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Visible-Light-Driven CO2 Reduction into Methanol Utilizing Sol-Gel-Prepared CeO2-Coupled Bi2O3 Nanocomposite Heterojunctions

Mohamed Mokhtar Mohamed Mostafa
Ahmed Shawky
Sharif Fakhruz Zaman
Katabathini Narasimharao
Mohamed Abdel Salam
Abdulmohsen Ali Alshehri
Nezar H. Khdary
Sulaiman Al-Faifi
1 and
Abhishek Dutta Chowdhury
Chemistry Department, Faculty of Science, King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia
Nanomaterials and Nanotechnology Department, Advanced Materials Institute, Central Metallurgical R&D Institute (CMRDI), P.O. Box 87, Cairo 11421, Egypt
Chemical and Materials Engineering Department, King Abdulaziz University, Jeddah 21589, Saudi Arabia
King Abdulaziz City for Science and Technology, Riyadh 11442, Saudi Arabia
College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, China
Authors to whom correspondence should be addressed.
Catalysts 2022, 12(11), 1479;
Submission received: 11 September 2022 / Revised: 16 November 2022 / Accepted: 16 November 2022 / Published: 19 November 2022
(This article belongs to the Special Issue Nanoparticles in the Catalysis)


Carbon dioxide (CO2) photoreduction into renewable fuels over semiconductor photocatalysts has emerged as a green and sustainable alternative for energy production. Consequently, tremendous efforts are being performed to develop robust and sustainable photocatalysts. Therefore, visible-light active nanocomposite photocatalysts composed of 5.0–20.0 wt.% bismuth oxide (Bi2O3) and cerium oxide (CeO2) were synthesized by a sol-gel-based process. The prepared nanocomposites were evaluated for the promoted photocatalytic reduction of CO2 into methanol (CH3OH). Various characterizations of the obtained photocatalysts exposed an outstanding development of crystalline structure, morphology, and surface texture due to the presence of Bi2O3. Moreover, the absorbance of light in the visible regime was improved with enhanced charge separation, as revealed by the exploration of optical response, photoluminescence, and photocurrent measurements. The overall bandgap calculations revealed a reduction to 2.75 eV for 15% Bi2O3/CeO2 compared to 2.93 eV for pure CeO2. Moreover, the adjusted 2.8 g L−1 dose of 15% Bi2O3/CeO2 selectively produced 1300 μmol g−1 CH3OH after 9 h of visible light irradiation. This photocatalyst also exhibits bearable reusability five times. The improved progression of 15% Bi2O3/CeO2 is denoted by significant charge separation as well as enhanced mobility. This study suggests the application of metal oxide-based heterojunctions for renewable fuel production under visible light.

Graphical Abstract

1. Introduction

With the development of the industrial revolution in the last two centuries, several industries have evolved, such as cement [1], fertilizers [2], and aerosols [3]. In addition, the rapid growth of the population with not enough environmental awareness has led to the release of massive waste materials through several activities [4]. The release of greenhouse gases proportionally increased and contributed to the significant rise in the earth’s temperature, with a threat value close to 1.5 °C [5,6]. Thus, reducing, capturing, or recycling greenhouse gases is a matter of life or death for humanity. Carbon dioxide gas (CO2) is the primary source of global warming that is currently humped due to various human activities ranging from burning fossil fuels to the rapid development of several industries [7,8]. There are several approaches for reducing CO2 emissions through the use of renewable energy sources or recycling the released CO2 into value-added products [9,10,11,12,13]. The photoconversion of CO2 into valuable and renewable hydrocarbon fuels on semiconductor photocatalysts has caused substantial apprehension toward the clean invention of alternative energy resources [14,15,16,17]. The photoreduction of CO2 either in the gas or liquid phase is an ecological pathway owing to its simplicity and sustainability, particularly regarding conservational topics [18,19]. The catalytic conversion of CO2 into value-added products, such as methanol (CH3OH) or other organic materials, is an additional benefit to the global warming issue [17,20,21]. As a consequence, plentiful research is currently being performed to advance this approach by utilizing simulated and natural solar light [22,23,24]. In the meantime, abundant ceramic-based photocatalysts demonstrate effectual progress in several fields, such as pollutant degradation and fuel production [25,26,27,28,29]. Notably, TiO2 and ZnO represent famous oxides for fuel production and the photocatalytic reduction of CO2 owing to their suitable reduction potential balanced within their bandgap energies (Eg) [30,31,32,33]. However, the extended Eg, photostability in solutions, and the rapid recombination of the photoinduced charges are key issues in the realization of these oxides [34,35]. Therefore, there are several efforts regarding the coupling of these oxides with more stable ceramics to enhance their stability [31,36,37,38]. The use of other stable ceramic oxides with comparable Eg represents an alternative path [39,40].
Interestingly, the use of rare-earth metal oxides such as cerium oxide (CeO2) is promising in photocatalysis. In a recent study by Hezam et al., the nanostructured CeO2 with controlled oxygen vacancies could be efficient for the photoreduction of CO2 into CH3OH [41]. Moreover, CeO2 has narrower Eg (2.8~3.1 eV) than TiO2 and ZnO, exhibiting an n-type character and representing a more stable ceramic oxide with potential applications in various photocatalytic reactions, including water splitting [42], organic transformation reactions [43], and pollutant degradation [44]. Moreover, the coupling with p-type oxides such as bismuth oxides (Bi2O3) showed a significant photocatalytic performance in dye degradation [37,45], self-cleaning coatings [46], and NO oxidation [47]. In a recent study performed by Yang et al., the β-Bi2O3-loaded CeO2 heterostructure photocatalyst exhibited a significant redox for the oxidation of stable antibiotic molecules due to the enhanced charge separation of the added β-Bi2O3 in the heterojunction system [48]. Moreover, Masula et al. investigated the improved photocatalytic performance of Bi2O3/CeO2 nanocomposites for the mineralization of wastewater pollutants in a short time due to their enhanced visible-light absorbance [49]. However, there are no studies regarding the photoreduction of CO2 as far as we know. It could be of great scientific and industrial benefit regarding the ability of this nanocomposite in the renewable reforming of CO2 into valuable and usable fuels. In this regard, nanostructured CeO2 was simply prepared using the polymeric surfactant-assisted sol-gel method and subsequently coupled with 5.0~20.0 wt.% of Bi2O3. The prepared Bi2O3/CeO2 heterojunctions featured a mesoporous surface texture, amended light harvesting, and superior photogenerated charge separations as concluded from the detailed characterization. The photoreduction of CO2 into selective CH3OH production over the optimized 15% Bi2O3/CeO2 presented a generation rate of 144.4 μmol g−1 h−1 and demonstrated robust stability for regeneration.

2. Results and Discussion

2.1. Structural, Surface, and Optical Investigations of the Produced Photocatalysts

The formed materials were identified by XRD graphs as described in Figure 1 for the pure sample in a compared to the Bi3+-added samples in b, c, d, and e for 5, 10, 15, and 20 wt.%. The pure sample a was identically identified as pure CeO2 by the coincident 2θ values of 28.5°, 32.8°, 47.5°, 56.0°, and 59.1° to the (1 1 1), (2 0 0), (2 2 0), (3 1 1), and (2 2 2) planes of cubic CeO2 as described in JCPDS card No. 34–0394, respectively [50]. Similarly, the 5% Bi3+ precursor addition (represented by b) did not alter the overall pattern due to the limited detection of minor contamination of the XRD instrument. However, the further addition of the Bi3+ precursor in c~e resulted in the appearance of new diffraction peaks, mainly existing at 27.6°, 35.2°, 39.1°, 45.1°, and 45.4° assigned to (201), (200), (321), (222), and (421) of the β-phase of Bi2O3 (JCPDS No. 76–0147) [20]. Thus, the Bi2O3/CeO2 nanocomposite formation was revealed by XRD depiction [51].
TEM images describing the morphology of the pure CeO2 compared with various Bi2O3-added CeO2 nanostructures can be seen in Figure 2. The pure CeO2 indicated the formation of irregular-shaped nanocrystals with well-defined edges (Figure 2a). The particle size of the CeO2 nanocrystals ranged between 20 and 40 nm. The magnified elected area at a high resolution (Figure 2b) confirmed the face-centered cubic phase of CeO2 owing to the d-spacing measurement of 0.31 nm representing the (111) plane [46]. The Bi2O3-loading of CeO2 revealed certain changes to the structure and morphology as seen in Figure 2c–h. In Figure 2c, representing 5% Bi2O3/CeO2, small spherical nanoparticles were seen side-by-side with the CeO2 nanocrystals of <10 nm as represented by black–white arrows [51]. The high-resolution TEM image of the selected part of this revealed the formation of Bi2O3 nanocrystals due to the measured d-spacing (0.2 nm) of the (222) plane as shown in Figure 2d [48]. On the other hand, the higher content of Bi2O3 at 10 wt.% (Figure 2e) and 15 wt.% (Figure 2g) showed an elongation of its Bi2O3 structure assigned by the representative black–white arrows. The Bi2O3 was grown in a rod-like morphology but maintained the same crystalline phase as confirmed by the d-spacing measurements in the high-resolution images (Figure 2f,h) without any effect on the CeO2 nanocrystals. This refers to the fact that the solution-based β-Bi2O3 is usually grown in a rod-like structure [20,40,49]. These findings indicate the successful fabrication of the Bi2O3/CeO2 heterojunction. The close contact between the Bi2O3 and CeO2 could enhance the charge transfer amongst them and explain the efficient photocatalytic activity toward the production of CH3OH [48,49,51].
To clarify the surface texture of the obtained Bi2O3/CeO2 heterojunctions, the N2 adsorption–desorption isotherms comparing pure CeO2 and 15% Bi2O3/CeO2 photocatalysts are shown in Figure 3. Both samples exhibited minor adsorption at a relative pressure (P/P0) of 0.65 and an H2-hysteresis loop with type IV until P/P0 = 0.94 [45]. This finding designates that the prepared samples parade a mesoporous structure. The inclusion of Bi2O3 has no significant effect on the superficial structure but slightly changes the SBET and pore diameters (inset of Figure 3). The determined SBET is comparatively decreased by including Bi2O3 to CeO2 from 191 to 169 m2 g−1 (for 15% Bi2O3/CeO2, Table 1) owing to the spread of smaller-sized Bi2O3 on the surface of CeO2. However, the pore size is almost the same (8.92 nm for CeO2 and 8.87 nm for 15% Bi2O3/CeO2) as seen in the inset of Figure 3.
The surface composition of the 15% Bi2O3/CeO2 sample was analyzed via XPS as seen in Figure 4. The Bi 4f the high-resolution spectrum (Figure 4a) was deconvoluted into two oxidation states for the majority of Bi3+ at binding energies of 164.2 and 159.2 eV for 4f5/2 and 4f7/2, respectively, and a minority of Bi5+ at 165.0 and 160.5 eV for the corresponding mentioned orbitals [49]. The Ce 3d spectrum (Figure 4b) was deconvoluted into the co-existing minority Ce3+ and the majority of Ce 4+ [44]. The Ce3+ is located at 902.1 and 886.5 for 3d3/2 and 3d5/2 orbitals [46]. On the other hand, the major peaks located at 917.2, 908.2, and 898.7 eV are attributed to the 3d3/2 binding of Ce4+ while the binding energies positioned at 890.2 and 883.1 eV correspond to the 3d5/2 of Ce4+ in CeO2. In Figure 4c, the O1s spectrum is deconvoluted into three peaks at 532.4, 530.1, and 529.0 eV assigned to the adsorbed oxygen species, the lattice bonding Bi–O of β-Bi2O3, and the lattice bonding Ce–O in CeO2, respectively [47]. Thus, the confirmation of the formed Bi2O3/CeO2 heterojunction is established by the XPS study. The quantitative analysis using the instrumental software showed the content of Bi2O3 in this sample at 13.9 wt.%.
The light harvesting by the synthesized photocatalysts was also investigated as displayed in Figure 5. The pristine CeO2 disclosed a minor visible-light absorbance with an absorption edge at 423 nm and an estimated Eg of 2.93 eV [43]. However, the synthesized Bi2O3/CeO2 showed an extension of the visible-light harvesting as the absorption edges shifted toward longer wavelengths due to the low-bandgap Bi2O3 [48]. The presence of 5, 10, 15, and 20 wt.% Bi2O3 in CeO2 redshifted the absorption edges to 427, 442, 453, and 454 nm, with corresponding Eg values of 2.89, 2.82, 2.75, and 2.74 eV, respectively (Table 1, inset of Figure 5). Thus, the presence of Bi2O3 could enhance light harvesting and associated photocatalytic performance [37,49].

2.2. Photoreduction of CO2 over Bi2O3/CeO2 Heterojunctions

The photocatalytic progression of the produced Bi2O3/CeO2 is evaluated for the reduction of CO2 as described in Figure 6 compared to the pure CeO2. In Figure 6a, the photocatalytic generation of CH3OH of Bi2O3/CeO2 is much higher than the pure CeO2 after 9 h of light irradiation as seen in Table 1. The production rate of CH3OH reached 120.0 and 122.2 μmol g−1 h−1 by 15 and 20% Bi2O3/CeO2, respectively, which is almost 15.4 and 15.7 times higher than the pure CeO2 at the same 2.0 gL−1 dose (7.8 μmol g−1 h−1, Figure 6b). As a result, the photoreduction of CO2 into CH3OH can be surged by governing the amount of Bi2O3. The 15 wt.% of Bi2O3 is chosen as the optimal included amount in the Bi2O3/CeO2 heterojunction based on the above results.
Photoluminescence (PL) spectral emission and photocurrent intensity are regarded as powerful means for understanding the photocatalytic activity promotion of the 15% Bi2O3/CeO2 as seen in Figure 7 [41,52,53]. In Figure 7a, we can see that the pure CeO2 exhibited an intensive PL peak positioned at 406.3 nm due to the large recombination of the photogenerated charges [44]. The introduction of 5, 10, 15, and 20 wt.% Bi2O3 amounts clearly quenched the PL intensity by 12, 21, 32, and 32.1%, respectively. These results indicate the suppression of the charge carrier’s recombination rates. So, the catalytic activity could be increased by the long-life charge carriers as proved in previous PL quenching due to the precise Bi2O3 occurrence in the 15% Bi2O3/CeO2 heterojunction. Moreover, transient photocurrent measurements have also established the best photocatalytic performance of the same 15% Bi2O3/CeO2 nanocomposite as presented in Figure 7b and Table 1. The 15% Bi2O3/CeO2 exhibited the largest photocurrent value (51.54 µA cm−2), which is almost 4.5 times higher than the photocurrent generated from pure CeO2. This finding reveals the promoted charge mobility due to the superior separation of the photoinduced charge carriers. Accordingly, the photoreduction rate of CO2 could be enhanced using this optimal photocatalyst.
The dose tuning plays a key role in the photoactivity of nanocomposite photocatalysts due to the upsurging of photoactive sites that are balanced with the adsorbed target molecules and light photons [54,55,56,57,58]. Figure 8a displays the impact of dose alteration (0.7~3.5 g L−1) of 15% Bi2O3/CeO2 on the photocatalytic generation of CH3OH. The dose-dependent generation of CH3OH parades the comparative upsurge from 608 (±19.5) to a maximum of 1300 (±41.6) µM g−1 for 0.7 and 2.8 g L−1, correspondingly, after 9 h of visible-light illumination. This outcome exhibits the improvement of the photocatalytic active spots bared to CO2 molecules in the case of 2.8 g L−1 compared to other doses. Nevertheless, the extra dose of the 15% Bi2O3/CeO2 at 3.5 g L−1 decreased the CH3OH generation to 1230 (±39.4) μmol g−1. Accordingly, the dosage accumulation could moderately impede the light photons from reaching the photocatalyst’s surface, resulting in a reduction of the overall activity [59,60]. Thus, 2.8 g L−1 is the optimized dose for the 15% Bi2O3/CeO2 that exhibits the highest generation rate of CH3OH as described in Figure 8b and proven by chromatograms in Figure S2 (supplementary materials). The spent 15% Bi2O3/CeO2 photocatalyst was also tested for recyclability as presented in Figure 8c. The outcomes showed that the optimal 15% Bi2O3/CeO2 validates five-time reusability for the CO2 reduction under visible light to CH3OH at 1274 μmol g−1, which is 98% of its initial value in the first run. The XRD and TEM investigations for the spent material did not show any significant structural changes (Figure S3, supplemental data). Therefore, the 15% Bi2O3/CeO2 exhibits a workable photocatalytic application for CO2 reduction beneath visible light.
Finally, the photoreduction pathway could be proposed as seen in Figure 9 based on previous discussions. The represented band diagrams of p-type Bi2O3 and n-type CeO2 can be calculated by the following formula (supplementary materials S-4):
ECB = χ − Ee − 0.5Eg
EVB = ECB + Eg
where EVB and ECB are the valence (VB) and conduction (CB) band potentials versus the standard hydrogen electrode (NHE), respectively. Ee is the energy of free electrons and χ is the electronegativity of the semiconductor photocatalyst, which is determined in supplementary materials [61,62,63]. Thus, upon illumination of both semiconductors by visible light, the photoinduced charge carriers, namely electrons (es), are generated at −0.405 and +0.535 eV corresponding to the CB of Bi2O3 and CeO2, respectively, while the holes (h+s) were located at +2.525 and +2.93, corresponding to VB of Bi2O3 and CeO2, respectively. Since the p-n heterojunction is formed between p-type Bi2O3 and n-type CeO2, an electric field (signed as E) is designed at the interface of the formed heterojunction and band-bending occurs due to the unified Fermi levels [64,65,66]. Thus, the excited es at the CB of p-type Bi2O3 could recombine with the h+s left at the VB of n-type CeO2 by the driving electric field effect [67]. On the other hand, the high redox potential es on the CB of CeO2 and h+s on the VB of Bi2O3 can contribute to the photoreduction of CO2 as follows: The H2 molecules could react with the h+s resulting in the splitting reaction to protons, oxygen, and free es as shown in Equation (3) [68,69].
3 H 2 O + h VB + ( Bi 2 O 3 ) 6 H + + 3 2 O 2 + 6 e
At last, at the CB of CeO2, which is very close to the reduction potential of CO2 into CH3OH [68,70], the produced protons and es in (3) can selectively convert the CO2 to CH3OH as in (4)
6 e + 6 H + + CO 2   ( CB   of   CeO 2 ) CH 3 OH + H 2 O
Therefore, the improved, recyclable, and efficient 15% Bi2O3/CeO2 heterojunctions reveal the promoted photoconversion of CO2 to an alternative renewable fuel under visible-light illumination.

3. Experiments

3.1. Chemicals

All materials were purchased from Merck in this work without subsequent treatment. The Ce and Bi precursors were Ce(NO3)3·6H2O (99.999% trace metals basis) and Bi(NO3)3·5H2O (99.999% trace metals basis). The polymeric non-ionic surfactant, namely Poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (Pluronic F-108, MW~14,600), was utilized as the growth-limiting reagent and micelle template [71]. Acetic (CH3COOH, ≥99.7%) and hydrochloric (HCl, ACS reagent, 37%) acids were used in the sol-gel process for hydrolysis and esterification procedures. Ethanol absolute (CH3CH2OH, ≥99.8%) and deionized water (DI-H2O, >18.2 MΩ) were also used for hydrolysis and washing purposes. Sodium carbonate (Na2CO3, ≥99.5%, ACS reagent) was used as the CO2 source in the photoreaction system.

3.2. Synthesis of CeO2 and Bi2O3/CeO2 Photocatalysts

Nanocrystalline CeO2 particles were synthesized by a modified sol-gel method in the presence of Pluronic F-108. In detail, 2.0 g of F-108 was disseminated in 40 mL of CH3CH2OH under vigorous stirring. After that, 12 mmol of HCl and 40 mmol CH3COOH were progressively poured under the same conditions and then left for 1h. Then, 1.63 g of Ce(NO3)3·6H2O was included in the above viscous solution and kept for another 2 h. The cloudy suspended mixture was then transferred into a clean Petri dish for 12 h of polymerization in a humidity chamber operated at 40 °C before drying at 65 °C for another 24 h. The obtained hard gel was crystallized for 5 h at 650 °C to obtain mesostructured CeO2 nanoparticles. The Bi2O3/CeO2 heterojunctions were prepared by impregnating the Bi2O3 into the CeO2 matrix. In brief, 1.0 g of synthesized CeO2 was sonicated in 100 mL of CH3CH2OH for 10 min. Next, 52 mg of Bi(NO3)3·5H2O was familiarized into the solution while stirring and maintained for 1 h. Then, CH3CH2OH was evaporated by increasing the temperature to 80 °C. After approximately 5 h, the calcination of the dried powder was performed at 600 °C for 3 h to crystallize the 5% Bi2O3/CeO2 heterojunction. The above procedure was repeated by increasing the Bi3+ precursor amount to 104, 156, and 208 mg to obtain 10, 15, and 20% Bi2O3/CeO2 nanocomposites.

3.3. Depictive Characterization of the Obtained CeO2 and Bi2O3/CeO2

Numerous instruments have been employed to characterize the produced sample. The Bruker AXS D8 X-ray diffraction (XRD, Billerica, MA, USA) unit explored the crystalline phases of the ground powders. The surface composition of the 15% Bi2O3/CeO2 was investigated by the Thermo VG Multilab 2000 X-ray photoelectron spectrometer (XPS, East Grinstead, UK). Regarding the morphology study, a high-resolving-power JEOL-JEM-1230 transmission electron microscope (TEM, Tokyo, Japan) examined the samples dispersed on carbon-coated grids. The Nova Quantachrome analyzer generated N2 adsorption–desorption (Ostfildern-Scharnhausen, Germany) isotherms to describe the surface texture of the degassed samples at 150 °C. The specific surface area (SBET) was calculated based on the Brunauer–Emmett–Teller (BET) formula, which estimates the specific surface area of the prepared structures. The light harvesting of CeO2 and Bi2O3/CeO2 was studied via a Jasco spectrophotometer (V-570, Tokyo, Japan) in the 200–800 nm spectral range. The bandgap energy (Eg) was determined based on the modified Tauc formula through their diffusive reflectance. Photoluminescence emission (PL) was investigated by an RF-5301 fluorescence spectrometer (Shimadzu, Kyoto, Tapan) in ambience. Lastly, the electrochemical workstation (Zahner Zennium, Ostfildern-Scharnhausen, Germany) measured the transient photocurrents of the 0.3 × 0.3 cm2 working electrodes composed of each sample on Ti foil at zero bias during the ON and OFF states of 1.0 mW cm−2 light illumination.

3.4. Setup for Photoreduction of CO2

The photoreduction experiments were performed in a homemade photoreactor containing a 250 mL two-neck quartz flask. The simulated visible light was provided by a 300 W Xe lamp (Newport, CA, USA, 45.2 mW cm−2) equipped with a 420 nm cut-off filter. In detail, a certain dose of the synthesized photocatalyst was dispersed in 125 mL of H2O before being heated to 80 °C. Then, 135 mg of Na2CO3 was inserted into the above suspension. Moreover, nitrogen gas was initially introduced for 30 min to ensure an inert atmosphere at 100 sccm. After that, 0.315 mL of 4 M HCl was gently injected to generate CO2. The whole system was kept in murky conditions for 1 h to settle the equilibrium. The photoreactor was then irradiated by visible light. At fixed one-hour intervals, a gas chromatograph (GC-FID, Shimadzu) equipped with a flame ionization detector was employed to sense the formed products through the photoreaction [68]. The produced spectra generated by the detection unit were compared with the standard GC spectra of 2.5 mM L−1 CH3OH solution (supplementary Figure S1) [21]. The produced CH3OH was recognized in the above-mentioned spectra under the same experimental conditions using the GC. Generally, neither CH3OH nor other products are detected before light illumination. The reusability of the spent photocatalyst for five independent cycles was performed via simple drying under ambient conditions for 1 h at 120 °C.

4. Conclusions

A facile sol-gel-based method was applied for the fabrication of the Bi2O3/CeO2 heterojunction photocatalyst. The precise addition of p-type Bi2O3 amended the absorbance in the visible light regime through the reduction of the overall bandgap of the produced Bi2O3/CeO2 heterojunction down to 2.74 eV. The various Bi2O3-included CeO2 were applied for the selective photocatalytic reduction of CO2 into methanol beneath visible light. The 2.8 g L−1 dose of 15% Bi2O3/CeO2 generated 1300 μmol/g of CH3OH within 9 h of illumination, nearly 18.6-fold higher than pure CeO2. This optimized heterojunction verified the five-time reusability at 98% of its initial performance. The substantial photoactivity of Bi2O3/CeO2 is attributed to the significant photogenerated charge separation in addition to the amended harvesting of light. This work provides plenty of room for the future of renewable fuel production from CO2 utilizing metal oxide-based heterostructure photocatalysts.

Supplementary Materials

The following supporting information can be downloaded at:, Figure S1: Gas chromatograph of standard 2.5 mM/L CH3OH solution showing a single peak at 2.46 min retention. The spectra is used as a reference to compare the selective photoreduction of CO2 over the prepared Bi2O3/CeO2 nanocomposites in the main manuscript; Figure S2: Gas chromatograph of the produced product over the optimized dose 2.8 g/L of 15% Bi2O3/CeO2 showing the accumulated production of CH3OH over time of irradiation; Figure S3: XRD patterns (a) and TEM images (b) of the spent nanocomposite after the first and the fifth runs displaying no substantial change in the crystalline phase or morphological structure after recycling for five times.

Author Contributions

Conceptualization, M.M.M.M., A.S and A.D.C.; methodology, M.A.S. and A.A.A.; software, M.M.M.M., A.S., S.F.Z. and S.A.-F.; validation, N.H.K.; formal analysis, S.F.Z., A.S. and K.N.; investigation, A.S., K.N. and A.A.A.; resources, S.A.-F.; data curation, A.S., M.A.S. and A.D.C.; writing—original draft, A.S. and A.D.C.; writing—review and editing, M.M.M.M. and A.S.; visualization, N.H.K.; supervision, A.S.; project administration, M.M.M.M.; funding acquisition, M.M.M.M. All authors have read and agreed to the published version of the manuscript.


This research was funded by Deanship of Scientific Research (DSR), King Abdulaziz University, Jeddah, Saudi Arabia (NO. IFPRC-108-130-2020).

Data Availability Statement

Data is contained within the article or supplementary material.


The authors extend their appreciation to the Deputyship for Research & Innovation, Ministry of Education in Saudi Arabia for funding this research work through project number IFPRC-108-130-2020 and the Deanship of Scientific Research (DSR), King Abdulaziz University, Jeddah, Saudi Arabia.

Conflicts of Interest

The authors declare no conflict of interest.


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Figure 1. XRD patterns of the produced powders representing the crystallization of pure CeO2 (as labeled in a compared with 5, 10, 15, and 20% Bi2O3/CeO2 as represented by b, c, d, and e, respectively.
Figure 1. XRD patterns of the produced powders representing the crystallization of pure CeO2 (as labeled in a compared with 5, 10, 15, and 20% Bi2O3/CeO2 as represented by b, c, d, and e, respectively.
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Figure 2. TEM investigations (up) and the corresponding high-resolution imaging for the selected area (down) of pure CeO2 (a,b), 5% Bi2O3/CeO2 (c,d), 10% Bi2O3/CeO2 (e,f), and 15% Bi2O3/CeO2 (g,h).
Figure 2. TEM investigations (up) and the corresponding high-resolution imaging for the selected area (down) of pure CeO2 (a,b), 5% Bi2O3/CeO2 (c,d), 10% Bi2O3/CeO2 (e,f), and 15% Bi2O3/CeO2 (g,h).
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Figure 3. N2 adsorption/desorption isotherms and corresponding pore size distribution (inset) for prepared CeO2 compared to 15% Bi2O3/CeO2 heterojunctions as indicated.
Figure 3. N2 adsorption/desorption isotherms and corresponding pore size distribution (inset) for prepared CeO2 compared to 15% Bi2O3/CeO2 heterojunctions as indicated.
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Figure 4. XPS analysis at high resolution for Bi 4f (a), Ce 3d (b), and O 1s (c) of the 15% Bi2O3/CeO2 heterojunction.
Figure 4. XPS analysis at high resolution for Bi 4f (a), Ce 3d (b), and O 1s (c) of the 15% Bi2O3/CeO2 heterojunction.
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Figure 5. Light absorption spectra of the prepared CeO2 compared with Bi2O3/CeO2 heterojunctions and their corresponding calculated energy gaps as seen in the inset.
Figure 5. Light absorption spectra of the prepared CeO2 compared with Bi2O3/CeO2 heterojunctions and their corresponding calculated energy gaps as seen in the inset.
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Figure 6. Photocatalytic production of CH3OH utilizing the produced CeO2 and Bi2O3/CeO2 heterojunctions as in (a). The influence of adding Bi2O3 on the generation rate of produced CH3OH is illustrated in (b).
Figure 6. Photocatalytic production of CH3OH utilizing the produced CeO2 and Bi2O3/CeO2 heterojunctions as in (a). The influence of adding Bi2O3 on the generation rate of produced CH3OH is illustrated in (b).
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Figure 7. PL spectra (a) and transient photocurrents (b) of pure CeO2 and Bi2O3/CeO2 heterojunctions as stated.
Figure 7. PL spectra (a) and transient photocurrents (b) of pure CeO2 and Bi2O3/CeO2 heterojunctions as stated.
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Figure 8. Photocatalytic production of CH3OH over different doses (in g L−1) of 15% Bi2O3/CeO2 heterojunctions as in (a). The influence of dosage on the generation rate of produced CH3OH is illustrated in (b). The reusability of the best-performing spent photocatalyst dose (2.8 g L−1) is illustrated in (c).
Figure 8. Photocatalytic production of CH3OH over different doses (in g L−1) of 15% Bi2O3/CeO2 heterojunctions as in (a). The influence of dosage on the generation rate of produced CH3OH is illustrated in (b). The reusability of the best-performing spent photocatalyst dose (2.8 g L−1) is illustrated in (c).
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Figure 9. Estimated band diagrams, charge transfer, and photocatalytic pathway for CO2 conversion into methanol over optimized Bi2O3/CeO2 heterojunctions.
Figure 9. Estimated band diagrams, charge transfer, and photocatalytic pathway for CO2 conversion into methanol over optimized Bi2O3/CeO2 heterojunctions.
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Table 1. Consequence of Bi2O3 inclusion on physicochemical characteristics of CeO2, namely, surface area (SBET), absorption edge (Abs. edge), and determined bandgap energies (Eg). The consistent photoreduction of CO2 to CH3OH after 9 h irradiation is listed. The corresponding photocurrent intensities are also stated in the last column.
Table 1. Consequence of Bi2O3 inclusion on physicochemical characteristics of CeO2, namely, surface area (SBET), absorption edge (Abs. edge), and determined bandgap energies (Eg). The consistent photoreduction of CO2 to CH3OH after 9 h irradiation is listed. The corresponding photocurrent intensities are also stated in the last column.
(m2 g−1)
Abs. Edge
CH3OH Generation
(μm g−1)
(µA cm−2)
CeO21914232.9370 (±2.8)11.56
5% Bi2O3/CeO21834272.89360 (±12.6)28.92
10% Bi2O3/CeO21744422.82720 (±21.6)43.52
15% Bi2O3/CeO21694532.751080 (±24.5)51.54
20% Bi2O3/CeO21634542.741100 (±38.4)51.81
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Mostafa, M.M.M.; Shawky, A.; Zaman, S.F.; Narasimharao, K.; Abdel Salam, M.; Alshehri, A.A.; Khdary, N.H.; Al-Faifi, S.; Chowdhury, A.D. Visible-Light-Driven CO2 Reduction into Methanol Utilizing Sol-Gel-Prepared CeO2-Coupled Bi2O3 Nanocomposite Heterojunctions. Catalysts 2022, 12, 1479.

AMA Style

Mostafa MMM, Shawky A, Zaman SF, Narasimharao K, Abdel Salam M, Alshehri AA, Khdary NH, Al-Faifi S, Chowdhury AD. Visible-Light-Driven CO2 Reduction into Methanol Utilizing Sol-Gel-Prepared CeO2-Coupled Bi2O3 Nanocomposite Heterojunctions. Catalysts. 2022; 12(11):1479.

Chicago/Turabian Style

Mostafa, Mohamed Mokhtar Mohamed, Ahmed Shawky, Sharif Fakhruz Zaman, Katabathini Narasimharao, Mohamed Abdel Salam, Abdulmohsen Ali Alshehri, Nezar H. Khdary, Sulaiman Al-Faifi, and Abhishek Dutta Chowdhury. 2022. "Visible-Light-Driven CO2 Reduction into Methanol Utilizing Sol-Gel-Prepared CeO2-Coupled Bi2O3 Nanocomposite Heterojunctions" Catalysts 12, no. 11: 1479.

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