A Mechanistic Approach on Oxygen Vacancy-Engineered CeO2 Nanosheets Concocts over an Oyster Shell Manifesting Robust Photocatalytic Activity toward Water Oxidation.

Lethargic kinetics is the foremost bottleneck of the photocatalytic water oxidation reaction. Hence, in this respect, the CeO2 coral reef made up of nanosheets is studied focusing on the oxygen vacancy that affects the water oxidation reaction. First, CeO2 was prepared in an oyster shell/crucible with the presence/absence of urea by a simple calcination technique to tune the oxygen vacancy. More oxygen vacancy was detected in CeO2 prepared from urea and oyster shell, which is evidenced from Raman and PL analyses. Further, the oyster shell-treated sample was found to be of nanosheet type with numerous pores as observed via TEM analysis. The theoretical approach was adopted to expose the role of oxygen vacancies and the fate of scavenging agents in the water oxidation mechanism. It was observed that an oxygen vacancy plays a vital role in minimizing the activation energy hump and opposes the reverse reaction. The apparent conversion efficiency of 7.1% is calculated for the oxygen evolution reaction. Oxygen vacancy, quantum confinement effect, and charge separation efficiency are mainly responsible for the better photocatalyzed water oxidation reaction and hydroxyl radical production. This investigation will help in providing valuable information toward designing cost-effective oxygen vacancy-oriented nanosheet systems and the importance of vacancy in the water-splitting reaction.

Introduction

The quest for economically and environmentally encouraging green energy production to surrogate the stereotype non-renewable and environmental catastrophic fossil fuel is the new challenging task faced by the scientific community. In this prospect, numerous techniques were projected, but conversion of solar energy to chemical energy via artificial photon-assisted heterogeneous catalysis driven by semiconductors is the most promising and interesting aspect from an environmental sustainability point of view.[1−5] Basically, semiconductor-propelled photocatalytic water splitting (H2 and O2) is considered as the most innovative and revolutionary technique toward resolving energy deficit without disturbing the ecological balance as these catalytic systems tend to tunnel the greener renewable energy (solar) into the chemical form.[6] Generally, complete water splitting is a very complicated and onerous task from a kinetic and thermodynamic point of view.[7] This is because water reduction to H2 goes via a two-electron mechanism, while oxidation leading to oxygen production requires four electrons and high activation energy, which make the conversion of H2O to O2 kinetically torpid.[8−12] Specifically, for this chemist’s nightmare, that is, water dehydrogenation/oxidation reaction, various approaches were made such as electrocatalytic, chemical, and photocatalytic water oxidation.[1] However, photon-directed metal oxide compounds corroborate to be the most propitious strategy for O2 evolution because the adopted method is green, and additionally, the preparation process of corrosion-resistant oxide materials is quite facile.[1−17] Therefore, there is an urgency to develop a novel and competent oxide-based catalyst for promoting robust oxygen evolution reactions.[18] So far, in this journey, back-bending efforts have been made, and many photocatalytic systems were examined like WO3, TiO2, Fe2O3, BiVO4, g-C3N4/Ag3PO4, and WO3/BiVO4, still the desired target is not achieved, that is, optimum conversion efficiency as these catalysts suffer faster electron–hole recombination and low active surface availability.[6,19−34] Therefore, it was concluded that the catalytic activity of these photocatalysts largely revolved around the morphology, that is, 0D, 1D, 2D, 3D, etc., and vacancy/anisotropic surface chemistry, which greatly influence the photo-oxidation process.[20,35] Especially, a morphology-oriented material equipped with an oxygen vacancy (OV) and porous framework exhibits rousing possibilities in the field of artificial photocatalysis due to a large exposed active site, more amount of surface atoms for better anchoring of substrate, light scattering feature, reactant activation, and above all trapping states resulting in effective charge separation.[35−38]

As the most abundantly found rare earth metal oxide, fluorite cerium oxide (CeO2) is considered to be a novel and assessed catalyst in many heterogeneous catalytic reactions including in the field of photocatalysis. Additionally, the strong oxidative nature of valence band, facile structural tailoring, non-toxicity, the potential to interchange its oxidation states (Ce4+ ↔ Ce3+) during the reaction process, high corrosion resistance, and oxygen storage ability are some of the adventitious properties that make CeO2 as the next-generation photocatalyst.[35] Further, as an n-type material with a more positive valence band position, CeO2 is believed to be the best competitor in a photocatalytic water oxidation reaction; however, due to the fast exciton recombination mechanism and a low percentage of active sites, photocatalytic O2 evolution is highly constricted.[39] To nullify these bottlenecks, various methods were implemented, but defect-oriented and dimension-controlled (1D or 2D) CeO2 is technically worthy for the water oxidation reaction under photon irradiation.[35,40] Further, oxygen vacancy in CeO2 is more emphasized as it regulates the inter-conversion of Ce4+ and Ce3+, and the higher ratio of Ce3+ leads to an increase in the catalytic capabilities of CeO2-based materials.[35,40] Adding more to the survey, 2D CeO2 with a surface vacancy and porous skeleton is quite superior and plays a crucial part in a photon-driven heterogeneous surface catalysis reaction as these defect states act as trapping points and resulting in a delay of exciton recombination process and also help in binding of substrates for effective catalysis.[35] In this context, our group has previously reported CeO2 nanosheets by a reflux method using SDS and hexamethylenetetramine toward water oxidation and pollutant degradation.[41] Further, several fascinating works exist on oxygen vacancy-based CeO2 systems toward photocatalytic energy production and energy storage, for example, Lavorato et al. synthesized a novel ceria/graphene photocatalyst for oxygen evolution.[42] Similarly, Marino and co-workers developed nanosized Au/TiO2 and Au/CeO2 for a total water-splitting reaction.[43] In another case, Wang and colleagues have designed CeO2– with OVs toward enhanced photocatalytic CO2 reduction.[44] Likewise, CeO2 nanocubes suffering oxygen defect have been constructed by Wang et al. for o-xylene oxidation.[45] Additionally, vacancy-oriented CeO2 was also tested for energy storage by Mofarah and researchers.[46] Above all, neat 2D porous CeO2 having oxygen defects within the lattice is less explored in this specified field of chemistry. Interestingly, the preparation of porous CeO2 sheets via a biomediated route is a novel and economically feasible one.

In the performed research, endeavor was made toward fabrication of OV-engineered CeO2 nanosheets with a porous framework over an oyster shell via a calcination method without any surface-directing chemicals, which are first of its kind to the best of authors’ knowledge. It was observed that the oyster shell- and urea-treated sample shows large spherical and elongated structures with uniform pore distribution as observed from TEM analysis. Further, effective charge separation leading to a high photocurrent density and activation of reactants favoring enhanced O2 evolution in the absence of co-catalyst is credited to the presence of surface defect, that is, Ce3+ and oxygen vacancy in the developed nanocatalyst (CeO2 nanosheet). The detailed role of oxygen vacancy and sacrificial agent toward O2 evolution is scripted in the manuscript. This investigation will be highly beneficial in designing vacancy-oriented photocatalysts via an economically viable biomediated pathway (oyster shell) to address green energy production and environmental sustainability.

Chemicals Used

Ce(NO3)·6H2O, urea, and acetone of high purity were bought from Sigma-Aldrich and hence used as such without further purification.

Photocatalyst Preparation Procedure

At first, the collected oyster shells were thoroughly washed with distilled water and then dipped in acetone in a beaker in closed fashion overnight. On the next day, the shells were properly cleaned with tissue paper and a dryer. A weighted amount of Ce(NO3)3·6H2O was placed carefully in the caved part of the oyster shell, which was previously rubbed with a sharp-pointed tool. Then, it was subjected to calcination at 700 °C in a muffle furnace for 7 h. After that, the sample was collected and grinded via a mortar and pestle and named as CO (CeO2 + oyster shell). Similarly, one more sample is prepared, which, along with Ce precursor urea, was mixed and acts as a fuel at the time of calcination, and the same procedure is followed as described above, and the obtained material is named as CUO (CeO2 + urea + oyster shell). Two more photocatalysts were synthesized in the same reaction condition in the same way, but this time, in place of the oyster shell, a silica crucible is used as the container, and the so-formed samples were designated as CC (CeO2 + crucible) and CUC (CeO2 + urea + crucible).

Oxygen Evolution

The photocatalytic potential of as-prepared materials was evaluated toward water oxidation reaction, that is, O2 generation in a quartz batch photoreactor in an oxygen-free atmosphere fitted with a chiller to maintain the reaction temperature. In the performed oxidation experiment, 2 mg of the photocatalyst was mixed with 20 mL of 0.05 M AgNO3 aqueous solution and subjected to stirring in order to prevent the process of catalyst agglomeration at the base of the reactor. Prior to light irradiation, the above suspension was purged with N2 gas (45 min) to remove all dissolved O2 so that the evolved oxygen will come only from the water-splitting process. Then, the suspension was illuminated with a 125 W (0.027 W cm–2) high-pressure mercury lamp as a UV light source. By using a downward displacement method, produced O2 was noticed and finally quantized and analyzed via an Agilent 7890b-series connected with a molecular sieve column (5 Å) and a conductive detector (TCD). The same reaction procedure is followed for other catalysts for O2 generation. Further, the apparent conversion efficiency (ACE) of O2 production under a Hg light source was calculated using eq , and more details are described in the Supporting Information:

Hydroxyl Radical Production

In the performed ·OH radical generation experiment, 20 mg of the synthesized photocatalyst (CUO) was suspended in the required amount of distilled water (20 mL), which in turn is composed of a measured quantity of 0.01 M NaOH and 3 mM terephthalate acid (TPA). Prior to the start of light irradiation, the above-mixed solution was kept in the dark under the slow stirring condition for nearly 45 min. Thereafter, the suspension was subjected to light illumination for about 2 h (the same light source and setup used for O2 evolution). Then, this light-exposed solution was centrifuged and filtered to remove the catalyst and then analyzed using a spectrofluorometer. The so-formed TPA-OH (2-hydroxyterephthalic acid) fluorescence complex was observed about 426 nm when excitation was carried out at around 330 nm.[47] All the characterization techniques used are compiled in Table .

Table 1

Characterization Details

sl. no.	instrument	description	model	company name
1	X-ray diffraction (XRD)	Cu Kα radiation source (λ = 0.154 nm), scanning window 2θ = 20–80° and 40 kV and 40 mA	Rigaku-Ultima IV	Rigaku
2	UV–Visible (UV–Vis) diffuse reflectance spectroscopy (DRS)	deuterium UV lamp and Xe visible light, BaSO4 as a standard	JASCO V-750	JASCO
3	Fourier transform infrared spectrometer (FTIR)	KBr pellet as a reference	JASCO FT/IR 4600	JASCO
4	photoluminescence Spectrofluorometer (PL)	deuterium UV lamp and Xe visible light	JASCO FP-8300 spectrofluorometer	JASCO
5	Raman spectrometer	332 nm laser	RENISHAW InVia Raman spectrometer	RENISHAW
6	field emission scanning electron microscopy (FESEM)	sample dispersed in ethanol and deposited over Al foil and Au coated	Carl Zeiss, Neon 40 instrument	ZEISS
7	electrochemical analyzer	three-electrode-based systems (Pt as the counter electrode, Ag/AgCl as the reference electrode, and sample-coated FTO as the working electrode), 0.1 M NaSO4 electrolyte	IVIUMnSTAT Multichannel electrochemical analyzer	IVIUM Technology
8	transmission electron microscopy (TEM)	acceleration voltage of 200 kV	TEM, JEOL-2100	JEOL

Results and Discussion

PXRD

Powder X-ray diffraction (PXRD) analysis of as-prepared photocatalysts was performed to verify the details of the crystallographic structure, that is, crystal orientation, phase purity, lattice strain, average crystallite size, lattice constant, and dislocation density. Figure shows the obtained PXRD results, which signify high crystallinity and phase purity without the presence of any adulterant phase, for example, Ce(OH)3 or Ce2O3. Additionally, for all samples, Bragg’s reflection was observed at 2θ = 28.7°, 33.1°, 47.8°, 56.4°, 59.2°, 69.5°, 76.7°, and 79.1°, which were indexed to the characteristic (111), (200), (220), (311), (222), (400), (331), and (420) crystal planes of CeO2, respectively.[40,48] On the other hand, a broad range of PXRD scan of all synthesized materials reveals the same type of diffractogram with a little bit of variation in the peak position and intensity. Adding more to this, the obtained pattern synchronizes well with JCPD file no. 34-0394, which confirms cubic fluorite CeO2 with Fm3̅m type space group.[48] Further, the average crystallite size of each sample was calculated by using the Williamson–Hall method.[49,50] Moreover, the calculated crystallite size, FWHM, lattice constant, dislocation density, and lattice strain of individual photocatalyst are depicted in Table .

Figure 1

XRD patterns of CeO2 prepared via oyster shells and crucible.

Table 2

Quantitative Information Derived from XRD Analysis

catalysts	average crystallite size (D) (nm) (W–H method)	FWHM	lattice constant (Å) (a = b = c)	lattice strain (ε)	dislocation density (δ)
CC	4.6189	0.412	3.0554	0.09108	0.002313
CO	4.0008	0.419	3.0599	0.05618	0.002393
CUC	8.1214	0.222	3.0559	0.01504	0.0006715
CUO	7.73	0.202	3.0581	0.01045	0.00055598

Further, based on the reported data, that is, the exciton Bohr radius of CeO2, which lies between 7 and 8 nm, we can claim that our prepared nanoparticles are quantum dots as their size lies within the quantum confinement zone. This is also well supported by UV–DRS and PL characterization. Again, it can be seen in the plot that urea-treated samples have an intense XRD peak, which implies that urea supports crystal nucleation and hence better crystal growth. The entire above claim shows good correlation with the reported literature.[51]

FTIR Spectrum

FTIR analysis of as-synthesized samples was carried out at room temperature to know the detailed textural structure, that is, different stretching and bending vibration modes associated with characteristic functional groups.

Figure demonstrates the FTIR spectrum of all prepared photocatalysts scanned within the wavenumber window of 400–4000 cm–1. All the samples display almost the same type of IR spectrum with just a little change in peak intensity and occurrence of the new band, which is due to the difference in the preparation method and reagent. In brief, IR bands seen at 510 cm–1 could be ascribed to Ce–O stretching vibration,[52] whereas those located around 870 and 1058 cm–1 represents the stretching and bending vibration of intercalated C–O species present in the precursor.[53] Further, the vibrational band appearing at 1350 cm–1 is due to the N–O stretching band.[54] This confirms the presence of N atom in the lattice of the CeO2 crystal. Again, the IR peak positioned at 1610 cm–1 is basically associated with the molecular H2O (H–O–H) bending frequency. Moreover, a broad IR band within the range of 3400–3500 cm–1 is because of the O–H group, which shows the stretching vibration of absorbed water on the surface of the cerium oxide nanoparticles.[54] The presence of nitrogen atom in the CeO2 skeleton is well aided by EDX and elemental mapping analyses.

Figure 2

FTIR spectra of (a) CO, (b) CC, (c) CUC, and (d) CUO.

Microscopic Scanning (FESEM)

For gathering information on the morphological and elemental composition of fabricated samples, FESEM analysis was performed. Figure displays the FESEM picture and EDX image of CUO, which appears to be like an elongated bean-shaped structure formed by the aggregation of many CUO particles. Further, these bean-shaped CUO agglomerates form a coral reef colony-type architecture as shown in Figure a,b. Additionally, the EDX image in Figure c confirms the presence of elements like Ce, O, and N atoms in the CUO skeleton. This elemental distribution is further proved via elemental color mapping of the CUO sample, and the result is depicted in Figure d–f. The above result clearly justifies the doping of N atom in the lattice of the CUO photocatalyst. The FESEM image along with EDX analysis data of the CO sample are shown in the Supporting Information (Figure S1), which indicates agglomerated pictures of CO particles and composed of Ce and O atoms, respectively. Again, the color mapping images of CO are included in the Supporting Information (Figure S2), and the results support the EDX data of CO.

Figure 3

(a, b) SEM images, (c) EDX images, and (d–f) elemental mapping of CUO.

TEM Imaging

In order to learn more about the morphology and crystal chemistry, TEM, HRTEM, HAADF, and SAED characterizations were undertaken.

Figure a,b represents the obtained micrograph images (TEM pictures) of CUO. The TEM images suggest the nanosheet coral reef-like morphology, which appears to be of spherical and elongated shape. Further, it can be seen that some white patches are there in these sheets, which indicate the formation of pores within the material (Figure a,c). The reason behind this porous framework is attributed to the decomposition of urea (as a combusting agent) and organic moieties that are present on oyster shells during the calcination process. This combustion at such an elevated temperature results in gas formation, which results in pores in the CeO2 nanosheet. Further, the CeO2 nanosheet is formed by the aggregation of small CeO2 crystals/particles as clearly observed from the HRTEM image shown in Figure d where a number of small crystals with a lattice fringe orientation in a different direction are seen. For a more clear view, the particles are encircled in yellow (Figure d). Additionally, the HRTEM picture in Figure discloses that the CeO2 particles are highly crystalline in nature, and the computed interplanar d-spacing value of 0.311 nm corresponds to the (111) crystal plane of fluorite cubic ceria.[48] Adding more to this microscopy characterization, the obtained discontinuous circular SAED pattern is due to the irregular distribution of CeO2 particles, which are not able to produce concentric ring patterns. However, for better understanding, manual circles are drawn, connecting bright spots that imply the polycrystalline feature of CeO2 with the rings indexed to respective crystal planes as shown in the Supporting Information (Figure S3). Further, Figure f stands for the HAADF picture of CUO. Extending the analysis to confirm the elemental content, EDX measurement (Figure g) was carried out, and it was visualized that the material is composed of Ce, O, and N atoms in its skeletal structure, and this data is well supported by FESEM analysis. Further, the inset in Figure g depicts the particle size distribution plot, which again confirms that a maximum number of particles are quantum particles and show a confinement effect.

Figure 4

(a, b) TEM images and (c, d) enlarged images of CUO. (e) Fringe related to the (111) plane of CUO. (f) HAADF image and (g) EDX patterns of CUO.

EIS and Bode Phase Analyses

Electron–hole separation, transport, and their lifetime/durability play an indispensable part in determining the proficiency of photocatalytic materials. Hence, to gain knowledge about all these parameters, EIS and Bode phase measurements were carried out at zero applied potential bias with all the fabricated materials, and details are narrated below (Figure ).

Figure 5

Nyquist plots of as-synthesized catalysts in (a) dark and (b) light conditions and (c) respective Bode phase plot.

In brief, electrochemical impedance spectroscopy (EIS) gives a handful of information regarding resistance originating around the electrolyte and electrolyte–electrode interface, that is, hindrance to charge flow, or to know more about the kinetics of the photocatalytic reaction, EIS analysis is conducted. Figure a,b shows the Nyquist graphs of all samples measured under dark and light conditions, measured at zero applied potential, respectively. The whole plot is fragmented into two: (i) the semicircular section in the high Hertz zone, which says about the amount of resistance to charge flow at the electrode–electrolyte interface and diffusion in the space charge region, that is, small arc means less resistance or high conductivity, and (ii) the loop in the low-frequency area, which indicates Warburg resistance, kinetics of charge carrier transport, and diffusion path length, viz., the more parallel the loop to the Y axis, the faster is the migration and the smaller is the diffusion journey.[55] In the present investigation, CUO shows the smallest arc diameter with a stiff loop in comparison to other samples, which suggests better separation and transfer of charge carriers. Further, characteristic equivalent circuit diagrams for the fitted Nyquist graph are placed as an inset in Figure a,b. The diagram contains specific symbols where R1 and R2 represent a solution and solution–electrode resistance, while C and W stand for capacitance and Warburg resistance, respectively. Additionally, the Bode phase plot in Figure c depicts the lifetime of the photoexcited electron–hole pair, and the calculation is made by following eq as highlighted below:where fmax is the maximum frequency and is inversely related to a lifetime (τ); hence, a lower value of fmax suggests a longer lifetime of excitons (delay recombination of charge carriers).[56] In our case, the CUO photocatalyst shows the best τ value, that is, 72.7 μs followed by 42.8, 34.2, and 29.7 μs for CO, CUC, and CC, respectively. This indicates that, in the CUO photocatalyst, exciton separation is effective, which leads to a longer life span of electrons and so the most efficient catalyst among other prepared samples. This is also well supported by its high photocatalytic activity, low PL peak intensity, and smaller Nyquist arc diameter.

Photoluminescence Spectroscopy (PL)

PL is a non-destructive and handy optical characterization technique used for measuring the fate of charge carriers (i.e., recombination, separation, and migration) of photocatalytic materials. In brief, PL peak intensity is directly linked with the kinetics of excitons recombination, viz., intense and narrow spectra means quick recombination, whereas broad and flat PL peaks suggest effective separation and high concentration of electrons and holes in the respective conduction and valence band of the material.[57] This PL band intensity has a solid effect on the catalytic activity of the semiconductor. Figure a shows the room-temperature PL spectra of as-prepared samples excited at 340 nm and scanned within the wavelength window of 360–600 nm. The observed PL peaks for all photocatalysts are almost identical with some variation in peak intensities, which may be attributed to the pace of electron–hole recombination, framework defect, and concentration of oxygen vacancies within the materials. From Figure a, it can be concluded that CUO has the best charge carrier separation potential as the PL peak intensity is low compared to others. This further suggests that the lifetime of the electron–hole pair is higher in CUO, which is well supported by the Bode phase plot and EIS analyses. Further, the cause of the intense PL signal in CC and then CUC is attributed to the presence of more Ce3+ and number of oxygen defect sites, which act as a trapping or recombination center resulting in faster exciton neutralization. The large amount of defect in these samples is well correlated with the sharpness of peak representing defect or oxygen vacancies (details narrated below). At the same time, these defects or oxygen voids in the control amount play a critical role in the charge separation process as it traps the photoexcited electrons and promotes them to the respective sites to execute the catalytic reaction. Such a type of control distortion (oxygen vacancies) is found in the CUO photocatalyst, which is clear from the low intense PL peak and characteristic defect featuring peak and also its enhanced photocatalytic property. Adding more to the above claim, the lifetime of charge carriers as calculated from the Bode phase plot for CUO shows a high value compared to the rest of the photocatalysts. Moreover, the PL emission observed between 360 and 600 nm represents oxygen defect and Ce3+ in the CeO2 lattice. In brief, the emission bands at 398 and 422 nm are due to electron relaxation from the Ce4f level to O2p, whereas the blue (450 and 468 nm) and blue-green (482 and 492 nm) PL signals correspond to surface defect in CeO2, respectively. The characteristic emission band due to oxygen vacancy is seen at 529 nm. Again, all the peaks visualized from 400–500 nm are linked to different defect states bound within the conduction band (Ce4f) and valence band (O2p) of CeO2.[50,52,58]

Figure 6

PL spectra of (a) prepared catalysts and (b–e) CC, CO, CUC, and CUO at various excitation wavelengths showing quantum confinement effect.

In order to have more information on the optical property, we have checked the photoluminescence at various excitation wavelengths, which revealed interesting science, that is, quantum confinement effect, surface state, and molecule state. CeO2 can be termed as quantum dots as their light emission shows a direct linkage with CeO2 particle size. In the present case, the PL emission bands undergo a bathochromic shift, which informs the distribution of CeO2 particles of various sizes having a bandgap within 3.1–2.9 eV. Additionally, for the first time, the quantum confinement effect property in CeO2 is developed by adopting the urea and oyster shell method of synthesis. Adding more to the study, it was figured out from the plotted PL result, as shown in Figure b–e, that the emission peaks suffer a red shift when the samples were excited at different wavelengths, that is, from 320 to 380 nm. This observation further indicates the quantum confinement concept, which is in good accordance with published articles.[50] Again, the higher wavelength shift in the fluorescence emission bands signifies particle distribution of different sizes.

Raman

Figure demonstrates the visible laser (532 nm) irradiated Raman polarization spectra of all fabricated catalysts in order to explore both bulk and surface textures, that is, crystal phase and defect presence. In the plotted Raman spectra of all samples, an intense and sharp vibrational mode is observed around 464 cm–1 (space group Fd3̅m), which stands for the characteristic F2g phonon band of the CeO8 unit in cubic fluorite CeO2 and matches well with XRD results.[48,59] Further, two low intense polarization peaks were seen at 252 and 598 cm–1, which correspond to 2TA vibrational mode (second-order transverse acoustic) or lattice dislocated oxygen and oxygen vacancy (OD) or defect-induced (D) levels, respectively.[59,60] The change in F2g peak position and intensity may be related to a number of parameters such as variation in the lattice constant, phonon confinement or phonon relaxation, size distribution, and strain (Figure b).[61] Similarly, the change in peak intensity of Raman mode at 598 cm–1 quantifies the conversion of Ce4+ to Ce3+ and induced defect/oxygen vacancy.[49] Additionally, the intensity ratio of ID/IF2g speaks about the percentage of defects present in the material.[62] It is clearly observed in Figure d that CUO contains a large amount of oxygen defect because the peak represented as OD is of high intensity compared to others. These defect points act as trapping sites for charge carriers and hence lengthen their lifetime, so excitons will be more available to carry out the photocatalytic reaction more effectively. These Raman results are well supported by PL and XRD analyses.

Figure 7

(a, b) Raman spectra of all prepared samples and their enlarged view. (c, d) Enlarged images of second-order transverse acoustic and oxygen defect.

UV–Visible DRS

Figure illustrates the optical response of the as-prepared photocatalysts, that is, light absorbance range and optical bandgap, which play a major role in artificial photocatalysis. CeO2 is known as a UV-active material and hence shows absorbance within 200–430 nm. The inset in the absorbance plot corresponds to the reflectance behavior of all prepared photocatalysts. From the UV–Visible DRS spectra (Figure a) of all samples, three types of broad photon absorption bands can be seen: (i) within 222–265 nm, which is due to O2 → Ce3+ charge transfer, (ii) between 277 and 308 nm, which is because of O2 → Ce3+ charge transfer, and (iii) that at 342 nm, which is attributed to inter-band transitions.[62] Interestingly, in both the oyster shell prepared samples, a blue/hypsochromic shift in the DRS spectrum is observed with respect to the samples synthesized via the crucible. This signifies the existence of a quantum confinement effect in oyster shell-treated photocatalysts and also indicates the presence of Ce3+ ions.[63,64] Additionally, the bandgap energy of semiconducting materials was calculated via eq :where α is the absorption coefficient, ν is the frequency of light, Eg is the bandgap energy, h is Planck’s constant, and A is the proportionality constant. The value of n decides the type of electronic transition taking place in the semiconductor, that is, n = 1 implies direct transition and n = 4 means indirect transition. In the present study, the prepared photocatalysts undergo indirect transition, and their respective optical bandgap is highlighted in Figure b–e. As a general concept, bandgap and crystal size are strongly related, which means that the smaller the size, the greater the optical band energy. It means that a blue shift in optical absorption spectra suggests the quantum size property, and similar types of effect in the CeO2 material are previously reported by various research groups.[63] Interestingly, CUO shows a larger bandgap compared to other synthesized materials pointing a smaller particle size and hence inherit the confinement effect.[63,64] Further, this confinement of electrons in these semiconductor quantum dots gets enhanced with a reduction of size. Again, it was observed that CUO shows the highest confinement property among all other synthesized materials (CC, CO, and CUC).

Figure 8

(a) Optical absorbance of as-synthesized samples. (b–e) Direct bandgap of CC, CUC, CO, and CUO.

Adding more to the investigation, the Urbach energy of all samples was estimated by following our previously published paper.[65] A high Urbach energy indicates more distortion/defect in the system. Again, it was seen from the plotted Urbach graph in Figure a–d that CUO has the highest value, that is, 0.34 eV followed by CUC (0.31 eV) > CO (0.26 eV) > CC (0.24 eV). This indicates more lattice distortion/defects in CUO and hence more charge carriers capturing sites resulting in better separation and high activity.[41] This concluded science shows good relevance with PL, EIS, Bode phase plot, and catalytic activity.

Figure 9

Urbach energy graphs of (a) CC, (b) CO, (c) CUC, and (d) CUO.

Mott–Schottky

In order to gain knowledge about the type of semiconducting material (p- or n-type) and band edge potential, Mott–Schottky (MS) study was performed in the dark at a biasing frequency of 500 Hz and pH = 6.8. Figure a,b depicts the MS graph of all prepared photocatalysts, and as we know, the flat band potential (Efb) value of the material is an essential parameter based on which the feasibility of the performed photocatalytic reaction depends. Further, the Efb position of each sample is calculated using eq :[65,66]where the symbols have a specific identity, that is, Csc is the space charge capacitance, ε and ε0 are the dielectric constant and dielectric constant in vacuum, respectively, Nd is the donor density, kB is the Boltzmann constant, E is the applied potential (V), q is the electronic charge, and T is the absolute temperature.

Figure 10

(a) Mott–Schottky plot of all prepared CeO2 samples. (b) Enlarged view of CeO2 oyster shell and CeO2 urea oyster shell.

The MS plot is drawn between 1/Csc[2] in farads versus applied potential (E) in volts. The intercept on the X axis gives the Efb result of each photocatalyst and also speaks about the type of semiconductor. In this case, all samples show an n-type character, which is in good correlation with reported articles, and the calculated Efb value is tabulated in Table . Moreover, in the case of the n-type semiconductor, Efb lies just 0.1 V below the conduction band (CB); hence accordingly, the CB and VB position of each sample is calculated by taking the optical bandgap. Additionally, the more negative Efb value or smaller slope of oyster shell-treated samples, that is, CUO and CO, suggests a high donor density, which is due to the presence of oxygen vacancies (justified by Raman, PL, and Urbach energy). This high electron density in CUO is well supported by the Bode phase plot (high electron lifetime), low PL peak intensity, smaller EIS arc diameter, and high photocatalytic activity.

Table 3

Flat Band Potential, Bandgap, CB, and VB Values of Prepared Samples

sample	Efb (Ag/AgCl) (V)	Efb (NHE) (V)	bandgap (eV)	conduction band (CB) NHE	valance band (VB) NHE
CC	–0.58	0.01	3.03	–0.08	2.95
CUC	–0.53	0.06	3.12	–0.03	3.09
CO	–0.78	–0.18	3.13	–0.28	2.85
CUO	–0.81	–0.21	3.15	–0.31	2.84

Linear Sweep Voltammetry (LSV)

Furthermore, to justify the production of a high concentration of photogenerated charge carriers due to better separation of exciton separation, LSV or I–V measurements, that is, photocurrent density of as-prepared samples, were carried out in light illumination within the potential window of 0.6–1.4 V. Figure a illustrates the plotted LSV curves of CUO, CO, CC, and CUC photocatalysts, which signify an increment in the current density of oyster shell-mediated systems along the anodic direction with the increase in basing potential. This indicates an n-type character of all prepared catalysts, which is further confirmed via Mott–Schottky measurement discussed in the respective section.[65] From the obtained data, it was found that CUO displays a high photocurrent value, that is, 494.8 μA/cm2 at a low onset potential compared to other samples. This result is also well supported by observed catalytic activities, low PL peak intensity, EIS, and Bode phase plot of the respective photocatalyst (CUO). Additionally, the cause of such a high current density in the case of CUO is attributed to the formation of controlled defect states (Raman, PL, and Urbach energy analyses) leading to effective separation of e––h+ pairs, which are proved through PL and EIS analyses. Adding more to the study of photocurrent characterization, all the designed photocatalysts were again subjected to transient photocurrent (OFF–ON current) measurements at a fixed applied potential, that is, 1.2 V (vs Ag/AgCl) for a time span of 300 s as displayed in Figure b. The obtained OFF–ON current graph suggests better charge mobility and effective charge separation in CUO.

Figure 11

(a) Polarization curves and (b) transient photocurrent plot of as-synthesized samples.

Photocatalytic O2 and ·OH Radical Generation

As we know, photocatalyst water oxidation or O2 generation is an uphill task and a complicated four-electron reaction. Hence, designing a promising photocatalyst toward water oxidation under light illumination is quite necessary, and at the same time, four electrons are evolved in this process, which can start a secondary reaction, that is, photocatalytic H2 production and CO2 reduction. In the present study, the fabricated samples were exposed for O2 evolution under light illumination as their valence band potential is enough positive (>0.82 V at pH 7 water oxidation potential) to make water oxidation feasible. Additionally, due to the presence of a controlled amount of oxygen defects (Raman, PL, and Urbach analyses), better exciton separation and longer lifetime of charge carriers occur, which leads to enhanced O2 evolution for CUO (312.2 μmol/2 h) compared to other photocatalysts, that is, CC (102.5 μmol/2 h), CUC (156.1 μmol/2 h), and CO (298.8 μmol/2 h) as shown in Figure a. Further, the longevity or photostability of CUO was examined for three consecutive cycles, and the obtained results are framed in Figure b. Additionally, the XRD plot of after and before treatment of the catalyst is included in the Supporting Information to support the stability (Figure S4). It was observed that the evolution pace gradually decreases with the increase in reaction cycle or irradiation time. This decrease in activity may be attributed to the settling of Ag or silver oxide nanoparticles of the active surface of the catalyst, which leads to attenuation of irradiated light that results in less light penetration and also because of the high solubility of formed O2.[67] Further, because of drawbacks associated with Ag scavenger, the best photocatalyst, that is, CUO, was again tested for O2 evolution under the same reaction condition but in FeCl3 solution as an electron trapper. In this case, Fe3+ captured electrons and transferred to Fe2+, and hence holes are readily available for water oxidation.[68] CUO shows an O2 generation of 365.7 μmol/2 h. This type of enhanced catalytic performance in the FeCl3 solution was also observed and reported by our previously published paper.[35] Again, the apparent conversion efficiency, which is a vital aspect in quantifying the potential of the photocatalyst, was calculated for photocatalytic O2 evolution under UV light irradiation over CUO, and the value was found to be 7.1% (detailed calculation given in the Supporting Information). Extending the investigation, the hydroxyl radical formation ability over different as-prepared samples (CO, CU, CUO, and CUC) experimented and the obtained results are presented in Figure c. The plotted graph indicates that CUO shows the best activity, that is, possess more capacity to generate ·OH radical as the PL peak intensity of the TPA-OH complex in the case of CUO is more intense. This high hydroxyl radical generation ability of CUO can be ascribed to effective separation exciton via oxygen vacancies leading to greater accumulation of highly oxidizable holes in the valence band of the material. Further, in detail, the calculated valance band potential of CUO is about 2.84 eV versus NHE, which is quite high compared to the standard reduction potential for hydroxyl generation, that is, OH/·OH = 1.99 eV versus NHE, and hence hydroxyl radical generation is feasible over CUO (Scheme ).[57] Additionally, to justify the superiority and nobility of the present work over other reported studies toward light-driven water oxidation reaction, a comparison table is provided in the Supporting Information (Table S1).

Figure 12

(a) O2 evolution rate over different photocatalysts. (b) Durability graph of CUO toward O2 production. (c) PL graph of the TPA-OH complex for CU, CO, CUO, and CUC.

Scheme 1

Hydroxyl Radical Generation over CUO under UV Light Irradiation

Proposed Pathway of O2 Formation (Theoretical Concept)

O2 formation pathway is a quite complicated and difficult chemistry to describe, but taking the idea and knowledge from the reported literature, we framed a mechanism toward the performed water oxidation reaction over oxygen-defective CUO, and the detail is narrated as such.[69−71] Two models were proposed for the formation of O=O: (i) water molecule breaks down to generate H and OH, and then OH interacts with the oxygen of another H2O to generate OOH, that is, H2O → H + OH, OH + H2O → OOH + 2H, and (ii) H2O combines with OH and H to produce HOOH (H2O + H + OH → HOOH + 2H). However, the second mechanism is not feasible as the process goes via a high activation energy barrier. So, the OOH intermediate mechanism with a low activation energy profile is accepted, and therefore the O2 evolution over oxygen vacancy-oriented CUO is linked as follows. The formed OOH species gets attached to the Ce site of defective CUO with a low energy barrier. Further, the O=O formation is the main step, that is, the rate-determining step in the whole oxidation process, and when it is carried out over oxygen defect-framed CUO, the activation energy input in this step is decreased to a notable level, which is attributed to frailty oxygen binding and restricted electron flow between Ce and O. Hence, water oxidation is more kinetically favorable over oxygen vacancy-oriented CeO2.

Possible Mechanism of O2 Evolution over CUO via Ag+ Sacrificial Agent

There are two possible mechanistic approaches to describe O2 evolution in Ag+ sacrificial agents via the photon irradiation process. In one case, Ag+ ion acts as an electron scavenger and gets reduced to Ag, and at the same time, photogenerated holes interact with water to form oxygen, whereas in another case, Ag+ gets oxidized to Ag2+ via holes and reacts with water to produce Ag2O2, which then finally decomposes to O2. So, it is essential to know the role of Ag+ in a water oxidation reaction to have a better mechanism description. In brief, the valence band of CUO is placed at a more positive potential, that is, 2.84 eV versus NHE, and hence the holes are readily available to carry out the oxidation process as depicted in Scheme . The two proposed mechanism for photocatalytic water oxidation is narrated below.

Scheme 2

Water Oxidation over Oxygen Vacancy-Oriented CUO under UV Light Irradiation

Mechanism 1:

Mechanism 2:

Further, from the obtained dark brownish black color solution after light irradiation, it suggests that the photo-oxidation of water in AgNO3 solution goes via mechanism 1 not via the peroxide decomposition pathway (mechanism 2), and interestingly, a similar type of conclusion was also reported by Bahnemann et al. over La-doped NaTaO3 toward photocatalytic water oxidation.[67]

Role in Exciton Separation

To ascertain the oxygen void and its importance in the physics of charge carrier separation, PL, Raman, and EIS analyses were conducted, and further the role of vacancies in enhancing the catalytic activity is briefly scripted as follows. In general, the lower energy valance band (VB) of CeO2 is made up of O 2p, and that of the conduction band (CB) is built with the Ce 4f orbital. In short, the fate or lifetime of photogenerated electron/hole pairs largely relies on the band structure and defect formation. Just like friction, vacancies are necessary evil, that is, if present in a higher amount, then it turns to a faster recombination site, and if found in a very low quantity or say negligible, then effective charge separation is hindered, and hence, in both cases, low catalytic performance is observed. So, defects/vacancy in a controlled concentration helps in trapping photoinduced electron and decreases the effective mass of electron due to the no-localization of the 4f level that results in the acceleration of electrons (small effective mass).[72] Further, due to this trapping states (oxygen vacancy), the faster-moving holes are abundantly available to initiate the oxidation process. It was reported by Zou et al. that the average charge of surface oxygen and Ce atoms is high in the case of oxygen defect-oriented CeO2, that is, the surface is more occupied with photoelectrons via these vacancies that lead to the creation of an inner electric zone, and hence fruitful electron–hole pair separation takes place resulting in significant photocatalytic activity.[40] Additionally, as stated at the start of the text, PL and Raman were used to diagnose oxygen vacancy, and PL, EIS, Bode phase plot, and catalytic performance were used to characterize the effective separation of excitons on CUO. In the present investigation, CUO shows a delay in the charge recombination process with a greater lifetime of electron holes, and this is well aided by performed characterization techniques and catalytic results.

Influence in Water Dehydrogenation/Oxidation

In the process of water dehydrogenation, the formation of the O intermediate is the crucial phase, and in oxygen vacancy-mediated systems, the generation of intermediate species goes via a low activation energy mountain. The whole chemistry of water adsorption and dissociation to O2 via defective CeO2 is elaborated likewise. First, H2O gets attached to the Ce center via a weak force followed by a spontaneous O–H bond cleavage through a negligible energy hump (which can be neglected) to generate OH and H, which are exothermic in nature.[73] Further, the OH part breaks down to H and O intermediates through a very low activation energy barrier, and this step is endothermic in character, and most importantly this OH cracking step in undefective CeO2 is very complicated as the reverse reaction, that is, a combination of formed H and O to produce OH back is very much feasible from an activation energy point of view. However, surprisingly, with vacancy-oriented CeO2, such reverse H and O binding is restricted due to strong bonding of O + H leading to surplus electron accumulation on Ce from the O atom.[40] The above-described science highlights the vital role of oxygen vacancy in the water oxidation reaction.

Conclusions

In this study, CeO2 nanosheets rich in oxygen vacancy are used to unveil the importance of oxygen vacancy in a photocatalytic water oxidation reaction and hydroxyl radical generation under photon irradiation. Further, the vacancy-framed CeO2 sheets were prepared economically via a biomediated route, that is, over an oyster shell through a calcination method. The prepared CeO2 samples possess a quantum confinement effect, which is evidenced by UV–DRS and PL analyses. CeO2 prepared from urea and oyster shell shows a high photocurrent density and electron life span of 494.8 μA/cm2 and 72.7 μs, respectively, which is significantly more compared to the other prepared samples. The role of oxygen vacancy in exciton separation and water dehydrogenation/oxidation is clearly explained. It is observed that, with the increase in oxygen vacancy, photocatalytic activity increases linearly. This work highlighting oxygen vacancy-engineered CeO2 nanosheets, hopefully, paves the path in designing a more efficient photocatalyst to overwhelm the bottleneck of the water oxidation reaction.