Band gap engineering of CeO₂ nanostructure using an electrochemically active biofilm for visible light applications

Abstract

Narrowing the optical band gap of cerium oxide (CeO₂) nanostructures is essential for visible light applications. This paper reports a green approach to enhance the visible light photocatalytic activity of pure CeO₂ nanostructures (p-CeO₂) through defect-induced band gap narrowing using an electrochemically active biofilm (EAB). X-ray diffraction, UV-visible diffuse reflectance/absorption spectroscopy, X-ray photoelectron spectroscopy, electron paramagnetic resonance spectroscopy, Raman spectroscopy, photoluminescence spectroscopy and high resolution transmission electron microscopy confirmed the defect-induced band gap narrowing of the CeO₂ nanostructure (m-CeO₂). The structural, optical, photocatalytic and photoelectrochemical properties also revealed the presence of structural defects caused by the reduction of Ce⁴⁺ to Ce³⁺ as well as an increase in the number of oxygen vacancies. The as-modified CeO₂ (m-CeO₂) nanostructure exhibited substantially enhanced, visible light-driven photoactivity for the degradation of 4-nitrophenol (4-NP) and methylene blue (MB) compared to the p-CeO₂ nanostructure. The enhancement in visible light performance was attributed to defects (Ce³⁺ and oxygen vacancy), resulting in band gap narrowing and a high separation efficiency of photogenerated electron–hole pairs. Photoelectrochemical investigations also showed a significantly-enhanced separation efficiency of the photogenerated electron–hole charge carriers in the m-CeO₂ nanostructure under visible light irradiation. The DC electrical conductivity of m-CeO₂ showed higher electrical conductivity than p-CeO₂ under ambient conditions. This study provides a new biogenic method for developing narrow band gap semiconductor nanostructures for efficient visible light driven photocatalysis and photoelectrode applications.

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1. Introduction

Over the past few years, considerable research effort has been devoted to the realization of efficient, economical and green sources for the treatment of pollutants for environment remediation processes.^1,2 Metal oxide nanostructure materials have played a major role in this endeavor because they possess an excellent combination of photochemical activity, thermal and mechanical stability. They have long been pursued for photocatalytic applications, such as hydrogen production through water splitting or decomposition of water pollutants, etc.³ The catalytic activities of metal oxide nanostructures are influenced significantly by the reactive sites present on the surface, which in turn may arise due to some type of defect in the crystal lattice.⁴ As one of the most reactive rare earth oxides, ceria (cerium oxide, CeO₂) has attracted considerable interest owing to its wide band gap (E_g = 3.19 eV) and high dielectric constant (ε = 24.5).⁵ These properties are beneficial for applications in emerging technologies for environmental and energy related applications,^2,6 such as solid oxide fuel cells,⁷ gas sensors,⁸ optics,⁹ UV light blockers, etc.¹⁰ On the other hand, the wide band gap of CeO₂ makes it active in ultraviolet (UV) radiation but not in visible light. Considerable efforts have been made to overcome this problem by modifying CeO₂ to enhance its photoactivity in the visible range.

Among the versatile modifications, transition metal ions of variable valency have been introduced to the CeO₂ lattice, which shifts the optical absorption edge of CeO₂ to lower energies, thereby increasing the photocatalytic activity in the visible region.^11,12 The usability of CeO₂ for most of these applications depends on its ability to release or uptake oxygen. CeO₂ has large oxygen storage capacity depending on Ce⁴⁺/Ce³⁺ redox cycles, which in turn depends on the concentration and types of oxygen vacancies in the lattice structure.¹³ In CeO₂, non-stoichiometric oxygen atoms are present at the grain boundary, and these vacancies play an important role in the stable grain boundary structure of CeO₂.¹⁴ Quantum mechanical calculations reported that the most favorable defect structure in CeO₂ is Ce³⁺–Ov–Ce³⁺ (Ov = oxygen vacancy), and the formation of Ce³⁺ has an effect on the red shift of the band gap of CeO₂ nanostructures.¹⁵ On the other hand, determining the type of defects responsible for the observed visible light catalytic activity remains a considerable challenge.

This study examined how oxygen vacancies and Ce³⁺ defects present in CeO₂ affect the band gap and related properties by changing its structure regularity and control the visible luminescence in the material. Recently, Ko et al.,¹⁶ extended the spectral response of CeO₂ sheets towards the visible light region using polydopamine-induced CaCO₃ vaterite microspheres. Similarly, Lu et al.,¹⁷ extended the spectral response of CeO₂ nanorods to visible light via an electrochemical assembly process. Most of these reported methods have used harmful chemicals, whose discharge in the biosphere is a major concern. Therefore, a greener route for the modification (band gap narrowing) of metal oxide nanostructures is needed.

In view of the above mentioned problems, an electrochemically active biofilm (EAB) has been employed on solid surfaces, which has shown potential in bioenergy and chemical production.¹⁸ Generally biofilms formed on solid surfaces by different type of microorganism for their mutual benefits. Electrochemically active biofilms (EABs) generated by electrically active microorganisms have properties that can be used to control the electrochemical reactions in a range of fields, such as bioenergy production, bioremediation and chemical/biological synthesis.¹⁸ Recently, through a series of studies, previous studies reported that EABs could be used for band gap narrowing of metal oxides, synthesizing noble metal nanoparticles and metal–semiconductor nanocomposites with higher efficiency than many other synthetic methods.^19–26 As part of an ongoing study of EAB, the present study focused on the use of EAB for the modification of p-CeO₂. This paper reports a very simple, biogenic and facile method for the modification of p-CeO₂ by EAB. The enhancement of the photocatalytic activity due to the presence of defects in m-CeO₂ was further confirmed by the degradation of 4-nitrophenol (4-NP) and methylene blue (MB) in an aqueous solution under visible light irradiation. A possible charge transfer mechanism was suggested to be involved in the degradation process. The photoelectrochemical result under dark and visible light further support the visible light activity of the m-CeO₂ photocatalyst, suggesting the applicability of CeO₂ in visible light as a photocatalyst and photoelectrode. m-CeO₂ showed high photocatalytic activity compared to p-CeO₂. The modification approach reported in this work does not involve the use of hazardous chemicals, capping agents, reducing agents, simple to perform, and has potential for commercialization.

2. Experimental

2.1. Materials

Cerium oxide (nano CeO₂) nanostructures and MB were purchased from Sigma-Aldrich. Sodium acetate, sodium sulfate (Na₂SO₄) and 4-NP were obtained from Duksan Pure Chemicals Co. Ltd South Korea. Ethyl cellulose and α-terpineol were supplied by KANTO Chemical Co., Japan, and fluorine-doped transparent conducting oxide glass (FTO; F-doped SnO₂ glass; 7 Ω sq⁻¹) was acquired from Pilkington, USA. Carbon paper (without wet proof, Fuel Cell Earth LLC, USA). All chemicals used in this study were of analytical grade and used as received. All solutions were prepared from deionized water obtained using a PURE ROUP 30 water purification system.

2.2. Methods

X-ray diffraction (XRD, PANalytical, X'pert PRO-MPD, Netherland) was carried out using Cu Kα radiation (λ = 0.15405 nm). The XRD peaks of the crystalline phases were compared with those of standard compounds reported in the JCPDS data file. The size of the pure CeO₂ (p-CeO₂) and EAB modified CeO₂ (m-CeO₂) nanostructures were observed by field emission transmission electron microscopy (FE-TEM, Tecnai G2 F20, FEI, USA) with an accelerating voltage of 200 kV combined with energy dispersive spectrometry (EDS) and selected-area electron diffraction (SAED). A UV-VIS-NIR spectrophotometer (VARIAN, Cary 5000, USA) was used to record the diffuse reflectance/absorption spectra (DRS) of the p-CeO₂ and m-CeO₂ nanostructures in the range of 200–800 nm. The photoluminescence (PL) properties of the p-CeO₂ and m-CeO₂ were recorded over the range, 200–800 nm, using a Kimon, 1 K, Japan. X-ray photoelectron spectroscopy (XPS, ESCALAB 250) was performed using monochromatized Al Kα X-rays (hν = 1486.6 eV) with a 500 μm spot size. The binding energy of C1s at 284.8 eV was used to calibrate the other binding energies. The XP spectra were fitted using the “Avantage program”. The electron paramagnetic resonance (EPR) measurements were performed using a Bruker EMX system at 20 K. The DC electrical conductivity (σ) was measured using a 4-in-line probe electrical conductivity measuring instrument (Scientific Equipment, India). The photoelectrochemical and photocatalytic experiments were carried out using a 400 W lamp (3 M, λ > 400 nm, USA). Electrochemical impedance spectroscopy (EIS) and linear scan voltammetry (LSV) were performed in a three electrode cell with a 0.2 M Na₂SO₄ aqueous solution as the electrolyte using a potentiostat (Versa STAT 3, Princeton Research, USA). The working electrodes were prepared using the technique reported elsewhere.²¹ Briefly, 100 mg of the p-CeO₂ and m-CeO₂ nanostructures were suspended thoroughly using a conditioning mixer by adding ethyl cellulose as a binder and α-terpineol as the solvent for the paste, and then coated on FTO glass electrode using the doctor-blade method. The p-CeO₂ and m-CeO₂ nanostructure-coated (FTO) glass substrates were used as the working electrode, Ag/AgCl (saturated with KCl) and Pt gauge were used as the reference and counter electrodes, respectively.

2.3. Development of electrochemically active biofilm

The electrochemically active biofilms (EABs) were developed on plain carbon paper according to previous reports.^18–26 In a typical procedure, carbon paper (without wet proof, Fuel Cell Earth LLC) with a size of 2.5 cm × 4.5 cm was dipped into a mineral salt medium containing sodium acetate (1 g L⁻¹) as the substrate and carbon source in a 250 mL bottle. 10 mL of anaerobic sludge (from a biogas plant in Paju, Korea) was added under strict anaerobic conditions by sparging N₂ gas for 5 min to remove the environmental oxygen. All media, including the bacterial inoculum, were changed every two days under strict anaerobic conditions. This process was repeated for two weeks to obtain the EAB on the surface of the carbon paper.

2.4. Modification of CeO₂ by the electrochemically active biofilm

In a typical modification process (Fig. S1†), 5 mM CeO₂ powder was dispersed in 200 mL of distilled water, and 0.2 g sodium acetate (1 g L⁻¹) was added as an electron donor with continuous magnetic stirring for 1 h at room temperature. Subsequently, the as-prepared EAB was hung under anaerobic conditions in the solution with slow magnetic stirring. The color of the solution turned light gray after 72 h. The pH of the above dispersion was checked before and after the reaction and was found to be 7.10 and 7.50 respectively. Subsequently, the dispersion was centrifuged and the product was obtained. The powder was dried and isolated for further characterization.

As a control experiment, one modification process was carried out in the absence of sodium acetate, i.e. in a 5 mM solution of CeO₂. The EAB was hung under anaerobic conditions by sparging N₂ gas. The other process was performed by adding 0.2 g sodium acetate (1 g L⁻¹) in a 5 mM CeO₂ solution in the absence of an EAB under anaerobic conditions by sparging N₂ gas. In both cases, no color change was observed, even after 72 h.

2.5. Evaluation of dye degradation

The photocatalytic activities of the p-CeO₂ and m-CeO₂ nanostructures were evaluated by measuring the degradation of 4-NP and MB under visible light irradiation (λ > 400 nm). In a typical photodecomposition process, 2 mg of each p-CeO₂ and m-CeO₂ nanostructure photocatalyst was added to two different 20 mL 4-NP (5 mg L⁻¹) and 20 mL MB (10 mg L⁻¹) solutions, and agitated for 30 min in the dark to achieve adsorption–desorption equilibrium. The above suspensions were irradiated with visible light and after a certain time (1 h). Subsequently, 2 mL of the solution was taken and the catalyst was separated from the solution by centrifugation to obtain a clear liquid, of which the UV-vis spectra were measured. The decolorization efficiency of the photocatalyst was determined using the equation reported elsewhere.²⁵

2.6. Photoelectrochemical studies (EIS and LSV)

To examine the photoelectrochemical response of the p-CeO₂ and m-CeO₂ nanostructures, EIS and LSV experiments were carried out under ambient conditions in the dark and under visible light irradiation in 50 mL of a 0.2 M Na₂SO₄ aqueous solution at room temperature. For each electrode, EIS was first performed in the dark and later under visible light irradiation (λ > 400 nm) at 0.0 V and with a frequency ranging from 1–10⁴ Hz. The photocurrent response was obtained by LSV under dark and visible light irradiation at a scan rate of 50 mV s⁻¹ over a potential range of −0.7 to 1.0 V.

2.7. DC electrical conductivity measurement

For the DC electrical conductivity measurements, a 0.2 g sample (p-CeO₂ and m-CeO₂ nanostructures) was pelletized at room temperature using a hydraulic press at 25 kN, applied for 10 min. The DC electrical conductivity (σ) was measured using a 4-in-line probe electrical conductivity measuring instrument, and calculated using the following equation:

σ = [ln 2(2S/W)]/[2πS(V/I)]

where I, V, W and S are the current (A), voltage (V), thickness of the pellet (cm) and probe spacing (cm), respectively, and σ is the DC electrical conductivity (S cm⁻¹).²⁷

3. Results and discussion

3.1. Proposed modification mechanism

EAB is used as a biogenic tool that provides excess of electrons and protons by biologically decomposing the carbon source (sodium acetate) as shown below:^26,28

CH₃COO⁻ + 4H₂O + EABs → 9H⁺ + 8e⁻ + 2HCO₃⁻

These electrons were used to modify the metal oxide nanostructure.^20,21 The advantage of the EAB method to modify the p-CeO₂ nanostructure is that it does not require any harmful chemicals, external energy input, capping or reducing agents, and tiresome treatments. The reaction medium for this protocol was water and the entire modification process took place at 30 °C. The product obtained in this synthesis is quite pure and free from impurities as the biofilm was supported on carbon paper. The change in pH from 7.10 to 7.50 is attributed to the dissociation of sodium acetate into CH₃COO⁻ and Na⁺. Formation of CH₃COO⁻ ions disturbs the water equilibrium by associating with some H⁺ and thereby reducing the concentration of H⁺. This increases the concentration of OH⁻, hence an increase in pH. The EAB-generated electrons and protons can interact with Ce⁴⁺ and surface oxygen, respectively (Fig. 1a). The coordination number of Ce⁴⁺ to O²⁻ decreases from eight to seven with the formation of Ce³⁺ ions in the crystal lattice due to the interactions of electrons with Ce⁴⁺.²⁹ The interaction of protons with the surface O atoms can produce –OH, resulting in the formation of Ce–OH linkages (Fig. 1b), which was confirmed by XPS. These interactions are responsible for inducing defects in the CeO₂ nanostructure, resulting in visible light absorption by the m-CeO₂ nanostructure. In our previous reports the interaction of EAB with ZnO and TiO₂ nanostructures was somewhat different because of different types of molecular structures and bonding patterns of ZnO and TiO₂.^20,21 In case of ZnO, oxygen vacancy was exclusively responsible for the band gap narrowing whereas in case of TiO₂, Ti³⁺ and –OH linkages were responsible for the band gap narrowing.^20,21

Fig. 1 (a) Proposed mechanism, and (b) reaction steps involved for modification of the p-CeO₂ nanostructure by EAB.

3.2. Structural analysis

The structures of the p-CeO₂ and m-CeO₂ nanostructures were analyzed by XRD. For the m-CeO₂ nanostructure (Fig. 2), the XRD peaks correspond to the planes that match well with that of the cubic fluorite CeO₂ structure (JCPDS no. 88-2326). Upon modification, the peak positions (2θ) of the m-CeO₂ nanostructure shifted toward a lower angle compared to p-CeO₂. According to Bragg's law, nλ = 2d sin θ, the d spacing increases with decreasing θ. Therefore, the d spacing of the m-CeO₂ nanostructure is slightly larger (3.22 Å) than those of the p-CeO₂ nanostructure (3.12 Å). The increasing lattice parameters in m-CeO₂ (a = b = c = 5.59 Å) compared to p-CeO₂ (a = b = c = 5.41 Å) can be explained in terms of the associated reduction of Ce⁴⁺ to Ce³⁺ and an increase in the number of oxygen vacancies, which leads to lattice expansion.^5,30 These were confirmed further by other techniques. This lattice expansion generates residual stress in the matrix, leading to a shift in the peak positions.¹² A similar XRD peak shift to a lower angle was observed for the CeO₂ nanoparticles with the formation of Ce³⁺ ions.³¹

Fig. 2 XRD patterns of the p-CeO₂ and m-CeO₂ nanostructures. The inset displays the shifting and broadening of the peaks.

3.3. Microstructural study

Fig. 3 shows the TEM and HR-TEM images of the p-CeO₂ and m-CeO₂ nanostructures before the photocatalytic reaction, whereas Fig. S2† shows the TEM and HR-TEM images of the p-CeO₂ and m-CeO₂ nanostructures after the photocatalytic reaction. Fig. 3a (p-CeO₂) and 3a′ (m-CeO₂) shows a mixture of spherical and cube shapes of CeO₂ nanostructure in the range of 10–50 nm, respectively. The inset in Fig. 3a and a′ corresponds to the SAED pattern, indicating the polycrystalline nature of CeO₂ with a fcc structure, which is in good agreement with the XRD patterns. The fringe spacing (Fig. 3c and c′) from the HR-TEM image was 0.31 nm for the p-CeO₂ and m-CeO₂ nanostructures, which corresponds to the interplanar distance of the (111) plane. The structural compositions of the particles were determined by HAADF-FETEM, as shown in Fig. S3 and S4† for p-CeO₂ and m-CeO₂, respectively. Fig. S5 and S6† show the EDX spectra of the p-CeO₂ and m-CeO₂ nanostructures, confirming the modification of p-CeO₂ and an absence of other impurities.

Fig. 3 TEM and HR-TEM images of the p-CeO₂ (a–c) and m-CeO₂ (a′–c′) nanostructures before photocatalytic reaction. The insets (a and a′) show the SAED patterns of the p-CeO₂ and m-CeO₂ nanostructures.

3.4. Diffuse absorption/reflectance study

UV-vis diffuse absorption/reflectance spectroscopy was used to evaluate the optical properties of the p-CeO₂ and m-CeO₂ nanostructures. Fig. 4a and S7† shows the absorption and reflectance spectra of the p-CeO₂ and m-CeO₂ nanostructures, respectively. The absorption spectra of the p-CeO₂ and m-CeO₂ nanostructures modified at different time intervals (i.e. 24 h, 48 h and 72 h) are shown in Fig. S8.† The absorption spectra reveal the m-CeO₂ nanostructure to have significantly higher absorption in the visible light region. The spectra show that the m-CeO₂ nanostructure possesses higher light absorption intensity and a red shifted absorption edge compared to the p-CeO₂ nanostructure. Mao et al.,³² in their computational study of nitrogen-doped CeO₂ nanoparticles, attributed the absorption (<500 nm) to the additional states caused by doping, whereas the absorption (>500 nm) was associated with defects and oxygen vacancies. The band gap was estimated from a plot of the transformed Kubelka–Munk function versus the energy of absorbed light (Fig. 4b).^21,25 The effective optical band gaps of the p-CeO₂ and m-CeO₂ nanostructures were 3.19 eV and 2.87 eV, respectively, indicating a 0.32 eV difference. This shows that the EAB acted as a band gap modifier.

Fig. 4 (a) UV-vis diffuse absorption spectra of the p-CeO₂ and m-CeO₂ nanostructures (the inset shows the color of p-CeO₂ and m-CeO₂ nanostructure), and (b) optical band gap obtained from the plots of (αhν)^1/2 versus the photon energy (hν) of the p-CeO₂ and m-CeO₂ nanostructure.

The red shift in the band gap of the m-CeO₂ nanostructure is affected by Ce³⁺ and oxygen defects.^12,17,33 Patsalas et al.,³⁴ reported that the red shifting in the band gap is due to an increase in the concentration of Ce³⁺ on the grain boundaries, and the reduction of the band gap increases with increasing Ce³⁺ concentration. The band gap narrowing of the m-CeO₂ nanostructure was attributed to the high concentration of Ce³⁺ induced by the defect levels generated in CeO₂ because of the interaction with electrons and protons. The higher light absorption intensity of the m-CeO₂ nanostructure also suggests that it could enhance the photocatalytic performance, which was confirmed by the photocatalytic degradation of the dyes. These observations suggest that the m-CeO₂ nanostructure can be photoexcited to generate more electron–hole pairs under visible light irradiation, which can result in higher photocatalytic performance.

3.5. XPS analysis

XPS of the p-CeO₂ and m-CeO₂ nanostructures was performed to examine the chemical composition and valance states of the elements present in the samples. Fig. S9† shows the XP survey spectra of p-CeO₂ and m-CeO₂, in which all the peaks were assigned to Ce, O and C. No extra peak for any other impurity was observed. The intensity of the C1s peak was higher for p-CeO₂ than for the m-CeO₂ nanostructures, which was attributed to the removal of surface carbon impurities (Fig. S10†). Fig. 5 shows the Ce 3d core-level fitted spectra with the corresponding deconvoluted Gaussian peaks. The deconvoluted Ce 3d core level spectra reveal the mixed valence states of both Ce³⁺ and Ce⁴⁺ owing to its nonstoichiometric nature and multiple d-splitting, such as 3d_3/2 and 3d_5/2.^5,35 The peak values for 3d_3/2 and 3d_5/2 were between 895 & 910 eV, and 875 & 890 eV, respectively. The characteristic peaks of the mixed valance, such as Ce⁴⁺ 3d_3/2 and Ce⁴⁺ 3d_5/2 were located at binding energies of 916.4 eV and 898.8 eV, respectively, whereas the peaks located at binding energies of 901.1 and 882.7 eV were assigned to Ce³⁺ 3d_3/2 and Ce³⁺ 3d_5/2, respectively.⁵ The mixed states of CeO₂, also determined by additional satellite peaks (which are called ‘shake-up’), were located at 907.2 eV for Ce³⁺ 3d_3/2, and 885.8 and 889.0 eV for Ce³⁺ 3d_5/2, respectively.⁵ Semiquantitative analysis of each deconvoluted peak area was used to calculate the concentration of Ce³⁺ in p-CeO₂ and m-CeO₂. The detailed notation and calculation of Ce 3d spectra of CeO₂ can be found elsewhere.^5,17,29,35 An analysis of the Ce 3d XP spectra of m-CeO₂ showed that the concentration of Ce³⁺ (0.489) increased compared to p-CeO₂ (0.431). The presence of a high concentration of Ce³⁺ in m-CeO₂ confirms the interaction of Ce⁴⁺ with the electrons produced by the EAB. During this process, oxygen vacancies can arise to maintain the charge neutrality of the system, which might have some additional contribution to band gap narrowing.³⁶ Passacantando et al.,³⁷ supports the present investigation in that when the concentration of Ce³⁺ ions becomes higher, their lattice parameters expanded because of the reduced electrostatic force derived from the increased concentration of Ce³⁺ ions. The correlation between the XPS and XRD data shows that the concentration of Ce³⁺ ions is proportional to the increase in the lattice parameter.

Fig. 5 XPS Ce 3d core level spectra of the p-CeO₂ and m-CeO₂ nanostructures.

Fig. 6 shows the O1s photoemission spectra of the p-CeO₂ and m-CeO₂ nanostructures. The O1s photoemission spectra also revealed an asymmetric profile and could be deconvoluted into a different symmetrical signal, which shows a different type of oxygen, such as absorbed oxygen, lattice oxygen, surface hydroxyl group and oxygen vacancies.^38,35 The low binding energy peak (528.0–530.5 eV) was assigned to lattice oxygen, whereas the high binding energy peak (531.0–532.8 eV) was assigned to oxygen vacancies.³⁹ The peak at 531.50 eV in p-CeO₂ shifted to a higher binding energy (531.80 eV), showing the formation of hydroxyl groups on the surface, which might be due to the interaction of protons produced by the EAB. On the other hand, the higher binding energy peak in the O1s spectrum could also be related to the presence of oxygen vacancies.^35,36 The reason for the existence of a higher binding energy peak of the O1s spectrum might be the more tightly bonded 1s electron to oxygen in the case of Ce³⁺ of ceria (O–Ce³⁺), which interacts with the protons produced by the EAB.³⁶ Despite this, the intensity is also a common way for qualitatively assessing the number of oxygen vacancies in ceria. In the case of m-CeO₂, the peak intensity also increases compared to p-CeO₂, which might be due to the presence of oxygen vacancies.⁴⁰

Fig. 6 XPS O1s core level spectra of the p-CeO₂ and m-CeO₂ nanostructure.

3.6. EPR measurement

Fig. 7a shows the X-band EPR signal of the p-CeO₂ and m-CeO₂ nanostructures. One signal at 3441 G was observed for p-CeO₂ and m-CeO₂. This signal at 3441 G has been studied widely and generally attributed to Ce³⁺ formed in the sample.^30,41,42 In this case, the intensity of the Ce³⁺ signal increases as defects are induced in the m-CeO₂ (Fig. 7). This increase in intensity is due to the reduction of Ce⁴⁺ ions to Ce³⁺ after the interaction of CeO₂ with electrons produced by EAB, leading to the formation of defects in p-CeO₂.⁴¹ Fig. S11† shows the g-value spectra of the p-CeO₂ and m-CeO₂ nanostructures, which indicates the presence of a narrow signal at g = 1.96 and a strong signal at g = 2.00. These signals are typical for Ce³⁺ in ceria-based materials.⁴² This also supports the presence of Ce³⁺ ions and oxygen vacancies in the sample.

Fig. 7 (a) X-band EPR spectra, and (b) PL spectra of the p-CeO₂ and m-CeO₂ nanostructures.

3.7. Photoluminescence characteristics

Photoluminescence spectroscopy is normally used to explore the efficiency of charge carrier migration and transfer, and is also helpful for understanding the fate of electron–hole pairs in semiconductor particles.^21,25 The band gap narrowing of CeO₂ was investigated further by observing the photoluminescence properties. Fig. 7b shows the room-temperature PL spectra of the p-CeO₂ and m-CeO₂ measured at 325 nm as the excitation source. The broad emission peak from 400 to 500 nm might be responsible for the electronic transition in the narrowed band gap of CeO₂. The emission intensity and broadening in m-CeO₂ is stronger than that of p-CeO₂, indicating the greater abundance of defects in m-CeO₂. When there is a higher concentration of Ce³⁺ states and a corresponding high concentration of oxygen vacancy states, a larger percentage of valence electrons can be excited to the defect state and a larger emission signal will be received.³³ The m-CeO₂ in this case has strong emission indicating a higher concentration of Ce³⁺ states compared to p-CeO₂. This suggests that m-CeO₂ contains more defects (Ce³⁺ and oxygen vacancy) than the commercial CeO₂ nanostructure (p-CeO₂), which might be helpful in improving the defect-related properties, such as photocatalytic activity.

3.8. Raman spectroscopy

Raman spectroscopy is also used as a complementary structural characterization technique because the responses are affected by a range of factors including strain and broadening associated with the size distribution and defects.^30,35 Fig. 8 shows the Raman spectra of p-CeO₂ and m-CeO₂. The Raman spectrum of the m-CeO₂ nanostructures was similar to that of p-CeO₂, which highlights the purity of CeO₂ after modification. Generally, dioxides with afluorite structure have a F_2g allowed mode, which is a Raman active mode.¹² The F_2g reveals the structure of the symmetric breathing pattern of the oxygen atoms around the Ce⁴⁺ ions. A peak shift of 1.97 cm⁻¹ and a broadening of the peak was observed in m-CeO₂. The peak shift and broadening observed in m-CeO₂ was attributed to the change in lattice parameter after the stoichiometric changes and defects (Ce³⁺ and oxygen vacancy).^12,17,30,31 XRD, XPS and DRS confirmed that the stoichiometric changes and defects are responsible for the minor changes in the Raman band.

Fig. 8 Raman spectra of the p-CeO₂ and m-CeO₂ nanostructures.

3.9. Visible light photocatalytic activity and its photoexcitation mechanism

The influence of defects (Ce³⁺ and oxygen vacancy) on the visible light photocatalytic activity of m-CeO₂ nanostructures was examined by measuring the degradation of a colorless aromatic compound (4-NP) and colored dye (MB). The degradation of 4-NP and MB by m-CeO₂ was evaluated by the change in the intensity of the absorption peaks. Fig. 9a and b shows plots of degradation (C/C₀) versus the irradiation time, where C is the absorption of a 4-NP and MB solution at each time interval of irradiation, and C₀ is the absorption of the initial concentration. Fig. 9a and b shows that the m-CeO₂ nanostructure has a higher photocatalytic activity than the p-CeO₂ nanostructure under similar conditions. Furthermore, the present photocatalytic activity result of the m-CeO₂ shows higher photoactivity compared to the EAB modified and commercially available metal oxide nanostructures (Degussa TiO₂ and ZnO).^20,21

Fig. 9 Kinetic profile of the photocatalytic degradation of (a) 4-NP, and (b) MB under visible light irradiation in the presence of the p-CeO₂ and m-CeO₂ nanostructures.

Based on the above results, Fig. 10 presents the general mechanism for the photoexcitation and electron transfer properties of the p-CeO₂ and m-CeO₂ nanostructures for the photocatalytic degradation of 4-NP and MB under visible light irradiation. m-CeO₂ showed higher activity than p-CeO₂. The UV-vis diffuse absorption spectra (Fig. 4a) and transformed Kubelka–Munk function plots (Fig. 4b) show that the photon absorption in visible light by m-CeO₂ increases significantly due to a decrease in band gap. On the other hand, because of the wide band gap of p-CeO₂ (3.19 eV), the generation of electron in the conduction bands and holes in the valence bands under visible light is not feasible (Fig. 10). When the band gap of m-CeO₂ is reduced significantly to 2.81 eV, electrons from the valance band can migrate easily to the conduction band by absorbing visible light.¹⁶ The electrons that accumulate on the surface of m-CeO₂ are then scavenged by oxygen molecules dissolved in water to yield highly oxidative species, such as superoxide radical anions and hydroxyl radicals, which are responsible for decomposing and mineralizing 4-NP and MB.^21,25 Following are the proposed reactions that occurred during the degradation of pollutants in the presence of m-CeO₂.^20,25,43,44

m-CeO₂ + hν (visible light) → e⁻ + h⁺

e⁻ + O₂ → O₂˙⁻

h⁺ + OH⁻ → ˙OH

O₂˙⁻ + H₂O → ˙OH

˙OH + pollutants → degradation of pollutants

Fig. 10 Proposed mechanism for the photodegradation of 4-NP and MB by the p-CeO₂ and m-CeO₂ nanostructures.

Generally, photocatalytic processes comprises the generation of electron–hole pairs through the absorption of suitable photon energy, separation of the photogenerated charge carriers, and surface redox reactions, which are responsible for the production of highly active oxidation species. On the other hand, expansion in the visible light absorption region alone cannot ensure high photocatalytic activity because the separations of the photogenerated electron–hole pairs and their migration to the surface reaction sites also plays an important role in determining the photocatalytic performance.^1,2,5 Upon the irradiation of visible light, the vacancy states might also promote visible-light absorption and the generation of photoexcited electron–hole pairs over the surface of m-CeO₂. The superior visible light photocatalytic activity of m-CeO₂ can be explained based on the defects (Ce³⁺ and oxygen vacancy) and band gap narrowing. The transformation of Ce⁴⁺ to Ce³⁺ and oxygen vacancies might have played a key role in increasing the photocatalytic activities of m-CeO₂.

3.10. Photoelectrochemical study

EIS was performed under dark and visible light irradiation to supply further evidence to support the enhanced visible light photocatalytic activity of m-CeO₂ (Fig. 11a). The semicircle in the Nyquist plot is the expected response of the simple circuit. EIS Nyquist plots were also employed to examine the charge transfer resistance and separation efficiency between the photogenerated electrons and holes.^21,45,46 The semicircular arcs in the EIS spectra were assigned to the contribution from the charge transfer resistance (R_ct) and constant phase element (CPE) at the photocatalyst/electrolyte interface. Fig. 11a shows that the arc radius in the EIS of m-CeO₂ under visible light irradiation is smaller than that of p-CeO₂. This suggests that m-CeO₂ has a lower resistance and effective separation of photogenerated electron–hole pairs than that of p-CeO₂. Moreover, the m-CeO₂ exhibits more effective separation of the photogenerated electrons–holes pairs and faster interfacial charge transfer.²¹ EIS showed that the photo active sample (m-CeO₂) with a lower charge transfer resistance has a higher photocatalytic activity for dye degradation.

Fig. 11 (a) Nyquist plots of the p-CeO₂ and m-CeO₂ nanostructures photoelectrodes under dark and visible light irradiation, and (b) linear scan voltammograms obtained for p-CeO₂ and m-CeO₂ nanostructures photoelectrodes under dark and visible light irradiation.

3.11. Photocurrent measurement

To examine the possible mechanisms for the enhanced photoreactivity, the LSV for the p-CeO₂ and m-CeO₂ nanostructures (Fig. 11b) were measured in the dark and visible light illumination.⁴⁷ The improvement in the photocurrent response of m-CeO₂ was attributed to the increase in light absorption ability due to defects that create a mid-gap density of states that help narrow the band gap of CeO₂. Owing to the sufficiently narrow band gap of m-CeO₂, the valence electrons can be excited to the conduction band state by absorbing visible light. The photoelectrochemical activity is determined by both the light-harvesting capacity and the separation of electron–hole pairs.^47–50 The electron–hole pairs are generated by absorbing the incident photons with energies greater than E_g, and recombination might occur unless they are not separated quickly. Generally, a high photocurrent suggests that the sample holds strong ability for generating and transferring the photoexcited charge carrier under irradiation.¹⁷ m-CeO₂ has a higher photocurrent than p-CeO₂ under the same conditions, suggesting that m-CeO₂ exhibits stronger ability in the separation of electron–hole pairs than p-CeO₂, as shown by PL analysis. The significant improvement in the photocurrent of m-CeO₂ suggests that it may be excited easily by visible light, and produce more photoinduced carriers, thereby possessing higher visible light photocatalytic activity.¹⁷ In addition, the EIS Nyquist plot (under visible light) also showed that the efficiency of charge separation of m-CeO₂ is much higher than that of p-CeO₂, indicating that m-CeO₂ has higher visible photocatalytic activity.

3.12. Defect-induced DC electrical conductivity

The defects created in m-CeO₂ by EAB and their effects on the electrical properties of m-CeO₂ were studied to understand the performance in terms of applications. Nano CeO₂, like most metal oxide nanostructures, exhibits conductivity in the semiconducting range. The poor electrical conductivity limits the application of CeO₂ in electrical devices.⁴ Many studies have reported that metal doping increases the electrical conductivity. Q. Li,⁵¹ in their Mo-doped CeO₂-materials, reported an increase in the electrical conductivity with increasing the Mo content. Similar results were also reported by L. Chen et al.,⁵² and Z. Khakpour et al.,⁵³ in their Gd-doped CeO₂ thin films and Sm-doped CeO₂, respectively. The preferential defect (oxygen vacancy) formation at the grain boundary sites is indicated as the major reason for the increase in electrical conductivity.⁵⁴ In the present case, the EAB induced defects (oxygen vacancy) in CeO₂, which is advantageous because of its greener route compared to the other methods reported. The DC electrical conductivity of p-CeO₂, as measured using the 4-in-line probe device, was found to be 3.142 × 10⁻⁶ S cm⁻¹, whereas that of m-CeO₂ was 3.142 × 10⁻³ S cm⁻¹, which is 1000 times higher than p-CeO₂. On the other hand, the measured DC electrical conductivity of the m-CeO₂ was too high to be attributed only to the defects created by EAB. Therefore, an EAB can be an effective tool for inducing defects, and increasing the electrical conductivity of semiconducting metal oxide materials.

3.13. Stability and reusability of the m-CeO₂

The reusability of m-CeO₂ was tested by centrifuging the catalyst from the dye solution, later washing with DI water and drying in an air oven at 100 °C. The reused catalyst showed a similar response to that of the fresh catalyst, highlighting the stability and reusability of m-CeO₂ nanostructures.

4. Conclusions

A pure CeO₂ nanostructure was modified by EAB for visible light applications. XRD, DRS, XPS, PL, Raman, EPR, EIS, visible light photocatalysis and photocurrent (LSV) showed that defect-induced properties developed in pure CeO₂ after the interaction with electrons and protons produced by EAB. A mechanism for modifying the p-CeO₂ nanostructure was proposed. The absorption spectrum of m-CeO₂ exhibited a red shift and remarkably enhanced PL intensity with respect to the p-CeO₂ nanostructure due to an increase in the number of defects in the m-CeO₂ nanostructure. m-CeO₂ also showed significantly higher photocatalytic activity towards 4-NP and MB degradation under visible light irradiation, which was attributed to an increase in the number of defects. These enhanced optical properties, visible light photocatalysis and resulting high photoelectrochemical performance of the m-CeO₂ nanostructures provide an interesting novel approach that can advance the field of photocatalysis. The DC electrical conductivity of m-CeO₂ was higher than that of p-CeO₂ under ambient conditions possibly due to defect formation. These results suggest that Ce³⁺ and oxygen vacancies can be produced in the p-CeO₂ nanostructure by a biogenic method. This novel concept to engineer the band gap of CeO₂ is expected to have a significant impact in a range of applications.

Acknowledgements

This study was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (Grant no.: 2012R1A1A4A01005951).

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Footnote

† Electronic supplementary information (ESI) available: Schematic representation of modification process, HAADF FE-TEM images, EDX, DRS spectra, XPS survey, C1s and EPR spectra of p-CeO₂ and m-CeO₂ nanostructures. See DOI: 10.1039/c4ra00861h

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