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Hydrogen evolution by templated cadmium indate nanoparticles under natural sunlight illumination
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Hydrogen evolution by templated cadmium indate nanoparticles under natural sunlight illumination Jason M. Thornton, Daniel Raftery* Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, IN 47907, USA
article info
abstract
Article history:
Carbon-doped cadmium indate (CdIn2O4) nanoparticles were synthesized by a solegel
Received 27 October 2012
templating method using the block co-polymer surfactant Pluronic F127 and evaluated for
Received in revised form
hydrogen generation activity under artificial and natural solar illumination. Each catalyst
1 April 2013
powder was loaded with platinum as a cocatalyst in order to promote charge carrier sep-
Accepted 8 April 2013
aration. BET surface area measurements indicated an increase in surface area with F127
Available online 18 May 2013
introduction of up to 5 times the area of the non-templated sample. Natural sunlight illumination experiments showed the hydrogen evolution rate of CdIn2O4 was 17 mmol h1
Keywords:
as compared to 2.1 mmol h1 for the Pt:TiO2 reference material. The H2 rate was determined
CdIn2O4
to be similar under both stirring and non-stirring conditions for the CdIn2O4 catalyst, which
Natural sunlight
resulted from 10 min irradiation exposure times. Laboratory experiments confirmed this
Solar hydrogen conversion
effect and showed that at longer, out to 60 min, irradiation times the evolution rate was 3
Photocatalysis
times greater for stirred over non-stirred samples. The templated nanoparticle CdIn2O4
Water splitting
catalysts are promising for solar hydrogen conversion.
C-doping
Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Methods for solar hydrogen conversion have been the focus of intensive investigation, which are driven by increases in energy demand and rising environmental concerns over global climate change. TiO2, examined by Fujishima and Honda in the early 1970s, was the first material studied for photocatalytic water splitting [1]. Solar generation of hydrogen from water splitting is one of many possible alternative energy solutions that include biomass, wind, geothermal and hydrothermal; however, solar energy is the only potential source capable of producing enough energy to meet the rising global demand over the long term [2]. In addition to TiO2 [3,4], there are numerous materials under investigation for use as solar photocatalysts, including Fe2O3 [5e7], WO3 [8e10], and In2O3
[11e13] and others. In addition to hydrogen generation, the oxygen evolution catalyst (OEC), Co-Pi, has been used in conjunction with other photocatalysts to create complete H2O splitting systems [14e18]. One key issue in solar hydrogen generation is efficiency, and high surface area materials have been reported to show increased water splitting activity [19e21]. Sreethawong et al. have shown an increase in NiO loaded TiO2 photocatalytic activity from 90 mmol h1 up to 160 mmol h1 by synthesizing a surface area enhanced material using a single step solegel process with a surfactant template [22]. The use of the surfactant Pluronic F127 as a structure-directing agent to synthesize templated TiO2 nanocrystals has been reported by Bahnemann [23,24]. F127 increased the BET surface of the TiO2 nanoparticles by a factor of 3.5 and required 60% less platinum
* Corresponding author. Current address: Departments of Chemistry and Anesthesiology & Pain Medicine, University of Washington, Seattle, WA 98109, USA Tel.: þ1 206 543 9709. E-mail addresses: [email protected], [email protected] (D. Raftery). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.04.039
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to achieve the material’s optimal water splitting potential [24]. Like other common surfactant templates, such as CTAB, PEG, or P123, Pluronic F127 directs the structural formation of the metal oxide lattice by providing a temporary scaffold that is removed during calcination [25]. While a large number of systems involving a variety of metal oxide photocatalysts have been reported, especially for degradation of organic toxins or dyes, but most reports use simulated solar irradiation exclusively. Still, there are a few reports on the natural light illumination of semiconductor metal oxide photocatalysts such as TiO2, ZnO, WO3, SnO2, and Fe2O3. These materials were examined for either photocatalytic degradation of organic toxins and dyes [26e32], reduction in parasite viability [33], or H2 evolution from water splitting [34e36]. Since most metal oxide photocatalysts have large band gap energies and require UV light for excitation, the lack of reports using natural sunlight illumination is not unexpected. Recently, our group has reported on the photocatalytic properties of carbon-doped cadmium indate, C-CdIn2O4 [37,38]. The mixed metal oxide was shown to have a band gap energy of 2.3 eV and capable of generating hydrogen under visible light only illumination [38]. In this study, we report on the use of Pluronic F127 as a templating agent for the synthesis of C-CdIn2O4 nanoparticles. We also investigate the evolution of hydrogen during natural sunlight illumination, as well as simulated solar irradiation. Some of the difficulties involved in making the solar measurements are also discussed.
2.
Experimental methods
2.1.
Catalyst preparation
Indium (III) nitrate hydrate (In(NO3)35H2O), cadmium (II) nitrate hydrate (Cd(NO3)26H2O), hexachloroplatinic acid hydrate (H2PtCl6XH2O) and the block copolymer surfactant EO106-PO70-EO106 (Pluronic F127, EO ¼ eCH2CH2Oe, PO ¼ eCH2(CH3)CHOe, Mw 12,600 g mol1) were obtained from SigmaeAldrich (St. Louis, MO). Glucose, ammonium hydroxide and potassium hydroxide were obtained from Mallinckrodt (St. Louis, MO). All chemicals were used without further purification. All aqueous solutions were prepared with ultrapure water (18 MU) from a Barnstead EASYpure II purification system (Waltham, MA). The catalyst powder synthesis for each of the four types of materials used in this study began with a solution containing the required metal ions. The C-doped cadmium indate solution was prepared using a 9:1 by volume solution that contained In(NO3)3, Cd(NO3)2, F127, glucose in the following molar ratio: 1.0 : 0.5 : 0.001 : 0.15, respectively. In a typical preparation of the templated doped catalyst (designated C/F127CdIn2O4), a solution of MeOH:H2O with initial total volume of 10 mL was used to dissolve 1.0 g In(NO3)3, 0.32 g Cd(NO3)2, 0.50 g F127 and 0.12 g glucose. The undoped template cadmium indate catalyst (designated F127-CdIn2O4) was prepared in the same manner but without the addition of glucose, and the untemplated C-doped cadmium indate catalyst (designated C-CdIn2O4) was prepared without the addition of F127.
Lastly, the untemplated/undoped cadmium indate catalyst (designated CdIn2O4) was also prepared in the same manner but without the addition of F127 and glucose. All solutions were thoroughly stirred for 30 min after the addition of reactants, and then the solvent was allowed to slowly evaporate on a hotplate overnight. The material that formed was converted to the metal oxide by calcination in air at 500 C for 3 h with an initial heating rate of 3 C/min. Platinum was deposited onto the surface for use as a cocatalyst using a modified photodeposition method [19]. A 1:1 solution by volume of MeOH:H2O (50 mL total volume) was used to suspend approximately 0.12 g unloaded catalytic powder and an appropriate volume of a hexachloroplatinic acid solution was added to give the desired loading of platinum by percent weight. The solution was stirred vigorously while being irradiated with UV light for 4 h to reduce the H2PtCl6 to platinum metal. The catalyst was then washed and dried overnight at 120 C, resulting in the final platinum loaded catalytic powder. For reference, a sample of 0.3 wt% platinum loaded P25 TiO2 (Degussa) was prepared using the same photodeposition method.
2.2.
Characterization
XPS high resolution scans were collected using a Kratos Axis Ultra X-ray photoelectron spectrometer employing monochromatic Al Ka excitation. Spectral processing was conducted using the CASA XPS software and binding energies were calibrated with respect to the residual C 1s peak at 284.6 eV. BET analysis was performed on a Micrometrics Tristar 3000 BET (Norcross, GA) using approximately 200 mg of catalyst that had been vacuum-dried overnight. The surface morphologies of the powdered photocatalysts were characterized using SEM imaging on a FEI NOVA nanoSEM (Hillsboro, OR) scanning electron microscope. Photocatalytic hydrogen generation experiments were carried out using a 150 W Xenon lamp (Oriel Co.) (Stratford, CT) as the light source with an irradiation power of 15 mW cm2 and an AM1.5 filter to match the solar spectrum. A water filter was used to remove IR energy and reduce overheating. The reaction chamber consisted of a sealed cylindrical aluminum cell with a quartz window on the top. The catalytic powder and aqueous solution were isolated from the cell in a Pyrex dish. Fig. 1 shows the hydrogen evolution chamber. For the natural sunlight illumination measurements, the solar irradiance was measured using a calibrated thermal head (Ophir Nova model 30-A detector, Jerusalem, Israel), which was then converted to photon flux using the solar spectral irradiance at AM1.5 obtained from NREL [39]. An excess amount of powder (w2 g) was used to allow for the maximum amount of light absorption possible. The natural sunlight experiments were performed in the months of August and September at Purdue University in West Lafayette, IN with longitude and latitude of 40 25.60 N and 86 54.70 W, respectively. For the laboratory experiments, 0.1 g of catalytic powder was used; this small amount allowed for the powder mass to be the limiting factor in the amount of hydrogen generated. For all experiments, the photocatalytic powders were suspended in 10 mL of ultrapure water; the pH of the solution was adjusted to 14 using KOH. The reaction solution was degassed
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Fig. 1 e Photocatalytic hydrogen evolution chamber. (A) Top view with the input and output ports, as well as the glass dish, inside the chamber, that contains the catalyst nanoparticles during the reaction. (B) Side view of the chamber emphasizing the reactor height.
prior to each experiment to remove any dissolved O2 and N2. No sacrificial agents were added to the solution with the exception of the TiO2 reference sample which used MeOH. Samples were taken from the gas-phase volume (headspace) above the aqueous solution containing the catalytic powder and analyzed using an inline SRI Instruments 310C gas chromatograph (Torrance, CA) with a molecular sieve 5 A column, column temperature of 30 C and a flow-gas pressure of 40 psi. The obtained peak area was used to calculate the number of moles of hydrogen produced at each time point; the area versus time plot was fitted using linear regression. The slope of the resulting line was used to estimate the photocatalytic generation rate.
Further, the peak in the full spectra corresponding to the F 1s region (688 eV) had increased area in the samples synthesized with F127. It should be noted that all samples show some evidence of fluorine; however the F127 synthesized samples contain considerably more. According to the full spectra, the F content has increased by 90% and 116% for F127-CdIn2O4 and C/F127-CdIn2O4, respectively, over undoped CdIn2O4. This increase is most likely due to an increase in reactive sites accessible to fluorine in the higher surface area materials that results from the templating action of F127. Fig. 2bed show the spectra for the C 1s, Cd 3d and In 3d regions individually. F127CdIn2O4 and C/F127-CdIn2O4 both show the presence of 2 additional carbon peaks at 292.9 and 295.8 eV. According to the NIST XPS spectral database, the two additional peaks correspond to a carbon atom bound to a fluorine [40]. The binding energies of the Cd 3d and In 3d peaks are in agreement with literature values [41,42]. Both showed no apparent shift in energy with the addition of F127 or glucose, indicating that the carbon from the surfactant or the dopant was located at interstitial sites in the lattice; this carbon is likely to cause the formation of oxygen vacancies in order to balance the additional charge. This situation is consistent with the results reported in our most recent publication on C-doped CdIn2O4 nanoparticles, where the addition of carbon resulted in no reduction of the material band gap energy but did increase the light absorption, an effect explained by the formation of oxygen vacancies in the lattice [38]. Peaks corresponding to the O 1s core levels showed no deviation from previously published XPS results on undoped and C-doped CdIn2O4 [37].
3.2.
Results and discussion
3.1.
X-ray photoelectron spectroscopy
Fig. 2 shows the XPS high resolution spectra of CdIn2O4, CCdIn2O4, F127-CdIn2O4, and C/F127-CdIn2O4. As seen in the full spectrum data (Fig. 2a), there are a number of subtle differences observed among the samples, including changes in the C 1s core levels between 182 and 196 eV, the Cd 3d between 402 and 414 eV, and the In 3d core levels between 442 and 454 eV.
SEM imaging
Fig. 3 shows the SEM images obtained from: (IeIV) unloaded CdIn2O4, C-doped CdIn2O4, F127-CdIn2O4, and C/F127-CdIn2O4; and (VeVI) Pt-loaded F127-CdIn2O4 and C/F127-CdIn2O4, respectively. The undoped (I) and C-doped (II) images are similar to those previously published [38]. The undoped sample exhibited a well-defined cubic structure while the Cdoped sample was much less defined. F127-CdIn2O4 (III) and C/ F127-CdIn2O4 (IV) appeared to have a more porous surface than the two samples synthesized without F127. The two samples also exhibit a structure similar to the C-doped sample. Pt-loaded F127-CdIn2O4 (V) and C/F127-CdIn2O4 (VI) appeared to have unchanged structures compared to the unloaded samples, including a lack of formation of nanofibers on the material surface, which was witnessed previously [38]. It is possible that the lack of nanofiber formation is a result of the change in morphology of the material surface.
3.3.
3.
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BET surface area
Fig. 4 shows the BET surface area of the various CdIn2O4 materials: (A) where the F127:In mass ratio was varied in undoped CdIn2O4; (B) where the glucose:In mass ratio was varied in F127-CdIn2O4; and (C) where the F127:In mass ratio was varied in C-doped CdIn2O4. Introducing F127 into undoped CdIn2O4 resulted in an increase in surface area from 2.8 m2 g1 to 14.5e17.0 m2 g1 (depending on the concentration), which represents an increase of at least 5 fold. For TiO2 templated with F127, the surface area was increased between 3 and 5 fold
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Fig. 2 e X-ray photoelectron spectra of (I) undoped CdIn2O4, (II) C-doped CdIn2O4, (III) F127-CdIn2O4 and (IV) C/F127-CdIn2O4. (A) Full spectra and (B) C 1s (C) Cd 3d and (D) In 3d core levels.
(175e270 m2 g1) again depending on calcination temperature, as compared to non-templated TiO2 (50 m2 g1 for P25) [24,43,44]. The addition of glucose into F127-CdIn2O4 resulted in no further increase in surface area. Altering the F127:In mass ratio of C-doped CdIn2O4 also resulted in no further increase from the range of 14.5e17.0 m2 g1. Another point of note is that carbon doping in non-templated CdIn2O4 also produced a material with a surface area in the same range, 15.1 m2 g1. Similar results were reported in a recent study, a carbon doping concentration of 20% (wt% in the precursor solution) in templated TiO2, the surface area resulted in the same range, approximately 4 times greater (209 m2 g1) than the P25 [45]. Increased C-doping to 30% and 50% resulted in incremental changes in surface area, 276 and 295 m2 g1, respectively; however 80% doping resulted in a dramatic increase to 465 m2 g1 [45].
3.4.
Natural sunlight illumination
Experiments under natural sunlight were performed to show the viability of cadmium indate for solar hydrogen conversion under practical experimental conditions. Under natural sunlight illumination, we observed that the reaction chamber heats up very quickly, due to the absence of an active cooling system, and this leads to a buildup of water vapor on the underside of the quartz window. To alleviate this situation, short exposure times were used to measure the H2 evolution rate. It was determined that an exposure period of 10 min was the maximum allowable time based on the observation of a noticeable moisture circle on the underside of the quartz window of the reactor cell. The hydrogen evolution rate of C/ F127-CdIn2O4 with natural sunlight illumination was measured under two experimental conditions, one in which
the catalyst solution was stirred and the other in which the catalyst solution was static. Both experiments used irradiation exposure times of 10 min. An observed H2 generation rate of 17 3 mmol h1 was obtained for both experiments. The TiO2 reference sample suspended in a 10% v/v aqueous methanol solution produced H2 at a rate of 2.1 0.1 mmol h1 while stirring under natural sunlight illumination. The much lower H2 evolution rate is expected due to the lack of visible light absorption by titanium dioxide. The apparent quantum efficiency can be calculated from the H2 measurements, using the following equation: Fð%Þ ¼ðno: of H2 molecules 2=no: of incident photonsÞ 100% The number of incident photons was calculated using the measured irradiation power that was then converted to a photon count using the solar irradiance spectrum. The calculated efficiency for H2 evolution under natural sunlight illumination was 0.025%. Since literature reports using natural sunlight are uncommon, a direct comparison is difficult to find. A WO3 film under natural light was able to achieve a solar-to-hydrogen efficiency of 3.76%, however oxalic acid was used as a sacrificial agent [34]. Khan et al. has reported on the performance of TiO2 and Fe2O3 thin film electrodes under natural sunlight conditions, with calculated efficiencies of 12% and 3%, respectively [35,36]. For our experiments, the same C/F217-CdIn2O4 catalyst nanoparticles were used for all natural sunlight experiments, and no degradation was observed over a period of 2 weeks. Performing natural sunlight experiments proved to be more difficult than anticipated and a number of issues arose that are worth noting. First, the previously discussed issue of sample stirring has shown to be a significant factor in
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Fig. 3 e SEM images of (I) undoped CdIn2O4, (II) C-doped CdIn2O4, (III) F127-CdIn2O4 and (IV) C/F127-CdIn2O4, (V) Pt-loaded F127-CdIn2O4 and (VI) Pt-loaded C/F127-CdIn2O4.
evolution rate. This can easily be rationalized since, without stirring, the catalyst quickly settled out of solution and thus drastically reduced the amount of active surface in contact with the water. Second, design of the hydrogen evolution chamber is of significant importance. Unfortunately, our chamber design is quite tall (300 interior height), such that even at solar noon a substantial portion of the interior was masked in the shadow of the chamber wall. This situation limited the amount of catalyst exposed directly to sunlight. Also, our reaction chamber has no cooling system, and as previously noted this leads to overheating and evaporation of the reaction solution. For the future, the chamber design should include some form of cooling; water-circulation would most
likely be the most effective but would further complicate performing the experiment outside of a laboratory setting. The inclusion of active cooling would allow for longer duration experiments and consequently more accurate evolution rate and quantum efficiency measurements. Alternatively, a very low profile cell design, or one with a separate gas collection chamber, could alleviate some of these issues.
3.5.
Simulated sunlight illumination
Similar hydrogen evolution experiments were also performed in the laboratory using the same apparatus and catalyst samples, but with a xenon lamp for irradiation. The effect of
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Fig. 4 e BET measurements of (A) Undoped CdIn2O4 when the F127:In mass ratio is varied, (B) F127-CdIn2O4 (synthesized at an F127:In ratio of 0.5:1) when the glucose:In ratio is varied, (C) C-doped CdIn2O4 (synthesized at a glucose:In ratio of 0.12:1) when the F127:In mass ratio is varied.
sample stirring was also investigated. Fig. 5 shows the difference in hydrogen evolution over time with and without stirring. Over a longer time period, stirring results in an increase in H2 evolution by approximately a factor of 3. However, at short irradiation times, the difference between the stirred and non-stirred samples is small. In Fig. 6, the difference between the amounts of hydrogen evolved after 10 min of irradiation is relatively small 0.50 0.07 and 0.22 0.03 for stirred and non-stirred respectively. Based on this result, the lack of observed difference in the stirred and non-stirred samples exposed to natural sunlight illumination is reasonable, since the short exposure times did not allow sufficient time for the accumulated amounts of H2 to diverge. The quantity of hydrogen evolved is less in the laboratory setting than the outdoor experiments due to a significant difference in irradiation power; 15 mW cm2 versus 70 mW cm2 for the laboratory and outdoor experiments, respectively. This is a
Fig. 5 e Effect of stirring on the hydrogen evolution rate using 2.0 g of C/F127-CdIn2O4 catalyst in the laboratory setting with 15 mW cmL2 irradiation, (A) With stirring and (B) Without stirring. Linear fits to the data are not shown for clarity purposes.
key point since it shows our material can be used in a more practical H2 evolution setup and does not require a highpower lamp with substantial UV output to generate hydrogen.
3.6.
Catalyst synthesis optimization
Fig. 6 shows the hydrogen evolution rate of the materials under simulated solar irradiation as a function of several synthetic variables: (A) F127:In mass ratio, (B) calcination temperature, (C) glucose:In mass ratio, and (D) Pt loading amount. The generation rate increased, from 0.9 mmol g1 h1e10.2 mmol g1 h1 with additional F127 until the mass ratio exceeded 0.5:1, at which point the rate decreased (Fig. 6A). The evolution rate had a strong dependence on the calcination temperature showing a maximum rate when the temperature was 500 C (Fig. 6B). This calcination temperature is lower than the previously determined optimal temperature using thin film electrodes [37], and is possibly a result of the direct relationship between calcination temperature and particle size [46e50]. Despite having no significant effect on the BET surface area, carbon doping of the templated nanoparticles had an effect on the H2 rate, initially decreasing the rate then gradually increasing the rate with the maximum value at a glucose:In mass ratio of 0.12:1 (Fig. 6C). The Pt loading amount showed an optimal value of 0.15 wt% (Fig. 6D), which represents a 40% reduction from the previously determined optimal value for carbon-doped CdIn2O4 [38]. Other factors that might affect the hydrogen evolution rate, including morphology as well as catalytic site geometry and adsorption/desorption energetics and kinetics provide interesting avenues to pursue, but were not explored in the current study. The reduction in cocatalyst required to obtain the maximum H2 evolution is an effect seen in other templated photocatalysts [24]. This is a desired result since the cocatalyst is usually an expensive noble metal.
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Fig. 6 e Optimization of the synthetic parameters shows the H2 evolution rate versus, (A) F127:In mass ratio, (B) Calcination temperature, (C) Glucose:In mass ratio and (D) Pt-loading concentration. All measurements were made under simulated solar irradiation.
4.
Conclusions
In summary, we have shown that C/F127-CdIn2O4 is capable of hydrogen evolution using natural sunlight illumination and water with no added sacrificial donors. While the H2 evolution rate of approximately 17 mmol h1 equates to a quantum efficiency of 0.025%, under natural sunlight C/F127-CdIn2O4 evolved hydrogen at a rate 8-fold greater than TiO2 suspended in an aqueous solution that required added methanol as a sacrificial regent. Synthetic parameters were optimized to an F127:In mass ratio of 0.5:1, glucose:In mass ratio of 0.12:1, calcination temperature of 500 C, and Pt-loading of 0.15 wt%. Despite a lack of increase in BET surface area over the C-doped material, introduction of F127 provided a small increase in the H2 evolution rate. More work needs to be done to fully understand and optimize the conditions for natural sunlight irradiated C/F127-CdIn2O4.
Acknowledgments Support for this work from National Science Foundation (DMR-0805096) is gratefully acknowledged. The authors also thank Dr. D. Zemlyanov of the Surface Analysis Laboratory, Birck Nanotechnology Center at Purdue University, for acquisition of the XPS spectra, and Chia-Ping Huang of the Purdue Life Science Microscopy Facility for acquisition of the SEM images. D.R. is a member of the Purdue Energy Center.
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electrodes functionalized with a cobalt oxygen-evolving catalyst. Proc Nat Acad Sci 2011;108(25):10056e61. Barroso M, Cowan AJ, Pendlebury SR, Gra¨tzel M, Klug DR, Durrant JR. The role of cobalt phosphate in enhancing the photocatalytic activity of a-Fe2O3 toward water oxidation. J Am Chem Soc 2011;133(38):14868e71. Steinmiller EMP, Choi SK-. Photochemical deposition of cobalt-based oxygen evolving catalyst on a semiconductor photoanode for solar oxygen production. Proc Nat Acad Sci 2009;106(49):20633e6. Seabold JA, Choi K. Effect of a cobalt-based oxygen evolution catalyst on the stability and the selectivity of photooxidation reactions of a WO3 photoanode. Chem Mater 2011;23:1105e12. Zhong DK, Cornuz M, Sivula K, Gra¨tzel M, Gamelin DR. Photo-assisted electrodeposition of cobaltephosphate (CoePi) catalyst on hematite photoanodes for solar water oxidation. Energy Environ Sci 2011;4(5):1759. Sreethawong T, Yoshikawa S. Enhanced photocatalytic hydrogen evolution over Pt supported on mesoporous TiO2 prepared by single-step solegel process with surfactant template. Int J Hydrogen Energy 2006;31(6):786e96. Lakshminarasimhan N, Bae E, Choi W. Enhanced photocatalytic production of H2 on mesoporous TiO2 prepared by template-free method: role of interparticle charge transfer. J Phys Chem C 2007;111:15244e50. Yi H, Peng T, Ke D, Ke D, Zan L, Yan C. Photocatalytic H2 production from methanol aqueous solution over titania nanoparticles with mesostructures. Int J Hydrogen Energy 2008;33(2):672e8. Sreethawong T, Suzuki Y, Yoshikawa S. Photocatalytic evolution of hydrogen over mesoporous supported NiO photocatalyst prepared by single-step solegel process with surfactant template. Int J Hydrogen Energy 2005;30(10):1053e62. Ismail AA, Bahnemann DW, Bannat I, Wark M. Gold nanoparticles on mesoporous interparticle networks of titanium dioxide nanocrystals for enhanced photonic efficiencies. J Phys Chem C 2009;113:7429e35. Kandiel TA, Ismail AA, Bahnemann DW. Mesoporous TiO2 nanostructures: a route to minimize Pt loading on titania photocatalysts for hydrogen production. Phys Chem Chem Phys 2011;13(45):20155e61. Ismail AA, Bahnemann DW. Mesoporous titania photocatalysts: preparation, characterization and reaction mechanisms. J Mater Chem 2011;21(32):11686e707. Nosrati R, Olad A, Maramifar R. Degradation of ampicillin antibiotic in aqueous solution by ZnO/polyaniline nanocomposite as photocatalyst under sunlight irradiation. Environ Sci Poll Res 2012;19(6):2291e9. Fenoll J, Hellı´n P, Flores P, Martı´nez CM, Navarro S. Photocatalytic degradation of five sulfonylurea herbicides in aqueous semiconductor suspensions under natural sunlight. Chemosphere 2012;87(8):954e61. Kuo C-Y, Pai C-Y, He C-C, Lin C-J, Cheng C-M. Photodegradation of aqueous reactive dye using TiO2/zeolite admixtures in a continuous flow reactor. Water Sci Technol 2012;65(11):1963e9. Lobedank J, Bellmann E, Bendig J. Sensitized photocatalytic oxidation of herbicides using natural sunlight. J Photoch Photobio A 1997;108:89e93. Fenoll J, Ruiz E, Hellı´n P, Flores P, Navarro S. Heterogeneous photocatalytic oxidation of cyprodinil and fludioxonil in leaching water under solar irradiation. Chemosphere 2011;85(8):1262e8. Karunakaran C, Senthilvelan S. Fe2O3-photocatalysis with sunlight and UV light: oxidation of aniline. Electrochem Comm 2006;8(1):95e101.
[32] Karunakaran C, Gomathisankar P, Manikandan G. Preparation and characterization of antimicrobial Ce-doped ZnO nanoparticles for photocatalytic detoxification of cyanide. Mater Chem Phys 2010;123(2e3):585e94. [33] Me´ndez-Hermida F, Ares-Maza´s E, McGuigan KG, Boyle M, Sichel C, Ferna´ndez-Iba´n˜ez P. Disinfection of drinking water contaminated with Cryptosporidium parvum oocysts under natural sunlight and using the photocatalyst TiO2. J Photoch Photobio B 2007;88(2e3):105e11. [34] Zhao W, Wang Z, Shen X, Li J, Xu C, Gan Z. Hydrogen generation via photoelectrocatalytic water splitting using a tungsten trioxide catalyst under visible light irradiation. Int J Hydrogen Energy 2012;37(1):908e15. [35] Shaban Y, Khan S. Photoelectrolysis of water at second generation carbon modified (CM)-n-Fe2O3 films in a photoelectrochemical cell under solar simulated light and sunlight illuminations. Sci Adv Mater 2012;4(2):356e64. [36] Shaban Y, Khan S. Carbon modified (CM)-n-TiO(2) thin films for efficient water splitting to H(2) and O(2) under xenon lamp light and natural sunlight illuminations. J Sol State Chem 2009;13(7):1025. [37] Sun Y, Thornton JM, Morris NA, Rajpura R, Henkes S, Raftery D. Photoelectrochemical evaluation of undoped and C-doped CdIn2O4 thin film electrodes. Int J Hydrogen Energy 2011;36(4):2785e93. [38] Thornton JM, Raftery D. Efficient photocatalytic hydrogen production by platinum-loaded carbon-doped cadmium indate nanoparticles. ACS App Mater Interfaces 2012;4(5):2426e31. [39] National Renewable Energy Lab. Solar spectral irradiance. http://rredc.nrel.gov/SOLAR/SPECTRA/AM1.5 n.d. [40] Gervais M, Douy A, Gallot B, Erre R. Surface studies of polypeptidic block copolymers by electron spectroscopy for chemical analysis: poly (N-trifluoroacetyl-l-lysine)polysarcosine diblock copolymers. Polymer 1986;27:1513e20. [41] Stambouli V, Zebda A, Appert E, Guiducci C, Labeau M, Diard J-P, et al. Semiconductor oxide based electrodes for the label-free electrical detection of DNA hybridization: comparison between Sb doped SnO2 and CdIn2O4. Electrochim Acta 2006;51(24):5206e14. [42] Wang W, Liao K, Liu A. X-ray photoelectron spectroscopy studies of CdIn2O4 films. Appl Surf Sci 1993;68:227e30. [43] Ismail AA, Bahnemann DW. One-step synthesis of mesoporous platinum/titania nanocomposites as photocatalyst with enhanced photocatalytic activity for methanol oxidation. Green Chem 2011;13(2):428e35. [44] Ismail AA, Bahnemann DW. Mesostructured Pt/TiO2 nanocomposites as highly active photocatalysts for the photooxidation of dichloroacetic acid. J Phys Chem C 2011;115:5784e91. [45] Liu R, Ren Y, Shi Y, Zhang F, Zhang L, Tu B, et al. Controlled synthesis of ordered mesoporous C-TiO2 nanocomposites with crystalline titania frameworks from organic-inorganicamphiphilic coassembly. Chem Mater 2008;20(8):1140e6. [46] Pawar MJ, Kaware PB, Bijewar RV. Ce3þ doped TiO2 nanoparticles; synthesis and photocatalytic activity. Int J Emerging Sci 2012;2:149e60. [47] Ramelan AH, Harjana H, Sakti LS. Titanium dioxide nanostructure synthesized by sol-gel for organic solar cells using natural dyes extracted from black and red sticky rice. Proc SPIE 2012;8435. 84352F-1. [48] Mguni LL, Meijboom R, Jalama K. Effect of calcination temperature and MgO crystallite size on MgO/TiO2 catalyst system for soybean transesterification. World Academy of Science. Eng Technol 2012;64:889e93.
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Hydrogen evolution by templated cadmium indate nanoparticles under natural sunlight illumination Jason M. Thornton, Daniel Raftery* Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, IN 47907, USA
article info
abstract
Article history:
Carbon-doped cadmium indate (CdIn2O4) nanoparticles were synthesized by a solegel
Received 27 October 2012
templating method using the block co-polymer surfactant Pluronic F127 and evaluated for
Received in revised form
hydrogen generation activity under artificial and natural solar illumination. Each catalyst
1 April 2013
powder was loaded with platinum as a cocatalyst in order to promote charge carrier sep-
Accepted 8 April 2013
aration. BET surface area measurements indicated an increase in surface area with F127
Available online 18 May 2013
introduction of up to 5 times the area of the non-templated sample. Natural sunlight illumination experiments showed the hydrogen evolution rate of CdIn2O4 was 17 mmol h1
Keywords:
as compared to 2.1 mmol h1 for the Pt:TiO2 reference material. The H2 rate was determined
CdIn2O4
to be similar under both stirring and non-stirring conditions for the CdIn2O4 catalyst, which
Natural sunlight
resulted from 10 min irradiation exposure times. Laboratory experiments confirmed this
Solar hydrogen conversion
effect and showed that at longer, out to 60 min, irradiation times the evolution rate was 3
Photocatalysis
times greater for stirred over non-stirred samples. The templated nanoparticle CdIn2O4
Water splitting
catalysts are promising for solar hydrogen conversion.
C-doping
Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Methods for solar hydrogen conversion have been the focus of intensive investigation, which are driven by increases in energy demand and rising environmental concerns over global climate change. TiO2, examined by Fujishima and Honda in the early 1970s, was the first material studied for photocatalytic water splitting [1]. Solar generation of hydrogen from water splitting is one of many possible alternative energy solutions that include biomass, wind, geothermal and hydrothermal; however, solar energy is the only potential source capable of producing enough energy to meet the rising global demand over the long term [2]. In addition to TiO2 [3,4], there are numerous materials under investigation for use as solar photocatalysts, including Fe2O3 [5e7], WO3 [8e10], and In2O3
[11e13] and others. In addition to hydrogen generation, the oxygen evolution catalyst (OEC), Co-Pi, has been used in conjunction with other photocatalysts to create complete H2O splitting systems [14e18]. One key issue in solar hydrogen generation is efficiency, and high surface area materials have been reported to show increased water splitting activity [19e21]. Sreethawong et al. have shown an increase in NiO loaded TiO2 photocatalytic activity from 90 mmol h1 up to 160 mmol h1 by synthesizing a surface area enhanced material using a single step solegel process with a surfactant template [22]. The use of the surfactant Pluronic F127 as a structure-directing agent to synthesize templated TiO2 nanocrystals has been reported by Bahnemann [23,24]. F127 increased the BET surface of the TiO2 nanoparticles by a factor of 3.5 and required 60% less platinum
* Corresponding author. Current address: Departments of Chemistry and Anesthesiology & Pain Medicine, University of Washington, Seattle, WA 98109, USA Tel.: þ1 206 543 9709. E-mail addresses: [email protected], [email protected] (D. Raftery). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.04.039
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to achieve the material’s optimal water splitting potential [24]. Like other common surfactant templates, such as CTAB, PEG, or P123, Pluronic F127 directs the structural formation of the metal oxide lattice by providing a temporary scaffold that is removed during calcination [25]. While a large number of systems involving a variety of metal oxide photocatalysts have been reported, especially for degradation of organic toxins or dyes, but most reports use simulated solar irradiation exclusively. Still, there are a few reports on the natural light illumination of semiconductor metal oxide photocatalysts such as TiO2, ZnO, WO3, SnO2, and Fe2O3. These materials were examined for either photocatalytic degradation of organic toxins and dyes [26e32], reduction in parasite viability [33], or H2 evolution from water splitting [34e36]. Since most metal oxide photocatalysts have large band gap energies and require UV light for excitation, the lack of reports using natural sunlight illumination is not unexpected. Recently, our group has reported on the photocatalytic properties of carbon-doped cadmium indate, C-CdIn2O4 [37,38]. The mixed metal oxide was shown to have a band gap energy of 2.3 eV and capable of generating hydrogen under visible light only illumination [38]. In this study, we report on the use of Pluronic F127 as a templating agent for the synthesis of C-CdIn2O4 nanoparticles. We also investigate the evolution of hydrogen during natural sunlight illumination, as well as simulated solar irradiation. Some of the difficulties involved in making the solar measurements are also discussed.
2.
Experimental methods
2.1.
Catalyst preparation
Indium (III) nitrate hydrate (In(NO3)35H2O), cadmium (II) nitrate hydrate (Cd(NO3)26H2O), hexachloroplatinic acid hydrate (H2PtCl6XH2O) and the block copolymer surfactant EO106-PO70-EO106 (Pluronic F127, EO ¼ eCH2CH2Oe, PO ¼ eCH2(CH3)CHOe, Mw 12,600 g mol1) were obtained from SigmaeAldrich (St. Louis, MO). Glucose, ammonium hydroxide and potassium hydroxide were obtained from Mallinckrodt (St. Louis, MO). All chemicals were used without further purification. All aqueous solutions were prepared with ultrapure water (18 MU) from a Barnstead EASYpure II purification system (Waltham, MA). The catalyst powder synthesis for each of the four types of materials used in this study began with a solution containing the required metal ions. The C-doped cadmium indate solution was prepared using a 9:1 by volume solution that contained In(NO3)3, Cd(NO3)2, F127, glucose in the following molar ratio: 1.0 : 0.5 : 0.001 : 0.15, respectively. In a typical preparation of the templated doped catalyst (designated C/F127CdIn2O4), a solution of MeOH:H2O with initial total volume of 10 mL was used to dissolve 1.0 g In(NO3)3, 0.32 g Cd(NO3)2, 0.50 g F127 and 0.12 g glucose. The undoped template cadmium indate catalyst (designated F127-CdIn2O4) was prepared in the same manner but without the addition of glucose, and the untemplated C-doped cadmium indate catalyst (designated C-CdIn2O4) was prepared without the addition of F127.
Lastly, the untemplated/undoped cadmium indate catalyst (designated CdIn2O4) was also prepared in the same manner but without the addition of F127 and glucose. All solutions were thoroughly stirred for 30 min after the addition of reactants, and then the solvent was allowed to slowly evaporate on a hotplate overnight. The material that formed was converted to the metal oxide by calcination in air at 500 C for 3 h with an initial heating rate of 3 C/min. Platinum was deposited onto the surface for use as a cocatalyst using a modified photodeposition method [19]. A 1:1 solution by volume of MeOH:H2O (50 mL total volume) was used to suspend approximately 0.12 g unloaded catalytic powder and an appropriate volume of a hexachloroplatinic acid solution was added to give the desired loading of platinum by percent weight. The solution was stirred vigorously while being irradiated with UV light for 4 h to reduce the H2PtCl6 to platinum metal. The catalyst was then washed and dried overnight at 120 C, resulting in the final platinum loaded catalytic powder. For reference, a sample of 0.3 wt% platinum loaded P25 TiO2 (Degussa) was prepared using the same photodeposition method.
2.2.
Characterization
XPS high resolution scans were collected using a Kratos Axis Ultra X-ray photoelectron spectrometer employing monochromatic Al Ka excitation. Spectral processing was conducted using the CASA XPS software and binding energies were calibrated with respect to the residual C 1s peak at 284.6 eV. BET analysis was performed on a Micrometrics Tristar 3000 BET (Norcross, GA) using approximately 200 mg of catalyst that had been vacuum-dried overnight. The surface morphologies of the powdered photocatalysts were characterized using SEM imaging on a FEI NOVA nanoSEM (Hillsboro, OR) scanning electron microscope. Photocatalytic hydrogen generation experiments were carried out using a 150 W Xenon lamp (Oriel Co.) (Stratford, CT) as the light source with an irradiation power of 15 mW cm2 and an AM1.5 filter to match the solar spectrum. A water filter was used to remove IR energy and reduce overheating. The reaction chamber consisted of a sealed cylindrical aluminum cell with a quartz window on the top. The catalytic powder and aqueous solution were isolated from the cell in a Pyrex dish. Fig. 1 shows the hydrogen evolution chamber. For the natural sunlight illumination measurements, the solar irradiance was measured using a calibrated thermal head (Ophir Nova model 30-A detector, Jerusalem, Israel), which was then converted to photon flux using the solar spectral irradiance at AM1.5 obtained from NREL [39]. An excess amount of powder (w2 g) was used to allow for the maximum amount of light absorption possible. The natural sunlight experiments were performed in the months of August and September at Purdue University in West Lafayette, IN with longitude and latitude of 40 25.60 N and 86 54.70 W, respectively. For the laboratory experiments, 0.1 g of catalytic powder was used; this small amount allowed for the powder mass to be the limiting factor in the amount of hydrogen generated. For all experiments, the photocatalytic powders were suspended in 10 mL of ultrapure water; the pH of the solution was adjusted to 14 using KOH. The reaction solution was degassed
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Fig. 1 e Photocatalytic hydrogen evolution chamber. (A) Top view with the input and output ports, as well as the glass dish, inside the chamber, that contains the catalyst nanoparticles during the reaction. (B) Side view of the chamber emphasizing the reactor height.
prior to each experiment to remove any dissolved O2 and N2. No sacrificial agents were added to the solution with the exception of the TiO2 reference sample which used MeOH. Samples were taken from the gas-phase volume (headspace) above the aqueous solution containing the catalytic powder and analyzed using an inline SRI Instruments 310C gas chromatograph (Torrance, CA) with a molecular sieve 5 A column, column temperature of 30 C and a flow-gas pressure of 40 psi. The obtained peak area was used to calculate the number of moles of hydrogen produced at each time point; the area versus time plot was fitted using linear regression. The slope of the resulting line was used to estimate the photocatalytic generation rate.
Further, the peak in the full spectra corresponding to the F 1s region (688 eV) had increased area in the samples synthesized with F127. It should be noted that all samples show some evidence of fluorine; however the F127 synthesized samples contain considerably more. According to the full spectra, the F content has increased by 90% and 116% for F127-CdIn2O4 and C/F127-CdIn2O4, respectively, over undoped CdIn2O4. This increase is most likely due to an increase in reactive sites accessible to fluorine in the higher surface area materials that results from the templating action of F127. Fig. 2bed show the spectra for the C 1s, Cd 3d and In 3d regions individually. F127CdIn2O4 and C/F127-CdIn2O4 both show the presence of 2 additional carbon peaks at 292.9 and 295.8 eV. According to the NIST XPS spectral database, the two additional peaks correspond to a carbon atom bound to a fluorine [40]. The binding energies of the Cd 3d and In 3d peaks are in agreement with literature values [41,42]. Both showed no apparent shift in energy with the addition of F127 or glucose, indicating that the carbon from the surfactant or the dopant was located at interstitial sites in the lattice; this carbon is likely to cause the formation of oxygen vacancies in order to balance the additional charge. This situation is consistent with the results reported in our most recent publication on C-doped CdIn2O4 nanoparticles, where the addition of carbon resulted in no reduction of the material band gap energy but did increase the light absorption, an effect explained by the formation of oxygen vacancies in the lattice [38]. Peaks corresponding to the O 1s core levels showed no deviation from previously published XPS results on undoped and C-doped CdIn2O4 [37].
3.2.
Results and discussion
3.1.
X-ray photoelectron spectroscopy
Fig. 2 shows the XPS high resolution spectra of CdIn2O4, CCdIn2O4, F127-CdIn2O4, and C/F127-CdIn2O4. As seen in the full spectrum data (Fig. 2a), there are a number of subtle differences observed among the samples, including changes in the C 1s core levels between 182 and 196 eV, the Cd 3d between 402 and 414 eV, and the In 3d core levels between 442 and 454 eV.
SEM imaging
Fig. 3 shows the SEM images obtained from: (IeIV) unloaded CdIn2O4, C-doped CdIn2O4, F127-CdIn2O4, and C/F127-CdIn2O4; and (VeVI) Pt-loaded F127-CdIn2O4 and C/F127-CdIn2O4, respectively. The undoped (I) and C-doped (II) images are similar to those previously published [38]. The undoped sample exhibited a well-defined cubic structure while the Cdoped sample was much less defined. F127-CdIn2O4 (III) and C/ F127-CdIn2O4 (IV) appeared to have a more porous surface than the two samples synthesized without F127. The two samples also exhibit a structure similar to the C-doped sample. Pt-loaded F127-CdIn2O4 (V) and C/F127-CdIn2O4 (VI) appeared to have unchanged structures compared to the unloaded samples, including a lack of formation of nanofibers on the material surface, which was witnessed previously [38]. It is possible that the lack of nanofiber formation is a result of the change in morphology of the material surface.
3.3.
3.
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BET surface area
Fig. 4 shows the BET surface area of the various CdIn2O4 materials: (A) where the F127:In mass ratio was varied in undoped CdIn2O4; (B) where the glucose:In mass ratio was varied in F127-CdIn2O4; and (C) where the F127:In mass ratio was varied in C-doped CdIn2O4. Introducing F127 into undoped CdIn2O4 resulted in an increase in surface area from 2.8 m2 g1 to 14.5e17.0 m2 g1 (depending on the concentration), which represents an increase of at least 5 fold. For TiO2 templated with F127, the surface area was increased between 3 and 5 fold
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Fig. 2 e X-ray photoelectron spectra of (I) undoped CdIn2O4, (II) C-doped CdIn2O4, (III) F127-CdIn2O4 and (IV) C/F127-CdIn2O4. (A) Full spectra and (B) C 1s (C) Cd 3d and (D) In 3d core levels.
(175e270 m2 g1) again depending on calcination temperature, as compared to non-templated TiO2 (50 m2 g1 for P25) [24,43,44]. The addition of glucose into F127-CdIn2O4 resulted in no further increase in surface area. Altering the F127:In mass ratio of C-doped CdIn2O4 also resulted in no further increase from the range of 14.5e17.0 m2 g1. Another point of note is that carbon doping in non-templated CdIn2O4 also produced a material with a surface area in the same range, 15.1 m2 g1. Similar results were reported in a recent study, a carbon doping concentration of 20% (wt% in the precursor solution) in templated TiO2, the surface area resulted in the same range, approximately 4 times greater (209 m2 g1) than the P25 [45]. Increased C-doping to 30% and 50% resulted in incremental changes in surface area, 276 and 295 m2 g1, respectively; however 80% doping resulted in a dramatic increase to 465 m2 g1 [45].
3.4.
Natural sunlight illumination
Experiments under natural sunlight were performed to show the viability of cadmium indate for solar hydrogen conversion under practical experimental conditions. Under natural sunlight illumination, we observed that the reaction chamber heats up very quickly, due to the absence of an active cooling system, and this leads to a buildup of water vapor on the underside of the quartz window. To alleviate this situation, short exposure times were used to measure the H2 evolution rate. It was determined that an exposure period of 10 min was the maximum allowable time based on the observation of a noticeable moisture circle on the underside of the quartz window of the reactor cell. The hydrogen evolution rate of C/ F127-CdIn2O4 with natural sunlight illumination was measured under two experimental conditions, one in which
the catalyst solution was stirred and the other in which the catalyst solution was static. Both experiments used irradiation exposure times of 10 min. An observed H2 generation rate of 17 3 mmol h1 was obtained for both experiments. The TiO2 reference sample suspended in a 10% v/v aqueous methanol solution produced H2 at a rate of 2.1 0.1 mmol h1 while stirring under natural sunlight illumination. The much lower H2 evolution rate is expected due to the lack of visible light absorption by titanium dioxide. The apparent quantum efficiency can be calculated from the H2 measurements, using the following equation: Fð%Þ ¼ðno: of H2 molecules 2=no: of incident photonsÞ 100% The number of incident photons was calculated using the measured irradiation power that was then converted to a photon count using the solar irradiance spectrum. The calculated efficiency for H2 evolution under natural sunlight illumination was 0.025%. Since literature reports using natural sunlight are uncommon, a direct comparison is difficult to find. A WO3 film under natural light was able to achieve a solar-to-hydrogen efficiency of 3.76%, however oxalic acid was used as a sacrificial agent [34]. Khan et al. has reported on the performance of TiO2 and Fe2O3 thin film electrodes under natural sunlight conditions, with calculated efficiencies of 12% and 3%, respectively [35,36]. For our experiments, the same C/F217-CdIn2O4 catalyst nanoparticles were used for all natural sunlight experiments, and no degradation was observed over a period of 2 weeks. Performing natural sunlight experiments proved to be more difficult than anticipated and a number of issues arose that are worth noting. First, the previously discussed issue of sample stirring has shown to be a significant factor in
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Fig. 3 e SEM images of (I) undoped CdIn2O4, (II) C-doped CdIn2O4, (III) F127-CdIn2O4 and (IV) C/F127-CdIn2O4, (V) Pt-loaded F127-CdIn2O4 and (VI) Pt-loaded C/F127-CdIn2O4.
evolution rate. This can easily be rationalized since, without stirring, the catalyst quickly settled out of solution and thus drastically reduced the amount of active surface in contact with the water. Second, design of the hydrogen evolution chamber is of significant importance. Unfortunately, our chamber design is quite tall (300 interior height), such that even at solar noon a substantial portion of the interior was masked in the shadow of the chamber wall. This situation limited the amount of catalyst exposed directly to sunlight. Also, our reaction chamber has no cooling system, and as previously noted this leads to overheating and evaporation of the reaction solution. For the future, the chamber design should include some form of cooling; water-circulation would most
likely be the most effective but would further complicate performing the experiment outside of a laboratory setting. The inclusion of active cooling would allow for longer duration experiments and consequently more accurate evolution rate and quantum efficiency measurements. Alternatively, a very low profile cell design, or one with a separate gas collection chamber, could alleviate some of these issues.
3.5.
Simulated sunlight illumination
Similar hydrogen evolution experiments were also performed in the laboratory using the same apparatus and catalyst samples, but with a xenon lamp for irradiation. The effect of
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Fig. 4 e BET measurements of (A) Undoped CdIn2O4 when the F127:In mass ratio is varied, (B) F127-CdIn2O4 (synthesized at an F127:In ratio of 0.5:1) when the glucose:In ratio is varied, (C) C-doped CdIn2O4 (synthesized at a glucose:In ratio of 0.12:1) when the F127:In mass ratio is varied.
sample stirring was also investigated. Fig. 5 shows the difference in hydrogen evolution over time with and without stirring. Over a longer time period, stirring results in an increase in H2 evolution by approximately a factor of 3. However, at short irradiation times, the difference between the stirred and non-stirred samples is small. In Fig. 6, the difference between the amounts of hydrogen evolved after 10 min of irradiation is relatively small 0.50 0.07 and 0.22 0.03 for stirred and non-stirred respectively. Based on this result, the lack of observed difference in the stirred and non-stirred samples exposed to natural sunlight illumination is reasonable, since the short exposure times did not allow sufficient time for the accumulated amounts of H2 to diverge. The quantity of hydrogen evolved is less in the laboratory setting than the outdoor experiments due to a significant difference in irradiation power; 15 mW cm2 versus 70 mW cm2 for the laboratory and outdoor experiments, respectively. This is a
Fig. 5 e Effect of stirring on the hydrogen evolution rate using 2.0 g of C/F127-CdIn2O4 catalyst in the laboratory setting with 15 mW cmL2 irradiation, (A) With stirring and (B) Without stirring. Linear fits to the data are not shown for clarity purposes.
key point since it shows our material can be used in a more practical H2 evolution setup and does not require a highpower lamp with substantial UV output to generate hydrogen.
3.6.
Catalyst synthesis optimization
Fig. 6 shows the hydrogen evolution rate of the materials under simulated solar irradiation as a function of several synthetic variables: (A) F127:In mass ratio, (B) calcination temperature, (C) glucose:In mass ratio, and (D) Pt loading amount. The generation rate increased, from 0.9 mmol g1 h1e10.2 mmol g1 h1 with additional F127 until the mass ratio exceeded 0.5:1, at which point the rate decreased (Fig. 6A). The evolution rate had a strong dependence on the calcination temperature showing a maximum rate when the temperature was 500 C (Fig. 6B). This calcination temperature is lower than the previously determined optimal temperature using thin film electrodes [37], and is possibly a result of the direct relationship between calcination temperature and particle size [46e50]. Despite having no significant effect on the BET surface area, carbon doping of the templated nanoparticles had an effect on the H2 rate, initially decreasing the rate then gradually increasing the rate with the maximum value at a glucose:In mass ratio of 0.12:1 (Fig. 6C). The Pt loading amount showed an optimal value of 0.15 wt% (Fig. 6D), which represents a 40% reduction from the previously determined optimal value for carbon-doped CdIn2O4 [38]. Other factors that might affect the hydrogen evolution rate, including morphology as well as catalytic site geometry and adsorption/desorption energetics and kinetics provide interesting avenues to pursue, but were not explored in the current study. The reduction in cocatalyst required to obtain the maximum H2 evolution is an effect seen in other templated photocatalysts [24]. This is a desired result since the cocatalyst is usually an expensive noble metal.
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Fig. 6 e Optimization of the synthetic parameters shows the H2 evolution rate versus, (A) F127:In mass ratio, (B) Calcination temperature, (C) Glucose:In mass ratio and (D) Pt-loading concentration. All measurements were made under simulated solar irradiation.
4.
Conclusions
In summary, we have shown that C/F127-CdIn2O4 is capable of hydrogen evolution using natural sunlight illumination and water with no added sacrificial donors. While the H2 evolution rate of approximately 17 mmol h1 equates to a quantum efficiency of 0.025%, under natural sunlight C/F127-CdIn2O4 evolved hydrogen at a rate 8-fold greater than TiO2 suspended in an aqueous solution that required added methanol as a sacrificial regent. Synthetic parameters were optimized to an F127:In mass ratio of 0.5:1, glucose:In mass ratio of 0.12:1, calcination temperature of 500 C, and Pt-loading of 0.15 wt%. Despite a lack of increase in BET surface area over the C-doped material, introduction of F127 provided a small increase in the H2 evolution rate. More work needs to be done to fully understand and optimize the conditions for natural sunlight irradiated C/F127-CdIn2O4.
Acknowledgments Support for this work from National Science Foundation (DMR-0805096) is gratefully acknowledged. The authors also thank Dr. D. Zemlyanov of the Surface Analysis Laboratory, Birck Nanotechnology Center at Purdue University, for acquisition of the XPS spectra, and Chia-Ping Huang of the Purdue Life Science Microscopy Facility for acquisition of the SEM images. D.R. is a member of the Purdue Energy Center.
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