**5. Characteristics of metal oxocomplex–surface-modified TiO2**

## **5.1. Optical property**

X-ray absorption near-edge structure (XANES) spectra for the iron oxide/TiO2 samples, and authentic Fe, Fe3O4, and α-Fe2O3 for comparison. The absorption edge of the iron oxide/TiO2 sample is in agreement with that of α-Fe2O3, indicating the oxidation number of the iron to be

structure (EXAFS) for the iron oxide/TiO2 samples. The peaks around 1.6 Ǻ and 2.8 Ǻ arise from the scattering from the nearest neighbor O and Fe, respectively. It is worth noting that the latter peak becomes very weak at *Γ* ≤ 0.5. Evidently, the iron oxides exist as a mononuclear

**Figure 3.** TEM images of Fe2O3/TiO2 with varying *Γ* : (A) *Γ* = 0 (P-25); (B) *Γ* = 0.23, (C) *Γ* = 0.38, (D) *Γ* = 0.54.

than the reciprocal number density of the Ti-OH groups on the TiO2 surface (~0.1 nm2

As illustrated in Scheme 2, the molecular size of Fe(acac)3 (~0.5 nm2

Consequently, mononuclear MOCs can be formed on the TiO2 surface.

Fourier transforms of the *k*<sup>3</sup>

In this manner, MOCs are formed on the TiO2 surface in a highly dispersed state by the CCC technique, whereas the conventional impregnation method usually yields metal oxide NPs.

−1) [25]. In the first step of the CCC process, Fe(acac)3 complexes are chemisorbed isolatedly each other owing to the bulky acac-ligands. Also, the strong bond between the complexes and the TiO2 surface suppresses the aggregation of the oxocomplexes during the second step.

**Figure 4.** XANES and EXAFS spectra. (A) XANES spectra for Fe, Fe3O4, α-Fe2O3, and Fe2O3/TiO2 with varying *Γ*. (B)



complex−1) is much larger

group

+3 (Table 1). Figure 4B shows the Fourier transforms of the *k*<sup>3</sup>

364 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

Fe oxocomplex on TiO2 designated as Fe2O3/TiO2 below.

The optical property is a fundamental factor affecting the photocatalytic activity. Figure 5 shows UV–visible absorption spectra for Fe oxocomplex–surface-modified mesoporous TiO2 nanocrystalline films (Fe2O3/mp-TiO2) with varying *Γ*. Impregnation samples usually have absorption approximately 470 nm in addition to absorption at 410 nm [16,17,26,27]. The former and latter absorptions can be attributed to the d–d transition and electronic transition from Fe3+ levels to the CB of TiO2, respectively [28]. A strong d–d absorption is also observed for a heavy-loading CCC sample (Fe2O3(*Γ* = 5.5)/mp-TiO2). In contrast, the absorption spectra of Fe2O3(*Γ* ≤ 2.1)/mp-TiO2 apparently show a marked bandgap narrowing from 3.3 to 2.85 eV with an increase in *Γ*, whereas the d–d transition absorption is very weak [24]. Similar spectra were previously observed for TiO2 doped with Cr [12] and N [13] prepared by ion implantation and magnetron sputtering. The weak d–d transition absorption is a common feature for the CCC samples including Co2O3/TiO2 [29], NiO/TiO2 [30,31], and CuO/TiO2 [32]. Clearly, the unique optical properties of the CCC samples originate from the highly dispersed MOCs on the TiO2 surface.

**Figure 5.** UV–Vis absorption spectra of Fe2O3/mp-TiO2 prepared by the CCC technique.

## **5.2. Fine-tuning of band energy**

The VB maximum determines the oxidation ability of the holes, and thus is a key factor for the decomposition of organic pollutants by semiconductor photocatalysts. The VB maximum level can be estimated from the VB-X-ray photoelectron spectroscopy (XPS) [24]. Since the VB maximum of TiO2 is almost independent of its crystal form and size, the VB maximum of MOCmodified TiO2 can be compared with respect to that of unmodified TiO2. Figure 6 shows the VB-XPS spectra for Fe2O3/TiO2 with varying *Γ*. The emission from the O 2*p*–VB extends from 2 to 9 eV. As a result of the surface modification, the top of VB rises, whereas the bottom remains invariant. Figure 6B compares the energy shift in the VB maximum level with respect to that of unmodified TiO2 (∆*E*VBM) as a function of *Γ* for the Fe2O3/TiO2, Co2O3/TiO2, NiO/TiO2, and SnO2/TiO2 systems. Interestingly, the ∆*E*VBM for the MOC-modified TiO2, except SnO2/TiO2, 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 using the CCC technique.

**Figure 6.** (A) VB-XPS spectra for Fe2O3/TiO2 with varying *Γ*. (B) Energy shift in the VB maximum level (∆*E*VBM) as a function *Γ* for various MOCs/TiO2. The figure (A) was taken from ref. 35.

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

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 distributed around 0.27 V below the CB of TiO2.

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

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‐ plexes. In the absence of O2, a small current due to water reduction is observed regardless of the surface modification. In the presence of O2, the current for ORR markedly increases with the surface modification (Fe2O3/mp-TiO2/FTO), whereas it remains weak without O2. In this manner, the surface Fe oxocomplex has an electrocatalytic activity for the ORR, and a similar ORR-promoting effect is also observed for the NiO/TiO2 [30] and Co2O3/TiO2 [29] systems. This is also the unique feature of the MOC/TiO2 systems.

**Figure 7.** (A) Dark current (*I*)–potential (*E*) curves for the Fe2O3/mp-TiO2/FTO electrodes. (B) Comparison of the elec‐ trocatalytic activity of the MOC/mp-TiO2/FTO electrodes for the ORR.
