Enhanced photocatalytic performance of TiO2 NTs decorated with chrysanthemum-like BiOI nanoflowers

Separation and Purification Technology 215 (2019) 565–572

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Enhanced photocatalytic performance of TiO2 NTs decorated with chrysanthemum-like BiOI nanoflowers Zhiyuan Liu, Qingyao Wang , Xinying Tan, Yujie Wang, Rencheng Jin, Shanmin Gao ⁎

T ⁎

School of Chemistry and Materials Science, Ludong University, Yantai 264025, China

ARTICLE INFO

ABSTRACT

Keywords: TiO2 nanotube arrays BiOI Nanoflowers Photoelectrochemical performance

The BiOI nanosheets/chrysanthemum-like nanoflowers were successfully deposited on the surface of TiO2 nanotube arrays (TiO2 NTs) by the successive ionic layer adsorption and reaction (SILAR) method, and the morphology and visible light response of samples with different SILAR deposition cycles were investigated in detail. The as-prepared BiOI/TiO2 NTs significantly enhanced photoelectrocatalytic (PEC) activity for the removal of Methyl orange (MO), Rhodamine B (RhB), Methylene blue (MB) and Cr(VI). The as-prepared Sample-7 with chrysanthemum-like nanostructures showed the high visible light photocurrent density of 120.06 μA/cm2, photovoltage of −203.61 mV/cm2, PEC efficiencies of 45%, 62%, 79% and 77% for the removal of MO, RhB, MB and Cr(VI), respectively. The high PEC performances could be ascribed to the excellent visible light response and charge carrier transportation in chrysanthemum-like BiOI nanoflowers. By further probing the charge separation and transportation behaviors, the experiments of the energy band structure and active species trapping were carried out. A possible p-n heterojunction photocatalytic mechanism was proposed, which not only benefited the efficient separation of photogenerated electrons but also demonstrated the advanced capacity for the PEC removal of organic dyes and heavy metal ions.

1. Introduction Semiconductor photocatalysts are considered as promising materials for solving issues of energy shortage and environmental pollution [1–3]. As well known that the n-type TiO2 semiconductor has the advantages of unique, non-toxicity and acid resistance in applications of electrochemical electrodes and photocatalysts [4,5]. TiO2 nanoparticles with various morphologies were investigated to enhance the electrochemical performance. Wang et al. used graphene-supported Si-TiO2 nanospheres as anode materials for Li-ion batteries [6]. Ahmad et al. prepared rutile phase TiO2 nanoflowers/nanorods to improve solar photovoltaic performance [7]. Li and his colleagues prepared TiO2 nanosheets to improve H2 production capacity [8]. Compared with TiO2 powders, TiO2 NTs have high specific surface area and highly ordered tubular structure prepared on a Ti substrate [9], showing superior photocatalytic and photoelectronic performances. As V-VI-VII ternary oxide and p-type semiconductors, bismuth oxyhalides (BiOX, X = Cl, Br, I) were previously used as ferroelectric materials and pigments [10–13]. BiOX belongs to aurivillius-related oxide family and has a special layer structure of X-Bi-O-Bi-X. The tetragonal layered structures containing [Bi2O2]2+ slabs are interleaved by double slabs of halogen atoms. The strong interlayer bonding and ⁎

weak interlayer van der Waals interactions lead to the excellent catalytic activity [14,15]. Recently, BiOX received intense attention because of its excellent photocatalytic activity in visible light and ultraviolet light [16]. First, BiOX has efficient electronic transmission because of special layer structure [17]. Furthermore, BiOX is a gap semiconductor that can effectively suppress the recombination of photogenerated electrons and holes. Meanwhile, different halide atoms in crystal structures can offer the high possibility to manipulate electronic structures, which is very important in photocatalytic reaction [18]. Among BiOX photocatalysts, BiOI has been recognized as the excellent photocatalyst due to the narrow band gap (1.72–1.92 eV) and efficient sunlight-harvesting nature [19,20]. For example, BiOI-sensitized TiO2 has high solar harvesting efficiency, however, the fast recombination of charges and poor conductivity lead to low photocatalytic degradation of phenol [21,22]. In order to enhance the electron transfer performance of BiOI, many methods have been investigated, such as doping, heterojunctions and structural design [23–26]. Fortunately, the effective electron transportation along the vectorial walls of TiO2 NTs would significantly reduce the recombination of electron/hole pairs in BiOI. Dai [27] and his colleagues prepared BiOI/TiO2 NTs p-n junction by a novel impregnating hydroxylation method, and the BiOI/TiO2 NTs sample showed strong visible light

Corresponding author. E-mail addresses: [email protected] (Q. Wang), [email protected] (S. Gao).

https://doi.org/10.1016/j.seppur.2019.01.046 Received 7 November 2018; Received in revised form 18 January 2019; Accepted 19 January 2019 Available online 22 January 2019 1383-5866/ © 2019 Elsevier B.V. All rights reserved.

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absorption, photocurrent response and PEC activity for the decolorization of MO dyes. Liu [28] prepared BiOI/TiO2 NTs p–n heterojunctions by loading BiOI nanoflakes on TiO2 NTs walls using the sequential chemical bath deposition, and the as-prepared BiOI/TiO2 NTs samples exhibited the high visible-light photocurrent response and PEC activity. Investigations indicated that the traditional deposition of BiOI nanoparticles on TiO2 NTs surface is uncontrollable, and the interface connection needs to be further improved. SILAR is designed based on chemical deposition and atomic layer epitaxy at the surface of TiO2 NTs to form a surface modification layer. The deposition could achieve nano-scale growth, and the interface connection and deposition amount are controlled by changing the reagent concentration and deposition cycles. However, there are few studies on the deposition of BiOI on TiO2 NTs for further enhancing photocatalytic efficiency. In this paper, BiOI nanosheets/chrysanthemum-like nanoflowers were prepared on the surface of TiO2 NTs by the SILAR deposition method. The TiO2 NTs/ BiOI photoelectrode showed excellent photoelectrochemical performances including photoelectric conversion and PEC removal of pollutants. The simple synthesis and high photoelectric activity of TiO2 NTs/ BiOI would provide the templet for the preparation and application of BiOX on TiO2 NTs.

microscopy (Hitachi SU 8010, Japan) at an accelerating voltage of 10 KV. The UV–Vis diffuse reflectance spectroscopy was applied to investigate the optical absorption of the as-prepared samples on a Cary 60 UV–Vis spectrophotometer using BaSO4 as the reference. The electrochemical properties of transient photocurrent, current-voltage (I-V) curves and open circuit potential (OCP) were measured by electrochemical workstation (CHI660E, China) with a standard three electrode system. The prepared samples with an active area of 1.8 cm2 was regarded as working electrode, and Ag/AgCl and Pt served as the reference and counter electrodes, respectively. The samples were illuminated with a solar simulator equipped with a 500 W Xe lamp (CELS500) with a visible-light filter (> 400 nm). 2.4. Measure of the photoelectrocatalytic activity

Ti substrate, bismuth nitrate (Bi(NO3)3·5H2O) and potassium iodide (KI) are purchased from Tianjin Bodi Chemical Industry Co., Ltd. and Tianjin Jinbei Fine Chemical Co., Ltd., and used without further purification.

The PEC degradation performances of MO, MB and RhB dyes were measured under solar irradiation simulated using a 500 W Xe lamp (CEL-S500). 0.2844 g of Na2SO4 was dissolved in dye solution, and served as supporting electrolyte. The potential was fixed at 1 V vs Ag/ AgCl in PEC measurement. The dye solution was adequately stirred in dark for 30 min for adsorption equilibrium. The dye concentration was determined with a UV–Vis spectrophotometer by detecting the maximum absorption wavelengths for MO, MB and RhB at 464, 664 and 552 nm, respectively. The PEC removal of Cr(VI) was performed similarly with the degradation of dyes. Differently, 0.5844 g of Na2SO4 was dissolved in Cr (VI) solution, and served as supporting electrolyte. The potential was fixed at 0.5 V vs Ag/AgCl in the PEC measurement. The Cr(VI) concentration in the solution was determined colorimetrically at 540 nm using the diphenylcarbazide method.

2.2. Preparation of TiO2 NTs/BiOI

3. Results and discussion

TiO2 NTs were prepared on a Ti substrate by a simple anodization method, which was similar with our previous report [29]. BiOI nanoparticles were deposited on the surface of TiO2 NTs by the SILAR method, and the experimental progress was illuminated in Scheme 1. Firstly, TiO2 NTs were immersed in 25 mL of 5 mmol L−1 Bi (NO3)3·5H2O solution, and soaked for 30 s. Then, the samples were cleaned with deionized water to remove excess ions. Shortly afterwards, the samples were immersed in 25 mL of 5 mmol L−1 KI solution for 2 min, and cleaned again with deionized water. The above two-step progress was repeated for several cycles. The samples prepared with 3, 7, 9 and 13 cycles were marked as Sample-3, Sample-7, Sample-9 and Sample-13. Lastly, samples were calcinated in air at 300 °C for 2.5 h.

The SEM images of TiO2 NTs were shown in Fig. 1 to investigate the morphology of TiO2 nanotubes arrays prepared by the two-step anodization method. The uniform and regular tube structures could be clearly observed, and the average diameter and wall thickness of TiO2 NTs was 150 and 15 nm, respectively. The morphology and microstructure of TiO2 NTs/BiOI composite were studied by SEM. As shown in Fig. 2, BiOI nanosheets were covered on the surface by the SILAR deposition, and BiOI nanosheets increased with the SILAR cycles. The typical SEM images of Sample-3 in Fig. 2a showed that the chrysanthemum-like nanoflowers were scattered on the surface of TiO2 NTs, and careful observation indicated that the nanoflowers were constituted by ultrathin BiOI nanosheets with the thickness of 10 nm. In Fig. 2b, further increasing SILAR deposition cycles, the BiOI nanosheets/flowers were evenly deposited on the surface of TiO2 NTs, displaying the formation of chrysanthemum-like nanoflowers with the size of ∼500 nm in Sample-7. Interestingly, when the SILAR cycles were increased up to

2. Experimental 2.1. Materials

2.3. Characterization The SEM images were observed by with scanning electron

Scheme 1. The SILAR growth diagram of TiO2 NTs/BiOI. 566

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Fig. 1. The SEM images of TiO2 NTs.

9 and 13 cycles, BiOI nanoflowers disappeared, and nanosheets were massively produced. As shown in Fig. 2c and d, a large number of chaotic nanosheets accumulated and completely covered the surface of TiO2 NTs. Sample-13 showed few thin nanosheets standing on the mouth of nanotubes, however, multiple thick nanosheets in Sample-9 laid the whole nanotubes. The previous results [14,15] indicated that the tetragonal layer structures containing [Bi2O2]2+ slabs made it easier to form nanosheets. This phenomenon is attributed to the fact that large amounts of BiOI make against the formation of complex nanoflowers. As shown in Fig. 3, the chemical composition of TiO2 NTs/BiOI was investigated by XPS spectra. The signal of Bi, I, Ti, O and C elements were found in Fig. 3a, and all the XPS data was calibrated with the banding energy of C 1s. The Bi 4f binding energy of TiO2 NTs/BiOI was located at 159.4 eV and 164.7 eV and the two strongest peaks were assigned to the orbital 4f7/2 and 4f5/2 of Bi3+. Typically, two characteristic peaks were displayed in Fig. 3c at 619.4 eV and 630.9 eV, which could be corresponded to I 3d5/2 and I 3d3/2, confirming the states of I− in TiO2 NTs/BiOI. As shown in Fig. 3d, the two strongest peaks were detected at 459.2 eV and 465.9 eV, corresponding to 2p3/2 and 2p1/2 of Ti element. The O 1s spectrum in Fig. 3d exhibited binding energy of 530.3 eV, 530.8 eV and 531.8 eV, which was ascribed to lattice oxygen of in BiOI and TiO2, surface hydroxyl groups and adsorbed water, respectively. Therefore, the XPS spectra confirmed the formation of BiOI nanoparticles on the surface of TiO2 NTs. The solar absorption activity of the as-prepared samples with

different BiOI SILAR deposition cycles was evaluated by UV–Vis absorption spectra in Fig. 4. It could be clearly noticed that the Sample-3 and Sample-7 showed strong visible light absorption performance, and the sharp harvesting peaks in 370–650 nm were ascribed to the chrysanthemum-like BiOI nanoflowers. However, the further increasement of BiOI deposition cycles, Sample-9 and Sample-13 showed low visible light response. The reason could be attributed that the visible light was reflected at the surface of BiOI nanosheets in Sample-9 and Sample-13, but the chrysanthemum-like microstructures could achieve multiple reflections. Therefore, the high visible light response of Sample-7 with chrysanthemum-like structures showed the high solar absorption, which would lay the foundations for the photoelectric conversion and PEC degradation of organic pollutants. The PEC performances of the as-prepared samples were determined by the removal of organic pollutants and heavy metal ions under simulated sunlight irradiation. The PEC results in Fig. 5 indicated that the as-prepared samples showed outstanding PEC performances under solar irradiation. In particular, Sample-9 showed the optimal PEC degradation of MO, and the final efficiency reached 52% after solar irradiation for 3 h. The PEC degradation rate showed the first-order function characterization, and the rate constant of Sample-9 achieved 6.03 × 10−3 min−1, which was 8.67, 2.15 and 1.33 folds of Sample-3, Sample-7 and Sample-13, respectively. In addition, Sample-7 exhibited high PEC activities for the removal of RhB, MB and Cr(VI), and the corresponding efficiencies reached 62%, 79% and 77%, respectively. The PEC rate constants of Sample-7 for the degradation of RhB and MB

Fig. 2. The SEM images of Sample-3 (a), Sample-7 (b), Sample-9 (c) and Sample-13 (d).

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Fig. 3. The XPS spectra of TiO2 NTs/BiOI: (a) a survey spectrum, (b) Bi 4f; (c) I 3d, (d) Ti 2p, (e) O 1s.

were 9.16 × 10−3 and 3.06 × 10−2 min−1. It could be observed that the Sample-9 also showed high PEC performance in the degradation of anionic dye (MO). Inversely, the cationic dyes (RhB and MB) could be easily decomposed by Sample-7, which could be speculated by the surface electrical behavior of samples [30]. The radical scavengers were added to degradation experiment to investigate main active groups. In the experiment, tertiary butyl alcohol (t-BuOH), benzoquinone (BQ) and EDTA-2Na were selected to capture %OH, %O2– and h+, respectively

[31]. Degradation efficiency of MO with the addition of EDTA-2Na was slightly enhanced. The enhanced PEC efficiency is attributed to the separation of electron-hole pairs, because holes were captured by EDTA-2Na. On the contrary, MO was hardly degraded after the addition of BQ. It is well known that BQ is an %O2– scavenger, and therefore it could be confirmed that %O2– is the main active group for PEC degradation of MO. Similar conclusions were obtained when scavengers were added into the dye solution. Except for the PEC oxidation 568

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77% was achieved by Sample-7 after solar irradiation for 3 h. It was 2.1, 2.9 and 1.3 folds of Sample-3, Sample-9 and Sample-13. The Cr(VI) removal mechanism was testified by the addition of dimethyl sulfoxide (DMSO) as the trapping agent of photoelectrons. When DMSO was added, the removal efficiency was drastically decreased from 58% to 19%. Moreover, the separation of electrons was affected by external voltage in the PEC progress, and the comparison experiments were carried out in Fig. 7a. It could be obviously observed that the PEC efficiencies were much higher than that of photocatalytic progress, and they were 1.05, 1.30, 1.08 and 3.85 folds of photocatalytic removal efficiencies of RhB, MO, MB and Cr(VI), respectively. The stability of the as-prepared photocatalysts was investigated by the repeated PEC degradation of MB dyes for 5 cycles. As shown in Fig. 7b, the photocatalyst showed excellent stability, and less than 5% of PEC efficiency was changed during the repeated PEC progress. The high PEC activity and stability of TiO2 NTs/BiOI would exhibit prospective applications in waste water treatment. The photoelectric conversion performance was evaluated by the characterization of visible light transient photocurrent and photovoltages. As shown in Fig. 8a, it clearly showed that Sample-7 had the highest photocurrent response among these samples prepared with different SILAR deposition cycles. The photocurrent densities of the Sample-3, Sample-7, Sample-9 and Sample-13 were 83.33, 120.06, 68.44 and 94.83 μA/cm2, respectively. The superhigh photocurrent density of Sample-7 indicated the excellent photoelectron generation and separation activity [38], which could be attributed to chrysanthemum-like morphology and synergetic effect of BiOI and TiO2

Fig. 4. UV–Visible diffuse reflectance spectra of the as-prepared samples.

decomposition of dye molecules, the reducibility of samples was evaluated by the PEC reduction of Cr(VI) into Cr(III) in Fig. 6. Compared with the traditional heavy metal ion adsorption [32–36], the photocatalytic reduction is an effective approach for treatment of water polluted by highly toxic heavy metal ions [37]. The highest efficiency of

Fig. 5. PEC degradation efficiencies, pseudo-first-order kinetics and adding different scavengers for MO (a), RhB (b) and MB (c) by photocatalysts.

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Fig. 6. The PEC removal curves of Cr(VI) (a) and adding DMSO scavenger (b) under solar irradiation.

nanoflowers, which was consistent with the previous PEC results. Based on the above discussion, a possible mechanism was proposed to explain the excellent PEC degradation of organic dyes by TiO2 NTs/ BiOI in Scheme 2. BiOI is a p-type semiconductor with narrow band gaps, and the conduction band (CB) of BiOI is ∼0.56 eV, and the electrons don’t have ability to oxidize O2 molecules on the surface to produce %O2– (EO2/%O2– = −0.33 eV). However, the BiOI-TiO2 heterojunction could be formed after SILAR deposition, the energy band locations would shift under solar irradiation, and the CB edge of BiOI dramatically shifts up [39,40]. Therefore, the p-type BiOI and n-type TiO2 can form a nested p-n heterojunction, and the high specific surface area and interface connection of TiO2 NTs/BiOI would significantly enhance the solar harvesting and charge carrier transfer. Under solar irradiation, the photogenerated electrons are excited to the CB, and the holes are left in the valence band (VB). The internal electric field of p-n heterojunction induces the electron transition from the CB of BiOI to that of TiO2 NTs, and the holes in the VB of TiO2 jump to the VB of BiOI. Therefore, photoelectrons and holes can be effectively separated, showing a positive effect on photodegradation. The electrons in the CB of titanium dioxide combine with the oxygen molecules to form superoxide radicals [40], and further transform into hydroxyl radicals with highly oxidative properties. The organic pollutants was oxidized by ·OH radicals and decomposed into small molecules and water. The main process is as follows: BiOI/TiO2 + hν → e− + h+ e %

−

+ O2 →·O2

O2–

+ 2H

H2O2 +

%

+

O2–

–

(2)

+e

−

→ H2O2

(3)

–

(4)

→ OH + OH + O2 %

MO + OH → CO2 + H2O %

(1)

(5)

4. Conclusions In summary, BiOI nanosheets/nanoflowers were successfully prepared on the surface of TiO2 NTs by the simple SILAR deposition. The DRS results indicated that the BiOI sensitization significantly extended the absorption of TiO2 NTs to the visible light region, and the Sample-7 showed the visible light photocurrent density as high as 120.06 μA/cm2 and photovoltage as high as −203.61 mV/cm2. In addition, the high PEC efficiencies of 52%, 62%, 79% and 77% were obtained by TiO2 NTs/BiOI photocatalysts for the PEC removal of MO, RhB, MB and Cr (VI), respectively. The efficient PEC performance was attributed to the chrysanthemum-like microstructure and rapid electron transfer at the

Fig. 7. The contrastive curves of PC and PEC removal of pollutants (a) and PEC repetitions of MB under solar irradiation (b).

NTs. As shown in Fig. 8b, Sample-7 photoelectrode showed the highest photocurrent density among these electrodes. The open-circuit voltages of Sample-3, Sample-7, Sample-9 and Sample-13 were −148.68, −203.61, −151.97 and −173.83 mV/cm2 in Fig. 7c, respectively. The excellent photoelectrochemical capability of Sample-7 was ascribed to the effective solar absorption and photoelectron transportation of BiOI 570

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Fig. 8. The transient photocurrent response (a), I-V curves (b), and OCP (c) of the as-prepared photoelectrodes.

Scheme 2. The Schematic diagram of PEC dye degradation by the as-prepared photocatalyst under solar irradiation.

interface of p-n heterojunctions. The novel TiO2 NTs/BiOI photoelectrode provides an efficient synthetic strategy for the preparation of photocatalysts in environmental treatment.

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