Engineered Oxygen Vacancies in NiCo2O4/BiOI Heterostructures for Enhanced Photocatalytic Pollutant Degradation

doi:10.21203/rs.3.rs-4806674/v1

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Engineered Oxygen Vacancies in NiCo₂O₄/BiOI Heterostructures for Enhanced Photocatalytic Pollutant Degradation

https://doi.org/10.21203/rs.3.rs-4806674/v1

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To address the bottleneck issue of poor carrier separation and transfer efficiency in NiCo₂O₄ photocatalyst, a novel 1D/2D-rod on rose like NiCO₂O₄/BiOI nanohybrid with abundant OV’s was successfully synthesized using a single step hydrothermal method and employed to the photocatalytic degradation of Rhodamine B (RhB). The study revealed that the optimized NiCo₂O₄-OV/BiOI hybrid could possess superior photocatalytic degradation efficiency towards RhB degradation under visible light with rate constant that was 3.8 and 3.03 times greater than that of BiOI and NiCo₂O₄-OV. Experimental findings indicated that the formation NiCo₂CO₄-OV/BiOI heterojunction significantly improved the charge separation efficiency and facilitated the formation of surface OV’s. These OV’s enhanced photogenerated e^--h⁺ separation and increased catalytic efficiency. Quenching experiments results confirmed that both holes and superoxide radicals are playing crucial roles in the degradation process. Thus, an oxygen vacancy and engineering NiCo₂CO₄-OV/BiOI heterojunction enhanced degradation mechanism was proposed, offering insights for the integration of advanced oxidation technologies and the development of catalytic materials to enhance pollutant degradation efficiency.

Oxygen Vacancy

NiCo2O4/BiOI

Rhodamine B

Visible light

Photocatalysts

Nowadays, energy and environmental issues are the two biggest concerns in our society. Though the revolution of industrialization in modern times has greatly improved our standard of living, it has additionally been implicated with a variety of ecological issues (Ismael et al. 2020; Arumugam et al. 2022; Byrne et al. 2018). Water pollution caused by dyes eluted from various industries has emerged as a major cause of water pollution, which requires immediate action. The use application of semiconductor photocatalysts for the degradation of pollutants is a potential strategy that has provoked the researchers. Utilizing innovative two-dimensional (2D) layered nanomaterials has attracted greater prominence because of recent accomplishments in studies pertaining to carbon nitride (Mani et al. 2020; Lv et al. 2022; Ahmad et al. 2022). In recent years, BiOX, a novel photocatalyst that resembles X = F, Cl, Br, and I, has gained significant attention in research on account of its distinctive layered structure, appropriate band gap, and potent photocatalytic properties under both UV and visible light illumination. Its structural framework comprises a layer of (Bi₂O₂)²⁺ slices layered between double layers of halogen ions X−. Non-bonding interactions caused by van der Waals forces between the halogen ions along the c axis produce stacked X-Bi-O-O-Bi-X– layers (Vinoth et al. 2022; Wei et al. 2021; Ali et al. 2023). The intrinsic electric field that it generates within the 2X and (Bi₂O₂)²⁺ layers may facilitate the separation of electrons and holes produced by photolysis, hence enriching the photocatalytic process. More specifically, researchers globally working in the area of photocatalysis have observed a great deal of fascination with bismuth oxyiodide (BiOI) (Lin et al. 2014; Zhang et al. 2020; Wang et al. 2020; Huang et al. 2021; Chakraborty et al. 2021). As a fundamental constituent of the Sillen family, it has become a renowned photocatalyst due to its remarkable features, including cost-effectiveness, friendliness to the environment, optical and chemical durability, as well as a narrow energy bandgap (∼1.8 eV) for exploiting an extensive solar spectrum. The main drawback of small bandgap catalysts is the rapid reconnection of e⁻/h⁺ pairs, as these charge carriers tend to quench before they reach the semiconductor surface to start the redox processes (Bavani et al. 2021a&b; Bavani et al. 2022). It is very appropriate to spatially separate these photoexcited e⁻/h⁺ to different active sites in order to lengthen their lives and solve this dilemma (Wu et al. 2022; Guo et al. 2021; Ha et al. 2022; Yu et al. 2024; Golrizkhatami et al. 2023; Teng et al. 2022). Hence, a variety of approaches have been developed for separating the photoinduced charges, such as doping, vacancy creation, and heterojunction formation. The heterojunctions have been considered to be one of the most promising ways to generate potent photocatalysts with excellent photocatalytic efficiency among the methods that were suggested. These tactics are favorable for controlling optimizing the surface physicochemical state, band structure, and producing an internal electric field, which improves the ability to absorb light, enhances the separation and migration of photoinduced carriers, and increases the potency of photocatalysis in eliminating pollutants.

Its widely affirmed aspect is that the nanointerface's strategic framework provides a vital tool for controlling the interfacial charge mechanisms in photocatalytic processes. The heterostructure building of BiOI with another energy band of compatible semiconductors appears to be an intriguing tactic for achieving this goal of controlling the destiny of these charge carriers (Sun et al. 2023). In this regard, NiCo₂O₄ (NCO) was selected to pair with BiOI via a heterojunction that was designed at the right moment. In this investigation, new oxygen-deficient NCO nanosheets with surface oxygen vacancy (NCO-OV) defects could be compiled with BiOI nanosheets. As demonstrated by prior investigations (Cong et al. 2022), OV defects in semiconductor photocatalysts facilitate the creation of close defect energy levels below the conduction band (CB), which can considerably enhance semiconductors potential to absorb light and efficiently hold photoinduced electrons. The unique oxygen-deficient NCO designed in this work is notable because it provides clearly defined 1D nanorod structures that can be combined with BiOI using 1D/2D coupling. When assessing the intimate line-to-face interaction denoted by the 1D/2D heterojunction compared to the 0D/2D heterojunction (point-to-face interaction) or 2D/2D heterojunction (face-to-face interaction), the large contact surface allows for quick relocation of photoinduced charges, leading to outstanding interfacial charge transfer (Wang et al. 2020).

In order to prevent charge carrier recombination, the OV defects on NCO-OV are involved; these surface OVs act as e⁻ quenching sites. A single-step solvothermal process was used to construct a 1D/2D surface defect-engineered NCO-OV/BiOI heterojunctions that effectively exploits the advantageous features and provides an adept depiction of the method used to generate, separate, transport, and utilize charges appropriately. We can alter and regulate these surface-active sites at the nanoscale level by artificially creating OV defects on NCO. As far as, this is the first study to effectively illustrate the heterojunction formation of oxygen-deficient NCO with BiOI to promote photocatalytic degradation towards RhB. This novel 1D/2D- rod-on-rose like NCO-OV/BiOI photocatalyst surpasses other existing BiOI and NCO-based photocatalytic materials over the course of photocatalytic activities and reveals our nanoscale-level control and design of electronic and architectural structures without the need of noble or rare earth metals. We contend that our investigations could provide fundamental mechanistic insights regarding the visible light-absorbing process for efficient charge carrier utilization and steering, opening up a new avenue for the investigation of extremely effective and low-cost photocatalysts for wastewater remediation.

2.1. Materials

Cobalt nitrate hexahydrate (Co(NO₃)₂·6H₂O), bismuth nitrate pentahydrate (Bi(NO₃)₃·5H₂O), nickel nitrate hexahydrate (Ni(NO₃)₂·6H₂O), cetyltrimethyl ammonium bromide (CTAB) were acquired from Sigma-Aldrich, India. Urea (CO(NH₂)₂), ethylene glycol (EG), potassium iodide (KI), benzoquinone (BQ), ammonium oxalate (AO), and isopropyl alcohol (IPA) were obtained from SD fine chemicals, India. There was no further purification performed on the aforementioned raw materials.

2.2. Synthesis of NCO and NCO-OV

The NCO-OV was prepared by a hydrothermal method followed by an annealing process. Herein, (2 mmol) Co(NO₃)₂·6H₂O and (1 mmol) Ni(NO₃)₂·6H₂O were dispersed in 30 mL of 3:1 ratio of EG and DI water. At that time, the 3 mmol of urea and CTAB in the 25 mL of water were added to the above mixture, stirring for 30 min. Following the above, the blend was moved into a Teflon-lined autoclave and kept at 120°C for 10 h. The attained product was rinsed with ethanol and water repeatedly. Lastly, it was dried at 60°C for 4 h and then calcined at 300°C for 2 h. Likely, the pure NCO was prepared by above procedure without addition of EG.

2.3. Synthesis of BiOI and NCO-OV/BiOI nanocomposites

For NCO-OV/BiOI nanocomposite preparation, 4 mmol of Bi(NO₃)₃·5H₂O and KI were added in a separate beaker containing 25 mL of H₂O, and then both solutions were mixed well by stirring for 45 min. Besides, different weight percentages (wt. %) such as 10, 20 and 30 wt. % of NCO-OV was added under stirring. Afterward, the final reaction mixture was transferred into a Teflon-lined autoclave heated at 120°C for 16 h. After attaining room temperature, the precipitate was repeatedly rinsed with ethanol and DI. water and left to dry at 60°C for 6 h in a vacuum oven. Different 10, 20 and 30 wt. % of NCO-OV in the NCO-OV/BiOI hybrid was represented as NCBI-1, NCBI-2 and NCBI-3. Similarly, the pure BiOI was prepared by the above procedure without addition of NCO-OV. Scheme 1 illustrates the pictorial representation of the synthesis of pure NCO, BiOI and NCBI nanohybrid.

2.4. Characterization

Using a Bruker D2 PHASER X-ray diffractometer with graphite monochromator and Cu Kα radiation (λ = 1.54184 Å) at room temperature, the phase of the materials was observed by X-ray powder diffraction (XRD). The spectra for energy dispersive X-ray spectroscopy (EDAX) has been identified using a field emission scanning electron microscope (FESEM, Hitachi, 30 S-4800). Using a monochromated Al Kα irradiation, X-ray photoelectron spectroscopy (XPS, PHI-5702, Physical Electronics) was conducted. The UV-vis diffuse reflectance spectroscopy (DRS) results were obtained using a UV-visible SHIMADZU 3101 spectrophotometer. Additionally, the photoluminescence (PL) of the materials was measured using a JASCO FP-8500 fluorescence spectrometer.

2.5. Photocatalytic degradation studies

The photocatalytic degradation reaction was carried out in a photocatalytic chamber that was operated at room temperature. A 500 W tungsten halogen lamp was employed as the light source for visible light. For this experiment, 75 mg of photocatalyst sample was evenly dispersed into the vessel containing 75 mL of RhB (1x10^− 5 M) dye solution. Prior to the light illumination, the suspension was placed in a dark environment and made sure to attain sorption equilibrium. Following the above, 5 mL of aliquots were collected at regular time intervals in the presence of light. The aliquots are subjected to centrifugation to remove the excess catalyst, and then the absorption maximum of the dye solution was observed at 554 nm using a UV-visible spectrometer (JASCOV-750).

3.1 Structure and morphological investigations

The photocatalytic material’s crystalline phase structures were investigated by XRD technique. Figure 1a demonstrates the XRD patterns of the bare NCO, NCO-OV, BiOI and NCBI nanohybrid photocatalysts. The XRD of BiOI was well coordinated to the tetragonal phase of BiOI (JCPDS No. 10–0445) for the peaks at 29.7, 31.9, 45.4, 55.3 and 68.8^o corresponds to (102), (110), (200), (212) and (221) planes, respectively (Sasikala et al. 2023). The XRD peaks of NCO and NCO-OV are assigned to the cubic phase of NCO (JCPDS No. 73-1702), while the characteristic peaks of NCO-OV are slightly shifted to lower direction owing to the OV compared from the NCO peaks at 2θ-18.8^o, 32.7^o, 38.1^o, 51.4^o, 58.6^o, 62.4^o and 69.6^o corresponds to the (111), (200), (311), (400), (422), (440) and (531) planes (Naz et al. 2024). In the case of NCO-OV/BiOI nanohybrid, both the peaks of BiOI and NCO-OV are in good agreement. Further, the distinctive peaks of NCO-OV in the composites are stronger as the loading of NCO-OV increases. The significant peaks of NCO-OV are apparent on the NCBI nanocomposite, which signifies the efficient formation of NCO-OV with BiOI.

The functional groups of the prepared photocatalytic materials have been determined using FT-IR spectroscopy. From Fig. 1b, the pure BiOI displays peaks at 517 and 779 cm^− 1 for Bi-O bond and peaks located at 872–1047 cm^− 1 corresponds to the I-O bond. At the time, the stretching and bending modes of surface adsorbed water molecules are identified at 1593 and 3739 cm^− 1 (Sasikala et al. 2023). Besides, as seen in Fig. 1b, the NCO possesses two bands at 1631 and 3543 cm^− 1 for the -OH bending and stretching vibrations (Katubi et al. 2024). Furthermore, the vibrations of O–M–O, M–O, and M–O–M (M- Co and Ni) can be attributed at 676–823 cm^− 1. The FT-IR spectra of NCBI nanohybrid affirms the presence of characteristics peaks of NCO-OV and BiOI.

The material’s structural morphology was investigated using SEM. The SEM images of bare BiOI (Fig. 2a) demonstrate rose-like hierarchical structures formed by an array of 2D-nanosheets. The BiOI flower is composed of several self-assembled irregular plates, resembling petals interconnected in a manner like to the formation of a Rieger Begonia. As illustrated in Fig. 2b, the oxygen-deficient NCO reveals its 1D-nanorods shaped structures. The SEM images of pure NCO possess aggregated like structure, results shown in Fig. S1 (a and b). Many of 1D-NCO-OV nanorods are observed to regularly grow on 2D-BiOI nanosheets. This intimate 1D/2D interface contact between NCO-OV/BiOI is prominently obvious within different regions of NCBI-2 rod-on-rose microflowers (see Fig. 2c), underscoring the formation of heterojunctions between NCO-OV and BiOI, facilitating effective contact. The formation of close 1D/2D contact in the composite is further confirmed from EDAX spectra, shown in Fig. 2d, which exhibits the existence of Bi, O, I, Co and Ni elements in the BiOI and NCO-OV nanohybrid. Fig. S2 (a-c), illustrates the EDAX spectra of NCO without and with defect and BiOI. The existence of Bi, O, I, Co and Ni elements in the composites can be observed by elemental mapping images (Fig. 2e).

XPS investigation was employed to identify the compound's elemental composition and atomic chemical environment. Figure 3 (a-f) depicts the XPS spectra of pure NCO, NCO-OV, BiOI and NCBI nanohybrids. The wide survey XPS spectra depicts the presence of Bi, O, Ni, Co, and I elements in the NCO, BiOI, and NCBI nanohybrids has been demonstrated correspondingly in the survey spectrum of XPS as shown in Fig. 3a. The pure NCO without OV was used to verify the existence of OV defects in the NCO-OV. High resolution XPS spectra of Ni 2p peaks around 856.7 and 874.1 eV for the respective Ni 2p_3/2 and Ni 2p_1/2 states, and the satellite peaks at 862.6 and 880.2 eV, which represents Ni²⁺ from the Ni 2p spectrum (Fig. 3b) were observed. The XPS peaks of NCBI-2 exhibit a negative shift, which implies an increase in electron density. This finding further validates the strong interaction and electron transfer between NCO-OV and BiOI. In Fig. 3c, the Co 2p_3/2 and Co 2p_1/2 doublets appears at 781.5 and 806.3 eV, and also the two satellite peaks at 785.7 and 798.4 eV, indicating Co²⁺ state. When compared with NCO-OV, NCBI-2 exhibits a towards higher binding energies (positive shift).

Two peaks, situated at 159.5 and 164.8 eV in the high resolution Bi 4f XPS spectrum of BiOI (Fig. 3d), are attributed to the Bi 4f_7/2 and Bi 4f_5/2 in the Bi³⁺ chemical states, correspondingly. According to I 3d spectrum of BiOI (Fig. 3e), the peaks at 619.3 and 630.8 eV, correspondingly, coincide with the I 3d_5/2 and 3d_3/2 of the I- chemical state. Furthermore, the binding energies of I 3d in BiOI are not significantly different from those observed in NCBI-2 hybrid. Moreover, O1s spectra (Fig. 3f) displayed three distinct peaks at 530.2, 531.8, and 533.0 eV, corresponding to lattice oxygen, oxygen vacancies, and surface adsorbed oxygen, correspondingly. When compared to NCO, the O1s spectra of NCBI-2 exhibited a slight shift towards higher binding energies, resulting in a drop in electron density. Also, compared to NCO-OV, NCBI-2 shows a notable reduction in the M–O peak, indicating that the OV defects primarily result from the removal of surface oxygen atoms from the M–O layers. Absorbed hydroxyl groups tend to dehydrate and react with the released Ni and the Bi substrate, forming Ni–O–Bi bonds. This bond serves as an efficient charge transport channel between BiOI and NCO-OV, significantly impacting the photocatalytic performance of the NCBI nanocomposites. This shift indicates the generation of more OV’s after formation of heterojunction between NCO-OV and BiOI. Notably, an increase in electron density is accompanied by a negative shift in binding energy, and vice versa. Hence, the shifting nature of XPS binding energy can be used to assess the electron transport mechanism in the composite. In this case, it appears that NCO-OV served as an e- acceptor in the NCBI composites.

UV-vis DRS was used to assess the optical characteristics of the fabricated materials. Figure 4a presents the absorption spectra of BiOI, NCO, NCO-OV and NCBI-2 hybrid where the absorption edges are determined to be 672, 750, 686 and 712 eV, respectively. Further investigation reveals that there was a red shift in the absorption edges of NCO-OV along with a decrease in the bandgap. When compared with pure NCO, the long absorption tail of NCO-OV demonstrated the existence of high-density surface defects in the material. From the Kubelka-Munk plots (Bavani et al. 2023; Sasikala et al. 2023; Bavani et al. 2022) (Fig. 4b), the bandgaps of NCO and NCO-OV, BiOI and NCBI-2 were determined to be 1.65, 2.05, 2.19 and 2.16 eV, respectively, [32–34]. The rod-on-rose-like 1D/2D binary NCO-OV/BiOI composite has been revealed to be photoresponsive throughout a broad solar spectrum, ranging from UV to NIR, by combining the advantageous properties of both BiOI and NCO-OV. This combination can lead to generate more e⁻/h⁺ pairs and taking part in the reactions, thereby improving the efficiency of photocatalytic processes.

The transfer capability and separation of charges is a crucial factor in assessing the performance of photocatalysts. Therefore, PL measurements are essential for this evaluation. Typically, a lower PL intensity signifies better electron-hole separation efficiency. The PL spectra of NCO, BiOI, NCO-OV and NCBI nanohybrid was shown in Fig. 4c. As illustrated in Fig. 5c, all samples exhibit a broad emission peak in the 500–600 nm range. Due to the abundance of oxygen vacancies on the surface of NCBI, the wavelength range of the hybrid has red shift from pure NCO, which can act as trapping sites for photogenerated charge carriers. Moreover, BiOI has the highest peak intensity, indicating the highest rate of charge carrier recombination. However, when BiOI is coupled with NCO-OV, its PL intensity decreases significantly, suggesting that the recombination rate is effectively reduced. Among the samples, NCBI-2 has the smallest PL intensity, representing the reduced electron-hole reconnection rate. As a result, NCBI extend their photogenerated charge carriers’ lifetime and enhances their utilization in photocatalytic reactions.

3.2. Photocatalytic degradation studies

The fabricated samples' photocatalytic activity was assessed by degrading the model pollutant RhB in the presence of light, as shown in Fig. 5a. RhB barely degrades under light without the use of a photocatalyst, so the direct photolysis of RhB is limited. From Fig. 5a, only 33 and 41% of RhB are degraded in 60 min under the influence of NCO-OV and BiOI, correspondingly. The degradation efficiency of NCBI composites with 10, 20, and 30 wt.% of NCO-OV exhibits 62, 89, and 74% degradation efficiency within 60 min of light irradiation. The degradation ability of NCBI-2 nanocomposites is significantly greater compared with pure BiOI and NCO-OV for RhB degradation.

Figure 5b illustrates the linear dependence with time and reveals the photocatalytic RhB decomposition follows apparent pseudo-first-order kinetics, Eq. (1) (Cui et al. 2009; Bavani et al. 2022; Khan et al. 2020; Zulmajdi et al. 2020; Kee et al. 2022),

lnC₀/C = kt (1)

Where C₀ and C are the initial RhB concentration and concentration of the RhB with different time intervals, k is the pseudo-first-order rate constant (min^− 1), and t is time (min). The pseudo-first-order kinetics plot with relevant k values is compiled in Fig. 5c; the 0.0073, 0.0092, 0.0148, 0.0279, and 0.0189 min^− 1 correspond to BiOI, NCO-OV, NCBI-1, NCBI-2, and NCBI-3 nanocomposites, respectively (Fig. 5c). The k value of the NCBI-2 nanocomposite is around 3.8 and 3.03 times greater than that of BiOI and NCO-OV.

To assess the significant impact of catalyst quantity on the decomposition, the optimized NCBI-2 catalyst was chosen for experimentation. Subsequent experiments were conducted to investigate the photocatalytic degradation of RhB under 60 min of visible light irradiation (VLI), utilizing varying amounts of catalyst ranging from 0.25 to 1.5 g/L. The results obtained, as displayed in Fig. 5d, highlight that the photocatalytic decomposition ability appears to gradually increase up to a catalyst concentration of 1.25 g/L, followed by a progressively reduce thereafter. The initial rise in degradation rate is attributed to the increased accessibility of numerous active sites accessible on the surface of the photocatalyst. However, the subsequent rise in catalyst content leads to a reduction in the decomposition rate due to the shielding effect of the photocatalysts, which impedes the passage of light through the solution. Further, the recycling ability of the NCBI-2 photocatalyst was shown in Fig. 6a, revealing catalyst stability and reusability even after four consecutive cycles.

3.3. Photocatalytic degradation mechanism

Radical trapping studies have identified that major active radicals, including O₂^•−, h⁺, and •OH, play crucial roles in the photocatalytic degradation of RhB. Various quenching agents, such as 1 mmol of benzoquinone (BQ), isopropyl alcohol (IPA), and ammonium oxalate (AO), were used to determine their effects (Worku et al. 2024). The results shown in Fig. 6b indicate that the existence of AO (h⁺) and BQ (O₂^•−) does not significantly alter the degradation rate compared to when no quenchers are present. However, IPA (•OH) significantly hinders the degradation efficiency. These findings imply that h⁺ and O₂^•−radicals are the primary the active species in charge of the degradation by photocatalysis of RhB (Fig. 6b).

The strategy for the photocatalytic degradation mechanism of the NCBI nanocomposite is proposed based on the attained results. Figure 7 illustrates the facilitated charge transfer mechanism for the photodecomposition of RhB on the NCBI system under VLI. To improve photocatalytic performance, the heterojunction formation between the two different types of photocatalysts at different energies is employed as an advantageous method, expediting separation capability, and reducing reconnection rates. Energy levels play a crucial role in this mechanism, and thus the valence band (VB) and conduction band (CB) potentials of the fabricated photocatalysts are determined employing the following equations (2) and (3) (Kong et al. 2019):

E_VB = X - E^e + 0.5E_g (2)

E_CB = E_VB - E_g (3)

Herein, E_CB and E_VB represent the CB and VB potentials, E^e denotes the energy of free electrons on the hydrogen scale (~ 4.5 eV), X signifies the electronegativity of the semiconductor, and E_g represents the bandgap energy value of the semiconductor. Figure 7 depicts the determined values of the VB and CB potentials of BiOI as 2.14 and − 0.05 eV, respectively, while the CB and VB potentials of NCO are noted as -0.615 eV and 2.26 eV, correspondingly.

When NCO-OV and BiOI come into contact to form NCBI composite, their individual semiconductor properties interact to achieve equilibrium (Fig. 7). Initially, NCO-OV, due to its oxygen vacancies, acts as an n-type semiconductor with its Fermi level (E_f) closer to the CB, while BiOI behaves as a p-type semiconductor with its E_f closer to the VB. The presence of oxygen vacancies in NCO-OV facilitates the formation of active sites for surface reactions, promoting the migration of charge carriers and extending their lifetime. Upon contact, electrons from the n-type NCO-OV diffuse into the p-type BiOI, and holes from the BiOI diffuse into the NCO-OV. This diffusion continues until the E_f of both materials align, creating a depletion region at the interface where no free charge carriers exist [40]. The band structure bends near the junction, establishing an internal electric field that reaches equilibrium when the Fermi levels are uniform across the NCBI composite.

Under light irradiation, the composite of NCO-OV and BiOI exhibits distinct behavior due to the generation of electron-hole pairs by photons with energy greater than the bandgap of the materials. Photons generate electron-hole pairs in both NCO-OV and BiOI as well as in the depletion region. The built-in electric field at the junction separates these photogenerated carriers, driving electrons to the n-type NCO-OV and h⁺ towards the p-type BiOI. Notably, the e⁻ migrate to the oxygen defect state and significantly impact the electronic properties and reactivity. Apart from that, these electrons also act as recombination centers for photogenerated charge carriers. However, they can participate in redox reactions, aiding RhB degradation into harmless byproducts. Consequently, the resulting potential difference and movement of charge carriers in NCBI composites enhances the efficiency of photocatalytic processes by utilizing the broad solar spectrum.

In conclusion, we have developed a novel 1D/2D-rod-on-rose-like NiCo₂O₄/BiOI nanohybrid that coexists with surface oxygen vacancy defects. The photocatalytic degradation ability of the prepared photocatalysts was assessed for the RhB degradation. The optimized NiCo₂O₄-OV/BiOI nanohybrid exhibited superior photocatalytic efficiency under visible light, with degradation rate constants 3.8 and 3.03 times greater than BiOI and NCO-OV, correspondingly. The heterojunction formation between NiCo₂O₄-OV and BiOI enhanced photogenerated electron-hole separation and increased surface oxygen vacancies. Quenching experiments confirmed the significant roles of holes and superoxide radicals in photocatalytic degradation of RhB using NiCo₂O₄-OV/BiOI nanohybrids. Based on these findings, we propose a photocatalytic degradation mechanism, offering valuable insights for the integration of advanced oxidation technologies and the development of efficient catalytic materials for pollutant degradation.

Author Contributions

Bavani Thirugnanam: Investigation, Original draft preparation, Visualization, Reviewing, Conceptualization, Methodology. Preeyanghaa Mani: Reviewing, Editing, Conceptualization, Methodology, Data curation, Investigation. Bader O. Almutairi, Kuppusamy Sathishkumar, Munusamy Settu: Data curation, Methodology, Investigation.

Conflicts of Interests

All authors declare that there is no conflict of interest.

Acknowledgement

Dr. T. Bavani feels grateful for their financial support granted by Karpagam Academy of Higher Education. This research was supported by researchers supporting project number (RSP2024R414), King Saud University, Riyadh, Saudi Arabia.

Competing Interests: All authors declare that there is no conflict of interest.

Ethical Approval: Not applicable

Consent to Participate: All authors have approved the final version of the manuscript and have given their consent for publication.

Consent to Publish: All authors have approved the final version of the manuscript and have given their consent for publication.

Data Availability: Data are available made on request.

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Engineered Oxygen Vacancies in NiCo₂O₄/BiOI Heterostructures for Enhanced Photocatalytic Pollutant Degradation

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Abstract

Figures

1. Introduction

2. Materials and Methods

2.1. Materials

2.2. Synthesis of NCO and NCO-OV

2.3. Synthesis of BiOI and NCO-OV/BiOI nanocomposites

2.4. Characterization

2.5. Photocatalytic degradation studies

3. Results and discussion

3.1 Structure and morphological investigations

3.2. Photocatalytic degradation studies

3.3. Photocatalytic degradation mechanism

4. Conclusions

Declarations

References

Supplementary Files

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