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Graphene Family Nanomaterials (GFN)-TiO2: Comparison
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Please note this is a comparison between Version 2 by Jessie Wu and Version 1 by Wei-Hsiang Chen.

In tThis review,e points of view on the intrinsic properties of TiO2, GFNs (pristine graphene, graphene oxide (GO), reduced GO, and graphene quantum dots (GQDs)), and GFN-TiO2 are presented. This review also explains practical synthesis techniques along with perspective characteristics of these TiO2- and/or graphene-based materials. The enhancement of the photocatalytic activity by using GFN-TiO2 and its improved photocatalytic reactions for the treatment of organic, inorganic, and biological pollutants in water and air phases are reported. It is expected that this review can provide insights into the key to optimizing the photocatalytic activity of GFN-TiO2 and possible directions for future development in these fields.

  • synthesis
  • graphene family nanomaterials (GFN)

1. GFN-TiO2

1.1. Synthesis

Many materials and methods can be used to synthesize TiO2-containing composites. It has been reported that these composites can be produced in many different forms, such as nanoparticles [121,122,123,124][1][2][3][4], nanofibers [125[5][6][7],126,127], and nanosheets [128,129][8][9]. The forms affect the physicochemical properties of these composites, such as the specific surface areas, influencing their photocatalytic activities. For example, the synthesis of TiO2-containing nanowires [130,131,132,133[10][11][12][13][14],134], nanorods [135,136[15][16][17][18],137,138], and nanotubes [139,140,141,142][19][20][21][22] with high specific areas that are associated with their improved efficiencies have been revealed. Besides the forms of the catalysts, the materials added in the synthesis of composites are another key. Among various materials, including carbonaceous materials and metal oxides, that are commonly used to enhance their photocatalytic performance [143[23][24],144], GFNs have aroused substantial attention recently due to their unique characteristics described above.  These methods include ion implantation, sintering at high temperatures, plasma processes, the hydrothermal method, the sol-gel method, hydrolysis, chemical modification, and low-temperature carbonization [32][25]. The hydrothermal method is the most frequently used method, given the advantages comprising the adjustable crystal form, GFN content, and variable reduction level of an rGO-TiO2 [145][26]. This method is known to avoid the high-temperature destruction of carbonaceous structures and successfully preserve stable and complete crystal forms.

1.2. Characterization

Different approaches have been used to study the different surface characteristics and chemical structures of GFN-TiO2 (Table 1). Scanning electron microscopy (SEM) [151[27][28][29][30][31],163,164,165,166], transmission electron microscopy (TEM) [163[28][30][31],165,166], and atomic force microscopy (AFM) [164][29] are typically used for morphological observation. The results indicated that GFN was well embedded or covered by TiO2. The composites with lower GFN ratios tended to aggregate, forming large spherical-shaped particles [151,163,164,165,166][27][28][29][30][31]. Adding graphene increased and then decreased the crystallite size of composites. The initial augmentation was caused by accelerating the crystallization of TiO2. Excess H2O by the dispersion of graphene promoted the hydrolysis of titanium isopropoxide. Continuously increasing the graphene content enhanced incorporation between the nucleation centers, delaying crystallization and decreasing the crystallite size [151,163,164,165,166][27][28][29][30][31]. Composites could exhibit non-spherical structures, such as platelet- or flower-like morphology with elevated GFN ratios [151,163,164,165,166,167][27][28][29][30][31][32]. The TEM studies indicated that GFN-TiO2 exhibited two-dimensional structures [163,165,166,167][28][30][31][32]. An AFM study showed a significant increase in the thickness when excess graphene was added during composite preparation [164][29].
Table 1. Methods and outcomes of characterization of TiO2-graphene composites.
Category Technology Description Ref.
Morphology
Table 3. Removal of water-phase pollutants by GFN-TiO2 in selected studies.
  Pollutant Catalyst
SEM Spherical and non-spherical (platelet- or flower-like) shapes were observed with low and high GFN contents, respectively. [151,163,164,165,166,167][27][28][29][30][31][32]
TEM A fine dispersion of TiO2 in GFN with low- and nano-dimensions was reported. [163,165,166,167][28][30][31][32]
AFM The thickness of GFN-TiO2 was increased to a scale of μm after preparation. [164][29]
Chemical constitution FTIR The peak of Ti-O-Ti at 400–900 cm−1 was broadened or shifted by the influence of Ti-O-C. The signals of carbonyl and epoxy groups were reduced. [151,165,168][27][30][33]
XPS The formation of C-Ti, O=C-O-Ti, and C-O-Ti bonds in GFN-TiO2 was observed. [163][28] [164][29]
XRD The signals due to the presence of anatase and rutile were reported. [151,27163,][28164,][29165,]166,168][[30][31][33]
Raman The signals of both TiO2 and GFN were reported. The D/G intensity ratio of GFN-TiO2 was higher than that of GFN. [163,164,165][28][29][30]
EPR The formation of hydroxyl and superoxide radical species was observed in GFN-TiO2. [166][31]
Physicochemical properties Zeta potential The zeta potential of GFN-TiO2 ranged between those of GFN and TiO2. [164][29]
TGA The irregular mass loss occurred at high temperatures. [164][29]
BET The surface area of GFN-TiO2 was significantly increased at a certain ratio of GFN to TiO2. [151,163,164,165,168][27][28][29][30][33]
ACM The current density of GFN-TiO2 was significantly increased at a certain ratio of GFN to TiO. [168][33]
PL The time dynamics of the TiO2-induced photoreduction of GO were observed. [169][34]
UV-Vis A shift to larger wavelengths in the absorption edge was observed, indicating bandgap narrowing. [151,164,165,166,168][27][29][30][31][33]
The chemical constitutions of GFN-TiO2 were investigated by using Fourier transform infrared spectrometry (FTIR) [151,165[27][30][33],168], X-ray photoelectron spectroscopy (XPS) [163,164][28][29], X-ray diffraction (XRD) [151,163,164[27][28][29][30][31][33],165,166,168], Raman spectrometry [163[28][29][30],164,165], and electron paramagnetic resonance (EPR) [166][31]. The FTIR results showed that the peak at 400–900 cm−1 was broadened or shifted due to the presence of Ti-O-C in the Ti-O-Ti adsorption peak. The original peaks of carbonyl (C=O, 1700 cm−1) and epoxy (C-O, 1230 cm−1) groups of GO became negligible in the results of GFN-TiO2 [151,165,168][27][30][33]. The XPS studies observed the bands of 463.2 and 458.5 eV in GFN-TiO2, indicating a chemical state of Ti4+ (TiO2) in GFN-TiO2 [163,164][28][29]. The identification of the peaks associated with Ti and GFN indicated the presence of Ti-C, O=C-O-Ti, and C-O-Ti in TiO2-GFNs, as the C1s spectrum showed peaks attributed to C=C/C-C, epoxy (C-O)/hydroxyl (C-OH), and carboxyl groups (C(=O)OH). [163,164][28][29]. The XRD studies have revealed the peak areas of anatase (25.3°) and a few rutile phases (27.4°), indicating TiO2 was well mixed with GFN with limited phase changes [151,163,164,165,166,168][27][28][29][30][31][33]. The Raman spectra of GFN-TiO2 exhibited bands of Eg(1) (149 cm−1), B1g(1) (395 cm−1), A1g+B1g(2) (517 cm−1), and Eg(2) (640 cm−1), attributable to the symmetric stretching and symmetric/asymmetric bending vibrations of the O-Ti-O group. The spectra also exhibited D (1384 cm−1) and G bands (1596 cm−1) of GFN, as the D/G intensity ratio was higher than that of GFN [163,164,165][28][29][30]. The EPR study showed increasing intensities of the hydroxyl and superoxide radicals by increasing the ratio of GFN to TiO2 [166][31].
Physicochemical properties including the surface charge, thermal stability, surface area, pore size, and pore volume of TiO2-GFN have been investigated by Zeta potential analysis [164[29][32],167], thermal gravity analysis (TGA) [164][29], and Brunauer–Emmett–Teller (BET) analysis [151[27][28][29][30][33],163,164,165,168], respectively. The nucleation of TiO2 on GFN masked the functional groups on the surface and lowered the zeta potential of GFN-TiO2 [164,167][29][32]. The TGA study showed a better heat resistance of GFN-TiO2, as TiO2 stabilized GO by the interaction between oxygen-containing groups of GFN and TiO2 [164][29]. Most studies have indicated a higher surface area of GFN-TiO2 compared to that of TiO2 [151,163,164,165[27][28][29][30][33],168], whereas an opposite trend has also been reported in a few studies [151,163,164,165,168][27][28][29][30][33]. GFN-TiO2 typically exhibited mesopore size distribution with averages near 10 nm [151,163,164,165,168][27][28][29][30][33].
Potentiostat, photoluminescence (PL), and ultraviolet-visible spectroscopy (UV-Vis) are useful tools to investigate the optical characteristics of GFN-TiO2 [151,164,165,166,168][27][29][30][31][33]. A study has reported that an optimal ratio of GFN to TiO2 increased the current density of GFN-TiO2, because the two-dimensional conjugation structure of GFN accepted and transported the excited electron from TiO2 [168][33]. Pallotti et al. used photoluminescence (PL) spectroscopy for real-time analysis to trace the time dynamics of the photoreduction of GO [169][34]. It was found in real-time that the photocatalysis induced by the presence of TiO2 contributed to GO photoreduction. By adding GFN into TiO2, the absorption edge of GFN-TiO2 displayed an increase in wavelength (known as redshift) that indicated a bandgap narrowing. Its light absorption intensity in the UV region was also increased [151,164,165,166,168][27][29][30][31][33]. Table 2 lists some examples of TiO2-GFN prepared for photocatalysis and battery storage.
Table 2. Properties of TiO2-GFN prepared for photocatalysis and battery storage in various studies.
Materials Average Size (nm) Functional Group Bandgap (eV) Wavelength (nm) Surface Area (m2/g) Reference
Light Source Removal Ref.
Graphene-TiO2 3.8 C-O, C=O, O=C-O, and O-Ti NA 1 600 176 [170][35]
Graphene-TiO2 ~6 C-O and O-C=O
Inorganic Cr(VI) (0.2 mM) GO-TiO2 (0.5 g/L) 254 nm, 20 W, UV lamp 90% [164][29]
NA NA 252 [158
Cr(VI)(10 mg/L) GO-TiO2 (0.5 g/L)] 365 nm, 8 W, UV lamp 99% [174][41][36] 2
GO-TiO2 NA C-O, Ti-O-Ti, Ti-O-C, and OH NA ~800 69.2 [151][27]
Organic Methylene blue (0.01 g/L) Graphene-TiO2 (0.75 g/L) 365 nm, 100 W, high-pressure Hg lamp

>400 nm, 500W, Xe lamp		 85%

65%		 [175][42] GO-Co-TiO2 NA C-O, C-N, O-C=O 2.77 421 206
Rhodamine B (20 mg/L) Graphene-TiO2 (0.1 g/L)[109][37]
11 W, low-pressure Hg lamp 91% [176][43] GO-Ti NA
Rhodamine B (20 mg/L)

Norfloxacin (20 mg/L)		NA

Aldicarb (10.5 mg/L)		 Graphene-TiO2 (1 g/L) >400 nm, Xe lamp2.9 ~550 68.9 79.7%

86.2%

36.8%		 [170][35][171][38]
rGO-TiO2 35 NA NA ~360 212.75
Malachite green oxalate (13.1 mg/L) GO-TiO2 (0.2 g/L)[ 450 W, water-cooled Hg lamp 80% [145][26]172][39]
rGO-TiO2 ~8 NA NA NA 229 [157][40] 2
1 NA denotes not available. 2 The materials were prepared for battery storage.

2. Photocatalytic Removal of Pollutants

2.1. Water-Phase Pollutants

GFN-TiO2 has been used for the photocatalytic removal of inorganic, organic, and biological pollutions in the water phase (Table 3). The photocatalytic reduction of inorganic pollutants such as metal ions was one example. Jiang et al. investigated the reduction of Cr(VI) to Cr(III) in water by using GFN-TiO2 [164][29]. The reduction rate constant was 0.0691 min−1, exceeding that of using pure TiO2 (0.0174 min−1) by a factor of 3.9. In another Cr(VI) removal study, the Cr(VI) concentration was adsorbed (~55%) by using TiO2-GO for 1 h, and with UV irradiation, nearly all Cr(VI) concentration was reduced in 7 h [174][41]. In the same system using TiO2 with UV irradiation, the Cr(VI) concentrations were limitedly adsorbed (23%) and reduced (30%).
Phenol (10 mg/L)
rGO-TiO
2
(5 g/L)
310-400 nm, UV lamp Not given [177][44]
2,4-D (15 mM) rGO-TiO2 (film) <320 nm, 450 W, Xe lamp ~87% [178][45]
Biological E. coli (106 CFU/mL), F. solani spores (103 CFU/mL) rGO-TiO2 (0.5 g/L) Sunlight ~100% [179][46]
E. coli, S.aureus, S.typhi, P. aeruginosa, B. subtilis, B. pumilus (106 CFU/mL) Graphene-Ag3PO4-TiO2 >420 nm, 350 W, Xe lamp ~100% [180][47]
E. coli (105–106 CFU/mL) GO-TiO2 (0.2 g/L) Xe lamp ~100% [181][48]
E. coli (106 CFU/mL) rGO-TiO2 (18 mg/L) >285 nm, UV-visible light; >420 nm, visible light ~100% [182][49]
Graphene-TiO2 has been frequently investigated for its potentials for photocatalytic degradation of organic pollutants. Homolytic cleavage is typically the first chemical step in photodegradation. Free radicals are formed in this step and rapidly react with any oxygen present in the system. Li et al. investigated the photocatalytic activity of graphene-TiO2 towards representative aqueous persistent organic pollutants (POPs) [170][35]. The POPs included rhodamine B, norfloxacin, and aldicarb. The presence of graphene-TiO2 significantly enhanced the removal of these POPs. While the compound concentrations were negligibly changed during pure photolysis, the presence of GFN-TiO2 (0.86% w/w of graphene) resulted in 79.7% and 86.2% of total organic carbon (TOC) removal in the experiments of rhodamine B and norfloxacin, respectively, after 10 h of simulated sunlight irradiation (λ > 320 nm). Only 36.8% of TOC removal was observed in the aldicarb experiment after 25 h of visible light irradiation (λ > 400 nm). Zhang et al. investigated photodegradation of methylene blue by using TiO2, carbon nanotube (CNT)-TiO2, and graphene-TiO2 as photocatalysts [175][42]. In 1 h of UV irradiation, the removal efficiency of graphene-TiO2 (85%) was significantly higher than TiO2 (25%) and CNT-TiO2 (71%). Using visible light reduced the performance of TiO2 by a factor of 2, whereas the removal efficiency of graphene-TiO2 (65%) was less affected.
GO represents another material that can work well with TiO2, forming an efficient photocatalyst. Perera et al. compared the photodegradation of malachite green by using TiO2, GO, and GO-TiO2 [145][26]. Pseudo-first-order reactions were found when TiO2 and GO-TiO2 were used as catalysts. The rate constant of GO-TiO2 (0.0674min−1) exceeded that of TiO2 (0.0281 min−1) by a factor of 3. No photodegradation of malachite green occurred in the presence of GO. Another study investigated the photodegradation of rhodamine B by using three different nanosphere catalysts (amine-modified TiO2–SiO2, graphene-TiO2, and GO-TiO2–SiO2) [176][43]. In 1.5 h of irradiation, the removal efficiencies of graphene-TiO2 (91%) and GO-TiO2–SiO2 (71%) were significantly higher than that of amine-modified TiO2–SiO2 (65%), indicating the synergistic effect between graphene or GO and TiO2 for the enhanced catalysis activity.
The use of rGO-TiO2 for the enhanced photocatalytic degradation of organic pollutants has also been demonstrated. Increasing the rGO content (from 0 to 1% w/w) in rGO-TiO2 enhanced the photocatalytic decomposition of phenol (the 1st-order rate constant was increased from 0.0039 to 0.0048 min−1) [177][44]. rGO-TiO2 exhibited fine photocatalytic performance after 5 cycles; however, a high rGO content (e.g., 5% w/w) potentially shielded the catalyst surface from light absorption, reducing the photocatalytic activity. Ng et al. investigated the removal of 2,4-dichlorophenolyxacetic acid (2,4-D), a commonly used herbicide, by photocatalytic reduction using TiO2 and rGO-TiO2 [178][45]. The pseudo-first-order rate constants of using TiO2 and rGO-TiO2 were 0.002 and 0.008 min−1, respectively. Adding rGO increased the response of the photocurrent by a factor of 2 and the availability of 2,4-D on the surface of rGO-TiO2, improving the whole photocatalytic reaction by a factor of 4.
Photocatalysis is capable of being adopted for use in many applications for disinfection in water matrices. Adding graphene in Ag3PO4-TiO2 effectively improved the synergistic photocatalytic disinfection of E. coli, S.aureus, S.typhi, P. aeruginosa, B. subtilis, and B. pumilus [180][47]. Fernández-Ibáñez et al. have reported effective solar photocatalytic disinfection of E. coli and F. solani spores by using rGO–TiO2. The presence of rGO significantly enhanced the performance of photocatalytic disinfection of E. coli. Increasing rGO–TiO2 from 0 to 500 mg/L accelerated the inactivation of E. coli (106 colony-forming units (CFU)/mL) from more than 100 to 10 min and reduced the solar UV dosage needed from 123 to 11 kJ/m2. Although both rGO-TiO2 and pure TiO2 exhibited excellent disinfection of F. solani spores, rGO significantly reduced the solar energy required from 336.2 to 42.1 kJ/m2 [179][46]. A certain ratio between rGO and TiO2 significantly enhanced the photocatalytic disinfection under UV and solar irradiation [182][49]. Another study has also demonstrated that GO, which effectively separated photo-generated e−−h+ pairs for more ▪OH production, improved the photocatalytic disinfection of E. coli. In 30 min, the disinfection efficiencies of using pure TiO2, GO, GO-TiO2 were 39.27%, 73.82%, 99.60%, respectively [181][48]. More detailed information concerning the removal of different inorganic, organic, and biological pollutants by using GFN-TiO2 is available in Table 8.

2.2. Air-Phase Pollutants

Similar to the removal of pollutants in the water phase, GFN-TiO2 has been adopted for use in removing a wide range of air pollutants. Shorter contact times and the complexity of the heterogeneous photocatalytic reactions (e.g., photon absorbance and radical reactions) between pollutants and catalyst surfaces represent two typical challenges in this field [151][27].
In the aspect of inorganic removal, the treatment efficiencies of gaseous NOx (from NO(g) to NO2(g) to NO3−(s)) by using pure TiO2, graphene-TiO2, and rGO-TiO2 were compared [165][30]. An appreciable level of GFN (e.g., 0.01–0.1% graphene or rGO) in TiO2 improved the removal of NOx under UV and visible light. The NOx removal efficiencies were 25.45%, 26.26–35.40%, and 39.38–42.86% by using TiO2, graphene-TiO2, and rGO-TiO2 under UV light, respectively, while under visible light the removal efficiencies using TiO2, graphene-TiO2, and rGO-TiO2 were 9.35%, 15.20–22.75%, and 19.88–22.34%, respectively. Giampiccolo et al. prepared graphene-TiO2 by using the sol-gel method for electrochemical sensing and photocatalytic degradation of NOx in the air [183][50]. Interestingly, the performances of graphene-TiO2 prepared by using the same method but with different step orders were compared (adding graphene to the reaction before initiating the sol-gel reaction followed by annealing (GTiO2S) and adding graphene to TiO2 which had already been annealed (GTiO2M)). The addition of graphene significantly improved the performance of the catalysts under solar irradiation (280–780 nm) (e.g., the pseudo-first-order rate constants of NOx removal by GTiO2S, GTiO2M, and TiO2 were 6.7, 5.6, and 4.3/min, respectively.). The thermal treatment helped synthesize graphene and TiO2 in more intimate contact and improved the exhibition. Besides NOx, photodegradation of CO by using GO-TiO2, which was functionalized by attaching a cobalt (Co)-imidazole (Im) complex on GO, was investigated [109][37]. The results revealed that the bandgaps of this functionalized GO-TiO2 (with Co and Im), GO-TiO2, and pure TiO2 were 2.78, 2.96, and 3.10 eV, respectively. The removal efficiencies of CO and NOx were improved from 10% to 46% and from 16% to 51% when the catalyst was changed from TiO2 to the functionalized GO-TiO2, respectively. Xu et al. added graphene into TiO2 to enhance the photocatalytic CO2 conversion to chemical fuels [184][51]. The addition of graphene inhibited the recombination of e−−h+ pairs and raised the surface temperature, improving the CO2 conversion efficiency. The conversion rates of CO2 to CH4 and CO by using graphene-TiO2 were higher than those using TiO2 by factors of 5.1 and 2.8, respectively.
Studies have demonstrated the photocatalytic degradation of organic pollutants in the air phase by using GFN-TiO2. Zang et al. have reported that adding graphene into TiO2 with a specific ratio (e.g., 0.5% w/w) exhibited a synergetic effect on the UV light photodegradation of benzene (the mineralization rates of GFN-TiO2 and TiO2 were 76.2% in 10 h and 1.2% in 28 h, respectively). The adsorption of benzene and intermediates during benzene degradation negatively affecting TiO2 adsorbing UV light was decreased by the presence of graphene. However, excess graphene could adsorb extra compounds and impact light absorption. Benzene removal was limitedly found when visible light was used [166][31]. Similarly, in a study that focused on the photocatalytic degradation of acetone in the air, graphene-TiO2 exhibited a better activity (the pseudo-1st-order rate constant was 10.2 × 10−3/min) exceeding that of pure TiO2 (5.99 × 10−3/min) by a factor of 1.7 and a good reproducibility after three cycles of illumination [163][28].
Adding other materials to graphene-TiO2 has been investigated to further enhance its photocatalytic activity. Photocatalytic degradation of formaldehyde by using graphene-TiO2-Fe3+ has been reported [168][33]. Under UV light, both graphene-TiO2-Fe3+ and graphene-TiO2 revealed better performances than pure TiO2, as only the photolytic activity of graphene-Fe3+-TiO2 was better under visible light irradiation. The photocatalyst with a TiO2/graphene ratio of 50 and a ratio of Fe3+/graphene-TiO2 of 0.12% revealed the optimal performance. Nitrogen has been doped into reduced graphene-TiO2 to change the polarity of the catalyst and to influence the adsorption and photodegradation of polar acetaldehyde and nonpolar ethylene [185][52]. Both reduced graphene-TiO2 and N-doped reduced graphene-TiO2 exhibited higher treatment efficiencies than pure TiO2. One explanation was that nitrogen doping improved the polarity of the catalyst, further enhancing the removal efficiency of polar acetaldehyde.
The feasibility of adding GO into TiO2 for the photocatalytic degradation of organic pollutants has been reported. A study used GO-TiO2 as a photocatalyst to accelerate the degradation of benzene, toluene, ethylbenzene, and xylene (BTEX) in the air [151][27]. Under UV irradiation, the removal of these compounds by using GO-TiO2 was higher than that of using pure TiO2 by a factor of 1.2, while GO-TiO2 exhibited an excellent treatment efficiency exceeding that of pure TiO2 by a factor of 12 under visible light irradiation. GO-TiO2 has also been used for the photocatalytic degradation of methyl ethyl ketone in indoor air [171][38]. The addition of GO in TiO2 has improved the removal efficiency from 32.7% to 96.8% under visible light irradiation. Proper humidity (e.g., 40%), flow rate (e.g., 50 mL/min), and pollutant concentration (e.g., 30 ppmv) were the key to optimal performance. Note that the use of nanostructured membranes based on polymeric nanofibers using TiO2 and GFNs, including GO, rGO, and few-layer graphene, for the photocatalytic oxidation of gas-phase methanol has been reported. As the photocatalytic activity was greatly changed by the membrane structure and affected by the affinity of GFN to the polymer matrix, rGO exhibited better performance due to its more enhanced electron mobility [186][53]. Table 4 summarizes the applications of GFN-TiO2 for the photodegradation of organic pollutants in the air in these studies.
Table 4 Removal of air-phase pollutants by GFN-TiO2 in selected studies.
  Pollutant Catalyst Light Source Humidity or Flow Rate Removal Ref.
Inorganic NOx (1 ppm) Graphene-TiO2

rGO-TiO2		 15 W, UVA

8 W, visible light		 50% humidity, 3 L/min 42%

49%		 [165][30]
NOx (200 ppb) Graphene-TiO2 280–780 nm, 300 W, solar lamp 1 L/min 77% [183][50]
CO (50 ppm)

NOx (1 ppm)		 Graphene-TiO2 8 W, UV lamp 0.2 L/min 46%

51%		 [109][37]
Organic Acetone (300 ± 20 ppm) Graphene-TiO2 365 nm, 15 W, UV lamp 1 L/min ~60% [163][28]
Acetaldehyde (500 ppm)

Ethylene (50 ppm)		 Graphene-TiO2 260 W, fluorescent lamp

500 W, Xenon lamp		 20 cm3/min ~82%

~90%		 [185][52]
Benzene (250 ppm) Graphene-TiO2 254 nm, 4 W, UV lamp 20 mL/min 6.4% [166][31]
Formaldehyde (3000 ppm) Graphene-TiO2 365 nm, 8 W, black light blue lamp

>420 nm, 8 W, fluorescent lamp		 Not specified 50.3%

25.5%		 [168][33]
Methanol (4,000 ppm) Graphene-TiO2

GO-TiO2

rGO-TiO2		 254 nm, 16 W, UV lamp 155 cm3/min 80%

99%

99%		 [186][53]
BTEX (1 ppm) GO-TiO2 400–720 nm, 8 W, daylight lamp 55% humidity, 1 L/min 96% [151][27]
  MEKT (30 ppm) GO-TiO2 80 W, Xe lamp 40% humidity, 50 mL/min 96.8% [171][38]

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