Solar Hydrogen Production Coupled with the Degradation of a Dye Pollutant Using TiO 2 Modified with Platinum and Nafion

The simultaneous production of molecular hydrogen (H 2 ) and degradation of rhodamine B (RhB) was successfully achieved using TiO 2 modified with platinum and nafion (Pt/TiO 2 /Nf) under visible light (λ > 420 nm). Pt/TiO 2 /Nf exhibited high activity for H 2 production in the presence of RhB and EDTA as a photosensitizer (also an organic dye pollutant) and an electron donor, respectively. However, the activity of TiO 2 modified with either platinum or nafion for H 2 production was negligible under the same experimental conditions. The negatively charged nafion layer enhances the adsorption of cationic RhB and pulls protons, a source of hydrogen, to the surface of TiO 2 through electrostatic attraction. On the other hand, platinum deposits on TiO 2 can act as an electron sink and a temporary electron reservoir for the reduction of protons. With the production of H 2 , RhB was gradually degraded throughN-deethylation, which was confirmed by the spectral blue shift of the maximum absorption wavelength (λmax) from 556 to 499 nm (corresponding to the λmax of rhodamine 110). With Pt/TiO2/Nf employed at [RhB] = 20 μM (0.6 μmol), approximately 70 μmol of H 2 was produced and RhB and its intermediates were completely removed over a 12 h period. A detailed reaction mechanism was discussed.

Organic dyes are one of the most serious pollutants in the aquatic environment due to their high production volumes from industry, toxicity, and low biodegradability.The annual global production of organic dyes is approximately one million tons, and a significant amount (10-15%) of wastewater containing dyes is discharged to the surface water without any treatment [16,17].In addition, some organic dyes are carcinogenic to humans and negatively affect aquatic organisms by interfering with their metabolic processes [16][17][18].However, the biological treatments widely employed in water treatment are usually inefficient for the degradation of organic dyes [19].
Recently, bifunctional TiO 2 photocatalysts have been developed, particularly for simultaneous hydrogen production and pollutant degradation.This bifunctionality was achieved through the surface modification of TiO 2 with metal nanoparticles or two different components (i.e., anions and metal nanoparticles) [20][21][22][23][24][25].However, the previously developed bifunctional TiO 2 photocatalysts worked only under UV light, limiting their practical applications because UV light accounts for only 3% of natural sunlight at ground level.Visible active photocatalysts can be used to achieve the simultaneous hydrogen production and pollutant degradation under visible light [26].Another approach to addressing this challenge would be to use the organic dyes to be treated as a photosensitizer for hydrogen production instead of expensive synthetic dyes, while simultaneously degrading these organic dyes under visible light (44% of natural sunlight).
In this work, we successfully achieved the simultaneous production of hydrogen and degradation of rhodamine B (RhB, an azo dye that accounts for more than 65% of total dye production [16]) using TiO 2 modified with platinum and nafion (Pt/TiO 2 /Nf) under visible light.The effects of various experimental parameters (e.g., initial pH (pH  ), RhB concentration, and EDTA concentration) on H 2 production were investigated.In addition, a detailed reaction mechanism for the simultaneous production of hydrogen and degradation of RhB was suggested.(18.3 MΩ cm) prepared by a water purification system (Barnstead) was used.

Catalyst Preparation.
Platinum (Pt) nanoparticles were deposited onto the surface of TiO 2 using a photodeposition method [27].An aqueous TiO 2 suspension (0.5 g/L, 500 mL) containing chloroplatinic acid as a Pt precursor (100 M) and methanol as an electron donor (1 M) was irradiated with a 200 W mercury lamp for 30 min.Next, the Pt-deposited TiO 2 (Pt/TiO 2 ) powder was collected by filtration through a 0.45 m PVDF disc filter (Pall), washed with distilled water, and dried in an oven at 70 ∘ C. The typical Pt loading on TiO 2 was estimated to be ca.3 wt% by measuring the concentration of unused chloroplatinic acid remaining in the filtrate solution after the photodeposition using inductively coupled plasma atomic emission spectroscopy (ICP-AES, Thermo Jarrell Ash Corp.).To obtain Pt/TiO 2 /Nf (TiO 2 modified with Pt and nafion), an aliquot of nafion solution (0.1 mL) was added to the Pt/TiO 2 powder (0.1 g), mixed well, and dried at room temperature overnight [28].

Photocatalysis.
The catalyst powder was dispersed in distilled water by sonication for 30 s in an ultrasonic cleaning bath.An aliquot of the RhB and EDTA stock solution was subsequently added to the suspension to yield the desired initial concentration.The initial pH (pH  ) of the suspension was adjusted with HClO 4 solution.The total volume of the suspension was 30 mL.Prior to visible light irradiation, N 2 gas (99.9%) was purged through the suspension for 1 h to remove dissolved oxygen, and then the reactor was sealed with a rubber septum.A 300 W Xe arc lamp (Oriel) was used as a light source.Light was passed through a 10 cm IR water filter and a cutoff filter ( > 420 nm), and then the filtered light was focused onto a cylindrical glass reactor with a quartz window.

Analysis.
The amount of photogenerated molecular hydrogen (H 2 ) in the headspace of the reactor was analyzed using a gas chromatograph (GC, HP6890A) equipped with a thermal conductivity detector and a 5 Å molecular sieve column.Sample aliquots were withdrawn from the visiblelight-irradiated reactor and filtered through a 0.45 m PTFE syringe filter (Millipore) to remove catalyst particles prior to the analysis of RhB.The color disappearance of RhB and its maximum absorption wavelength ( max ) shift resulting from the stepwise -deethylation [29] were monitored using a UV-visible spectrophotometer (Shimadzu UV-2401PC).It should be noted that RhB (colored) is transformed to leuco RhB (colorless) through the addition of H • (formed from the reduction of H + ) in the absence of oxygen, but leuco RhB is dehydrogenated back to RhB in the presence of oxygen [30].To exclude the color disappearance through the formation of leuco RhB, the absorption spectrum of samples was measured after exposure to air.The concentrations of RhB adsorbed on the catalyst surface were calculated by subtracting the equilibrated concentrations (with catalyst, after 30 min in the dark) from the initial concentrations (without catalyst).The concentration of RhB was determined by measuring the absorbance at 556 nm.

Effect of Nafion
Coating on H 2 Production.RhB, a representative azo dye pollutant, was selected as a photosensitizer, and the production of H 2 in the presence of EDTA (electron donor) was compared between Pt/TiO 2 and Pt/TiO 2 /Nf (Figure 1(a)).The production of H 2 in the suspension of Pt/TiO 2 was very low but was markedly enhanced by the nafion coating of Pt/TiO 2 .
Under visible light, RhB adsorbed on the TiO 2 surface is excited (see, reaction (1)), and then electrons are transferred from the excited RhB to protons (or water molecules) through the TiO 2 CB and Pt nanoparticles (see, reactions (2)-( 4)): Pt (2e tr In these processes, Pt nanoparticles enhance the interfacial electron transfer from the TiO 2 CB to protons as an electron sink (Schottky-barrier electron trapping) and a temporary electron reservoir that enables the two-electron reduction of protons (see, reactions (3) and ( 4)) [32,33].In the absence of Pt nanoparticles (i.e., in the cases of TiO 2 and TiO 2 /Nf), the production of H 2 was negligible (data not shown).The coating of nafion, an anionic perfluorinated polymer with a sulfonate group (-SO 3 − ), changes the surface charge of TiO 2 (pH zpc = 6.0 [9]) from positive to negative under  acidic conditions [27,28,34].Therefore, the adsorption of cationic RhB on the TiO 2 surface should be enhanced, which accelerates the electron transfer from the excited RhB to the TiO 2 CB and, eventually, the production of H 2 .Figure 1(b) shows the adsorption of RhB on the surfaces of Pt/TiO 2 and Pt/TiO 2 /Nf under the same conditions ([RhB] = 20 M and pH  = 5).The amount of RhB adsorbed on Pt/TiO 2 was very low (0.7 M, 3.5%) because the positively charged surface of Pt/TiO 2 at pH  = 5 repels the cationic RhB molecules.Under this condition, the interfacial electron transfer from the excited RhB to the Pt/TiO 2 CB should be limited.On the other hand, the adsorption of RhB on the surface of Pt/TiO 2 /Nf was significant (18.0 M, 90.0%) because the negatively charged surface induced by nafion attracts cationic RhB molecules.This result can help to explain why the production of H 2 is markedly enhanced by the nafion coating.

Effect of Various
Parameters on H 2 Production.In dyesensitized TiO 2 systems, the solution pH has a significant effect on the production of H 2 because the adsorption kinetics of dyes on the TiO 2 surface strongly depends on the pHdependent surface charge of TiO 2 .Figure 2 shows the production of H 2 and the adsorption of RhB in the suspension of Pt/TiO 2 and Pt/TiO 2 /Nf as a function of pH  .The pHdependent H 2 production trends for Pt/TiO 2 and Pt/TiO 2 /Nf were opposite: as the pH  increased, H 2 production gradually increased for Pt/TiO 2 but decreased for Pt/TiO 2 /Nf (Figure 2(a)).However, it should be noted that the H 2 production in the suspension of Pt/TiO 2 /Nf was much higher than that in the suspension of Pt/TiO 2 over the whole pH range of 2.3 to 5.0.
The surface charge of Pt/TiO 2 is strongly positive at pH  = 2.3 because most of the surface hydroxyl groups are protonated.Under this condition, the adsorption of cationic RhB on TiO 2 should be inhibited, which results in the negligible production of H 2 .However, the number of surface hydroxyl groups that are not protonated increases as the pH increases.This makes the surface charge of TiO 2 less positive and enables the adsorption of cationic RhB on TiO 2 (see, reaction (5)) and H 2 production.As expected, the adsorption of RhB on the surface of Pt/TiO 2 was negligible at pH  = 2.3 but was clearly observed at pH  = 5.0, albeit in small amounts (Figure 2(b)): (2/3)+ + RhB (p  = 3.9 [35]) (the surface charge of TiO 2 is determined by assuming that the surface Ti having +4 formal charge is located at the octahedral site surrounded by five lattice oxygen atoms and one surface group).
On the other hand, the surface charge of Pt/TiO 2 /Nf is highly negative, even at acidic pH, because the anionic sulfonate groups (-SO 3 − ) in the nafion layer outnumber the protonated surface hydroxyl groups of TiO 2 (>Ti-OH 2 ] (2/3)+ ) [27,28].Therefore, sufficient RhB to have little effect on H 2 production can be adsorbed on the surface of Pt/TiO 2 /Nf at both pH  = 2.3 and 5.0 (Figure 2(b)).Under this condition, the decrease in the production of H 2 with increasing pH implies the existence of another mechanism for H 2 production with Pt/TiO 2 /Nf.It has been reported that the concentration of protons in the nafion layer is much higher than that in the aqueous bulk phase due to the electrostatic attraction between the positively charged protons and anionic sulfonate groups [36].Because the electron transfer from Pt nanoparticles to protons is kinetically more favorable than that to undissociated water molecules [(e , the locally concentrated protons within the nafion layer provide good conditions for H 2 production.At lower pH, more protons can be trapped within the nafion layer, which enhances the electron transfer from Pt nanoparticles to protons.Therefore, the production of H 2 with Pt/TiO 2 /Nf increases as the pH decreases, in contrast to the case of Pt/TiO 2 .Overall, the nafion-enhanced H 2 production in the RhB-sensitized Pt/ TiO 2 /Nf system is ascribed to two factors.First, the negatively charged nafion layer enhances the adsorption of cationic RhB.Second, protons, which are more favorably reduced to H 2 than undissociated water molecules, are concentrated within the nafion layer.Figure 3 shows the effect of RhB and EDTA concentrations on the production of H 2 in the suspension of Pt/TiO 2 / Nf.The production of H 2 was negligible in the absence of either RhB or EDTA, which clearly indicates that RhB and EDTA act as a photosensitizer and an electron donor in the RhB-sensitized Pt/TiO 2 /Nf system, respectively.The production of H 2 rapidly increased with increasing RhB and EDTA concentrations and then saturated at [RhB] = 10 M and [EDTA] = 2 mM. The fact that H 2 was not produced in the absence of EDTA (electron donor) indicates that the electron transfer from the TiO 2 CB to RhB •+ (i.e., charge recombination, reaction (6)) is much faster than that from the TiO 2 CB to Pt nanoparticles (see, reaction (3)), which is consistent with other dye-sensitized TiO 2 systems using expensive synthetic dyes for H 2 production [3,7].However, EDTA can prevent the charge recombination process by regenerating RhB •+ to RhB (i.e., electron transfer from EDTA to RhB •+ , reaction (7)), which enables the electron transfer from the TiO 2 CB to Pt nanoparticles and, eventually, the production of H 2 : EDTA

Mechanism of RhB Degradation.
The stability of dye molecules is an important parameter for the evaluation of dye-sensitized TiO 2 systems using expensive synthetic dyes.
On the other hand, the degradation of dye during the course of H 2 production should increase the utility of dyesensitized TiO 2 systems if organic dye pollutants are used as a photosensitizer.This is conceptually similar to bifunctional photocatalysis (i.e., the simultaneous production of hydrogen and degradation of pollutants) [20][21][22][23][24][25][26].
Figure 4 shows the absorption spectral change (i.e., degradation) of RhB in the suspension of Pt/TiO 2 and Pt/TiO 2 /Nf as a function of irradiation time.In accordance with the absence of H 2 production with Pt/TiO 2 at pH  = 2.3 (Figure 2(a)), the degradation of RhB was negligible (Figure 4(a)).On the other hand, with Pt/TiO 2 /Nf, the intensity of the RhB absorption spectrum decreased and its position shifted to shorter wavelengths with irradiation time (Figure 4(b)).This result clearly indicates that RhB is degraded with H 2 production in the suspension of Pt/TiO 2 /Nf.Some dyes including RhB can be degraded through two pathways: the cleavage of the chromophoric ring and -dealkylation [29,38,39].Between these pathways, the cleavage of the chromophoric ring is not favored under our experimental conditions (i.e., in the absence of oxygen) because it is primarily initiated by the reaction between reactive oxygen species (e.g., O 2 •− and • OH) and RhB •+ in the bulk phase [29,38].On the other hand, it has been reported that the -deethylation proceeds through the electron transfer from RhB to the TiO 2 CB and subsequent hydrolysis of RhB •+ [29], leading to a blue shift of the maximum absorption wavelength ( max ) because -de-ethylated RhB intermediates exhibit  max at shorter wavelengths than RhB ( max = 556 nm) [29,38].With Pt/TiO 2 /Nf, the  max shifted from 556 to 502 nm after 4 h of visible light irradiation.This significant spectral shift of  max (Δ max ) clearly indicates that the degradation of RhB is primarily initiated by -deethylation.
RhB was not degraded in the absence of EDTA (data not shown), which implies that EDTA •+ , not the hydrolysis of RhB •+ , plays a critical role in the -deethylation of RhB in our system.This seems to be because the -deethylation through the hydrolysis of RhB •+ is much slower than the reduction of RhB •+ by the TiO 2 CB electron (see, reaction (6)) in the absence of oxygen.In this situation, the -deethylation of RhB should be due to the oxidation of RhB (or more preferentially RhB •+ ) by EDTA •+ (see, reaction (8)) generated from the oxidation of EDTA by RhB •+ (see, reaction (7)): → EDTA + -de-ethylated RhB intermediate It has been reported that EDTA •+ can oxidize , ,   ,  tetramethyl--phenylenediamine (TMPD), which has electron-donating alkyl groups [40].Likewise, EDTA •+ could react with the ethyl group of RhB •+ to generate -de-ethylated RhB intermediates.It should be noted that RhB is regenerated when RhB •+ accepts one electron from the TiO 2 CB or EDTA (see, reactions ( 6) and ( 7)).However, -de-ethylated RhB intermediates generated by further oxidation of RhB   Figure 5(a) shows the spectral shift of  max depending on [EDTA] and pH  .In all cases,  max shifted from 556 to 499 nm and then remained constant.Rh-110 (the fully de-ethylated form of RhB) exhibits an absorption maximum at 499 nm [28].Therefore, the generation of Rh-110 further confirms that RhB is degraded through -deethylation in the suspension of Pt/TiO 2 /Nf with EDTA.
The -deethylation rate, which is proportional to the Δ max rate, was greatly dependent on both [EDTA] and pH  .The -deethylation at [EDTA] = 4 mM was slower than that at [EDTA] = 0.4 mM, as the reaction of RhB •+ with EDTA (regeneration of RhB, reaction (7)) becomes more favored than that with EDTA •+ (-deethylation of RhB, reaction (8)) as the concentration of EDTA increases.On the other hand, the -deethylation at pH  = 5.0 was faster than that at pH  = 2.3 at the same [EDTA], which is related to the pHdependent speciation of EDTA.At pH  = 5.0, EDTA primarily exists as H 2 EDTA 2− , which can be repelled from the negatively charged surface of TiO 2 induced by the nafion (Figure 5(b)).On the other hand, H 4 EDTA and H 3 EDTA − , which can more favorably approach the negatively charged surface, are the main species at pH  = 2.3 (Figure 5(b)).Therefore, the concentration of EDTA within the nafion layer at pH  = 5.0 should be lower than that at pH  = 2.3 despite the concentration of EDTA added being the same.A lower concentration of EDTA within the nafion layer at higher pH reduces the regeneration of RhB and therefore enhances the degradation of RhB.The lower production of H 2 at higher pH (see Figure 2(a)) should also be related to the faster degradation of RhB at higher pH (see Figure 5

Long-Term Experiment.
In the suspension of Pt/TiO 2 /Nf, H 2 was continuously produced up to 12 h at [RhB] = 20 M (0.6 mol), [EDTA] = 4 mM, and pH  = 2.3, although the RhB (i.e., absorbance at  = 556 nm) was completely removed within 4 h (Figure 6).This result implies that not only RhB but also the intermediates generated from the degradation of RhB can act as a photosensitizer.However, the rate of H 2 production gradually decreased (9.0 mol/h for 0-3 h, 6.4 mol/h for 3-6 h, 3.6 mol/h for 6-9 h, and 2.3 mol/h for 9-12 h) as RhB and its intermediates were degraded (Figure 6(a)).The degradation of Rh-110 ( max = 499 nm) resulting from the complete -deethylation of RhB proceeded (i.e., the absorbance at  = 499 nm continuously decreased) after 4 h and was completed after 12 h (Figure 6(b)).In accordance with the complete degradation of RhB and its intermediates after 12 h, the production of H 2 stopped after 12 h.
Using Pt/TiO 2 /Nf under visible light, approximately 70 mol of H 2 was produced and 20 M (0.6 mol) of RhB and its intermediates were completely degraded over a 12 h period without the use of external energy or chemical oxidants (Figure 6).Although high concentrations of EDTA (millimolar levels) are required in the RhB-sensitized Pt/ TiO 2 /Nf system for the simultaneous production of H 2 and degradation of RhB, many industrial wastewaters (e.g., pulp and paper, textile, and cosmetics wastewaters) contain high concentrations of EDTA [41].In addition, EDTA can be easily degraded through conventional biological treatment, unlike RhB [42,43].Therefore, the application of Pt/TiO 2 /Nf for the simultaneous production of H 2 and degradation of organic dye pollutants could become practicable by using  industrial wastewaters containing high concentrations of EDTA.

Conclusions
The present study introduces a new strategy for visible-lightinduced bifunctional photocatalysis (i.e., the simultaneous production of hydrogen and degradation of pollutants) using TiO 2 modified with platinum and nafion (Pt/TiO 2 /Nf) and organic dye pollutants.In the presence of RhB (as both a photosensitizer and an organic dye pollutant) and EDTA (as an electron donor), Pt/TiO 2 /Nf exhibited considerably higher activity for H 2 production than Pt/TiO 2 .This is ascribed to the negatively charged surface of TiO 2 induced by the nafion, which enhances the adsorption of cationic RhB and pulls protons, a source of hydrogen, to the surface of TiO 2 .In addition, the degradation of RhB was accompanied by the concurrent production of H 2 .The intensity of the RhB absorption spectrum decreased and its position shifted to shorter wavelengths, which indicates that RhB is primarily degraded through -deethylation.EDTA •+ , which is generated from the oxidation of EDTA by RhB •+ , is found to be involved in the degradation mechanism, as RhB was not degraded in the absence of EDTA.Rh-110, a fully -de-ethylated form of RhB, was further degraded and the production of H 2 continued until RhB and its intermediates were completely degraded.Based on its high efficiency, bifunctionality, and visible light activity, this organic dye pollutant-sensitized TiO 2 system using Pt/TiO 2 /Nf can be proposed as a viable photocatalytic system for the simultaneous production of hydrogen and degradation of organic dye pollutants.
Figure5(a) shows the spectral shift of  max depending on [EDTA] and pH  .In all cases,  max shifted from 556 to 499 nm and then remained constant.Rh-110 (the fully de-ethylated form of RhB) exhibits an absorption maximum at 499 nm[28].Therefore, the generation of Rh-110 further confirms that RhB is degraded through -deethylation in the suspension of Pt/TiO 2 /Nf with EDTA.The -deethylation rate, which is proportional to the Δ max rate, was greatly dependent on both [EDTA] and pH  .The -deethylation at [EDTA] = 4 mM was slower than that at [EDTA] = 0.4 mM, as the reaction of RhB •+ with EDTA (regeneration of RhB, reaction(7)) becomes more favored than that with EDTA •+ (-deethylation of RhB, reaction (8)) as the concentration of EDTA increases.On the other hand, the -deethylation at pH  = 5.0 was faster than that at pH  = 2.3 at the same [EDTA], which is related to the pHdependent speciation of EDTA.At pH  = 5.0, EDTA primarily exists as H 2 EDTA 2− , which can be repelled from the negatively charged surface of TiO 2 induced by the nafion (Figure5(b)).On the other hand, H 4 EDTA and H 3 EDTA − , which can more favorably approach the negatively charged surface, are the main species at pH  = 2.3 (Figure5(b)).Therefore, the concentration of EDTA within the nafion layer at pH  = 5.0 should be lower than that at pH  = 2.3 despite the concentration of EDTA added being the same.A lower concentration of EDTA within the nafion layer at higher pH reduces the regeneration of RhB and therefore enhances the degradation of RhB.The lower production of H 2 at higher pH (see Figure2(a)) should also be related to the faster degradation of RhB at higher pH (see Figure5(a)).The degradation of RhB reduces the number of RhB molecules adsorbed on the surface of Pt/TiO 2 /Nf and eventually decreases the production of H 2 .Overall reactions occurring in the suspension