Boron-Doped TiO₂ from Anodization of TiB₂ for Efficient Photocatalysis

A PAS TiB₂ monolith (TiB₂ > 98.4%, O < 0.8%, C < 0.5% and Fe < 0.1% by weight, Longjitetao New Materials, China) with a 95% density was cut into two orthogonal directions: one was long while the other was normal to the hot press direction, and the two samples were noted as S_∥ and S_⊥, respectively. The sample is 10 cm × 10 cm × 1 mm in dimensions, giving an active electrode area of 1 cm². The samples were ground using sandpapers down to 1000 grit, cleaned using deionized water, and dried under flowing air for further use. Silver wires were attached to the back surfaces of TiB₂ before they were sealed in epoxy resin for the anodization and electrochemical measurement.

75 ml aqueous solution containing 0.1 M Na₂SO₄ (99.9%, Keshi, China) and 0.1 M NH₄F (99.99%, Macklin, China) or NaF (99.9%, Keshi, China) was used as the electrolyte and a Ti⁰ foil (99.99%, Goodfellow, UK) as the counter electrode to avoid any contamination from heterogeneous cations. The anodization was performed using a DC power source (M8811, Maynuo power source, China) with a cutting voltage and current of 30 V and 100 mA, respectively. After the anodization, S_∥ with a coating on the surface was cleaned using deionized water and annealed at 600 °C for 1 h in the ambient air to increase the crystallinity.²⁶ The thickness of the scale was measured using an ellipsometer (Sopra SE 200 Ellipsometer), while the soluble element after the anodization is measured with an inductively coupled plasma-optical emission spectrometer (ICP-OES, Angilent 5110, USA). The suspension containing the anodic product from S_⊥ was transported to a 100 ml PTFE lined autoclave and then treated hydrothermally at 180 °C for 10 h to obtain a precipitated sample that was washed and dried at 80 °C in an oven.

Electrochemistry including cyclic voltammetry (CV) and electrochemical impedance (EIS) was performed on an Admiral Squidstat Plus workstation using a conventional three-electrode system containing an Ag/AgCl reference and Au counter electrode. The scanning rate of CV is 25 mV S^−1, and the frequency range of the 10 mV sine wave of the EIS is 1 MHz to 0.1 Hz. A 250 W Xe lamp (HDL-II, Bobei, China, 1 mW cm⁻²) with a filter to remove the photons under 400 nm and longer than 800 nm was used as the light source for the PEC cell and dye degradation.²⁷ An additional filter was used to remove photons between 400 and 420 nm to evaluate the effect of photon energy on the dye degradation. 100 ml aqueous solution of RhB (10 μM) was mixed with 100 mg powered catalysts from S_⊥ via sonication and the mixture was transported to a water-cooling reactor for the illumination. The absorbance at 586 nm was used to determine the concentration of the dye left behind.¹⁰

Optical absorption data of PEC electrode and sample powders were collected on a UV–vis spectrophotometer coupled with an integrating sphere (UV-1800, MACY, China). The reference non-absorbing material is BaSO₄. Optical microscopy (OM) and scanning electron microscopy (SEM) were performed on an Aosvi metallurgical microscope and a field emission Hitachi S-4800 microscope, respectively. X-ray diffractions (XRDs) of the electrode or powers were measured on a diffractometer (Persee, XD-3, China) with a monochromated Cu K_α target (λ = 1.5406 Å) in a reflective mode. The anisotropy degree (an absolute value)²⁸ and the lotgering orientation factor²⁹ were calculated by XRD analysis to evaluate the crystallographic orientation. X-ray photoelectron spectroscopy (XPS) data were obtained on a ESCALAB 250Xi spectrometer (Thermofischer Scientific, USA) using monochromatic Al Kα X-rays (1486 eV). All binding energies were adjusted according to the energy shift of the adventitious carbon C 1s peak at 284.6 eV.

Orientation and morphology

TiB₂ contains metal-metal bonding inside the (100) plane and Ti-B covalent bonding along the 〈001〉 direction, and thus the crystal orientation would be important to the physical properties.^24,30 The morphology of TiB₂ crystals has a significant effect on their physical and chemical properties but is rather difficult to be controlled for specific applications.³¹ The PAS TiB₂ in this study was pressed in two orthogonal directions and the sample cut along the pressing direction, S_∥, was found to show extraordinarily high peak intensity of (100) plane from the XRD (Fig. 2). On the other hand, the surface of the sample cut along the free sintering direction, S_⊥, was dominated by (001) facet. The orientation of the TiB₂ can be validated by the OM images, where darker areas were found in the S_⊥ because the ionic bonding in (001) direction would cause the brittleness³² and enhance the roughness of the surface for the dark area, the crystallographic orientation degree along [001] direction (A_[001]) was calculated for the S_⊥ and S_∥ as an absolute value to be 10.13 and 0.84, respectively, and the lotgering orientation factor along [001] direction (f_[001]) was 0.697 and 0.072 for the S_⊥ and S_∥,respectively, which evidence more grains orientation forms for S_⊥ in [001] direction.

Zoom In Zoom Out Reset image size

Corrosion electrochemistry

As a PEC anode, the corrosion resistance under working conditions is important for durability. For the Ti (Fig. 3), S_∥ and S_⊥ showed relatively high current densities in the scan from −0.5 to 1.2 V in the first cycle as a result of the oxidation on the surface. From the second cycle to the fifth cycle, the current densities, j, are much lower than the first cycle indicating a passivation region between 0.2–0.8 V vs Ag/AgCl. The addition of NH₄F dramatically changes the corrosion behavior of S_∥ and S_⊥ but has little influence on that of Ti. Specifically, the S_⊥ showed an increased current density in the 2nd ∼ 5th cycles than the first one as a result of the active corrosion. Although S_∥ showed a smaller peak current density than S_⊥, it did not change the corrosion behavior in F⁻ containing solution, which is in accord that both samples can produce a soluble gel after prolonged corrosion times under anodic current. Nonetheless, the rough surface of S_⊥ does not allow the formation of a TiO₂ layer for the preparation of the PEC electrode. TiB₂ was subjected to localized corrosion on the grain boundary in NaCl solution³³ and the fast corrosion of S_⊥ was reasonable in our case as it showed finer crystallite due to the brittles of B–Ti bonds and finer grains.

Zoom In Zoom Out Reset image size

After the CV, EIS (Fig. 4) was used to study the surface property and corrosion behavior of the electrode. The EIS of the S_∥, S_⊥, and Ti showed only one arc in Na₂SO₄ electrolyte. The EIS of S_⊥ was much smaller than those of Ti or S_∥ in a neutral Na₂SO₄ solution. When F⁻ was added to the electrolyte, Ti electrode showed insignificant variation, but the impedance values of S_∥ and S_⊥ were much smaller than those without F⁻, indicating an accelerated corrosion rate induced by the F⁻.^34,35 The EIS of S_∥ with (001) surface showing two semi-arcs could be related to the formation of a porous film on the surface, while S_∥, whose EIS showed only one semi-arc, experienced active corrosion. Therefore, the EIS for samples in electrolyte without and with NH₄F can be fitted with an equivalent circuit graphically represented in Fig. 4, and the fitting parameters are listed in Table I.

Zoom In Zoom Out Reset image size

Table I.  Parameters of the simulated data.

	0.1 M Na2SO4				0.1 M Na2SO4 + NH4F
	Re	Ydl	ndl	Rct	Re	Ydl	ndl	Rct	Yf	nf	Rf
	Ω cm2	Ω−1 Sn cm2		Ω cm2	Ω cm2	Ω−1 Sn cm2		Ω cm2	Ω−1 Sn cm2		Ω cm2
Ti	13.9	1.1 × 10−4	0.77	1.4 × 105	14.0	1.3 × 10−4	0.79	1.0 × 105	—	—	—
S⊥	20.9	3.3 × 10−4	0.76	7.2 × 103	19.5	3.0 × 10−3	0.73	603.0	—	—	—
S∥	12.2	8.4 × 10−5	0.80	3.5 × 104	15.7	4.5 × 10−3	0.72	1.3 × 103	2.2 × 10−3	0.67	50.1

In the model circuit, R_e represents the electrolyte resistance, C_dl the double-layer capacitance, R_ct the charge-transfer resistance, C_f the oxide capacitance, and R_f the oxide resistance. Taking into account the dispersion effect, a constant phase angle element (CPE) Q is used to describe the element C_dl and C_f in the fitting procedure. The Y_dl and n_dl, and Y_f and n_f are the constants representing the element Q_dl and Q_f.^34,35 As the corrosion rate is proportional inversely to R_ct, the comparison of R_ct indicates that the corrosion behavior of Ti⁰ varied little with the addition of F⁻ while the S_⊥ corrodes 10 times faster in F⁻ containing electrolyte than the one without. S_∥ was the only sample that showed an impedance response from the porous oxide film.

PEC electrode from S_∥

TiB₂ is very resistant to high-temperature oxidation in air. The oxidation of TiB₂ at temperatures below 1000 °C will produce a thin layer of TiO₂ covered by B₂O₃,³⁶ but the direct oxidation at 600 °C for 1 h would be insufficient to grow a film for the photocatalysis.^21,36 In our study, XRD (Fig. 5a) of the anodized S_∥ sample after annealing at 600 °C for 1 h showed very distinct peaks of rutile- and anatase-type TiO₂ without B₂O₃, indicating that the oxides are produced during the anodization process. According to the ellipsometry, the thickness of the TiO₂ on S_∥ layer is around 5 μm, which is around 10 times thicker than that on Ti⁰. Thicker TiO₂ (<6 μm) can be produced in ethylene glycol electrolyte with 2 vol% water and NH₄F,³⁷ but the thickness of the TiO₂ layer is subjected to the operation condition of anodization voltage and its sweeping rate, and F⁻ concentration.¹⁵ The retained TiO₂ layer during anodization is dependent on the oxidation rate of the substrate and the dissolution process. Generally, the dissolution rate of the produced TiO₂ decreases with pH near the substrate/TiO₂ interface. The thicker TiO₂ layer on TiB₂ could be attributed to (1) TiB₂ in our case showed much higher anodization rate than Ti⁰ as indicated in the electrochemical investigation and (2) the concentration of NH₄F is 0.1 M, the rate of TiO₂ dissolution on Ti⁰ could be higher than that on TiB₂ as the formation of B₂O₃ in latter could be able to scavenger the H⁺ near the TiB₂/TiO₂ interface.

Zoom In Zoom Out Reset image size

The XPS spectra (Fig. 5b) of the outgrown oxides show an insignificant peak of F1s, but the B1s peak at 191.8 eV clearly indicates the presence of minor interstitial B forming [BO₄] units in TiO₂.⁷ The detailed analysis of Ti2p indicates that a small fraction (3.0%) of Ti³⁺ is produced on the anodic coating on S_∥ after the calcination. The O1s peak at the binding energy of 529.5 eV is unequivocally attributed to the O²⁻ in Ti–O bonds while the one at 531 eV is possible resultant from the weakly adsorbed hydroxyl group in Ti–OH.^38–40 The oxide layer on S_∥ is very rough and porous (Fig. 5c), but the produced TiO₂ crystals in the magnified image are in the size of 100–300 nm and show sharp edges (Fig. 5d). The anodic coating on TiB₂ did not consist of nanotubes, but Zhen et al.⁴¹ prepared orientated TiO₂ pillars on Ti⁰ foil and found that the exposed high-energy reactive facets in rutile boosted the water oxidation reaction by a factor of 2.5.

The TiO₂ layer on TiB₂ showed a light absorbance edge at 408 nm (Fig. 6a) and the broad arc in the wavelength from 400 nm to 1100 nm could be related to the absorption induced by Ti³⁺ and defects as shown in the XPS results.^16,42–44 The light absorbance edge is 26 nm red-shifted comparing with the TiO₂ from Ti⁰ foil [e.g. 382 nm as reported by Chen et al.⁴⁵], which could be related to the B doping. The space charge capacitance, C_sc, of the TiO₂ layer under bias was measured using EIS, and the Mott-Schottky plot (C_sc⁻² vs bias, Fig. 6b) showed a positive slope and an intersection of −0.57 V vs Ag/AgCl with −0.37 V vs SHE, indicating the TiO₂ layer is an n-type semiconductor with a flat band energy, E_fb, of −0.37 V vs SHE.^46–48 The E_fb of the TiO₂ on TiB₂ is close to with the values of TiO₂ nanotubes (−0.23 V vs SHE) from the anodization of Ti.^49,50

Zoom In Zoom Out Reset image size

In accordance with the previous study, Ti⁰ is more corrosion-resistant under an anodic bias than S_∥ in neutral Na₂SO₄ electrolyte, so S_∥ showed a slightly higher current density under dark condition (Fig. 6c). However, the PEC anode from S_∥ showed a much higher current density than the counterpart on Ti⁰ foil if they were illuminated under a 250 W Xe lamp with >400 nm photons. Specifically, PEC anode on S_∥ under a bias of 0.5 V showed a photocurrent of 1.96 mA cm⁻² (Figs. 6c and 6d), while the one on Ti showed only a photocurrent density of 0.10 mA cm⁻². The photocurrent generation with the PEC on S_∥ is stable for at least two dark-light cycles. The high photocurrent from PEC on S_∥ sample could be related to the thick TiO₂ film and B doping to enhance the light absorption or the presence of Ti³⁺ to ease the charge transfer resistance.⁵¹

Anodization of metals has been used for the production of monocrystalline oxides under the ambient condition^52,53 and the dissolution of the metal and precipitation of the ionic species in the electrolyte are generally involved for the growth of oxides.⁵⁴ Under the anodic condition, S_⊥ suffers from active corrosion in F-containing electrolytes without any oxide deposition on the surface. A gel in the electrolyte instead as a result of the dissolution and precipitation of Ti and B species was produced in the electrolyte. The concentrations of soluble B and Ti in the electrolyte are measured to be 13.5 mM l⁻¹ and 1.98 mM l⁻¹, respectively, if the solid content of the gel is filtered out.

In order to study the reaction mechanism, XPS of the sample from the drying and washing of the gel, along with those of Degussa P25, hydrothermally-treated solids from electrolytes containing NaF (T-NaF) and NH₄F (T-NH₄F) (Fig. 7). The solid from the gel after mild drying at 60 °C showed the obvious peak of Ti³⁺ and this could be related to the oxidation of TiB₂ under anodic according to the following reaction:⁵⁵

$$\begin{eqnarray*}&&{{\rm{TiB}}}_{2}+6{{\rm{H}}}_{2}{\rm{O}}\to {{\rm{Ti}}}^{3+}+2{{\rm{H}}}_{3}{{\rm{BO}}}_{3}+6{{\rm{H}}}^{+}+9{e}^{-}\end{eqnarray*}$$

Where nine electrons are involved in the reaction. This reaction is in contrast with the ten-electron reaction for the production of soluble Ti⁴⁺-containing species:

$$\begin{eqnarray*}&&{{\rm{TiB}}}_{2}+6{{\rm{F}}}^{-}+6{{\rm{H}}}_{2}{\rm{O}}\to {{{\rm{TiF}}}_{6}}^{2-}+2{{\rm{H}}}_{3}{{\rm{BO}}}_{3}+6{{\rm{H}}}^{+}+10{e}^{-}\end{eqnarray*}$$

The presence amount of Ti³⁺ is conditional and can reach 40% of the anodic reaction in deaerated conditions.²⁵ However, the Ti³⁺ in the solid gel is estimated to be 4.0% of (Ti³⁺ + Ti⁴⁺) though the fitting of Ti_2p in XPS. The low Ti³⁺ on the surface of gel could be related to the hydrolysis and oxidation in ambient air:

$$\begin{eqnarray*}&&{{\rm{Ti}}}^{3+}+{{\rm{H}}}_{2}{\rm{O}}\to {{\rm{TiO}}}_{2}+2{{\rm{H}}}^{+}+{e}^{-}\end{eqnarray*}$$

While NH₄F was used in the hydrothermally prepared TiO₂ with F-doped TiO₂,⁵⁶ Sun et al.⁵⁷ used boric acid (H₃BO₃), tetrabutyl titanate and ammonium fluoride (NH₄F) as the precursor for the synthesis of B/N co-doped TiO₂. As the electrolyte contains F⁻ and ammonia, the possible F- and N-doping in T-NH₄F was also studied through XPS (Fig. 7). Because the binding energy of F1s of T-NaF and T-NH₄F peaked at 684.0 eV was much lower than that of TiOF₂ (685.3–685.5 eV)⁵⁸ or TiO_2−xF_x (689.0 eV),⁴² therefore the F⁻ could be only assigned to the absorbed F⁻ replacing the –OH groups on the surface. The O1s peak for T-NaF and T-NH₄F was identical to that of Degussa P25 and the formation of oxide-ion vacancies is minor. N1s peaks at 399.7 eV were found on the XPS spectra for the Degussa P25 and T-NaF, and thus they could be related to the adsorbed N species.⁵⁹ Nonetheless, we cannot rule out the trace N doping in T-NH₄F since the core-level XPS of N is affected by the absorbed species. T-NH₄F was the only sample showing a significant peak of B1s at 191.9 eV, corresponding to interstitial B in the tetragonal unit [BO₄].^7,60 The [B]/([B]+[Ti]) on the surface of T-NH₄F was calculated to be 0.5 at% from XPS. The interstitial B doping in the TiO₂ crystal will induce the formation of Ti³⁺ in the oxide lattice:

$$\begin{eqnarray*}&&{\rm{B}}+{{\rm{Ti}}}^{4+}\to 1/{\rm{n}}\,{{\rm{B}}}^{{\rm{n}}+}+{{\rm{Ti}}}^{3+}\end{eqnarray*}$$

Zoom In Zoom Out Reset image size

The anodic product of TiB₂ was able to produce Ti³⁺, while the B was oxidized to be B^IIIO₃³⁻ in the electrolyte. During the hydrothermal treatment, B needs to be reduced slightly if the charge of B is smaller than 3. In preparing the B doped TiO₂, Lambert et al.⁶¹ used TiCl₄ under a dry, O₂-free nitrogen atmosphere and BH₃ in tetrahydrofuran as the reactant to reduce the oxidation of B species or the generated Ti³⁺ species. The reducing agent for B³⁺ in the aqueous phase could be Ti³⁺ and the replacing of NaF with NH₄F would assist the reduction process as the latter can scavenge the residual oxygen in the autoclave and reduce the oxidation of Ti³⁺. The preservation of Ti³⁺ in the autoclave could in the end increase the B incorporation in titania lattice. The B doping in T-NaF was too low to be detected in XPS.

Comparing to the white Degussa P25 whose UV–vis spectra, T-NaF and T-NH₄F showed a light absorption tail stretching to 500 and 600 nm (Fig. 8a), respectively. T-NH₄F showed deeper color and stronger light absorption in the visible light region than T-NaF as a result of higher B doping in the former. According to the XRD, the crystal structure of both samples was found to be anatase-type TiO₂ with a clear decrease in peaks intensity and slight shifting to lower angle suggesting the incorporating of B (Fig. 8b). Anatase is the low-temperature polymorph of TiO₂ in contrast with the mixed rutile and anatase in the PEC anode calcined at 600 °C. Uniform particles within the scale of 20–30 nm were found in T-NaF according to the SEM images (Fig. 8c), but the T-NH₄F sample showed aggregate of finer crystals (10–20 nm in diameter). The B doping in TiO₂ was found to inhibit the crystallinity of TiO₂⁶² and this can be also implied from the wide shoulder peak in the XRD of T-NH₄F. The RhB removal efficiency from T-NH₄F was the highest among the three photocatalysts (Fig. 8d): the removal efficiency of T-NH₄F is 2.64 and 4.0 times higher than that of Degussa P25 under 400–800 nm and 420–800 nm irradiation, respectively.

Zoom In Zoom Out Reset image size

TiB₂ as the most common 2-dimensional transition metal boride was found to be able to produce B-doped TiO₂ coating or nanocrystals for efficient photocatalysis. The use of TiB₂ instead of Ti⁰ was able to vary the anodization kinetics under F-containing electrolyte and a porous film of 5 μm in thickness can be obtained in 2-hour anodization. The TiO₂ PEC on TiB₂ achieved a high photo-current of 1.96 mA cm⁻² under irradiation in between 400 to 800 nm. The fast dissolution of TiB₂ with exposed (001) facet during the anodization process was able to produce a gel containing Ti³⁺, which is a metastable state for the synthesis of oxygen-deficient TiO_2−x.⁶³ Boron-doped TiO₂ after hydrothermal treatment boasted interstitial B that increased the light absorption in the visible light region for efficient RhB degradation. As MBene and Maxene^64,65 comprise a large family of electric conductive ceramics, their anodization would be of great importance to the synthesize of cation/anion doped semiconducting oxides for efficient photocatalysis.

We would like to acknowledge the support from Natural Science Foundation of China (NSFC, 51702264, 41371275), the Fundamental Research Funds for the central universities (XDJK2020B066). Y.T. thanks to the undergraduate innovation project from Chongqing (S20201063517). C.N. also thanks to the support of Bayu Young Scholar from Chongqing.