Functionalization of ZnO aggregate films via iodine-doping and TiO₂ decorating for enhanced visible-light-driven photocatalytic activity and stability

Abstract

A multifunctional visible-light-driven photocatalyst composed of an iodine-doped ZnO aggregate (ZnO:I) film post-decorated by TiO₂ (ZnO:I@TiO₂) on fluorine-doped tin oxide-coated (FTO) glass via a hydrothermal method and subsequent wet-chemical process is demonstrated. A series of ZnO:I with various iodine concentrations was firstly prepared to study iodine dopant amount dependent-photocatalytic activity. The photocatalytic measurement results showed that the ZnO:I film with an optimum I-doping ratio of 5.0 mol% achieved a degradation efficiency of 93.7% toward RhB, which is much higher than that of undoped ZnO (only ∼54.3%). Furthermore, the ZnO:I@TiO₂ exhibited enhanced light absorption and a charge separation efficiency of photogenerated e⁻–h⁺, as characterized by UV-vis absorption spectra and the photoelectrochemical characterization by EIS, which originates from iodine doping and TiO₂ post-modification. Owing to these synergic advantages, the ZnO:I@TiO₂ composite photocatalyst exhibited significantly enhanced decomposition activity for RhB (∼97% after 4 h of irradiation, a 79% increase over pure ZnO) under visible irradiation. Additionally, the ZnO:I@TiO₂ films exhibited enhanced chemical stability in both acidic and alkaline solutions in comparison to ZnO:I, which was further verified by repeated photodegradation experiments under visible light irradiation. These results indicate that the prepared ZnO:I@TiO₂ could serve as an efficient photocatalytic material to degrade organic pollutants in aqueous eco-environments.

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Introduction

Photocatalytic degradation of organic pollutants using nanostructured semiconductors offers great potential for environmental purification.^1,2 To date, a large number of semiconductor materials, such as metal oxides and sulfides (such as TiO₂, WO₃, CdS, ZnS, ZnO), have been identified as active photocatalysts for photodegradation of organic pollutants.^3–7 In the past decade, ZnO has attracted extensive interest owing to its wide band gap (E_g = 3.37 eV), large exciton binding energy (60 meV), high carrier mobility (∼205–1000 cm² V⁻¹ s⁻¹), non-toxicity, abundant availability, and low cost.⁸ In general, high inner specific surface area, fast electron transport and outstanding light-scattering capability are prerequisites for an excellent photocatalyst in efficient environmental purification.⁹ For ZnO, its unique merit, namely simple tailoring of the nanostructures, and easy modification of the surface structure, opens wide possibilities to accommodate all of the above favorable characteristics through control of morphology and structure.¹⁰ To this end, several exceptional structures, such as 1D nanorods and 2D nanosheets, are fabricated and used as photocatalyst for enhancing the photocatalytic performance because of their high charge transportation capability.^11,12 However, such 1D and 2D nanostructures seems to possess low specific surface area, and consequently results in insufficient photocatalytic performance. Recently, the 3D ZnO nanocrystalline aggregate assembled with high-specific-surface-area nanocrystals, as one of the hotspot nanostructures, is thought to be able to efficiently harness sunlight because of their superior light-scattering effect arisen from the sub-micrometer size and large surface area provided by the nanocrystals.¹³ Unfortunately, very poor response to visible light and high recombination ratio of photoinduced electron–hole pairs and photocorrosion have hindered the application of ZnO in photocatalysis.¹⁴ The long-standing challenge remains to improve the fundamental disadvantages of ZnO as a photocatalyst for diverse applications.

A plenty of methods such as ion doping (Mg²⁺, Eu³⁺, Cr⁵⁺, La³⁺, Ce⁴⁺),^15–18 noble metal loading (such as Au, Ag, Cu, Al, and Pt),^19–23 or combining ZnO with another semiconductor (such as C₃N₄, CuO, Cu₂O, and TiO₂),^24–27 have been applied to improve the visible light response of ZnO based photocatalysts. In recent years, ZnO doped or incorporated with non-metal (such as S, C, I, and N) has been proved to be effective way to broaden its light absorption from the UV region to the visible region.^28–30 In particular, iodine is thought to be the most suitable dopant for extending the light response of n-type semiconductor into the visible region.³¹ Furthermore, it has been demonstrated that iodine anchored on the ZnO acts as a conduction-band electron scavenger capable of inhibiting the rapid recombination of photoinduced electron–hole pairs.³² Boukherroub et al. synthesized iodine-doped ZnO (ZnO:I) nanoflower photocatalytic films for dye degradation and found that the photocatalytic performance of ZnO:I under visible light irradiation is obviously higher than that of undoped ZnO.³⁰ Our previous work has also demonstrated that the iodine doping enabled increased light absorption in the visible range and more efficient charge separation of photogenerated e⁻–h⁺ and thus enhanced the photocatalytic performance of ZnO nanorod array (1D ZnO).¹²

On the other hand, pure ZnO still suffers from insufficient electron transport and chemical instability in practical photocatalytic systems. In previous work, coupling with another semiconductor to construct ZnO based heterostructures such as ZnO@CdS, ZnO@CuO, ZnO@GaN, and ZnO@TiO₂ showed the ability to improve the photocatalytic activity of ZnO by promoting the photoinduced charge carrier transfer process.^25,27,33,34 Meanwhile, the chemical stability of ZnO can be enhanced by coating an inert semiconducting layer, such as SiO₂, Al₂O₃, or TiO₂, on the surface of the ZnO.^35–37 Notably, the band gap energies of ZnO and TiO₂ are similar (3.37 eV for ZnO, 3.22 eV for anatase TiO₂), the conduction and valence bands of ZnO are located a little above (by about 0.48 eV, i.e. more negative on the electrochemical scale) the corresponding bands of TiO₂. For ZnO@TiO₂, the electrons photogenerated in ZnO can be transferred into TiO₂ to enhance catalytic reactions on surface, simultaneously the holes can be injected into the valence band of ZnO to prevent the unwanted charge recombination.²⁷ Thus, a combination of all these favorable features mentioned above will likely produce improved photocatalytic and photostable activity for ZnO-based materials applicable in environmental purification.

In this paper, a visible light-responsive iodine-doped ZnO nanocrystalline aggregates (ZnO:I) film post decorated by TiO₂ on fluorine-doped tin oxide-coated (FTO) glass was developed. Hierarchical ZnO aggregate was selected as the photocatalyst matrix owing to their multi-merits: large surface area, efficient scattering centers for enhanced light-harvesting efficiency, and porous structure. The fabricated I-doped ZnO aggregates film incorporated with TiO₂ coating (ZnO:I@TiO₂) was found to exhibit enhanced light absorption intensity, effective separation of photogenerated e⁻–h⁺, and excellent chemical stability. Owing to these synergic advantages, the degradation efficiency of the ZnO:I@TiO₂ sample toward rhodamine B (RhB) reached ∼97% after irradiation for 4 h, an efficiency 79% higher than that of pure ZnO aggregate film. More importantly, the ZnO:I aggregates films decorated by TiO₂ coating possessed high stability and photocatalytic activity both in acidic and alkaline medium. Furthermore, the recycled degradation results showed that the ZnO:I@TiO₂ could serve as an efficient photocatalytic material to degrade organic pollutants in aqueous eco-environments. To clarify the effect of the dopant amount on the iodine doped ZnO photocatalyst, a series of ZnO:I with varying iodine amounts were fabricated and utilized as photocatalyst in photocatalytic degradation of RhB. The photocatalytic measurement results showed that the ZnO:I film with an optimizing I-doping ratio of 5.0 mol% achieved a degradation efficiency of 93.7% toward RhB under visible-light irradiation.

Experimental

2.1. Sample preparation

All chemicals used were analytical grade reagents without any further purification steps. Uniform ZnO:I or ZnO nanocrystalline aggregates were synthesized by the solvothermal process of zinc salt in a polyol medium. As an example, 5.0 mol% ZnO:I (I/Zn molar ratio of 5.0%) were obtained as follows: 0.09 g iodic acid (HIO₃) and 2.19 g zinc acetate dihydrate was added to diethylene glycol (100 mL) with vigorous agitation. The mixed solution was heated in an oil bath at a rate of 5 °C min⁻¹ and the hydrolysis was performed under reflux at 160 °C for 8 h. The monodispersed ZnO:I colloids were formed by repeated centrifugation and sonication to remove the supernatant. Finally, the obtained products were calcined at 450 °C for further characterizations. In parallel, other samples with different I-doping ratios (I/Zn molar ratio of 2.5%, 5.0%, 7.5%) and pure ZnO aggregates were also synthesized by following the same procedure as mentioned above. For synthesizing ZnO:I film, a colloid solution containing ZnO:I aggregates, ethyl cellulose and terpineol with a weight ratio of 1 : 0.15 : 1.5 was prepared and subsequently coated on a fluorine-doped tin oxide (FTO) glass substrate via a typical doctor blading method. Afterwards, the ZnO:I@TiO₂ aggregate films were obtained by directly soaking the ZnO:I aggregate films in a solution of 40 mM titanium(IV) isopropoxide dissolved in 2-propanol, followed by heat-treating at 400 °C for 30 min.

2.2. Characterization

The crystalline structure of the as-synthesized ZnO based nanocrystalline aggregate films were identified by X-ray diffraction analysis (XRD) (X'Pert PRO MPD, Panalytical) using Cu Kα radiation. The surface morphology of the ZnO:I@TiO₂ aggregate films was determined from scanning electron microscope (SEM) images on a Hitachi S-4700 microscope. An energy-dispersive spectroscopy (EDS) measurement was performed with an X-ray energy dispersive spectrometer installed on a JEOL-6701F microscope. The absorption spectra were measured using UV-vis spectrophotometry (Lambda 950, Perkin Elmer). The chemical compositions of the I-doped ZnO and ZnO:I@TiO₂ aggregate samples were determined by X-ray photoelectron spectroscopy (XPS) performed on a thermo ESCALAB250 XPS system using an Al Kα X-ray source. The overall composition and spatial distribution of different elements in a selected area were further investigated by scanning transmission electron microscopy elemental mapping with an X-ray energy dispersive spectrometer installed on a JEOL JEM-3010 microscope. Electrochemical and photoelectrochemical measurements were performed in a three-electrode quartz cell with a 0.1 M Na₂SO₄ electrolyte solution; a platinum wire was used as the counter electrode, a saturated calomel electrode (SCE) was used as the reference electrode, and the as-prepared photocatalyst film was used as the working electrode. The photoelectrochemical experiment results were recorded with an electrochemical system. Samples were irradiated in the visible range using a 500 W Xe lamp (Institute for Electric Light Sources, Beijing) with a 420 nm cut-off filter. Potentials were given with reference to the SCE. Electrochemical impedance spectra (EIS) were measured at 0.0 V. A sinusoidal AC perturbation of 5 mV was applied to the electrode over the frequency range of 0.05 to 10⁵ Hz.

2.3. Photocatalytic activity test

The photocatalytic activity was evaluated by the decomposition of rhodamine B (RhB) dyes under visible light irradiation (λ > 420 nm) using a 500 W Xe lamp with a 420 nm cut-off filter, and the average visible light intensity was 100 mW cm⁻². 6.0 cm² of prepared films were immersed in a 40 mL aqueous solution of RhB (initial concentration = 5 × 10⁻⁶ mol L⁻¹). Before illumination, the RhB aqueous solution was stirred for 30 min in the dark to ensure an adsorption/desorption equilibrium was established. During degradation testing, the RhB solution with the photocatalyst film was continuously stirred using a dynamoelectric stirrer and the concentration of RhB was monitored by colorimetry with a UV-vis spectrophotometer.

Results and discussion

3.1. Characterization of ZnO:I@TiO₂ aggregates films

Fig. 1a shows the scanning electron microscopy (SEM) images of photocatalyst films that consist of pure ZnO aggregates. It is observed that the as-synthesized hierarchically-structured ZnO is made of polydisperse spherical aggregates with sizes from 50 to 750 nm. From high-magnification SEM images (inset of Fig. 1a), one can see that each aggregate consists of ∼15 nm-sized nanoparticles. Such hierarchical architecture indeed may provide more reaction space allowing substrate molecules to readily diffuse into or out of the nanoparticle aggregates based ZnO. The ZnO:I film exhibits almost the same hierarchical-structure and morphology with pure ZnO aggregates (Fig. 1b), which indicates that the introduction of HIO₃ have no impact on the formation of ZnO structure. The cross-sectional SEM image in inset of Fig. 1b shows that the average thickness of the ZnO:I aggregate film is ∼15 μm. The as-synthesized ZnO:I aggregate subsequently post-treated via titanium alkoxide to obtain ZnO:I@TiO₂ composite film. The schematic diagram of ZnO:I@TiO₂ aggregate film grown on the FTO glass substrate is shown in Fig. 1c. It is found that the morphology of the ZnO:I@TiO₂ film structure remains intact (Fig. 1d). No obvious difference in structure and morphology of as-prepared films can be observed upon I doping and TiO₂ surface modification. To detect the nice distinction among the three ZnO based films, 3D AFM images were measured to give the surface RMS roughness of 89.7 nm for ZnO, 75.1 nm for ZnO:I, and 106.0 nm for ZnO:I@TiO₂, respectively (see Fig. 2). Note that the increment in the surface RMS roughness of ZnO:I@TiO₂ indicates that TiO₂ nanoparticles are formed on ZnO:I aggregate surface owing to titanium alkoxide post treatment.

Fig. 1 (a) SEM image of ZnO (inset is SEM of single ZnO aggregate), (b) SEM image of ZnO:I (inset is cross-sectional SEM image of as-prepared ZnO:I film), (c) schematic diagram and (d) SEM image of ZnO:I@TiO₂ (inset is cross-sectional SEM image of as-prepared ZnO:I@TiO₂ film).

Fig. 2 3D AFM images of ZnO, ZnO:I and ZnO:I@TiO₂ film.

The representative XRD patterns, as shown in Fig. 3, provide the information of crystal phase and crystalline of ZnO and ZnO:I and ZnO:I@TiO₂ samples. All the clear diffraction peaks of both ZnO and ZnO:I samples can be indexed to the standard pattern of hexagonal ZnO phase (JCPDS no. 36-1451).³⁸ No iodine peaks can be detected in the XRD pattern indicating that no pure or composite iodine exists in the sample. To confirm the iodine state in the ZnO:I, fined scanned X-ray diffraction (XRD) patterns (Fig. S1†) were measured. One can see that the strong diffractive peaks of ZnO:I with the index of (100), (002) and (101) shift toward lower 2θ values in comparison to the pristine ZnO, suggesting the successful doping of I in ZnO crystal structure.¹¹ As for ZnO:I@TiO₂, a typical peak at around 25.5° corresponding to the (101) peak of anatase TiO₂ (JCPDS no. 21-1272) can be observed.³⁹ The EDS demonstrates the presence of Ti and I in the ZnO-based film (Fig. S2†). By scanning the ZnO:I@TiO₂ aggregate films in different areas, the average I and Ti concentrations are estimated to be 1.2 mol% and 1.5 mol%, respectively. The overall composition and spatial distribution of I and Ti in an individual ZnO nanocrystalline aggregate is further investigated by scanning transmission electron microscopy (STEM) elemental mapping. Fig. 4a–d shows the bright-field STEM image of ZnO:I@TiO₂ aggregates, and the elemental mapping for Zn, Ti and I atoms, respectively. The homogeneous spatial distribution of I dopant and Ti within the sample is clearly discernible. The uniform distribution of I and Ti within the ZnO nanocrystalline aggregate is also confirmed by line scanning analysis (Fig. 4f).

Fig. 3 XRD patterns of ZnO, ZnO:I and ZnO:I@TiO₂.

Fig. 4 (a) Bright-field STEM image of ZnO:I@TiO₂ aggregates. (b–d) The corresponding elemental mapping of Zn (b), Ti (c) and I (d); (e and f) STEM line scans for a ZnO:I@TiO₂ aggregate, with Zn, Ti, and I data plotted as blue, green, and pink lines, respectively.

The composition and chemical valence status of the ZnO:I@TiO₂ aggregates film was measured using XPS. Fig. S3† shows a typical XPS spectrum of the ZnO:I@TiO₂ aggregates containing the Zn, O, I and Ti elements. The two apparent I 3d peaks located at 619.3 and 631.2 eV can be assigned to I 3d_3/2 and I 3d_5/2 states, respectively (Fig. 5a). This demonstrates that I₂ molecule in zinc oxide matrix partially substituted of oxidic sites in the ZnO lattice.⁴⁰ The binding energy peaks of Zn 2p of the ZnO:I@TiO₂ observed at around 1021.3 and 1044.5 eV are attributed to those of Zn 2p_3/2 and Zn 2p_1/2, respectively (Fig. 5b). The XPS core level spectra of O²⁻ in the ZnO:I@TiO₂ sample are displayed in Fig. 5c. It can be seen that an intense peak ascribed to O 1s of ZnO:I@TiO₂ centers at 530.3 eV. Fig. 5d shows the narrow range scans of the Ti 2p core level peaks of the ZnO:I@TiO₂ aggregate film. The binding energies of 458.4 eV for Ti 2p_3/2 and 463.5 eV for Ti 2p_1/2 provide further evidence of the formation of a TiO₂ in ZnO:I system. Moreover, the binding energy of the Ti 2p core peak of the ZnO:I@TiO₂ films is assigned as a Ti–O bond (the main peak in the XPS spectra), suggesting that the chemical state of the Ti on the ZnO:I@TiO₂ aggregates is Ti⁴⁺.⁴¹

Fig. 5 High resolution XPS spectrum (a) I 3d, (b) Zn 2p, (c) O 1s and (d) Ti 2p of ZnO:I@TiO₂ film.

3.2. Photocatalytic activity

The photocatalytic activity of as-prepared films was evaluated by the degradation of RhB solution under visible light irradiation (420 < λ < 800). For each run, before irradiation, the reaction solution was first magnetically stirred in dark for 30 min to achieve adsorption and/desorption equilibrium. Comparative experiments were also carried out to investigate the photocatalytic activity of the films with different composites under identical conditions (Fig. 6a). The degradation curve for all the samples is estimated as C/C₀ versus time, where C₀ is the initial concentration of dye and C is the concentration after photo irradiation. It can be seen that no obvious degradation can be observed without catalyst, indicating that the self-photolysis of RhB under visible light is negligible. Pure ZnO aggregate film presents a poor photoactivity, that is even worse than P25. After doping iodine into ZnO aggregates, the degradation rate of ZnO:I film for RhB increases markedly, with 77.5% for 2.5 mol% ZnO:I and 93.7% for 5.0 mol% ZnO:I, respectively. Afterwards, upon continuing increasing the I-doping ratio up to 7.5 mol%, the degradation efficiency decreases obviously to 87.0%. Moreover, the ZnO:I@TiO₂ aggregate film not only exhibits a further improved degradation activity, with 97% after 4 h of irradiation for RhB, which is superior to that of the pure ZnO (only ∼54.3%), but also displays greatly improved photocatalytic stability in both acidic or alkaline medium in comparison with the ZnO:I aggregate films without the TiO₂ decoration (see experimental data and detailed discussion in next section).

Fig. 6 (a) Comparison of the photocatalytic degradation of RhB solution in the presence of pure ZnO, 2.5% ZnO:I, 5.0% ZnO:I, 7.5% ZnO:I, ZnO:I@TiO₂ and no photocatalyst (blank) under visible light irradiation. C₀ and C are the initial concentration after adsorption equilibrium and temporal concentration of RhB at given irradiation time, respectively. (b) The linear fitting of ln(C₀/C) versus irradiation time (t). (c) The temporal spectrum changes of RhB taking place in the presence of ZnO:I@TiO₂ aggregates film.

The photocatalytic kinetics of heterogeneous photocatalysis can be analyzed using the Langmuir–Hinshelwood model with first-order reaction:

ln(C₀/C) = k_at,

where k_a is reaction (photodegradation) rate constant and t is irradiation time. The photocatalytic degradation can be quantitatively evaluated by comparing the apparent reaction rate constants k_a, which can be obtained from the plots of ln(C₀/C) versus irradiation time (t). Fig. 6b shows the ln(C₀/C) versus irradiation time curves of the RhB photodegradation using various ZnO-based photocatalysts. All curves shown in Fig. 6b are nearly linear, which reveals that the kinetic data of the RhB photodegradation fit well to the first order reaction kinetic model. Table S1 (see ESI†) summarizes the calculated k_a and the corresponding coefficient of determination (R²) values. It is apparent that the photodegradation rate of the ZnO:I is much greater than that of ZnO film. The photodegradation rate of ZnO:I photocatalysts increases gradually with the increase of the I doping content. In particular, the rate constant of the RhB photodegradation using the ZnO:I@TiO₂ (0.855 h⁻¹) shows 4.4-fold larger than that using the bare pure ZnO (0.194 h⁻¹). In addition, temporal changes in the concentrations of RhB are monitored by examining the variations in the maximal absorption peak at 554 nm in the UV-vis spectra. The temporal evolution of the spectral changes of RhB mediated by the ZnO:I@TiO₂ aggregate films, as shown in Fig. 6c, exhibits that the absorption peak at 554 nm decreases gradually with irradiation time and approaches zero after 4 h. Also the absorbance values of RhB in the whole spectrum range from 200 to 800 nm are gradually reduced with increasing irradiation time, indicating the effectiveness and completeness of the photodegradation of pollutants.

3.3. Photocatalytic stability

In practical applications, the stability and recycling of the photocatalyst are also critical evaluation criteria. Because pH is an important operational variable in wastewater treatment, the photocatalytic degradation of RhB catalyzed by both ZnO:I, P25 and ZnO:I@TiO₂ aggregate films at different pHs are performed, and the results are shown in Fig. 7. It can be observed that degradation efficiency for the ZnO:I aggregate films is 65% higher than that of P25 in the neutral solution. Unfortunately, the ZnO:I films presents gradually decreased degradation rates with increasing or decreasing the pH of reaction medium, with only 42% (pH = 5) and 59% (pH = 10), which are inferior to that of the P25 (62% at pH = 5 and 61% at pH = 10). The decrease in the degradation efficiency may be ascribed to intrinsic instability of ZnO, including photocorrosion in long-term photocatalytic processes and decomposition of the ZnO aggregates in acidic or alkaline photoreactive media.⁴¹ However, the ZnO:I@TiO₂ aggregate films show excellent degradation efficiency of RhB both in acidic and alkaline solutions (pH range from 5 to 10), especially in alkaline solutions. One can see that the final degradation efficiency of the ZnO:I@TiO₂ sample in alkaline solutions (pH = 10) reaches up to 89% after 4 h for RhB, which is 51% higher than that of ZnO:I. These results suggest that the TiO₂ post decoration either on the surface or within of the ZnO:I aggregates plays an important role in inhibiting photocorrosion of ZnO aggregate and consequently improving the stability of the ZnO:I aggregate films during photocatalysis.

Fig. 7 The photocatalytic degradation rates of RhB with different initial pHs in the presence of ZnO:I, P25 or ZnO:I@TiO₂ aggregate films under visible light irradiation.

To study the lifetime of the photocatalyst, we also carried out repeated photodegradation experiments of the ZnO:I@TiO₂ aggregate films in this study (Fig. 8). One can see that the high photocatalytic efficiency of the ZnO:I@TiO₂ aggregate films during RhB degradation is effectively maintained after 5 runs, indicating that ZnO:I@TiO₂ aggregate films have high stability and durability under visible light irradiation.

Fig. 8 Cycling runs for the photodegradation of RhB in the presence of ZnO:I@TiO₂ aggregates film under visible light irradiation.

3.4. Photoelectrochemical performance

To reveal the light-harvesting capability of the fabricated ZnO-based films in the visible light range, UV-vis absorption spectra of the pure ZnO, ZnO:I, and ZnO:I@TiO₂ aggregate films were measured (Fig. 9). An undeniable red shift following the addition of iodine is observed. This enhanced absorption in the visible region is more likely to be induced by a sub-band-gap transition corresponding to the excitation from the valence band of ZnO (E_vb(ZnO) = 2.89 eV, NHE) to the doped I₂ molecule in zinc oxide matrix.³¹ As a result, such enhanced absorption in the visible light region for the ZnO:I explains the increase in visible light photocatalytic activity during RhB degradation as evidenced in Fig. 6a. It is noted that the TiO₂-decorated ZnO:I also shows enhanced absorption in the whole visible light region, indicating that the TiO₂ layer does not hinder the visible absorption of the ZnO:I nanocrystalline aggregate films.

Fig. 9 UV-vis absorption spectra of ZnO, ZnO:I and ZnO:I@TiO₂ aggregate films.

The broadened absorption is not the most important characteristic for improving the photocatalytic activity of the as-grown films; effective separation of the photogenerated electron–hole pairs plays a more important role. The electrochemical impedance spectroscope (EIS) Nyquist plots of as-prepared samples both with and without light irradiation were carried out to investigate the interface charge separation efficiency. The semicircle at high frequency is characteristic of the charge transfer process and the smaller arc radius on the EIS Nyquist plot indicates an effective separation of photo-generated electron–hole pairs and a fast interfacial charge transfer process.⁴² As shown in Fig. 10, the impedance plots of the pure ZnO, ZnO:I, and ZnO:I@TiO₂ aggregate electrodes cycled in a 0.1 M Na₂SO₄ electrolyte solution all exhibit one arc/semicircle within the measured frequencies, suggesting that only the surface charge transfer step is involved in the photocatalytic reaction. Clearly, I doping leads to a significantly faster charge transfer rate indicated by a decrease in the radius of the plot compared to the pure ZnO aggregate, both with and without light irradiation. The ZnO films present slightly lower charge transfer resistance under radiation than in dark, which is also due to the disable visible response of pure ZnO. Furthermore, the smaller plot radius of the ZnO:I@TiO₂ aggregate films than that of the ZnO:I aggregate indicates an efficiently improved electron transport process in ZnO:I@TiO₂ composite, which most likely, to some extent, comes as a result of TiO₂ modification widening electron pathways by cementing the ZnO:I particles together to favor electron transport across the ZnO:I film,⁴³ in turn, this facilitates the photocatalytic degradation of RhB.

Fig. 10 EIS Nyquist plots of ZnO, ZnO:I, and ZnO:I@TiO₂ aggregate films with light on/off cycles under the irradiation of visible light (λ > 420 nm).

Conclusion

In summary, versatile ZnO:I@TiO₂ aggregate films synchronously functionalized via iodine-doping and TiO₂ decorating were developed for use as photocatalysts to study the pollutant photodegradation. The results demonstrated that the degradation efficiency of the ZnO:I/@TiO₂ nanocrystalline aggregates films was significantly enhanced (∼97% after 4 h of irradiation, a 79% increase over pure ZnO). The improved efficiency is attributed to the increased light absorption in the visible range and more efficient charge separation of photogenerated e⁻–h⁺ generated by iodine doping and TiO₂ post modification. Besides, the ZnO:I@TiO₂ aggregate films exhibited enhanced chemical stability both in acidic and alkaline solutions in comparison to ZnO:I, which was further verified by repeated photodegradation experiments under visible light irradiation. This work demonstrates the possibility of fabricating high efficiency photocatalytic materials based on structural and compositional manipulation of semiconducting nanomaterials for photocatalysts with high activity and stability.

Acknowledgements

The work was supported by the National Natural Science Foundation of China (No. 21176019, 21377011, 21476019) and Beijing Higher Education Young Elite Teacher Project (YETP0487).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra00903d

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