DOI: 10.3303/CET25117144 Paper Received: 6 January 2025; Revised: 1 April 2025; Accepted: 15 May 2025 Please cite this article as: Ogbeifun O., Tichapondwa S.M., Chirwa E.M.N., 2025, Photocatalytic Degradation Enhancement of Dye on Bi12O17Cl2 through Er³⁺ and Yb³⁺ Doping , Chemical Engineering Transactions, 117, 859-864 DOI:10.3303/CET25117144 CHEMICAL ENGINEERING TRANSACTIONS VOL. 117, 2025 A publication of The Italian Association of Chemical Engineering Online at www.cetjournal.it Guest Editors: Fabrizio Bezzo, Flavio Manenti, Gabriele Pannocchia, Almerinda di Benedetto Copyright © 2025, AIDIC Servizi S.r.l. ISBN 979-12-81206-17-5; ISSN 2283-9216 Photocatalytic Degradation Enhancement of Dye on Bi12O17Cl2 through Er³⁺ and Yb³⁺ Doping Osemeikhian Ogbeifun*, Shepherd M. Tichapondwa, Evans M. N. Chirwa Water Utilization and Environmental Engineering Division, Department of Chemical Engineering, University of Pretoria, Pretoria, 0002, South Africa osemeikhianosi@yahoo.com Photon upconversion effect employing rare earth lanthanides Ytterbium (Yb3+) and Erbium (Er3+) was harnessed in Bi12O17Cl2 to achieve greater light utilisation targeting the large near-infrared (NIR) region of the solar radiation. In this regard, Er3+ and Yb3+ were doped into Bi12O17Cl2, and the best doping level was determined by measuring the decolourisation rate of rhodamine B on the materials. The photocatalytic decolourisation of rhodamine B dye on Yb3+ and Er3+ doped Bi12O17Cl2 materials was assessed under visible light and natural sunlight. The best-performing materials were 0.12%Er- and 0.48%Yb-Bi12O17Cl2, which showed 2.84 and 6.46 folds better than pristine Bi12O17Cl2. As expected, the materials displayed enhanced performance under natural sunlight compared to visible light, the reason being the conversion of a significant portion of NIR (52 % – 55 % of the solar spectrum) into UV and visible light, achieving greater solar light utilisation. The upconversion effect conferred by rare earth intensifies the irradiation strength on the material, which enhances the photocatalytic processes. The degradation mechanism was investigated, revealing the involvement of O2 •– and h+ in the decolourization of the dye. The study findings underscore the potential of utilising natural sunlight, rich in NIR, to activate Bi12O17Cl2 and propel the photocatalytic degradation of pollutants. 1. Introduction The transition from fossil fuel to renewable energy is necessary because of global warming and the associated detrimental impacts on the globe. Remediation and pollution control take a significant portion of carbon footprint. For example, removing pollutants from water matrix such as advanced oxidation, filtrations (ultra, micro or nano), reverse osmosis, and other non-traditional techniques, takes significant energy inputs, which in most cases come from fossil energy. Using renewable energy sources for remediation purposes is desirable to reduce greenhouse gas emissions from burning fossil fuels. Photocatalytic process for removal of organic contaminant can address global warming. Photocatalysis can be based on renewable solar energy if semiconductor material is driven by solar energy spectrum. Materials are available that can utilized the major wavelengths from ultra volet (UV) through visible light (vis) to near infra-red (NIR). The preference for vis photocatalysis stems from the need to reduce energy requirement in generating UV and using solar radiation. Powering a UV process consumes more energy compared to VL driven process. The VL process can be driven by abundant solar light. Furthermore, extending the process to cover NIR will be beneficial since VL (42 % – 43 %) and NIR (52 % – 55 %) make up to 94 % – 98 % of the solar spectrum against UV, which makes up only 3%–5% of the solar spectrum (Hassaan et al., 2023). The solar radiation spectrum has the following composition of UV, vis and NIR as < 5 %, < 50 % and > 50 %, respectively (Sang et al., 2015, Sheng et al., 2023). This indicates that NIR contributes more than half of the total solar radiation and is a worthwhile effort to utilise the NIR portion of the radiation. Upconversion luminescence converts NIR photons into vis (Hao et al., 2021), making the full utilization of the solar spectrum possible. Upconverting materials amongst other applications have been applied in photocatalysis for degradation of dyes (Zhou et al., 2015). 859 There are suitable materials that can be activated by VL and in few instances by NIR. Materials that can operate up to the NIR region of the of the spectrum is desirable and implies that such materials can utilize almost the entire wavelengths of abundant renewable solar spectrum. To achieve almost full solar spectrum utilization, semiconductor materials have been doped with lanthanides such as Sm3+, Yb3+ and Er3+, to achieve up- conversion (UC ) effect. In UC, low-energy infrared is converted to high-energy VL and UV radiation (Marimuthu et al., 2019). When the material is excited in infrared, there is luminesce in the VL and UV regions. The effect means the full use of the spectrum for photocatalyst activation. Bi12O17Cl2 is an ideal visible light photocatalyst absorbing significantly at the UV region up till 550 nm with its suitable band gap energy for VL irradiation. A stretch into the NIR portion of the spectrum will take advantage of the most significant portion of solar spectrum, the NIR, which makes (NIR) 55 % – 60 % of sunlight (Hao et al., 2021). This study explores the doping of Yb3+ and Er3+ in Bi12O17Cl2 material and testing for the photocatalytic activity under NIR. It has been shown that co-doping with Yb3+ and Er3+ significantly enhances the infrared–to-visible conversion in the host material (Pisarski et al., 2017). In Yb3+/Er3+ co-doped material, Yb3+ serves as the sensitiser and Er3+ as the activator ion (Zhang et al., 2019, Regmi et al., 2017). Yb3+ ions absorb at the NIR region (973 nm) with only two states. The energy difference between these two states is almost equal to the energy states of Er3+, making energy shift from Yb3+ ions to Er3+ ions possible (Marimuthu et al., 2019). Thus, Yb3+–Er3+ couple is a very efficient UC system. 2. Materials and method 2.1 Chemical Bismuth (III) nitrate pentahydrate (Bi (NO3)3·5H2O), Ytterbium (III) nitrate pentahydrate (Yb(NO3)3·5H2O), Erbium (III) nitrate pentahydrate (Er(NO3)3·5H2O) (Sigma Aldrich, South Africa), KCl, KIO3, ethylene glycol, rhodamine B (Rh B), ethanol (Glassworld Pty, South Africa), deionized water. 2.2 Synthesis and Er3+/Yb3+ doping of Bi12O17Cl2 The synthesis and doping of Bi12O17Cl2 with Er3+ and Yb3+ is illustrated in Figure 1, in which 2 mmol () of Bi(NO3)3·5H2O was dissolved in 20 mL of ethylene glycol and 0.33 mmol () of KCl was dissolved separately in 5 mL of ethylene glycol. The two solutions were mixed by means of a magnetic stirrer for 1 h. The resultant mixture was transferred to an autoclave and placed in an oven for hydrothermal treatment at 160 °C for 12 h. The autoclave was allowed to cool down and the product was collected and washed with water and ethanol. The product was dried at 60 °C and then calcined at 400 °C for 1 h to obtain yellow Bi12O17Cl2. For Er3+ and Yb3+ doped BOC, the preparation step introduced the right amount of Er(NO3)3·5H2O and Yb(NO3)3·5H2O. The following doped levels of Er3+ and Yb3+ in Bi12O17Cl2 were obtained and the materials are labelled as 0.12%ErBOC, 0.24%ErBOC, 0.47%ErBOC, 1.42%ErBOC, 4.16%ErBOC, 0.48%YbBOC, 1.34%YbBOC, 2.87%YbBOC and 4.3%YbBOC and 5.7%YbBOC. Figure 1: Synthesis and doping Bi12O17Cl2 with Er3+ and Yb3+ Transfer Bi(NO3)3·5H2O + Ethylene glycol Hydrothermal treatment at 160oC Transfer Bi12O17Cl2 KCl + Ethylene glycol KCl + Ethylene glycol Bi(NO3)3·5H2O + Ethylene glycol + Yb(NO3)3·5H2O OR Er(NO3)3·5H2O Transfer Er/Yb doped Bi12O17Cl2 Calcination at 400oC 860 3. Characterisation of Photocatalyst 3.1 XRD and UV-Vis DRS XRD and UV-Vis DRS were applied to determine the phase and optical properties of the synthesized materials. Figures 2a and b confirm the successful synthesis of Bi1217Cl2 and doped materials. The XRD pattern of the doped materials showed that the crystalline nature of the material was not lost in the process, as there is consistency in the peak positions. The peak intensity, however, weakened as the doping amounts of Er3+ and Yb3+ increased. All the peaks of synthesized Bi12O17Cl2 can be assigned to tetragonal Bi12O17Cl2 phase (JCPDS No. 37-0702) which appeared at 2θ = 23.23°, 24.3°, 26.37°, 29.20°, 30.42°, 32.88°, 45.42°, 47.18°, 54.95°, 56.45°, 57.19° and 58.56°, corresponding to (111), (113), (115), (117), (0012), (200), (2012), (220), (307), (315), (317) and (319) crystal planes respectively. (Zhao et al., 2023). For the Er3+ and Yb3+-doped Bi12O17Cl2 materials, the peaks were also assigned to the Bi12O17Cl2 phases, indicating that despite doping the material with Er3+ and Yb3+, no additional phase(s) were generated. However, a reduction in peak intensity was observed, with increased doping from 0.12%Er/0.48%Yb to 4.16%Er/5.77%Yb. The Bi3+ in Bi12O17Cl2 were partly replaced by Er3+ and Yb3+, having smaller atomic radii. UV-Vis DRS was performed to investigate the optical properties of the doped material; the result is presented in Figures 2c, and d. As shown, the absorption properties of the material were improved with Er3+ and Yb3+ doping. The shift in the absorption edge of the material, by doping, altered the band gap energy of doped materials. Figure 2: XRD pattern of materials (a), (b), and respective UV Vis DRS spectra (c),(d) 4. Photocatalytic degradation studies The photocatalytic performances of the material were measured by the rate of decolourisation of rhodamine, which served as the model pollutant. The batch reactor was a 50 mL beaker containing 20 mL of 20 mg/L rhodamine B and dispersed 0.02 g of photocatalyst material. The content was stirred for 1.5 h in the dark to provide absorption desorption equilibrium between the photocatalyst material and the dye. The beaker was irradiated with visible light (λ > 400 nm), and at intervals of 1.5 h, 2 mL aliquot was withdrawn. The absorbance of rhodamine B at 540 nm was used to monitor the concentration of rhodamine B over time. a b dc 861 Figures 3 a and d provide the absorbance peak and concentration of rhodamine B over time. The results in Figure 3 b, c, e and f, showed that the 0.12%ErBOC and 0.48%YbBOC from each category performed best. At 4.5 h hours of light irradiation, the 0.12%ErBOC and 0.48%YbBOC reduced the concentration of rhodamine B from the 20 mg/L initial concentration to 2 mg/L and 0.5 mg/L, respectively. This translates to a degradation efficiency of 80% on 0.12%ErBOC and 95% degradation on 0.48%YbBOC. The degradation results were fitted to the first-order reaction model, -ln(Ct/C0) = t, and the rate constants, k, are shown in Table 1. Important k values are 0.37 h-1 for 0.12%Er, 0.80 h-1 for 0.48%ErBOC and 0.13 h-1 for BOC, which indicate a significant shift in the reaction rate due to Er3+ and Yb3+ doping. Doping expanded the utilisation of light beyond visible light, generating more charge carriers and leading to improved photocatalytic ability in the doped materials. The UC effect conferred by doping rare earth in the material also leads to broader light utilisation, availing more charged carriers and radicals for degradation. Figure 3: Absorption spectra of rhodamine over 0.12%ErBOC, Inset: calibration curve (a), 0.48%YbBOC (d). Degradation of rhodamine B on x%ErBOC (b), x%YbBOC (c). The pseudo-first-order reaction kinetics of rhodamine B over x%ErBOC (c), and x%YbBOC materials (f) Table 1: First-order kinetics data Material k (h-1) R2 BOC 0.12%ErBOC 0.24%ErBOC 0.48%ErBOC 1.42%ErBOC 4.16%ErBOC 0.48%YbBOC 1.34%YbBOC 2.87%YbBOC 4.3%YbBOC 5.77%YbBOC 0.13 0.37 0.30 0.27 0.083 0.094 0.84 0.78 0.23 0.18 0.18 0.87 0.95 0.94 0.94 0.89 0.90 0.93 0.93 0.84 0.98 0.95 5. Degradation mechanism Radical scavenging experiments were conducted to determine the reactive species involved in the photocatalytic process. AgNO3 (e⁻ scavenger 2 mM; 5.1 mg), Ethylenediaminetetraacetic acid, EDTA-Na (h+ scavenger 2 mM, 8.8 mg), para-benzoquinone, p-BQ( O₂•⁻ radical scavenger 1 mM, 1.6 mg), Isopropyl alcohol e a b c d f 862 (IPA) (·OH radicals scavenger, 5.7 µL 5 mM) (Wang et al., 2022). At the instant of activation of the material by light energy, electrons (e-) and holes (h+) are produced. The (e-) reacts with dissolved oxygen (O2) in the solution to produce superoxide (O2 •–). The h+ reacts with water (H2O) to produce hydroxyl radicals. In this instance, the trapping experiment, Figure 4, showed the following percentage decolorization of the dye: No scavenger (100%), AgNO3 (96 %), IPA (85 %), p-QB (40 %), EDTA (4.9 %). The results point to h+ and O2 •– as the main species in the degradation, while e− and OH• played supporting roles. Figure 4: Radical trapping experiment showing the extent of radical involvement in degradation. The following equation proposes the reaction that occurs in the system. (1) 0.48%YbBOC + hv → h+ + e- (2) e− + O2 → O2 •– (3) h+ + H2O → OH• + H+ (4) h+ + OH−→ OH• (5) Rhodamine dye + (OH•/e−/OH•/O2 •–) → degradation products 6. Conclusions The photocatalytic strength of Bi12O17Cl2 was improved by doping with rare earth Er3+ and Yb3+. The rare earth converts low-energy irradiation in the NIR to visible light. The improved photocatalytic activity of Bi12O17Cl2 by rare earth upconversion effect is significant for solar activation, which has approximately a 45 % NIR, implying a sustainable process that requires less energy. The optimum doping at 0.12%ErBOC and 0.48%YbBOC was determined. Decolourisation/degradation of rhodamine dye on the Er3+ - and Yb3+ doped Bi12O17Cl2 was 2.84 and 6.46 times faster than on pure Bi12O17Cl2 material. The doped materials are also valuable in removing other organic pollutants from the environment. In addition, the ability of the doped materials to function not only in the visible region of solar radiation but also in NIR implies a reduction in cost in running a photocatalytic system. The target is to rely on natural solar light to activate the photocatalytic degradation of contaminants. The study further highlights using clean energy, i.e. solar, in photocatalytic systems. References Hao S., Shang Y., Hou Y., Chen T., Lv W., Hu P., Yang C., 2021. 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