Synthesis and photocatalytic activity of BiOI particles: An efficient visible-light-driven degradation of organic pollutants in aqueous environments

Bismuth oxyiodide (BiOI) particles were successfully fabricated through solvothermal activation using bismuth (III) nitrate pentahydrate and potassium iodide as precursors. The photocatalytic activities of BiOI were investigated under the influence of synthesis temperature (120–200 °C) with a fixed duration of 12 h. The physical-chemical properties of the samples were systematically investigated using X-ray diffraction, scanning electron microscopy, N2-sorption isotherm techniques, and pH of the point of zero charge (pHpzc). The prepared BiOI exhibited a single phase with a non-uniform 2D morphology with different sizes, an average crystallite size of 30 nm, and a BET surface area of 20.71 m²g−1. The zero-point charge was determined to be 7.7. The photocatalytic performance under simulated solar light irradiation was investigated by examining the effects of dye initial concentration, catalyst dose, and dye pH. Remarkably, BiOI synthesized at 160 °C demonstrated an impressive 85% removal efficiency of 10 ppm MO (methyl orange) at normal pH levels (pH 6.8) within 120 min under visible light irradiation, along with the highest rate constant of 0.01418 min−1. The quenching effects of different scavengers indicate the significant role of reactive O2•– and a minor role of hVB+ and •OHads in the photocatalytic process. These findings underscore the potential of BiOI particles as efficient photocatalysts for environmental applications, particularly in the degradation of organic pollutants in water using solar light irradiation.


Introduction
The discharge of synthetic dyes into water bodies from industrial effluents poses a grave environmental concern known as dye pollution.With a global production of approximately 1,000,000 tons, various industries, especially textiles, contribute significantly to dye emissions, releasing around 7.5 metric tons annually [1].These dyes exhibit persistence, toxicity, and pose severe threats to both aquatic ecosystems and human health.The release of dyes into aquatic ecosystems leads to reduced water quality, disrupted food chains, and toxicity, posing significant threats to the health and balance of the ecosystem [2].Dye 1340 (2024) 012009 IOP Publishing doi:10.1088/1755-1315/1340/1/012009 2 pollution, in addition, can have detrimental effects on human health, causing skin irritation, respiratory issues, hormonal disruption, and being associated with long-term health risks such as carcinogenicity and reproductive disorders [3].In recent years, various alternative approaches have been employed to tackle this specific issue, including ozonation [4], electrochemical treatment [5], membrane filtration [6], adsorption method [7], coagulation [8], bioadsorption [9], photocatalysis [10,11], and ion exchange removal [12].These investigations have underscored the remarkable efficiency for the removal and treatment of dye contaminants in wastewater.Among these approaches, photocatalysis has gained significant attention due to its ability to break down organic pollutants under light exposure, offering the potential for a sustainable and energy-efficient approach.Notably, BiOX has gained prominence as a promising photocatalyst, highlighting its exceptional performance in the degradation of dyes.
The crystal structure of BiOX was initially reported by Bannister et al. in 1935 [13].The BiOX materials, categorized as ternary oxide semiconductors of the V-VI-VII group, highlight a tetragonal matlockite structure.They are comprised of [Bi2O2] layers positioned between double halide [X] layers, forming [X-Bi-O-Bi-X] layers.These layers are interconnected through van der Waals interactions facilitated by the halogen atom along the [001] direction [14].Among various BiOX semiconductors, BiOI photocatalyst has drawn considerable attention in recent years due to its superior photocatalytic activity and stability, since its narrow band gap and specific layered structure are in favor of visible light absorption and electron-hole pair separation [15].BiOI is a semiconductor material characterized by a layered crystal structure consisting of alternating layers of bismuth oxide (Bi2O2) and iodine (I).This unique structure endows BiOI with excellent photocatalytic properties, making it an attractive candidate for the treatment of dye pollution.The photocatalytic activity of BiOI primarily stems from its narrow bandgap, which facilitates efficient absorption of visible light.Upon photon absorption, electrons in the valence band of BiOI become excited, transitioning to the conduction band and generating electron-hole pairs.These photoexcited charge carriers play a vital role in the degradation of dye molecules.Electrons in the conduction band can either reduce oxygen or directly react with the dye molecules, while the holes in the valence band can oxidize water or organic compounds, thereby generating reactive oxygen species.
To achieve enhanced photocatalytic efficiency, it is essential to tailor the properties of BiOI particles by adjusting experimental parameters.Recent studies have extensively explored the influence of various solvothermal conditions, particularly temperature and duration, on the improved photoactivity of BiOI.Adriana et al. [16] synthesized BiOI microspheres for varying durations of 2, 6, 12, 24, 30, and 48 h by using a solvothermal method with 1-butyl-3-methylimidazolium iodide as an ionic liquid at a constant autoclave temperature of 120 °C.Meanwhile, Zhu et al. [17] investigated the impact of synthesis temperature (ranging from 160 to 200 °C for 18 h) on the crystallinity, morphology, and photocatalytic activity of BiOI-Bi/graphene under solvothermal conditions.These findings underscore the significant role of synthesis temperature in BiOI performance.In contrast, this study specifically focuses on the effect of solvothermal temperature within a range of 120 to 200 °C, with a fixed duration of 12 h.This research aims to shed light on the influence of solvothermal time on the photocatalytic performance of BiOI materials and to provide fresh insights into the relationship between solvothermal parameters and degradation efficiency under simulated visible light.Our primary objective is to determine the optimal synthesis temperature for BiOI using the solvothermal method, with the goal of achieving the highest possible photocatalytic degradation efficiency for the targeted dye, MO.Additionally, comprehending the mechanism behind the photocatalytic activity of BiOI is crucial for optimizing its performance.By conducting scavenger studies, a mechanism was proposed that sheds light on the involvement of reactive species in the process of photocatalytic degradation.This proposed mechanism enriches the current understanding of BiOI's photocatalytic behavior and contributes to the development of more efficient and sustainable photocatalytic systems.

Synthesis of BiOI catalyst
BiOI was synthesized via the solvothermal method using the following procedure.Solution A was initially prepared by dissolving 5 mmol of Bi(NO3)3•5H2O in 30 mL EG, while solution B was prepared by 5 mmol of KI in 10 mL of water.The solution B was then gradually added dropwise to the solution A while continuously stirring for 1 h to attain a homogeneous suspension.The resulting mixture was then transferred to a 50 mL Teflon-lined stainless autoclave and subjected to solvothermal treatment in an oven for 12 h.The solvothermal synthesis process involved conducting a series of experiments within a temperature range of 120 ℃ to 200 ℃.After the completion of the solvothermal process, the autoclave was cooled to room temperature.The resulting precipitate was washed with deionized water and ethanol, followed by drying at 60 ℃.The denotation of the products as BiOIx (x = 120, 140, 160, 180, 200 o C) was in accordance with the solvothermal temperature.

Material characterization
X-ray diffraction (XRD) patterns were recorded on the Malvern Panalytical machine's radiation source (United Kingdom, Empyrean) using Cu-Ka radiation (λ = 1.54184 nm) with a diffraction angle (2θ) from 20 o to 80 o , a step size of 0.013 o at a step time 100 s for studying the crystal phases of the obtained products.The N2 desorption isotherms of the samples were obtained using a Quantachrome NOVA 1000E pore size and surface area analyzer at a liquid nitrogen temperature of -196 °C.Brunauer-Emmett-Teller (BET) surface area and pore size distribution of the obtained powders were performed using relative pressure (P/Po) data between 0.046 and 1.0 to define a specific surface area of 50 points.Scanning electron microscopy (SEM) images were acquired using Hitachi Fe-SEM S4800.The particle size of samples is determined by using Scherrer's equation ( 1): (1) where, d is crystallite diameter (nm), λ is the wavelength of the radiation (λ=0.15418nm), β is peak width at half maximum (radian), θ is the Bragg angle (radian), k is shape factor equal to 0.9 The degree of crystallinity (DC) was calculated by using equation ( 2): where, Ac and Ac +Aa refer to the area of crystalline peaks and the area of all peaks (crystalline and amorphous) in the XRD pattern.
To investigate the point of zero charge (pHpzc) of the photocatalyst, an acid-base titration method was employed.A background electrolyte solution was prepared by combining KCl at a concentration of 0.1 M, and 25 ml of this solution was transferred into a series of 250 ml erlenmeyer flasks.The pH values of the solution were then adjusted precisely to targeted levels (pHi) of 2, 4, 6, 7, 8, 10, and 12 using either HCl 0.1 M or NaOH 0.1 M, along with distilled water, to achieve a final volume of 100 mL.Subsequently, 0.1 g of the BiOI160 photocatalyst was added to each flask, followed by 24 h of agitation using an orbital shaker at a stirring speed of 180 rpm.Each suspension's final pH (pHf) was recorded, and a graph was plotted by plotting the difference in pH values (ΔpH) against the initial pH for each photocatalyst.The pH at the intersection point of the curve was considered as the pHpzc of the photocatalyst.

Photocatalytic MO degradation experiments
MO degradation was conducted by using Compact 26W -110V (Natural light 26W Exo-Terra PT 2191, America) as light source, and cooling water was also used to ensure a constant reaction temperature.First, 0.200 g of photocatalyst was dispersed and magnetically stirred in 200 mL of 10 ppm MO solution in a quartz reactor.An equilibrium of adsorption-desorption was established in the dark for 60 min before turning on the light.A zero-time point is the moment when light irradiation started.Then, the solution was irradiated for the next 2 h.During irradiation, around 5 mL aliquots were removed from the reaction solution after given time intervals by a Nylon filter (d = 0.45 μm).Subsequently, the collected suspension was analyzed using a UV-vis spectrophotometer (Hitachi U-2910) with a wavelength range of 300 to 600 nm for calculating the degradation efficiency of the catalyst.The evaluation of dye degradation was based on measuring the change in intensity of the main absorption peak.The photocatalytic degradation efficiency was determined using the equation (3): (3) where, Ao: absorbance of solution at t=0, At: absorbance of solution at irradiation time of 120 min A simplified pseudo-first-order kinetic model based on Langmuir-Hinshelwood equation was used to estimate the apparent rate constant of the dye degradation process, as shown in equation (4): where, Co: concentration when t=0, Ct: concentration when irradiation time t=120 min, k: the pseudofirst-order rate degradation constant (min -1 ).

Characterizations
BiOI samples prepared at various temperatures ranging from 120 to 200 °C were investigated using XRD method.The XRD patterns in Figure 1 clearly demonstrated the formation of the single tetragonal phase of BiOI, as referenced in JCPDS card No. 073-2062.Detailed analysis revealed almost identical 2θ diffraction peaks indexed to BiOI at 19.3°, 29.7°, 31.7°,37.2°, 39.5°, 45.5°, 51.5°, 55.3°, and 61.7°.The intense peaks at 29.7°, 31.7°,45.8°, and 55.3° corresponded to (012), (110), (020), and (122) planes, respectively.Synthesizing BiOI at temperatures lower than 160 °C may introduce impurities or secondary phases, as indicated by unidentified peaks at 15.6° and 22.4°.This phenomenon can be attributed to the slower reaction rates at lower temperatures, leading to intermediate compounds that may not completely convert into the desired BiOI phase.Notably, the particle sizes for all BiOI samples are approximately in the same range (approximately 27 to 33 nm), as determined using the Scherrer equation and higher synthesis temperatures correlated with increased crystallinity.Table 1.Average crystallite size and % crystallinity of prepared BiOI samples.

Figure 1. XRD patterns of BiOI samples with different solvothermal temperatures
To investigate the impact of solvothermal temperature on the specific surface area of the samples, nitrogen adsorption-desorption measurements were conducted, as illustrated in Figure 2 (a), accompanied by the corresponding multipoint BET plots and Barret-Joyner-Halenda (BJH) pore size distribution curves presented in Figure 2 (b).
The samples are clearly exhibited by type-IV nitrogen isotherms according to the IPUAC classification.The hysteresis loops of the isotherms are categorized as type H3-type hysteresis loops (P/P0 = 0.8-1.0),shifting to a high absorption at high relative pressure (approaching 1.0), suggesting the existence of large mesopores and macropores.All samples possess mesopores (∼2-20 nm) and macropores (~85-90 nm) and tend to have more mesopores than macropores.The BET specific surface areas were estimated to be 12.76, 20.71, and 6.95 m 2 g -1 for BiOI140, BiOI160, and BiOI180, respectively.The BiOI160, with the largest surface area among the samples, highlights the enhanced surface area, contributing to its excellent photocatalytic properties, resulting in a faster reaction rate and superior photocatalytic activity.
Pores with diameters ranging between 2.5 and 4.5 nm are the most appropriate for adsorbing the MO molecule [18].Analysis of structural parameters (Table 2) revealed that BiOI160 has a pore diameter of 35.92 Å, falling within the optimal range for adsorbing MO molecules and making it highly suitable for this purpose.SEM analysis of the synthesized BiOI160 product, as depicted in Figure 3, revealed non-uniform BiOI particles with diverse sizes and morphologies, suggesting agglomeration or uneven growth during synthesis.The irregular shapes and varying dimensions indicate the formation of heterogeneous structures.The semiconductor BiOI material exhibits a distinct and non-uniform morphology, characterized by variations in plate dimensions.
The surface charge accumulation on a catalyst affects its adsorption properties and catalytic efficiency [19].The pHPZC, determined using the pH drift method, indicates the point where surface charge is neutralized.Below the pHPZC, the surface carries a negative charge, attracting protons from the solution.Above the pHPZC, the surface is naturally positive, attracting hydroxyl ions, resulting in an overall negative charge [20].BiOI160 exhibited a pHPZC value of 7.7, as shown in Figure 4.

Photocatalytic activities 3.2.1 Effect of solvothermal temperature
Figure 5 illustrates photocatalytic activities at different solvothermal temperatures.BiOI's photoactivity increased up to 160°C but declined beyond.BiOI160 achieved 84.5% MO degradation in 120 min, with the highest rate constant (k) of 0.01418 min -1 , surpassing BiOI140 and BiOI180 by 1.2 and 1.6 times, respectively.However, at a higher solvothermal temperature (180 C), its surface area becomes smaller, which is a negative factor affecting photocatalytic performance.This reduction in surface area can limit the availability of active sites for the photocatalytic reaction and, as a result, decrease the overall efficiency.Thus, the solvothermal temperature of 160 °C was chosen for further investigation.

Effect of different photocatalytic conditions
The study analyzed the impact of varying initial MO concentrations (10 to 40 mgL -1 ) on photocatalysis.Figure 6 (a) illustrates a decrease in degradation efficiency (from 85.53% to 31.20%) due to increased light absorption by MO, hindering radical formation.This prevented the light from effectively reaching the photocatalyst surface to form radicals [21].As dye concentration rose, degradation decreased as dye molecules covered active sites, limiting availability, and reducing catalyst activity [22].This decline in catalyst activity is reflected in the reduction of the rate constant (k) values for dye degradation, as depicted in Figure 6 (b).
To assess the impact of catalyst concentration on BiOI's photoactivity, varying amounts of catalyst (ranging from 0.25 gL -1 to 2.5 gL -1 ) were tested in a 10 ppm MO solution.Figure 7 shows higher photocatalytic activity with increased catalyst, reaching nearly 90% degradation in 120 min.However, it is worth noting that once the catalyst concentration exceeded 1.00 gL -1 , the photodegradation efficiency reached a plateau and remained constant.Several factors could account for this phenomenon, such as light interception by the suspension and the agglomeration of catalyst particles, making their surfaces less available for photon absorption.Moreover, this decrease can be attributed to MO molecules absorbing a substantial amount of light instead of the catalyst.Consequently, less light flux reached the catalyst surface, resulting in reduced efficiency.Therefore, the optimum catalytic dosage is 1.00 gL -1 .Based on the experimental findings, it was observed that at pH 9.0, the degradation efficiency was lower compared to pH 5.0, 7.0, and 6.8 (Figure 9).Notably, at pH 5.0, a higher efficiency was achieved compared to pH 7.0 and 6.8.This can be attributed to the strong interaction between the positive charge on the catalyst surface and the negative charge in the MO molecule's structure, leading to a higher rate of degradation.

Proposed mechanism
To precisely determine the primary reactive radicals that contribute to the degradation of MO when utilizing the BiOI catalyst, free radical trapping experiments were conducted.In this study, IPA and EtOH were employed to quench •OHbulk, while KI was used to quench hVB + and/or •OHads.Additionally, a BQ/EtOH mixture was employed to quench O2 •− radicals.As depicted in Figure 10, the addition of p-BQ notably suppressed the degradation efficiency to 30%, underscoring the predominant role played by O2 •− radicals in the photocatalytic process.Furthermore, the photocatalytic activity exhibited evident inhibition, with an efficiency reduction to 70% upon the addition of KI, suggesting that hVB + and OHads also contributed to the photodegradation process.In contrast, the inclusion of EtOH or IPA resulted in an insignificant reduction of 3-5% in the removal rate, indicating that •OHbulk had a negligible impact on the photodegradation process.Therefore, the photodegradation of MO is primarily caused by O2 •− .

Conclusions
In summary, BiOI particles were successfully fabricated using the solvothermal method.The standout performance was observed in BiOI particles synthesized at 160 °C for 12 h.These particles exhibited exceptional photocatalytic activity, achieving an impressive 85% degradation efficiency and a pseudofirst-order rate constant of 0.01418 min⁻¹ at pH 6.8.Moreover, the optimal catalyst concentration for MO degradation was determined to be 1.00 gL -1 .However, as the MO concentration increased, the degradation efficiency showed a decreasing trend.Additionally, the zero-point charge of the BiOI particles was identified as 7.7, and under conditions with a pH of 5.0, the highest degradation efficiency of 100% was attained.The results of free radicals trapping experiments further elucidated the active species responsible for the photodegradation process with O2•− was found to be the dominant species, followed by hVB + and •OHads.These results highlight BiOI particles' remarkable potential as highly efficient photocatalysts for combating water pollution, making significant strides in sustainable environmental solutions.

Figure 2 .
Figure 2. (a) Nitrogen adsorption-desorption isotherms of BiOI140, BiOI160 and BiOI180 and (b) pore size distribution plot of the interparticle present

Figure 4 .
Figure 4. Point of zero charge of the BiOI160 sample

Figure 8 .
Figure 8.The structure of MO

Table 2 .
The specific area, pore volume and average pore diameter of BiOI140, BiOI160 and BiOI180 samples.