Influence of the Preparation Temperature on the Photocatalytic Activity

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Influence of Preparation Temperature on Photocatalytic Activity of 3D Ordered Macroporous Anatase Formed with Opal Polymer Template María José Torralvo-Fernández, Eduardo Enciso, Sandra Martinez, Isabel Sobrados, Jesús Sanz, Dino Tonti, Javier Soria, Sedat Yurdakal, Giovanni Palmisano, and Vincenzo Augugliaro ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00253 • Publication Date (Web): 17 May 2018 Downloaded from http://pubs.acs.org on May 17, 2018

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ACS Applied Nano Materials

Influence of Preparation Temperature on Photocatalytic Activity of 3D Ordered Macroporous Anatase Formed with Opal Polymer Template María José Torralvo-Fernández,# Eduardo Enciso,# Sandra Martínez,§ Isabel Sobrados,§ Jesús Sanz,§ Dino Tonti,§ Javier Soria,ǂ Sedat Yurdakal,† Giovanni Palmisano,‡ and Vincenzo Augugliaro*║ #

Facultad de Ciencias Químicas, Universidad Complutense de Madrid, 28040 Madrid, Spain. E-

mails: [email protected] [email protected] §

Instituto de Ciencia de Materiales, CSIC, C/ Sor Juana Inés de la Cruz, Cantoblanco, 28049

Madrid,

Spain.

E-mails:

[email protected]

[email protected]

[email protected] [email protected] ǂ

Instituto de Catálisis y Petroleoquímica, CSIC, C/ Marie Curie 2, Cantoblanco, 28049 Madrid,

Spain. E-mail: [email protected] †

Kimya Bölümü, Fen-Edebiyat Fakültesi, Afyon Kocatepe Üniversitesi, Ahmet Necdet Sezer

Kampüsü, 03100, Afyonkarahisar, Turkey. E-mail: [email protected] ‡

Department of Chemical and Environmental Engineering, Masdar Institute of Science and

Technology, PO BOX 54224, Abu Dhabi (UAE). E-mail: [email protected] ║

“Schiavello-Grillone”

Photocatalysis

Group,

Dipartimento

di

Energia,

Ingegneria

dell’Informazione e Modelli Matematici (DEIM), University of Palermo, Viale delle Scienze, 90128 Palermo, Italy E-mail: [email protected]

*Corresponding author mailing address: DEIM, University of Palermo, Viale delle Scienze, 90128 Palermo, Italy Tel.: 00393204328574

FAX: 0039091488452

E-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT Even if many aspects of three dimensionally ordered macroporous (3DOM) titania have been extensively studied, the interplay between the amorphous and crystalline components of their skeleton wall has not attracted too much attention, though it should strongly influence the properties of these materials. In order to get new insights on this interplay, we have studied in detail the wall structure of three 3DOM titania samples prepared by heating the 3DOM titania precursor at 673, 773, and 873 K by X-ray diffraction, thermogravimetric analysis, differential thermal analysis, N2 adsorption/desorption, pores distribution, HRTEM and 1H-MAS NMR. In addition, their photoreactivity towards the partial oxidation and mineralization of 4methoxybenzyl alcohol was determined and compared with that of commercial anatase and P25 samples. The results show that when the 3DOM sample heated at 673 K, whose skeleton wall is formed by very small anatase nanoparticles covered by relatively thick layers of amorphous titania, is heated at 773 K, most amorphous titania is eliminated, leaving a very thin amorphous titania layer covering the slightly grown anatase nanoparticles. The photoreactivity results show that the sample heated at 773 K has the highest overall photoreactivity that was lower than those of commercial anatase and P25 samples; however, its reactivity towards the alcohol partial oxidation was the highest compared with those obtained with all the other samples. These results indicate that amorphous titania negatively affects the overall photocatalytic activity; however, its particular distribution in 3DOM samples enhances the partial oxidation reaction.

KEYWORDS: 3DOM Titania, Anatase Nanoparticles Boundaries, Amorphous Titania, Partial Oxidation Photoreactivity, 1H MAS-NMR, TEM.


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1. Introduction The relatively high specific surface area, the macroporosity and the novel optical and electronic properties of the three-dimensionally ordered (3DOM) titania have made this structure a promising material for applications in fields such as (photo)catalysis, solar energy conversion or lithium batteries.1-9 This structure has demonstrated to significantly increase the lightharvesting efficiency of TiO2, by enhancing the light absorption and then improving its photoreaction efficiency;10-14 moreover, in these last years the 3DOM photocatalytic efficiency has been improved by means of multi-component doping able to promote the photogenerated charge separation and also to harvest visible light.15-19 In 3DOM structures, the framework made of sub-micrometric particles provides a large number of active sites for charge-transfer and the interconnected voids constitute a continuous and accessible pathway for reactants and for ionic and electronic conduction. Despite those beneficial characteristics, some obstacles can hamper further applications of these TiO2 -based materials, particularly, the relatively low electronic conductivity and thermal stability.20 The initially obtained titania is usually amorphous or poorly crystalline and it needs thermal and/or aging treatments to produce both the morphology and the atomic-scale structure; by that treatment the material becomes denser and undergoes a total or partial transformation from amorphous to crystalline materials. In the case of incomplete crystallization the resulting titania structures are formed by amorphous material and crystalline nanoparticles aggregate, whose properties are influenced by the nanoparticles morphology and composition but, mainly, by their surface properties, that determine the characteristics of the assembled particles boundaries. In order to determine the influence of amorphous TiO2 on the activity performance, we have recently studied the photocatalytic oxidation of 4-methoxybenzyl alcohol (MBA) in water using nanocrystalline titania powder, which was prepared through a precursor solution obtained by slowly adding TiCl4 into a beaker containing distilled water.21-23 After the TiCl4 hydrolysis at room temperature, the solution underwent an aging treatment at 373 K obtaining a white suspension of freely precipitated particles. The resulting semicrystalline powder (HP sample) was constituted by anatase nanocrystals (mean size of 5 nm) diluted into an amorphous titania matrix, which results from the precipitation of the excess titania as a disordered structure. The reactivity results indicated that MBA degradation proceeds according to two parallel pathways: partial oxidation to p-anisaldehyde (PAA) and total oxidation to CO2 (mineralization). The


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amorphous titania at the anatase surface is mainly providing the active sites for MBA partial oxidation while most of the sites active for mineralization were located on the anatase surface, the overall photoactivity being limited by the incorporation of amorphous titania to the nanoparticles boundaries. This incorporation hampers the inter-particle electron transport thus favouring charge carriers recombination in anatase particles where they are photogenerated.24-26 In a further investigation27 the HP sample preparation was carried out through precursor solutions obtained by hydrolysis of different amounts of TiCl4 in distilled water containing anatase crystals (BDH TiO2) which acted as precipitation seeds. In these BDH-HP samples, the HP species, constituted by anatase nanocrystals and amorphous titania, were incorporated to the surface of BDH nanoparticles. The morphological and photocatalytic characteristics of BDH-HP samples markedly depended on the HP amount. If this amount is small, the resulting amorphous phases are poorly condensed and they appear dispersed on the BDH anatase nanoparticles, forming part of their surface disordered layer. In this case the transfer of photogenerated holes is fast originating high photoactivity towards the partial photocatalytic oxidation. When the incorporated HP amount is high, the amorphous titania condenses eventually producing dense amorphous phases covering the crystal surfaces as thick layers. This incorporation produces a strong decrease of the partial and overall oxidation photoactivity. In the present work, the titania samples were prepared by performing the hydrolysis of a solution of titanium isopropoxide and isopropanol in an opal arrangement of spherical latex beads with size in the 200-300 nm range. After infiltration by capillarity, the latex/precursor composites were kept at room temperature to complete the hydrolysis with atmospheric moisture; then the resulting material was heated at 673, 773 or 873 K. This complex preparation method was chosen in order to study the features of titania prepared in a very constrained space and in the presence of polymeric template.28 In the course of the thermal treatment different processes are occurring in sequence: anatase particles nucleation in the bulk of infiltrated solution and at polymer surface, densification of structural walls and anatase particles growth with creation of internal stresses detrimental to the walls stability. All these processes are affected by the treatment temperature which eventually determines the 3DOM final features. The textural and structural characteristics of 3DOM titania samples have been investigated by XRD, TGA, DTA, N2 adsorption-desorption, 1H MAS-NMR and HRTEM. In order to get information on the influence of the anatase nanoparticles boundaries on the 3DOM photocatalytic


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performance,25-27,29-31 the behaviour of 3DOM samples has been tested for the MBA photocatalytic oxidation in water. The 3DOM photoreactivity results have been compared with those obtained on commercial (BDH) anatase and P25 (Degussa) TiO2, the classical titania reference.

2. Experimental methods Figure 1 reports the scheme illustrating the structure synthesis. A batch of spherical latex beads with size in the 200-300 nm range was prepared by an emulsifier-free copolymerization reaction of styrene and methacrylic acid, according to a procedure detailed elsewhere.32,33 The obtained colloidal solution was dialyzed against water for 15 days. Face-centered cubic (fcc), or opal, arrangements were produced by evaporating in a Petri dish approximately 10% (w/w) suspensions in water at room temperature. The opals were infiltrated by capillarity with titanium isopropoxide dissolved in isopropanol (alkoxide:alcohol molar ratio of 1:4). After infiltration the latex/precursor composites were kept 24 hours at room temperature to let complete hydrolysis with atmospheric moisture. To eliminate the polymeric template, the infiltrated opals were heated in static air with a 1 K·min-1 ramp up to 573 K and with a 0.5 K·min-1 ramp up to 673, 773, or 873 K and kept at those temperatures for three hours. The resulting materials are hereafter indicated as 3DOM673, 3DOM773, and 3DOM873; these last two samples were colourless, indicating the almost complete removal of organics during calcination, while the 3DOM673 one was greyish indicating the presence of residual carbon species. Commercial (BDH) anatase and P25 (Degussa) TiO2 were used as received from the factory. Thermogravimetric analyses were performed using a Perkin-Elmer apparatus (model STA 6000). Samples were put on an open Pt crucible and heated at 5 K min-1 up to 870 K in a 20 ˑmin cm3 min-1 N2 flow. XRD patterns were recorded at 1º

-1

in a Philips PanalyticalX’Pert MPD

diffractometer using the Cu Kα radiation. For structural analyses, the PowderCell 2.4 program was used.34 Prior to structure refinements, a pattern matching without structural model was performed to obtain suitable profile parameters. Then, the structural model was refined with the Rietveld method keeping constant profile parameters. The mass percentages of samples crystallinity were evaluated following the procedure reported by Jensen et al.35 XRD diffractograms were recorded for mixtures of 3DOM TiO2 and CaF2 (50% by weight) and the


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areas of the 100% peaks of anatase (101) and CaF2 (220) were determined. By comparing the ratio between the peaks areas of two phases, the degree of sample crystallinity was estimated. It is important to outline that the simple CaF2 method,35 even if it may underestimate the sample crystallinity and is less accurate than a recently proposed one,36 has been used in this work only with the aim to establish a correct sequence of crystallinity in 3DOM samples treated at different temperatures. [Poly[styrene-methacrylic acid]

Infiltration of the precursor solution Ti(OPri )/PrOH

Hydrolysis and condensation reactions

Thermal treatment

Macroporous TiO2

Figure 1. Scheme illustrating the structure synthesis.

Transmission electron microscopy (TEM) with a JEOL JEM 2000FX microscope (operating at 200 kV) and High-Resolution TEM (HR-TEM) with a JEOL 3000 (operating at 300 kV) were used to characterize the latex particles and the morphology and microstructure of 3DOM samples. The samples were suspended in butanol and a drop of the suspension was deposited over a copper grid coated with a holey carbon support film. Specimens were reoriented in the electron beam to achieve different projections of 3DOM TiO2 assemblies. Samples for scanning electron microscopy (SEM) were coated with a gold film before observation in a JEOL JSM 6335F field emission microscope operating at 15 kV. The adsorption-desorption measurements were made in an ASAP 2020 Micromeritics equipment. Prior to the adsorption experiments, the sample was outgassed at 383 K for 3 hours. For BET calculations the value of 0.162 nm2 has been used for the area occupied by a nitrogen


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molecule in a monolayer on flat surface.37 The pore size distribution was obtained by the BJH method38 using the Kelvin equation with the assumption of cylindrical pores.39 1

H magic angle spinning (MAS) NMR spectra were obtained at 400.13 MHz (9.4 T

magnet) in an Avance 400 (Bruker) spectrometer after irradiation of samples with π/2 pulses. The number of accumulations was chosen to get S/N ratios above 20. To preserve relaxation conditions, interval between accumulations was 10 s. Spectra were recorded at room temperature on samples spun at 10 kHz around an axis inclined 54º44’ with respect to the magnetic field (MAS technique). In these experiments the cancelation of dipolar H-H interactions improved the experimental resolution of OH groups. To analyze the influence of adsorbed water, spectra were recorded on natural stored samples after outgassing at room temperature or at 373 K. A conventional vacuum line (residual pressure: 1·10-6 Torr) was used to evacuate samples; to preserve the evacuation conditions, the rotors were filled under nitrogen atmosphere inside a globe box. Chemical shift values were referred to the tetramethylsilane signal and NMR lines intensities to the line originated by the rotor cap used. Spectra deconvolution was carried out with the Winfit (Bruker) software package. A cylindrical Pyrex batch photoreactor with immersed lamp, containing 0.5 L of aqueous suspension, was used to perform the MBA oxidation runs. The photoreactor was provided with ports in its upper section for the inlet and outlet of oxygen and for sampling. A magnetic stirrer guaranteed a satisfactory suspension of the photocatalyst and the homogeneity of the reacting mixture. A 125 W medium pressure Hg lamp (Helios Italquartz, Italy), axially positioned within the photoreactor, was cooled by water circulating through a Pyrex thimble; the temperature of the suspension was 300±2 K. The radiation energy impinging on the suspension, 10 mW·cm-2, was measured at 360 nm by using a radiometer UVX Digital. For all the runs the initial MBA concentration was 1 mM and the catalyst amount was 0.1 g/L. A 0.2 M NaOH solution was used to adjust the initial pH to 7. Before switching on the lamp, pure oxygen was bubbled into the suspension for 30 min at room temperature to reach the thermodynamic equilibrium; the bubbling was maintained during the course of all the runs. Adsorption of the alcohol under dark conditions was always quite low, i.e. ca. 5% for 3DOM samples and ca. 3% for commercial TiO2. During the runs samples of suspension were withdrawn at fixed time intervals and immediately filtered through a 0.45 μm hydrophilic membrane (HA, Millipore) before analysis.


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The identification and quantitative determination of the species present in the reacting suspension were performed by means of a Beckman Coulter HPLC (System Gold 126 Solvent Module and 168 Diode Array Detector), equipped with a Luna 5μ Phenyl-Hexyl column (250 mm long × 2 mm i.d.), using Sigma-Aldrich standards. Retention times and UV spectra of the compounds were compared with those of standards. The eluent consisted of 17.5% acetonitrile, 17.5% methanol and 65% 40 mM KH2PO 4 aqueous solution. Retention times for MBA and PAA are 8.7 and 17.1 min, respectively. Total organic carbon (TOC) content was measured by using a 5000 A Shimadzu TOC analyzer. All the used chemicals were purchased from Sigma-Aldrich with a purity >99.0%.

3. Results and Discussion 3.1 Textural and structural characteristics The thermogravimetric (TG) and differential thermal analysis (DTA) profiles of the 3DOM TiO2 precursor are compared with those of the polymer opal template in Figure 2. Differences in the profiles indicate that the TiO2 precursor, formed by heating the infiltrated dispersion, interacts with the polymeric template. The TG profile of opals structure heated between 473 and 873 K shows a strong and sharp weight loss (87%) starting at ~540 K and a small one (4%) starting at ~720 K. The 3DOM TiO2 precursor profile presents a less marked first weight loss (67%), starting at ~560 K and mostly attributed to partial decomposition of latex templates; this loss is slowed down in the 630-660 K range, then enhanced in the 700-730 K one and, finally, slowed down to a nearly constant value in the 730-770 K range. When temperature is higher than 770 K, no weight loss was observed, indicating that the template was completely removed. The differences between the two profiles indicate that the TiO2 precursor interaction with the polymeric template delays the start of the template combustion. The DTA profile of the opals structure shows three exothermic peaks at ~ 570, 610 and 740 K, stronger the former ones than the latter, Figure 2. These peaks should be originated by the combustion of the polymeric template and some more stable intermediates, respectively. The 3DOM precursor DTA profile shows four exothermic peaks at ~600, 630, 715 and 730 K. The 600, 630 and 730 peaks correspond to the opal structure peaks but with a significantly lower intensity than in the opal structure profile; the first two ones appear at higher temperature


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indicating that the combustion temperature of part of the polymeric template has been modified by its interaction with the titania precursor species. The new exothermic peak at ~715 K corresponds to anatase crystallization and it determines the displacement of opal 740 K peak to 730 K.40

100

25

80 60

0

0

5

20

40

10

Weight [%]

15

Heat flow [W/g]

20

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


373

473

573

673

773

873

Temperature [K] Figure 2. Profiles of thermogravimetric and differential thermal analyses of the infiltrated TiO2 precursors (blue lines) and of opal template structures (grey lines).

Table 1 reports some features of 3DOM samples; more details on textural and structural characteristics are given in Supporting Information (SI). The XRD pattern of the opal structure, infiltrated with the titania precursor dispersion and calcined at 473 K, shows that the sample is almost completely amorphous, Figure 3. After calcination at 673, 773 and 873 K the patterns show broad bands characteristic of amorphous phases but also all reflections expected for anatase (JCPDS n. 78-2486). In samples heated at 773 and 873 K, diffractograms exhibit a small reflection at 27.45º characteristic of a small amount of rutile (JCPDS, No. 76-1940). The anatase


9


crystal size, calculated by the Scherrer equation, slightly increases with the treatment temperature (Table 1).

A

LATEX + TiO2 precursor

A

R

A

A600°C A 3DOM873 500°C

I I[A.U.] (A.U.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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3DOM773 400°C

3DOM673 200°C 20

20

30

30

40

50

40

θ−2θ

50

60

60

70

70

2θ

Figure 3. XRD patterns of 3DOM sample heated at different temperatures, showing characteristic peaks of anatase (A) and rutile (R) structures. The green line shows the pattern of the opal structure, infiltrated with the titania precursor dispersion and calcined at 473 K.

The XRD pattern of the 3DOM673 sample shows that the narrow peaks overlap broader ones, indicating the presence of anatase nanoparticles with very small crystal size. The growth of these nanocrystals should be limited by their interaction with template polymeric species. This interaction should also originate the slowing down of the weight loss observed in the TG sample pattern at the 630-660 K range. In addition, this profile shows a very broad band, under the first anatase peak, that can be attributed to amorphous titania species. The 3DOM773 pattern shows a small contribution of the broad peaks overlapped by the narrow anatase peaks and the presence of the very broad band, though with decreased intensity. These modifications suggest that the calcination at 773 K has eliminated most of the polymeric species that were hindering the growth of the very small anatase nanoparticles; above 773 K the nanoparticles growth is limited by a


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surface layer of amorphous titania. The appearance of rutile peak in 3DOM773 and 3DOM873 samples, while the narrow anatase peaks slightly broaden, should correspond to the first steps of the transformation of anatase into rutile. The values of the amorphous titania mass percentage (Table 1) were estimated by following the procedure reported by Jensen et al.35 This method is not appropriate for quantitative studies but it has proved to be useful for comparative purposes. So, the high crystallinity increase, produced by calcination at 773 K, is determined by the elimination of polymeric species hindering the titania crystallization while the low increase, observed after the sample calcination at 873 K, may be produced by the crystallization of residual thin amorphous layers covering anatase particles. 3DOM structures are formed by filling the cavities localized among polymer template spheres with titania suspensions. At the end of the crystallization process, the elimination of the template reduces the volume occupied by 3DOM structures by 25%, forcing anatase particles to interact each other and thus favoring the thin walls consolidation.41 SEM micrographs indicate, as expected, that the 3DOM TiO2 samples show inverse opal structure with interconnected macropores, Figure 4. TEM micrographs (Figure 5a-c) indicate that the highly ordered framework is defined by thin walls made of anatase nanoparticles whose dimension ranges increase by increasing the treatment temperature: 4-10 nm for 3DOM673, 10-20 nm for 3DOM773 and 15-30 nm for 3DOM873; this observation agrees with peaks narrowing detected in XRD patterns of these samples. The nanoparticles aggregate tightly by means of disordered mesoporous layers that form thin walls in 3DOM samples.41 The void system of 3DOM structures, in contrast with that in the aggregates of non-structured nanoparticles,42 can be described as formed by: i) macropores with fcc packing connected through large windows (Lw) left by the contact areas between de latex particles; ii) the voids corresponding to partially mineralized octahedral (Oh) and tetrahedral (Th) holes that are connected by triangular windows (Tw); and iii) walls porosity consisting of the free space among the anatase nanoparticles and inside amorphous titania, Figure 5. The void volume decreases by increasing the 3DOM treatment temperature, the highest decrease being that between 3DOM773 and 3DOM673 (Table 1). The nitrogen adsorption-desorption isotherms of 3DOM samples are of type IV, with type H1 hysteresis loop extended down to P/P0 value of 0.6, typical for tubular pores in the


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macro-mesopore region,37-39 inset of Figure 6. The BET area values, calculated from the isotherm data and reported in Table 1, decrease by increasing the treatment temperature, the highest decrease being that between 3DOM673 and 3DOM773; the 3DOM773 area is 2 times lower than that of 3DOM673 but 1.5 times higher than that of 3DOM873. The BET decrease may be related to the amorphous titania percentage decrease; it may be noted that the percentage variations of the BET area values (Table 1) are similar to those exhibited by the percentage values of amorphous phases with increasing calcination temperature.

Table 1. Some features of 3DOM samples and commercial TiO2. Crystal

SBET

VP

ATMP

D

phase

(m2/g)

(cm3/g)

(%)

(nm)

3DOM673

A

92

0.34

85

5

3DOM773

A

44

0.25

50

6

3DOM873

A

30

0.23

46

9

BDH TiO2

A

9

-

34

90

P25 TiO2

A+R

50

-

12

48

Note. A, anatase; R, rutile; SBET, BET surface area; VP, pore volume; ATMP, amorphous titania mass percentage; D, mean crystal size calculated by the Scherrer equation.

The curves showing the variation of the samples pore size distribution and their cumulative pore area (CPA) are presented in Figures 6a and 6b, respectively, as function of the pore size. For pores greater than 15 nm, the pore distribution and CPA profiles are not significantly affected by the calcination temperature; this porosity should correspond to the pore system of the 3DOM structure. On the contrary, the samples show different patterns in the 3-15 nm range. 3DOM673 shows a broad pores distribution which suffers a marked decrease in 3DOM773 sample, pores at ~ 4 nm being those mainly affected. This decrease is probably favored by the increased density of amorphous titania43 and it is consistent with the growth of the smallest anatase nanoparticles, induced by the elimination of the polymeric species. The 3DOM873 profile shows the almost elimination of pores around 6 nm; this elimination, parallel


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with the amorphous titania percentage decrease, suggests that those pores were formed in this sample component. Figure 6b shows that calcination at 773 K produces a marked CPA decrease with respect to the 3DOM673 CPA. This decrease is determined by pores with size lower than 15 nm, increasingly marked with decreasing pore size; CPA contribution by pores greater than 15 nm does not change as those pores are not significantly affected. So, the decreased contribution of the smallest pores to the sample CPA indicates that the strong decrease produced on 3DOM773 BET area is originated by the elimination of those pores; this elimination, without significant modification of 3DOM773 structure, is a further indication that the smallest pores belong to amorphous titania. Calcination at 873 K diminishes the contribution of smallest pores to the CPA until their near elimination; this elimination without formation of new pores should suggest an increase of amorphous titania density. The HRTM images allow to analyze the structure of walls that surround big cavities, Figure S1. In agreement with XRD data, HRTEM images of 3DOM samples show that their nanoparticles exhibit a well-faceted crystal habit, showing sets of parallel fringes with spacing of ~0.343 nm corresponding to anatase (101) planes.44-46 In the case of the 3DOM673 sample, a big amount of individualized anatase particles is disposed near main walls. In the 3DOM773 sample, anatase particles are better sinterized, reducing layers that separate neighboring particles. Finally, in 3DOM873 sample, anatase particles size and the thickness of walls increase. In analyzed samples, some of the contiguous nanoparticles appear bonded together keeping the same orientation of lattice fringes. This coincident orientation indicates that the nanoparticles have been approaching one each other in a face-to-face oriented self-attachment mechanism, during structural network formation, and merged along specific crystallographic directions47-49 by following a mechanism observed in crystals of different phases.50,51 Oriented attachment of nanocrystals is a well known process in the solution-phase growth of nanostructures and a likely explanation of this oriented aggregation is that it occurs when surface hydroxyls combine with protons by eventually producing interparticle water which is eliminated by the heating treatment.52,53 In the present case the contiguous nanoparticles with the same orientation are separated by a thin layer of amorphous titania indicating that these anatase nanoparticles have been approaching one another by H-bonding interactions of bridging and terminal hydroxyls of the amorphous titania layer attached to the surface of those nanoparticles.


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The thickness of these amorphous superficial layers must be small in order to allow the same orientation of contiguous nanoparticles. In the case of thick layers, the nanoparticles do not merge along specific crystallographic directions and their lattice fringes do not show the same orientations. Isolated anatase particles display important surface amorphous layers (Figure 7a), and, in general, amorphous layers remaining at external surfaces of anatase network are considerably thicker than amorphous boundaries that connect contiguous anatase particles inside walls (Figure 7b-c); this fact enhances interparticles electron mobility and therefore photocatalytic performance. The width of the amorphous titania layer54,55 interconnecting nanoparticles is responsible of the worm-like features detected at the surface of separated particles.54 The borders of anatase particles interfaces decrease considerably in merged particles. In 3DOM773 and 3DOM873 samples, amorphous titania patches, that were observed in anatase nanoparticles of HP samples,31 are not produced, underlining the high dispersion of amorphous titania achieved on anatase surface particles.

Figure 4. SEM micrographs of 3DOM sample illustrating inverse opal framework formed after polymer template combustion at 773 K. Hollow spheres are replica of the latex template spheres.


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Figure 5. TEM micrographs of 3DOM673 (a), 3DOM773 (b) and 3DOM873 (c) titania samples. Images correspond to the projection along the (111) direction of the fcc sphere particle packing.


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b

a

Figure 6. Nitrogen adsorption-desorption isotherm (inset); pore size distribution (a) and cumulative pore area (b) calculated from the desorption isotherm.


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Thick amorphous layer

b

c

Figure 7. In 3DOM673 separated particles, amorphous layers are considerably thicker (a) than in cemented particles constituting 3DOM773 walls (b,c) where crystalline particles are connected by very thin amorphous boundaries. The circles show nanoparticles with the same orientation of lattice fringes.


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(c)

(b)

Anatase

Condensed Amorphous Titania

Amorphous Layers

W

(a)

20

10

ppm

0

-10

Figure 8. 1H-MAS-NMR spectra of 3DOM773 sample rehydrated (a), outgassed at RT (b), and outgassed at 373 K (c).


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The presence of amorphous titania at the surface of anatase particles is justified by the mild conditions of the 3DOM preparation method32 which starts by producing amorphous titania precursors in the spaces between polymer particles. Carboxylic groups at the surface of latex particles can interact with titanium atoms of the TiO2 precursor to promote polymerization of this inorganic precursor at the template surface.33 Thus, carboxylic groups modify the relative rates of hydrolysis and condensation of titania precursors, becoming nucleation sites for dispersed amorphous titania. The amorphous titania evolution to crystalline phases has been described as a condensation process which initially determines the assembly of short staggered chains of defective octahedral-like Ti-O units.56 The Ti4+ cations adopt four, five or six oxygen coordination sphere42 and, depending on the condensation process, the initially formed low coordinated Ti4+ cations should increase their coordination during anatase nucleation. 3.2 NMR results The NMR spectroscopy has permitted the investigation of adsorbed water and surface hydroxyls in titania samples.29,57-60 The 1H-MAS-NMR spectra of hydrated samples are mainly formed by a narrow line of highly mobile water interacting with OH groups, but spectra recorded on samples evacuated above 373 K mainly correspond to hydroxyl proton species. In previously investigated HP samples, formed by 6 nm anatase particles surrounded by amorphous TiO2, 1H-MAS-NMR spectra showed the presence of three different bands at 10, 6 and 1 ppm.27 These bands were ascribed to OH groups of: (i) amorphous segregated phases; (ii) anatase particles; and (iii) thin amorphous layers covering anatase particles. In crystalline BDH samples, the 6 ppm band splits in two bands at 6.5 and 5.5 ppm, that were ascribed to bridge and terminal OH groups of crystallized anatase. In HP samples, 1H-MAS-NMR spectra also showed that amorphous titania hydroxyls interact/react with those of anatase.29 In 3DOM samples, the elimination of polymer template favours the consolidation of structure walls (Figure 5). During 673-873 K calcination, most OH groups were eliminated; however, adsorption of water produced the formation of surface OH groups. In rehydrated 3DOM samples, 1H MAS-NMR spectra are mainly formed by an asymmetric broad band centred at 5.4 ppm (denoted line W, hereafter) and a 1 ppm band, Figures 8a and S2a-c. Line W width values, measured at half height of the three samples spectra, were 1.3 (DOM673), 2.1 (3DOM773) and 1.6 (3DOM873) ppm. By taking into account that the signal of the liquid water


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is near 4.8 ppm,58 the line W shift detected at 5.4 ppm has been ascribed to water interacting with acid OH groups, or to formation hydrated proton structures.59 In both cases, the mobility of water is limited owing to its interaction with amorphous or crystalline titania surfaces. The absence of exchange processes explains the line-width of NMR lines.57,59 The evacuation at RT of 3DOM samples determines the removal of adsorbed water, what originates the line W shift from 5.4 to 6.6 ppm, Figures 8b and S2a-c. Hydrated excess proton structures, formed by solvation of strongly acid hydroxyls protons, produce NMR lines centred at ~ 7 ppm.61 In parallel, the line W intensity decreases and broadens, indicating the partial water removal. This shift, not observed in the spectra of non-structured anatase nanoparticles evacuated at RT,57,59 suggests that the acid character of bridging hydroxyls protons is stronger than that of the non-structured nanoparticles. A similar shift was detected during the progressive incorporation of amorphous titania to the surface of BDH particles.27 The deconvolution of 1 H MAS-NMR spectra of 3DOM samples is given in Table S1, which gives the values of chemical shift and relative intensity of components ascribed to anatase, condensed amorphous titania and amorphous layers interacting with anatase particles. The bands centred at ~ 6.5 and 5.5 ppm have been attributed to bridging and terminal OH groups of anatase and those at ~ 9.5 and 3.5 ppm to those of condensed amorphous titania. In the last case, the shift of hydroxyl groups towards higher and lower values suggests a stronger interaction with surface basic oxygens. For samples evacuated at 373 K the sum of anatase bands is near 75 %. The percentage of OH groups on condensed amorphous titania is 15 % while that in inner parts of particles is 65 %. The bands centred at ~ 1 ppm, considerably shifted with respect to those of anatase phase, have been ascribed to amorphous species attached to anatase particles. The narrow lines have been ascribed to terminal hydroxyls of less condensed titania chains, where Ti cations could display four- or five-fold coordination. These chains condense to give amorphous layers that give broader lines detected near 1 ppm. The broad band represents the 85 % and the narrow ones the 15 % of intensity of bands near 1 ppm. In evacuated samples, intensity of amorphous layers inversely varies with respect to that of condensed amorphous titania, indicating that both species are affected by amorphous-anatase interaction. In particular, intensity of amorphous phase hydroxyls decreased with treatment temperature, suggesting that the formation of Ti-O-Ti bonds between amorphous titania and anatase particles reduces the role played by amorphous layers.


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The intensity of narrow 1 ppm components increases with the calcination temperature, suggesting that amorphous layers could be broken producing less condensed chains during heating at 873 K. The sum of chains and layer species is about 25 % of OH groups. After evacuation at 373 K, the 3DOM spectra show the presence of two resolved lines at 5.5 and 7.3 ppm, particularly in the 3DOM873 sample, Figures 8c and S2a-c. Chemical shift values of both peaks are similar to those observed in the spectrum of BDH-HP particles covered with 15% of amorphous titania particles.47 In this sample, where amorphous chains were interacting with anatase, the shift of hydroxyl bands towards more positive values was attributed to hydrogen bonds or formation of proton hydrated structures interacting with anatase or amorphous layers. The absence of a significant band at ~10 ppm, assigned to segregated amorphous titania in HP samples, is consistent with high dispersion of amorphous titania achieved in 3DOM samples. This dispersion, which limits the formation of amorphous agglomerates, may be determined by the interaction of spherical latex particles with the titania precursor. At this point, it is interesting to compare NMR data to TEM results. In general, the formation of thick disordered layers is detected in anatase nanoparticles not involved in formation of 3DOM walls, Figure S1. These layers are often detected in sample prepared at 673 K, where a significant amount of non connected particles was detected. In 3DOM samples heated at 773 K, larger walls are formed by 6 nm particles cemented by thin amorphous layers that impede the growth of anatase particles. The anatase particles size increases slightly in samples calcined at 873 K. TEM images are not able to detect easily amorphous phases,36 suggesting that 3DOM structures are predominantly crystalline. However, the Jensen method,35 used to estimate the sample crystallinity, revealed that a considerable part of titania is still amorphous in 3DOM samples. 3.3 Photocatalytic activity The photocatalytic activity of 3DOM samples has been studied for MBA oxidation and the results compared with those obtained using BDH and P25 TiO2 samples. Figure S3 reports the experimental results obtained in a representative run. For all the photoreactivity runs, the MBA disappearance rate, (-rMBA ), standing for partial oxidation and mineralization reactions, showed to obey first-order kinetics with respect to MBA


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concentration; the same kinetic law was followed by PAA and CO2 production rates. By considering that the MBA concentration is the parameter experimentally measured, the (-r MBA) term assumes the following form:

(− rMBA ) ≡ − 1 dN = − V dC = k Ov C = (k PO + k Min ) C S dt

S dt

in which S is the surface area of the catalyst, N the MBA moles, t the irradiation time, V the reaction volume, C the MBA concentration, and kOv, kPO and k Min are the first-order kinetic constants of MBA overall, partial oxidation and mineralization reactions, respectively. Even if oxygen is an essential reactant for the occurrence of MBA photodegradation, its influence on the reaction rate may be omitted owing to the fact that oxygen concentration in the suspension was constant during the occurrence of MBA photooxidation. The deduced kinetic law held for MBA conversions lower than 0.5; at higher conversions, PAA molecules do compete with MBA ones for adsorption onto the catalyst surface sites for mineralization and partial oxidation. By fitting the integrated equation to the experimental data, the values of kOv, kPO, and k Min were obtained; they are reported in Table S2 together with the reaction selectivity towards aldehyde formation. It must be noted that all the photoreactivity runs have been carried out at equal conditions of the operative parameters (temperature, pH, dissolved oxygen concentration, irradiation, absorbed photon flow, etc.) so that obtained kinetic constant values, which are normalized with respect to the catalyst surface area, can be used for comparing the specific surface reactivity of the tested solids. Figure 9 reports the values of kOv, kMin and kPO kinetic constants of 3DOM samples. The highest overall photoreactivity is showed by the 3DOM773 sample. By comparing the value obtained with the commercial (BDH) anatase with those obtained with 3DOM samples, it is clear that the commercial crystalline anatase shows overall reactivity two times higher than that of 3DOM773, three times higher than that of 3DOM873 and more than 20 times higher than that of 3DOM673. With respect to the P25 TiO2, the 3DOM773 sample shows overall reactivity three times lower; this comparison, however, must be taken with a great care as the two samples have different phases composition.


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4,5E-05 kOv

4,0E-05 Kinetic constant [m/h]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


kMin

kPO

3,5E-05 3,0E-05 2,5E-05 2,0E-05 1,5E-05 1,0E-05 5,0E-06 0,0E+00

3DOM673

3DOM773

3DOM873

Figure 9. First-order kinetic constant values of MBA overall, kOv , partial oxidation, kPO , and mineralization, k Min, reactions.

In terms of reactivity towards partial oxidation, the 3DOM773 sample shows the highest activity of all studied samples. Indeed the highest selectivity towards the partial oxidation is showed by the 3DOM673 sample but its overall reactivity is the lowest one so that this feature has a lesser importance. Considering that a fast transport of charges among particles is a very efficient way of hampering photogenerated electron-hole recombination and that this transport is limited by electron traps in the periphery of the amorphous titania layer at the anatase nanoparticles interface,25,26,31 the decrease of overall photoreactivity in anatase samples with increasing content of amorphous titania can be attributed to the hindered inter-particles electron transport, strongly depending on the width of the periphery of the amorphous titania layer at anatase nanoparticles interfaces. This higher reactivity of 3DOM773 towards partial oxidation than that of commercial anatase confirms that the active sites for MBA partial oxidation are located in the amorphous titania networks, where low coordinated Ti4+ and O2- ions are far more abundant than in the


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anatase surface. This kPO increase, while kMin value shows a three times decrease, indicates that, while the inter-particles electron transport is decreasing, the number of active sites for MBA partial oxidation in the nanoparticles inter-faces increases with increasing amorphous titania content. For 3DOM673 sample the great kPO decrease, simultaneous with that of the kOv and k Min, shown with increasing amorphous titania content, when the inter-particles electron transport has been strongly hindered, indicates that the increasing amount of amorphous titania incorporated to the anatase nanoparticles surface is hampering their photo-activity by favouring photo-generated charge carriers recombination. The faster decrease of the kPO constant than that of the k Min indicates a faster elimination of the active sites for partial than for total oxidation. These results indicate that, when the anatase particles are covered by strongly condensed amorphous titania layers, their crystal size does not significantly influence the photocatalytic properties. Taking into account the strong influence of the accumulation of amorphous titania on the samples photoactivity, the significantly high value of the 3DOM kOv and kMin kinetic constants can be attributed to a homogeneous distribution of amorphous titania on the 3DOM surface. Previous HRTEM studies of HP sample have shown that most of its anatase nanoparticles boundaries are covered by thick patches of condensed amorphous titania, due to the strongly acidic conditions of the sample preparation.31 On the other hand, the HRTEM images of 3DOM show that its anatase nanoparticles are covered by a thin layer of amorphous titania, but very few anatase nanoparticles boundaries were covered by amorphous titania patches, The low effect of the amorphous titania thin layer on 3DOM photoactivity, also observed on BDH surface, suggests that this layer, owing to its low condensation, is strongly affected by the 3DOM anatase nanoparticles. By taking into account that a large part of the amorphous titania on the 3DOM surface is modified by strong interactions with the anatase surface, the 3DOM kPO value can be attributed to weakly condensed amorphous titania on the anatase nanoparticles boundaries. By taking the 3DOM773 sample as reference, the increase of the thermal treatment temperature from 773 to 873 K determines a photoreactivity decrease. Indeed 3DOM873 sample exhibits a decrease of BET surface area (Table 1) and amorphous character and a slight increase of anatase crystal size. The temperature increase determines a further densification of amorphous phases with a consequent decrease of walls porosity and eventually the overall photoreactivity decreases.43 The photoreactivity results, however, indicate that the decrease is mainly linked to a modification of


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amorphous phases; in fact, while the kinetic constant for mineralization (linked to anatase surface) only decreases of 8% that for partial oxidation (linked to thin disordered layers) decreases of 41%. The almost invariance of mineralization reactivity also suggests that the exposed faces of anatase particles do not change in a significant way with the temperature increase. The photoactivity results indicate that the 3DOM773 shows an overall reactivity lower than that of commercial anatase sample; this feature may be justified by the greater content of amorphous phases contained in 3DOM773 than in commercial BDH TiO2. By considering, however, the activity towards the partial oxidation reaction, the 3DOM773 sample shows quite better performance than the almost crystalline anatase; in this case the presence of amorphous phases is beneficial for improving the reaction selectivity. In conclusion 3DOM773 sample is a better catalyst for partial oxidation reactions than the commercial sample and its performance greatly compensates for its complex preparation method.

4. Conclusions This study shows that the 3DOM titania material is formed by crystalline and amorphous materials and that structure properties depend on both component characteristics, that are determined by the sample preparation conditions.32 In amorphous layers that cover anatase particles, the NMR investigation shows that local structure and Ti coordination change with respect to anatase phase. The sample heating produces the elimination of water and the formation of Ti-O-Ti bonds between anatase and amorphous layers, favouring the formation of 3DOM walls with separated anatase nanoparticles. The characterization results show that the wall of 3DOM673 sample is formed by anatase nanoparticles with crystal size of about 5 nm, covered by a very thin layer of amorphous titania, and by smaller anatase particles, covered by amorphous titania and agglomerated by polymeric species. The thin amorphous titania layer on these anatase nanoparticles originates the smallest mesopores, mainly of about 4 nm size. The heating at 773 K eliminates most of the polymeric species, favouring the growth of the smallest anatase nanoparticles agglomerated by those species. So, sample 3DOM773 is mainly formed by nanoparticles with crystal size of about 6 nm, covered by a thin layer of amorphous titania originating mesopores of about 6 nm. In


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3DOM873 sample, the heating originates the fragmentation of the thin amorphous titania layer, with the formation of small amorphous titania agglomerates, leading to the removal of the small mesopores and the growth of the anatase nanoparticles to about 9 nm. The remaining porosity corresponds to the 3DOM crystalline structure. Meanwhile, the photocatalytic results of MBA degradation in water show that, though the condensed amorphous titania forms agglomerates containing very active sites for the recombination of photogenerated charge carriers, the amorphous titania thin layer can have some positive effects when the photocatalyst is in contact with water. In this case, the water filling the smallest mesopores can solvate the proton of the strongly acid bridging hydroxyls of the thin amorphous titania layer, forming hydrated excess proton structures.62,63 Under UV irradiation, the fast positive charge transport taking place in these structures favours the holes transfer from deprotonated bridging hydroxyls of the anatase particles surface to those of the thin amorphous titania layer, generating active sites for partial oxidation processes there. This holes transfer also favours the photogenerated charge carriers separation in the anatase particles. The low content of amorphous titania agglomerates and the thin amorphous titania layer in 3DOM773 sample determine its higher photoactivity for total and partial oxidation than those of the two other 3DOM samples. These effects are probably generalizable to other nanostructured titania systems and it is therefore important to the materials chemistry/materials science community. In nanoparticles prepared at low temperature, amorphous oxide is always present and it may drastically affect the bulk and surface properties of samples. To ignore this presence and its effects may determine mistakes and misunderstandings in explaining physico-chemical properties of prepared materials.

Acknowledgement J.Sa. and D.T. thank the Spanish Agency MINECO (project MAT2016-78362-C4-2R) and the regional Government (project S2013/MIT-2753) for financial support.

Supporting Information Available: Additional information on textural and structural characteristics. HRTEM micrographs of 3DOM structures illustrating the disordered character of


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particles surface and the relative arrangement of disordered domains inside agglomerate. particles. Table with chemical shift and relative intensity values of 1H NMR-MAS spectra components. Figures of 1H NMR-MAS spectra. Reactivity results of a representative run. Table with the values of kinetic constants and selectivity towards partial oxidation product.

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TOC

In separated particles (a) the amorphous layer thickness is considerably bigger than in connected particles forming 3DOM walls (b).


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Influence of the Preparation Temperature on the Photocatalytic Activity

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