Synthesis of Porous Crystalline Doped Titania Photocatalysts Using

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Synthesis of porous crystalline doped titania photocatalysts using modified precursor strategy Michal Marszewski, Jowita Marszewska, Svitlana Pylypenko, and Mietek Jaroniec Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b03429 • Publication Date (Web): 11 Oct 2016 Downloaded from http://pubs.acs.org on October 12, 2016

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Chemistry of Materials

Synthesis of porous crystalline doped titania photocatalysts using modified precursor strategy Michal Marszewski,† Jowita Marszewska,† Svitlana Pylypenko,‡ Mietek Jaroniec†* †

Department of Chemistry and Biochemistry, Kent State University, Kent, OH 44242, USA

‡

Department of Chemistry, Colorado School of Mines, Golden, CO 80401, USA

ABSTRACT: We propose a new strategy for the synthesis of porous crystalline doped titania materials—dubbed the modified precursor strategy. The modified precursors are prepared by reacting generic titania precursors with organic acids in order to introduce “carbonizable” groups into the precursor’s structure, so that carbon–titania composites can form upon carbonization. The resulting carbon framework serves as a scaffold for TiO2, and supports the structure during crystallization. Afterwards, removal of the carbon scaffold through calcination results in titania with a well-developed structure and high crystallinity. The titanias synthesized according to this strategy, using common organic acids as the modifiers, have specific surface areas reaching 100 m2 g–1 and total pore volumes exceeding 0.20 cm3 g–1, even after crystallization at temperatures from 500 °C to 1000 °C. The materials possess high crystallinity and tunable phase composition, and some show visible light absorption and significantly narrowed band gaps (2.3–2.4 eV). Photocatalytic degradation of methylene blue proved that these photocatalysts are active under visible light. All tested titanias show an excellent photocatalytic performance due to the combined effect of the well-developed structure, high crystallinity, and narrow bandgap. This strategy can easily be adopted for the preparation of other porous crystalline materials.

INTRODUCTION

driven by photocatalytic process, restricted only by oxidation/reduction potentials.

Titania is a well-known transition metal oxide that has been produced and used commercially for over a century now, as pigment and ingredient in paints, ointments, toothpaste, and other everyday products and items.1,2 The latter, especially, proves that TiO2 is an inexpensive, abundant, widely available, and non-toxic material. Recently, titania has been explored in plenty of novel applications, including photocatalysis,3–7 energy conversion,8–10 energy storage,11–14 and others.15–21 The research on the photocatalytic application of titanium dioxide was sparked by Fujihima and Honda,22 who in 1978 showed that TiO2 could split water into H2 and O2 when illuminated with UV light. This kind of application is very interesting, considering all nowadays efforts to develop green and renewable energy sources.23 Hydrogen production, however, is only a single example of possible photocatalytic reactions.

For instance, in the case of photocatalytic water splitting, hydrogen is reduced and oxygen is oxidized, producing fuel in an inexpensive fashion.23 Alternatively, carbon dioxide—a greenhouse gas, heavily emitted in the combustion of fossil fuels and industrial processes—can be used as a reactant and subject to reduction in order to form methane, methanol, or other useful hydrocarbons, to be used as fuels or as a chemical feedstock.25 Lastly, photocatalytic activity can be used for decontamination.26–29 In this case, a photocatalyst is employed to decompose, typically through oxidation, organic or biological contaminants present in water or air. Overall, photocatalysis is a powerful process, where desired chemical reactions can be driven simply by the presence of photocatalyst exposed to light. If the sun is used as the light source, the supply of energy is virtually unlimited, and the process can be considered as green and renewable.

Photocatalysis is a process where electrons in the semiconductor’s valence band are excited to the conduction band by absorption of light.24 These electrons, and holes created in the valence band (the positive charge carriers), can then freely travel throughout the photocatalyst, until they reach the surface and take part in useful chemical reactions. Transfer of electrons to the species adsorbed on the surface constitutes a reduction reaction, and transfer of holes constitutes an oxidation reaction. By changing reactants and reaction type, numerous reactions can be

The ideal photocatalyst would possess large surface, large pore volume, high crystallinity, and narrow band gap.25 A large surface can accommodate more charge carriers and reactants, resulting in faster reaction rates, because more reactants stored at the surface means more molecules can undergo reaction at any given time, while it also means charge carriers can find reactants faster and have less time for recombination. Porosity, on the other hand, facilitates diffusion of reactants and products be-

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tween the bulk and surface phases. This ensures constant and fast supply of reactants from the bulk phase to the surface phase, and a swift withdrawal of the products, improving the process’ efficiency. High crystallinity is important to maximize charge carriers’ mobility and minimize their trapping or scattering.30 The faster charge carriers get to the surface, the lower are chances of their recombination. Crystal defects and impurities will slow down the charge carriers’ transport because they either trap or scatter them. Both factors increase the probability of charge recombination. Lastly, visible-light-driven photocatalysis is important because the visible light accounts for 44 % of the solar energy reaching the Earth’s surface;23 on the other hand, the UV light constitutes only ca. 4 %. A photocatalyst responsive only to the UV light will be accordingly less effective if the solar light is used to drive the process. Crystalline porous photocatalyst would be an ideal solution to maximize photocatalytic activity. Unfortunately, the well-developed surface and porosity are easily attained only for amorphous or partially crystallized materials.31 The commonly used synthesis strategies: softtemplating, sol-gel, hydrothermal etc. usually result in the materials with good structural properties, but poor crystallinity. Calcination of these materials at high temperatures improves their crystallinity, but at the same time results in the deterioration or complete collapse of their nanostructures.32,33 The reason is that the pore walls are made of amorphous titania, which during crystallization undergoes a phase transition and densification. In addition, the crystallites only grow at the expense of the amorphous material. Overall, both these processes weaken and strain the framework, leading to the loss of integrity and collapse. Porosity and crystallinity are thus, mutually exclusive features and preparation of porous and crystalline titania is a challenging task. Another challenge in the development of titania photocatalyst is to assure the visible light driven photocatalysis. Titania is a wide bandgap semiconductor, with Eg = 3.2 eV for anatase phase. This means TiO2 absorbs only UV light with a wavelength < 380 nm. Commonly, titania is doped with metal or nonmetals to narrow its band gap and achieve visible light absorption.34–36 So far, non-metals have been shown to be more effective than metal dopants. Among non-metals, nitrogen has been heavily investigated.37–41 More recently, carbon has been shown to be an effective dopant too.42–44 Similarly to crystallinity and porosity conflict, a simultaneous achievement of crystallinity and doping is also a difficult task because crystallization process tends to remove any “impurities” from the crystal lattice, including dopants.39,45 Increasing crystallization, thus, is often associated with removal of the dopants introduced to narrow the band gap. Currently, there is no strategy deliberately designed to achieve titania with all of these features. The commonly used approaches focused either on the development of structure or improving crystallinity or introducing dopants.

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In this paper, we propose, outline, and test a new strategy for the synthesis of porous crystalline doped titania. The strategy—dubbed the modifier precursors strategy— is demonstrated through the synthesis of highly effective titania photocatalysts, but it can easily be adopted for the preparation of other materials too. We hope the modified precursor strategy will solve some of the problems concerning synthesis of titania photocatalysts, and help to synthesize other important materials.

EXPERIMENTAL SECTION Chemicals Titanium (IV) oxyacetylacetonate (90%, CAS# 14024-647), acetic acid (97.7+%, ACS reagent, CAS# 64-19-7), monochloroacetic acid (99%, CAS# 79-11-8), benzoic acid (CAS# 65-85-0), isophthalic acid (CAS# 121-91-5), and trimesic acid (98%, CAS# 554-95-0) were acquired from Aldrich Chemical Company, Inc. (Milwaukee, WI, USA). Titanium isopropoxide (TIPO; 98+%, CAS# 546-68-9) was acquired from Acros Organics (Fair Lawn, NJ, USA). Citric acid (anhydrous, 99.5+%, CAS# 77-92-9) was acquired from Fisher BioReagents (Fair Lawn, NJ, USA). Formic acid (CAS# 64-18-6) was acquired from Eastman Kodak Company (Rochester, NY, USA). Oxalic acid (dihydrate, technical grade, CAS# 6153-56-6) was acquired from Fisher Scientific Company (Fair Lawn, NJ, USA). Phthalic anhydride (CAS# 85-44-9) was acquired from TCI America (Portland, OR, USA). Ethanol (200 proof, CAS# 64-175) was acquired from Decon Laboratories, Inc. (King of Prussia, PA, USA). Deionized water from IonPure Plus 150 water system was used for all experiments.

Materials Titania materials prepared from TiO(acac)2 The first batch of titania materials was prepared by carbonization and subsequent calcination of commercially available titanium salt: titanium (IV) oxyacetylacetonate (TiO(acac)2). First, carbonization was applied to transform TiO(acac)2 into carbon–titania composite and let titania crystalize. The in-situ formed carbon scaffold was used to support titania during crystallization and to prevent the structure from collapsing. The subsequent calcination removed the carbon scaffold, resulting in a pure crystalline and mesoporous titania. The exact procedure was as follows: 3.0 g of TiO(acac)2 was placed in a quartz boat and carbonized in flowing nitrogen at either 500 °C, 600 °C, 700 °C, or 800 °C for 2 h; the initial temperature ramp was 5 °C min–1. The carbonization yielded 1.2–1.3 g (40–43 % step yield) of each carbon–titania composite. Subsequently, 0.7 g of the composite was placed in a quartz boat and calcined in flowing air at 450 °C for 0.5 h; the initial temperature ramp was 5 °C min–1. The calcination yielded ≈ 0.56 g of each final titania material (80 % step yield, 32–34 % total yield). The carbon–titania composites derived from TiO(acac)2 were labeled ta-x* and the titania materials derived from these composites were labeled ta-x, where x denotes the carbonization temperature.


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Figure 1. Schematic illustration of the modified precursor strategy.

Titania materials prepared from Ti(citr)3 The second batch of titania materials was prepared in the similar fashion to the first, but using a self-prepared titanium–citric acid complex as the carbon–titania precursor and with slightly different carbonization and calcination steps. The exact procedure was as follows: first, 7.20 g (37.5 mmol) of citric acid was dissolved in 25 mL of absolute ethanol during continuous stirring (≈ 0.5 h). Then, 3.7 mL (12.5 mmol) of titanium isopropoxide was mixed in (the resulting solution remained transparent) and the stirring was continued for 1 h. Afterwards, the solution was transferred to a Petri dish and into an oven for evaporation and drying at 60 °C overnight. The resulting compound, viscous at temperatures exceeding the room temperature but solid when cooled, was transferred to a quartz boat and carbonized in flowing nitrogen at either 550 °C, 700 °C, 850 °C, or 1000 °C for 2 h; the initial temperature ramp was 1 °C min–1. The resulting carbon–titania composite was placed in a quartz boat and calcined in flowing air at 450 °C for 0.5 h; the initial temperature ramp was 1 °C min–1. The carbon–titania composites were labeled tcx* and the final titania materials were labeled tc-x, where x denotes the carbonization temperature.

was dissolved in 25 mL of absolute ethanol during continuous stirring (≈ 0.5 h). Then, 3.7 mL (12.5 mmol) of titanium isopropoxide was mixed in (for some acids this resulted in immediate precipitate) and the stirring was continued for 1 h. Afterwards, the solution was transferred to a Petri dish and into an oven for evaporation and drying at 60 °C overnight. The resulting compound was transferred to a quartz boat and carbonized in flowing nitrogen at 850 °C for 2 h; the initial temperature ramp was 1 °C min– 1 . The resulting carbon–titania composite was placed in a quartz boat and calcined in flowing air at 450 °C for 0.5 h; the initial temperature ramp was 1 °C min–1. In the case of phthalic acid, 25 mmol of phthalic anhydride and 25 mmol of water were added to 25 mL of absolute ethanol during continuous stirring to hydrolyze the anhydride and form phthalic acid prior to the TIPO addition. The remaining steps were identical to those outlined above. In the case of isophthalic and trimesic acids, 50 mL of ethanol was used instead of 25 mL, due to their poor solubility. These acids have not dissolved completely in this amount either, but the precipitation of the forming TIPO–acid complex was assumed to have pushed the reaction to the completion.

Titania materials prepared from formic, acetic, chloroacetic, oxalic, benzoic, phthalic, isophthalic, and trimesic acids

Finally, one material was prepared according to the same procedure but without the addition of any modifying acid.

The third batch of titania materials was prepared in the exactly the same fashion as the second, but using different carboxylic acid modifiers and in 1:2 TIPO:acid ratio instead of 1:3. The reason for 1:2 ratio is that while, Ti– citric acid complex is well characterized and known to have 1:3 stoichiometry,46 many of the explored modifiers are not so well, or at all, characterized.47 Previous studies reported that acetic acid, and similar simple carboxylic acids, form complexes with 1:2 stoichiometry.48 Based on that study, we settled on using 1:2 ratio to simplify the screening and comparison of a larger group of modifiers for the purpose of this work.

None of these materials have been labeled and are referred to simply by the name of the modifying carboxylic acid.

The exact procedure was as follows: first, 25 mmol of either formic, acetic, chloroacetic, oxalic, or benzoic acid

Measurements Nitrogen adsorption–desorption isotherms were measured at –196 °C on a surface area and porosity analyzer ASAP 2010 manufactured by Micromeritics Instrument Corporation (Norcross, GA, USA). All samples were degassed at 200 °C for 2 h in vacuum before every measurement. Powder X-ray diffractograms were collected on a powder X-ray diffractometer X’Pert PRO manufactured by PANalytical Inc. (Westborough, MA, USA). The system



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Table 1. Characterization of the materials prepared from TiO(acac)2 and Ti(citr)3 precursors.* Carbonized materials Precursor

TIPO

TiO(acac)2

Ti(citr)3

Sample

Carbonization temperature (°°C)

Calcined materials

Carbon content (wt%)

Sample

Specific surface area 2 –1 (m g )

Total pore volume 3 –1 (cm g )

R/A peak ratio

Band gap (eV)

—

850

0

—

2

0.01

—

—

ta-500*

500

23.7

ta-500

57

0.04

0.11

3.01

ta-600*

600

23.4

ta-600

84

0.08

0.29

3.03

ta-700*

700

22.5

ta-700

92

0.13

0.67

3.03

ta-800*

800

20.7

ta-800

78

0.15

1.55

2.38

tc-550*

550

38.9

tc-550

71

0.07

1.15

2.95

tc-700*

700

36.5

tc-700

81

0.09

1.28

2.97

tc-850*

850

31.6

tc-850

86

0.15

1.53

2.26

tc-1000*

1000

17.9

tc-1000

62

0.21

2.03

2.27

*Notation: Carbon content was calculated based on thermogravimetric profile recorded in air. Specific surface area was calculated based on low-temperature nitrogen adsorption using the Brunauer–Emmett–Teller method. Total pore volume was calculated based on the nitrogen amount adsorbed at the relative pressure ≈ 0.99. R/A peak ratio was calculated as a ratio of the most intense peaks for rutile and anatase. Band gap values were calculated by extrapolation of the absorption onset in Tauc plots.

used the Bragg–Brentano theta–theta configuration and CuKα radiation (copper as the radiation source and a diffracted-beam curved-crystal monochromator to eliminate CuKβ). All scans were taken using a continuous scan mode in the 2theta range of 20.00°–80.00° and with a step size of 0.05°. Thermogravimetric curves were recorded on a Q500 thermogravimetric analyzer manufactured by TA Instruments (New Castle, DE, USA). All measurements were done in flowing air with at least 10 mg of the sample placed in a platinum pan. Diffuse reflectance UV–Vis absorption spectra were collected on a UV-3600Plus UV–Vis–NIR spectrometer equipped with an ISR-603 integrating sphere accessory (d = 6 cm) manufactured by Shimadzu (Columbia, MD). Photocatalytic measurements were done using 20 mg of the photocatalyst dispersed in 50 mL of 1.5 mg L–1 methylene blue aqueous solution. The solution was allowed to achieve adsorption equilibrium for 1 h in dark before starting irradiation and was stirred continuously during the experiment. The light source was a SlimStyle LED light bulb (continuous spectrum between 400 nm and 800 nm, 10.5 W, color temperature 5000 K, model# 9290002709) manufactured by Philips Lighting (Somerset, NJ, USA). 3 mL aliquots of the solution were withdrawn every 15 min (except Degussa P25, where the interval was 1 hour) and analyzed using a Cary 300 Bio UV–Vis spectrometer manufactured by Varian (Palo Alto, CA, USA). X-ray photoelectron spectra were measured using a Kratos Nova X-ray photoelectron spectrometer equipped with a monochromatic Al Kα source operating at 300 W. Survey and high-resolution C 1s, O 1s, and Ti 2p spectra were acquired at 160 eV and 20 eV, respectively, providing charge compensation using low-energy electrons. Two areas per sample were analyzed. Data analysis was done

using CasaXPS software. A linear background was applied to C 1s and O 1s regions, and Shirley background was used for Ti 2p regions. Quantification was performed using sensitivity factors supplied by the manufacturer. The analysis included charge referencing to the internal aromatic carbon signal at 284.6 eV.

Methods The specific surface areas were calculated using the Brunauer–Emmett–Teller method49 based on the lowtemperature nitrogen adsorption in the range 0.05–0.30 and assuming a nitrogen cross-section area of 0.162 nm2.50,51 The total pore volumes were calculated by conversion of the nitrogen amount adsorbed at the relative pressure ≈ 0.99 to the volume of liquid nitrogen at the experiment’s conditions (e.g. using conversion factor of 0.0015468).50,52 The carbon content was calculated from thermogravimetric run in air as a difference between the mass of the outgassed sample (i.e. at 150 °C) and the residual mass. Figures S1, S2, and S3 in the Supporting Information show TG curves for the selected carbon–titania composites. The rutile/anatase (R/A) peak ratio was calculated as a ratio of net peak heights for rutile (≈ 27.5°, 2θ CuKα) and anatase (≈ 25.4°, 2θ CuKα) phases. The band gap values were calculated using Tauc plots by extrapolation of the absorption onset on the plot of [hυ F(R)]1/n vs. hυ,53 where h is the Planck constant, υ is light’s frequency, n is a constant related to transition type in the semiconductor (equal to 2 for titania’s indirect bandgap), and F(R) is the Kubelka–Munk function calculated as: 1− = 2


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Figure 2. Nitrogen adsorption–desorption isotherms measured at –196 °C for the titania materials prepared from (A) TiO(acac)2 3 –1 3 –1 and (B) Ti(citr)3. ta-600 isotherm shifted up by 10 cm STP g , ta-700 isotherm shifted up by 30 cm STP g , ta-800 isotherm 3 –1 3 –1 3 –1 shifted up by 60 cm STP g ; tc-700 isotherm shifted by 15 cm STP g , tc-850 isotherm shifted by 40 cm STP g , tc-1000 iso3 –1 therm shifted by 100 cm STP g .

where R is sample’s reflectance. Figures S4 and S5 in the Supporting Information show the Tauc plots used to calculate the band gap values. The normalized concertation of methylene blue was calculated based on the Beer–Lambert law and the UV– Vis absorption measurements by dividing adsorption at 664 nm for the measured sample and for the sample measured at the initial time (t = 0:00). Figure S6 shows the UV–Vis absorption measurements of methylene blue as a function of time during the photocatalytic measurements, used to calculate the normalized concentration.

RESULTS AND DISCUSSION In this work, we propose a novel strategy for the synthesis of porous crystalline doped titania photocatalysts— dubbed the modified precursor strategy (see Figure 1 for the general scheme). First, a modified precursor is prepared by reacting a generic titania precursor (e.g., TIPO) with an organic acid, resulting in the substitution of some or all alkoxide groups in the TIPO structure. The modification step is a simple, yet powerful, chemistry: titanium atom has a very high affinity toward carboxylic moiety.46– 48 This chemistry is exploited to quickly and reliably mod-

ify TIPO, as long as the modifier has a carboxylic moiety in its structure. The number of substituted groups and the coordination type depend on the acid and the experimental conditions. For instance, acetic acid substitutes either one or two isopropoxide groups, and forms either monodentate, bidentate chelating, or bidentate bridging bonds;48 whereas citric acid substitutes all alkoxide groups and forms a complex with the stoichiometry Ti(citr)3.46 As the result of this rich chemistry, the modified precursors can be custom made, and the precursor’s properties tailored, by selecting different modifiers and by controlling the modification process. The modification’s goal is to introduce “carbonizable” groups in a generic precursor’s structure. The resulting modified precursor can then serve as both titania and carbon precursor simultaneously. After the modification, the precursor undergoes two thermal treatment steps: first, carbonization, then, calcination. This order is crucial and integral part of this strategy. In the first step, the precursor degrades, forming titania domains, while the introduced organic groups carbonize, forming carbon domains. The resulting carbon–titania composite consists of intertwined titania and carbon frameworks, intimately



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Figure 3. Powder X-ray diffractograms for the titania materials prepared from (A) TiO(acac)2 and (B) Ti(citr)3. ta-600, ta-700, ta800, tc-550, tc-700, and tc-1000 diffractograms shifted up for clarity. Letters “a” and “r” indicate diffractions matched to anatase (ICDD PDF# 00-021-1272) and rutile (ICDD PDF# 00-021-1276), respectively.

mixed at small length scale. This in-situ formed carbon framework, uniformly distributed and spanning the whole structure, is created specifically to act as a scaffold for the titania framework. During the crystallization, taking place later at high temperatures, the carbon scaffold protects titania, supports the structure, and prevents its collapse. After crystallization, the carbon scaffold is removed by calcination, creating/opening porosity and enlarging surface area, resulting in a porous crystalline titania. The following paragraphs contain an in-depth discussion of each step of the modified precursor strategy. First, we tested the concept of in-situ formation of the carbon scaffold to protect titania during its crystallization. We used a commercially available, organic titanium salt, titanium oxyacetylacetonate (TiO(acac)2) as a model modified precursor, to avoid the layer of complexity associated with the modification step itself. Table 1 shows characterization of the resulting materials (ta-x* for the carbon–titania composites, and ta-x for the pure titanias, where x denotes the carbonization temperature). Carbonization of TiO(acac)2 indeed produced carbon–titania composites, with the carbon content in the range 21–24 wt% (based on the TG measurements). This carbon amount decreased slightly with the increasing temperature, probably due to the removal of volatile groups and

other thermal degradations. The composites were then calcined to produce the final titania materials. Figure 2 (A) shows low-temperature nitrogen adsorption– desorption isotherms for all resulting titanias. All isotherms are of type IV according to the IUPAC classification, indicating the presence of mesopores.51 The specific surface areas range from 57 to 92 m2 g–1 and the pore volumes range from 0.04 to 0.15 cm3 g–1, showing these materials have well-developed structures. In comparison, the material prepared from unmodified TIPO had a barely measurable surface area of 2 m2 g–1 and a pore volume of 0.01 cm3 g–1. This proves the concept of a single carbon– titania precursor, and use of the in-situ formed carbon framework as a scaffold. Encouraged by these favorable results, we prepared and tested an actual modified precursor. The precursor was prepared by reacting TIPO with citric acid, in 1:3 molar ratio, to form Ti(citr)3,46 next carbonized, and then calcined, in a fashion similar to TiO(acac)2. Table 1 shows characterization of the resulting materials and Figure 2 (B) shows low-temperature nitrogen adsorption– desorption isotherms (tc-x* for the carbon–titania composites, and tc-x for the pure titanias, where x denotes the carbonization temperature). Similarly as in the case of TiO(acac)2., carbonization of Ti(citr)3 resulted in the for-


mation of carbon–titania composites. These composites possessed higher carbon content ranging from 18 wt% to 39 wt%, but also showed a more pronounced decrease in the carbon content due to the higher carbonization temperatures. All isotherms are of type IV as well, indicating the presence of

compared with the ta-x series (5 °C min–1 heating rate). This shows that both, the temperature and the time of the carbonization, determine the phase composition. Notably, this effect can be exploited to tailor the phase composition by controlling carbonization conditions.

2.2

100

2

90

Specific surface area (m2 g–1)

1.8

Rutile-to-anatase peak ratio

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


1.6 1.4

1 °C min–1

1.2 1 0.8 0.6 0.4

5 °C min–1

0.2

Ti(citr) Series2 3 TiO(acac) Series1 2

0 500

600 700 800 900 Carbonization temperature (°C)

1000

Figure 4. Phase composition (expressed as the rutile-toanatase peak ratio) as a function of the carbonization temperature for the titania samples prepared from TiO(acac)2 –1 (full circles) and Ti(citr)3 (hollow circles). 1 °C min and 5 °C –1 min designate the initial heating rate. 51

mesopores according to the IUPAC classification. The sur2 –1 2 –1 face areas range from 62 m g to 86 m g and the pore vol3 –1 3 –1 umes range from 0.07 cm g to 0.21 cm g , indicating that structures of these materials are well-developed. Overall, this modified precursor performed similarly as TiO(acac)2. The latter proves that modification of TIPO with organic acids is a facile method to prepare a single carbon–titania precursor.

Crystallinity wise, all materials had a high degree of crystallinity, with tall and sharp peaks in their diffractograms (Figure 3). Moreover, the materials showed a clear progression of the crystallization and phase transition with the carbonization temperature; for instance, ta-500 titania was almost exclusively anatase phase, while ta-600, ta-700, and ta-800 gradually progressed toward rutile phase. The latter is apparent from the decreasing intensity of the anatase peaks and increasing intensity of the rutile peaks. More quantitatively, this observation is reflected by the ratio of the rutile and anatase’s most intensive peaks (listed in Table 1). The R/A values range from 0.1 for ta-500 (almost exclusively anatase) to over 2.0 for tc-1000 (dominating rutile phase). Figure 4 shows the R/A values for both series as a function of carbonization temperature. Clearly, the phase composition is a function of carbonization temperature. Interestingly, the R/A values for the tc-x series are noticeably higher than the R/A values of the ta-x series at similar temperatures. This is attributed to the longer temperature program used for the preparation of the tc-x series (1 °C min–1 heating rate) as

Trimesic tc-850

80

Isophthalic

ta-800

70

Phthalic

60 50 Oxalic

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40

Chloroacetic

30

Acetic Benzoic Formic TIPO

20 10 0 0

10

20 30 40 Carbon content (wt%)

50

Figure 5. Specific surface area as a function of the carbon scaffold amount for the titania samples prepared from TIPO modified with: formic, acetic, chloroacetic, oxalic, benzoic, phthalic, isophthalic, and trimesic acid, in 1:2 ratio. ta-800, tc-850, and TIPO included for comparison. The red dashed line as a guide only.

A comparison of the diffraction patterns of the carbon– titania composites (Figure S7 in the Supporting information) and the final titanias shows that crystallinity evolves during the calcination step too. For instance, ta500* and ta-600* composites show exclusively anatase phase, with no detectable rutile phase, yet after the calcination, ta-500 and ta-600 titanias have small amounts of rutile phase (R/A values 0.1 and 0.3, respectively). This can be a problem if exclusively anatase phase is desired, but we suspect that a lower calcination temperature can be a solution. The 450 °C calcination temperature used in this experiment was high enough to facilitate titania’s crystallization, but 350–400 °C temperature can well be used for the calcination and should not induce the undesired phase transition. On the other hand, obtaining exclusively rutile phase should be a matter of long enough carbonization. Ultimately, the calcination step can be just considered as a secondary way to tune crystallinity, and we believe that titania with any phase composition—pure anatase, mixed anatase and rutile, or pure rutile—can be prepared through the modified precursor strategy by a careful control of the thermal treatments. All these results prove that i) a modified precursor can serve as a simultaneous source of titania and carbon and ii) that carbon framework can act as a scaffold, protecting the titania framework and iii) the resulting titanias have



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Figure 6. Normalized diffuse reflectance UV–Vis absorption spectra of the titania materials prepared from (A) TiO(acac)2 and (B) Ti(citr)3. The insets show photographs of the actual samples.

high surface areas, large pore volumes, and high degree of crystallinity. Following this initial proof of concept, we explored other modified precursors. These were prepared from the common organic acids: formic, acetic, chloroacetic, oxalic, benzoic, phthalic, isophthalic, and trimesic. The synthesis procedure was the same as for the tc-x materials, except 1:2 TIPO to acid ratio was used instead. Table S1 in the Supporting Information shows characterization of the resulting materials. Clearly, the carbon-rich modifiers resulted in higher carbon content, with trimesic, isophthalic, and phthalic delivering the highest carbon amounts (53 wt%, 36 wt%, and 16 wt%, respectively). On the other hand, formic and oxalic acids delivered the smallest amounts of carbon (< 2 wt%). Interestingly, this is only a general trend and there were a few noticeable exceptions. For instance, even though acetic acid and chloroacetic acids have the same number of carbon atoms, the latter delivered a roughly twice higher carbon content. We suspect this is due to the presence of chlorine, which can increase the “carbonizability” of organic molecules. A similar example would be carbonization of polyethylene and polyvinyl chloride, which differ only by the presence of chlorine, but only the latter can be carbonized under normal conditions.54 Oxalic acid, on the other hand, resulted in a surprisingly small amount of carbon. It produced even less carbon than formic acid, which is perhaps due to the well-known tendency of oxalic acid for decomposition to CO2. Finally, functionality and crosslinking have a pronounced effect on the amount of the created carbon. Among the aromatic carboxylic acids, benzoic acid (monocarboxylic) had the smallest carbon content, while trimesic acid (tricarboxylic) the highest, and phthalic acid and isophthalic acid in between (both dicarboxylic). Interestingly, there was a noticeable difference even between these two isomers (16 wt% vs. 36 wt%). We suspect that probably phthalic acid, due to its structure, coordinates to a single Ti atom, while

isophthalic acid, probably favors a bidentate bridging bonding. As a result, the latter results in more crosslinked structure and the higher carbonization. Overall, the amount of carbon scaffold can be controlled and depends on: 1) number of carbon atoms in the modifier, 2) number of introduced modifier groups, 3) functionality and crosslinking, and 4) chemistry of the modifier. The higher amount of scaffold resulted in better protection and better structural parameters of the final titania materials. Figure 5 shows the specific surface area values as a function of the carbon scaffold amount for the materials prepared with: formic, acetic, chloroacetic, oxalic, benzoic, phthalic, isophthalic, and trimesic acid (ta850 and tc-800 included for comparison). Although the red dashed line serves only as a guide, it is clear that the surface area follows closely the carbon scaffold amount. Notably, the surface area depends on the thermal treatments too. For instance, ta-800 titania was carbonized faster and calcined at a lower temperature than all other samples in Figure 5. The lower heat impact resulted in a slightly higher surface area than predicted. The general trend, however, is clear: the higher specific surface area is obtained for materials corresponding to the larger amount of carbon scaffold. A similar, but much less pronounced, dependence is observed for the pore volume (see Figure S8 in the Supporting Information). The samples having higher amounts of carbon scaffold generally possessed larger pore volumes; however, a significant stray in the plot is noticeable. This is attributed to the dispersed nature of the carbon scaffold, which was shown to only slightly influence pore volume.55 Ultimately, the structural parameters can be controlled by the amount of carbon scaffold, which in turn can be controlled by the structure and chemistry of the modified precursor. Interestingly, some materials—ta-800, tc-850, and tc1000—had yellow color instead of the typical white color observed for titania. Figure 6 shows the diffuse reflec-


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Figure 7. Band gap of the titania materials as a function of the carbonization temperature. The red dashed curve as a guide only. The error bars represent 0.05 eV estimated uncertainty.

tance UV–Vis spectra and the optical photographs of all prepared titanias. Five materials show typical “titania” spectra: no visible light absorption and a single absorption edge in the UV region. The absorption edge is a bit shifted with respect to the typical 380 nm observed for a pure titania but still acceptable. The yellow-colored materials, on the other hand, show significant absorption in the visible light region. ta-800 starts absorbing at ca. 525 nm, whereas tc-850 and tc-1000 start absorbing at 575 nm. Based on this data, the plot of (F(R) hυ)1/2 vs. hυ was constructed to calculate the band gap values for all materials (Figure S4 and S5 in the Supporting Information). Table 1 lists all calculated band gap values and Figure 7 plots the bandgap values as a function of the carbonization temperature. All materials carbonized at temperatures up to 700 °C have band gaps of ca. 3 eV, which is close to 3.2 eV expected for pure titania. On the other hand, all materials carbonized at temperatures above 800 °C have bandgaps in the range of 2.3–2.4 eV. This significant reduction in band gap is extremely interesting and can translate into a tremendous improvement in photocatalytic efficiency. We have performed the XPS analysis to further infer upon the composition of the doped materials and elucidate the possible reason for their yellow color. Figures S9 and S10 show Ti 2p, O 1s, and C 1s XPS spectra for both ta and tc series. For the Ti 2p spectra, we observe two peaks at 458.5 eV and 464.2 eV, separated by 5.7 eV as expected due to the spin-orbit splitting. The binding energy for the first peak matches that for Ti–O bond in TiO2. Moreover, no other peaks, peak shifts or peak broadening are observed when comparing all samples in ta and tc series. Therefore, the Ti XPS spectra indicate that titanium is exclusively present as Ti4+ in TiO2, and no Ti3+ species or other titanium species are formed. This is further corroborated by the O 1s XPS spectra that show a strong peak at

Figure 8. Normalized concentration as a function of time in a visible-light-driven photocatalytic degradation of methylene blue by the selected titania materials prepared from TiO(acac)2 and Ti(citr)3. The error bars represent 0.05 (normalized concentration units) estimated uncertainty.

529.8 eV corresponding to oxygen bound in a metal oxide, i.e. TiO2. The C 1s spectra show peaks at 284.6 eV, 285.2 eV, 286.1 eV and 287.2 eV due to C=C, C–C, C–O and C=O species, but also peaks at ca. 288.6 eV and 289.3 eV that have been previously attributed to carbon doping in TiO2 materials.36,42,56 The former peak can also have contribution from O–C=O species. Additionally, survey spectra (not shown) confirm that no other elements that could result in the narrowed band gap (e.g. nitrogen) were present in these materials. Based on these findings, we propose that the significant band gap reduction observed for the yellow samples is due to a combined effect of 1) formation of mixed anatase–rutile phase and 2) carbon doping. Based on the previously published reports each of these mechanisms can decrease titania’s band gap by up to 0.4 eV. In the first case, the formation of rutile from anatase creates a heterojunction between the two phases, and because rutile has ca. 0.4 eV higher valence band edge than anatase, the effective band gap is reduced by ca. 0.4 eV.57 In the second case, carbon doping has been shown to narrow TiO2 bandgap by ca. 0.4 eV too,36 by introducing tails in both valence and conduction bands. We believe that the combination of these two mechanisms results in the observed band gap narrowing. The carbon doping is most probably introduced during the carbonization step, when titania and carbon are in an extremely intimate mix, with the extended interface, and small domains sizes. We hypothesize that at sufficiently high temperatures, carbon atoms can diffuse into TiO2 lattice, resulting in doping. This mechanism is supported by the fact that doping is observed regardless of the precursor, with materials prepared from either TiO(acac)2 or Ti(citr)3 showing the same reduced band gaps. The



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threshold temperature, between 700 °C and 800 °C in this case, may be related to the energy necessary for the diffusion of carbon atoms (kinetic restriction) or to the entropic term of the system’s energy (energetic restriction). Ultimately, further studies are necessary to determine the exact mechanism of this process, the doping concentration, and the doping type. We carried out photocatalytic experiments in order to test how the narrowed bandgap translates into photocatalytic activity. Figure 8 shows results of the visible-lightdriven photocatalytic decomposition of methylene blue (selected as a model contaminant). All three colored materials showed photocatalytic activity under visible light irradiation. The best results were observed for tc-850, followed by tc-1000 and ta-800 in the end. This order correlates with the amount of additional visible light absorbed by these materials, as judged from the UV–Vis absorption spectra. tc-850 shows the largest and furthest extending absorption shoulder in the visible range. The tc-1000’s absorption shoulder extends roughly the same, but it is roughly 30% less intense. Finally, ta-800 has much less intensive and shorter absorption shoulder (reflected by the slightly larger band gap). A blank measurement was also done to show the extent of small but noticeable photolysis reaction.58,59 The narrowed band gap clearly translated into photocatalytic activity, with all doped titanias showing very fast decomposition rates. Specifically, tc-850 reduced the methylene blue’s concentration by ≈ 85%, tc-1000 by ≈ 70 %, and ta-800 by ≈ 40%, within the first 90 min. The corresponding quasi-firstorder kinetic constants are 0.015 min–1 for tc-850, 0.0099 min–1 for tc-1000, and 0.0045 min–1 for ta-800. These constants were calculated based on the first 3 points in each dataset in order to minimize the effect of the volume change that is apparent toward the experiment’s end and affects the linearity of the first-order kinetic plots. Figure S11 shows the loss of photocatalytic activity for the tc-850 photocatalyst during five consecutive runs. The material showed a good stability during the photocatalytic testing. The quasi-first-order-kinetics constant decreased only 13% after the fifth run. However, we would like to stress that the inactivation of (photo)catalyst depends on many factors, including the reaction, reactants, products and conditions; thus, the observed good stability of the sample in this model reaction might not necessarily translate to other photocatalytic reactions. Overall, all tested titanias showed activity under visible light—due to the narrowed band gaps—and a superior photocatalytic performance—due to the combined effect of the well-developed structure, high crystallinity, and narrow bandgap.

CONCLUSIONS We proposed, outlined, and tested a new strategy for the synthesis of porous crystalline doped titania materials— dubbed the modifier precursor strategy. The modified precursors are compounds prepared by reacting generic titania precursors, e.g., TIPO, with organic acids. The reaction hinges on the highly oxophilic nature of Ti atom that forces substitution of the alkoxide groups with the carboxylic moiety. This chemistry is ge-

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neric in nature and most organic acids can serve as the modifiers. The modification’s goal is to introduce “carbonizable” groups in the precursor’s structure. The modified precursor is then carbonized, which transforms it into a composite mixture of titania and carbon. The carbon framework then serves as a scaffold for TiO2 and supports the structure during crystallization. Afterwards, the carbon scaffold is removed through calcination, which creates/opens porosity and enlarges the surface area. The final titania has a well-developed structure and high crystallinity. Multiple titania materials were synthesized according to this strategy. Various organic acids were used as modifiers, including: formic, acetic, chloroacetic, oxalic, citric, benzoic, phthalic, isophthalic, and trimesic; in addition to the commercially-available titania salt TiO(acac)2 used initially as a model modified precursor. We showed that modification of TIPO can indeed produce “carbonizable” precursors. The carbon content ranged from < 1 wt% for oxalic acid to > 50 wt% for trimesic acid. In contrast, carbonization of TIPO using the same conditions did not produce any carbon. Overall, the amount of carbon depended on, and can be controlled by: 1) number of carbon atoms in the modifier, 2) number of introduced modifier groups, 3) functionality and crosslinking, and 4) chemistry of the modifier. This carbon framework indeed acted as a scaffold, supporting titania during its crystallization. The resulting titanias (after calcination) achieved specific surface areas reaching 100 m2 g–1 and total pore volumes exceeding 0.2 cm3 g–1, even after crystallization at temperatures from 500 °C to 1000 °C. The larger amounts of scaffold resulted in better protection and better structural parameters of the final materials. Moreover, the structural parameters could be controlled by the amount of carbon scaffold, especially specific surface area showed a very strong correlation with the amount of carbon scaffold. The final materials possessed high crystallinity and tunable phase composition. The phase composition depended on the temperature and time of the heat treatments, and ranged from almost exclusively anatase, through mixed anatase–rutile, to almost rutile. Both carbonization and calcination can be exploited to tailor the phase composition; however, carbonization is the primary, and calcination a secondary way to tune crystallinity. We believe that titania with any phase composition can be prepared through the modified precursor strategy by a careful control of the thermal treatments. Some prepared titanias, carbonized at temperatures > 800 °C, showed yellow color instead of white. We stipulate that the yellow color is due to a combined effect of 1) formation of mixed anatase–rutile phase and 2) carbon doping, each of which, based on the previously published reports, can decrease titania’s band gap by up to 0.4 eV. The diffuse reflectance UV–Vis absorption proved that these materials feature significant absorption in the visible region, with the absorption shoulders extending up to 575 nm. The calculated band gap values are in the range of 2.3–2.4 eV, showing a significant reduction as com-


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pared with the typical 3.2 eV for titania. In the photocatalytic degradation of a model pollutant, all tested titanias were active under visible light. The best photocatalyst reduced the concentration of methylene blue by 85 % in the first 90 min. Overall, all tested titanias showed an excellent photocatalytic performance—due to the combined effect of the well-developed structure, high crystallinity, and narrow bandgap. The modified precursor strategy was developed specifically to achieve titania with all features necessary for an ideal photocatalyst: high surface area, large pore volume, high crystallinity and narrow band gap. All of these goals have been achieved in this work. Still, further optimization is possible to achieve even better structural, crystalline, and optical properties, and to tune them for applications other than photocatalysis. Finally, we believe that this strategy, although shown here for titania precursors and materials, can be extended to other metal oxides, ceramics, and materials. The simplest example would be to take analogs of TiO(acac)2 such as numerous metal acetylacetonates and use them as precursors. In addition, most metals readily form organiccontaining compounds such as acetates or citrates, which again can be used as precursors. Finally, the last possibility would be to look for and exploit chemistries similar to that of titanium and carboxylic acids, which would give the most flexibility and the best results. Overall, we hope that future extensions of this strategy will help produce other important materials with desired properties.

ASSOCIATED CONTENT Supporting Information. Characterization of the samples prepared from TIPO modified with other common organic acids (table), thermogravimetric curves for the carbon– titania composites, Tauc plots used to calculate band gaps for the titania materials, UV–Vis absorption spectra of methylene blue as a function of time during the photocatalytic measurements, powder X-ray diffractograms for the carbon– titania composites, a plot of total pore volume as a function of the carbon scaffold amount, high-resolution XPS spectra of Ti 2p, O 1s and C 1s regions for titania materials, and plot of the loss of photocatalytic activity as a function of run number. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Corresponding Author: Mietek Jaroniec, Department of Chemistry and Biochemistry, Kent State University, Kent, OH 44242, USA; tel: +1 (330) 672-2032; fax: +1 (330) 672-3816; e-mail: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT The authors thank Professor John West, Dr. Jakub Kolacz, and Junren Wang for the help with the diffuse-reflectance

UV–Vis absorption measurements and Dr. Chilan Ngo for the help with the XPS data.

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Insert Table of Contents artwork here Modification Titanium isopropoxide

Titania preparation

Ti Ti

+ Ti

Organic acid Modified precursors

Carbon–titania composite

Porous crystalline doped titania


Synthesis of Porous Crystalline Doped Titania Photocatalysts Using

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