Structural, optical and photocatalytic properties of erbium (Er³⁺) and yttrium (Y³⁺) doped TiO₂ thin films with remarkable self-cleaning super-hydrophilic properties

Abstract

The self-cleaning and super hydrophilic properties of pristine TiO₂ and of TiO₂ doped with Er³⁺ or Y³⁺ transparent thin films deposited onto glass substrates were investigated. The thin films prepared by multiple dipping and drying cycles of the glass substrate into the pristine TiO₂ sol and Er³⁺ or Y³⁺-doped TiO₂ sol were characterized by X-ray diffraction, UV-vis spectrophotometry, and atomic force microscopy (AFM). The self-cleaning photocatalytic activity of the thin films towards the removal of oleic acid deposited on the surface under UVA irradiation was evaluated. A remarkable enhancement was observed in the hydrophilic nature of the TiO₂ thin films under irradiation. The optical properties and wettability of TiO₂ were not affected by Er³⁺ or Y³⁺ doping. However, the photocatalytic degradation of oleic acid under UVA irradiation improved up to 1.83 or 1.95 fold as the Er³⁺ or Y³⁺ content increased, respectively, due to the enhanced separation of the photogenerated carriers and reduced crystallite size. AFM analysis showed that the surface roughness increased by increasing the Er³⁺ or Y³⁺ content due to the formation of large aggregates. This in turn contributes to the increase of the active surface area enhancing the photodegradation process. This study demonstrates that TiO₂ doped with low amounts of Er³⁺ or Y³⁺ down to 0.5 mol% can produce transparent, super-hydrophilic, thin film surfaces with remarkable self-cleaning properties.

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Introduction

Heterogeneous photocatalysis is a process initiated by the photogeneration of conduction band electrons and valence band holes in a semiconductor material irradiated with band gap photons. The electron–hole charges, after migration to the catalyst interphase, can initiate coupled reduction and oxidation reactions, respectively with electron acceptors and electron donors species adsorbed on or near the surface of the semiconductor.^1,2 Such a process has been exploited in environmental decontamination, production of renewable energy carriers and in the manufacture of smart and self-cleaning materials.

The most common semiconductor photocatalyst is titanium dioxide (TiO₂), due to its physico-chemical properties and ability to promote efficient photoinduced phenomena. In the absence of light irradiation, under darkness, the surface of TiO₂ thin films exhibits hydrophobic properties. However, under UV light irradiation the surface becomes super-hydrophilic and such process is exploited in the manufacture of self-cleaning materials, such as glass windows. The mechanism of light induced hydrophilicity of TiO₂ relies on the hydroxylation of the catalyst surface under irradiation, which is driven by the conduction band electrons reduction of Ti⁴⁺ that yields Ti³⁺ and by the generation of oxygen vacancies via the hole-oxidation of the bridging O²⁻ to O₂. Thus, hydroxyl ions from water can be preferentially adsorbed onto the TiO₂ oxygen vacancies inducing hydrophilicity of the TiO₂ surface.¹ Under these conditions, water wetting on the TiO₂ surface forms a continuous thin water film that carries away dirt particulates from the substrate surface, cleaning the surface.

Photocatalytic, thin-film, self-cleaning surfaces are also able to kill and prevent the attachment of bacteria on the coated surfaces, and this has been exploited to prevent the spread and diffusion of infections.^2,3 TiO₂ is one of few low-cost materials known to show photoinduced hydrophilicity and switchable wettability and photocatalytic activity under light illumination.⁴ Thin films of TiO₂ have been immobilized on different support materials, such as wool fabrics, glass and polymers.^4–7 Doping of TiO₂ with metals has been used as a strategy to improve the intrinsic photocatalytic activity of TiO₂.^8–17 The doping of TiO₂ with rare earths, such as erbium (Er)¹¹ and yttrium (Y)¹² has shown a 2 to 3-fold increase in the photocatalytic degradation of dyes in aqueous catalyst suspensions. This higher activity is ascribed to the transition of 4f electrons of Er³⁺ or Y³⁺ and to the red shifts of the optical absorption edge of TiO₂ due to the rare earth doping,^10,12 which reduces the bandgap and improves separation of electron–hole pairs.¹²

Only a few studies have investigated the properties of immobilized TiO₂ thin films doped with rare earths or polymer modified-TiO₂ thin films¹³ to improve the photocatalytic activity. A study on the structural and spectroscopic properties of ytterbium (Yb) and thulium (Tm) doped TiO₂ thin films deposited by the ultrasonic spray pyrolysis technique demonstrated that rare earth doping prevented the phase transition from anatase to rutile up to annealing temperatures of 800 °C.¹⁴ Other studies on thin films of Yb doped TiO₂ ^15,16 and Y doped TiO₂ ¹⁷ prepared by spin coating and by the magnetron sputtering technique revealed adverse effects of rare earth doping on photocatalytic activity at concentrations of 2 wt% or higher but a positive effect at very low concentrations of 0.5 wt% was observed. On the other hand, doped TiO₂ thin films produced by magnetron sputtering¹⁷ were composed of fully oxidized states of the two elements (Y or Ti), Y₂O₃–TiO₂ composite oxides. The growth of the TiO₂ crystal in the films was inhibited by the presence of the Y₂O₃ phase and the films were essentially amorphous. However, the UV-vis transmittance of the films decreased whereas their reflectance increased slightly, causing a decrease of the photocatalytic activity of Y-doped TiO₂ films.

In this study, the structural, optical and photocatalytic properties of Er³⁺ and Y³⁺ doped TiO₂ thin films exhibiting remarkable self-cleaning super-hydrophilic properties under light irradiation were investigated. The transparent, thin films deposited on glass were prepared by the simpler sol–gel dip coating method to allow accurate control of the chemical composition of the films. Using this deposition technique very thin film were formed that were subsequently thermally treated to eliminate the excess solvent present, causing shrinkage and densification of the photocatalytic film.^6,18 The impact of Er and Y loading was assessed in order to determine the lowest amount necessary to impart remarkable self-cleaning and super-hydrophilic properties to the films.

Experimental section

Y³⁺ or Er³⁺-doped and undoped TiO₂ thin films: preparation and characterization

Thin films were prepared on soda-lime microscopy slide glass surfaces by the sol–gel dip coating method. In the synthesis process, 0.61 mL of the stabilizing agent acetylacetone (A.R., Neon) was added to 69 mL of isopropanol (A.R., Vetec) to control the hydrolysis and condensation reactions. Then, 6 mL of the titanium alkoxide pre-titrating agent titanium tetraisopropoxide (TTIP, 97.0%, Sigma-Aldrich) was introduced into the same mixture. After 30 min magnetic stirring, 6.86 mL of acetic acid (A.R., Dinâmica) was added and the solution was kept under magnetic stirring for a further 30 min.

The Er³⁺ or Y³⁺-doped TiO₂ samples were produced by dissolving the desired molar amount of erbium(III) pentahydrate nitrate (99.9%, Sigma-Aldrich) or yttrium(III) hexahydrate nitrate (99.9%, Sigma-Aldrich) in distilled water and dropwise transferring the solution to the previously synthesized sol solution. The Er³⁺ or Y³⁺ content in the samples for this study were 0; 0.25 mol%; 0.50 mol%; and 1.00 mol%.

The glass substrates (25 mm × 85 mm) were pre-cleaned ultrasonically with acetone for 15 min, rinsed with ethanol and distilled water, and then dipped into the sol solution. The withdrawing speed was 2.1 mm s⁻¹. The coated substrates were then thermally treated at 450 °C or 500 °C to remove the organic compounds from the glass surface. This procedure was repeated up to 3 times on the same glass substrate to produce thin and transparent films, designated as TxCw + zRE³⁺, where x is the temperature used in the thermal treatment, w is the number of immersion/emersion cycles, z is the molar percentage of rare earth, and RE³⁺ is Y³⁺ or Er³⁺.

The morphological properties of the films were obtained by combining several techniques. The surface relief of the films was examined by the atomic force microscopy (AFM) technique using an Asylum Research microscope (Model MFP 30) with amplitude modulation AFM imaging (tapping mode), a resonance frequency of 30 kHz and a spring constant of 42 N m⁻¹. The root mean square (RMS) average roughness was determined using the Gwyddion software program, version 2.22.

The crystallinity was determined by X-ray diffraction (XRD) analysis of the powders previously dried at 100 °C for 24 h, using a Philips X ′Pert X-ray diffractometer equipped with Cu Kα at 40 kV and 30 mA, θ–2θ and λ = 1.54056 Å, with a scanning speed of 2θ from 0° to 80°. The thermal stability of the dried powders was determined in a Shimadzu DTG-60 analyzer, under N₂ atmosphere with a flow rate of 50 mL min⁻¹ and at a constant temperature gradient of 10 °C min⁻¹. To establish the surface area and porous structure of the powdered photocatalysts, the corresponding N₂ adsorption–desorption isotherms were measured at 77 K using an automatic absorptometer (Autosorb 1C, Quantachrome, USA).

The optical transmittance in the wavelength of 300–800 nm was examined using a UV spectrophotometer (Hach DR 5000, Germany).

Determination of the apparent water contact angle on glass surface

Standard protocols were used to evaluate the apparent water contact angle and photocatalytic activity.¹⁹ Prior to the measurement of the contact angle, all samples were cleaned in acetone and irradiated for 24 h under UVA irradiation (light source 8 W, GENERIC; UV intensity 20.38 W m⁻²). The apparent water contact angle in air for the samples was measured with a contact angle meter (Ramé-Hart Inst. Co. 250-F1 goniometer) at ambient temperature (water temperature 20–25 °C). For each measurement a 2 μL water was used and at least 3 different positions on the surface of the sample were examined and the average value was determined.

Photocatalytic degradation of oleic acid

Six identical pieces of coated glass were rinsed with acetone and then irradiated for 24 h under UVA irradiation (light source 8 W, GENERIC; UV intensity 20.38 W m⁻²). A sufficient fixed amount of oleic acid (OA) was applied with a loading of 21.8 μg cm⁻², which corresponds to approximately 240 nm or 120 monolayers of a uniform film.²⁰ The samples were placed inside a UV chamber at room temperature and, at regular time intervals, the apparent water contact angle on the coated glass surface was measured using a contact angle meter (Ramé-Hart Inst. Co. 250-F1). Five measurements were carried out at five different positions on the surface of each piece at regular time intervals for 100 h.

As previously reported by Manole et al.,²¹ an exponential decay of the water/oleic acid contact angle can be used to describe the self-cleaning properties and the removal of oleic acid from a photoactive surface. The photocatalytic degradation of oleic acid was therefore described as a pseudo-first-order reaction (eqn (1)):

where t is the UV irradiation time, θ_o is the initial contact angle of the water on the oleic acid-coated surface, θ is the contact angle of the water on the oleic acid-coated surface at time t, and k is the pseudo-first-order rate constant.

Results and discussion

Thermal analysis of powdered samples

The results for the thermogravimetric analysis of the powdered samples (uncalcined) are shown in Fig. 1.

Fig. 1 TGA (a) and DTA (b) curves for undoped (TiO₂) and powdered doped samples (TiO₂ + 0.25Y and TiO₂ + 1.00Er).

The mass loss of undoped TiO₂ in the temperature range of 250–450 °C is attributed to combustion of non-hydrolyzed isopropoxide binders and other organic substances bound to the titania particles.¹⁸ Thus, temperatures at or above 450 °C were required to obtain titanium dioxide (TiO₂) in the gel form. At temperatures close to 600 °C, the phase transition anatase to rutile occurs in the undoped TiO₂, without mass loss.²² This crystalline phase change can also be observed on the DTA curve for the undoped TiO₂ in Fig. 1b, with the exothermic peak starting at approximately 550 °C.

The Er³⁺ or Y³⁺ doped TiO₂ samples also exhibited a mass loss between 250–450 °C, ascribed to the combustion of non-hydrolyzed isopropoxide binders and other organic substances bound to the titania particles and to the thermal decomposition of yttrium nitrate²³ or erbium nitrate,²⁴ as shown in Fig. 1a. The endothermic peaks associated with the anatase-rutile phase transition at 600 °C for the undoped sample, were not observed on the Y³⁺–TiO₂ or Er³⁺–TiO₂ samples, indicating that these rare earths thermally stabilized the TiO₂ crystallite transition phase. Such behavior has also been observed in Yb and Tm doped TiO₂ thin films.¹⁴

Fig. 2 shows the diffractograms of the powdered doped samples calcined at 450 °C (T450 + 0.25Y; T450 + 0.50Y; T450 + 0.25Er; T450 + 0.50Er; and T450 + 1.00Er) and undoped TiO₂ calcined at 450 °C (T450) or 500 °C (T500). All undoped samples peaks are attributed to the anatase phase, with no peaks related to rutile or any other phases being observed. The samples shows diffraction peaks characteristic of the anatase TiO₂ crystal planes (101), (004), (200), (105), (211), (204), (116), (220) and (215) at 2θ = 25.3°, 37.8°, 48.1°, 53.9°, 55.0°, 62.7°, 68.7°, 70.3° and 75.2°.²⁵ The Y³⁺ or Er³⁺-doped TiO₂ samples were also composed of the anatase phase only.

Fig. 2 XRD diffractograms for powdered undoped and Y³⁺ or Er³⁺ doped TiO₂ samples (symbols: TiO₂ anatase).

In the doped samples, the variation in the XRD peak intensity was not pronounced because the dopant was below 1 mol%. The apparent crystallite size determined by the Scherrer equation using the (101) anatase plane²⁴ decreased with increasing amount of yttrium or erbium (Table 1). Crystallite sizes along other orientations could not be estimated due to lack of adequate intensity.

Table 1 Size of crystallites of undoped and Y³⁺ or Er³⁺-doped TiO₂ powdered samples

Sample powdered	Average size of crystallites (Å)
T450	161.0 ± 12.0
T500	190.0 ± 17.0
T450 + 0.25Y	110.6 ± 14.3
T450 + 0.50Y	109.6 ± 5.2
T450 + 0.25Er	89.4 ± 13.6
T450 + 0.50Er	98.8 ± 9.1
T450 + 1.00Er	81.2 ± 9.7

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It is assumed that Y³⁺ or Er³⁺ doping hinders the growth of the crystallite due to the segregation of the dopant cations at the grain boundary.^26,27 Moreover, the replacement of Ti⁴⁺ in TiO₂ by RE³⁺ ions results in the loss of one electron, which would be compensated for by the formation of one oxygen vacancy. As a result, the photocatalytic activity might be improved.¹¹ Moreover, the inhibition of phase transition from anatase to rutile, TiO₂ lattice distortion, and forming smaller grains, could result in a significant improvement in the hydrophilicity and photoreactivity of Y³⁺ or Er³⁺ doped TiO₂.

The presence of the Er₂Ti₂O₇ phase in the Er³⁺–TiO₂ was not found at 2θ equal to 32°,²⁸ since its formation would be expected only when the sample is heated at temperatures higher than >500 °C.^29,30 The existence of Er₂Ti₂O₇ crystallites would be the key factor for upconversion transforming visible light into UV light.¹⁵

The UV-vis spectra of the Y³⁺, Er³⁺ and bare TiO₂ samples are shown and discussed in Fig S1†.

Optical transmittances of doped and undoped TiO₂ thin films

High visible light transmittance is an indispensable requirement for the application of these TiO₂ thin films on transparent window glass. High light transmittance of the undoped and of the Er³⁺ or Y³⁺ doped TiO₂ thin films on glass exceeding 60% in the visible range was observed (Fig. 3).

Fig. 3 Optical transmittance spectra of doped and undoped TiO₂ films and uncoated glass: (a) T450C1, T450C2, T450C3; (b) T500C1, T500C2, T500C3; (c) Er³⁺-doped TiO₂ with different Er³⁺ contents; and (d) Y³⁺-doped TiO₂ with different Y³⁺ contents.

The optical transmittance was nearly the same considering the number of thin film layers deposited on the glass (Fig. 3a), the temperature of the thermal treatment (Fig. 3a and b), and the yttrium (Fig. 3c) or erbium (Fig. 3d) content. The thickness of the TiO₂ films is expected to increase linearly by repeating the dipping and heat-treatment cycles.³¹ Film thickness obtained by single, double or triple dipping should be sufficiently high to achieve effective self-cleaning properties without deterioration of the film optical quality.

The transparency of thin films is influenced by several factors, such as the scattering of light by the surface and volume defects, however, the associated mechanisms are not yet clearly understood. In general, the film thickness should not be higher than 1 μm to maintain a high light transmittance in the range of 300–800 nm.¹⁰ The dip coating deposition technique produced transparent coated glass, in a simple and non-expensive method, that may meet a wide range of practical applications.

Contact angle and surface roughness

Hydrophilicity is an extremely important characteristic for the analysis of self-cleaning materials, since it regulates the interactions between dirt particulate and the substrate surface. Hydrophilic surfaces have a strong interaction with water and water vapor and a thin transparent aqueous film able to spread easily over the entire surface could remove dirt (self-cleaning) and impart anti-fog properties.

Fig. 4 shows the apparent water contact angles of the coated and uncoated glass samples. The apparent water contact angle was zero for all coated glass samples produced in this study, indicating a superhydrophilic character. The superhydrophilicity of TiO₂ coatings is a result of the reaction of adsorbed water on the illuminated surface (reactions (2) to (5)).

TiO₂ + hυ → h⁺ + e⁻ (charge pair generation) (2)

Ti⁴⁺ + e⁻ → Ti³⁺ (surface trapped electron) (3)

H₂O + oxygen vacancy → ˙OH (5)

Fig. 4 Apparent water contact angles for the doped and undoped TiO₂ thin film glass after 24 h UVA irradiation.

Rare earth doping can provide a dopant level close to the valence band of TiO₂. The electronic transition from the modified dopant band to the conduction band effectively improves, and the hole–electron recombination decreases. The electrons tend to reduce Ti⁴⁺ to Ti³⁺, oxygen atoms are ejected and oxygen vacancies are created, producing more Ti defects. Water molecules thus can occupy these oxygen vacancies, producing adsorbed hydroxyl groups. The increase in the hydroxyl content on the surface of TiO₂ thin films makes the surface hydrophilic, since the hydroxyl can form hydrogen bonds with water. Moreover, the chemisorbed hydroxyl groups can stabilize the structure of Ti³⁺–OH, which also results in the enhancement of hydrophilicity. Although the superhydrophilicity of TiO₂-coated glass has been widely reported,¹² this is the first time that the super hydrophilic character and zero-water contact angle is reported for Y³⁺–TiO₂ or Er³⁺–TiO₂ coated glass.

The surface structure, morphology and roughness of the undoped and doped TiO₂ thin films were analyzed by AFM (Fig. 5). The images of the scanned surface reveal that the films were uniform, homogeneous and densely packed with small grains. A columnar structure was observed in the 3D AFM images, which indicated that growth occurred in the (101) plane of the films. Agglomerated particles were observed in the Y³⁺ or Er³⁺ doped samples. These agglomerated particles increased in size with the Y³⁺ or Er³⁺ content, resulting in increased surface roughness. The undoped TiO₂ thin film (T450C3) was the smoothest sample, with a roughness of 2.0 ± 0.3 nm, and uniform distribution of TiO₂ on the substrate (Fig. 5a). The AFM images for the doped samples show a change in topography with increasing Y³⁺ or Er³⁺ content. The increasing roughness is a result of the aggregation of the nanoparticles.

Fig. 5 AFM images of the thin films on the glass: (a) T450C3; (b) T450C3 + 0.25%Y; (c) T450C3 + 0.50%Y; (d) T450C3 + 1.00%Y; (e) T450C3 + 0.25%Er; and (f) T450C3 + 0.50%Er; (g) uncoated glass.

The root mean square (RMS) roughness values are shown in Table 2 shows a direct correlation of surface roughness within increasing Er³⁺ or Y³⁺ doping level, as well as the presence of large aggregates at higher Y³⁺ or Er³⁺ content. The increased roughness in the Er³⁺ or Y³⁺ doped TiO₂ samples increases the active surface area and thus enhance the photodegradation process.³² As observed from the AFM results, the roughness of the TiO₂ film increases with Y³⁺ or Er³⁺ doping, that is in agreement with the apparent water angle contact results for hydrophilic surfaces.^33,34 In fact, the BET surface of the powdered TiO₂ samples increased from 27.5 m² g⁻¹ to 93.4 m² g⁻¹ when³³ the Er³⁺ content increased from zero to 1.0 wt%.

Table 2 RMS roughness

Sample	RMS (nm)
Uncoated glass	1.0 ± 0.7
T450C3	2.0 ± 0.3
T450C3 + 0.25Y	2.2 ± 0.2
T450C3 + 0.50Y	3.3 ± 1.9
T450C3 + 1.00Y	31.0 ± 4.5
T450C3 + 0.25Er	2.4 ± 2.2
T450C3 + 0.50Er	4.8 ± 0.8

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Photocatalytic degradation of oleic acid and self-cleaning effect

The kinetics of the conversion of oleic acid (C₁₈H₃₄O₂) provide the basis for the ISO 27448 method¹⁹ to evaluate the performance of photocatalytic and self-cleaning surfaces.

The average initial water contact angle on the oleic acid-coated surface, taken at five different points, for 5 different samples of each coated glass did not depend on the nature of the surface since the surface was fully covered with oleic acid (Fig. 6).

Fig. 6 Contact angle of water on the oleic acid-coated surface.

The mechanism associated with the photodegradation of oleic acid on TiO₂ has not been completely described, but it is known that some organic aldehydes and acid compounds (9-oxononanoic acid, azelaic acid, nonanoic acid, heptanal, octanal, nonanal and decanal) are the main intermediates of this photocatalytic reaction.³⁵ Thus, the water contact angle of the irradiated photocatalytic coated glass decreased as the oleic acid on the surface degraded.

While the majority of reports in the literature provide only initial and final contact angle values, the monitoring of the contact angle over the reaction time could be used as an indicator of the surface cleaning process.^20,21,36,37 This analysis method can produce quantitative results, which are closely related to those obtained from the traditional techniques of FTIR, AFM or gravimetry.³⁶ When the surface was irradiated, a continuous decrease in the contact angle was observed on both undoped and doped TiO₂-coated surface which is indicative of the oleic acid photocatalytic degradation (Fig. 7).

Fig. 7 Time evolution of contact angle for water on different coated glass with UV light.

In contrast, the uncoated glass surface did not exhibit oleic acid degradation. The pseudo-first-order rate constant (k) for the oleic acid photocatalytic degradation on TiO₂, Y–TiO₂ or Er–TiO₂ coated glass is shown in Fig. 8.

Fig. 8 Pseudo-first-order rate constant for oleic acid degradation of different TiO₂-coated glass samples.

The results show that as the yttrium or erbium content increased up to 1 wt%, the photocatalytic activity and self-cleaning properties of the coated glass also increased. This effect may be attributed to a series of factors in the Er³⁺ or Y³⁺ doped samples, including the reduction of the crystallite size TiO₂, the increased roughness and surface area (Table 1) the reduced band-gap and the enhanced separation of the photogenerated carriers.³⁸ Er³⁺ doped TiO₂ performed far better than Y³⁺ doped TiO₂, since Er³⁺ doped thin films present higher roughness and smaller crystallite sizes.

As shown in Tables 1 and 2, the doping using very small amount of Y³⁺ (0.25%), no significative difference was observed on the roughness (2.0 ± 0.3 nm and 2.2 ± 0.2 nm, for T450C3 and T450C3 + 0.25Y, respectively), although there is a significative difference on the crystallite size (161.0 ± 12.0 A and 110.6 ± 14.3 A, for T450C3 and T450C3 + 0.25Y, respectively). Thus, an insignificant effect on oleic acid degradation was observed comparing the T450C3 and T450C3 + 0.25T thins films (see Fig. 8).

Despite Zhang et al.¹⁷ has shown that the photocatalytic activity of Y₂O₃ doped TiO₂ thin film decreases for Y dopant concentration higher than 2 wt%, the homogeneity and low rare earth metal dopant used in this study proved that it is a suitable way to improve the superhydrophilic character of glass surface.

Conclusions

The rough undoped TiO₂ or doped Y³⁺–TiO₂ and Er³⁺–TiO₂ films are efficient photocatalysts for the decomposition of thin layers of oleic acid deposited on their surfaces and exhibit a remarkable superhydrophilic character. The kinetics of the oleic acid film degradation was studied by monitoring the apparent water contact angle and this process was described by a pseudo-first-order reaction. It was observed that the photocatalytic activity and self-cleaning character of the coated glass increases with the yttrium or erbium content. The increased roughness with Er³⁺ or Y³⁺ doping greatly enhances the surface area and thus the photodegradation process and self-cleaning process. The anatase crystal phase was identified, however, the diffraction peaks of TiO₂ became broader and weaker with increasing Er³⁺ or Y³⁺ concentration, due to a reduction in the crystallite size. Thus, Er³⁺ or Y³⁺ doping can hinder the growth of the crystallite due to the segregation of the dopant cations at the grain boundary. This study demonstrates that low loadings of Er³⁺ or Y³⁺ down to 0.5 mol% can produce transparent, super-hydrophilic surfaces with remarkable self-cleaning properties.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors would like to acknowledge the Brazilian government agencies CAPES/Brazil (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior, Project number 88887.310560/2018-00) and CNPq/Brazil (Conselho Nacional de Desenvolvimento Cientifíco e Tecnológico, Grants 405.223/2018-8 and 301.479/2018-6) for the financial support.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0ra02242j

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