Photocatalytic Activity of Cu–TiO₂ Nanopowder Under UVA and Sunlight Illumination: Influence of Composition and Calcination Temperature on Charge Transfer

Abstract

Cu–TiO₂ nanoparticles of a broad range of compositions with 0, 0.002, 0.005, 0.02, 0.05, 0.2, 0.5, 1.0, 2.0, 3.0, 5.0, 7.0 and 10.0 mol% Cu were synthesized via the sol–gel method using copper (II) acetate and titanium tetraisopropoxide (TTIP) precursors at a low hydrolysis ratio of H = 1.25, which favours homogeneous TiO₂ nucleation and Cu dispersion in the host matrix at nanoscale. The precipitated materials were dried at 80 °C and calcined at 450, 500, and 550 °C to form crystalline nanopowders, whose photocatalytic activity was evaluated on the decomposition of a representative pollutant, methylene blue (MB), in aqueous solutions under UVA and sunlight illuminations. The compositions with small Cu content of ~0.05 mol% showed the highest activity. A gain of activity over pure titania of 4 times after calcination at 450 °C, 2.5 times at 500 °C and 20% at 550 °C was measured under UVA illumination. Even higher gain of activity observed under sunlight illumination might be due to an extension of action spectrum to the visible range due to intra-gap defect states produced by Cu²⁺ insertion. The time-resolved microwave conductivity (TRMC) measurements of the photoinduced charges relaxation suggest that both excessive calcination temperature and Cu content decrease the activity due to Cu-defects clustering. Modelling relates the activity to the photoinduced electron-hole pair separation; the optimal Cu content is explained by accessibility of the recombination centre by a conduction band (CB) electron. Accordingly, an increase in calcination temperature resulted in a longer pathlength of CB electron.

1. Introduction

The cationic doping of TiO₂ has received much attention in recent decades [1,2,3,4]. This material modification has helped improve the overall photocatalytic process efficiency as compared to pure titania due to a better separation of the photoinduced charges and a bandgap reduction, the latter of which shifts the action spectrum from UVA to the visible spectral range, enabling the reaction activation by sunlight. As a result, an effective photocatalytic process under sunlight illumination is under development worldwide (see, e.g., [5,6]). Among the investigated cations, Cu²⁺ appeared to be promising because of a similar ionic radius permitting Ti⁴⁺ substitution. At the same time, charge imbalance can promote anion (O²⁻) removal accompanied by the appearance of oxygen vacancy Vo, which leads to the formation of reduced Ti³⁺ small-polaron centres absorbing in the visible spectral range, whose contribution to the material activity is under debate [7,8,9]. The CuO–TiO₂ composites commonly possess strong absorption in the visible spectral range and enhanced charge separation efficiency at the common interface [10,11].

The mixed metal oxides Cu–TiO₂ have been synthesized by various methods: sol–gel, solvothermal, hydrothermal, microwave, impregnation, chemical reduction, mechanochemical, precipitation, thermal decomposition, low temperature hydrolysis, micro-arc oxidation, electrochemical, electrodeposition, atomic layer deposition, electrochemical anodization, etc. Among them, the sol–gel method, being versatile and inexpensive, has become the most widely used. The synthesized Cu–TiO₂ materials have been prepared in powder form and as films. The relevant literature concerning optimal composition (x, mol%) and activity gain (with respect to pure TiO₂) of Cu–TiO₂ photocatalyst is summarized in Table 1.

Table 1. Synthesis and activity of Cu–TiO₂ photocatalysts.

Synthesis Method	Optimal x, mol%	Illumination	Enhancement	Pollutant	Ref.
Microwave Solvothermal	6	UV (Xe 300 W)	1.6	2,4-dichlorophenoxyacetic acid	[12]
Wet impregnation	0.63	Vis	8.8	Phenol	[13]
Sol–gel	17	Vis (LED, 50 kW)	14.7	Rhodamine-B	[14]
Sol–gel	1	UV-Vis (350–450 nm)	4.9	Methyl Orange (MO)	[15]
Electrospinning	0.05	UV	2	Amaranth Dye	[16]
Sol–gel	0.05	UV and Vis	2	Methylene blue (MB), MO	[17]
Sol–gel	0.25	Vis	1.8	Rhodamine B dye	[18]
Sol–gel	5	Vis	1.1	MB	[19]
Microwave-assisted sol–gel	0.65	(1) UV (2) Vis	8 MB 5 MO 3.4 MB 2.4 MO	MB, MO	[20]
Electrochemical oxidation	-	Vis	2.4	MB	[21]
Atomic Layer deposition	1	UV	1.5	MB, phenol	[22]
Surfactant aided ultrasonication	3	Vis	Enhancement	MB, reduction of Cr6+	[23]
Denotation	17	UV	1.6	MO	[24]
Equilibrium deposition filtration	1	UV/Vis (300–800 nm)	From 0.3 to 1.8	Antibiotics: isoniazid, metronidazole, sulfadiazine, sulfamethoxazole, trimethoprim, norfloxacin, moxifloxacin, lincomycin	[25]
E-beam evaporation	3.2	Vis	1.3	MB	[26]
Hydrothermal	3.7	UV-Vis (300–600 nm)	1.5	Diclofenac	[27]
Sol–gel	1	Sunlight	1.3	Dissolved organic nitrogen	[28]
Aerosol-assisted self-assembly	0.075	Vis	3	Paracetamol	[29]

Studies have confirmed the effectiveness of Cu-doped TiO₂ in environmental photocatalysis under sunlight illumination [30,31,32]. Cu doping has been reported as more effective for Cr(VI) removal, as compared to Fe- and Zn-doped TiO₂ [33]. The enhanced reactivity as compared to doping with Ag, Co, Au, Cr, Fe, Pd, Ni, Pt, Zn, and Y for hydrogen generation [34] and better hydrogen production as compared to doping with Fe, Ni, Co, and Zn [35] (however, smaller than with Mo and W doping [18]) have been reported. At the same time, Table 1 indicates a significant discrepancy in the optimum photocatalyst composition, varying in different studies from 0.05 mol% to 17 mol% Cu. Moreover, while most studies have shown an efficiency gain after Cu insertion into TiO₂, some works have reported a decrease in the photocatalytic decomposition effectiveness of pollutants [36,37] and bacteria [38]. The large spread of optimal material composition seems to be due to a spatial inhomogeneity of the elemental distribution. In fact, this important aspect of the material synthesis has been discussed in previous papers on TiO₂ doping with Fe [39], V [40], and W [41]. In particular, poor mixing at the preparation stage and attaining the solubility limit of the added cation to the relevant anatase TiO₂ matrix must be considered. Otherwise, the synthesis can result in local clustering of the inserted atoms, thus perturbing functional properties of the material. Since the material composition can also be affected at the heat-treatment stage, the influence of the calcination temperature has to be taken into account as well in the evaluation of material properties.

In this paper, we report on the investigation of Cu–TiO₂ mixed oxide photocatalyst with largely varying Cu content from 0 up to 10 mol%, covering the range of the available literature data about optimal photocatalyst composition (see Table 1). The nanopowders were prepared via the sol–gel method in conditions favourable for the elemental micromixing at the nanoscale. The decomposition of a representative pollutant, methylene blue (MB), in aqueous solutions was used for the evaluation of material activity under UVA and sunlight illuminations. The influence of calcination temperature on the activity was analyzed. A physical mechanism of the activity enhancement/suppression was proposed, relating to the photoinduced charge transfer and recombination, and confronted with the experimental data.

2. Materials and Methods

2.1. Chemicals

All chemicals were analytical grade and used without further purification. Titanium (IV) isopropoxide (TTIP) (≥98% purity) and copper (II) acetate (99% purity) were purchased from Acros Organics (Shanghai, China and Geel, Belgium, respectively). Methylene blue (MB) was from Sigma Aldrich (Bangalore, India), 2-propanol (99.5% purity) and water for chromatography were from Sigma Aldrich (Darmstadt, Germany), and copper standard solution (1000 ppm Cu(NO₃)₂ in 0.5 mol/L nitric acid) was from Merck (Darmstadt, Germany).

2.2. Synthesis

The colloidal solutions of Cu-doped titanium oxo-alkoxy (TOA) nanoparticles were prepared at room temperature via the sol–gel method at a low hydrolysis ratio H = C_w/C_pre = 1.25 (C_w is the concentration of water) and total precursor concentration C_pre = C_Cu + C_Ti = 0.3 mol/L in a volume of 100 mL. For the synthesis of pure TOA nanoparticles, TTIP precursor was dissolved in isopropanol solvent of total volume 50 mL, labelled as solution A. Water in isopropanol of 50 mL volume was labelled as solution B. Both solutions were stirred by a magnetic stirrer (RCT basic, IKA, Staufen, Germany) at 500 rpm for 10 min until homogenized, then solution B was slowly dropped into solution A. According to our previous studies [42,43], nucleation of TOA nanoparticles takes place in these conditions while prohibiting nucleus growth and agglomeration; this permits homogeneity at the nanoscale of the guest cations distribution in host titania matrix [40,41]. The mixed metal copper-titanium-oxo-alkoxo (CTOA) nanoparticles were prepared after the addition of solution A (as that was used for pure TOA) to solution B, prepared by dissolving copper (II) acetate and water in isopropanol. The following steps were the same as those used for the preparation of pure TOA colloids. The initially stable colloidal solutions were allowed to absorb atmospheric moisture and precipitate over several days, which were then dried in an oven at 80 °C overnight. The nanopowders were calcined at temperatures of 450, 500, and 550 °C for 4 h to form crystalline Cu–TiO₂ photocatalysts. The elemental composition of nanopowders was labelled as x (mol% Cu) = 100 × C_Cu/(C_Cu + C_Ti), with Cu and Ti molar concentrations respectively C_Cu and C_Ti.

2.3. Characterization

X-ray diffraction (XRD) patterns were collected using an EQUINOX 1000 diffractometer (INEL, Orlean, France) equipped with a Cu-Kα source (λ = 1.541 Å). Phase quantification was estimated from the XRD patterns with a rutile-to-anatase ratio calculated from the respective (110) and (101) peaks intensities via equation F_r = 1 − (1 + 1.265 × I_r110/I_a101)⁻¹ [44]. The diffraction patterns were processed using the Rietveld method implemented in MAUD (Material Analysis Using Diffraction) software (V2.998). Peaks were fitted according to a standardized procedure. The refined parameters were background parameters, cell parameters, NP size, and presence of microstrain. The crystallographic information file for TiO₂ anatase and rutile were obtained from Inorganic Crystal Structure Database ICSD 9852 (TiO₂ anatase, I41/amd S, a = 3.7842 Å, c = 9.5146 Å) and ICSD 36412 (TiO₂ rutile, P 42/mnm, a = 4.593 Å, c = 2.959 Å). In the final step, an arbitrary texture option was used to optimize the quality of the Rietveld pattern refinement (uniquely for monophasic samples).

The relaxation of the photoinduced charges was monitored via the time-resolved microwave conductivity (TRMC) method by measuring the reflected microwave power by the semiconductor surface after illumination with pulsed laser light. The experimental installation has been described in Ref. [45]. In these experiments, the samples were irradiated with 360 nm light pulses of 8 ns duration and 1 mJ energy delivered by a cw OPO Nd:Yag laser (EKSPLA) operating at 10 Hz repetition rate. The laser photon energy hν > E_g (E_g = 3.2 eV is anatase TiO₂ band gap energy) enabled intergap electronic transition, creating conduction band (CB) electron e^- and valence band (VB) hole h⁺, whose concentration is proportional to the microwave reflection. A circular spot on the sample with a 1 cm² area was illuminated, and the temporal evolution of the TRMC signal was accumulated over 200 laser pulses. The measurements were performed at room temperature in an ambient atmosphere. The experimental series was repeated to guarantee reproducibility of measurements.

In the photocatalytic experiments, 1 g/L Cu-doped TiO₂ nanopowders were added to a beaker with a 5 ppm MB aqueous solution of 250 mL volume under stirring at 500 rpm with a magnetic stirrer in the dark. No significant variation in the pollutant adsorption capacity by Cu–TiO₂ nanopowders was observed amounting for 12.6 ± 3.6 mol% of the initial MB concentration, as shown in Figure 1. After 30 min of intense stirring, the solution was illuminated with a UVA lamp (365 nm) of 6 W power (model VL-6.LC, VWR International, Rosny-sous-Bois, France) at a distance of 25 cm. Alternatively, the photocatalytic experiments were performed under natural sunlight illumination in Phnom Penh city (Cambodia) between 9 am and 12 pm. The photocatalytic activity under UVA and sunlight illuminations was tested with the same nanoparticle concentration; the full series of Cu–TiO₂ photocatalysts with different elemental compositions was placed in identical aqueous solutions of MB pollutant and simultaneously exposed. The MB concentration was monitored by absorbance measurements of the reactive solution, after 5 mL sampling every 15 min during 2 h. The samples were transferred into tubes and centrifuged at 5000 rpm to separate the catalyst and solution. Then, the pollutant solution was collected and absorbance measured by a UV-Vis spectrophotometer GENESYS 10S (Thermo Fisher Scientific, Waltham, MA, USA) at 664 nm wavelength, which corresponds to the maximum of MB absorbance. The first-order rate constants of MB decomposition were derived from the MB degradation kinetics and used as a measure of the materials activity.

Figure 1. MB stationary absorption in photocatalytic experiments before illumination.

3. Results and Discussion

3.1. Structural Properties of Cu–TiO₂ Nanopowders

Before photocatalytic measurements, structural properties of the prepared nanopowders were accessed by analysis of XRD patterns, representatively shown in Figure 2. Their first evaluation indicated pure crystalline anatase at Cu content below 10 mol%, at calcination temperature T < 550 °C (Figure 2a,b). In contrast, after calcination at T = 550 °C (Figure 2c), the rutile phase was observed, increasing to a rutile ratio F_r ~ 10 mol%.

Figure 2. X-ray diffraction patterns of selected Cu–TiO₂ nanopowders with different Cu content (indicated in mol%) heat-treated at 450 °C (a), 500 °C (b), and 550 °C (c). Peaks indexation of anatase and rutile phases is shown respectively in (a) and (c).

The appearance of the rutile phase at 550 °C after calcination for 4 h is a common feature of pure TiO₂. Furthermore, Byrne et al. [46] have reported on the inhibition of the anatase to rutile phase transformation with an increase in the Cu content. The phase transformation is known to be a kinetically limited process, and the anatase-rutile phase transformation has already been observed in TiO₂ nanopowders at temperatures as low as 465 °C [47]. Therefore, the appearance of the rutile phase at 550 °C in pure TiO₂ in this work is in line with that at 650 °C with a shorter calcination time of 2 h [46]. Accordingly, a small addition of ~0.5 mol% Cu resulted in the anatase lattice. However, we do not confirm the pure anatase phase of Cu–TiO₂ nanopowders (heat treated at 550 °C) with Cu loading of 1 mol% and larger. No CuO phase inclusion was observed.

The Rietveld analysis shown in Figure 3 for two selected nanopowders from Figure 2 confirmed lattice parameters close to those of anatase TiO₂ structure, as indicated in Table 2. Powders form large crystalline domains (D) of several tens of nanometers in size, i.e., much larger than the anatase crystalline cell.

Figure 3. Rietveld analysis of Cu–TiO₂ nanopowders with 0.5 mol% (a) and 10 mol% (b) Cu heat-treated at 500 °C (b).

Table 2. Rietveld analysis of selected Cu–TiO₂ nanopowders ¹.

Sample		a (Å)	c (Å)	Size (Å)
x (mol%)	T (°C)
0	450	3.775	9.484	237
0	500	3.755	9.438	287
0	550	3.777	9.500	538
0.05	450	3.776	9.486	262
0.05	500	3.778	9.495	315
0.05	550	3.777	9.498	448
0.5	450	3.772	9.471	237
0.5	500	3.782	9.509	384
0.5	550	3.773	9.490	429
1	450	3.778	9.489	293
1	500	3.778	9.494	418
1	550	3.773	9.491	978
10	450	3.775	9.481	243
10	500	3.782	9.505	311
10	550	3.783	9.511	388

¹ Structure references: ICSD 9852, TiO₂ anatase, a = 3.7842 Å, c = 9.5146 Å. ICSD 36412, TiO₂ rutile, a = 4.593 Å, c = 2.959 Å.

3.2. Photocatalytic Activity of Cu-Doped TiO₂

The photocatalytic degradation of MB using Cu–TiO₂ photocatalyst under UVA radiation is shown in Figure 4, in the semi-logarithmic frame. The first-order reaction kinetics $\mathrm{L}\mathrm{n}\left({\mathrm{C}}_0/\mathrm{C}\right)=\mathrm{k}\mathrm{t}$ (where C₀ and C are initial and current MB concentrations, respectively) was confirmed by the linear regression of experimental data, which enabled evaluation of the material activity [48]. By consequence, the strongest activity (k) corresponds to the compositions with Cu content in the range of 0.005–0.2 mol% and depends on the heat treatment conditions. In comparison with previous studies, our results match optimal values of the lowest Cu addition [16,17,29]. In contrast, the overestimated compositions with x ≥ 1 mol% Cu from Table 1 can be explained by the elemental inhomogeneity of the materials and, probably, guest (Cu) cation clustering. While the solubility limit of Cu²⁺ ions in anatase lattice is about C_s = 6 mol% [49], an insufficient mixing of the reacting products may lead to their local overloading and clustering, at the still lower mean copper concentration C < C_s. According to these data, a significant gain in activity over pure TiO₂ was observed.

Figure 4. MB degradation on Cu–TiO₂ nanopowders calcined at 450 °C (a), 500 °C (b), and 550 °C (c) with different compositions (5 ppm MB, 1 g/L catalyst) under UVA illumination.

In addition, photocatalytic experiments were completed with the sunlight illumination. The results in Figure 5 (in semi-logarithmic frame) confirmed first-order reaction kinetics and provided optimal composition for heat-treated material with T = 500 °C in the range of 0.002 ≤ x ≤ 0.05 mol% Cu. This is in agreement with the measurements under UVA illumination.

Figure 5. MB degradation on Cu–TiO₂ nanopowders calcined at 500 °C with different compositions (5 ppm MB, 1 g/L catalyst) under sunlight illumination.

The photocatalytic experiments in Figure 4 and Figure 5 indicate a gain of the photocatalytic activity with the Cu addition to TiO₂, defined as the ratio of MB decomposition rate constants using, respectively, Cu–TiO₂ and TiO₂ nanopowders: $\mathrm{A}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{v}\mathrm{i}\mathrm{t}\mathrm{y}\mathrm{g}\mathrm{a}\mathrm{i}\mathrm{n}={\mathrm{k}}_{{\mathrm{C}\mathrm{u}-\mathrm{T}\mathrm{i}\mathrm{O}}_2}/{\mathrm{k}}_{{\mathrm{T}\mathrm{i}\mathrm{O}}_2}$), as Figure 6 shows. The gain clearly appeared at a small Cu insertion of x ≤ 0.1 mol%, decreasing with an increase in the calcination temperature. The rates of MB decomposition under sunlight vs. UVA illumination depend on the light source’s power and spectrum. In this connection, the gain of activity over pure titania increased up to 4 times under sunlight illumination. This is significantly stronger than under UVA illumination, where the activity gain over pure titania was ~2.5 times at the same calcination temperature of 500 °C. Further experiments with sunlight illumination are required to confirm this tendency, which might be due to an extension of the action spectrum of Cu–TiO₂ photocatalyst compared to pure TiO₂, related to the lattice intra-gap defect states produced by Cu²⁺ insertion.

Figure 6. Gain of photocatalytic activity with Cu addition to TiO₂.

Since the material crystallinity depends on the calcination temperature, the activity gain due to the Cu insertion in TiO₂ is also expected to change. An increase in the CB electron path length before trapping with calcination temperature will be discussed below in the framework of a model.

3.3. Charge Transfer

Since no significant variation in the pollutant adsorption capacity was observed in the present study (see Figure 1), we relate the strong activity variation with Cu content to the photoinduced charge transfer and recombination. A model describing the influence of the Cu cation insertion into the TiO₂ lattice on the photocatalyst activity has been introduced earlier in connection with Fe³⁺ doping of TiO₂ [39]. This model assumes that the photoexcited charges undergo a random-walk displacement before localization: the CB electron e^- localizes more rapidly than the VB hole h⁺ [50] and in a smaller region of radius r_e. Furthermore, the electron localizes on Ti⁴⁺, forming a polaron-like Ti³⁺ centre [8], on which the hole can effectively recombine. The guest cation in TiO₂ lattice can also attract a photoinduced hole, which thus avoids annihilation with the localized electron on Ti³⁺ and can trigger photocatalytic reactions.

The defect energy levels of Cu²⁺ are not explicitly established. However, Cu²⁺ is expected to substitute Ti⁴⁺ in TiO₂ because of quite similar cationic radii, creating a charge imbalance compensated by oxygen vacancy; the adjacent Cu and Ti cations in the absence of the removed oxygen atom have five-fold coordination forming a Ti³⁺ state [7]. The theoretical calculations have shown that the substitutional Cu_Ti is placed 0.65 eV above the VB maximum for 0/1− transition and 1.57 eV above the VB maximum for 1−/2− transition, while the interstitial Cu_i is placed 0.37 eV below the CB minimum for 2+/1+ transition (Cu⁺) and ~0.05 eV above the CB minimum for 1+/0 transition (Cu°); the valence states Cu⁺ and Cu²⁺ thus coexist in the anatase TiO₂ lattice [51]. We notice in this connection a progressive coloration of the prepared Cu-TiO₂ nanopowders from white (0 mol% Cu) to grey (10 mol% Cu), which is in agreement with the Vo/Ti³⁺ defects formation and Cu²⁺ reduction to Cu⁺ or even Cu° [52]. We therefore assume that the mobile VB hole can be trapped on Cu_Ti or, alternatively, on complex defect clusters of type Vo/Ti³⁺, thus suppressing annihilation with the photoinduced CB electron. In the following, we will call this defect centre {Cu}.

The principal relevant processes running at the UVA illumination of the doped TiO₂ are given below (see Ref. [39]):

$${\mathrm{T}\mathrm{i}\mathrm{O}}_2\stackrel{\mathrm{h}\mathrm{v}}{\to }{\mathrm{h}}^{+}+{\mathrm{e}}^{-}$$

(1)

$${\mathrm{h}}^{+}\stackrel{{\mathrm{\tau }}_{\mathrm{h}0}}{\to }{\mathrm{h}}_{\mathrm{t}\mathrm{r}}^{+}$$

(2)

$${\mathrm{e}}^{-}+{\mathrm{T}\mathrm{i}}^{4+}\to {\mathrm{T}\mathrm{i}}^{3+}$$

(3)

$${\mathrm{h}}^{+}+{\mathrm{T}\mathrm{i}}^{3+}\stackrel{{\mathrm{k}}_{\mathrm{r}1}}{\to }{\mathrm{T}\mathrm{i}}^{4+}$$

(4)

$${\mathrm{h}}^{+}+\left\{\mathrm{C}\mathrm{u}\right\}\stackrel{{\mathrm{k}}_{\mathrm{h}}}{\to }{\left\{\mathrm{C}\mathrm{u}\right\}}^{+}$$

(5)

$${\mathrm{e}}^{-}+{\left\{\mathrm{C}\mathrm{u}\right\}}^{+}\stackrel{{\mathrm{k}}_{\mathrm{e}}}{\to }\{\mathrm{C}\mathrm{u}\}$$

(6)

where k and τ with subscripts are, respectively, rate constants and lifetimes. The rate Equations (1)–(6) propose an analytic solution of the electron-hole separation probability:

$$\mathrm{\gamma }={\displaystyle \frac{1}{1+\frac{{\mathrm{k}}_{\mathrm{r}1}{\mathrm{\tau }}_{\mathrm{h}0}}{1+{\mathrm{k}}_{\mathrm{h}}{\mathrm{\tau }}_{\mathrm{h}0}\mathrm{x}}+\frac{{\mathrm{k}}_{\mathrm{r}2}{\mathrm{\tau }}_{\mathrm{h}0}\mathrm{x}}{1+{\mathrm{k}}_{\mathrm{h}}{\mathrm{\tau }}_{\mathrm{h}0}\mathrm{x}}\mathrm{e}\mathrm{x}\mathrm{p}\left(-{\left(\frac{{\mathrm{r}}_{\mathrm{T}\mathrm{i}}}{{\mathrm{r}}_{\mathrm{e}}}\right)}^2{\mathrm{x}}^{-2/3}\right)}}$$

(7)

where r_Ti ≈ 0.3 nm is the mean Ti–Ti distance in the anatase lattice. We tentatively relate γ to the photocatalytic yield.

The model curves (7), traced over experimental photocatalytic rates k in Figure 7, successfully describe the experiment. The values of r_e = 1.39 (a), 1.61 (b), and 4.02 (c) nm were obtained for Cu–TiO₂ nanopowders calcined respectively at 450 °C, 500 °C, and 550 °C. The increase in CB electron pathlength before localisation r_e supports crystallinity improvement, vanishing defect states, and thus, the decrease in activity gain over pure TiO₂.

Figure 7. Photocatalytic rate constant of Cu–TiO₂ catalyst calcined at 450 °C (a), 500 °C (b), and 550 °C (c) with UVA lamp (■, ●, ▲) and sunlight (○) illumination (5 ppm MB, 1 g/L catalyst). Solid lines trace model curves corresponding to electron localisation distance of 1.39 (a), 1.61 (b) and 4.02 (c) nm. Relative error bars from the least-squares fit are included.

We notice that although a small presence of rutile phase was evidenced in Cu–TiO₂ by XRD measurements, the highest activity of anatase compared to rutile titania and large crystalline domains D >> r_e suggest the dominant contribution of anatase phase to the photocatalytic reactions that allows the comparison.

3.4. TRMC Analysis

The relaxation kinetics of photoinduced charges after UV-laser pulsed excitation of Cu–TiO₂ nanopowders with x = 0.005 calcined at different temperatures are shown in Figure 8. The experimental curves of the materials with T < 550 °C match the power law kinetics I = At^−α related to the photoexcited electrons released from shallow traps [48,53]. In the materials calcined at T = 550 °C, a complementary short component appeared, which might be due to the rutile phase appearance, in agreement with Colbeau-Justin et al. [45]. However, its duration ~10 ns is much shorter compared to that of the plasma-like free charges recombination of ~200 ns duration, according to Ref. [53]; it was therefore ascribed to the laser pulse scattering by ${\mathrm{C}\mathrm{u}}_{\mathrm{N}}^{\mathrm{o}}$ cluster defects accumulating in the sample under UVA irradiation.

Figure 8. TRMC decay curves of Cu–TiO₂ nanopowders with 0.005 mol% Cu calcined at 450 (a), 500 (b), and 550 (c) °C. Least-squares fits with power law function I = At^−α are shown by straight lines.

The power decay constant α is connected to the charge trap binding energy and does not directly reflect the photocatalyst activity of a material. However, slow relaxation of the photoinduced charges generally contributes to the effective charge-transfer reactions at contact with pollutant molecules. The power constants α obtained from TRMC measurements of the powders with low Cu content and different calcination temperatures are shown in Figure 9.

Figure 9. Power coefficient α of TRMC decay of Cu–TiO₂ nanopowders calcined at 450 (■), 500 (▲), and 550 (▼) °C, and of reference P25 TiO₂ (□) powder.

The result indicates a significant shortening of the lifetime of the CB electron population, which, according to the above discussion of the activity gain of Cu–TiO₂ over pure TiO₂ catalysts, can be related to a decrease in the lattice defects concentration with an increase in the calcination temperature, which promotes long-lasting migration and recombination of charges [53]. This can support a contribution to the activity gain under sunlight illumination of the sub-band defect states (e.g., Cu_Ti and/or Vo/Ti³⁺) absorbing visible light photons. Defect engineering in application to the TiO₂-based photocatalysts has attracted much interest in the last few decades [54,55], and those associated with Cu insertion have already allowed the activity enhancement under UV illumination [56]. Although no control of the defect chemistry was proposed in our study, their easy and effective formation at the sol–gel synthesis in low-hydrolysis ratio conditions could be of interest for the elaboration process scale-up.

We notice the appearance of a short 10 ns decay component of the TRMC kinetics in Cu–TiO₂ nanopowders calcined at 450 °C and 500 °C with compositions respectively x ≥ 3 and x ≥ 0.2 mol% Cu. This can be connected to the 1+/0 transition of Cu_i, which manifests at large Cu content and/or increased calcination temperature. Complementary, amplitudes of the TRMC signal strongly weaken in nanopowders calcined respectively at 450, 500, and 550 °C. Summing up, both excessive calcination temperature and Cu content decrease the material activity due to the Cu-defect clustering. Furthermore, lowering the calcination temperature promotes the formation of inter-gap defect states capable of enlarging the action spectrum of the Cu–TiO₂ photocatalyst.

4. Conclusions

This study concerned the investigation of the photocatalytic activity enhancement of TiO₂ after Cu cation insertion into the crystalline lattice. The nanoparticles of copper titanium oxo-alkoxide (CTOA) were successfully synthesized with the sol–gel method by using TTIP and copper (II) acetate precursors in 2-propanol solvent, in low hydrolysis ratio conditions of H = 1.25. The precipitated particles were dried at 80 °C and calcined at 450, 500, and 550 °C for four hours to form a crystalline product with a large variation of elemental compositions: x = 0, 0.002, 0.005, 0.02, 0.05, 0.2, 0.5, 1.0, 2.0, 3.0, 5.0, 7.0, and 10.0 mol% Cu. The crystalline nanopowders were used to decompose a representative pollutant, MB, in aqueous solutions. The compositions, with x between 0.002 and 0.05 mol% Cu, showed the highest activity towards MB decomposition both under UVA and sunlight illuminations. It was observed that both excessive calcination temperature and Cu content decrease the material activity due to the Cu-defect clustering. The gain of activity over pure titania of 4 times after calcination at 450 °C, 2.5 times at 500 °C, and only 20% at 550 °C was measured under UVA illumination. An even stronger gain of activity under sunlight illumination, observed at the calcination temperature of 500 °C, can be due to an extension of the action spectrum of Cu–TiO₂ photocatalyst to the visible range. The TRMC measurements of the photoinduced charge relaxation support the conclusion about the negative influence of both excessive calcination temperature and Cu content on the material activity. Modelling allowed for the relation of the photocatalytic activity to the photoinduced charge separation, owing to the localization of a lighter charge (h⁺) on distant Cu⁺ cation and heavier charge (e⁻) migrating through Ti⁴⁺ sites by hopping. The electron localisation distances of 1.39 (a), 1.61 (b), and 4.02 (c) nm were obtained in Cu–TiO₂ nanopowders calcined respectively at 450 °C, 500 °C, and 550 °C. Since the material crystallinity improvement is expected with an increase in the calcination temperature, the gain in activity due to defect states is expected in moderate heat treatment conditions. An effective formation of relevant defect states at the sol–gel synthesis in low-hydrolysis ratio conditions could be of interest for effective photocatalyst elaboration.