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Article

Enhancing the Removal Efficiency of Rhodamine B by Loading Pd onto In2O3/BiVO4 Under Visible Light Irradiation

by
Yuanchen Zhu
1,
Shivam Parekh
1,
Shiqian Li
1,
Xiangchao Meng
2 and
Zisheng Zhang
1,*
1
Department of Chemical and Biological Engineering, University of Ottawa, Ottawa, ON K1N 6N5, Canada
2
College of Chemistry and Chemical Engineering, Ocean University of China, Qingdao 266100, China
*
Author to whom correspondence should be addressed.
Processes 2025, 13(7), 1983; https://doi.org/10.3390/pr13071983
Submission received: 13 May 2025 / Revised: 16 June 2025 / Accepted: 20 June 2025 / Published: 23 June 2025
(This article belongs to the Special Issue Wastewater and Waste Treatment: Overview, Challenges and Current Trends (3rd Edition))
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Versions Notes

Abstract

A simple method for synthesizing novel Pd-In2O3/BiVO4 composites by using a hydrothermal technique is proposed. The synthesized samples showed a monoclinic phase and featured homogeneously dispersed Pd and BiVO4 dopants on In2O3, as confirmed by XRD, SEM, and XPS analyses. The Pd-In2O3/BiVO4 composite exhibited notable improvements, such as broadened visible-light absorption (up to 596.1 nm) and a narrowed band gap (2.08 eV vs. 2.82 eV for pure In2O3), a more compact and integrated morphology observed by SEM, which are expected to promote improved light harvesting and facilitate charge separation during photocatalysis. Under visible-light irradiation, the optimized 1 wt% Pd-In2O3/BiVO4 achieved 99% degradation of Rhodamine B (10 mg/L) within 40 min, while pure In2O3 showed less than 10% removal after 60 min—highlighting the strong synergistic effect of dual doping. Additionally, the composite demonstrated excellent stability and reusability over multiple cycles. A plausible photocatalytic mechanism for this process is proposed, providing insights into the design of efficient photocatalysts for wastewater treatment.

1. Introduction

Recent decades have marked a notable escalation in modern industrial growth. Many organic pollutants have been discharged into water bodies, especially wastewater from the textile industry, which accounts for nearly half of the total [1]. The persistence of some organic substances in the environment makes it difficult to decompose, thereby creating ecological hazards and posing potential harm to human health. Therefore, finding an effective solution for treating industrial wastewater remains a considerable challenge [2]. The development of advanced oxidation processes (AOPs) has provided new approaches for water treatment and gained increasing attention [3]. The activity and structural properties of catalyst materials have a significant impact on this. Among AOPs, photocatalytic degradation is considered a highly promising approach because of its operational simplicity, excellent efficiency, and minimal risk of generating secondary pollutants [4]. In the textile industry, Rhodamine B (RhB) is extensively applied in multiple fields such as textile dyeing, water flow tracing, and fluorescence labeling for microscopic analysis, as well as in photosensitization and laser dye technologies [5,6]. Recognized for its harmful effects, RhB dye is linked to cancer risk and neurological damage, along with other adverse impacts such as irritation to the respiratory and digestive systems, eye infections, and developmental toxicity in living organisms [7]. Its strong stability and persistence in the environment make it a commonly used benchmark compound in evaluating photocatalytic degradation performance.
Many scientists have explored the use of various semiconductor photocatalysts to degrade RhB, such as TiO2 [8], SnO2 [9], ZnO [10], WO3 [11], and In2O3 [12]. Indium oxide (In2O3) has gained significant attention from scientists. In2O3 has attracted considerable attention for its diverse range of potential applications, such as hydrogen production, CO2 reduction, water remediation, and other fields [13,14]. Due to its intrinsic properties, In2O3 has a wide band gap ranging from 2.7 to 3.6 eV, which shows the potential to be reconstructed with other semiconductors [15,16]. In2O3 exhibits advantages in visible light photocatalysis and photoelectrochemical protection, particularly excelling in applications related to organic pollutant degradation wastewater effluents [5,17]. Compared to TiO2, In2O3 shows higher photocatalytic activity under visible light, though its response to ultraviolet light is relatively weak. Nevertheless, pure In2O3 exhibits reduced photocatalytic activity due to the fast recombination of photoinduced charge carriers [18]. Thus, facilitating the dissociation of photogenerated electrons and holes is vital for optimizing the photocatalytic behavior of In2O3. BiVO4 has a narrower band gap (approximately 2.4–2.5 eV), thus permitting absorption across a wider segment of the visible spectrum, representing nearly 40% of solar radiation) [19]. This characteristic gives BiVO4 remarkable activity under visible-light-driven photocatalysis. The advantage of BiVO4 lies in its response to visible light, making it particularly attractive for applications such as pollutant degradation, especially under natural outdoor lighting conditions [20]. Therefore, the combination of BiVO4 and In2O3 was selected to boost the photocatalytic activity. Furthermore, various photocatalytic composites have been explored for the removal of organic dyes, including Pd/TiO2 [21], Pt/TiO2 [22], Ag/ZnO [23], and Ti-doped Mn3O4 [24]. These studies demonstrate that doping metal oxides with various metals can form electronic structures that act as electron traps, thereby enhancing charge separation by accelerating the movement of electrons created under light irradiation to adjacent materials and minimizing recombination with holes. As a result, the synergistic enhancement boosts the overall photocatalytic performance and accelerates the treatment of dye-contaminated water through degradation processes [25].
In this paper, we detail the synthesis of a heterojunction structure comprising In2O3 and BiVO4, doped with palladium, using a straightforward hydrothermal method. This combination significantly enhances the optical and electronic behavior, characterized by enhanced charge carrier separation, suppressed electron-hole recombination, and a narrowed band gap [26,27]. The efficiency of the developed photocatalysts was tested through the visible illumination degradation of Rhodamine B as a model compound. Experimental outcomes confirm superior photocatalytic behavior due to the addition of BiVO4 and Palladium to In2O3. Furthermore, the novel photocatalyst exhibited excellent recyclability over multiple cycles. Additionally, to better understand the photocatalytic process of Pd-In2O3/BiVO4 under visible light irradiation, a plausible reaction mechanism is suggested.

2. Materials and Methods

2.1. Photocatalyst Synthesis

2.1.1. Synthesis of In2O3

Pristine indium oxide was prepared using the hydrothermal method. Typically, a total of 7.20 mmol of InCl3 (Fisher Scientific, Waltham, MA, USA, Certified ACS) was added to 45 mL of DDW (Distilled Deionized Water) and was mixed under magnetic stirring for 30 min. Then, a total of 21.63 mmol NaOH (Sigma-Aldrich, St. Louis, MO, USA) was induced into the solvent and blended under magnetic stirring for an additional 30 min. The hydrothermal treatment was conducted at 180 °C for 6 h in a 45 mL stainless steel autoclave lined with Teflon (Parr Instrument Company, Moline, IL, USA) after transferring the solution from the previous step. Following the thermal treatment, the autoclave was left to return to room temperature. The treated mixture, In(OH)3 was isolated by filtering and washed once with absolute ethanol (Fisher Scientific, Waltham, MA, USA) and then twice with double-distilled water (DDW) to remove residual impurities. The obtained product was loaded into crucibles and annealed at 500 °C for 2 h, increasing the temperature by 3 °C per minute under ambient air in a muffle furnace (Thermo Scientific, Waltham, MA, USA). As a result, pure crystalline indium oxide (In2O3) nanoparticles were obtained.

2.1.2. Synthesizing Heterojunction of In2O3/BiVO4

The In2O3/BiVO4 composite with an In:Bi molar ratio of 2:8 (In2Bi8) exhibited the highest photocatalytic efficiency compared to the samples with ratios of 1:9, 3:7, 5:5, and 8:2, as shown in Figure S1. Therefore, the heterojunction of In2O3 and BiVO4 with a molar ratio of 2:8 was synthesized by a one-pot hydrothermal route. A total of 2.54 mmol of Bi(NO3)3·5H2O (Fisher Scientific, Waltham, MA, USA, Certified ACS) was added to 90 mL of DDW under magnetic stirring for a period of 30 min. Then, a total of 2.48 mmol of NH4VO3(ACROS ORGANICS, Waltham, MA, USA) was induced into the solvent under magnetic stirring for 30 min. The color turned red as soon as NH4VO3 was added, and the color turned yellow after 30 min. An amount of 1.27 mmol of InCl3 (Fisher Scientific, Waltham, MA, USA, Certified ACS) was transferred to the current solvent and mixed for 30 min. Then, a total of 3.75 mmol of NaOH was poured into the solvent and mixed under magnetic stirring for an additional 30 min. The solution turned light yellow once the stirring was complete.
The obtained mixture was placed into a 100-mL autoclave (Parr Instrument Company, Moline, IL, USA), which is made of Teflon-lined stainless steel, and subjected to hydrothermal treatment at 180 °C for 6 h. After finishing the reaction process, cooling of the autoclave was carried out to standard room conditions under ambient conditions, the resulting product was collected by filtering, and then the obtained solid was washed once with absolute ethanol followed by two rinses with double-distilled water (DDW). The filtered products were transferred to crucibles and calcined at 500 °C for 2 h with a temperature ramp of 3 °C per minute, under ambient air in the muffle furnace (Thermo Scientific, Waltham, MA, USA). An In2O3/BiVO4 heterojunction composite with a 2:8 molar ratio was successfully obtained.

2.1.3. Preparation of Palladium Doped In2O3/BiVO4 Heterojunctions

Half a gram of as-prepared In2O3/BiVO4 with a certain molar ratio was added to the crucible. Pd ionic was provided by a pre-made palladium chloride (PdCl2) (Fisher Scientific, Waltham, MA, USA, Certified ACS) solvent with a concentration of 20 g/L. Each time, the PdCl2 solvent was ultrasonicated for 15 min. A volume of 0.25 mL of pre-made PdCl2 was added to a crucible using a glass bar to mix the photocatalyst with the solvent. DDW was added to the catalyst powder and heated at 120 °C for 2 h. The weight percentage of Pd doping was maintained at 1 wt% relative to the total mass of the composite. Therefore, the Pd-In2O3/BiVO4 composite was successfully synthesized via the described method.

2.2. Characterization of Prepared Samples

In the characterization, X-ray powder diffraction (XRD) patterns were recorded by a Rigaku Ultima IV X-Ray diffractometer (Rigaku Americas Corp., The Woodlands, TX, USA), equipped with Cu Kα (λ = 0.15418 nm) as the radiation source, operated at 40 kV and 40 mA under Bragg–Brentano geometry. The morphologies of the prepared samples were observed under JEOL JSM-7500F field emission scanning electron microscopy (SEM) (JEOL USA, Peabody, MA, USA). The operating voltage is 3.0 kV. An X-ray photoelectron spectroscopy (XPS) analysis was conducted through an Axis Ultra DLD spectrometer (Kratos Analytical Ltd., Manchester, UK), outfitted with a 140 W monochromatic Al Kα X-Ray source, was employed for the XPS analysis. UV–Vis diffuse reflectance spectra (DRS) were obtained for the prepared materials using the Thermo Evolution 300 equipment (Thermo Scientific, Waltham, MA, USA). A scan rate of 240 nm min−1 was selected for the measurement.

2.3. Evaluation of Photocatalytic Performance

The photocatalytic efficiencies of the synthesized materials were assessed through the degradation of RhB dye in aqueous solution. A 500-mL beaker was used as the reaction vessel, and the temperature was controlled at approximately 20 °C all the time using a cooling jacket. Firstly, a 200 mL solution of RhB (Sigma-Aldrich, St. Louis, MO, USA) at a level of 10 mg/L was prepared for the photocatalytic experiments, and 0.1 g of the photocatalysts were introduced into the solution and mixed thoroughly in the dark for 30 min to establish adsorption equilibrium. Irradiation of the suspension was carried out at a stable temperature of 20 °C and maintained using a circulating water-cooling system. The reactant solution was exposed to visible light illumination provided by a 300 W halogen tungsten lamp (Ushio, Tokyo, Japan), delivering a spectral output predominantly between 310 and 800 nm; there is a 410 nm UV cut-off filter (Kenko Zeta, Tokyo, Japan, transmittance > 90%) under the lamp to eliminate UV components and allow only visible light to reach the reaction system. For each degradation test, RhB solvent and photocatalysts were mixed in a dark environment for 30 min to facilitate reaching an adsorption–desorption equilibrium between pollutant and catalyst. Photocatalysis began once the lamp was turned on.
The photocatalytic removal of Rhodamine B (RhB) was monitored by recording the absorbance at 554 nm, which corresponds to the maximum absorption peak of Rhodamine B (RhB). As described by the Beer–Lambert law, absorbance increases proportionally with dye concentration. Therefore, changes in absorbance were used to calculate the degradation efficiency, which was determined by the formula below [28]:
η ( % ) = C 0 − C C 0 × 100 %
In this equation, C0 is the starting concentration of Rhodamine B (RhB), and C is the remaining concentration of RhB. The degradation efficiencies can be calculated from the obtained data. There are three graphs we can plot: C0 versus irradiation time, −ln(C/C0) versus sunlight illumination time, and degradation efficiency versus different photocatalysts. The degradation of RhB was modeled as a pseudo-first-order reaction, and the corresponding rate constant was calculated by fitting the experimental data to a linear relationship between −ln(C/C0) and reaction time.

3. Results and Discussion

3.1. Characterization

3.1.1. XRD

XRD graphs for the In2O3, Pd/In2O3, In2O3/BiVO4, and Pd-In2O3/BiVO4 samples are shown in Figure 1a–d, respectively. The signature diffraction peaks of In2O3 shown in Figure 1a are located at 2θ = 21.47°, 30.65°, 35.54°, 51.10°, and 60.75°, and corresponded to the (2 1 1), (2 2 2), (4 0 0), (4 4 0) and (6 2 2) planes. These peaks align with the standard XRD pattern of cubic In2O3 (JCPDS #06-0416) which confirms the successful formation of In2O3 [29]. The molar ratio of indium and bismuth is 2:8, the peak’s position representing BiVO4 is shown in Figure 1c,d. Diffraction peaks are located at 2 [30,31], which confirms the presence of BiVO4 in the composites. Due to the minor Pd amount (1 wt%), which is too low to be detected by X-ray diffraction, there is no obvious peak in Figure 1b,d.

3.1.2. XPS

The XPS results of Pd doped on In2O3/BiVO4 are presented in Figure 2, verifying the presence of Pd along with In, Bi, V, and O elements. The peaks detected at 448.9 eV and 441.3 eV are attributed to the In 3d3/2 and In 3d7/2 binding energies, respectively (Figure 2a). The binding energy peak located at 526.5 eV is assigned to the O1s orbital, confirming the presence of oxygen species in the sample (Figure 2b). Hence, the presence of indium and oxygen in the Pd-In2O3/BiVO4 photocatalyst was confirmed [32]. The two peaks at 513.4 and 521 eV match the binding energies of V 2p3/2 and V 2p1/2, indicating the successful incorporation of vanadium (Figure 2b). Additionally, the peaks detected at 334 and 339.4 eV are indicative of binding energy positions associated with Pd 3d3/2 and Pd 3d5/2 orbitals, respectively (Figure 2d), providing evidence for the successful deposition of metallic Pd [33]. The two distinct peaks located at 155.6 and 161 eV in Figure 2c correspond to the Bi 4f7/2 and Bi 4f5/2 orbitals, respectively (Figure 2c). These peaks are indicative of Bi3+ species, supporting the formation of BiVO4 instead of merely indium metal [34,35].

3.1.3. SEM

SEM images of In2O3, Pd-In2O3, In2O3/BiVO4, and Pd-doped In2O3/BiVO4, obtained at a magnification of ×8500, are displayed in Figure 3. The image of pure In2O3 displays well-defined, uniformly dispersed cubic-shaped particles, consistent with its known crystalline structures shown in Figure 3a. In Figure 3b, the image reveals irregularly shaped aggregates with a rough and porous surface texture. The red-circled region highlights small particulate clusters on the surface, which are attributed to the deposition of Pd nanoparticles. As proved in Figure 3c, in the In2O3/BiVO4 sample, plate-shaped BiVO4 crystals are observed, with In2O3 particles visibly deposited on their surfaces, particularly within the red square region, suggesting successful heterojunction formation. As shown in Figure 3d, the Pd-doped In2O3/BiVO4 exhibits a more compact structure with finer partile distribution and better integrated composite structure. The red-marked region indicates areas where In2O3 cubes are embedded within the BiVO4 microplates, forming a tightly integrated heterojunction. The overall surface appears more porous and rougher, which could provide a larger active surface area beneficial for photocatalytic reactions. This improved microstructure is expected to play a critical role in reducing electron-hole recombination and enhancing photocatalytic activity under visible-light irradiation.

3.1.4. UV–Vis-NIR Spectroscopy

Figure 4 illustrates the UV–Vis absorption profiles of In2O3, Pd-In2O3, In2O3/BiVO4, and Pd-In2O3/BiVO4 samples, confirming that all the prepared samples possess visible-light absorption capability. The absorption edges for the pure In2O3, Pd-In2O3, In2O3/BiVO4, and Pd-In2O3/BiVO4 samples are 439.60, 481.7, 540.8, and 596.1 nm, respectively. The highest light absorption was observed for the Pd-In2O3/BiVO4 sample. Additionally, the band gap of each of those samples exhibits differences due to Pd doping on the photocatalyst surface. This allows the samples to generate mid-gap states that act as electron traps in In2O3, thus reaching a lower Eg value [32]. Table 1 shows the Eg values estimated from UV–Vis spectral response by using the Tauc method [36]. The band gap was changed from 2.82 eV for the pure In2O3 to 2.08 eV upon co-doping with Pd and BiVO4.

3.2. Assessment of the Photocatalytic Properties of the Developed Photocatalysts

3.2.1. Influence of Doping Metal

The photocatalytic decomposition of RhB using various photocatalysts is shown in Figure 5, which was tested with a catalyst dosage of 0.5 g/L and a RhB concentration of 10 mg/L. The results from Figure 5 show that the activity can be ranked: Pd-In2O3/BiVO4 > In2O3/BiVO4 > Pd-In2O3 > In2O3. An enhanced degradation of RhB was observed from the constructed In2O3/BiVO4 composites, attributed to the cooperative interaction between the two semiconductors. After 60 min of UV light exposure, In2O3 alone degraded only 10% of the RhB. In contrast, the In2O3/BiVO4 composite achieved a 71% degradation of RhB, significantly boosting photocatalytic efficiency and leading to a much more effective breakdown of the dye. Furthermore, with the addition of a small amount of Pd loading on In2O3/BiVO4 composites, the photodegradation efficiency reached 99% within 40 min, which achieved the best degradation capability. This suggests that the addition of Pd to In2O3/BiVO4 significantly improves the photoactivity, primarily caused by the bandgap tuning effect introduced through surface doping, where donor impurities contribute additional electronic states close to the conduction band, whereas acceptor impurities generate energy levels adjacent to the valence band [37]. Therefore, the choice of metal doping is crucial for the photocatalytic performance of In2O3/BiVO4.

3.2.2. Effect of Pd-In2O3/BiVO4 Loading on Photocatalytic Degradation Efficiency

The influence of Pd-In2O3/BiVO4 dosage on the photodecomposition of Rhodamine B under visible light is shown in Figure 6. To evaluate the effect of catalyst loading, the optimized photocatalyst was tested at concentrations ranging from 0.3 to 0.7 g/L. The increased dosage from 0.3 g/L to 0.5 g/L enhanced the photocatalytic removal efficiency from 74 to 100%, respectively. This may result from the increased contact between the RhB dye and the photocatalyst’s reactive sites [38]. However, when the amount of Pd-In2O3/BiVO4 photocatalyst increased to 0.7 g/L, the photocatalytic degradation dropped to 92% after 40 min. This was attributed to the significant aggregation of the photocatalyst inhibiting the light penetration onto the surface of Pd-In2O3/BiVO4, thus, decreasing the accessible active sites for the reaction [39,40].
It should also be noted that the enhanced Rhodamine B removal capability under photocatalysis was achieved without the introduction of any hole scavengers or through pH modification of the medium. Our optimized Pd-In2O3/BiVO4 photocatalyst demonstrated the highest degradation capability among all compared materials, confirming the potential of this photocatalyst in future research work.

3.2.3. Recyclability and Durability of Pd-In2O3/BiVO4

The recycle performance of Pd-In2O3/BiVO4 is shown in Figure 7. The catalysts were separated via centrifugation after each run. A total of four recycling experiments were performed, and at each run, the concentration of RhB decreased, as shown in Figure 7. It was recorded that 93% degradation of RhB was achieved within 40 min of the fourth run. This indicates that the photoactivity of the catalyst remains high, though some of the active sites may be blocked by RhB intermediate, resulting in a slightly lower activity rate [41]. Overall, the data clearly demonstrate the superior photocatalytic stability of the Pd-In2O3/BiVO4 composite over multiple cycles.

3.2.4. RhB Degradation Pathway

The RhB degradation pathway was assessed using UV–Vis spectroscopy. The color of the RhB solution gradually fades from deep pink to colorless over 40 min, and a set of photographs taken during the RhB photodegradation experiment using the Pd-In2O3/BiVO4 photocatalyst under visible light irradiation is shown in Figure S2. The A UV–Vis absorption curve of the reaction solution at various irradiation times during a photodegradation process is presented in the plot below.
The degradation of RhB occurs through two primary pathways. The first involves the breakdown of the central conjugated structure, where •OH radicals attack the chromophore rings of the RhB molecules [42]. This pathway can be identified when there is no shift in the peak wavelength in the absorbance spectrum. However, the plot above indicates a noticeable shift in the peak wavelength throughout the degradation process. Thus, it was concluded that 1% of Pd-In2O3/BiVO4 degrades RhB through the N-de-ethylation process, generating multiple intermediates such as N,N,N′-triethyl Rhodamine (TER) and N,N′-diethyl Rhodamine, Rhodamine (DER), N-ethyl Rhodamine (ER), and Rhodamine with peaks observed at 539 nm, 520 nm, 510 nm, and 500 nm, respectively [43]. From Figure 8, there is a characteristic blue shift in the wavelength of maximum absorbance coinciding with lower absorbance, indicating the presence of various intermediates over time.

3.2.5. Proposed Photocatalytic Mechanism

The results found in the previous passage can help in understanding the photocatalytic mechanism of RhB degradation with Pd-In2O3/BiVO4. To achieve this, it is important to first use the band gap of In2O3 and BiVO4 to ascertain the band positions, using the electronegativity theory proposed by Mulliken in the following equations [44,45]:
E CB =   χ p − E e − 0.5 E g
E VB = E CB + E g
Here, ECB and EVB refer to the conduction band and valence band potentials, χ p is the absolute Mulliken electronegativity of the semiconductor, Ee represents the standard energy of free electrons relative to the normal hydrogen electrode (NHE), typically taken as 4.5 eV, and Eg refers to the semiconductor’s energy gap, typically measured in eV, which separates the valence and conduction bands.
As shown in Table 2, the band gap energies of In2O3 and BiVO4 were determined to be 2.59 and 2.31 eV, respectively. From the data, a plausible mechanism for the photocatalytic degradation of RhB dye using our optimized Pd-In2O3/BiVO4 photocatalyst is illustrated in Figure 9.
The initiation of photocatalysis occurs when visible light photons are absorbed by both In2O3 and BiVO4, resulting in excitation across the band gap, thereby generating photogenerated electron-hole pairs in each semiconductor. Based on the valence and conduction layers of In2O3 and BiVO4, both the EVB and ECB of BiVO4 are larger than In2O3. It was determined that In2O3/BiVO4 formed a type II heterojunction. In type II heterojunctions, under illumination, photon energy excites electrons to the CB, concurrently generating holes in the VB [47,48]. Due to the band alignment, electrons flow toward In2O3 with a lower CB edge, while holes accumulate in BiVO4 due to its elevated VB position. This spatial separation inhibits the direct charge carrier recombination, thereby extending the lifetime of the photogenerated charges. Moreover, the introduction of a Schottky junction between Pd metal and BiVO4 induces an electromagnetic field in the charge-separation zone, allowing for increased electron-hole separation [49]. This allows the Pd nanoparticles to act as charge traps for e− in the CB of In2O3, thereby preventing recombination. Additionally, the surface plasmon resonance effect helps to capture a significant amount of incoming visible light photons, allowing for more conduction band electrons to be produced [37,50].
From the diagram above, photocatalysis is triggered when the energy of the incident photons matches or exceeds the band gap (2.08 eV) of the catalyst, and electrons from the valence bands of both In2O3 and BiVO4 are excited by their respective conduction bands, leaving holes behind. The electrons from the CBIn2O3 subsequently move to Pd, which acts as a charge trap, mitigating recombination. These electrons are then able to participate in the reduction of O2 to form •O2−. Holes accumulated on VBIn2O3 react with OH to form •OH, leading to the degradation of RhB. Moreover, some of the holes were considered to be directly attacking the pollutants as well. This is because of the high redox potential of the two semiconductors in the heterojunction. As such, a synergistic effect between h+ and •OH provides an effective degradation of RhB molecules. The proposed mechanism was further supported by radical quenching experiments shown in Figure S3, indicating that the photocatalytic process was mainly driven by h+ and •O2− as the key oxidative species under visible light. Furthermore, a series of control experiments were conducted under varying light and catalyst conditions, as shown in Figure S4. The results can be concluded that the degradation of RhB is primarily driven by the photocatalytic process, rather than by simple redox reactions or dye photolysis.

4. Conclusions

The successful co-doping of In2O3 with Pd and BiVO4 was achieved via a facile hydrothermal approach. The notable improvements of the synthesized samples on the structure, morphology, and capability of light harvesting owing to the incorporation of BiVO4 and trace amounts of Pd. To evaluate the individual and combined effects of the dopants on the photocatalytic activity, Rhodamine B (RhB) was employed as a model organic contaminant. Compared to pure In2O3, which achieved only 10% RhB degradation after 60 min, the co-doped photocatalyst, containing 1 wt% Pd, exhibited significantly improved performance, and achieved 99% degradation of RhB within 40 min upon exposure to visible light. The improved photocatalytic performance is primarily attributed to the synergistic effects of Pd and BiVO4, which collectively improved surface texture, increased harvesting of visible light, decreased band gap energy, and reduced the charge carrier recombination rate. The 1 wt% Pd-In2O3/BiVO4 photocatalyst indicated reliable catalytic activity with impressive stability and regeneration capacity. The proposed reaction process suggests that co-doping leads to highly separated photogenerated charge carriers, enhancing photocatalytic behavior. This synthesized photocatalyst holds great promise for practical applications in industrial wastewater treatment driven by visible light.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/pr13071983/s1, Figure S1. Photocatalytic degradation of RhB dye using In2O3/BiVO4 composites with varying In:Bi molar ratios under visible light irradiation (Catalyst dosage: 0.5 g/L degradation temperature 20 °C, and initial RhB concentration of 10 ppm). Figure S2. Photodegradation of RhB (10 ppm) by Pd-In2O3/BiVO4 under visible light irradiation. Figure S3. Effect of various scavengers on RhB degradation (10 ppm) using 0.5 g/L catalyst at 20 °C. Figure S4. Photodegradation of RhB(10 mg/L) under different conditions: with/without catalyst and with/without light irradiation.

Author Contributions

Y.Z.: methodology, formal analysis, writing—original draft, writing—review, and editing; S.P.: software; S.L.: investigation; X.M.: supervision and writing—review and editing; Z.Z.: supervision, writing—review and editing, and financial support. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Natural Sciences and Engineering Research Council of Canada (NSERC) 06917/RGPIN/2018.

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Materials. Further inquiries can be directed to the corresponding authors.

Conflicts of Interest

The authors declare no conflicts of interest.

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Processes 13 01983 g001
Figure 1. (a–d) displays the XRD patterns of (a) pure In2O3, (b) Pd-In2O3, (c) In2O3/BiVO4, and (d) Pd doped In2O3/BiVO4, respectively.
Figure 1. (a–d) displays the XRD patterns of (a) pure In2O3, (b) Pd-In2O3, (c) In2O3/BiVO4, and (d) Pd doped In2O3/BiVO4, respectively.
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Processes 13 01983 g002
Figure 2. (a–d) presents the high-resolution XPS spectra of In 3d, Bi 4f, V 2p, O 1s, and Pd 3d in the Pd-In2O3/BiVO4 sample.
Figure 2. (a–d) presents the high-resolution XPS spectra of In 3d, Bi 4f, V 2p, O 1s, and Pd 3d in the Pd-In2O3/BiVO4 sample.
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Processes 13 01983 g003
Figure 3. SEM images (acquired at a magnification of ×8500) of prepared samples: (a) pure In2O3, (b) Pd-In2O3 (the red circle indicates regions where Pd nanoparticles are deposited), (c) In2O3/BiVO4 (the red boxes highlight In2O3 deposited on BiVO4 particles), and (d) Pd-In2O3/BiVO4 (the red box marks areas with finer particle dispersion and better intergrated composite structure).
Figure 3. SEM images (acquired at a magnification of ×8500) of prepared samples: (a) pure In2O3, (b) Pd-In2O3 (the red circle indicates regions where Pd nanoparticles are deposited), (c) In2O3/BiVO4 (the red boxes highlight In2O3 deposited on BiVO4 particles), and (d) Pd-In2O3/BiVO4 (the red box marks areas with finer particle dispersion and better intergrated composite structure).
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Processes 13 01983 g004
Figure 4. UV–Vis diffuse reflectance spectra of pure In2O3, Pd-In2O3 and In2O3/BiVO4, and Pd-In2O3/BiVO4 heterojunctions.
Figure 4. UV–Vis diffuse reflectance spectra of pure In2O3, Pd-In2O3 and In2O3/BiVO4, and Pd-In2O3/BiVO4 heterojunctions.
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Processes 13 01983 g005
Figure 5. Photocatalytic degradation of RhB dye using In2O3, Pd/BiVO4, In2O3/BiVO4, and Pd-In2O3/BiVO4 photocatalysts versus visible light illumination time.
Figure 5. Photocatalytic degradation of RhB dye using In2O3, Pd/BiVO4, In2O3/BiVO4, and Pd-In2O3/BiVO4 photocatalysts versus visible light illumination time.
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Figure 6. Effect of Pd-In2O3/BiVO4 catalyst dosage on the degradation of RhB dye.
Figure 6. Effect of Pd-In2O3/BiVO4 catalyst dosage on the degradation of RhB dye.
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Processes 13 01983 g007
Figure 7. Assessment of the recycling performance of Pd-In2O3/BiVO4 for Rhodamine B removal with visible light induction.
Figure 7. Assessment of the recycling performance of Pd-In2O3/BiVO4 for Rhodamine B removal with visible light induction.
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Processes 13 01983 g008
Figure 8. UV–Vis absorption curve of RhB under various irradiation times (Pd-In2O3/BiVO4 dosage: 0.5 g/L, degradation temperature 20 °C, and an initial RhB concentration of 10 ppm).
Figure 8. UV–Vis absorption curve of RhB under various irradiation times (Pd-In2O3/BiVO4 dosage: 0.5 g/L, degradation temperature 20 °C, and an initial RhB concentration of 10 ppm).
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Processes 13 01983 g009
Figure 9. Schematic illustration of the RhB photodecomposition process over the Pd-In2O3/BiVO4 photocatalyst.
Figure 9. Schematic illustration of the RhB photodecomposition process over the Pd-In2O3/BiVO4 photocatalyst.
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Table 1. Summary of band energies (Eg) of as-prepared samples.
Table 1. Summary of band energies (Eg) of as-prepared samples.
SamplesWavelength (nm)Eg (eV)
In2O3439.62.82
Pd-In2O3481.72.57
In2O3/BiVO4540.82.29
Pd-In2O3/BiVO4596.12.08
Table 2. Estimated band positions for In2O3 and BiVO4 photocatalysts. Electronic properties ( χ p and Ee) were collected from the CRC handbook [46].
Table 2. Estimated band positions for In2O3 and BiVO4 photocatalysts. Electronic properties ( χ p and Ee) were collected from the CRC handbook [46].
Photocatalystχp (eV) Ee (eV) Eg (eV) ECB (eV) EVB (eV)
In2O35.274.502.82−0.632.18
BiVO46.034.502.350.362.71
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Zhu, Y.; Parekh, S.; Li, S.; Meng, X.; Zhang, Z. Enhancing the Removal Efficiency of Rhodamine B by Loading Pd onto In2O3/BiVO4 Under Visible Light Irradiation. Processes 2025, 13, 1983. https://doi.org/10.3390/pr13071983

AMA Style

Zhu Y, Parekh S, Li S, Meng X, Zhang Z. Enhancing the Removal Efficiency of Rhodamine B by Loading Pd onto In2O3/BiVO4 Under Visible Light Irradiation. Processes. 2025; 13(7):1983. https://doi.org/10.3390/pr13071983

Chicago/Turabian Style

Zhu, Yuanchen, Shivam Parekh, Shiqian Li, Xiangchao Meng, and Zisheng Zhang. 2025. "Enhancing the Removal Efficiency of Rhodamine B by Loading Pd onto In2O3/BiVO4 Under Visible Light Irradiation" Processes 13, no. 7: 1983. https://doi.org/10.3390/pr13071983

APA Style

Zhu, Y., Parekh, S., Li, S., Meng, X., & Zhang, Z. (2025). Enhancing the Removal Efficiency of Rhodamine B by Loading Pd onto In2O3/BiVO4 Under Visible Light Irradiation. Processes, 13(7), 1983. https://doi.org/10.3390/pr13071983

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