Isopropoxy Tetramethyl Dioxaborolane on TiO₂: Reaction Pathway and Formation of a Visible-Light-Sensitive Photocatalyst

Abstract

Borate toxicity is a concern in agriculture since a high level of borates may likely exist in irrigation water systems. In this research, transmission infrared spectroscopy and X-ray photoelectron spectroscopy are employed to study the thermal and photochemical reactions of isopropoxy tetramethyl dioxaborolane (ITDB) on TiO₂, with the aid of density functional theory calculations. In addition, the possibility for the formation of a boron-modified TiO₂ (B/TiO₂) surface, using ITDB as the boron source, is explored and the photocatalytic activity of the B/TiO₂ is tested. After adsorption of ITDB on TiO₂ at 35 °C and heating the surface to a temperature higher than ∼200 °C in a vacuum, the surface is found to be covered with both the organic components of OC(CH₃)₂–C(CH₃)₂O and OCH(CH₃)₂ and the inorganic components of (TiO₂)BO and Ti–B–O. The organic intermediates can be further thermally transformed into pinacolone and acetone; however, the inorganic parts exist at 400 °C, forming a boron-modified surface. The thermal decomposition of ITDB is proposed to be initiated by breaking one B–O bond, forming −OC(CH₃)₂–C(CH₃)₂O–B–OCH(CH₃)₂ on the surface. In the case of photoreaction, the ITDB on TiO₂ decomposes under photoirradiation at 325 nm to form acetone. The boron-modified TiO₂ surface can absorb visible light, likely due to the presence of new states in the band gap, and shows a photocatalytical activity in degrading methylene blue, under 500 nm irradiation in air.

Introduction

Borate esters are boron-containing organic compounds, which have two main classes of B(OR)₃ and B₃O₃(OR)₃. The R can be an alkyl or an aryl group. Borate esters, trimethyl borate [B(OCH₃)₃] for example, have a variety of applications as precursors for boric acid derivatives, as boron sources to manufacture biocides and corrosion inhibitors, as catalysts for the production of resins, or as lubricants to reduce friction and wear.^1,2

Recently, the tribological properties of isopropoxy tetramethyl dioxaborolane (ITDB), with the structure of [B(OCH(CH₃)₂)(OC(CH₃)₂(CH₃)₂CO)] shown in Scheme S1, on copper surfaces have been investigated, which is related to the lubrication of a sliding Cu_(s)–Cu_(s) contact in an electric rotor and to the molecular structure tribological chemistry.^3,4 In addition to the usage for forming tribofilms, trialkyl borates have been employed as a boron source in preparation of boron-doped TiO₂ materials to study their photoactivities, using a sol–gel method or via thermal decomposition over existing TiO₂.⁵⁻⁷ Zhang et al. used Ti(OC₄H₉)₄ and B(OC₄H₉)₃ as the starting compounds to prepare boron-doped TiO₂ by the sol–gel method.⁵ It was found that this photocatalyst could decompose methyl orange under UV irradiation. Zaleska et al. prepared B-doped TiO₂, using Ti(OC₃H₇)₄, B(OC₂H₅)₃, and B(OH)₃, and found its photocatalytic activity in phenol decomposition under visible light irradiation.⁶ Lu’s group deposited B on a nanotube-like TiO₂ film through B(OCH₃)₃ decomposition and showed its activity in photocatalytic degradation of pentachlorophenol, with UV and visible light exposure.⁷

Although borate esters are possible precursors to prepare boron-doped TiO₂, their reactions in the preparation processes on TiO₂, as well as on other metal-oxide surfaces, have not been revealed. Besides, borate toxicity is also a concern in agriculture since a high level of borates may likely be present in irrigation water systems.⁸ Remedies of environmental air and water pollution using TiO₂ to degrade organic contaminants have attracted great attention. In boron-doped TiO₂, theoretical calculations suggest that the boron atom may be present at an interstitial site or as a substituent to an oxygen atom.⁹⁻¹¹ The interstitial boron atoms behave as an electron donor that can reduce the Ti⁴⁺ to Ti³⁺. In the latter case, it creates new states in the band gap and becomes a potential, visible-light-sensitive photocatalyst. In the present research, we investigate the thermal and photochemical chemistry of ITDB on TiO₂, explore its possibility to form boron-modified TiO₂, and evaluate the visible-light-sensitive photocatalytic activity of the modified surface. It is found that boron atoms are left on the TiO₂ surface upon thermal reaction of ITDB at 400 °C under a vacuum, forming a B-modified TiO₂ surface, which induces photocatalytic degradation of methylene blue deposited in the irradiation of the light at 500 nm.

Results and Discussion

We have calculated the structure of ITDB and its vibrational frequencies because they are unavailable in the literature and help in the identification of the surface species after adsorption of this molecule on TiO₂. Scheme 1 shows the calculated ITDB structure, with selected bond angles and lengths. It is found that the ∠OBO in the ring (113.8°) is less than another two ∠OBO angles (121.2 and 125.0°), implying a possible strain present in the 5-membered ring. However, the two B–O bond lengths in the ring (1.388 and 1.393 Å) are elongated by ∼0.025 Å as compared to that of the other B–O bond (1.365 Å). These structure parameters suggest that ITDB on TiO₂ is more likely to decompose through B–O bond breakage in the ring due to the ring strain and weaker B–O bond strength.

Scheme 1. Calculated ITDB Structure and Selected Bond Angles and Lengths.

Table 1 shows the major calculated vibration frequencies (≥1000 cm^–1) of a single ITDB molecule. From this calculation showing coupling of vibrational modes, it can be roughly summarized that the δ_as(BO₃) contributes to the absorptions in the range of ∼1320–1480 cm^–1, ρ(CH₃) to ∼1010–1250 cm^–1, δ(CH) to ∼1320–1480 cm^–1, δ_s(CH₃) to ∼1360–1380 cm^–1, δ_as(CH₃) to ∼1440–1480 cm^–1, ν(CC) to ∼1130–1380 cm^–1, ν(CO) to ∼1110–1210 cm^–1, ν(CH) to ∼2960–3050 cm^–1, ν_s(CH₃) to ∼2960–2980 cm^–1, and ν_as(CH₃) to ∼3050–3085 cm^–1.

Table 1. Vibrational Frequencies (cm^–1) of an Isolated ITDB Molecule and Its Dissociative Form of OC(CH₃)₂–C(CH₃)₂O–B–OCH(CH₃)₂ on TiO₂.

		dissociateda		ITDB/SiO2c	ITDB/TiO2c
ITDBa	modea,b	a ITDB/TiO2(101)	modea,b	35 °C	35 °C	200 °C
1019	ρ(CH3), ν(CO)	1030	ρ(CH3), νs(BO3), ν(CO)		1027	1026
1113	ν(CO), ρ(CH3)	1105	ρ(CH3), νs(BO3), νas(CCO)
		1126	ρ(CH3)		1124	1124
1133	νas(CCC), ρ(CH3)	1133	ρ(CH3), νs(BO3), ν(CO)
1145	ν(CO), ρ(CH3)	1151	ρ(CH3), νas(COB), ν(CC)		1151	1151
1159	ρ(CH3)	1162	νs(BO3), ρ(CH3), ν(CO)
		1183	ρ(CH3), ν(CC), ν(BO)
1206	ν(CC), ν(CO), ρ(CH3)	1217	δ(CH), νas(BO3)
1250	νR(CC), ρ(CH3)	1244	νas(BO3), δ(CH)		1255	1244
1323	δ(CH), νas(BO3)	1320	δ(CH), νas(BO3)	1321
1333	νas(BO3), δ(CH)	1340	δs(CH3), νas(CCC), νas(BO3)
1339	δ(CH), δs(CH3)	1345	δs(CH3), νas(CCC), νas(BO3)
		1356	δs(CH3), νs(CCC), νas(BO3)
		1360	δs(CH3), νs(CCC), νas(BO3)
1368	δs(CH3), νas(CCC), δ(CH)	1371	δs(CH3), νs(CCC), νas(BO3)			1375
1378	δs(CH3), νs(CCC), νas(OBO)
1380	δs(CH3), νs(CCC), δ(CH)			1386
		1408	δas(CH3), ν(CC)
		1424	δas(CH3)		1395	1398
1440	νas(BO3), δas(CH3), δ(CH)	1439	δas(CH3)
1442	δas(CH3), νas(BO3)
1445	νas(BO3), δas(CH3)
1447	νas(BO3), δas(CH3), δ(CH)	1447	δas(CH3)			1444
		1455	δas(CH3)		1450
1463	δas(CH3), νas(BO3)	1463	δas(CH3)	1456
1467	δas(CH3), νas(BO3)	1471	δas(CH3)
1479	δas(CH3), νas(BO3), δ(CH)			1477	1477	1475
					2875	2873
2961	νs(CH3), ν(CH)	3001	νs(CH3), ν(CH)	2940	2936	2939
2974	νs(CH3), ν(CH)	3005	νs(CH3), ν(CH)	2980	2975	2975
2981	νs(CH3)	3016	νs(CH3)
3053	νas(CH3), ν(CH)	3020	νs(CH3)
3062	νas(CH3)	3022	νs(CH3)
3068	νas(CH3)	3027	νs(CH3)
3071	νas(CH3)	3081	νas(CH3)
3085	νas(CH3)	3082	νas(CH3)
		|
		3139	νas(CH3)

^aCalculations of this work. Only the prominent peaks with frequencies ≥ 1000 cm^–1 are listed.

^bν_as: antisymmetric stretching; ν_s: symmetric stretching; δ_as: antisymmetric bending; δ_s: symmetric bending; ρ: rocking.

^cExperimental results of this work.

Adsorption of ITDB on TiO₂ and SiO₂

Three experimental infrared spectra of TiO₂ after adsorption of ITDB are shown in Figure 1. The labeled 0.1 Torr (Figure 1a) denotes the vapor pressure of ITDB introduced into the infrared cell for adsorption on TiO₂ at 35 °C, followed by cell evacuation. The infrared bands in the range of 1000–1600 cm^–1 overlap considerably. Figure 1b,c shows the spectra obtained after further introducing 0.1 and 0.2 Torr (0.2 and 0.4 Torr in total) of ITDB molecules into the cell. From 0.1 to 0.2 Torr, the increasing band intensities, without accompanying obvious peak shifts in frequency, indicate the increase of surface coverage. However, the bands are not proportionally enhanced. The peak intensities of 1151, 1386, and 2975 cm^–1 increase by ∼2.45, 2.45, and 1.9 times, respectively. This result suggests that adsorption of ITDB on TiO₂ at 35 °C may not generate just one surface species; for example, they can be ITDB itself and its dissociative form through B–O bond breakage. From 0.2 to 0.4 Torr, the absorptions in the range of 2800–3000 cm^–1 are similar, in terms of the peak positions and intensities. However, a very different absorption behavior is observed between 1000 and 1550 cm^–1. In addition to the increase of overall absorbance, changes of relative peak intensities and slight peak shifts are found. For example, the 1386 cm^–1 peak is shifted to 1395 cm^–1, together with the largely increased intensity at 1151 cm^–1. From 0.2 to 0.4 Torr, the relative amounts of surface species can vary, resulting in the quite similar absorptions between 2800 and 3000 cm^–1 (CH_x stretching region) in contrast to the changes in the range 1000–1550 cm^–1. These can be explained if the surface species have a similar absorption behavior or if one of the surface species has relatively very small absorptions in the CH_x stretching region. In the range of 3400–3800 cm^–1 in Figure 1, the presence of negative peaks indicates that the adsorbed ITDB can interact with the surface isolated OH groups, presumably via hydrogen bonding of Ti–OH···O–B.

Figure 1.

Infrared spectra of TiO₂ obtained after successively introducing ITDB vapor [from 0.1 to 0.4 Torr in total, (a–c)] into the IR cell for adsorption at 35 °C, followed by cell evacuation.

Figure 2 shows three infrared spectra of SiO₂ (35 °C) after adsorption of ITDB in different amounts. The SiO₂ used is much less reactive than the TiO₂.^12,13 The major bands appear at 1321, 1386, 1456, 1477, and 2984 cm^–1. The absorptions of ITDB below ∼1300 cm^–1 have been cut off due to the absorptions of SiO₂ itself. In contrast to the case of TiO₂ (Figure 1), the absorption frequencies of ITDB on SiO₂ are basically the same, without changing with the dosing pressures. Moreover, all of the peak intensities increase by the same ratio, indicating that only one surface species exists from 0.2 to 0.6 Torr. It is proposed that ITDB is molecularly adsorbed on SiO₂ at 35 °C. Note that, in Figure 2, the intensities of 1386 and 2984 cm^–1 are much smaller that of 1456 cm^–1. However, in Figure 1a,b, the absorptions at 1386 and 2975 cm^–1 are stronger than or comparable to that of 1458 cm^–1, suggesting the possible decomposition of ITDB on TiO₂. The observed infrared bands of ITDB/SiO₂ are listed in Table 1 and can be assigned as follows: 1321 cm^–1 to ν(CH) + ν_as(BO₃); 1386 cm^–1 to δ_s(CH₃) + ν_s(CCC) + ν_as(OBO); 1456 cm^–1 to δ_as(CH₃) + ν_as(BO₃); and 2984 cm^–1 to ν(CH_x). For comparison, the observed infrared absorptions of ITDB/TiO₂ at 35 °C (0.4 Torr, Figure 1) are also included in Table 1.

Figure 2.

Infrared spectra of SiO₂ obtained after successively introducing ITDB vapor into the IR cell for adsorption at 35 °C, followed by cell evacuation. The spectra in the range of 2800–4000 cm^–1 have been multiplied by a factor of 3.

Adsorption of 2-Propanol, Pinacol, Pinacolone, and Boric Acid on TiO₂

To explore the reaction pathways of ITDB on TiO₂, we have investigated the adsorption of 2-propanol and pinacol on TiO₂, which contain the organic parts of OCH(CH₃)₂ and OC(CH₃)₂C(CH₃)₂O of ITDB, respectively, attempting to identify the reaction intermediates generated from the ITDB decomposition. Moreover, adsorption of boric acid (B(OH)₃) on TiO₂ has been studied as well, to reveal the destiny of the inorganic BO₃ in the ITDB reaction. Figure 3a shows the infrared spectra of 2-propanol adsorbed on TiO₂ at 35 °C, followed by heating the surface to 200 °C under a vacuum. In general, aliphatic alcohols adsorbed on TiO₂ at room temperature exist both in intact, molecular form and dissociative form of alkoxys.^14,15 The adsorbed alcohols can desorb or further deprotonate, leaving only alkoxys on the surface at higher temperatures. The 200 °C spectrum of Figure 3a is due to adsorbed 2-propoxy, with a relatively strong C–O stretching vibration at 1134 cm^–1. Figure 3b shows the temperature-dependent infrared spectra of TiO₂ after adsorption of pinacol and water. Note that the pinacol molecules were co-deposited with water by spraying an aqueous pinacol solution onto the TiO₂ catalyst at room temperature. In the 35 °C spectrum, the main infrared absorptions appear at 1132, 1155, 1374, 1467, 1560, 1621, 2938, and 2977 cm^–1. Except for the 1560 and 1621 cm^–1, which could be attributed to adsorbed carbonate (from background CO or CO₂ adsorption) and water, respectively, the other peaks are found to agree with the gaseous pinacol infrared absorptions reported previously.¹⁶ For the adsorbed pinacol, the peaks in the range of 2915–2992 cm^–1 are due to CH₃ stretching modes and the peaks from 1365 to 1467 cm^–1 are due to CH₃ bending modes. The strongest 1155 cm^–1 peak and the shoulder of 1132 cm^–1 are related to C–O and C–C–C stretching vibrations.¹⁶ The intensity ratio of 2977/1155 cm^–1 (I₂₉₇₇/I₁₁₅₅) is ∼0.50. This ratio increases at higher temperatures and can be used to strongly support the formation of pinacolone (explained later). As shown in Figure 3b, the carbonate (1560 cm^–1) and water (1621 cm^–1) peaks largely decrease or disappear, as the surface temperature is increased to 200 °C under a vacuum. A small, broad band appears at 1675 cm^–1, assignable to a carbonyl stretching vibration. Upon heating to 250 °C, the 1675 cm^–1 peak grows considerably, at the sacrifice of the ∼1150 cm^–1 intensity. The intensity ratio of 2977/1150 cm^–1 becomes ∼0.70. These infrared changes with temperature indicate the thermal decomposition of pinacol, forming a C = O-containing species on the surface.

Figure 3.

Temperature-dependent infrared spectra of (a) 2-propanol, (b) pinacol, and (c) pinacolone on TiO₂. 2-Propanol and pinacolone were dosed at 35 °C, followed by heating the surface to the indicated temperatures under vacuum. Pinacol and water were coadsorbed on TiO₂. All of the spectra were recorded at 35 °C in a vacuum.

Figure 3c shows the infrared spectrum of pinacolone adsorbed on TiO₂ at 35 °C, followed by surface heating to 150 °C. Because both pinacol and pinacolone possess several CH₃ groups, they have significantly overlapping peaks near 1150, 1360, 1470, and 2975 cm^–1. However, the adsorbed pinacolone has the strongest, characteristic C=O stretching peak at 1680 cm^–1 and a much smaller C–O stretching peak at 1145 cm^–1. The former peak matches the observed 1675 cm^–1 peak in the 250 °C spectrum of Figure 3b, signifying the formation of pinacolone from the pinacol thermal reaction on TiO₂. For the adsorbed pinacolone (Figure 3c), the intensity ratio of 2975/1680 cm^–1 (I₂₉₇₅/I₁₆₈₀) is ∼0.50. At 250 °C of the pinacol/TiO₂ system (Figure 3b), the surface is covered with pinacolone and residual pinacol, showing the relatively strong peaks at 1675 and 1150 cm^–1, respectively. Both the surface species contribute to the CH₃ stretching absorption at 2977 cm^–1 in the 250 °C spectrum. In other words, the 2977 cm^–1 intensity at 250 °C can be roughly estimated from the 1675 cm^–1 intensity of pinacolone and the 1150 cm^–1 intensity of pinacol, with the known intensity ratios of I₂₉₇₅/I₁₆₈₀ = 0.5 for pinacolone and I₂₉₇₇/I₁₁₅₅ = 0.5 for pinacol. It is found that the estimated 2977 cm^–1 intensity is close to the measured intensity, within a 5% difference. This result further supports the formation of pinacolone from pinacol thermal decomposition on TiO₂.

The chemical transformation of pinacol to pinacolone in the presence of strong acids, H₂SO₄, for example, is called pinacol–pinacolone rearrangement. This organic reaction is initiated by the protonation of one of the OH groups in a pinacol molecule. A carbenium ion [HO(CH₃)₂CC⁺(CH₃)₂] is thus formed after the detachment of a H₂O molecule. The migration of a methyl group and dissociation of the O–H bond for the carbenium ion would generate a pinacolone molecule. This rearrangement reaction can also proceed on acidic solid catalysts.¹⁷ On TiO₂, the surface Ti⁴⁺ ions can play an acid role and promote the C–OH bond dissociation of pinacol and generation of a carbenium-like ion on the surface, eventually resulting in pinacolone formation.

Boric acid [B(OH)₃] adsorption on hydrous ferric oxide has been carried out in aqueous solutions at pH 10.4 and investigated with ATR-FTIR spectroscopy.⁸ This previous study reported three broad bands of 1150 (shoulder), 1250, and 1350 cm^–1, which were assigned to the BOH bending and B–O stretching modes of an inner-sphere trigonal (FeO)₂BOH species. We have studied the adsorption of boric acid on TiO₂, with the infrared result shown in Figure 4. In the 35 °C spectrum, boric acid adsorption causes infrared absorptions near 1326 cm^–1, with the coadsorbed H₂O bending peak at 1624 cm^–1. After heating the surface to 200 °C, two broad absorptions (1211 and 1334 cm^–1) below 1500 cm^–1 are observed, which might be due to tightly bound BO₃ or O₂BOH species.⁸

Figure 4.

Infrared spectra of TiO₂ after adsorption of boric acid and water, followed by heating the surface to 200 °C in a vacuum. The spectra were recorded at the indicated temperatures.

Thermal Decomposition of ITDB on TiO₂

Figure 5a shows the temperature-dependent infrared spectra of TiO₂ after adsorption of ITDB molecules. The ITDB adsorption on TiO₂ (35 °C) was performed by directly introducing 0.4 Torr of the vapor pressure into the cell, followed by cell evacuation. The infrared spectra were recorded after heating the TiO₂ surface to the temperatures indicated in a vacuum. The 35 °C spectrum is similar to that shown in Figure 1c. Upon heating to 100 °C, the absorption at 1398 cm^–1 is relatively enhanced, although no significant changes are observed in the entire spectral range. The peak intensity of 1398 cm^–1, relative to that of 1444 cm^–1, continuously increases, as the surface is further heated to 200 °C. In the 200 °C spectrum, the major absorptions are located at 1026, 1124, 1151, 1244, 1375, 1398, 1444, 1475, 2939, and 2975 cm^–1, which are listed in Table 1. To further analyze the detailed changes from 35 to 200 °C, the 35 °C spectrum has been subtracted from the 200 °C one and the differential spectrum is shown in Figure 5b. The main negative peak at 1480 cm^–1, together with the smaller ones at 1161, 2929, and 2966 cm^–1, can be explained by the decrease of adsorbed ITDB molecules due to dissociation and/or desorption as the TiO₂ is heated from 35 to 200 °C. The positive peaks in Figure 5b, representing the increases in intensity, appear near 1024, 1137, 1217, 1329, and 1400 cm^–1, which can be related to reaction intermediates of ITDB on TiO₂. Continuously heating the surface to 250 °C causes a considerable change in the infrared absorption pattern. The absorptions between 2800 and 3000 cm^–1 decrease largely, together with the disappearance or almost vanishing of the sharp peaks at 1151 and 1398 cm^–1. A new set of peaks at 1026, 1140, 1332, 1381, and 1664 cm^–1 appear in the 250 °C spectrum. The former four peaks seem to sit on broad features. As the surface is further heated to 300 or 400 °C, only three broad absorptions are observed near 1122, 1269, and 1369 cm^–1. To more clearly show the infrared changes from 250 to 300 °C, a difference spectrum has been obtained by subtracting the 250 °C spectrum from the 300 °C one, with a correction factor of 0.9, as shown in Figure 5c, which shows the loss of the peaks at 1026, 1137, 1331, 1383, 1466, 1664, 2873, 2939, and 2976 cm^–1. The two strongest, negative bands at 1137 [ν(CO)] and 2976 [ν(CH_x)] cm^–1 are similar to the observed strong bands in the 200 °C spectra of 2-propanol/TiO₂ (1134 and 2974 cm^–1) and pinacol/TiO₂ (1150 and 2977 cm^–1). This resemblance suggests that the B–O bonds in ITDB have broken on TiO₂ at 250 °C, forming the organic groups of (CH₃)₂CHO– and −OC(CH₃)₂–C(CH₃)₂O– [or HOC(CH₃)₂–C(CH₃)₂O−] directly bonding to the surface. The latter two species can further be transformed into pinacolone on TiO₂, which is responsible for the negative 1664 cm^–1 peak in Figure 5c. The 1331 cm^–1 peak in Figure 5c also strongly suggests the presence of 2-propoxy on TiO₂, as supported by the 200 °C spectrum of Figure 3a. 2-Propoxy can further react on TiO₂ to form acetone.^18,19

Figure 5.

(a) Temperature-dependent infrared spectra of ITDB on TiO₂ and (b, c) two differential spectra. The ITDB was dosed at 35 °C, followed by progressively heating the surface to the indicated temperatures under a vacuum. All of the spectra were recorded at 35 °C. Both the differential spectra have been multiplied by a factor of 3.

As shown in Figure 5a, organic components, such as CH₃, have been removed from the surface at 300 °C. The observed broad features (1122, 1269, and 1369 cm^–1) in the 300 °C spectrum are from inorganic absorptions, suggesting the presence of B–O-containing inorganic species on TiO₂. (TiO)₂BOH is a possible form. However, the existence of a Ti–B–O bonding form cannot be ruled out, which can be generated in the thermal decomposition of ITDB on TiO₂ in a vacuum, by evolving O-containing reaction products, such as acetone and pinacolone.

It has been shown that heating the ITDB/TiO₂ from 35 to 200 °C in a vacuum can cause desorption and/or further decomposition of the boron-containing molecules. The 200 °C infrared spectrum of ITDB/TiO₂ (Figure 5a) is considered to be originated from a reaction intermediate, and its further decomposition forms the organic groups of (CH₃)₂CHO– and −OC(CH₃)₂–C(CH₃)₂O– on TiO₂. It is proposed that this intermediate is generated by breaking one of the B–O bonds of ITDB, to remove the ring strain. We have theoretically obtained the optimized structure of the dissociative ITDB on anatase(101) and its vibrational frequencies, as shown in Scheme 2 and Table 1. In this optimized adsorption geometry, the B atom is chemically attached to a bridging oxygen atom and the O atom is bonded to a 5-fold coordinated titanium atom after the B–O bond dissociation. Moreover, the calculated vibrational frequencies (1000–1500 cm^–1) of the dissociated ITDB agree with the 200 °C spectrum of ITDB/TiO₂ (Figure 5a). On Cu(111), ITDB is suggested to decompose primarily by B–O bond scission, forming two surface intermediates of [−OCH(CH₃)₂] and [−B(OC(CH₃)₂–C(CH₃)₂O)].⁴ The isopropoxy continuously reacts on the surface and generates acetone at 54 °C. The boron-containing intermediate also reacts to form acetone (118 °C) and 2,3-dimethyl-2-butene (91 °C), leaving B and BO_x on the surface, respectively.⁴

Scheme 2. Optimized Dissociative Structure of ITDB on Anatase TiO₂(101).

Photoreaction of ITDB on TiO₂

Figure 6 shows the infrared spectra recorded during the photoirradiation (325 nm) process of ITDB/TiO₂ in the presence of 10 Torr of O₂. Initially, the TiO₂ surface is covered with intact and dissociated ITDB. Along with the photoirradiation, the peaks relating to ITDB decrease in intensity gradually. A new peak at 1693 cm^–1 emerges at around 10 min and increases with photoirradiation time. This peak is attributed to adsorbed acetone. The carbonyl stretching absorption of acetone on TiO₂ has been reported to be at ∼1700 cm^–1.^18,19Figure 7 shows the infrared spectra recorded during the photoirradiation process of ITDB on SiO₂ using the same condition as that of TiO₂. It is found that the peak intensities of ITDB do not change after 120 min photoirradiation. New peaks from photoproducts are not observed either. The SiO₂ result indicates that the ITDB degradation on TiO₂ under the photon exposure is not through a direct photodissociation mechanism but is mediated by TiO₂. Electron–hole pairs can be quickly generated as the TiO₂ absorbs photons with energy higher than the band gap. The photogenerated electrons can be captured by O₂, forming reactive oxygen anions, such as O₂^–. The lifetime of the holes is therefore increased. These reactive species of O₂^– and h⁺ would initiate an oxidation reaction for the organic parts in ITDB, i.e., (CH₃)₂CHO– and/or −OC(CH₃)₂–C(CH₃)₂O–, to form acetone.

Figure 6.

Infrared spectra of ITDB on TiO₂ as a function of photoirradiation time, at 325 nm and 10 Torr of O₂. The spectra in the range of 2050–2500 cm^–1 have been multiplied by a factor of 10.

Figure 7.

Infrared spectra of ITDB on SiO₂ as a function of photoirradiation time, at 325 nm and 10 Torr of O₂.

Characterization and Visible-Light-Sensitive Photoactivity of the Boron-Modified TiO₂ Surface

The boron-modified TiO₂ was prepared by heating an ITDB/TiO₂ surface to 400 °C in a vacuum. The infrared result of Figure 5 suggests the presence of BO₃ and/or Ti–B–O species on the surface. X-ray photoelectron spectroscopy was employed to study the states of B 1s, O 1s, and Ti 2p for the ITDB adsorbed on TiO₂ at 35 °C, followed by heating the surface to 400 °C under a vacuum. The XPS results are shown in Figure 8. The peaks at 458.7 and 464.4 eV are due to Ti⁴⁺ ions from the TiO₂ substrate, with its O 1s at 529.9 eV. No Ti³⁺ state at lower binding energies is observed. The B 1s state appears at 191.7 eV. The B 1s binding energy of B₂O₃ or B(OH)₃ has been reported to be ∼193.0 eV, in contrast to the ∼187.5 eV of TiB₂. Besides, B 1s binding energy in the range of 191.6–192.0 eV has been attributed to Ti–B–O species in the previous boron-doped TiO₂ studies.²⁰⁻²²

Figure 8.

X-ray photoelectron spectra of B 1s, O 1s, and Ti 2p recorded after dosing ITDB onto TiO₂ at 35 °C and subsequent surface heating to 400 °C under a vacuum.

Figure 9 compares the UV–visible diffuse reflectance spectra of the B-modified TiO₂ from ITDB/TiO₂ and the pristine TiO₂ (Degussa P25) itself. For the latter case, it is transparent to visible light due to the band gap of ∼3.2 eV corresponding to ∼400 nm. With the TiO₂ surface modified with boron, the absorptions between 400 and 800 nm are enhanced, showing its potential to be a visible-light-sensitive photocatalyst. Theoretical calculations have pointed out that boron substitution to the oxygen of TiO₂ can create new states in the band gap.⁹⁻¹¹ In the present case, the boron is likely in the forms of Ti–B–O and/or (TiO₂)BO on the B-modified TiO₂ surface. The states in the band gap may be originated from the boron atom and/or from TiO₂ surface defects generated by the boron-containing species. The acceptor states in the band gap of TiO₂ can serve as stepping sites to relay the valence electrons to the conduction band, upon absorbing visible light. In Figure 9, the band gap of P25 is estimated to be 3.05 eV (407 nm), which is approximately the onset of the sharp B/TiO₂ absorption. This result suggests that the surface-modified TiO₂ with B (B/TiO₂) has a similar band-gap energy, as compared to the bare TiO₂ (P25).

Figure 9.

UV–visible diffuse reflectance spectra of (a) P25-TiO₂ and (b) B-modified TiO₂. The onset of the sharp increase in absorbance for both samples is approximately estimated to be 407 nm (3.05 eV).

The photoactivities of the B-modified TiO₂ and TiO₂ (P25) are compared in methylene blue (MB) decoloration in an air–solid system,²³ which could be employed in self-surface-cleaning. Figure 10 shows the spectral changes of MB molecules on the B/TiO₂ and P25 particles as a function of photoirradiation time. The initial absorbances of MB on both TiO₂ samples before photoirradiation are not exactly the same, which are probably due to different MB coverages and/or different surface textures of the measured samples that can affect light scattering of TiO₂. Under illumination at 500 nm, the MB/TiO₂ (P25) absorption is shifted toward lower wavelengths but without inducing a large change in integrated peak intensity between 400 and 800 nm (Figure 10a). The blue shift can be explained by the change in the interaction of MB molecules with P25 particles after the photoillumination. MB molecule itself can absorb visible light and becomes an electronically excited state (MB*), with a different electron distribution from that of the ground state, therefore producing a MB*–TiO₂ interaction not the same as MB–TiO₂ one. Even as the excited MB falls back to the ground state, the interaction of MB with TiO₂ may not completely return to the original situation. The slightly reduced peak area after 150 min photoirradiation reveals that the extent of MB photodegradation at 500 nm is small. As a contrast, the B/TiO₂ causes a larger photocatalytic degradation of MB, as shown in Figure 10b. In addition to a blue shift, the absorbance of MB is continuously reduced with photoillumination time. The visible wavelength absorption of the B/TiO₂ could generate electron–hole pairs due to new states present in the band gap, resulting in the formation of OH^• and O₂^–. OH^•, O₂^–, and h⁺ are active species and can react with MB, leading to its decomposition. Figure 10c shows the normalized MB integrated peak intensities in the range of 400–800 nm as a function of photoirradiation time for the B/TiO₂ and TiO₂ (P25). The initial peak area in each case is scaled to 1.0. The MB on B/TiO₂ decreases continuously with irradiation time, down to ∼40% after 150 min.

Figure 10.

(a, b) Change of the absorbance of methylene blue on the P25-TiO₂ and B/TiO₂ with photoirradiation (500 nm) time. (c) Normalized peak areas (400–800 nm) as a function of photoirradiation time.

Summary

As shown in Scheme 3, ITDB can be dissociatively adsorbed in –OC(CH₃)₂–C(CH₃)₂O–B–OCH(CH₃)₂ form on TiO₂, by breaking one of the B–O bonds. Continuous decomposition of this surface species at a temperature higher than ∼200 °C would generate the organic moieties of OC(CH₃)₂–C(CH₃)₂O and OCH(CH₃)₂ directly bonding on the TiO₂ and the inorganic components of (TiO₂)BO and Ti–B–O. The organic groups can further decompose, forming pinacolone and acetone. The boron-modified TiO₂ surface (B/TiO₂) absorbs visible light, possibly due to the formation of new states in the band gap, and can photocatalytically, at 500 nm, degrade methylene blue on TiO₂ in air. Besides, acetone is found to be the reaction product in the photoirradiation (325 nm) of ITDB/TiO₂ in O₂.

Scheme 3. Proposed Surface Reaction of ITDB on TiO₂.

Experimental and Computational Methods

The TiO₂ powder (Degussa P25, ∼50 m² g^–1) containing mixed phases of anatase (70%) and rutile (30%) was supported on a tungsten fine mesh for infrared measurements. The powder was first uniformly dispersed in a water/acetone solution and then sprayed onto a tungsten mesh (1.5 × 2.2 cm²). After that, the TiO₂/W was mounted inside the IR cell. The W mesh was fully covered with TiO₂ of approximately 0.05 g. The detailed TiO₂/W preparation and the IR cell structure have been reported previously.^24,25 The IR cell had two KBr windows for IR transmission and was connected to a gas manifold and a turbomolecular pump for a base pressure of ∼1.0 × 10^–7 Torr. The TiO₂ sample in the cell was heated to 450 °C for 24 h in a vacuum by passing an electric current through the W mesh. The TiO₂ temperature was measured by a K-type thermocouple spotwelded on the tungsten support. To further remove possible organic contaminants on TiO₂, before each run of the experiment, its temperature was held at 450 °C in a vacuum for 2 h and at 350 °C for 0.5 h in the presence of 3.0 Torr of O₂. After the heating treatment, the cell was evacuated when the TiO₂ temperature was decreased to 70 °C and 10 Torr of O₂ was introduced into the cell to compensate for the possible oxygen loss of the TiO₂ caused by the heating treatment. When the TiO₂ temperature reached 35 °C, the cell was evacuated again for gas or vapor dosing, and an infrared spectrum was recorded as the reference background. The procedure to prepare SiO₂ (Cab-O-Sil)/W was similar to that of TiO₂/W, but the SiO₂ powder was dispersed in H₂O. Isopropoxy tetramethyl dioxaborolane (97%, Sigma-Aldrich), 2-propanol (99%, Sigma-Aldrich), pinacol (98%, Sigma-Aldrich), pinacolone (98%, Sigma-Aldrich), boric acid [B(OH)₃, 99.5%, Sigma-Aldrich], and O₂ (99.998%, Matheson) were used as received without further purification. Each liquid reagent was transferred to a sample tube as quickly as possible to avoid possible contamination. Moreover, the air in the sample tube was removed by three cycles of freeze–pump–thaw with liquid nitrogen. The adsorption of these reagents was performed by introducing their vapors into the IR cell with the TiO₂/W. The reagent vapor was first introduced into a segment (∼60 mL for the volume) of the gas manifold, with a Baratron capacitance manometer to measure the pressure. Then, the cell valve separating the segment and the infrared cell was opened, allowing the compound to be in contact with the TiO₂ surface at ∼35 °C. The volume for the cell was ∼300 mL. Without TiO₂ in the cell, the pressure was assumed to be dropped down to one-sixth after opening the cell valve. The reduced pressure was reported in the experimental results of this study. Because of the insufficient volatility of pinacol and boric acid, their deposition on TiO₂ was carried out differently. First, the high-temperature-treated TiO₂/W sample was removed from the cell after the reference spectrum was recorded. An aqueous pinacol (0.02 M) or boric acid (0.01 M) solution was then sprayed onto the TiO₂/W surface. Afterward, the TiO₂/W sample with coadsorbed pinacol and water (or boric acid and water) was remounted inside the cell, followed by evacuation overnight to partially remove adsorbed water prior to temperature-dependent infrared measurements. Infrared spectra were recorded with a 4 cm^–1 resolution by a Bruker FTIR spectrometer (Vector 22) with an MCT detector. The entire optical path was purged with CO₂-free dry air. The spectra presented here have been ratioed against the TiO₂ background spectrum. In addition, the adsorption state of ITDB on TiO₂ at 35 °C, followed by surface heating to 400 °C in a vacuum, has been analyzed by X-ray photoelectron spectroscopy using the ULVAC-PHI spectrometer (PHI 5000 Versaprobe II), calibrated at the Ag 3d_5/2 peak and with an excitation beam of 1486.7 eV. In the photochemistry study, both the UV and IR beams were 45° to the normal of the TiO₂ sample. The UV light source used was a combination of a 350 W Hg arc lamp (Oriel Corp), a water filter, and a band-pass filter with a bandwidth of ∼100 nm centered at ∼325 nm. The photon power at the position of the TiO₂ sample was ∼240 mW/cm² determined by an optical power meter (Newport 1917R). The boron-modified TiO₂ (B/TiO₂) was obtained by adsorption of ITDB on TiO₂ at 35 °C, followed by heating the surface to 400 °C in a vacuum to remove the organic moieties from the surface. The visible light absorption of B/TiO₂ has been confirmed by UV–visible reflectance using the Hitachi U-3010 spectrophotometer. In the study of methylene blue (MB) photodegradation of the air–B/TiO₂ system, 0.1 g of the B/TiO₂ was mixed with 200 μL of an aqueous MB solution (1.56 mM, 500 ppm). The MB/B/TiO₂ slurry was then kept in dark for 24 h and pressed into an indented circular area (ϕ ≈ 1.0 cm) in a glass plate. The MB coating process on B/TiO₂ was conducted in a dim or dark environment to avoid possible photocatalysis. No color fading was found for the blue MB/B/TiO₂ film by the naked eye after 24 h storage in the dark. The loaded MB/B/TiO₂ was subjected to photoillumination at 500 nm. The blue film gradually became white along with the photon exposure, which was further examined by UV–visible reflectance measurements. Photoirradiation of MB/B/TiO₂ was carried out for 150 min, but the light was turned off every 30 min for the measurements of UV–visible spectra. In terms of the following two reasons, we chose 500 nm as the irradiation wavelength for the photocatalytic degradation of MB on B/TiO₂. The absorption of B/TiO₂ at 500 nm is relatively high and the absorption of MB on B/TiO₂ at 500 nm is relatively low, shown later, that can avoid visible light bleaching of MB.²⁶ The light source was a combination of a mercury lamp, a water filter, and a band-pass filter at 500 nm (Newport, 10BPF25-500). The light power at the MB/B/TiO₂ surface was measured to be ∼28.6 mW/cm².

The structure of an isolated ITDB molecule and the dissociative adsorption structures of ITDB [−OC(CH₃)₂–C(CH₃)₂O–B–OCH(CH₃)₂] on the anatase(101) model surface were calculated in the framework of density functional theory using DMol³ package. In these calculations, the generalized gradient approximation (GGA) with Perdew–Burke–Ernzerhof (PBE) formulation was employed. A double-numeric quality basis set with polarization functional (DNP), a Monkhorst–Pack k-point set at 2 × 3 × 1, and a supercell of 24 [TiO₂] units with dimensions of 7.55 × 10.89 × 21.35 ų were used in this study. All of the TiO₂ slabs were separated by a 12.0 Å vacuum space. The positions of the Ti and O atoms in the first and second layers of the slabs were allowed to be varied in the optimized structure calculations. In addition, the vibrational frequencies of ITDB and its dissociative form on anatase(101) were also calculated. No scaling factor was used for the reported frequencies. The mode assignments were based on the animated molecular vibrations.

Supporting Information Available

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.9b02197.

• Chemical structures of isopropoxy tetramethyl dioxaborolane, pinacol, and pinacolone (Scheme S1) (PDF)

The authors declare no competing financial interest.