Gas-generated thermal oxidation of a coordination cluster for an anion-doped mesoporous metal oxide.

Central in material design of metal oxides is the increase of surface area and control of intrinsic electronic and optical properties, because of potential applications for energy storage, photocatalysis and photovoltaics. Here, we disclose a facile method, inspired by geochemical process, which gives rise to mesoporous anion-doped metal oxides. As a model system, we demonstrate that simple calcination of a multinuclear coordination cluster results in synchronic chemical reactions: thermal oxidation of Ti8O10(4-aminobenzoate)12 and generation of gases including amino-group fragments. The gas generation during the thermal oxidation of Ti8O10(4-aminobenzoate)12 creates mesoporosity in TiO2. Concurrently, nitrogen atoms contained in the gases are doped into TiO2, thus leading to the formation of mesoporous N-doped TiO2. The mesoporous N-doped TiO2 can be easily synthesized by calcination of the multinuclear coordination cluster, but shows better photocatalytic activity than the one prepared by a conventional sol-gel method. Owing to an intrinsic designability of coordination compounds, this facile synthetic will be applicable to a wide range of metal oxides and anion dopants.

Geochemical process coupled with gas generation is of great importance to the evolution of natural porous minerals. The porosity in the minerals is created by evaporation of gas bubbles. The gases comprised mostly of water steam, carbon dioxide but also contains a small amount of hydrogen sulphide, hydrogen fluoride and ammonia1. The anions in those gases react with minerals to be incorporated as anionic partners for metal ions2. Consequently, incorporation of anions and void formations in the minerals simultaneously occur, giving rise to natural porous minerals containing anions such as sulphur, fluorine and nitrogen.

Porous metal oxides represent promising materials for energy storage3, photocatalysis45, and photovoltaics67 because of the large active surface area. By contrast, the control of chemical composition in metal oxides is also vital to these applications. In particular, incorporation of another anion into metal oxides, i.e. anion doping, provides excellent performance in ion-storage8 and photocatalytic reaction910. However, synthesis of porous metal oxides and anion doping have been individually developed. In that context, a crucial challenge in this research field is to coherently integrate these two processes. These considerations inspire us to mimic the geochemical process to establish a facile synthetic method for anion-doped porous metal oxides.

Coordination compounds, wherein metal ions and organic ligands are rationally varied111213141516, are candidates for precursor to apply the gas-generated thermal oxidation. Indeed, coordination compounds are thermally oxidized into metal oxides by calcination17181920. On the other hand, organic molecules are fragmented to generate gases by intense heating2122. In particular, gases containing reactive anions are generated by the fragmentation of organic functional groups, which potentially act as dopant sources. In general, however, the organic ligands of coordination compounds are removed by heating before reaching temperatures where metal oxides are formed. Because of the temperature gap, a calcination of coordination compounds gives metal oxides even without anion doping.

Our strategy to overcome the problem is to improve thermal stability of organic ligands by robust coordination bonding23 of carboxylates with a multinuclear metal cluster. As a model system, we design a multinuclear titanium coordination cluster comprised of a carboxylate ligand with a pendant amino-group. The carboxylate ligand is anchored by coordination bonding with the multinuclear titanium cluster until formation of metal oxides. Therefore, fragmentation of amino-group overlaps with thermal oxidation of the titanium coordination cluster. Consequently, TiO2 is formed under evaporation of gases containing nitrogen atoms, giving rise to N-doped TiO2242526 with permanent porosity. In other words, the porous N-doped TiO2 can be obtained by a simple calcination of the coordination cluster.

Metal oxides doped with anion272829 has attracted much attention because of potential applications of visible-light photocatalyst for water splitting30, pollutant degradation3132 and solar energy conversion3334. Porosity further improves the photocatalytic activity by increasing a surface area and improving the accessibility to catalytic active sites35. The mesoporous metal oxide has been fabricated by elaborate protocols, including templating method3637, or particle assembly3839. Sol-gel method is rather simple to synthesize mesoporous metal oxide, which can be easily combined with anion doping4041. However, synthesis of mesoporous metal oxides via sol-gel method requires precise control of hydrolysis and condensation rates, which would conflict with anion doping approach. From simplicity of the protocol, calcination of coordination clusters will be an attractive strategy to fabricate anion-doped porous metal oxides (Fig. 1). Notably, coordination compounds can be rationally designed by a judicious choice of metal ions and organic ligands42. Therefore, the strategy presented here will be applicable to other types of metal oxides and anion dopants.

Figure 1

Schematic illustration of gas-generated thermal oxidation of a coordination cluster.

Results

We synthesized a titanium coordination cluster with 4-amino benzoic acid. A solvothermal reaction of titanium isopropoxide and 4-amino benzoic acid in acetonitrile gave cuboid crystals with a size of several hundred μm. The resulting compound of Ti8O10(4-aminobenzoate)12 (1) consists of Ti8O10 cluster, where octanuclear titanium is linked by ten μ2-oxo bridges. The carboxyl groups of twelve 4-aminobenzoate further bridge each titanium to each of its neighbouring titanium in a bidentate fashion (Fig. 2a–c).

Figure 2

Crystal structures of titanium coordination clusters.

Crystal structures of 1 and 2: (a) coordination geometry of 1. (b) view of 1 along a axis and (c) c axis. (d) coordination geometry of 2. (e) view of 2 along b axis and (f) c axis. The hydrogen atoms and solvent molecules are omitted for clarity. Each atoms of Ti, oxygen and carbon are coloured by blue, red and white. Ti is shown as a cation centred octahedral geometry.

As a reference, another titanium coordination cluster without amino-group, Ti8O8(benzoate)1643 (2), was synthesized by a solvothermal reaction of titanium isopropoxide and benzoic acid. The compound (2) consists of Ti8O8 ring-shaped cluster, where octanuclear titanium is linked by eight μ2-oxo bridges. The carboxyl groups of sixteen benzoate binds to titanium in a bidentate fashion from the axial and equatorial positions (Fig. 2d–f). Eight equatorial benzoate point up and down alternatively from the plane of Ti8O8 ring cluster, whereas the eight other axial benzoate point up and down perpendicularly.

Dozens of crystals of 1 and 2 were calcined at 480 °C in air for 3 hours (heating rate: 8 °C/min). The tiny crystalline particles with the size of 5 μm were obtained by calcination of 1 and 2 (Figure S1). X-ray diffraction (XRD) pattern of calcined 1 and 2 corresponded to anatase TiO2, suggesting that 1 and 2 were converted into TiO2 (denoted as TiO2-(1) and TiO2-(2), respectively) (Fig. 3a).

Figure 3

Spectroscopic characterization of TiO2-(1) and TiO2-(2).

(a) PXRD of (i) TiO2-(1), (ii) TiO2-(2), (iii) Simulated TiO2, (iv) 1, (v) 2, (vi) simulated 1 and (vii) simulated 2. (b–c) XPS spectra of TiO2-(1) for Ti2p and N1s with fitting curves (red). The whole spectra is shown in supplementary information. (d) Photograph of TiO2-(1) and TiO2-(2), (e) UV-vis absorption spectra of TiO2-(1) (red) and TiO2-(2) (black).

X-ray photon spectroscopy (XPS) was carried out to clarify the incorporation of nitrogen atoms in TiO2. A broad XPS peak of N1s was observed in TiO2-(1) but not in TiO2-(2), suggesting that nitrogen in TiO2-(1) is originating from the amino group of 4-aminobenzoate (Figure S2). The binding energy of N1s (398 eV) corresponded to anionic N− in Ti-O-N which is in the range typically observed for substitutional nitrogen doping into TiO2444546. Furthermore, the binding energies of Ti2p1/2 (464 eV) and Ti2p3/2 (459 eV) well matched with those of Ti in N-doped TiO247 (Fig. 3b,c). As shown in Figure S4, Raman spectra of TiO2-(1) and TiO2-(2) showed the characteristic blue shift of Eg(1) band by nitrogen doping (139.6 cm−1 for TiO2-(2) and 144.0 cm−1 for TiO2-(1))48. These results suggested that nitrogen originating from amino group was incorporated into TiO2 as a dopant, giving rise to N-doped TiO2. The nitrogen concentration in TiO2-(1) was estimated as 0.96%.

The resulting TiO2-(1) is yellow because of the nitrogen doping, whereas non-doped TiO2, including TiO2-(2), is white (Fig. 3d). As expected, TiO2-(1) showed absorption in the visible-light region (400–500 nm), but TiO2-(2) absorbs only light in ultraviolet (UV) region (Fig. 3e). This is because nitrogen doping into TiO2 created a new energy level (N2p level) above the valence band maximum. The new absorption band in 400–450 nm corresponds to the energy gap between conductance band and N2p level (2.7 eV). These results suggest that TiO2-(1) is able to work as photocatalyst under visible light.

Besides the nitrogen dope into TiO2, the porosity of TiO2-(1) and TiO2-(2) was evaluated by N2 adsorption (Figure S5a). The adsorption/desorption hysteresis was observed for TiO2-(1) and TiO2-(2) in the relative pressure (P/P0) range of 0.4–0.9. This characteristic hysteresis is attributed to the mesopores of TiO2. The gradual adsorption in the hysteresis region, classified as H2 type adsorption, suggested mesopores with ununiform size and shape. The pore-size distribution, based on the desorption branch of the isotherm, was estimated by Barret, Joyner, and Halender (BJH) method, assuming a cylindrical pore model. The pore sizes of TiO2-(1) and TiO2-(2) were calculated to be around 4 nm (Figure S5b). The mesopores of TiO2-(1) were also observed by TEM (Figure S6). BET surfaces of TiO2-(1) and TiO2-(2) were estimated as 170.6 m2/g and 139.8 m2/g, which were relatively large compared to those of metal oxides prepared by calcination of coordination compounds4950.

The series of measurements indicated that simple calcination of the coordination cluster allows the synthesis of mesoporous N-doped TiO2. To investigate the formation mechanism of mesoporous N-doped TiO2, variable-temperature XRD (VT-XRD) and thermogravimetry with differential thermal analysis (TG-DTA) were carried out. As seen in VT-XRD, 1 was decomposed and the formation of TiO2 began over 400 °C (Fig. 4a). This result of VT-XRD was well matched with that of TG-DTA. TG-DTA showed the weight loss over 250 °C because of evaporation of acetonitrile. Note that the exothermic peak was observed in DTA over 350 °C (Fig. 4b). The exothermic peak is ascribed to the oxidation of titanium coordination clusters to form TiO2. The results of VT-XRD and TG-DTA suggested that TiO2 began to be crystallized over 350–400 °C.

Figure 4

Time course experiments on calcination of the coordination cluster.

(a) PXRD of 1 at variable temperature 20–480 °C. (b) TG analysis showing weight loss of 1 upon heating (black solid). DTA analysis showing exothermal peak around after 350 °C (black dots). (c) Q-MS analysis with heating of 1: aniline (m/z: 95, blue), benzene (m/z: 79, red), CO2 (m/z: 44, black) and HNO3 (m/z: 63, green) were observed. Black dot line shows temperature of the sample cell. The yellow back ground indicates the temperature region for the formation of TiO2.

Quadrupol mass spectroscopy (Q-MS) of 1 under heating further gave the insight into gas generation and mechanism of nitrogen doping. As seen in Fig. 4c, the gases of benzene, aniline, HNO3 and CO2 were generated in the temperature region of 300–480 °C, suggesting the decomposition of 4-aminobenzoate. The organic ligand was decomposed to generate gases concurrently with the formation of TiO2. In other words, TiO2 was crystalized during the generation of gases.

The decomposition of 4-aminobenzoate into benzene indicates that the covalent bond between the amino-group and phenyl ring was cleaved to generate the fragments containing nitrogen atoms (N-fragment). The generation of N-fragment was also confirmed by the detection of HNO3. HNO3 was most likely formed by the oxidation of N-fragments. The rest of N-fragments reacted with TiO2 and nitrogen atoms were incorporated into TiO2 as a dopant.

This synchronic reaction was also observed in 2. 2 was decomposed to begin the formation of TiO2 over 400 °C, which was characterized by VT-XRD and DT-XRD (Figure S7 and Figure S8). Q-MS measurement of 2 showed that benzoate was decomposed into gases of benzene and CO2 in 300–480 °C. Gas generation and formation of TiO2 were overlapped in the temperature range of 350–480 °C (Figure S9). Nitrogen was not doped into TiO2-(2) because of no nitrogen source (amino group) in the starting material of 2. However, gas generation during the formation of TiO2 also resulted in the formation of mesoporous TiO2 (Figure S10).

Based on VT-XRD, TG-DTA, and Q-MS, we propose following the reaction mechanism of nitrogen doping. Ti8O10(4-aminobenzoate)12 was decomposed to form TiO2 over 350 °C. 4-aminobenzoate of 1 was decomposed into the gases of aniline, benzene, CO2 and N-fragments. Nitrogen atoms in N-fragments reacted with TiO2 to be incorporated into TiO2 as a dopant, forming N-doped TiO2 (Fig. 5(i) molecular scale). The gases, including CO2, benzene, were generated concurrently with the formation of TiO2. Thus, gas evaporation during the formation of TiO2 created internal voids, leading to the formation of mesoporous N-doped TiO2 (Fig. 5(ii) mesoscale). As mentioned above, the surface area of TiO2-(1) and TiO2-(2) are larger than the metal oxides synthesized by calcination of extended coordination frameworks50. This is because the gas generation synchronized with formation of TiO2 created mesopores and significantly increased the surface area.

Figure 5

Schematic illustration of the reaction mechanism.

Reaction scheme at (i) molecular scale and (ii) mesoscale: Coordination cluster of 1 is converted to mesoporous N-doped TiO2.

To evaluate the advantage of the new synthetic method, we synthesized mesoporous N-doped TiO2 by a sol-gel method as a reference (TiO2-sg)4041. Isopropanol solution of titanium isopropoxide was mixed with aqueous solution of urea and nitric acid to prepare precursor sol. The resulting sol was calcined to synthesize mesoporous N-doped TiO2. The nitrogen originating from urea was doped into TiO2. The mesoporosity and BET surface were evaluated by N2 adsorption (Figure S10). The mesoporosity is attributed to the interparticle voids as described in previous literatures41. As shown in Table S1, The BET surfaces of TiO2-(1) and TiO2-(2) were more than twice as large as that of TiO2-sg (TiO2-(1): 170.6 m2/g, TiO2-(2): 139.8 m2/g, TiO2-sg: 59.24 m2/g). The crystallinity of TiO2-(1) and TiO2-(2) is nearly same as TiO2-sg (crystallite size; TiO2-(1): 13.4 nm, TiO2-(2): 15.8 nm, TiO2-sg: 16.4 nm) (Figure S11 and Table S2). However, the concentration of nitrogen in TiO2-sg was slightly higher than TiO2-(1) (Figure S12-13).

We evaluated the visible-light photocatalytic activity of TiO2-(1), TiO2-(2) and TiO2-sg by degradation of methylene blue (MB)5152. The crystals of TiO2-(1), TiO2-(2) or TiO2-sg were placed in a solution of MB and vigorously stirred under visible-light irradiation (> 410 nm). The absorption intensity of MB decreased over time, showing the photocatalytic activity of TiO2-(1) and TiO2-sg for the degradation of MB (Fig. 6 and Figure S14). The decrease rate of TiO2-(2) and no catalysts were nearly same, suggesting that the intensity decrease of MB was attributed to not the adsorption of MB on TiO2 particles but the photocatalytic decomposition of MB. Although the nitrogen concentration of TiO2-sg was higher than TiO2-(1), TiO2-(1) decomposed MB much faster than TiO2-sg. MB was completely decomposed by TiO2-(1) in 150 min, while only a half amount of MB was decomposed by TiO2-sg. Since nitrogen concentration of TiO2-(1) is lower than TiO2-sg, the rapid degradation of MB is most likely attributed to the large surface area of TiO2-(1). The mesoporous N-doped TiO2 can be easily prepared by calcination of the coordination cluster, but shows better photocatalytic activity than the one synthesized by a conventional sol-gel method.

Figure 6

Photocatalytic activity of N-doped TiO2.

(a) UV-visible spectroscopic changes of methylene blue solution over TiO2-(1), (b) chronological change of adoption intensity upon various photocatalysts under visible-light (>410 nm) irradiation: no catalyst (circle), TiO2-(1) (black dot), TiO2-(2) (black square) and TiO2-sg (black triangle).

Conclusion

In this contribution, we demonstrate a facile method for the synthesis of mesoporous anion-doped metal oxides. As a model system, we synthesized a multinuclear titanium coordination cluster with a pendant amino-group. A simple calcination of the coordination cluster resulted in synchronic reactions: thermal oxidation of the coordination cluster into TiO2 and gas generation including N-fragments. The gas generation during the formation of TiO2 allows the introduction of mesopores. Furthermore, nitrogen atoms in N-fragments reacted with TiO2 to be incorporated as nitrogen dopant, thus leading to the formation of mesoporous N-doped TiO2. The resulting mesoporous N-doped TiO2 showed photocatalytic activity under visible light better than TiO2 prepared by a conventional sol-gel method, because of its larger surface area.

Notably, coordination clusters can be rationally designed by a choice of metal ions and organic ligands. Besides, doping amount can be potentially controlled by optimizing calcination conditions of coordination clusters (Figure S15). The synthetic and calcination protocols of the coordination clusters do not require specialized instruments. Therefore, coordination clusters as precursors will be a promising method for anion-doped porous metal oxides, which will offer significant benefits for the fabrication of light emitting diodes, ion storage batteries and heterogeneous catalysts.

Methods

Synthesis of Ti8O10(4-aminobenzoate)12

A mixture of titanium(IV) isopropoxide (5.1 × 10−2 mL, 1.72 × 10−1 mmol) and benzoic acid (284 mg, 2.33 mmol) was suspended in acetonitrile (3 mL) and heated in a teflon-lined steel bomb at 100 °C for 1 day. The resulting crystals of Ti8O10(4-amino benzoate)12 (1) were harvested by centrifuge and washed with acetonitrile three times.

Synthesis of Ti8O8(benzoate)16

A mixture of titanium(IV) isopropoxide (2.55 × 10−2 mL, 0.86 × 10−1 mmol) and benzoic acid (142 mg, 1.66 mmol) was suspended in acetonitrile (3 mL) and heated in a teflon-lined steel bomb at 100 °C for 1 day. The resulting crystals of Ti8O8(benzoate)16 (2) were harvested by centrifuge and washed with acetonitrile three times.

Calcination of 1 and 2

Crystals of 1 or 2 are placed in an Al2O3 boat (Sansho, SAB-995). The crystals are heated up to 480 °C and kept at the temperature for 3 hours.

Synthesis of N-doped TiO2 by Sol-Gel Method

N-doped TiO2 was prepare by a reported protocol with slight modifications4041. Titanium(IV) isopropoxide (5.94 × 10−1 mL, 2.0 mmol) was added to 10 ml of isopropanol. Subsequently, urea (120 mg, 2.0 mmol) and nitric acid (25 μl) were mixed with deionized water (0.36 mL). The solution of urea was dropped into the solution of titanium(IV) isopropoxide under stirring. The resulting sol was dried at 70 °C and calcined at 400 °C in air for 4 hours.

Photocatalytic Activity Test

TiO2 (3 mg) was added to a quartz cell with 3 ml of MB solution (20 ppm). A halogen lamp (SX-UI502M, USHIO SPAX INC.) was used as the light source. 400 nm cut-off filter was placed in front of the reactor.

X-ray Photon Spectroscopy (XPS)

Dried powders of TiO2-(1) and TiO2-(2) were placed on a carbon conductive tape to avoid the powders from swirling in the air. XPS data were collected by JEOL Ltd. JPS-9200.

N2 Gas Adsorption

N2 adsorption measurements were carried out by Quantachrome Autosorb 6AG. The BET surface area was determined by the multipoint BET method using the adsorption branch in the relative pressure (P/P0) range of 0.05–0.3. The pore-size distribution was estimated by applying Barret, Joyner, and Halender (BJH) method to the desorption branch of the isotherms.

Powder X-ray Diffraction (XRD)

PXRD data were collected by Bruker D8 Advance ECO. Scherrer equation is applied to 110 diffraction of anatase TiO2 to estimate the average size of crystallite for TiO2-(1), TiO2-(2) and TiO2-sg. The instrumental broadening estimated by a standard sample (Al2O3) is 0.042.

Single Crystal X-ray Diffraction

Single-crystal XRD data collection (5° < 2θ < 55°) was conducted at 223 K on Rigaku ACR-7R diffractometer Mo-Kα radiation (λ = 0.7105 Å) with Rigaku Mercury CCD system. The structures were solved by a direct method (SHELXS) and expanded using Fourier techniques. All calculations were performed using Yadokari-XG. Crystal data for 1: C44H24N7O17Ti4, monoclinic, space group P21/n (no. 14), a = 12.430(5) Å, b = 24.443(9), c = 16.163(6) Å, β = 93.367(6), V = 4902.27 Å3, Z = 4, T = 223 K, ρcalcd = 1.510 gcm−3, μ(Mo-Kα) = 0.706 cm−1; R1 = 0.0957, wR2 = 0.1812, GOF = 1.055. The hydrogen are severely disorder. (CCDC: 1406003).

Quadrupole Mass Spectrometer (Q-MS)

The mass spectra of gases were collected by ULVAC APS-001 under heating of titanium coordination clusters (1) and (2).

Other Apparatus

SEM images were collected by Phenom ProX. UV-vis absorption was measured by JASCO V-570. TEM image was collected by JEM-2100.

Additional Information

How to cite this article: Hirai, K. et al. Gas-generated thermal oxidation of a coordination cluster for an anion-doped mesoporous metal oxide. Sci. Rep. 5, 18468; doi: 10.1038/srep18468 (2015).