1. Introduction Ever-growing problems of our modern world are environmental and atmospheric pollution, emerging antibiotic-resistant bacteria, and persistent organic pollutants (POPs) that are not easily eliminated in waste water treatments [1–3]. Amongst the available technologies, semiconductor photocatalysis is one of the most promising to fight those problems, because it represents a simple, viable and sustainable way to take advantage, not only of the primary source of energy freely available on the Earth, solar light, but also of artificial indoor / outdoor illumination. Therefore, photocatalysts are a truly environmentally-friendly solution to combat most soil, sediment, air (indoors or outdoors) or biota pollution agents, as well as harmful micro-organisms [4–6]. Titanium dioxide (titania, TiO2) is one of the most widely used photocatalysts. Its three best known polymorphs are (in order of abundance) rutile, anatase and brookite, all of which are chemically inert, nontoxic and photocatalytically active (with no additives) in mild conditions. The range of wavelengths and photocatalytic performance are highly dependent on the polymorph, the energy band-gaps (Eg) of anatase and rutile being 3.2 and 3.0 eV, respectively [7], with anatase widely considered a better photocatalyst than rutile [8]. Anatase and rutile are able to degrade both volatile organic compounds (VOCs) [9,10], and recalcitrant / persistent pollutants [11], and they are also reported to be active against several bacterial strains and yeasts [12–14]. They are also employed in applications other than photocatalysis, i.e. H2 production [15], sensors [16], biomedicine [17] and photovoltaic applications [18]. The wide Eg of titania is widely recognised as its biggest drawback, limiting its photocatalytic activity (PCA) to the UVA range, which represents <5% of the total solar spectrum [19]. A number of approaches have been taken to overcome that Eg limitation, and thus enhance the visible-light absorption: amongst these are doping with transition metals, with anions / cations, or producing semiconductor hetero-structures [20–23].

2

Both copper and zinc have previously been used individually as TiO2 dopants/modifying agents, to improve the photocatalytic properties and/or impart antibacterial activity [24,25]. However, no study was ever performed showing a direct comparison between the two elements, and with varying degrees of comodification. Indeed, to our knowledge, only one study has been published previously showing the effects of co-doping with both metals [26], and in that study there was no antibacterial testing and the PCA was assessed only in the liquid-solid phase, monitoring the degradation of an organic dye (methyl orange). Such dye degradation tests can represent subtle ``pseudo-photocatalytic'' systems, masking the actual noncatalytic nature of the reaction involved [27–31], and for this reason we carried out gas-solid photocatalytic tests. In this work, we present for the first time a systematic study of copper and zinc modification and comodification of titania. TiO2 was synthesised via a green aqueous sol-gel method [32,33] – whose preparation process was shown not to be dangerous for workers in laboratory conditions [34] – and modified / co-modified with copper and zinc. The paper reports on the effect of these two elements (alone and together) have on the PCA and antibacterial activities of TiO2, this being still an unexplored field.

2. Experimental 2.1

Sample preparation

Aqueous titanium(IV)hydroxide sols were made via the carefully controlled hydrolysis and peptisation of titanium(IV)isopropoxide (Ti-i-pr, Ti(OCH(CH3)2)4), with distilled water diluted in isopropyl alcohol (IPA, propan-2-ol), following a procedure previously reported in detail by the authors [32]. Briefly, one part of Tii-pr (Sigma Aldrich, 97%) was added to four parts of isopropyl alcohol to make a 20 vol% Ti-i-pr solution, that was afterwards hydrolysed by the dropwise addition of an excess of water (5:1 water:Ti-i-pr) employed as a 20 vol% solution in IPA. The acid necessary to peptise the sol (concentrated HNO 3, Aldrich, 65%) was also added to this water-IPA solution, in a molar ratio of Ti4+:acid of 2.5:1. This water-IPA-acid solution was added dropwise to the Ti-i-pr solution at room temperature, whilst being stirred. The precipitated mixture was evaporated to a white jelly-like mass on a rotary evaporator, removing the IPA. Distilled water was

3

added to restore the mixture to the original volume, and this was then dried once more to a dried gel at 60 °C. 1 mol% Zn modified, 1 mol % Cu modified and mixed 1 mol% Cu-Zn co-modified sols (with Cu:Zn in various ratios) were prepared, with the Cu:TiO2, Zn:TiO2 and (Cu/Zn):TiO2 molar ratios listed in Table 1. This was achieved using stoichiometric amounts of copper(II) nitrate trihydrate (Sigma Aldrich, ≥ 98.5%), and zinc(II) nitrate hexahydrate (Sigma Aldrich, > 98%) that were added to, and dissolved in, the prepared sols. 2 mol% modified sols were also made, and some results for those are shown in the ESI. The dried gels were thermally treated at 450 °C under a static air flow, using a muffle furnace, with a heating / cooling rate of 5 °C min–1, with a 2 hour dwell time at the selected temperature. Table 1 indicates the naming system (nomenclature) for the samples. 2.2

Sample characterisation

Semi-quantitative phase analysis (QPA) of the crystalline phases in the various specimens (not taking into account the likely presence of any residual amorphous phase) was obtained by X-ray powder diffraction (XRPD), using the Rietveld method as implemented in GSAS-EXPGUI [35,36]. Data were collected on a PANalytical X’Pert Pro (NL) / diffractometer equipped with 0.5° divergence slit, 0.5° anti-scattering slit, 0.04 rad Soller slits, a 15 mm copper mask in the incident beam pathway and a fast RTMS detector (PIXcel 1D, PANalytical) on the diffracted arm. The 20−80 °2θ range was investigated using Cu Kα radiation (45 kV and 40 mA) with a virtual step scan of 0.02 °2θ and virtual time per step of 200 s. The starting atomic parameters for anatase, rutile and brookite, described in the space groups I41/amd, P42/mnm and Pbca respectively, were taken from the literature [37,38]. The instrumental broadening was measured by means of the NIST SRM 660b standard (LaB6) − data were collected in the same conditions as those used for the modified TiO2 samples – and was taken into account in the refinements. The refined parameters were: scale-factors, zero-point, specimen displacement, six coefficients of the Chebyshev background polynomial and unit cell parameters, and two Lorentzian terms and one angle independent Gaussian term as the profile coefficients.

4

Microstructural analysis (i.e. crystalline domain shape, size and size distribution) was solved by means of whole powder pattern modelling (WPPM) [39], as implemented in the PM2K software [40]. Data were collected using the same instrument and set-up described above, but in the 20-115 °2θ range, with a virtual step of 0.1 °2θ, and virtual time per step of 500 s – so to have a high signal-to-noise ratio. The instrumental contribution was also obtained, modelling the profile of 14 reflections from the NIST SRM 660b standard (LaB6), according to the Caglioti et al. relationship [41]. The following parameters were then refined: background (6th-order Chebyshev polynomial function), peak intensities, specimen displacement, lattice parameters, mean and variance of the size distributions. Crystalline domains were approximated to be spherical, and their diameter distributed according to a lognormal curve. Raman spectra of the samples were acquired in the 50–700 cm−1 wavenumber range, with 4 cm−1 resolution, on a Bruker RFS 100/S (DE), equipped with a Nd:YAG laser (1064 nm) as the excitation source. FT-IR analysis was also performed, with the aim of detecting the occurrence of OH groups and/or water adsorbed on the photocatalyst surface. This was carried out on a Bruker Tensor 27 (DE) spectrometer. The measurements were made over the wavenumber range of 4000–350 cm−1, in attenuated total reflectance (ATR) mode. Diffuse reflectance spectroscopy (DRS) was used to assess the optical properties of the samples. DRS spectra were collected in reflectance mode, on a Shimadzu UV 3100 (JP) spectrometer, equipped with an integrating sphere made of BaSO4, in the UV-Vis range (250–825 nm), with 0.2 nm step-size, and using BaSO4 as a reference. The optical Eg of the samples was evaluated via the differential reflectance method [22,42]; the resulting curves describing the first derivative of reflectance versus the wavelength λ were successfully fitted with a Gaussian function (OriginPro Lab, version 8.5), and the maximum values, together with the experimental errors, were consequently calculated from the fitting. Specific surface area (SSA) of the prepared samples was measured with the Brunauer–Emmett–Teller (BET) method (Micromeritics Gemini 2380, US), using N2 as the adsorbate gas, and degassing the samples at 120 °C. 2.3

Functional properties evaluation 5

2.3.1

Antibacterial activity

One Gram-positive and one Gram-negative microorganism were tested, Methicillin resistant Staphylococcus aureus (MRSA ATCC 29213) and Escherichia coli (E. coli NCTC 9001), respectively. The strains were grown in Mueller Hinton Agar at 37 °C for 24 hours. Liquid cultures of the strains were prepared in saline solution (0.85 % NaCl), having an approximate concentration of 10+6−10+7 CFU/mL; 500 L of each culture was pipetted into a sterile petri dish, containing 4.5 mL of sterile saline solution and a weighted amount of the solid sample. The powders were sterilized by heating them at 100 °C overnight, in the dark. The closed petri dishes were shaken at 80 rpm, either in the dark or under UV irradiation (XX-15 BLB UVP lamp,  = 365 nm, irradiation density: 8 W m−2 – see literature for detailed spectral emission [43]. At regular intervals, 50 μL aliquots were taken from the plates and diluted using 0.85 % NaCl solution. The diluted solutions were plated on plate counting agar and incubated at 37 °C for 24 hours; the colonies were then counted. Control experiments were also done in parallel, with bacterial solution either irradiated or in the dark without any powder sample. Each experiment was performed in triplicate; an average value of the bacterial counts was calculated, with the corresponding standard deviation.

2.3.2

Gas-solid phase photocatalytic activity

PCA of the samples was assessed in the gas-solid phase employing two different reactors, and, as such, two different model pollutants. One reactor (described previously in detail, cf ref. [33]) was made of a stainless steel cylinder (35 L, internal volume), and operated in continuous conditions. This reactor was used to monitor the NOx abatement (NOx = NO + NO2). Samples were prepared in the form of a thin layer of powder, with a constant mass (~0.10 g), and consequently approximately constant thickness, in a 6 cm diameter Petri dish (irradiated surface ~28.3 cm2). A solar lamp (Osram UltraVitalux, its emission spectrum being reported in the literature [44]), placed 85 cm from the surface of the photocatalyst, was used. Light intensity reaching the surface of the samples was estimated (through a radiometer) to be ~ 3.6 W m −2 in the UVA range, and 25 W m−2 in the visible-light range. 6

Tests were performed at 22±1 °C (temperature inside the reactor) with a relative humidity of 35% − these parameters, controlled through a thermocouple placed inside the chamber, and a humidity sensor placed in the inlet pipe, remained stable throughout the tests. The outlet concentration of the pollutant gas was measured using a chemiluminescence analyser (AC–30 M, Environment SA, FR). After having placed the photocatalyst inside the reactor, and covered the glass window, the inlet gas mixture (prepared using cylinders with synthetic air and NOx gas) was allowed to flow until it stabilised at a concentration of 0.2 ppmv. Two mass flow controllers were used to prepare a mixture of air with this concentration of NO x, and with a flow rate of 1 L min−1. Once the desired concentration of 0.2 ppmv was reached inside the reactor, the window glass was uncovered, the lamp turned on, and the photocatalytic reaction was supposed to start – total irradiation time was set at 20 min. The photocatalytic efficiency was evaluated as the ratio of the removed concentration of NOx. The conversion rate (%) of the initial NOx concentration was calculated as:

NOx conversion% 

 NOx 0   NOx S  NOx 0

(1)

where (NOx)0 and (NOx)S are, respectively, the initial NOx and the NOx concentration after a certain irradiation time [45]. The other reactor used was operated in batch conditions, and was used to monitor the degradation of IPA, taken as pollutant representing VOCs [46]. IPA can be actually converted first into acetone and then into further smaller molecules/fragments. The reactor used was previously described in detail [22,47]. Samples were illuminated by a 300 W Xe lamp (Newport Oriel Instruments, USA). The lamp emits both UV and visible light, imitating the solar spectrum; the light intensity reaching the samples was 40 W m −2. Tests were performed using both UV and visible-light, or visible light alone; these will be referred to in the paper as (UV+Vis) and (Vis) respectively. The visible-light only irradiation was achieved using a filter cutting all light under 400 nm.

3. Results and discussion 7

3.1

X-ray diffraction analysis

XRD patterns of prepared samples are depicted in Fig. S1a (Electronic Supplementary Information), whilst a graphical output of a Rietveld refinement is reported in Fig.S1b. Semi-QPA data are also reported in Table

2. Ti450, contains 55.6 wt% anatase, 20.1 wt% rutile, and 24.3 wt% brookite [48]. The large presence of this latter TiO2 polymorph is not to be unexpected, and has been attributed to the acidic conditions of the synthesis, which do not favour its conversion into rutile, as has been widely reported in the literature [49– 51]. No metallic copper, copper oxides, zinc titanates or zinc oxides were detected in the XRD patterns. This is likely because the copper and/or zinc levels were too low (≤ 2 mol%). Also, the addition of copper to TiO 2 (Cu-Ti450) greatly delayed the anatase-to-rutile phase transformation (ART) because, of a grain-boundary pinning mechanism caused by Cu modification of titania, as we have shown in a recently published work [48]. Zinc addition (Zn-Ti450) likewise retarded the ART, though to a lesser extent compared to copper modification: clustering around TiO2 NPs, it plays a role in retarding anatase grain-growth, and the ART as a consequence [52]. This is consistent with literature reports [52–55], and in the case of Zn-modification, brookite quantities are higher in both samples (27.8 and 30.5 wt%), most likely because of a more acidic environment. Similarly, copper-zinc co-modification had a retarding effect on the ART. However, greater copper molar amounts resulted in a smaller anatase weight percentage in the specimen (i.e. 77.7 wt% in 0.25Cu/0.75ZnTi450, and 73.9 wt% in 0.75Cu/0.25Zn-Ti450, in favour of a slightly higher brookite amount in the specimens). This opposes the findings of Zhang and Zeng, who found that copper and zinc co-modification favoured the ART [26]. For microstructural data, an example of WPPM modelling output is shown in Fig. S1c (for CuZn-Ti450), while WPPM data are listed in Tables 3 and 4. Negligible differences, within the experimental errors, were observed in the unit cell volumes of anatase, rutile and brookite present in all the Cu/Zn modified samples (Table 3). From these data, we can reasonably assume that neither Cu nor Zn significantly entered the TiO 2 lattice (ionic radii of hexa-coordinated Cu1+/2+, Zn2+ and Ti4+ are: 0.77/0.73 Å, 0.74 Å, and 0.61 Å, respectively 8

[56]), and thus they clustered as oxides on the surface of the TiO2 NPs, creating a nano-heterojunction between these semiconductor materials. Furthermore, as all the diffraction line profiles are quite large, this supports the nanocrystalline nature of all the TiO2 polymorphs. Information on the crystalline domain sizes is shown in Table 4, and in Fig. S2a-c. We know from a previous published work that the unmodified TiO2, Ti450, has average domains of 10.4 nm for anatase, 14.4 nm for rutile, and 7.0 nm for brookite [48]. Addition of Cu and / or Zn to the samples generally led to a decrease in the average size of anatase domains (the exception was 0.75Cu/0.25Zn-Ti450, its average diameter being close to that of anatase in unmodified Ti450). This demonstrated their limiting effect on anatase crystal growth and, as a consequence, on the ART. A similar consideration can be made for brookite: copper and copper + zinc additions limited its crystal growth, as shown by both the smaller brookite crystalline domain diameters in the samples with Cu and CuZn, and also by the lower amounts of this TiO2 polymorph in these samples. Modification with 1 mol% Zn had an opposite effect, favouring both brookite crystallisation and crystal growth, compared to Cu and CuZn addition. This is shown by the greater brookite amount in Zn-Ti450, and by the bigger brookite crystalline domain sizes, comparable to those in Ti450 [48]. The average size of rutile decreased with copper addition, but when zinc was also present (in Cu-Zn mixed additives), rutile had a slightly larger average size

3.2

Spectroscopic Analyses

DRS data are shown in Fig. 1, and the calculated optical Eg values are reported in Table 6. As can be seen from the reflectance spectra, all the samples exhibit an absorption band at around 400 nm, assigned to the Ti4+−O2– metal-ligand charge transfer (MLCT) in titania [57]. On the other hand, all of the copper-containing samples also display other absorptions besides this MLCT band, in the visible region. The wide absorption band centred at around 825 nm belongs to Cu2+ d−d transitions [57]. Another band is also seen in these samples at around 450 nm, assigned to the interfacial charge transfer (IFCT) from the valence band of TiO2 to that of CuxO clusters on the surface of the titania [58].

9

The calculated optical Eg values are assigned to the rutile phase present in the mixtures, as the values are consistent with those expected for rutile (413 nm). However, it has to be underlined that with the method we used, the curve of anatase might be overlapping that of rutile, hence the observed Eg of rutile may ``mask'' that of anatase. FT-IR spectra are depicted in Fig. 2a-b, and they all show similar characteristics. In general, the band in the 400–600 cm−1 range is caused by Ti–O–Ti vibrations, the peak at around 1630 cm−1 corresponding to the bending vibrations of O–H, while the broad band centred at approximately 3250 cm−1 is attributed to the surface-adsorbed hydroxyl groups [33]. Thus, from the FT-IR spectra, we can infer that samples with zinc caused more surface hydroxyl groups to attach to the TiO2 surface. In principle, this should favour an enhanced PCA, because, being able to accept any photogenerated holes, they create hydroxyl radicals that oxidise the pollutant molecules adsorbed on the photocatalyst surface [59]. Raman analysis, Fig. 3a-b, essentially confirmed the XRD data. The only Raman vibration modes detected were those of TiO2 polymorphs (anatase, rutile, and brookite); no signal of copper oxide or zinc titanate was detected, confirming the wide dispersion of copper and zinc on the TiO2 surface [60,61]. Table 7 reports the position and full width at half maximum (FWHM) of the symmetric vibrational Eg (O−Ti−O) Raman mode of anatase in the samples, at around 145 cm−1. The general trend, compared to that of our unmodified Ti450 (see Ref [44] for unmodified TiO2 Raman data), was a red-shift in the Eg Raman mode of the anatase for the Cu- and Zn-modified samples, together with an increase in the FWHM. It is accepted that this is caused by the generation of grain boundaries, increases in disorder, and the lowering of the crystalline domain size [62,63]. This effect is more pronounced in the mono Cu- or Zn-modified TiO2 than in the Cu-Zn co-modified samples, implying there is a stronger effect of copper or zinc alone in perturbing the TiO2 crystal structure / microstructure. Interestingly, Zn-Ti450 exhibited a higher Raman intensity compared to the unmodified Ti450 (Fig. 3b). This clearly indicates that Zn modification has a great influence on the surface enhanced Raman scattering (SERS) of the TiO2 NPs. Such an improved performance of SERS has been attributed to the enriched surface states (e.g. defects) of TiO2 and improved separation efficiency of the photo-generated charge carriers related to surface states in TiO2 [64]. 10

3.3

Functional properties

Liquid-solid PCA was measured only under UV-light with methylene blue (MB) discoloration as the indicator – the experimental details are given in the ESI, and results shown in Fig. S3.

3.3.1

Photocatalytic activity – NOx abatement

PCA results in the gas-solid phase, using nitrous oxides as a model pollutant, are reported in Fig. 4a, and they are shown as the pseudo-first-order kinetic constant (k’app) for the initial 20 min of reaction time. All the samples were photocatalytically active in the gas-solid phase regime, using a light source simulating the solar spectrum. Furthermore, Cu and Zn modifications gave the specimen a higher BET SSA compared to the unmodified Ti450 (cf Table 6), which had favoured an increased PCA, compared to the unmodified TiO2.Zn-Ti450 possessed the highest PCA of all the mono-modified Cu-TiO2 and Zn-TiO2 samples. This can be attributed partly to the higher percentage of anatase phase in them, but also to the greater number of surface hydroxyl groups attached to their surface, as demonstrated by FT-IR measurements. Furthermore, Fu et al. recently stated that a TiO2/ZnO heterojunction can improve the separation efficiency of the charge carriers, i.e. delay the h+/e− recombination, thus improving the PCA of the final products compared with that of pure TiO2 [65]. For the Cu-Zn co-modification, no particular trend could be inferred from PCA results. Indeed, 0.25Cu/0.75Zn-Ti450 (Cu:Zn molar ratio equal to 1:3) was the best performing specimen in degrading NOx, most likely because it represents the optimum combination of these two modifying agents in the TiO 2 electronic structure. This may help delay the recombination of the photogenerated h+/e− pair, which is often the weakness of TiO2 as a photocatalytic material [66,67]. On the other hand, the opposite ratio of Cu:Zn = 3:1 in 0.75Cu/0.25Zn-Ti450 had the opposite effect on the PCA.

3.3.2

Photocatalytic activity – Isopropanol degradation

PCA of the samples in gas-solid mode against this model VOC, under UV+VIS irradiation, is shown in Figure 4b-c. A trend shows that samples modified prevalently with copper are generally less active than their zincmodified cousins. In fact, the relationship between the modifying elements ratio seems to be quite straightforward – the greater the Zn:Cu ratio, the higher the PCA, with Zn-Ti450 having the highest PCA, 11

even greater than that of Ti450. Thus, it is also no surprise that Cu-Ti450 exhibited the lowest PCA, and ZnTi450 the highest. As has been already stated, and is clear from FT-IR results, the higher PCA of the zinc-modified sample can be at least partially attributed to higher OH group populations on the surface of the particles. Scanlon et al. recently showed that in anatase–rutile nano-composite materials, an energetic band alignment (type II) is present with a value of 0.4 eV at the band edges of the rutile and anatase polymorphs. This significantly lowers the effective Eg, facilitating an effective electron–hole separation, with respect to their individual counterparts [68]. PCA of the samples under visible-light is not so straightforward. On one hand, Ti450 itself possesses a significant PCA (7 ppm h–1 acetone formation), because it is composed of both anatase and rutile, and the latter is well-known to act as an ``antenna'', able to extend the PCA into visible wavelengths [69]. 0.75Cu0.25Zn-Ti450 stands out as the most active sample (20 ppm h–1 acetone formation). Its PCA was twice as high as that of the next-best sample, Zn-Ti450, and the PCA of the remaining samples was lower than that of Ti450. 0.75Cu0.25Zn-Ti450 could be an example of a synergistic effect of the two modifying agents – copper nanoparticles that decorate the titania act as antennae for visible-light absorption, while zinc contributes to lowering recombination probability, the overall effect being the observed higher PCA.

3.3.3

Possible mechanism of photocatalysis

It is well established that the redox reactions that might be achieved on the surface of a semiconductor material are ``limited'' by the positions of the band edges of a given photocatalyst [70]. Our materials are a mixture of anatase and rutile (with small amounts of brookite), cf Table 2. The Eg of anatase and rutile is ~3.2 and ~3.0 eV, respectively [8], with the conduction-band bottom (ECB) of anatase being 0.4 eV more positive (lower) than that of rutile, as reported by very recent literature about anatase-rutile junctions [68,71]. This leads to a superior separation of charge carriers in the mixed material compared to the individual phases, as stated above, and also explains the high PCA of Ti450. The higher PCA of Zn-Ti450 can be explained considering the Eg, and the energy levels of the ECB, with respect with the normal hydrogen electrode (NHE) scale, as shown in scheme 1. 12

In such a picture, as suggested by Serpone et al. [72], an electron transfer occurs from the conduction band (CB) of light-activated ZnO to the CB of light-activated TiO2; vice versa, hole transfer can arise from the valence band (VB) of TiO2 to the VB of ZnO. This might limit the electron-hole recombination, and consequently improve the PCA of the ZnO/TiO2 junction. Conversely, copper modified specimens experienced a lower PCA because both the VB and CB of CuO (assuming copper is present as CuO) are sandwiched between those of TiO2. In this case, a charge transfer from both the CB and VB of TiO2 to those of CuO happens, favouring a charge recombination, that is reflected by the lower PCA [74].

3.3.4

Antibacterial activity

Following the photocatalysis results, the UV-light induced antibacterial activity of some selected samples was also measured. Powders modified with either 1 mol% Cu or Zn were tested (Cu-Ti450 and Zn-Ti450), and compared to Ti450; moreover, a sample modified with both elements (0.25Cu0.75Zn-Ti450) was also considered. The results are shown in Fig. 5a-h. Two strains were tested – Gram-negative E. coli and Grampositive MRSA. In all experiments, no significant bacteria population decrease was observed for either microorganism if they were not in contact with the samples, either in the dark or under UV illumination. Cu-Ti450 (Fig. 5c) shows excellent antibacterial behaviour towards E. coli. Indeed, after just 1 hour of irradiation, no viable bacterial colonies could be detected (<50 CFU/mL); this corresponds to an inactivation rate of 99.9999 %. If compared to Ti450 (Figure 5a), Cu-Ti450 shows better efficiency – in fact, for the unmodified TiO2, a complete inactivation of the Gram-negative strain is achieved only after 2 hours of contact. With just 1 hour, some viable colonies can still be observed. Moreover, results show that Cu-Ti450 also has some significant activity without any UV illumination, as after 2 hours contact in the dark the viable bacterial population has decreased by about 4 orders of magnitude (inactivation rate of 99.99 %). This indicates that the antimicrobial action is partially due to the copper released from the sample. This is reasonable considering the known antibacterial activity of this element [75]. Gram-positive MRSA (Fig. 5d) was also inactivated by Cu-Ti450, as the bacterial population was more than three orders of magnitude lower after two hours of UV illumination (inactivation of 99.96 %). Such activity, although inferior than that towards E. coli, is still remarkable; in fact, a compound is generally considered 13

antibacterial towards a species if can inactivate at least 99 % of its population [76]. Without light illumination, on the other hand, no effect can be seen on the bacterial population. It has to be highlighted, however, that the activity of Cu-Ti450 is lower than that of unmodified TiO2 towards MRSA. With 2 hours of contact, Ti450 had an inactivation rate of 99.9999 %. This result is surprising, considering that Cu-Ti450 was much more effective than Ti450 on Gram-negative E. coli. These differences can be due to possible different and parallel mechanisms of the bactericidal action, and also to the differences in the morphology of Gram-negative and positive bacteria. Gram-positive micro-organisms, in fact, can show higher resistance due to their thicker peptidoglycan layer, which is less likely to be affected by light-induced reactive oxide species and radicals [43,77]. Literature reports detail contrasting results for Cu-modified TiO2-based materials; in some cases, higher activity towards Gram-negative strains was observed [24,77], while in other studies higher inactivation rates for Gram-positive micro-organisms were seen [78,79]. 0.25Cu0.75Zn-Ti450 was also tested, to determine whether the presence of both constituents could have a positive effect on the antibacterial properties. For E. coli (Fig. 5e), a complete inactivation was observed under UV irradiation after 2 hours; the intermediate sampling at 1 hour, however, shows that the strain is more resistant to this sample. In fact, the bacterial population had decreased by just 3 orders of magnitude, which is comparable to the performance of Ti450, but less than that of Cu-Ti450. Figure 5e also shows that 0.25Cu0.75Zn-Ti450 has only moderate antibacterial activity in the dark; after two hours incubation, the bacterial population had decreased by almost 96 % (data statistically different from the control experiments). 0.25Cu0.75Zn-Ti450 was moderately effective towards MRSA under UV irradiation (Fig. 5f), as a bacteria inactivation of about 90 % was registered. In the dark, however, no significant effect was observed. This behaviour is comparable to that observed for photocatalytic NOx abatement (Figure 5a),. Overall these data showed that copper and zinc co-modification does not improve the anti-microorganism activity. Tests performed on the other samples gave similar results (data not shown), that is, a lower effectiveness in bacterial strain inactivation. Zinc and copper co-modification was previously reported for materials such as 14

hydroxyapatite and Ni-Mn-O spinel structures [80,81]. However, to our knowledge, no data are available in the literature for the antibacterials effects of zinc and copper co-modification in TiO2. As this is the first report of its kind, it is not possible, to perform any direct comparison. Figures 5g and 5h show the tests for Zn-Ti450. For E. coli, the antibacterial activity is much lower than that of the other samples; in fact, with UV irradiation, the inactivation rate is only about 98 %, while no effect is seen in the dark. For MRSA, the behaviour of Zn-Ti450 is comparable to 0.25Cu0.75Zn-Ti450. These results indicate that the anti-microorganism properties of TiO2 do not benefit from zinc modification, and that the use of copper should be preferred instead. Literature does not report direct comparisons of these two kinds of materials; for other materials/applications, however, literature data indicate that copper has higher antibacterial activity than zinc [79], and this study seems to confirm this. Conclusions TiO2 nanopowders, modified with copper or zinc (up to 1 mol%) or co-modified with both the metals, were prepared via a green sol–gel route and thermally treated at 450 °C. This is the first time that a direct comparison between these two constituents has been performed on a TiO2 based material. The product of the synthesis gave anatase, rutile and brookite; advanced X-ray methods showed that neither copper nor zinc entered the TiO2 lattice, but clustered as oxides around the titania NPs, hence forming a heterojunction between these semiconductor materials, and favouring the delay of the ART. Raman spectroscopy showed that Zn had a great influence on the SERS performance of the TiO2 NPs. The results showed that zinc-containing samples had higher PCA than the corresponding copper-containing ones; this was seen for catalysis in gas phase. Considering the antibacterial activity, on the contrary, copper had a much better effect than zinc. Co-modification with both ions had mixed effects, depending on both the concentrations of the metals and their ratio. Indeed, in some cases an increase in the photocatalytic activity was observed, while in others the performance was worse. The antimicrobial activity, on the other hand, was always worse for the comodified samples.

15

These results show that modifying an anatase-rutile junctions – that possess superior separation of charge carriers compared to the individual phases, because of a type (II) staggered band alignment of ~0.4 eV – can be quite tricky, and counter-productive in terms of photocatalytic activity. TiO2 modification is thus a very complex subject, whose results are difficult to predict. Tailored conditions should be selected depending on the applications of each material. Acknowledgements D.M. Tobaldi is grateful to the ECO-SEE project (funding from the European Union’s Seventh Framework Programme for research, technological development and demonstration under grant agreement no 609234). R.C. Pullar and C. Piccirillo acknowledge the support of FCT grants SFRH/BPD/97115/2013 and SFRH/BPD/86843/2012, respectively. N. Rozman and A. Sever Škapin thank Slovenian research agency (ARRS) for the financial support. This work was developed in the scope of the project CICECO−Aveiro Institute of Materials (Ref. FCT UID/CTM/50011/2013), financed by national funds through the FCT/MEC and when applicable co-financed by FEDER under the PT2020 Partnership Agreement. References

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Figure 1 – UV-Vis spectra (825-250 nm wavelength range) of the samples. 21

Figure 2 – FT-IR spectra of: a) Cu or Zn-modified specimens; b) Cu and Zn-modified samples.

Figure 3 – a) Raman spectra of Ti450 and Cu-Ti450. In the inset a magnification between 110–190 cm−1 is shown, to highlight the peak broadening and red shift of the Raman Eg active mode of anatase at 144 cm−1 (the vertical dashed line indicates the position of that peak in the unmodified TiO2 sample); b) Raman spectra of Ti450 [44], and Zn-Ti450 specimens, to show the influence of Zn-modification on the surface enhanced Raman scattering (SERS). The vertical bars represent the Raman peak shifts of: anatase (red), rutile (black), and brookite (blue).

Figure 4 – a) Photocatalytic NOx abatement of the prepared samples, reported as the pseudo-first-order kinetic constant (k’app) for the initial 20 min of reaction time; b) photocatalytic isopropanol degradation, reported as ppm h–1 of acetone formation, using the UV+Vis-light source; c) photocatalytic isopropanol degradation, reported as ppm h–1 of acetone formation, using the visible-light source.

Fig. 5 – Bacteria inactivation for samples Ti450 (a,b), Cu-Ti450 (c,d) and 0.25Cu0.75ZnTi450 (e,f) and Zn-Ti450 (g,h); graphs on the left (a,c,e,g): E. coli; graphs on the right (b,d,f,h): MRSA. Full squares: control dark; full triangles: control light; open circles: sample dark; open triangles: sample light.

Scheme 1 – Simplified schematic diagram depicting the redox potentials of the valence and conduction bands and the Eg for ZnO, TiO2 and CuO. The electrochemical potentials of the band edges for TiO2, ZnO and CuO are from the literature [72–74]; the Eg of TiO2 is from this work (Zn-Ti450).

TABLES Table 1 – Sample labelling and copper and zinc doping and co-doping molar amounts. Composition (mol% modifying Sample

agent)

Cu-Ti450

1 Cu

0.5Cu0.5Zn-

0.5 Cu + 0.5 Zn

Ti450 0.75Cu0.25Zn-

0.75 Cu + 0.25 Zn

Ti450 0.25Cu0.75Zn-

0.25 Cu + 0.75 Zn

Ti450 Zn-Ti450

1 Zn

22

Table 2 – Rietveld agreement factors and crystalline phase composition of the unmodified and Cu/Zn-modified TiO2. Sample No. of Agreement factors Phase composition (wt%) variables R(F2) Rwp χ2 anatase rutile brookite (%) (%) 27 2.30 4.15 1.37 55.6±0.2 20.1±0.2 24.3±0.9 Ti450† † 20 3.16 3.60 2.03 82.2±0.1 9.0±0.2 8.8±0.8 Cu-Ti450 22 3.20 4.32 1.51 77.7±0.1 12.0±0.2 10.3±1.0 0.25Cu/0.75ZnTi450 22 4.04 4.35 1.60 74.1±0.1 11.0±0.2 14.8±0.9 0.50Cu/0.50ZnTi450 22 2.95 4.06 1.42 73.9±0.1 12.6±0.2 13.5±1.2 0.75Cu/0.25ZnTi450 22 2.59 4.35 1.40 61.3±0.2 10.8±0.2 27.8±0.8 Zn-Ti450 Note: there were 2285 observations for every refinement; the number of anatase, rutile and brookite reflections was 32, 31 and 153, respectively. † From Ref. [48]. Table 3 – WPPM agreement factors and unit cell parameters for the three titania phases in the synthesised samples. Agreement factors Rwp (%) Rexp (%) 2 6.17 2.53 5.63 5.50 4.75 4.96

1.96 2.09 2.10 2.12 2.16 2.12

3.15 1.21 2.68 2.60 2.20 2.34

Unit cell parameters Rutile

Anatase a=b (nm) 0.3791(1) 0.3790(1) 0.3789(1) 0.3800(2) 0.3794(1) 0.3795(2)

c (nm) 0.9515(1) 0.9510(3) 0.9511(2) 0.9511(4) 0.9512(1) 0.9509(4)

V (nm3) 0.137(1) 0.137(1) 0.137(1) 0.137(1) 0.137(1) 0.137(1)

a=b (nm) 0.4598(1) 0.4600(1) 0.4597(1) 0.4599(1) 0.4597(1) 0.4594(1)

c (nm) 0.2959(1) 0.2959(1) 0.2960(1) 0.2960(1) 0.2961(1) 0.2961(1)

V (nm3) 0.063(1) 0.063(1) 0.063(1) 0.063(1) 0.063(1) 0.062(1)

Brookite a (nm) 0.5440(2) 0.5459(4) 0.5452(3) 0.5443(3) 0.5453(2) 0.5473(5)

b(nm) 0.9206(4) 0.9129(29) 0.9184(8) 0.9210(9) 0.9184(6) 0.9139(8)

† From Ref. [48]. Table 4 – Mean crystalline domain size of anatase (ant), rutile (rt) and brookite (brk) – defined as the mean of the lognormal size distribution; maximum values of the lognormal size distribution. Sample Ti450† Cu-Ti450†

Mean crystalline domain diameter 〈Dant〉 〈Drt〉 〈Dbrk〉 (nm) (nm) (nm) 10.4±0.7 14.4±0.6 7.0±0.1 8.0±0.2 12.2±1.1 4.3±0.5

Mode of the size distribution Ant Rt (nm) Brk (nm) (nm) 9.4±0.6 9.9±0.4 5.3±0.1 6.6±0.2 8.6±0.8 3.8±0.4 23

0 0 0 0 0 0

0.25Cu/0.75ZnTi450 0.50Cu/0.50ZnTi450 0.75Cu/0.25ZnTi450 Zn-Ti450

7.4±0.3

19.7±1.7

3.7±0.2

5.9±0.2

16.8±1.5

2.0±0.1

7.9±0.5

17.9±1.6

7.3±0.3

6.7±0.4

15.2±1.4

5.8±0.2

10.7±0.5

20.1±1.7

4.4±0.2

9.4±0.4

18.5±1.6

2.8±0.1

9.0±0.9

18.9±1.6

7.0±0.2

7.9±0.8

16.1±1.4

6.1±0.2

† From Ref. [48].

Table 6 − Optical energy band-gap (Eg) – assigned to the rutile phase in the mixtures − and specific surface area (SSA) of the specimens. Sample Ti450† Cu-Ti450 0.25Cu/0.75ZnTi450 0.50Cu/0.50ZnTi450 0.75Cu/0.25ZnTi450 Zn-Ti450

Optical Eg (eV) 3.06±0.01 3.14±0.01 3.12±0.01

SSABET (m2 g−1) 41.9±1.0 59.9±1.1 63.2±1.3

3.14±0.01

59.5±1.2

3.13±0.01

64.2±1.4

3.10±0.01

56.7±0.6

† From Ref. [44]. Table 7 – Position and full width at half maximum (FWHM) of Raman Eg mode of anatase. Sample Ti450† Cu-Ti450 0.25Cu/0.75ZnTi450 0.50Cu/0.50ZnTi450 0.75Cu/0.25ZnTi450 Zn-Ti450

Anatase Eg mode (cm−1) 145.3±0.1 145.9±0.1 144.7±0.1

FWHM (cm−1) 15.1±0.4 17.0±0.4 15.4±0.4

145.1±0.1

15.6±0.5

145.3±0.1

15.9±0.4

146.2±0.1

17.1±0.6

† From Ref. [44].

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