Optimized synthesis of Bi4NbO8Cl perovskite nanosheets for enhanced visible light assisted photocatalytic degradation of tetracycline antibiotics

Journal Pre-proof Optimized synthesis of Bi4 NbO8 Cl perovskite nanosheets for enhanced visible light assisted photocatalytic degradation of tetracycline antibiotics Ankush Majumdar (Conceptualization) (Methodology) (Software) (Validation) (Formal analysis) (Investigation) (Data curation) (Writing - original draft) (Writing - review and editing) (Visualization) (Project administration), Anjali Pal (Conceptualization) (Resources) (Writing - review and editing) (Supervision) (Project administration) (Funding acquisition)

PII:

S2213-3437(19)30768-7

DOI:

https://doi.org/10.1016/j.jece.2019.103645

Reference:

JECE 103645

To appear in:

Journal of Environmental Chemical Engineering

Received Date:

19 August 2019

Revised Date:

22 November 2019

Accepted Date:

28 December 2019

Please cite this article as: Majumdar A, Pal A, Optimized synthesis of Bi4 NbO8 Cl perovskite nanosheets for enhanced visible light assisted photocatalytic degradation of tetracycline antibiotics, Journal of Environmental Chemical Engineering (2019), doi: https://doi.org/10.1016/j.jece.2019.103645

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Optimized synthesis of Bi4NbO8Cl perovskite nanosheets for enhanced visible light assisted photocatalytic degradation of tetracycline antibiotics Ankush Majumdar, Anjali Pal* Department of Civil Engineering, Indian Institute of Technology, Kharagpur 721302, India

Corresponding author at: Department of Civil Engineering, Indian Institute of Technology,

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Kharagpur 721302, India; Tel.: 91-3222-281920, Fax: 91-3222-282254, E-mail addresses: [email protected] (A. Pal)

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Graphical Abstract

Highlights

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Multivariate optimization of flux method synthesis of Bi4NbO8Cl nanosheet was done

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Photocatalytic degradation of OTC and TCH was done by Bi4NbO8Cl under visible light

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TCH and OTC degradation was highest for the optimized Bi4NbO8Cl photocatalyst

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Visible light photocatalysis by a wide bandgap catalyst was observed

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Abstract

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A novel 3D architecture of perovskite Bi4NbO8Cl (BNOC) nanosheet was synthesized by molten salt flux method and was employed in the photocatalytic degradation of tetracycline

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hydrochloride (TCH) and oxytetracycline (OTC), under visible Light Emitting Diode (LED)

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irradiation. A multivariate approach using response surface methodology (RSM) coupled with face-centered central composite design was performed to model and optimize the synthesis

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process parameters (solute concentration, temperature, and duration of calcination) targeting the enhancement of degradation efficiency. The significant role of these parameters in the present

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study was exhibited from the wide variation of degradation efficiencies ranging from 28.53 to 80.12% for OTC and from 25.46 to 78.36% for TCH, respectively. The physicochemical properties of BNOC prepared at various synthesis conditions were explored using XRD, FESEM, BET, FTIR, DRS, and PL spectroscopy to delineate the influence of several conditions

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of the preparation process. A pronounced variation in the crystal structures, morphologies, surface area, and optical properties of the photocatalysts further confirmed the significant influence of synthesis process condition. The maximum degradation of OTC and TCH was 82.07% and 79.28%, respectively, when the optimum values of calcination temperature, duration, and solute concentration were 500.1 °C, 10.44 h, and 1.3 mol %, respectively. The

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catalyst efficiently mineralized 30.4 % OTC in 60 min and was also stable up to four cycles of degradation experiments. Despite having a wide bandgap, the visible light response shown by the BNOC nanosheet photocatalyst evoked its promising future in the field of photocatalysis.

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Keywords: Perovskite nanosheet; Bi4NbO8Cl; RSM; Photocatalysis; Visible light; Tetracycline.

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Abbreviations: ANOVA, Analysis Of Variance; AAD, Absolute Average Deviation; BNOC, Bi4NbO8Cl; CV, Coefficient of Variance; DRS, Diffuse Reflectance Spectra; FCCD, Face-centered Central Composite Design; FESEM, Field Emission Scanning Electron Microscope; FTIR, Fourier Transform Infrared Spectroscopy; FWHM, Full-Width At Half-Maximum; LED, Light Emitting

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Diode; MAE, Mean Absolute Error; MPE, Model Predictive Error; OTC, Oxytetracycline; PL, Photoluminescence; RMSE, Root Mean Square Error; TC, Tetracycline; TCH, Tetracycline

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Hydrochloride; XRD, X-Ray Diffraction.

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1

Introduction

Among various advanced oxidation processes, semiconductor photocatalysis is considered as a promising green and sustainable technology to eliminate recalcitrant organic pollutants from aqueous matrices, including antibiotics [1–4]. To use the maximum potential of solar energy for practical applications, researchers are constantly striving for developing photocatalysts that work

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under visible light (~48 % of sunlight) [5]. However, in practical situations, the intensity and availability of sunlight vary with the time and geographical location, creating a huge challenge

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for photocatalytic reactors operating in a continuous mode [6]. Thus the use of energy-efficient

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artificial visible light source is one part of the solution, while the other part is to develop a photocatalyst that works efficiently under low-intensity visible light. However, developing a

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stable photocatalyst that can efficiently operate under low-intensity visible light is a challenge.

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Several visible-light-responsive photocatalysts like metal oxyhalides [7–10], ternary metal oxides [11,12], and layered perovskites [13–15] have been used for photocatalytic pollutant

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degradation. Among these photocatalysts, perovskites have the most intriguing physicochemical features that allow researchers a wide range of tunability to obtain high photocatalytic efficiency, stability, and low rates electron-hole recombination [15]. The Sillen-Aurivillius perovskite Bi4NbO8Cl (BNOC) has a layered structure, consisting of single-layer NbO4 blocks that are

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separated by (Bi2O2)2Cl blocks [16]. It is a stable visible light-reactive photocatalyst with a narrow bandgap of ~2.4 eV [17]. BNOC has been reported as an effective visible light photocatalyst, mostly in the field of water splitting [16,18–20] and the degradation of a few dyes [17,21]. However, the photocatalytic efficiency of bulk BNOC is limited by its small specific surface area, and high electron-hole recombination. The physicochemical characteristics like morphology, size, and surface area of a material have a profound impact on its photocatalytic 5

properties like bandgap and charge carrier separation rate [22]. Change in morphology with the formation of nanostructures can increase its photocatalytic efficiency by increasing its surface area and inhibiting the electron-hole recombination [10,21]. To obtain faster photocatalyst synthesis at low temperatures, and better control over particle size and morphology, molten salt or flux method was chosen over the conventional solid-state method [23]. Recent studies have

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reported the use of flux method to synthesize BNOC nanosheets for environmental remediation [24,25]. However, there is no study on the multivariate optimization of flux method for the

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controlled synthesis of BNOC nanosheets to increase its photocatalytic performance. Moreover, an insight into the effect of the synthesis parameters on the physicochemical and photocatalytic

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properties of BNOC by correlating the outcome of optimization studies and material

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characterization is also required. Finally, there is no report on the use of optimized BNOC nanosheets towards visible-light-assisted photocatalytic degradation of antibiotics.

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Antibiotics are a category of emerging pharmaceutical pollutants that are globally used to treat bacterial infections and diseases in humans and animals, and also as livestock growth promoters

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[26]. Among various antibiotics, tetracycline hydrochloride (TCH) and oxytetracycline (OTC) of the tetracycline (TC) group are of particular concern [27]. The broad-spectrum antibacterial activity and low cost of TCH and OTC result in their profuse application as both humans and veterinary medicines, which cause their relative abundance in the aquatic environment

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worldwide [28]. Besides, these antibiotics cannot be treated by conventional biological treatment methods due to their refractory nature [29]. Thus antibiotics continuously persist in the aquatic environment in low concentrations (ng L−1 to μg L−1), sufficient to cause bacterial antibiotic resistance and ecotoxicity, which are an imposing threat to the ecology and human health [30].

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Hence, both TCH and OTC has been selected as the target pollutants to determine the efficiency of the synthesized photocatalysts. In this work, a novel optimized flux method was attempted for the synthesis of BNOC nanosheet with 3D architecture, aiming maximum photocatalytic degradation of OTC and TCH under visible LED light irradiation. Multivariate optimization of synthesis parameters (viz. calcination

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temperature, duration of calcination, and solute concentration) influencing the photocatalytic efficiency of BNOC, was done using response surface methodology (RSM). As one of the

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experimental design techniques, RSM was effectively used for multivariate optimization of

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parameters for material synthesis to achieve maximum pollutant removal efficiency [31–34]. A 33 face-centered central composite design (FCCD) was used to ascertain the single and collective

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effects of the key synthesis parameters on the photocatalytic efficiency of BNOC, and the response surfaces were generated to obtain the optimum condition. A wide range of

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characterization studies for the photocatalysts synthesized under various conditions was conducted. The outcome of the material characterization studies was compared with the results

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of the RSM study to gain insight into the effect of the synthesis parameters on the physicochemical and photocatalytic properties of BNOC.

Materials and Methods

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2.1

Materials

The antibiotics TCH and OTC acquired from HiMedia were 98 % pure. The stock solutions of the antibiotics were prepared in Millipore water. AR grade CsCl, Bi2O3, BiOCl, and Nb2O5 (Loba Chemicals) and NaCl (Merck) were used for BNOC synthesis. All other chemicals used were of AR grade. Millipore water was used in the entire study. 7

2.2

Experimental design for preparation of BNOC

The photocatalytic efficiency of a material is largely influenced by its physicochemical characteristics that can be controlled by varying the different synthesis conditions. In this study, the effects of three independent variables (solute concentration, temperature, and duration of calcination) on the synthesis of BNOC photocatalysts were comprehensively studied using

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FCCD with RSM approach. The photocatalytic efficiency of BNOC towards the degradation of aqueous OTC and TCH was selected as the response. The three selected variables were denoted

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as A, B, C, with the coded values at three levels: −1, 0, +1. The levels of temperature (A) and duration (B) of calcination were varied from 500 to 700 °C and 5 to 15 h respectively, and solute

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concentration (C) was varied from 0.1 to 10 mol % (expressed as Log values) as presented in

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Table 1. The determined values of the variables were selected based on literature and preliminary experiments [24].

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The sequence of experiments was computed using Design Expert (Trial version 7.0.0, Stat-Ease, Inc. USA), which provides 15 experimental runs along with 5 central runs, as shown in Table 2.

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As per the design, photocatalytic experiments were performed under different conditions by changing the variables. For a 3-factor, 3-level (33) FCCD, the mathematical relationship between the coded variables (Xi and Xj) and response (R) can be represented by a second-order

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polynomial model given in Eq. (1).

𝑅 = 𝛽0 + ∑𝑘𝑖=1 𝛽𝑖 𝑋𝑖 + ∑ ∑𝑖<𝑗 𝛽𝑖𝑗 𝑋𝑖 𝑋𝑗 + ∑𝑘𝑖=1 𝛽𝑖𝑖 𝑋𝑖2 + 𝑒

(1)

Where, β0 is a constant coefficient; k is the number of studied factors; βi, βii and βij are respectively the coefficients of the linear, quadratic and second-order interactions; and e is the error. Regression models were fitted with Eq. (1), and analysis of variance (ANOVA) was used to examine the implication of individual model terms. The effects of the individual parameters 8

on the OTC and TCH degradation process were addressed using a Pareto chart. F-value from Fisher's F test and p-value (probability) were used to express the adequacy of the model and the significance of the parameters. The coefficient of determination (R2), adjusted R2, predicted R2, adequate precision, and coefficient of variation (CV) values were used to display the model fitness. The three-dimensional (3D) response surface plots and perturbation curves were

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produced to elucidate, respectively, the interactive and individual effects of the variables on the response. Finally, the optimal parameters for the synthesis of BNOC photocatalyst with the

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highest efficiency for OTC and TCH degradation were obtained. The predicted and experimental values were used to determine the precision of the models via error analyses like absolute

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average deviation (AAD), root mean square error (RMSE), model predictive error (MPE), and

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mean absolute error (MAE), as presented in Eq. (2-5). 𝑛

𝑌𝑖,𝑝 − 𝑌𝑖,𝑒 1 𝐴𝐴𝐷 = ( ∑ ( ) ) × 100 𝑛 𝑌𝑖,𝑒

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𝑖=1

𝑛

1/2

1 𝑅𝑆𝑀𝐸 = ( ∑(𝑌𝑖,𝑝 − 𝑌𝑖,𝑒 )2 ) 𝑛

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(2)

(3)

𝑖=1

𝑛

𝑌𝑖,𝑒 − 𝑌𝑖,𝑝 100 𝑀𝑃𝐸(%) = ∑| | 𝑛 𝑌𝑖,𝑝

(4)

𝑖=1

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𝑛

1 𝑀𝐴𝐸 = ∑|𝑌𝑖,𝑒 − 𝑌𝑖,𝑝 | 𝑛

(5)

𝑖=1

where, n is the number of experiments, and Yi,e, and Yi,p are the experimental and predicted value of the ith experiment, respectively.

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2.3

Synthesis of BNOC

BNOC samples were prepared following the molten salt method or flux method, as reported in a previous study [24]. A molten salt of the eutectic mixture (65:35) of CsCl and NaCl was used as a flux. Bi2O3, BiOCl, and Nb2O5 were taken in a stoichiometric molar ratio (3:2:1), as precursors of BNOC. The flux and BNOC precursors were mixed at a solute concentration (BNOC/(BNOC

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+flux)) as required (0.1, 1, and 10 mol %). The mixture was taken in an alumina crucible and calcined in a muffle furnace in the presence of air to the required calcination temperature (500,

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600, and 700 °C) and kept for the desired duration (5, 10 and 15 h). After the calcination process, the products were naturally cooled, then ground using an agate mortar pestle and washed

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thoroughly with deionized water. Finally, the samples were oven-dried at 60 °C for 24 h and

Characterization

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stored for future use.

The X-ray diffraction (XRD) patterns were determined by a diffractometer (Panalytical,

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Netherlands) within a range of 5 to 85°. The Fourier transform infrared spectroscopy (FTIR) was performed using a Nicolet 6700 spectrometer (Thermo Fisher, USA) by the standard KBr disk method. Thermo-gravimetric analysis (TGA) was performed in a thermo-gravimetric analyzer (Perkin Elmer Pyris Diamond). The surface morphology was observed using a field emission

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scanning electron microscopy (FESEM, Gemini-2, Zeiss-Merlin, Germany). The N2 adsorptiondesorption analysis was conducted in an AutosorbiQ automated gas sorption analyzer of Quantachrome instrument (USA) for evaluation of Brunauer-Emmett-Teller (BET) surface area of the catalyst. The UV-visible diffuse reflectance spectra (DRS) were obtained by a UV-visible spectrophotometer (Cary 5000, Varian). Photoluminescence (PL) spectra were recorded in a fluorescence spectrophotometer (Cary Eclipse, Agilent). 10

2.5

Photocatalytic Experiment

The photocatalytic activities of the synthesized samples were evaluated by the degradation of antibiotics OTC and TCH in aqueous solution. In a typical photocatalytic experiment, 10 mg of a prepared BNOC photocatalyst was added to 10 mL of an aqueous solution of the respective antibiotics having a concentration of 20 mg L−1 in a glass beaker. A glass cover was placed over

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the glass beaker to minimize the evaporation. The mixture was initially stirred for 30 min in the dark to attain adsorption-desorption equilibrium. The reactor was then irradiated for 1 h with an

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18-W household LED bulb (Crompton, India) as the visible light source, kept at a distance of 6 cm from the solution surface. The average intensity of the incident light was 10 mW cm−2, as

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measured by a digital solar power meter (Kusam-Meco, India). After the reaction was complete,

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the solution was filtered through a 0.22 µm syringe filter, and the filtrate was analyzed for OTC and TCH using a UV-visible spectrophotometer (Cary 60, Agilent) at wavelengths of 354 nm

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and 357 nm, respectively. The calibration equations for OTC and TCH were as follows: Absorbance = 0.0283 × Conc. (mg L−1) + 0.0665, (R2 = 0.9995), and Absorbance = 0.0327 ×

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Conc. (mg L−1) + 0.0535, (R2 = 0.9997), respectively. The antibiotic degradation was determined using Eq. (7):

𝐴𝑛𝑡𝑖𝑏𝑖𝑜𝑡𝑖𝑐 𝑑𝑒𝑔𝑟𝑎𝑑𝑎𝑡𝑖𝑜𝑛 (%) =

(𝐶0 −𝐶𝑡 ) 𝐶0

× 100

(7)

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where C0 and Ct were the concentrations of the antibiotic before and after the photocatalytic reaction, respectively. The mineralization efficiency of the catalyst has also been studied with the help of a total organic carbon (TOC) analyzer (OI Analytical Aurora 1030 W).

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3

Results and discussions

3.1 3.1.1

Characterization of BNOC XRD

The XRD patterns of the synthesized catalysts under various conditions is shown in Fig. 1a-c.

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The diffraction peaks of all the catalysts were indexed as the orthorhombic phase of BNOC (JCPDS No. 84-0843). The absence of impurity peaks indicates that the synthesized

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photocatalysts have a pure phase. However, it can be observed that the diffraction peaks of materials synthesized at a calcination temperature of 700 °C (Fig. 1a) and with a solute

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concentration of 10 mol % (Fig. 1c) are narrower with a higher intensity as compared to the other

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materials shown in Fig.1. The narrow diffraction peaks with high intensity indicate a larger crystal size of the materials. The average crystallite sizes were estimated from the corresponding

𝐾𝜆

(8)

𝛽𝑐𝑜𝑠𝜃

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𝐷=

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XRD peaks by using the Scherer formula as given in Eq. 8 [9]:

where 𝜆 (0.15405 nm) is the radiation wavelength, K (0.9) is a constant, 𝜃 is the XRD angle, and 𝛽 is the full-width at half-maximum (FWHM) of the (1 1 6) plane. The estimated crystal size of

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the materials is presented in Table S1. 3.1.2

FTIR and TGA

The FTIR spectra of BNOC prepared under various conditions (Fig. 2a-c) revealed the presence of functional groups in the photocatalysts. The bands appearing below 600 cm−1 with a peak at 556 cm−1 was attributed to Bi–O stretching vibrations [35]. The peaks appearing at 605, 722, and 845 cm−1 are assigned to the characteristic Nb–O stretching vibration in the NbO4 layers [36]. 12

The increase in intensity of Nb–O, and Bi–O peaks observed in the case of BNOC prepared at a temperature of 700 °C (Fig. 2a) and a solute concentration of 10 mol % (Fig. 2c) can be attributed to the strong bonding of functional groups due to sufficient ionic diffusion of BNOC precursors. The peaks at 1390, 1471, and 1629 cm−1 were identified as C–O [37]. The peak at 3422 cm−1 was identified as OH group, which may be attributed to atmospheric moisture

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absorbed on the material surface as reported in previous studies [32]. To confirm the presence of surface moisture and examine the thermal stability of the samples,

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TGA of BNOC synthesized at 500 °C is presented in Fig. S1. From TGA and differential thermal

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analysis (DTA) curves, it can be confirmed that negligible mass loss at low temperature indicates the presence of surface moisture (Fig. S1a). TGA also reveals that the catalyst is thermally stable

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up to 700 °C, after which significant mass loss occurs (Fig. S1a). From the differential scanning

3.1.3

FESEM

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calorimetry (DSC) curve (Fig. S1b), the heat flow indicates that the process is endothermic.

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The effect of variation of synthesis conditions on the surface morphology of BNOC was analyzed by FESEM, as shown in Fig. 3. At a fixed solute concentration of 1 mol % and 10 h duration of calcination, the decrease of temperature resulted in the significant change in shape and size of the catalysts from thicker and bigger square plates to 3D architecture of nanosheets as

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observed from Fig. 3a. Production of BNOC nanosheets at the temperatures close to the melting point of CsCl/NaCl flux (486 °C) may be a result of the deficient ionic diffusion of precursors. FESEM images of the samples synthesized under varying solute concentrations reveal that increasing solute concentration results in the increase of particle size (Fig. 3b). This can be attributed to the increased frequency of the ionic collision of BNOC precursors in the flux mixture [24]. The morphology does not vary substantially with the duration of calcination, 13

although the catalyst synthesized at 10 h has a slightly larger size than the other two, as seen in Fig. 3c. The observations made from FESEM are in good agreement with the crystallite sizes calculated from the XRD peaks, as given in Table S1. 3.1.4 BET The surface area, pore volume, and pore diameter of the synthesized materials were analyzed by

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BET method and summarized in Table S2. The surface area of the catalyst synthesized at 500 °C is 15.73 m2g-1, which is not only greater than the catalysts synthesized at 600 and 700 °C but also

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greater than Bi4NbO8Cl synthesized by solid-state method as reported earlier [38]. It can be seen

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that the morphology (Fig. 3a) and crystal size (Table S1) has a direct relationship with the surface area and pore volume (Table S2) of the catalysts prepared at different temperatures. With

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the decrease of temperature, there is a significant change in morphology of the catalysts from thicker and bigger square plates to nanosheet-like 3D architecture. This results in a consistent

UV–visible DRS

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3.1.5

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decrease of crystal size and an increase in surface area and pore volume of the catalyst.

The UV–visible DRS and corresponding Tauc plots of materials synthesized under various conditions are shown in Fig. 4. A direct relationship can be observed between the crystal sizes and bandgap energies of the catalysts, as reported in Table S1 and seen from their Tauc plots

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(Fig. 4b, d, f). It can be seen that with the decrease in crystal size, the bandgap increases. The blue shift in the absorption edge and the consequent increase in the bandgap of the materials with decreasing crystal size might be due to quantum size effect [39]. A similar result was reported in previous literature [21]. It is interesting to note that the catalysts synthesized at 5 h and 15 h duration have wide band gaps (3.38 and 3.27 eV, respectively), as seen from their Tauc plots (Fig. 4d), although they are degrading TC under visible light. The reason is that despite having 14

the main absorption peak in the UV region, they have a shoulder peak, which extends their absorption band to ~500 nm as observed from their DRS (Fig. 4c). This is also the case with BNOC synthesized at 0.1 mol % solute concentration. (Fig. 4e and f). However, the reason behind the photocatalytic activity of BNOC synthesized at 500 °C, which has a bandgap of 3.4 eV, and no such shoulder peak (Fig. 4a and b) is discussed later in sub-section 3.2.4. PL spectroscopy

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The PL spectrum gives a clear indication of the charge migration, transfer, and recombination

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rate of the photocatalyst. Lower PL intensity suggests a lower rate of photogenerated charge

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carrier recombination. Thus the photocatalytic efficiency of a catalyst is high when its PL intensity is low. The PL spectra of the catalysts synthesized at various conditions are given in

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Fig. 5. The PL emission peaks were observed at around 313 nm when the catalysts were excited with monochromatic light of 260 nm wavelength. It can be seen from Fig. 5 that the intensity

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decreases as the calcination temperature decreases. However, the intensity is lowest for the central values of duration (10 h) and solute concentration (1 mol %). Thus the catalysts

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synthesized at 500 °C, 10 h, and 1 mol % have the lowest charge recombination rate and, therefore, the highest photocatalytic efficiency. The quantum confinement effect is also responsible for increasing the electron transport in the direction of the plane and also the lifetime

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of electrons and holes [40].

3.2

3.2.1

Material optimization by RSM Development and validation of the regression model

A polynomial model equation was formulated using FCCD that represented the dependency of the response on the influencing factors. The design matrix and the corresponding experimental 15

observations and predicted outcomes of RSM study are given in Table 2. There was a significant variation in the degradation efficiencies OTC (28.53 to 80.12%) and TCH (25.46 to 78.36%), which exhibited the strong influence of synthesis parameters on the photocatalytic efficiency of BNOC. In the present study, the quadratic model with lower standard deviation, higher R2, and insignificant lack of fit values was suggested as the best fit (Table S3 and S4). The mathematical

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relationship between the coded variables and the responses are expressed as the following best fitted quadratic polynomial equations (Eq. 9 and 10):

(9)

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− 12.16𝐵2 − 10.48𝐶 2

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𝑅𝑂𝑇𝐶 = 68.03 − 6.91𝐴 + 0.28𝐵 + 12.81𝐶 − 3.78𝐴𝐵 − 1.30𝐴𝐶 + 0.11𝐵𝐶 + 4.54𝐴2

𝑅𝑇𝐶𝐻 = 65.97 − 6.08𝐴 + 0.61𝐵 + 13.17𝐶 − 3.78𝐴𝐵 − 0.80𝐴𝐶 − 0.19𝐵𝐶 + 4.53𝐴2 (10)

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− 13.21𝐵2 − 10.68𝐶 2

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where A, B, and C represent the calcination temperature, duration, and solute concentration, respectively, and ROTC and RTCH are OTC and TCH degradation efficiencies, respectively.

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The test for adequacy and significance of the proposed model was done by ANOVA and presented in Tables 3 and 4. The high F-values (i.e., 36.11 for OTC and 42.03 for TCH) and low p-values (< 0.0001 for both OTC and TCH) have validated the significance of the proposed models. The fitness of both the models is confirmed by their high R2 values, which are in good

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agreement with the adjusted and predicted R2 values. The narrow difference between the experimental and predicted values seen in Table 2 leads to lower CV values. The reliability of the executed experiments is evident from the higher adequate precision values in both the models.

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The regression model co-efficient presented in Eqn. 9 and 10, help to understand the significance of the effect of variables on the response in terms of both magnitude and signs. Thus the positive co-efficient of variables like B, C, BC, and A2 (for ROTC) and B, C, and A2 (for RTCH) signified that these variables were favorable for the degradation efficiencies. On the contrary, negative coefficient of variables like A, AB, AC, B2, and C2 (for ROTC) and A, AB, AC, BC, B2, and C2 (for RTCH) indicated their unfavorable effect on degradation efficiencies. The terms A, C, AC, BC,

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and A2, exhibited a significant effect on ROTC with a confidence interval of 95% (p < 0.05)

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(Table S5). RTCH was significantly affected by the linear terms (A and C), interactive terms (AB, AC, and BC) and square term (A2) with a 95 % confidence interval (p < 0.05) (Table S6). The

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Pareto chart (Fig. 6) also demonstrates the effect of the parameters on the TC degradation

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efficiencies using the following Eq. (11) [41]:

(𝑖 ≠ 0)

(11)

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𝛽𝑖2 𝑃𝑖 = ( ) × 100 ∑ 𝛽𝑖2

where βi denotes the coefficient of the quadratic model. The individual and interactive effect of

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the parameters on the OTC and TCH degradation efficiencies are expressed quantitatively as a percentage. As observed in Fig. 6 a and b, it is clear that the factors C, AC, and BC have the highest contribution to the photocatalytic efficiencies of BNOC on both OTC and TCH. The factors A, AB, and A2, had a moderate effect while B2, B, and C2 showed the least effect on the

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degradation efficiencies. Additionally, the effects of quadratic terms on the response are represented in the ascending order as follows: C2 < B2 < A2. Thus, the proposed models represented the relationship between the process parameters and degradation efficiencies well enough.

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3.2.2

Graphical analysis

The adequacy of the fitted quadratic models was further demonstrated graphically by normal probability plots of studentized residual (Fig. S2) and parity plots of observed vs. predicted values (Fig. S3). The normal probability plots displayed the normal distribution of error since the scattering points were closely placed on a straight line. Furthermore, from the parity plots, it may

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be seen that the data points are near the diagonal line, demonstrating a good agreement between experimental results and model-predicted values. Thus the significance and adequacy of the

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proposed quadratic models are justified.

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The response surface plots revealed the collaborative effects of the synthesis parameters and determined the optimal conditions for the maximum degradation efficiencies of OTC and TCH.

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From Fig. 7a, it can be observed that high OTC degradation was achieved when BNOC was synthesized at lower calcination temperature and moderate duration when solute concentration

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was kept constant. On the other hand, as shown in Fig. 7b, at a constant duration, medium to high solute concentration and low temperature produces BNOC with high photocatalytic

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efficiency. Again, medium to high solute concentration and low-temperature results in the synthesis of BNOC with high OTC degradation efficiency (Fig. 7c). Similar response surface plots were also obtained for TCH degradation, as shown in Fig. S4. The perturbation curves, as shown in Fig. S5 demonstrated the individual effects of parameters on TC degradation

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efficiency. 3.2.3

Optimization of synthesis parameters

The desirability function was used to optimize the OTC and TCH degradation efficiency simultaneously. The value of the desirability index varied from 0 to 1, i.e., undesirable to desirable. Keeping the synthesis parameters within the studied experimental ranges, the aim was 18

to obtain the maximum desirability for the responses. The maximum OTC and TCH degradation efficiencies (i.e., 81.3% and 78.4% respectively) were simultaneously attained when the optimum calcination temperature, duration, and solute concentration were found to be 500.1°C, 10.44 h, and 1.3 mol %, respectively (Fig. S6). Under the optimum synthesis conditions, the photocatalytic degradation efficiencies of BNOC were 82.07 % and 79.28 % for OTC and TCH,

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respectively. Thus the validity of the models was confirmed by the good agreement between the predicted value and the experimental result. The high prediction accuracy of the models was

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supported by the calculated error function values, viz. RMSE, ADD, MAE, and MPE (%), which were 2.41, 0.092, 1.725, and 2.95 for OTC, and 2.29, 0.085, 1.555, and 2.65 for TCH

Correlation between material characterization and RSM study

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3.2.4

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degradation, respectively.

Degradation experiments were conducted using BNOC photocatalysts prepared with varying

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temperatures, at constant duration (10 h) and solute concentration (1 mol %), to investigate the effect of calcination temperature on the photocatalytic degradation efficiency of TC antibiotics.

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From Table 2, it is found that with the increase in the temperature from 500 to 700 °C, the OTC degradation efficiency decreases steadily from 80.12 to 61.4 %. The RSM plots (Fig. 7a and b) again indicate that the highest degradation efficiency is obtained when the calcination

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temperature is kept at its minimum. The XRD patterns (Fig. 1a), calculated crystal sizes (Table S1), FESEM images (Fig. 3a) and BET analysis (Table S2), demonstrate that temperature has a major effect on the crystal structure, size, morphology, and surface area of the BNOC photocatalysts. The formation of 3D nanosheet architecture of BNOC at 500 °C increases the surface area and pore volume. This facilitates the adsorption of more TC molecules onto the catalyst surface and provides more active sites for the photocatalytic reactions. Thus the highest 19

degradation efficiency obtained for BNOC catalyst prepared at 500 °C can be attributed to its nanosheet-like structure with the increased surface area. The enhanced photocatalytic efficiency is also contributed by the inhibition of the electron-hole recombination rate as it has the lowest PL intensity than the other two catalysts, as seen from its PL spectra (Fig. 5). However, one stimulating observation is that the bandgap of the catalyst is 3.4 eV, and it has no

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absorption band in the visible spectra (Fig. 4a and b) although it is showing the highest degradation efficiency under a white LED, which irradiates light only in the visible range as

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shown in Fig. S7. Adsorption experiment in the dark with the catalyst resulted in 13 % removal

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of TC, while only 1% was degraded by photolysis without a catalyst. Thus BNOC is degrading TC by photocatalysis. This is an interesting observation as this result appears to violate the

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fundamental principle of photocatalysis, i.e., photon energy larger than the bandgap of the

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catalyst is capable of generating electron-hole pairs that fuel the redox photocatalytic reactions. Hence we probed further into the matter to study whether TC affected the absorption band of BNOC. Thus, we conducted a DRS of BNOC saturated with the same initial concentration of

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OTC solution as in the degradation studies. The obtained DRS were compared to the spectra of BNOC alone, as shown in Fig. S8. In the presence of the OTC, a red shift and formation of a new weak absorption band in the visible region was observed, which was absent in the catalyst alone.

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Therefore we suggest that catalyst-TC surface interaction and subsequent self-sensitization of OTC is the reason behind the visible-light-induced photocatalytic activity of the wide bandgap catalyst in our case. Similar observations have been reported in recent studies [42,43]. The effect of duration of calcination on the photocatalytic degradation efficiency of the synthesized BNOC photocatalysts towards TC was studied while the temperature and solute concentration were kept constant. It can be seen from Table 2 that as the duration of calcination 20

is increased from 5 to 10 h, the OTC degradation efficiency increases from 52.56 to 74.47 %, and then decreased to 55.57 % upon a further increase in duration to 15 h. The RSM plots (Fig. 7a and c) also indicate that the highest degradation efficiency is obtained when the duration is around its central value, i.e., 10h. Minor changes can be observed in the crystal structure, size, and morphology of the BNOC photocatalysts due to variation of duration as seen from their

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XRD patterns (Fig. 1b), calculated crystal sizes (Table S1) and SEM images (Fig. 3c). The highest degradation efficiency obtained for BNOC catalyst prepared at 10 h duration (600 °C and

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1 mol %) may be attributed to the slowest electron-hole recombination rate, as it has the least PL intensity among the three materials (Fig. 5). The narrow bandgap (2.77 eV) of the catalyst, as

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seen from the UV-visible DRS and Tauc plots (Fig. 4c and d), support its visible-light-induced

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photocatalytic activity.

Solute concentration has a significant effect on the TC degradation efficiency of the synthesized

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BNOC photocatalysts. As solute concentration is increased from 0.1 to 1 mol %, the OTC degradation efficiency increases from 41.78 to 74.47 % and then decreases slightly to 69.7 %

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upon further increase to 10 mol % (Table 2). This result is also reflected in the RSM plots (Fig. 7b and c), which show that the highest OTC degradation occurs near the central value of solute concentration. Substantial variation can be observed in the crystal structure, size, and morphology of the BNOC photocatalysts due to change in solute concentration as seen from their

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XRD patterns (Fig. 1c), calculated crystal sizes (Table S1) and SEM images (Fig. 3b). The highest degradation efficiency was obtained for BNOC nanosheets prepared at a solute concentration of 1 mol %, 600 °C temperature, and 10 h duration, with the lowest PL intensity and narrow bandgap of 2.77 eV.

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A similar trend can also be seen when the RSM study for the effect of the synthesis parameters on the degradation efficiency of BNOC towards TCH is compared with the characterization results. Thus the outcome of the material characterization has a good correlation with the result of the RSM study, which validates the significant impact of the synthesis parameters on the visible light photocatalytic degradation of the TC antibiotics.

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3.2.5 Reusability and stability of BNOC For practical applications, reusability and stability of photocatalysts are important parameters

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other than its photocatalytic activity. As shown in Fig. 8, recycling runs for the photodegradation

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of OTC over the BNOC catalyst (500 °C, 10 h, 1 mol%) were performed to evaluate the reusability of the catalyst. No significant reduction of photocatalytic efficiency was observed up

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to the fourth cycle, implying that the photocatalyst can be effectively reused. The XRD and FTIR of the BNOC catalyst were also studied after degradation experiments with OTC. There are no

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phase and structural changes in BNOC before and after reaction, as observed from the XRD and FTIR patterns given in Fig. S9a and S9b. Thus, it can be affirmed that the BNOC catalyst

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remains stable during the photocatalytic degradation process. 3.2.6 Mineralization efficiency

Mineralization of OTC (concentration 20 mgL−1) by the BNOC (500 °C, 10 h, 1 mol%)

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photocatalyst (dosage 1 gL−1) during photodegradation experiment was studied through TOC analysis according to Eq (12): % 𝑀𝑖𝑛𝑒𝑟𝑎𝑙𝑖𝑠𝑎𝑡𝑖𝑜𝑛 = (1 −

𝑇𝑂𝐶𝑓 ) × 100 𝑇𝑂𝐶0

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(12)

where TOCf and TOC0 are final and initial TOC concentrations (mgL−1), respectively. From the experimental results, it can be seen that while 80.12% OTC was photodegraded, 30.4% TOC removal was achieved after 60 min (Fig. S10).

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Conclusion

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A novel 3D architecture of BNOC nanosheets was successfully synthesized by an optimized flux method in the present study. FCCD, combined with RSM, was used for the multivariate

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optimization of synthesis parameters aiming for the visible LED light assisted photocatalytic degradation of OTC and TCH. The temperature and duration of calcination and solute

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concentration were selected as the influential synthesis parameters. The influence of synthesis

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parameters on the physicochemical and photocatalytic properties of BNOC is evident from the variation in the degradation efficiencies towards OTC and TCH from 28.53 to 80.12% and 25.46

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to 78.36%, respectively. The maximum degradation efficiencies of OTC and TCH were 82.07% and 79.28%, respectively, when the optimum values of calcination temperature, duration, and

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solute concentration were 500.1 °C, 10.44 h, and 1.3 mol %, respectively. Both the RSM study and material characterization validate these physicochemical changes in BNOC under the individual and interactive influence of the synthesis parameters. The FESEM and BET analysis reveal the formation of 3D architecture of BNOC nanosheets with increased surface area and

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pore volume at 500 °C, 10 h, and 1 mol%. This aids increased adsorption of TC molecules onto the catalyst surface for enhanced photocatalytic degradation. The enhanced degradation efficiency of the photocatalyst is also contributed by the inhibition of the electron-hole recombination rate as it has the lowest PL intensity. The catalyst efficiently mineralized 30.4 % OTC in 60 min. Reusability tests, FTIR, and XRD patterns indicate that the catalyst is also stable

23

after repeated degradation experiments. However, despite having the highest visible-lightinduced photocatalytic efficiency, DRS shows that this photocatalyst has the widest bandgap of 3.4 eV. Such finding is reported for the first time and explained by the catalyst-TC surface interaction and subsequent self-sensitization of OTC that enabled visible light absorption. The use of a multivariate optimization tool may be helpful in the controlled synthesis of the desired

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photocatalyst for specific environmental applications.

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Acknowledgments

Authors are thankful to IIT Kharagpur for providing the instrumental facility and Ministry of

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Human Resource Development, Government of India, for financial support. We are grateful to Prof. Sudha Goel and Mr. Ved Prakash Ranjan (Department of Civil Engineering, IIT

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Kharagpur) for the spectrophotometric measurements of TC samples. We also gratefully

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acknowledge Prof. Partha Sarathi Ghosal (School of Water Resources, IIT Kharagpur) for helpful discussion. The authors are thankful to the Departments of Chemistry and Chemical

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Engineering, IIT Kharagpur, for UV-Visible DRS and XRD analysis, respectively. Conflict of interests

The authors declare no conflicts of interest.

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Authors contribution

Ankush Majumdar: Conceptualization, Methodology, Software, Validation, Formal analysis, Investigation, Data Curation, Writing - Original Draft, Writing - Review & Editing, Visualization, Project administration

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Dr. Anjali Pal: Conceptualization, Resources, Writing - Review & Editing, Supervision, Project administration, Funding acquisition

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Supplementary Information Supplementary information associated with this article has been provided.

A. Majumdar, A. Pal, Recent advancements in visible-light-assisted photocatalytic

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Figure captions Fig. 1. XRD pattern of BNOC prepared at (a) Different calcination temperature (at 10 h, 1 mol %) (b) Different duration (at 600 °C, 1 mol %) and (c) Different solute concentration (at 600 °C,

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Fig. 2. FTIR spectra of BNOC prepared at (a) Different calcination temperature (at 10 h, 1 mol %) (b) Different duration (at 600 °C, 1 mol %) and (c) Different solute concentration (at 600 °C,

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10 h).

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Fig. 3. SEM images of BNOC prepared at (a) Varying calcination temperature (at 10 h, 1 mol %)

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(b) Varying solute concentration (at 600 °C, 10 h) (c) Varying duration (at 600 °C, 1 mol %).

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Fig. 4. UV-visible DRS and corresponding Tauc plot of BNOC prepared at (a,b) Different calcination temperature (at 10 h, 1 mol %) (c,d) Different duration (at 600 °C, 1 mol %) and (e,f)

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Different solute concentration (at 600 °C, 10 h).

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Fig. 5. PL spectra of the BNOC photocatalysts synthesized at various conditions.

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Fig. 6. Pareto chart of the individual and interactive effect of parameters on the degradation

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efficiency of BNOC towards (a) OTC and (b) TCH.

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Fig. 7. 3D response surface plots displaying collaborative effect of (a) calcination temperature and duration, (b) calcination temperature and solute concentration, and (c) duration and solute

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Fig. 8. Reusability test of BNOC photocatalyst for degradation of OTC. (Experimental conditions: initial OTC concentration = 20 mgL-1, BNOC dose = 1 gL-1, 30 min in dark followed

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by visible light irradiation).

Fig S1. (a) TGA, DTA and (b) DSC curves of BNOC synthesized at 500°C. Fig. S2. Normal probability plot of studentized residual for degradation of (a) OTC and (b) TCH in RSM study. 42

Fig. S3. Parity plot showing the correlation between the predicted and actual values for (a) OTC and (b) TCH degradation. Fig. S4. Three-dimensional response surface plot showing interactive effect of, (a) duration and calcination temperature, (b) solute concentration and calcination temperature and (c) solute concentration and duration on TCH degradation efficiency (%). Fig. S5. Perturbation curve for individual effect of process parameters (A: calcination

of

temperature, B: Duration, C: Solute concentration) on degradation efficiency of (a) OTC and (b) TCH degradation in RSM study.

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Fig. S6. Desirability function graph for affecting parameters on the OTC and TCH degradation

-p

in RSM study. Fig. S7. Spectrum of white LED light source.

re

Fig. S8. Comparison of UV-visible DRS of BNOC (500 °C, 10 h, 1 mol %) in the presence and absence of OTC (Inset: Magnified image showing the new absorption band formed due to

lP

BNOC-OTC surface interaction).

Fig. S9. (a) XRD and (b) FTIR of BNOC photocatalyst before and after OTC degradation.

ur na

Fig. S10. Mineralization of OTC (concentration: 20 mgL−1) by the optimized BNOC

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photocatalyst (dosage: 1 gL−1).

43

Table captions Table 1. Influencing parameters and their levels used in RSM study. Table 2. FCCD matrix with response outcomes from experimental observation and model prediction.

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Table 3. ANOVA results of the quadratic model for OTC degradation by BNOC Table 4. ANOVA results of the quadratic model for TCH degradation by BNOC

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Table S1. Crystal sizes and bandgap values of BNOC photocatalysts synthesized at different

-p

conditions

Table S2. Surface area, pore size and pore volume parameters for BNOC samples prepared at

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different calcination temperatures (duration 10h, solute concentration 1 mol%)

lP

Table S3. Statistical analyses of the adequacy of different models for OTC degradation in RSM study

study

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Table S4. Statistical analyses of the adequacy of different models for TCH degradation in RSM

Table S5. ANOVA table of CCD model for OTC degradation in RSM study

Jo

Table S6. ANOVA table of CCD model for TCH degradation in RSM study

44

Table 1. Influencing parameters and their levels used in RSM study. Parameters

Notation

Level -1

0

+1

A

500

600

700

Duration (h)

B

5

10

15

Log of solute concentration

C

-1

0

+1

Jo

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lP

re

-p

ro

of

Calcination temperature (°C)

45

Table 2. FCCD matrix with response outcomes from experimental observation and model prediction. Run

Parameters

OTC degradation

TCH degradation

efficiency (%)

efficiency (%)

B

C

Observed Predicted Observed Predicted

1

600

5

0

52.56

55.59

50.69

52.15

2

700

15

-1

28.53

27.9

25.46

25.18

3

600

10

0

69.88

68.03

65.98

65.97

4

700

15

1

52.03

51.14

50.69

49.53

5

600

10

1

69.70

70.35

67.36

68.46

6

600

10

0

69.35

68.03

66.44

65.97

7

700

5

-1

37.19

35.11

32.49

31.15

8

500

15

-1

46.91

46.67

43.04

43.32

9

500

15

1

73.94

75.11

69.95

70.87

10

600

10

0

63.52

68.03

60.63

65.97

11

600

10

0

12

600

10

0

13

600

10

14

500

5

15

500

5

16

700

10

17

600

18

re

-p

ro

of

A

68.03

69.5

65.97

67.93

68.03

64.6

65.97

lP

70.23

41.78

44.73

41.51

42.12

1

67.05

66.77

62.61

62.46

-1

38.78

38.77

33.41

34.14

0

61.40

65.66

60.93

64.42

15

0

55.57

56.15

53.13

53.38

500

10

0

80.12

79.47

78.36

76.58

19

600

10

0

74.47

68.03

72.1

65.97

20

700

5

1

58.57

57.91

56.96

56.26

Jo

ur na

-1

46

Table 3. ANOVA results of the quadratic model for OTC degradation by BNOC DF

Model

Sum of Squares 3786.54

9

Mean Square 420.73

Residual

116.51

10

11.65

Lack of Fit

53.33

5

10.67

Pure Error

63.19

5

12.64

Total

3903.05

19

F Value 36.11

Prob> F < 0.0001

significant

0.84

0.5716

not significant

of

Source

Jo

ur na

lP

re

-p

ro

R2 = 0.9701 Adjusted R2 = 0.9433 Predicted R2 = 0.8949 Adequate Precision = 21.365 C.V. % = 5.79

47

Table 4. ANOVA results of the quadratic model for TCH degradation by BNOC Source

Sum of Squares

DF

Mean Square

Model

3969.62

9

441.07

Residual

104.94

10

10.49

Lack of Fit

26.24

5

5.25

Pure Error

78.69

5

15.74

Total

4074.56

19

F Value

Prob> F

42.03

< 0.0001

significant

0.33

0.8733

not significant

Jo

ur na

lP

re

-p

ro

of

R2 = 0.9742 Adjusted R2 = 0.9511 Predicted R2 = 0.9266 Adequate Precision = 22.441 C.V. % = 5.75

48