Enhancement of Optical Activity and Properties of Barium Titanium Oxides to Be Active in Sunlight Through Using Hollandite Phase Instead of Perovskite Phase

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Article Enhancement of Optical Activity and Properties of Barium Titanium Oxides to Be Active in Sunlight through Using Hollandite Phase Instead of Perovskite Phase

Adil Alshoaibi 1,* , Osama Saber 1,2 and Faheem Ahmed 1

1 Department of Physics, College of Science, King Faisal University, P.O. Box 400, Al-Ahsa 31982, Saudi Arabia; [email protected] (O.S.); [email protected] (F.A.) 2 Egyptian Petroleum Research Institute, Nasr City, P.O. Box 11727, Cairo 11765, Egypt * Correspondence: [email protected]; Tel./Fax: +966-013589-9417

Abstract: The present study aims to enhance the optical properties of barium titanate through narrowing its band gap energy to be effective for photocatalytic reactions in sunlight and be useful for solar cells. This target was achieved through growth of the hollandite phase instead of the perovskite phase inside the barium titanate crystals. By using solvent thermal reactions and thermal treatment at different temperatures (250 ◦C, 600 ◦C, and 900 ◦C), the hollandite phase of barium titanate was successfully obtained and conﬁrmed through X-ray diffraction (XRD), Raman spectra and scanning electron microscopy techniques. XRD patterns showed a clear hollandite phase of barium titanium oxides for the sample calcined at 900 ◦C (BT1-900); however, the samples at 600 ◦C showed the presence of mixed phases. The mean crystallite size of the BT1-900 sample was found to be 38 nm. Morphological images revealed that the hollandite phase of barium titanate consisted of a mixed Citation: Alshoaibi, A.; Saber, O.; morphology of spheres and sheet-like features. The optical properties of barium titanate showed Ahmed, F. Enhancement of Optical that its absorption edge shifted to the visible region and indicated band gap energy tuning ranging Activity and Properties of Barium from 1.75 eV to 2.3 eV. Photocatalytic studies showed the complete and fast decolorization and Titanium Oxides to Be Active in mineralization of green pollutants (naphthol green B; NGB) in the prepared barium titanate with Sunlight through Using Hollandite hollandite phase after illumination in sunlight for ten minutes. Finally, it can be concluded that the Phase Instead of Perovskite Phase. Crystals 2021, 11, 550. https:// low band gap energy of barium titanate having the hollandite phase introduces beneﬁcial structures doi.org/10.3390/cryst11050550 for optical applications in sunlight.

Academic Editors: Alexander Keywords: low band gap energy; hollandite structure; barium titanate; optical properties; photocat- S. Novikov and Assem Barakat alytic degradation

Received: 2 April 2021 Accepted: 8 May 2021 Published: 15 May 2021 1. Introduction In order to remove pollutants at low concentrations, as well as for the removal of Publisher’s Note: MDPI stays neutral harmful chemicals, photocatalysis has attracted great attention as an effective method [1,2]. with regard to jurisdictional claims in Titanium dioxide (TiO2) has attracted great attention as a photocatalyst [1–8] because of its published maps and institutional afﬁl- strong oxidative power activated under the illumination of ultraviolet (UV) [4,5] or visible iations. light [6–8]. However, to deal with a variety of harmful chemicals under different conditions, a diverse range of photocatalysts is needed, which could resolve the problems. Among the oxides, perovskite-type materials possess photostability and outstanding photocatalytic activity. This outstanding property results from the unique structures with larger lattice Copyright: © 2021 by the authors. distortion and defects by trapping holes, thus preventing the formation of electron-hole Licensee MDPI, Basel, Switzerland. pairs [9–11]. Additionally, stimulation of the adsorption of oxygen on the surface of cations, This article is an open access article due to the vacancy of metal in the perovskite-type structure, resulted in enhancing the distributed under the terms and photocatalytic reaction [11]. conditions of the Creative Commons In particular, BaTiO is a typical perovskite photocatalyst with unique physical and Attribution (CC BY) license (https:// 3 chemical properties, which could be governed by its surface morphology and particle size; creativecommons.org/licenses/by/ 4.0/). hence, nanostructured materials with high purity are greatly needed [12]. By changing

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the chemical composition or performing thermal treatment of these ceramics, the physical properties of these ceramics change drastically, and they transform to other forms such as barium hollandites or titanate hollandites. Hollandites can be deﬁned by using unit-cell formula AxB8O16, where A denotes ions in the tunnel cavities of the structure (e.g., Ba, Cs), while B represents smaller cations including Al3+, Ga3+, Cr3+ and Ti4+, which reside in octahedral sites. Tetragonal (space group I4/m) or monoclinic (C2/m) structures could be assigned to the hollandites [13,14]. 4+ More specifically,titanate hollandites, which have the compositions Ax(Ti , B)8O16 (where A = Ba2+, Cs+, Sr2+, Rb+, etc.; B = Al3+, Fe3+, Mg2+, Ti3+, Ga3+, Cr3+, Sc3+, etc.) [15–21], 4+ 3+ are structural analogs of the prototype hollandite, Ba(Mn , Mn )8O16 [22]. In the structure, [TiO6] and [BO6] octahedral are linked via edge- and corner-sharing to form a three- dimensional framework, in which Ti4+ and B3+ are disordered over one or two crystal- lographically distinct sites. A+ or A2+ cations occupy the box-shaped cavity sites, each coordinated by eight oxygen atoms, within tunnels parallel to the c-axis. The adoption of either a tetragonal (space group I4/m) or monoclinic (space group I2/m) symmetry usually depends on the ratio of mean radius of cations in the A and Ti/B sites [15,21]. The catalytic properties of thermally activated hollandite compounds have been investigated and reported previously [23–25]. In addition, the photocatalytic properties on the hollandites by selective decomposition of nitric oxide in the gas phase and nitrate ions in water have also been examined [26–29]. These observations suggested that hollandites might prove to be promising photocatalysts. In this study, the enhancement in the optical properties of barium titanium oxides with hollandite phase, in order to make them active in visible light for the utilization of the hollandite-type compound as an efﬁcient photocatalyst, is reported.

2. Experimental Details Barium carbonate (99.9%), titanium(IV) isopropoxide (97%) and zinc acetate dehy- drate (98%) were acquired from Sigma Aldrich. Barium carbonate (7 g) was reacted with 10 mL HCL (37%) and 20 mL water. Titanium isopropoxide (10.3 g) was reacted with 180 mL ethanol. The two solutions were mixed together inside an autoclave. An appropri- ate amount of water was used for forming a milky solution. An excess amount of methanol (350 mL) was added to the milky solution. A closed and pressurized vessel (autoclave) was used for keeping the mixture at a temperature = 250 ◦C and under high pressure (10 bar) for 2 h. After 2 h, the system was shut down and the pressure was released with an inert gas (Argon). The product was called BT1. By adding zinc acetate (0.08 g) as a dopant for the above solution, another nanocomposite was prepared by the same technique. This product was coded as BT2. These two different crystalline nanostructures of barium titanium oxides, BT1 and BT2, were prepared by using a high pressure of alcohol. By thermal treatment of these two samples at 600 ◦C and 900 ◦C, four types of crystalline nanostructures were produced and coded as BT1-600, BT1-900, BT2-600 and BT2-900, respectively. Commercial nanoparticles of barium titanium oxide were used for the comparison. To characterize the prepared samples, the powder X-ray diffraction technique was used for detecting the crystalline structure and the different phases of barium titanate by Bruker-AXS, Karlsruhe, Germany with Cu-K radiation (λ = 0.154 nm). The morphology and size of the synthesized product were measured by ﬁeld emission scanning electron microscopy (FESEM) using a JEOL electron microscope (JSM-7500, Akishima, Tokyo, Japan). HORIBA Jobin Yvon Lab RAM HR was used for determining the Raman spectrum of the prepared samples. The excitation for the prepared samples depended on the source of a 633 nm laser. For measuring the optical properties of the prepared materials, a UV/VIS/NIR Shimadzu 3600 spectrophotometer with an attached integrating sphere (ISR-603) (Shimadzu, Columbia, MD, USA) was used to determine the diffuse reﬂectance of the solid materials. Photocatalytic degradation reactions of naphthol green B (NGB) in sunlight were used to determine the photoactivity of the prepared materials. In the current study, the prepared Crystals 2021, 11, x FOR PEER REVIEW 3 of 14

Photocatalytic degradation reactions of naphthol green B (NGB) in sunlight were used to determine the photoactivity of the prepared materials. In the current study, the prepared aqueous solution of NGB (4 × 10−4 M) was mixed with 0.1 g of the prepared Crystals 2021, 11, 550 material and illuminated in sunlight through a 10 cm2 radiation area. Depending3 of 14 on the intensity of the measured spectrum of the dye, the concentration of the dye was determined according to the law of Beer-Lambert. The absorbance of the samples was meas- × −4 uredaqueous after solution ten minutes of NGB (4 of 10sunlM)ight was irradiation mixed with using 0.1 g of the the preparedUV–Vis material spectrophotometer. and By illuminated in sunlight through a 10 cm2 radiation area. Depending on the intensity of the followingmeasured spectrumthe integrated of the dye, area the of concentration the characteristic of the dye peak was determinedof NGB at according714 nm, tothe decomposi- tionthe lawof the of Beer-Lambert. green dyes Thewas absorbance calculated. of theThe samples photocatalytic was measured experiments after ten minutes of NGB were car- riedof sunlight out during irradiation the summer using the UV–Visseason spectrophotometer. (July) in Saudi Arabia By following under the irradiation integrated by sunlight inarea the of period the characteristic of 9:30 A.M. peak ofand NGB 10:30 at 714 A.M. nm, the decomposition of the green dyes was calculated. The photocatalytic experiments of NGB were carried out during the summer season (July) in Saudi Arabia under irradiation by sunlight in the period of 9:30 A.M. and 3.10:30 Results A.M. and Discussion X-ray diffraction (XRD) was used to characterize the crystalline structures and chem- 3. Results and Discussion ical formulae of the prepared barium titanium oxides. The XRD patterns of BT1 are shown X-ray diffraction (XRD) was used to characterize the crystalline structures and chemi- incal Figure formulae 1. ofIt shows the prepared clear barium peaks titanium for BT1-900 oxides. at The 2θ XRD = 17.65, patterns 25.14, of BT1 27.94, are shown 31.58, 36.36, 39.90, 40.52,in Figure 48.20,1. It 54.66, shows clear58.21 peaks and for67.135°, BT1-900 which at 2 θ =are 17.65, in accordance 25.14, 27.94, 31.58, with 36.36, the hollandite 39.90, phase of ◦ barium40.52, 48.20, titanium 54.66, 58.21oxides and with 67.135 chemical, which are formulae in accordance Ba1.12 with(Ti8 theO16 hollandite), as shown phase in of JCPDS file No. 77-0883.barium titanium Figure oxides1 shows with that chemical the peaks formulae of BaBT1-9001.12(Ti8O 16are), asindexed shown in to JCPDS the diffraction ﬁle No. planes of 77-0883. Figure1 shows that the peaks of BT1-900 are indexed to the diffraction planes of hollandite phase [200], [220], [130], [101], [121], [240], [301], [411], [600], [251] and [541]. hollandite phase [200], [220], [130], [101], [121], [240], [301], [411], [600], [251] and [541].

Figure 1. XRD patterns of BT1 at different temperatures. Figure 1. XRD patterns of BT1 at different temperatures.

The basic formula of hollandite is AB8O16, with the A-site often occupied by larger mono-and/orThe basic divalent formula cations of andhollandite the B-site is by AB smaller8O16, tri-and/or with the tetravalent A-site often cations. occupied The by larger mono-and/orformation of the divalent hollandite cations phase resulted and the from B-site a framework by smaller composed tri-and/or of BO tetravalent6 octahedra cations. The with tunnels along the C-axis in which the larger A-cations are located. Due to the barium formation of the hollandite phase resulted from a framework composed of BO6 octahedra deﬁciency in the stoichiometry, barium titanium hollandite is formed. The symmetry of with tunnels along the C-axis in which the larger A-cations are located. Due to the barium the crystal structure of Ba1.12(Ti8O16) can be tetragonal. Therefore, the crystal structure deficiencyconsists of octahedrain the stoichiometry, of TiO6, and partially barium occupied titanium Ba-cations hollandite on the tunnel is formed. sites. Barium The symmetry of thehollandites crystal maystructure exhibit of long-range Ba1.12(Ti ordering8O16) can of be barium tetragonal. cations and Therefore, vacancies inthe the crystal tunnels structure con- and have the possibility to be commensurate [30]. sists of octahedra of TiO6, and partially occupied Ba-cations on the tunnel sites. Barium In addition, the diffraction peaks with remarkable broadening revealed the crys- hollanditestallinity in the may nanoscale exhibit regime long-ran of thege sample ordering of BT1-900 of barium particles. cati Therefore,ons and vacancies the crys- in the tun- nelstalline and size have was calculatedthe possibility using the to Debye–Schererbe commensurate formula: [30]. In addition, the diffraction peaks with remarkable broadening revealed the crystallinity in the nanoscale regime of the sample of BT1-900 particles. Therefore, the crystalline size was calculated using the Debye–Scherer formula:

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D = kλ/βcosθ where k is constant (shape factor around 0.9), λ corresponds to the wavelength of the X-ray used (1.5418 × 10−10 m), β is the FWHM of the diffraction line, and θ is the angle of the diffraction. The mean crystallite size of the BT1-900 particles measured from XRD peaks with widths of [220], [130] and [411] was calculated to be 38 nm. However, the samples BT1 and BT1-600 showed mixed phases. The XRD patterns of the sample BT1 show the peak at 2θ = 15.98◦, indicating the reflection of plane [111] of barium hydroxide, as shown in JCPDS file No. 26-0155. In addition, it shows a clear peak at 2θ = 38.13◦, indicating the presence of the rutile phase of titanium oxide, in accordance with JCPDS file No. 75-1753. The other peaks are weak and due to barium titanium oxides. The sample BT1-600 showed the disappearance of the peak of barium hydroxide and appearance of a clear peak at 2θ = 25.14◦, indicating the presence of anatase phase of titanium oxide, in accordance with JCPDS file No. 84-1286. The other peaks are due to the barium titanium oxide. According to these XRD data, the thermal treatment at 900 ◦C is considered necessary to obtain the pure phase of barium titanium oxides Ba1.12(Ti8O16). In the case of BT2, the XRD patterns showed that BT2-900 consists of two phases of barium titanium oxides: perovskite phase BaTiO3 and hollandite Ba1.12(Ti8O16). Figure2 Crystals 2021, 11, x FOR PEER REVIEWshows two series of peaks for BT2-900. The first series revealed the peaks at 2θ = 17.86, 4 of 14

20.14, 23.99, 26.38, 28.57, 35.33, 40.0, 49.05, 54.25 and 59.24. These peaks were well matched with the standard diffraction planes of Ba1.12(Ti8O16) phase as shown in JCPDS file No. 77-0883. The second series showed peaks at 2θ = 22.33, 31.58, 38.86, 45.1, 56.22 and 66.1. These peaks were in accordanceD = k withλ/βcos theθ standard entire diffraction planes of the BaTiO3 phase, as shown in JCPDS file No. 74-1964. By analyzing the main peak of both phases, wherethe mean k is crystallite constant sizes (shape of both factor BaTiO around3 and Ba 1.120.9),(Ti 8λO 16corresponds) were estimated to the by thewavelength Debye– of the X- rayScherer used equation. (1.5418 The× 10 particle−10 m), sizesβ is the of BaTiO FWHM3 and of Ba the1.12 diffraction(Ti8O16) were line, 44 nm and and θ 47is nm,the angle of the diffraction.respectively. The The mean samples crystallite BT2 and BT2-600,size of the which BT1-900 were thermally particles treated measured at a lowerfrom XRD peaks temperature, showed only one phase of barium titanium oxide, BaTiO3, mixed with the withcrystalline widths phase of [220], of barium [130] chloride, and [411] as shown was incalculated JCPDS file to No. be 25-1135 38 nm. and in Figure2.

Figure 2. XRD patterns of BT2 at different temperatures. Figure 2. XRD patterns of BT2 at different temperatures.

However, the samples BT1 and BT1-600 showed mixed phases. The XRD patterns of the sample BT1 show the peak at 2θ = 15.98°, indicating the reflection of plane [111] of barium hydroxide, as shown in JCPDS file No. 26-0155. In addition, it shows a clear peak at 2θ = 38.13°, indicating the presence of the rutile phase of titanium oxide, in accordance with JCPDS file No. 75-1753. The other peaks are weak and due to barium titanium oxides. The sample BT1-600 showed the disappearance of the peak of barium hydroxide and appearance of a clear peak at 2θ = 25.14°, indicating the presence of anatase phase of titanium oxide, in accordance with JCPDS file No. 84-1286. The other peaks are due to the barium titanium oxide. According to these XRD data, the thermal treatment at 900 °C is considered necessary to obtain the pure phase of barium titanium oxides Ba1.12(Ti8O16). In the case of BT2, the XRD patterns showed that BT2-900 consists of two phases of barium titanium oxides: perovskite phase BaTiO3 and hollandite Ba1.12(Ti8O16). Figure 2 shows two series of peaks for BT2-900. The first series revealed the peaks at 2θ = 17.86, 20.14, 23.99, 26.38, 28.57, 35.33, 40.0, 49.05, 54.25 and 59.24. These peaks were well matched with the standard diffraction planes of Ba1.12(Ti8O16) phase as shown in JCPDS file No. 77- 0883. The second series showed peaks at 2θ = 22.33, 31.58, 38.86, 45.1, 56.22 and 66.1. These peaks were in accordance with the standard entire diffraction planes of the BaTiO3 phase, as shown in JCPDS file No. 74-1964. By analyzing the main peak of both phases, the mean crystallite sizes of both BaTiO3 and Ba1.12(Ti8O16) were estimated by the Debye–Scherer equation. The particle sizes of BaTiO3 and Ba1.12(Ti8O16) were 44 nm and 47 nm, respectively. The samples BT2 and BT2-600, which were thermally treated at a lower temperature, showed only one phase of barium titanium oxide, BaTiO3, mixed with the crystalline phase of barium chloride, as shown in JCPDS file No. 25-1135 and in Figure 2. Although, zinc acetate has been used as a dopant in the BT2 samples, there are no peaks observed in the X-ray pattern, which indicates that zinc is homogenously dispersed inside the structure. In addition, it has a positive role in growing the perovskite phase, in

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accordance with the resultsAlthough, of Gao zinc et acetate al., which has been indicated used as a the dopant role in of the zinc BT2 dopants samples, there for the are no peaks observed in the X-ray pattern, which indicates that zinc is homogenously dispersed crystallization of BaTiOinside3 perovskite the structure. [31]. In addition, it has a positive role in growing the perovskite phase, in The microstructuralaccordance studies with of the the results hollandite of Gao et titanate al., which were indicated performed the role of by zinc FESEM. dopants The for the morphological featurescrystallization of the BT1 of BaTiO sample3 perovskite are depicted [31]. in Figure 3. In Figure 3a, micro- graphs of the as-preparedThe sample microstructural reveal studies a dense of the microstructure, hollandite titanate werecomposed performed of by spherical FESEM. The morphological features of the BT1 sample are depicted in Figure3. In Figure3a, micro- particles of micron size.graphs With of the an as-prepared increase samplein temperature reveal a dense to 600 microstructure, °C, Figure composed 3b depicts of spherical that the well-defined sphereparticles shape, of micron with size.a smooth With an surface increase and in temperature clear grain to boundary, 600 ◦C, Figure was3b depicts observed. These clear thatspheres the well-deﬁned showed spherethat a shape, good with crystalline a smooth surfacephase and was clear observed grain boundary, at this was observed. These clear spheres showed that a good crystalline phase was observed at this temperature. With atemperature. further increase With a further in temperature increase in temperature to 900 °C, to a 900 mixed◦C, a mixedmorphology morphology of of spheres and sheet-likespheres features and sheet-like was observed features was(Figure observed 3c). (Figure 3c).

Figure 3. 3.FESEM FESEM images images of BT1 samples:of BT1 (samples:a) as-prepared, (a) (as-prepared,b) calcined at 600 (◦bC,) (calcinedc) calcined at 900600◦C. °C, Scale (c) bar calcined in (a–c) is at 2 µ 900m. °C. Scale bar in (a), (b) and (c) is 2 µm. Figure4 depicts the morphology of the BT2 samples calcined at different temperatures. It is clear from Figure4a that as-prepared samples produced a dense, spherical particle-like morphology similar to that of the BT1 samples with smaller-sized particles. With the increase in temperature, BT2 samples at 600 ◦C (Figure4b) showed improved crystalline features of spherical particles assembled in a sphere and distributed all over the surface with high density. BT2 samples calcined at 900 ◦C, as shown in Figure4c, consisted of a very clear sphere with excellent crystallinity having small spheres attached to their surfaces, which makes them unique. The morphological studies revealed that by increasing the temperature, the morphology of hollandite titanate was changed, with improved crystal quality.

Figure 4. FESEM images of BT2 samples: (a) as-prepared, (b) calcined at 600 °C, (c) calcined at 900 °C. Scale bar in (a), (b) and (c) is 2 µm.

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accordance with the results of Gao et al., which indicated the role of zinc dopants for the crystallization of BaTiO3 perovskite [31]. The microstructural studies of the hollandite titanate were performed by FESEM. The morphological features of the BT1 sample are depicted in Figure 3. In Figure 3a, micro- graphs of the as-prepared sample reveal a dense microstructure, composed of spherical particles of micron size. With an increase in temperature to 600 °C, Figure 3b depicts that the well-defined sphere shape, with a smooth surface and clear grain boundary, was observed. These clear spheres showed that a good crystalline phase was observed at this temperature. With a further increase in temperature to 900 °C, a mixed morphology of spheres and sheet-like features was observed (Figure 3c).

Crystals 2021Figure, 11, 550 3. FESEM images of BT1 samples: (a) as-prepared, (b) calcined at 600 °C, (c) calcined at 9006 of 14 °C. Scale bar in (a), (b) and (c) is 2 µm.

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Figure 4 depicts the morphology of the BT2 samples calcined at different temperatures. It is clear from Figure 4a that as-prepared samples produced a dense, spherical particle-like morphology similar to that of the BT1 samples with smaller-sized particles. With the increase in temperature, BT2 samples at 600 °C (Figure 4b) showed improved crystalline features of spherical particles assembled in a sphere and distributed all over the surface with high density. BT2 samples calcined at 900 °C, as shown in Figure 4c, consisted of a very clear sphere with excellent crystallinity having small spheres attached to their surfaces, which makes them unique. The morphological studies revealed that by increas- FigureFigure 4. FESEM 4. FESEM images ofimages BT2ingsamples: the of temperature, BT2 (a) as-prepared,samples: the ( (morphologyab)) as-prepared, calcined at 600of hollan◦ C,(b ()c )calcined calcineddite titanate at at 900 600 ◦wasC. °C, Scale changed, ( barc) calcined in ( awith–c) is improved 2atµ m.900 °C. Scale bar in (a), crystal(b) and quality. (c) is 2 µm. Figure 55 showsshows thethe RamanRaman spectraspectra ofof BT1BT1 samplessamples calcined at different temperatures, while Figure 66 shows shows RamanRaman spectraspectra ofof BT2BT2 samples.samples. ItIt couldcould bebe clearlyclearly seenseen fromfrom thethe Raman spectra of BT1 and BT2 samples that Raman active vibration modes were observed −1 in the frequencyfrequency rangerange 100–800 100–800 cm cm−1,, whichwhich areare in in close close agreement agreement with with the the compounds compounds of ofthe the hollandite hollandite type type [32 [32].].

Figure 5. Raman spectra of BT1 sample calcined at different temperatures. Figure 5. Raman spectra of BT1 sample calcined at different temperatures.

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Figure 6. Raman spectra of BT2 sample calcined at different temperatures. Figure 6. Raman spectra of BT2 sample calcined at different temperatures. For as-prepared BT1 samples, as shown in Figure5, there are Raman peaks that appear at 152.6,For 432.7,as-prepared 511.3, 656.8 BT1 and samples, 711.1 cm as− 1shown. The Raman in Figure spectrum 5, there of BT1 are samples Raman calcined peaks that ap- ◦ pearat 600 at 152.6,C showed 432.7, modiﬁcations 511.3, 656.8 in and the 711.1 peaks cm which−1. The are Raman positioned spectrum at 147.0, of 432.1, BT1 519.5,samples cal- −1 ◦ cined663.0 andat 600 705.2 °C cmshowed. With modifications the further increase in the peaks in temperature which are to 900positionedC, the BT1-900at 147.0, 432.1, −1 519.5,sample 663.0 showed and peaks 705.2 positionedcm−1. With at the 137.0, further 365.5, increase 436.3, 588.3 in temperature and 705.2 cm to. The900 band°C, the BT1- near 136–152 cm–1 was assigned to the B1g mode, which occurs due to the rotational 900 sample showed peaks positioned at 137.0, 365.5, 436.3, 588.3 and 705.2 cm−1. The band vibration of oxygen atoms in TiO6 octahedra [33–35]. It was noted that with an increase –1 nearin the 136–152 calcination cm temperaturewas assigned to 900 to◦ theC, this B1g band mode, shifted which towards occurs a lower due wavenumberto the rotational vi- 6 −1 brationwith a change of oxygen in intensity. atoms Thein TiO band octahedra positioned [33–35]. near 432 cmIt wasis noted assigned that to with the Eg anmode, increase in thewhich calcination resulted from temperature the symmetric to 900 stretching °C, this vibration band shifted of the Ti–O towards bond [a36 lower,37]. wavenumber −1 with Thea change peaks nearin intensity. 365, 511, The 663 andband 705 positioned cm might near correspond 432 cm to−1 is the assigned bending to mode the ofEg mode, whichTiO6 octahedra resulted [from38–40 ].the One symmetric can see that stretchi a shiftng and vibration the appearance of the Ti–O of some bond peaks [36,37]. in the RamanThe spectra peaks of near BT1 365, samples 511, with 663 increasingand 705 cm temperature−1 might correspond were observed, to the which bending implies mode of that the hollandite system undergoes a structural transition. In the Raman spectra of BT2 TiO6 octahedra [38–40]. One can see that a shift and the appearance of some peaks in the samples calcined at different temperatures, as shown in Figure6, as-prepared samples Raman spectra of BT1 samples with increasing temperature were observed, which implies showed peaks appearing at 150.90, 298, 434.90 and 665.81 cm−1. With the increase in thattemperature the hollandite to 600 ◦ systemC, BT2 samples undergoes showed a struct onlyural two peakstransition. near 147.09In the and Raman 643.09 spectra cm−1. of BT2 samplesWith a further calcined increase at different in temperature temperatures, to 900 ◦asC, shown the Raman in Figure spectra 6, of as-prepared BT2 samples samples showedconsisted peaks of peaks appearing positioned at at150.90, 141.45, 298, 268.18, 434.90 306, and 432.90, 665.81 520 andcm− 7201. With cm− 1the. increase in tem- peratureIt is clearto 600 that °C, there BT2 are samples various showed changes only in the two Raman peaks spectra near that 147.09 indicated and 643.09 the phase cm−1. With atransition further increase as temperature in temperature increased. Theto 900 modiﬁcation °C, the Raman in the intensityspectra of and BT2 position samples of the consisted −1 ofband, peaks ranging positioned from 136 at 141.45, to 152 cm 268.18,, with 306, the 432.90, increase 520 in temperature,and 720 cm−1 resulted. from the changesIt is inclear the TiOthat6 octahedrathere are various in the symmetric changes stretching in the Raman modes spectra [40]. It wasthat observedindicated that the phase by the phase transition, TiO octahedra’s bending mode was also affected, which could be transition as temperature 6increased. The modification in the intensity and position of the explained by the observed changes in the intensity of the band and peak positions near 300, −1 band,520 and ranging 720 cm− from1. The 136 appearance to 152 cm of some, with peaks, the suchincrease as at 665in temperature, cm−1, and their resulted extinction from the changeswith increasing in the temperature,TiO6 octahedra indicated in the the symmetric phase transition stretching with themodes temperature [40]. It [was40]. observed that byThe the effect phase of the transition, hollandite TiO phase6 octahedra’s of Ba1.12(Ti8 Obending16) on the mode optical was properties also affected, of the which couldperovskite be explained phase of barium by thetitanium observed oxides changes was studiedin the intensity by UV–Vis of absorption. the band Figureand peak7 posi- tionsshows near the UV–Vis300, 520 absorption and 720 cm spectra−1. The for appearance the pure phase of ofsome BaTiO peaks,3, BT1-900 such andas at BT2-900. 665 cm−1, and theirThe strong extinction increase with in absorbance,increasing temperature, which was observed indicated as cut-off the phase behavior transition at the blue with end the tem- peratureof the spectrum, [40]. is owing to the excitonic transition through direct electronic transitions from the valence band to the conduction band. The effect of the hollandite phase of Ba1.12(Ti8O16) on the optical properties of the perovskite phase of barium titanium oxides was studied by UV–Vis absorption. Figure 7 shows the UV–Vis absorption spectra for the pure phase of BaTiO3, BT1-900 and BT2-900. The strong increase in absorbance, which was observed as cut-off behavior at the blue end of the spectrum, is owing to the excitonic transition through direct electronic transitions from the valence band to the conduction band.

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c b

Figure 7. UV–Vis absorbance of: (a) BT nano commercial, (b) BT2-900 and (c) BT1-900. Figure 7. UV–Vis absorbance of: (a) BT nano commercial, (b) BT2-900 and (c) BT1-900. Figure7a displays the UV–Vis absorbance of the pure perovskite phase of BaTiO 3 nanoparticles. An absorption edge was observed at 400 nm with one maximum at 300 nm. By growing the hollandite phase, the absorption edge of the barium titanate perovskite phase was shifted to the visible region through the formation of mixed phases between perovskite and hollandite crystals to be sensitive in sunlight, as seen in Figure7b. The absorption edge of BT2-900 was observed at 650 nm and its main absorption band was centered at 400 nm. This observation became clearer through preparing barium titanium oxides with only the hollandite phase, causing a red shift of barium titanate. Figure7c shows that the absorption edge of the sample BT1-900 was observedc at 800 nm, with two maxims at 430 nm and 650 nm. In addition, this red shift was further confirmedb by calculatinga the energy band gap. In order to calculate the energy band gap (Eg), the main absorption has been used according to the relation between the absorption coefficient (α) and the incident photon energy [41–44]: m (αhν) = A(hν−Eg) where the value (m) describes the optical absorption process and equals 2 or 1/2 for allowed direct and allowed indirect transitions, respectively, and A is constant. In the current materials, the value (m) is 2 because of the allowed direct transitions. Therefore, the optical band gap energy of the samples can be determined when (αhν)2 is zero. By plotting (αhν)2 and (E = hν), and extending the straight line to the (E) axis, as shown in Figure8, the optical band gap of the samples becomes clear. Figure8a shows that the band gap energy of barium titanium oxide nanoparticles with the perovskite phase is 3.2 eV. By growing hollandite phases, the optical properties of barium titaniumoxide nanopar- Figureticles are 8. stronglyBand gap modified energy through of: ( aa) reduction BT nano in commercial, band gap energy, (b as) BT2-900 shown in and (c) BT1-900. Figure8. In the case of BT2-900, the band gap energy decreased to 2.3 eV through growing the hollandite phase with the perovskite phase. This behavior continued through preparing barium titanium oxide nanoparticles with the hollandite phase only, as seen in the sample BT1-900. The band gap energy of BT1-900 was 2 eV. These results indicate that the narrowing of the band gap energy of the barium titanium oxide nanoparticles from 3.2 eV to 2.0 eV leads to the introduction of photocatalysts suitable to act and work in sunlight. This finding was confirmed by studying the optical behavior of the other samples, BT1-600 and BT2-600. Figure9 reveals the absorbance spectrum of both BT1-600 and BT2-600 in the wavelength range of 200 to 800 nm. It means that these materials become active in b a

Figure 9. UV–Vis absorbance of: (a) BT2-600 and (b) BT1-600.

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c b c b a a

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solar energy because of their broad absorbance in visible light. Figure 10 conﬁrmed the improvement of optical properties through narrowing the band gap energy, indicating 1.75 eV and 2.3 eV for BT1-600 and BT2-600, respectively. Similar behavior was observed Figurefor the samples 7. UV–Vis BT1 and absorbance BT2. Figure 11 revealedof: (a) BT wide nano absorbance commercial, in the same wavelength(b) BT2-900 and (c) BT1-900. rangeFigure of 2007. UV–Vis to 800 nm. absorbance The band gap energyof: (a) for BT both nano samples commercial, was 2.2 eV and(b) 2.4BT2-900 eV, and (c) BT1-900. respectively, as shown in Figure 12.

c c b a b a

FigureFigure 8. Band8. Band gap energy gap of: energy (a) BT nano of: commercial, (a) BT nano (b) BT2-900 commercial, and (c) BT1-900. (b) BT2-900 and (c) BT1-900. Figure 8. Band gap energy of: (a) BT nano commercial, (b) BT2-900 and (c) BT1-900.

b a b a

Figure 9. UV–Vis absorbance of: (a) BT2-600 and (b) BT1-600. Figure 9. UV–Vis absorbance of: (a) BT2-600 and (b) BT1-600. Figure 9. UV–Vis absorbance of: (a) BT2-600 and (b) BT1-600.

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Figure 10. Band gap energy of: (a) BT2-600 and (b) BT1-600.

(αhν)m = A(hν−Eg) where the value (m) describes thea optical absorption process and equals 2 or 1/2 for allowed direct and allowed indirect transitions, respectively, and A is constant. In the current materials, the value (m) is 2 because of the allowed direct transitions. Therefore, the optical band gap energy of the samples can be determined when (αhν)2 is zero. By plotting (αhν)2 and (E = hν), and extending the straight line to the (E) axis, as shown in Figure 8, the optical band gap of the samples becomes clear. Figure 8a shows that the band gap energy of barium titanium oxide nanoparticles with the perovskite phase is 3.2 eV. Figure 10. Band gap energy of: (a) BT2-600 and (b) BT1-600. Figure 10. Band gap energy of: (a) BT2-600 and (b) BT1-600.

Figure 7a displays the UV–Vis absorbance of the pure perovskite phase of BaTiO3 nanoparticles. An absorption edge was observed at 400 nm with one maximum at 300 nm. By growing the hollandite phase, the absorption edge of the barium titanate perovskite phase was shifted to the visible region through the formation of mixed phases between perovskite and hollandite crystals to be sensitive in sunlight, as seen in Figure 7b. The absorption edge of BT2-900 was observedb at 650 nm and its main absorption band was centered at 400 nm. This observationa became clearer through preparing barium titanium oxides with only the hollandite phase, causing a red shift of barium titanate. Figure 7c shows that the absorption edge of the sample BT1-900 was observed at 800 nm, with two maxims at 430 nm and 650 nm. In addition, this red shift was further confirmed by calculating the energy band gap. In order to calculate the energy band gap (Eg), the main absorption has been used accordingFigure 11. UV–Vis to absorbancethe relation of: (a) BT2 between and (b) BT1. the absorption coefficient (α) and the incident photon FigureThe 11. solar UV–Vis activity of absorbance BT1-600, BT2-600, of: ( BT2-900a) BT2 andand BT1-900 (b) BT1. was tested through energyphotocatalytic [41–44]: degradation of the green dyes (naphthol green B; NGB) from water in the presence of sunlight. (αhν)m = A(hν−Eg) At the same time, the commercial barium titanium oxide nanoparticles BaTiO3 were usedwhere for comparison. the value The intensity (m) of describes the absorption the of naphthol optical green absorption B at λmax = 720 process nm and equals 2 or 1/2 for allowedhas been used direct for expressing and allowed the concentration indirect of transitions, the green pollutants. respectively, High stability and A is constant. In the cur- of NGB in sunlight was observed because there were no changes in the absorption of rentthe greenmaterials, dye. The mixturethe value between (m) the is green 2 because dye and one of of the the solidallowed materials direct was transitions. Therefore, the irradiated for ten minutes in sunlight. After that, the absorbance of the green solution was 2 opticalmeasured band and used gap as theenergy concentration of the for samples the green dyes. can These be determined results are revealed when in (αhν) is zero. By plotting (αFigureshν)2 13and and (E 14. = In h theν), presence and extending of BT1-900 or BT2-900,the straight the absorbance line ofto NGB the was (E) axis, as shown in Figure 8, strongly reduced after irradiation for 10 min in sunlight, as seen in Figure 13. Figure 14 the optical band gap of the samples becomes clear. Figure 8a shows that the band gap energy of barium titanium oxide nanoparticles with the perovskite phase is 3.2 eV.

b a

Figure 11. UV–Vis absorbance of: (a) BT2 and (b) BT1.

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shows that BT1-900 and BT2-900 achieved 100% and 98% degradation of the pollutants, respectively. By using BT1-600 and BT2-600, the removal decreased to 91% and 84%, respectively, in the same period. This means that the complete removal and decolorization of the green pollutants were attained after using BT1-900. In the case of using the nanoparticles of barium titanium oxides with perovskite, a minor difference was observed for the absorbance of the pollutants after 10 min irradiation in sunlight, as revealed in Figure 13. Additionally, Figure 14 shows low performance for the perovskite phase because Crystals 2021, 11, x FOR PEER REVIEW it caused 40% removal of the green dyes. This indicated that the perovskite phase is inactive 10 of 14 in sunlight. In comparison with barium titanate having the perovskite phase, the samples containing the hollandite phase became very effective in the visible region. The presence of the hollandite phase caused broad absorbance and narrowing of the band gap energy to 2.0 eV, leading to complete removal of NGB in 10 min under sunlight.

b a

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Figure 12. Band gap energy of: (a) BT2 and (b) BT1. Figure 12. Band gap energy of: (a) BT2 and (b) BT1.

By growing hollandite phases, the optical properties of barium titanium oxide nanoparticles are strongly modified through a reduction in band gap energy, as shown in Fig- ure 8. In the case of BT2-900, the band gap energy decreased to 2.3 eV through growing the hollandite phase with the perovskite phase. This behavior continued through preparing barium titanium oxide nanoparticles with the hollandite phase only, as seen in the sample BT1-900. The band gap energy of BT1-900 was 2 eV. These results indicate that the narrowing of the band gap energy of the barium titanium oxide nanoparticles from 3.2 eV to 2.0 eV leads to the introduction of photocatalysts suitable to act and work in sunlight. This finding was confirmed by studying the optical behavior of the other samples, BT1- 600 and BT2-600. Figure 9 reveals the absorbance spectrum of both BT1-600 and BT2-600 in the wavelength range of 200 to 800 nm. It means that these materials become active in solar energy because of their broad absorbance in visible light. Figure 10 confirmed the

improvement of optical properties through narrowing the band gap energy, indicating Figure 13. Absorbance spectra of naphthol green B after 10 min in sunlight (a) in the absence and in Figure 13. Absorbance spectra of naphthol green B after 10 min in sunlight (a) in the absence and 1.75 eVthe and presence 2.3 of eV (b) BTfor nano BT1-600 commercial, and (c) BT2-600,BT2-600, (d) BT2-900, respectively. (e) BT1-600 andSimilar (f) BT1-900. behavior was observed in the presence of (b) BT nano commercial, (c) BT2-600, (d) BT2-900, (e) BT1-600 and (f) BT1-900. for the samples BT1 and BT2. Figure 11 revealed wide absorbance in the same wavelength range of 200 to 800 nm. The band gap energy for both samples was 2.2 eV and 2.4 eV, respectively, as shown in Figure 12. The solar activity of BT1-600, BT2-600, BT2-900 and BT1-900 was tested through photocatalytic degradation of the green dyes (naphthol green B; NGB) from water in the presence of sunlight.

Figure 14. Percentage removal of naphthol green B after reacting for 10 min in sunlight.

At the same time, the commercial barium titanium oxide nanoparticles BaTiO3 were used for comparison. The intensity of the absorption of naphthol green B at λmax= 720 nm has been used for expressing the concentration of the green pollutants. High stability of NGB in sunlight was observed because there were no changes in the absorption of the green dye. The mixture between the green dye and one of the solid materials was irradiated for ten minutes in sunlight. After that, the absorbance of the green solution was measured and used as the concentration for the green dyes. These results are revealed in Fig- ures 13 and 14. In the presence of BT1-900 or BT2-900, the absorbance of NGB was strongly reduced after irradiation for 10 min in sunlight, as seen in Figure 13. Figure 14 shows that BT1-900 and BT2-900 achieved 100% and 98% degradation of the pollutants, respectively. By using BT1-600 and BT2-600, the removal decreased to 91% and 84%, respectively, in the same period. This means that the complete removal and decolorization of the green

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Figure 13. Absorbance spectra of naphthol green B after 10 min in sunlight (a) in the absence and Crystals 2021, 11, 550 in the presence of (b) BT nano commercial, (c) BT2-600, (d) BT2-900, (e) BT1-600 and (12f) ofBT1-900. 14

FigureFigure 14. 14. Percentage removal removal of of naphthol naphthol green green B after B after reacting reacting for 10 for min 10 in min sunlight. in sunlight.

4. Conclusions At the same time, the commercial barium titanium oxide nanoparticles BaTiO3 were In summary, BaTiO3 with the hollandite phase was successfully prepared by the usedchemical for comparison. route with heat The treatment, intensity usingof the different absorption temperatures of naphthol to tune green the B phasesat λmax of= 720 nm hashollandites. been used X-ray for diffractedexpressing patterns the concentratio conﬁrmedn that of the preparedgreen pollutants. product calcinedHigh stability at of NGB900 ◦ Cin was sunlight the hollandite was observed type, which because is well there matched were with no standardchanges data.in the However, absorption the of the ◦ greensamples dye. treated The atmixture 600 C werebetween found the to green be in mixed dye and phases. one Inof addition, the solid with materials the increase was irradi- atedin calcination for ten minutes temperature, in sunlight. the crystalline After that, size ofthe the absorbance barium hollandite of the green was tuned. solution Raman was meas- uredspectra and of used the samples as the concentration furthermore conﬁrmed for the green the formation dyes. These of the results hollandite are revealed phase in in Fig- BaTiO with characteristic peaks of hollandite structures. With an increase in calcination ures 133 and 14. In the presence of BT1-900 or BT2-900, the absorbance of NGB was strongly temperature, the peak intensity of Raman modes was found to be shifted. UV–Vis studies reducedshowed thatafter the irradiation tuning of for band 10 gap min could in sunlight, be accomplished as seen in by Figure changing 13. theFigure calcination 14 shows that BT1-900temperatures and BT2-900 of BaTiO 3achieved. The energy 100% band and gap 98% was degradation found to decrease of the from pollutants, 3.2 eV to 2.0respectively. eV Byand using the samples BT1-600 showed and BT2-600, a visible the light removal response de withcreased enhanced to 91% optical and properties.84%, respectively, The in thephotocatalytic same period. capabilities This means of BaTiO that3 with the hollanditecomplete phaseremoval were and tested decolorization by means of UV–Vis of the green absorption for the photo-degradation of green dye. The results showed that the samples having the hollandite phase became very effective in the visible region and degraded the green pollutant completely (100%) within 10 minutes. The obtained results have shown enhanced photocatalytic properties in visible light, which indicated that BaTiO3 with hollandite phase is a suitable candidate for a novel photocatalyst for pollutant degradation.

Author Contributions: Conceptualization, A.A., O.S.; Data curation, O.S., F.A.; Formal analysis, F.A., O.S.; Funding acquisition, A.A.; Methodology, O.S., F.A.; Resources, A.A.; Visualization, A.A., F.A., O.S.; Writing—original draft, O.S., F.A.; Writing—review and editing, A.A., O.S., F.A. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by Deanship of Scientific Research at King Faisal University for supporting this research through the Ra’ed track grant number 187010 and the APC was funded by [Ra’ed track]. Institutional Review Board Statement: Not Applicable. Informed Consent Statement: Not Applicable. Data Availability Statement: Data could be available on request. Acknowledgments: The authors would like to thank the Deanship of Scientific Research at King Faisal University for supporting this research through the Ra’ed track (Grant # No. 187010). Conflicts of Interest: The authors declare that they have no competing interests. Crystals 2021, 11, 550 13 of 14

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