Effective Inclusion of Sizable Amounts of Mo within TiO2 Nanoparticles Can Be Obtained by Reverse Micelle Sol-Gel Synthesis.

Six Mo/TiO2 samples (with 0, 1.0, 2.5, 5.0, 7.5, and 10 wt % Mo nominal contents) were obtained by reverse micelle sol-gel synthesis, followed by calcination at 500 °C. The samples were characterized by means of powder X-ray Diffraction (PXRD), quantitative phase analysis as obtained by Rietveld refinement, field-emission scanning electron microscopy (FE-SEM) coupled with energy-dispersive X-ray analysis, N2 adsorption/desorption at -196 °C, X-ray photoelectron spectroscopy, and diffuse reflectance (DR) UV-vis spectroscopy. As a whole, the adopted characterization techniques showed the inclusion of a sizeable Mo amount, without the segregation of any MoO x phase. Specifically, PXRD showed the occurrence of anatase and brookite with all the studied samples; notwithstanding the mild calcination temperature, the formation of rutile occurred at Mo wt % ≥2.5 likely due to the presence of brookite favoring, in turn, anatase to rutile transition. DR UV-vis and XP spectroscopies allowed determining the samples' band gap energy (E g) and valence band energy, respectively, from which the conduction band energy was calculated; and the observed E g value increase at 10 wt % Mo was ascribed to the Moss-Burstein effect.

Introduction

TiO2 is one of the most studied oxides due to its unique physicochemical properties, including the low toxicity and its availability in several morphologies, polymorphic composition, and nanoparticle (NP) size. Such reasons, along with its chemical stability and light absorption properties, allow its use in various fields, namely pigments, food additives, PPCPs (pharmaceuticals and personal care products), DSSC (dye-sensitized solar cells), and heterogeneous catalysis, particularly photocatalysis.[1−5]

Improving the ability to exploit solar light is particularly useful for photocatalytic applications because TiO2 mostly absorbs in the UV range,[6−8] and doping with heteroatoms (either metals or nonmetals) is a widely adopted strategy to shift the absorption edge toward the vis range.[9−14] Doping may occur by different means leading to different results, and sometimes disappointing, i.e., when high dopant levels induce the formation of defects that, in turn, favor the electron/hole recombination, ultimately lowering the photocatalytic activity.[15−17]

Many literature methods that allow doping TiO2, while simultaneously controlling the NP shape/size and type of polymorphic phase, imply the use of the sol–gel technique and the presence of a (soft or hard) template in order to induce (either intra- or interparticle) mesoporosity because the occurrence of mesopores is a desirable property, especially for catalytic and photocatalytic applications requiring a facile diffusion of reagents/products.[4,18−23]

Other doping methods provided fair results as well, including spray pyrolysis and magnetron sputtering,[24,25] laser ablation in solution,[26] hydrothermal synthesis,[27] etc. However, the availability of both diblock and triblock copolymers (acting as nonionic surfactants) and the EISA[28] (evaporation-induced self-assembly) technique (allowing the obtainment of uniform thin films) contributed to the widespread of template-assisted sol–gel techniques.[29] The use of a diblock copolymer in an organic solvent allows, indeed, the formation of reverse micelles having a hydrophobic shell and a hydrophilic core that acts as a nanoreactor, where NP nucleation and growth occur also in the presence of heteroatoms (here, Mo), facilitating their effective inclusion in large(r) amounts and the size of NPs being determined by the micelle itself.[30,31] It has been previously reported that reverse micelles provide an acidic environment, leading to the occurrence of brookite-containing Mo-doped NP TiO2:[13] the occurrence of brookite was found to play a role in the stabilization of electron/hole pairs during the photodegradation of rhodamine B under simulated solar light. Furthermore, the positive photocatalytic effect of brookite was also demonstrated with a set of (undoped) TiO2 NPs toward the solar photodegradation of the emerging pollutant N-phenylurea.[18]

Concerning the doping mechanism(s), DFT (density functional theory) calculations have shown[32] that Mo 4d orbitals form impurity levels below the conduction band (CB) of TiO2, whereas the Fermi level should not shift, finally improving the absorption of vis light in the resulting material. As mentioned before, the type of doping sites, i.e., substitutional versus interstitial ones, is crucial[33,34] because the former has a prevailing role on the resulting TiO2 electronic structure, red shift, and light absorption abilities. Both MoVI6+ and MoVI5+ species have an ionic radius (0.59 and 0.61 Å, respectively) allowing the isomorphic substitution of TiVI4+ ions (0.605 Å).[35] In order to balance the charge, the formation of Mo5+ species usually occurs (although a MoVI6+ precursor is used), otherwise, oxygen vacancies and/or Ti3+ species form.

In this work, a set of six samples with nominal Mo contents in the 0–10 wt % range was considered in order to provide an insight into the doping mechanism (interstitial vs substitutional); and the type of TiO2 polymorphs and how, in turn, such polymorphs may affect the Mo inclusion or vice versa. Indeed, according to the literature, the rutile structure may accommodate heteroatoms/defects more easily than anatase, while the ATR (anatase to rutile) transition may be favored by brookite, whereas high valence dopant cations (charge >+4) disfavor ATR.[36]

The aim of this work is to effectively introduce, by reverse micelle sol–gel technique, sizable amounts of Mo in TiO2 NPs in order to redshift their absorption onset and to lower the band gap energy. Furthermore, several complementary characterization techniques have been adopted in order to gain an insight into the type of Mo doping, the nature of Mo species and, consequently, the structural and surface properties of the obtained materials.

Results and Discussion

Textural Properties of the Samples

Figure a shows the powder XRD patterns of the samples in the 20–90 2θ range. The TiO2 sample showed the peaks of anatase (labeled A) at the 2θ values of 25.2 (101), 37.8 (004), 47.9 (200), 53.8 (105), 54.9 (211), 62.6 (204), 68.7 (116), 70.1 (220), 74.9 (125), and 82.5 (224); and the same sample also showed a broad and weak signal centred at ca. 30.7 2θ (labeled B), readily ascribed to the (121) diffraction peak of brookite, and the formation of which is favored at pH values as low as 2.0[18,37] and, here, was likely favored by the adopted synthesis procedure, implying extremely acidic conditions in the reverse micelle core. The Mo_1 sample showed very similar XRD patterns, albeit the (121) diffraction of brookite seemed less intense. At a higher Mo content (samples Mo_2.5, Mo_5, Mo_7.5, and Mo_10), two additional peaks (labeled R) were observed at 27.2 and 54.4 2θ values, respectively, assigned to the (110) and (211) diffraction peaks of rutile. Concerning the effect of Mo6+/Mo5+ doping, especially on the reticular plane of the various phases, a careful analysis of the 35–40 2θ range was carried out. The inset in Figure a shows a magnification of the 35–40 2θ range for the XRD patterns of the TiO2, Mo_5, and Mo_10 samples only, in order to highlight possibly relevant strain/stress phenomena, as reported by the literature in similar TiO2-based materials.[38−40] After a careful analysis of the shape of the anatase (004) peak profiles, no relevant differences were, however, observed, likely due to the small difference in the cationic radii of octahedrally coordinated Ti4+, Mo5+, and Mo6+ ions (0.605, 0.61, and 0.59 Å, respectively).

Figure 1

Powder XRD patterns of the studied samples (a); amounts of anatase, brookite, and rutile phases (wt %, as obtained by the Rietveld refinement) vs the samples’ nominal composition (b); anatase cell volume (Å3, as obtained by the Rietveld refinement) vs the samples’ nominal composition (c). Inset to section (a) reports a magnification of the 35–40 2θ range for a selected set of samples (namely, TiO2, Mo_5, and Mo_10) for the sake of clarity.

Table reports the corresponding quantative phase analysis (QPA) results, as obtained by the Rietveld refinement. Besides anatase, QPA showed the occurrence of brookite with all the samples, whereas the formation of rutile occurred at Mo wt % ≥2.5. Interestingly, no signals ascribable to any crystalline MoO phase were detected, neither with the Mo_10 sample: on the one side, amorphous and/or extremely dispersed MoO phases, if present, could escape the XRD detection, and on the other side, micro-Raman spectroscopy (Figure S1) shows the occurrence of the surface polymolibdate species (i.e., Mo7O246– and Mo8O264– species, where Mo was octahedrally coordinated) at a Mo content ≥5 wt %, without the MoO3 segregation. Previous literature results[41] concerning samples at a comparable Mo content that were obtained by a microemulsion method showed the formation of both Mo surface species and MoO phases, indicating that the reverse micelle sol–gel synthesis described here was more effective toward the inclusion of Mo in the TiO2 matrix, likely because of the adopted surfactant.[42]

Table 1

Textural Properties of the Studied Samples as Obtained by Powder XRD, FE-SEM, and EDX Analyses, N2 Isotherms at −196 °C

		anatase
sample	Mo wt %a	wt %b	crystal size (nm)b	brookite wt %b	rutile wt %b	average NP size (nm)c	BET SSA (m2 g–1)	total pore volume (cm3 g–1)
TiO2		91.00	16.3	9.00		10.7	71	0.084
Mo_1	0.83	90.00	18.7	10		20.6	42	0.064
Mo_2.5	3.2	87.56	17.1	6.04	6.40	25	92	0.161
Mo_5	5.8	85.46	18.5	7.96	6.58	25.1	74	0.133
Mo_7.5	7.7	90.30	22.1	5.41	4.27	19.7	65	0.122
Mo_10	9.75	93.47	18.6	3.06	3.47	16.1	96	0.127

As obtained by the EDX analysis

As obtained by the Rietveld refinement

As obtained by the FE-SEM analysis

Figure b shows the trend of anatase and brookite (and rutile) wt % as a function of the sample nominal composition and the actual composition, as determined by the energy-dispersive X-ray (EDX) analysis, being indeed very close to the nominal one (Table ), indicating the successful inclusion of Mo in the TiO2 matrix by the adopted synthesis method. Interestingly, with the TiO2, Mo_1, Mo_2.5, and Mo_5 samples, the anatase wt % decreases and, correspondingly, the brookite wt % increases, and the formation of rutile starts with the Mo_2.5 sample. At a higher Mo content (i.e., the Mo_7.5 and Mo_10 samples), the anatase wt % starts to increase again and, correspondingly, the overall amount of the other two phases decreases. The formation of rutile in the studied samples at the adopted calcination temperature (low for the ATR transition) is likely favored by the presence of brookite: the acidic environment of the reverse micelle core should indeed favor the formation of brookite which, in turn, favors the ATR transition according to the literature.[18,43] On the contrary, Mo6+ (and Mo5+) species should inhibit the ATR transition: as reported by Hanaor et al.,[36] high charge cations (i.e., >+4) inhibit the rutile formation, whereas low charge cations (i.e., <+4) act as ATR transition promoters. Here, brookite likely promoted the ATR transition, being able to overcome the effect of Mo ions up to a 5 wt % Mo nominal content; and then, the effect of Mo ions became prominent and, indeed, less rutile formed in the Mo_7.5 and Mo_10 samples, as it can be appreciated in Figure b.

Figure c reports, instead, the (Rietveld refinement determined) values of the anatase cell volume (Å3), which first increase upon the inclusion of Mo, and then decrease (with the Mo_7.5 and Mo_10 samples). The increase of the cell volume with the increasing Mo content is reported by the literature,[44] as well as the cell volume decrease at a sufficiently higher Mo content.[45,46] Such a behavior can be explained by considering that, due to the similar ionic radii of Mo6+/Mo5+ and Ti4+, a substitutional solid solution forms, leading to an expansion of the unit cell. At a higher Mo concentration, the excess dopant ions cannot enter the TiO2 lattice anymore, being therefore forced into interstitial sites, with the formation of defects, which, in turn, may lead to a unit cell volume decrease at a higher Mo concentration.[45] The Mo content, instead, does not affect the anatase crystallite size much, as shown by the corresponding values reported in Table . Unfortunately, the samples’ low crystallinity did not allow the calculation of brookite and the rutile cell volume and crystallite size and thus, we could not monitor the occurrence of Mo doping in the other (less abundant) phase(s), although we are aware that the rutile structure is able to accommodate defects more efficiently than anatase.[47,48]

The morphological analysis, as carried out by FE-SEM, showed the occurrence of aggregated rounded particles (Figure S2). The corresponding particle size distributions and the NP average size values are reported in Figure a and in Table , respectively. As a whole, the TiO2 sample shows smaller NPs than the Mo-doped ones; the NP size increases progressively with the Mo_1 and Mo_2.5 samples, and the latter having an average NP size very close to the Mo_5 sample. The Mo_5 sample, however, is characterized by a broader NP size distribution, as compared to the other samples. With both Mo_7.5 and Mo_10 samples, the NP average size decreases. The trend of NP average size as a function of the Mo nominal content is compared to that of the anatase cell volume (as obtained by the Rietveld refinement) in Figure b and, interestingly, the trends are similar, i.e., both parameters increase and, then, decrease with the Mo content, supporting the idea that Mo doping mainly occurs in the (most abundant) anatase phase.

Figure 2

Particle size distribution as obtained by the FE-SEM analysis (a); average NP size (black curve, as obtained by the FE-SEM analysis) and anatase cell volume (red curve, as obtained by the Rietveld analysis,) vs the samples’ nominal composition (b).

Figure a shows the N2 adsorption/desorption isotherms obtained at −196 °C on the studied samples, which showed type IV isotherms and type H2 hysteresis loop, typical of N2 condensation within interparticle mesopores, and the corresponding BET SSA values are reported in Table . Figure b shows the corresponding PSD (pore size distribution) as obtained by applying the DFT method: as a whole, mesoporous samples were obtained, and their cumulative pore volume values are reported in Table . The Mo_2.5, Mo_5, and Mo_7.5 samples show broader PSD curves, in agreement with the fact that we are dealing with interparticle mesoporosity, and the same samples also showed broader NP size distributions (Figure a).

Figure 3

N2 adsorption/desorption isotherms at −196 °C on the studied samples (a, full and empty symbols refer to adsorption and desorption run, respectively); PSD (pore size distribution) as obtained by applying the DFT method (b).

Bulk and Surface Composition

Table reports, inter alia, the samples’ bulk and surface composition, as determined by EDX and XPS analyses, respectively. As mentioned before, the EDX values were in fair agreement with the nominal ones, and the EDX maps (Figure S3) showed a uniform Mo distribution also with the Mo_10 sample. Although EDX is a semiquantitative technique, the adopted synthesis technique allows an effective inclusion of Mo, without the segregation of other phases, likely due to an optimized mixing of the Mo precursor and the Ti precursor within the reverse micelle core.

Table 2

Nominal Mo/Ti At. % Are Compared to the Total and Surface Mo/Ti At. % of the Studied Samples. XPS Determined Values of Surface Mo6+ and Mo5+ At. % and Valence Band Energies Are Also Reported (eV), along with DR UV–Vis Determined Band Gap Values (Eg, eV). pHIEP Values Have Been Determined from Electrophoretic Measurements

				surface % of Mo6+ and Mo5+ speciesb
sample	nominal Mo/Ti at. %	totala Mo/Ti at. %	surfaceb Mo/Ti at. %	Mo6+	Mo5+	VB energyb (eV)	Eg (eV)c	pHIEP
TiO2	0	0	0			2.2	3.07	3.6
Mo_1	0.84	0.70	0.04	84	16	2.3	2.86	2.3
Mo_2.5	2.13	2.75	0.09	80	20	2.35	2.76	2.3
Mo_5	4.38	5.12	0.15	89	11	2.4	2.58	2.1
Mo_7.5	6.75	6.97	0.17	85	15	2.5	2.48	1.6
Mo_10	9.24	9.01	0.19	84	16	2.7	2.69	1.4

As determined by the EDX analysis

As determined by the XPS analysis

As determined by applying the Tauc’s plot method to the Kubelka–Munk DR spectra.

The XPS analysis was used to determine the samples’ surface composition and allows us inferring possible differences between the surface and bulk composition. The XPS determined (surface) Mo/Ti at. % as a function of the EDX determined (bulk) Mo/Ti at. % (Figure ) does not show a linear trend, but the tendency of Mo to sit at the NP surface and with the Mo_10 sample, the surface Mo/Ti at. % seems to reach a plateau. Such a “surface enrichment” is in agreement with micro-Raman spectroscopy (Figure S1 in the Supporting Information) showing the formation of surface polymolibdates at a higher Mo loading. Such a phenomenon could also affect the surface acidity, which was studied by measuring the samples’ electrophoretic mobility in water, with the determination of their ζ potential (Figure ) and pH at isoelectric point (pHIEP, Table ). The inspection of the curves in Figure allows some considerations to be drawn: first of all, the TiO2 sample shows a lower pHIEP value as compared to the reported literature values for P25 (6.2–6.9)[49] and other types of TiO2.[50−52] Such a phenomenon in the TiO2 sample can be ascribed to the peculiar synthesis, which leads to the formation of an acidic surface, as already reported in the literature.[18,51] The (XPS determined) Mo enrichment of the samples’ surface brings about a further increase in surface acidity, with progressive lowering of the pHIEP values, as the amount of Mo increases. Another point of interest is the measured value of ζ potential, which can be used to evaluate the stability of the suspension: usually, NP suspensions in water are stable with ζ potential values above +30 mV or below −30 mV.[53] In the examined pH range, stability is reached at basic pH with TiO2 NPs and, to a minor extent, with Mo_2.5 NPs, indicating a tendency of the NPs to aggregate in water as the Mo content increases, indicating a modification of the NP surface. In Figure b, the measured pHIEP has been plotted as a function of the (XPS determined) surface Mo/Ti at. %: indeed, the pHIEP does not change sizably from sample Mo_1 to sample Mo_5, notwithstanding the progressive Mo surface enrichment. This likely means that the Mo is forming highly coordinated polymolibdate units, whereas at higher Mo contents, the pH decreases, indicating the formation of more acidic species, likely Mo–OH groups at the end of polymolibdate chains.

Figure 4

XPS determined surface Mo/Ti at. % vs EDX determined bulk Mo/Ti at. %.

Figure 5

ζ potential curves for the studied samples (a); pHIEP values vs XPS determined surface Mo/Ti at. % (b).

The oxidation state of the surface Mo species was also studied by XPS by applying a curve-fitting procedure to the Mo3d doublet of all the studied samples. Figure a shows the comparison of the Mo3d spectral regions, whereas Figure b shows a typical curve-fitting procedure for the Mo_2.5 sample. In all the studied cases, two doublets were used to properly interpret the spectral lines: the obtained binding energy (BE) values were compatible with the presence of both Mo6+ and Mo5+ surface species (BE Mo3d5/2 = 231.3 ± 0.3 eV and BE Mo3d5/2 = 232.5 ± 0.2 eV, respectively).[28]

Figure 6

Comparison of the Mo3d XP spectra of the Mo-doped samples (a); Mo3d XP spectrum and related curve-fitting results as obtained with the Mo_2.5 sample (b); Ti2p3/2 BE peak position vs surface Mo/Ti at. % (c); O1s XP spectrum and the related curve-fitting for the Mo_7.5 sample (d).

After the curve-fitting procedure, the relative abundance of Mo6+/Mo5+ species was obtained, and the corresponding values are reported in Table . The occurrence of surface Mo5+ species is not surprising because when isomorphic substitution occurs, the positive extra charges induced by the Mo6+ species have to be balanced by different means, namely the formation of oxygen vacancies, formation of (reduced) Mo5+ species, and reduction of Ti4+ to Ti3+ species. If those phenomena occur at the surface, they can be studied by XPS. Concerning the formation of reduced Ti3+ species, Figure c reports the position of the Ti2p3/2 peak component as the surface Mo content (Mo/Ti at. %) increases: the Ti4+ peak position is seen to shift with progressive doping, but the observed Ti2p spin–orbit splitting is constant, indicating the unlikely formation of Ti3+ species at the surface.[54] To evaluate the occurrence of oxygen vacancies, the O1s spectral region was investigated and subjected to a curve-fitting procedure (Figure d shows, as an example, the results obtained with the Mo_7.5 sample). With all the samples, two peak components were found at BE = 529.8 ± 0.2 eV and BE = 530.8 ± 0.2 eV, ascribable to the lattice oxide species (M–O) and to oxygen species in the oxygen-deficient region, respectively.[55,56] The relative abundance of the oxygen vacancy component amounts to 15 ± 2% and is almost constant with the Mo content, with the exception of the Mo_5 sample in which the peak is less intense, and the amount of oxygen vacancies is equal to 9 ± 1%. The same sample also showed a lower amount of surface Mo5+ species; because XPS allows the study of surface species, (further) oxygen vacancies and/or Mo5+ species could be located in the bulk of Mo_5 NPs, finally escaping XPS detection.

Furthermore, by XPS it was also possible to measure the position of the valence band (VB) energy (the corresponding values being reported in Table ) that, in turn, was used to determine the CB position, by considering the Eg values determined by DR UV–vis spectroscopy (vide infra).

DR UV–vis spectra (Figure a) show a redshift of the onset of absorption with Mo doping, along with a broad absorption band in the d-d transition range. As expected, the TiO2 sample absorbed below 400 nm, whereas the introduction of Mo brought about two effects: a slight redshift of the absorption edge and the appearance of a broad absorption centered at ca. 550 nm, readily assigned to the sub-band transitions related to midband gaps formed by Mo doping.[57]

Figure 7

DR UV–vis spectra of the studied samples in the 200–1000 nm range (a); energy of the VB, as determined by XPS and of the CB, as obtained by (VB + Eg).

The redshift of the absorption edge shows that Mo doping is modifying the materials’ Eg values that were here calculated by the Tauc’s plot method (not shown) for the indirect semiconductor, anatase being the most abundant polymorph (Table ). The smallest Eg value was obtained with the Mo_7.5 sample, then an Eg increase occurred at a higher Mo content. Figure b shows the Eg values along with the position of VB samples, as determined by the XPS analysis and CB values calculated as (VB + Eg). Interestingly, when a template-free sol–gel method was adopted to obtain Mo-doped TiO2 with a similar composition, the effect on the Eg was ca. 10% smaller, especially at higher Mo contents, showing that here the reported reverse micelle sol–gel synthesis may lead to a remarkable decrease in Eg.[33] The figure also shows that the Mo introduction modifies the position of both the VB and the CB and that at a higher Mo content, an increase in Eg is observed. The latter phenomenon can be due to the occurrence of the Moss–Burstein (or Burstein–Moss) effect, i.e., the apparent increase of a semiconductor band gap, due to the fact that at high doping levels, some states close to the CB are being populated.[58] When the electron carrier concentration exceeds the CB edge density of the states, degenerate doping in semiconductors occurs: here, it shows that the adopted synthesis method allowed such an effective inclusion of Mo in the TiO2 matrix that the Moss–Burstein effect was observed.

Indeed, such an effect, i.e., an (unexpected) increase in the Eg at a higher amount of dopant had been already observed in Mo-doped TiO2 NPs[13] which, at an intermediate Mo content, were active toward the photocatalytic degradation of rhodamine B (a model water pollutant) under simulated solar light, showing that this type of doped TiO2 has indeed appealing photocatalytic properties, in agreement with the literature in the field.[59−61] In perspective, such an effective doping procedure could be applied not only to introduce other types of doping elements within TiO2 matrices but also to synthesize other types of materials, like doped ZnO, which is along with TiO2, the most studied semiconductors with many and diverse applications in photocatalysis and biology as an antibacterial, etc., as acknowledged by the literature.[62−64]

Conclusions

The obtained results allowed inferring a prominent role of the adopted synthesis technique on the samples’ physicochemical properties in that reverse micelle sol–gel synthesis allowed the inclusion of a sizeable amount of Mo in the bulk, with a consequent modification of the VB and CB of the final material. The type of synthesis also affected the kind of mixed phases occurring in the final product (anatase, brookite, and rutile, the last one at Mo wt % ≥2.5) as well as the surface species in the samples’ surface is extremely acidic especially when Mo doping occurs.

The surface species were also studied by the XPS analysis, showing the occurrence of both Mo6+ and Mo5+ species, the latter forming, along with oxygen vacancies, in order to balance the extra positive charge induced by doping.

The energies of both the VB and CB were determined by means of the joint use of DR UV–vis and XPS, showing the occurrence of the Moss–Burstein effect at the highest Mo content, as a consequence of the extremely effective inclusion of Mo in the NPs due to the type of synthesis. The reverse micelles containing the system likely allowed the ultimate mixing of both Ti and Mo precursors and could be used to include other types of metals, which instead tend to form defects and/or aggregates (e.g., Fe), with a detrimental effect on photocatalytic applications.

Experimental Section

Synthesis Method

Undoped and Mo-doped TiO2 NPs were prepared by a sol–gel reverse micelle microemulsion method[65] by using polyoxyethylene (20) oleyl ether, Brij O20 as a surfactant, cyclohexane as an oil phase, titanium(IV) butoxide 97% (Ti-(BuO)4), and ammonium heptamolybdate tetrahydrate ((NH4)6Mo7O24*4H2O, Sigma-Aldrich, purum p.a. > = 99.9%) as Ti and Mo precursors, respectively.

The proper precursor amounts were used in order to obtain Mo wt % nominal contents of 0.0, 1.0, 2.5, 5.0, 7.5, and 10. A typical synthesis involves the following steps: the surfactant is dispersed in cyclohexane by stirring at 50 °C, while the salt precursor is dissolved in Milli-Q water at the same temperature. Afterward, the salt solution is added to the oil/surfactant mixture and stirred for 45 min, with the formation of a water-in-oil (w/o) microemulsion of surfactant nanoreactors. The TiO2 precursor is then dropwise added to the emulsion. The mixture is stirred for 2 h at the constant temperature of 50.0 °C and finally, the emulsion is broken by the addition of 2-propanol, followed by sonication. The solid is collected by centrifugation and dried at 100 °C for 24 h, followed by calcination in air at 500 °C for 2 h with a temperature ramp of 2.5 °C min–1 to remove the surfactant.

Characterization Methods

Powder X-ray diffraction (PXRD) patterns were obtained by using an X’Pert Phillips diffractometer operating at 40 kV and 40 mA with Cu Kα radiation (scanning resolution 2θ = 0.026°, time per step: 2 s). The samples’ phase composition and structural parameters were determined by the Rietveld analysis as implemented in the MAUD software.[66] Instrumental broadening was characterized by the NIST standard 660a (LaB6).

The Raman spectra were taken by a Renishaw InVia Reflex micro-Raman spectrometer equipped with a cooled charge-coupled device (CCD) camera; the source was a diode laser ( λex = 514 nm); and powders were pressed into self-supporting wafers to ensure the inspection of a flat surface by means of a microscope objective (100X) in a backscattering light collection mode (laser power: 0.5 mW; exposure time: 10 s; 1 accumulation).

The field-emission scanning electron microscopy (FE-SEM) images were taken by a ZEISS Supra 40 FESEM instrument equipped with an energy-dispersive X-ray (EDX) probe; the latter was used for the (semiquantitative) analysis of the sample chemical composition on 5 μm3 volume spots; and EDX maps were also measured in order to determine the dispersion of Ti, O, and Mo in the NPs.

N2 adsorption/desorption isotherms at −196 °C were measured on samples previously outgassed at 120 °C for 1 h and then at 150 °C for 2 h (Micromeritics ASAP 2020 Plus instrument). The samples’ specific surface area (SSA) was calculated by applying the Brunauer–Emmett–Teller (BET) method; pore total volume was measured at P/P0 = 0.99; and the pore size distribution and cumulative pore volumes were determined by the density functional theory (DFT) method through a N2/oxide surface kernel for the cylindrical pores.

The diffuse reflectance (DR) UV–visible spectra were recorded by a UV–vis Varian Cary 5000 spectrophotometer equipped with an integration sphere for the DR measurements. The samples’ band gap energy (Eg) was then calculated by applying the Tauc’s plot method for the indirect semiconductors.

The surface chemical composition and speciation were investigated by X-ray photoelectron spectroscopy (XPS) using a Versa Probe II scanning XPS microprobe spectrometer (Physical Electronics GmbH), equipped with a monochromatized Al Kα source (X-ray spot = 100 μm) at a power of 26.7 W. Both wide scans and high-resolution XP spectra were acquired using a fixed analyzer transmission (FAT) mode with pass energies of 117.40 and 29.35 eV, respectively. An electron gun was used for the charge compensation (1.0 V 20.0 μA). The data processing was performed by using MultiPak software version 9.9.0.8.

The samples’ electrophoretic mobility was measured using a Zetasizer Nano ZSP instrument (Malvern Instruments Ltd., GB) on mixtures prepared by adding ca. 5 mg of powder to 10 mL of bi-distilled water and then sonicating for 5 min; during the measurements, pH was adjusted by the addition of either HCl or NH4OH aqueous solutions. The samples’ ζ potential was obtained from the electrophoretic mobility by applying the Smoluchowski’s approximation.