Study of Structural Properties of BiOI on ZnO Thin Films and ZnO Nanorods

We report the study of ZnO nanorods (ZnO-NR) decorated with BiOI nanoplatelets by the simple precipitation of BiOI in aqueous solution containing ZnO-NR powders. Likewise, as a reference for the decoration of ZnO-NR, BiOI thin films were prepared by the SILAR technique on RF magnetron-sputtered ZnO thin films.-e scanning electron microscopy analysis showed that BiOI grew on the ZnO-NR surfaces in nanoplatelet shape with an order of magnitude smaller in size, with respect to the platelets observed in BiOI thin films. Analysis by X-ray diffraction exhibited the hexagonal P63mc phase of ZnO and the tetragonal P4nmm crystal phase of BiOI. On the other hand, BiOI thin films are nonuniform and strongly adherent and, besides they exhibited the P4nmm tetragonal crystal structure of BiOI, they have highly preferential orientation along (001) direction (9.66°). Optical characterization of ZnO-NR/BiOI by absorbance showed a widened peak slightly shifted to infrared compared with the ZnO-NR absorbance peak, suggesting the increase of the absorption wavelength ranges from the ultraviolet to visible for ZnO-NR. Similarly, the results of the optical characterization by absorbance, for BiOI thin films, indicated the bandgap energy of the samples was between 1.78 eV and 1.88 eV, values previously reported for BiOI.


Introduction
e importance of ZnO material is well known due to its excellent physicochemical properties, which we can mention, and its thermal and mechanical stability that allows ZnO to crystallize in different morphologies, microstructured and nanostructured; its wide bandgap energy (3.37 eV), its high optical absorption (300 cm −1 ), its large exciton bond power of 60 meV, and its high speed to generate electron-hole pairs under optical excitation make the ZnO have excellent photocatalytic properties as an oxidant under UV irradiation, when it is synthesized in nanostructures [1][2][3].However, a limiting factor in its photocatalytic performance is the narrow range of energies of the electromagnetic spectrum required to activate the photocatalysis [4,5].In this way, in order to take advantage of a greater range of the solar spectrum, it is necessary to combine ZnO with other materials with photocatalytic properties that require a lower energy to work.An excellent candidate to extend the energy range of photocatalytic activation is BiOI, which has a narrow bandgap energy from 1.6 to 1.9 eV [6][7][8] and has a tetragonal crystal structure, belonging to the P 4 nmm group, which offers important advantages such as the formation of internal electric fields that separate the photogenerated electron-hole pairs and prevent their recombination.is field is formed by the way BiOI is structured, according to its Bravais cell exhibited in Figure 1, in which layers of (Bi 2 O 2 ) 2+ and iodide ion bilayers (I − ) are intercalated [9,10].
In this research, we report for the rst time a systematic study of the structural properties of BiOI synthesized entirely in aqueous medium at room temperature.Our study begins with BiOI thin lms grown by the SILAR method on RF magnetron-sputtered ZnO thin lms and continues with powder samples, obtained by the precipitation of BiOI in aqueous medium.Likewise, we report a relatively simple process to decorate ZnO nanorods by precipitin BiOI from a simple chemical reaction between two aqueous solutions, the rst one of Bi(NO 3 ) 2 and the second one a solution containing powders of ZnO nanorods dispersed in aqueous solution of KI.

Synthesis of BiOI.
In order to characterize the structural and optical properties of BiOI, we prepared a group of samples as thin lms by employing the SILAR technique [20].e BiOI thin lms were grown on ZnO, deposited by the sputtering technique on commercial glass substrates by 40, 90, and 120 immersion cycles.e ZnO thin lm was initially immersed in 20 mL of an aqueous solution of 0.2 M Bi(NO 3 ) 2 for 2 min; afterwards, the sample was removed, washed with deionized water, and dried with N 2 .en, the sample was immersed in 20 mL of an aqueous solution of 0.2 M KI, also for 2 min.Immediately after, the sample was removed, washed, and dried with deionized water and N 2 , respectively, in order to restart a new cycle of immersion.
A second group of samples was prepared as powders; these samples were synthesized by mixing, under strong magnetic agitation, 0.2 M Bi(NO 3 ) 2 (2 mL) and a solution of 0.2 M KI (2 mL) dissolved in 0 mL, 40 mL, 80 mL, and 160 mL (powder samples 1, 2, 3, and 4, respectively) of deionized water.e precipitated solid was separated from the solution and dried in an oven at 100 °C.

Synthesis of ZnO Nanorods.
ZnO-NR powders were synthesized by the hydrothermal method based on an equimolar reaction of 0.01 M Zn(NO 3 ) 3 •6H 2 O and hexamethylenetetramine in aqueous medium, at 90 °C for 4 h.e solid phase was separated by centrifugation and dried in an oven (80 °C).

Decoration of ZnO-NR.
Decorated ZnO-NR were prepared by the precipitation of BiOI on ZnO-NR powders at room temperature.A sample of 29 mg ZnO-NR was initially dispersed in 2 mL of an aqueous solution of 0.2 M KI under ultrasonic agitation.Deionized water (38 mL) was added, and immediately after, 2 mL of an aqueous solution of 0.02 M Bi(NO 3 ) 2 was supplied.
e mixture turned to a yellow turbid and was maintained at room temperature and under magnetic agitation for 5 min.
e solid phase was separated and dried in an oven (100 °C).

Characterization. Morphology of BiOI thin lms and
ZnO-NR/BiOI nanostructures were analyzed by scanning electron microscopy (SEM) employing a Jeol JSM 5200 microscope.e structural analysis was realized by X-ray di raction by using a PANalytical X'Pert Powder system at 45 kV and 40 mA, and the optical characterization was realized in an Evolution 600 UV-visible ermo Fisher Scienti c spectrometer.

BiOI Samples.
Based on SEM analysis, BiOI thin lms exhibit a nonuniform surface predominantly formed by microplatelets grown in perpendicular direction with respect to the substrate surface as we can see in Figures 2(a e di erent crystalline shapes observed in the micrographs of Figures 2(a) and 2(b) suggest di erent growth stages of a BiOI thin lm on a ZnO deposition.According to this observation, in its rst stage, BiOI precipitates in hemispherical grains formed by platelets as exhibited in the inset of Figure 2(a) with a mean size of 3 μm.A second stage can be explained on the main micrograph of Figure 2(a), where we can observe areas with a greater concentration of these hemispherical grains, which continue to grow overlapping each other until the surface is built, as shown in e thin films showed a dependence with the grown temperature both in size and shape.According to other authors, in all depositions realized at different temperatures of substrates, growth of nanoplatelets was observed vertically.Although the highly oriented growth is not verified by X-ray diffraction analysis, TEM micrography exhibits the nanoplatelets oriented in normal direction to the substrate [17].A similar surface was obtained by Bhachu et al. [23].In the present work, we have obtained a BiOI thin film formed by highly oriented microplatelets by employing the SILAR technique at room temperature through the reaction of Bi(NO 3 ) 2 and KI on the substrate surface.

Decoration of ZnO-NR.
e result of decorating ZnO-NR with BiOI nanoplates is shown in Figures 3(a) and 3(b).According to these micrographs, we could assume that ZnO-NR surfaces are serving as seeds for the growth of BiOI nanoplatelets.We can verify that practically all nanorods prepared by the hydrothermal method exhibit a high concentration of BiOI nanoplatelets on their surfaces, and the nanoplatelets, such as in BiOI thin films, are oriented in perpendicular direction with respect to nanorod surfaces.
In the inset of Figure 3(b), we calculated the extension of these nanostructures and found that these were reduced by an order of magnitude with respect to growth of BiOI plates on ZnO thin films since the surface in which actually BiOI grows is micrometric.us, we have obtained micrometric ZnO nanorods decorated by BiOI nanoplates vertically oriented with respect to ZnO nanorod surface.
Kuang et al. [19] reported the growth of ZnO-NR on fluorine-doped tin oxide which was decorated by BiOI nanoplatelets through a solvothermal method at 126 °C, for 6 h.Other reports have published the preparation of ZnO/BiOI nanocomposites by one-pot synthesis of ZnO and BiOI precursors heated in an autoclave, also around 125 °C [24] or by microwave irradiation [25]; however, reports about nanocomposites formed by ZnO nanorods decorated with BiOI micro/nanoplatelets, or with a shell of BiOI, are rare.Hence, we are reporting a simple process of preparation at room temperature of decorated ZnO nanorods by BiOI nanoplatelets through the simple dispersion of ZnO-NR in aqueous solution of 0.2 M KI and adding an aqueous solution of 0.02 M Bi(NO 3 ) 2 .

X-ray Diffraction.
e X-ray diffraction patterns exhibited in Figure 4 correspond to BiOI samples grown on ZnO thin films by 40, 90, and 120 SILAR immersion cycles (labeled as 40, 90, and 120 cycles).One of these samples was annealed at 100 °C, as is indicated in Figure 5.In addition, we also have the X-ray diffraction patterns for powder samples synthesized according to the method described in Section 2.2.All diffraction patterns were obtained for 2Θ values from 5 to 60 °.In the diffractogram corresponding to 40 cycles sample, it is possible to observe a highly intense peak around 9.66 °and a series of lower intensity peaks in 19.39, 29.74, 39.49, 45.78, 51.63, and 55.42 °.All peaks correspond to reflections of the crystalline planes (001), (002), (004), ( 200), (211), and (105)/(212) of the tetragonal phase (group P 4 nmm) of BiOI [26].e same crystalline phase was also observed for all samples, both thin film and powder.e intensity of the observed peaks is reduced for the 90 and 120 cycle samples, possibly due to the dissolution of thin film surface with the successive SILAR cycles, which causes the reduction of their thickness.e thermal treatment of the sample prepared by 40 SILAR cycles has reduced the intensity of the (001) peak and has been increasing slightly the intensity of the remaining peaks.According to the texture coefficient, f(hkl) [27], where I 0 (hkl) and I(hkl) are the standard X-ray intensity of BiOI and the measured X-ray intensity corresponding to the hkl crystalline plane; the f(001) factor is 2.47 for the sample of 40 SILAR cycles and f(hkl) < 1, for the other crystalline planes.e f(001) factor is reduced to 0.95 with annealing; however, the preferential orientation of thin lms crystal structure is highly directed along the plane (001).However, for all powder samples, the preferential orientation like that exhibited by thin lm samples was not observed.On the other hand, the (001) re ection, exhibited by all samples, rarely has been reported by other authors, and the high intensity showed in thin lm samples has not been reported [21,28].Hence, the highly oriented BiOI microplatelets observed in the micrographs of Figure 2 is a consequence of the highly oriented crystal structure along (001) direction.erefore, BiOI grows preferably perpendicular to the direction of the ZnO thin lm surface, along (001) direction.e X-ray di raction pattern exhibited in Figure 5 corresponds to a powder sample of decorated ZnO-NR.We can easily distinguish two crystalline phases through two-line shapes, the rst one, formed by broadened diffraction peaks of minor intensity, and the second one, formed by narrowed di raction peaks of high intensity.e di raction peaks of minor intensity correspond to BiOI   shows the size measurements of some nanoplatelets. 4 Advances in Materials Science and Engineering nanoplatelets grown on ZnO-NR surface.Unlike the diffraction pattern of BiOI thin lm showed in Figure 4, the di raction peaks corresponding to BiOI nanoplatelets grown on the ZnO-NR surface are widened and shifted slightly to minor 2Θ values, possibly due to tensions in their crystal structure and the mismatch lattice parameters between the BiOI and ZnO-NR crystalline structures.Likewise, there is no preferential growth along (001) direction.
In order to compare the in uence of the sample preparation method on the structural properties of BiOI, X-ray di raction patterns were analyzed to obtain interplanar spacing, crystallite size, and lattice parameters.e interplanar spacing of tetragonal lattice was obtained by using the formula [29]: where h, k, and l, are the Miller indexes and a and c are the lattice constants of the tetragonal lattice.e crystallite size was measured by the Debye-Scherrer formula [29]: where K is the shape factor with value 0.9, λ is the wavelength of X-ray used (1.5418Å), β is the full width at the maximum (FWHM) of the di raction major line, and Θ is the Bragg angle.All values for lattice parameters, interplanar spacing, and crystallite size are listed in Table 1.In general, we can observe that BiOI has a crystal structure considerably stable, but interplanar spacing and the lattice parameters experienced only slight changes.ese variations are observed mainly as a compression of the lattice cell through the axis a and b, with a slight elongation of the axis c.Important changes were not observed with other samples, including the ZnO-NR/BiOI, which means that BiOI is a compound relatively simple to obtain with a crystal structure highly stable.is characteristic allows its application as a photocatalyst both in aqueous and gas media, maintaining the same properties such as BiOI thin lm, powder, and decorative on ZnO nanorod surface without employing  Advances in Materials Science and Engineering ethylene glycol or another organic compound.Furthermore, the crystallite size increased when BiOI is grown homogeneously, as powder, reaching its maximum value for the growth of BiOI on ZnO-NR surface.

Optical Properties.
e optical absorption spectra for some representative BiOI thin lms are shown in Figure 6.
e samples were prepared by SILAR technique with 20, 30, and 120 immersion cycles on ZnO thin lms.e bandgap of samples was calculated by the extrapolation of the straight line portion of the curves to zero absorbance value [30].
e bandgap value practically remained unchanged, within the range from 1.78 eV to 1.88 eV, which is close to values reported in other works [31]. is means that the wavelength interval of the optical absorption of ZnO-NR can be increased from the ultraviolet to visible band.
Figure 7 shows the absorbance spectrum of decorated and nondecorated ZnO-NR.We can observe the e ect of BiOI growth on ZnO nanorods through its optical absorption since the main peak of the spectrum of ZnO is broadened and lightly shifted towards the infrared range of wavelength. is means that ZnO nanorods extended their spectrum of optical absorption from the ultraviolet to the visible.

Conclusions
We have reported the growth of BiOI nanostructures on ZnO nanorod surface through a simple precipitation of BiOI in aqueous solution containing ZnO nanorod powder at room temperature.e morphology analysis by scanning electron microscopy showed that nanorod surfaces worked as seed coatings for the growth of BiOI structures in nanoplatelet shape.Analysis by X-ray di raction con rmed the observation by electron microscopy; the di raction pattern of ZnO nanorods/BiOI powders exhibited the crystalline phases P6 3 mc (hexagonal) and P 4 nmm (tetragonal) of ZnO and BiOI, respectively.Absorbance spectroscopy showed that ZnO nanorods extended their spectrum of optical absorption from the ultraviolet to the visible.BiOI thin lms were nonuniform and formed mostly by microplatelets highly oriented in perpendicular direction with respect to the surface of ZnO lm where they were grown.e preferential orientation of these structures was veri ed by X-ray di raction technique, whose analysis indicated that BiOI lms grew highly oriented in z direction, speci cally along the (001) direction, and with a stable crystalline structure.

Figure 1 :
Figure 1: Schematic representation of the Bravais cell for BiOI.

Figure 2 :
Figure 2: SEM micrographs corresponding to a BiOI thin film grown on a ZnO thin film.It is possible to observe different stages of growth: (a) the growth of rounded grains and their overlapping and (b) microplatelets grown in perpendicular direction to the substrate surface.

Figure 4 :
Figure 4: X-ray di raction patterns corresponding to BiOI samples grown on ZnO thin lms and BiOI powder samples.

Figure 3 :
Figure 3: SEM micrograph obtained from di erent positions of a powder sample of decorated ZnO nanorods.e inset in Figure 2(b)shows the size measurements of some nanoplatelets.

Table 1 :
Lattice parameters, interplanar spacing, and crystallite size measured for all samples.