Preparation of porous 3D Ce-doped ZnO microflowers with enhanced photocatalytic performance

Abstract

Porous 3D Ce-doped ZnO microflowers were prepared by a hydrothermal method followed by a low temperature annealing process. The effects of Ce doping on the structural and photocatalytic properties of porous ZnO microflowers were investigated in detail. The samples were characterized using XRD, SEM, EDS, XPS, DRS, PL spectra and BET surface area measurements. According to the XRD analysis, both of the crystalline structures of the synthesized pure ZnO and Ce-doped ZnO samples are hexagonal wurtzite. The XPS results demonstrated the successful synthesis of Ce⁴⁺ doped ZnO. In addition, the SEM morphologies showed the unique porous 3D flower-like structure of the Ce-doped ZnO. Compared with the porous ZnO microflowers, the Ce-doped ZnO samples exhibit improved photocatalytic performance in the decomposition of Rhodamine B (RhB). It is proposed that the special structural feature with a porous 3D structured and Ce modification lead to the rapid photocatalytic activity of the Ce-doped ZnO microflowers.

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1. Introduction

Environmental pollution is one of the main problems and challenges that human beings have to face in contemporary society, and much basic and applied research on environmental remediation has been carried out, especially on photocatalysts of wide band gap semiconductors.^1,2 Semiconductor photocatalysts, as environmentally friendly catalysts, have the potential ability to remove contaminants in air and water without causing additive damage to our environment.^3–5 Under light irradiation, pollutants could be decomposed into non-toxic substances on the surface of the photocatalyst. ZnO, a wide direct band gap (3.37 eV) metal-oxide semiconductor, is a promising photocatalytic material owing to its low cost, excellent catalytic efficiency, high physical and chemical stability, and environmental sustainability.⁶ Additionally, the ZnO photocatalyst can be easily manipulated with desirable micro and nanoscale structures, which are proven to be important factors affecting the photocatalytic activity. Among numerous structures, the 3D porous structure of ZnO has attracted tremendous interest due to superior photocatalytic properties as compared with various low-dimensional ZnO structures.^7–9 Such a special structure not only avoids the self-aggregation of small particles, but also gives a larger surface area and provides more active sites during the reaction, facilitating the diffusion and mass transportation of organic molecules and increasing the photocatalytic reaction rate.¹⁰

However, the rapid recombination of photogenerated electron–hole pairs formed in photocatalytic processes is a major obstacle for increasing the photocatalytic efficiency of ZnO.¹¹ To solve this problem, one of the efficient methods is the modification of semiconductors with electron scavenging agents, such as noble metals,^12–14 metal oxides,^15,16 carbon materials,^17,18 and so on. Recently, there have been some reports of using some dopants that can act as trapping sites to retard the electron–hole recombination rate and increase the photocatalytic activity.^19,20 Among a variety of dopants, rare-earth doped ZnO materials have potential applications in many technologies such as degradation of pollutants and optoelectronic devices.^21,22 Cerium is a major element in the useful rare earth family. Some attention has been paid to the synthesis of Ce-doped zinc oxide for applications in various technologies. Li et al. synthesized Ce-doped ZnO hollow nanofibers via facile single capillary electrospinning and measured their gas sensing properties.²³ Rezaei et al. prepared Ce-doped ZnO nanoparticles in water by refluxing for 3 h at about 90 °C and applied them to a photodegradation reaction.²⁴ Jung et al. studied the optical properties of Ce-doped ZnO nanorods fabricated via a hydrothermal method on a Si (100) substrate.²⁵ Although there are a few reports about Ce-doped ZnO materials which have been successfully synthesized, there is little literature reported for the fabrication of the architectures of porous 3D flower-like Ce-doped ZnO composites and the study of their photocatalytic properties.

In the present research, we prepared porous 3D Ce-doped ZnO microflowers using a low temperature-hydrothermal method followed by a heat treatment process for the first time. The effects of cerium doping on the structural and optical properties of ZnO microflowers were also investigated in detail. Finally, the photocatalytic activity of the as-prepared composites is studied, and a possible mechanism of photocatalysis is also discussed and proposed.

2. Experimental section

2.1 Chemicals

All reagents, including zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O, AR), hexamethylenetetramine (HMT, AR), cerium nitrate hexahydrate (Ce(NO₃)₃·6H₂O, 99.95%) and oxalic acid dehydrate (H₂C₂O₄·2H₂O, AR) were purchased from Beijing Chemical Reagent Research Company and Shanghai Aladdin Industrial Corporation. Distilled water was used for all experimental processes.

2.2 Synthesis of porous 3D Ce-doped ZnO microflowers

3 mmol of Zn(NO₃)₂·6H₂O, 3 mmol of HMT and different amounts of Ce(NO₃)₃·6H₂O were dissolved in 80 mL of distilled water. After stirring for 15 min, 0.3 mmol of H₂C₂O₄·2H₂O was added into the above solution. Then, after additional agitation for 30 min, the mixture was transferred to a 100 mL Teflon-lined autoclave and sealed. The autoclave was heated up to 90 °C and maintained at this temperature for 3 h. After naturally cooling down to room temperature, the resulting precipitates were collected by centrifugation, washed three times with distilled water and ethanol, and then dried at 70 °C in air for 12 h. The samples were retrieved through a heat treatment of the precursors at 400 °C in air for 1 h with a heating rate of 1 °C min⁻¹. A variety of porous 3D Ce-doped ZnO microflower composites was prepared by adding different molar concentrations of Ce(NO₃)₃·6H₂O to the Zn(NO₃)₂·6H₂O precursor. The molar ratio of Ce(NO₃)₃·6H₂O/Zn(NO₃)₂·6H₂O was changed in the range of 0 to 5% and the samples were denoted as ZnO, CZ-0.25%, CZ-0.5%, CZ-1%, CZ-3%, and CZ-5%, in which the last number corresponds to the molar ratio of Ce(NO₃)₃·6H₂O/Zn(NO₃)₂·6H₂O.

2.3. Characterization

The crystallographic structures of the porous 3D Ce-doped ZnO microflowers were collected using powder X-ray diffraction using a D/Max-IIIC (Rigaku, Japan) with Cu Kα radiation. The morphology and dimension measurements of the as-synthesized microstructures were carried out using an S-4800 field emission scanning electron microscope (Hitachi, Japan) equipped with an energy-dispersive X-ray spectrometer (JEOL JXA-840). X-ray photoelectron spectroscopy (XPS) analysis was performed on a PHI-5000CESCA system with Mg Kα radiation (hν = 1253.6 eV). The X-ray anode was run at 250 W and the high voltage was kept at 14.0 kV with a detection angle at 540°. All the binding energies were calibrated using the containment carbon (C 1s = 284.6 eV). The Brunauer–Emmett–Teller (BET) surface area of the powders was analyzed using nitrogen adsorption on a Micromeritics ASAP 2020 nitrogen adsorption apparatus (U.S.A.). The sample was degassed under vacuum at 150 °C for 7 h before the nitrogen adsorption measurement. The BET surface area was determined using a multipoint BET method using the adsorption data in the relative pressure (P/P₀) range of 0.01–0.99. Adsorption branches of the isotherms were used to determine the pore size distributions for the samples studied via the Barrett–Joyner–Halenda (BJH) method. The volume of nitrogen adsorbed at the relative pressure (P/P₀) of 0.99 was used to determine the pore volume. The photoluminescence (PL) spectra of porous 3D Ce-doped ZnO microflowers were recorded using a Hitachi F-7000 spectrophotometer equipped with a 150 W xenon lamp at 341 nm excitation wavelength. All measurements were performed at room temperature.

2.4 Photocatalytic experiments

The photocatalytic activity of the as-prepared samples was evaluated by using RhB as the targeted dye pollutant. A high-pressure Hg UV lamp (GGZ175, 175 W) with a maximum emission at 365 nm served as the UV light source. In a typical procedure, 0.04 g of catalyst was dispersed into 20 mL of the aqueous solution of RhB (8 mg L⁻¹). After stirring in the dark for 30 min, the suspensions were placed under UV-light irradiation. After the photoreaction, the samples were collected at regular intervals and centrifuged to remove the catalyst. The concentration of the target organic solution before and after degradation was measured using a UV-vis spectrometer (UV-2550, Shimadzu, Japan).

3. Results and discussion

3.1. Morphological and structural analysis

The phase characteristics of the synthetic samples were investigated using powder X-ray diffraction (XRD). Fig. 1 shows the XRD patterns of the porous 3D ZnO microflowers and Ce-doped ZnO microflowers with different amounts of cerium ions, respectively. For pure ZnO, the diffraction peaks are consistent with (100), (002), (101), (102), (110), (103), (200), (112), (201), and (202) planes in agreement with the wurtzite hexagonal crystalline phase of ZnO (JCPDS no. 36-1451). The sharp diffraction peak shown in the XRD patterns illuminates that ZnO was highly crystallized. However, for Ce-doped ZnO, with increasing doping concentration, the intensity of the diffraction peaks decrease, the crystalline quality of ZnO degrades, suggesting that increasing the doping concentration deteriorated the crystal structure of ZnO. Similar dopant-induced changes in crystallization have been observed in nickel-doped ZnO.²⁶ We also found that for a lower concentration of Ce-doping no other crystalline impurities were detected in the XRD pattern. This may be due to the fact that at low concentrations of cerium ions, these ions would uniformly substitute Zn²⁺ sites or interstitial sites in the ZnO lattice.²⁷ However, at higher concentrations of Ce doping (3% and 5%), a small peak located at about 2θ = 28.4° attributed to the (111) reflection plane of CeO₂ showed up. Similar behavior of cerium doped ZnO nanocrystals was reported by Kannadasan et al.²⁸

Fig. 1 The XRD patterns of ZnO and Ce-doped ZnO.

Fig. 2a and b demonstrate the SEM images of the synthesized CZ-1% sample. As is shown in Fig. 2a, the CZ-1% exhibits a well-defined 3D morphology which resembles a peony flower pattern. The detailed observation of a single CZ-1% microflower is shown in Fig. 2b, the entire flower-like structure is assembled from numerous porous nanoplates with an average thickness of ∼15 nm. It can be found that these nanosheets intersect with each other and result in flower-like structures. In addition, we can obviously observe that there are numerous pores in the nanoplates, which are shown in the inset of Fig. 2b. The large amount of nanopores in the nanosheets should be beneficial to enlarge the contact area for enhanced reactant diffusivity, which is highly favorable for catalytic activity.²⁹ The EDS analysis was done and the result is mentioned in Fig. 2c. In the EDS spectrum, numerous well-defined peaks were evident and related to Zn, O and Ce which clearly support that the synthesized CZ-1% is composed of Zn, O and Ce. No other peak related to impurities was detected in the spectrum which further confirms that Ce-doped ZnO was successfully synthesized.

Fig. 2 SEM (a and b) and EDS (c) patterns of the porous flower-like CZ-1% sample.

To further assess the effect of the amount of Ce doping on the morphology, Fig. 3 shows the SEM micrographs of the as prepared products with different Ce doping concentrations. As can be seen from Fig. 3a, the undoped ZnO is composed of 3D flower-like structures with smooth surfaces. It is clear that the degree of damage to the product morphology is smaller when the Ce doping concentration is lower. ZnO remains a flower-like structure when the Ce doping concentration is 3% (Fig. 3b–e). With the incorporation of Ce ions into ZnO, the flower-like structure partly broke up and the morphology became rougher. When the doping concentration is 5% (Fig. 3f), some branches of the flower-like aggregate decrease and the nanosheets of the hierarchical porous microflowers become severely damaged. This phenomenon is more obvious, and the reason is that the Ce ionic radius is larger than the Zn ionic radius and Ce can replace the Zn ion when it enters into the lattice of ZnO and may damage the crystal structure. The greater the proportion of Ce ions entering the ZnO lattice, the more Zn ions are replaced by Ce. Therefore the ZnO original crystal structure is destroyed gradually.³⁰

Fig. 3 SEM pattern for (a) undoped ZnO, (b) CZ-0.25%, (c) CZ-0.5%, (d) CZ-1%, (e) CZ-3%, and (f) CZ-5%.

3.2. XPS analysis

In order to determine the surface composition and chemical state of the sample, CZ-1% was studied using the XPS technique. The survey result in Fig. 4a demonstrates that all peaks are ascribed to Zn, Ce, O and C (due to air contamination and CO/CO₂ adsorption on the surface), which confirms that CZ-1% is composed of the three elements of Zn, Ce and O. The results are in good agreement with XRD and EDS as described above. The high-resolution scans of Zn 2p, Ce 3d, and O 1s were carried out. As can be seen from Fig. 4b, the XPS spectrum of Zn 2p gives peaks centered at 1021.8 eV (Zn 2p3/2) and 1044.6 eV (Zn 2p1/2) respectively, which are similar to that of pure ZnO. This finding confirms that Zn exists mainly in the Zn²⁺ chemical state on the surface of the sample.³¹ In Fig. 4c, the O 1s profile can be fitted to two peaks, indicating that two different kinds of O species existed in the sample. The peaks are attributed to the crystal lattice oxygen in ZnO (located at 530.2 eV) and the adsorbed oxygen on the catalyst surface (located at 531.5 eV), respectively.³² In the Ce 3d spectrum (Fig. 4d), six peaks corresponding to three pairs of spin–orbit doublets could be identified as the Ce 3d5/2 (v₀, v₁, v₂) and Ce 3d3/2 (v′₀, v′₁, v′₂), which clearly indicates that the Ce ions have a +4 oxidation valence state.³³ The high binding energy doublet v₂, v′₂ at 898.9 and 916.5 eV originates from the final state of Ce(IV) 3d⁹4f⁰O2p⁶, doublet v₁, v′₁ at 888.7 and 907.5 eV is attributed to the state of Ce(IV) 3d⁹4f¹O2p⁵, and doublet v₀, v′₀ at 882.5 and 901.7 eV corresponds to the state of Ce(IV) 3d⁹4f²O2p⁴. All this confirmed that the samples were doped with cerium in the form of Ce(IV).³⁴ Besides, these values of the Ce 3d binding energy for the Ce-doped ZnO microflowers slightly shifted compared with the standard XPS energy peak locations of Ce 3d.³⁵ The slight shift indicates that the Ce–O bond length in ZnO has changed due to Ce⁴⁺ doped on the ZnO sample, which is similar to ZnO:Tb thin films.³⁶ In addition, no signals of other impurities are detected from Fig. 4, implying the synthesis of Ce⁴⁺-doped ZnO microflowers.

Fig. 4 The XPS spectra of the porous CZ-1% microflower: (a) the XPS full spectra of the samples, and the high resolution scans of (b) Zn 2p, (c) O 1s and (d) Ce 3d.

3.3. BET surface measurements

To investigate the specific areas and the porous nature of the ZnO and Ce-doped ZnO microflowers, Brunauer–Emmett–Teller (BET) gas sorption measurements were performed. The nitrogen adsorption/desorption isotherms and the pore size distribution plots of the porous flower-like ZnO and CZ-1% samples are shown in Fig. 5. All the samples display the type IV curve accompanied by a type H3 hysteresis loop, which is attributed to the predominance of mesopores, according to the IUPAC classification.³⁷ The pore size distributions of ZnO and CZ-1% are given in the inset of Fig. 5. The samples have a bimodal pore distribution using the Barrett–Joyner–Halenda (BJH) method. Clearly, the smaller-sized pores are intrinsic within the nanoplates of ZnO and CZ-1%, whereas the larger-sized pores are mainly due to interstices between the nanoplates.³⁸ In addition, the BET surface area, pore size and crystal size of all the samples are summarized in Table 1. It can be seen that the specific surface area of the Ce-doped ZnO is higher than that of native ZnO. This increase in surface area can be attributed to a decrease in the crystal size of the nanoparticles. By applying the Scherrer formula for the broadening of the (101) peak reflection of ZnO, the crystal sizes of the samples of pure ZnO and Ce-doped ZnO with different Ce doping concentrations were found to be 23.4, 19.2, 18.1, 15.6, 22.2, and 21.1 nm (shown in Table 1), respectively, indicating that doping with Ce creates surface defect sites and surface disorder, and has a depression effect on the growth of crystal size. Systematically when the Ce concentrations were increased from 0 to 1%, the specific surface area increased, which might be due to the decrease in the crystallite size. However, the specific surface area decreased when the Ce concentration was further increased to 3%. This might be because the structure of the porous microflowers was damaged and these particles aggregate, increasing the crystallite size. It is consistent with the results of SEM. Thus, it is evident that the cerium amount influences significantly the surface area of photocatalysts, and our conclusion is supported by the literature.^39–41

Fig. 5 N₂ adsorption/desorption isotherms and BJH pore diameter distribution of the porous ZnO and CZ-1% microflowers.

Table 1 BET surface area of the porous flower-like ZnO and Ce-doped ZnO with different Ce doping concentrations

Samples	Surface area (m^2 g^−1)	Pore size (nm)	Crystal size (nm)
ZnO	10.47	18.62	23.4
CZ-0.25%	11.30	20.53	19.2
CZ-0.5%	12.10	15.19	18.1
CZ-1%	21.07	16.15	15.6
CZ-3%	13.44	17.22	22.2
CZ-5%	16.62	16.20	21.1

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3.4. UV-vis diffuse reflectance and photoluminescence spectrum

Diffuse reflectance UV-visible spectra of the undoped ZnO and Ce-doped ZnO samples with different Ce contents (0.25%, 0.5%, 1%, 3% and 5%) are presented in Fig. 6. The strong absorption in the UV region (200–400 nm) is assigned to ZnO. It can be seen that the pristine ZnO possesses the most light absorption between 200–400 nm. This indicates that our uniform 3D microstructure of ZnO has excellent light trapping characteristics. Both the hierarchical organized structure itself and the voids between and within the nanosheets are advantageous for enhanced light absorption due to the reflection of trapped incident light within the samples,^7,8 and the hierarchical ZnO microflowers ensure a large specific surface which is favorable for enhancing the light absorption and light propagation; as confirmed by the SEM and BET surface area results. Undoped porous ZnO microflowers had no absorption in the visible region (>400 nm). However, the Ce-doped ZnO microflowers exhibited a broad absorption tail band between 400 and 500 nm. By comparing ZnO with Ce-doped ZnO, it can still be found that the doping leads to a noticeable red-shift in the absorption band in the visible light region. The reason may be attributed to the appearance of a new electronic state in the ZnO band-gap. The distance of charge transfer between the f electrons of the cerium ions and the conduction band (CB) or valence band (VB) of ZnO is narrowed, thus allowing visible light absorption. When the doping amount of Ce is increased from 0 to 1%, the absorbance intensity of the Ce-doped ZnO microflowers increases in the visible light region. It is due to the fact that the Ce doped into the ZnO crystal grains can greatly increase the visible light absorption ability. When the doping concentration was increased to 3% and 5%, the visible light absorption ability of the Ce-doped ZnO decreased. As we know, doping is usually accompanied by the formation of defects, which can play the role of trap centers to photoelectrons, but excessive doping may lead to some surface defects acting as recombination centers for electron–hole pairs and thus decrease the photocatalytic activity. Similar optical spectra were observed in some previous literature.^42,43 It reveals that the Ce-doped ZnO microflowers may have photocatalytic properties under sunlight irradiation.^44–46 Besides, the optical absorption of the Ce-doped ZnO microflowers in the UV region was enhanced. The band gap value of the samples is calculated using the formula E_g (eV) = 1240/λ_g (nm), where λ_g stands for the wavelength value corresponding to the intersection point of the vertical and horizontal parts of the spectra, and a narrowing of the band gap value is observed with an increase in cerium content (3.06 eV for ZnO, 3.04 eV for CZ-0.25%, 3.01 eV for CZ-0.5%, 2.98 eV for CZ-1%, 2.96 eV for CZ-3%, and 2.93 eV for CZ-5%).

Fig. 6 UV-vis diffuse reflectance spectra of the porous 3D ZnO and Ce-doped ZnO with various Ce content.

The photoluminescence (PL) spectra of the undoped and Ce-doped ZnO samples were evaluated to observe the electronic structure and fate of electron and hole separations.⁴⁷ The PL is excited at a wavelength of 334 nm. The excitation wavelength was determined from the absorption spectra. As seen in Fig. 7, two types of major emission peaks were observed, i.e. one sharp peak near the UV region and a broad peak in the visible light region. The sharp UV emission can be ascribed to the near-band-edge emission of ZnO originating from the electron–hole recombination.^48,49 It is worth noting that the PL spectra of Ce-doped ZnO shows a decrease compared to the pure ZnO in the intensity of the UV emission peak, which indicates the reduction of the recombination centers for the electrons and holes in the samples when doping Ce on ZnO. Moreover, it can be seen clearly that the sample CZ-1% shows the minimum PL intensity. When the amount of Ce increased further, up to 3%, the intensity of the spectra increased, and indicated that the recombination of the electrons and holes increased. The increase of the recombination of electrons and holes can be attributed to the surface defects created by the excess dopant ions, which act as the recombination centers, and consequently, increase the charge recombination.⁵⁰ The charge separation and recombination impose a direct impact on the photocatalytic activity of the catalyst. According to the above analysis, it can be seen that doping Ce can make the intensity of PL decrease. The weaker the PL response, the better the separation efficiency of the photoinduced electron–hole pairs, and the higher the photocatalytic performance of the Ce-doped ZnO composites will be, which will be discussed in Section 3.5.

Fig. 7 The PL spectra of the porous 3D ZnO and Ce-doped ZnO with various Ce doping amounts.

3.5. Photocatalytic performance

The photocatalytic degradation of RhB has been chosen as a representative reaction to evaluate the photocatalytic activities of the as-synthesized Ce-doped ZnO under UV irradiation. As a contrast, the curves showing the degradation of RhB by undoped ZnO and the blank experiment (without any catalyst added) were also presented. As shown in Fig. 8, there is only a little decrease of RhB concentration in the blank experiment after UV irradiation for 120 min, indicating that the photolysis of the RhB solution with the blank is negligible in our experimental conditions. The porous Ce-doped ZnO catalysts exhibit higher photocatalytic activities compared to the undoped ZnO microflowers, indicating that the Ce-dopant played a significant role. Furthermore, the photocatalytic efficiency varied with the amount of Ce in the samples, and the CZ-1% exhibited the highest photocatalytic activity, which was also corroborated by the PL behavior. When the Ce amount increases from 0.25% to 1%, there is more of the dominant Ce⁴⁺ to capture electrons, resulting in a greater photocatalytic activity. But, if the amount of Ce⁴⁺ ions is higher than the optimum amount (x > 1%), the high concentration of dopant ions act as electron and hole recombination centers and hence the photocatalytic activity decreases.⁵¹ The photocatalytic degradation of RhB can be considered a pseudo-first-order reaction. The efficiency of RhB photodegradation by the photocatalyst was determined quantitatively using the pseudo-first-order model as follows: ln(C/C₀) = −kt, where C₀ and C are the concentrations of dye at time 0 and t, respectively, and k is the apparent rate constant (min⁻¹). The pseudo-first-order rate constants, k, of the various photocatalysts are shown in Table 2. The results clearly demonstrate that the CZ-1% sample shows the highest catalytic activity with a rate constant k = 1.2173 min⁻¹ that is about 2 times higher than that of pure ZnO (0.6520 min⁻¹). These results show that the photocatalytic activity of ZnO can be obviously improved in the presence of a Ce-dopant.

Fig. 8 The photodegradation curves of RhB by different catalysts under UV-light irradiation.

Table 2 The apparent rate constant, k, calculated for different catalyst systems

Photocatalysts	Kinetic constants k (min^−1)
Blank	0.0021
ZnO	0.6520
CZ-0.25%	0.7529
CZ-0.5%	0.8951
CZ-1%	1.2173
CZ-3%	1.0210
CZ-5%	0.7881

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3.6. The photocatalytic mechanism

A mechanistic scheme diagram of the charge separation and photocatalytic reaction for Ce-doped ZnO photocatalyst is shown in Fig. 9. For the undoped ZnO, the photocatalyst was irradiated using UV-light and the electrons (e⁻) in the valence band (VB) were excited to the conduction band (CB) with the generation of holes (h⁺) in the VB (eqn (1)). Generally, these electrons and holes recombine quickly, leading to a decrease in the catalytic activity of ZnO. For the Ce-doped ZnO, the Ce 4f level has crucial influences on the photo-excited charge generation and transfer, together with the inhibition of electron–hole recombination. Ce⁴⁺ could act as an effective electron scavenger to trap the conduction band (CB) electrons of the Ce-doped ZnO photocatalysts. Ce⁴⁺ acts as a stronger Lewis acid than O₂. The Ce⁴⁺ ion is superior to O₂ in its capability of trapping electrons (eqn (2)). The electrons can transfer to the adsorbed O₂ via an oxidation process (eqn (3)), and produce a superoxide radical anion (˙O₂⁻).⁵² Meanwhile, the photoinduced holes in ZnO can be also readily scavenged by the immanent H₂O molecules to yield ˙OH radicals (eqn (4)). The highly reactive superoxide radical anion (˙O₂⁻) and hydroxyl radical (˙OH) are responsible for the degradation of the organic chemicals, which can be destroyed into mineral acids, CO₂ and H₂O (eqn (5) and (6)). This process can be proposed as follows:⁵³

ZnO + hν → e⁻ + h⁺ (1)

Ce⁴⁺ + e⁻ → Ce³⁺ (2)

Ce³⁺ + O₂ → Ce⁴⁺ + ˙O⁻₂ (3)

h⁺ + H₂O → ˙OH + H⁺ (4)

RhB + ˙O⁻₂ → mineral acids + CO₂ + H₂O (5)

RhB + ˙OH → mineral acids + CO₂ + H₂O (6)

Fig. 9 UV-light-induced charge separation and the photocatalytic mechanism of the porous Ce-doped ZnO.

4. Conclusions

In summary, we successfully synthesized porous 3D Ce-doped ZnO microflowers using a hydrothermal method followed by heat treatment for the first time. The influence of the cerium dopant on the structural and photocatalytic properties of the synthesized porous ZnO microflower samples has been investigated in detail. According to the XPS analysis the cerium doped ZnO microflowers contain mainly Ce⁴⁺ ions. The BET surface area measurements showed an increase in the specific surface area of the samples after the incorporation of Ce⁴⁺. All Ce-doped samples showed a significantly higher photocatalytic activity compared to the pure porous ZnO for the degradation of RhB. Moreover, the CZ-1% microflower exhibits the highest photocatalytic activity among the prepared samples. Ce, with a moderate amount doped on ZnO, can act as an electron trap and suppress the photogenerated electron–hole pair recombination and result in the improvement of the photocatalytic performance of the samples. The PL spectra reveal the inhibition of the recombination of the photogenerated electron and hole pairs by the samples, which is consistent with the photocatalytic performance results.

Acknowledgements

This work was supported by the Mineral and Ore Resources Comprehensive Utilization of Advanced Technology Popularization and Practical Research (MORCUATPPR) founded by the China Geological Survey (grant no. 12120113088300). It was also supported by the Key Technology and Equipment of Efficient Utilization of Oil Shale Resources (no. OSR-05) and the National Science and Technology Major Projects (no. 2008ZX05018).

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