Highly Efficient Temperature-Induced Visible Light ... - ACS Publications

Page 1 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Highly Efficient Temperature-induced Visible Light Photocatalytic Hydrogen Production from Water Bing Han and Yun Hang Hu* Department of Materials Science and Engineering, Michigan Technological University, 1400 Townsend Drive, Houghton, MI 49931-1295, USA

Abstract: Intensive effort has led to numerous breakthroughs for photo-processes. So far, however, energy conversion efficiency for the visible-light photocatalytic splitting of water is still very low. In this paper, we demonstrate (1) surface-diffuse-reflected-light can be two orders of magnitude more efficient than incident light for photocatalysis, (2) the inefficiency of absorbed visible light for the photocatalytic H2 production from water with a sacrificial agent is due to its kinetic limitation, and (3) the dispersion of black Pt/TiO2 catalyst on the light-diffuse-reflection-surface of a SiO2 substrate provides a possibility for exploiting a temperature higher than H2O boiling point to overcome the kinetic limitation of visible light photocatalytic hydrogen production. Those findings create a novel temperature-induced visible light photocatalytic H2 production from water steam with a sacrificial agent, which exhibits a high photo-hydrogen yield of 497 mmol/h/gcat with a large apparent quantum efficiency (QE) of 65.7% for entire visible light range at 280 oC. The QE and yield are one and two orders of magnitude larger than most reported results, respectively. *Corresponding Author. Phone: 906-4872261. Email: [email protected]

1 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 26

1. Introduction Hydrogen is considered as one of the most promising fuels for the future. Currently, most hydrogen is produced from steam-reforming of natural gas, which requires intensive thermal energy input. The efficient photo-conversion of H2O to H2 is a long-time goal of energy science and engineering. Fujishima and Honda discovered the electrochemical photolysis of water on TiO2 electrodes, which created a new era in heterogeneous photocatalysis in 1972.1 This prompted worldwide research efforts on photocatalytic production of H2 from water.2-15 TiO2 has a relatively large band gap energy (3.0~3.2 eV) and thus it can absorb only ultraviolet (UV) light (about 4% of the total solar energy), leading to a low photo-conversion efficiency (less than 2% under AM 1.5 global sunlight illumination).3-6 In the past 20 years, numerous pioneering contributions were made to extend the working spectrum of TiO2 to visible-light region (which constitutes about 45% of the total solar energy) via three approaches: (a) the non-metal ions (such as C, N, and S) were introduced into TiO2 to tune its valence band (VB), 16-20, (b) the metal ions were incorporated into TiO2 to generate states below its conduction band (CB),21 and (c) the hydrogenation of TiO2 was exploited to invent black TiO2.3,

6, 22

Furthermore, breakthroughs were also obtained using other types of semiconductor catalysts.2, 4, 8-15

However, although impressive progresses were made to develop novel semiconductor water-

splitting photocatalysts for solar lights beyond UV,4,

23-26

solar-to-hydrogen efficiency of

photocatalytic water splitting is still very low under visible light illumination. Furthermore, redirection of incident light was employed to increase the power conversion efficiency of solar cells

27

, but the scattering and reflection of light on catalysts are generally expected to be

minimized for the improvement of light absorption efficiency in photocatalytic processes.4 Herein, however, we report that the combination between the dispersion of black Pt/TiO2 catalyst 2 ACS Paragon Plus Environment

Page 3 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


on a light-diffuse-reflection-surface of a SiO2 substrate and an elevated reaction temperature created a highly efficient temperature-induced visible light photocatalytic hydrogen production from water steam (with methanol as a sacrificial agent), leading to high photo-hydrogen yield and quantum efficiency for entire visible light range at 280 oC.

2. Experimental Section 2.1 Preparation and characterization of a SiO2 substrate Silicon dioxide and quartz wool were mixed with water at weight ratio of 30:30:1 and then heated to 1100 °C for 2h to form a piece of white solid. The solid piece was shaped as a round disk substrate (diameter of 1.6 cm and thickness of 2 mm) or a rectangle disk substrate (1.4 cm × 2.9 cm). The surface roughness of the SiO2 substrate was measured using Mahr Federal Pocket Surf III profilometer. Furthermore, its light scatter coefficient was determined using a Shimadzu UV-Vis spectrophotometer. Its scattered light intensities at different scattering angles were obtained with a Newport light intensity detector. 2.2 Black Pt/TiO2 catalyst and its dispersion on light-diffuse-reflection-surface 1wt% Pt/TiO2 catalyst was synthesized by impregnating TiO2 powder (Degussa P25 with surface area of 48 m2/g) in aqueous solution of H2PtCl6·6H2O at 25 oC overnight, followed by calcination at 500 oC for 2 hours. The obtained Pt/TiO2 powder, which was located in a ceramic tube reactor (diameter of 3 cm), was vacuumed for 6 hours. Then, hydrogen was introduced into the reactor, followed by increasing temperature to 200 oC in 20 min and remaining the temperature for 24 hours. Consequently, black Pt/TiO2 catalyst was generated. To disperse the



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

catalyst on the light-diffuse-reflection-surface, black Pt/TiO2 powder was mixed with water as a paste, followed by coating on the surface of the SiO2 substrate and then drying at 25 oC overnight. The dispersion of the catalyst on the light-diffuse-reflection-surfaces was evaluated by a Hitachi-4700 field emission scanning electron microscope (FESEM). The light absorbance was determined using a Shimadzu UV-Vis spectrophotometer. The existence of Ti3+ in the black catalyst was examined with electron paramagnetic resonance (EPR) spectra, which were recorded at 77K using Bruker CW X-band (9-10 GHz) spectrometer (ESP-300 E). 2.3 Photocatalytic hydrogen production from H2O at room temperature A quartz reactor (diameter: 1.5 cm) was filled with 10 ml water (containing 30% methanol as a sacrificial agent) and 1 Sun (100 mW/cm2) was simulated by a solar simulator (Newport), in which AM 1.5 global sunlight was generated from 150 W xenon lamp coupled with an AM 1.5G filter. To generate visible light, UV light (wavelength < 420 nm) was filtered from the simulated sunlight. For photocatalytic hydrogen production from H2O over black Pt/TiO2 catalyst without light-diffuse-reflection, the catalyst powder was directly dispersed in the liquid with slight stirring. To evaluate the photocatalytic performance of the catalyst with surface-light-diffuse-reflection, a black Pt/TiO2 coated on the light-diffuse-reflection-surface of SiO2 round disk substrate (or filter paper) was immersed in the liquid with vertical illumination of incident light on the substrate surface. Hydrogen production was monitored by pressure change and analyzed by a gas chromatograph. O2 from H2O was not detected in all cases. 2.4 Temperature-induced photocatalytic hydrogen production from H2O Black Pt/TiO2 catalyst (6 mg) on the light-diffuse-reflection-surface of a SiO2 rectangle disk substrate (1.4 cm × 2.9 cm) was located in a quartz reactor (diameter: 1.5 cm). To carry out 4 ACS Paragon Plus Environment

Page 4 of 26

Page 5 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


temperature-induced photocatalytic hydrogen production, the reactor was heated to a selected temperature (arranged from 150 to 300 oC) by an electrical furnace, water (with 30% methanol as a sacrificial agent) pumped into the reactor, and the catalyst irradiated by 1 Sun (100 mW/cm2, simulated by AM 1.5G Newport solar simulator) with irradiation area of 4 cm2. For visible light photocatalytic reaction test, UV light (wavelength < 420 nm) was filtered from the simulated sunlight. Argon was exploited to carry products into on-line gas chromatograph (GC) equipped with a Porapak Q column and a 5A column for composition analysis. Furthermore, the condensed liquid products were analyzed by GC-Mass spectrometer. 2.5 Band potential measurements: The band potentials of pristine (white) TiO2 (P25) and the black Pt/TiO2 catalyst were measured as follows: (1) Flat-band potentials (Efb) were determined by Mott-Schottky method with 0.1 M LiClO4 as electroloyte, Pt as a counter electrode, and a saturated calomel electrode (SCE) as a reference electrode for both 1 cm2 pristine (white) TiO2 coated on a fluorine-doped tin oxide (FTO) glass plate and 1 cm2 black Pt/TiO2 coated on a FTO glass plate. Argon (>99.99%) was bubbled through the electrolyte to remove oxygen before the test. The capacitance was measured by electrochemical impedance spectroscopy. Conduction band edge potentials (Ecb) were calculated from flat-band potentials with a method described in ref. 28. (2) The energy gaps (Eg) between conduction band (CB) and valence band (VB) for white TiO2 and black Pt/TiO2 were measured using a Shimadzu UV-Vis spectrophotometer and the energy gap between conduction band (CB) and the generated donor level (Ti3+) for black Pt/TiO2 was determined using a Bomem Fourier transform near infrared (FTNIR) spectrophotometer. (3) The valence band edge potential (Evb) was obtained by combining the conduction band edge potential with the VB-CB energy gap. 5 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 26

2.6 Apparent quantum efficiency: The photoproduction of H2 from water with a sacrificial agent (CH3OH) can be expressed as a general reaction: H2O + xCH3OH → reduction product (yH2) + oxidation products (zCO, nCO2, or mCH2O) (1) The apparent quantum efficiency for the reaction is equal to the ratio of the number of reacted electrons (or holes) to the number of incident photons. Because the formation of one photo H2 molecule (produced by photocatalysis) is associated with two reacted electrons, the apparent quantum efficiency (Q.E.) can be calculated using following equation:

=

! "#×% &!&'( )*("

(2)

The measurements of incident photons were carried out using Newport Quantum Efficiency Test Instrument under the same irradiations (visible light or AM 1.5 global sunlight) as used for the photocatalytic reactions.

3. Results and Discussion To obtain a light-diffuse-reflection-surface, we synthesized a SiO2 substrate via mixing silicon dioxide and quartz wool with water, followed by heating to 1100°C for 2 hours (to form a piece of white solid) and then shaping as a disk substrate. The surface roughness and the light scattering coefficient of the substrate are 0.078 µm and 385 cm2/g, respectively. Furthermore, it was shown that incident light was reflected at various angles instead of just a specular reflection angle (Fig. 1). Those demonstrate that the synthesized SiO2 substrate possesses an excellent 6 ACS Paragon Plus Environment

Page 7 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


light-diffuse-reflection-surface. It is well-known that a paper with rough surface can exhibit strong light-diffuse-reflection. Therefore, we selected a filter paper (from Whatman) as another substrate with a light-diffuse-reflection-surface. Its light-diffuse-reflection was confirmed by measuring scattered-light-intensity at various angles (Fig. 1) and its high light-scattering coefficient of 422 m2/g with a big roughness of 0.268 µm. This indicates that the paper exhibited even better performance for light-diffuse-reflection than the SiO2 substrate. For the photocatalytic hydrogen production without light-diffuse-reflection, black 1wt% Pt/TiO2 catalyst (about 20 nm in diameter) was homogeneously dispersed in H2O (with 30% methanol as a sacrificial agent) in a quartz tube reactor with slight stirring that ensures the liquid being transparent. Under illumination of simulated AM 1.5 global sunlight (100 mW/cm2) at room temperature, a low hydrogen yield of 0.20 mmol/h/gcat was obtained with an apparent quantum efficiency of 0.02% (Fig.2A). However, if the catalyst was coated on the light-diffusereflection-surface of the SiO2 substrate, its hydrogen yield and the apparent quantum efficiency increased to 20.11 mmol/h/gcat and 2.74%, respectively (Fig.2A). As shown in Movie S1 (Supplementary Information), one can watch such an impressive process of hydrogen generation. This indicates that the photocatalytic efficiency of the Pt/TiO2 catalyst on light-diffusereflection-surface is 100 times larger than that without a light-diffuse-reflection-surface. In other words, the surface-diffuse-reflected-light is two orders of magnitude more efficient than the incident light for photocatalytic production of H2 from water. Namely, the scattered lights play a sole role in the photocatalytic process, which contributes 99% hydrogen yield. Furthermore, the utilization of a filter paper as a substrate for the Pt/TiO2 resulted in a hydrogen yield of 23.32 mmol/h/gcat and apparent quantum efficiency of 3.18%, which are even larger than those with black Pt/TiO2 on a SiO2 substrate (Fig.2A). This happened because the paper possesses larger 7 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 26

light-diffuse-reflection than the SiO2 substrate. It would be noted that no hydrogen was produced by the SiO2 substrate (or the filter paper) without black Pt/TiO2 catalyst. This confirms that the SiO2 substrate (or the filter paper) played a role of light-diffuse-reflecting-surface instead of a photocatalyst. The importance of surface-diffuse-reflected-light for photocatalysis was further supported by UV/visible spectra. As shown in Fig. 2B, one can see that the black Pt/TiO2 on the filter paper and the SiO2 substrate exhibited much larger light-absorbance than that dispersed in liquid. This clearly proves that the absorbance efficiency of the photocatalyst is much larger for surface-diffuse-reflected-light than for incident light, contributing to a significant increase in photocatalytic efficiency. This happened because the light-diffuse-reflection on the substrate surface can remarkably increase the light irradiation of black Pt/TiO2 particles, which are located at the substrate (Fig.2C and D). Furthermore, in contrast to the conventional photocatalytic splitting of water, in which catalyst powder is dispersed in liquid water and thus reaction temperature must be below water boiling point (100oC), the dispersion of catalyst on the lightdiffuse-reflection-surface of a substrate can allow the photocatalytic reaction at a temperature higher than water boiling point. The significance of the photocatalyitc reaction at an elevated temperature was demonstrated in the following sections. Although UV/visible spectrum shows that black Pt/TiO2 can remarkably absorb visible light (Fig.2B), hydrogen yield and apparent quantum efficiency were almost negligible in the photocatalytic hydrogen production (with 30% methanol as a sacrificial agent) at room temperature when the UV light of the full solar lights was filtered (Fig. 3). This indicates that the enhanced visible light absorption of black TiO2 has negligible contribution to hydrogen production. This observation is consistent with previous reports.3,

22

Furthermore, Li et al.

revealed that black TiO2 nanowires (obtained by hydrogen treatment) possess the same VB as 8 ACS Paragon Plus Environment

Page 9 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


white TiO2.22 Therefore, they attributed the absorption of visible and infrared lights to the formation of oxygen vacancies (generating Ti3+) in hydrogenated rutile TiO2. We determined the CB edges of pristine (white) TiO2 and black Pt/TiO2, which are -0.51 and -0.52 eV (vs. SHE at pH=7), respectively, from flat-band potentials. This indicates that the white and black TiO2 possesses almost same CBs. Furthermore, it was found that VB edges (2.69 eV for white TiO2 and 2.68 eV for black Pt/TiO2 vs. SHE) were also almost same. In addition, our electron paramagnetic resonance (EPR) spectra confirmed the existence of Ti3+ in the black Pt/TiO2 (Fig. S1 in Supplementary Information) and our Fourier transform near infrared spectroscopy (FTNIR) measurement showed 1.30 eV of the generated donor level (Ti3+) below TiO2 CB, which is in a good agreement with reported results.29,

30

Therefore, the visible and near-infrared light

absorptions could be attributed to the excitation of electrons from the Ti3+ level to TiO2 CB. This can allow us to obtain a relationship between band structure of black TiO2 and redox potentials of water splitting. As shown in Fig. 4, one can see that those energies meet the thermodynamic requirement for visible light photocatalytic hydrogen production (with methanol as a sacrificial agent): (a) TiO2 CB is more negative than H2O/H2 redox potential, (b) the energy level of the Ti3+ is more positive than the redox potential of CH2O/CH3OH (CO/CH3OH or CO2/CH3OH), and (c) energy gap (1.3 eV) between the Ti3+ level and TiO2 CB ensures the absorption of visible light (and even near IR). This indicates that no contribution of visible light to the photocatalytic splitting of water with a black Pt/TiO2 is due to kinetic limitation instead of thermodynamics. In UV light absorption, electrons are excited from TiO2 VB to its CB, followed by the electron donation to TiO2 VB from the oxidation of CH3OH to CH2O (CO or CO2). Therefore, the driving force (energy difference between TiO2 VB and CH2O/CH3OH redox potential) is 2.78 eV for the electron donation. In contrast, for the absorption of visible (or near IR) light, the excitation of



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 26

electrons occurs from Ti3+ level to TiO2 CB, and then the electrons (generated from the oxidation of CH3OH to CH2O) are donated to the oxidized Ti3+, namely, the driving force (energy difference between Ti3+ level and CH2O/CH3OH redox potential) is 0.88 eV. This clearly indicates that the driving force for the electron donation from the oxidation of a sacrificial agent is much larger for the UV light absorption than for the visible (or near IR) light absorption. In other words, no contribution of the absorption of visible (or near IR) light to photocatalytic hydrogen production is due to the insufficient driving force for the electron donation from CH3OH to the oxidized Ti3+. A simple approach to increase the driving force for a reaction would be to increase the temperature of reactants, because the obtained thermal energy of reactants at an elevated temperature can enhance their reactivity. Therefore, if methanol can obtain enough thermal energy to drive its electron donation to the oxidized Ti3+ level, the photocatalytic reduction of water to H2 should take place under visible (or even near IR) light illumination. This would create a novel temperature-induced visible light photocatalytic production of hydrogen from water. Furthermore, the feasibility of this approach was confirmed by the following experiments. The black Pt/TiO2 catalyst (6 mg) dispersed on the light-diffuse-reflection-surface of the SiO2 substrate (4 cm2) was located in a quartz tube reactor and heated to a selected temperature by an electrical furnace (Fig. 5). Water (with 30% methanol as a sacrificial agent) was introduced into the reactor. All products were analyzed by Gas Chromatograph (GC) and Mass Spectrometer (MS). As shown in Fig. 6A, the reaction without light illumination started to produce hydrogen at 200 oC. This indicates that when temperature is 200 oC or higher, the obtained thermal energy of methanol is sufficient to drive its oxidation. This was further approved by the formation of oxidation products (CO, CO2 and CH2O) (Fig. S2 Supplementary 10 ACS Paragon Plus Environment

Page 11 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Information). The hydrogen yield increased with increasing reaction temperature. Furthermore, when AM 1.5 global sunlight was illuminated on the catalyst, H2 yield is much larger than that without light illumination (Fig. 6A). Particularly, when reaction temperature was 280oC, the hydrogen yield reached 688 mmol/h/gcat with AM 1.5 global sunlight illumination, which is almost 6 times larger than that (117 mmol/h/gcat) without light illumination. There are two possibilities for light to increase hydrogen yield: (1) raising temperature to accelerate thermocatalytic process and (2) creating photocatalysis. Because the same temperature for the reactions with and without light irradiation was ensured by an in-situ thermocouple, the effect of light illumination on thermocatalytic process was eliminated. This was confirmed using Pt/Al2O3 catalyst. Because Al2O3 is an insulator with a large energy gap (about 8.8 eV), Pt/Al2O3 may not be a photocatalyst. In other words, the reaction over Pt/Al2O3 is only a thermocatalytic process. As shown in Fig. 6B, one can see that the hydrogen yield over Pt/Al2O3 catalyst (dispersed on the light-diffuse-reflection-surface of a SiO2 substrate) is almost the same for the processes with and without light illumination. This clearly demonstrates that the light illumination could not affect the thermocatalytic process. Therefore, the large difference of hydrogen yield between the processes with and without light illumination on black Pt/TiO2 catalyst is due to the photocatalytic reaction. Namely, the difference of H2 yields with and without light irradiation is photo H2 yield, which was shown with the apparent quantum efficiency in Fig. 6C. As temperature increased, the photo H2 yield and the apparent quantum efficiency increased to 571 mmol/h/gcat and 78.0% at 280 oC and then decreased to 352 mmol/h/gcat and 48.0% at 300 oC, respectively. This indicates that the photo H2 yield and apparent quantum efficiency can reach the maximum values (571 mmol/h/gcat and 78.0%) for entire AM 1.5G sunlight range at 280 oC (Fig. 6C), which are 28 times higher than those at room temperature (Fig. 2A). Such an increase-



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 26

then-decrease performance in photo H2 yield with increasing temperature can be explained by the competition between thermocatalytic and photocatalytic processes. As temperature increased from 200 to 280 oC, thermocatalytic production of H2 slowly increased (such as only 13.6% enhancement in thermocatalytic H2 yield by increasing temperature from 270 to 280 oC), whereas photocatalytic process rapidly increased, leading to an increase in photo H2 yield (Fig. 6C). In contrast, when temperature reached 290 oC, thermocatalytic hydrogen production jumped (namely, thermocatalytic H2 yield was doubled with increasing temperature from 280 to 290 oC), causing a decrease in selectivity of reactants to photo H2 and thus a reduction in quantum efficiency. This indicates that 280 oC is the optimum temperature for photocatalytic hydrogen production from H2O with methanol over black Pt/TiO2 catalyst. Furthermore, when UV light was completely filtered from AM 1.5 global sunlight, the photo H2 yield and apparent quantum efficiency can still reach 497 mmol/h/gcat and 65.7% at 280oC (Fig. 6D), whereas they were almost zero at room temperature (Fig. 3). Such impressive quantum efficiency and hydrogen yield are one and two orders of magnitude larger than reported results for entire visible light range.4 The relationship between quantum efficiencies and wavelengths further demonstrated that all wavelengths (from 395 to 950 nm) can contribute to photo hydrogen production at 280 oC (Fig. 7). Furthermore, the excellent stability of black Pt/TiO2 catalysts for the process was demonstrated by the reaction under AM 1.5 global sunlight illumination at 280 oC, in which hydrogen yield remained almost unchanged for 30 hours (Fig. 8). The above results clearly show that the significant enhancement of photocatalytic H2 production from H2O with a sacrificial agent (methanol) over the black Pt/TiO2 catalyst at an elevated temperature is due to the temperature-induced visible light photocatalysis. This can be explained as follows: It is well known that excited electrons in TiO2 can easily recombine with 12 ACS Paragon Plus Environment

Page 13 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


holes, causing its very poor performance for H2O splitting to H2 4. This is the reason why the photocatalytic H2 production from H2O over TiO2 needs a sacrificial agent (such as methanol) to inhibit such a recombination via its electron-donation to holes. Therefore, it is important to ensure the oxidation of the sacrificial agent. As discussed above, for the absorption of visible (or near IR) light, the excitation of electrons occurs from Ti3+ level to TiO2 CB, generating to holes at Ti3+ level. The driving force (energy difference between Ti3+ level and CH2O/CH3OH redox potential) is 0.88 eV, which is insufficient for the oxidation of methanol to donate electrons to the oxidized Ti3+ (Fig. 4). However, as shown in Fig.S2, when temperature reached 200 oC or higher, the thermal energy was large enough to drive the oxidation of methanol, which can donate electrons to the oxidized Ti3+. As a result, the photocatalytic reduction of water to H2 can take place under visible (or even near IR) light illumination, leading to the significant enhancement of H2 production at an elevated temperature. 4. Conclusions In conclusion, the findings in this work demonstrated: (a) surface-diffuse-reflected-light was 100 times more efficient than incident light for photocatalysis; (b) the negligible efficiency of absorbed visible light for the photocatalytic H2 product from water with a sacrificial agent was due to its kinetic limitation; and (c) the combination between the dispersion of black Pt/TiO2 catalyst on a light-diffuse-reflection-surface and an elevated reaction temperature created a highly efficient temperature-induced visible light photocatalytic hydrogen production from water (with methanol as a sacrificial agent), leading to a photo hydrogen yield of 497 mmol/h/g and an apparent quantum efficiency of 65.7% for entire visible light range at 280 oC. This novel approach opens a new door to develop highly efficient visible light photocatalytic processes.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 26

Acknowledgements This work was supported by the U.S. National Science Foundation (NSF-CBET-0931587). Hu also thanks Charles and Carroll McArthur for their great support. Supporting Information EPR spectra, product yield vs temperature, effect of H2O/methanol ratio on photocatalytic hydrogen production, results of control experiments (comparing Pt, TiO2, and SiO2) for photocatalytic hydrogen production, and a movie for photocatalytic hydrogen production process. This information is available free of charge via the Internet at http://pubs.acs.org.

References 1. Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238, 37-38. 2. Liao, L.; Zhang, Q.; Su, Z., Zhao Z.; Wang, Y.; Li, Y.; Lu, X.; Wei, D.; Feng, G.; Yu, Q. et al. Efficient Solar Water-splitting Using a Nanocrystalline CoO Photocatalyst. Nat. Nanotech. 2014, 9, 69–73. 3. Chen, X.; Liu, L.; Yu, P. Y.; Mao, S. S. Increasing Solar Absorption for Photocatalysis with Black Hydrogenated Titanium Dioxide Nanocrystals. Science 2011, 331, 746-750. 4. Chen, X.; Shen, S.; Guo, L.; Mao, S. S. Semiconductor-based Photocatalytic Hydrogen Generation. Chem. Rev. 2010, 110, 6503-6570. 5. Khaselev, O.; John, J.A. A Monolithic Photovoltaic-Photoelectrochemical Device for Hydrogen Production via Water Splitting. Science 1998, 280, 425-427. 14 ACS Paragon Plus Environment

Page 15 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


6. Hu, Y. H. A Highly Efficient Photocatalyst—Hydrogenated Black TiO2 for the Photocatalytic Splitting of Water. Angew. Chem. Int. Ed. 2012, 51, 12410-12412. 7. Schultz, D. M.; Yoon, T. P. Solar Synthesis: Prospects in Visible Light Photocatalysis. Science 2014, 343, 1239176. 8. Kenney, M. J.; Gong, M.; Li, Y.; Wu, J. Z.; Feng, J.; Lanza, M.; Dai, H. High-performance Silicon Photoanodes Passivated with Ultrathin Nickel Films for Water Oxidation. Science 2013, 324, 836-840. 9. Kato, H.; Asakura, K.; Kudo, A. Highly Efficient Water Splitting into H2 and O2 over Lanthanum-doped NaTaO3 Photocatalysts with High Crystallinity and Surface Nanostructure. J. Amer. Chem. Soc. 2003, 125, 3082-3089. 10. Wang, D.; Hisatomi, T.; Takata, T.; Pan, C.; Katayama, M.; Kubota, J.; Domen, K. Core/Shell Photocatalyst with Spatially Separated Co-Catalysts for Efficient Reduction and Oxidation of Water. Angew. Chem. Int. Ed. 2013, 52, 11252–11256. 11. Khan, S. U. M.; Al-Shahry, M.; Ingler Jr. W. B. Efficient Photochemical Water Splitting by a Chemically Modified n-TiO2. Science 2002, 297, 2243-2245. 12. Onda, K.; Li, B.; Zhao, J.; Jordan, K. D.; Yang, J.; Petek, H. Wet Electrons at the H2O/TiO2 (110) Surface. Science 2005, 308, 1154-1158. 13. Zou, Z.; Ye, J.; Sayama, K.; Arakawa, H. Direct Splitting of Water under Visible Light Irradiation with an Oxide Semiconductor Photocatalyst. Nature 2001, 414, 625-627. 14. Maeda, K.; Teramura, K.; Lu, D.; Takata, T.; Saito, N.; Inoue, Y.; Domen, K. Photocatalyst Releasing Hydrogen from Water. Nature 2006, 440, 295-297. 15 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 26

15. Murdoch, M.; Waterhouse, G. I. N.; Nadeem, M. A.; Metson, J. B.; Keane, M. A.; Howe, R. F.; Llorca, J.; Idriss, H. The Effect of Gold Loading and Particle Size on Photocatalytic Hydrogen Production from Ethanol over Au/TiO2 Nanoparticles. Nat. Chemistry 2012, 3, 489492. 16. Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Visible-light Photocatalysis in Nitrogen-doped Titanium Oxides. Science 2001, 293, 269-271. 17. Chen, X.; Burda, C. The Electronic Origin of the Visible-light Absorption Properties of C-, N- and S-doped TiO2 Nanomaterials. J. Amer. Chem. Soc. 2008, 130, 5018-5019. 18. Park, J. H.; Kim, S.; Bard, A. J. Novel Carbon-doped TiO2 Nanotube Arrays with High Aspect Ratios for Efficient Solar Water Splitting. Nano Lett. 2006, 6, 24−28. 19. Umebayashi, T.; Yamaki, T.; Itoh, H.; Asai, K. Band Gap Narrowing of Titanium Dioxide by Sulfur Doping. Appl. Phys. Lett. 2002, 81, 454−456. 20. Hoang, S.; Guo, S.; Hahn, N. T.; Bard, A. J.; Mullins, C. B. Visible Light Driven Photoelectrochemical Water Oxidation on Nitrogen-modified TiO2 Nanowires. Nano Lett. 2012, 12, 26−32. 21. Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Environmental Applications of Semiconductor Photocatalysis. Chem. Rev. 1995, 95, 69-96. 22. Wang, G.; Wang, H.; Ling, Y.; Tang, Y.; Yang, X.; Fitzmorris, R. C.; Wang, C.; Zhang, J. Z.; Li, Y. Hydrogen-treated TiO2 Nanowire Arrays for Photoelectrochemical Water Splitting. Nano Lett. 2011, 11, 3026–3033.


Page 17 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


23. Kudo, A.; Miseki, Y. Heterogeneous Photocatalyst Materials for Water Splitting. Chem. Soc. Rev. 2009, 38, 253–278. 24 Maeda, K.; Teramura, K.; Domen, K. Effect of Post-calcination on Photocatalytic Activity of (Ga1–xZnx)(N1–xOx) Solid Solution for Overall Water Splitting under Visible Light. J. Catal. 2008, 254, 198–204. 25 Mubeen, S.; Lee, J.; Singh, N.; Krämer, S.; Stucky, G. D.; Moskovits, M. An Autonomous Photosynthetic Device in Which All Charge Carriers Derive from Surface Plasmons. Nat. Nanotech. 2013, 8, 247–251. 26. Liu, C.; Tang, J.; Chen, H. M.; Liu, B.; Yang, P. A Fully Integrated Nanosystem of Semiconductor Nanowires for Direct Solar Water Splitting. Nano Lett. 2013, 13, 2989–2992. 27. Isabella, O.; Krč, J.; Zeman, M. Modulated Surface Textures for Enhanced Light Trapping in Thin-film Silicon Solar Cells. Appl. Phys. Lett. 2010, 97, 101106. 28. Katz, M. J.; Vermeer, M. J. D.; Farha, O. K.; Pellin, M. J.; Hupp, J. T. Effects of Adsorbed Pyridine Derivatives and Ultrathin Atomic-layer-deposited Alumina Coatings on the Conduction Band of TiO2 and on Redox-shuttle-derived Dark Currents, Langmuir 2013, 29, 806-814. 29. Cronemeyer, D. C. Infrared Absorption of Reduced Rutile TiO2 Single Crystals. Phys. Rev. 1959, 113, 1222–1226. 30. Kim, W. T.; Kim, C. D.; Choi, Q. W. Sub-band-gap Photoresponse of TiO2−x Thin-film— electrolyte Interface. Phys. Rev. B 1984, 30, 3625–3628.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 26

Fig. 1. Scattered-light intensity vs scattering angle. The scattering angle is defined as an angle between the directions of the incident light and the scattered light. Because of the limitation of the test unit, scattering light intensity was not measured at scattering angles between 155 and 205o (Green for SiO2 substrate, blue for filter paper, and red for clear H2O/CH3OH liquid).


Page 19 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig. 2. Surface-diffuse-reflected-light for photocatalytic hydrogen production from H2O with methanol as a sacrificial agent over black Pt/TiO2 catalyst at room temperature. (A) Hydrogen yield and apparent quantum efficiency under illumination of simulated AM 1.5 global sunlight, (B) UV-vis spectra, (C) Field emission scanning electron microscope (FESEM) image of black Pt/TiO2 coated on SiO2 substrate, and (D) FESEM image of black Pt/TiO2 coated on filter paper.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 26

Fig. 3. Photocatalytic hydrogen production from H2O with 30% methanol as a sacrificial agent over black Pt/TiO2 catalyst dispersed on the light-diffuse-reflection-surface of SiO2 substrate at room temperature. Hydrogen yield and apparent quantum efficiency are almost negligible when visible light is exploited for illumination at room temperature.


Page 21 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig. 4. Relationship between band structure of black TiO2 and redox potentials of water splitting with methanol as a sacrificial agent. The absorption of UV generates the excitation of electrons from valence band (VB) of TiO2 to its conduction band (CB), whereas the absorption of visible light is associated with the excitation of electrons from the donor (Ti3+) level to TiO2 CB.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 26

Fig. 5. Test unit schematic for temperature-induced photocatalytic hydrogen production from H2O with methanol as a sacrificial agent. (1) Thermocouple, (2) Black Pt/TiO2 on SiO2 substrate, (3) Quartz wool, (4) Quartz tube reactor, and (5) Electrical tube furnace.


Page 23 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig. 6. Temperature-enhanced photocatalytic hydrogen production from H2O with a sacrificial agent (30% methanol) over a catalyst dispersed on light-diffuse-reflection-surface of a SiO2 substrate: (A) Hydrogen yield vs reaction temperature over black Pt/TiO2 catalyst without light illumination (blue bar) and with AM 1.5 global sunlight illumination (red bar); (B) Hydrogen yield vs reaction temperature over Pt/Al2O3 catalyst without light illumination (blue bar) and with AM 1.5 global sunlight illumination (red bar); (C) Photo hydrogen yield (equal to the difference of hydrogen yields with and without

light irradiation) and apparent quantum

efficiency vs reaction temperature over black Pt/TiO2 catalyst under AM 1.5 global sunlight illumination; (D) Photo hydrogen yields and apparent quantum efficiencies of visible light and AM 1.5 global sunlight over black Pt/TiO2 catalyst at 280 oC.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 26

Fig. 7. Quantum efficiencies vs wavelengths of incident light for photocatalytic hydrogen production from water containing 30 % methanol over black Pt/TiO2 dispersed on light-diffuse-reflection-surface

of a SiO2 substrate at 280 °C.


Page 25 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig. 8. Hydrogen yield (from water with 30% methanol) vs reaction time over black Pt/TiO2 catalyst dispersed on light-diffuse-reflection-surface of a SiO2 substrate under AM 1.5 global sunlight illumination at 280 oC.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table of Contents (TOC) Image


Page 26 of 26

Highly Efficient Temperature-Induced Visible Light ... - ACS Publications

Recommend Documents