ZnO-embedded BiOI hybrid nanoflakes: Synthesis, characterization, and improved photocatalytic properties

Accepted Manuscript ZnO-embedded BiOI hybrid nanoflakes: Synthesis, characterization, and improved photocatalytic properties

Yanhua Tong, Chu Zheng, Wenjing Lang, Fan Wu, Tao Wu, Wenqin Luo, Haifeng Chen PII: DOI: Reference:

S0264-1275(17)30161-2 doi: 10.1016/j.matdes.2017.02.033 JMADE 2773

To appear in:

Materials & Design

Received date: Revised date: Accepted date:

6 November 2016 11 February 2017 13 February 2017

Please cite this article as: Yanhua Tong, Chu Zheng, Wenjing Lang, Fan Wu, Tao Wu, Wenqin Luo, Haifeng Chen , ZnO-embedded BiOI hybrid nanoflakes: Synthesis, characterization, and improved photocatalytic properties. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Jmade(2017), doi: 10.1016/j.matdes.2017.02.033

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ACCEPTED MANUSCRIPT

ZnO-embedded BiOI hybrid nanoflakes: Synthesis,

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characterization, and improved photocatalytic properties

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Yanhua Tong,*a Chu Zheng,b Wenjing Lang,b Fan Wu,a Tao Wu,a Wenqin Luo,a and

a

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Haifeng Chena

Department of Materials and Chemistry, Huzhou University, Huzhou 313000,

School of Chemistry and Pharmacy, Guangxi Normal University, Guilin, 541000,

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b

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China.

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D

China

Tong

–

Tel:

+86-0572-2320685;

Fax:

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Yanhua

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* Corresponding author:

[email protected]

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+86-0572-2320685;

Electronic

mail:

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Abstract BiOI-based p–n heterostructures with enhanced visible-light photocatalytic activity have been investigated in detail. However, photocatalytic activity of most of heterostructures usually drops step by step with increasing irradiation time. The main reason for this deactivation is the

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instability of the loaded heterostructures. In this study, ZnO-embedded BiOI hybrid nanoflakes were fabricated using Zn5(CO3)2(OH)6 ultrathin nanosheets for BiOI deposition followed by

different

from those

of

ZnO-loaded BiOI heterostructures.

The

visible-light

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BiOI,

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calcination. This embedded hybridized nanostructure showed strong coupling between ZnO and

photodegradation experiments demonstrate that the ZnO-embedded BiOI hybrid nanoflakes not

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only exhibited remarkably enhanced and sustainable photocatalytic activity, but also showed good recyclability, comparing with the pristine ZnO, BiOI and ZnO/BiOI heterostructures. Integrated

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measurements of electrochemistry and photoelectrochemistry, photoluminescence and reactive species during the photodegradation process, substantial enhancement of photocatalytic activity for ZnO-embedded BiOI hybrid nanoflakes is probably attributed to the raised potential of

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valence-band edge, good conductivity, and quenching of deep-level defects. This study provides

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an embeddedly hybridized route to enhance photocatalytic activity and simultaneously improve

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their sustainability and recyclability.

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Key words: Hybrid nanostructure; ZnO-BiOI; Visible light; Photocatalyst

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ACCEPTED MANUSCRIPT 1. Introduction Photocatalysts have received considerable attention owing to their potential applications in environmental remediation and renewable resource production such as water purification and hydrogen production [1–4]. Over 40 years, various types of semiconductor materials, including metal oxides [4], sulfides [5], nitrides [6], solid solutions [7], silver-based compounds [Ag3VO4,

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Ag3PO4, Ag2MoO4, AgCrO4, Ag/AgX (X = Cl, Br, I), etc.] [8–13] and bismuth-based compounds [Bi2O3, NaBiO3, BiOX (X = Cl, Br, I), BiVO4, Bi2WO6, Bi2MoO6, etc.] [14–18] have been

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explored as photocatalysts responsive to both UV and visible light ranges. However, the practical application of pristine semiconductor for photocatalysts are limited because of two important

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factors: the fast recombination of photogenerated electron–hole pairs for narrow band-gap semiconductors and the limited visible-light response for wide band-gap semiconductors.

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Significant attempts have been made to solve the above-mentioned problems by modifying semiconductor surfaces such as loading noble metal nanoparticles, doping transition-metal ions, or

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combinations with other semiconductors. Of them, the coupling of two semiconductors is making headlines in the research area because of their promising properties such as improvement of photo

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absorption of the UV active materials in the visible region and also suppression of the

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recombination rate of charge carriers [19,20]. Particularly, the coupling of p- and n-type semiconductors is preferable for many researchers, because an internal electric field can be built

[21].

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up in the p–n junction, efficiently enhancing the separation of photoinduced electron–hole pairs

Among p-type narrow band-gap semiconductors, BiOI have shown strong photoresponse in the

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visible light region due to its band gap energy (1.73–1.92 eV). It is of layered structure, [Bi2O2]2+ slabs interleaved by double slabs of iodine atoms in the tetragonal matlockite structure. The large space of BiOI layered structure can polarize the related atoms and orbitals. The induced dipole can efficiently separate electron–hole pairs. Apart from that, BiOI has an indirect-transition band gap; therefore, the excited electrons have to travel a certain k-space distance to be emitted, which reduces the recombination probability of the excited electrons and holes [22,23]. Owing to these attributes, BiOI is not only an effective photocatalyst under visible light irradiation, but also a potential sensitizer to sensitize wide band gap semiconductors [24,25]. By now, a series of BiOI-based p–n heterostructures, such as TiO2/BiOI [26], ZnO/BiOI [27], BiOI/Bi2O3 [28], 3

ACCEPTED MANUSCRIPT Bi2S3/BiOI [29], AgI/BiOI [30], Fe2O3/BiOI [31], Pt/BiOI [32], Ag/BiOI [33], Bi2O2CO3/BiOI [34,35], BiOI/ZnTiO3 [36], ZnWO4/BiOI [37], and GO/BiOI [38] have been reported. These studies illustrate that the BiOI-based heterostructures perform enhanced photocatalytic activities under visible light irradiation. However, photocatalytic activity of these heterostructures usually decreases step by step with

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the prolongation of irradiation time. For TiO2/BiOI heterostructures [26], the percentage of photodegradation of methyl orange was 93% after an initial irradiation of 2 h; however, it dropped

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to 3% in the next 2 h. In another example for ZnWO4/BiOI heterostructures [37], the percentage of photodegradation of methyl orange reached 73% in starting 2 h and decreased to 13% in the next 2

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h. Bi2O2CO3/BiOI heterojunction [35] also performed a high photocatalytic activity followed by a decrease. The main reason for this deactiviation is the instability of their heterostructures. In view

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of this, most heterostructures are also limited for photocatalytic application. Therefore, hurdles related to not only improving photocatalytic activity but also maintaining high activity in

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applications represent considerable challenges.

In this study, two-dimensional ZnO-BiOI combined nanomaterials with different weight

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percentages of BiOI were fabricated by using Zn5(CO3)2(OH)6 nanosheets for BiOI deposition

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followed by calcination. When BiOI content was in the range 20–50%, ZnO-BiOI combined nanomaterials exhibited a ZnO/(ZnO-embedded BiOI) heterostructure, with high initial photocatalytic activity followed by a decrease with the extension of visible-light irradiation time,

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similar to the general heterostructure. However, when BiOI content was ≥ 60%, they evolved into ZnO-embedded BiOI hybrid nanoflakes, performing not only remarkably enhanced photocatalytic

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activity but also good activity maintained during the photocatalytic process in comparison with ZnO/(ZnO-embedded

BiOI)

heterostructure.

The

substantially

enhanced

photocatalytic

mechanism for ZnO-embedded BiOI hybrid nanoflakes was discussed in details by the measurements of electrochemistry, photoelectrochemistry, and photoluminescence as well as active species during the photocatalytic process.

2. Experimental section 2.1 Reagents All major chemicals of analytical grade were purchased from Aladdin Reagent Co., Ltd., Shanghai China and used as received without further purification. All the solutions were prepared with doubly distilled water. 4

ACCEPTED MANUSCRIPT 2.2 Synthesis of Zn5(OH)6(CO3)2 nanosheets Zn(CH3COO)2·2H2O (0.23 g) was dissolved in distilled water (75 mL), followed by adding urea (0.29 g) and 2, 2'-bipyridine (0.18 g). The resulting mixture was magnetically stirred for 1 h to form transparent solution and then transferred into a 100 mL Teflon-lined autoclave. The autoclave was then screwed up and kept inside an electric oven at 120oC for 12 h. After cooling to

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room temperature naturally, white cotton-shaped precipitate suspending in the solution was observed. The precipitate of Zn5(OH)6(CO3)2 was collected by centrifugation and then washed

2.3 Synthesis of ZnO-embedded BiOI hybrid nanoflakes

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with deionized water and absolute ethanol several times, respectively.

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ZnO-embedded BiOI hybrid nanoflakes were prepared by a precipitation–deposition method in combination with low-temperature calcination. A measured amount of KI was dissolved in 100

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mL distilled water to get clear solution. Then, the prepared Zn5(CO3)2(OH)6 nanosheets as substrates were added into that clear solution and the mixture was vigorously stirred for 30 min so

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that part of I- ions can be adsorbed on the surface of Zn5(CO3)2(OH)6 nanosheets by electrostatic interaction. To maintain the pH value of the mixture in the range 6–7, buffer solution composed of

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NaOH and CH3COONa (2 mL) was added. Subsequently, required amount of Bi(NO3)3·5H2O

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dissolved in ethylene glycol was dropwise added to the above-made mixture, the amount of which was varied to obtain serial samples with varying weight percentages of BiOI. The above-mentioned whole process was carried out at room temperature under magnetic stirring. The

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resulting Zn5(CO3)2(OH)6-BiOI combined samples were centrifuged, washed thoroughly with distilled water, and then dried at 65oC for 12 h.

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Final ZnO-BiOI combined nanostructures were obtained after calcination of a series of Zn5(CO3)2(OH)6-BiOI combined samples at 300oC for 2 h. The color of ZnO-BiOI combined nanomaterials was gradually deepened depending on the weight percentage of BiOI. White, green, yellow, deep yellow, orange, orange red, and deep red combined nanomaterials were obtained at the weight percentages of 10%, 20%, 30%, 40%, 50%, 60%, and 70%, respectively. For comparison, pristine BiOI was prepared by the same process without Zn5(CO3)2(OH)6 precursor. 2.4 Characterization The phase composition and crystal structure of the as-fabricated samples were determined by powder X-ray diffraction (PXRD, XD-6 with Cu K alpha radiation at a scan rate of 0.02 2θs-1). 5

ACCEPTED MANUSCRIPT The morphologies and microstructures of the samples were studied by high-resolution transmission electron microscopy (HRTEM, FEI-Tecnai F20) at an accelerating voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) measurements were carried out on a Thermo Fisher Scientific Escalab 250 spectrometer with monochromatized Al K alpha excitation, and C1s (284.6 eV) was used to calibrate the peak positions of all the elements. UV–vis diffuse reflectance spectra

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(DRS) were measured using a UV–Vis spectrophotometer (EVOLUTION 220) with an integrating sphere under ambient conditions. The photoluminescence (PL) spectra were acquired on a

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spectrofluorometer (fluoroSENS-9000). A Nicold-5700 FTIR spectrophotometer was used for recording IR spectra by the KBr pellet method. Total organic carbon (TOC) measurements were

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carried out using a TOC analyzer (SHIMADZU TOC-V CPH).

2.5 Electrochemical and photoelectrochemical measurement and

photoelectrochemical

measurements

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Electrochemical

were

performed

using

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three-electrode cell equipped with a Pt counter electrode and saturated calomel (SCE) reference

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electrode. Commercial indium tin oxide (ITO) served as the working electrode. The ITO electrode was cleaned by sonication sequentially for 20 min each in acetone, 10% KOH in ethanol, and

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doubly deionized water. Films of the resulting semiconductors were fabricated on this electrode by

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drop-coating and drying. The coated area was fixed at 1 cm × 1 cm. Electrolyte consisted of aqueous 0.1 M Na2SO4/H2SO4 solution (pH = 6). Oxygen was removed from the solutions by degassing with N2 prior to the measurements. Cyclic voltammogram curves were recorded at a

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scan rate of 0.05 V/s with a CHI 660C workstation (CH Instruments, Chenhua, Shanghai, China) controlled by a personal computer. Flat potentials for n-semiconductor and p-semiconductor were

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determined by applying anodic and cathodic scans, respectively. Light for photoelectrochemical measurements was produced by a 400 W Xe lamp, filtered with a water IR filter, and directed onto the working electrode. 2.6 Measurement of photocatalytic degradation Rhodamine B (Rh B) was chosen as a photo-degradation object. Glass beakers (capacity ca. 100 mL) were used as photoreactor vessels. The reaction system containing Rh B (3 mg/L, 10 mL) with a pH of 6.56 and photocatalysts (5 mg) was sonicated for 20 min to reach the adsorption equilibrium of Rh B on the surface of photocatalysts. Afterwards, it was placed into cylinder-shaped tank filled with circulating water and irradiated by visible light under magnetic 6

ACCEPTED MANUSCRIPT stirring for a given period. The visible light with wavelength ranging from 420 nm to 770 nm was produced from a 300 W Xenon lamp (Model CEL-HXF300) with a UVIRCUT 420 filter. The power density of light at the position of reactor was 1900 mW/cm2, measured using an optical power meter (Model CEL-NP2000). The temperature of photocatalytic reaction was kept at room temperature. In order to keep photocatalytic conditions (the volume of the dye solution, the

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amount of photocatalyst) invariable at different irradiation times, required aliquots (each one containing 10 ml Rh B solution and 5 mg photocatalyst) were taken to be irradiated for differently

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given periods. After the photocatalytic reaction, the degradation suspensions under different illumination periods were centrifuged to separate the photocatalysts. The absorbance spectra of Rh

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B solutions under different photocatalytic periods were estimated using a UV–vis spectrophotometer (UV–vis 8500). The absorbance at 553 nm represents the aromatic content of

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Rh B and its decrease indicates the degradation of dye.

The recycle adsorption and photodegradation of the 60% ZnO-BiOI hybrid nanoflakes were

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evaluated by repeating experiments under the above-mentioned conditions. After each run, photocatalysts were collected after washing with deionized water and absolute ethanol several

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times, separately, in order to remove the adsorbed degradation products.

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2.7 Examination of reactive species

Certain amounts of scavengers were introduced into the Rh B solution prior to the addition of the photocatalyst and the following procedures were the same as those in photodegradation.

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3. Results and discussion

3.1 Characterization of ZnO-BiOI combined nanostructures

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Fig. 1 (Left) shows the PXRD patterns of ZnO-BiOI samples with varying BiOI contents along with the pristine ZnO and BiOI for comparison. All the diffraction peaks in Fig. 1a were perfectly assigned as the hexagonal phase of ZnO, coinciding well with the standard data of ZnO (JCPDS card 36-1451). The broadening of diffraction peaks reveals that the as-prepared ZnO is of small size. All the diffraction peaks of the pristine BiOI are consistent with the tetragonal phase of BiOI (JCPDS card 10-0445), as shown in Fig. 1g. The absence of any impurity related peaks in the PXRD patterns of ZnO and BiOI samples confirms the high purity of their phases. In the PXRD patterns of ZnO-BiOI combined samples as shown in Figs. 1b–1h, with an increasing amount of BiOI from 20% to 70%, the tetragonal BiOI diffraction peaks began to form and gradually 7

ACCEPTED MANUSCRIPT intensified. On the contrary, the hexagonal ZnO diffraction peaks gradually weakened and completely disappeared at 60% although these samples contain the same amount of ZnO. This result suggests that ZnO has embedded into the BiOI phase. Fig. 1 (Right) shows the main diffraction peak of BiOI (102) for 30–70% ZnO-BiOI combined samples and the pristine BiOI. Compared to the pristine BiOI, the (102) peaks of BiOI in

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ZnO-BiOI combined samples shifted to a low angle region, indicating that the lattice parameters of BiOI enlarged. This result confirms that in the ZnO-BiOI combined samples, BiOI has been

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embedded by ZnO. It was further observed that all the (102) peaks for samples with 30–60% BiOI have the largest shift of 0.60 to the left. When the BiOI content increases to 70%, the shift of

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BiOI (102) peak begins to reduce. It signifies that BiOI with ≤ 60% can be maximumly embedded with available amount of ZnO, and BiOI with > 60% is merely able to be partly embedded by the

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same amount of ZnO. This is because for the preparation of a series of ZnO-BiOI combined samples, BiOI content was changed while keeping ZnO at fixed quantity. However, no shift in the

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diffraction peaks of ZnO-loaded BiOI heterostructures was reported by Jiang et al. [27]. This suggests that the coupling between ZnO and BiOI in the embedded nanostructure is stronger than

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those in the loaded heterostructures.

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XPS measurements can provide further information on the chemical composition and surface chemical states. The binding energies in the XPS spectra presented were calibrated by that of C 1s (284.6 eV). Fig. 2a shows the survey XPS spectrum of 60% ZnO-BiOI combined sample and all

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the peaks can be indexed into Zn, Bi, O, and I elements, except for the C 1s peak of carbon present in the instrument. Figs. 2b–2e show the high-resolution XPS spectra of Bi 4f, Zn 2p, O 1s, and I

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3d. Two strong peaks located at 159.1 and 164.4 eV are assigned to Bi 4f7/2 and Bi 4f5/2, respectively (Fig. 2b); the peak separation between them is 5.3 eV, indicating +3 oxidation state of bismuth. The absence of satellite peaks around Bi 4f7/2 and Bi 4f5/2 indicates no any other oxidation state of element Bi in this sample. Two strong peaks at 1022.2 and 1045.2 eV are attributed to Zn 2p1/2 and Zn 2p3/2, respectively (Fig. 2c). The peak separation between Zn 2p1/2 and Zn 2p3/2 is 23.0 eV, ascribed to the Zn2+ cations in ZnO-BiOI combined nanostructure. Two peaks at 619.8 and 631.3 eV are assigned to I 3d5/2 and I 3d3/2, respectively (Fig. 2d). The peak difference of 11.5 eV is corresponding to I- anions. The unsymmetrical peak at 531.5 eV in the fine XPS spectrum of O 1s (Fig. 2e) can be de-convolved into two peaks: one at 532.1 eV being 8

ACCEPTED MANUSCRIPT ascribed to the O–H bonds of the surface-adsorbed water (Oa) [39] and another at 531.0 eV being attributed to the lattice oxygen ( O 1s). However, compared to the pristine ZnO [40] and BiOI [37], the binding energies of both Zn 2p and I 3d have a shift to the high energy region by 0.5 and 0.9 eV, respectively, confirming that strong interaction between ZnO and BiOI in the embedded nanostructure. It is possibly

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attributable to the formation of Zn–I bonds in the ZnO-embedded BiOI nanostructures. Much to our attention, the fine XPS spectrum of O 1s shows only one kind of lattice oxygen in the

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ZnO-embedded BiOI hybrid nanostructure, whereas two kinds of lattice oxygens from BiOI and ZnO have been detected in the reported ZnO-loaded BiOI heterostructures [27]. It is because the

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weakening of Zn–O bonds caused by the formation of Zn–I bonds and dispersion of ZnO nanocrystalline result into difficult detection of the O1s of ZnO in this embedded nanostructures.

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The binding energy of O1s at 531.0 eV is larger than that of the pristine BiOI (529.5 eV) [28]. This can be explained by the fact that ZnO embedding increases the interlamellar spacing among

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[Bi2O2] slabs in the BiOI phase and then reduces the outer-shell electron density of the O element. These XPS results reveal the intimate coupling between two phases in the ZnO-embedded BiOI

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nanostructures again.

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In order to investigate the morphologies and microstructures of ZnO precursor [Zn5(OH)6(CO3)2], ZnO and two typical ZnO-BiOI combined samples were characterized by TEM and HRTEM. Figs. 3a and 3b show the TEM images of the Zn5(OH)6(CO3)2. It consists of a large

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number of nanosheets of 1μm in width. The enlarged TEM image shows a thickness roughly in the range of several nanometers denoted by arrows in Fig. 2b. These nanosheets evolved into a

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porous structure after calcination at 300oC for 2 h as shown in Fig. 3c. Fig. 3d displays the HRTEM image of the calcined precursor. The lattice spacing of 0.261 nm corresponds to the (0002) facet of hexagonal-phase ZnO, affirming that Zn5(CO3)2(OH)6 after calcination has evolved into ZnO. Fig. 4 shows the TEM and HRTEM images of two typical ZnO-BiOI combined samples. For 30% ZnO-BiOI sample, the TEM image shown in Fig. 4a displays some thin BiOI nanoflakes loaded on the porous ZnO nanosheet. An interface region between BiOI and ZnO was enlarged by HRTEM technique, and its image is shown in Fig. 4b. The lattice spacing of 0.307 nm corresponds to the (102) plane of BiOI, which is swelled compared to that of the pristine BiOI 9

ACCEPTED MANUSCRIPT (0.298 nm). The swollen BiOI lattice corresponds to the left-shift XRD diffraction peaks. As expected, the lattice spacing of 0.244 nm corresponds to the (10-11) plane of ZnO, agreeing with the value of the pristine ZnO (0.246 nm). The swollen BiOI lattice and the unchanged ZnO lattice show that 30% ZnO-BiOI sample is a typical ZnO/(ZnO-embedded BiOI) heterostructure. When the weight percentage of BiOI increased to 60%, ZnO-embedded BiOI nanoflakes grew up and

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meanwhile porous ZnO nanosheets disappeared (Fig. 4c). Fig. 4d shows the HRTEM image taken from one nanoflake. The lattice spacing of 0.286 and 0.288 nm corresponding to the (110) plane of

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BiOI and that of 0.269 nm corresponding to the (0002) plane of ZnO in the nanoflake are larger than that of the pristine BiOI (JCPDF, 0.28 nm) and pure ZnO (JCPDF, 0.26 nm), respectively.

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The enlarged lattice spacing of both BiOI and ZnO phases indicate that merely ZnO-embedded BiOI nanoflakes exist in 60% ZnO-BiOI combined sample. The TEM elemental mapping images

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(Fig. 5) of 60% ZnO-BiOI combined sample show the presence of Zn, Bi, I and O elements, furthermore confirming a hybrid structure. Based on the XRD and HRTEM characterizations, we

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can conclude that 20–50% ZnO-BiOI samples are ZnO/(ZnO-embedded BiOI) heterostructures and those of 60–70% are ZnO-embedded BiOI hybrid nanostructures.

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3.2 Formation mechanism of ZnO-embedded BiOI hybrid nanoflakes

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Layered hydrolyzed zinc carbonate (LHZC) is commonly thought to have a bottom-center monoclinic crystal symmetry, but more important is its layered structure, as shown in Fig. 6a. The layered structure is formed by holding complex sheets by CO32- groups, and the complex sheet,

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Zn-OH layer, is composed of two kinds of Zn(OH)6O2 octahedra and one kind of Zn(OH)3O tetrahedra sharing edges alternately for running [41]. The surface of Zn-OH layer including CO32-

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groups performs hydrophobicity [42]. Thus, in the presence of organic molecules with hydrophobic group, the layered structure is easily cleaved among layers, assisting the formation of LHZC nanosheets. Our ZnO-embedded BiOI hybrid nanoflakes were fabricated on the basis of the LHZC nanosheets playing the role of precursor and template. Typical formation mechanism of ZnO-embedded BiOI hybrid nanoflakes was proposed as follows. LHZC nanosheets obtained via a hydrothermal route were dispersed in water under magnetic stirring. After the addition of I- ions, the surfaces of LHZC nanosheets with deficient CO32- groups would adsorb a certain number of I- ions (Fig. 6b). Subsequently, the solution of Bi(NO3)3 dissolved in ethylene glycol (EG) was added into the above system. It needs to be 10

ACCEPTED MANUSCRIPT pointed out that complex reaction would occur as Bi(NO3)3 solid dissolves in the EG solution, mainly producing [Bi(OCH2CH2OH)2]+ ([Bi(m-EG)2]+) and [Bi2(OCH2CH2OH)3]+ ([Bi2(m-EG)3]+) complexes as well as H+ cations [43]. The complex reaction can be written as 2Bi3+ + 5EG → [Bi(m-EG)2]+ + [Bi2(m-EG)3]+ + 7H+. When this complex solution was dropped into the above-mentioned system, on one hand, H+ cations could react with a part of CO32- and OH- groups in LHZC nanosheets and leave O and OH vacancy. These vacant locations could be quickly

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occupied by I- anions or hydroxyls of the EG molecules. Accordingly, the LHZC nanosheets

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would be swelled under a part CO32- and OH- groups missing, and meanwhile insertion of EG molecules and I- anions (Fig. 6c). On the other hand, [Bi(m-EG)2]+ and [Bi2(m-EG)3]+ complexes

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were hydrolyzed into [Bi2O2]2+ groups and EG molecules under neutral solution. These nascent [Bi2O2]2+ groups react with I- ions adsorbed on the surface of Zn-OH layers to produce BiOI

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nucleus and subsequently grow into nanoflakes by further extending of [Bi2O2]2+ slabs along with I- ions. As a result, BiOI nanoflakes embeddedly hybridized with Zn-OH layers are generated (Fig.

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6d). Once annealed at 300oC, the Zn-OH layers evolved into ZnO fragments, ultimately forming ZnO-embedded BiOI hybrid nanoflakes (Fig. 6e). The formation mechanism of ZnO-embedded

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3.3 Optical absorption property

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BiOI hybrid nanoflakes is schematically showed in Fig. 6.

Fig. 7a shows the UV–vis diffuse reflectance spectroscopy of neat ZnO, neat BiOI and ZnO-BiOI combined nanostructures. Optical absorption edges (position at which reflectance

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decreases) of ZnO and BiOI were roughly estimated from the absorption onset located at 400 and 668 nm, respectively. The observed result indicates that the pristine ZnO has no significant

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absorption in the visible region, whereas the pristine BiOI exhibits a broad absorption in the visible region. ZnO-BiOI combined nanostructures with 20–70% BiOI display steep absorption edges with similar gradient to that of the pristine BiOI. Different from the reported ZnO-loaded BiOI heterostructures [27], these optical absorption edges are orderly red-shifted with increasing BiOI content. Fig. 7b shows a photo of them, also illustrating that the color is gradually deepened from green to orange red with increasing BiOI weight percentage from 10% to 70%. The sequential red shift of optical absorption edges for ZnO-BiOI combined samples with increasing BiOI content is because of the quantum size effect - an increase in the size of ZnO-embedded BiOI hybrid nanoflakes with increasing BiOI content results into descending in their band gap. 11

ACCEPTED MANUSCRIPT The band gap energy of a semiconductor could be determined from diffuse reflectance spectra by Tauc plot [44]. The Kubelka–Munk relational expression is as follows. (hνF(R∞))1/n = A(hν  Eg)·······························································(1) where h, ν, A, and Eg are the Plank constant, light frequency, a proportionality constant, and band gap energy, respectively. F(R∞) could be calculated by (1  R)2/2R (R delegates the reflectivity).

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The value of the exponent n denotes the nature of the sample transition (i.e., n = 1/2 for direct allowed transition or n = 2 for indirect allowed transition). For ZnO [45] and BiOI [23], the value

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of n is 1/2 and 2, respectively. Thus, using the Kubelka–Munk function, (hνF(R∞))2 was plotted against the photon energy (hν) for ZnO and (hνF(R∞))1/2 was plotted against hν for BiOI. By the

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point of intersection of the tangent line to the point of inflection on the plotted curve with the horizontal axis, the Eg values for ZnO and BiOI were estimated to be 3.29 eV and 1.86 eV,

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respectively, as shown in Figs. 8a and 8b. According to the absorption expression [α = A(hν−Eg)n], samples with different transition types (i.e., various value for n) should display the curves of

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absorption edge in different shapes. Owing to the continuous phase in ZnO-embedded BiOI hybrid nanostructures being BiOI phase and their absorption-edge shapes similar to that of pristine BiOI,

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ZnO-embedded BiOI hybrid nanoflakes could be regarded as an indirect allowed transition, and

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the n value for them is also 2. As a result, the Eg value of the ZnO-embedded BiOI hybrid nanoflakes with 60% BiOI is equal to 2.03 eV, according to the plot of (hνF(R∞))1/2 versus hν as shown in Fig. 8b.

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3.4 Electrochemical and photoelectrochemical behaviors To further investigate the variation of photoresponse and conductivity in ZnO,

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ZnO/(ZnO-embedded BiOI) heterostructure, ZnO-embedded BiOI hybrid nanostructure and BiOI, electrochemical and photoelectrochemical measurements were carried out on the films of these materials deposited on an ITO substrate as the working electrode. Cyclic voltammograms of these materials measured in the air and under light irradiation at a velocity of 0.05 V/s are shown in Fig. 9. A weak reductive peak is observed for ZnO (Fig. 9a), and two oxidation peaks are observed for ZnO-BiOI combined nanostructures (Figs. 9b and 9c) and BiOI (Fig. 9d). It’s worth noting that the current density of the oxidation peaks in the air for ZnO-embedded BiOI hybrid nanostructure is 30 times larger than that of ZnO/(ZnO-embedded BiOI) heterostructure and roughly 10 times larger than that of the pristine BiOI. This demonstrates that ZnO-embedded BiOI hybrid 12

ACCEPTED MANUSCRIPT nanoflakes have the best conductivity among the measured samples. For ZnO-BiOI combined nanostructures and pristine BiOI under light irradiation (λ ≤ 700 nm), the current density of their oxidation peaks enhances in varying degrees compared to those in the air (Figs. 9b–9d). It reveals that these materials can be activated by UV–visible light, among which ZnO-embedded BiOI hybrid nanostructure performs the strongest photoresponse. It was

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further observed that for ZnO-embedded BiOI hybrid nanoflakes under light illumination, two oxidation peaks have a shift to the oxidizing direction relative to those in the air. Apart from this, a

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new broad oxidation peak was observed at an applied potential of 0.26 V (vs. SCE), as shown in Fig. 9c. These results suggest that photoinduced holes from ZnO-embedded BiOI hybrid

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nanostructure have stronger oxidizing activity than those from ZnO/(ZnO-embedded BiOI) heterostructure and pristine BiOI. Based on the results of optical and photoelectrochemical

visible-light photocatalytic oxidizing activity.

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3.5 Photocatalytic performance

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measurements, ZnO-BiOI combined samples with different structures should have different

The photocatalytic activities of pristine ZnO, ZnO-BiOI combined nanostructures and pristine

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BiOI were evaluated by degradation of Rh B solutions under visible light irradiation (λ ≥ 420 nm).

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The UV–vis spectra of Rh B being degraded for different periods for all the catalysts and in the absence of photocatalysts are shown in Fig. S1. The percentage of Rh B degradation over all the photocatalysts as a function of exposure time is shown in Fig. 10. Photocatalytic experiments were

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carried out under four different conditions. (i) Rh B aqueous solutions with photocatalysts under magnetic stirring in the dark for 20 min. Relative concentration (C/C0) of Rh B at the moment of

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irradiation can show that the adsorption capacity of ZnO-BiOI combined nanostructures for Rh B molecules increases with increasing BiOI content and reaches the maximum at 60%. This is because BiOI has stronger adsorption ability than ZnO. (ii) Rh B aqueous solutions under visible light without photocatalysts. This experiment indicates that Rh B molecules are difficult to decompose under visible light irradiation in the absence of photocatalysts. (iii) Rh B solutions under visible light irradiation in the presence of all the chemically synthesized samples. In order to distinguish the photocatalytic performance of the prepared samples, the plots for samples with different photodegradation performance and the plots for samples with similar photodegradation performance are shown in the top and bottom of Fig. 10, respectively. ZnO and 13

ACCEPTED MANUSCRIPT 10% ZnO-BiOI combined samples performed certain photocatalytic activity under visible light irradiation although they can’t absorb visible light, which has to be attributable to their dye-sensitized photodegradation. It is further confirmed by another fact that the stronger the adsorption capability of a sample, the higher is the photocatalytic activity, comparing the C/Co vs. t plot for ZnO with that for 10% ZnO-BiOI sample in the top of Fig. 10. When BiOI content

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increases from 10% to 20%, ZnO-BiOI combined nanomaterial performs the first-stage substantial enhancement of photocatalytic activity. In the first illumination of 20 min, the RhB-photodegraded

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percentage was 58.6% deducting the adsorption amount, which was 6 times higher than BiOI (9.8%) and 4 times higher than ZnO (14.8%). It should be ascribed to visible-light absorption and

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formation of ZnO/(ZnO-embedded BiOI) hetero interface which can separate photoinduced carriers. However, in the second illumination of 20 min, the RhB-photodegraded percentage

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decreased to 12.0%. With the BiOI content ranging from 20% to 50%, ZnO-BiOI combined samples perform similar photocatalytic behaviors - high starting activity followed by a gradual

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decrease with the extension of irradiation time, shown in the bottom of Fig. 10. This poor sustainability of photocatalytic activity for these samples mainly results from the instability of

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their ZnO/(ZnO-embedded BiOI) heterostructure. While the BiOI content increases from 50% to

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60%, the initial adsorption reaches 63.0%. In order to make a valid comparison of the degradation efficiency, the initial adsorption was ruled out. Once the BiOI content increases to 60%, the photocatalytic activity of ZnO-BiOI combined nanomaterial is enhanced in the second stage. In

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the illumination of 5 min, the RhB-photodegraded percentage was 37.2% deducting the adsorption amount, which was 8.6 times higher than BiOI (4.3%) and 8 times higher than ZnO (4.6%).

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Further increasing the BiOI content up to 70%, the RhB-photodegraded percentage was 21% in the first 10 min and 18.5% in the second 10 min. Although the photocatalytic activity began to decrease compared to 60% ZnO-BiOI hybrid, they showed good sustainability of photocatalytic activity. The results based on the photocatalytic measurements indicate that ZnO-embedded BiOI hybrid nanostructures showed both remarkably enhanced and sustainable photocatalytic activity in comparison with the pristine ZnO, BiOI and ZnO/BiOI heterostructures. IR technique is a valid means to measure whether the dye molecules adsorbed on the surface of photocatalysts are degraded or not [46]. Complete degradation for the 60% ZnO-BiOI photocatalyst was estimated from the IR spectra shown in Fig. S2. The IR spectrum (Fig. S2b) of 14

ACCEPTED MANUSCRIPT 60% ZnO-BiOI after adsorption shows the characteristic absorption peaks [47] of Rh B molecules marked with vertical lines. The IR spectrum (Fig. S2c) of 60% ZnO-BiOI collected after irradiation of 10 min is similar to that of fresh 60% ZnO-BiOI (Fig. S2a), demonstrating that all the adsorbed dye molecules have been completely degraded. To further confirm the mineralization of Rh B, the degradation solution was also analyzed by TOC measurements. The TOC value of

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2.85 mg·L-1 for 3 mg·L-1 Rh B solution decreased to 1.86 mg·L-1 (65.2% mineralization percentage) after irradiation of 10 min in the presence of 60% ZnO-BiOI, indicating that dye

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molecules were not completely mineralized.

The recyclability of the 60% ZnO-BiOI hybrid nanoflakes was evaluated by repeating

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experiments on the degradation of Rh B under visible light irradiation. After each run, the photocatalysts were washed with deionized water followed by absolute ethanol several times to

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remove degradation products adsorbed on the surface of photocatalysts. As shown in Fig. 11, 60% ZnO/BiOI has 62.9% initial adsorption in the first run and 59.5% and 54.7% adsorption in

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the second and third run, respectively. In the irradiation of 5 min, the degradation percentages in the first, second, and third runs are 36.7%, 39.6% and 42.8% after deducting initial adsorption

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amount, respectively. The results suggest that there is no significant decrease in the adsorption and

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photocatalytic efficiency after three successive cycles, indicating that ZnO-embedded BiOI hybrid nanostructure not only has high and sustainable photocatalytic activity but also good recyclability. The pH-dependent photodegradation and adsorption of 60% ZnO-BiOI were studied by

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adjusting the pH of the dye solution in the range 3–9 before irradiation. The effect of the initial pH on the Rh B degradation rate constant is shown in Fig. S3. The pseudo-first order rate constants

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for 60% ZnO-BiOI at pH of 3.01, 5.52, 6.56, 7.53, and 9.03 were 0.313, 0.104, 0.520, 0.004, and 0.003 min-1, respectively. The results demonstrate that the photocatalytic efficiency in the acid reaction system is better than that in the basic reaction system, the best being in the solution with a pH of 6.56. The effect of initial pH on the adsorption capacity (qe) of 60% ZnO-BiOI photocatalysts is shown in Fig. S4. The results suggest that the qe of 60% ZnO-BiOI in an acid medium is also higher than that in a basic medium. However, the highest qe (5.6 mg/g) is observed in Rh B solution at the lowest pH of 3.01. The pH-dependent photodegradation and adsorption experiments show that the adsorption is one but not a sole factor affecting photodegradation efficiency for our ZnO-embedded BiOI hybrid nanostructure. 15

ACCEPTED MANUSCRIPT 3.6 Enhanced photocatalytic mechanism for ZnO-embedded BiOI hybrid nanoflakes The internal electric field in a p–n heterostructure would not establish until the particle size is large enough for band bending (not less than 100 nm) [48,49]. Since the thickness of ZnO-embedded BiOI hybrid nanoflakes is not more than 10 nm, substantial enhancement in the photocatalytic activity could not be explained by internal electric field separating photoinduced

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electrons and holes [27]. It has to be ascribable to the formation of ZnO-embedded BiOI hybrid nanostructure that gives rise to other important effects on photocatalytic activity.

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3.6.1 Role of reactive species

To evaluate the roles of these reactive species, their corresponding scavengers were used as

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probes in the degradation of dyes. Benzoquinonene (BQ) [35], triethanolamine (TEOA) [50], and isopropyl alcohol (IPA) [51] were adopted as the traps for ∙O2−, hvb+, and ∙OH, respectively, during

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the Rh B degradation with ZnO-embedded BiOI hybrid nanoflakes with 60% BiOI under visible-light irradiation. The concentrations of BQ, TEOA, and IPA in the photodegradation

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system were 0.1, 1.0, and 1.0 mmol/L, respectively.

Fig. 12 illustrates the variation of Rh B degradation with different scavengers against irradiation

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time. In the presence of BQ or TEOA, the photodegradation of Rh B was inhibited almost

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completely, only adsorption occurred. This signifies that ∙O2− and hvb+ played crucial roles in the visible-light photodegradation system. ∙O2− could be generated through reacting photoinduced electrons (ecb-) with O2 adsorbed on the surface of photocatalyst [35]. The addition of IPA showed

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that the photocatalytic degradation of Rh B was inhibited to a certain degree, indicating that ∙OH species also play a part in the photodegradation process. Hence, we proposed that both ∙O2− and h+

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are the main reactive species for ZnO-embedded BiOI nanoflakes as visible-light photocatalyst. 3.6.2 Effect of band-edge potential The potential of valence band edge (Evb) or potential of conduction band edge (Ecb) can be evaluated by flat band potential in combination with the optical band gap. For an n-semiconductor, the flat band potential is at which the cathodic current begins, close to the bottom of conduction band; for a p-semiconductor, the flat band potential is at which the anodic current begins, close to the top of valence band [52]. Therefore, the conductivity type of the as-fabricated materials needs to be judged before their flat band potentials are estimated. An n-semiconductor can produce an anodic photocurrent, which increases in the anodic 16

ACCEPTED MANUSCRIPT direction (towards more oxidizing potentials); a p-semiconductor can produce a cathodic photocurrent, which increases in the cathodic direction (towards more reducing potentials). Figs. 9a and 9c illustrate that both ZnO and 60% ZnO-BiOI combined nanostructures perform anodic photocurrent. Fig. 9d illustrates that the pristine BiOI performs cathodic photocurrent. Based on these results, we proposed that both ZnO and ZnO-embedded BiOI hybrid nanoflakes are

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n-semiconductors, and pristine BiOI is a p-semiconductor. The flat band potential for an n-semiconductor (or p-semiconductor) can be obtained from the

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point of intersection of the tangent line to the curve of cathodic current (or anodic current) with the horizontal base line [52]. The insets in Figs. 9a, 9c, and 9d represent the Efb values (vs. SCE)

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for ZnO, ZnO-embedded BiOI hybrid, and BiOI, respectively. The Efb values (vs. normal hydrogen electrode (NHE)), obtained by subtracting the value of ESCEθ (0.24 V) from the Efb

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value (vs. SCE), are equal to 0.49 V, 0.55 V, and 1.11V for ZnO, ZnO-embedded BiOI hybrid, and BiOI, respectively. Equating these potentials with the conduction band edges for

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n-semiconductors or the valence band tops for p-semiconductors and then using the optical band gaps (Figs. 8a and 8b) to calculate the other band gap edge (EVB or ECB) for each sample, the

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energy band scheme can be derived, as shown in Fig. 13.

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In Fig. 13, one can also observe that for samples except for ZnO, the EVB values are less oxidizing than the formation potential of ·OH (hvb+ + H2O = ·OH + H+, 2.23 V) [53], and ECB values of these three samples are more reducing than the formation potential of ·O2- species (O2 +

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ecb- = ·O2-, 0.33 V) [54,55]. Therefore, for ZnO-BiOI embedded hybrid and pristine BiOI under visible-light irradiation, ·O2- and photogenerated hvb+ are the main active species during our

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photodegradation reactions. This result is consistent with the measurement of the roles of reactive species. It is noticeable that the EVB of ZnO-embedded BiOI hybrid (1.48 V) is more oxidizing than that of BiOI (1.11 V), revealing that photoinduced hvb+ from ZnO-embedded BiOI hybrid has higher photo-oxidation activity than the pristine BiOI. 3.6.3 Effect of deep-level defects PL spectra originate from the migration, transfer, and separation efficiency of the photogenerated charge carriers in a semiconducting material [56–58]. There is a strong correlation between the PL intensity and photocatalytic performances. The higher PL intensity indicates the higher recombination of charge carriers, resulting in an decrease in photocatalytic activity [36,59]. 17

ACCEPTED MANUSCRIPT To obtain more insight into the mechanism of enhanced photocatalytic activity for ZnO-embedded BiOI hybrid nanoflakes under visible-light irradiation, additional PL spectra were recorded for ZnO, ZnO/(ZnO-embedded BiOI) heterostructure, ZnO-embedded BiOI hybrid nanoflakes, and pristine BiOI excited by visible light with a wavelength of 420 nm (Fig. 14). The PL spectrum of the pristine ZnO shows a blue emission centered at 466 nm and a strong and

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wide emission centered at 625 nm. As reported, the blue emission originates from the electronic transition from the interstitial-zinc-related defect levels to the valence band, and the orange

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emission corresponds to the electron transition from the interstitial-zinc-related defect levels to the deep defect levels [60]. By this statement, there is a great deal of interstitial-zinc-related defects

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and deep-level defects in the as-prepared ZnO. For the pristine BiOI, the PL spectrum doesn’t show any emission peaks; it is probably because BiOI is a semiconductor of indirect allowed

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transition, where stimulated electrons tend to return to ground states via heat emission or other ways. For ZnO-BiOI combined materials, their blue emission slightly strengthens with respect to

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that of the pristine ZnO, whereas their orange emission substantially diminishes with increasing BiOI content and vanishes up to 60%. These results indicate that the ZnO-embedded BiOI hybrid

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nanostructure can efficiently inhibit the generation of deep-level defects in ZnO; this could explain

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the enhancement in the conductivity of ZnO-embedded BiOI hybrid nanoflakes. In addition, the sharp decrease of deep-level defects in the ZnO-embedded BiOI hybrid nanoflakes can reduce the recombination centers of the photogenerated electrons and holes and is another reason for further

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enhancement of their photocatalytic activity. On the basis of the above-mentioned discussion, using ZnO-embedded BiOI hybrid

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nanostructure as photocatalysts, the photodegradation process was carried out not only by generated ·O2- oxidation but also via direct hvb+ oxidation route. There are two important reasons for the enhanced photocatalytic activity of the ZnO-embedded BiOI hybrid nanostructure. One is the raised EVB, causing photogenerated hvb+ to possess stronger oxidation activity. The other is the quenching of deep-level defects in ZnO-embedded BiOI hybrid nanostructure, which not only can increase the transition of photoinduced electrons and holes but also reduce the recombination centers of the photoinduced electrons and holes, ultimately promoting the photocatalytic degradation process. In other words, the raised EVB and quenching of deep-level defects cause the remarkably enhanced photocatalytic activity of ZnO-embedded BiOI hybrid nanostructure. 18

ACCEPTED MANUSCRIPT 4. Conclusions In summary, ZnO-BiOI combined photocatalysts were prepared using Zn5(CO3)2(OH)6 nanosheets for BiOI deposition followed by calcination. The characterizations by PXRD and HRTEM indicate that the ZnO-BiOI combined nanomaterials with 20–50% BiOI are ZnO/(ZnO-embedded BiOI) heterostructure and those with 60–70% BiOI are the ZnO-embedded

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BiOI hybrid nanostructures. The PXRD and XPS measurements show intimate coupling between ZnO and BiOI in the ZnO-embedded BiOI hybrid nanostructures, compared to ZnO-loaded BiOI

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heterostructures. The light absorption range in the visible light region for ZnO-BiOI combined nanostructures gradually enlarges with increasing BiOI content, up to nearly that of the pristine

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BiOI. The visible-light photocatalytic experiments show that ZnO-embedded BiOI hybrid nanostructures not only perform higher photocatalytic activity but also show better sustainability

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than ZnO/(ZnO-embedded BiOI) heterostructure. The electrochemical and photoelectrochemical data together with the PL spectrum prove that remarkably enhanced photocatalytic activity is

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attributed to the formation of ZnO-embedded BiOI hybrid nanostructure leading to high potential of hvb+, good conductivity, and quenching of deep-level defects. Our work suggests that further

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optimization of photocatalytic activity with good sustainability and recyclability should be

Acknowledgements

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possible by hybridizing BiOI with a semiconductor with a more oxidizing valence band edge.

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This work was supported by National Natural Science Foundation of China (21607041), Science and Technology Planning Project of Zhejiang Province, China (2017C33240), Natural

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Science Foundation of Zhejiang Province, China (LQ14F040003 and Y15B070018), Natural Science Foundation of Huzhou City, China (2015YZ03), and Special Commission Project of the National Social Science Foundation of China (16@ZH005).

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ACCEPTED MANUSCRIPT Figure Captions Fig. 1 (Left) X-ray diffraction patterns of (a) ZnO, (b) 10% ZnO-BiOI, (c) 20% ZnO-BiOI, (d) 30% ZnO-BiOI, (e) 40% ZnO-BiOI, (f) 50% ZnO-BiOI, (g) 60% ZnO-BiOI, (h) 70% ZnO-BiOI, (i) BiOI; (Right) Enlarged (102) diffraction peaks of BiOI, which shifts to the low angle region with respect to pristine BiOI.

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Fig. 2 XPS patterns of 60% ZnO-BiOI hybrid nanoflakes: (a) typical XPS survey, (b) Zn 2p, (c) Bi 4f, (d) O 1s, and (e) I 3d fine spectra.

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Fig. 3 (a and b) TEM image and magnified TEM image of precursor before annealing, displaying

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Fig. 4 (a and b) TEM and HRTEM images of 30% ZnO-BiOI, TEM image represents porous ZnO nanosheet loading BiOI lamella designated by arrows, and HRTEM image shows a general

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heterojunction; (c and d) TEM and HRTEM images of 60% ZnO-BiOI, showing both swelled ZnO (0002) lattice fringes and BiOI (110) lattice fringes. Fig. 5 TEM elemental mapping images of 60% ZnO-BiOI, showing nanoflakes with a hybrid

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Fig. 6 Schematic diagram for possible formation mechanism of ZnO-embedded BiOI hybrid nanoflakes.

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Fig. 7 (a) UV–vis diffuse reflectance spectra of ZnO, 10% ZnO-BiOI, 20% ZnO-BiOI, 30% ZnO-BiOI, 40% ZnO-BiOI, 50% ZnO-BiOI, 60% ZnO-BiOI, 70% ZnO-BiOI and BiOI, the

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absorption edges of these samples having a red shift with the increasing BiOI content. (b) Photos of samples corresponding to (a), showing sequential variation in color. Fig. 8 (a) Plot of (hνF(R∞))2 vs. photon energy (hν) for ZnO; (b) plots of (hνF(R∞))0.5 vs. hν for pristine BiOI and ZnO-embedded BiOI hybrid nanoflakes with 60% BiOI. All the bang gap energy (Eg) are marked with arrows. Fig. 9 Cyclic voltammograms curves in aqueous 0.1 M Na2SO4/H2SO4 solution (pH = 6) in the air and under light (λ ≤ 700 nm): (a) ZnO, (b) 30% ZnO-BiOI, (c) 60% ZnO-BiOI and (d) BiOI. Insets in a–d represent the flab potential value of the corresponding sample (vs. SCE). All the scanning directions are marked with arrows.

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ACCEPTED MANUSCRIPT Fig. 10 Variation of the relative content of Rh B (C/C0) in the dark and then under visible-light irradiation (λ ≥ 420 nm) in the absence/presence of photocatalysts: (top) plots for comparison among different photodegradation performance; (bottom) plots for comparison among similar photodegradation performance. Fig. 11 Cyclic adsorption and degradation curve for ZnO-hybridizing BiOI nanoflakes with 60%

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Fig. 13 Band edge potentials obtained from CV curves in Fig. 9 combined with the Eg values in

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Fig. 8. The potential of O2/·O2- (-0.33 V) and the potential of H2O/·OH (+2.23 V) at neutral pH are shown by the borders of the gray band.

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Fig. 14 Photoluminescence emission spectra of (a) ZnO, (b) 30% ZnO-BiOI, (c) 60% ZnO-BiOI

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Fig. 1 (Left) X-ray diffraction patterns of (a) ZnO, (b) 10% ZnO-BiOI, (c) 20% ZnO-BiOI, (d) 30% ZnO-BiOI, (e) 40% ZnO-BiOI, (f) 50% ZnO-BiOI, (g) 60% ZnO-BiOI, (h) 70% ZnO-BiOI,

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(i) BiOI; (Right) Enlarged (102) diffraction peaks of BiOI, which shifts to the low angle region

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Fig. 2 XPS patterns of 60% ZnO-BiOI hybrid nanoflakes: (a) typical XPS survey, (b) Zn 2p, (c) Bi

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4f, (d) O 1s, and (e) I 3d fine spectra.

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porous structures. (d) HRTEM image of ZnO, showing the lattice fringes of (0002) crystal faces.

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Fig. 4 (a and b) TEM and HRTEM images of 30% ZnO-BiOI, TEM image represents porous ZnO

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ZnO (0002) lattice fringes and BiOI (110) lattice fringes.

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Fig. 5 TEM elemental mapping images of 60% ZnO-BiOI, showing nanoflakes with a hybrid

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Fig. 6 Schematic diagram for possible formation mechanism of ZnO-embedded BiOI hybrid

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nanoflakes.

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Fig. 7 (a) UV-vis diffuse reflectance spectra of ZnO, 10% ZnO-BiOI, 20% ZnO-BiOI, 30%

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absorption edges of these samples having a red shift with the increasing BiOI content. (b) Photos

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Fig. 8 (a) Plot of (hνF(R∞))2 vs photon energy (hν) for ZnO; (b) plots of (hνF(R∞))0.5 vs hν for pristine BiOI and ZnO-embedded BiOI hybrid nanoflakes with 60% BiOI. All the bang gap

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energy (Eg) are marked with arrows.

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Fig. 9 Cyclic voltammograms curves in aqueous 0.1 M Na2SO4/H2SO4 solution (pH = 6) in the air

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Fig. 10 Variation of the relative content of Rh B (C/C0) in the dark and then under visible-light

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Fig. 12 Effects of different scavengers on RhB degradation in the presence of ZnO-embedded

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BiOI hybrid nanostructure with 60% BiOI.

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Fig. 13 Band edge potentials obtained from CV curves in Fig. 9 combined with the Eg values in Fig. 8. The potential of O2/·O2- (-0.33 V) and the potential of H2O/·OH (+2.23 V) at neutral pH are

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Fig. 14 Photoluminescence emission spectra of (a) ZnO, (b) 30% ZnO-BiOI, (c) 60% ZnO-BiOI

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Graphical abstract

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ZnO-embedded BiOI hybrid nanoflakes with enhanced and sustainable visible-light photocatalytic activity have been fabricated using Zn5(CO3)2(OH)6 ultrathin nanosheets as template for BiOI deposition followed by calcination.

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ACCEPTED MANUSCRIPT Highlights  ZnO-embedded BiOI hybrid nanoflakes showed intimate coupling between ZnO and BiOI.  This embedded hybrid nanostructure exhibited remarkably enhanced and

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sustainable photocatalytic activity.

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 The raised potential of valence-band edge, good conductivity and quenching of

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deep-level defects attributed to enhanced photocatalytic activity.

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