Abstract

Photodegradation processes are of great interest in a range of applications, one of which is the photodecomposition of pollutants. For this reason, analysing nanoparticles that improve the efficiency of these processes under solar radiation are very necessary. Thus, in this study, TiO2 was doped with Mo and Cu using low-temperature hydrolysis as the method of synthesis. Pure TiO2 and x%MoS2/Cu/TiO2 nanoparticles were prepared, where x is the theoretical quantity of MoS2 added (0.0%, 1.0%, 5.5%, 10.0%), setting the nominal quantity of Cu at 0.5 wt.%. The samples obtained were characterized by X-ray diffraction, Raman spectroscopy, X-ray electron spectroscopy and UV-Vis spectroscopy in diffuse reflectance mode. The results suggest that the TiO2 structure was doped with the Mo6+ and Cu2+ ions in the position of the Ti4+. The x%MoS2/Cu/TiO2 samples presented lower band gap energy values and greater optical absorption in the visible region than the pure TiO2 sample. Lastly, the photocatalytic activity of the samples was assessed by means of the photodegradation of methylene blue under visible light. The results show that when the quantity of Mo in the co-doped samples increased (x%MoS2/Cu/TiO2) there were significant increases of up to 93% in the photocatalytic activity.

INTRODUCTION

A great deal of research has been performed in the last few decades into TiO2 thanks to its magnificent electronic and photocatalytic properties, low toxicity, low cost and high chemical stability (Szczepanik 2017). Photocatalytic efficiency depends on a range of factors including crystalline phases, morphology, specific surface, chemical surface, defects, band gap energy, type of dopant and the level of doping (Vattikuti et al. 2015; Fu et al. 2016; Munir et al. 2016). However, the studies performed have raised controversy regarding the influence of these properties on photocatalytic activity.

The electron-gap pairs generated in the bulk material as a consequence of the light absorbed must reach the surface of the photocatalyst to react and begin the photodegradation. The valence band establishes the electron-gap pairs available and the conductivity of the gaps, while the conduction band determines the mobility of the electrons. Consequently, the density of these states and their energy will determine the efficiency so that charged particles reach active centres on the surface of the material (Wojtaszek et al. 2016). Therefore, for a photocatalyst to be really efficient it is essential to minimize the recombination of photogenerated charge carriers.

Studies exist reporting that doping with molybdenum is an effective way of improving photoactivity thanks to the creation of low conduction band states that act as electron donors (Stengl & Bakardjieva 2010). Doping with copper has also been shown to enhance photocatalytic activity under visible light due to the creation of new states in the valence band that lead to a smaller band gap (Navas et al. 2014). However, there are not many studies of photocatalytic applications using Mo and Cu as dopants. Chaudhary et al. (2014) reported the effect of co-doping of TiO2 with Mo and Cu using different synthesis methods and proportions of the dopants and evaluated the photoactivity of TiO2 under UV light. Thus, it is reasonable to think that co-doping with Cu and Mo will have a synergic effect on photocatalytic efficiency under irradiation with visible light. This study presents the effect of co-doping with copper and molybdenum on the properties of TiO2 using MoS2 and CuCl2·2H2O as precursors. More specifically, a study was performed into the influence of the percentage of co-doping on the optical and structural properties of the photocatalysts synthesized. Finally, the effect of these properties on the enhanced photocatalytic efficiency was assessed in terms of the photodegradation of methylene blue (MB) under visible radiation.

EXPERIMENTAL

Chemicals

The reagents were from commercial sources and used without further purification. Titanium(IV) n-butoxide (97%) and MoS2 powder were supplied by Sigma-Aldrich; nitric acid (HNO3, 65%) was from Merck; and CuCl2·2H2O (purity 98%) and MB (purity 82%) were supplied by Panreac.

Synthesis

The MoS2/Cu/TiO2 samples were synthesized using the low-temperature hydrolysis method described in previous studies (Alcantara et al. 2011). Titanium n-butoxide, MoS2 and CuCl2·2H2O were used as precursors of TiO2, Mo and Cu respectively. Also, the cost of reagents is low, and the procedure is not expensive.

The stoichiometric amount of MoS2 was added to obtain nominal proportions of MoS2 of 1.0%, 5.5%, and 10% wt. And a stoichiometric amount of CuCl2·2H2O was added to obtain nominal proportions of Cu/TiO2 of 0.5%. This Cu concentration makes it possible to produce a decrease in the band gap energy of TiO2, which leads to enhanced light harvesting in the visible range. This concentration is also low enough so as not to generate distortions in the anatase phase of TiO2. Distortions in the crystalline phase of Cu-doped TiO2 have been reported previously at higher levels of doping (Navas et al. 2014). Finally, as-prepared MoS2/Cu/TiO2 samples were annealed in air for 1 h at 973 K. Since the theoretical quantity of Cu was identical in all the samples synthesized (0.5% wt.) and only the amount of the source of Mo added was modified, from here on the samples will be named x%MoS2/Cu/TiO2, where x is the theoretical quantity of MoS2 added (0.0%, 1.0%, 5.5%, 10.0%). Also, pure TiO2 was synthesized using the same procedure to compare the results obtained.

Characterization

Pure TiO2 and x%MoS2/Cu/TiO2 samples were characterized to understand how CuCl2·2H2O and MoS2 affected their photocatalytic properties. Several instrumental techniques were used in order to obtain the proportion of Mo and Cu, their crystalline phases and band gap energy.

Plasma atomic emission spectroscopy (ICP-AES) was used to study the composition of the x%MoS2/Cu/TiO2 samples using an Iris Intrepid spectrometer, supplied by Thermo Elemental. To identify the different crystalline phases and estimate the crystallite size of synthetized samples, X-ray diffraction (XRD), model D8Advanced diffractometer supplied by Bruker, was used. The XRD spectra were recorded with 2θ in a range from 20 to 70° with a resolution of 0.02°, 40 kV and 30 mA. The structural characterization was accomplished by Raman spectroscopy using a Jobin Yvon U1000 double monochromator equipped with a Hamamatsu R-943 photomultiplier, using a DPSS 532 nm laser supplied by CNI, model MSL-III-532 nm–50 mW. On the other hand, to analysis the oxidation states of the samples X-ray photoelectron spectroscopy (XPS) experiments were performed using a Kratos Axis UltraDLD spectrometer. Furthermore, the optical properties of the samples and the band gap energy were determined by means of UV-Vis spectroscopy in diffuse reflectance mode (DR-UV-Vis). The equipment, assembled in our laboratory, was composed of an ASB-XE-175 Xenon lamp supplied by Spectral Products, as the illumination source; a USB2000+ spectrometer supplied by Ocean Optics; and an integrating sphere from Spectra Tech. Finally, the photocatalytic activity of the MoS2/Cu/TiO2 samples was analysed. The photodegradation of MB was assessed using equipment assembled in our laboratory, composed of a chamber (750 × 400 × 300 mm) wherein stable temperature (20 °C) was achieved during all the experiments. Inside of the chamber, at the top, a halogen lamp (model SL500R, 230 V/50 Hz, 500 W max.), used as the visible irradiation source, is located. A manual lab lifting platform was used to control the distance between the reactor and the irradiation source. The initial concentration of the aqueous solution of MB (purity 82%, Panreac) was 5·10−5 M, and the amount of photocatalyst was 0.3 g L−1. The reaction time was 6 h, and the mixture of photocatalyst and the MB solution was kept in darkness for 3 h before the visible irradiation to reach the adsorption–desorption equilibrium. The photodegradation of MB was studied by absorbance measurements. A calibration curve was used to determine the evolution of the MB concentration and the kinetics of the photodegradation. The absorbance was measured using a spectrometer (Ocean Optics, USB2000+) with a UV-Vis-NIR light source (Ocean Optics, DH-2000-BAL).

RESULTS AND DISCUSSION

ICP-AES

ICP-AES was used to determine the amount of Mo and Cu incorporated into the structure of the samples. The theoretical percentage of Cu by weight added to the samples was 0.5% wt., while the theoretical quantities of MoS2 by weight added were 0.0%, 1.0%, 5.5% and 10.0% wt.

The percentages of Cu and Mo obtained in the ICP-AES analysis of the samples synthesized are shown in Table 1. The results show that 80–100% of the precursor of Cu and 37–59% of the sources of Mo were successfully incorporated into the TiO2 structure. As the percentage of MoS2 added increased there was a slight decrease in the percentage of Cu incorporated, as Figure 1 shows.

Table 1

Theoretical percentage of x%MoS2/Cu/TiO2 versus real weight percentage of Cu and Mo incorporated determined by ICP-AES

Samples Cu ICP-AES (%w/w) Mo ICP-AES (%w/w) 
Pure TiO2 ≤0.10a ≤0.10a 
0.0%MoS2/Cu/TiO2 0.560 ± 0.050 ≤0.10a 
1.0%MoS2/Cu/TiO2 0.480 ± 0.020 0.370 ± 0.011 
5.5%MoS2/Cu/TiO2 0.464 ± 0.003 3.240 ± 0.020 
10.0%MoS2/Cu/TiO2 0.400 ± 0.010 5.200 ± 0.200 
Samples Cu ICP-AES (%w/w) Mo ICP-AES (%w/w) 
Pure TiO2 ≤0.10a ≤0.10a 
0.0%MoS2/Cu/TiO2 0.560 ± 0.050 ≤0.10a 
1.0%MoS2/Cu/TiO2 0.480 ± 0.020 0.370 ± 0.011 
5.5%MoS2/Cu/TiO2 0.464 ± 0.003 3.240 ± 0.020 
10.0%MoS2/Cu/TiO2 0.400 ± 0.010 5.200 ± 0.200 

aPercentage lower than the detection limit of analytical method.

Figure 1

Percentage of Mo and Cu incorporated into the TiO2 structure obtained by ICP-AES versus the nominal percentage added in the samples of x%MoS2/Cu/TiO2 (x = 0.0%, 1.0%, 5.5%, 10.0%).

Figure 1

Percentage of Mo and Cu incorporated into the TiO2 structure obtained by ICP-AES versus the nominal percentage added in the samples of x%MoS2/Cu/TiO2 (x = 0.0%, 1.0%, 5.5%, 10.0%).

On the other hand, the percentage of Mo incorporated was seen to increase when the percentage of MoS2 added increased, but only up to a point, a slight decrease being observed in the sample with the highest concentration (10.0%MoS2/Cu/TiO2).

XRD

Figure 2 shows the diffraction patterns of the x%MoS2/Cu/TiO2 samples, where x = 0.0%, 1.0%, 5.5%, 10.0% wt of MoS2. The diffractograms do not reveal the presence of any phases associated with the formation of any copper species (JCPDS 45-0937, JCPDS 065-3288) and no peaks are observed related to the presence of MoS2 (Reference JCPDS 87-2416); only anatase and rutile phase peaks can be seen and these have been assigned in the figure. Thus, the diffraction patterns obtained suggest that the Mo and Cu ions were mainly distributed within the crystalline lattice of the TiO2.

Figure 2

XRD patterns of the pure TiO2 and x%MoS2/Cu/TiO2 (x = 0.0%, 1.0%, 5.5%, 10.0%) samples.

Figure 2

XRD patterns of the pure TiO2 and x%MoS2/Cu/TiO2 (x = 0.0%, 1.0%, 5.5%, 10.0%) samples.

The XRD patterns of the samples show that the peak width corresponding to the plane (101) of the anatase phase and to the plane (110) of the rutile phase increased as the percentage of Cu and MoS2 added increased. This suggests that the structure becomes distorted with the increase in Cu and Mo incorporated. The ionic radii of the Mo ion (r(Mo6+) = 0.62 Å) and Cu ion (r(Cu2+) = 0.72 Å) are very similar to those of the Ti ion (r(Ti4+) = 0.68 Å). The distortion observed may be produced by Ti ions being replaced by Cu and Mo ions in the TiO2 lattice (Weast 1979; Wang et al. 2013).

Regarding the phases present, in the pure TiO2 sample, a mixture of anatase–rutile phase is present, rutile phase being predominant. In the TiO2 0.0%MoS2/Cu/TiO2 samples (containing 0.5%Cu and 0.0%MoS2) the presence of Cu is seen to increase the proportion of rutile with regard to the pure sample; in other words, it promotes the transformation of anatase phase to rutile. In the remaining samples (x%MoS2/Cu/TiO2) the quantity of anatase increases as the amount of MoS2 added increases, which suggests that the Mo inhibits the transition of anatase to rutile, anatase being the predominant phase. However, sample 10%MoS2/Cu/TiO2 does not follow this trend, showing instead a diffraction pattern similar to the pure TiO2 sample but with a shift in the peaks. This difference suggests the structures are co-doped with Mo and Cu. However, as the quantity of Mo increases it tends to diffuse towards the surface in the case of the sample with the highest concentration (10%MoS2/Cu/TiO2), producing a structural reorganization in which the internal part presents higher levels of Cu doping, which would justify the appearance of rutile, while the part nearer the surface is enriched with Mo.

From the XRD patterns, a semi-quantitative assessment was performed of several crystalline properties, such as the percentage of anatase (A) and rutile (R) phases, the average crystallite size (t) and the unit cell volume (V).

The average crystallite size (t) was calculated according to the Scherrer equation (Landmann et al. 2012): t = (0.9·λ)/β·cosθ, where λ is the wavelength of the X-ray radiation (1.5406 Å) and β is the full width at half-maximum height of the most intense peak of the sample. To calculate the proportion of anatase (WA) and rutile (WR) phases, given in mass fraction, the relative intensity of (101) and (110) peaks of anatase and rutile, respectively, was used. The relative content of anatase was estimated by the equation WA = 1/(1 + 1.26·IR/IA), where IA and IR are the intensities of the reflection of the (101) and (110) planes for the anatase and rutile phases, respectively (Landmann et al. 2012) and relative content of rutile as WR= 100 − WA; that is, only anatase and rutile phases were considered. Furthermore, the volume of the unit cell was calculated from the values of the lattice constants a, b and c corresponding to the maximum intensity peak as V = a2·c (a=bc, for anatase and rutile phases).

Table 2 shows that the percentage of anatase increased in the MoS2/Cu/TiO2 samples with regard to the pure TiO2 sample, except in the case of 10.0%MoS2/Cu/TiO2, in which a higher percentage of rutile was formed. In this sample, similar percentages of anatase and rutile to the unmodified TiO2 sample were obtained. The same trend is observed in the lattice parameters with slight modifications in a and c and a very similar cell unit volume for the pure TiO2 10.0%MoS2/Cu/TiO2 samples. Due to the effect of the Cu, the predominant phase in the 0.0%MoS2/Cu/TiO2 sample was rutile. Thus, this likeness suggests that the Cu doping of the crystal may be responsible for the higher proportion of rutile generated. And as discussed above, at high concentration of Mo, it tends to diffuse towards the surface. This diffusion process leads to a reorganization where the inner part of the crystal shows an enrichment in Cu doping. This can justify the presence of a high percentage of rutile in this phase. Moreover, the outer part of the crystallites shows an enrichment in Mo, which is of great interest for photocatalytic processes as is discussed below.

Table 2

Semi-quantitative values of anatase and rutile mass fraction, average crystallite size, unit cell volume obtained from XRD patterns of the samples synthesized

Samples WA/WR/% t/nm a/Å c/Å V3 
Pure TiO2 18.89 81.11 113.24 4.59 2.71 57.20 
0.0%MoS2/Cu/TiO2 100 64.51 4.64 2.89 62.22 
1.0%MoS2/Cu/TiO2 58.05 41.95 22.12 3.77 9.51 135.29 
5.5%MoS2/Cu/TiO2 72.50 27.50 24.86 3.79 9.40 134.77 
10.0%MoS2/Cu/TiO2 19.21 80.79 108.39 4.56 2.73 57.36 
Samples WA/WR/% t/nm a/Å c/Å V3 
Pure TiO2 18.89 81.11 113.24 4.59 2.71 57.20 
0.0%MoS2/Cu/TiO2 100 64.51 4.64 2.89 62.22 
1.0%MoS2/Cu/TiO2 58.05 41.95 22.12 3.77 9.51 135.29 
5.5%MoS2/Cu/TiO2 72.50 27.50 24.86 3.79 9.40 134.77 
10.0%MoS2/Cu/TiO2 19.21 80.79 108.39 4.56 2.73 57.36 

Furthermore, analysing the evolution of the average crystallite size (t) in accordance with the percentage of Mo and Cu in the structure of the TiO2, t is seen to decrease with an increased percentage of co-doping. This smaller crystallite size may be justified by the incorporation of Cu and Mo into the structure. This distorts the structure and breaks the crystal continuity, resulting in smaller average crystallite sizes, as reported for others dopants (Xu et al. 2012; Aguilar et al. 2013). In the samples with the highest proportions of Mo (10.0%MoS2/Cu/TiO2), the increase in the crystallite size is due to the appearance of rutile. The decrease in the crystal particle size observed in the co-doped samples helps to enhance photocatalytic activity (Zhang et al. 2012), possibly thanks to their higher specific surface area which leads to an increase of adsorption centres, which favours the adsorption of species onto the surface, as is shown below from XPS results.

Raman

Figure 3 shows the Raman spectra of the x%MoS2/Cu/TiO2 samples synthesized. The Raman spectra recorded only show the active Raman modes of the anatase and rutile phases. The active modes of the anatase phase appear at 144 cm−1 (Eg), 197 cm−1 (Eg), 399 cm−1 (B1g), 519 cm−1 (A1g + B1g) and 639 cm−1 (Eg) (Patel et al. 2011) while those of the rutile phase can be seen at 143 cm−1 (B1g), 447 cm−1 (Eg) and 612 cm−1 (A1g) (Mathews et al. 2009). None of the spectra show evidence of any crystalline phase formed by Mo or Cu species, which suggests that the Mo and Cu ions were incorporated into the inside of the crystal.

Figure 3

Raman spectra of the pure TiO2 and x%MoS2/Cu/TiO2 samples (x = 0.0%, 1.0%, 5.5%, 10.0%).

Figure 3

Raman spectra of the pure TiO2 and x%MoS2/Cu/TiO2 samples (x = 0.0%, 1.0%, 5.5%, 10.0%).

Rutile was the predominant phase in the pure TiO2, 0.0%MoS2/Cu/TiO2 and 10.0% MoS2/Cu/TiO2 samples, but anatase was the predominant phase in the remaining MoS2/Cu/TiO2 doped samples.

Furthermore, as the quantity of MoS2 added to the co-doped samples increases, the bands widen suggesting that the TiO2 structure becomes distorted because of the co-doping with Cu and Mo. In addition, the Raman spectra obtained reveal a blue-shift in the band of the Eg(1) vibrational mode of the anatase phase. The substitution of the Ti ions by Mo generates tensions and distortions in the crystalline lattice and these distortions lead to the shift in the Raman bands (Kang 2005; Zhan et al. 2013). Thus, the results obtained from the Raman spectra are in agreement with those from XRD given above.

XPS

The oxidation states of the samples were determined using XPS spectroscopy. As an example, the spectra are shown of the pure TiO2, 0.0%MoS2/Cu/TiO2 and 5.5%MoS2/Cu/TiO2 samples (Figure 4). In the spectra obtained for the Ti 2p signal (Figure 4(a)), two peaks can be seen at 464 and 459 eV corresponding to Ti 2p1/2 and Ti 2p3/2, consistent with the Ti4+ values in the TiO2 structure (Wang et al. 2013). The typical values reported for the Ti 2p3/2 signal with a +4 oxidation state is 458.66 eV, while the Ti signal with +3, +2, and 0 oxidation state appears at 457.12, 455.34, and 453.86 eV (Biesinger et al. 2010). In addition, Figure 4(b) shows the spectrum of the Mo 3d signal. The peaks of Mo 3d5/2 and Mo 3d3/2 are located at 233.17 and 236.17 eV, which are assigned to Mo6+ (Houng et al. 2013; Chaudhary et al. 2014; Huang et al. 2015). The typical values reported for the Mo 3d5/2 signal with +4, +5, and +6 oxidation states appear at 229.7, 231.4, and 232.5 eV, respectively (Nguyen et al. 2015; Erdogan et al. 2016). No signal can be found related to the presence of Mo+4 and Mo+5, indicating that the main oxidation state of molybdenum is +6. In turn, Figure 4(c) shows the Cu 2p signal of the 0.0%MoS2/Cu/TiO2 and 5.5%MoS2/Cu/TiO2 samples. Two signals appear at 932 and 953 eV attributed to Cu 2p3/2 and Cu 2p1/2, respectively. These signals may correspond to a Cu2+ oxidation state, which is coherent for the procedure used (Duke et al. 2015).

Figure 4

XPS spectra of pure TiO2, 0.0% MoS2/Cu/TiO2 and 5.5%MoS2/Cu/TiO2: (a) Ti 2p, (b) Mo 3d, (c) Cu2p and (d)–(f) O 1s.

Figure 4

XPS spectra of pure TiO2, 0.0% MoS2/Cu/TiO2 and 5.5%MoS2/Cu/TiO2: (a) Ti 2p, (b) Mo 3d, (c) Cu2p and (d)–(f) O 1s.

Furthermore, in the O 1s signal of the pure TiO2 sample (Figure 4(d)) an asymmetric peak can be seen that could be composed of the peak at 529.0 eV, which is usually assigned to the oxygen in the TiO2 lattice, together with another peak in the 530–532 eV region, which is indexed to absorbed oxygen species (Luo et al. 2012). The 0.0%MoS2/Cu/TiO2 sample shows a slight shift of the O signal of the lattice with regard to the pure TiO2 sample due to the surroundings of the Ti being modified because of the doping with Cu. It is also possible to observe an increase in the signal belonging to absorbed species after Cu doping (Figure 4(d)), possibly due to the increase in the absorption centres produced to compensate for the oxygen vacancies created when the Ti4+ is replaced by Cu2+ (Yang et al. 2016). In turn, in the 5.5%MoS2/Cu/TiO2 sample, the peak centred at 529.0 eV shifts to 530.43 eV, indicating that the surface oxygen of the lattice is bonded to Mo (Lu et al. 2014; Li et al. 2017). Furthermore, as Figure 4(e)–4(f) show, the signal corresponding to absorbed species is more intense in the co-doped sample than in the one doped only with copper. The results suggest that substituting Ti4+ with Mo6+ generates an O deficiency. This may be compensated for with more oxygen adsorbed onto the surface. Thus, Cu-Mo co-doping promotes the adsorption of species onto the surface, which may capture the electrons necessary to form H2O2, HO2 and O2 species that enhance photocatalytic activity (Simonsen et al. 2009; Wang et al. 2013).

DR-UV-VIS

The diffuse reflectance spectra of the pure TiO2 sample and the x%MoS2/Cu/TiO2 samples were recorded to analyse the absorption of light in the samples synthesized (Figure 5(a)). The x%MoS2/Cu/TiO2 samples absorbed 40% more radiation in the visible range than the pure TiO2. These enhanced optical properties in the x%MoS2/Cu/TiO2 samples is one of the factors affecting their improved photocatalytic activity, offering more valence electrons that can easily be converted into free electrons as a result of photonic absorption (Erdogan et al. 2016).

Figure 5

(a) UV-Vis spectra in diffuse reflectance mode and (b) Kubelka–Munk function for estimating the band gap energy of pure TiO2 and x%MoS2/Cu/TiO2 samples.

Figure 5

(a) UV-Vis spectra in diffuse reflectance mode and (b) Kubelka–Munk function for estimating the band gap energy of pure TiO2 and x%MoS2/Cu/TiO2 samples.

Furthermore, the band gap energy was calculated from the Kubelka–Munk (f(R)) function and Tauc plot (Murphy 2007). For TiO2, the Tauc plot satisfies the equation (Serpone et al. 1995): [f(R]1/2 = K·(Eg), where is the photon energy, Eg is the band gap energy, and K is a characteristic constant of each semiconductor. The Tauc plots for the samples analysed in this study are shown in Figure 5(b). The linear segment of the graph of [f(R]1/2 versus was extrapolated to intersect the axis to obtain the indirect band gap value of the samples under study (Serpone et al. 1995; Sasca & Popa 2013). Table 3 shows the band gap energy values obtained.

Table 3

Band gap energy values for the x%MoS2/Cu/TiO2 samples synthesized (x = 0.0%, 1.0%, 5.5%, 10.0% wt. of MoS2)

x%MoS2/Cu/TiO2 Pure TiO2 0.0% 1.0% 5.5% 10.0% 
Eg (eV) 2.90 2.88 2.25 2.70 2.83 
x%MoS2/Cu/TiO2 Pure TiO2 0.0% 1.0% 5.5% 10.0% 
Eg (eV) 2.90 2.88 2.25 2.70 2.83 

The evolution of the band gap shown in Table 3 indicates that a red-shift takes place in the x%MoS2/Cu/TiO2 samples with regard to the pure TiO2 sample. However, as the quantity of MoS2 added to the %MoS2/Cu/TiO2 samples increases, there is a slight increase in the band gap value. The tendency for the band gap value to increase with the increase in Mo doping may be explained by the well-known Burstein–Moss effect (Moss 1954), whereby the lower states of the conduction band are blocked and transitions can only take place at higher energy levels in the conduction band. This effect is often observed in degenerated semiconductors in which the Fermi level coincides with or exceeds one of the edges of the permitted bands, something that often occurs when there is a high concentration of impurities. Thus, the shift observed towards a higher band gap suggests an increase in the carrier density when the Mo content increases (Bharti et al. 2016; Munir et al. 2016). Nevertheless, in all the %MoS2/Cu/TiO2 samples the band gap value was lower than that of the pure TiO2 sample, which benefits photocatalytic activity under visible light.

Photocatalytic activity

Tests were performed on the photodegradation of MB in order to study the effect of the Mo-Cu co-doped TiO2 on photocatalytic activity. A reference experiment was performed without a catalyst and no significant degradation of the MB was found. Furthermore, the samples of MB with photocatalyst were kept in the dark for 3 hours to ensure they reached the adsorption–desorption equilibrium. The concentration of MB used for the tests was 5·10−5 M. Figure 6 shows the time evolution of the degradation of MB using the different samples studied as a photocatalyst.

Figure 6

Photodegradation of MB of the different photocatalysts under visible light.

Figure 6

Photodegradation of MB of the different photocatalysts under visible light.

Studies about photodegradation routes of MB have been reported in the literature (Houas et al. 2001; Sinha & Ahmaruzzaman 2015; Bakre et al. 2016; Ray et al. 2017). The hydroxyl radical shows high capability for degrading MB, generating different products. These products can be generated following several routes. One of these routes is the oxidation of S following by N-demethylation, hydroxylation and mineralization (Bakre et al. 2016; Ray et al. 2017). Another possible route is the attack of OH radical to the group C─S+ = C of MB. In this stage, O atom is joined to S atom (S = O) and H atom is joined to N atom (N-H). After, two methyl groups joined to N can be lost. Also, these products can be degraded generating several compounds with simple rings. Thus, the breakup of the bonds C-S and N-C by OH radical can generate several structures with single rings, which can finally lead to mineralization in NO3−, SO42−, Cl, NH4+, CO2 and H2O.

In accordance with the results obtained (Figure 6), the photocatalytic degradation of MB, using the samples studied as a photocatalyst, is a pseudo-first order reaction and its kinetics can be described by ln(C0/C) = −kKt = kappt, where C0 is the initial concentration of MB, C is the concentration of MB at the time of irradiation t, k is the reaction velocity constant, K is the adsorption constant of the reactant and kapp is the apparent constant rate (Hwang et al. 2012). The kapp values and the calculation of the percentage of degradation are shown in Table 4. The constant rate values show that the co-doped x%MoS2/Cu/TiO2 samples were 3.7–10.4 times faster than the pure TiO2 samples. If we compare the co-doped x%MoS2/Cu/TiO2 samples with the Cu-doped sample (0.0%MoS2/Cu/TiO2) or with the commercial sample (P25 TiO2), the rate can be seen to be 1.4 times higher, the 10.0%MoS2/Cu/TiO2 sample presenting the fastest rate (up to 3.7 times faster).

Table 4

Percentage of degradation and reaction rate constant of the degradation of MB under visible light

Photocatalyst Degradation/% kapp/h−1 
Pure TiO2 22.3 0.0403 
P25 TiO2 51.5 0.1139 
0.0%MoS2/Cu/TiO2 52.6 0.1251 
1.0%MoS2/Cu/TiO2 61.3 0.1582 
5.5%MoS2/Cu/TiO2 72.7 0.1869 
10.0%MoS2/Cu/TiO2 92.9 0.4222 
Photocatalyst Degradation/% kapp/h−1 
Pure TiO2 22.3 0.0403 
P25 TiO2 51.5 0.1139 
0.0%MoS2/Cu/TiO2 52.6 0.1251 
1.0%MoS2/Cu/TiO2 61.3 0.1582 
5.5%MoS2/Cu/TiO2 72.7 0.1869 
10.0%MoS2/Cu/TiO2 92.9 0.4222 

Regarding the percentage of photodegradation of MB, the P25 TiO2 (51.5%) and 0.0%MoS2/Cu/TiO2 (52.6%) samples presented higher degradation than the pure TiO2 sample (22.3%). In turn, as the percentage of Mo increased in the co-doped samples, so did the percentage of degradation, reaching 92.9% in the 10.0%MoS2/Cu/TiO2 sample.

Photocatalytic activity depends on many of the catalyst's properties including its band gap energy, crystallite size, surface structure, extent of the crystallinity and the structure of the material. The band gap values obtained for all the co-doped samples were lower than that of the pure TiO2 sample. However, as the percentage of doping with Mo increased, the band gap value rose slightly; thus, although the band gap values have an influence, they alone cannot explain the enhanced photoactivity observed. In turn, analysing the differences in the size of the crystallite and in the proportion of anatase–rutile obtained for the pure TiO2 sample and the co-doped samples (Table 2), the MoS2/Cu/TiO2 samples are seen to have a smaller crystallite size than the pure sample. Also, the amount of O species adsorbed increased in doped samples, as is deduced from XPS results, which favours photocatalysis. In addition, regarding the phases, although all the samples presented a mixture of anatase and rutile phases, the co-doped samples generated a higher proportion of anatase phase, which presents greater photoactivity (Scanlon et al. 2013; Chimupala et al. 2016). However, the 10.0%MoS2/Cu/TiO2 sample had very similar values, in terms of the crystallite size and the proportion of anatase–rutile phase, to the pure sample but was the most efficient sample.

Typically, anatase phase shows higher photoactivity than rutile phase; however, Luttrell et al. (2014) reported a study on photocatalytic activity as a function of film thickness for anatase and rutile and showed that rutile has a similar slightly higher photocatalytic activity for very thin films (less than 2 nm). Therefore, surface properties can differ widely for the same material and, as a consequence, surface effects contributing significantly to the photocatalytic activity can be observed. This greater efficacy as a photocatalyst could be explained by the higher concentration of localized d states from the transition metal, which may act as traps that capture electrons from the conduction band and gaps from the valence band. The captured electrons and gaps may be transferred to the surface of the photocatalyst and promote the photocatalytic reaction (Gupta & Tripathi 2011). In addition, as reported by Khan & Berk (2014), Mo6+ may act as an electron trap and promote charge separation and consequently decrease the recombination of photogenerated charge carriers. Thus, dopant enrichment close to the surface of the photocatalyst along with increased movement of the charge carriers would explain the increase in photocatalytic activity observed in the co-doped samples with the highest percentage of dopant.

CONCLUSIONS

The results from XRD and Raman spectroscopy suggest that the co-doping of the TiO2 with Mo and Cu using MoS2 and CuCl2·2H2O as the precursor of the dopant took place internally. This is supported by the data obtained using XPS, which also indicate the possible presence of oxygen adsorbed onto the surface of the catalyst to compensate for the internal O deficiencies. In turn, the percentage of Cu added favours the transition from anatase to rutile. However, increasing amounts of Mo have the opposite effect, inhibiting the transformation of anatase to rutile. Furthermore, in every case the band gap of the x%MoS2/Cu/TiO2 samples was lower than that of the pure TiO2 sample. However, the increase in Mo in the co-doped samples generated a slight shift towards higher values that can be explained by the Burstein–Moss effect. Finally, the x%MoS2/Cu/TiO2 samples presented a significant improvement in photocatalysis processes compared with pure TiO2 samples. Of the co-doped samples studied, the 10.0%MoS2/Cu/TiO2 sample was the most efficient photocatalyst with a percentage of degradation above 92% after 6 hours of irradiation under visible light. The results indicate that this is a promising material for use in photodegradation reactions under conditions of solar irradiation.

ACKNOWLEDGEMENTS

We thank the Ministerio de Economia y Competitividad (MINECO) of the Spanish Government for funding under Grant No. ENE2014-58085-R. We gratefully acknowledge the Science and Technology Center of University of Cádiz for the equipment supplied.

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