Efficient H2 production by photocatalytic water splitting under UV or solar light over variously modified TiO2-based catalysts

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Efficient H2 production by photocatalytic water splitting under UV or solar light over variously modified TiO2-based catalysts R. Fiorenza a,b, S. Scir
e a,*, L. D'Urso a, G. Compagnini a, M. Bellardita c, L. Palmisano c  di Catania, Viale Andrea Doria 6, 95125, Catania, Italy Dipartimento di Scienze Chimiche, Universita CNR-IMM, Via Santa Sofia 64, 95213, Catania, Italy c  di Palermo, Viale delle Scienze, 90128, Palermo, Italy Dipartimento di Ingegneria, Universita a

b

article info

abstract

Article history:

This paper focused for the first time on the comparison between three different approach

Received 10 February 2019

to modify the chemico-physical properties of TiO2-based photocatalysts and their effect in

Received in revised form

the H2 production by photocatalytic water splitting both under UV and solar light irradi-

29 March 2019

ation, under the same experimental conditions. The application of pulsed laser irradiation

Accepted 4 April 2019

to aqueous TiO2 suspensions (first approach) induced structural transformations both on

Available online 27 April 2019

the bulk and on the surface of TiO2, boosting the H2 production, under UV light irradiation, of almost three times (20.9 mmol/gcat$h) compared to bare TiO2 (7.7 mmol/gcat$h). The

Keywords:

second strategy was based on a templating method to obtain TiO2 with a macroporous

Hydrogen

structure to favour an efficient light absorption process inside the material pores, thus

Titania

allowing a high H2 production (0.64 mmol/gcat$h) under solar light irradiation. This per-

Ceria

formance was further enhanced when the macroporous TiO2 was coupled with CeO2 or W

Tungsten

(third approach). In the latter case the H2 production increased to 0.72 mmol/gcat$h for

Laser treatment

macroporous TiO2eCeO2 and to 0.82 mmol/gcat$h for macroporous TiO2eW. This work

Macroporous structure

highlights how it is possible to tune the TiO2 photocatalytic properties with easy and green procedures to obtain environmental friendly catalyst for hydrogen production. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction The rapid growth over the past century of a large-scale economy and industrialization has given rise to serious concerns regarding both the depletion of natural resources and the global warming. Despite these troubles, fossil fuels still remain the main source of energy. This prompted the research toward clean renewable energy sources.

After its first application in water cleavage, with hydrogen production using a TiO2 photoanode in 1972 [1], photocatalysis attracted an immediate interest, because of the 1973 and 1979 energy crises. Since then, the attention of the scientific community raised continuously in so as photocatalysis represents the ideal green technology which uses a renewable and available energy resource as solar light and mild reaction conditions [2e4]. Photodegradation of pollutants and

* Corresponding author.
). E-mail address: [email protected] (S. Scire https://doi.org/10.1016/j.ijhydene.2019.04.035 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

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production of clean fuels by photocatalytic water splitting or CO2 photoconversion were the hottest subjects in photocatalysis [5e7]. Since its appearance on the market in the early 20th century, titanium dioxide (TiO2) underwent an extensive use in several applications (sunscreens, paints, ointments, toothpaste, etc). Moreover after the first report in 1972 by Fujishima and Honda [1], TiO2 has been largely employed in solar cells, lithium, biomedical devices and to-date is by far the most used photocatalyst exploiting its non-toxicity, good stability and low cost [8e11]. However, there are still some inherent disadvantages restraining the wide use of TiO2 in the various photocatalytic applications. In fact, due to the wide band gap (3.2 and 3.0 eV for anatase and rutile, respectively) TiO2 suffers a low exploitation of the solar electromagnetic spectrum. Moreover, a fast recombination of photo-produced electronhole pairs and a great overpotential for the splitting of water leads to low photoefficiency. Therefore, in the last years, several efforts have been pursued in order to enhance the TiO2 photoefficiency, by increasing the photocatalytic surface, by extending the absorption in the visible region and by tailoring the structure of bands to match specific energy levels suitable with the redox potentials of the desired reactions [12e23]. These objectives have been achieved through different approaches such as synthesis of porous structure, realization of peculiar structure (inverse opal photonic crystals), structural or chemical modifications, addition of sacrificial reagents, dye sensitization, doping with metal and non-metal species, and coupling of different semiconductors, giving rise to Schottky junctions or heterojunctions. The addition of sacrificial reagents or hole scavengers has proved to be a good strategy for reducing the recombination rate of photogenerated charges by reacting with the holes and allowing the electrons to reduce Hþ ions [2,24,25]. Moreover, the overall process efficiency can be optimized if waste biomass derivatives are used as sacrificial agents, affording high added value compounds [2,3,18]. Another way to separate the e/hþ pairs has been the deposition of noble metals (as Pt, Ag, Au, Pd, Rh, Ni, Cu) onto the photocatalyst surface [26e28]. In this case, being the Fermi level of the metal lower than that of TiO2, electrons move from the conduction band toward the metal particles, whereas the holes remain in the photocatalyst valence band. The metal can also act as a co-catalyst providing additional reactions sites on the catalyst surface, and a plasmonic effect can occur, depending on the particles size [29]. Doping and dye sensitization were effective in shifting absorption towards the visible region allowing the solar light exploitation [21,30,31]. Different systems have been investigated, but a general conclusion could not be reached, due to the various experimental conditions selected. Another key issue of the photocatalytic water splitting is the low H2 yield due to different properties that the photocatalysts should have for the global reaction to take place. In particular, it is reported that some materials are efficient for H2 evolution and others for O2 evolution. In this contest, coupling different photocatalysts, each suitable for a specific reaction, can improve the efficiency of the process [14,15,32,33]. In addition, by coupling TiO2 with a smaller band-gap semiconductor, it is possible to extend the

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absorption spectrum range toward the visible region [20,27]. Recently, good results for H2 production under cocatalyst-free conditions have been reported by using colored TiO2 nanomaterials containing oxygen vacancies and Ti3þ centres obtained with different technologies [34,35]. In this paper we compare different strategies applied by us to adjust the chemico-physical properties of photocatalysts, pointing out the specific influence that each modification caused on the performance of H2 production from water splitting. The experiments were carried out either under UV or solar light irradiation. In particular, (i) structural changes in the surface and bulk TiO2 were induced by laser treatment in water, a simple and green method to obtain reduced highly photoactive TiO2 species [36], (ii) a template strategy was used to obtain TiO2 with a highly ordered macroporous structure (Macro-TiO2) [37], and (iii) chemical modifications to introduce defects in both anatase TiO2 and Macro-TiO2 were performed by addition of CeO2 or W species, acting as photosensitizer or doping agents, respectively [38]. The home prepared photocatalysts were characterized by scanning electron microscopy (SEM), UVevis diffuse reflectance spectroscopy (DRS), BET specific surface area (SSA), Raman spectroscopy and photoluminescence (PL) to correlate some of their properties to the observed photoactivity. The main novelty of this work consists in the comparison of the photocatalytic performance for H2 production of various samples modified by using different techniques and tested under the same experimental conditions. Previous literature data are also compared with the results presented here.

Experimental Catalysts preparation Concerning with untreated oxides used in this work, TiO2 was commercial anatase (Sigma Aldrich, prod. Nos. 637254) and CeO2 was home synthesized by KOH (0.1 M) precipitation from Ce(NO3)3$6H2O aqueous solution and subsequent filtration and calcination in air at 450  C for 4 h [39,40]. Anatase TiO2-10 wt% CeO2 composite (coded TiO2eCeO2) and anatase TiO2-10% at. W (coded TiO2eW) were prepared by wet-impregnation using the proper amount of Ce(NO3)3$6H2O and tungsten (VI) chloride as metal salt precursor, then stirring the slurry for 3 h, drying in an oven at 100  C for 24 h and treating in air at 350  C for 3 h. For laser treated samples (TiO2 LT), TiO2 was dispersed in distilled water and sonicated, then irradiated under stirring using the second harmonic radiation (532 nm) of a Nd:YAG nanosecond pulsed laser, following the methodology described in ref. [36]. In detail, the laser beam size used was around 28 mm2, and it was focused toward the titania solution without any focusing lens. The titania suspension was irradiated homogeneously at a constant laser power up to 1.45 W (0.5 J/cm2) for 30 min. The highest used fluence was far below the ablation threshold (around 0.8 J/cm2). No plasma emission has been detected during the synthesis. Immediately after the laser treatment, the initially white suspension became deep blue.

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Macroporous TiO2 (coded Macro-TiO2) was synthesized by adopting the templating strategy [37,38]. Briefly, polystyrene (PS) spheres (300 nm) were impregnated by drop-wise addition of titanium isopropoxide solution. The precursor-template mixture was first dried for 24 h and then calcined in air at 550  C for 12 h. Macroporous anatase TiO2-10 wt% CeO2 composite (coded Macro-TiO2eCeO2) was prepared by wetimpregnation of Macro-TiO2 using the appropriate amount of Ce(NO3)3$6H2O, followed by 100  C drying for 24 h and 350  C air treating for 3 h. Macroporous TiO2-10at.% W (coded MacroTiO2eW) was prepared with the same procedure of bare Macro-TiO2 adding a proper amount of tungsten (VI) chloride to the titanium isopropoxide solution.

Catalysts characterization The morphology and the texture of samples were assessed by a scanning electron microscopy (SEM) with a Jeol JSM-7500 F microscope and by adsorption-desorption of N2 at 196  C using a Sorptomatic series 1990 (Thermo Quest) with prior overnight outgassing of samples at 120  C. The characterization of the optical behaviour was carried out by: a) a WITec alpha 300 confocal Raman apparatus exciting with a 532 nm laser line of a Coherent Compass Sapphire Laser; b) UltravioleteVisible-Diffuse Reflectance Spectroscopy (UVeVis DRS) in the range of 200e800 nm using a Jasco UVevisible spectrometer, with a Labsphere for diffuse reflectance analysis, and BaSO4 as the reference; c) Photoluminescence (PL), using a PerkinElmer LS45 apparatus and 320 nm as the excitation wavelength.

Photocatalytic activity tests Photocatalytic water splitting tests were carried out in a jacketed Pyrex reactor, kept at 30  C, and by irradiating the suspension of the catalyst (25 mg) in deionized water (45 ml)ethanol (5 ml, used as sacrificial agent) with a 100 W UV mercury lamp (Blak-Ray B 100 A, 365 nm) or with a 300 W sunlight simulating lamp (Osram Ultra Vitalux). Prior to irradiation the stirred suspension was purged 1 h in Ar to get rid of dissolved air. H2 evolution was determined by an online gas chromatograph according to Ref. [36].

Results and discussion As discussed in the introduction of this paper, we applied different strategies in order to modify the properties of anatase TiO2 and to assess the role of the changes on the photocatalytic H2 generation by water splitting either under UV or solar light.

Laser treatment in water Reduced TiO2 (TiO2-x) materials, also known as black titania, have received considerable attention in the last years, on account of the high photocatalytic and photoelectrochemical performance [34,36,41], caused by peculiar properties such as broadened absorption in the solar light region, surface and bulk structural changes with creation of Ti3þ species and

oxygen vacancies. The most used methods to reduce TiO2 (thermal treatment in H2 flow, treatment under vacuum, or using hydrogen plasmas) entail high temperatures (400e700  C), the use of vacuum arrangement and multistep procedures with long processing times. Here we considered an alternative, easy and green approach to obtain reduced TiO2 by laser irradiation in aqueous dispersion [36,42]. According to the above literature, laser treatment of anatase TiO2 determined significant structural modifications of the TiO2. It was confirmed by the characterization techniques used (Raman and PL), which are gathered in Fig. 1 for TiO2, TiO2 LT and Macro TiO2 samples. An indirect qualitative indication of the structural modification is the slight grey/blue coloration observed for the TiO2 treated sample. In particular the Raman characterization (Fig. 1A) clearly shows similar spectra for TiO2 and Macro-TiO2 samples, pointing to the presence of the typical features of anatase, namely a main band at 140 cm1 (Eg), due to the OeTieO vibration, and three other ones at 395 cm1 (A1g), 515 cm1 (B1g) and 635 cm1 (Eg). It can be noticed that the laser irradiation treatment in water caused a partial crystalline phase change from anatase to rutile, as demonstrated by the appearance of the bands at 445 and 600 cm1 in the TiO2 LT sample, distinctive of rutile (Eg). This is also well discernible in the Raman maps reported in Fig. 1B showing in blue the signals at 140 cm1 for anatase and in red those at 445 cm1 for rutile. A similar trend of partial phase transformation (anatase / rutile) has been already reported by some of us in the case of laser treatment of TiO2 P25 [36]. In Fig. 1C PL spectra of TiO2, TiO2 LT and Macro TiO2 samples are abridged. It can be seen that for all three samples the spectrum is quite broad, with several components in the 350e600 nm range, reasonably related to the typical PL features of anatase titania, namely at 370e390 nm (band to band emission of the TiO2 [43]), 430 (self-trapped excitons confined on TiO6 octahedral [44]), 475, 525 and 560 nm (electrons in defective Ti centres connected with oxygen vacancies [45]). Notably, the TiO2 LT sample exhibits a spectrum with a significant higher intensity in the 450e600 nm region, pointing to a higher number of defects, whose number has been reported to be somewhat correlated to the PL intensity in this region [45e47]. The band gap energies values for the investigated samples, calculated using the modified Kubelka-Munk spectral function [F (R∞0 )hn]1/2 on the basis of UVevis DRS spectra are displayed in Table 1. No substantial variations of the Eg values were calculated for TiO2, TiO2 LT and Macro TiO2 samples. On account of the above characterization it can be concluded that laser treatment of TiO2 results in the introduction of lattice distortion with Ti3þ species and oxygen vacancies and in a moderate change in the crystal phase of TiO2 with a small increase in the BET surface area (Table 1). These changes probably concur to the relevant enhancement of the photoactivity of TiO2 LT with a significant increase of H2 production by photocatalytic water splitting, especially under UV irradiation (Fig. 2). The occurrence of defects in the crystalline structure of TiO2 is the key factor explaining the increase of the H2 production. The defects, in fact, act as hole traps owing to the creation of a Ti3þ donor level just below the bottom of the CB [36], thus strongly decreasing the recombination rate between the photoholes and the electrons.

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Fig. 1 e Characterization data of TiO2, Macro TiO2 and TiO2 LT samples. (A) Raman spectra; (B) Raman map of TiO2 LT sample showing in blue the signal at 145 cm¡1 for anatase phase and in red that at 445 cm¡1 for rutile; (C) PL spectra for TiO2, Macro TiO2 and TiO2 LT materials respectively.

Templating strategy and chemical modifications The second approach was based on the combination of a template strategy, aimed to obtain TiO2 with a macroporous structure (Macro TiO2), and chemical modifications achieved by adding CeO2 as host component or W as doping agent. TiO2 catalysts with macro-mesoporous structures have attracted the interest of the researchers working in heterogeneous photocatalysis due to the highly available surface

Table 1 e BET Surface areas and band-gap energies (Eg) for investigated TiO2-based samples. Catalysts TiO2 TiO2 LT TiO2eCeO2 TiO2eW Macro TiO2 Macro TiO2eCeO2 Macro TiO2eW

SBET (m2 g1)

Eg (eV)

55.0 61.1 57.5 56.1 28.3 23.3 28.8

3.24 3.23 3.19 3.25 3.23 3.18 3.26

area and the regular size of the pores with intrinsic connectivity, which help both efficacious charge transfer and reactants mass flow [48]. The higher photoactivity of these systems was attributed to the specific porous structure of TiO2 leading to a high efficient light absorption process inside the material pores [36,37,49,50]. In Fig. 3 the SEM images, at the same magnification, of Macro TiO2, Macro TiO2eCeO2 and Macro TiO2eW are reported. It can be seen that all macroporous TiO2-based samples have a specific porous macrostructure made of interconnected cavities. This is in accordance with the lower surface area of these latter samples (23e28 m2/g vs. 55e61 m2/g of the non-Macro TiO2 samples), as shown in Table 1. Interestingly the Macro TiO2 sample was more active than TiO2 in the H2 production (Fig. 2). In particular the activity enhancement effect of the macroporous TiO2 was much higher under simulated solar than UV light irradiation. In fact, Macro TiO2 exhibited 2 and 6 times higher H2 production than the anatase TiO2 sample under UV and solar light, respectively. While the Raman spectra of TiO2 and Macro TiO2 are similar (Fig. 1A), the Macro TiO2 shows instead a PL spectrum

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Fig. 2 e H2 production over TiO2, TiO2 LT and Macro TiO2 samples: (A) under UV lamp; (B) under solar lamp.

more intense than the TiO2 one in the band to band emission region (Fig. 1C), pointing to the presence of more excitons species. The peculiar structure of these materials (macroporous inverse opal materials) allows the enhancement of the light harvesting efficiency increasing the light path length and enabling light waves to easily enter the photocatalyst [36,37,48e50]. The highly porous structure, moreover, increases the active surface area and allows an easier mass transportation of the reagent to the active sites of the catalysts. This results in an increase of the path length of light and in an improved photoactivity.

Chemical modification (doping with CeO2 or W) It has been broadly accounted that photocatalytic activity of titania can be strongly affected by coupling TiO2 with other oxides (ZnO, Cu2O, Bi2O3, BiVO4, CeO2) [36,40,51e53] or doping with metallic (V, W, Fe) [21,38,54,55] or non-metallic elements (C, N, S, F) [38,56e59]. In particular the combination of titania with an appropriate amount of specific oxides (as CeO2) can be highly beneficial for the photoactivity in so as the oxide can favour the electron transfer from TiO2 to the other oxide or vice versa, leading to a lower e/hþ recombination rate and therefore to an enhancement of the charge separation [14,37,40]. Doping with metallic or non-metallic elements may instead boost the generation of oxygen vacancies, propping up the activity via excitation of electrons from the VB to the

Fig. 3 e SEM images of macroporous TiO2-based photocatalysts (Macro-TiO2, Macro TiO2eCeO2 and Macro TiO2eW). oxygen vacancies energy levels [21,60]. In accordance with the above considerations in this work we investigated how CeO2 or W modified the physicochemical and photocatalytic properties of anatase TiO2 (TiO2 sample) and macroporous TiO2 (Macro TiO2 sample). Interestingly, Raman spectra (Fig. 4A) show that chemical treatments with CeO2 do not significantly affect the distribution and the position of the TiO2 bands, pointing out that anatase remains the only phase present. It can be noted that

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Fig. 4 e Characterization data of TiO2 and macroporous TiO2-based photocatalysts. (A) Raman spectra of TiO2 and macroporous TiO2-based photocatalysts (the spectra in the inset follow the same order), (B) PL spectra of TiO2-based photocatalysts; (C). PL spectra of macroporous TiO2-based photocatalysts. ceria-modified samples (TiO2eCeO2 and Macro TiO2eCeO2) show an Eg value 0.05 eV lower than anatase TiO2 and Macro TiO2, in agreement with previous literature data [14,37] and ascribed to the substitution of the lattice Ti4þ species with Ce4þ or Ce3þ ones [61,62]. The addition of tungsten results in a different behaviour depending on the TiO2 used (TiO2 or Macro TiO2). Notably, in fact, the Raman spectra of TiO2eW (Fig. 4A) exhibited two bands at 273 cm1 and 718 cm1 respectively, attributed to vibration modes of orthorhombic WO3 [63,64]. In particular, the peak at 273 cm1 is related to the OeWeO bending vibrations of the bridging oxygen, and the peak at 718 cm1 to the corresponding stretching mode. Furthermore, in this latter sample a red shift of the main peak of the anatase phase (at around 150 cm1) was observed (inset of Fig. 4A). By considering that this band is attributed to the main vibrational mode of anatase (OeTieO) [14], the presence of WO3 in the TiO2 lattice probably produces a bond distortion which causes the measured shift. However, the above peaks and the red shift of the OeTieO band of anatase are not observed in the spectra of Macro TiO2eW. Reasonably, the porous backbone of TiO2 allows a higher dispersion of tungsten in this sample, with chance of interstitial substitution of titanium by tungsten

ions. As reported in the literature [65e67], in fact, tungsten ions can replace Ti4þ in the lattice of TiO2 because of the correspondence of the ionic radius of W4þ/W6þ and Ti4þ. Accordingly, nonstoichiometric solid solution of WxTi1xO2 or TiO2-amorphous WOx composites would form, with potential generation of tungsten impurity energy levels. The PL spectra of investigated TiO2-based catalysts are reported in Fig. 4B, C . It must be reminded that the bare CeO2 displays the band to band emission peak at around 455 nm. It can be noted that doping TiO2 with CeO2 or W (TiO2eCeO2 and TiO2eW samples) causes an increase of the intensity of the band to band emissions (region at around 375 cm1), but a decrease in the intensity of bands at higher wavelengths (>470 cm1), associated with the recombination rate of charges. Consequently, a lower recombination rate of electron-hole pairs due to a better charge separation efficiency of the doped samples can occur [45e47]. The combination of ceria or W with the Macro TiO2 (Macro TiO2eCeO2 and Macro TiO2eW samples) caused a drop of the intensity of all of the bands. This indicates a restrained radiative recombination process in the Macro TiO2 doped sample [68], which can be attributed to a change of the defective state at a thin level of the Macro TiO2 surface [69], probably due to a synergistic

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effect of the doping and the photonic effect (light harvesting) of macroporous TiO2. These features enhance the chance of interfacial transport of photo-induced electrons and holes favouring a higher photo-activity [68e70]. The presence of tungsten didn't change the band-gap extension of TiO2 (Table 1). The observed changes in the TiO2 structure due to the coupling with CeO2 or W have a key importance to interpret the improvement of the photocatalytic activity of this mixed oxide system for the water splitting. In fact, the photoactivity of ceria- or W-doped TiO2 samples as for H2 production through water splitting (Figs. 5 and 6) was found to be enhanced both for non-macroporous (Fig. 5) and, even more, for macroporous (Fig. 6) TiO2 modified samples. In both cases the extent of this effect appeared more relevant under solar light than under UV irradiation. Therefore the application of a mixed approach, consisting in modifying TiO2 or Macro TiO2 with CeO2 or W, was highly effective in the enhancement of the titania photoactivity. In this case the introduction of shallow level defects in a macroporous structure contemporaneously boosted up the light absorption ability and the electron-hole charge disjointing of TiO2, thus favouring the water reduction for which CB electrons are crucial. Noteworthy, regarding the non-macroporous TiO2 samples, the most important effects could be observed mainly under solar light irradiation, with an H2 production of about 4 and 6 times higher with TiO2eCeO2 and TiO2eW compared to the bare

Fig. 6 e H2 production over macroporous TiO2-based photocatalysts: (A) under UV lamp; (B) under solar lamp.

TiO2. In the case of UV irradiation the modifications with cerium oxide and tungsten led to comparable performances being the H2 production 12.5 and 10 mmol h1$g1 cat with TiO2e CeO2 and TiO2eW, respectively, anyhow higher than the unmodified TiO2 sample (7.7 mmol h1$g1 cat).

General comparison of data and discussion

Fig. 5 e H2 production over TiO2, TiO2eCeO2 and TiO2eW photocatalysts: (A) under UV lamp; (B) under solar lamp.

Fig. 7 summarizes the comparison of the results obtained in this work in terms of H2 production by photocatalytic water splitting carried out at the same experimental conditions under UV (Fig. 7A) and simulated solar (Fig. 7B) irradiation over the representative TiO2-based catalysts used. It can be easily observed that the TiO2 sample treated by laser ablation in liquid (TiO2 LT) is notably the most active one under UV light (Fig. 7A), with H2 generation rate almost triple than that of the untreated sample and about twice those of the other two modified TiO2-based catalysts (TiO2eCeO2 and TiO2eW). On the basis of the characterization data, laser treatments lead to a more defective titania sample with the growth of Ti3þ and oxygen vacancies. The formation of undercoordinated titanium ions, the stimulated lattice distortion together with the moderate increase of surface area and the formation of rutile phase (see Raman spectra) are the key factors to explain the higher H2 production under UV irradiation compared to the other modified samples. The undercoordinated Ti ions act, in fact, as anchored and charge

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Fig. 7 e General comparison of H2 production by photocatalytic water splitting over TiO2-based catalysts used in this work: (A) under UV lamp; (B) under solar lamp.

injection/recombination sites, changing in a decisive way the electron/hole recombination dynamics and favouring an easier reduction of proton from water [71,72]. The H atoms will replace the locations of oxygen vacancies in the TiO2 crystalline structure, thus attracting the electrons from the nearby Ti and O atoms, resulting in an enhanced H2 production [73e75]. Reasonably, these effects influence the photocatalytic performance mainly under UV irradiation. In fact, considering that the Eg value of TiO2 LT is comparable to that of the bare TiO2 (Table 1), the formation of photoelectrons was much smaller when the reaction was performed under solar than under UV irradiation, even though the laser treatment can induce mid-gap states within the band gap of TiO2 [76]. Moreover, it cannot be excluded the positive role in the photoactivity played by the contact between the formed rutile phase under laser irradiation and the pre-existing anatase phase [72]. On the contrary, for the TiO2eCeO2, Macro TiO2eCeO2, TiO2eW and Macro TiO2eW catalysts, the hydrogen yields under solar light simulation are lower compared to those obtained under UV irradiation, in accord with the band gap energies reported in Table 1. Nevertheless, the differences in terms of hydrogen production with respect to the bare TiO2, are higher under solar light than UV light irradiation (Fig. 7). Reasonably, it is possible to explain these results considering that only a smaller fraction of efficient UV photons are present in the 300 W lamp simulating solar light (with respect to the high number of photons deriving from the 100 W UV lamp) which could give rise to a levelling effect in the accumulation/ recombination of charge pairs carriers. Moreover, a beneficial

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effect could be attributed to the presence of host agents as CeO2 or W which can absorb light in the visible range. In particular, the use of a template strategy to obtain macroporous TiO2 leads to a raise of the photocatalytic H2 production of about 1.7 under UV irradiation and 6 times higher under solar irradiation than bare TiO2. The presence of a macroporous structure enhances the absorption of photons, boosting the optical path length and the transfer rate of e/hþ pairs on the titania surface. This allows an easier Hþ ions reduction over Macro TiO2 by the photoproduced electrons [77e79]. This behaviour well agrees with the PL spectrum of the Macro TiO2, exhibiting a higher intensity of the band due to the excitons (375 nm), according to the higher light adsorption on the macroporous surface and a lower intensity of the bands generally related to the electron-holes recombination rate, as a consequence of the enhanced electron-hole pairs migration rate [37,38]. The combination of a structural (Macro TiO2) and a chemical (addition of CeO2 or W) modification permits to further increase the photoactivity. In particular, with the addition of cerium oxide, the photo-generated electrons can be transferred from CeO2 to TiO2 while holes, owing to the more positive valence band of titania, can proceed reversely giving rise to an efficient charge separation and a higher hydrogen production [14,37,80]. Moreover, the presence of the Ce3þ/Ce4þ redox couple gives rise to a scavenging effect in the TiO2 conduction band further enhancing the charge carrier separation. CeO2 acts as photosensitizer favouring also a remarkable activity also under solar irradiation. The contemporaneous presence of these effects, namely the macroporous structure with the increase of the light path length and the photosensitizer action of cerium oxide, are essential to rise the hydrogen production with respect to both un-modified TiO2 and Macro TiO2 samples. Notably, the Macro TiO2eW sample is the most active catalyst under solar light irradiation (Fig. 7). As reported in the literature [81e83] tungsten can enter the TiO2 lattice and partly replace the titanium ions. Moreover, the doping with W can generate new electronic states above the TiO2 valence band. These new states may facilitate the visible light absorption by TiO2, thus favouring the activity under solar light irradiation. Similarly to the cerium oxide effect, the addition of tungsten aids in detrapping of charge carriers to the surface of the photocatalyst, being the energy level of Ti 3d electrons close to that of W 5d electrons [84,85]. Noteworthy, TiO2eW and Macro TiO2eW showed quite different Raman and PL spectra (Fig. 4). Probably, the porous structure promotes the migration of tungsten ions into the crystalline lattice of TiO2 thus allowing the formation of more electronic states between the valence and the conduction bands of TiO2. The very low intensity of Macro TiO2eW PL bands is a further evidence of a higher charge carriers separation. This well agrees with the higher H2 production observed on the Macro TiO2eW sample compared to TiO2 and Macro TiO2 samples, both under UV and solar light. In the non-macroporous TiO2 system, the W ions can segregate more easily on the surface of TiO2 with formation of WO3 species, pointed out by Raman characterization (Fig. 4A), which can partially block the light absorption process of titania and then contributing to explain the lower photoactivity of TiO2eW compared to Macro TiO2eW.

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Table 2 e Catalyst, H2 evolution rate, irradiation type and sacrificial agent for H2 production under UV irradiation. Catalyst

1 H2 production (mmol g1 cat h )

Irradiation source

Sacrificial agent

Ref.

20.9 4.7 1.15 9.6 7.5 11.5 0.5

100 W Hg lamp (l ¼ 365 nm) UV lamp (l ¼ 365 nm) UVC lamp (l ¼ 254 nm) Hg lamp (l ¼ 254 nm) 250 W Hg lamp 450 W Hg lamp 450 W Hg lamp

ethanol 2-propanol ethanol methanol methanol methanol Na2CO3

this work [86] [87] [88] [89] [90] [91]

Laser modified TiO2 Rh-WO3/TiO2 (nanotubes) Pd/TiO2 Au/TiO2 Pt/TiO2-activated carbon Pt/TiO2 Pt/TiO2e Cl and Br co-doped

Table 3 e Catalyst, H2 evolution rate, irradiation type and sacrificial agent for H2 production under solar/vis irradiation. Catalyst Macro-TiO2eW TiO2eMoSe2 TiO2 doped carbon RueTiO2 GdeNeTiO2 FeeNi/Ag/TiO2 Cu2O/TiO2 Au/TiO2 Fe/NieTiO2 NeTiO2/Pt

H2 production (mmol 1 g1 cat h )

Irradiation source

Sacrificial agent

Ref.

0.82 0.26 0.37

300 W sunlight simulating lamp Xe arc lamp 300 W Xe lamp

ethanol methanol lactic acid

this work [92] [93]

0.85 1.08 0.79 0.25 0.13 0.36 0.57

500 W Xe lamp with a visible light: (420e680 nm) cut-off filter Solar simulator/natural sunlight 500 W Xe lamp 300 W Xe lamp 450 W Hg lamp with NaNO2 filter 500 W Xe lamp with a 400 nm cut-off filter 500 W Xe lamp

methanol methanol ethanol ethanol ethanol ethanol methanol

[94] [95] [96] [97] [98] [99] [100]

It is very difficult to compare the performance of different systems used for H2 production, due to the various experimental setups used (i.e. light source, sacrificial agent, type, amount and preparation methods of the photocatalyst, presence of different electrolytes, etc). However, some results obtained under similar working conditions in the presence of differently modified TiO2 photocatalysts, together with some experimental data, under UV or solar/visible light irradiation, are reported in Tables 2 and 3, respectively. The laser treated TiO2 samples allowed the highest H2 production under UV light irradiation (Table 2), therefore the use of these systems for practical applications is encouraging. Different systems allowed to obtain good H2 production even under solar/vis light irradiation (Table 3), and MacroTiO2eW showed a photocatalytic activity comparable with the best system reported in the literature.

Conclusions On the basis of the results discussed in this paper we can conclude that the induction of structural and chemical modifications in TiO2 based photocatalysts by various approaches is a promising strategy to achieve a more efficient hydrogen production through water splitting. The main results of the paper can be summarized below: a) laser treatment of TiO2 resulted in a series of structural and morphological changes as introduction of Ti3þ species and oxygen vacancies, modification of surface area and crystal phase of TiO2, which all together concurred to the relevant

enhancement of the photoactivity of TiO2 LT sample, especially under UV irradiation. b) the use of a macroporous ordered structure of TiO2 led to slow photon effects with high light absorption, strongly enhancing the TiO2 photoactivity mainly under solar light irradiation. The positive effect was further increased by the presence of a photosensitizer as CeO2 or by a doping agents as W, which enhanced the light absorption and the charge separation between electrons and holes of the photocatalyst. The possibility to tune the chemico-physical properties of titanium dioxide with easy and green procedures, is a promising approach for an environmental friendly scale-up catalysts synthesis for the production of hydrogen starting from sustainable materials. In this contest, the future works will be focused on the modifications of some experimental parameters both in the photocatalytic tests (changing the sacrificial agent, the time of irradiation and the irradiation source) and in the synthesis (exploring other solvents instead of water for the laser treatment or using other host components as Fe2O3 or Bi2O3 for the Macro-porous TiO2) to improve the H2 yields under solar simulation irradiation, a key factor by a practical point of view.

Acknowledgements Authors would like to thank Giuseppe Francesco Indelli, Laboratory Technician at Bio-Nanotech Research and Innovation Tower, (BRIT) of University of Catania for technical support.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 1 4 7 9 6 e1 4 8 0 7

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