Hierarchically Porous, Biphasic, and C-Doped TiO2 for Solar Photocatalytic Degradation of Dyes and Selective Oxidation of Benzyl Alcohol.

Macroporous TiO2 monoliths were synthesized by self-sustained combustion reactions of molded pellets made up of a mixture of TiCl4 as a precursor, urea as a fuel, ammonium nitrate as an oxidizer, and starch as a binder. The porous TiO2 monoliths were found to be a heterostructure of anatase and rutile phases, in addition to being doped with carbon. Variation in the amount of starch yielded porous monoliths of different anatase-rutile ratios (increasing rutile component from 0 to 40%) but comparable Brunauer-Emmett-Teller (BET) surface area (∼30 m2 g-1). The porous monoliths obtained, where the TiCl4/starch mass ratio was 2.17, exhibit exceptional photocatalytic activity in the degradation of dyes (methylene blue and methyl orange) and selective oxidation of benzyl alcohol to benzaldehyde under natural sunlight. The synergistic combination of high surface area, porous network, lowered band gap due to heterostructured anatase-rutile polymorphs, and the presence of doped carbon renders the macroporous TiO2 an efficient photocatalyst.

Introduction

TiO2 photocatalysts[1] have been of interest, as they facilitate the photocatalytic degradation of organic pollutants,[2−6] selective organic transformations,[7−11] and hydrogen generation by photocatalytic water splitting.[12−14] Adsorbed radicals formed by photogenerated electron–hole pairs at the photocatalyst surface cause photocatalytic reactions.[15,16] TiO2 exists in three polymorphic forms—anatase, rutile, and brookite.[1,15,16] The anatase phase has been found to exhibit higher photocatalytic efficiency.[17,18] While anatase with a band gap of 3.2 eV is confined to absorption of UV light, rutile with a lower band gap (3.0 eV) can absorb visible light. However, low surface area, low redox potential, and faster electron–hole recombination render rutile ineffective.[19] There has been immense interest to modify TiO2 to enable it to absorb a wide range of wavelengths of solar energy,[1,15,16] including the visible region.[2,7,15,16,19] This has been achieved through various methods—metal[20,21] or nonmetal doping,[21,22] creating oxygen deficiency to form TiO2–,[23] dye sensitization[15,16] or making TiO2–quantum dots heterostructures.[15,16,24] Factors such as the concentration and nature of the dopant, crystallite size, surface area, and anatase–rutile ratio[2,4−6,14,19] control the photocatalytic efficiency of TiO2.

Commercially available TiO2 (Degussa P25) is a biphasic interfacial heterojunction of anatase–rutile (80:20).[6,19,25] Bickley et al.[25] were the first to propose a synergetic effect between anatase and rutile to be responsible for the relatively high photoreactivity of Degussa P25, in comparison to pure anatase or rutile.[25] This is because the close proximity of anatase and rutile polymorphs results in two key processes:[6,19,25−28] (1) the rutile phase with a lower band gap not only facilitates the production of charges in the visible light region but also transfers these charges to the conduction band of the anatase phase and eventually to the surface sites and (2) photogenerated holes from the valence band of anatase can be effectively transferred to that of rutile, thus leading to slow electron–hole recombination.[6,26] It would be ideal to improve the efficiency of materials similar to Degussa P25.[6,19] Major concerns in the preparation of compositions similar to Degussa are: (1) anatase–rutile mixtures are generally formed above 600 °C, yielding TiO2 samples that deviate from nanoregime, and hence reduce the surface area, leading to a decrease in photocatalytic efficiency[29] and (2) nitrogen doping stabilizes the anatase phase at higher temperatures, thus preventing the formation of a biphasic mixture.[30]

Anatase–rutile mixtures of TiO2 have been synthesized by calcination of the products obtained by solvothermal[5,17] and sol–gel[6,19] methods or through direct combustion.[29] Rutile–anatase branched heterostructures were prepared by a combination of electrospinning and the hydrothermal reaction.[31] Patterned anatase–rutile junctions have been formed by calcination of patterned TiO2 gel films.[32] Pulsed-pressure MOCVD has been used to fabricate anatase–rutile heterojunctions.[14] Porous films of TiO2 mixtures have been prepared through plasma electrolytic oxidation (PEO).[33] Heterogeneous nanostructures of anatase nanoparticles on rutile nanorods were synthesized through layer-by-layer, electrostatic deposition.[34] Though the existing synthesis routes yield anatase–rutile mixtures/heterojunctions, in most cases, multistep reactions are involved and only a few of them yield porous products. While it is important to find simpler scalable methods for anatase–rutile mixtures of suitable ratios, it would also be of interest to obtain these in the form of light-weight porous monoliths so that these could be used as floats[35] in water bodies for pollutant degradation applications. Our group has developed a self-sustained combustion synthesis of metal oxide foams using starch-based molded pellets.[36,37] For example, t-ZrO2 monolithic foams could be obtained by subjecting a pellet made up of ZrO(NO3)2, urea, and starch.[37] Adapting this method,[36,37] in this work, we have synthesized macroporous monoliths of anatase–rutile TiO2. The excess of carbon in the reaction mixture ensured carbon doping (C-doping) of TiO2. These monoliths are found to be quite efficient in natural sunlight photocatalysis.

Experimental Section

Synthesis of Porous TiO2 by the Combustion Method

Macroporous TiO2 was prepared by the combustion method using TiCl4 as a precursor, urea as a fuel, ammonium nitrate as an oxidizer and starch as a binder. Urea (0.011 mol, 0.7125 g) was mixed with 0.8435 g of ammonium nitrate (0.01 mol), starch, and 0.5 mL of TiCl4 (0.86 g, 0.004 mol). The resulting mixture was ground into a dough-like consistency that could be molded into pellets of the desired shape. The mass of starch was varied from 100 to 2000 mg. The pellet was placed in a preheated crucible (800 °C) in an electric Bunsen. Instantly, a vigorous reaction was observed with the formation of an oxide foam. The oxide foam was heated in air at 800 °C for 20 min to burn away the organic remnants.

Photocatalytic Degradation of Dyes

Macroporous TiO2 (10 mg) was dispersed in 100 mL of the dye solution (10 mg L–1). The solution was stirred in the dark for 1 h. After attaining the equilibrium, the solution was irradiated with natural sunlight. Aliquots were collected periodically, and the catalyst was removed by centrifugation. The dye concentration was monitored by measuring the absorbance of methylene blue (MB) at 664 nm and that of methyl orange (MO) at 464 nm.

Photocatalytic Selective Oxidation of Alcohol

Macroporous TiO2 (10 mg) and 10 μL of benzyl alcohol (0.1 mmol) were added to 1.5 mL of oxygen saturated benzotriflouride (BTF). The solution was stirred in the dark for 0.5 h. The solution was then transferred into a Pyrex glass filled with oxygen. The solution was irradiated with direct sunlight. After the reaction, the catalyst was removed by centrifugation. High-performance liquid chromatography (HPLC) was used to monitor the oxidation of the alcohol.

Characterization

The samples were characterized by X-ray diffraction (XRD) using a PANalytical X’pert pro diffractometer (Cu Kα radiation, secondary graphite monochromator, scanning rate of 1° 2θ/min). IR spectroscopic studies were carried out in a PerkinElmer FTIR spectrophotometer (spectrum two) in the range from 4000 to 550 cm–1 with a resolution of 4 cm–1. X-ray photoelectron spectra (XPS) of the samples were recorded using a Kratos axis Ultra DLD. Scanning electron microscopy (SEM) images were recorded using a Zeiss, Ultra 55 field emission scanning electron microscope. A PerkinElmer LS 35 spectrometer was used to record the UV–visible spectra. The catalytic oxidation of benzyl alcohol was monitored by HPLC (Jasco) using a C18 column and a UV detector at 253 nm. A mixture of water and acetonitrile in a 70:30 volume ratio and 0.2 M phosphoric acid was used as the mobile phase. The mobile phase flow rate was maintained at 0.8 mL min–1. The nitrogen sorption analysis was performed in a BELsorp mini-II instrument at liquid nitrogen temperature. The surface area of the material was determined by employing the Brunauer–Emmett–Teller equation. The pore sizes and pore volumes of the materials were obtained by the Barrett–Joyner–Halenda (BJH) method.

Results and Discussion

The processes involved in the formation of porous TiO2 monoliths by the starch pellet combustion method are schematically depicted in Figure . Once the combustion is initiated, the pellet catches fire and grows into a voluminous cylindrical foamy product, with its radius comparable to the radius of the initial pellet, within a few seconds. Partial burning of starch leaves behind a lot of carbonaceous impurities at this stage. Further heating in air burns away these impurities leaving behind a porous monolith of TiO2.

Figure 1

Schematic of the processes involved in the formation of porous C-doped TiO2.

Catalyst Characterization

The XRD patterns of macroporous TiO2 obtained by combustion synthesis with varying amounts of starch are shown in Figure . All of the peaks could be assigned to the anatase phase in the case of TiO2 prepared in the presence of 100 mg of starch (Figure a). Peaks due to the rutile phase of TiO2 are observed on increasing the mass of starch to 250 mg and above (Figure b–g). With an increase in the mass of starch used in the synthesis (100–2000 mg), the percentage of the rutile phase also increases (0–40%) gradually. Though the reaction has been carried out at 800 °C (the actual local temperature in the reaction mixture would be even higher due to the exothermic combustion reaction), the stable anatase phase is observed in contrast to studies that indicate stabilization of the rutile phase at temperatures above 600 °C. Even with 2 g of starch (TiCl4/starch ratio of 0.38), the anatase phase is largely stabilized with only 40% of the rutile phase being present in the product. (Figure , right hand side panel).

Figure 2

XRD patterns of porous TiO2 synthesized using (a) 100, (b) 250, (c) 350, (d) 500, (e) 750, (f) 1000, and (g) 2000 mg of starch. Peaks indicated as (*) are due to the anatase phase coexisting with the rutile phase. The expanded region (20–30°) is shown in the right hand side panel. The percentages of the anatase and rutile phases were obtained from the relative intensities of the 101 reflection of anatase and 110 reflection of rutile, respectively.

The Spurr equation FR = 1/{1+0.8[IA(101)/IR(110)]} was employed for the precise calculation of the amount of rutile in the sample,[19] where FR is the mass fraction of rutile and IA (101) and IR (110) are the integrated main peak intensities of anatase and rutile, respectively. The rutile content increases with an increase in the amount of starch used (Figure S1, Supporting Information), suggesting that an increase in starch content leads to the destabilization of the anatase phase. Thus, the starch-based combustion method allows one to easily synthesize an array of TiO2 materials with different anatase–rutile ratios.

The chemical composition of TiO2 was further probed using X-ray photoelectron spectroscopy (XPS). The Ti 2p spectrum (Figure a) shows peaks (458.77 and 464. 47eV) due to Ti4+. The core-level C 1s spectrum (Figure b) exhibits peaks at 284.95, 286.47, and 288.86 eV that are ascribed to adventitious carbon from the internal standard, −C–O, and −C=O, respectively. No peaks due to Ti–C (281 eV) were observed, indicating that carbon is not in the substitutional lattice position.[29,38] These features suggest that carbon could be in the interstitial position of the TiO2 lattice or as carbonate species at the surface.[39,40] The infrared spectrum (Figure S2, Supporting Information) further corroborates the existence of −C–O species along with strong −O–H stretching and bending absorptions, indicating the interaction of moisture with surface carboxylate species.[29,38,39,41] The N 1s spectrum (not shown) indicates the absence of nitrogen in the sample. These observations are in accordance with the literature for C-doped TiO2.[29,38,39]

Figure 3

XPS spectra showing (a) Ti 2p and (b) C 1s core-level peak regions. (c) UV–visible reflectance spectrum of porous TiO2 (synthesized using 350 mg of starch).

The UV–visible absorbance spectrum (Figure c) of porous TiO2 (350 mg of starch) exhibits a broad absorption range in the visible region with a band gap of 2.51 eV calculated from the Tauc plot (the inset of Figure c). In comparison to theoretical 3.00 and 3.20 eV for rutile and anatase, respectively, narrowing of the band gap in porous TiO2 is due to anatase–rutile heterojunctions in addition to contributions of dopants and defects as indicated by XPS studies.[19,38]

The SEM images (Figure a,b) of TiO2 (350 mg of starch) show an irregularly shaped, coral-like, highly porous network with macropores of diameters of ∼1 μm. The porous nature is further corroborated by the BET surface area measurements (Figure e,f). The surface area is measured to be 33.4 m2 g–1 with a pore volume of 0.12 cm3 g–1 and a mean pore diameter of 14.6 nm, suggesting that TiO2 obtained using 350 mg of starch exhibits macro- and mesopores. In comparison, the SEM image of TiO2 (2000 mg of starch) comprises sheets (Figure c,d) that are in μm size laterally without any visible macropores. The surface area is measured to be 28.5 m2 g–1, with a pore volume of 0.009 cm3 g–1 and a mean pore diameter of 14.0 nm, indicating that the material is mesoporous.

Figure 4

Bright-field SEM images of porous TiO2 prepared using 350 mg of starch (a, b) and 2000 mg of starch (c, d). Nitrogen adsorption–desorption isotherms of porous TiO2 prepared using 350 mg of starch (e) and 2000 mg of starch (f). The insets in (e, f) are the corresponding BJH plots.

Mechanism of Formation of Anatase–Rutile Biphasic Porous TiO2

The anatase–rutile transformation is reconstructive, wherein the transformation involves the breaking and reforming of bonds.[42] As suggested in the previous studies,[36,37] the amount of starch and ΔG°(f) of the metal oxide are expected to control the porosity of the material. The amount of gases evolved depends on the mass of starch used. The energy released (ΔG°) during the reaction decides the local temperature. All of the reactions, using different amounts of starch, have been carried out under similar heating conditions. During combustion, the internal temperature is expected to increase when a higher mass of starch is being burnt, thus resulting in anatase–rutile transformation. However, experimental observations suggest that the internal temperature decreases with increasing amounts of starch.[37] With a higher amount of starch, burning of starch through self-sustained combustion is slow, resulting in the lowering of the overall temperature. The reason for lower rutile content in the products seems to be purely kinetic. The total reaction time is ∼20 min, which is insufficient to convert the initially formed anatase into rutile. The increased rutile content with an increase in starch may be attributed to the reductive atmosphere created by starch. During combustion, starch is dehydrated to give carbon, which being reductive in nature results in oxygen vacancies, leading to an increased rate of transformation of anatase to rutile.[42]

As the ΔG°(f) of TiO2 is the same for reactions using 350 and 2000 mg of starch, the surface area and porosity solely depend on the nature of combustion of starch. With lower amounts of starch, the temperature is sufficient to cause high rates of the reaction, resulting in the local pressure build up and coral-like combusted products. As the starch content increases, the reaction proceeds at a lower rate, analogous to the smoldering of a cigarette stick.[37] This is conducive to the formation of anisotropic products, as starch–sponge template could burn rapidly, uniformly leading to interconnected thin, porous sheets.

Photocatalytic Performance of Anatase–Rutile Biphasic Porous TiO2 in Dye Degradation

The photocatalytic efficiency of porous TiO2 prepared with varying amounts of starch was examined by monitoring the degradation of methylene blue under solar irradiation. The photocatalytic activity of porous TiO2 summarized in Table suggests that TiO2 prepared with lower starch content degrade the dye with minimum adsorption in comparison to TiO2 prepared using a higher amount of starch. The UV–visible spectra of the dye extracted from the catalytic sample indicate that the reaction in the dark is adsorption and not degradation. The increased adsorption with the use of increased amounts of starch could be attributed to the increased surface −O–H and −C–O as the starch content increases, though the surface areas of all of the samples are comparable.

Table 1

Summary of the Solar Photocatalytic Degradation of Methylene Blue (100 mL, 10 ppm) using 10 mg of Porous TiO2 Synthesized using Varying Amounts of Starch

	% composition		MB degradation
starch (mg)	anatase	rutile	adsorption (%)	degradation (%)	time (min)
100	100	0	0	0	120
250	96	4	4	80	120
350	85	15	6	100	80
500	82	18	12	100	110
750	70	30	19	100	120
1000	67	33	30	100	120
2000	60	40	42	100	120

The photocatalytic activity of porous TiO2 (350 mg of starch) in the degradation of MB and MO was evaluated under direct solar light. In all of the cases, prior to photocatalytic degradation, adsorption–desorption equilibrium in the dark indicates negligible or low adsorption. Figure a,b represents the time-resolved UV–visible absorption spectra of photocatalytic degradation of MB and MO, respectively. In both cases, the intensity of the prominent absorption decreases with time. The log (absorbance) versus time plots (insets of Figure a,b) of MB and MO degradation indicate pseudo-first-order kinetics with rate constants of 0.034 and 0.0511 min–1, respectively. Over four degradation cycles (the inset of Figure c), porous TiO2 (prepared with 350 mg of starch) exhibits consistent catalytic activity. Adsorbed water and oxygen on the surface of TiO2 produce reactive •OH and •O2– in the presence of sunlight. Degradation takes place only in the presence of a catalyst and sunlight (Table S1, Supporting Information) due to the increased concentration of net oxidizing species in solution. Nonlinear dependency on the amount of the catalyst appears to be exponential with saturation beginning at 10 mg of the catalyst (Figure c).

Figure 5

Degradation of dyes (10 mg L–1) was traced through UV–visible absorption spectra of reaction mixtures containing 10 mg of porous TiO2 heterojunctions as a photocatalyst. Evolution of absorption spectra with time in the case of (a) MB and (b) MO, and the insets correspond to log (absorbance) versus time plots. Variation of the photocatalytic MB (100 mL, 10 mg L–1) degradation efficiency with (c) mass of porous TiO2 heterojunctions as a catalyst and (d) pH. Catalytic efficiency over repeated cycles in the case of MB degradation is shown as the inset in (c).

One of the requisites of an ideal catalyst is its ability to catalyze the degradation of organics under all pH conditions. The effect of pH on the degradation of MB (Figure d) clearly indicates that porous TiO2 effectively catalysize dye degradation in a wide pH range (1–11) and more efficiently in basic pH due to the higher concentration of intermediate radicals.[6,41]

Photocatalytic Performance of Anatase–Rutile Biphasic Porous TiO2 in Selective Oxidation of Benzyl Alcohol

The photocatalytic oxidation of many organic molecules by optically excited TiO2 is thermodynamically feasible at room temperature in the presence of oxygen.[11] The photogenerated holes with an oxidation potential of 3.0 V renders TiO2 with considerable oxidizing capability.[11,43] Benzaldehyde is a widely used raw material in pharmaceutical industries. Benzaldehyde is synthesized through selective oxidation of benzyl alcohol using liquid-phase chlorination, a toxic and corrosive process.[11,44] It is of importance to develop alternative chlorine-free routes to produce benzaldehyde.[44,45] In the presence of noble metal[46] or transition metal complexes[47] as catalysts, molecular oxygen has emerged as a primary oxidant in the oxidation of alcohols to carbonyl compounds.[11,44−46] Photocatalytic selective organic transformations,[44−47] utilizing renewable solar energy, have garnered interest as a greener and efficient route.[44−46] Environmentally benign, economically viable, and naturally abundant TiO2 has been explored as a potential photocatalyst in selective oxidation of benzyl alcohol.[11,43−47]

Benzyl alcohol interacts with the surface hydroxyl groups of the photoexcited porous TiO2. The photogenerated holes in TiO2 abstract the protons from benzyl alcohol, while the alcohol loses an electron and gets oxidized to aldehyde. The longer surface adsorption of the reactant (alcohol) or the product (aldehyde) not only leads to a decrease in the photocatalytic activity of TiO2 but also leads to further oxidation of aldehyde to acid, thus minimizing the selectivity of the oxidation process.

The porous TiO2 (prepared using 350 mg of starch) heterostructure exhibits excellent photocatalytic activity toward selective oxidation of benzyl alcohol to benzaldehyde. The HPLC chromatograms of the aliquots of the reaction mixture taken at different time intervals (Figure A) show a gradual decrease in the concentration of alcohol and a corresponding increase in the concentration of aldehyde with the concentration of the acid being zero at all times, indicating 100% selectivity. A conversion of 80% with 100% selectivity was achieved in 60 min (Figure B). The conversion and selectivity are defined as followsCo is the initial concentration of benzyl alcohol, Calcohol and Caldehyde are the concentrations of benzyl alcohol and benzyaldehyde, respectively, at a given reaction time. However, beyond 60 min, the progress of the reaction was quite slow and a conversion percentage of 87 was observed after 120 min. The catalyst could be recovered and reused to get similar results over four cycles.

Figure 6

(A) HPLC chromatograms of (a) standard equimolar solution of benzyl alcohol, benzoic acid, and benzaldehyde, and solutions obtained by photooxidation of benzyl alcohol catalyzed by porous TiO2 (prepared using 350 mg of starch) in the presence of oxygen saturated benzotriflouride (BTF) at (b) 1st, (c) 30th, and (d) 60th min, respectively. (B) Plot of the percentage conversion of benzyl alcohol with respect to time.

Reasons for the Enhanced Photocatalytic Performance of Anatase–Rutile Biphasic Porous TiO2

Adsorbed water and oxygen on the surface of porous TiO2 monoliths produce reactive •OH and •O2– in the presence of sunlight. These radicals cause degradation of the dye. The porous monolith architecture of TiO2 facilitates the formation of more heterojunctions, thereby increasing the activity. With the conduction band edge of rutile ∼0.2 eV lower than that of anatase, the photoexcited electrons are effectively transferred from the conduction band of rutile to that of anatase at the interface between anatase and rutile.[6,26] While this promotes photoreduction at the anatase site (Figure ), photooxidation takes place either on the anatase or rutile surface.

Figure 7

Schematic of the processes involved in the sunlight photocatalytic activity of porous biphasic C-doped TiO2 monoliths.

The photocatalytic performance of all of the porous TiO2 monoliths is higher than that of commercial Degussa P25. This is attributed to the following reasons: (1) a porous network provides channels for faster reactant diffusion and more active catalytic sites, (2) anatase–rutile heterojunctions aid in electron–hole separation and a wider range of solar energy absorption, and (3) doped carbon not only contributes to the lowering of the absorption band gap of TiO2 but also renders the TiO2 surface hydrophilic due to increased interaction with moisture through hydrogen bonding. The heterojunctions obtained using 350 mg of starch exhibit higher photocatalytic activity in comparison to all other TiO2 monoliths (Table ). The highest activity could be as a result of optimum anatase–rutile composition (85:15) and the presence of interstitial carbon as −C–O that aids in better wettability of the catalyst and thus faster interaction and degradation of organics.[6,38,41] TiO2 prepared using 100 and 200 mg of starch exhibit lower photoactivity (Table ) due to the low percentage of anatase–rutile heterojunctions (Figure ). On the other hand, increasing the percentage of anatase–rutile heterojunctions and carbon doping using higher starch content (Table ) also does not improve the performance of porous TiO2. Further, the surface areas of TiO2 prepared using 350 and 2000 mg of starch are comparable. Therefore, the increased solar photocatalytic activity for the heterojunctions can be correlated to the combined effect of the porous network, band gap narrowing due anatase–rutile heterojunctions, and carbon doping. Porous TiO2 prepared using 350 mg of starch shows better performance in dye degradation under solar light compared to the already known TiO2-based catalysts (Table ). In fact, its catalytic activity under sunlight is better than some of the catalysts under UV irradiation. For selective oxidation of benzaldehyde, our catalyst under sunlight performs better than known catalysts under UV light. The enhanced photocatalytic activity under ambient conditions in natural sunlight renders porous TiO2 monoliths a superior catalyst and a desired material for environmental amelioration.

Table 2

Comparison of Photocatalytic Activities of Porous TiO2 (350 mg of Starch) with other TiO2 Catalysts Reported in the Literature

				degradation
catalysta	BET surface area (m2 g–1)	mass of the catalyst (mg)	pollutant conc. in ppm and (volume)	irradiation source	time (min)	%	ref
Methylene Blue Degradation
porous TiO2(A/R = 85:15)	33.37	10	10 (100 mL)	sunlight	80	100	present work
		10	100 (100 mL)	sunlight	150	43
		10	500 (100 mL)	sunlight	150	25
		10	1000 (100 mL)	sunlight	150	12
TiO2 nanoflower (A/R = 99:1)	36.9	30	12.79 (150 mL)	UV light	30	100	(17)
TiO2 A/B nanoparticles	183.6	60	10 (600 mL)	UV light	180	97	(48)
TiO2 rods (A = 100%)	40.5	30	12.79 (150 mL)	UV light	40	80	(49)
mesoporous TiO2 (A/B = 80:20)	186	120	32 (200 mL)	UV light	70	100	(50)
TiO2 anatase/rutile nanoflowers	106.29	10	15 (20 mL)	350 W xenon lamp	120	72	(51)
graphene-wrapped anatase–rutile hierarchical TiO2 nanoflower	33.2	30	500 (40 mL)	500 W Hg lamp	30	100	(52)
porous TiO2 (A/R = 55:45)	−	−	1.28 (30 mL)	UV light	100	100	(33)
TiO2–xNx heterojunction (A/R = 90:10)	79.6	60	3 (50 mL)	solar simulator	180	90	(19)
TiO2 (A = 100) hierarchical mesoporous	159	1.6	10 (8 mL)	UV light	180	90	(53)
nanocrystalline TiO2(A = 100)	246	100	100 (100 mL)	sunlight	225	100	(38)
TiO2 (A = 100)	29	20	10 (40 mL)	UV light	180	100	(18)
TiO2 nanofilms (A = 100%)	−	100	4 (100 mL)	UV light	50	67	(54)
Methyl Orange Degradation
porous TiO2(A/R = 85:15)	33.37	10	10 (100 mL)	sunlight	105	91	present work
core–shell rutile–anatase TiO2	−	20	10 (150 mL)	UV light	150	100	(55)
porous TiO2–Pt–rGO	61.7	20	10 (20 mL)	300 W xenon lamp	120	80	(56)
TiO2 nanoflower (A/R = 82:18)	165	300	10 (300 mL)	500 W xenon lamp	60	98	(57)

A = anatase; R = rutile; and B = brookite.

Conclusions

Solar light active macroporous C-doped TiO2 heterojunction photocatalysts were prepared through simple, single-step, self-sustained combustion reactions. Porous monoliths of different anatase–rutile ratios (increasing rutile component from 0 to 40%) were obtained by varying the amount of starch. Monoliths with an anatase–rutile ratio of 85:15 exhibit exceptional photocatalytic activity in the degradation of dyes (methylene blue and methyl orange) and selective oxidation of benzyl alcohol to benzaldehyde under natural sunlight. The synthesis route could be used as a general strategy to synthesize economically viable TiO2 monoliths with multiple features for efficient natural sunlight photocatalytic applications such as H2 production and CO2 conversion.