Multifunctional property exploration: Bi4O5I2 with high visible light photocatalytic performance and a large nonlinear optical effect

Exploration of the versatility of materials is very important for increasing the utilization of materials. Herein, we successfully prepared Bi4O5I2 powders via a facile solvothermal method. The Bi4O5I2 photocatalyst exhibited significantly higher photocatalytic activity as compared to the common BiOI photocatalyst in the degradation of methyl orange, methylene blue and rhodamine B under visible light irradiation. Especially, for the degradation of methyl orange, the photocatalytic activity of Bi4O5I2 is about 10 times that of BiOI. Moreover, Bi4O5I2 exhibits an extremely high second harmonic generation response of about 20 × KDP (the benchmark) estimated by the unbiased ab initio calculations. The coexisting multifunction of Bi4O5I2 is mainly because of the increased dipole moment due to the stereochemical activity of lone pairs that promotes separation and transfer of photogenerated carriers, then enhances the photocatalytic activity and results in a high second harmonic generation response. This indicates that Bi4O5I2 may have good potential applications in photocatalytic and nonlinear optical fields.


Introduction
Green, efficient, harmonious and civilized society is the development trend of future life. Ever since Fujishima and Honda have reported the photocatalytic water splitting on the TiO 2 surface, 1 TiO 2 has been extensively studied for the degradation of pollutants or splitting of water. 2 However, the wide bandgap (3.0-3.2 eV) of TiO 2 limits its application range. Therefore, it is essential to nd visible light-driven photocatalysts. Methylene blue (MB), methyl orange (MO) and rhodamine B (RhB) are the main components in common water pollutants. Scientists have invented many methods for the degradation of organic pollutants. [3][4][5][6][7][8] Bismuth-containing materials are attracting signicant attention of researchers due to their unique electronic, magnetic, optical, and catalytic properties; 9-13 however, the existence of high photogenerated carrier recombination rate restricts the development of Bi-based photocatalysts.
To enhance the photocatalytic activity, scientists have carried out a lot of research and exploration. [14][15][16][17][18][19][20][21] To the best of our knowledge, in 2002, Bi 4 O 5 I 2 was successfully synthesized by changing the stoichiometry of bismuth oxyhalide. 22 Aer this, non-stoichiometric ratio halogenated bismuth compounds, for example, Bi 4 O 5 I 2 , Bi 4 O 5 Br 2 , and Bi 5 O 7 Br, have been studied constantly. 23,24 Ye et al. have committed to the research and development of non-stoichiometric Bi-based photocatalysts. They made a series of innovative achievements such as photocatalytic water splitting for hydrogen (H 2 ) evolution, 25 selective photoreduction of CO 2 into CO, 26 molecular oxygen activation and RhB degradation. 27,28 Xiao et al. 29,30 have successfully synthesized Bi 4 O 5 I 2 nanoakes, which had better photocatalytic activities in the degradation of 4-tert-butylphenol, and predicted that they could be applied to other elds as well.
Non-stoichiometric ratio halogenated bismuth compounds have become the most promising second-order nonlinear optical (NLO) and photovoltaic (PV) candidate materials owing to their special structural features, rich chemical compositions, and suitable optical band gaps. In fact, most of these noncentrosymmetric structures could be applied in more than one of the aforementioned elds. It is known that nonlinear optical (NLO) materials have played an important role in laser science and technology eld and are rapidly developing. [31][32][33][34] The NLO crystals are laser frequency conversion materials, and their SHG coefficients are the focus of attention, which may reect the ability of the NLO crystal to generate harmonic laser emission. 35,36 Among them, uoroborate is responsive to ultraviolet light and is widely used in military, medical, aviation and other elds. 37 It is known that lower SHG coefficient limits their applications. Therefore, the search for new NLO materials with SHG response larger than 10 Â KDP has attracted signicant attention.
In general, noncentrosymmetric crystal structure materials may possess very good NLO properties. 28 In 2014, Fan et al. [38][39][40] found a series of materials, such as K 3 B 6 O 10 Br, Na 3 VO 2 B 6 O 11 , and M 2 B 5 O 9 Cl (M ¼ Ca, Sr, Ba, Pb), with noncentrosymmetric structures. Due to intrinsic large polarization effects, they not only have good nonlinear optical properties, but have also been found to exhibit signicantly enhanced photocatalytic activities.
To the best of our knowledge, the noncentrosymmetric polarized Bi 4 O 5 I 2 belongs to the space group P2 1 . 22 Then, does it have the multifunctional property? To research the potential properties of Bi 4 O 5 I 2 , we carried out photodegradation experiments and theoretical calculations of the optical properties. The results showed that Bi 4 O 5 I 2 had high visible light photocatalytic performance. Moreover, the theoretical calculation showed that Bi 4 O 5 I 2 exhibited large second harmonic generation (SHG) response. At present, the versatility of materials is a hot research topic that scientists have been exploring. Based on the abovementioned ndings, herein, we successfully prepared BiOI and Bi 4 O 5 I 2 materials, and some characterizations and calculations were performed to verify the versatility of these materials.

Materials and methods
Ethylene glycol (EG), bismuth nitrate pentahydrate (Bi(NO 3 ) 3 -$5H 2 O), and potassium iodide (KI) were purchased commercially and used without further purication. Moreover, deionized water was used throughout this study.
Bi 4 O 5 I 2 powders were synthesized via a facile solvothermal method. A mixture of 2 mmol Bi(NO 3 ) 3 $5H 2 O, 4 mmol KI and 35 mL EG was prepared and transferred to a 45 mL Teon-lined stainless steel autoclave, and the pH of the suspension was adjusted to 10 using 2 mol L À1 NaOH. Subsequently, it was maintained at 150 C for 12 h followed by naturally cooling to room temperature. The precipitate was centrifugally washed with distilled water and absolute ethanol for several times and then dried at 80 C for 5 h under an air atmosphere. BiOI was obtained through the abovementioned procedure without the addition of NaOH.

Characterization
Phase identication was performed using the Bruker D8 ADVANCE X-ray diffractometer (XRD) equipped with a diffracted-beam monochromator set for Cu and Ka (l ¼ 0.154059 nm) as the radiation sources. The acceleration voltage and acceleration current are 40 kV and 40 mA, respectively. The results were obtained by scanning the sample through a range of 2 theta values (10-85 ). UV-visible diffuse reectance spectra (DRS) were obtained via the Shimadzu UV-3600 UV-vis spectrophotometer using BaSO 4 as a reference. The morphologies of the samples were observed via a scanning electron microscope (SEM) using a Zeiss SUPRA 55VP apparatus, and energy dispersive X-ray (EDX) spectroscopic analysis was conducted using EDX-8000. The photocurrents (PC) of UV-vis light on and off studies were measured via the CHI660E electrochemical system (Shanghai, China) using a standard three-electrode cell containing a working electrode (1.0 cm Â 1.0 cm), a platinum plate as the counter electrode, and a standard calomel electrode (SCE) as the reference electrode. Electrochemical impedance spectroscopy (EIS) measurements were carried out using an electrochemical workstation (Zahner Im6ex) with the working electrode in the frequency range from 100 kHz to 0.01 Hz at open circuit potential in a 0.1 M KCl solution.

Photocatalytic reaction
The photocatalytic activities of BiOI and Bi 4 O 5 I 2 samples were tested using a 300 W Xe lamp as the light source and by the degradation of MO, MB, RhB and noroxacin (NOR) under the visible-light irradiation. Optical lters were used to eliminate the UV light (l # 420 nm). The power density at the position of the reactor is about 7.4 mW cm À2 . In each of the experiment, 50 mg catalyst powder was dissolved in an aqueous solution of MO (100 mL, 10 mg L À1 ) in a beaker. Prior to irradiation, the suspensions were magnetically stirred in the dark for 30 min to achieve an adsorption-desorption equilibrium. Reduction in the concentrations of dyes was analyzed by the Cary 500 UV-vis spectrophotometer at the wavelength of maximal absorption (464 nm, 664 nm, and 554 nm). The samples of the reaction solution were taken out and then centrifuged and ltered at regular intervals. Finally, the ltrates obtained were analyzed.

Numerical calculation details and methods
The electronic structures and NLO properties of BiOI and Bi 4 O 5 I 2 were simulated by the CASTEP package. 41 The experimental crystal data were used in density functional theory (DFT) calculation via a plane wave pseudo-potential method. During the calculation, geometry optimization was performed using the BFGS minimization technique. The local density approximation (LDA) with the Ceperley-Alder-Perdew-Zunger functional and the ultraso pseudopotentials (USPs) were chosen as the exchange correlation functional and pseudopotential, respectively. 42 In the electronic structure and optical property calculations, the ultra-ne pseudopotentials were set up for BiOI and Bi 4 O 5 I 2 . Bi 5d 10 6s 2 6p 3 , I 5s 2 5p 5 , and O 2s 2 2p 4 were chosen as the valence electrons. The k-point separation of each material was set to 0.04Å À1 in the Brillouin zone.  (Fig. 1b). These ndings are consistent with the previously reported data. Therefore, it can be initially determined that BiOI and Bi 4 O 5 I 2 samples have been successfully synthesized.

SEM and EDS analyses
To identify the morphological information and atomic compositions of samples, BiOI and Bi 4 O 5 I 2 samples were investigated by SEM and EDS. The results are shown in Fig. 2. As clearly observed from Fig. 2a-d, the architecture and morphology of Bi 4 O 5 I 2 nanosheets are distinctly different from those of BiOI. The aked structure of the Bi 4 O 5 I 2 nanosheet has been successfully prepared. The EDS results are shown in Fig. S1a and b, † and the Bi : I ratios of Bi 4 O 5 I 2 and BiOI are determined to be 2.2 : 1 and 1.08 : 1, respectively. The EDS results further demonstrate the conclusion drawn based on the XRD results.

UV-vis diffuse reectance spectral analysis
As shown in Fig. 3a where a, h, hn, A, and E g are the absorption coefficient, Planck's constant, photon energy, a constant value, and bandgap energy, respectively. In addition, n is determined by the transition type of a semiconductor (n ¼ 1 for direct transition, whereas n ¼ 4 for indirect transition). According to the previously reported studies, 48 the n value of BiOI and Bi 4 O 5 I 2 is 4, which is consistent with the band structure results. According to the formula, the band gap energies for BiOI and Bi 4 O 5 I 2 samples are found to be 1.83 and 2.39 eV, respectively.

Photocatalytic degradation analysis
As shown in Fig. S2a †, an adsorption-desorption equilibrium has been achieved in dark for 30 min. To better understand the photocatalytic effect of the samples, MO solutions at different initial concentrations were used for comparison. It is observed from Fig. S2b † that lower concentrations of MO degrade more efficiently. As the initial concentration of MO increases, additional organic molecules are able to adsorb onto the photocatalysts surface, thereby reducing the photogeneration of the reactive oxygen species. In addition, it can be clearly seen that Bi 4 O 5 I 2 displays relatively better degradation efficiency than TiO 2 (P25) (Fig. S2a †). Herein, MO, RhB and MB dyes were selected as model pollutants to evaluate the photocatalytic activities of BOI and Bi 4 O 5 I 2 (Fig. S2c †). It is clearly seen that Bi 4 O 5 I 2 displays relatively better degradation efficiency than BiOI. Moreover, as can be seen, under the light irradiation for 100 min, about 29%,   This journal is © The Royal Society of Chemistry 2019 50%, and 90% of RhB, MO and MB were degraded over the BOI samples, respectively. However, under the light irradiation for 60 min, 94% of MO and MB were degraded over the Bi 4 O 5 I 2 samples. Note that under the light irradiation for 40 min, 99% of RhB was degraded over the Bi 4 O 5 I 2 samples. In Table ESI-1, † the results show that the photocatalytic activity of the Bi 4 O 5 I 2 photocatalyst is about 10 times that of BiOI for the degradation of MO. In addition, to minimize the effect of dye sensitization, degradation of colorless solution has been performed (Fig. S2d †). As can be seen, under light irradiation for 100 min, about 43% of NORs were degraded over the Bi 4 O 5 I 2 samples.

Free-radical trapping experiment analysis
Trapping experiments may conrm the main active species during the photodegradation process. 4,30 Ar, IPA, and KI purging were employed to quench the active species such as $O 2À , $OH and h + , respectively. At rst, trapping agents were dissolved in a Bi 4 O 5 I 2 solution before the reaction. The free-radical trapping experiment results are shown in Fig. S3a. † It can be seen that the photoinduced h + and $O 2À radicals are the primary reactive species. To evaluate the repeatability of the Bi 4 O 5 I 2 sample, recycling experiments for the photocatalytic degradation of MO over the Bi 4 O 5 I 2 sample were performed (Fig. S3b †). Note that even aer 4 cycles, the photocatalytic performance of Bi 4 O 5 I 2 was retained. These results imply the high stability of the Bi 4 O 5 I 2 sample, which is signicant for its practical application.

Band structure analysis
The electronic structures of BiOI and Bi 4 O 5 I 2 were calculated. As shown in Fig. 4a, the highest valence band (VB) is located between the R and Z points and the lowest conduction band (CB) is located between G and Z points for BiOI. As shown in Fig. 4b, the highest valence band (VB) is located between D and E points and the lowest conduction band (CB) is located at G point for Bi 4 O 5 I 2 . Therefore, the calculated indirect band gaps of BiOI and Bi 4 O 5 I 2 are about 1.65 and 2.34 eV, which are close to the experimental values of 1.83 and 2.39 eV, respectively.

Dipole moment analysis
To identify what promotes the effective separation of photogenerated electron-hole pairs of Bi 4 O 5 I 2 , the dipole moment calculation was carried out. 39 The calculations of the Bi-O and Bi-I bond valences and dipole moments of the BiO m I n (m ¼ 4, n ¼ 4;  Table ESI-2. † It proves that the direction of the dipole moment of Bi 4 O 5 I 2 powder is along [0 À1 0]. Due to the increased dipole moments, Bi 4 O 5 I 2 has better separation efficiency of electron-hole pairs. The polarization promotes the separation of photogenerated electron-hole pairs and mitigates the charge recombination, thus resulting in high photocatalytic activity. The calculated results are in accordance with those obtained via the photocatalytic degradation experiments.

PC and EIS analyses
The separation of photoinduced electron-hole pairs and the charge transfer processes in Bi 4 O 5 I 2 were investigated by transient photocurrent (TPC) response and electrochemical impedance spectra (EIS), respectively. Fig. 5a shows the instantaneous photocurrent response of BiOI and Bi 4 O 5 I 2 catalysts. As can be seen from Fig. 5a, due to the action of dipole moments, the photocurrent of Bi 4 O 5 I 2 is signicantly higher than that of BiOI, which promotes the effective separation of photogenerated carriers, thereby resulting in an increase in photocurrent. To better understand the PC measurement results, the EIS spectrum in the form of a Nyquist plot are shown in Fig. 5b. It reects the recombination rate of electrons and holes during the photoreaction. The smaller arc radius reects the lower resistance of the interface charge transfer. 49 It is clearly shown that Bi 4 O 5 I 2 has a smaller arc radius, lower chemical impedance and faster charge transport rate. This is consistent with the abovementioned characterization results.

PL analysis
Photoluminescence (PL) technique was employed herein to further conrm the transfer and recombination of photogenerated electrons and holes for BiOI and Bi 4 O 5 I 2 during the process of photocatalysis. Fig. S3c † shows that Bi 4 O 5 I 2 sample exhibits much lower intensity than BiOI sample, which suggests that Bi 4 O 5 I 2 sample possesses lower recombination rate of the photogenerated charge carriers. It is consistent with the results of PC and EIS analyses.

Mott-Schottky plot analysis
Mott-Schottky plots (vs. SCE) for Bi 4 O 5 I 2 are shown in Fig. S3d. † It is known that the conduction bands of the n-type semiconductors are 0-0.1 eV higher than that of the at potentials,  which could be estimated with Mott-Schottky plots. The at potential is calculated to be À0.75 V versus the saturated calomel electrode (SCE), which is equivalent to À0.51 V versus the normal hydrogen electrode (NHE). Therefore, the E CB of Bi 4 O 5 I 2 was estimated to be À0.61 eV vs. NHE. E VB vs. NHE can further be calculated from the equation E VB ¼ E g + E CB , which is estimated to be 1.78 eV vs. NHE. Fig. 6 shows a schematic diagram of the band structure and charge migration and separation caused by visible light irradiation. Due to the action of dipole moments, Bi 4 O 5 I 2 noncentrosymmetric polarized structure promotes the separation of photogenerated electron-hole pairs, which leads to efficient photocatalytic activity. On the other hand, based on the trapping and Mott-Schottky experiment, h + and $O 2À play the role of major active radicals during photodegradation. The possible reason is that a part of $O 2À is converted into $OH. In addition, E CB of Bi 4 O 5 I 2 is estimated to be À0.61 eV vs. NHE, which is lower than that of E 0 (O 2 /O 2 c À ¼ À0.046 eV vs. NHE). Therefore, the photogenerated electrons on the CB of Bi 4 O 5 I 2 could be trapped by molecular oxygen to generate $O 2À under visible light.

Mechanism analysis
However, its valence band is more negative than E 0 (OH À / $OH ¼ 2.38 eV vs. NHE). The result is corresponding to the trapping experiments. The mechanism of the active radical production is proposed as follows: In addition, we used the band-resolved method to further explain the contributions of orbitals to the SHG response. The virtualelectron (VE) and virtual-hole (VH) contributions to the total SHG coefficients were obtained. Based on the band-resolved results shown in Fig. 7, these VE and VH contributions are 61.6% and 38.4%, respectively. It indicates that the SHG effect may be due to the VE and VH processes.

Conclusions
In summary, exploration of the versatility of materials to increase the utilization of materials is a hot research topic nowadays. Herein, Bi 4 O 5 I 2 powders have been fabricated via a facile solvothermal method. Under the light irradiation for 60 min, 94% of MO and MB were degraded over Bi 4 O 5 I 2 samples. It is noteworthy that for the degradation of methyl orange, the photocatalytic activity of Bi 4 O 5 I 2 is about 10 times that of BiOI. Due to the action of dipole moments, charge separation of the photogenerated electron-hole pairs occurs and thus enhances the photocatalytic activity. In addition, increased dipole moment due to the stereochemical activity of lone pairs induces a high second harmonic generation response. Moreover, theoretical calculation and experiment results reveal that Bi 4 O 5 I 2 would have potential applications in photocatalytic and NLO elds.

Conflicts of interest
There are no conicts of interest to declare.