**6. Photocatalytic activity**

goes up with an increase in *Γ*, which means that the oxidation ability of the VB holes can be tuned by *Γ.* This is the most unique and important feature of the TiO2 modification with MOCs

**Figure 6.** (A) VB-XPS spectra for Fe2O3/TiO2 with varying *Γ*. (B) Energy shift in the VB maximum level (∆*E*VBM) as a

These results were further simulated by the density functional theory (DFT) calculations [7]. In the DFT-optimized structure for a model Fe2O3 cluster-adsorbed TiO2 system, plural Fe–O– Ti interfacial bonds were observed. The PEDOS (projected electronic density of states) plots showed that states from the adsorbed Fe2O3 clusters lie above the VB of TiO2, that is, the iron oxide-derived states make it to the top of the VB. This changes the nature of the VB edge that moves the top of the VB to higher energy. The offsets between the TiO2 VB edge and the iron oxide states around the VB are ~0.3 eV for Fe2O3-modified TiO2, which is comparable with the experimental value. The effective mixing between the surface Fe2O3 levels and O 2*p* through the Tis–O–Fe interfacial bond is considered as yielding a d-band overlapping the VB of TiO2. Thus, the excitation of Fe2O3/TiO2 by the visible light with wavelength below 500 nm can induce

Conversely, the information about empty levels can be obtained by photoluminescence spectroscopy. TiO2(ST-01) has a broad emission band centered at 538 nm (*E*1) [22]. The *E*1 signal intensity remarkably weakens with heating ST-01 at 773 K for 1 h in air. This PL band is assignable to the emission from the surface oxygen vacancy levels of anatase TiO2 [33]. On modifying ST-01 with the Fe oxocomplexes, the intensity further decreases to disappear at *Γ* > 0.044 ions nm−2, while a new emission appears at 423 nm (*E*2). The *E*<sup>2</sup> signal can be attributed to the emissions from extrinsic levels. These findings strongly suggest that the excited electrons in the CB of TiO2 are transferred to the empty surface Fe oxocomplex levels with the energy

As stated above, the ORR is frequently the key process in the TiO2-photocatalyzed reactions as well as low-temperature polymer electrolyte membrane fuel cells (PEMFCs) [34,35]. Figure 7 shows dark current (*I*)–potential (*E*) curves of the mp-TiO2 film-coated F-doped tin oxide (FTO) electrodes (mp-TiO2/FTO) with and without the surface modification by Fe oxocom‐

function *Γ* for various MOCs/TiO2. The figure (A) was taken from ref. 35.

366 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

distributed around 0.27 V below the CB of TiO2.

**5.3. Electrocatalytic activity for oxygen reduction reaction**

the interfacial electron transfer from the surface d-band to the CB of TiO2.

using the CCC technique.

Acetaldehyde is a toxic volatile organic compound (VOC), while 2-naphthol is widely used as the starting material of azo dyes. Both of them are optically transparent in the visible region, and then, acetaldehyde and 2-naphthol were used as model air and water pollu‐ tants, respectively. The relative photocatalytic activities of various MOCs/TiO2 were evaluated with respect to that of highly active commercial TiO2 particles with a crystal form of anatase (ST-01, Ishihara Sangyo Co.). The photocatalytic degradation of 2-naphthol and acetaldehyde apparently follows the first-order rate law, and the rate constants for irradiation of UV light (330 < *λ* < 400 nm, *k*UV) and visible light (*λ* > 400 nm, *k*vis) were used as the indicators for the photocatalytic activities. Figure 8 shows the *k*vis and *k*UV of Fe2O3/ TiO2 for the degradations of 2-naphthol (A) and acetaldehyde (B) as a function of *Γ*. Surprisingly, the surface modification of TiO2 by the Fe-oxocomplex gives rise to a high level of visible-light activity and a concomitant increase in the UV-light activity of ana‐ tase TiO2 [24]. Each plot exhibits a volcano-shaped curve, which is a general feature in the activity-*Γ* plots for the MOC/TiO2 systems. By using an atomic layer deposition techni‐ que, Libera et al. have recently prepared Fe(III) oxospecies–surface-modified TiO2 show‐ ing a reactivity for the decoloration of methylene blue under visible-light irradiation [18].

Particulate systems have high photocatalytic activity due to the large surface area, but needs the troublesome separation of the particles from purified water. Oppositely, in supported films, the photocatalytic activity is generally much lower due to the smaller surface area,

**Figure 8.** (A) UV-light activity (*k*UV, blue) and visible-light activity (*k*vis, red) of Fe2O3/TiO2(ST-01) for the liquid-phase decomposition of 2-NAP as a function of *Γ*. (B) UV-light activity (*k*UV, blue) and visible-light activity (*k*vis, red) of Fe2O3/TiO2(ST-01) for the gas-phase decomposition of CH3CHO as a function of *Γ*.

**Figure 9.** TiO2 NTA parepared by a two-step anodization of Ti plate (first anodization 40 V, 0.5 h/second anodization 40 V-1 h/heating temperature, *T*c = 773 K). The figure was taken from Ref. 35.

while the separation process is unnecessary. TiO2 nanotube array (NTA) has the advantag‐ es of the particulate and film systems is promising. Figure 9 shows TEM image for TiO2 NTA prepared by two-step anodization. The application of the CCC technique to the TiO2 NTA led to a high visible-light activity for 2-naphthol degradation comparable with that of the particulate system [36].

Figure 10 compares the relative visible-light activity (*k*vis) and UV-light activity (*k*UV) of 3d MOCs/TiO2(ST-01) with respect to the activities of unmodified TiO2(ST-01) (*k*vis0 and *k*UV0 ) for the 2-naphthol degradation under the same conditions. Each *Γ* shows the optimum value for visible-light activity in each MOC/TiO2 system. Among MOCs, the surface modifica‐ tion by Fe2O3 [24], Co2O3 [25], and NiO [30] is effective in the visible-light activation. Particularly, the Co2O3/TiO2 system exhibits a very high level of visible-light activity [29]. The activity is on the order of Co2O3 > Fe2O3 > NiO > CuO > V2O5 ≈ Mn2O3 > SnO2 ≈ unmodified TiO2. However, the surface modification with Fe2O3, NiO, and Co2O3 by the CCC technique can endow anatase TiO2 with high levels of visible-light activity, with the high UV-light activity further increased (Fe2O3) or maintained (Co2O3, NiO). Although the effect of the surface modification by SnO2 was small for anatase, a significant increase in the UV-light activity was induced for rutile [37]. Interestingly, Boppana and Lobo have recently reported that loading of SnOx clusters on ZnGa2O4 by the impregnation method causes visible-light activity for the decomposition of p-cresol [38]. Besides metal oxides, the surface modification of TiO2 with halogeno complexes of rhodium(III) and platinum(IV) on the TiO2 surface is known to induce visible-light activity [39,40].

**Figure 10.** Comparison of the visible-light activities (*k*vis) and UV-light activities (*k*UV) of MOCs/TiO2(ST-01) with re‐ spect to those of unmodified TiO2 for the 2-naphthol degradation under the same conditions.

The degradation of formic acid was further carried out in the aqueous phase with Co2O3/ TiO2 at 298 K under visible-light irradiation. The Co2O3 surface modification greatly enhanced the decomposition of formic acid to CO2. The visible-light activity reached a maximum at *Γ* = 0.17 with the conversion to CO2 reaching ~100% within 5 h [29] (Eq. 3).

$$\text{HCOOH} + 1/2\text{O}\_2 \xrightarrow[\text{IV} \times 400 \text{nm})} \text{CO}\_2 + \text{H}\_2\text{O} \tag{3}$$

Also, prolonging irradiation decomposed 2-naphthol to CO2, but the conversion was only ~6% at 96 h. The decomposition of 2-naphthol to CO2 would proceed stepwise via oxidative cleavage of the naphthalene ring.

while the separation process is unnecessary. TiO2 nanotube array (NTA) has the advantag‐ es of the particulate and film systems is promising. Figure 9 shows TEM image for TiO2 NTA prepared by two-step anodization. The application of the CCC technique to the TiO2 NTA led to a high visible-light activity for 2-naphthol degradation comparable with that

**Figure 9.** TiO2 NTA parepared by a two-step anodization of Ti plate (first anodization 40 V, 0.5 h/second anodization

**Figure 8.** (A) UV-light activity (*k*UV, blue) and visible-light activity (*k*vis, red) of Fe2O3/TiO2(ST-01) for the liquid-phase decomposition of 2-NAP as a function of *Γ*. (B) UV-light activity (*k*UV, blue) and visible-light activity (*k*vis, red) of

Fe2O3/TiO2(ST-01) for the gas-phase decomposition of CH3CHO as a function of *Γ*.

368 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

40 V-1 h/heating temperature, *T*c = 773 K). The figure was taken from Ref. 35.

Figure 10 compares the relative visible-light activity (*k*vis) and UV-light activity (*k*UV) of 3d MOCs/TiO2(ST-01) with respect to the activities of unmodified TiO2(ST-01) (*k*vis0 and *k*UV0

for the 2-naphthol degradation under the same conditions. Each *Γ* shows the optimum value for visible-light activity in each MOC/TiO2 system. Among MOCs, the surface modifica‐ tion by Fe2O3 [24], Co2O3 [25], and NiO [30] is effective in the visible-light activation. Particularly, the Co2O3/TiO2 system exhibits a very high level of visible-light activity [29]. The activity is on the order of Co2O3 > Fe2O3 > NiO > CuO > V2O5 ≈ Mn2O3 > SnO2 ≈ unmodified TiO2. However, the surface modification with Fe2O3, NiO, and Co2O3 by the CCC technique can endow anatase TiO2 with high levels of visible-light activity, with the high UV-light activity further increased (Fe2O3) or maintained (Co2O3, NiO). Although the effect of the surface modification by SnO2 was small for anatase, a significant increase in the UV-light activity was induced for rutile [37]. Interestingly, Boppana and Lobo have

)

of the particulate system [36].

On the basis of the energy band diagram, the action mechanism of MOCs in the TiO2 photo‐ catalysis can be explained. In the nanoscale Fe2O3–TiO2 coupling system, Fe2O3 NP with a band gap of 2.2 eV is excited by the visible-light irradiation. However, the potential of the CB electrons is more positive than the TiO2 CB minimum of TiO2 (−0.48 V) and the standard redox potential of O2 (*E*<sup>0</sup> (O2/O2 − ) = −0.284 V). Thus, the electron transfer from the CB electrons of Fe2O3 to neither TiO2 nor O2 can occur. Consequently, nano-coupling does not show visiblelight activity [41].

Scheme 3 illustrates the surface modification effects of the Fe oxocomplex on the TiO2 photocatalytic decomposition of organic pollutants. In this case, the surface modification raises the VB maximum with the CB minimum unchanged, due to the effective electronic coupling through the Fe–O–Ti interfacial bonds (Effect 1). The resulting decrease in the band gap shifts the light absorption to the visible region (Effect 2). The visible-light absorption triggers electronic excitation from the highest-energy oxocomplex-derived VB states to the empty CB

**Sheme 3.** Surface modification effects of the Fe oxocomplex on the TiO2-photocatalyzed decomposition of organic pol‐ lutants. The levels around −0.2 V show the vacant Fe oxocomplex.

of TiO2 in order to generate charge carriers. This surface-to-bulk interfacial electron transfer enhances charge separation (Effect 3). The surface modification permits the electron transfer from the CB of TiO2 to shallow vacant surface oxocomplex levels, which distribute around ca. −0.2 V [22]. The formation of O2 − radicals was confirmed by chemiluminescence photometry in the Cu2+-grafted TiO2 system under visible-light irradiation [42]. In this cathodic process, the electrons efficiently reduce adsorbed O2 with the aid of the electrocatalytic activity of the surface-adsorbed oxocomplex (Effect 4). This effect should also contribute to the increase in the UV-light activity. In the anodic process, the holes generated in the VB could take part in the oxidation process without diffusion (Effect 5) [15]. Consequently, Fe2O3/TiO2 as well as NiO/TiO2 and Co2O3/TiO2 satisfy the three requirements of the "solar environmental catalyst."
