Enhanced photocatalytic activity of silver vanadate nanobelts in concentrated sunlight delivered through optical fiber bundle coupled with solar concentrator
To date, sunlight driven photocatalytic process for wastewater treatment is a great challenge. Herein, the photocatalytic methylene blue (MB) dye degradation activity of silver vanadate nanobelts has been enhanced using concentrated sunlight irradiation. The MB dye degradation factor in normal and concentrated sunlight irradiation is 0.57 and 0.25 respectively after 120 min of light exposure. Therefore, degradation of MB dye in concentrated sunlight occurs more than two times faster than normal sunlight. The silver vanadate nanobelts have been prepared by simple hydrothermal method. The prepared nanobelts are very thin having length ranging between 5 and 10 μm and width is in the 100–300 nm range. The optical band gap of the synthesized silver vanadate nanobelts is 1.96 eV (632.5 nm), which indicates strong absorption of visible light. This work will open new technological aspects for cost-effective sustainable wastewater treatment using solar energy.
KeywordsSilver vanadate Sunlight driven photocatalysis Concentrated sunlight Solar concentrator Optical fiber Water purification
Water is an essential constituent for the survival of human beings, although millions of people worldwide are suffering from lack of fresh and clean drinking water. The ground water and surface water have been polluted by rapid speed of industrialization, expansion of population, unplanned urbanization and environmental pollution [1, 2, 3, 4]. The discharge of untreated sanitary and toxic industrial wastes, dumping of industrial effluent, and run off from agricultural fields are the main sources of water pollution [5, 6]. The agriculture and pharmaceutical effluents release pesticides and other chemicals that are responsible for some chronic diseases [5, 6, 7, 8]. Various industries such as textiles, dyeing and printing discharge large amounts of synthetic organic dyes as effluents. According to the World Health Organization’s (WHO) 2017 report, around 844 millions of people worldwide have no scope to utilize fresh and clean water. In developing countries, almost 70% of all the illness are related to water contamination [9, 10]. Therefore, it is the greatest challenges in twenty-first century for the researchers to develop an eco-friendly, cost-effective and fast processing technology to remove the pollutants from wastewater in order to protect the environment and human beings. To address this serious issue, semiconductor photocatalysis has been believed as a low cost, sustainable and environmentally friendly approach by making use of solar energy .
The semiconductor photocatalysts have been studied extensively for purification of wastewater. Among the semocinductor photocatalysts, titanium dioxide (TiO2) is widely used owing its outstanding photocatalytic activity, non-toxicity and high photostability [12, 13, 14]. But, TiO2 is only sensitive under UV-light since it has large bandgap energy of 3.2 eV and utilizes only 4% of available solar energy, which highly restricts its potential application as photocatalyst [14, 15, 16, 17]. Therefore, numerous efforts have been made to enhance photocatalytic activity of TiO2 in visible-light, including anion doping, metal doping, and oxygen deficiency creation [18, 19, 20]. Although the light absorption of doped TiO2 was extended to visible light region, these processes often suffered from thermal instability or carrier-recombination [21, 22]. These methods are not appropriate to develop highly efficient visible-light active photocatalysts. Hence, researchers have devoted much attention to fabricate visible-light driven photocatalysts to enhance the use of solar energy in the process of wastewater purification.
Recently, researchers have developed band structure control technique to fabricate non-TiO2-based visible-light active photocatalysts, such as BiVO4 , AgNbO3 , Bi2WO6  Ag3VO4 , AgVO3 , CoFe2O4 , Zr2Ni2Cu7 . Among thses photocatalysts, silver vanadate has drawn immense interest due to their excellent antibacterial, electrochemical and photophysical properties along with photocatalytic activity [25, 28, 29]. The researchers are still trying to enhance photocatalytic activity of silver vanadate nanomaterials using various techniques [30, 31, 32, 33]. In this work, we report the synthesis of silver vanadate nanobelts by a simple hydrothermal method without using any template or surfactant and their photocatalytic activity in the degradation of Methylene Blue under normal and concentrated sunlight irradiation. We have used a solar concentrator coupled with optical fiber bundle to deliver concentrated sunlight to the photocatalytic reactor. Most of the researchers use Xenon arc lamp or fluorescent lamp or LED lamp or solar simulator as visible light source to study photocatalytic activity [23, 24, 25, 34, 35]. Recently, some of them studied photocatalysis process in direct sunlight irradiation [26, 27, 36]. This is the first time in literature we are using a novel technology to use concentrated sunlight as visible light source to study photocatalytic activity. The utilization of solar concentrator coupled optical fiber bundle as the concentrated sunlight source provides a promising direction for future applications of low cost and fastest sustainable wastewater purification process.
2.1 Preparation of photocatalysts
Silver vanadate nanobelts were prepared using simple hydrothermal method. In a typical procedure, 2 mmol of NH4VO3 and 2 mmol of AgNO3 was dissolved in 40 mL distilled water seperately and then AgNO3 aqueous solution was mixed dropwise into NH4VO3 aqueous solution under magnetic stirring. The resulting suspension was transferred into a 100 mL Teflon-lined stainless steel autoclave and kept at 150 °C for 4 h and then cooled to room temperature. The resulting sample was collected and washed with distilled water and ethanol several times and dried overnight at 70 °C.
2.2 Sample characterization
The morphology of the prepared sample was investigated using field emission scanning electron microscope (JEOL, JSM 7500F) and transmission electron microscope (JEOL, JEM 1230). Structural properties were studied using Selected Area Electron Diffraction (SAED) and X-ray diffraction (XRD). The XRD pattern of the powder silver vanadate nanobelts was studied using Bruker D8 Advance X-ray diffractometer. Raman spectroscopy was recorded using Horiba Jobin–Yvon (HR 800) Micro-Raman under 632.8 nm laser irradiation. The Raman spectrum was recorded in the 100–1200 cm−1 wavenumber range. The optical absorption characteristic of the sample was investigated using the Varian Cary 5000 UV–Vis–NIR spectrophotometer. Optical power of normal and concentrated sunlight was measured using Gentec-eo UP55N-300F-H12-D0 power meter. An agilent 1200 Liquid Chromatography Mass Spectrometer (LCMS) was used to identify the intermediates of MB dye during degradation.
2.3 Photocatalytic activity evaluation
For photocatalytic degradation of MB dye, 20 mg of silver vanadate was dispersed in 50 ml of 20 ppm MB dye solution under ultrasonication for 2 min, and was kept in the dark place for 2 h to achieve adsorption–desorption equilibrium. At regular intervals of time, aliquots of the suspension were collected and analyzed in UV–Vis–NIR spectrophotometer (Varian Cary 5000).
3 Results and discussion
3.1 Morphology and structure
3.2 Optical properties
3.3 Photocatalytic activity
Variation of solar irradiance power with wavelength for normal and concentrated sunlight
Irradiance power (mW/cm2)
3.4 Photocatalysis mechanism
Photocatalytic activity of silver vanadate nanobelts enhances in presence of concentrated sunlight because concentrated sunlight has more irradiance power and contains more number of photons. These excess photons create more electron–hole pairs and hence more hydroxyl and superoxide radicals are formed to degrade MB dye faster.
The nanobelts of β-AgVO3 have been prepared by simple hydrothermal method. The prepared thin nanobelts are of well crytalline and uniform having length between 5 and 10 μm with width 100–300 nm. These nanobelts have high capacity to absorb visible light and have optical band gap 1.96 eV. The β-AgVO3 nanobelts exhibited high visible-light photocatalytic activity in the degradation of Methylene Blue dye under sunlight irradiation. The dye degradation rate becomes more than two times faster under concentrated sunlight irradiation. The concentrated sunlight has been utilized to degrade MB dye using solar concentrator coupled with optical fiber bundle. This new technology can be implemented to purify industrial wastewater using solar energy in a sustainable and cost-effective way.
This work is financially supported by Sao Paulo Research Foundation (FAPESP), Brazil (Grant No. 2015/22828-6). Author J.S. Roy is grateful to FAPESP for providing postdoctoral research fellowship (Grant No. 2017/16826-6).
Compliance with ethical standards
Conflict of interest
On behalf of all authors, the corresponding author states that there is no conflict of interest.
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