Skip to main content
NCBI home page
ACS Omega logoLink to ACS Omega
. 2020 Jun 2;5(23):14147–14156. doi: 10.1021/acsomega.0c01827

Hollow Square RodLike Microtubes Composed of Anatase Nanocuboids with Coexposed {100}, {010}, and {001} Facets for Improved Photocatalytic Performance

Yi-en Du †,§,∥,*, Xianjun Niu †, Jing He †, Leng Liu †, Yufang Liu †, Changdong Chen ‡,*, Xiaojing Yang §,*, Qi Feng ∥
PMCID: PMC7301601  PMID: 32566882

Abstract

graphic file with name ao0c01827_0007.jpg

In this study, hollow square rodlike microtubes composed of anatase nanocuboids with coexposed {100}, {010}, and {001} facets were successfully synthesized via a mild hydrothermal treatment method in the presence of NH4F by using layered H2Ti3O7 ribbons as the precursor. The precursor H2Ti3O7 ribbons were prepared from H+/Na+ ion-exchanged Na2Ti3O7. The suspension solution of protonated H2Ti3O7 ribbons was adjusted to desired pH values (0.5–13.0) prior to hydrothermal treatment. The elongated direction of the microtubes is along the b axis, according to the profile of the H2Ti3O7 ribbons. The transformation from staggered [Ti3O7]2– sheets to hollow square rodlike microtubes contained the formation and recombination of the dispersed octahedral [Ti(OH)2(OH2)4]2+ monomers, the formation and growth of the initial anatase nuclei, and the reassembly of the anatase nanocuboids along the b-axis direction during the continuous hydrothermal process. The degradation rate of pH 0.5-TiO2 was the highest at 1.66 × 10–2 min–1, which was 1.3, 1.5, 2.0, 2.3, and 18.4 folds higher than that of pH 3.0-TiO2 (1.27 × 10–2 min–1), pH 7.0-TiO2 (1.11 × 10–2 min–1), pH 5.0-TiO2 (0.83 × 10–2 min–1), P25–TiO2 (0.73 × 10–2 min–1), and the blank sample (0.09 × 10–2 min–1), respectively. Compared with P25–TiO2 and the other anatase TiO2 samples, pH 0.5-TiO2 exhibited the best photocatalytic activity, which was mainly attributed to its larger proportion of {010} (or {100}) facets, smaller crystalline size, higher band gap, and larger specific surface area.

Introduction

Nowadays, water pollution caused by many water-soluble organic dyes (such as methylene blue, methyl orange, and rhodamine B (RB), etc.) is becoming more serious.1,2 However, the traditional sewage treatment methods, such as biodegradation and activated carbon adsorption, are invalid to solve the problem of water-soluble organic dyestuff wastewater.3 It is found that semiconductor photocatalysis can replace the abovementioned traditional methods. The dyes in dyestuff wastewater can be decomposed into small organic molecules and eventually mineralized into H2O, CO2, and other inorganic ions under the irradiation of visible or ultraviolet light because of its high catalytic performance, nonselective oxidation, nontoxicity, low expenses, and secondary pollution.3−6 Therefore, it is very important to design and synthesize novel photocatalysts with high catalytic activity to solve the problem of highly polluted textile wastewater. In a variety of semiconductor photocatalysts, titanium dioxide (TiO2) is considered to be the most widely used catalyst for the photocatalytic degradation of organic pollutants because of its outstanding chemical and thermal stability, great oxidizing power, excellent photochemical activity, harmlessness, and nontoxicity.7−9 Among the four confirmed polymorphs of TiO2 (anatase, rutile, brookite, and TiO2-B), the anatase phase has been widely regarded to exhibit the most photoactive reactivity in photocatalysis applications, which can be attributed to the lower rate of charge recombination and the stronger interaction between the organic molecules and the anatase surfaces.10,11 However, the rapid recombination rate of photogenerated electrons and holes leads to a reduction in quantum efficiency and limits the photocatalytic ability of anatase.10 Therefore, the design and synthesis of the anatase TiO2 nanocrystal with a high photocatalytic performance is the key to reduce the recombination of the photogenerated charge carriers. It is well known that the photocatalytic performance of the semiconductor photocatalyst greatly depends on the crystallinity, particle size, specific area, morphology, and crystal facets.12 Particularly, the morphology and crystal facets of anatase have a significant effect on the enhancement of the photocatalytic performance.13 Hence, the design and synthesis of anatase TiO2 nanocrystals with tailor-made exposed crystal facets and well-defined morphologies is necessary. The surface energies of anatase TiO2 are 0.44, 0.53, 0.90, 1.09, and 1.61 J·m–2 for the {101}, {010} (or {100}), {001}, {110}, and {111} crystal facets, respectively.14,15 However, the crystal facets with higher surface energies usually decrease rapidly to reduce the total surface free energy during the crystal growth process, resulting in the formation of slightly truncated {101}-faceted tetragonal bipyramid.16 Since {010}-faceted single anatase TiO2 nanocrystals with different morphologies were first reported by Wen,17 there have been increasing interest to synthesize anatase TiO2 crystals with exposed high-energy planes or special morphologies. Particularly, using fluorine as a capping agent, truncated tetragonal bipyramid anatase crystals with 47 and 89% {001} facets exposed were prepared by Yang,16,18 which was regarded as a groundbreaking discovery. Following these breakthroughs, more and more synthetic strategies were used to synthesize well-defined anatase TiO2 crystals with high-energy crystal facets. For instance, using HF as a controlling agent, anatase TiO2 single crystals with a high percentage of {001} facets were synthesized.19,20 Without using fluorine and organic capping agents, bipyramid-shaped, nanosheet-shaped, and rodlike anatase TiO2 nanocrystals with about 95% {101}, 91% {001}, and 90% {010} facets exposed were synthesized, respectively.21 Anatase nanorods with dominant {010} facets and porous anatase TiO2 microspheres composed of high-energy {010}-faceted nanobelts were prepared by using H0.68Ti1.83O4 and titanium glycerolate as precursors under hydrothermal conditions, respectively, which exhibited enhanced photovoltaic and photocatalytic performance.22,23 Using layered titanate-delaminated nanosheets or nanoribbons as precursors, {010}-faceted and [111]-faceted anatase TiO2 nanocrystals with various morphologies were prepared.24−30 Using HAc as the solvent and [bmim][BF4] as the capping agent, respectively, wholly coexposed {100}-faceted and {001}-faceted anatase TiO2 nanocuboids were synthesized through the hydrolysis of titanium tetraisopropoxide.31 Hierarchical anatase TiO2 microspheres consisting of 45% {001}-faceted nanodecahedrons were synthesized through a facile hydrothermal technique, which showed superior photoreactivity.32 Truncated tetragonal bipyramid anatase TiO2 nanocrystals with coexposed high-energy {001}, {010}, and [111] facets were also prepared by the hydrothermal treatment of the protonated titanic acid at different pH values.33 Recently, nanotubes/nanowires composed of high-energy {111}-faceted anatase nanoflakes were prepared by the solvothermal method, which showed a high CO2 reduction activity.9 {101}-Faceted anatase TiO2 truncated bipyramid and {001}-faceted TiO2 nanosheet were prepared by a hydrothermal procedure using HF as the capping agent, respectively, and the {001}-faceted anatase TiO2 exhibited higher stability.34 M-{001}TiO2/Ti photoelectrodes with different surface exposure ratios of {001}/{101} facets were assembled, which could improve the photoelectrochemical oxidation performance of DMP wastewater.35

In this work, we report the synthesis of hollow square rodlike microtubes composed of anatase nanocuboids with coexposed {100}, {010}, and {001} facets via a mild hydrothermal treatment method in the presence of NH4F by using layered H2Ti3O7 ribbons as the precursor. The elongated direction of microtubes is along the b axis according to the profile of the H2Ti3O7 ribbons. The transformation mechanism from the staggered [Ti3O7]2– sheets to the hollow square rodlike microtubes was investigated. Compared to P25–TiO2 and the other anatase TiO2 samples, pH 0.5-TiO2 with a larger proportion of {010} (or {100}) facets, smaller crystalline size, higher band gap, and larger specific surface area exhibited the best photocatalytic activity.

Results and Discussion

Structure and Morphology

The precursor, monoclinic Na2Ti3O7, is confirmed by the powder X-ray diffraction (XRD) pattern (JCPDS no. 14-0085, a = 0.8571, b = 0.3804, c = 0.9135 nm, and β = 101.9°), as shown in Figure 1a. After the Na+/H+ ion-exchange reaction, the basal spacing was changed from 0.817 nm [(100) facet] for Na2Ti3O7 to 0.760 nm [(200) facet] for H2Ti3O7 (JCPDS no. 41-0192, a = 1.599, b = 0.3738, c = 0.9172 nm, and β = 101.5°), indicating that the H2Ti3O7 specimen maintains the basic layered framework (Figure 1b). The analyzed compositions of Na1.976H0.024Ti3O7 and Na0.065H1.935Ti3O7 are approximate to the stoichiometric Na2Ti3O7 and H2Ti3O7, respectively (Table 1).

Figure 1.

Figure 1

Open in a new tab

XRD patterns of (a) Na2Ti3O7 and (b) H2Ti3O7 specimens.

Table 1. Chemical Analysis Results.

  content found (calculated)/wt %
  
specimen Na Ti chemical formula
Na2Ti3O7 14.96 (15.09) 46.54 (47.70) Na1.976H0.024Ti3O7
H2Ti3O7 0.54 (0.58) 54.75 (55.44) Na0.065H1.935Ti3O7
Open in a new tab

The chemical analysis also shows that almost all the Na+ ions were replaced after Na+/H+ ion-exchange, which is consistent with the reduction of basal spacing from 0.817 nm of Na2Ti3O7 to 0.760 nm of H2Ti3O7.

The finally obtained specimens through the hydrothermal treatment of the H2Ti3O7 ribbons were denoted as pHx-TiO2, where x corresponds to the desired pH value of the solution (x = 0.5–13.0). For all specimens, only the diffraction peaks of the anatase phase were observed, indicating that the H2Ti3O7 ribbons were completely transformed to the anatase phase at 180 °C, and the structural transformation had nothing to do with the pH value of the solution, as shown in Figure 2. Generally speaking, the pH value in the reaction medium has an important effect on the crystal structure of the resulting TiO2. The strong acidic medium is conductive to the formation of rutile TiO2. With the increase of the pH value, rutile TiO2 disappeared and anatase TiO2 increased.36 However, the crystalline form of TiO2 obtained in the present work is different from those reported with the exfoliated nanosheets as the precursor, usually rutile, or the coexistence of anatase and rutile are obtained at a low pH value.17,37 Compared with the standard card of anatase TiO2 (JCPDS no. 21-1272, tetragonal system, a = b = 0.3785 nm, c = 0.9514 nm), all the diffraction peaks of the obtained anatase products shift slightly to the right as a whole, implying that the crystal plane spacing of the synthesized anatase TiO2 specimens decreases slightly. As the XRD patterns above were measured with almost the same content of TiO2 samples, the intensity of the XRD peaks increased with the increasing pH value of the solution, which was probably caused by the increase of crystallinity and crystal size. The influence of the pH value of the reaction medium on the crystallinity and crystal size of the resulting TiO2 could be attributed to the influence of the H2Ti3O7 precursor. The shrinkage of the interlayer and the reassembly of the H2Ti3O7 precursor, which made it difficult to destroy the structure of H2Ti3O7, resulted in poor crystallinity and smaller crystal size of the resulting TiO2 specimens.38

Figure 2.

Figure 2

Open in a new tab

XRD patterns of (a) pH 0.5-TiO2, (b) pH 1.0-TiO2, (c) pH 3.0- TiO2, (d) pH 5.0-TiO2, (e) pH 7.0-TiO2, (f) pH 9.0-TiO2, (g) pH 11.0-TiO2, and (h) pH 13.0-TiO2 specimens obtained from the H2Ti3O7 square rod.

The morphologies of Na2Ti3O7 and H2Ti3O7 are confirmed by the field emission scanning electron microscopy (FESEM) images, as shown in Figure 3a–d. The FESEM images show the square rodlike morphology of Na2Ti3O7 with ∼1–4 μm in length, ∼0.5–1.5 μm in width, and ∼0.15–0.6 μm in thickness (Figure 3a,b). After the ion-exchange reaction, the obtained H2Ti3O7 specimen retains the morphology of the original Na2Ti3O7 precursor, and the H2Ti3O7 extension direction corresponds to the b-axis direction of the crystal (Figure 3c,d). Figure 3e–t shows the anatase crystals obtained in the hydrothermal system with different pH values, whereas the other synthesis parameters remain unchanged. When the pH is 0.5, many rodlike particles about 160 nm in length and 80 nm in width and many entangled amorphous particles were obtained, as shown in Figure 3e,f. In addition, many hollow secondary square rodlike microtubes with about 1–4 μm in length, 0.5–1.7 μm in width, and 0.5–1.0 μm in thickness were formed by the oriented aggregation of the above particles, and the profiles are very similar to those of the H2Ti3O7 precursor, implying that the long-axis direction of the secondary microparticles is along the b axis. When the pH value is 1.0, in addition to many entangled amorphous particles, some well-faceted anatase TiO2 nanocuboids about 170 nm in length were also observed, which formed hollow secondary square rodlike microparticles along the b-axis direction (Figure 3g,h). When the pH value continues to increase to 3 or above, larger well-defined anatase TiO2 nanocuboids were observed, which also aggregated into hollow secondary square rodlike microparticles along the b-axis direction (Figure 3i–t). Based on the above discussion, we can see that the anatase TiO2 nanoparticles are formed by the splitting of the square rodlike H2Ti3O7 crystals, and the increase of the pH value of the solution can improve the crystallinity and crystal size of the nanoparticles (Figure 2) but has little effect on morphology. The effect of pH value on morphology is different from that previously reported. Generally speaking, the pH value in the reaction medium has an important effect on the crystal morphology of the resulting TiO2. The strong acidic medium is in favor of the formation of rodlike rutile TiO2. With the increase of the pH value, it is favorable to form rodlike, cuboid, rhombic, and spindle-shaped anatase TiO2.17,25−27

Figure 3.

Figure 3

Open in a new tab

FESEM images of (a,b) Na2Ti3O7, (c,d) H2Ti3O7, and the obtained anatase TiO2 products (e,f) pH 0.5-TiO2, (g,h) pH 1.0-TiO2, (i,j) pH 3.0-TiO2, (k,l) pH 5.0-TiO2, (m,n) pH 7.0-TiO2, (o,p) pH 9.0-TiO2, (q,r) pH 11.0-TiO2, and (s,t) pH 13.0-TiO2.

Transmission electron microscopy (TEM), high-resolution TEM (HRTEM) images, and the corresponding fast Fourier transform (FFT) patterns of the specimens are shown in Figure 4. For the specimen prepared at pH 0.5, aggregation of rodlike, nanocuboid, and entangled amorphous anatase TiO2 particles along the b axis were obtained (Figure 4a), as confirmed by the corresponding FESEM investigation of the pH 0.5-TiO2 specimen (Figure 3e,f). Figure 4b exhibits the HRTEM image of a nanocuboid anatase TiO2 crystal; the clear lattice fringes (0.190 nm) can be well assigned to the (200) facets of the anatase TiO2 nanocrystals. The lateral plane of the nanocuboid anatase TiO2 crystal is parallel to the (200) facings, indicating that the exposed crystal facets of the side is (200) facets, that is to say, the lateral plane exposed is {100} facets. This can also be confirmed by the corresponding fast FFT image (inset in Figure 4b); the observed angles of 69 and 42° are identical to the theoretical values for the angles between the (200) and (116) facets and those between the (116) and (1–16) facets, respectively. The HRTEM image (Figure 4c) of the nanocrystal clearly shows two types of nonparallel lattice fringes, and the measured lattice spacing of 0.167 and 0.190 nm of anatase TiO2 confirmed the lattice distances of the anatase (211) and (200) planes, respectively. Further, the 28.3° angle between the (211) and (200) facets agrees with the calculated results from the lattice constants of anatase (Tetragonal crystal system, JCPDS 21-1272, I41/amd, a = b = 0.37582 nm, c = 0.95139 nm).39 The FFT image (inset in the Figure 4c) taken from the nanoparticle exhibits a “single-crystal-like” diffraction, further confirming that the nanoparticle is a single nanocrystal. The TEM image shows that the as-obtained pH 3.0-TiO2 nanocrystals consist of a large amount of anatase TiO2 nanocuboids, with lengths ranging from 90 to 230 nm and widths ranging from 40 to 150 nm, respectively (Figure 4d). Figure 4e,f shows the corresponding HRTEM images of the marked area with red dotted boxes in Figure 4d. The lattice spacing of 0.190 and 0.239 nm correspond to the distances between two adjacent (200) and (004) planes, respectively, and the internal angle between the (200) and (004) planes is 90°, as shown in Figure 4e,f. The lateral and basal planes of the nanocuboid anatase TiO2 crystals are parallel to the (200) and (004) facets, respectively, indicating that the exposed crystal facets of the lateral and basal planes are the (200) and (004) facets, that is, the exposed crystals are the {100} (or {010}) and {001} facets. The FFT patterns shown in Figure 4e and f reveal that the nanocuboid TiO2 is single-crystalline. Figure 4g–i exhibits the TEM and HRTEM images of the as-obtained pH 7.0-TiO2 specimen. Evident lattices fringes could be observed by HRTEM (Figure 4h,i). The lattice fringe was 0.237 nm in size, corresponding to the (004) crystal plane of anatase TiO2. The FFT pattern shown in Figure 4i reveals that the exposed crystal surface at the bottom of the hexagon is {010} facets. Figure 4j is a low-TEM image of pH 13.0-TiO2. As the image shows, the secondary square rodlike microparticles are composed of many nanoparticles with 180–230 nm in length and 100–170 nm in width, which are consistent with the FESEM observation (Figure 3s,t). Figure 4k,l shows the HRTEM image of the nanocuboids and irregular nanocrystals obtained at pH 13.0. The corresponding lattices of 0.351 and 0.236 nm are ascribed to the (101) and (004) facets, respectively. The interfacial crystal angle between the (101) and (004) crystal planes is 68.3°, which can confirm that the exposed crystal facet is {010} facets. The FFT patterns of the marked region (Figure 4k,l inset) can be indexed to the diffraction spots of the [010] zone, which further confirm that the exposed crystal facet of the nanocuboids and irregular nanocrystals is {010} facets. It can also be seen from Figure 5k that the lateral plane of the nanocuboid anatase TiO2 crystals is parallel to the (004) facets, indicating that the exposed crystal facets of the lateral plane is (004) facets, that is to say, the exposed crystal surface is {001} facets. The above observations and analysis of the as-obtained anatase TiO2 surface structure and characterization illustrate that the anatase TiO2 nanocuboids have four {100} (or {010}) facets exposed on the lateral planes and two {001} facets on the basal planes. It is noteworthy that NH4F was used as the capping agent instead of the extremely corrosive HF in the synthesis of anatase TiO2 nanocuboids. NH4F is a strong electrolyte, which can instantaneously dissociate NH4+ ions and the self-etching agent F– ions in the reaction system (NH4F = NH4+ + F–).40 Furthermore, the capping agent F– ions adsorbed on the {001} facets of the anatase nanocrystals, which can stabilize the {001} facets by reducing the surface energy.41 Thus, anatase TiO2 nanocuboids with the coexposed {100}, {010}, and {001} facets were prepared in the presence of NH4F by using a modified hydrothermal method.

Figure 4.

Figure 4

Open in a new tab

TEM and HRTEM images of the anatase TiO2 nanocrystals obtained from different pH values under hydrothermal conditions: (a–c) pH 0.5-TiO2, (d–f) pH 3.0-TiO2, (g–i) pH 7.0-TiO2, (j–l) pH 13.0-TiO2. The insets in (b,c,e,f,h,k,l) are the corresponding FFT patterns.

Figure 5.

Figure 5

Open in a new tab

Transformation reaction from the layered protonic trititanate to anatase TiO2 nanocrystals.

Transformation Reaction Mechanism from Trititanate to Anatase TiO2 Nanocrystal

The basic crystal structure units of H2Ti3O7 ribbons are [Ti3O7]2- sheets that consist of three side-by-side aligned TiO6 octahedra, as shown in Figure 5. Three TiO6 octahedra are connected to each other by sharing the edges at one level to form a unit. The unit is connected with similar units located at the same level by sharing the corners. The periodic repetition of the above structure forms a single chain of TiO6 octahedra. Furthermore, the single chains at different levels are connected above and below by sharing additional apical edges, forming zigzag strings extending in the directions of the b axis, as well as the axis direction of the H2Ti3O7 ribbons. The zigzag strings are assembled into staggered sheets by sharing the corners of the TiO6 octahedra. The staggered sheets are stacked together in the c direction in the H2Ti3O7 ribbons, forming a layered framework. The H+ ions are located in the interlayers of the layered framework to balance the negative charge of the [Ti3O7]2– sheets. During the hydrothermal process, the staggered [Ti3O7]2– sheets are split into dispersed octahedral [Ti(OH)2(OH2)4]2+ monomers by the dissolution reaction along the corner-shared and edge-shared oxygen. The transformation from the staggered [Ti3O7]2– sheets to [Ti(OH)2(OH2)4]2+ monomers can be described as follows

graphic file with name ao0c01827_m001.jpg 1
graphic file with name ao0c01827_m002.jpg 2

Subsequently, the dispersed octahedral [Ti(OH)2(OH2)4]2+ monomers are combined together through oxolation or olation under hydrothermal conditions, forming different polymeric units of anatase, such as dimers, trimers, tetramers, and so forth. The different polymeric units nucleate at the reaction temperature to form the initial anatase nuclei. The transformation process can be expressed simply as follows

graphic file with name ao0c01827_m003.jpg 3
graphic file with name ao0c01827_m004.jpg 4

Further, the initial anatase nuclei gradually grow into crystals during the continuous hydrothermal reaction. Many nanocuboids are gathered in a certain direction, leading to the formation of a hollow anatase microtube. The profile of the microtube is very similar to that of the H2Ti3O7 ribbons, implying that the elongated direction of the microtube is along the b axis. It should be noted here that the crystal phase and morphology in the present work are not dependent on the pH value of the solution, which is different from those reported in the literature.37

Photocatalytic Activity of the Anatase TiO2 Crystals with Dominant {100}, {010}, and {001} Facets

The photocatalytic activity of the hollow square rodlike microparticles composed of anatase TiO2 nanocrystals with the dominant {100}, {010}, and {001} facets synthesized in the present work and the benchmark P25–TiO2 (∼87% anatase and ∼13% rutile) was measured via bleaching 8.35 × 10–6 mol/L RB aqueous solution. The RB solution containing TiO2 was performed for 2 days, so as to achieve better adsorption–desorption equilibrium of RB on the surface of TiO2 nanocrystals. The adsorption values [mol(RB)/g(TiO2)] of RB on the surface of TiO2 nanocrystals were 6.5 × 10–7, 1.3 × 10–6, 8.5 × 10–7, 1.1 × 10–6, and 7.5 × 10–7 mol/g for the P25–TiO2, pH 0.5-TiO2, pH 3.0-TiO2, pH 5.0-TiO2, and pH 7.0-TiO2 samples, respectively. These results indicated that the enhancement order of adsorption binding of RB to TiO2 was P25–TiO2 < pH 7.0-TiO2 < pH 3.0-TiO2 < pH 5.0-TiO2 < pH 0.5-TiO2 and that the strong anchoring of RB onto the surface of pH 0.5-TiO2 could improve the photocatalytic activity.24Figure 6a shows the UV–vis spectral changes of the centrifuged RB aqueous solution with pH 0.5-TiO2 as the photocatalyst. It can be seen that the absorbance intensity of the visible region (390–780 nm) gradually decreased with the increase of UV irradiation time. Moreover, the adsorption peaks shifted from 554 to 552, 551, 550, 549, 548, and 547 nm at 0, 15, 30, 45, 60, 75, and 90 min, respectively. The movement of the adsorption peak position can be attributed to the de-ethylation of RB.11Figure 6b shows the photocatalytic degradation activity plot of the as-prepared anatase TiO2 nanocrystals with dominant high-energy facets and the benchmark P25–TiO2 nanocrystals with a tiny proportion of the exposed [111] facets for the photodegradation of RB under UV light irradiation. It can be seen that the tested anatase TiO2 nanocrystals and the commercial P25–TiO2 nanocrystals exhibited a heterogeneous photocatalytic activity for the bleaching of the RB solution. Comparing the curves of the as-prepared anatase TiO2 nanocrystals, the commercial P25–TiO2, and the blank, one can find that the pH 0.5-TiO2 nanocrystals exhibited improved degradation properties toward RB under UV light irradiation. The degradation efficiency of RB was as follows: pH 0.5-TiO2 (76.0%) > pH 3.0-TiO2 (68.0%) > pH 7.0-TiO2 (63.5%) > pH 5.0-TiO2 (51.5%) > P25–TiO2 (48.1%) > the blank sample (8.5%). It needs to mention that the as-synthesized TiO2 and the commercially available P25–TiO2 cannot degrade RB completely, which is not because of the inactivation of the photocatalysts but because of more RB solution (250 mL), less photocatalyst (50 mg), and greater irradiation distance (25 cm). If the RB aqueous solution was given enough irradiation time, it should be able to be degraded completely. The pseudo-first-order kinetics equation and the corresponding photocatalytic rate constants of RB degradation were obtained by taking the experiment data of photocatalysis into–ln(ct/c0) = kt,42 as shown in Figure 6c,d. The degradation rate of pH 0.5-TiO2 was the highest at 1.66 × 10–2 min–1, which was 1.3, 1.5, 2.0, 2.3, and 18.4 folds higher than that of pH 3.0-TiO2 (1.27 × 10–2 min–1), pH 7.0-TiO2 (1.11 × 10–2 min–1), pH 5.0-TiO2 (0.83 × 10–2 min–1), P25–TiO2 (0.73 × 10–2 min–1), and the blank sample (0.09 × 10–2 min–1), respectively.

Figure 6.

Figure 6

Open in a new tab

(a) Spectral changes of the centrifuged RB aqueous solution with pH 0.5-TiO2 as the photocatalyst; (b) photocatalytic degradation activity plot of the as-prepared anatase TiO2 nanocrystals with dominant high-energy facets for the photodegradation of RB under UV light irradiation; (c) photocatalytic kinetic plot of the as-prepared anatase TiO2 nanocrystals with dominant high-energy facets for the degradation of RB under UV light irradiation; (d) degradation rate constants of RB in different photocatalysts.

The photocatalytic activity of TiO2 nanocrystals is influenced by many factors, for example, crystalline phase, heterogenous structure, exposed facets, crystal morphology, crystalline size, specific surface area, preparation method, and so forth.43,44 Among the four different polymorphs of TiO2 (anatase, rutile, brookite, and TiO2-B), the anatase phase has been considered to exhibit the most photoactive reactivity for the degradation of wastewater contaminated by organic dyes.10,11 Compared with the as-prepared anatase TiO2, the commercially available P25–TiO2 exhibited lower photocatalytic activity, although its heterojunction structure could improve the separation efficiency of the photogenerated electrons and photogenerated holes, indicating that the heterojunction structure is not the main reason for the improvement of photocatalytic efficiency in the present study.7 The pH 0.5-TiO2, pH 3.0-TiO2, pH 5.0-TiO2, and pH 7.0-TiO2 nanocrystals with cuboid morphology have a large proportion of coexposed {100}, {010}, and {001} facets on their surfaces according to the FESEM and TEM analyses; however, the commercial P25–TiO2 (∼87% anatase and 13% rutile) nanocrystals have only a few exposed high-energy [111] facets. For anatase TiO2, the order of surface energy increase is {101} facets (0.44 J·m–2) < {010} facets (or {100} facets) (0.53 J·m–2) < {001} facets (0.90 J·m–2) < {110} facets (1.09 J·m–2) < [111]- facets (1.61 J·m–2).14,15 Because of the cooperative mechanism of the surface atomic structure and surface electronic structure, the {010} (or {100}) facets show the highest photoreactivity, although the surface energy of {010} facets (or {100} facets) is not the highest.45 Hence, the pH 0.5-TiO2, pH 3.0-TiO2, pH 5.0-TiO2, and pH 7.0-TiO2 nanocrystals with the coexposed {100}, {010}, and {001} facets exhibit higher photocatalytic activities than that of P25–TiO2 with only a few exposed [111] facets. For the as-prepared anatase TiO2 (pH 0.5-TiO2, pH 3.0-TiO2, pH 5.0-TiO2, and pH 7.0-TiO2) nanocrystals, the morphology and crystal facets are similar, indicating that other factors also affect the photocatalytic activity. As we know, the smaller crystalline size and larger specific surface can offer a powerful redox capability and more active sites in the photochemical reaction, respectively, resulting in an improvement of the photocatalytic activity in the decomposition of RB.46,47 Further, the specific surface areas increase with the decrease of crystalline size. The specific surface areas increase in the order of P25–TiO2 (52.5 m2/g) > pH 0.5-TiO2 (33.6 m2/g) > pH 3.0-TiO2 (30.7 m2/g) > pH 7.0-TiO2 (27.1 m2/g) > pH 5.0-TiO2 (25.4 m2/g). The specific surface areas decreased slightly with the increase of the pH values, which could be attributed to the increase of the crystal size of the prepared anatase TiO2. It has been reported that the valence band maxima of anatase TiO2 with different exposed facets are similar, whereas the conduction band minima increase in the order of {001} < {101} < {010} < {111} facets of anatase TiO2.15 Also, the conduction band minimum edges of anatase TiO2 increased with the decrease of the crystalline size.22 In other words, the band gaps of the as-prepared anatase TiO2 samples with the coexposed {100}, {010}, and {001} facets on their surfaces are likely to be different because of the different crystalline sizes and the different proportions of high-energy facets. In has been reported that the anatase TiO2 nanocrystals with the lowest conduction band minimum edges could promote the enhancement of the generated electrons, which would exhibit superior photocatalytic activity.15 Therefore, pH 0.5-TiO2 exhibited the best photocatalytic activity, mainly because of the larger proportion of {010} (or {100}) facets, smaller crystalline size, higher band gap, and larger specific surface area.

Conclusions

In summary, the hollow square rodlike microtubes composed of anatase nanocuboids with the coexposed {100}, {010}, and {001} facets were successfully prepared via a mild hydrothermal treatment method in the presence of NH4F by using layered H2Ti3O7 ribbons as the precursor. The elongated direction of microtubes is along the b axis according to the profile of the H2Ti3O7 ribbons. The transformation from the staggered [Ti3O7]2– sheets to the hollow square rodlike microtubes included the formation and recombination of the dispersed octahedral [Ti(OH)2(OH2)4]2+ monomers, the formation and growth of the initial anatase nuclei, and the reassembly of the anatase nanocuboids along the b-axis direction during the continuous hydrothermal process. Compared with P25–TiO2 and the other anatase TiO2 samples, pH 0.5-TiO2 exhibited the best photocatalytic activity, mainly attributed to its larger proportion of {010} (or {100}) facets, smaller crystalline size, higher band gap, and larger specific surface area. Moreover, the hollow square rodlike microtubes composed of anatase nanocuboids with high-energy facets in the present work may have potential applications in dye-sensitized solar cells.

Experimental Section

Preparation of Na2Ti3O7 and H2Ti3O7 Square Rods

Layered sodium trititanate Na2Ti3O7 was prepared by a conventional solid-phase reaction method.48 In a typical synthesis procedure, titanium dioxide (TiO2, 98.5%, Tianjin Bodi Chemical Co., Ltd.) and sodium carbonate (Na2CO3, 99.0%, Tianjin Hengxing Chemical Reagent Manufacturing Co., Ltd.) were mixed in a stoichiometric ratio according to the reaction Na2CO3 + 3TiO2 → Na2Ti3O7 + CO2↑ and grinded in an agate mortar until mixed well. Then, the well-mixed powder (18.0 g) was placed in a corundum crucible and calcined in a high-temperature box furnace at 900 °C for 24 h with 5 °C/min heating rate. After cooling to room temperature, the obtained white Na2Ti3O7 specimens (15.0 g) were treated with an aqueous solution of HCl (1.0 mol/L, l.5 L) with continuous stirring for 72 h at ambient temperatures to be converted into an acid-exchanged form, H2Ti3O7.

Preparation of Anatase TiO2 Crystals with Dominant {100}, {010}, and {001} Facets

The protonated trititanate square rods (1.0 g) and deionized water (60 mL) were added to eight Teflon-lined autoclaves (80 mL), respectively, and then the mixtures were stirred by a magnetic stirrer and adjusted to the set pH value (0.5–13.0) with different concentrations of HCl aqueous solution (0.5–3.0 mol/L) and NaOH aqueous solution (0.5–3.0 mol/L). After this, NH4F (0.5 g) was added to the pH-adjusted H2Ti3O7 suspension solutions and stirred at room temperature for 30 min. Subsequently, the autoclave was sealed and heated to 180 °C for 24 h in a constant-temperature blast drying oven. After the reaction is finished, the autoclaves were naturally cooled to room temperature, and the final products were separated from the solution by filtration, washed by deionized water, and dried at room temperature.

Specimen Characterization

The crystal structure of the obtained Na2Ti3O7, H2Ti3O7, and anatase TiO2 specimens was examined by powder XRD (XRD-6100, Shimadzu, Kyoto, Japan) with monochromated Cu Kα radiation (λ = 0.15406 nm). The data were collected for scattering angles (2θ) from 5 to 80° with a scanning speed of 8°/min. The size and morphology of the specimens were observed using FESEM (FEI, Quanta FEG 250 FEI, America). The crystalline nanostructures were investigated using TEM, HRTEM, and FFT patterns (JEM-2100-F, JEOL, Japan). The specific surface area was determined by the Brunauer–Emmett–Teller method, using an automatic gas adsorption analyzer (Autosorb-IQ3, Quantachrome, America) by nitrogen gas adsorption. The UV–vis spectrophotometer (model TU-1901) manufactured by Beijing Purkinje General Instrument Co., Ltd. was used for measuring the absorbance of RB solution in the wavelength range of 350–650 nm.

Evaluation of Photocatalytic Activity

The photocatalytic performance of the as-prepared TiO2 photocatalyst was evaluated by the photodegradation of RB (obtained from Sinopharm Chemical Reagent Co., Ltd.) in a self-assembled photochemical apparatus. A 50 mg of as-synthesized TiO2 (pH 0.5-TiO2, pH 1.0-TiO2, pH 3.0-TiO2, pH 5.0-TiO2, and pH 7.0-TiO2) photocatalyst and the best commercially available Degussa P25 TiO2 (∼87% anatase and ∼13% rutile) photocatalyst were suspended in 250 mL of an aqueous solution containing 4 mg/L RB (8.35 × 10–6 mol/L) in a 300 mL of quartz glass beaker. A 175 W low-pressure mercury lamp (with a peak emission at 365 nm, Shanghai Mingyao Glass Hardware Tools) was used as the analogue light source, and the dark adsorption experiment was performed for 2 days in the dark to reach the adsorption–desorption equilibrium before the above lamp was turned on, where the lamp was located 25 cm away from the surface of the RB solution. The irradiation area of the lamp is 56.7 cm2 and the depth of the solution is 5.5 cm. Every 15 min, 5 mL of the suspension was removed from the quartz glass beaker and centrifuged at 2500 rpm for 5 min to remove the solid photocatalysts. Finally, the supernatant obtained was taken for the measurement of absorbance at λ = 554 nm using a TU-1901 spectrophotometer (Beijing Purkinje General Instrument Co., Ltd.) with deionized water as the reference solution.

Acknowledgments

This work was financially supported by the Applied Basic Research Project of Shanxi (no. 201901D111303), the Doctor Research Funds of Jinzhong University, the Shanxi “1331 Project” Key Innovation Team (no. PY201817), the Jinzhong University “1331 Project” Key Innovation Team (nos. jzxycxtd2017004 and jzxycxtd2019005), Scientific and Technological Innovation Programs of Higher Education Institutions in Shanxi (no. 2019L0881), the Educational Department of Liaoning Province (no. L2019036), the Liaoning Provincial Natural Science Foundation (no. 20170540583), the National Science Foundation of China (nos. 51272030 and 51572031), and the Grants-in-Aid for Scientific Research (B) (no. 26289240) from the Japan Society for the Promotion of Science and Kagawa University.

Author Contributions

The paper was written through the contributions of all authors. All authors have given approval to the final version of the paper.

The authors declare no competing financial interest.

References

  1. Xu H.-Q.; Hu J. H.; Wang D. K.; Li Z. H.; Zhang Q.; Luo Y.; Yu S.-H.; Jiang H.-L. Visible-light photoreduction of CO2 in a metal-organic framework: boosting electron-hole separation via electron trap states. J. Am. Chem. Soc. 2015, 137, 13440–13443. 10.1021/jacs.5b08773. [DOI] [PubMed] [Google Scholar]
  2. Cao F.; Xiong J.; Wu F.; Liu Q.; Shi Z.; Yu Y.; Wang X.; Li L. Enhanced photoelectrochemical performance from rationally designed anatase/rutile TiO2 heterostructures. ACS Appl. Mater. Interfaces 2016, 8, 12239–12245. 10.1021/acsami.6b03842. [DOI] [PubMed] [Google Scholar]
  3. Zhang T.; Oyama T.; Aoshima A.; Hidaka H.; Zhao J.; Serpone N. Photooxidative N-Demethylation of methylene blue in aquepus TiO2 dispersions under UV irradiation. J. Photochem. Photobiol., A 2001, 140, 163–172. 10.1016/s1010-6030(01)00398-7. [DOI] [Google Scholar]
  4. Yan Y.; Yang H.; Yi Z.; Xian T.; Li R.; Wang X. Construction of Ag2S@CaTiO3 heterostructure photocatalysts for enhanced photocatalytic degradation of dyes. Desalin. Water Treat. 2019, 170, 349–360. 10.5004/dwt.2019.24747. [DOI] [Google Scholar]
  5. Wei X.-X.; Cui B.; Wang X.; Cao Y.; Gao L.; Guo S.; Chen C.-M. Tuning the physicochemical property of BiOBr via solvent adjustment: Towards an efficient photocatalyst for water treatment. CrystEngComm 2019, 21, 1750–1757. 10.1039/c8ce02072h. [DOI] [Google Scholar]
  6. Yan Y.; Yang H.; Yi Z.; Wang X.; Li R.; Xian T. Evolution of Bi nanowires from BiOBr nanoplates through a NaBH4 reduction method with enhanced photodegradation performance. Environ. Sci. Eng. 2020, 37, 64–77. 10.1089/ees.2019.0284. [DOI] [Google Scholar]
  7. Liao Y.; Que W.; Jia Q.; He Y.; Zhang J.; Zhong P. Controllable synthesis of brookite/anatase/rutile TiO2 nanocomposites and single-crystalline rutile nanorods array. J. Mater. Chem. 2012, 22, 7937–7944. 10.1039/c2jm16628c. [DOI] [Google Scholar]
  8. Wu W.; Zhu J.; Deng Y. H.; Xiang Y.; Tan Y. W.; Tang H. Q.; Zou H.; Xu Y. F.; Zhou Y. TiO2 nanocrystals with the {001} and {101} facets co-exposed with MIL-100(Fe): an egg-like composite nanomaterial for efficient visible light-driven photocatalysis. RSC Adv. 2019, 9, 31728–31734. 10.1039/c9ra06359e. [DOI] [PMC free article] [PubMed] [Google Scholar]
  9. Liu J.; Liu B.; Ren Y.; Yuan Y.; Zhao H.; Yang H.; Liu S. Hydrogenated nanotubes/nanowires assembled from TiO2 nanoflakes with exposed {111} facets: excellent phoyo-catalytic CO2 redunction activity and charge separation mechanism between (111) and (1(_)1(_)1(_)) polar surfaces. J. Mater. Chem. A 2019, 7, 14761–14775. 10.1039/c9ta00736a. [DOI] [Google Scholar]
  10. Liu B.; Khare A.; Aydil E. S. TiO2-B/anatase core-shell heterojunction nanowires for photocatalysis. ACS Appl. Mater. Interfaces 2011, 3, 4444–4450. 10.1021/am201123u. [DOI] [PubMed] [Google Scholar]
  11. Liu X.; Li Y.; Deng D.; Chen N.; Xing X.; Wang Y. A one-step nonaqueous sol-gel route to mixed-phase TiO2 with enhanced photocatalytic degradation of Rhodamine B under visible light. CrystEngComm 2016, 18, 1964–1975. 10.1039/c5ce02322j. [DOI] [Google Scholar]
  12. Chen T.; Bao L.; Zheng Y.; Yang X.; Ruan L.; Liu Y.; Xu G.; Han G. Hydrothermal synthesis of perovskite CaTiO3 tetragonal microrods with vertical V-type holes along the [010] direction. CrystEngComm 2019, 21, 4763–4770. 10.1039/c9ce00726a. [DOI] [Google Scholar]
  13. Yang Z.; Wang B.; Cui H.; An H.; Pan Y.; Zhai J. Synthesis of crystal-controlled TiO2 nanorods by a hudrothermal method:rutile and brookite as highly active photocatalysts. J. Phys. Chem. C 2015, 119, 16905–16912. 10.1021/acs.jpcc.5b02485. [DOI] [Google Scholar]
  14. Han X.; Kuang Q.; Jin M.; Xie Z.; Zheng L. Synthesis of titania nanosheets with a high percentage of exposed {001} facets and related photocatalytic properties. J. Am. Chem. Soc. 2009, 131, 3152–3153. 10.1021/ja8092373. [DOI] [PubMed] [Google Scholar]
  15. Xu H.; Reunchan P.; Ouyang S.; Tong H.; Umezawa N.; Kako T.; Ye J. Anatase TiO2 single crystals exposed with high-reactive {111} facets toward efficient H2 evolution. Chem. Mater. 2013, 25, 405–411. 10.1021/cm303502b. [DOI] [Google Scholar]
  16. Yang H. G.; Sun C. H.; Qiao S. Z.; Zou J.; Liu G.; Smith S. C.; Cheng H. M.; Lu G. Q. Anatase TiO2 single crystals with a large percentage of reactive facets. Nature 2008, 453, 638–641. 10.1038/nature06964. [DOI] [PubMed] [Google Scholar]
  17. Wen P.; Itoh H.; Tang W.; Feng Q. Single nanocrystals of anatase-type TiO2 prepared from layered titanate nanosheets: Formation mechanism and characterization of surface properties. Langmuir 2007, 23, 11782–11790. 10.1021/la701632t. [DOI] [PubMed] [Google Scholar]
  18. Yang H. G.; Liu G.; Qiao S. Z.; Sun C. H.; Jin Y. G.; Smith S. C.; Zou J.; Cheng H. M.; Lu G. Q. Solvothermal Synthesis and photoreactivity of anatase TiO2 nanosheets with dominant {001} facets. J. Am. Chem. Soc. 2009, 131, 4078–4083. 10.1021/ja808790p. [DOI] [PubMed] [Google Scholar]
  19. Liu M.; Li H.; Zeng Y.; Huang T. Anatase TiO2 single crystals with dominant {001} facets: Facile fabrication from Ti powders and enhanced photocatalytical activity. Appl. Surf. Sci. 2013, 274, 117–123. 10.1016/j.apsusc.2013.02.125. [DOI] [Google Scholar]
  20. Li B.; Zhao Z.; Gao F.; Wang X.; Qiu J. Mesoporous microspheres composed of carbon-coated TiO2 nanocrystals with exposed {001} facets for improved visible kight photocatalytic activity. Appl. Catal. B Environ. 2014, 147, 958–964. 10.1016/j.apcatb.2013.10.027. [DOI] [Google Scholar]
  21. Ye L. Q.; Mao J.; Liu J. Y.; Jiang Z.; Peng T. Y.; Zan L. Synthesis of anatase TiO2 mamocrystals with {101}, {001} or {010} single facets of 90% level exposure and liquid-phase photocatalytic reduction and oxidation activity orders. J. Mater. Chem. A 2013, 1, 10532–10537. 10.1039/c3ta11791j. [DOI] [Google Scholar]
  22. Pan J.; Wu X.; Wang L.; Liu G.; Lu G. Q.; Cheng H.-M. Synthesis of anatase TiO2 rods with dominant reactive {010} facets for the photoreduction of CO2 to CH4 and use in dye-sensitized solar cells. Chem. Commun. 2011, 47, 8361–8363. 10.1039/c1cc13034j. [DOI] [PubMed] [Google Scholar]
  23. Zhao J.; Zou X.-X.; Su J.; Wang P.-P.; Zhou L.-J.; Li G.-D. Synthesis and photocatalytic activity of porous anatase TiO2 microspheres composed of {010}-faceted nanobelts. Dalton Trans. 2013, 42, 4365–4368. 10.1039/c3dt33053b. [DOI] [PubMed] [Google Scholar]
  24. Wen P.; Xue M.; Ishikawa Y.; Itoh H.; Feng Q. Relationships between cell parameters of dye-sensitized solar cells and dye-adsorption parameters. ACS Appl. Mater. Interfaces 2012, 4, 1928–1934. 10.1021/am3001693. [DOI] [PubMed] [Google Scholar]
  25. Du Y.-E.; Feng Q.; Chen C.; Tanaka Y.; Yang X. Photocatalytic and dye-sensitized solar cell performances of {010}-faceted and [111]-faceted anantase TiO2 nanocrystals synthesized from tetratitanated nanoribbons. ACS Appl. Mater. Interfaces 2014, 6, 16007–16019. 10.1021/am503914q. [DOI] [PubMed] [Google Scholar]
  26. Chen C.; Xu L.; Sewvandi G. A.; Kusunose T.; Tanaka Y.; Nakanishi S.; Feng Q. Microwave-assisted topochemical conversion of layered titanate nanosheets to {010}-faceted anatase nanocrystals for high performance photocatalysis and dye-sensitized solar cells. Cryst. Growth Des. 2014, 14, 5801–5811. 10.1021/cg501062r. [DOI] [Google Scholar]
  27. Du Y.-E.; Du D.; Feng Q.; Yang X. Delithiation, exfoliation, and transformation of rock-salt-structured Li2TiO3 to highly exposed {010}-faceted anatase. ACS Appl. Mater. Interfaces 2015, 7, 7995–8004. 10.1021/acsami.5b00227. [DOI] [PubMed] [Google Scholar]
  28. Du Y.-E.; Bai Y.; Liu Y.; Guo Y.; Cai X.; Feng Q. One-pot synthesis of [111]-/{010} facets coexisting anatase nanocrystals with enhanced dye-sensitized solar cell performances. Chemistryselect 2016, 1, 6632–6640. 10.1002/slct.201601326. [DOI] [Google Scholar]
  29. Du Y.-E.; Niu X.; Li W.; An J.; Liu Y.; Chen Y.; Wang P.; Yang X.; Feng Q. Microwave-assisted synthesis of high-energy faceted TiO2 nanocrystals derived from exfoliated porous metatitanic acid nanosheets with improved photocatalytic and photovoltaic performance. Materials 2019, 12, 3614. 10.3390/ma12213614. [DOI] [PMC free article] [PubMed] [Google Scholar]
  30. Du Y. e.; Niu X.; Bai Y.; Qi H.; Guo Y.; Chen Y.; Wang P.; Yang X.; Feng Q. Synthesis of anatase TiO2 nanocrystals with defined morphologies from exfoliated nanoribbons: photocatalytic performance and application in dye-sensitized solar cell. ChemistrySelect 2019, 4, 4443–4457. 10.1002/slct.201900257. [DOI] [Google Scholar]
  31. Zhao X.; Jin W.; Cai J.; Ye J.; Li Z.; Ma Y.; Xie J.; Qi L. Shape- and size-controlled synthesis of uniform anatase TiO2 nanocuboids enclosed by active {100} and {001} facets. Adv. Funct. Mater. 2011, 21, 3554–3563. 10.1002/adfm.201100629. [DOI] [Google Scholar]
  32. Zheng Z.; Huang B.; Lu J.; Qin X.; Zhang X.; Dai Y. Hierarchical TiO2 microspheres: synergetic effect of {001} and {101} facets for enhanced photocatalytic activity. Chem.—Eur. J. 2011, 17, 15032–15038. 10.1002/chem.201101466. [DOI] [PubMed] [Google Scholar]
  33. Niu X.; Du Y.-E.; Liu Y.; Qi H.; An J.; Yang X.; Feng Q. Hydrothermal synthesis and formation mechanism of the anatase nanocrystals with co-exposed highenergy {001}, {010} and [111]-facets for enhanced photocatalytic performance. RSC Adv. 2017, 7, 24616–24627. 10.1039/c7ra03707d. [DOI] [Google Scholar]
  34. Zhang H.; Wang W.; Zhao H.; Zhao L.; Gan L.-Y.; Guo L.-H. Facet-mediated interaction between humic acid and TiO2 nanoparticles: implications for aggregation and stability kinetics in aquatic environments. Environ. Sci.: Nano 2019, 6, 1754–1764. 10.1039/c9en00231f. [DOI] [Google Scholar]
  35. Han S. N.; Niu Q. Y.; Qin N.; Gua X. T.; Zhang Y.-N.; Zhao G. H. In-situ growth of M-{001}TiO2/Ti phototelectrode: synergetic dominated {001} facet and ratio-optimally surface junctions for effective oxidation of environment pollutants. Chem. Commun. 2020, 56, 1337–1340. 10.1039/c9cc09296j. [DOI] [PubMed] [Google Scholar]
  36. Cheng H.; Ma J.; Zhao Z.; Qi L. Hydrothermal preparation of uniform nanosize rutile and anatase particles. Chem. Mater. 1995, 7, 663–671. 10.1021/cm00052a010. [DOI] [Google Scholar]
  37. Chen C.; Ikeuchi Y.; Xu L.; Sewvandi G. A.; Kusunose T.; Tanaka Y.; Nakanishi S.; Wen P.; Feng Q. Synthesis of [111]- and {010}-faceted anatase TiO2 nanocrystals from tritianatae nanosheets and their photocatalytic and DSSC performances. Nanoscale 2015, 7, 7980–7991. 10.1039/c5nr00069f. [DOI] [PubMed] [Google Scholar]
  38. Suzuki S.; Miyayama M. Lithium intercalation properties of octatitanate synthesized through exfoliation/reassembly. J. Phys. Chem. B 2006, 110, 4731–4734. 10.1021/jp056034g. [DOI] [PubMed] [Google Scholar]
  39. Howard C. J.; Sabine T. M.; Dickson F. Structural and thermal parameters for rutile and anatase. Acta Crystallogr. B 1991, 47, 462–468. 10.1107/s010876819100335x. [DOI] [Google Scholar]
  40. Feng J.; Yin M.; Wang Z.; Yan S.; Wan L.; Li Z.; Zou Z. Facile synthesis of anatase TiO2 mesocrystal sheets with dominant {001} facets based on topochemical conversion. CrystEngComm 2010, 12, 3425–3429. 10.1039/c0ce00070a. [DOI] [Google Scholar]
  41. Ong W.-J.; Tan L.-L.; Chai S.-P.; Yong S.-T.; Mohamed A. R. Highly reactive {001} facets of TiO2-based composites: synthesis, formation mechanism and characterization. Nanoscale 2014, 6, 1946–2008. 10.1039/c3nr04655a. [DOI] [PubMed] [Google Scholar]
  42. Liu J.; Olds D.; Peng R.; Yu L.; Foo G. S.; Qian S.; Keum J.; Guiton B. S.; Wu Z.; Page K. Quantitative analysis of the morphology of {101} and {001} faceted anatase TiO2 nanocrystals and its implication on photocatalytic activity. Chem. Mater. 2017, 29, 5591–5604. 10.1021/acs.chemmater.7b01172. [DOI] [Google Scholar]
  43. Li J.-G.; Ishigaki T.; Sun X. Anatase, brookite, and rutile nanocrystals via redox reactions under mild hydrothermal conditions: phase-selective synthesis and physicochemical properties. J. Phys. Chem. C 2007, 111, 4969–4976. 10.1021/jp0673258. [DOI] [Google Scholar]
  44. Zhang H.; Liu P.; Li F.; Liu H.; Wang Y.; Zhang S.; Guo M.; Cheng H.; Zhao H. Facile fabrication of anatase TiO2 microspheres on solid substrates and surface crystal facet transformation from {001} and {100}. Chem. Eur J. 2011, 17, 5949–5957. 10.1002/chem.201002433. [DOI] [PubMed] [Google Scholar]
  45. Pan J.; Liu G.; Lu G. Q. M.; Cheng H.-M. On the true photoreactivity order of {001}, {010}, and {101} facets of anatase TiO2 crystals. Angew. Chem. Int. Ed. 2011, 50, 2133–2137. 10.1002/anie.201006057. [DOI] [PubMed] [Google Scholar]
  46. Sitthisang S.; Komarneni S.; Tantirungrotechai J.; Dong Noh Y.; Li H.; Yin S.; Sato T.; Katsuki H. Microwave-hydrothermal synthesis of extremely high specific surface area anatase for decomposing NOx. Ceram. Int. 2012, 38, 6099–6105. 10.1016/j.ceramint.2012.04.057. [DOI] [Google Scholar]
  47. Hu D.; Zhang W.; Tanaka Y.; Kusunose N.; Peng Y.; Feng Q. Mesocrystalline nanocomposition of TiO2 polymorphs: Topochemical mesocrystal conversion, characterization, and photocatalytic response. Cryst. Growth Des. 2015, 15, 1214–1225. 10.1021/cg501616k. [DOI] [Google Scholar]
  48. Izawa H.; Kikkawa S.; Koizumi M. Ion exchange and dehydration of layered titanate, Na2Ti3O7 and K2Ti4O9. J. Phys. Chem. 1982, 86, 5023–5026. 10.1021/j100222a036. [DOI] [Google Scholar]

Articles from ACS Omega are provided here courtesy of American Chemical Society

RESOURCES