Weak Field Tuning of Transition Metal Dopant Hybridization in Solid

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C: Physical Processes in Nanomaterials and Nanostructures

Weak Field Tuning of Transition Metal Dopant Hybridization in Solid Hosts Pragathi Darapaneni, Orhan Kizilkaya, Zhen Wang, and James A. Dorman J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b06069 • Publication Date (Web): 10 Sep 2018 Downloaded from http://pubs.acs.org on September 10, 2018

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The Journal of Physical Chemistry

Weak Field Tuning of Transition Metal Dopant Hybridization in Solid Hosts Pragathi Darapaneni,a Orhan Kizilkaya,b Zhen Wang,c,d James A. Dorman*,a a

Cain Department of Chemical Engineering, Louisiana State University, Baton Rouge, Louisiana

70803, United States b

Center for Advanced Microstructures and Devices, Louisiana State University, Baton Rouge,

Louisiana 70806, United States c

Department of Physics and Astronomy, Louisiana State University, Baton Rouge, Louisiana

70803, United States d

Brookhaven National Laboratory, Condensed Matter Physics and Materials Science

Department, Upton, New York 11973, United States

ABSTRACT: Transition metal (TM) doped solids are one of the most extensively studied compounds in the fields of catalysis, magnetism, solar cells, etc., due to their tunable optoelectronic properties that stem from TM energy level hybridization. In this work, the hybridization of the Ni-O bond in TiO2:Ni films was controlled in a stable, reversible manner via surface functionalization with polarized molecules. The Ni-doped TiO2 surface was functionalized with para-benzoic acid groups to modify the electron density distribution within

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the film. The dopant distribution and elemental composition at the interface are probed via highresolution TEM images coupled with EELS composition mapping. The effect of the surface modification on the dopant, Ni2+, is studied via surface sensitive electronic characterization techniques such as x-ray photoelectron spectroscopy (XPS) and soft x-ray absorption spectroscopy (XAS). The electron density in the valence orbitals of the dopant was observed to be a function of the dipole moment of the para-substituted benzoic acid. The resulting XAS spectra of the Ni2+ after surface modification of TiO2:Ni films were modeled (CTM4XAS) and indicated ligand-dependent symmetry breaking around the Ni2+ at the functionalized interface. Therefore, the modified electron density at the interface due to the polarized molecules is observed to impact the hybridization of the TM dopant energy levels in solid hosts. This phenomenon of adaptive dopant hybridization in a solid host (TiO2) can be exploited to obtain tunable optical responses from TM doped inorganic phosphors, which have an impact in various fields such as luminescent displays, solar cells, sensors, telecommunications, counterfeit technologies and bio-detection.

1. Introduction Metal oxide semiconductor devices are extensively employed in many fields such as photocatalysis,1 photovoltaics,2 and spintronics3 owing to their unique optoelectronic and magnetic transport properties. These properties arise from the manipulation of the hybridization of the metal-oxygen orbitals4-5 by using metal/non-metal doping,6-7 defect creation,8-9 and surface modification,10-11 etc., to tune the physical and chemical properties for selected applications. The result of these above mentioned methods is the formation of interband gap/trap states, which hybridize with the valence (anionic) and conduction (cationic) band orbitals, giving rise to delocalized electron density in those hybrid molecular orbitals. Doping with first row transition


2

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metals (TM) has received a large amount of attention due to the enhancement of the optical (and magnetic) properties,12 while simultaneously introduce lattice defects, as seen when doping with different valent metals.13 The hybridization of TM 3d orbitals with oxygen 2p orbitals is a key parameter controlling the properties in a TM doped solid, analogous to that of rare earth nickelates, wherein the extent of Ni 3d - O 2p hybridization determines the electrical and magnetic transport properties.14 Previous reports focusing on tailoring the hybridization in a crystal are based on the application of strong electric/magnetic fields15-16, mechanical stress17, or crystal composition18-19 to distort local symmetries or crystal field splitting. Strain engineering has been commonly implemented to indirectly control the metal-oxygen hybridization by changing the Ni-O bond distance in Ni-doped SrTiO3 thin films20 and organo-metallic complexes.21 However, these methods are limited due to the constant application of strong fields or the irreversible modification of the composition, which can limit device performance via space charges accumulation or unwanted geometric distortion.22 Therefore, it remains a challenge for the scientific community to modify the hybridization of atomic orbitals in a stable but reversible manner. Reversible tuning of crystal field splitting energy for controlling the TM 3d – O 2p hybridization in a TM-doped solid is possible using weak external fields (surface dipoles).23 While the overall effect of these external fields is limited primarily at the surface (∆o ∝ ܴ ିହ, where R is the metal-ligand distance in octahedral TM complexes)24 they can be reversibly manipulated.25 It is possible to manipulate the interfacial electron density with polarized molecules, potentially modifying the TM dopant 3d orbitals/p-d hybridization to control the electronic properties of a film. This response has been previously used to tune the open circuit


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voltage/short circuit current in photovoltaic devices by systematically controlling the recombination kinetics and charge injection.23 To study the relationship between TM-O hybridization in the presence of surface dipoles, Nidoped TiO2 films were chosen for their chemical stability and strong optical response. Ni was incorporated into a TiO2 thin film by spin coating a dilute sol-gel solution and annealing. The thin films were characterized using high-resolution transmission electron microscope (HRTEM), with elemental electron loss spectroscopy (EELS) chemical mapping, X-ray Diffraction (XRD), and UV-Vis spectroscopy to determine the surface composition, bulk crystal structure, and crystal field splitting energy of the solid. The surface of the TiO2:Ni films was functionalized with para-substituted benzoic acid ligands which can modify the dipole moment over 8 D. The influence of these external chemical fields on the electronic structure of the interfacial dopants is probed via surface sensitive electronic characterizations such as X-ray Photoelectron Spectroscopy (XPS) and soft X-ray Absorption Spectroscopy (XAS). The relationship between the dopant electron density in the valence 3d orbitals and the electronegativity of the benzoic acid

substituent

has

been

studied.

The

characterization

results

suggest

that

the

covalency/hybridization of the dopant (Ni) – oxygen (O) bond increases for electronegative substituents and vice versa for electropositive substituents. 2. Experimental Section 2.1. Materials: Titanium (IV) Isopropoxide (TTIP, Acros Organics, >98%), nickel (II) chloride hexahydrate (NiCl2.6H2O, BTC, > 99%), hydrochloric acid (HCl, 36-38.5% purity, ACS grade), p-nitrobenzoic acid (NO2-BZA, Alfa Aesar, 99%), p-aminobenzoic acid (NH2BZA, VWR Chemicals), reagent alcohol (99.8%) were obtained commercially. All the materials were used without further purification.


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2.2. TiO2:Ni Film Deposition: The sol required for coating TiO2:Ni film was synthesized by employing sol-gel chemistry.26 NiCl2·6H2O was dissolved in 5 mL of ethanol and then 1.5 mL of TTIP was added dropwise to form homogeneous TiO2:Ni sol after 3-4 h of continuous stirring. 125 µL of HCl was used as a catalyst in this process to control the rapid hydrolysis of TTIP precursor. The concentration of Ni precursor to TTIP was varied from 0 to 15 mol%. The prepared sol was aged for 24 h before spin coating. The sol was diluted (1:2, v/v) with ethanol prior to spin coating onto Si(100) substrates at 3000 rpm for 60 s. The spin-coated samples were dried at 100 ºC for 5 min with subsequent annealing at 450 ºC for 2.5 h under low vacuum (~100 mtorr). 2.3. Surface Functionalization of TiO2:Ni Film: The surface of the inorganic film was modified with benzoic acid (BZA) ligands via carboxylic acid chemistry,23 wherein the carboxylic groups chemisorb onto the hydrophilic surface. Two para-substituted BZA groups were chosen to act as an electron withdrawing group (NO2, µ = 3.8 D) and an electron donating group (NH2, µ = -4.5 D). The TiO2 films were immersed in 1 mM acid solution in acetonitrile. After 2-3 h, the samples were rinsed with ethanol and isopropanol before drying in air. 2.4. Structural and Optical Characterization: The thickness of the film was measured using a Filmetrics (F3-UV) reflectometer tool. A standard Si(100) wafer was used as a reference to account for the native oxide layer. HRTEM images were obtained (sensitive to light elements) using the 200 kV JEOL-ARM electron microscope equipped with double aberration correctors, a dual-energy-loss spectrometer, and a cold FEG source. Scanning EELS spectra were obtained with a convergence semi-angle of 20 mrad, and a collection semi-angle of 88 mrad. Dual EELS mode was used to remove the intrinsic energy shifts of the electron beam introduced in the EELS scanning process. The EELS spectra were background subtracted with a power-law function, and


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multiple scattering was removed by a Fourier deconvolution method. The elemental maps were determined from Ni-LII,III, Ti LII,III, O K, and Si L edges. The crystal structure was identified by performing Gracing Incidence (GI)-XRD using PANalytical X-ray diffractometer operating at 45 kV and 40 mA. The θ-2θ radial scan was performed over the range 15-70º with a step size of 0.04º and dwell time of 60 s, using Cu Kα1 (λ=1.54 Å) as radiation source. The absorption spectra of TiO2:Ni was recorded using a Perkin-Elmer Lambda 900 UV/Vis/NIR spectrometer equipped with an integrating sphere and a center-mounted sample holder. The absorption scans ranging from 300-1300 nm with a scan rate of 0.5 nm/s were obtained on the thin films deposited on glass substrates before annealing. The change in monochromators was set to occur at 900 nm. Fourier-transform infrared (FTIR) spectroscopy was performed on the surface functionalized TiO2 thin films using an ATR Germanium crystal in a Thermo Scientific Nicolet 6700 FTIR equipped with an MCT detector cooled to liquid N2 temperatures. The data was collected in absorbance (log (1/R)) mode, with air as background, and resolution being 4 cm-1 in the region going from 400-1400 cm-1. The incident angle of the laser was kept at 50º to collect the total internally reflected light. 2.5. Electronic Characterization: The oxidation states of the TiO2:Ni film were determined from XPS measurements performed using Scienta Omicron ESCA 2SR XPS system. A monochromatic Al Kα1 X-ray source and a hemispherical analyzer with a 128 channel detector were used for all samples. The pressure inside the chamber was maintained at 1.5 × 10-9 torr. The XPS spectra were calibrated to adventitious C 1s peak at 284.6 eV. The step size of these measurements was 0.05 eV and the inherent Gaussian width of the source was 0.167 eV. All peaks were fit (using CasaXPS software27) to symmetric Voigt line shapes that were 70% Gaussian and 30% Lorentzian product functions.


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X-ray Absorption Near Edge Spectroscopy (XANES) measurements were taken at two beamlines: High Energy X-ray Absorption Spectroscopy (HEXAS: 5-30 keV) beamline on a 11pole, 7.5 T multi-pole wiggler and Vacuum Line Spacing-Plane Grating Monochromatic (VLSPGM: 0.2-1 keV) at the Center for Advanced Microstructures and Devices (CAMD). Ni K edge in TiO2:Ni (15 mol%) thin film was measured in the HEXAS beam line in fluorescence mode of detection. The L edges of Ti, Ni and K edge of O were measured in the VLSPGM beam line with photon energy resolution of about 0.1 eV. The data was collected in total electron yield (TEY) mode with the sampling depth being less than 10 nm.28 The samples are loaded onto a stage before transferring them to the vacuum chamber via load lock. The pressure inside the sample chamber was maintained around 2×10-9 torr. The vertical slit width used for these low energy XAS measurements was 100 µm for Ti and O; and 50 µm for Ni L edge to enhance the resolution. The spectra reported is obtained after averaging the data from multiple scans. TiO2 and NiO powders were used as reference for calibration purposes. XANES data is normalized and analyzed using Athena software. 3. Results and Discussion TiO2 films were spin coated onto Si substrates from an aged solution produced via sol-gel chemistry. The thickness was controlled based on spin speeds and number of coatings with the minimum thickness of a pure TiO2 film measured to be 40 ± 5 nm (Table S1, Supporting Information). Film thickness with Ni dopants was expected to be similar based on the processing but could not be quantified due to the formation of a non-conformal film as observed via AFM (Figure S1, Supporting Information). To verify the Ni incorporation, UV-Vis absorption spectra was collected for pure TiO2 and TiO2:Ni sol deposited on glass substrates (Thickness ~500 nm) before annealing. Characteristic Ni2+ absorption peaks were identified, as shown in Figure 1(a),29


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and assigned to their respective electronic transitions between the t2g and eg levels according to theory for d8 electronic configuration in octahedral symmetry 30-33 (Figure S2, and inset of Figure 1(a)). An extracted 10 Dq value of 1.10 eV was obtained from the UV-Vis spectrum (Table S2), which is less than pure TiO2 (10 Dq = 1.8 eV) 34 and is attributed to the amorphous nature of the film. Figure 1(b) shows the GI-XRD pattern of the pure TiO2 film annealed at 450 °C with all peaks indexed to anatase TiO2 (JCPDS# 12-1272). In TiO2:Ni film (Figure S3), the crystal structure was observed to be amorphous, which was attributed to the inability to measure the non-conformal film, instead probing the native SiO2 layer. Therefore, in order to probe the local crystal structure and the distribution of Ni dopants in the TiO2:Ni film after annealing, aberration-corrected HRTEM imaging was performed in conjunction with EELS chemical mapping. Bright field HRTEM images (Figure 1(c)) indicate the presence of two phases which were identified as TiO2 and an oxide phase formed by Ni and Ti (yellow outline). Lattice spacings of 3.54 Å and 1.91 Å were identified for (101) and (020) planes of anatase TiO2 (Figure S4).35 Furthermore, the slight expansion observed in the lattice of TiO2 is attributed to the larger Ni ion incorporated as a substitutional dopant.36 The yellow outlined region is the Ni dense phase in the TiO2 film. The lattice fringe spacing in this region was extracted as 2.12 Å, which matches to the (002) plane of NiTiO3.37 Although, the formation of NiTiO3 phase is not favored at temperatures below 600 °C,38-40 the lower solubility of the dopant (Ni) and higher annealing temperatures (450 ºC) resulted in the agglomeration and growth of the NiTiO3 clusters in the TiO2 network.38 The EELS compositional mapping (Figure 1(d-f)) shows the chemical maps for Ni, Ti, and O elements in the highlighted region (Ni dense) of Figure 1(c). The elemental maps also indicate the formation of a mixed phase of Ni, Ti, and O and sparsely distributed Ni in TiO2 film. Furthermore, the electronic characterizations such as XPS and XAS (discussed below)


8

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suggest the presence of NiTiO3 clusters. In either case of TiO2:Ni or NiTiO3, the local symmetry of Ni remains unchanged, i.e., Ni dopant ions are bonded to six O ions in an octahedral (Oh) symmetry (Figures S5 and S6).


9


(a)

(b) 3

TiO2:Ni (15 mol%)

T1g(3P)

TiO2

1

3

TiO2

T2g T1g(3F) 1 Eg

Anatase TiO2 (JCPDS# 12-1272)

3

3

Intensity (a.u)

Eg - 3A2g 1

A2g

3

T2g - 3A2g

Ni

T1g - 3A2g

3

T2g - 3A2g

T2g

1

Absorbance (a.u)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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400

(c)

600

800

1000

1200

λ (nm)

25

30

(d)

35

40

45

50

55

60

2θ (degrees)

Ni

(f)

(e)

Ti

O

Figure 1. TiO2:Ni (15 mol%) thin films imaged and quantified using (a) UV-Vis absorption spectra, inset shows the Oh coordination of Ni2+ and electronic transitions according to theory (b) GI-XRD pattern of TiO2 thin film is indexed to anatase TiO2 (JCPDS# 12-1272) (c) Bright field HRTEM image of the TiO2:Ni films showing TiO2 and Ni dense regions (highlighted in yellow), and (d-f) EELS chemical mapping of Ni, Ti, and O elements, demonstrating the high and low concentration Ni phases. The scale bar in Figures 1(c-f) is 5 nm.


10

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In order to investigate the Ni 3d-O 2p hybridization in the TiO2:Ni film, surface sensitive XPS and XAS were performed. Due to the nature of these methods, the resulting response is a statistical representation of the film and not an individual atom, describing the overall chemical and physical properties of the film. The survey scans (Figure 2(a)), Ti 2p, O 1s, and Ni 2p detailed spectra for the pure TiO2 and TiO2:Ni are shown in Figure 2. The O 1s XPS spectra (Figure 2(b)) is comprised of a strong lattice oxygen peak (O1, Ti-O-TM)39 along with the shoulder peak (O2). The positive shift in O1, is attributed to the presence of a higher electronegative dopant (Ni2+) in the TiO2 lattice41, suggesting the formation of a -Ni-O-Ti- bond. The area under the shoulder peak (O2), ascribed to the oxygen bonded to under coordinated cations, is observed to increase with Ni doping. When Ni2+ substitutes Ti4+ ions, the charge in the lattice is compensated by the creation of oxygen vacancies13, 39 resulting in under coordinated cations as shown in the EELS maps (Figures 1(d-f)). Additionally, this peak can be attributed to surface hydroxyl (-OH) groups bonded to the hydrophilic TiO2:Ni surface.42 Next, Ti 2p detailed spectra (Figure 2(c)) is performed and the characteristic Ti4+ spin-orbital splitting, with energies at 458.8 eV (2p3/2) and 464.4 eV (2p1/2), was observed for both the pure and doped films.43 The broadening of the Ti 2p3/2 main peak in TiO2:Ni films is due to the Ti3+ shoulder, which is again attributed to the charge compensation TiO2:Ni lattice.13 Ni 2p detailed scans (Figure 2(d)) indicate the presence of Ni2+.44 The spin-orbital splitting energy was measured as 17.48 eV, lower than that of NiO (18.4 eV),45 suggesting that the NiO clusters are not present. Furthermore, the peak position of the Ni 2p3/2 main peak at 855.04 eV corresponds to either Ni2+ doped in TiO2 lattice or to NiTiO3.26,

40, 46

The two satellite peaks (6 eV, 9.5 eV) in Ni 2p spectra are

attributed to the screening effects of various core-hole and ligand hole states by the 3d and 4s bands.47 Deconvoluted spectra are shown in Figure S7 and tabulated in Table S3.


11


TiO2:Ni (15 mol%)

TiO2:Ni (15 mol%)

200

Intensity (a.u)

O2

530

528

(d TiO2:Ni (15 mol%)

Ti 2p1/2

Intensity (cps)

TiO2:Ni (15 mol%)

532

470 468 466 464 462 460 458 456

Ni2+ 2p3/2

TiO2

534

Binding Energy (eV)

Ti 2p3/2

(c)

536

0

Ni2+ 2ps,3/2

400

Ni2+ 2p1/2

600

Binding Energy (eV)

Ni2+ 2ps,1/2

800

Ti 3p

C 1s

Ni 2p

O KLL

1000

Ti 2p

TiO2 O 1s

TiO2

-Ti-O-TM- (O1)

(b

Intensity (cps)

(a)

Intensity (cps)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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890 885 880 875 870 865 860 855 850

Binding Energy (eV)

Binding Energy (eV)

Figure 2. X-ray photoelectron spectra for pure TiO2 and TiO2:Ni (15 mol%) thin films (a) survey scans, (b) Ti 2p spectra, (c) O 1s spectra, and (d) Ni 2p spectrum. A shift in intensities from the O1 (lattice oxygen) to the O2 (sub-lattice oxygen) with Ni doping is observed and attributed to oxygen defects for charge compensation.

In order to understand the effect of the external dipole moment on the TM hybridization in a solid host, the as-prepared TiO2:Ni films were functionalized with p-substituted benzoic acid ligands via carboxylic acid chemistry.23 These carboxylic acids bridge to the surface of TiO2 in a bidentate fashion resulting in a dipole moment normal to the surface, changing the electron


12

affinity of the organic (ligand) - inorganic (film) interface.48 The C-H stretches were identified with FTIR (Figure S8) for both the ligand-bonded samples around 2800-3000 cm-1 indicating that the organic group was bound to the surface.49 Furthermore, the NO2 symmetric stretching mode was also observed at 1370 cm-1 for NO2-BZA functionalized sample.50 After surface modification, the Ti 2p spectra was expected to remain unchanged as the spectra is dominated by the bulk of the crystal, whereas the adsorbate dipole moment acts near the

TiO2:Ni (15 mol%)

(Figure 3) of Ni in TiO2:Ni shifted less than

screening of the two-hole state, core hole and

Ni2+ 2ps,3/2

asymmetric 6 eV satellite peak is due to the

NO2-BZA

Ni2+ 2p1/2

NH2-BZA

half an eV, suggesting no change in the oxidation state after ligand bonding. The

Ni2+ 2p3/2

surface.51 The 2p3/2 and 2p1/2 main peaks

Ni2+ 2ps,1/2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Intensity (cps)

Page 13 of 28

3d hole, by the wide 4s band.47 Furthermore, the interatomic wave function mixing of the 890 885 880 875 870 865 860 855 850

Binding Energy (eV)

Ni 3d states and the ligand p states influence the screening of these multiplet effects.47 In the present case of surface modified TiO2:Ni

Figure 3. XPS scans of Ni 2p in TiO2:Ni (15 mol%) film after surface modification with benzoic acid (BZA) ligand (offset).

films, the aromatic surface ligands are expected to delocalize the hole wave functions, i.e, conduction band 3d orbitals of the inorganic layer.52 Despite the slight changes in the 6 eV satellite peak positions of the ligand-bonded TiO2:Ni films, it is hard to decipher the effect of electronegativity of the ligand on the core states of Ni 2p, due to various factors such as instrument resolution, interatomic wave function mixing (Ni 3d – O 2p),53 and atomic multiplet coupling. Ni 2p deconvoluted spectra are shown in Figure S9 and tabulated in Table S4.


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The XPS spectra of Ti 2p and O 1s in TiO2:Ni films indicate that the formation of the new oxide phase NiTiO3 has not affected the structure of TiO2, suggesting that the surface of TiO2:Ni film has octahedrally coordinated Ni2+ and Ti4+ ions. However, there is no strong evidence from XPS that suggests the perturbation of site symmetry or core level energy of these ions upon ligand bonding. Therefore, in order to understand the subtle changes in the electronic and geometric structure of the inorganic film at the hybrid interface, XANES was employed. The O K edge spectra (Figure 4(a)) demonstrates the electronic transitions from O 1s to the derived states of O 2p. The t2g and eg splitting of the Ti 3d states hybridized with O 2p states is evident in the low energy region of TiO2:Ni film, whereas the higher energy spectral features indicated the formation of Ni-O bond. The broadening of these spectral features also indicates the absence of long-range order of the local lattice,54 as observed in XRD. Figure 4(b) shows the TEY spectrum of Ti LIII (2p3/2 – 3d, 460.8 eV) and LII (2p1/2 – 3d, 468.4 eV) edges of pure and TiO2:Ni films. The doublet of the eg band in TiO2 LIII edge is attributed to the non-degenerate ݀௭ మ and ݀௫ మ ି௬ మ , which is a signature of anatase crystal structure.55 This splitting dampens upon Ni doping due to the oxygen vacancies altering the Ti-O bonding environment, causing non-cubic structural distortion in TiO2:Ni film.56-57 Moreover, the onset of the absorption edge for TiO2:Ni films is shifted to lower binding energies due to the presence of Ti3+ ions as observed in Ti 2p XPS of TiO2:Ni (15 mol%) films.58 After surface modification, the O K edge (Figure 4(c)) and Ti LIII/II edge (Figure 4(d)) spectra for TiO2:Ni films indicated a change in the crystal field splitting energy (10 Dq). The XANES spectra of Ti and O in the surface modified TiO2 films did not show any difference with surface functionalization due to the weak penetration of these ligand fields. The energy splitting between the t2g and eg hybridization peaks in the O K edge54 of TiO2:Ni films is proportional to the adsorbate dipole moment of the ligand (Figure S10(a)).59 In


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addition to the spectral differences in the lower energy region, the higher energy spectral features of O K edge, which correspond to the O 2p

Table 1. ∆E of O K edge and FWHM of eg peak in Ti LII edge for surface modified TiO2:Ni (15 mol%) films, indicating the crystal field splitting shifts as a function of the surface dipole moment.

hybridization with the Ti/Ni valence levels also Thin Film

∆E (eV)

FWHM e g (LII Edge)

TiO2 :Ni

2.36

2.11

onset of the second peak in the O 1s – O 2p (4sp)

NH2 -BZA

2.28

2.01

region suffered a shift of about 0.1 eV from the

NO 2 -BZA

2.44

2.35

indicate significant differences with the ligand. The

reference TiO2:Ni (15 mol%), suggesting a change in the hybridization of the O 2p orbitals with the ligand. Moreover, the broadening of these peaks for the NO2-BZA bonded films, indicate the covalent nature of the metal-oxygen bond accompanied with slight geometric distortion at the hybrid interface.54 In the Ti L edge spectra, while the Ti LIII/II edge peak positions are indicative of the crystal field splitting, de Groot and co-workers showed that the FWHM of the eg peak in the Ti LII edge is also proportional to the 10 Dq.60 The deconvoluted Ti LII edge spectra (Figures S10(b-d)) show a systematic increase in the FWHM of the eg peak with an increasing ligand dipole moment. The ∆E in O K edge peak positions and FWHM of the eg peak in Ti LII edge are tabulated in Table 1.


15


TiO2

TiO2

TiO2:Ni

TiO2:Ni

TiO2 ref

TiO2 ref dz

(c)

540

Energy (eV)

545

550

455

TiO2:Ni NH2-BZA NO2-BZA

535

540

545

LIII (2p3/2-3d)

Norm µ(E)

1s - 2p (3d) 530

t2g

1s - 2p (4sp) 550

465

470

Energy (eV)

NO2-BZA

∆E

eg 460

TiO2:Ni

t2g

eg

t2g

(d

NH2-BZA eg

2

455

Energy (eV)

LII (2p1/2-3d)

535

dx -y 2

t2g 530

2

LIII (2p3/2-3d)

Norm µ(E)

1s - 2p (4sp)

NiO ref eg

t2g

LII (2p1/2-3d)

(b)

1s - 2p (3d)

Norm µ(E)

(a)

Norm µ(E)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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FWHM eg

t2g

eg 460

465

Energy (eV)

470

Figure 4. XAS scans of pure TiO2 and TiO2:Ni (15 mol%) films (a) O K edge, (b) Ti LIII/II edge. The redshift of the Ti LIII/II edge in TiO2:Ni (15 mol%) films suggests the formation of Ti3+ ions. For surface modified TiO2:Ni (15 mol%) films (c) O K edge, (d) Ti LIII/II edge. The change in the O K edge ∆E and the eg Ti LII edge FWHM are proportional to ligand dipole moment. The higher energy 1s-2p region in the O K edge is shifted according to the ligand dipole moment.

To acquire the fingerprint analysis on the 3d electronic states of Ni which are influenced by surface ligands, XANES spectra for Ni K and LIII/II edges in the TiO2:Ni film were collected. The K edge XANES spectra was compared with standard NiO reference powder (Figure S11) to identify the oxidation state of Ni as 2+ in TiO2:Ni film. The Ni LIII/II edge spectra (Figure 5(a))


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for ligand bonded TiO2:Ni films are split into LIII and LII edges due to 2p spin-orbit coupling. The two peaks in LII edge correspond to the 3d states (t2g, eg) of Ni2+ ion bonded to O2- in Oh symmetry.57, 61 A clear shift is observed in the LII edge t2g/eg peak intensities as a function of the adsorbate dipole moment. This difference is attributed to the change in the Ni eg – O p hybridization with the ligands.60, 62 The well-resolved multiplet structure on the LIII and LII edges of NH2-BZA bonded films compared to that of NO2-BZA films suggest the ionic nature of the Ni-bond,63 which is attributed to the weaker metal-oxygen orbital overlap. The LIII/II edge XANES spectra of Ni2+ in TiO2:Ni was simulated using a ligand field dependent simulation software, CTM4XAS.64-65 These structures have been modeled in past with distorted symmetries (D3d/D2d) demonstrating no difference with the Oh symmetry.57 The key parameters involved in these calculations are ligand field parameter (10 Dq), charge transfer energy (∆), Hubbard core-hole potentials (Upp, Upd), slater integrals (Fpp, Fpd), and the hopping parameters ((T(t2g), T(eg)). The values for these parameters were obtained from previous calculations and are summarized in Table S5.64, 66 The value of 10 Dq used in these calculations was obtained from the UV-Vis absorption spectra. Furthermore, it was observed that changing the value of 10 Dq does not simulate the experimentally observed variation in Ni LII edge (t2g/eg) branching ratio. Therefore, to understand the effect of electronegative/electropositive ligand on the hybridization of the Ni2+ 3d states, all the parameters except the hopping parameters were based on literature. The Ni LIII/II edge spectra of NH2-BZA bonded TiO2:Ni film (Figure 5(b)) was modeled using standard hopping parameters (T(eg) = 1.8, T(t2g) = 1) in Oh environment.67 Simulations show that shift in the onset of the absorption edge in NH2-BZA bonded TiO2:Ni films to higher energies is due to the spin exchange interactions. The Coulomb (U) and charge transfer energies (∆) increase for electron donating ligands (NH2-BZA) bonded TiO2:Ni. This is


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attributed to an electron addition to the high-spin Ni2+ 3d8 state requiring additional spin exchange stabilization energy, pushing the leading absorption edge to higher energies.68 On the other hand, for NO2-BZA bonded films, the simulated spectra matched the experimental results by considering strong ligand character in the ground state 3d8L orbitals. This is done by setting the values of the hopping parameters of t2g and eg equal to 1.8, indicating strong mixing between the t2g, eg orbitals.64-65, 67 Furthermore, these non-standard values of hopping parameters suggest geometric distortion at the interface, i.e. the transformation of an octahedron (Figure 5(c)) into a low symmetry structure such as square planar, as shown in Figure 5(d). From an experimental standpoint, the mixing of the t2g and eg energy levels in NO2-BZA bonded films can be attributed to the strong covalent nature of the Ni-O bond. Additionally, the increased intensity of the eg peak in NO2-BZA bonded films corresponds to the reduced electron density in the Ni 3d states. Both of these effects were observed in the O K edge spectra of the NO2-BZA bonded films (Figure 4(c)). However, as the model suggests, the transformation of an octahedron to square planar structure involves distinct structural changes accompanied by oxygen vacancies,69 which was not observed with the other characterization techniques. Therefore, the observed branching ratio in NO2-BZA bonded TiO2:Ni film can be interpreted as a slight breaking of the Oh symmetry due to the elongation along the axial (eg orbital) direction of the Ni atom as shown in Figure 5(e).


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850

(b) TiO2:Ni

NH2-BZA

NH2-BZA

T(eg=1.8, t2g = 1)

855

865

t2g eg

LII(2p1/2 - 3d)

LII (2p1/2-3d)

NiO ref

Norm µ(E)

NO2-BZA

LIII(2p3/2 - 3d)

LIII (2p3/2-3d)

(a)

Norm µ(E)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


NO2-BZA T(t2g=eg=1.8)

870

875

850

855

Energy (eV)

(c)

t2g eg

865

870

875

Energy (eV)

(d)

(e) 2+

Ni2O

Figure 5. (a) Experimental Ni LIII/II edge spectra of surface modified TiO2:Ni (15 mol%) films showing a change in the branching ratio (t2g/eg) in LII edge as a function of the ligand. (b) A comparison between calculated and experimental data for the Ni LIII/II edge spectra using computer program CTM4XAS for Oh/NH2-BZA (top) and D4h/NO2-BZA (bottom) Ni2+ symmetry. (c) Octahedral (Oh) symmetry, (d) Square planar symmetry (D4h), and (e) Distorted octahedral symmetry of Ni2+ are shown to highlight the change in local symmetry. The calculated spectra were shifted by a constant (1 eV) to match the absolute values of the experimental spectra.


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The interplay between the surface dipole, (a)

CB

electronic states, p-d hybridization, and the

Ov

3+

Ti EF

crystal field splitting energy of Ni2+ 3d

Ni 3d

orbitals is illustrated in Figure 6. The electronic structure of TiO2:Ni (Figure 6(a))

VB (b)

shows the formation of interband gap Ni 3d

eg

states, which resulted in lowering of the

t2g

+ µL

Fermi energy.70 The energy of these eg t2g

interband gap Ni 3d states is manipulated by

- µL

the ligand due to hybridization (Figure 6(b)) with the neighboring O atoms resulting in a

(c)

shift in orbitals energy proportional to the crystal field splitting. This ability to tune the local crystal field allows for control of the optical states via the modified hybridization of TM eg orbitals as shown in Figure 6(c). Unfortunately, the shift was not detectable in the UV-Vis measurements due to the weak absorption of the thin, non-conformal films. However,

the

surface

sensitive

XAS

Figure 6. Schematic showing the change in the (a) electronic density of states in TiO2:Ni, (b) energy of the Ni 3d states, and (c) crystal field splitting energy (10 Dq) of the solid with the ligand.

measurements identified the change in the electron density in the valence 3d states of Ni2+ with the ligand. Effectively, the surface dipole manipulates the energy of Ni 3d more than bulk Ti 3d


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orbitals, impacting the electron density in those orbitals. In particular, the eg orbitals, which are pointed towards X, Y, and Z axes60 overlap with the p orbitals of the neighboring O atoms, wherein the strength of the hybridization is determined by the nature of the dipole. As seen with the XAS spectra, the change in the hybridization of the Ni-O bond in TiO2:Ni film is quantified by the shift in the Ni LII edge branching ratio (t2g/eg), FWHM of the eg peak in the Ti LII edge, and the ∆E of the O K edge. This congruency among the valence level spectra of all the elements reinforces the fact that the electronic structure of TM dopant in a solid host is a function of the dipole moment of the surface ligand. 4. Conclusions In summary, TiO2:Ni films were spin-coated on Si (100) substrates using sol-gel chemistry. Initial structural and optical characterization results confirmed the crystal structure and crystal field splitting energy. The crystalline nature of the TiO2:Ni (15 mol%) film was locally determined using HRTEM to identify the presence of TiO2:Ni and NiTiO3 phases. Furthermore, spatial mapping of these films using EELS confirmed those two phases. Surface functionalization of these inorganic films was performed with weak benzoic ligands via carboxylic acid chemistry to apply a weak external field in the form of a dipole moment. The surface dipoles were observed to show no effect on the pure TiO2 films owing to the bulk-like characteristics of the elements present in the film. The influence of the surface dipole on core and valence electronic states of the TM dopant in TiO2:Ni2+ was systematically investigated by surface sensitive characterization techniques such as XPS and XAS. The results from these characterization methods point to the change in the ligand character of the Ni 3d orbitals. It is implicitly proven that the overlap between the Ni 3d orbitals and O 2p orbitals is a function of the dipole strength of the surface ligand. This ability to control the hybridization of TM ion in a


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solid host via weak external fields can be utilized to engineer the optical and magnetic responses in a device. Specifically, the adaptive optical properties of TM doped solids can be coupled with the steady rare earth emissions in inorganic phosphors to obtain dynamic luminescence and thereby, minimize the usage of multiple rare earth doped Red-Green-Blue (RGB) phosphors. Furthermore, these hybrid luminescent materials due to their tunable properties will have potential applications in flexible electronics, biosensors, solar cells5, etc.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: HRTEM images, SAED and FFT patterns, EELS spectra, Crystal field splitting energy calculations, FTIR spectra, Deconvoluted XPS and XAS spectra, and CTM4XAS calculation parameters (PDF). AUTHOR INFORMATION Corresponding Author *

Email: [email protected]

Author Contributions P.D completed the preparation of TiO2 and TiO2:Ni (15 mol%) samples, XRD, UV-Vis, XPS, and XAS analysis. P.D. and O.K. did the XAS measurements. Z.W. performed the HRTEM analysis and EELS chemical mapping. J.A.D. conceived the project of modifying the transition metal hybridization through weak surface ligands and contributed to major points in the article.


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ACKNOWLEDGMENT P.D would like to acknowledge the Louisiana Board of Regents (LEQSF(2016-19)-RD-A-03) for financial support. Discussions with Dr. E.W. Plummer and Dr. William Shelton are gratefully acknowledged. The authors would like to thank Dr. Yuanbing Mao and Dr. Mohammad Saghayezhian for their initial support during the preliminary XPS measurements. We also acknowledge the support of the staff of the CAMD synchrotron light source. REFERENCES 1. Schneider, J.; Matsuoka, M.; Takeuchi, M.; Zhang, J.; Horiuchi, Y.; Anpo, M.; Bahnemann, D. W., Understanding TiO2 Photocatalysis: Mechanisms and Materials. Chem. Rev. 2014, 114, 9919-9986. 2. Bai, Y.; Mora-Seró, I.; De Angelis, F.; Bisquert, J.; Wang, P., Titanium Dioxide Nanomaterials for Photovoltaic Applications. Chem. Rev. 2014, 114, 10095-10130. 3. Rossella, F.; Galinetto, P.; Mozzati, M. C.; Malavasi, L.; Diaz Fernandez, Y.; Drera, G.; Sangaletti, L., TiO2 Thin Films for Spintronics Application: A Raman Study. J. Raman Spectrosc. 2010, 41, 558-565. 4. Minasian, S. G.; Keith, J. M.; Batista, E. R.; Boland, K. S.; Bradley, J. A.; Daly, S. R.; Kozimor, S. A.; Lukens, W. W.; Martin, R. L.; Nordlund, D., Covalency in Metal–Oxygen Multiple Bonds Evaluated Using Oxygen K-Edge Spectroscopy and Electronic Structure Theory. J. Am. Chem. Soc. 2013, 135, 1864-1871. 5. Lorenz, M.; Rao, M. R.; Venkatesan, T.; Fortunato, E.; Barquinha, P.; Branquinho, R.; Salgueiro, D.; Martins, R.; Carlos, E.; Liu, A., The 2016 Oxide Electronic Materials and Oxide Interfaces Roadmap. J. Phys. D: Appl. Phys. 2016, 49, 433001. 6. Peng, B.; Meng, X.; Tang, F.; Ren, X.; Chen, D.; Ren, J., General Synthesis and Optical Properties of Monodisperse Multifunctional Metal-Ion-Doped TiO2 Hollow Particles. J. Phys. Chem. C 2009, 113, 20240-20245. 7. Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y., Visible-Light Photocatalysis in Nitrogen-Doped Titanium Oxides. Science 2001, 293, 269-271. 8. Chen, C. S.; Chen, T. C.; Chen, C. C.; Lai, Y. T.; You, J. H.; Chou, T. M.; Chen, C. H.; Lee, J.-F., Effect of Ti3+ on TiO2-Supported Cu Catalysts Used for Co Oxidation. Langmuir 2012, 28, 9996-10006. 9. Nakamura, I.; Sugihara, S.; Takeuchi, K., Mechanism for NO Photooxidation over the Oxygen-Deficient TiO2 Powder under Visible Light Irradiation. Chem. Lett. 2000, 29, 12761277. 10. Liu, H.; Ma, H.; Li, X.; Li, W.; Wu, M.; Bao, X., The Enhancement of TiO2 Photocatalytic Activity by Hydrogen Thermal Treatment. Chemosphere 2003, 50, 39-46. 11. Shin, J.-Y.; Joo, J. H.; Samuelis, D.; Maier, J., Oxygen-Deficient TiO2− δ Nanoparticles Via Hydrogen Reduction for High Rate Capability Lithium Batteries. Chem. Mater. 2012, 24, 543-551.


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Weak Field Tuning of Transition Metal Dopant Hybridization in Solid

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