**3. Results and discussion**

#### **3.1. Characterization**

(ii) In the second method, 2.0 g of the TiO<sup>2</sup>

284 Titanium Dioxide - Material for a Sustainable Environment

B-LDH).

analysis was performed prior to the photocatalytic tests.

B, and TiO<sup>2</sup>


**2.5. Phenol photodegradation and adsorption tests**

I, TiO<sup>2</sup>

100°C for 20 hours (TiO<sup>2</sup>


3.5 hours (TiO<sup>2</sup>

**2.4. Characterization**

The LDH, CLDH, TiO<sup>2</sup>

**Figure 1.** Methodologies for TiO<sup>2</sup>

of TiO<sup>2</sup>

by N<sup>2</sup>

B catalyst and 0.2 g of the CLDH solid were mixed

I-LDH, TiO<sup>2</sup>

B-LDH, and TiO<sup>2</sup>

T-

in 20 mL of ethanol with mechanical agitation for 3 hours; the resulting paste was dried at

(iii) In the preparation of the third composite, 9 mL of TTIP were mixed with 0.5 g of CLDH, and the mixture agitated for 30 minutes in the air. The samples were then calcined at 550°C for

The materials obtained were characterized by XRD using Siemens D500 diffractometer (Cu kα λ = 1.54 Å) at a scanning speed of 2(°2θ)/min. The specific surface area was determined

 adsorption using the BET method on BELSORP-max equipment. The surface analysis of the materials by the AFM technique was performed in an Oxford Asylum Research Cipher AFM at room conditions with noncontact mode with a Si tip of 10 nm radius and resonance frequencies from 180 to 240 kHz. The measurements were taken in a range of 500 × 500 nm. The composites were also analyzed by XPS using Thermo Scientific K-Alpha X-ray photoelectron spectrometer, using the Al kα radiation line (1487 eV) in standard mode, with 10 scans, tip size of 400 μm, step voltage of 200.0 eV, and pass energy of 1.0 eV. All the characterization

T precursors and TiO<sup>2</sup>

LDH composites were mixed separately with a phenol solution (Baker) (C0 = 10 mg/L). Air was pumped through each mixture to maintain constant agitation, and the solution was stabilized for 20 minutes. Three different conditions were evaluated: (i) under UV light with a UVS-18 EL (λ = 264 nm, 8 w) lamp and in the absence of solid (photolysis), (ii) in the dark without UV irradiation and with the synthesized materials (adsorption), and (iii) under UV light with the presence of the synthesized solids (photocatalysis). The experiments were conducted for 120 minutes at room temperature (20°C) and without external pH variation. The effect of different concentrations of the prepared composites was also evaluated at concentrations of 0.5, 1.0, 1.5, and 2.0 g/L; aliquots were extracted at 0, 5, 10, 30, 50, 80, and 120 minutes during

T-LDH). **Figure 1** illustrates the processes described above in the preparation

**Figure 2** shows the X-ray diffraction patterns for the synthesized TiO<sup>2</sup> precursors along with the LDH and its calcined product CLDH. For the photocatalysts, the TiO<sup>2</sup> I sample is composed mostly of the crystalline structure related to the TiO<sup>2</sup> anatase phase, presenting peaks at 25.4, 37.9, 48.1, 54.1, 55.2, and 62.6 o 2θ (JCPDS 01-089-4921), in addition to a peak related to the TiO<sup>2</sup> brookite phase at 30.9 o 2θ (JCPDS 00-029-1360). The diffractogram for the TiO<sup>2</sup> T sample shows a bigger variety in the different crystalline structures that this photocatalyst possesses, with a main peak at 25.4 o 2θ and smaller peaks at 37.9, 48.1, 54.0, and 55.2 o 2θ referring to the TiO<sup>2</sup> anatase phase (JCPDS 01-089-4921). In smaller proportion characteristic, reflections of the TiO<sup>2</sup> phase with rhombohedral structure are observed in 32.9, 35.7, and 40.8 o 2θ (JCPDS 01-071-0146), a peak at 43.7 o 2θ of TiO with monoclinic structure (JCPDS 01-072-0020) and one lower peak associated with TiO<sup>2</sup> rutile phase with tetragonal coordination at 27.6 o 2θ (JCPDS 01-089-4920). The TiO<sup>2</sup> B sample shows peaks referring to the TiO<sup>2</sup> anatase phase at 25.4, 37.9, 48.1, 54.0, and 55.2 o 2θ (JCPDS 01-089-4921) and a signal at 30.9 o 2θ of the TiO<sup>2</sup> brookite with orthorhombic crystalline formation (JCPDS 00-029-1360), as well as peaks related to the TiO<sup>2</sup> rutile phase at 27.6 and 36.2 o 2ϴ (JCPDS 01-089-4920).

In a photocatalyst, the pure anatase phase is considered photocatalytically superior to the rutile phase, which, although more heat stable, at the same time has a higher rate of electron–hole recombination (e− BC-h<sup>+</sup> BV) and a lesser affinity for the adsorption of organic compounds like phenol [19]. A key factor in the photocatalytic activity of TiO<sup>2</sup> is to obtain a mixed anatase-rutile material at an optimal ratio of about 80:20%, which has lower recombination rates (e− BC-h<sup>+</sup> BV) due to the interconnection of the electronic bands, in which the rutile phase acts as e− BC collector. In the meanwhile, the anatase phase is the photocatalytically active part causing oxidation and reducing reactions, which are carried out separately, maximizing the photocatalytic mechanism [8]. The composition of the precursor photocatalysts in the anatase-rutile phases was determined by the Spurrs and Myers equation [20], finding that the optimal anatase-rutile phase ratio (80:20%) was close to be found in samples TiO<sup>2</sup> T (82:18%) and TiO<sup>2</sup> B (89:11%). Meanwhile, the rutile phase of TiO<sup>2</sup> was not observed in the TiO<sup>2</sup> I photocatalyst; only the anatase and brookite phases were present. These results occasionally influence the performance of these materials and the composites used in the photocatalytic tests, as discussed later.

**Figure 3** shows the X-ray diffraction patterns for the synthesized TiO<sup>2</sup>

(JCPDS 01-003-0998) corresponding to CLDH component and another peak at 32.9 o

the MgTi mixed oxide with rhombohedral crystalline structure (JCPDS 01-079-0831), thus

T-LDH composite, the TiO<sup>2</sup>

achieving the diffusion of the photocatalyst in the composite [21, 23].

I-LDH composite mostly shows diffraction of the MgTiO<sup>3</sup>

monoclinic TiO presents two diffractions with peaks located at 42.1 and 43.1 o

and LDH compounds, with A (TiO<sup>2</sup>

brookite), O (MgO), and M (MgTiO<sup>3</sup>

anatase), R (TiO<sup>2</sup>

rutile), TiO<sup>2</sup>

rombohedric crystalline structure) peaks.

m (TiO<sup>2</sup>

monoclinic

observed separately; the reflections of the TiO<sup>2</sup>

bohedral structure, with a main peak located at 32.9 o

confirming the immobilization of TiO<sup>2</sup>

24.1, 35.7, 40.8, 49.3, 53.7, 62.1, and 63.8 o

the case of the TiO<sup>2</sup>

The TiO<sup>2</sup>

37.9, 48.1, 54.0, and 55.2 o

**Figure 3.** XRD patterns for TiO<sup>2</sup>

crystalline structure), B (TiO<sup>2</sup>


mixed oxide with rhom-

2θ and secondary peaks at 19.3, 21.3,

2ϴ

287

2θ of

2θ (JCPDS

anatase phase and the mixed oxides can be

http://dx.doi.org/10.5772/intechopen.72279

anatase phase are predominant at 25.4, 37.0,

in the composite through the calcination process and

Influence of the Synthesis Method on the Preparation Composites Derived...

2θ (JCPDS 01-079-0831). The crystal structure of

2θ (JCPDS 01-089-4921). A less intense peak can be seen at 42.9 o

**Figure 2.** XRD patterns for TiO<sup>2</sup> and LDH compounds, with A (TiO<sup>2</sup> anatase), R (TiO<sup>2</sup> rutile), TiO m (TiO monoclinic crystalline structure), TiO<sup>2</sup> r (TiO<sup>2</sup> rombohedric crystalline structure), B (TiO<sup>2</sup> brookite) H (LDH), and O (MgO) peaks.

The diffraction pattern of LDH (**Figure 2**) shows the typical rhombohedral structure of an MgAl layered double hydroxide, where the diffraction at 11.3 o 2θ corresponds to the basal plane (003), defined as the distance between two adjacent layers. Harmonic reflections corresponding to planes (006), (009), (012), (015), and (110) can also be observed at 22.0, 34.6, 38.2, 47.8, and 61.0 o 2θ, respectively (JCPDS 00-014-0191) [21]. The diffraction pattern for CLDH (**Figure 2**) shows the characteristic peaks of periclase MgO with crystallographic planes (200) and (220) at 42.9 and 62.3 o 2θ, respectively (JCPDS 00-003-0998), which correspond to crystals of a MgAl mixed oxide associated with the collapse of the laminar structure of the LDH [21, 22].

**Figure 3** shows the X-ray diffraction patterns for the synthesized TiO<sup>2</sup> -LDH composites. In the case of the TiO<sup>2</sup> T-LDH composite, the TiO<sup>2</sup> anatase phase and the mixed oxides can be observed separately; the reflections of the TiO<sup>2</sup> anatase phase are predominant at 25.4, 37.0, 37.9, 48.1, 54.0, and 55.2 o 2θ (JCPDS 01-089-4921). A less intense peak can be seen at 42.9 o 2ϴ (JCPDS 01-003-0998) corresponding to CLDH component and another peak at 32.9 o 2θ of the MgTi mixed oxide with rhombohedral crystalline structure (JCPDS 01-079-0831), thus confirming the immobilization of TiO<sup>2</sup> in the composite through the calcination process and achieving the diffusion of the photocatalyst in the composite [21, 23].

The TiO<sup>2</sup> I-LDH composite mostly shows diffraction of the MgTiO<sup>3</sup> mixed oxide with rhombohedral structure, with a main peak located at 32.9 o 2θ and secondary peaks at 19.3, 21.3, 24.1, 35.7, 40.8, 49.3, 53.7, 62.1, and 63.8 o 2θ (JCPDS 01-079-0831). The crystal structure of monoclinic TiO presents two diffractions with peaks located at 42.1 and 43.1 o 2θ (JCPDS

The diffraction pattern of LDH (**Figure 2**) shows the typical rhombohedral structure of an

and LDH compounds, with A (TiO<sup>2</sup>

rombohedric crystalline structure), B (TiO<sup>2</sup>

plane (003), defined as the distance between two adjacent layers. Harmonic reflections corresponding to planes (006), (009), (012), (015), and (110) can also be observed at 22.0, 34.6, 38.2,

(**Figure 2**) shows the characteristic peaks of periclase MgO with crystallographic planes

crystals of a MgAl mixed oxide associated with the collapse of the laminar structure of the

2θ, respectively (JCPDS 00-014-0191) [21]. The diffraction pattern for CLDH

2θ, respectively (JCPDS 00-003-0998), which correspond to

anatase), R (TiO<sup>2</sup>

2θ corresponds to the basal

rutile), TiO m (TiO monoclinic

brookite) H (LDH), and O (MgO) peaks.

MgAl layered double hydroxide, where the diffraction at 11.3 o

r (TiO<sup>2</sup>

286 Titanium Dioxide - Material for a Sustainable Environment

47.8, and 61.0 o

**Figure 2.** XRD patterns for TiO<sup>2</sup>

crystalline structure), TiO<sup>2</sup>

LDH [21, 22].

(200) and (220) at 42.9 and 62.3 o

**Figure 3.** XRD patterns for TiO<sup>2</sup> and LDH compounds, with A (TiO<sup>2</sup> anatase), R (TiO<sup>2</sup> rutile), TiO<sup>2</sup> m (TiO<sup>2</sup> monoclinic crystalline structure), B (TiO<sup>2</sup> brookite), O (MgO), and M (MgTiO<sup>3</sup> rombohedric crystalline structure) peaks.

01-072-0020). The TiO<sup>2</sup> anatase phase in this composite is seen in a small peak of (110) plane at 25.4 o 2θ (JCPDS 01-089-4921) suggesting that only a small part of Ti exists in this phase and the rest is dispersed over the MgAl mixed oxide [21]. Supporting the above statement, no diffraction spikes attributed to the MgAl mixed oxide can be observed, suggesting that the impregnated TiO<sup>2</sup> particles are disaggregated [24] when mixed with the LDH gel and prior to the heat treatment of the composite, achieving a chemical interaction between the composites resulting in the formation of the MgTiO<sup>3</sup> phase [25].

is almost double that the TiO<sup>2</sup>

photodegradation process [19].

properties of the solids; TiO<sup>2</sup>

the TiO<sup>2</sup>

observed in TiO<sup>2</sup>

the surface area for TiO<sup>2</sup>

calcined LDH (54.61 and 77.47 m<sup>2</sup>

in the images, the particles for TiO<sup>2</sup>

On the other hand, in the TiO<sup>2</sup>

I-LDH and TiO<sup>2</sup>

comparing TiO<sup>2</sup>

composites.

them mesoporous materials [22]. The N<sup>2</sup>

In the prepared composites, the presence of TiO<sup>2</sup>

tribution of the surface area of the photocatalyst TiO<sup>2</sup>

The AFM characterization in noncontact mode of TiO<sup>2</sup>

I and LDH precursors (62.99 and 77.47 m<sup>2</sup>

sible pores, reflected in a slight reduction in pore volume [24].

the two- and three-dimensional (2D and 3D) images for the TiO<sup>2</sup>

synthesis [28–29].

may enable a more homogeneous interaction with TiO<sup>2</sup>

I sample. Similarly, the generation of nanometric crystals in

Influence of the Synthesis Method on the Preparation Composites Derived...

adsorption-desorption isotherms that were obtained

induces profound changes in the textural

/g, respectively), attributed to the increase

and CLDH precursors and the com-

B precursors preserve their shape, while their

T used as precursor (1.48 m<sup>2</sup>

/g) compared to the TiO<sup>2</sup>

/g, respectively), attributed to the reduced number of acces-

T-LDH composites, a more uniform morphology can be seen on

B-LDH, where in the latter, it can be more clearly appreciated

/g) compared to

/g), whereas

B precursors and

are spread over

/g), attributed to the con-

T photocatalyst consists of

compared to other methods of LDH

http://dx.doi.org/10.5772/intechopen.72279

289

LDH (11 and 43 nm of LDH and CLDH, respectively) that are obtained by sol-gel synthesis

The textural properties of a photocatalyst affect its contact with pollutants. The heterogeneous photocatalysis process performs better in photocatalysts with high surface area, which increases the probability that the molecules of the pollutant and its oxidation intermediates are in direct contact with the photogenerated holes during irradiation, thus enhancing the

**Table 1** shows the results of surface area, total volume, and mean pore diameter of the synthesized solids. The mean diameter of the samples is in the range of 2 to 50 nm assigning

for the materials in all cases were type IV according to IUPAC classification, where a slow increase in the adsorption process can be observed, followed by a rapid adsorption typical of mesoporous materials. Furthermore, the hysteresis loop for all materials correspond to H3 type, which is associated with the filling and emptying of the mesopore by capillary condensation; this type of hysteresis is usually found in materials that form particle aggregates, which indicate the presence of asymmetric pores, with nonuniform size and shape [4, 30].

of the pore volume in samples [4]. The preparation of this composite entirely by a sol–gel route also accounts for the greater specific surface area of this sample. This behavior is not

posites obtained is shown in the images in **Figure 4**. The surface morphology observed in

spherical particle agglomerates with diameters in the range of 35 to 175 nm. As can be seen

size decreases considerably to diameters in the range of 15 to 50 nm as a result of the sol-gel process used to synthesize these two photocatalysts in particular [31], which is related to the crystal size obtained for these materials. The image for CLDH shows a topography formed by densely packed plate-shaped particles whose horizontal dimensions vary from 40 to 100 nm.

the surface of the larger plates of the LDH component, thus confirming the formation of the

I and TiO<sup>2</sup>

how the components contrast where the smaller spherical particles of TiO<sup>2</sup>

T-LDH, since it has a smaller surface area (70.44 m<sup>2</sup>

B-LDH is reduced (45.29 m<sup>2</sup>

I-LDH presents a bigger surface area (90.12 m<sup>2</sup>

The diffraction pattern of the TiO<sup>2</sup> B-LDH composite is mostly composed of diffractions with the TiO<sup>2</sup> anatase phase at 25.4, 37.9, 48.1, 54.1, 55.2, and 62.6 o 2θ (JCPDS 01-089-4921); similarly, it is possible to observe a peak relating to the TiO<sup>2</sup> rutile phase at 27.6 o 2θ (JCPDS 01-089- 4920), a signal at 30.9 o 2θ of the TiO<sup>2</sup> brookite phase (JCPDS 00-029-1360), and a lesser intense peak at 42.9 o 2θ associated with the MgO oxide (JCPDS 00-003-0998). Reflections related to the formation of MgTiO<sup>3</sup> mixed oxide are absent.

These results confirm the addition of TiO<sup>2</sup> in the composites. In the case of TiO<sup>2</sup> I-LDH, there are significant changes in the structure with respect to the precursors, whereas in the TiO<sup>2</sup> T-LDH and TiO<sup>2</sup> B-LDH samples, the components remain segregated.

Properties such as the photocatalytic crystalline phases, the proportion of each one, and the size of the crystals in photocatalysts are influential in the generation and/or recombination of electron–hole pairs. The formation of the different crystalline phases of a photocatalyst is related to the atomic arrangement and the facet that shows the crystals during irradiation in photocatalytic processes [16]. The crystal sizes of the synthesized samples are calculated using the Debye-Scherrer equation [26] and are given in **Table 1**. The preparation temperature on TiO<sup>2</sup> synthesis affects the formation of the anatase and rutile phases, which is reflected in the size of the crystal formed [27]. The crystal size for the composites is from 26 to 36 nm, observing that the different methodologies used to prepare the composites influence the size of the crystals formed. The TiO<sup>2</sup> T-LDH and TiO<sup>2</sup> B-LDH composites present a similar crystal size to the TiO<sup>2</sup> T and TiO<sup>2</sup> B photocatalysts, while the crystal size for TiO<sup>2</sup> I-LDH


**Table 1.** Crystal size and textural characteristics of the composites.

is almost double that the TiO<sup>2</sup> I sample. Similarly, the generation of nanometric crystals in LDH (11 and 43 nm of LDH and CLDH, respectively) that are obtained by sol-gel synthesis may enable a more homogeneous interaction with TiO<sup>2</sup> compared to other methods of LDH synthesis [28–29].

01-072-0020). The TiO<sup>2</sup>

impregnated TiO<sup>2</sup>

4920), a signal at 30.9 o

formation of MgTiO<sup>3</sup>

temperature on TiO<sup>2</sup>

TiO<sup>2</sup>

TiO<sup>2</sup>

TiO<sup>2</sup>

TiO<sup>2</sup>

TiO<sup>2</sup>

TiO<sup>2</sup>

lar crystal size to the TiO<sup>2</sup>

resulting in the formation of the MgTiO<sup>3</sup>

288 Titanium Dioxide - Material for a Sustainable Environment

These results confirm the addition of TiO<sup>2</sup>

the size of the crystals formed. The TiO<sup>2</sup>

**Sample Crystal size (nm) ASBET (m2**

The diffraction pattern of the TiO<sup>2</sup>

at 25.4 o

the TiO<sup>2</sup>

peak at 42.9 o

LDH and TiO<sup>2</sup>

anatase phase in this composite is seen in a small peak of (110) plane

particles are disaggregated [24] when mixed with the LDH gel and prior to

2θ associated with the MgO oxide (JCPDS 00-003-0998). Reflections related to the

B-LDH composite is mostly composed of diffractions with

rutile phase at 27.6 o

brookite phase (JCPDS 00-029-1360), and a lesser intense

in the composites. In the case of TiO<sup>2</sup>

synthesis affects the formation of the anatase and rutile phases, which is

B photocatalysts, while the crystal size for TiO<sup>2</sup>

2θ (JCPDS 01-089-4921); simi-

B-LDH composites present a simi-

**/g) Average pore diameter (nm)**

2θ (JCPDS 01-089-

I-LDH, there

T-

I-LDH

2θ (JCPDS 01-089-4921) suggesting that only a small part of Ti exists in this phase

and the rest is dispersed over the MgAl mixed oxide [21]. Supporting the above statement, no diffraction spikes attributed to the MgAl mixed oxide can be observed, suggesting that the

the heat treatment of the composite, achieving a chemical interaction between the composites

phase [25].

are significant changes in the structure with respect to the precursors, whereas in the TiO<sup>2</sup>

Properties such as the photocatalytic crystalline phases, the proportion of each one, and the size of the crystals in photocatalysts are influential in the generation and/or recombination of electron–hole pairs. The formation of the different crystalline phases of a photocatalyst is related to the atomic arrangement and the facet that shows the crystals during irradiation in photocatalytic processes [16]. The crystal sizes of the synthesized samples are calculated using the Debye-Scherrer equation [26] and are given in **Table 1**. The preparation

reflected in the size of the crystal formed [27]. The crystal size for the composites is from 26 to 36 nm, observing that the different methodologies used to prepare the composites influence

T-LDH and TiO<sup>2</sup>

**/g) Total pore volume (cm3**

B-LDH samples, the components remain segregated.

anatase phase at 25.4, 37.9, 48.1, 54.1, 55.2, and 62.6 o

mixed oxide are absent.

T and TiO<sup>2</sup>

T 34 1.48 0.003 8.16

I 16 62.99 0.180 11.40

B 25 54.61 0.113 8.30 LDH 11 87.09 0.169 7.74 CLDH 43 77.47 0.151 7.80

T-LDH 36 70.44 0.175 9.95

I-LDH 31 90.12 0.239 10.61

B-LDH 26 45.29 0.111 9.82

**Table 1.** Crystal size and textural characteristics of the composites.

larly, it is possible to observe a peak relating to the TiO<sup>2</sup>

2θ of the TiO<sup>2</sup>

The textural properties of a photocatalyst affect its contact with pollutants. The heterogeneous photocatalysis process performs better in photocatalysts with high surface area, which increases the probability that the molecules of the pollutant and its oxidation intermediates are in direct contact with the photogenerated holes during irradiation, thus enhancing the photodegradation process [19].

**Table 1** shows the results of surface area, total volume, and mean pore diameter of the synthesized solids. The mean diameter of the samples is in the range of 2 to 50 nm assigning them mesoporous materials [22]. The N<sup>2</sup> adsorption-desorption isotherms that were obtained for the materials in all cases were type IV according to IUPAC classification, where a slow increase in the adsorption process can be observed, followed by a rapid adsorption typical of mesoporous materials. Furthermore, the hysteresis loop for all materials correspond to H3 type, which is associated with the filling and emptying of the mesopore by capillary condensation; this type of hysteresis is usually found in materials that form particle aggregates, which indicate the presence of asymmetric pores, with nonuniform size and shape [4, 30]. In the prepared composites, the presence of TiO<sup>2</sup> induces profound changes in the textural properties of the solids; TiO<sup>2</sup> I-LDH presents a bigger surface area (90.12 m<sup>2</sup> /g) compared to the TiO<sup>2</sup> I and LDH precursors (62.99 and 77.47 m<sup>2</sup> /g, respectively), attributed to the increase of the pore volume in samples [4]. The preparation of this composite entirely by a sol–gel route also accounts for the greater specific surface area of this sample. This behavior is not observed in TiO<sup>2</sup> T-LDH, since it has a smaller surface area (70.44 m<sup>2</sup> /g), attributed to the contribution of the surface area of the photocatalyst TiO<sup>2</sup> T used as precursor (1.48 m<sup>2</sup> /g), whereas the surface area for TiO<sup>2</sup> B-LDH is reduced (45.29 m<sup>2</sup> /g) compared to the TiO<sup>2</sup> B precursors and calcined LDH (54.61 and 77.47 m<sup>2</sup> /g, respectively), attributed to the reduced number of accessible pores, reflected in a slight reduction in pore volume [24].

The AFM characterization in noncontact mode of TiO<sup>2</sup> and CLDH precursors and the composites obtained is shown in the images in **Figure 4**. The surface morphology observed in the two- and three-dimensional (2D and 3D) images for the TiO<sup>2</sup> T photocatalyst consists of spherical particle agglomerates with diameters in the range of 35 to 175 nm. As can be seen in the images, the particles for TiO<sup>2</sup> I and TiO<sup>2</sup> B precursors preserve their shape, while their size decreases considerably to diameters in the range of 15 to 50 nm as a result of the sol-gel process used to synthesize these two photocatalysts in particular [31], which is related to the crystal size obtained for these materials. The image for CLDH shows a topography formed by densely packed plate-shaped particles whose horizontal dimensions vary from 40 to 100 nm.

On the other hand, in the TiO<sup>2</sup> T-LDH composites, a more uniform morphology can be seen on comparing TiO<sup>2</sup> I-LDH and TiO<sup>2</sup> B-LDH, where in the latter, it can be more clearly appreciated how the components contrast where the smaller spherical particles of TiO<sup>2</sup> are spread over the surface of the larger plates of the LDH component, thus confirming the formation of the composites.

**Figure 4.** AFM 3D–2D images and height profiles for (a) TiO<sup>2</sup> T, (b) TiO<sup>2</sup> I, (c) TiO<sup>2</sup> B, and (d) CLDH samples and (e) TiO<sup>2</sup> T-LDH, (f) TiO<sup>2</sup> I-LDH, and (g) TiO<sup>2</sup> B-LDH composites.

Surface roughness can be quantitatively identified using the data obtained from the AFM analysis through the definition of the quadratic mean, as follows [32]:

$$R\_{ms} = \sqrt{\frac{\sum\_{n=1}^{N} (z\_n - z)^2}{N - 1}} \tag{1}$$

As observed in the surface profiles in **Figure 4**, all the samples presented roughness, the TiO<sup>2</sup>

**B CLDH TiO2**

Rrms (nm) 32.54 6.72 4.56 18.44 18.75 40.84 39.71

**Table 2.** Roughness values of quadratic mean (*Rrms*) for the precursors and synthesized composites.

coincides with the reduced particle size observed for these last photocatalysts, whereas the CLDH precursor presents a roughness with intermediate value compared to that obtained

of the particles [33], suggesting that the photocatalyst is diffused inside the composite, while the surface is constituted mostly by the CLDH component. This result is not observed in the

into energy peaks at 529.48, 530.01, 531.77, and 532.87 eV attributed to the bonds in TiO<sup>2</sup>

The resolution of the XPS spectrum of Ti 2p3/2 region shows spikes with binding energies of

this latter as a result of the chemical interaction between the LDH oxides and the impregnated

529.20, 529.99, 531.80, and 532.75 eV, respectively. The binding energies of the Ti 2p3/2 region

Similarly, in the XPS spectra of **Figure 5** corresponding to the Ti 2p region, a traditional Ti 2p spectrum can be observed, where the intensity of the Ti 2p3/2 peak is higher than for the Ti 2p1/2 peak. It can also be seen that in all the composites the neighboring distances between the main Ti 2p3/2 and Ti 2p1/2 peaks are close to 5.9+/−0.2 eV. This value indicates that the

The XPS analysis shows the different types of Ti coordination in the samples; the octahedral

coordination is predominant in all the samples. The chemical interaction of Ti with the

found at 457.34, 458.74, and 460.34 eV indicate their presence forming the Ti<sup>2</sup>

(tetragonal) composites, respectively [29, 34].

(octahedral coordination) and the TiO<sup>2</sup>

related to the binding energies 456.90, 458.15, and 458.89 eV, respectively [21, 34].

, and CO, whereas the Ti 2p spectrum is resolved in the coordinations that form

O3

, Al<sup>2</sup> O3

particles in the composite have an octahedral coordination typical of the anatase

I and TiO<sup>2</sup>

**T-LDH TiO2**

Influence of the Synthesis Method on the Preparation Composites Derived...

T-LDH composite shows a decrease in the *Rrms* value compared

B-LDH composites, indicating that in these materials

/Al<sup>2</sup> O3

O3

B-LDH composite in the O 1 s region shows the

(octahedral) and TiO<sup>2</sup>

, and CO composites with peaks at

O3 , TiO<sup>2</sup>

I-LDH is resolved in spikes with binding energies of 529.66,

, MgO, Al<sup>2</sup>

and TiO<sup>2</sup>

T; a reduction in the roughness value is representative of the homogeneity

photocatalysts having a higher value compared to the TiO<sup>2</sup>

**I TiO2**

**T TiO2**

I-LDH and TiO<sup>2</sup>

are presented in **Figure 5**. The O 1 s spectra signals for the TiO<sup>2</sup>

The high-resolution XPS spectra for the O 1 s and Ti 2p regions of the TiO<sup>2</sup>

the photocatalyst remains more superficially exposed.

530.43, 532.10, and 533.37 eV which correspond to TiO<sup>2</sup>

456.93, 458.34, and 459.05 eV corresponding to Ti<sup>2</sup>

contributions of O forming bonds in the MgO, TiO<sup>2</sup>

The resolution of the XPS spectra for the TiO<sup>2</sup>

in the photocatalysts. The TiO<sup>2</sup>

to the value of TiO<sup>2</sup>

**Muestra TiO2**

MgO, Al<sup>2</sup>

TiO<sup>2</sup>

the Ti in Ti<sup>2</sup>

AFM images for the TiO<sup>2</sup>

O3

[12, 30, 34].

dral), and TiOx

charged TiO<sup>2</sup>

TiO<sup>2</sup>

phase [12, 21, 35].

O3

and TiO<sup>2</sup>

The XPS O 1 s spectrum for TiO<sup>2</sup>

T

291

,

B precursors. This trend

**B-LDH**

**I-LDH TiO2**

http://dx.doi.org/10.5772/intechopen.72279


T-LDH material decompose

interaction, which are

, and CO composites.

/Al<sup>2</sup> O3 ;

(octahe-

where *Rrms* is the roughness of the mean quadratic value, *zn* is the height of the nth data point, *z* is equal to the average height of the *zn* values obtained by AFM topography, and *N* is the number of data points. The results of statistical analysis for the materials used are shown in **Table 2**.


**Table 2.** Roughness values of quadratic mean (*Rrms*) for the precursors and synthesized composites.

As observed in the surface profiles in **Figure 4**, all the samples presented roughness, the TiO<sup>2</sup> T photocatalysts having a higher value compared to the TiO<sup>2</sup> I and TiO<sup>2</sup> B precursors. This trend coincides with the reduced particle size observed for these last photocatalysts, whereas the CLDH precursor presents a roughness with intermediate value compared to that obtained in the photocatalysts. The TiO<sup>2</sup> T-LDH composite shows a decrease in the *Rrms* value compared to the value of TiO<sup>2</sup> T; a reduction in the roughness value is representative of the homogeneity of the particles [33], suggesting that the photocatalyst is diffused inside the composite, while the surface is constituted mostly by the CLDH component. This result is not observed in the AFM images for the TiO<sup>2</sup> I-LDH and TiO<sup>2</sup> B-LDH composites, indicating that in these materials the photocatalyst remains more superficially exposed.

The high-resolution XPS spectra for the O 1 s and Ti 2p regions of the TiO<sup>2</sup> -LDH composites are presented in **Figure 5**. The O 1 s spectra signals for the TiO<sup>2</sup> T-LDH material decompose into energy peaks at 529.48, 530.01, 531.77, and 532.87 eV attributed to the bonds in TiO<sup>2</sup> , MgO, Al<sup>2</sup> O3 , and CO, whereas the Ti 2p spectrum is resolved in the coordinations that form the Ti in Ti<sup>2</sup> O3 and TiO<sup>2</sup> (octahedral coordination) and the TiO<sup>2</sup> /Al<sup>2</sup> O3 interaction, which are related to the binding energies 456.90, 458.15, and 458.89 eV, respectively [21, 34].

The XPS O 1 s spectrum for TiO<sup>2</sup> I-LDH is resolved in spikes with binding energies of 529.66, 530.43, 532.10, and 533.37 eV which correspond to TiO<sup>2</sup> , MgO, Al<sup>2</sup> O3 , and CO composites. The resolution of the XPS spectrum of Ti 2p3/2 region shows spikes with binding energies of 456.93, 458.34, and 459.05 eV corresponding to Ti<sup>2</sup> O3 and TiO<sup>2</sup> (octahedral) and TiO<sup>2</sup> /Al<sup>2</sup> O3 ; this latter as a result of the chemical interaction between the LDH oxides and the impregnated TiO<sup>2</sup> [12, 30, 34].

The resolution of the XPS spectra for the TiO<sup>2</sup> B-LDH composite in the O 1 s region shows the contributions of O forming bonds in the MgO, TiO<sup>2</sup> , Al<sup>2</sup> O3 , and CO composites with peaks at 529.20, 529.99, 531.80, and 532.75 eV, respectively. The binding energies of the Ti 2p3/2 region found at 457.34, 458.74, and 460.34 eV indicate their presence forming the Ti<sup>2</sup> O3 , TiO<sup>2</sup> (octahedral), and TiOx (tetragonal) composites, respectively [29, 34].

Surface roughness can be quantitatively identified using the data obtained from the AFM

number of data points. The results of statistical analysis for the materials used are shown in

\_\_\_\_\_\_\_\_\_\_ ∑*<sup>n</sup>*=<sup>1</sup> *<sup>N</sup>* (*zn* − *z*) 2

T, (b) TiO<sup>2</sup>

I, (c) TiO<sup>2</sup>

\_\_\_\_\_\_\_\_\_\_

*<sup>N</sup>* <sup>−</sup> <sup>1</sup> (1)

values obtained by AFM topography, and *N* is the

is the height of the nth data point,

B, and (d) CLDH samples and (e) TiO<sup>2</sup>

T-

analysis through the definition of the quadratic mean, as follows [32]:

B-LDH composites.

*Rrms* = √

**Figure 4.** AFM 3D–2D images and height profiles for (a) TiO<sup>2</sup>

I-LDH, and (g) TiO<sup>2</sup>

290 Titanium Dioxide - Material for a Sustainable Environment

*z* is equal to the average height of the *zn*

**Table 2**.

LDH, (f) TiO<sup>2</sup>

where *Rrms* is the roughness of the mean quadratic value, *zn*

Similarly, in the XPS spectra of **Figure 5** corresponding to the Ti 2p region, a traditional Ti 2p spectrum can be observed, where the intensity of the Ti 2p3/2 peak is higher than for the Ti 2p1/2 peak. It can also be seen that in all the composites the neighboring distances between the main Ti 2p3/2 and Ti 2p1/2 peaks are close to 5.9+/−0.2 eV. This value indicates that the charged TiO<sup>2</sup> particles in the composite have an octahedral coordination typical of the anatase phase [12, 21, 35].

The XPS analysis shows the different types of Ti coordination in the samples; the octahedral TiO<sup>2</sup> coordination is predominant in all the samples. The chemical interaction of Ti with the

**Figure 5.** XPS spectra for Ti 2p and O 1s regions for TiO<sup>2</sup> -LDH composites.

LDH also presents [12]. From these results, it can be deduced that the composites obtained are not mixtures of unrelated components, rather than there is a chemical interaction between them. According to the literature [36], chemical interaction reduces photocatalytic efficiency since the Ti is incorporated into the structure, remaining a lesser extent on the surface. This occurs with greater frequency in the TiO<sup>2</sup> I-LDH composite where, based on the different Ti coordinations presented (see **Figure 6**), the chemical bond between the TiO<sup>2</sup> -Al<sup>2</sup> O3 components occurs in 25.9%, whereas this contribution is lower (19.6%) for TiO<sup>2</sup> T-LDH and is not observed in TiO<sup>2</sup> B-LDH. However, TiO<sup>2</sup> B-LDH is possible to observe the tetragonal TiOx coordination which may enhance photocatalytic efficiency in the degradation of phenolic composites [21], directly relating these results to the photocatalytic degradation rates of each of these materials as it is shown below.

The elementary XPS study in terms of atomic percentage produces the results shown in **Table 3**.


**Muestra Mg Al Ti C O Ti/(Mg + Al)**

T-LDH 14.95 6.83 9.83 24.01 44.38 0.45

I-LDH 30.54 10.06 5.31 9.11 44.98 0.13

B-LDH 9.74 0.95 15.29 22.40 51.62 1.44

odology used in the preparation of this composite is not conducive to its incorporation on the

ratio [36] through elementary XPS analysis, finding values of 1.44, 0.45, and 0.13 for TiO<sup>2</sup>

methods used in the preparation of the materials, finding that the direct mixing of the TiO<sup>2</sup>

enabling increased activity per unit of mass, reducing the agglomeration of photocatalytically active particles and the screening phenomenon, as well as allowing the easy separation and recovery of the solid at the end of its use in photocatalytic processes [36]. Contrary to the

tocatalyst to remain on the external surface, instead of being diffused in the interior causing

I-LDH, leading us to say that the meth-

B-

pho-

B-LDH, the percentage of surface Ti is higher (9.83 and


Influence of the Synthesis Method on the Preparation Composites Derived...

http://dx.doi.org/10.5772/intechopen.72279

293

over the LDH surface can be determined by means of the Ti/(Mg + Al)

I-LDH, respectively. The results are attributed to the synthesis

B-LDH leads to a lower Ti propagation inside this composite,

I-LDH sample presents a lower value in Ti loading, attributed to

solid, causing fewer particles of the TiO<sup>2</sup>

The presence of Ti in the composites is lower in TiO<sup>2</sup>

T-LDH, and TiO<sup>2</sup>

**Figure 6.** Contributions of Ti coordinations in TiO<sup>2</sup>

the direct mixing of the LDH gel with the TiO<sup>2</sup>

T-LDH and TiO<sup>2</sup>

**Table 3.** Elemental analysis by XPS (% atom.) and relation Ti/(Mg+Al) for TiO<sup>2</sup>

surface, while for TiO<sup>2</sup>

15.29%, respectively).

The loading of TiO<sup>2</sup>

and LDH solids as in TiO<sup>2</sup>

aforementioned, the TiO<sup>2</sup>

lower photodegradation rates [12].

LDH, TiO<sup>2</sup>

TiO<sup>2</sup>

TiO<sup>2</sup>

TiO<sup>2</sup>

Influence of the Synthesis Method on the Preparation Composites Derived... http://dx.doi.org/10.5772/intechopen.72279 293

**Figure 6.** Contributions of Ti coordinations in TiO<sup>2</sup> -LDH composites.

LDH also presents [12]. From these results, it can be deduced that the composites obtained are not mixtures of unrelated components, rather than there is a chemical interaction between them. According to the literature [36], chemical interaction reduces photocatalytic efficiency since the Ti is incorporated into the structure, remaining a lesser extent on the surface. This


coordination which may enhance photocatalytic efficiency in the degradation of phenolic composites [21], directly relating these results to the photocatalytic degradation rates of each

coordinations presented (see **Figure 6**), the chemical bond between the TiO<sup>2</sup>

nents occurs in 25.9%, whereas this contribution is lower (19.6%) for TiO<sup>2</sup>

B-LDH. However, TiO<sup>2</sup>

I-LDH composite where, based on the different Ti

B-LDH is possible to observe the tetragonal TiOx


T-LDH and is not

compo-

occurs with greater frequency in the TiO<sup>2</sup>

**Figure 5.** XPS spectra for Ti 2p and O 1s regions for TiO<sup>2</sup>

292 Titanium Dioxide - Material for a Sustainable Environment

of these materials as it is shown below.

observed in TiO<sup>2</sup>


**Table 3.** Elemental analysis by XPS (% atom.) and relation Ti/(Mg+Al) for TiO<sup>2</sup> -LDH composites.

The elementary XPS study in terms of atomic percentage produces the results shown in **Table 3**. The presence of Ti in the composites is lower in TiO<sup>2</sup> I-LDH, leading us to say that the methodology used in the preparation of this composite is not conducive to its incorporation on the surface, while for TiO<sup>2</sup> T-LDH and TiO<sup>2</sup> B-LDH, the percentage of surface Ti is higher (9.83 and 15.29%, respectively).

The loading of TiO<sup>2</sup> over the LDH surface can be determined by means of the Ti/(Mg + Al) ratio [36] through elementary XPS analysis, finding values of 1.44, 0.45, and 0.13 for TiO<sup>2</sup> B-LDH, TiO<sup>2</sup> T-LDH, and TiO<sup>2</sup> I-LDH, respectively. The results are attributed to the synthesis methods used in the preparation of the materials, finding that the direct mixing of the TiO<sup>2</sup> and LDH solids as in TiO<sup>2</sup> B-LDH leads to a lower Ti propagation inside this composite, enabling increased activity per unit of mass, reducing the agglomeration of photocatalytically active particles and the screening phenomenon, as well as allowing the easy separation and recovery of the solid at the end of its use in photocatalytic processes [36]. Contrary to the aforementioned, the TiO<sup>2</sup> I-LDH sample presents a lower value in Ti loading, attributed to the direct mixing of the LDH gel with the TiO<sup>2</sup> solid, causing fewer particles of the TiO<sup>2</sup> photocatalyst to remain on the external surface, instead of being diffused in the interior causing lower photodegradation rates [12].

#### **3.2. Photodegradation and phenol adsorption tests**

In heterogeneous photocatalysis processes, several phenomena can occur. Direct photodegradation is where the adsorption of the organic pollutant on the surface of the catalyst promotes its decomposition by the action of the photogenerated holes. On the other hand, indirect photodegradation is based on the generation of ●OH radicals which react with the organic matter degrading it. Other processes that may occur in phenol photodegradation are direct photolysis due to the presence of UV irradiation and photooxidation by the action of UV radiation and the oxidizing agent, but without any involvement from the photocatalyst [19].

charges [39]. Considering the abovementioned, the particle size based on the AFM analysis is directly related to the percentage of photodegradation found in the photocatalysts, with the

The adsorption rates found in the photocatalysts are lower and unrelated to the surface area

methodology as the most efficient for achieving maximum phenol degradation of 5.5 mg/L (54.6%). According to the XRD study, this is attributed mainly to the optimal anatase-rutile ratio of 88:12% and to smaller crystal size (25 nm), which is reflected in an optimal particle

There is evidence that although LDH and CLDH are not semiconductors, these materials can act as photocatalysts since, due to the presence of Mg2+ and Al3+ cations, other materials with photoinduced defects on the oxide surface can be obtained, which can act as active centers for

ized toward the deficient charge of the Al3+ and the hole oxidizes the surrounding hydroxyl groups allowing the formation of ●OH radicals [41, 30]. This statement is not reinforced by the results obtained in this study, where it can be observed (**Figure 7**) that the percentage of phenol degradation by photocatalysis for LDH and CLDH reaches 15.9 (1.6 mg/L) and 17.1%, (1.7 mg/L), respectively, after 120 minutes of irradiation. This behavior is mostly attributed to the concurrence of the adsorption and photolysis phenomena. Furthermore, in the adsorption process, lower percentages of phenol removal 3.9 (0.4 mg/L) and 2.7% (0.3 mg/L) were observed, with the value achieved with LDH being higher than with mixed oxides (CLDH),

The photodegradation curves obtained for the prepared composites are shown in **Figure 8**. It can be seen that the composites present higher phenol degradation rates than the LDH and CLDH precursors. The photodegradation curves in the composites show bigger photocatalytic activity during the first minutes of the reaction, when there is less competition between the phenol molecules to be degraded; as the passing of time, the photodegradation intermediates produced limit the elimination of phenol by the supported photocatalyst. Over time, there is no appreciable decrease in the photodegradation rate with the exception of the TiO<sup>2</sup>

LDH solid, where it can be observed that after the first few minutes of irradiation, the gradient of the curve remains stable, leading to higher percentages of phenol photodegradation. The

causing a blocking of the radiant energy to the molecules of the photocatalyst [13]. The above-

The effect of synergy between a support material and a catalyst is based on the active surface exposed during the photocatalytic process [24, 36]. Higher degradation percentages

the composite, which effectively reduces agglomeration and at the same time minimizes the screening phenomenon of the photocatalyst during UV irradiation. This is supported by the XPS results obtained, where it can be observed that the proportion of chemical interaction

T-LDH and TiO<sup>2</sup>

B-LDH sample, attributed to the methodology used in preparing

formation of ●OH radicals can be affected by excess coverage of LDH over the TiO<sup>2</sup>

I (1.5%), 0.2 and 0.1 mg/L, respectively. These results confirm the TiO<sup>2</sup>

BC-h<sup>+</sup>

T sample having the highest particle size (in the range of 35–175 nm), which is reflected

B (3.2%) 0.3 mg/L compared to TiO<sup>2</sup>

Influence of the Synthesis Method on the Preparation Composites Derived...

BV pairs where the electron is delocal-

http://dx.doi.org/10.5772/intechopen.72279

BV recombination to be minimized [40].

BC-h<sup>+</sup>

T (1.8%)

295

B-

particles,

I-LDH samples, coinciding with

B synthesis

TiO<sup>2</sup>

and TiO<sup>2</sup>

in a lower photoactivity.

size range (15–50 nm), allowing e−

of the materials, being slightly higher for TiO<sup>2</sup>

the surface reactions, promoting the generation of e−

mentioned is manifested mainly in the TiO<sup>2</sup>

the results of the AFM characterization.

were obtained for the TiO<sup>2</sup>

attributed to the greater surface area and pore volume of LDH.

As observed in **Figure 7**, after 120 minutes of photolytic reaction, degradation of phenol reaches 11.0% (1.1 mg/L). Since there is no absorption or light dispersion by the presence of any solid, majority of the photon flux was used for the photolytic reaction. This process reaches higher speed during the first 30 minutes of irradiation, and then the rate of photodegradation decreases but without reaching equilibrium, attaining the lowest level in all tests due to the lack of a material to act as photocatalyst and/or adsorbent [37]. For precursor photocatalysts, phenol degradation rates of 38.2 (3.8 mg/L), 41.8 (4.2 mg/L), and 54.6% (5.5 g/L) were obtained for TiO<sup>2</sup> T, TiO<sup>2</sup> I, and TiO<sup>2</sup> B, respectively, during 120 minutes of irradiation. Although the TiO<sup>2</sup> anatase phase predominates in all the photocatalysts, as shown in the DRX analysis, the higher performance of the TiO<sup>2</sup> B sample is attributed to the anatase-rutile ratio of 88:12%, which is the closest to the optimal one (80:20%); the rutile phase acts as a e− BC collector reducing recombination rates with h<sup>+</sup> BV and transferring the pollutant particles to the active TiO<sup>2</sup> anatase phase [38]. Another important difference is observed in the formation of crystalline phases of TiO<sup>2</sup> without photocatalytic properties [39], finding the presence of monoclinical TiO and rhombohedral TiO<sup>2</sup> in the TiO<sup>2</sup> T sample, while the TiO<sup>2</sup> brookite phase forms in TiO<sup>2</sup> I, resulting in lower photocatalytic efficiency and in TiO<sup>2</sup> B the anatase and rutile phases they are present.

In addition, based on the particle size obtained for these photocatalysts, it is known that the optimum size in a TiO<sup>2</sup> photocatalyst is in the range of 20 to 30 nm, since this gives an optimum balance between the production of e− BC-h<sup>+</sup> BV pairs and the recombination process due to the surface/volume ratio becoming larger, enabling the timely use of the photogenerated

**Figure 7.** (a) Adsorption, (b) phenol photolysis, and photocatalytic degradation, with TiO<sup>2</sup> and LDH.

charges [39]. Considering the abovementioned, the particle size based on the AFM analysis is directly related to the percentage of photodegradation found in the photocatalysts, with the TiO<sup>2</sup> T sample having the highest particle size (in the range of 35–175 nm), which is reflected in a lower photoactivity.

**3.2. Photodegradation and phenol adsorption tests**

294 Titanium Dioxide - Material for a Sustainable Environment

for TiO<sup>2</sup>

T, TiO<sup>2</sup>

performance of the TiO<sup>2</sup>

the optimum size in a TiO<sup>2</sup>

nation rates with h<sup>+</sup>

bohedral TiO<sup>2</sup>

I, and TiO<sup>2</sup>

in the TiO<sup>2</sup>

lower photocatalytic efficiency and in TiO<sup>2</sup>

optimum balance between the production of e−

closest to the optimal one (80:20%); the rutile phase acts as a e−

T sample, while the TiO<sup>2</sup>

**Figure 7.** (a) Adsorption, (b) phenol photolysis, and photocatalytic degradation, with TiO<sup>2</sup>

In heterogeneous photocatalysis processes, several phenomena can occur. Direct photodegradation is where the adsorption of the organic pollutant on the surface of the catalyst promotes its decomposition by the action of the photogenerated holes. On the other hand, indirect photodegradation is based on the generation of ●OH radicals which react with the organic matter degrading it. Other processes that may occur in phenol photodegradation are direct photolysis due to the presence of UV irradiation and photooxidation by the action of UV radiation

As observed in **Figure 7**, after 120 minutes of photolytic reaction, degradation of phenol reaches 11.0% (1.1 mg/L). Since there is no absorption or light dispersion by the presence of any solid, majority of the photon flux was used for the photolytic reaction. This process reaches higher speed during the first 30 minutes of irradiation, and then the rate of photodegradation decreases but without reaching equilibrium, attaining the lowest level in all tests due to the lack of a material to act as photocatalyst and/or adsorbent [37]. For precursor photocatalysts, phenol degradation rates of 38.2 (3.8 mg/L), 41.8 (4.2 mg/L), and 54.6% (5.5 g/L) were obtained

anatase phase predominates in all the photocatalysts, as shown in the DRX analysis, the higher

[38]. Another important difference is observed in the formation of crystalline phases of TiO<sup>2</sup> without photocatalytic properties [39], finding the presence of monoclinical TiO and rhom-

In addition, based on the particle size obtained for these photocatalysts, it is known that

to the surface/volume ratio becoming larger, enabling the timely use of the photogenerated

BC-h<sup>+</sup>

BV and transferring the pollutant particles to the active TiO<sup>2</sup>

B, respectively, during 120 minutes of irradiation. Although the TiO<sup>2</sup>

B sample is attributed to the anatase-rutile ratio of 88:12%, which is the

brookite phase forms in TiO<sup>2</sup>

photocatalyst is in the range of 20 to 30 nm, since this gives an

B the anatase and rutile phases they are present.

BV pairs and the recombination process due

and LDH.

BC collector reducing recombi-

anatase phase

I, resulting in

and the oxidizing agent, but without any involvement from the photocatalyst [19].

The adsorption rates found in the photocatalysts are lower and unrelated to the surface area of the materials, being slightly higher for TiO<sup>2</sup> B (3.2%) 0.3 mg/L compared to TiO<sup>2</sup> T (1.8%) and TiO<sup>2</sup> I (1.5%), 0.2 and 0.1 mg/L, respectively. These results confirm the TiO<sup>2</sup> B synthesis methodology as the most efficient for achieving maximum phenol degradation of 5.5 mg/L (54.6%). According to the XRD study, this is attributed mainly to the optimal anatase-rutile ratio of 88:12% and to smaller crystal size (25 nm), which is reflected in an optimal particle size range (15–50 nm), allowing e− BC-h<sup>+</sup> BV recombination to be minimized [40].

There is evidence that although LDH and CLDH are not semiconductors, these materials can act as photocatalysts since, due to the presence of Mg2+ and Al3+ cations, other materials with photoinduced defects on the oxide surface can be obtained, which can act as active centers for the surface reactions, promoting the generation of e− BC-h<sup>+</sup> BV pairs where the electron is delocalized toward the deficient charge of the Al3+ and the hole oxidizes the surrounding hydroxyl groups allowing the formation of ●OH radicals [41, 30]. This statement is not reinforced by the results obtained in this study, where it can be observed (**Figure 7**) that the percentage of phenol degradation by photocatalysis for LDH and CLDH reaches 15.9 (1.6 mg/L) and 17.1%, (1.7 mg/L), respectively, after 120 minutes of irradiation. This behavior is mostly attributed to the concurrence of the adsorption and photolysis phenomena. Furthermore, in the adsorption process, lower percentages of phenol removal 3.9 (0.4 mg/L) and 2.7% (0.3 mg/L) were observed, with the value achieved with LDH being higher than with mixed oxides (CLDH), attributed to the greater surface area and pore volume of LDH.

The photodegradation curves obtained for the prepared composites are shown in **Figure 8**. It can be seen that the composites present higher phenol degradation rates than the LDH and CLDH precursors. The photodegradation curves in the composites show bigger photocatalytic activity during the first minutes of the reaction, when there is less competition between the phenol molecules to be degraded; as the passing of time, the photodegradation intermediates produced limit the elimination of phenol by the supported photocatalyst. Over time, there is no appreciable decrease in the photodegradation rate with the exception of the TiO<sup>2</sup> B-LDH solid, where it can be observed that after the first few minutes of irradiation, the gradient of the curve remains stable, leading to higher percentages of phenol photodegradation. The formation of ●OH radicals can be affected by excess coverage of LDH over the TiO<sup>2</sup> particles, causing a blocking of the radiant energy to the molecules of the photocatalyst [13]. The abovementioned is manifested mainly in the TiO<sup>2</sup> T-LDH and TiO<sup>2</sup> I-LDH samples, coinciding with the results of the AFM characterization.

The effect of synergy between a support material and a catalyst is based on the active surface exposed during the photocatalytic process [24, 36]. Higher degradation percentages were obtained for the TiO<sup>2</sup> B-LDH sample, attributed to the methodology used in preparing the composite, which effectively reduces agglomeration and at the same time minimizes the screening phenomenon of the photocatalyst during UV irradiation. This is supported by the XPS results obtained, where it can be observed that the proportion of chemical interaction

**Figure 8.** (a) Adsorption and (b) phenol photodegradation with TiO<sup>2</sup> -LDH composites.

between Ti and mixed oxides (CLDH) is in direct relation to the photodegradation percentages, as observed for TiO<sup>2</sup> T-LDH and TiO<sup>2</sup> I-LDH, where there is a chemical interaction between Ti and the mixed oxides (CLDH), causing the impregnated Ti to be diffused to a greater degree inside the composite, with a lesser proportion to be spread over the surface [36]. The opposite can be seen in the TiO<sup>2</sup> B-LDH sample, reflected in better photocatalytic performance.

Another possible cooperative effect between the TiO<sup>2</sup> mixed with LDH in the composites can be explained by the CLDH reconstruction process, since, when these are put in contact with an aqueous solution, they form highly hydroxylated species on the surface which can react with the photogenerated holes to promote ●OH radical production, enabling them to attack the phenol more effectively [13].

The adsorptive capacity of the composites in general is minimal and is unrelated to the surface area of the materials; in the case of the TiO<sup>2</sup> I-LDH sample, the percentage of phenol adsorbed is 1.5% (0.15 mg/L), in TiO<sup>2</sup> T-LDH is 1.8% (0.18 mg/L), and in TiO<sup>2</sup> B-LDH is 3.2% (0.32 mg/L).

Based on the results obtained, it can be assumed that, in general, phenol removal in the composites is attributed to indirect photocatalytic degradation through oxidation by ●OH radicals as opposed to direct degradation by photogenerated holes due to the low adsorption rates of these materials [19].

The photocatalytic efficiency in the synthesized TiO<sup>2</sup> -LDH composites depends mainly on the degree of chemical interaction of the impregnated Ti, which is in direct relation to the proportion of Ti diffused into the composite and the presence of photocatalytically active phases found on the surface of the material leaving the catalyst more exposed to UV irradiation, avoiding agglomeration and the screening phenomenon.
