Influence of hydrogenation conditions on the structural, optical, and magnetic properties of degenerated Ni/Al co-doped anatase nanocomposite

In the present work, pristine and Ni 2+ /Al 3+ - codoped anatase (TiO 2 ) nanocomposite powders were synthesized by the thermal co-precipitation method. The samples were characterized by several techniques. The X ‐ Ray diffraction (XRD) was used for structural characterization and the optical absorption spectroscopy was used for the optical characterization. The structural/optical studies proved the formation of a substitutional solid solution (SSS). The magnetization measurements were performed to investigate the magnetic properties of the synthesized samples. In the present work, nickel (Ni 2+ ) dopant ions were used to introduce stable ferromagnetic (FM) properties into the synthesized anatase, while Al 3+ dopant ions were utilized to supply itinerant electrons necessary to support and boost the created FM properties. The roadmap of the present work was to establish the hydrogenation conditions necessary to create ferromagnetic (FM) properties in the host codoped samples and study the effect of hydrogenation temperature on the parameters of the created FM properties, especially the magnetic energy (U mag ) and saturation magnetization (M sat ). It was found that the created FM properties in Ni/Al-doped TiO 2 nanocomposite powder by using the hydrogenation effect could be enhanced and controlled via the temperature of the hydrogenation (T H ). The experimental results revealed that U mag of TiO 2 :Ni:Al system increased by ~241% and the saturation magnetization by ~140% with increasing of T H by 100 o C (from 400 o C to 500 o C). The obtained saturation magnetization (M sat ) of 1.09 emu/g and magnetic energy (U mag ) of 42.6 erg/g were higher than the previously obtained values for created Ni-doped TiO 2 by ~50 times. Such great novel results were obtained due to dealing with two factors; the Al 3+ ions as co-dopant, which can supply an excess of itinerant electrons that boost the S.S Heisenberg interactions in addition to choosing a suitable temperature of hydrogenation.


Introduction
Titanium dioxide belongs to the transparent conducting oxide (TCO) family. It has received great consideration as a multifunctional material widely used for various devices, such as optoelectronics, solar cells, transparent electrodes, gas sensors, and photocatalysis, because of its excellent electrooptical properties [1][2][3][4][5][6]. Moreover, the developments in the properties of TiO2-based materials are the subject of nowadays.
Pristine Ti dioxide is a wide-band semiconductor with an indirect bandgap of ~ 3 -3.2 eV [7,8]. It crystallizes in three possible crystalline structures: anatase [A], rutile [R], and brookite [B] depending on the preparation conditions and temperature [9,10]. The photocatalytic, electrical, optical, and other properties of Ti oxide can be controlled through doping with different ions [11][12][13][14]. Besides, the properties of Ti oxide can be managed by organizing its natural point defects, like oxygen (O) vacancies and Ti interstitials. Moreover, it was found that doping could create exotic properties. For example, doping with transition metal (TM) or rare-earth (RE) ions could create ferromagnetic (FM) properties converting the host TiO2 material into diluted magnetic semiconductors (DMS), which widen its applications.
It is noteworthy that, weak FM behaviour could be easily created in synthesized TCO samples even without doping, which symbolized as a d 0 FM phenomenon [15,16]. The d 0 FM phenomenon is attributed to the presence of structural point defects like O-vacancies in the nanocrystallites. The Ovacancies can capture itinerant electrons, creating local moments, which lead to some kind of weak magnetic spin-spin (S.S) Heisenberg responses [17][18][19][20][21]. The d 0 FM phenomenon was proved to be unstable and could be removed thermally [22].
In the present work, anatase TiO2 nanopowder samples codoped with Ni 2+ and Al 3+ ions were synthesized to construct DMS. Nickel dopant ions are used to introduce stable ferromagnetic (FM) properties into the anatase powder sample, while Aluminum dopant ions are used to supply itinerant electrons necessary to support and boost the created FM properties. According to the 6-coordination ionic radii table [23], the radii difference between Ni 2+ (0.069 nm) and Ti 4+ (0.06 nm) ions is ~13% and between Al 3+ (0.054 nm) and Ti 4+ is ~10%. Thus, the possible ionic substitutions NiTi and AlTi cannot cause a significant geometrical distortion in the TiO2 host lattice, according to the Hume-Rothery (H-R) rules. Therefore, ions Ni 2+ and Al 3+ can easily be doped in the TiO2 crystal forming a substitutional solid solution (SSS). Moreover, it is also possible to believe that some amount of Ni and Al dopant ions might occupy interstitial positions and reside in crystallite boundaries.
The strength of the spin-spin (S.S) interaction between Ni 2+ dopant ions that could create the FM properties significantly depends on the effective interionic separation (R) as well as the local permittivity of the host TiO2 crystalline medium (ECM) necessary to switch on the S.S Heisenberg interaction. Therefore, it is possible to control the created magnetic properties of the DMSs through 3 the dopant kind and concentration and by catalyzing the ECM to permit/boost the S.S interaction. In the present work, the action used to catalyze the S.S interaction is the hydrogenation process. The H2 molecules should be dissociated into H-species under the impact of Ni 2+ dopant ions [24]. The Hspecies can create O-vacancies, which stimulate the ECM to permit the S.S interaction. The Al ions dopant can supply the itinerant electrons that support and boost the S.S interaction. The effect of hydrogenation temperature on the created FM parameters especially the magnetic energy (Umag) and saturation magnetization (Msat) is the purpose to measure in the present study.

Experimental details
The road map of the present investigation began with the synthesis of pristine and Ni/Al-codoped TiO2 (TiO2:Ni:Ga) nano-powders by thermal co-precipitation method. The starting analar materials for the synthesize were titanium tetrachloride (TiCl4) liquid, nickel (II) chloride (NiCl2.6H2O), and aluminum nitride (Al(NO3)3) (from Sigma-Aldrich Chemical Company). The molar ratios of Ni/Ti and Al/Ti were ~ 4.8 %. The procedure of synthesis was discussed elsewhere. In brief, the precursor was prepared from a mixture solution of 3 ml TiCl3, ~ 30 ml ethanol, ~ 120 ml bi-distilled deionized water, with the appropriate amounts of NiCl2.6H2O, and Al(NO3)3 in a glass beaker. The solution was kept under magnetic stirring for ~2-3h at room temperature, and then the temperature was slowly increased to ~ 60-80 o C with continuous stirring until a precipitate powder was obtained. Finally, the powder was flash sintering in a closed oven at 500 o C for 1 hr, followed by a slow cool to the room temperature. The present syntheses lead to the formation of a single phase of the TiO2 structure.
Finally, the powder was palletized under ~8 tons for characterization. The reference un-doped TiO2 powder was also synthesized by a similar procedure. Amounts from each synthesized fine powder were annealed in the H2-gas atmosphere at the 400 o C and 500 o C for 20 min. It is noteworthy that such hydrogenation at high-temperature 500 o C could not reduce TiO2 to Ti since TiO2 is very stable oxide and it is difficult to reduce it. The prepared samples were referred to as; TiO2-H, TiO2:Ni:Al-H (for 400 o C), and TiO2:Ni:Al-HH (for 500 o C), respectively.
The crystalline structures were investigated by the X-ray diffraction (XRD) method that carried out with a Rigaku Ultima-VI X-ray diffractometer (Cu K  ). The obtained XRD results were analyzed by

Structural analysis
The crystal structure of the synthesized samples (TiO2, TiO2:Ni:Al, TiO2-H, TiO2-Ni-Al-H, and TiO2-Ni-Al-HH) were investigated by the X-ray diffraction (XRD) method and the results are shown in Fig.1a. The built-in "Rietveld refinements fitting -PDXL2" program was used to analyze the obtained XRD patterns. The starting models for the structural analysis were anatase [A], rutile [R], and brookite [B]. However, the main crystal structure of the pristine and codoped Ti oxide, shown in  Table:1. The first observation is that the calculated CS reveals that the synthesized samples consist of nanometric crystallite size in the range of 5 -7 nm.
The absence of any XR-reflection related to the pure or compound of Ni and Al dopant ions confirms that the dopant ions dissolved in TiO2 lattice and did not aggregate or reacted with the sample's mother ions. Thus, the dopant Ni and Al ions formed SSS within TiO2, which confirms the prediction mentioned in the "introduction" that based on the geometrical H-R rule. 5 The XRD patterns of the hydrogenated samples were demonstrated in Fig.1a

Optical analysis
The It is worth noting that such an increase in free (itinerant) electrons concentration should support the ECM to switch on the S.S interaction and, hence create/enhance the FM properties.

5.1-Investigation of the as-prepared samples:
The pristine anatase TiO2 was known to have diamagnetic (DM) properties [38,39]. However, the synthesized un-doped TiO2 of the present work has PM behaviour of small susceptibility (~10 -7 cgs/g), as presented in Table:2. Similar behaviour was also observed earlier [38,39]. The PM properties were attributed to the created structural point defects, including O-vacancies. In general, the point defects can generate PM or weak FM properties [20] depending on their concentration, distribution, and separations.
The PM behaviour of TiO2 nano-powder in the present work maintained even with codoping.
However, the incorporation of Ni/Al ions in the host TiO2 caused a remarkable increase in the susceptibility by ~ 10 times. The PM susceptibility of TiO2:Ni:Al sample can be used to estimate the average effective para-magnetic moment per dopant Ni ion interacted paramagnetically with the applied field by using Curie law for mass susceptibility (per g): is the para-magnetic moment per dopant Ni ion, kB is the Boltzmann constant,  is the density of host TiO2, and T is the working temperature. Thus  i is equal to 3.38B for TiO2:Ni:Al, where B is the Bohr magneton. However, the known experimental paramagnetic moment of Ni(II) ion in some octahedral complexes was 2.9-3.3B [40]. Therefore, almost all the incorporated Ni 2+ dopant ions participated in the observed PM interaction. To attain the spin-spin S.S Heisenberg exchange interaction between dopant Ni 2+ ions, the average interionic separation (R) between them should be less than (1.5×a), where (a) is the TiO2 lattice parameter [41,42] in addition to the proper local 7 permittivity of the host TiO2 crystalline medium (ECM), which intermediate the S.S interaction. For uniform distribution of Ni 2+ dopant ions throughout the host TiO2 lattice, the average interionic separation (R) can be estimated by; NionV=1, where Nion is the Ni 2+ concentration (1.37×10 21 cm -3 ) and V= (4/3)R 3 , thus R ~ 0.56 nm, which almost equal to 1.5×a. Moreover, the concentration of Ni 2+ ions is much less than that of anatase unit cells, Ccell= 1 cell V − = 7.40x10 21 cm -3 . Therefore, due to the distance separation between dopant Ni 2+ ion, the S.S interaction between neighbouring Ni 2+ -Ni 2+ dopant ions could not be switched-on confirming the observed results unless stimulate the anatase ECM for the super-exchange interaction between the Ni 2+ -Ni 2+ dopant ions. Such stimulation was successfully achieved in the present work by the hydrogenation process.

5.2-Investigation of hydrogenated samples
The magnetic study of the hydrogenated synthesized un-doped TiO2-H sample at room temperature (RT) revealed the creation of weak FM properties superimposed on the major DM behavior. The FM component of TiO2-H behavior is shown in Fig.3. Such a weak d 0 FM phenomenon (explained in the introduction) could be generated by the internal structural point defects, like O-vacancies, which mainly created by hydrogenation [20].
The FM parameters (Hc: coercive force, Mr: remanence, and Msat: saturation magnetization) extracted from hysteresis loops are given in table:2. Moreover, the magnetic energy (Umag) was calculated to compare the quality of the FM hysteresis behaviours for one magnetic system and considered as a figure-of-merit, FOM=Umag=HcMr (erg/g) [43]. The magnetic energy Umag of undoped TiO2 was weak (0.12 erg/g) and it is unstable and could be thermally removed.  [44], 0.020 emu/g [45], 0.021emu/g [46], and for (Ni+Fe)-codoped TiO2 sample the result was 0.160 emu/g [47].

Conclusions
Pristine and Ni/Al-codoped TiO2 nanocomposites were successfully synthesized by the thermal co- Umag of the hydrogenated TiO2:Ni:Al system increased by 241% and the magnetization increased by 140% with increasing of TH by 100 o C (400 o Cto -500 o C).
Such great novel results obtained due to two factors: using Al 3+ ions co-doping as a supplier of itinerant electrons that boost the S.S Heisenberg interactions and choosing the appropriate temperature of hydrogenation.
No conflict of interest exists: I wish to confirm that there are no conflicts of interest associated with this publication and there has been no significant financial support for this work that could have influenced its outcome.