Effect of Au nanoparticle loading on the photo-electrochemical response of Au–P25–TiO2 catalysts

Journal of Solid State Chemistry 281 (2020) 121051

Contents lists available at ScienceDirect

Journal of Solid State Chemistry journal homepage: www.elsevier.com/locate/jssc

Effect of Au nanoparticle loading on the photo-electrochemical response of Au–P25–TiO2 catalysts Anirban Das a, Preeti Dagar b, Sandeep Kumar a, Ashok Kumar Ganguli a, * a b

Department of Chemistry, Indian Institute of Technology, Hauz Khas, New Delhi, 110016, India Institute of Nano Science and Technology, Habitat Center, Phase-10, Sector-64, Mohali, India

A R T I C L E I N F O

A B S T R A C T

Keywords: Au–P25–TiO2 5 nm Photoelectrochemistry Mott-Schottky Flatband potential Charge carrier density

Very low concentration of surface plasmon resonance (SPR) active and photocorrosion resistant Au nanoparticles (NPs) have been used to sensitize a wide bandgap semiconductor (TiO2) and extend its photocatalytic activity to the visible region. Au–P25–TiO2 nanocomposites with Au NPs (less than 5 nm) with varying concentration (0.5–1 wt %) were synthesized and characterized. Their electrochemical response in presence and absence of light irradiation was evaluated and it was observed that at lower Au loadings the photocurrent increases with Au concentration and maximizes at 0.7 wt % (wt%), and subsequently on higher loadings the photocurrent decreases monotonically. The Mott-Schottky analysis indicates that a balance between charge carrier density (Nd, a kinetic parameter) and the flatband potential (VFB, a thermodynamic parameter) is potentially responsible for the observed change in photocurrent response with Au loading. This report is useful for the design and performance optimization of Au–TiO2 catalyst based photoelectrochemical devices.

1. Introduction TiO2 nanoparticles (NPs) and their composites/heterostructures have been widely used as photocatalyts due to their abundance, stability, and non-toxicity [1]. However, their large bandgap requiring UV radiation to activate as well as the high recombination rate of photogenerated charge carriers results in low quantum efficiency of photochemical reactions catalyzed by these TiO2 based materials. This has led to numerous efforts to enhance their light harvesting ability, predominantly by use of appropriate sensitizers that extend their activity to the visible region. One of the methods to achieve this is to make composites with Au and other noble metal NPs having surface plasmon resonance (SPR) band in the visible region [2]. It was generally seen that catalysts based on Au NP composites with TiO2 exhibit superior photo(electro)catalytic activity than bare TiO2 [3–9], though some recent reports have argued that Au-SPR has no influence on the activity of anatase TiO2 [10]. In addition, Au being a noble metal, Au based catalysts, do not undergo photocorrosion [11,12]. Au NPs can also act as co-catalysts and hence Au/P25 TiO2 particles have been reported to be active catalysts for various chemical and photo(electro)catalytic transformations [13,14]. For example, CO oxidation [15,16] and water gas shift reactions [17] were reported to be catalyzed by Au/TiO2 systems. The first report on

photocatalytic splitting of water on Au/P25–TiO2 catalysts was by Garcia et al. [18] who determined that the Au NP loading, particle size and calcination temperature governed the catalytic activity. The most active catalysts had 0.2 wt% loading of Au NPs whose average size was 1.87 nm. Additionally, it was observed that the catalysts calcined at 200 C had the best activity. It was demonstrated that photocatalytic splitting of water under visible light irradiation proceeded by excitation of the Au surface plasmon. Reichert et al. [14]. explored the role of Au NPs as cocatalysts in the photoelectrocatalytic (PEC) water splitting and photocatalytic H2 production. It was reported that in the PEC water oxidation reaction, the role of the Au NPs was to catalyze the recombination of photogenerated holes and electrons as well as parasitic side reactions [19,20]. The most commonly reported method for the synthesis of these catalysts is by the incipient wetness method where the Au precursor (usually HAuCl4) is introduced on the support and reduced in-situ. However, the main disadvantage of this method is inadequate control on the particle size and further it has been shown that decoration of preformed Au NPs over supports results in better catalytic activity [21]. It was also previously reported that smaller size (around 5 nm and smaller) particles exhibit higher photo-, electro-, as well as chemical catalytic activity than larger sized particles. For example, it was shown that Au NPs of size 2–5 nm show a especially high activity for oxidation of CO and propylene

* Corresponding author. E-mail address: [email protected] (A.K. Ganguli). https://doi.org/10.1016/j.jssc.2019.121051 Received 22 July 2019; Received in revised form 27 October 2019; Accepted 2 November 2019 Available online 9 November 2019 0022-4596/© 2019 Elsevier Inc. All rights reserved.

○

A. Das et al.

Journal of Solid State Chemistry 281 (2020) 121051

[22,23]. In this report we have studied the photoelectrochemical properties of P25–TiO2 decorated with 5 nm Au NPs. Our studies show that an optimal Au loading of 0.7 wt% results in the maximum current density. Such behavior on Au loading is reported for photocatalytic activity too (for chemical transformations) [19,24–28], however very few reports exist for photo-electrochemical studies in an Au/TiO2 system [29]. It is known that higher efficiency may be observed under photoelectrocatalytic conditions as compared to the same process carried out under photocatalytic conditions [30–41]. Hence our study looks in detail the variation of photoelectrochemical activity with Au loading in TiO2 based catalysts with low loading of small (~5 nm) Au NPs for which not much is reported previously.

Table 1 Details of the catalyst, volume of Au colloidal solution added to each and size of Au NPs (determined by TEM analysis). * Bare P25–TiO2 sample calcined under the same conditions as the Au loaded catalysts.

2. Materials and methods

electrolyte solution. For photo-electrochemical studies, linear sweep voltammetry (LSV) scan in dark and under light illumination was performed in the potential range of 0 to þ1.2 V versus Ag/AgCl reference electrode with a scan rate of 10 mV/s. EIS experiments were performed at 200 mV using 1 kHz AC frequency. Mott-Schottky analysis was carried out at a frequency of 1 kHz. Zeta potential was measured on a zeta-sizer Nano-ZS90 instrument by manufactured by Malvern.

HAuCl4 and tetrakis(hydroxymethyl)phosphoniumchloride (THPC) were purchased from Sigma Aldrich and diluted to the appropriate concentration using double distilled water. P25–TiO2 nanoparticles were purchased from Evonik Industries. NaOH was purchased from Chemlabs (India). All chemicals were used as received without any further purification. The colloidal solution of Au NPs was prepared by a method previously reported by Duff et al. [42]. Briefly, to a RB flask containing 45.5 ml double distilled water, 1.5 ml of 0.2 M NaOH and 1 ml of 0.0164 M THPC were added in sequence. The THPC acts both as a reducing agent as well as a stabilizer. The solution was stirred for 2 min, and then 2 ml of 25 mM chloroauric acid (HAuCl4) was added. The color of the colloidal solution changed to brownish orange indicating the formation of Au NPs. The contents of the reaction were stirred for 2 min and then stored at 4 C for further use. To 155 mg of P25–TiO2 dispersed in 50 ml water by sonication, the desired volume (as governed by the loading) of the Au colloidal solution was added; the color of the suspended solid changed from white to reddish-pink. The dispersion was stirred for 4 h at room temperature. Thereafter the solids were separated out by centrifugation at 6500 rpm for 7 min and calcined at 200 C for 2h under a H2 flow. Five different Au loadings of these catalysts (0.5, 0.6., 0.7, 0.8 and 1 wt%) were synthesized, characterized and their response to photoelectrochemical stimuli were evaluated. Powder x-ray diffraction analysis was carried out on a Bruker D8 Advance diffractometer equipped with Ni-filtered Cu Kα radiation (λ ¼ 1.5418 Å) in the 2θ range of 10–80 at a scan rate of 0.02 per second. UV–visible spectroscopy was performed on a Shimadzu UV-2450 instrument equipped with a tungsten and deuterium lamp scanning the absorption from 200 nm to 850 nm. Diffuse reflectance spectroscopy (DRS) was performed on the same instrument with appropriate accessory using dried BaSO4 as a standard. Fourier transform IR spectra were measured on an Agilent Carey 660 instrument equipped with an ATR accessory. Raman data was obtained on a Horiba Jobin-Yvon, XPLORA confocal Raman instrument equipped with a 532 nm laser and a resolution of 1 cm1. The dry solid samples were smeared (with the aid of a spatula) onto an aluminum foil supported on a glass slide. Samples for TEM measurements were obtained by dispersing a 1 mg of sample in 1 ml ethanol (Merck) and ultrasonication for 10 min and then drop casting 4 μl onto a carbon-coated 200-mesh copper grid. Images were acquired on a FEI Tecnai TF20 High Resolution Transmission Electron Microscope and recorded at an accelerating voltage of 200 kV. Photoelectrochemical experiments were performed on a CHI electrochemical analyzer (Model 608E) equipped with a Xe arc lamp (150W, 200–1000 nm range). 8 mg of the catalyst was dispersed in 150 μl water and 210 μl isopropanol followed by addition of the binder Nafion (20 μl). 120 μl of this suspension was deposited onto a ITO glass (SnO2: In) substrate of an area approximately 1 cm2. Three electrode photoelectrochemical cell was used to obtain the current-voltage (I–V) characteristics. Mott-Schoktty analysis and Electrochemical Impedance Spectroscopy (EIS) studies were performed by using the as-synthesized material deposited onto ITO as a working electrode, platinum wire as a counter electrode and Ag/AgCl as reference electrode in 0.5 M Na2SO4

Notation

Au loading (wt%)

Volume (ml) of Au colloidal solution per 155 mg P25–TiO2

Size of Au NPs (nm)

PA3 PA4 PA5 PA6 PA7 P25C*

0.5 0.6 0.7 0.8 1 0

3.5 4.25 5.0 6.0 7.0 0

4.71  1.29 4.75  1.25 4.46  0.82 4.89  1.46 4.86  1.40 NA

3. Results and discussion TiO2 NPs “sensitized” with metal NPs having appropriate SPR band can enable photoactivity in the visible region for applications in harvesting solar energy. Several reports are available where Au NPs composites with TiO2 have been reported [3–9]. However, most of the reports feature complex preparation methods which may not be commercially feasible. This report investigates an Au/P25–TiO2 composite that was synthesized in high yield using a simple method, using chemicals which are either commercially available or can be synthesized fairly easily. Au NPs of small sizes (~5 nm) were synthesized by following the method reported by Duff et al. [42]. This method is the most commonly used method to yield these small sized particles and is superior in terms of reaction time (~2min reaction) and extended stability (stable for several months if stored at 4ΟC) as compared to the commonly used citrate method [43]. The low concentration of Au NPs used in this report makes this catalyst viable for large scale applications. Different concentrations of pre-synthesized Au NPs colloidal solution were deposited onto P25–TiO2 supports as listed in Table 1. The notation PAX corresponds to P25–Au –X, where X corresponds to the volume (in ml) of the as prepared Au colloidal solution that was deposited onto a fixed mass of P25–TiO2. After stirring for 4h, the solid was separated out by centrifugation. The supernatant was evaluated (by UV–Visible spectroscopy) for residual Au and none was detected (Fig. S1). It should be noted that P25–TiO2 disperses very well in water and cannot be precipitated out of suspension even on prolonged centrifugation (1 h) at high rotation velocities (~9000 rpm). However, we observed that addition of Au NPs (over a certain concentration of Au NPs as we shall describe below) results in the composite precipitating out under relatively mild centrifugation conditions (5 min, 4000 rpm). The minimum concentration chosen in this study was 3.5 ml Au colloidal solution per 155 mg P25–TiO2 (PA3); lower concentration leads to incomplete separation of the composite from the suspension by centrifugation. This phenomenon was investigated by recording the zeta potentials which are key indicators of stability of a colloidal dispersion. A larger magnitude of the zeta potential implies that a greater repulsive force has been introduced into the system and the probability of flocculation occurring is smaller [44]. We recorded the zeta potential values for Au colloidal solution as well as for P25–TiO2 (155 mg dispersed in 50 ml water) with varying volumes of Au colloidal solution added (a) 0 ml b) 1.5 ml c) 2.5 ml and d) 3.5 ml ). The zeta potential values are provided in Table S1. For bare P25–TiO2 the value is þ29.9 mV and the system is stable and does not precipitate out of suspension. However, on progressive addition of Au colloidal solution (1.5 ml, 2.5 ml and 3.5 mL), the value of zeta potential decreases (þ21.2, þ19.2 and þ 16.7 mV respectively) and dispersion becomes more and

○

○

2

A. Das et al.

Journal of Solid State Chemistry 281 (2020) 121051

Fig. 1. (a) Powder X-ray diffraction patterns of calcined bare P25-TiO2 and P25-TiO2 with different loadings of Au. A and R refer to the anatase and rutile phase respectively (b) Diffused reflectance spectra of calcined bare P25-TiO2 and P25-TiO2 with different loadings of Au.

more unstable, thus the Au–TiO2 composite NPs separate out of the dispersion on centrifugation. The maximum concentration chosen was 7.5 ml Au colloidal solution per 155 mg of P25–TiO2 (PA7); as increasing the concentration further leads to Au particles remaining in the supernatant (as determined by UV–Visible spectroscopy). Thus, using PA3 and PA7 as the lower and upper limits of Au loadings, three other intermediate concentration were chosen for evaluation of photoelectrocatalytic activity. For UV–visible spectroscopy, the standard was prepared by addition of exactly the same amount of Au NPs colloidal solution into the same amount of DI water as were used for loading Au onto a TiO2 suspension. The calcined catalysts were characterized by powder X-ray diffraction (PXRD). The PXRD patterns (Fig. 1a) of all the catalysts indicated the presence of both anatase and rutile phases of titania as is typical of P25–TiO2. The samples were calcined at 200 ΟC where, as expected, conversion of the anatase phase to rutile is not observed. Due to the small loading, Au is not detectable by PXRD. Further, no change is detected in the PXRD pattern of the calcined and uncalcined P25–TiO2 catalyst (Fig. S2). Characterization of the series of catalysts by UV–Visible diffuse reflectance spectra (DRS) is reported in Fig. 1b. A surface plasmon resonance (SPR) peak at 550 nm due to Au NPs is clearly seen in all the DRS spectra of Au loaded catalysts. Additionally, as expected, this peak is not observed in the DRS spectrum of the bare P25C. The bandgap calculated from the DRS spectrum was 3.75eV for bare uncalcined P25–TiO2 and 3.69 for P25C while that for the Au loaded samples was found to be 3.08eV. This phenomenon of decrease in bandgap of TiO2 on making a composite with Au NPs was reported previously in literature [3, 22,45] and attributed to the simultaneous downward shift in the conduction band and an upward shift in the valence band. FTIR spectra of the as synthesized catalysts were also obtained (Fig. 2) and is analogous to other reports of Au/TiO2 composites [46]. A number of differences are seen in the spectra of the Au loaded catalysts as compared to the bare calcined catalyst. It is seen that the peak at 3425 cm1 corresponding to the hydrogen bonded hydroxyl groups on the surface of the catalyst is prominently observed in the Au loaded catalysts while it is absent in the bare P25C catalyst. Also at wavenumbers around 2852 to 2921 cm1 the peak resulting from the water of crystallization is clearly seen in the Au loaded catalysts, while it is not observed in the bare catalyst. The origin of the water of crystallization may be attributed to the stabilizer THPC. The FTIR spectrum of THPC and Au colloidal solution is presented in Fig. S3. Peaks are seen in the region in these two spectra, however the peaks in the Au–TiO2 catalysts are shifted ~15 and 58 cm1 respectively as compared to. THPC and the Au NPs colloidal solution possibly resulting from a different co-ordination environment. A set of peaks around 1630 cm1 are also seen in the Au loaded catalysts and they correspond to the surface adsorbed water. Additionally, the peak at 649 cm1, corresponding to the Ti–O stretching

Fig. 2. FTIR spectra of calcined bare P25-TiO2 and P25-TiO2 with different loadings of Au (Table 1) catalysts.

and Ti–O–Ti bridging vibrations broadens and shifts to lower wavenumbers in Au loaded catalysts. Raman analysis was carried out for the different catalysts as presented in Fig. 3. In calcined P25 TiO2, the peaks corresponding to pure anatase phase are seen at 139 cm1 (Eg), 199 cm1 (Eg), 393 cm1 (B1g), 515 cm1 (B1g/A1g) and 635 cm1 (Eg) while a peak at 442 cm1 is observed that corresponds to the A1g mode of the rutile phase. On loading Au NPs a significant shift (5-19 cm1) to higher wavenumbers is seen in the Eg mode at 139 cm1; additionally, all peaks are broadened. The shifting and broadening was previously reported in literature [45,47] and was attributed to creation of surface defects as would be expected on loading Au NPs. Representative TEM images of PA5 are presented in Fig. 4 and they clearly show the Au particles with the (200) planes exposed on the surface. TEM analysis of the other catalysts (not shown) also show similar characteristics. There is not much variation in the particle size (~5 nm) of the Au NPs on variation in the loading (Table 1). Linear sweep voltammograms (LSV) were recorded in a three electrode setup with Ag/AgCl reference electrode, a Pt counter electrode and a working electrode comprised of ITO coated glass onto which the catalyst was deposited. The approximate surface area of the catalyst coated ITO glass was 1 cm2. Current densities reported were normalized per actual surface area. Na2SO4 (0.5 M) was used as the electrolyte. The potential was varied from 0 to þ1.2V (vs Ag/AgCl). From the LSV plots (Fig. 5) it is seen that the current increases with increasing potential. At a potential of 0.8 V, the difference in the light and dark currents are 3

A. Das et al.

Journal of Solid State Chemistry 281 (2020) 121051

Fig. 3. Raman spectra of calcined bare P25-TiO2 and P25-TiO2 with different loadings of Au. Region between 100 to 300 cm1 is shown on the left, while the region between 350 to 750 cm1 is shown on the right.

Fig. 4. (a) TEM image of PA5. (b) and (c) corresponding HRTEM images.

compared as depicted in Fig. 6a. The potential 0.8 V was chosen as at potentials higher than this, water splitting is expected to take place. At potentials lower than this, adequate separation of the light and dark currents is not observed. Photocurrents were reproducible and stable as indicated by recording current output (at applied potential of 0.8V vs Ag/ AgCl) at several cycles of light on and light off conditions (Fig. 6b). The current density at 0.8V is in the range of 10 μAcm2. This magnitude of this current density is comparable to other reports of Au/TiO2 catalysts (Table 2). Au NPs capped titania nanotube arrays [48] with mean diameter of the Au NPs of 1.51 nm produced photocurrent in the range of 100 μAcm2. In a recent report, a composite comprising of in-situ generated Au NPs on layer by layer deposited 2D TiO2 films [49] the maximum current density obtained was 0.106 μAcm2, about 2 orders of magnitude lower than reported by us. Photoelectrochemical studies on semi-transparent composite films of Au loaded TiO2 nanotubes prepared by sputtering Au nanoparticles on highly transparent TiO2 nanotubes films resulted in maximum current density of 8 μAcm2 [50], which is comparable to the present report. In another report of a RGO/meso-TiO2/Au ternary composite [51] the maximum current density obtained was 4 μAcm2 , again comparable to this report. Thus, low Au loadings and relatively easy synthesis procedure using a commercially available support, is capable of producing current densities comparable to a

Fig. 5. Linear sweep voltammograms of bare P25-TiO2 those with different loadings of Au, in light (solid lines) as well as in the dark (dashed lines). 4

A. Das et al.

Journal of Solid State Chemistry 281 (2020) 121051

Fig. 6. (a) Variation of photocurrent as a function of Au loadings (b) Light on-off cycles depicting reproducibility and stability of the catalysts. The experiments were carried out at 0.8V vs Ag/AgCl. Table 2 Comparison of reported photocurrent densities of some Au–TiO2 catalysts. S No

Au NPs size

Architecture of TiO2 support

1 2 3 4 5 6

1.51 nm 29–42.5 nm 5–10 nm 6 nm 2.5 nm 5 nm

Nanotubes 2D nanotube films with Pt co-catalyst Nanotube films RGO/Mesoporous TiO2 Nanotubes P25–TiO2

Au loading

Photocurrent

Reference

13.3 wt% ~0.3 wt% 1.14 wt% 0.7 wt%

100 μAcm2 0.106 μAcm2 8 μAcm2 4 μAcm2 20 μAcm 10 μAcm2

[48] [49] [50] [51] [19] This work

photoelectrochemical performance and comparable to our results, a current density of the order of 20 μAcm2 was obtained. In other studies, it was shown [24] that the activity for the photocatalytic decomposition of thiophene, thiol, Rhodamine B and phenol over Au/TiO2 catalysts increased for Au loadings from 0.1 to 0.5 wt% and then decreased at higher loadings (1 and 2 wt%). The size of the Au NPs (0.5 wt% loading) was 10.7 nm, much larger than those in our present study. It was proposed, that on higher Au loading, more active sites on the surface of semiconductor would be covered and this may prevent the contact of reacting species with the semiconductor surface thus leading to decrease in photocatalytic efficiency. For photocatalytic benzene hydroxylation it was reported [25] that on increasing the Au loading from 0.25 to 2.2 wt %, the catalytic activity was maximum at 1 wt% Au. Contrary to our present report (Table 1), in the abovementioned study [25], the size of the Au NPs (in the composite) increases with increase in Au loading. For the photocatalytic decomposition of 4-cholorophenol [26] Au loadings (0.25–1.5 wt%) were evaluated and again the catalytic activity was found to be maximum at an optimal 0.5 wt%. The particle size (prepared by different methods) ranged from 6.3 to 10.3 nm and it was found that the amount of Au NPs loading and the size influenced the catalytic activity with the smaller size resulting in better activity. In another study [27] Au/TiO2 nanocomposites (0.5–5 wt% Au) were evaluated as electrode materials for dye-sensitized solar cells and it was observed that an optimal loading of 0.5 wt% of catalyst shows the best photocatalytic activity. The particle sizes varied from 5 to 10 nm, larger than those in our report. Consistent with our observations, no Au peaks could be detected in the XRD pattern of the composites that contained 1 wt% Au or below. The optimal amount of Au NPs in the photoelectrode was attributed, to the beneficial plasmonic effect of increasing light harvesting on lower Au loadings and the detrimental effect of metallic NPs as recombination and trapping centers at higher Au loadings. In a report of Au–TiO2 based dye sensitized solar cells, the loading of Au particles (size 8–20 nm) was varied from 0.5 to 7.5 wt% and it was found that the

catalyst obtained by complicated routes and high Au loadings. In our study an optimal Au loading (0.7 wt%) was observed (Fig. 6a) to produce the highest photocurrent. At this loading the photocurrent produced (in presence of light) is approximately 9 times the current produced in dark. Yang et al. [29]. have reported a similar phenomenon in Au –TiO2 mesospheres where the Au loadings were 0, 0.1.0.25, 0.5 and 1 wt%. However, in their samples, the size of the Au NPs also varied from 10 to 50 nm depending on the loadings. The primary phase was anatase, though they observed that there are structural changes in TiO2 octahedra on increasing the Au loading. This may however be attributed to the large size of the Au NPs. Though they observed that the photocurrent density increased on Au loading to 0.5% and then decreases, (with the 1 wt% loading performing equivalent to 0 wt% loading). The size of the Au NPs (with smaller particles exhibiting better catalytic performance) and the phase composition of the support (TiO2) are important parameters influencing catalytic behavior, and hence the work of Yang et al. [29] cannot be strictly compared to ours, as in our case the size of the Au NPs does not change. There have been several studies where similar behavior was observed in photocatalytic reactions (without applied potential) where increasing the Au NPs loading on TiO2 increases the photocatalytic activity and on higher loadings the activity decreases. For example, photocatalytic activity as a function of Au loading was studied in Au NPs decorated TiO2 nanotubes [19]. The loadings evaluated were 0.57, 1.14, 2.28 and 2.82 wt% and it was observed that the activity increased with the Au loading and then decreased, with the sample with 1.14 wt% performing the best. It was postulated that since the photocatalytic mechanism involved the reaction of the oxidative and reductive species produced on the surface of the photocatalysts with neighboring molecules, therefore, increased weight percentage loading of Au NPs would possibly reduce the exposure of active sites on the TiO2 NT/Au composite surface and cause agglomeration of nanoparticles, resulting in a lower photocatalytic activity. Only one of the catalysts (1.14 wt% loading) was evaluated for

5

A. Das et al.

Journal of Solid State Chemistry 281 (2020) 121051

Table 3 Effect of Au loading on catalytic activity of Au–TiO2 catalysts. S No

Particle size of Au

Nature of TiO2

Au loading variation

Activity measured

Conclusions

Reference

1

1.87–6.4 nm

P25– TiO2

2.5 nm

Nanotubes

Photocatalytic water splitting to generate H2 Photocatalytic

3

10.7 nm

4

5

1.8 nm–5.7 nm depending on Au loading 6.3–10.3 nm

P25– TiO2 functionalized with SO24 P25– TiO2

0.2 wt% loading of 1.87 nm Au exhibits best performance 1.14 wt% exhibits best performance PEC 20 μAcm2 Activity increases from 0.1 to 0.5 wt % then decreases

[18]

2

0.25, 1.5 and 2.2 wt % 0.57, 1.14,2.28,2.82 wt% 0.1, 0.5, 1 and 2 wt%

6

0.25 to 2.2 wt%

P25– TiO2

0.25 to 1.5 wt%

5–10 nm

TiO2 nanofilm

0.5 to 5 wt%

7

8–20 nm

P25– TiO2

0.5 to 7.5 wt%

8

5 nm

P25– TiO2

0.5, 0.6.0.7.0.8 and 1 wt%

Photocatalytic decomposition of thiophene, thiol, Rhodamine B and phenol Photocatalytic benzene hydroxylation

Photocatalytic decomposition of 4chlorophenol Photocatalytic dye sensitized solar cells (DSSC) Photocatalytic DSSC Photoelectrochemical activity

1 wt% has the maximum activity. Size of the particles increases on loading 0.5 wt% shows best activity, Smaller size results in better activity 0.5 wt% shows best photocatalytic activity 2.5 wt% shows best photocatalytic activity 0.7 wt% exhibits highest photocurrent

[19] [24]

[25]

[26] [27] [28] This work

Fig. 7. Mott-Schottky plots for (a) P25C and (b) PA3, PA4, PA5, PA6 and PA7.

dielectric constant was determined as described elsewhere [55] based on the percentage of anatase and rutile phases (85% and 15% respectively) in the sample as calculated from the PXRD pattern. The slopes of the 1/C2 vs V curve (Mott- Schottky plot) for the Au loaded as well as the unloaded catalysts are all positive indicating that the n-type behavior of TiO2 is retained in Au loaded catalysts (Fig. 7). To gain insight into the effect of Au loading on the photocurrent density, analysis of Nd and VFB was also undertaken. The slope Mott- Schottky plots (Table S2) also yield the charge carrier (dopant) density (Nd) as presented in (Table 4) and Fig. 8a. The Nd increases on increasing the Au loadings. Within experimental error Nd for the PA3 was almost identical to that of unloaded P25C, however, at larger loadings, as expected the value is substantially higher and in general increases with increase in Au loading. The magnitudes reported are comparable to those of similar catalysts reported in literature [54]. Thereafter, analyzing the intercepts of the Mott-Schottky plots (Table S2), the VFB were also calculated (Table 4, Fig. 8b) and it is seen that the obtained values are in the range reported for similar catalysts in literature [52,54]. The values of VFB were compared with respect to the Fermi level of ~ 5 nm Au particles deposited on TiO2, as reported by Kiyonaga et al. [56]. It is seen that at lower Au loadings, the VFB are similar but at higher loadings (PA6 and PA7) the values become more negative. These data indicate that a balance between Nd, a kinetic parameter and VFB, a thermodynamic parameter governs the photocurrent at a particular Au loading. At loadings lower than 0.7 wt%, the VFB are almost identical, but the Nd increases with increasing Au loading, thus the photocurrent increases. However, at higher loadings of Au despite the Nd rising monotonically, the VFB is more

catalyst with 2.5 wt% Au loading exhibits the maximum photocurrent [28]. Studies have also indicated that excess Au on the surface of TiO2 is detrimental to photoelectrocatalytic activity. For example, in a report [52] on photoelectrochemical performance of Au–TiO2 thin films (5 mol % Au) prepared by both sputtering and sol-gel methods followed by annealing to 600ΟC, it was observed that the films prepared by the former method exhibited lower performance. It was found (via XPS and SEM analysis) that these films had a larger concentration of Au on the surface and thus the authors argued that more Au concentration on the surface is detrimental to the photoelectrochemical performance. In this present report too, we find that higher loading of Au on the surface of TiO2 leads to decreased photoelectrochemical performance. Table 3 summarizes the effect of variation of Au loading on the catalytic activity of reported Au–TiO2 catalysts . Despite there being reports on the effect of loading on the photocatalytic and photoelectrocatalytic activity in Au–TiO2 catalysts all of which show similar trends of increase and subsequent decrease in activity with the loading, very little analysis has been reported earlier. In this report we attempt to explain this behavior in photoelectrochemical systems in terms of the Mott-Schottky analysis (Equation 1) [53,54]. 1/C2 ¼ (2/e0 ε0 ε Nd) [(V  VFB)  kT/e0] .

(1)

where, C is the differential capacitance, e0 is the electron charge,

ε0 the permittivity of vacuum, ε the dielectric constant of P25–TiO2 (57)

[55], Nd the dopant density, V the applied electrode potential, VFB the flatband potential, k is the Boltzman constant and T is the absolute temperature (kT/e0 is a temperature-dependent correction term). The 6

A. Das et al.

Journal of Solid State Chemistry 281 (2020) 121051

Fig. 8. (a) Nd and (b) VFB trends for P25C, PA3, PA4, PA5, PA6 and PA7.

Acknowledgement

Table 4 The charge carrier density (Nd) and the flatband potential (VFB) of the different catalysts obtained from the Mott-Schottky analysis. Catalyst

Au Loading wt%

Nd

VFB (vs Ag/AgCl)

PA3 PA4 PA5 PA6 PA7 P25C*

0.5 0.6 0.7 0.8 1.0 0

7.91  1022 1.27  1020 1.40  1020 1.70  1020 2.55  1020 1.18  1021

7.99  1001 8.21  1001 8.08  1001 9.73  1001 9.28  1001 8.04  1001

AKG acknowledges the support of nanoscale research facility, IIT Delhi for part of the work described. AD acknowledges the support from the Council of Scientific and Industrial Research (CSIR-India) for financial support in form of SRA (Senior Research Associate – Pool Scheme) fellowship. Appendix A. Supplementary data Supplementary data to this article can be found online at https://do i.org/10.1016/j.jssc.2019.121051.

negative. Since for n-type semiconductors, the VFB is directly proportional to the energy of the conduction band edge [57], thus it is conceivable that for a more negative VFB, thermodynamically the process of electron transfer from the Fermi level of Au to the conduction band of TiO2 [58] becomes less favorable, thus the photocurrent decreases. EIS analysis (Fig. S4) indicated that, as expected, the impedance in presence of light (for a particular catalyst) is much lesser than in the dark. Additionally, the overall impedance experienced in presence of light was minimum for PA5 and PA6 catalysts and this correlates well with the former having the highest photoelectrocatalytic activity. To investigate the effect of calcination on the photoelectrochemical response, we performed LSV on the calcined and uncalcined 0.7% Au loaded catalysts (Fig. S5). It was observed that under light illumination, at a potential of 0.8 V (vs Ag/AgCl) the photocurrent is approximately 4 times higher for the calcined catalyst (PA5) as compared to the uncalcined catalyst.

References [1] C. Po Chun, C. Chien Chon, C. Shih Hsun, A review on production, characterization, and photocatalytic applications of TiO2 nanoparticles and nanotubes, Curr. Nanosci. 13 (2017) 373–393. [2] V. Subramanian, E.E. Wolf, P.V. Kamat, Influence of metal/metal ion concentration on the photocatalytic activity of TiO2-Au composite nanoparticles, Langmuir 19 (2003) 469–474. [3] S.P. Lim, Y.S. Lim, A. Pandikumar, R. Ramaraj, N.M. Huang, H.N. Lim, Y.H. Ng, D.C.S. Bien, O.K. Abou-Zied, Gold-silver@TiO2 nanocomposite-modified plasmonic photoanodes for higher efficiency dye-sensitized solar cells, Phys. Chem. Chem. Phys. 19 (2017) 1395–1407. [4] S.H. Nam, H.-S. Shim, Y.-S. Kim, M.A. Dar, J.G. Kim, W.B. Kim, Ag or Au nanoparticle-embedded one-dimensional composite TiO2 nanofibers prepared via electrospinning for use in lithium-ion batteries, ACS Appl. Mater. Interfaces 2 (2010) 2046–2052. [5] T. Sakamoto, D. Nagao, M. Noba, H. Ishii, M. Konno, Dispersed-Nanoparticle loading synthesis for monodisperse Au-titania composite particles and their crystallization for highly active UV and visible photocatalysts, Langmuir 30 (2014) 7244–7250. [6] J. Yu, G.A. Rance, A.N. Khlobystov, Electrostatic interactions for directed assembly of nanostructured materials: composites of titanium dioxide nanotubes with gold nanoparticles, J. Mater. Chem. 19 (2009) 8928–8935. [7] S. Kundu, A. Kafizas, G. Hyett, A. Mills, J.A. Darr, I.P. Parkin, An investigation into the effect of thickness of titanium dioxide and gold-silver nanoparticle titanium dioxide composite thin-films on photocatalytic activity and photoinduced oxygen production in a sacrificial system, J. Mater. Chem. 21 (2011) 6854–6863. [8] D.K. Hwang, Y.-G. Shul, K. Oh, Photocatalytic application of Au-TiO2 immobilized in polycarbonate film, Ind. Eng. Chem. Res. 52 (2013) 17907–17912. [9] S. Kamimura, T. Miyazaki, M. Zhang, Y. Li, T. Tsubota, T. Ohno, (Au@Ag)@Au double shell nanoparticles loaded on rutile TiO2 for photocatalytic decomposition of 2-propanol under visible light irradiation, Appl. Catal., B 180 (2016) 255–262. [10] J. Nie, J. Schneider, F. Sieland, S. Xia, D.W. Bahnemann, The role of Au loading for visible-light photocatalytic activity of Au-TiO2 (anatase), J. Photochem. Photobiol. A Chem. 366 (2018) 111–117. [11] E. Pakdel, W.A. Daoud, L. Sun, X. Wang, Photostability of wool fabrics coated with pure and modified TiO2 colloids, J. Colloid Interface Sci. 440 (2015) 299–309. [12] J. Shi, J. Chen, G. Li, T. An, H. Yamashita, Fabrication of Au/TiO2 nanowires@ carbon fiber paper ternary composite for visible-light photocatalytic degradation of gaseous styrene, Catal. Today 281 (2017) 621–629. [13] M. Meire, P. Tack, K. De Keukeleere, L. Balcaen, G. Pollefeyt, F. Vanhaecke, L. Vincze, P. Van Der Voort, I. Van Driessche, P. Lommens, Gold/titania composites: an X-ray absorption spectroscopy study on the influence of the reduction method, Spectrochim. Acta, Part B 110 (2015) 45–50. [14] R. Reichert, Z. Jusys, R.J. Behm, Au/TiO2 photo(electro)catalysis: the role of the Au cocatalyst in photoelectrochemical water splitting and photocatalytic H2 evolution, J. Phys. Chem. C 119 (2015) 24750–24759.

4. Conclusions A series of catalysts were prepared by variation in loading (0.5–1 wt %) of preformed Au NPs (~5 nm) onto commercially available P25–TiO2 and our studies show that an optimal loading of 0.7 wt% yields the maximum photocurrent. Mott-Schottky analysis indicated that the charge carrier density (Nd) increases on increasing the Au loading. However, the flatband potentials (VFB) are more negative for higher loadings. It is thus proposed that at lower Au loadings the Nd increases, but VFB are similar, thus the photocurrent increases due to increase in Nd. However at higher loadings, though the Nd increases with increasing loading, the VFB are more negative, and since the VFB is directly proportional to the conduction band energy in n-type semiconductors, the process of electron transfer from the Fermi level of Au to the conduction band of TiO2 becomes thermodynamically more unfavorable leading to decrease in photocurrent. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

7

A. Das et al.

Journal of Solid State Chemistry 281 (2020) 121051 [37] S. Ghasemian, D. Nasuhoglu, S. Omanovic, V. Yargeau, Photoelectrocatalytic degradation of pharmaceutical carbamazepine using Sb-doped Sn80%-W20%-oxide electrodes, Separ. Purif. Technol. 188 (2017) 52–59. [38] X. Zhao, H. Liu, J. Qu, Photoelectrocatalytic degradation of organic contaminant at hybrid BDD-ZnWO4 electrode, Catal. Commun. 12 (2010) 76–79. [39] X. Zhao, L. Guo, C. Hu, H. Liu, J. Qu, Simultaneous destruction of Nickel (II)-EDTA with TiO2/Ti film anode and electrodeposition of nickel ions on the cathode, Appl. Catal., B 144 (2014) 478–485. [40] C. Zhai, M. Zhu, Y. Lu, F. Ren, C. Wang, Y. Du, P. Yang, Reduced graphene oxide modified highly ordered TiO2 nanotube arrays photoelectrode with enhanced photoelectrocatalytic performance under visible-light irradiation, Phys. Chem. Chem. Phys. 16 (2014) 14800–14807. [41] D. Wang, X. Li, J. Chen, X. Tao, Enhanced visible-light photoelectrocatalytic degradation of organic contaminants at iodine-doped titanium dioxide film electrode, Ind. Eng. Chem. Res. 51 (2012) 218–224. [42] D.G. Duff, A. Baiker, P.P. Edwards, A new hydrosol of gold clusters. 1. Formation and particle size variation, Langmuir 9 (1993) 2301–2309. [43] J. Turkevich, P.C. Stevenson, J. Hillier, A study of the nucleation and growth processes in the synthesis of colloidal gold, Discuss. Faraday Soc. 11 (1951) 55–75. [44] E. Dickinson, in: D.H. Everett (Ed.), Basic Principles of Colloid Science, vol. 45, Royal Society of Chemistry, London, 1988, pp. 328–329. Paperback, pp. xv þ 243, price £9.95, $ 19.50. ISBN 0-85186-443-0 1989 J. Chem. Technol. Biotechnol. [45] A. Kafizas, S. Kellici, J.A. Darr, I.P. Parkin, Titanium dioxide and composite metal/ metal oxide titania thin films on glass: a comparative study of photocatalytic activity, J. Photochem. Photobiol., A 204 (2009) 183–190. [46] X. Liu, J. Zhu, X. Huo, R. Yan, D.K.Y. Wong, An intimately bonded titanate nanotube-polyaniline-gold nanoparticle ternary composite as a scaffold for electrochemical enzyme biosensors, Anal. Chim. Acta 911 (2016) 59–68. [47] I. Oja Acik, N.G. Oyekoya, A. Mere, A. Loot, L. Dolgov, V. Mikli, M. Krunks, I. Sildos, Plasmonic TiO2:Au composite layers deposited in situ by chemical spray pyrolysis, Surf. Coat. Technol. 271 (2015) 27–31. [48] F.-X. Xiao, S.-F. Hung, J. Miao, H.-Y. Wang, H. Yang, B. Liu, Metal-cluster-decorated TiO2 nanotube Arrays: a composite heterostructure toward versatile photocatalytic and photoelectrochemical applications, Small 11 (2015) 554–567. [49] Q. Zhang, Y. Zhang, K. Xiao, Z. Meng, W. Tong, H. Huang, Q. An, Plasmonic gold particle generation in layer-by-layer 2D titania films as an effective immobilization strategy of composite photocatalysts for hydrogen generation, Chem. Eng. J. 358 (2019) 389–397. [50] H. Wang, W. Liang, Y. Liu, W. Zhang, D. Zhou, J. Wen, Asymmetric photoelectric property of transparent TiO2 nanotube films loaded with Au nanoparticles, Appl. Surf. Sci. 386 (2016) 255–261. [51] Y. Yang, Z. Ma, L. Xu, H. Wang, N. Fu, Preparation of reduced graphene oxide/ meso-TiO2/AuNPs ternary composites and their visible-light-induced photocatalytic degradation n of methylene blue, Appl. Surf. Sci. 369 (2016) 576–583. [52] N. Naseri, P. Sangpour, A.Z. Moshfegh, Visible light active Au:TiO2 nanocomposite photoanodes for water splitting: sol-gel vs. sputtering, Electrochim. Acta 56 (2011) 1150–1158. [53] N. Shahzad, F.Y. Chen, Reductant-assisted synthesis, characterization and photovoltaic characteristics of ligand-protected gold nanoparticles, RSC Adv. 5 (2015) 81093–81102. [54] H. Chen, G. Liu, L. Wang, Switched photocurrent direction in Au/TiO2 bilayer thin films, Sci. Rep. 5 (2015) 10852. [55] J.Y. Kim, H.S. Jung, J.H. No, J.R. Kim, K.S. Hong, Influence of anatase-rutile phase transformation on dielectric properties of sol-gel derived TiO2 thin films, J. Electroceram. 16 (2006) 447–451. [56] T. Kiyonaga, M. Fujii, T. Akita, H. Kobayashi, H. Tada, Size-dependence of Fermi energy of gold nanoparticles loaded on titanium(iv) dioxide at photostationary state, Phys. Chem. Chem. Phys. 10 (2008) 6553–6561. [57] G. Pastor-Moreno, Electrochemical Applications of CVD Diamond, University of Bristol, Bristol, UK, 2002. [58] A. Furube, L. Du, K. Hara, R. Katoh, M. Tachiya, Ultrafast plasmon-induced electron transfer from gold nanodots into TiO2 nanoparticles, J. Am. Chem. Soc. 129 (2007) 14852–14853.

[15] M. Haruta, S. Tsubota, T. Kobayashi, H. Kageyama, M.J. Genet, B. Delmon, Lowtemperature oxidation of CO over gold supported on TiO2, α-Fe2O3, and Co3O4, J. Catal. 144 (1993) 175–192. [16] M. Valden, X. Lai, D.W. Goodman, Onset of catalytic activity of gold clusters on titania with the appearance of nonmetallic properties, Science 281 (1998) 1647. [17] A.A. Hakeem, Z. Zhao, F. Kapteijn, M. Makkee, Revisiting the synthesis of Au/TiO2 P25 catalyst and application in the low temperature water–gas shift under realistic conditions, Catal. Today 244 (2015) 19–28. [18] C. Gomes Silva, R. Ju arez, T. Marino, R. Molinari, H. García, Influence of excitation wavelength (UV or visible light) on the photocatalytic activity of titania containing gold nanoparticles for the generation of hydrogen or oxygen from water, J. Am. Chem. Soc. 133 (2011) 595–602. [19] F. Xiao, Self-assembly preparation of gold nanoparticles-TiO2 nanotube arrays binary hybrid nanocomposites for photocatalytic applications, J. Mater. Chem. 22 (2012) 7819–7830. [20] H. Elbohy, M.R. Kim, A. Dubey, K.M. Reza, D. Ma, J. Zai, X. Qian, Q. Qiao, Incorporation of plasmonic Au nanostars into photoanodes for high efficiency dyesensitized solar cells, J. Mater. Chem. 4 (2016) 545–551. [21] Y. Tai, K. Tajiri, Preparation, thermal stability, and CO oxidation activity of highly loaded Au/titania-coated silica aerogel catalysts, Appl. Catal., A 342 (2008) 113–118. [22] G. Melinte, M. Baia, D. Georgescu, L. Baia, V. Iancu, L. Diamandescu, T. Popescu, L.C. Cotet, L. Barbu-Tudoran, V. Danciu, S. Simon, The influence of the Au nanoparticles dimension on the photocatalytic performances of TiO2-Au porous composites, Acta Phys. Pol., A 121 (2012) 208–210. [23] R. Liu, A. Sen, Controlled synthesis of heterogeneous metal-titania nanostructures and their applications, J. Am. Chem. Soc. 134 (2012) 17505–17512. [24] F. Lin, B. Shao, Z. Li, J. Zhang, H. Wang, S. Zhang, M. Haruta, J. Huang, Visible light photocatalysis over solid acid: enhanced by gold plasmonic effect, Appl. Catal., B 218 (2017) 480–487. [25] T. Marino, R. Molinari, H. Garcia, Selectivity of gold nanoparticles on the photocatalytic activity of TiO2 for the hydroxylation of benzene by water, Catal, Today 206 (2013) 40–45. [26] S. Oros-Ruiz, J.A. Pedraza-Avella, C. Guzman, M. Quintana, E. Moctezuma, G. Del Angel, R. Gomez, E. Perez, Effect of gold particle size and deposition method on the photodegradation of 4-chlorophenol by Au/TiO2, Top. Catal. 54 (2011) 519–526. [27] F. Zheng, Z. Zhu, Preparation of the Au@TiO2 nanofibers by one-step electrospinning for the composite photoanode of dye-sensitized solar cells, Mater. Chem. Phys. 208 (2018) 35–40. [28] S.P. Lim, A. Pandikumar, N.M. Huang, H.N. Lim, Facile synthesis of Au@TiO2 nanocomposite and its application as a photoanode in dye-sensitized solar cells, RSC Adv. 5 (2015) 44398–44407. [29] K.-S. Yang, Y.-R. Lu, Y.-Y. Hsu, C.-J. Lin, C.-M. Tseng, S.Y.H. Liou, K. Kumar, D.H. Wei, C.-L. Dong, C.-L. Chen, Plasmon-Induced visible-light photocatalytic activity of Au nanoparticle-decorated hollow mesoporous TiO2: a view by X-ray spectroscopy, J. Phys. Chem. C 122 (2018) 6955–6962. [30] H.S. Kushwaha, N.A. Madhar, B. Ilahi, P. Thomas, A. Halder, R. Vaish, Efficient solar energy conversion using CaCu3Ti4O12 photoanode for photocatalysis and photoelectrocatalysis, Sci. Rep. 6 (2016) 18557. [31] L. Ozcan, P. Yalcin, O. Alagoz, S. Yurdakal, Selective photoelectrocatalytic oxidation of 5-(hydroxymethyl)-2-furaldehyde in water by using Pt loaded nanotube structure of TiO2 on Ti photoanodes, Catal. Today 281 (2017) 205–213. [32] M.I. Carreno-Lizcano, A.F. Gualdron-Reyes, V. Rodriguez-Gonzalez, J.A. PedrazaAvella, M.E. Nino-Gomez, Photoelectrocatalytic phenol oxidation employing nitrogen doped TiO2-rGO films as photoanodes, Catal. Today 341 (2020 February 1) 96–103 (Ahead of Print). [33] Y. Wang, N. Lu, M. Luo, L. Fan, K. Zhao, J. Qu, J. Guan, X. Yuan, Enhancement mechanism of fiddlehead-shaped TiO2-BiVO4 type II heterojunction in SPEC towards RhB degradation and detoxification, Appl. Surf. Sci. 463 (2019) 234–243. [34] T. An, Y. Xiong, G. Li, C. Zha, X. Zhu, Synergetic effect in degradation of formic acid using a new photoelectrochemical reactor, J. Photochem. Photobiol., A 152 (2002) 155–165. [35] Y.-H. Liu, W.-J. Huang, C.-T. Wang, Photoelectrocatalytic oxidation of phenol by UV-assisted electrogenerated Ce(IV) in aqueous solution, J. Taiwan Inst. Chem. Eng. (2019) (Ahead of Print). [36] J.C. Cardoso, G.G. Bessegato, M.V. Boldrin Zanoni, Efficiency comparison of ozonation, photolysis, photocatalysis and photoelectrocatalysis methods in real textile wastewater decolorization, Water Res. 98 (2016) 39–46.

8