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Plasmon-enhanced photocatalytic hydrogen production over visible-light responsive Cu/TiO2
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2015 Nanotechnology 26 125402 (http://iopscience.iop.org/0957-4484/26/12/125402) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 129.173.72.87 This content was downloaded on 05/03/2015 at 02:30
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Nanotechnology Nanotechnology 26 (2015) 125402 (5pp)
doi:10.1088/0957-4484/26/12/125402
Plasmon-enhanced photocatalytic hydrogen production over visible-light responsive Cu/TiO2 Jong Min Kum, Yang Jeong Park, Hyun Jin Kim and Sung Oh Cho Department of Nuclear and Quantum Engineering, Korea Advanced Institute of Science and Technology (KAIST), 373-1 Guseong, Yuseong, Daejeon 305-701, Korea E-mail: [email protected] Received 4 October 2014, revised 3 February 2015 Accepted for publication 14 February 2015 Published 4 March 2015 Abstract
Cheap and visible-light responsive Cu/TiO2 photocatalysts were fabricated by illuminating ultraviolet (UV) to a mixture of TiO2 nanoparticles (NPs) and Cu2O NPs in an evacuated reaction chamber. The Cu2O NPs were reduced by UV in an oxygen-free reaction chamber, and hence, metallic Cu NPs with size less than 5 nm were uniformly loaded on TiO2. Due to the plasmon resonance of the Cu NPs, the Cu/TiO2 exhibited a good performance of water-splitting hydrogen production under visible light in the presence of glycerol as a hole scavenger. The optimum hydrogen production rate of Cu/TiO2 was 0.24 mmol h−1 g−1. The Cu/TiO2 also showed high stability of the photocatalytic performance in the evacuated chamber; however, the visible-light responsive photocatalytic properties dramatically and rapidly decreased when exposed to air. S Online supplementary data available from stacks.iop.org/NANO/26/125402/mmedia Keywords: copper, TiO2, photocatalyst, plasmon resonance, hydrogen production (Some figures may appear in colour only in the online journal) 1. Introduction
efficiency: the metal NPs suppress the recombination of photo-generated electron–hole pairs [13–16]. Generally, noble metal NPs such as Au, Ag, and Pt NPs are used for the co-catalyst [17–20]. One of the additional effects of the noble metal NPs is that they can have plasmon resonance absorption property in visible light region [21]. As a consequence, TiO2 coupled with the noble metal NPs can exhibit plasmonenhanced photocatalytic activities. Therefore, noble metal NPs attached to TiO2 play dual roles: (i) enhancement of visible light absorption and (ii) separation of photo-generated charges in TiO2. However, the practical applications of photocatalytic hydrogen production based on the noble metal NPs co-catalyst are restricted due to high cost of the noble metal. Here, we present that cheap Cu NPs coupled with TiO2 also exhibit enhanced photocatalytic hydrogen production under visible light due to the plasmon resonance of the Cu NPs. It has been reported that Cu NPs can also exhibit plasmon resonance behavior in visible light region [22], but plasmon-enhanced photocatalysis based on Cu NPs has never
Various approaches to fabricate efficient photocatalysts for water-splitting hydrogen production have been developed since Fujisima and Honda firstly discovered photocatalytic hydrogen production [1]. Photocatalytic water splitting provides a clean and sustainable energy generation method because hydrogen is produced from water using sunlight [2]. Due to the advantages of high stability and low cost, TiO2 is most widely used as a photocatalyst for water splitting [3]. However, photocatalytic activity of pure TiO2 is limited because only ultraviolet (UV) can drive TiO2 due to its wide band gap of ∼3.2 eV and a large fraction of solar energy (∼96%) should be wasted [4]. To overcome this limitation, several methods such as doping of ions [5, 6] and sensitization with quantum dots [7–9] or organic dyes [10–12] have been studied, which enables the utilization of visible light. In addition, metal nanoparticles (NPs) should be loaded on the surface of TiO2 as a co-catalyst to improve the catalytic 0957-4484/15/125402+05$33.00
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© 2015 IOP Publishing Ltd Printed in the UK
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Nanotechnology 26 (2015) 125402
Figure 1. Schematic illustration displaying the fabrication procedure of the Cu/TiO2 photocatalysts.
been reported. In addition, the plasmon resonance of Cu NPs was observed only when Cu NPs were embedded in other materials [23]. This might be attributed to the fact Cu, particularly in nanometer-sized particles, tend to be easily and rapidly oxidized in air or in aqueous solution [24]. To overcome this problem, we tried to fabricate and load Cu NPs on TiO2 in an evacuated reaction chamber. The prepared Cu NPloaded TiO2 (hereafter abbreviated to Cu/TiO2) show a good performance of photocatalytic hydrogen production under visible light. Many papers has been published on Cu2O and CuO supported TiO2 [25–27]. However, practically no photocatalytic hydrogen production results using oxidation-free Cu supported TiO2 was reported. Although few papers claim that they produced hydrogen by photocatalytic reaction of Cu/ TiO2 [28, 29], we believe that they used cupper oxides because Cu NPs are very easily oxidized both in the air and the aqueous solution. We synthesized pure Cu/TiO2 and carried out whole experiments in vacuum condition to avoid oxidation of Cu. Cu/TiO2 NPs is much cheaper than noble metal-loaded photocatalysts and visible-light absorption property of Cu NPs allows efficient utilization of sunlight for photocatalytic water splitting.
reaction chamber was cooled by water to remove heat produced during the Xe lamp illumination. In order to confirm the photocatalytic reactivity of Cu/ TiO2 under visible light, a UV cut-off filter which passes only visible light with the wavelength larger than 420 nm was installed at the top surface of the reaction chamber. To demonstrate the stability of the reactivity of Cu/TiO2, photocatalytic experiments were carried out for 48 h. The reaction chamber was evacuated every 12 h to maintain the pressure inside the reaction chamber below atmospheric pressure. Hydrogen evolution was confirmed by gas chromatography (GC, hp 5890) equipped with a thermal conductivity detector. The hydrogen production rate was characterized by measuring the pressure change in the reaction chamber using a pressure gauge (PIZ100, Ilmvac Gmbh). The recorded pressure change was converted to hydrogen production rate based on the ideal gas law. When the aqueous solution was irradiated with a Xe lamp, the pressure was slightly increased at an initial stage. Although the reaction chamber was cooled with water, the water temperature was increased by ∼5 °C when the water was illuminated with a Xe lamp. In addition, some dissolved gases can still remain in water even though most of the gases are removed by the evacuation process. As a result, gases dissolved in the aqueous solution can be evolved into the reaction chamber due to the increase of water temperature. However, the gas evolution and accordingly the pressure increase was observed only at an initial stage (99.9%) was added as a hole scavenger. Distilled water (DI water, ⩾18.3 MΩ cm) obtained from a water purification system (Human Power I+, Human Corporation) was used in the experiments. All the chemicals and materials were used in their as-received form without further purification. A closed-loop photocatalytic hydrogen production device [30] was used for the photocatalytic hydrogen production experiments. The reaction chamber was made of pyrex and the volume of the chamber was 250 mL. A 500 W Xe lamp with average power density of 1.5 mW cm−2 was used as a light source. The light intensity of Xe lamp was measured by radiometer (ILT 1400-A, International Light Technologies, USA) on the top of the reaction chamber. The Xe lamp was installed on the top of the reaction chamber for the illumination of light into water. The distance between the Xe lamp and the top surface of the reaction chamber was 30 cm. The
3. Results and discussion Figure 1 displays the fabrication procedure of the Cudeposited TiO2 NPs. Cu2O NPs (average size: 50 nm) were mixed with P25 TiO2 NPs (average size: 25 nm) in a reaction chamber containing 100 mM glycerol aqueous solution. The mixture of the two NPs was dispersed by a magnetic stirrer followed by evacuation. The evacuation process removes not only air inside the reaction chamber but also gases dissolved 2
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Nanotechnology 26 (2015) 125402
Figure 3. Hydrogen production rates under visible light over the Cu/ TiO2 fabricated from a different amount of Cu2O: (a) bare TiO2, (b) 20 mg, (c) 33 mg, (d) 50 mg, (e) 100 mg and (f) 200 mg. The amount of TiO2 is fixed to 100 mg.
TiO2 NPs increased, the hydrogen production rate gradually increased. The maximum hydrogen production rate of the Cu/ TiO2 NPs under visible light was 0.24 mmol h−1 g−1 when the mass ratio of Cu2O and TiO2 NPs were 1:3. However, excessive addition of Cu2O decreased the hydrogen production rate, suggesting that Cu NPs formed on the surfaces of TiO2 NPs interrupt the absorption of incident light [28]. Therefore, these results confirm that the Cu/TiO2 NPs can be good visible-light responsive photocatalysts for water-splitting hydrogen production. Although the light intensity has spatial gradient and the energy of incident photons with various wavelength are not uniform, the approximate quantum yield can be estimated from the incident energy of the light and the outcome energy of the produced hydrogen. Considering the light intensity 1.5 mW cm−2 and the reaction chamber diameter of 8 cm, 75.4 mJ of light energy is supplied to the reaction chamber every second. The optimum hydrogen production rate, 0.24 mmol g−1 h−1 was achieved from 130 mg of photocatalysts. The enthalpy of hydrogen combustion is about 286 KJ mol−1. Therefore, we obtained 2.48 mJ s−1 by visible light-driven photocatalytic hydrogen production. From the calculation, the estimated quantum yield is 3.29%. Regardless of the ratio of Cu and TiO2, all the Cu/TiO2 NPs exhibited stable hydrogen production with almost constant production rate for 12 h ((b)–(f) in figure 3). To further investigate the hydrogen production stability of the Cu/TiO2 NPs, photocatalytic hydrogen production experiments were carried out for 48 h. Figure 4 shows that the hydrogen production rate does not change during the four-cycle photocatalytic experiments. This indicates that no photo-corrosion or photo-oxidation of the Cu/TiO2 catalysts occurred during the recycled experiments, and these results demonstrate the high-stable characteristics of the Cu/TiO2 photocatalysts. Generally, Cu NPs tend to be immediately oxidized when exposed to air or dispersed in aqueous solution. However, since the reaction chamber in our studies was evacuated, and hence, oxygen gas including the dissolved gas in the aqueous solution was removed from the chamber. Consequently, the Cu NPs formed inside the solution can exist in the solution
Figure 2. Electron microscope characterization of the fabricated Cu/
TiO2: (a) TEM image and (b) high-resolution TEM image.
in the aqueous solution. After the evacuation process, light from a Xe lamp was illuminated into the reaction chamber. In ∼10 min illumination, the color of the solution was changed from gray to dark red (figure S1), suggesting the formation of Cu NPs on TiO2 NPs. TEM images show that Cu NPs with the diameter less than 5 nm are uniformly loaded on P25 TiO2 NPs (figure 2(a)). High-resolution TEM image clearly confirms that the two NPs are Cu and TiO2, respectively (figure 2(b)). The lattice distance of the bigger NPs is 0.35 nm, which corresponds to the (101) plane of anatase TiO2 [31]. Smaller particles adsorbed on the TiO2 surface have the lattice distance of 0.21 nm, which is consistent with the lattice distance of Cu (110) plane [32]. Therefore, TiO2 NPs decorated with small Cu NPs were simply fabricated by illuminating UV into an aqueous glycerol solution containing TiO2 NPs and Cu2O NPs. TiO2 itself cannot absorb visible light due to its wide band gap. As a consequence, no hydrogen was produced from an aqueous glycerol solution over bare TiO2 under visible light ((a) in figure 3). However, the hydrogen production rate was drastically enhanced when Cu/TiO2 NPs were irradiated with visible light ((b)–(f) in figure 3). Various Cu/TiO2 NPs were prepared from the Cu2O NPs and TiO2 NPs with different mass ratio of the two NPs. As the ratio of Cu2O NPs to 3
J M Kum et al
Nanotechnology 26 (2015) 125402
Figure 4. Performance of the Cu/TiO2 photocatalysts showing a long-term stability of the water-splitting hydrogen production under visible light.
without oxidation or without the transformation into copper oxides. It was reported that UV illumination can reduce copper oxides such as CuO and Cu2O to Cu [33]. When a solution comprising Cu2O NPs and P25 TiO2 NPs is illuminated with a UV lamp, Cu NPs generated by the reduction of Cu2O can be deposited on TiO2, thereby leading to the formation of Cu/ TiO2 NPs. In addition, hydrogen is produced inside the reaction chamber during the formation of Cu/TiO2 NPs because TiO2 NPs mixed with Cu2O and Cu/TiO2 NPs can induce water splitting under UV [29]. This hydrogen-rich environment can accelerate the reduction of Cu2O to Cu [33]. However, the small Cu NPs formed on TiO2 can be easily reoxidized to copper oxides if oxygen exists in the solution. In our study, since the reaction chamber was evacuated, oxygen in the solution and in the chamber was almost removed, preventing the re-oxidation of the produced Cu NPs. Consequently, evacuation of the reaction chamber is crucial for the successful formation of Cu/TiO2. This fact was further confirmed by our experiments. If Cu NPs were formed on TiO2 in a vacuum reaction chamber, the color of the solution was dark red (figure S1). However, when the vacuum state of the reaction chamber is broken by exposing the reaction chamber to air, the color of the NPs was immediately converted from dark red to gray in 1 min (figure S1). The color change to gray suggests that the Cu NPs on TiO2 were oxidized. We observed that this bleached Cu/TiO2 NPs did not produce hydrogen from water under visible light anymore, even though they could produce hydrogen under UV (figure S2). Figure 5 shows the UV/vis absorption spectra of P25 TiO2 NPs and the Cu/TiO2 NPs. As is well known, TiO2 absorbs only UV light with the wavelength less than ∼320 nm (figure 5(a)). However, the Cu/TiO2 NPs absorbs the whole range of visible light. Particularly, a broad peak centered at around 450 nm was observed, which is attributed to the surface plasmon resonance (SPR) peak of Cu NPs [34]. The broadening of the SPR peak can be explained as follows. First, aggregation of Cu NPs can broaden the SPR peak. If metal NPs are aggregated, neighboring NPs induces a destructive interference of the plasmon resonance and broaden the absorption peak [35]. The HRTEM image of the
Figure 5. UV/vis characterization of the fabricated Cu/TiO2: (a)UV/ vis absorption spectra of P25 TiO2 NPs and the Cu/TiO2 photocatalysts and (b) the magnified UV/vis absorption spectrum of the Cu/TiO2 photocatalysts.
Cu/TiO2 NPs shows that most of the Cu NPs were dispersed on the TiO2 surface but some of the Cu NPs were in contact with other Cu NPs. Second, NPs attached to other particles show broader plasmon peaks than isolated NPs [36]. The Cu NPs are not dispersed in the solution but are loaded on TiO2 NPs, thereby leading to a broadening of the plasmon peak. Due to the broadening of the SPR peak, a large portion of visible light spectrum can be utilized for the photocatalytic water splitting of the Cu/TiO2.
4. Conclusions We have presented a route to fabricate visible-light responsive Cu/TiO2 photocatalysts. The Cu/TiO2 photocatalysts were prepared by UV illumination on a mixture of Cu2O and TiO2 NPs in an evacuated reaction chamber. Cu2O was reduced to Cu by UV and, consequently, small-sized Cu NPs were uniformly loaded on TiO2. Cu NPs tend to be rapidly oxidized to copper oxides, but the vacuum environment prevents the oxidation of the synthesized Cu NPs. Cu NPs enhance visible light absorption due to the plasmon resonance effect and also act as co-catalysts to separate photo-generated charges in TiO2. As a result, the Cu/TiO2 exhibited enhanced photocatalytic hydrogen production under visible light, splitting water with a help of a glycerol hole scavenger. The Cu/TiO2 photocatalysts also show high stability in the evacuated chamber. Cu NPs are much cheaper than noble metals that are generally used as co-catalysts. Moreover, the Cu/TiO2 4
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Nanotechnology 26 (2015) 125402
photocatlaysts were fabricated by mixing two commercially available Cu2O and TiO2 NPs under UV, allowing mass production of the photocatlaysts. Therefore, the presented route can be very useful for the practical applications to water-splitting hydrogen production.
[12] Law M, Greene L E, Johnson J C, Saykally R and Yang P 2005 Nat. Mater. 4 455–9 [13] John M R, Furgals A J and Sammells A F 1983 J. Phys. Chem. 87 801–5 [14] Bamwenda G R, Tsubota S, Nakamura T and Haruta M 1995 J. Photochem. Photobiol. A 89 177–89 [15] Li F B and Li X Z 2002 Chemosphere 48 1103–11 [16] Kim S and Choi W 2002 J. Phys. Chem. B 106 13311–7 [17] Sakthivel S, Shankar M V, Palanichamy M, Arabindoo B, Bahnemann D W and Murugesan V 2004 Water Res. 38 3001–8 [18] Du J M, Zhang J L, Liu Z M, Han B X, Jiang T and Huan Y 2006 Langmuir 22 1307–12 [19] Hirakawa T and Kamat P V 2005 J. Am. Chem. Soc. 127 3928–34 [20] Yu J G, Yue L, Liu S W, Huang B B and Zhang Y 2009 J. Colloid Interface Sci. 334 58–64 [21] Anker J N, Hall W P, Lyandres O, Shah N C, Zhao J and Van Duyne R P 2008 Nat. Mater. 7 442–53 [22] Ghodselahi T, Vesaghi M A and Shafiekhani A 2009 J. Phys. D: Appl. Phys. 42 015308–14 [23] Chan G H, Zhao J, Hicks E M, Schatz G C and Van Duyne R P 2007 Nano Lett. 7 1947–52 [24] Cheng G and Hight Walker A R 2010 Anal. Bioanal. Chem. 396 1057–69 [25] Xu S and Sun D D 2009 Int. J. Hydrog. Energy 34 6096–104 [26] Bandara T, Udawattab C and Rajapaksea C 2005 Photochem. Photobiol. Sci. 2005 857–61 [27] Jin Z, Zhang X, Li Y, Li S and Lu G 2007 Catal. Commun. 8 1267–73 [28] Sreethawong T and Yoshikawa S 2005 Catal. Commun. 6 661–8 [29] Choi H J and Kang M S 2007 Int. J. Hydrog. Energy 32 3841–8 [30] Kum J M, Yoo S H, Ali G and Cho S O 2013 Int. J. Hydrog. Energy 38 13541–6 [31] Lee J S, You K H and Park C B 2012 Adv. Mater. 24 1084–8 [32] Nath A and Khare A 2011 J. Appl. Phys. 110 043111 [33] Lalitha K, Sadanandam G, Kumari V D, Subrahmanyam M, Sreedhar B and Hebalkar N Y 2010 J. Phys. Chem. C 114 22181–9 [34] Yeshchenko O A 2013 Ukr. J. Phys. 58 249–59 [35] Dahmen C, Schmidt B and von Plessen G 2007 Nano Lett. 7 318 [36] Dozzi M V, Saccomanni A, Altomare M and Selli E 2013 Photochem. Photobiol. Sci. 12 595–601
Acknowledgments This work was supported by the National Research Foundation (NRF) of Korea grant funded by the Korea Ministry of Science, ICT and Future Planning (No. 2013M2A8A1041415). Supporting Information Available Photos of the Cu/TiO2 catalysts before and after the exposure to air, the results of the photocatalytic hydrogen production experiments using the bleached Cu/TiO2 under UV and visible light.
References [1] Fujishima A and Honda K 1972 Nature 238 37–8 [2] Liao C H, Huang C W and Wu J C S 2012 Catalysts 2 490–516 [3] Ramachandran R, Sathiya M, Ramesha K, Prakash A S, Madras G and Shukla A K 2011 J. Chem. Sci. 123 517–24 [4] Umebayashi T, Yamaki T, Tanaka S and Asai K 2003 Chem. Lett. 32 330–1 [5] Choi W, Termin A and Hoffmann M R 1994 J. Phys. Chem. 98 13669–79 [6] Choi J, Park H and Hoffmann M R 2010 J. Phys. Chem. C 114 783–92 [7] Lee Y L, Huang B M and Chien H T 2009 Chem. Mater. 20 6903–5 [8] Xie Y, Ali G, Yoo S H and Cho S O 2010 ACS Appl. Mater. Interfaces 2 2910–4 [9] Chen C, Ali G, Yoo S H, Kum J M and Cho S O 2011 J. Mater. Chem. 21 16430 [10] O’regan B and Grätzel M 1991 Nature 353 737–9 [11] Bach J, Lupo D, Comte P, Moser J E, Weissörtel F, Salbeck J, Spreitzer H and Grätzel M 1998 Nature 395 583–5
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Plasmon-enhanced photocatalytic hydrogen production over visible-light responsive Cu/TiO2
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2015 Nanotechnology 26 125402 (http://iopscience.iop.org/0957-4484/26/12/125402) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 129.173.72.87 This content was downloaded on 05/03/2015 at 02:30
Please note that terms and conditions apply.
Nanotechnology Nanotechnology 26 (2015) 125402 (5pp)
doi:10.1088/0957-4484/26/12/125402
Plasmon-enhanced photocatalytic hydrogen production over visible-light responsive Cu/TiO2 Jong Min Kum, Yang Jeong Park, Hyun Jin Kim and Sung Oh Cho Department of Nuclear and Quantum Engineering, Korea Advanced Institute of Science and Technology (KAIST), 373-1 Guseong, Yuseong, Daejeon 305-701, Korea E-mail: [email protected] Received 4 October 2014, revised 3 February 2015 Accepted for publication 14 February 2015 Published 4 March 2015 Abstract
Cheap and visible-light responsive Cu/TiO2 photocatalysts were fabricated by illuminating ultraviolet (UV) to a mixture of TiO2 nanoparticles (NPs) and Cu2O NPs in an evacuated reaction chamber. The Cu2O NPs were reduced by UV in an oxygen-free reaction chamber, and hence, metallic Cu NPs with size less than 5 nm were uniformly loaded on TiO2. Due to the plasmon resonance of the Cu NPs, the Cu/TiO2 exhibited a good performance of water-splitting hydrogen production under visible light in the presence of glycerol as a hole scavenger. The optimum hydrogen production rate of Cu/TiO2 was 0.24 mmol h−1 g−1. The Cu/TiO2 also showed high stability of the photocatalytic performance in the evacuated chamber; however, the visible-light responsive photocatalytic properties dramatically and rapidly decreased when exposed to air. S Online supplementary data available from stacks.iop.org/NANO/26/125402/mmedia Keywords: copper, TiO2, photocatalyst, plasmon resonance, hydrogen production (Some figures may appear in colour only in the online journal) 1. Introduction
efficiency: the metal NPs suppress the recombination of photo-generated electron–hole pairs [13–16]. Generally, noble metal NPs such as Au, Ag, and Pt NPs are used for the co-catalyst [17–20]. One of the additional effects of the noble metal NPs is that they can have plasmon resonance absorption property in visible light region [21]. As a consequence, TiO2 coupled with the noble metal NPs can exhibit plasmonenhanced photocatalytic activities. Therefore, noble metal NPs attached to TiO2 play dual roles: (i) enhancement of visible light absorption and (ii) separation of photo-generated charges in TiO2. However, the practical applications of photocatalytic hydrogen production based on the noble metal NPs co-catalyst are restricted due to high cost of the noble metal. Here, we present that cheap Cu NPs coupled with TiO2 also exhibit enhanced photocatalytic hydrogen production under visible light due to the plasmon resonance of the Cu NPs. It has been reported that Cu NPs can also exhibit plasmon resonance behavior in visible light region [22], but plasmon-enhanced photocatalysis based on Cu NPs has never
Various approaches to fabricate efficient photocatalysts for water-splitting hydrogen production have been developed since Fujisima and Honda firstly discovered photocatalytic hydrogen production [1]. Photocatalytic water splitting provides a clean and sustainable energy generation method because hydrogen is produced from water using sunlight [2]. Due to the advantages of high stability and low cost, TiO2 is most widely used as a photocatalyst for water splitting [3]. However, photocatalytic activity of pure TiO2 is limited because only ultraviolet (UV) can drive TiO2 due to its wide band gap of ∼3.2 eV and a large fraction of solar energy (∼96%) should be wasted [4]. To overcome this limitation, several methods such as doping of ions [5, 6] and sensitization with quantum dots [7–9] or organic dyes [10–12] have been studied, which enables the utilization of visible light. In addition, metal nanoparticles (NPs) should be loaded on the surface of TiO2 as a co-catalyst to improve the catalytic 0957-4484/15/125402+05$33.00
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© 2015 IOP Publishing Ltd Printed in the UK
J M Kum et al
Nanotechnology 26 (2015) 125402
Figure 1. Schematic illustration displaying the fabrication procedure of the Cu/TiO2 photocatalysts.
been reported. In addition, the plasmon resonance of Cu NPs was observed only when Cu NPs were embedded in other materials [23]. This might be attributed to the fact Cu, particularly in nanometer-sized particles, tend to be easily and rapidly oxidized in air or in aqueous solution [24]. To overcome this problem, we tried to fabricate and load Cu NPs on TiO2 in an evacuated reaction chamber. The prepared Cu NPloaded TiO2 (hereafter abbreviated to Cu/TiO2) show a good performance of photocatalytic hydrogen production under visible light. Many papers has been published on Cu2O and CuO supported TiO2 [25–27]. However, practically no photocatalytic hydrogen production results using oxidation-free Cu supported TiO2 was reported. Although few papers claim that they produced hydrogen by photocatalytic reaction of Cu/ TiO2 [28, 29], we believe that they used cupper oxides because Cu NPs are very easily oxidized both in the air and the aqueous solution. We synthesized pure Cu/TiO2 and carried out whole experiments in vacuum condition to avoid oxidation of Cu. Cu/TiO2 NPs is much cheaper than noble metal-loaded photocatalysts and visible-light absorption property of Cu NPs allows efficient utilization of sunlight for photocatalytic water splitting.
reaction chamber was cooled by water to remove heat produced during the Xe lamp illumination. In order to confirm the photocatalytic reactivity of Cu/ TiO2 under visible light, a UV cut-off filter which passes only visible light with the wavelength larger than 420 nm was installed at the top surface of the reaction chamber. To demonstrate the stability of the reactivity of Cu/TiO2, photocatalytic experiments were carried out for 48 h. The reaction chamber was evacuated every 12 h to maintain the pressure inside the reaction chamber below atmospheric pressure. Hydrogen evolution was confirmed by gas chromatography (GC, hp 5890) equipped with a thermal conductivity detector. The hydrogen production rate was characterized by measuring the pressure change in the reaction chamber using a pressure gauge (PIZ100, Ilmvac Gmbh). The recorded pressure change was converted to hydrogen production rate based on the ideal gas law. When the aqueous solution was irradiated with a Xe lamp, the pressure was slightly increased at an initial stage. Although the reaction chamber was cooled with water, the water temperature was increased by ∼5 °C when the water was illuminated with a Xe lamp. In addition, some dissolved gases can still remain in water even though most of the gases are removed by the evacuation process. As a result, gases dissolved in the aqueous solution can be evolved into the reaction chamber due to the increase of water temperature. However, the gas evolution and accordingly the pressure increase was observed only at an initial stage (99.9%) was added as a hole scavenger. Distilled water (DI water, ⩾18.3 MΩ cm) obtained from a water purification system (Human Power I+, Human Corporation) was used in the experiments. All the chemicals and materials were used in their as-received form without further purification. A closed-loop photocatalytic hydrogen production device [30] was used for the photocatalytic hydrogen production experiments. The reaction chamber was made of pyrex and the volume of the chamber was 250 mL. A 500 W Xe lamp with average power density of 1.5 mW cm−2 was used as a light source. The light intensity of Xe lamp was measured by radiometer (ILT 1400-A, International Light Technologies, USA) on the top of the reaction chamber. The Xe lamp was installed on the top of the reaction chamber for the illumination of light into water. The distance between the Xe lamp and the top surface of the reaction chamber was 30 cm. The
3. Results and discussion Figure 1 displays the fabrication procedure of the Cudeposited TiO2 NPs. Cu2O NPs (average size: 50 nm) were mixed with P25 TiO2 NPs (average size: 25 nm) in a reaction chamber containing 100 mM glycerol aqueous solution. The mixture of the two NPs was dispersed by a magnetic stirrer followed by evacuation. The evacuation process removes not only air inside the reaction chamber but also gases dissolved 2
J M Kum et al
Nanotechnology 26 (2015) 125402
Figure 3. Hydrogen production rates under visible light over the Cu/ TiO2 fabricated from a different amount of Cu2O: (a) bare TiO2, (b) 20 mg, (c) 33 mg, (d) 50 mg, (e) 100 mg and (f) 200 mg. The amount of TiO2 is fixed to 100 mg.
TiO2 NPs increased, the hydrogen production rate gradually increased. The maximum hydrogen production rate of the Cu/ TiO2 NPs under visible light was 0.24 mmol h−1 g−1 when the mass ratio of Cu2O and TiO2 NPs were 1:3. However, excessive addition of Cu2O decreased the hydrogen production rate, suggesting that Cu NPs formed on the surfaces of TiO2 NPs interrupt the absorption of incident light [28]. Therefore, these results confirm that the Cu/TiO2 NPs can be good visible-light responsive photocatalysts for water-splitting hydrogen production. Although the light intensity has spatial gradient and the energy of incident photons with various wavelength are not uniform, the approximate quantum yield can be estimated from the incident energy of the light and the outcome energy of the produced hydrogen. Considering the light intensity 1.5 mW cm−2 and the reaction chamber diameter of 8 cm, 75.4 mJ of light energy is supplied to the reaction chamber every second. The optimum hydrogen production rate, 0.24 mmol g−1 h−1 was achieved from 130 mg of photocatalysts. The enthalpy of hydrogen combustion is about 286 KJ mol−1. Therefore, we obtained 2.48 mJ s−1 by visible light-driven photocatalytic hydrogen production. From the calculation, the estimated quantum yield is 3.29%. Regardless of the ratio of Cu and TiO2, all the Cu/TiO2 NPs exhibited stable hydrogen production with almost constant production rate for 12 h ((b)–(f) in figure 3). To further investigate the hydrogen production stability of the Cu/TiO2 NPs, photocatalytic hydrogen production experiments were carried out for 48 h. Figure 4 shows that the hydrogen production rate does not change during the four-cycle photocatalytic experiments. This indicates that no photo-corrosion or photo-oxidation of the Cu/TiO2 catalysts occurred during the recycled experiments, and these results demonstrate the high-stable characteristics of the Cu/TiO2 photocatalysts. Generally, Cu NPs tend to be immediately oxidized when exposed to air or dispersed in aqueous solution. However, since the reaction chamber in our studies was evacuated, and hence, oxygen gas including the dissolved gas in the aqueous solution was removed from the chamber. Consequently, the Cu NPs formed inside the solution can exist in the solution
Figure 2. Electron microscope characterization of the fabricated Cu/
TiO2: (a) TEM image and (b) high-resolution TEM image.
in the aqueous solution. After the evacuation process, light from a Xe lamp was illuminated into the reaction chamber. In ∼10 min illumination, the color of the solution was changed from gray to dark red (figure S1), suggesting the formation of Cu NPs on TiO2 NPs. TEM images show that Cu NPs with the diameter less than 5 nm are uniformly loaded on P25 TiO2 NPs (figure 2(a)). High-resolution TEM image clearly confirms that the two NPs are Cu and TiO2, respectively (figure 2(b)). The lattice distance of the bigger NPs is 0.35 nm, which corresponds to the (101) plane of anatase TiO2 [31]. Smaller particles adsorbed on the TiO2 surface have the lattice distance of 0.21 nm, which is consistent with the lattice distance of Cu (110) plane [32]. Therefore, TiO2 NPs decorated with small Cu NPs were simply fabricated by illuminating UV into an aqueous glycerol solution containing TiO2 NPs and Cu2O NPs. TiO2 itself cannot absorb visible light due to its wide band gap. As a consequence, no hydrogen was produced from an aqueous glycerol solution over bare TiO2 under visible light ((a) in figure 3). However, the hydrogen production rate was drastically enhanced when Cu/TiO2 NPs were irradiated with visible light ((b)–(f) in figure 3). Various Cu/TiO2 NPs were prepared from the Cu2O NPs and TiO2 NPs with different mass ratio of the two NPs. As the ratio of Cu2O NPs to 3
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Nanotechnology 26 (2015) 125402
Figure 4. Performance of the Cu/TiO2 photocatalysts showing a long-term stability of the water-splitting hydrogen production under visible light.
without oxidation or without the transformation into copper oxides. It was reported that UV illumination can reduce copper oxides such as CuO and Cu2O to Cu [33]. When a solution comprising Cu2O NPs and P25 TiO2 NPs is illuminated with a UV lamp, Cu NPs generated by the reduction of Cu2O can be deposited on TiO2, thereby leading to the formation of Cu/ TiO2 NPs. In addition, hydrogen is produced inside the reaction chamber during the formation of Cu/TiO2 NPs because TiO2 NPs mixed with Cu2O and Cu/TiO2 NPs can induce water splitting under UV [29]. This hydrogen-rich environment can accelerate the reduction of Cu2O to Cu [33]. However, the small Cu NPs formed on TiO2 can be easily reoxidized to copper oxides if oxygen exists in the solution. In our study, since the reaction chamber was evacuated, oxygen in the solution and in the chamber was almost removed, preventing the re-oxidation of the produced Cu NPs. Consequently, evacuation of the reaction chamber is crucial for the successful formation of Cu/TiO2. This fact was further confirmed by our experiments. If Cu NPs were formed on TiO2 in a vacuum reaction chamber, the color of the solution was dark red (figure S1). However, when the vacuum state of the reaction chamber is broken by exposing the reaction chamber to air, the color of the NPs was immediately converted from dark red to gray in 1 min (figure S1). The color change to gray suggests that the Cu NPs on TiO2 were oxidized. We observed that this bleached Cu/TiO2 NPs did not produce hydrogen from water under visible light anymore, even though they could produce hydrogen under UV (figure S2). Figure 5 shows the UV/vis absorption spectra of P25 TiO2 NPs and the Cu/TiO2 NPs. As is well known, TiO2 absorbs only UV light with the wavelength less than ∼320 nm (figure 5(a)). However, the Cu/TiO2 NPs absorbs the whole range of visible light. Particularly, a broad peak centered at around 450 nm was observed, which is attributed to the surface plasmon resonance (SPR) peak of Cu NPs [34]. The broadening of the SPR peak can be explained as follows. First, aggregation of Cu NPs can broaden the SPR peak. If metal NPs are aggregated, neighboring NPs induces a destructive interference of the plasmon resonance and broaden the absorption peak [35]. The HRTEM image of the
Figure 5. UV/vis characterization of the fabricated Cu/TiO2: (a)UV/ vis absorption spectra of P25 TiO2 NPs and the Cu/TiO2 photocatalysts and (b) the magnified UV/vis absorption spectrum of the Cu/TiO2 photocatalysts.
Cu/TiO2 NPs shows that most of the Cu NPs were dispersed on the TiO2 surface but some of the Cu NPs were in contact with other Cu NPs. Second, NPs attached to other particles show broader plasmon peaks than isolated NPs [36]. The Cu NPs are not dispersed in the solution but are loaded on TiO2 NPs, thereby leading to a broadening of the plasmon peak. Due to the broadening of the SPR peak, a large portion of visible light spectrum can be utilized for the photocatalytic water splitting of the Cu/TiO2.
4. Conclusions We have presented a route to fabricate visible-light responsive Cu/TiO2 photocatalysts. The Cu/TiO2 photocatalysts were prepared by UV illumination on a mixture of Cu2O and TiO2 NPs in an evacuated reaction chamber. Cu2O was reduced to Cu by UV and, consequently, small-sized Cu NPs were uniformly loaded on TiO2. Cu NPs tend to be rapidly oxidized to copper oxides, but the vacuum environment prevents the oxidation of the synthesized Cu NPs. Cu NPs enhance visible light absorption due to the plasmon resonance effect and also act as co-catalysts to separate photo-generated charges in TiO2. As a result, the Cu/TiO2 exhibited enhanced photocatalytic hydrogen production under visible light, splitting water with a help of a glycerol hole scavenger. The Cu/TiO2 photocatalysts also show high stability in the evacuated chamber. Cu NPs are much cheaper than noble metals that are generally used as co-catalysts. Moreover, the Cu/TiO2 4
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photocatlaysts were fabricated by mixing two commercially available Cu2O and TiO2 NPs under UV, allowing mass production of the photocatlaysts. Therefore, the presented route can be very useful for the practical applications to water-splitting hydrogen production.
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Acknowledgments This work was supported by the National Research Foundation (NRF) of Korea grant funded by the Korea Ministry of Science, ICT and Future Planning (No. 2013M2A8A1041415). Supporting Information Available Photos of the Cu/TiO2 catalysts before and after the exposure to air, the results of the photocatalytic hydrogen production experiments using the bleached Cu/TiO2 under UV and visible light.
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