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carbon nanocomposites for photocatalytic degradation of a reactive textile dye
Materials Research Bulletin 48 (2013) 581–586
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Multivariate optimisation of TiO2/carbon nanocomposites for photocatalytic degradation of a reactive textile dye Elias da Costa, Aldo J.G. Zarbin, Patricio Peralta-Zamora * Departamento de Quı´mica, Universidade Federal do Parana´. CP 19081, 81531-990, Curitiba-PR, Brazil
A R T I C L E I N F O
A B S T R A C T
Article history: Received 20 June 2012 Received in revised form 12 September 2012 Accepted 6 November 2012 Available online 15 November 2012
In this study, the effect of synthesis variables on the photocatalytic activity of TiO2 nanoparticles and TiO2/C nanocomposites was evaluated by factorial design using a reactive dye as a model substrate. The most significant result demonstrated a significant effect of the pyrolysis temperature on the photocatalytic performance of both materials. For TiO2 nanoparticles, the lower temperatures of pyrolysis enhanced the photocatalytic activity because of the lower rutile content. Conversely, for nanocomposites, the optimum condition was represented by higher temperatures of pyrolysis, which resulted in greater concentrations of the rutile phase. In the latter case, the higher activity observed for the rutile phase was clear evidence of the favourable effect of the presence of carbon. ß 2012 Elsevier Ltd. All rights reserved.
Keywords: A. Semiconductors B. Sol–gel chemistry D. Catalytic properties
1. Introduction Because of its high degradation efficiency against many substrates of environmental relevance, heterogeneous photocatalysis occupies a prominent place within the group of advanced technologies for the treatment of liquid wastes. In general, advanced oxidation processes based on the use of semiconductors are efficient in the degradation of resistant pollutants and often result in their complete mineralisation. Within this context, the use of titanium dioxide (TiO2) is particularly relevant, especially in its anatase crystalline form, which has allowed for efficient photocatalyst degradation of pollutants using either artificial or solar radiation [1–3]. The principles of photocatalysis [4], along with the practical problems that preclude their application in large-scale treatment systems, are well established. Attention has also been given to the difficulties in separating the photocatalysts, which are usually of nanometric size. The need for artificial radiation sources has also drawn attention because the band gaps of most of the photocatalysts are in the ultraviolet region. To overcome these drawbacks, new approaches that involve the use of immobilised [5] and modified [6] photocatalysts have been devised. Admittedly, the use of nanocomposites based on TiO2 and carbonaceous materials can improve the efficiency of the photocatalytic process through a synergistic effect, which can be explained by the previous adsorption of the substrate onto the
* Corresponding author. Tel.: +55 41 33613297; fax: +55 41 33613186. E-mail address: [email protected] (P. Peralta-Zamora). 0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2012.11.045
carbonaceous materials followed by mass transfer to the photoactive TiO2 [6]. Moreover, the high electrical conductivity of carbon allows for an alternative path for the electron in the conduction band, thus preventing the electron–hole recombination [6]. Therefore, the number of published works in this area has grown substantially since the mid-1990s [6]; most of these reports concern sol–gel processes, which are followed by the polymerisation of a carbon precursor and a thermal treatment. For obvious reasons, nanoparticles (TiO2) and carbon nanocomposites (TiO2/C) exhibit characteristics that strongly depend on the synthesis conditions. Thus, the effects of relevant operational parameters (e.g., synthesis temperature of TiO2 nanoparticles, TiO2/polymeric precursor ratio, pyrolysis temperature, etc.) on the main characteristics of the nanocomposites (e.g., crystalline phase, particle size, etc.) are commonly investigated, along with the photocatalytic activity of the nanocomposite against model substrates. With this objective, most researchers in this subject area use univariate optimisation strategies to assess the effect of one variable at a time; these strategies thus neglect the existence of the effects of interactions between the variables [7]. In the current literature, a few papers have considered multivariable systems (factorial design of experiments) to evaluate the effect of relevant experimental variables on the properties of the synthesised materials; thus, highlighting the production of films and other TiO2 hybrid materials (TiO2/quitosane [8], TiO2/ SiO2 [9] and TiO2/paper [10]) by sol–gel processes. In general, these studies expose the existence of strong interaction effects that cannot be observed by conventional optimisation systems. For this study, factorial design systems were used to evaluate the effects of experimental variables on the main characteristics of
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TiO2 nanoparticles and TiO2/C nanocomposites as well their effects on the photocatalytic efficiency of the nanocomposites towards the degradation of a model textile dye. 2. Materials and methods 2.1. Preparation of TiO2 nanoparticles TiO2 nanoparticles were prepared by the sol–gel method according to the procedures described by Oliveira et al. [11]. Under an argon atmosphere, 20.0 mL of 2-propanol (Merck) were added to 20.0 mL of titanium isopropoxide (TTIP, Ti[OCH(CH3)2]4, Strem) and to 120 mL of 0.2 mol L1 HCl solution (Carlo Erba). After precipitation, the mixture was kept under reflux at 60 8C for 8 h. The excess water and solvent were subsequently removed by vacuum drying at room temperature (rotary evaporator) and then incubating at 55 8C. The TiO2 nanoparticles obtained in this way were labelled as TiO2 0.2 M. 2.2. Preparation of TiO2/poly furfuryl alcohol (PFA) and TiO2/C nanocomposites The overall procedure for the synthesis of nanocomposites from TiO2 and furfuryl alcohol (FA) was based on the synthesis route proposed by de Almeida filho et al. [12]. Two grams of TiO2 0.2 M nanoparticles were dispersed in 74.4 mL of deionised water and submitted to ultrasound (154 W, frequency of 37 kHz) for 8 min. Over this dispersion, 900 mL of FA was added, the ultrasound was maintained for 1 min, after which magnetic stirring was continued for 1 h. The water and the unreacted monomer were subsequently removed under vacuum at 40 8C. The resulting solid material was subjected to heat treatment at 70 8C for 24 h to promote the complete polymerisation of the FA. The nanocomposites obtained using this method were dark brown in colour and were labelled as TiO2/PFA 0.2 M-04, which indicated the use of a 1:0.4 TiO2:PFA ratio. Finally, these samples (1.0 g) were pyrolysed under an argon stream (260 mL min1) using an EDGECON 5P tubular furnace. The temperature was increased to 900 8C at a heating rate of 10 8C min1 and maintained for 2 h. The temperature was subsequently reduced to room temperature. The materials obtained in this way exhibited a characteristic black colour and were denominated TiO2/C 0.2 M-04.
2.3. Characterisation techniques X-ray diffraction (XRD) experiments were performed on a Shimadzu XRD-6000 equipped with Cu Ka radiation source (l = 1.5418 A˚) operated at a voltage of 40 kV and a current of 40 mA. The XRD patterns were collected at a scan rate of 28 min1 in a 2u range of 10–608. The compositions of the crystalline phases of TiO2 were calculated from the equations proposed by Zhang [13]. 2.4. Photocatalytic degradation of the model dye Photocatalytic degradation studies were performed using anthraquinone blue QR 19 dye aqueous solutions at a concentration of 50 mg L1. All tests were performed in a conventional bench photochemical reactor (250 mL capacity) equipped with a watercooling system (temperature: 25 2 8C), a magnetic stirrer and an oxygenation system (O2: 50 mL min1). Radiation was provided by a 125 W mercury vapour lamp immersed in the solution with the aid of a quartz bulb. In this reactor, samples that contained 200 mL of dye solution at pH 7 were added to 50 mg of catalyst (TiO2 and TiO2/C) and submitted to adsorption in the absence of light and oxygen for 30 min followed by 60 min of photocatalysis. Samples were collected at regular intervals (typically every 10 min), filtered through a cellulose acetate membrane (0.45 mm) and analysed by UV–Vis spectroscopy. These measurements were performed on a Hewlett Packard 8452-A (180–800 nm) spectrophotometer using 1 cm quartz cells. 2.5. Factorial design Initially, the effect of the experimental variables (synthesis temperature, H+ concentration, Ti:FA ratio and pyrolysis temperature) on the photocatalytic degradation capacity of TiO2/C nanocomposites was evaluated by a 241 factorial design, which was increased by a central point assayed in triplicate (Table 1). The response was processed using the degradation of the model dye at a reaction time of 30 min. For comparison, a second factorial design was performed (23) that involved the variables of synthesis temperature, H+ concentration, pyrolysis temperature and the photocatalytic degradation ability of TiO2 nanoparticles (Table 2). In both cases, the variables and the levels studied were defined according to the preliminary results obtained in the univariate optimisation studies.
Table 1 Fractional factorial design (241) to study the effect of experimental variables on the photocatalytic degradation capacity of TiO2/C nanocomposites (Blue QR 19 dye: 250 mL, 50 mg L1; O2: 50 mL min1; reaction time: 30 min). Variable
Level
Synthesis temperature (Ts, 8C) Ti:FA proportion (P) Acid concentration ([H+], mol L1) Pyrolysis temperature (Tp, 8C)
0
+
40 1:0.2 0.10 600
60 1:0.4 0.25 750
80 1:0.6 0.40 900
Run
Ts
P
[H+]
Tp
QR-19 degradation (%)
1 2 3 4 5 6 7 8 9 10 11
+ + + + 0 0 0
+ + + + 0 0 0
+ + + + 0 0 0
+ + + + 0 0 0
52.9 73.7 74.2 61.9 82.3 54.2 49.7 76.3
Main effects: Ts: 1.7; P: 0.3; [H+]: 0.1; Tp: 21.9. Second order effects: Ts P = [H+] Tp: 5.4; Ts [H+] = P Tp: 2.5; Ts Tp: 5.0 = P [H+]: 5.0. Third order effects: Tp P [H+] = Tp: 21.9; Tp P Tp = [H+]: 0.1; Ts [H+] Tp = P: 0.3; P [H+] Tp = Ts: 1.7.
73.8 3.4
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Table 2 Additional characteristic of TiO2/C nanocomposites. Run
Pyrolysis temperature (8C)
TiO2 phase (%)a A
B
R
1 2 3 4 5 6 7 8 9 10 11
600 900 900 600 900 600 600 900 750 750 750
68.4 4.4 7.9 72.7 13.7 62.8 45.0 0.4 26.1 8.9 13.8
26.1 11.3 26.1 51.3 35.7 13.2 6.3 4.7 3.1
5.5 84.3 92.1 1.2 34.9 1.5 41.8 99.6 67.6 86.4 83.1
a b c
Adsorption (%)b
Photocatalytic degradation (%)c
0.6 18.1 9.9 5.0 18.5 5.8 2.0 22.4 12.5 7.6 15.4
52.9 73.7 74.2 61.9 82.3 54.2 49.7 76.3 73.7 70.5 77.2
A: anatase; B: brookite; R: rutile. Blue QR 19 adsorption at contacting time of 30 min. Blue QR 19 photocatalytic degradation at contacting time of 30 min.
3. Results and discussion To illustrate the adsorptive and photocatalytic capacities of the nanocomposites, a spectral sequence was obtained during the degradation of the model dye using the TiO2/C 0.2 M-04 material, as shown in Fig. 1. In the absence of light, the adsorption process may result in the removal of approximately 15% of the initial load of dye during a 30 min time period, which is a substantially larger amount of removed dye than that reported in similar studies involving the use of Degussa P25 TiO2 [14]. In this regard, the photocatalytic process can be considered a surface phenomenon that is highly influenced by the preliminary adsorption of the substrates. Thus, with an increase in the adsorption capacity of the material, the presence of carbon can provide significant improvement in the photocatalytic degradation, as shown in several studies involving the degradation of dyes [15,16]. This photocatalytic process allowed for a rapid degradation of the characteristic chromophoric group (max l at 592 nm), which removed the colour almost completely at reaction times of 60 min. The aromatic structure of the dye, which manifested itself as an intense absorption in the ultraviolet region (200–300 nm), was also efficiently degraded by photocatalysis which suggests a high degree of mineralisation. The small residual signal observed during the 60 min treatment corresponded to the short-chain carboxylic SEQUENCE -1
5
Blue QR 19 Dye (20 mg L ) Adsorption (10, 20 and 30 min) Photocatalysis (5, 10, 15, 30 and 60 min)
Absorbance
4
acids (typically oxalic acid) that tended to accumulate at the end of the process [17]. For a comparative evaluation of the degradation performance of the materials produced under different experimental conditions, the discolouration rate, which was evaluated from the decay of the spectral band centred at 592 nm with a reaction time of 30 min, was used. 3.1. TiO2/C nanocomposites Initially, the effect of H+ concentration, synthesis temperature, Ti:FA ratio and pyrolysis temperature on the degradation capacity of the nanocomposites was investigated using a 241 fractional factorial design, which was increased by a central point assayed in triplicate (Table 1). In this kind of factorial designs, eight experiments are necessaries to evaluate the effect of three variables at two levels, which configures a 23 full factorial design. To study the fourth variable it is necessary to establish a defining relation (or generator) that determines which effects are confounded with each other. In this case, a 4 = 1 2 3 defining relation was used, which implies that the main effect of variable 4 (pyrolysis temperature) is confounded with the third-order effect (1 2 3). Similarly, second-order effects are confounded with other second-order effects, as shown in Table 1. Normally, responses are affected by a small number of main effects and lower-order interactions. Consequently, fractional factorial designs can be used to study a large number of factors, focusing the main effects and two-factor interactions. In Table 1, the plus signal represent all the runs were the variable is at the high level, while the minus signal symbolise the low studied level. Thus, the main effect of the variable can be calculated by averaging high and low levels, according to the follow expression [18]:
3
P Effect ¼
2
1
0 200
300
400
500
600
700
800
Wavelength (nm) Fig. 1. Spectrophotometric monitoring of the adsorption and photocatalytic degradation of blue QR 19 dye by the TiO2/C 0.2M-04 nanocomposite.
Yþ Y nþ n
where n represent the number of data points assayed at each level. Interaction effects are calculated of the same way, using the signal that result from the multiplication of the parent terms. Thus, the second-order AB effect is calculated from the signals that result of the product of A and B columns. The effects summarised in Table 1 were calculated according to this procedure and were compared with the standard deviation observed in the analysis of the central point. Thus, it was possible to deduce that the variables H+ concentration,
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Table 3 Factorial design (23) to study the effect of experimental variables on the photocatalytic degradation capacity of TiO2 nanoparticles (Blue QR 19 dye: 250 mL, 50 mg L1; O2: 50 mL min1; reaction time: 30 min). Level
Variable
Synthesis temperature (Ts, 8C) Acid concentration ([H+], mol L1) Pyrolysis temperature (Tp, 8C)
0
+
40 0.10 600
60 0.25 750
80 0.40 900
Run
Ts
Tp
[H+]
QR-19 degradation (%)
1 2 3 4 5 6 7 8 9 10 11
+ + + + 0 0 0
+ + + + 0 0 0
+ + + + 0 0 0
51.0 54.5 14.9 20.8 45.6 48.8 38.1 27.4 32.8 3.0
Main effects: Ts: 0.5; [H+]: 4.7; Tp: 24.7. Second order effects: Ts [H+]: 4.2; Ts Tp: 2.9; [H+] Tp: 10.2. Third order effects: Ts [H+] Tp: 4.1.
synthesis temperature and Ti:FA ratio did not exert any significant effect on the monitored response because the main effect values were similar to the experimental standard deviation (3.4%) evaluated in the triplicate. In contrast, the pyrolysis temperature had a significant positive effect (approximately 22 percentage points) that favoured the photocatalytic degradation process in which catalysts pyrolysed at a higher temperature were used. In the first analysis, the third-order effect involving the variables synthesis temperature (1), Ti:FA ratio (2) and acid concentration (3) appeared significant (Tp P [H+]: 21.9 percentage points). However, it was necessary to consider that, for fractional factorial design involving the I = 1234 defining relation, the third-order effect (1 2 3) was confounded with the main effect of variable 4 (4 = 123). For similar reasons, the 1 4 second-order effect was confounded with the 2 3 effect. However, the fact that variables 2 and 3 did not show any significant effect suggested that this second-order effect was a function of variables 1 and 4.
o Pyrolysis temperature ( C)
900
-3
78 +/- 2
75 +/- 2
To examine this interaction, the preliminary 241 fractional factorial design was transformed into a new 22 factorial design, which allowed each test to be represented by a duplicate. The results shown in Fig. 2 confirmed the existence of a secondorder effect because an increase in the synthesis temperature improved the catalytic activity of the nanocomposites produced using low-temperature pyrolysis (+7 percentage points) while decreasing the efficiency of materials prepared using hightemperature pyrolysis (3 percentage points). According to these observations, the synthesis temperature became insignificant when the pyrolysis was performed at elevated temperatures. This fact was well-illustrated by the higher rates of discolouration observed in experiments 2, 3, 5 and 8 (Table 1), all of which correspond to a pyrolysis temperature of 900 8C. Other relevant characteristics of the TiO2/C nanocomposites are summarised in Table 2. According to these features, the best adsorption and photocatalytic activity were observed in the nanocomposites produced at a higher pyrolysis temperature (900 8C, experiments 2, 3, 5 and 8), which modified the structure of TiO2 (anatase–rutile transformation), the size of the TiO2 particles and the structure of the carbonaceous material. In addition, these materials predominantly showed the rutile crystalline phase, which is usually considered to have low photocatalytic efficiency. 3.2. TiO2 nanoparticles
750
27
600
51 +/- 2 40
73 +/- 2
17
+7
58 +/- 5 60
80 o
Synthesis temperature ( C) Fig. 2. Geometric representation of the 22 factorial design used to identify a secondorder effect between synthesis and pyrolysis temperature on the photocatalytic efficiency of TiO2/C nanocomposites.
For comparative purposes, a new 23 factorial design was developed to evaluate the effect of the synthesis temperature, H+ concentration and pyrolysis temperature on the photocatalytic degradation efficiency of TiO2 nanoparticles (Table 3). In this study each variable was evaluated at two levels, denoted as +1 (high level) and 1 (low level). The left column of Table 3, numbers 1 through 8, specifies a non-randomized run order. The calculated effects shown in Table 3, which are based on a 30 min reaction time for the removal of colour, were compared with the typical deviation observed in the central conditions and tested in triplicate. In view of these values, the synthesis temperature and the concentration of acid did not exert any significant effect on the monitored response because the main effect values were similar to the experimental standard deviation (3.0%).
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Table 4 Additional characteristic of TiO2 nanoparticles. Run
Pyrolysis temperature (8C)
TiO2 phase (%)a A
B
R
1 2 3 4 5 6 7 8 9 10 11
600 600 900 900 600 600 900 900 750 750 750
6.9 5.9 – – 5.6 6.3 – – – – –
– – – – – – – – – – –
93.1 94.1 100.0 100.0 94.4 93.7 100.0 100.0 100.0 100.0 100.0
Adsorption (%)b
Photocatalytic degradation (%)c
ND 1.5 0.2 0.7 1.7 1.8 1.8 1.0 2.0 2.0 2.0
51.0 54.5 14.9 20.8 45.6 48.8 38.1 27.4 31.0 31.1 36.3
ND: not detected. a A: anatase; B: brookite; R: rutile. b Blue QR 19 adsorption at contacting time of 30 min. c Blue QR 19 photocatalytic degradation at contacting time of 30 min.
On the contrary, the pyrolysis temperature had a significant negative effect (approximately 25 percentage points), which indicated the disadvantage of the photocatalytic degradation process with samples prepared at a high pyrolysis temperature. These observations were illustrated by the higher rates of discolouration observed in experiments 1, 2, 5 and 6 (Table 3), all of which correspond to a lower pyrolysis temperature (600 8C). Furthermore, a significant second-order effect of the variables was found between the acid concentration and pyrolysis temperature (approximately 10 percentage points), which was clearly observed in the geometric representation shown in Fig. 3. These results (Fig. 3) confirmed the greater catalytic efficiency of photocatalysts produced at lower pyrolysis temperature, especially for materials synthesised at low acid concentrations (Experiments 1 and 2, Table 3). Other relevant information pertaining to the TiO2 nanoparticles produced in this study are summarised in Table 4. According to these results, all of the materials produced at a pyrolysis temperature greater than 750 8C consisted of 100% rutile phase, which could explain their low photocatalytic efficiency. However, the best-performing materials were produced at the lower pyrolysis temperature (600 8C, experiments 1, 2, 5 and 6, Tables
900
+15
18 +/- 4
2 and 4), which produced materials with approximately 6–7% anatase phase of higher photocatalytic activity. 3.3. TiO2 nanoparticles vs. TiO2/C nanocomposites In this work, several relevant observations were made, particularly those that pertain to the effect of synthesis variables on the characteristics and photocatalytic efficiency of the produced materials. First, the presence of carbon retarded the rutilisation process of TiO2, which allowed the permanence of the anatase phase even at pyrolysis temperatures of 900 8C (see Table 2). This effect was clearly evidenced in the sequence of XRD spectra shown in Fig. 4, which indicate the virtual absence of anatase phase in TiO2 nanoparticles pyrolysed at 600 8C and the preservation of this phase in the photoactive TiO2/C nanocomposite pyrolysed at the same temperature. According to Slimen et al., the carbon matrix, which has a high surface area, baffles the anatase-to-rutile phase transformation because of its large interfacial energy, thereby creating an anti-calcination effect [19]. Moreover, the results presented in Fig. 5 show a low photocatalytic degradation capacity of TiO2 nanoparticles produced at a pyrolysis temperature of 600 8C, which was justified by the preponderant presence of the rutile phase of lower photochemical activity. For their part, the TiO2/C nanocomposites that
33 +/- 8
o
Pyrolysis temperature ( C)
R A
600
33 +/- 3
-35
-14
-6
53 +/- 3 0.10
0.25
Relative intensity
750
R
R
A
TiO2/C (600°C) TiO2/C (400°C)
47 +/- 2
TiO2 (600°C)
0.40
TiO2 (400°C)
+
[H ] Concentration Fig. 3. Geometric representation of the 22 factorial design used to identify a secondorder effect between acid concentration and pyrolysis temperature on the photocatalytic efficiency of TiO2 nanoparticles.
10
20
30
o
40
50
60
2θ
Fig. 4. XRD spectra of TiO2/C nanocomposites and TiO2 nanoparticles obtained at different pyrolysis temperature.
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ADSORPTION TiO2 , 600 °C
0.6
TiO2/C, 900 °C
PHOTOCATALYSIS TiO2 , 600 °C
Absorbance (592 nm)
0.5
TiO2/C, 900 °C 0.4
0.3
0.2
0.1
0.0 0
10
20
30
40
50
60
70
80
90
Reaction time (min) Fig. 5. Adsorption and degrading capacity of TiO2/C nanocomposites and TiO2 nanoparticles obtained at the best synthesis conditions.
exhibited better photocatalytic performance were produced at pyrolysis temperature of 900 8C, which, as previously discussed (see Table 2), also led to a majority of rutile phase in the composite. The degradation efficiency of the rutile-phase nanocomposites can only be explained by the favourable effect of the presence of carbon. In general, the presence of carbon-based adsorbents allowed a high concentration of substrate molecules in the vicinity of TiO2 particles. The high concentration of substrate molecules not only sped up the process of photodegradation, but contributed to the adsorption and subsequent oxidation of byproducts of the reaction, which additionally inhibited the release of species with the highest toxicity [20]. However, the presence of defects, especially oxygen vacancies, are well known to control many of the surface properties of semiconductor oxides [21], which highlights, for example, the sequestration of electrons and the consequent preservation of the positive holes [22]. According to recent studies [23], the presence of carbonaceous materials during the heat treatment of the samples induced the reduction of the TiO2 anatase with the formation of TiO2x and oxygen vacancies. Heat treatments at higher temperatures led to the formation of rutile-phase TiO2 that contained Ti3+ and exhibited high photocatalytic activity, as well as to the concomitant diffusion of vacancies from the surface to the bulk of the particles [23,24]. In recent years, the characteristic synergy observed between semiconductors and graphene [25–27] and carbon nanotubes [28], which usually improve the performance of specific applications, such as heterogeneous photocatalysis, have been unquestionably demonstrated. Finally, the use of TiO2/C nanocomposites in photocatalytic processes aimed at the remediation of liquid wastes appears to be convenient because of the facilitated sedimentation of the material. These results represent an important precedent because it would allow for the development of fluidised bed reactors, which would enable the continued treatment of high volumes of liquid wastes. 4. Conclusions According to the results reported in this study, the convenience of using multivariate optimisation methods has been demonstrated.
In the synthesis of TiO2/C nanocomposites was observed that the variables H+ concentration, synthesis temperature and Ti:FA ratio did not exert any significant effect on the photocatalytic activity of the material. On the other hand, a second-order effect (synthesis and pyrolysis temperature) showed that increased activity of photocatalytic materials can be obtained at higher pyrolysis temperatures with little or no influence of the synthesis temperature. In the synthesis of TiO2 nanoparticles the factorial design demonstrates that H+ concentration and synthesis temperature did not influence the degradation capacity of the photocatalyst. In contrast, the pyrolysis temperature had a significant negative effect, which indicated the disadvantage of the photocatalytic degradation process with samples prepared at a high pyrolysis temperature. Furthermore, a second-order effect (pyrolysis temperature and acid concentration) was also found in the synthesis of TiO2 nanoparticles, which showed little influence of the acid concentration, because the pyrolysis temperature was maintained at values near the lower temperature level (600 8C). The most relevant conclusion is probably represented by the effect of the pyrolysis temperature on the photocatalytic activity of both materials. Whereas the degradation capability of the TiO2/C nanoparticles was maximised at a high pyrolysis temperature (main effect of 21.9 percentage points), which resulted in products that contain mostly rutile-phase TiO2, the TiO2 nanoparticles were more effective when synthesised at a low temperature of pyrolysis (main effect of 24.7 percentage points), which tended to maintain the anatase phase. The beneficial effect of the presence of carbon was evident because of the unusually low photocatalytic degradation capacity of the rutile phase. References [1] U.I. Gaya, A.H. Abdullah, J. Photochem. Photobiol. C: Photochem. Rev. 9 (2010) 1–12. [2] M.N. Chong, B. Jin, C.W.K. Chow, C. Saint, Water Res. 44 (2010) 2997–3027. [3] S. Malato, J. Blanco, A. Vidal, C. Richter, Appl. Catal. B: Environ. 37 (2002) 1–15. [4] A. Fujishima, X. Zhang, D.A. Tryk, Surf. Sci. Rep. 63 (2008) 515–582. [5] A.Y. Shan, T.I. MohdGhazi, S.A. Rashid, Appl. Catal. A: Gen. 389 (2010) 1–8. [6] R. Leary, A. Westwood, Carbon 49 (2011) 741–772. [7] S.M. Tracey, S.N.B. Hodgson, A.K. Ray, Z. Ghassemlooy, J. Mater. Process. Technol. 77 (1998) 86–94. [8] Y. Tao, J. Pan, S. Yan, B. Tang, L. Zhu, Mater. Sci. Eng. B: Solid State Mater. Adv. Technol. 138 (2007) 84–89. [9] L. Luo, A.T. Cooper, M. Fan, J. Hazard. Mater. 161 (2009) 175–182. [10] S. Ko, J. Pekarovic, P.D. Fleming, P. Ari-Gur, Mater. Sci. Eng. B: Solid State Mater. Adv. Technol. 166 (2010) 127–131. [11] M.M. Oliveira, D.C. Schnitzler, A.J.G. Zarbin, Chem. Mater. 15 (2003) 1903–1909. [12] C.D. Almeida filho, A.J.G. Zarbin, Carbon 44 (2006) 2869–2876. [13] H. Zhang, J.F. Banfield, J. Phys. Chem. B 104 (2000) 3481–3487. [14] M. Saquib, M. Muneer, Dyes Pigment. 53 (2002) 237–249. [15] Y. Yu, J.C. Yu, C.Y. Chan, Y.K. Che, J.C. Zhao, L. Ding, W.K. Ge, P.K. Wong, Appl. Catal. B: Environ. 61 (2005) 1–11. [16] D. Huang, Y. Miyamoto, T. Matsumoto, T. Tojo, T. Fan, J. Ding, Q. Guo, D. Zhang, Sep. Purif. Technol. 78 (2011) 9–15. [17] P. Bansal, D. Singh, D. Sud, Sep. Purif. Technol. 72 (2010) 357–365. [18] G.E.P. Box, W.G. Hunter, J.S. Hunter, Statistics for Experimenters: An Introduction to Design, Data Analysis, and Model Building, John Wiley and Sons, New York, 1978. [19] H. Slimen, A. Houas, J.P. Nogier, J. Photochem. Photobiol. A: Chem. 221 (2011) 13–21. [20] H. Yoneyama, T. Torimoto, Catal. Today 58 (2000) 133–140. [21] H. Cheng, A. Selloni, Phys. Rev. B 79 (2009) 1–4. [22] G. Colo´n, M.C. Hidalgo, G. Munuera, I. Ferino, M.G. Cutrufello, J.A. Navı´o, Appl. Catal. B: Environ. 63 (2006) 45–59. [23] S. Yang, W. Tang, Y. Ishikawa, Q. Feng, Mater. Res. Bull. 46 (2011) 531–537. [24] C. Xiao-Quan, L. Huan-Bin, G. Guo-Bang, Mater. Chem. Phys. 91 (2005) 317–324. [25] Y. Zhang, Z.R. Tang, X. Fu, Y.J. Xu, ACS Nano 4 (2010) 7303–7314. [26] Y. Zhang, Z.R. Tang, X. Fu, Y.J. Xu, ACS Nano 5 (2011) 7426–7435. [27] Y. Zhang, N. Zhang, Z.R. Tang, Y.J. Xu, Phys. Chem. Chem. Phys. 14 (2012) 9167–9175. [28] Y.J. Xu, Y. Zhuang, X. Fu, J. Phys. Chem. C 14 (2010) 2669–2676.
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Multivariate optimisation of TiO2/carbon nanocomposites for photocatalytic degradation of a reactive textile dye Elias da Costa, Aldo J.G. Zarbin, Patricio Peralta-Zamora * Departamento de Quı´mica, Universidade Federal do Parana´. CP 19081, 81531-990, Curitiba-PR, Brazil
A R T I C L E I N F O
A B S T R A C T
Article history: Received 20 June 2012 Received in revised form 12 September 2012 Accepted 6 November 2012 Available online 15 November 2012
In this study, the effect of synthesis variables on the photocatalytic activity of TiO2 nanoparticles and TiO2/C nanocomposites was evaluated by factorial design using a reactive dye as a model substrate. The most significant result demonstrated a significant effect of the pyrolysis temperature on the photocatalytic performance of both materials. For TiO2 nanoparticles, the lower temperatures of pyrolysis enhanced the photocatalytic activity because of the lower rutile content. Conversely, for nanocomposites, the optimum condition was represented by higher temperatures of pyrolysis, which resulted in greater concentrations of the rutile phase. In the latter case, the higher activity observed for the rutile phase was clear evidence of the favourable effect of the presence of carbon. ß 2012 Elsevier Ltd. All rights reserved.
Keywords: A. Semiconductors B. Sol–gel chemistry D. Catalytic properties
1. Introduction Because of its high degradation efficiency against many substrates of environmental relevance, heterogeneous photocatalysis occupies a prominent place within the group of advanced technologies for the treatment of liquid wastes. In general, advanced oxidation processes based on the use of semiconductors are efficient in the degradation of resistant pollutants and often result in their complete mineralisation. Within this context, the use of titanium dioxide (TiO2) is particularly relevant, especially in its anatase crystalline form, which has allowed for efficient photocatalyst degradation of pollutants using either artificial or solar radiation [1–3]. The principles of photocatalysis [4], along with the practical problems that preclude their application in large-scale treatment systems, are well established. Attention has also been given to the difficulties in separating the photocatalysts, which are usually of nanometric size. The need for artificial radiation sources has also drawn attention because the band gaps of most of the photocatalysts are in the ultraviolet region. To overcome these drawbacks, new approaches that involve the use of immobilised [5] and modified [6] photocatalysts have been devised. Admittedly, the use of nanocomposites based on TiO2 and carbonaceous materials can improve the efficiency of the photocatalytic process through a synergistic effect, which can be explained by the previous adsorption of the substrate onto the
* Corresponding author. Tel.: +55 41 33613297; fax: +55 41 33613186. E-mail address: [email protected] (P. Peralta-Zamora). 0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2012.11.045
carbonaceous materials followed by mass transfer to the photoactive TiO2 [6]. Moreover, the high electrical conductivity of carbon allows for an alternative path for the electron in the conduction band, thus preventing the electron–hole recombination [6]. Therefore, the number of published works in this area has grown substantially since the mid-1990s [6]; most of these reports concern sol–gel processes, which are followed by the polymerisation of a carbon precursor and a thermal treatment. For obvious reasons, nanoparticles (TiO2) and carbon nanocomposites (TiO2/C) exhibit characteristics that strongly depend on the synthesis conditions. Thus, the effects of relevant operational parameters (e.g., synthesis temperature of TiO2 nanoparticles, TiO2/polymeric precursor ratio, pyrolysis temperature, etc.) on the main characteristics of the nanocomposites (e.g., crystalline phase, particle size, etc.) are commonly investigated, along with the photocatalytic activity of the nanocomposite against model substrates. With this objective, most researchers in this subject area use univariate optimisation strategies to assess the effect of one variable at a time; these strategies thus neglect the existence of the effects of interactions between the variables [7]. In the current literature, a few papers have considered multivariable systems (factorial design of experiments) to evaluate the effect of relevant experimental variables on the properties of the synthesised materials; thus, highlighting the production of films and other TiO2 hybrid materials (TiO2/quitosane [8], TiO2/ SiO2 [9] and TiO2/paper [10]) by sol–gel processes. In general, these studies expose the existence of strong interaction effects that cannot be observed by conventional optimisation systems. For this study, factorial design systems were used to evaluate the effects of experimental variables on the main characteristics of
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TiO2 nanoparticles and TiO2/C nanocomposites as well their effects on the photocatalytic efficiency of the nanocomposites towards the degradation of a model textile dye. 2. Materials and methods 2.1. Preparation of TiO2 nanoparticles TiO2 nanoparticles were prepared by the sol–gel method according to the procedures described by Oliveira et al. [11]. Under an argon atmosphere, 20.0 mL of 2-propanol (Merck) were added to 20.0 mL of titanium isopropoxide (TTIP, Ti[OCH(CH3)2]4, Strem) and to 120 mL of 0.2 mol L1 HCl solution (Carlo Erba). After precipitation, the mixture was kept under reflux at 60 8C for 8 h. The excess water and solvent were subsequently removed by vacuum drying at room temperature (rotary evaporator) and then incubating at 55 8C. The TiO2 nanoparticles obtained in this way were labelled as TiO2 0.2 M. 2.2. Preparation of TiO2/poly furfuryl alcohol (PFA) and TiO2/C nanocomposites The overall procedure for the synthesis of nanocomposites from TiO2 and furfuryl alcohol (FA) was based on the synthesis route proposed by de Almeida filho et al. [12]. Two grams of TiO2 0.2 M nanoparticles were dispersed in 74.4 mL of deionised water and submitted to ultrasound (154 W, frequency of 37 kHz) for 8 min. Over this dispersion, 900 mL of FA was added, the ultrasound was maintained for 1 min, after which magnetic stirring was continued for 1 h. The water and the unreacted monomer were subsequently removed under vacuum at 40 8C. The resulting solid material was subjected to heat treatment at 70 8C for 24 h to promote the complete polymerisation of the FA. The nanocomposites obtained using this method were dark brown in colour and were labelled as TiO2/PFA 0.2 M-04, which indicated the use of a 1:0.4 TiO2:PFA ratio. Finally, these samples (1.0 g) were pyrolysed under an argon stream (260 mL min1) using an EDGECON 5P tubular furnace. The temperature was increased to 900 8C at a heating rate of 10 8C min1 and maintained for 2 h. The temperature was subsequently reduced to room temperature. The materials obtained in this way exhibited a characteristic black colour and were denominated TiO2/C 0.2 M-04.
2.3. Characterisation techniques X-ray diffraction (XRD) experiments were performed on a Shimadzu XRD-6000 equipped with Cu Ka radiation source (l = 1.5418 A˚) operated at a voltage of 40 kV and a current of 40 mA. The XRD patterns were collected at a scan rate of 28 min1 in a 2u range of 10–608. The compositions of the crystalline phases of TiO2 were calculated from the equations proposed by Zhang [13]. 2.4. Photocatalytic degradation of the model dye Photocatalytic degradation studies were performed using anthraquinone blue QR 19 dye aqueous solutions at a concentration of 50 mg L1. All tests were performed in a conventional bench photochemical reactor (250 mL capacity) equipped with a watercooling system (temperature: 25 2 8C), a magnetic stirrer and an oxygenation system (O2: 50 mL min1). Radiation was provided by a 125 W mercury vapour lamp immersed in the solution with the aid of a quartz bulb. In this reactor, samples that contained 200 mL of dye solution at pH 7 were added to 50 mg of catalyst (TiO2 and TiO2/C) and submitted to adsorption in the absence of light and oxygen for 30 min followed by 60 min of photocatalysis. Samples were collected at regular intervals (typically every 10 min), filtered through a cellulose acetate membrane (0.45 mm) and analysed by UV–Vis spectroscopy. These measurements were performed on a Hewlett Packard 8452-A (180–800 nm) spectrophotometer using 1 cm quartz cells. 2.5. Factorial design Initially, the effect of the experimental variables (synthesis temperature, H+ concentration, Ti:FA ratio and pyrolysis temperature) on the photocatalytic degradation capacity of TiO2/C nanocomposites was evaluated by a 241 factorial design, which was increased by a central point assayed in triplicate (Table 1). The response was processed using the degradation of the model dye at a reaction time of 30 min. For comparison, a second factorial design was performed (23) that involved the variables of synthesis temperature, H+ concentration, pyrolysis temperature and the photocatalytic degradation ability of TiO2 nanoparticles (Table 2). In both cases, the variables and the levels studied were defined according to the preliminary results obtained in the univariate optimisation studies.
Table 1 Fractional factorial design (241) to study the effect of experimental variables on the photocatalytic degradation capacity of TiO2/C nanocomposites (Blue QR 19 dye: 250 mL, 50 mg L1; O2: 50 mL min1; reaction time: 30 min). Variable
Level
Synthesis temperature (Ts, 8C) Ti:FA proportion (P) Acid concentration ([H+], mol L1) Pyrolysis temperature (Tp, 8C)
0
+
40 1:0.2 0.10 600
60 1:0.4 0.25 750
80 1:0.6 0.40 900
Run
Ts
P
[H+]
Tp
QR-19 degradation (%)
1 2 3 4 5 6 7 8 9 10 11
+ + + + 0 0 0
+ + + + 0 0 0
+ + + + 0 0 0
+ + + + 0 0 0
52.9 73.7 74.2 61.9 82.3 54.2 49.7 76.3
Main effects: Ts: 1.7; P: 0.3; [H+]: 0.1; Tp: 21.9. Second order effects: Ts P = [H+] Tp: 5.4; Ts [H+] = P Tp: 2.5; Ts Tp: 5.0 = P [H+]: 5.0. Third order effects: Tp P [H+] = Tp: 21.9; Tp P Tp = [H+]: 0.1; Ts [H+] Tp = P: 0.3; P [H+] Tp = Ts: 1.7.
73.8 3.4
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Table 2 Additional characteristic of TiO2/C nanocomposites. Run
Pyrolysis temperature (8C)
TiO2 phase (%)a A
B
R
1 2 3 4 5 6 7 8 9 10 11
600 900 900 600 900 600 600 900 750 750 750
68.4 4.4 7.9 72.7 13.7 62.8 45.0 0.4 26.1 8.9 13.8
26.1 11.3 26.1 51.3 35.7 13.2 6.3 4.7 3.1
5.5 84.3 92.1 1.2 34.9 1.5 41.8 99.6 67.6 86.4 83.1
a b c
Adsorption (%)b
Photocatalytic degradation (%)c
0.6 18.1 9.9 5.0 18.5 5.8 2.0 22.4 12.5 7.6 15.4
52.9 73.7 74.2 61.9 82.3 54.2 49.7 76.3 73.7 70.5 77.2
A: anatase; B: brookite; R: rutile. Blue QR 19 adsorption at contacting time of 30 min. Blue QR 19 photocatalytic degradation at contacting time of 30 min.
3. Results and discussion To illustrate the adsorptive and photocatalytic capacities of the nanocomposites, a spectral sequence was obtained during the degradation of the model dye using the TiO2/C 0.2 M-04 material, as shown in Fig. 1. In the absence of light, the adsorption process may result in the removal of approximately 15% of the initial load of dye during a 30 min time period, which is a substantially larger amount of removed dye than that reported in similar studies involving the use of Degussa P25 TiO2 [14]. In this regard, the photocatalytic process can be considered a surface phenomenon that is highly influenced by the preliminary adsorption of the substrates. Thus, with an increase in the adsorption capacity of the material, the presence of carbon can provide significant improvement in the photocatalytic degradation, as shown in several studies involving the degradation of dyes [15,16]. This photocatalytic process allowed for a rapid degradation of the characteristic chromophoric group (max l at 592 nm), which removed the colour almost completely at reaction times of 60 min. The aromatic structure of the dye, which manifested itself as an intense absorption in the ultraviolet region (200–300 nm), was also efficiently degraded by photocatalysis which suggests a high degree of mineralisation. The small residual signal observed during the 60 min treatment corresponded to the short-chain carboxylic SEQUENCE -1
5
Blue QR 19 Dye (20 mg L ) Adsorption (10, 20 and 30 min) Photocatalysis (5, 10, 15, 30 and 60 min)
Absorbance
4
acids (typically oxalic acid) that tended to accumulate at the end of the process [17]. For a comparative evaluation of the degradation performance of the materials produced under different experimental conditions, the discolouration rate, which was evaluated from the decay of the spectral band centred at 592 nm with a reaction time of 30 min, was used. 3.1. TiO2/C nanocomposites Initially, the effect of H+ concentration, synthesis temperature, Ti:FA ratio and pyrolysis temperature on the degradation capacity of the nanocomposites was investigated using a 241 fractional factorial design, which was increased by a central point assayed in triplicate (Table 1). In this kind of factorial designs, eight experiments are necessaries to evaluate the effect of three variables at two levels, which configures a 23 full factorial design. To study the fourth variable it is necessary to establish a defining relation (or generator) that determines which effects are confounded with each other. In this case, a 4 = 1 2 3 defining relation was used, which implies that the main effect of variable 4 (pyrolysis temperature) is confounded with the third-order effect (1 2 3). Similarly, second-order effects are confounded with other second-order effects, as shown in Table 1. Normally, responses are affected by a small number of main effects and lower-order interactions. Consequently, fractional factorial designs can be used to study a large number of factors, focusing the main effects and two-factor interactions. In Table 1, the plus signal represent all the runs were the variable is at the high level, while the minus signal symbolise the low studied level. Thus, the main effect of the variable can be calculated by averaging high and low levels, according to the follow expression [18]:
3
P Effect ¼
2
1
0 200
300
400
500
600
700
800
Wavelength (nm) Fig. 1. Spectrophotometric monitoring of the adsorption and photocatalytic degradation of blue QR 19 dye by the TiO2/C 0.2M-04 nanocomposite.
Yþ Y nþ n
where n represent the number of data points assayed at each level. Interaction effects are calculated of the same way, using the signal that result from the multiplication of the parent terms. Thus, the second-order AB effect is calculated from the signals that result of the product of A and B columns. The effects summarised in Table 1 were calculated according to this procedure and were compared with the standard deviation observed in the analysis of the central point. Thus, it was possible to deduce that the variables H+ concentration,
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Table 3 Factorial design (23) to study the effect of experimental variables on the photocatalytic degradation capacity of TiO2 nanoparticles (Blue QR 19 dye: 250 mL, 50 mg L1; O2: 50 mL min1; reaction time: 30 min). Level
Variable
Synthesis temperature (Ts, 8C) Acid concentration ([H+], mol L1) Pyrolysis temperature (Tp, 8C)
0
+
40 0.10 600
60 0.25 750
80 0.40 900
Run
Ts
Tp
[H+]
QR-19 degradation (%)
1 2 3 4 5 6 7 8 9 10 11
+ + + + 0 0 0
+ + + + 0 0 0
+ + + + 0 0 0
51.0 54.5 14.9 20.8 45.6 48.8 38.1 27.4 32.8 3.0
Main effects: Ts: 0.5; [H+]: 4.7; Tp: 24.7. Second order effects: Ts [H+]: 4.2; Ts Tp: 2.9; [H+] Tp: 10.2. Third order effects: Ts [H+] Tp: 4.1.
synthesis temperature and Ti:FA ratio did not exert any significant effect on the monitored response because the main effect values were similar to the experimental standard deviation (3.4%) evaluated in the triplicate. In contrast, the pyrolysis temperature had a significant positive effect (approximately 22 percentage points) that favoured the photocatalytic degradation process in which catalysts pyrolysed at a higher temperature were used. In the first analysis, the third-order effect involving the variables synthesis temperature (1), Ti:FA ratio (2) and acid concentration (3) appeared significant (Tp P [H+]: 21.9 percentage points). However, it was necessary to consider that, for fractional factorial design involving the I = 1234 defining relation, the third-order effect (1 2 3) was confounded with the main effect of variable 4 (4 = 123). For similar reasons, the 1 4 second-order effect was confounded with the 2 3 effect. However, the fact that variables 2 and 3 did not show any significant effect suggested that this second-order effect was a function of variables 1 and 4.
o Pyrolysis temperature ( C)
900
-3
78 +/- 2
75 +/- 2
To examine this interaction, the preliminary 241 fractional factorial design was transformed into a new 22 factorial design, which allowed each test to be represented by a duplicate. The results shown in Fig. 2 confirmed the existence of a secondorder effect because an increase in the synthesis temperature improved the catalytic activity of the nanocomposites produced using low-temperature pyrolysis (+7 percentage points) while decreasing the efficiency of materials prepared using hightemperature pyrolysis (3 percentage points). According to these observations, the synthesis temperature became insignificant when the pyrolysis was performed at elevated temperatures. This fact was well-illustrated by the higher rates of discolouration observed in experiments 2, 3, 5 and 8 (Table 1), all of which correspond to a pyrolysis temperature of 900 8C. Other relevant characteristics of the TiO2/C nanocomposites are summarised in Table 2. According to these features, the best adsorption and photocatalytic activity were observed in the nanocomposites produced at a higher pyrolysis temperature (900 8C, experiments 2, 3, 5 and 8), which modified the structure of TiO2 (anatase–rutile transformation), the size of the TiO2 particles and the structure of the carbonaceous material. In addition, these materials predominantly showed the rutile crystalline phase, which is usually considered to have low photocatalytic efficiency. 3.2. TiO2 nanoparticles
750
27
600
51 +/- 2 40
73 +/- 2
17
+7
58 +/- 5 60
80 o
Synthesis temperature ( C) Fig. 2. Geometric representation of the 22 factorial design used to identify a secondorder effect between synthesis and pyrolysis temperature on the photocatalytic efficiency of TiO2/C nanocomposites.
For comparative purposes, a new 23 factorial design was developed to evaluate the effect of the synthesis temperature, H+ concentration and pyrolysis temperature on the photocatalytic degradation efficiency of TiO2 nanoparticles (Table 3). In this study each variable was evaluated at two levels, denoted as +1 (high level) and 1 (low level). The left column of Table 3, numbers 1 through 8, specifies a non-randomized run order. The calculated effects shown in Table 3, which are based on a 30 min reaction time for the removal of colour, were compared with the typical deviation observed in the central conditions and tested in triplicate. In view of these values, the synthesis temperature and the concentration of acid did not exert any significant effect on the monitored response because the main effect values were similar to the experimental standard deviation (3.0%).
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Table 4 Additional characteristic of TiO2 nanoparticles. Run
Pyrolysis temperature (8C)
TiO2 phase (%)a A
B
R
1 2 3 4 5 6 7 8 9 10 11
600 600 900 900 600 600 900 900 750 750 750
6.9 5.9 – – 5.6 6.3 – – – – –
– – – – – – – – – – –
93.1 94.1 100.0 100.0 94.4 93.7 100.0 100.0 100.0 100.0 100.0
Adsorption (%)b
Photocatalytic degradation (%)c
ND 1.5 0.2 0.7 1.7 1.8 1.8 1.0 2.0 2.0 2.0
51.0 54.5 14.9 20.8 45.6 48.8 38.1 27.4 31.0 31.1 36.3
ND: not detected. a A: anatase; B: brookite; R: rutile. b Blue QR 19 adsorption at contacting time of 30 min. c Blue QR 19 photocatalytic degradation at contacting time of 30 min.
On the contrary, the pyrolysis temperature had a significant negative effect (approximately 25 percentage points), which indicated the disadvantage of the photocatalytic degradation process with samples prepared at a high pyrolysis temperature. These observations were illustrated by the higher rates of discolouration observed in experiments 1, 2, 5 and 6 (Table 3), all of which correspond to a lower pyrolysis temperature (600 8C). Furthermore, a significant second-order effect of the variables was found between the acid concentration and pyrolysis temperature (approximately 10 percentage points), which was clearly observed in the geometric representation shown in Fig. 3. These results (Fig. 3) confirmed the greater catalytic efficiency of photocatalysts produced at lower pyrolysis temperature, especially for materials synthesised at low acid concentrations (Experiments 1 and 2, Table 3). Other relevant information pertaining to the TiO2 nanoparticles produced in this study are summarised in Table 4. According to these results, all of the materials produced at a pyrolysis temperature greater than 750 8C consisted of 100% rutile phase, which could explain their low photocatalytic efficiency. However, the best-performing materials were produced at the lower pyrolysis temperature (600 8C, experiments 1, 2, 5 and 6, Tables
900
+15
18 +/- 4
2 and 4), which produced materials with approximately 6–7% anatase phase of higher photocatalytic activity. 3.3. TiO2 nanoparticles vs. TiO2/C nanocomposites In this work, several relevant observations were made, particularly those that pertain to the effect of synthesis variables on the characteristics and photocatalytic efficiency of the produced materials. First, the presence of carbon retarded the rutilisation process of TiO2, which allowed the permanence of the anatase phase even at pyrolysis temperatures of 900 8C (see Table 2). This effect was clearly evidenced in the sequence of XRD spectra shown in Fig. 4, which indicate the virtual absence of anatase phase in TiO2 nanoparticles pyrolysed at 600 8C and the preservation of this phase in the photoactive TiO2/C nanocomposite pyrolysed at the same temperature. According to Slimen et al., the carbon matrix, which has a high surface area, baffles the anatase-to-rutile phase transformation because of its large interfacial energy, thereby creating an anti-calcination effect [19]. Moreover, the results presented in Fig. 5 show a low photocatalytic degradation capacity of TiO2 nanoparticles produced at a pyrolysis temperature of 600 8C, which was justified by the preponderant presence of the rutile phase of lower photochemical activity. For their part, the TiO2/C nanocomposites that
33 +/- 8
o
Pyrolysis temperature ( C)
R A
600
33 +/- 3
-35
-14
-6
53 +/- 3 0.10
0.25
Relative intensity
750
R
R
A
TiO2/C (600°C) TiO2/C (400°C)
47 +/- 2
TiO2 (600°C)
0.40
TiO2 (400°C)
+
[H ] Concentration Fig. 3. Geometric representation of the 22 factorial design used to identify a secondorder effect between acid concentration and pyrolysis temperature on the photocatalytic efficiency of TiO2 nanoparticles.
10
20
30
o
40
50
60
2θ
Fig. 4. XRD spectra of TiO2/C nanocomposites and TiO2 nanoparticles obtained at different pyrolysis temperature.
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ADSORPTION TiO2 , 600 °C
0.6
TiO2/C, 900 °C
PHOTOCATALYSIS TiO2 , 600 °C
Absorbance (592 nm)
0.5
TiO2/C, 900 °C 0.4
0.3
0.2
0.1
0.0 0
10
20
30
40
50
60
70
80
90
Reaction time (min) Fig. 5. Adsorption and degrading capacity of TiO2/C nanocomposites and TiO2 nanoparticles obtained at the best synthesis conditions.
exhibited better photocatalytic performance were produced at pyrolysis temperature of 900 8C, which, as previously discussed (see Table 2), also led to a majority of rutile phase in the composite. The degradation efficiency of the rutile-phase nanocomposites can only be explained by the favourable effect of the presence of carbon. In general, the presence of carbon-based adsorbents allowed a high concentration of substrate molecules in the vicinity of TiO2 particles. The high concentration of substrate molecules not only sped up the process of photodegradation, but contributed to the adsorption and subsequent oxidation of byproducts of the reaction, which additionally inhibited the release of species with the highest toxicity [20]. However, the presence of defects, especially oxygen vacancies, are well known to control many of the surface properties of semiconductor oxides [21], which highlights, for example, the sequestration of electrons and the consequent preservation of the positive holes [22]. According to recent studies [23], the presence of carbonaceous materials during the heat treatment of the samples induced the reduction of the TiO2 anatase with the formation of TiO2x and oxygen vacancies. Heat treatments at higher temperatures led to the formation of rutile-phase TiO2 that contained Ti3+ and exhibited high photocatalytic activity, as well as to the concomitant diffusion of vacancies from the surface to the bulk of the particles [23,24]. In recent years, the characteristic synergy observed between semiconductors and graphene [25–27] and carbon nanotubes [28], which usually improve the performance of specific applications, such as heterogeneous photocatalysis, have been unquestionably demonstrated. Finally, the use of TiO2/C nanocomposites in photocatalytic processes aimed at the remediation of liquid wastes appears to be convenient because of the facilitated sedimentation of the material. These results represent an important precedent because it would allow for the development of fluidised bed reactors, which would enable the continued treatment of high volumes of liquid wastes. 4. Conclusions According to the results reported in this study, the convenience of using multivariate optimisation methods has been demonstrated.
In the synthesis of TiO2/C nanocomposites was observed that the variables H+ concentration, synthesis temperature and Ti:FA ratio did not exert any significant effect on the photocatalytic activity of the material. On the other hand, a second-order effect (synthesis and pyrolysis temperature) showed that increased activity of photocatalytic materials can be obtained at higher pyrolysis temperatures with little or no influence of the synthesis temperature. In the synthesis of TiO2 nanoparticles the factorial design demonstrates that H+ concentration and synthesis temperature did not influence the degradation capacity of the photocatalyst. In contrast, the pyrolysis temperature had a significant negative effect, which indicated the disadvantage of the photocatalytic degradation process with samples prepared at a high pyrolysis temperature. Furthermore, a second-order effect (pyrolysis temperature and acid concentration) was also found in the synthesis of TiO2 nanoparticles, which showed little influence of the acid concentration, because the pyrolysis temperature was maintained at values near the lower temperature level (600 8C). The most relevant conclusion is probably represented by the effect of the pyrolysis temperature on the photocatalytic activity of both materials. Whereas the degradation capability of the TiO2/C nanoparticles was maximised at a high pyrolysis temperature (main effect of 21.9 percentage points), which resulted in products that contain mostly rutile-phase TiO2, the TiO2 nanoparticles were more effective when synthesised at a low temperature of pyrolysis (main effect of 24.7 percentage points), which tended to maintain the anatase phase. The beneficial effect of the presence of carbon was evident because of the unusually low photocatalytic degradation capacity of the rutile phase. References [1] U.I. Gaya, A.H. Abdullah, J. Photochem. Photobiol. C: Photochem. Rev. 9 (2010) 1–12. [2] M.N. Chong, B. Jin, C.W.K. Chow, C. Saint, Water Res. 44 (2010) 2997–3027. [3] S. Malato, J. Blanco, A. Vidal, C. Richter, Appl. Catal. B: Environ. 37 (2002) 1–15. [4] A. Fujishima, X. Zhang, D.A. Tryk, Surf. Sci. Rep. 63 (2008) 515–582. [5] A.Y. Shan, T.I. MohdGhazi, S.A. Rashid, Appl. Catal. A: Gen. 389 (2010) 1–8. [6] R. Leary, A. Westwood, Carbon 49 (2011) 741–772. [7] S.M. Tracey, S.N.B. Hodgson, A.K. Ray, Z. Ghassemlooy, J. Mater. Process. Technol. 77 (1998) 86–94. [8] Y. Tao, J. Pan, S. Yan, B. Tang, L. Zhu, Mater. Sci. Eng. B: Solid State Mater. Adv. Technol. 138 (2007) 84–89. [9] L. Luo, A.T. Cooper, M. Fan, J. Hazard. Mater. 161 (2009) 175–182. [10] S. Ko, J. Pekarovic, P.D. Fleming, P. Ari-Gur, Mater. Sci. Eng. B: Solid State Mater. Adv. Technol. 166 (2010) 127–131. [11] M.M. Oliveira, D.C. Schnitzler, A.J.G. Zarbin, Chem. Mater. 15 (2003) 1903–1909. [12] C.D. Almeida filho, A.J.G. Zarbin, Carbon 44 (2006) 2869–2876. [13] H. Zhang, J.F. Banfield, J. Phys. Chem. B 104 (2000) 3481–3487. [14] M. Saquib, M. Muneer, Dyes Pigment. 53 (2002) 237–249. [15] Y. Yu, J.C. Yu, C.Y. Chan, Y.K. Che, J.C. Zhao, L. Ding, W.K. Ge, P.K. Wong, Appl. Catal. B: Environ. 61 (2005) 1–11. [16] D. Huang, Y. Miyamoto, T. Matsumoto, T. Tojo, T. Fan, J. Ding, Q. Guo, D. Zhang, Sep. Purif. Technol. 78 (2011) 9–15. [17] P. Bansal, D. Singh, D. Sud, Sep. Purif. Technol. 72 (2010) 357–365. [18] G.E.P. Box, W.G. Hunter, J.S. Hunter, Statistics for Experimenters: An Introduction to Design, Data Analysis, and Model Building, John Wiley and Sons, New York, 1978. [19] H. Slimen, A. Houas, J.P. Nogier, J. Photochem. Photobiol. A: Chem. 221 (2011) 13–21. [20] H. Yoneyama, T. Torimoto, Catal. Today 58 (2000) 133–140. [21] H. Cheng, A. Selloni, Phys. Rev. B 79 (2009) 1–4. [22] G. Colo´n, M.C. Hidalgo, G. Munuera, I. Ferino, M.G. Cutrufello, J.A. Navı´o, Appl. Catal. B: Environ. 63 (2006) 45–59. [23] S. Yang, W. Tang, Y. Ishikawa, Q. Feng, Mater. Res. Bull. 46 (2011) 531–537. [24] C. Xiao-Quan, L. Huan-Bin, G. Guo-Bang, Mater. Chem. Phys. 91 (2005) 317–324. [25] Y. Zhang, Z.R. Tang, X. Fu, Y.J. Xu, ACS Nano 4 (2010) 7303–7314. [26] Y. Zhang, Z.R. Tang, X. Fu, Y.J. Xu, ACS Nano 5 (2011) 7426–7435. [27] Y. Zhang, N. Zhang, Z.R. Tang, Y.J. Xu, Phys. Chem. Chem. Phys. 14 (2012) 9167–9175. [28] Y.J. Xu, Y. Zhuang, X. Fu, J. Phys. Chem. C 14 (2010) 2669–2676.