Introduction

Photocatalyst is a semiconductor material (Kumar et al. 2008) activated by adsorbing photon and has the capability of accelerating a reaction without being consumed (Fox 1988). These semiconductor materials are used for heterogeneous photocatalysis, which is an advanced oxidation process of organic compound that leads its degradation to carbon dioxide, water and mineral ions (Garza-Tovar et al. 2006; Dai et al. 2013). This advance oxidation process is based on generation of hydroxyl and superoxide anion radicals being responsible for the photocatalytic degradation of organic pollutants (Saeed et al. 2015a, b). It was also reported that the photocatalytic technique is the most promising technique for the wastewater treatment because it has advantages over the traditional techniques like quick oxidation, no formation of polycyclic products, and oxidation of pollutants up to the parts per billion (ppb) level (Chen et al. 2008).

Various photocatalysts were used for the degradation of dyes such as copper hexacyanoferrate(II) (Sharma and Sharma 2013), ZnO and Mn-doped ZnO (Ullah and Dutta 2008), TiO2 (Shrivastava 2012), etc. Among these photocatalysts, TiO2 has received greater attention due to its cheap availibilty, photoreactivity, non-toxicity, strong oxidizing power, chemical and biological inertness, and longterm photo-stability (Tolia et al. 2012; Mukhlish et al. 2013). Also it has the ability to degrade organic pollutants in water and atmosphere due to its chemical stability and suitable band gap energy (Zori 2011). However, the low separation efficiency of charges and relatively high recombination of photo-generated carriers limit the photocatalytic efficiency of TiO2 (Su et al. 2013).

In the present study, an approach is introduced to retard such charges recombining deficiencies by depositing noble metal on TiO2 surface. The loading of noble metals on TiO2 surface increases its photocatalytic activity due to acceleration of hydroxyl radical formation and inhibits electron-hole recombination because it acts as an electron accepting species (Sakthivel et al. 2004). In our work, we prepared TiO2, TiO2/Pd and TiO2/Pt nanoparticles and used them as photocatalysts for the photodegradation of methyl violet dye. The methyl violet (triphenylmethane dye) was selected because it has a wide range of applications in different industries and in the staining of bacteriological and histopathological work. It imparts color to water even at very low concentration, which is harmful to aquatic and terrestrial life including human beings (Sarnaik and Kanekar 1999). The dye also causes severe skin and eye irritation, and gastrointestinal tract irritation if swallowed (Mittal et al. 2008). The morphological study and percentage of elements were determined by SEM and EDX, respectively, while photodegradation in the study was carried out by UV/VIS spectrophotometer. The efficacy of the recovered photocatalysts for degradation of dye in aqueous medium was also studied.

Experimental work

Materials

Titanium tetrachloride (TiCl4), platinum (IV) chloride (PtCl4) and palladium (II) chloride (PdCl2) were purchased from BDH. The sulphuric acid (H2SO4) and NH3 were supplied by Merck and Sigma-Aldrich, respectively. Methyl violet dye was purchased from Scharlau.

Preparation of photocatalyst

TiO2 was prepared by treating TiCl4 with ice cold diluted H2SO4 solution. The mixture was stirred vigorously for 30 min and then heated at 60 °C. The heated mixture was then cooled to room temperature for ammonolysis. A white precipitate of TiO2 was obtained at pH 7, filtered and then washed with distilled water. The TiO2 was dried and then calcined at 400 °C with heating rate 1 °C/min and the temperature was retained for 6 h.

TiO2 supported Pd and Pt photocatalysts with metal contents ranging from 0.1 to 1.2 wt% were prepared by the incipient wetness method. A paste of TiO2 was formed in the aqueous solution of PdCl2 and PtCl4 (containing calculated amount of Pt/Pd) and dried in an oven at 105 °C overnight. The sample was then subjected to calcination at 400 °C (1 °C/min) and maintained for 6 h at the same temperature. The calcined photocatalyst was pulverized and finally reduced in a mixture of H2 and N2 gases at a flow rate of 40 mL/min for 9 h at 240 °C.

Photodegradation of methyl violet dye

10 mL of methyl violet (15 ppm) and 0.02 g of TiO2, TiO2/Pd and TiO2/Pt were taken separately in beakers and kept under UV-light (UV lamp having 254 nm, 15 W) with constant stirring. After specific irradiation time, the catalyst was separated by centrifugation (1200 rpm) and dye degradation was measured using UV–VIS spectrophotometry. The percent degradation of methyl violet in aqueous medium was calculated by the following equation (Saeed et al. 2015a).

$${\text{Degradation}}\;{\text{rate}}\; (\% )= \left( {\frac{{C_{0} - C}}{{C_{0} }}} \right) \times 100$$
$${\text{Degradation}}\;{\text{rate}}\; (\% )= \left( {\frac{{A_{0} - A}}{{A_{0} }}} \right) \times 100$$

where C o is the initial dye concentration, C is the dye concentration after UV irradiation, A o shows initial absorbance, and A shows the dye absorbance after UV irradiation.

Characterization

The morphological study of gold-coated TiO2/Pd and TiO2/Pt was carried out by JEOL, JSM-5910 SEM. The EDX analyses of GNs/Sn-Pt were performed by EDX (model INCA 200/Oxford Instruments, UK). The photodegradation study of methyl violet was performed using a UV–VIS spectrophotometer (UV-1800, Shimadzu, Japan).

Results and discussion

SEM and EDX study

Figure 1 shows the SEM images of prepared TiO2/Pd and TiO2/Pt nanoparticles’ photocatalysts. The SEM images showed that the Pd and Pt nanoparticles  appeared on the surface of TiO2. The size range of both TiO2/Pd and TiO2/Pt nanoparticles were below 500 nm as depicted from the micrographs. The presence of Pd and Pt on TiO2 surface was also confirmed by EDX (Fig. 2), which show that Pd and Pt nanoparticles are deposited onto titania. The result shows that about 0.24% Pd and 0.16 Pt % by weight were deposited on TiO2.

Fig. 1
figure 1

SEM images of a TiO2/Pd, b TiO2/Pt

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Fig. 2
figure 2

EDX study of a TiO2/Pt and b TiO2/Pd

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XRD analysis

Figure 3a and b shows the X-ray diffraction (XRD) pattern of the prepared TiO2/Pd and TiO2/Pt, respectively. The characteristic peaks at 2θ = 25° and 48° are for the anatase phase of TiO2 nanoparticles, while no peaks were observed for brookite and rutile phases of titania. The peaks appearing at 2θ = 40.1° and 46.7° indicate Pd, while the peaks appearing at 2θ = 40° and 46° indicate Pt (Rashid et al. 2016).

Fig. 3
figure 3

XRD patterns of a TiO2/Pt and b TiO2/Pd

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Photodegradation study of methyl violet

Figure 4 shows the efficiencies of both TiO2/Pd and TiO2/Pt photocatalysts in methyl violet degradation under UV-light irradiation. By incresing the radiation time the degradation of methyl violet dye can be increased as depicted by the UV/VIS spectra. The results also (Fig. 4) illustrated that the degradation efficiency of TiO2/Pd nanoparticles was higher than TiO2/Pt and pure TiO2 nanoparticles. Figure 4c shows the %degradation of methyl violet photodegraded by TiO2, TiO2/Pd and TiO2/Pt, which present that TiO2/Pd and TiO2/Pt nanoparticles degraded about 95 and 78% of dye within 20 min while pure TiO2 degraded about 40% dye within the same irradiation time. The greater difference in such degradation is the deposition of noble metals on TiO2 surface, which inhibits recombining of separated charges. The general mechanism for photodegradation is that when photon is absorbed by TiO2, excitation of electrons (e−) occurs from valence band to the conduction band and that results in positive holes (h+) in valence band. The e− in the conduction band is accepted by the loaded noble Pd and Pt metals, which inhibit recombining deficiencies of created charges. The generated h+ reacts with H2O molecules and generates.OH radicals. While the e− that is absorbed by loaded metals, reacts with adsorbed O2 molecules and form superoxide ions (·O2 −). These generated hydroxyl and superoxide radicals are strong oxidants, which attack the dye molecules and as a result degradation of dye molecules occur (Su et al. 2013; Sakthivel et al. 2004; Chnirheb et al. 2012). The major possible reactions are believed to be as follows, which is also illustrated in the Fig. 5.

$${\text{TiO}}_{2} + {\text{hv}} \to {\text{e}}^{ - } + {\text{ h}}^{ + }$$
$${\text{Pd}}/{\text{Pt }} + {\text{ e}}^{ - } \to {\text{ Pd}}^{ - } /{\text{Pt}}^{ - }$$
$${\text{Pd}}^{ - } /{\text{Pt}}^{ - } + {\text{O}}_{2} \to{\!^{ \cdot} {\text{O}}_{2}^{ - }}$$
$${\text{H}}_{2} {\text{O}}/{\text{OH}}^{-} + {\text{h}}^{ + } \to^{ \cdot} {\text{OH}}$$
$${\text{Dye}} +{\!^{ \cdot} {\text{O}}_{2}^{ - }} \to {\text{Degradable}}\;{\text{products}}$$
$${\text{Dye}} +{\!^{ \cdot} {\text{OH}}} \to {\text{Degradable}}\;{\text{products}}$$
Fig. 4
figure 4

UV–VIS absorbance spectra of MV degraded by a TiO2/Pd, b UV–VIS absorbance spectra of MV degraded by TiO2/Pt and c comparison of %degradation by both photocatalysts

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Fig. 5
figure 5

Mechanism of photodegradation of methyl violet

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Effect of photocatalysts dosage

The effect of photocatalysts dosage was also studied by loading different amount of photocatalyst (0.010, 0.015, 0.020 and 0.025 g) during photocatalytic reaction, where dye concentration (15 ppm) and irradiation time (15 min) remains constant. Figure 6 shows the UV/VIS spectra of methyl violet in aqueous media using different amount of TiO2/Pd and TiO2/Pt photocatalysts. The spectra illustrated (Fig. 6a, b) that the photodegradation of methyl violet increased as increased the quantity of catalyst and then level off. The enhanced photodegradation of dye is due to increase in catalyst and as result active sites increased, which in turn to increase the number of hydroxyl and superoxide radicals. After that limit, degradation rate remains decreases or remains constant, which might be due to interception of light by suspension via increasing catalyst dosage from optimum limit. The possibility is the agglomeration of catalyst particles due to which surface for photon absorption become unavailable (Akpan and Hameed 2009). Figure 6c represents the comparison of %degradation of methyl violet dye using different amount of photocatalyst. The results show that 0.01 g of TiO2/Pd nanoparticles degraded 86.3% dye, which increased gradually and about 90% of dye was degraded by using 0.025 g catalyst. Similarly, 0.01 g of TiO2/Pt degraded 49% dye, which also increased gradually and highest degradation (72.6%) was obtained by adding 0.025 g of catalyst.

Fig. 6
figure 6

UV–VIS absorbance spectra of methyl violet degraded different catalyst dosage of a TiO2/Pd, b TiO2/Pt and c comparison of %degradation by different catalyst dosage

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Effect of dye concentration

The effect of dye concentration on photodegradation rate of methyl violet was also evaluated by studying photodegradation at various concentrations (5, 10, 15, 20 and 25 ppm) of dye. Figure 7 shows the %degradation of methyl violet using constant catalyst dosage and irradiated time (15 min). The results revealed that the rate of photodegradation of dye decreased as increased the dye concentration. The possible reason is that as the initial concentration of dye increases, the molecules of dye get adsorb on catalyst surface, which absorb significant amount of UV-light rather than TiO2 particles. It also decreases the formation of hydroxyl radicals as dye molecules occupy active sites of photocatalyst (Reza et al. 2015). It is clear from the Fig. 7 that TiO2/Pd and TiO2/Pt decomposed about 86.6 and 74.5% dye at concentration of 5 ppm while at 25 ppm about 68.5 and 46.7% of dye degraded within 15 min.

Fig. 7
figure 7

Comparison of %degradation by TiO2/Pd and TiO2/Pt at various dye concentrations

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Effect of recovered catalyst on photodegradation of methyl violet

The efficiency of recovered catalysts was also studied by using it again for the degradation of methyl violet in aqueous medium under the same experimental conditions. The recovered catalysts show less catalytic activity as compared to original catalysts due to blocking of its active site by deposition of photosensitive hydroxides on the photocatalysts’ surface (Ong et al. 2012). Figure 8a and b shows the UV–VIS spectra of methyl violet using recovered TiO2/Pd and TiO2/Pt photocatalysts. It was found that the photodegradation of dye increases by increasing the irradiation time. Figure 8c shows the %degradation of dye by recovered catalysts, which presented that recovered TiO2/Pd and recovered TiO2/Pt degraded methyl violet 85 and 74%, respectively, within 20 min.

Fig. 8
figure 8

UV–VIS spectra of methyl violet degraded by recovered a TiO2/Pd, b TiO2/Pt and c comparison of %degradation by recovered TiO2/Pd and TiO2/Pt

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Effect of tap water

The effect of tap water on the photodegradation of methyl violet dye was also studied by preparing dye solution in tap water. Figure 9 shows the %degradation of methyl violet dye in tap water degraded by TiO2/Pd and TiO2/Pt photocatalysts. Results data show that TiO2/Pd and TiO2/Pt degraded less dye in tap water as compared to dye degraded in deionized water under the same experimental conditions. The possible reasons for such deviations might be due to the presence of additional species in tap water such as organic, inorganic, and metallic ions, serve as competing species against catalyst active sites, hence reduce its activity (Banegas and Hartmann 2014). Results also illustrated that TiO2/Pd is less active than TiO2/Pt in tap water, which is against the normal condition as in deionized water. This might be due to the presence of chloride ions in tap water, which acting as competing species against catalyst active sites instead of dye molecules. The results indicated that TiO2/Pt degraded 62% dye while TiO2/Pd degraded about 47% dye under the same experimental conditions (time 20 min).

Fig. 9
figure 9

Comparison of %degradation of methyl violet dye in tap water by TiO2/Pd and TiO2/Pt

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Conclusion

It was concluded that both TiO2/Pd and TiO2/Pt nanoparticles are efficient catalysts for photodegradation of methyl violet dye in aqueous medium. It was also found that TiO2/Pd nanoparticles were more efficient as compared to TiO2/Pt nanoparticles. It was found that the rate of degradation is increased by increasing the irradiation time and catalyst dosage. While the rate of degradation is decreased by increasing the initial dye concentration. The methyl violet dye in aqueous medium was also significantly degraded by recovered catalysts.