Investigation to the Structural, Optical, and Magnetic

Investigation to the Structural, Optical, and Magnetic Properties of Synthesized NiDoped Anatase Nanoparticles: Essential Role of Treatment in Hydrogen on LongRange Ferromagnetic Order A. A. Dakhel, H. Hamad & Adnan Jaafar

Journal of Superconductivity and Novel Magnetism Incorporating Novel Magnetism ISSN 1557-1939 J Supercond Nov Magn DOI 10.1007/s10948-018-4945-8

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Author's personal copy Journal of Superconductivity and Novel Magnetism https://doi.org/10.1007/s10948-018-4945-8

ORIGINAL RESEARCH

Investigation to the Structural, Optical, and Magnetic Properties of Synthesized Ni-Doped Anatase Nanoparticles: Essential Role of Treatment in Hydrogen on Long-Range Ferromagnetic Order A. A. Dakhel 1 & H. Hamad 2 & Adnan Jaafar 1 Received: 26 September 2018 / Accepted: 9 November 2018 # Springer Science+Business Media, LLC, part of Springer Nature 2018

Abstract Nanocomposite powders of anatase TiO2 doped with Ni ions (TiO2:Ni) were synthesized by facile thermal co-decomposition of a mixture of metals complexes. The X-ray diffraction (XRD) analyses using the Rietveld method confirm the formation of almost single nanocrystalline anatase structure. Anatase TiO2 incorporated with Ni ions (TiO2:Ni) illustrated the formation of a substitutional solid solutions (SSS). The diffuse reflection spectroscopy (DRS) method used to study the optical properties of the prepared samples revealed a redshift associated with Ni doping and hydrogenation, which is explained by the creation of point defects including O-vacancies. This study showed the essential importance of hydrogenation process in order to create or boost room-temperature ferromagnetism (RT-FM). The magnetic measurements indicated that anatase TiO2 doped with 4.8 mol% Ni produce a magnetic saturation of 0.84 emu/g, which is a remarkable result compared with that related to the similar studies on doped TiO2. Keywords Ni-doped TiO2 . Room-temperature ferromagnetism . Hydrogenation

1 Introduction Titania (TiO2), the widely used semiconductor crystallizes into three different crystalline phase structures: anatase (A), rutile (R), and brookite (B) depending on the preparation conditions and temperature [1, 2]. It has been extensively exploit as multifunctional semiconductor in many applications, including gas sensing, photovoltaic cells, environmental treatments, and photocatalysis, because of its adequate properties [3–5]. Its excellent photocatalytic properties depend on synthesis detail [5, 6].

* A. A. Dakhel [email protected] H. Hamad [email protected] Adnan Jaafar [email protected] 1

Department of Physics, College of Science, University of Bahrain, P. O. Box 32038, Zallaq, Kingdom of Bahrain

2

University of Abu Dhabi, P. O. Box 59911, Abu Dhabi, United Arab Emirates

Pristine anatase has a large energy band gap (~ 3.2 eV) in which the photocatalytic activity works under UV range. Therefore, from practical point of view, it is necessary to extend the range of response spectrum to the visible light by narrowing the band gap. The latter is usually done by the wide-used method of doping by different ions [7–10]. The objective of the present work is to investigate the effect of doping process, in controlling the physical properties of anatase titania. It has been established that a certain doping can improve the photocatalytic, electrical, and optical properties of titania depending on the dopant quality and quantity [11, 12]. Moreover, annealing of titania in hydrogen gas (hydrogenation) was used to elevate its photocatalytic activity [13] that attributed to the creation of oxygen vacancies. It was observed that the doping of semiconductors with magnetic ions like transition-metal (TM) ions (Fe, Co, and Ni) [14] or rare-earth ions (Gd, Tb, etc.) can built-up dilutemagnetic semiconductors (DMS) of many applications especially in the field of spintronics. Therefore, efforts have been devoted to understand the origin of the created stable ferromagnetism (FM) with the doping of semiconductors by only few percent of magnetic impurities. Some authors explained the creation of DMS in Ni-doped TiO2 to the formation of Ni clusters [15], while another authors explained it by the

Author's personal copy J Supercond Nov Magn

creation of lattice strain with nanocrystallite size [2, 16]. The present work focused on the hydrogenation of synthesized nanograins anatase titania in order to create oxygen vacancies for the fabrication of DMS properties. Various methods are used to synthesize titania nanoparticles, such as sol–gel process, electrochemical coating, hydrothermal process, hydrolysis, and chemical vapor deposition. However, in the present work, a facile hydrothermal method extracted from reference [17] was used to synthesize undoped and Ni-doped anataseTiO2 nanocomposite powders. Systematically, structural, optical, and magnetic properties of the as-synthesized and hydrogenated samples were studied focusing on the capability of having FM properties with host TiO2(A).

Kα radiation and operated at 40 kV, 40 mA. Optical properties were studied by diffuse reflectance spectroscopy (DRS) method. The spectrophotometric measurements of Rdiff (λ) were performed over the wavelength range 200–1000 nm with Shimadzu UV-3600 double beam spectrophotometer equipped with an integrating sphere. Magnetic characterizations were measured by using PMC MicroMag 3900 model Vibrating Sample Magnetometer (VSM) with a sensitivity of 0.5 μemu at a step field of 25 Oe and an averaging time of 1 s. Magnetization curves were measured at room temperature of T = 294 K in the field ranges + 10 kOe to − 10 kOe.

3 Results and Discussion 2 Experimental Procedure Pristine anatase and Ni-doped TiO2 nanopowders were synthesized by a facile chemical procedure of thermal codecomposition of metal complexes. Briefly, initial precursor of 3 mL of high purity titanium tetrachloride has been supplied by (Sigma-Aldrich products) and 30 mL of pure ethanol were put into a glass beaker. The solution was magnetically stirred for ~ 30 min at room temperature. During this period, a yellow sol phase was formed. Then, ~ 120 mL of deionized bidistilled water was added with a continuous stirring for ~ 1 h and the solution became almost colorless (referred to as mother solution). Next, the temperature of the mother solution was increased slowly up to ~ 60 °C with continuous stirring for ~ 20 h, i.e., until a dry powder was formed on the bottom of the beaker. The powder was collected and tested and then, a flash sintering in air at 500 °C for 1 h followed by natural cooling in a closed oven up to the room temperature was done to obtain undoped TiO2. However, to dope TiO2 with Ni ions, a controlled amount of nickel(II) chloride hexahydrate (NiCl2· 6H2O) was added to the mother solution. This preparation procedure is almost identical with that discussed in reference [17]. Two (Ni/Ti) mass ratios were synthesized: 2.6% and 5.9%, which were referred respectively as TiO2:Ni-1 and TiO2:Ni-2 powders. Then, some amounts from those two synthesized powders were hydrogenized at 400 °C for 20 min, cooled outside the oven, and re-referred as TiO2:Ni-1-H and TiO2:Ni-2-H, respectively. Finally, the resulting composite powders were pelletized for characterization. Undoped and hydrogenated titania powders synthesized by the same procedure to serve as references were referred to as TiO2 and TiO2:H. The elemental contents and purity of the synthesized samples were studied by the X-ray fluorescence (XRF) method carried with an Amptek XR-100CR (USA) X-ray detector of energy resolution 180 eV controlled by a built-in MCDWIN 3.1 program. The structural analyses were performed using a Rigaku Ultima IV θ-2θ X-ray diffractometer equipped with Cu

3.1 Structural Characterization The elemental analysis for each sample in a form of a pressed disc powder was carried out using the XRF method. Figure 1 shows the XRF spectrum of synthesized undoped and Nidoped TiO2 (TiO2:Ni). The spectrum shows Ti K-band (4.51 keV and 4.93 keV for Kα and Kβ, respectively) and Ni Kα-signal (7.47 keV). Therefore, the spectra confirmed the purity of the prepared samples. Figure 2a reveals the formation of TiO2 crystalline structure for the as-prepared undoped and Ni-doped TiO2 before sintering. The structural analyses showed that it has mainly anatase (A) structure of tinny sized (3.9 nm) nanocrystallites, as tabulated in the first row of Table 1. The observed diffraction peaks were indexed according to anatase TiO2 phase of early data; PDF-00-001-0562 card; tetragonal [I41/amd(No.141)] with lattice parameters: a = 0.373 nm, c = 0.937 nm [18]. Weak reflections from little formatted amounts of rutile (R) and brookite (B) phases were observed. Figure 2b shows the XRD patterns of the undoped and Ni-doped TiO2 samples after sintering at 500 °C for 1 h. The patterns revealed that the major crystalline structure was

Fig. 1 XRF spectra of the studied samples


Fig. 2

a XRD patterns of the as-prepared-before sintering samples. b XRD patterns of the as-synthesized samples. c XRD patterns of the hydrogenated samples

anatase of tetragonal structure of space group of [I41/amd] with traces of rutile (R) of tetragonal [P42/mnm] structure and brookite (B) of orthorhombic Pbca structure. The traces of R and B phases vanished with increasing of Ni content. No diffraction peaks arising from pure dopant Ni metal or any related phases were detected so that crystalline traces of secondary phases were not found within the XRD detection limit. In addition, it was noticed that there is no structural transformation from major anatase phase to any one of other TiO2 phases. Figure 2c illustrates the XRD patterns of the hydrogenated samples, which shows no additional phases or changes in the crystalline structure have been occurred. The possible dissolving of Ni ions in TiO2 structure can be explained according to the following geometrical rules, as the ionic radius of Ti4+ (coordination VI) is 0.0605 nm and that of Ni2+ (coordination VI) is 0.069 nm [19], then Ni2+ ions could substitute for Ti4+ ions in TiO2 structure. This substitution is taking place without inducing a strong geometrical distortion in the crystalline structure of the host TiO2. Therefore, Ni ions and TiO2 can form substitutional solid solution (SSS), which follows Hume-Rothery rule [20] as the ionic radii difference is ~ 14%. It might be reasonable to suppose that some amount of Ni atoms/ions could occupy interstitial positions and might be accumulated on crystallite boundaries (CBs) in a form of amorphous clusters. The results of structural analyses (lattice parameters and crystallite size) are presented in Table 1. Rietveld refinements were carried out using PDXL program, where the starting model was based on TiO2 of tetragonal structure (I41/amd) [18]. The Rietveld refinements quality parameters (Rwp (%) weighted profile R-factor; Rp (%) profile R-factor; Re (%) expected R-factor; and S = Rwp/Re goodness-of-fit) were all indicate a good fit; (for a good fitting, the S parameter value should be between 1 and 2, or close to 1 [21]). Figure 3a shows graphically the Rietveld refinements results for the TiO2:Ni-2 sample, as example of excellent single-phase fitting (S = 1.12), where the blue solid line (up) is the experimental data, the red solid line (up) is the calculated pattern, and the solid pink line (down) is the intensity difference. The structural analysis including crystallite size (CS) was calculated by using the Williamson-Hall (W-H) method, which was done by the built-in software of the used XRD apparatus. The used W-H equation was [22]: ðβhkl =tanθhkl Þ2 ¼ ðk=DÞ ðβhkl =tanθhkl sinθhkl Þ þ k 0

ð1Þ

where θ is the Bragg’s angle, k ~ 0.75, D is the CS, βhkl is the peak width at half maximum, and k' is a constant for each sample which depends on the internal strain. The plot of Y (rad 2) = (β hkl / tan θhkl ) 2 versus X(rad) = (β hkl / tan θ hkl

Author's personal copy J Supercond Nov Magn Table 1 Phase composition, mean crystallite size (CS), lattice parameters (a, c), volume of unit cell (Vcell), refinement factors, and bandgaps Eg (eV) for the as-synthesized and hydrogenated samples Sample

CS (nm)

a, c (Å)

Vcell (Å3)

Refinements factors Rwp (%)

Rp (%)

Re (%)

S

Eg (eV)

TiO2(A)*

3.9

a = b = 3.775, c = 9.450

134.7

24.33

18.54

20.21

1.20

–

TiO2(A) TiO2:Ni-1(A) TiO2:Ni-2(A)

5.8 4.8 6.5

a = b = 3.773, c = 9.484 a = b = 3.772, c = 9.463 a = b = 3.785, c = 9.490

135.0 134.6 135.9

27.7 30.72 19.68

21.61 24.01 15.02

21.27 20.88 17.55

1.30 1.47 1.12

3.07 2.67 2.43

Hydrogenated samples 4.8 TiO2-H(A) 5.8 TiO2:Ni-1-H(A) 7.2 TiO2:Ni-2-H(A)

a = b = 3.779, c = 9.499 a = b = 3.768, c = 9.499 a = b = 3.787, c = 9.499

135.7 134.9 136.2

25.39 34.22 18.71

19.03 26.60 14.09

17.47 18.22 16.5

1.45 1.87 1.13

3.09 2.44 2.36

sin θhkl)gave a straight line from which, the CS can be determined, as shown in Fig. 3b for sample TiO2:Ni-2. The nanoCS, given in Table 1 was less than ~ 10 nm.

The Ni ions doping, occupation on interstitial positions, accumulation on CBs, creation of O-vacancies, and redistribution of static charges, all of these factors caused variation in Vcell in some complicated manner. Nevertheless, the hydrogenation caused increase in Vcell for all samples Table 2. This can be attributed to the increase of concentration of created O-vacancies in addition to homogenize the Ni dopant spatial distribution throughout the sample. It is worth mentioning that the effect of hydrogenation is more obvious on the optical and magnetic properties rather than the geometrical XRD measurements.

3.2 Optical Characterization The synthesized powders were characterized optically by using DRS technique. The DR spectra are usually analyzed according to the Kubelka–Munk (K–M) equation [23]: ðK=S Þ ¼ F ðλÞ ¼ ð1−R∞ Þ2 =2R∞

Fig. 3 a Graphical representation of Rietveld refinements for TiO2:Ni-2 sample, where the blue solid line (up) is the experimental data, the red solid line (up) is the calculated pattern, and the solid pink line (down) is the intensity difference. b W-H plot for TiO2:Ni-2 sample

ð2Þ

where S is the scattering coefficient, K is the K–M absorption coefficient or K = 2α, α is the linear absorption coefficient, F(λ) is the K–M absorption function, and R∞ = Rsample/ Rstandard is the spectral diffuse reflectance of thick powder sample (Rsample) measured relative to standard reflector (R s tand ard ), in which a BaSO 4 powder (supplied by Schemadzu) serve as the standard reflector. In the studied wavelength range, S was approximately considered constant, thus F(λ) ~ α. Figure4a shows the variation of the spectral function F(λ) with the wavelength of the synthesized powder samples. For long-wavelengths region, the absorption was almost constant with the wavelength and increased with Ni dopant content level. Obviously, the major peaks in Fig. 4a in the range 300–400 nm are not due to the presence of Ni ions. Figure 4b demonstrates the spectral function F(λ) of

Author's personal copy J Supercond Nov Magn Table 2 Magnetic parameters; coercivity (Hc), remanence (Mr), saturation magnetization (Ms), and PM susceptibility of the assynthesized and hydrogenated samples

Sample

Behavior

Hc (Oe)

Mr (memu/g)

Ms (memu/g)

χ (cgs/g)

TiO2-H TiO2:Ni-1-H TiO2:Ni-2-H TiO2 TiO2:Ni-1

DM+weak FM Super PM FM Weak PM PM

216.4 – 22.8 – –

0.559 – 77.6 – –

0.426 147.4 833.9 – –

– – – 3.4 × 10−7 1.43 × 10−6

TiO2:Ni-2

PM

–

–

–

1.74 × 10−6

hydrogenated powder samples, which shows a great changes in the inter-band energy region. These changes were attributed to the creation of high density of point defects, like

Fig. 4 a Spectral absorption function F(λ) of as-synthesized powder samples. b Spectral absorption function F(λ) of hydrogenated powder samples

O-vacancies (VOs). The VOs are created by removing some structural oxygen ions by hydrogen H ion/atom. Therefore, to create VOs, molecules of H2 must be dissociated into H species (atoms/ions) in the presence of Ni ion dopants, which can play a role of a catalyst in the dissociation of H2 molecules [24]. Due to the high density of inter-band levels, it is not possible to find the exact value of the energy bandgap (Eg) through Tauc technique. Therefore, Eg can be estimated by measuring the threshold value of λg, as shown in Fig. 4, then using the formula Eg(eV) = 1242.3/λg(nm) to estimate its value as presented in Table 1. The Eg of undoped synthesized TiO 2 (~ 3.1 eV) is close to the known value (3.2 eV). The redshift or bandgap narrowing (BGN) with doping of TiO2 was also observed in many references [25, 26]. The BGN is attributed to the created structural defects like VOs that can be represented energetically by impurity levels at the bottom and overlapped with the conduction band of host TiO2 causing decrease of Eg. Moreover, the hydrogenation of TiO2:Ni samples redshifted the bandgaps, as presented in Table 1. Such effect was also observed in reference [16]. That redshift is explained by increasing the density of created VO s caused by the hydrogenation at the presence of dopant Ni ions. On the other hand, it was known that annealing in H2 atmosphere would strike off much type of structural defects, imperfections, and dangling atoms/ions. Therefore, the hydrogenation, in general, could also decrease the density of structural defects resulting in an increase of Eg. Moreover, the hydrogenation would homogenize the Ni ions distribution throughout TiO2 lattice via tossing them, which should have influence on E g . Therefore, observed results are the resultant consequences of the hydrogenation. Furthermore, it should be pointed out that the graphical procedure of estimation of Eg from the F(λ) and the limitation of the DRS method makes the estimation of Eg not enough accurate and sensitive to detect much smaller variations. It is worth mentioning here that the BGN could enhance the photoelectrical responses, which increases the number of photo-generated carriers that will be beneficial for the improvement of photocatalytic activity of TiO2 under sunlight irradiation.


3.3 Magnetic Properties It was known that pristine anatase phase of TiO2 has diamagnetic (DM) properties [27, 28], however nanocrystalline TiO2 may show weak magnetism if defects are formed during synthesis process [29]. In the present work, the synthesized undoped pure TiO2 has paramagnetic (PM) properties with small susceptibility, as given in Table 2. Those magnetic properties are considered to be due to nanocrystalline sizes effect and different defects including induce lattice strain. With the hydrogenation, the undoped pure TiO2 gained weak ferromagnetic (FM) properties superimposed on DM behaviors, as shown in Fig. 5a. It seems that the partial FM behavior was created with the aid of the structural point defects (Ovacancies) that created by the hydrogenation. Data of Table 2 show that the incorporation of Ni ions in host TiO2 lattice increase the PM susceptibility of the host samples without switching on the Heisenberg spin–spin (S.S) exchange interaction necessary to create FM properties.

In general, the realization and strength of the S.S exchange interaction between dopant Ni2+ ions depend on their average interionic separation (R), in addition to the properties of the electronic crystalline medium (ECM), which conduct or assist the S.S interaction. For uniform distribution of dopant Ni2+ ions within the host TiO2 lattice, R could be estimated by; NNiV = 1, where NNi is the dopant Ni2+ ions concentration and V = (4/3)πR 3 , thus R ~ 0.73 nm and 0.55 nm for TiO2:Ni-1 and TiO2:Ni-2 samples, respectively. However, the nearest neighbor distance in pure Ni crystal is 0.25 nm. Therefore, direct spin interaction between neighboring Ni-Ni ions could not be switched-on with the used dopant concentrations, unless stimulating the ECM between the Ni-Ni ions in order to assist the realization of that interaction. In the present work, such stimulation was done by the hydrogenation, which creates VOs that assist realize the super exchange interaction necessary for S.S exchange interactions. The obtained PM susceptibility in Table 2 can be used to estimate the average effective magnetic moment per dopant Ni ion interacted paramagnetically with the applied field by using Curie equation for volume susceptibility χ(vol) in cgs units: χðvolÞ ¼ nion μ2 =3kB T

Fig. 5 a M-H relationship of the hydrogenated powder samples. b lowfield magnified region M-H relationship of the hydrogenated powder samples

ð3Þ

where μ is the magnetic moment per dopant ion, kB is the Boltzmann constant, χ(vol) = Dχ(mass), χ(mass) is the mass susceptibility, D is the density of host TiO2, and T is the working temperature. Thus, μNi is equal to 3.56 μB and 2.61 μB for TiO2:Ni-1 and TiO2:Ni-2, respectively, where μB is the Bohr magneton. However, the known experimental magnetic moment of Ni(II) ion in some octahedral complexes was 2.9– 3.3 μB [30]. Therefore, all Ni ions incorporated in TiO2:Ni-1 sample participated in the PM interaction, while only ~ 53% of Ni ions incorporated in TiO2:Ni-2 sample participated in the PM interaction. This means that ~ 47% of incorporated Ni ions in TiO2:Ni-2 sample were inactive paramagnetically. In general, not all dopant Ni ions must participate in magnetic responses. This might be due to the medium bonds, constrains, distribution, and location of Ni ions like occupation of interstitial positions with oxidation number different from + 2, and accumulation on CBs as atoms or small clusters, etc. [31, 32]. Figure 5 and Table 2 illustrated the following magnetic parameters: coercive force (Hc), remanence (Mr), saturation magnetization (Ms) as determined from M-H hysteresis loops. However, the magnetic behaviors of the hydrogenated samples indicate RT-FM properties. It was clear that the hydrogenation had a remarkable effect on the magnetic properties, which create RT-FM by influencing on ECM and redistributing the Ni2+ ions through tossing. The Ms data of Table 2 can be used to estimate the effective concentration (n∗) of Ni2+ ions participated in S.S RT-FM interaction by using the following equation: μ (N i2+ ) = Ms (e m u/g) × D/n∗,


where D is the density of host TiO2 (3.9 g cm−3) and μ ( N i2+ ) is the magnetic moment of Ni2+ ions (μNi2þ ¼ 3:4 μB [20]). Thus, only ~ 3% and 7% of the total doped Ni ions in TiO2:Ni-1-H and TiO2:Ni-2-H, respectively participated in S.S RT-FM interaction. This small participation fraction is attributed to the nature of the synthesized powder samples, which consists of nanocrystallites. Finally, it is interesting to compare the magnetization (Ms) of the present samples to those doped TiO2 reported recently by other researchers. The present results for Ms were obviously higher than previous results obtained from sol–gel of TiO2:3%Co (7.42 memu/g), TiO2:3%Mn (9.46 memu/g) [28], TiO 2 :5%Ni (21.79 memu/g) [2], and TiO 2 :5%Fe (20 memu/g) [33]. It is obvious that the importance of hydrogenation is to support the embedded magnetic properties by its dissociation into H species.

4 Conclusions Pure and Ni-doped TiO2 nanocomposites were successfully synthesized by thermal co-decomposition of metal complexes. Structural study confirms the formation of solid solutions (SSS) with anatase structure. This work focused on the hydrogenation effect especially induction of RT-FM. Remarkable controllable diverse magnetic behaviors were obtained. The results showed that doping with Ni2+ ions is necessary but not sufficient to create stable RT-FM in host TiO2; the hydrogenation was also essential in order to create RT-FM. The saturation magnetization of ~ 0.83 emu/g was obtained in the present work and compared with the results obtained by different scientists. Therefore, the possibility of preparing Nidoped TiO2 powders with tailored magnetic properties is proved, hence becoming a potential candidate for future DMS applications. Moreover, the results indicated the significance importance of the ECM through which the S.S exchange interaction can take place. The results of the present work demonstrate a potential candidate medium of TiO2:Ni-H for future DMS applications associated with other practical multifunctional applications of host anatase-TiO2.

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