the basis of the successful design and implementation of novel routes for synthesizing pristine and heterojunction photocatalysts [[18], [19], [20]].
Several photocatalytic studies have reported effective dye degradation using pristine and BiVO4-based heterojunctions [4,[11], [12], [13],[21], [22], [23], [24]]. Nevertheless, those reports using Xe, Hg, and tungsten halide lamps as irradiation sources present significant drawbacks, such as high-power consumption, high-temperature operation, complex photoreactor design, short lifetime, expensive process, and Hg toxicity. Further, a large part of the light emitted by these lamps falls out of the effective wavelength to activate BiVO4. On the contrary, the use of visible light-emitting diodes (LEDs) as irradiation source during the photocatalytic process is attributed to the following outstanding advantages: compact size, high reliability, low-operation temperatures (∼40 °C), longer lifetime (50,000 h), low cost, higher current-to-light conversion efficiency (40%), low voltage (1.5–5 V), environmental safety. LEDs are available in different shapes granting flexibility to the photoreactor geometry and design. Another remarkable feature of LEDs is their operation using a DC power supply, which enables operating the water treatment by photocatalysts to remote and isolated areas.
Nowadays, there are few studies related to dye photodegradation over BiVO4-based catalysts using visible LEDs as the irradiation source. BiVO4 nanoparticles (NPs) synthesized by hydrothermal method achieved MB percentage degradation of 78% in 60 min using white LEDs (60 W). The photodegradation rate of MB increased by adding H2O2, acting as an external electron acceptor, and promoting the separation of e- and h+ [15]. The RhB percentage degradation over BiVO4 was 62% after 200 min of LEDs irradiation (60 W), and the BiVO4 catalyst synthesized by hydrothermal/calcination at 450 °C showed the highest photocatalytic activity [16]. BiVO4 NPs prepared by a microwave-assisted hydrothermal method achieved a crystal violet degradation of 100% in 120 min using 21 blue LEDs. The synthesis parameters affected the morphology and polymorphism of BiVO4, being the tetragonal/m-BiVO4 (30/70) the most photoactive heterojunction [17]. Recently, BiVO4/Ag/Ag2O Z-scheme catalyst enhanced the RhB degradation compared to pristine BiVO4 and confirming the efficacy of applying blue LEDs as visible light in photocatalytic processes [25].
This study presents a novel, straightforward and green synthesis of BiVO4-based photocatalysts using metathesis and metathesis-assisted molten salts methodologies. The effect of the synthesis conditions on the physical characteristics and photocatalytic properties of BiVO4 were examined thoroughly. The synthesis of GO/BiVO4 through metathesis-assisted molten salts has not been studied previously. Photoactivity under light illumination from low-cost LEDs and the simplicity of the synthesis procedures employed are outstanding features of the BiVO4-based catalysts studied in this work. BiVO4-based catalysts under LEDs irradiation exhibited higher photodegradation of RhB dye than those reported for other BiVO4 compounds using high-power lamps as irradiation sources.
Section snippets
Photocatalysts synthesis
BiVO4-based photocatalysts were prepared through a mechanically activated metathesis reaction:
Bi(NO3)3, NaVO3 and NaOH reagents, supplied by Merck, were weighed using a molar ratio of 1:1:2, respectively. Reagents and balls of 12.7 mm diameter were loaded in a ZrO2 vial of 45 mL (balls-to-powder mass ratio of 10:1). Milling was conducted in a high-energy ball mill, Mixer/Mill 8000 M, SPEX, at a speed of 1725 rpm during three cycles of 20 min. The
Results and discussion
Fig. 1 (a) shows the XRD patterns of the as-prepared photocatalysts. The monoclinic scheelite BiVO4 phase (JCPDS 75–2480) was detected in all samples, and the Miller index of the principal planes was included in brackets. Herein, the intensity and sharpness of reflections decreased as follows: BV-COM > BV350/12 > GO/BV (350/12) > BV350/2 > BV-RT, which is related to the crystallinity of compounds as well as with the sizes of the obtained particle. The samples prepared at 350 °C showed high
Conclusions
A straightforward, high-yield, efficient, room and intermediate-temperature, novel, and green methodology was successfully applied to synthesized visible light-activated BiVO4-based photocatalysts. The as-prepared BiVO4 catalysts showed different crystallinity levels, crystallite size, morphology, particle size, Eg, and SBET area, exhibiting a remarkable effect in their photocatalytic performance. The BV-RT showed spherical-like particles with sizes <34 nm (79% cumulative) and SBET of 19 m2/g.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
Financial support for the work was provided by Consejo Nacional de Ciencia y Tecnología (CONACYT) through Projects CB-2016-285350 and INFRA-2018-294130. E. Mendoza-Mendoza thanks CONACYT for Cathedra's program, project 864. Also, thanks to SENER-CONACYT-Newton Funds-291564. The authors thank CONACYT through Laboratorio Nacional de Micro y Nanofluídica and Laboratorio Nacional de Materiales Grafénicos.
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