Characterization and Application of Nanostructured Films Containing

Article pubs.acs.org/JPCC

Characterization and Application of Nanostructured Films Containing Au and TiO2 Nanoparticles Supported in Bacterial Cellulose Nicolle Dal’Acqua,† Alessandra Batista de Mattos,‡ Israel Krindges,† Marcelo Barbalho Pereira,§ Hernane da Silva Barud,∥ Sidney José Lima Ribeiro,∥ Gian Carlos Silva Duarte,‡ Claudio Radtke,§ Luciano Costa Almeida,⊥ Marcelo Giovanela,† Janaina da Silva Crespo,† and Giovanna Machado*,‡,⊥ †

Universidade de Caxias do Sul, UCS, Caxias do Sul, Rio Grande do Sul, Brazil Centro de Tecnologias Estratégicas do Nordeste, CETENE, Recife, Pernambuco, Brazil § Universidade Federal do Rio Grande do Sul, UFRGS, Porto Alegre, Rio Grande do Sul, Brazil ∥ Universidade Estadual Paulista, UNESP, Araraquara, São Paulo, Brazil ⊥ Universidade Federal de Pernambuco, UFPE, Recife, Pernambuco, Brazil ‡

ABSTRACT: In the last several years, the use of renewable energy sources has increased; consequently, the number of studies regarding their efficiency has also increased. It is well known that fossil and atomic fuels will not last forever and that their use contributes to environmental pollution. Thus, nanostructured thin films have attracted attention due to numerous applications, including construction of photovoltaic energy generating and photoluminescence materials. Therefore, in this study, we prepared and characterized thin films supported on bacterial cellulose that were produced using the layer-by-layer (LbL) technique. The weak polyelectrolytes, such as poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA), combined with titanium dioxide (TiO2) and gold nanoparticles (Au NPs) were used to produce flexible devices capable of producing hydrogen gas (H2) by photocatalysis. The presence of the Au NPs and TiO2 in the films was confirmed using UV− vis spectroscopy, Rutherford backscattering spectrometry, and X-ray diffraction. Scanning electron microscopy was used to evaluate the surface morphology of the films, and the distribution and average size of the Au NPs were analyzed using transmission electron microscopy, which revealed sizes in the nanometer range. Finally, the thin films were analyzed using gas chromatography to evaluate the H2 production by photocatalysis. Overall, the system with (PAH + TiO2) and PAA solutions at pH = 4.0 in the presence of gold salt that were reduced with ultraviolet light were more efficient due to their greater interactions with the TiO2 during multilayer deposition.

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INTRODUCTION Hydrogen is considered an ideal fuel for the future, and its use represents a visionary strategy because it is abundant, clean, flexible, and secure. Currently, approximately 95% of hydrogen (H2) is produced from fossil fuels by processes that release greenhouse gases, especially carbon dioxide. The other 5% of H2 produced is obtained using a renewable water electrolysis process. Thus, technological advances regarding production of this efficient, economical, and environmentally friendly fuel are happening worldwide.1,2 Among the various technologies for obtaining H2, the photocatalytic water-splitting method using titanium dioxide (TiO2) as a semiconductor stands out because it offers a way to produce clean energy through the low-cost and environmentally friendly production of hydrogen using solar energy.3,4 Thus, the production of hydrogen by water dissociation, which was shown in 1972 by Honda and Fujishima,5 is one alternative process that has been recently studied. This process is based on the use of solar radiation with a small potential applied over one © XXXX American Chemical Society

photocatalyst metal oxide and a counter electrode (metallic) to generate oxygen at the anode and hydrogen at the cathode. Through this process, it is possible to produce renewable clean energy and hydrogen for use in fuel cells. TiO2 semiconductors have been applied as power converters and received special attention due to their strong chemical stability over a wide pH range, their photostability, and their potential for activation by sunlight. This semiconductor exhibits interesting photocatalyst properties like hydrogen production from water. However, its activity is limited to the ultraviolet (UV) region, which accounts for only 4−5% of the solar radiation. When this semiconductor is irradiated with energy equivalent to or greater than the band gap of the semiconductor, electron−hole are created. The electrons in the valence band (VB) are excited to the conduction band (CB), Received: September 15, 2014 Revised: November 28, 2014

A

DOI: 10.1021/jp509359b J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

LbL technique using weak polyelectrolytes (PAH and PAA) combined with TiO2 and Au NPs supported in BC to produce flexible devices that are capable of producing H2 by photocatalysis.

leaving a hole in the VB. These photogenerated electrons and hole cause reduction and oxidation reactions in the CB and VB, respectively. Therefore, TiO2 is more effective when it is incorporated into thin films containing visible light sensitizers. In order to get better photocatalytic activity, noble metals, such as Au, Pt, and Ni, have been used for enhancement of TiO2 photocatalytic. These noble metals have a Fermi level lower than that of TiO2, and the photoexcited electrons can be transferred from the CB to metal deposited on surface of TiO2, while the VB hole remains on the TiO2. This results in efficient charge separation which improves the photocatalytic activity during production of H2, reducing the possibility of electron− hole recombination.6,7 Currently, photocatalysis research is focused on developing methods to modify semiconductors by incorporating materials that absorb light in the visible region8,9 for better use of solar radiation. In 2007, Ni3 and colleagues attempted to improve the efficiency of TiO2 and extend its absorption from the UV to the visible light region through chemical modification with carbon and through doping with anions (sulfur, nitrogen). In 2010, Chatterjee10 studied the use of dyes, such as rhodamine and erythrosine, in TiO2 semiconductors to achieve light absorption in the visible region for photocatalytic degradation of organic pollutants. In addition to these surveys, in 2011, Machado et al.11 described a method in which the degree of incorporation of Au NPs that were stabilized with sodium citrate in poly(allylamine hydrochloride) (PAH)/poly(acrylic acid) (PAA) thin films strongly depended on the pH of the colloidal gold solution. The study by Popiolski et al.12 evaluated the incorporation and distribution of Au NPs that were stabilized with poly(vinylpyrrolidone) in PAH/PAA films that were produced using the layer-by-layer (LbL) method. In 2013, Dal’Acqua et al.13 studied the incorporation of polymeric thin films with TiO2 and Au NPs and generated hydrogen in the bulk of the multilayer thin film. Faria et al.14 produced hydrogen fuel using solar irradiation and thin films composed of [(poly(diallyldimethylammonium chloride) (PDDA) + cadmium selenide (CdSe)/PAA + TiO2]. These thin films can be obtained using the LbL technique with sequential adsorption of polyelectrolytes based on the electrostatic interactions between opposite charges.15 In all these studies rigid glass substrates were used.16,17 In the current study, cationic polyelectrolyte PAH and anionic polyelectrolyte PAA were chosen for formation of thin films. Currently, it is challenging to prepare thin films of polyelectrolytes combined with semiconductors and metal nanoparticles on flexible substrates. Compared to rigid materials such as glass and silicon, flexible substrates have certain advantages including light weight and thin profiles. Moreover, these substrates can easily be bent and flexed, which allowed construction of supports with microchannel structures, thereby increasing the surface area. One example of a flexible substrate is bacterial cellulose (BC), which can be obtained through biosynthesis of the Gluconacetobacter xylinus (G. xylinus) bacterium with high crystallinity, elasticity, durability, absorption capacity, and water retention.18 These chemical and structural properties confer strong interactions between the hydroxyl groups and provide a stable and high tensile structure. Generally, BC is considered as an ideal hydrophilic matrix for incorporating metals.19 In this context, the main objective of this study was to prepare and characterize thin films that were produced by the

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MATERIALS AND METHODS The BC that was used as a support in the self-assembled thin films was produced from biosynthesis of the bacterium G. xylinus. Cultivation of G. xylinus was accomplished in trays of 30 cm × 50 cm with a cultivation time of 96 h at 28 °C. The culture medium consisted of glucose 2% (w/v), peptone 0.5% (w/v), yeast extract 0.5%, disodium phosphate anhydrous 0.27% (w/v), and citric acid monohydrate 0.115% (w/v). After 69 h, highly hydrated BC films were produced with a thickness of 4 mm. The BC hydrogels were treated with a 1% (w/v) NaOH solution at 70 °C to remove the bacteria. 20 Subsequently, the BCs were washed with distilled water to remove the basic solution. Next, washing was conducted until the final pH of the washing water reached 5.5. The wet BCs were cut to a size of 5.5 cm × 2.5 cm × 0.15 cm and fixed on glass substrates with a size of 7.5 cm × 2.5 cm × 0.1 cm to subsequently produce self-assembled thin films. The polyelectrolyte PAH (Mw = 70 000 g mol−1/SigmaAldrich) was used as the polycation, while PAA (Mw = 90 000 g mol−1, 25 wt %/Polysciences) was used as the polyanion. All polyelectrolytes were prepared as 0.01 mol L−1 solutions (based on the repeat-unit molecular weight). For the TiO2 semiconductor (STS-100, Mw = 80 g mol−1, 15.4 wt % in titanium/Ishihara Sangyo Kaisha Ltd.), a solution with a concentration of 0.0375 mol L−1 was used. In addition, a 0.005 mol L−1 tetrachloroauric acid (HAuCl4) (Merck) solution in Au with a pH of 2.3 was used. The Au NPs that were reduced by sodium citrate were prepared using a solution of 0.005 mol L−1 HAuCl4 in Au. Next, 180 mL of deionized water was added to 10 mL of the previously prepared Au solution, and the solution was heated to 100 °C, followed by addition of 10 mL of 0.5% (w/v) sodium citrate. Heating was continued until the color changed to purple. The final solution had a pH of 5.4. This procedure was based on the method proposed by Turkevich (1951).21 Self-assembled thin films were prepared using NanoStracto Sequence equipment. In this case, substrates were immersed in the polycation aqueous solution with the semiconductor aqueous solution (PAH + TiO2) for 15 min before rinsing twice for 2 min and once for 1 min in deionized water. Next, the substrate containing the adsorbed polycation was dipped into the polyanion solution (PAA) for 15 min and the rinsing process was repeated. The multilayers were obtained using aqueous solutions composed of {(PAH + TiO2)x(z:w)/PAAy}i, where x is the pH of the polycation and semiconductor solution, z and w are the volumetric proportions of the PAH and TiO2 solutions, y is the pH of the polyanion solution, and i is the number of layers. Finally, multilayers were prepared for two combinations: combination 1 (S7), {(PAH + TiO2)7.0(1:1)/PAA4.0}21, and combination 2 (S4), {(PAH + TiO2)4.0(1:1)/PAA4.0}21. After formation of the multilayer, the Au NPs were incorporated using two different methods. First, the films were immersed into a HAuCl4 solution for 1 h at pH 2.3 before rinsing for 1 min in deionized water. After incorporation of the Au salt, the films were exposed under UV light for 48 h at a wavelength of 365 nm to promote reduction of the metal ions and formation of Au NPs. B


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The Journal of Physical Chemistry C In the second method, the films were immersed for 1 h but in an Au NPs solution that was reduced with sodium citrate at pH 5.4, followed by rinsing for 1 min in deionized water. Combinations of 1 and 2 with polyelectrolytes were called P7 and P4, respectively. Combinations S7 and S4 after impregnation with Au NPs are presented during the discussion as Au NPs and reduced by citrate (S7-ctt and S4-ctt) or UV light (S7-UV and S4-UV). Here, the numbers 7 and 4 represent the pH for the system (PAH + TiO2). Table 1 shows the conditions of the Au NPs that were incorporated in the systems with different pH values.

0784 for Au and the Inorganic Crystal Structure Database (ICSD) no. 78-2486 for TiO2 100% anatase. Scanning Electron Microscopy (SEM). Micrographs of the BC and films were obtained using an accelerating voltage of 5−15 kV on a Shimadzu SSX-550 Superscan. A gold coating was used with a deposition time of 3 min. Elemental qualitative analysis of the samples was performed using energy-dispersive spectroscopy (EDS) to confirm the presence of the Ti and Au in the films. Transmission Electron Microscopy (TEM). For TEM the self-assembled thin films that were impregnated with Au and deposited in the cellulose membrane were cut and embedded in epoxy resin that was cured at 60 °C for 48 h. Next, these specimens were trimmed with a glass knife before obtaining ultrathin cross sections using a Diatome diamond knife with an angle of 45° at room temperature. The ultrathin sections that were approximately 80 nm thick were collected and immediately mounted onto 200 mesh copper grids before drying in a desiccator. Finally, samples were examined using a Morgagni 268D (FEI) that was operated at an accelerating voltage of 80 kV. Particle sizes were measured using the ImageTool software by counting approximately 500 nanoparticles per studied system. Measurements of H2 Production by Gas Chromatography (GC). Photocatalytic reactions for hydrogen production by solar irradiation were performed in a double-walled quartz photochemical reactor under continuous magnetic stirring. Films were fixed inside a quartz reactor and submersed in a water/ethanol solution with a ratio of 1.0/0.25 (v/v). Prior to irradiation, the photochemical reactor was deaerated by bubbling Ar through it under a vacuum for 20 min. The temperature of the reaction system was maintained at 25 °C by circulating water from a thermostatic bath through the photochemical reactor. A 150 W Xe lamp (ScienTech Inc.) was used as the light source. The produced hydrogen gas was quantified using a Shimadzu GC-2014ATF/SPL gas chromatograph with a molecular column sieve 5A, which was equipped with a thermal conductivity detector (TCD) and argon as carrier gas. The evolution of H2 production was monitored by collecting 500 μL aliquots of the gas at intervals of 0.5 h for 3 h. These samples were collected using a Hamilton syringe with a valve (model 1005SL Sample Lock Syringe) that was connected through a septum in the lid of the reactor.

Table 1. Systems with Different pH Values without and with Incorporation of Au NPs by Different Methods pH

system S7 S4 S7-ctt S7-UV S4-ctt S4-UV P7 P4

solution PAH + TiO2

solution PAH

solution PAA

7.0 4.0

4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0

7.0 4.0 7.0 7.0 4.0 4.0

solution Au NPs reduced citrate

solution Au NPs reduced UV light

5.4 2.3 5.4 2.3

Ultraviolet−Visible Spectroscopy (UV−vis). Absorbance spectra of the thin films were calculated from the transmittance (%T) and total reflectance (%R) measurements, which were performed using a CARY 5000 (Agilent Technologies) spectrophotometer over a wavelength range of 250−800 nm. To ensure a more stable signal, measurements were made using a double light beam configuration and an integrating sphere accessory was used with the spectrophotometer. These measurements were performed for the doped films with Au and TiO2 nanoparticles (S) and for the blank films without Au and TiO2 nanoparticles (P). Then, the percentage of light absorbed by these films was calculated using the expression %Abs = 100% − %T − %R

Finally, the absorbance corresponding only to the doping particles (Au and TiO2) was calculated as

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⎞ ⎛ 100 Abs = log⎜ ⎟ ⎝ 100 − (%AbsS − %Abs P) ⎠

RESULTS AND DISCUSSION To prepare thin films produced by LbL with a bacterial membrane support it is important to understand the ionization conditions of these polyelectrolytes at certain pH values. Thus, for the S7 system {(PAH + TiO2)7.0(1:1)/PAA4.0}21, the PAA chains have a low degree of ionization relative to the PAH chains.16,23 Under these pH conditions (PAH pH = 7.0; PAA pH = 4.0), approximately 70% of the PAA functional groups are in the carboxylate form (−COO−). Thus, most of the amine (−NH3+) groups of the PAH will neutralize the charged −COO− groups of the PAA; as a result, few −NH3+ groups are available on the multilayer system.14,16 Therefore, PAH was used as the last deposited layer because it facilitates the interactions of the charged −NH3+ groups with the Au NPs that are reduced by citrate and the Au NPs that are reduced by UV light from the anion of the salt (AuCl4−). In the S4 system {(PAH + TiO2)4.0(1:1)/PAA4.0}21, the pH of the dipping solutions was set at 4.0 and a high degree of ionization was observed for the PAH (95%). Consequently, a thin film was

Rutherford Backscattering Spectrometry (RBS). Gold (Au) and titanium (Ti) loadings were determined using Rutherford backscattering spectrometry (RBS) with a He+ ion beam with an incident energy of 2.0 MeV. This method is based on determining the number and energy of the detected particles that are elastically scattered by the Coulombic field of the atomic nuclei in the target.22 In this study, the Au/C or Ti/ C atomic ratios were determined from the heights of the signals that corresponded to each of the elements in the spectra. X-ray Diffraction (XRD). X-ray diffraction patterns were obtained using an X-ray diffractometer (model BRUCKERADVANCED 8- XRD) with Cu Kα radiation (λ = 1.5406 Å) with a scan range of 10° ≤ 2θ ≤ 50°, a step size of 0.02°, and a measuring time of 2 s per point. Diffraction patterns were defined by comparing them with the crystallographic Joint Committee on Powder Diffraction Standards (JCPDS) No. 4C


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Figure 1. Electrostatic interactions of the films: (a) S7-UV, (b) S4-UV, (c) S7-ctt, and (d) S4-ctt.

produced in which the −NH3+ and COO− groups were completely interconnected electrostatically. The absence of free −NH3+ groups explains the difficulty of incorporating the salt (AuCl4−) (observed by Rubner et al.24) and the Au NPs that are stabilized by citrate (as observed by Machado et al.11). Regarding the aqueous TiO2 solution, incorporation of these species in the multilayer depends on the charges of the hydrated surface and is determined by reactions of TiO2 with

the H+ or OH− ions from water. The following scheme represents variations in the TiO2 surface at different pH values. At low pH, the surface is expected to be protonated and positively charged (reaction a). At high pH, the surface is expected to be deprotonated and negatively charged (reaction b). At some intermediate pH, the surface has a net charge of zero, which is called the point of zero charge (PZC) or the isoelectric point (reaction c). This situation corresponds to D


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Figure 2. UV−vis absorbance spectra of the systems described in the legend.

interactions between the TiO2 and Au NPs may result in shifts in the plasmon absorption band λmax. The S7-UV and S4-UV systems show more intense SPR absorbance in relation to other samples. At pH = 7.0 (S7-UV), TiO2 does not interact or interacts partially with the polyelectrolytes because TiO2 has a neutral surface charge. Consequently, a small part of free −NH3+ groups can bind to the gold complex ions, which results in formation of a higher incorporation of Au NPs after suitable UV reduction (Figure 1a). In contrast, in the S4-UV system interactions of −COO− and Ti−OH2+ are established during formation of multilayers. Thus, in acidic conditions (AuCl4− pH = 2.3) the carboxylate groups are converted to protonated carboxylic acid groups and cause the breakup of COO−−NH3+ electrostatic linkages that are formed during multilayer assembly (Figure 1a and 1b). This process renders the ammonium groups available for gold salt binding. In the S4ctt system, incorporation of the Au NPs was more efficient than in the S7-ctt system (see Figure 2). This result occurred because of the pH of the TiO2 nanoparticles solution, which promoted formation of additional free −NH3+ groups that could bind to the Au NPs that were reduced by citrate (Figure 1d) relative to the S7-ctt system (Figure 1c). However, when PAH had a pH of 7.0, only a relatively small fraction of free −NH3+ groups can bind to the Au NPs, which results in a low incorporation of gold nanoparticles (Figure 2). The absorption band of TiO2 at ∼275 nm was more intense in the S4-ctt and S4-UV films, which demonstrated greater incorporation of TiO2 due to its interactions with PAA. These results correspond with the results obtained by the RBS (Table 2).

immersion of TiO2 in water, and this pH solution has an equilibrium between the H+ and the OH− concentrations.25,26 The TiO2 nanoparticles used in this study had an isoelectric point near pH = 7.0, which is consistent with the reported values for the TiO2 nanoparticles.27−29 In our studies, the surface charge of the TiO2 corresponded with the following reaction scheme. pH < PZC: Ti−OH + H+ → TiOH 2

(a)

pH > PZC: Ti−O− + H+ → TiOH

(b)

pH = 7 ≈ PZC: Ti−OH+−Ti ↔ Ti−O−Ti + H+ (zero charge neutral )

(c)

Thus, the TiO2 solution at pH 4.0 (S4) will facilitate formation of electrostatic interactions between the Ti−OH2+ and the −COO− groups (Figure 1b and 1d). However, in TiO2 solution at pH 7.0 (S7), equilibrium exists between the H+ and the OH− concentrations and the charge is neutral, which impedes the electrostatic interactions of TiO2 with both polyelectrolytes (Figure 1a and 1c). Figure 2 shows the absorbance spectra of the S7-ctt, S7-UV, S4-ctt, and S4-UV films. In this case, it is possible to observe the characteristic absorption band from the Au NPs due to surface plasmon resonance (SPR).30−32 For the Au NPs spectra that were reduced with sodium citrate, HAuCl4, and TiO2 (not show here), the absorption bands were observed at ∼520, ∼ 220, and ∼275 nm, respectively.33−35 For the Au NPs systems that were reduced by citrate (S7-ctt and S4-ctt), the maximum absorbance intensity (λmax) occurred at ∼520 nm, while in the systems with reduced Au NPs and UV light a bathochromic shift to λmax ≈ 580 nm occurred (S7-UV and S4-UV), most likely due to agglomeration or to the larger size of the Au NPs.36,37 The λmax values of the SPR bands correlate with the nanoparticle sizes and concentrations.38−42 In addition, this bathochromic shift could indicate an efficient interaction between the TiO2 and Au NPs43,44 because the Au NPs are partially incorporated into these films that contain TiO2 NPs where a dielectric core is present, such as TiO2. Thus, the

Table 2. Au/C and Ti/C Atomic Ratios of the LbL Samples

E

samples

Au/C

Ti/C

S7-ctt S7-UV S4-ctt S4-UV

0.0003 0.0672 0.0060 0.0626

0.1129 0.0473 0.2590 0.3027 DOI: 10.1021/jp509359b J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 3. XRD of the LbL films with TiO2 and Au NPs.

A SEM image of the BC is shown in Figure 4. This micrograph reveals a random network of many thick cellulose

To identify the characteristic peaks of BC, TiO2, and Au NPs, XRD was used (Figure 3). Qualitative interpretations of the XRD patterns were made using the Au and TiO2 patterns from the database of the JCPDS and ICSD (card numbers 4-0784 and 78-2486) to identify the Au and TiO2, respectively. In the XRD patterns, BC peaks were observed at 14.6° and 22.6°.45,46 The most representative Au peaks occurred at 38.2° and 44.4° 2θ, which corresponded to the (111) and (200) planes, respectively. In addition, TiO2 was identified by the Bragg reflection at 25.3° 2θ, which corresponded to the (101) plane. Scherrer’s equation was applied to the peak broadening to determine the crystalline size as follows47,48 Dhkl =

Kλ β1/2 cos θ

where K is a constant of nearly unity (which was estimated as 0.9), θ is the Bragg angle, λ is the X-ray wavelength, and β is the full width at half-maximum (fwhm) of the peak. Crystalline sizes and average diameters of the Au NPs in the LbL films are shown in Table 3. These results were obtained using Scherrer’s equation and TEM histograms (discussed later), respectively.

Figure 4. Morphology of the BC surface by SEM.

fibers tangled together to form a flat and slightly rough surface. Figures 5a and 6a show the surface morphologies of the P7 and P4 films. Greater surface roughness was observed for the P7 film because the last PAH layer at pH = 7.0 tends to be thicker and less linear, resulting in minimal interpenetration of the PAA chain.23 Images of the S7 and S4 films (Figures 5b and 6b), which were obtained using EDS analysis (Figures 5c and 6c), revealed the presence of titanium (Ti) on the surface of the sample based on the peak corresponding to an energy of Kα = 4.5 keV. Micrographs (Figures 5e and 6e) show structures with triangular and hexagonal shapes that are characteristic of Au salt crystals, and when these films are exposed to UV irradiation, reduction of Au3+ to Au (0) occurs. In Figures 5f and 6f the presence of various sized clusters that are distributed throughout the film can be observed, which demonstrates that irradiation by UV light is effective for reducing Au3+.

Table 3. Crystalline Size by Scherrer’S Equation and by TEM for Au NPs in LbL Films system

DRX (nm)

S7-ctt S7-UV S4-ctt S4-UV

14.4 36.9 10.4 30.9

± ± ± ±

0.3 0.1 0.1 0.3

TEM (nm) 15.0 25.0 12.0 40.0

± ± ± ±

0.5 0.5 0.5 0.5

The presence of the TiO2 peak at 25.3° 2θ was only observed in samples S4-ctt and S4−UV. The absence of this phase in the other samples (S7-ctt and S7-UV) can be explained by the pH conditions of these samples. As previously mentioned, TiO2 has a neutral charge at pH = 7.0 and presents minimal interactions with the polyelectrolytes during the LbL process. F


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Figure 5. SEM of the self-assembled films: (a) P7, (b) S7, (c) EDS of S7, (d) S7-ctt, (e) S7 + gold salt (HAuCl4), and (f) S7-UV.

Figure 6. SEM of the self-assembled films: (a) P4, (b) S4, (c) EDS of S4, (d) S4-ctt, (e) S4 + gold salt (HAuCl4), and (f) S4-UV.

In Figure 8, the H2 production results (in μmol of H2 per cm2) are shown as a function of irradiation time. The H2 production rates of the films are shown in Table 4. In the films (S7 and S4) in the presence of TiO2, H2 production was approximately twice, compared to P7 and P4 samples (benchmark systems) because the photocatalytic activity exhibits by the TiO2. Incorporation of gold (confirmed by RBS analysis, Table 2) on the surface of TiO2 improves the photocatalytic activity for hydrogen generation due to the interfacial transfer of electron from TiO2 to the gold when irradiated, increasing the electron− hole separation and consequently minimizing the charge recombination. The electrons on gold can be transferred to the protons absorbed on the surface and further reduce the H+ into H2. Furthermore, gold nanoparticles when prepared by the UV method presented better photocatalytic efficiency than the citrate method as observed for the S7-UV (Figure 8a) and S4UV (Figure 8b) films. The variation can be explained by better contact of gold nanoparticles with TiO2 active sites and, consequently, charge transfers become more efficient. The Au NPs also facilitate charge stabilization within TiO2 and play an

To understand the morphology and distribution of TiO2 and Au NPs in the LbL films, the TEM technique was used as shown in Figure 7. The histograms presented in Figure 7a1, 7b1, 7c1, and 7d1 show the average diameters of the NPs (15, 12, 25, and 40 nm, respectively), which corresponded with the XRD results (Table 3). However, a small discrepancy was observed in the mean diameter values that were obtained using XRD and TEM for samples that were reduced by UV, which can be explained by the XRD results that provided information regarding the observed mean volume and the TEM images that were observed for specific regions. According to the sample micrographs of the Au NPs that were reduced by UV light, the Au NPs were not distributed along the film as observed for the Au NPs that were reduced by citrate. However, the RBS results (Table 2) showed that incorporation of Au NPs was higher in the samples reduced by UV light. In this case, the acidity of the gold salt (pH = 2.3) favors protonation of the −COO− groups, which breaks the electrostatic interactions between the polyelectrolytes. Consequently, more −NH3+ groups become free to promote electrostatic interactions with the gold salt and as a result better incorporation of gold nanoparticles. G


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Figure 7. TEM images of the self-assembled films for samples (a) S7-ctt, (b) S4-ctt, (c) S7-UV, and (d) S4-UV with their respective histograms.

the surface of the films and to the penetration resistance of the films to radiation. After 1.5 h of irradiation, the reaction speed and rate of H2 production increase. The S4-UV film introduced a better photocatalytic potential. The evolution in the H2 production (as in this film) results from greater TiO2 concentrations due to the electrostatic interactions of the TiO2 with the polyelectrolytes in formation of multilayers.

important role in increasing the photovoltage and as a result H2 production, improving the interfacial charge-transfer kinetics.42 However, H2 production of S4-ctt and S4-UV increased 22 and 76 times, respectively, relative to the films with the polyelectrolytes + TiO2 (S4). The slopes of the curve for the S4-ctt and S4-UV films for up to 1.5 h were 0.025 and 0.13 μmol h−1 cm−2, respectively. Between 1.5 and 3.0 h the declivities increased to 0.12 and 0.33 μmol h−1 cm−2, respectively, for the same films. This result likely occurred because the reaction speed is slower at the beginning of the reaction due to elimination of probable impurities adsorbed on

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CONCLUSIONS This study demonstrated that the LbL technique is useful for depositing thin films of polyelectrolytes and TiO2 on BC and H


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Figure 8. H2 production of the systems described in the legend.

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Table 4. H2 Production Rate by GC system

rate (μmol h−1 cm−2)

P7 S7 P4 S4 S7-ctt S7-UV S4-ctt S4-UV

0.0015 0.0028 0.0014 0.0031 0.0030 0.0730 0.0700 0.2330

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for subsequently incorporating Au NPs for production of H2. According to the characterization techniques that were performed in this study, the polyelectrolytes multilayer pH was important for formation of the films. Electrostatic interactions of the gold salt with the −NH3+ groups of PAH were also relevant. When these interactions occurred, greater light absorption was achieved and greater incorporation of the Au NPs was observed according to UV−vis and XRD. Films produced in this study indicated that TiO2 is important in formation of the polyelectrolytes multilayers because the film formation process differs depending on the pH. The best production of H2 photocatalysis was measured using gas chromatography for the films and corresponded to 0.70 μmol cm−2 of H2 for the S4-UV film when irradiated for 3 h. This film showed the H2 production rate increase 76 times relative to the films with the polyelectrolyte and TiO2 colloidal solutions.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Phone: +55 81 33347232. Fax: + 55 81 33347200. E-mail: [email protected] or giovanna.machado@cetene. gov.br. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research described herein was supported by CETENE, FACEPE, CNPq, and FAPERGS and also scholarships granted by CNPq and CAPES/FAPERGS. I


Article

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Characterization and Application of Nanostructured Films Containing

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