Journal of Alloys and Compounds 771 (2019) 505e512
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Nitrogen doping on the core-shell structured Au@TiO2 nanoparticles and its enhanced photocatalytic hydrogen evolution under visible light irradiation Gautam Kumar Naik a, Sanjit Manohar Majhi a, d, Kwang-Un Jeong b, In-Hwan Lee c, Yeon Tae Yu a, * a
Division of Advanced Materials Engineering and Research Center for Advanced Materials Development, College of Engineering, Chonbuk National University, Jeonju, 54899, South Korea Polymer Materials Fusion Research Center & Department of Polymer-Nano Science and Technology, Chonbuk National University, Jeonju, 54896, South Korea c Department of Materials Science and Engineering, Korea University, Seoul, 02841, South Korea d Center for Advanced Membranes for Porous Materials (AMPM), Computer Electrical and Mathematical Sciences & Engineering Division, King Abdullah University of Science and Technology (KAUST), Thuwal, 23955-6900, Saudi Arabia b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 May 2018 Received in revised form 20 August 2018 Accepted 28 August 2018 Available online 31 August 2018
The current study concerns about the large band gap of TiO2 for its use as photocatalysts. The photocatalytic activity of core-shell structured Au@TiO2 nanoparticles were enhanced by the doping of nitrogen. The nitrogen doping has been done by simple hydrothermal method taking ethylenediamine as the precursor for nitrogen. The crystals structure of TiO2 shell remained unaltered even with the introduction of nitrogen. The photocatalytic activity of the prepared samples were evaluated towards the hydrogen evolution from photocatalytic water splitting under solar light irradiation. It was found that nitrogen doped core-shell structured Au@TiO2 nanoparticles (Au@N-TiO2) showed higher photocatalytic activity with an average H2 evolution rate of 4880 mmol h1g1, which is 3.79 times more than that of bare TiO2 in 4 h under xenon light irradiation. The relationship among the other samples was in order of Au@N-TiO2 > Au@TiO2 > N-TiO2 > TiO2. This enhanced photocatalytic activity of Au@N-TiO2 can be responsible for the formation of an plasmonic photocatalyst and the formation of an impurity band between the conduction band (CB) and the valence band (VB) of TiO2. © 2018 Elsevier B.V. All rights reserved.
Keywords: TiO2 Core-shell Nitrogen doped Hydrogen evolution Water splitting
1. Introduction Hydrogen, one of the primary candidates as a future energy carrier, has recently attracted increasing attention due to the increasing energy demands. Since the pioneer discovery of Fujishima and Honda, production of hydrogen by catalytic water splitting on the surface of Titanium dioxide (TiO2) has been regarded as the most economical and environmental friendly way [1e6]. Since then TiO2 have largely been studied as a photocatalyst for its nontoxic, high thermal stability and high crystalline nature [7e13]. However its large band gap energy has restricted its photocatalytic activity only in UV region of solar spectrum. Further the higher electronhole recombination rate on the surface of TiO2 is still a challenge
* Corresponding author. E-mail address:
[email protected] (Y.T. Yu). https://doi.org/10.1016/j.jallcom.2018.08.277 0925-8388/© 2018 Elsevier B.V. All rights reserved.
for its photocatalytic activity. To overcome these shortcomings, a heterostructure with noble metals (Au, Ag, Pt) is essential [14e17]. The role of Au nanoparticles (NPs) on Au decorated TiO2 (Au/TiO2) photocatalyst for hydrogen evolution was investigated by Reichert et al. [18] They found that the Au NPs helps in separation of photogenerated electrons and holes by the formation of a Schottky barrier at the Au-TiO2 interface. The activity of photocatalysts largely depends on their stability and morphology. Recently TiO2 supported gold catalysts have increased a great attention for photocatalysis [19,20]. But due to instability of gold NPs against thermal sintering and growing of small size Au NPs in heat treatment process, the catalytic activity of the supported gold catalyst decreases rapidly [21e23]. To overcome these drawbacks various strategies have been developed such as structural modification and doping of other suitable metals. Out of various structures, core-shell NPs is one of
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the most effective materials, for their unique structure, large surface area, high thermal and chemical stability and higher light harvesting ability [24e27]. The core-shell structure has been also proved to be beneficial for the stability of core metal NPs. In metal@metal oxide (MO) core-shell structure, the encapsulation of metal NPs into core-shell structure is regarded as most promising structure for photocatalysis. In core-shell nanostructures, the interfacial area is maximized and the interaction between noble metal and metal oxide is intensified, resulting in enhancement of their catalytic activity. So many researches have been done to modify the morphology of core-shell NPs to meet the required properties [28e32]. Many researchers have synthesized metal@TiO2 core-shell NPs by various synthetic routes to achieve the maximum photocatalytic capability of TiO2 based photocatalysts. Recently Ngaw et al. have studied the hydrogen evolution activity of Au@TiO2 core-shell and found that Au@TiO2 core-shell shows 7 times higher evolution rate than that of commercial TiO2 (P-25) [33]. They attributed this higher photocatalytic activity towards the higher surface area of Au@TiO2 core-shell and the higher light absorption due to localized surface plasmon resonance (LSPR) of Au NPs. Wedge-shaped Au@TiO2 core-shell NPs have been prepared by a flexible hydrothermal route taking TiF4 as precursor by Xiao-Feng Wu et al. [34] They also studied the photocatalytic activity of the prepared Au@TiO2 core-shell NPs towards the degradation of acetaldehyde and found that the wedge-shaped morphology helped to narrow the band gap of anatase type TiO2. They also stated that the combined Fermi level existed just below the conduction band (CB) of TiO2, as the Fermi level of Au NPs is higher than that of anatase type TiO2. So the photoexcited electron can transfer from the CB of TiO2 to the Fermi level of gold, which is the reason for the enhanced catalytic activity. The catalytic activity of Pd@TiO2 was investigated by Zhou et al. [35] They compared the catalytic efficiency of Pd@TiO2 with that of with that of hollow TiO2. The catalytic activity of Pd@TiO2 core-shell towards the reduction of 4-nitrophenol is much better. They found that the TiO2 shell does not only avoided Pd NPs leaching from support but also improved the homogeneous distribution of Pd NPs on the support and their catalytic activity for the reduction [36]. Li et al. prepared the eccentric Au@TiO2 coreshell NPs via a sol-gel process in the presence of Au NPs with block copolymer shells as templates for photocatalysts [37]. They found that the photodegradation rate of methylene blue was the fastest in presence of Au@TiO2 core-shell NPs when it compared with that of bare TiO2 and Au decorated TiO2 (Au/TiO2) NPs. The high surface area of core-shell structure, stability of Au NPs due to TiO2 shell and the heterointerface between Au and TiO2 are the reasons behind the high catalytic activity of Au@TiO2 core-shell. In recent time Sun et al. have successfully synthesized N-doped anatase TiO2 nanobelts by simple hydrothermal method [38]. They have found that the photocatalytic activity towards hydrogen evolution is much higher and steady than that of nascent TiO2 under visible light irradiation. Many theoretical calculations and researches have proved that the anion doping on TiO2 can alter the band gap energy which is the key reason for the enhancement of catalytic activity [39,40]. There are lot of reports on the effect of nitrogen doping on TiO2 NPs or TiO2 nano rod for improved photocatalysis. But there is need for the study of the effect of nitrogen doping on the other heterostructure like core-shell and yolk-shell structure. Yet the photocatalytic hydrogen evolution on metal@MO core-shell NPs has not vastly investigated till now. In this work, we tried to modify the properties of TiO2 shell on Au@TiO2 core-shell NPs for enhancement of their photocatalytic properties. Here we used a facile modified microwave assisted hydrothermal method for the doping of nitrogen on Au@TiO2 coreshell NPs for the first time. Our synthesis method is a low
temperature and two steps process. Here we studied the effect of nitrogen loading on the catalytic activity of Au@TiO2 core-shell towards photocatalytic water splitting for the generation of hydrogen. We also compared the catalytic activity of Au@N-TiO2 with that of bare TiO2, nitrogen doped TiO2 (N-TiO2) and bare Au@TiO2 core-shell. In addition the mechanism for the hydrogen evolution on Au@N-TiO2 under visible light irradiation and reusability of all the photocatalyst prepared is discussed in details. 2. Experimentals 2.1. Chemicals All chemicals were analytical grade and used without further purification. Hydroaurochloric acid (HAuCl4) and tri-sodium citrate (Na3C6H5O7) used for the synthesis of Au NPs were obtained from SHOWA chemicals. Titanium fluoride (TiF4) used as TiO2 precursor were obtained from ACROS organics and ethylenediamine used as nitrogen precursor was obtained from Junsei chemicals. Analytical grade methanol was used as sacrificial agent for the photocatalytic hydrogen evolution reaction. Milli-Q water was used throughout the experiment. 2.2. Synthesis of Au@N-TiO2 core-shells NPs The gold nanoparticles (Au NPs) were synthesized according to sodium citrate reduced method [33]. In a typical reaction, 500 mL solution of HAuCl4 (1 mM) was heated with mild stirring until boiling. A solution of freshly prepared tri-sodium citrate (25 mL, 34 mM) was added with rapid stirring. The resulting solution was kept at 97 C for 15 min with constant stirring. After that the solution was allowed to cool. This solution was directly used as precursor for Au core NPs. Au@N-TiO2 core-shell was synthesized by a two steps ex-situ microwave assisted hydrothermal process. For the synthesis of Au@TiO2 core-shell NPs, in a typical reaction 8 mL of TiF4 solution (0.04 M) were mixed with 16 mL of prepared Au NPs colloids drop wise under moderate stirring. Then the volume of the mixture solution was made up to 60 mL with Milli-Q water. Then the solutions were transferred to a Teflon-lined autoclave. The microwave assisted hydrothermal reaction was conducted at 180 C for 1 h. The heating rate was 15 C/min. After that the products were cooled to room temperature and separated by centrifugation (12,000 rpm, 12 min). Then the products were dried at 60 C for 12 h and named as Au@TiO2. For nitrogen doping Au@TiO2 coreshell, 10 mg of prepared Au@TiO2 were dispersed in 15 mL of ethanol. 2 mL of ethylenediamine were added drop wise with vigorous stirring and the volume was made up to 60 mL with MilliQ water. The above solution underwent microwave assisted hydrothermal reaction at 120 C for 1 h. Then the solution was cooled down to room temperature and the products were separated by centrifugation (12,000 rpm, 12 min) followed by washing with water and ethanol. The products were dried at 60 C for 12 h and named as Au@N-TiO2. 50 mg of each prepared catalysts were taken for the photocatalytic hydrogen evolution reaction. Bare TiO2 was also prepared following same procedure taking TiF4 as precursor without taking Au colloids. Nitrogen doped TiO2 (N-TiO2) also prepared as that of Au@N-TiO2 described above, taking preprepared TiO2 instead of Au@TiO2 core-shell NPs. 2.3. Characterizations The morphology and microstructure of the prepared photocatalysts were studied by TEM (JEM-2010, JEOL) and FESEM (Hitachi, S-4800). The selected area electron diffraction (SAED) patterns and high resolution TEM were performed by HRTEM (HRTEM, JEM-
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2200FS JEOL). The X-ray diffraction (XRD) pattern of all samples were recorded with the help of an X-ray diffractometer (D/Max2005, Rigaku) having Cu Ka source (l ¼ 1.54178 A ) with a scan rate of 3 /min. A UVevisible spectrophotometer (UV-2550, Shimadzu) was used to record diffuse reflectance UVevisible spectra (DRS UVeVis) of the prepared samples taking BaSO4 as the standard material. 2.4. Photocatalytic hydrogen evolution The photocatalytic reaction for hydrogen production was carried out with the help of a compact xenon light source (ASAHI SPECTRA Co. Ltd, MAX-303, 300 W). The photocatalytic reaction was performed by adding 50 mg of powder photocatalyst into 50 mL of aqueous solution containing 25% methanol as sacrificial reagent. To prevent the particles from settling down at the bottom of the reactor, the solution was kept under continuous stirring with help of the magnetic stirrer. Before light irradiation, air was completely removed from the sacrificial solution by purging with nitrogen for 15 min. The generated gas was collected by water displacement technique and was analysed in a gas chromatograph (GC-2010, Shimadzu corporation) using a 5 A molecular sieve column and thermal conductivity detector (TCD). The retention time of the peak that appeared on the chromatogram was compared with standard, which confirmed the amount of evolved hydrogen gas. The time duration for each cycles of reaction was kept for 4 h. After completion of each cycles, the photocatalyst were separated from the sacrificial reagent and dried at 60 C for 12 h and used for the next cycles. For comparison and mechanism study the control reaction for photocatalytic hydrogen evolution was carried out for pure TiO2. 3. Results and discussion 3.1. Morphological studies To investigate the structure and surface morphology of the prepared Au@N-TiO2 core-shell NPs, TEM, HAADF and FESEM images of the samples were observed. Fig. 1a and Fig. 1b show the TEM image of Au@TiO2 and Au@N-TiO2 core-shell NPs. The surface of Au core and TiO2 layers are visible based on the contrast difference in the images. The average thickness of TiO2 shell is in the range of 20e40 nm, whereas the diameter of Au core diameter is about 15e20 nm. All the Au@N-TiO2 core-shell NPs are uniform and well distributed. To establish the structure and morphology, these samples are further investigated by HAADF-STEM image analysis. Fig. 1c and d shows the HAADF image of the Au@TiO2 and Au@NTiO2 core-shell. Apparently they have brighter contrast at the core and darker contrast in the shell indicating different chemical composition of the core and shell. As Au has much higher atomic number than Ti, so it scatters electrons very intensively, therefore it will appear brighter in the HAADF image. For confirmation of the loading of nitrogen, both Au@TiO2 and Au@N-TiO2 core-shell were further examined by line scanning elemental mapping analysis. Fig. 2 shows the line scanning elemental mapping profile of Au@TiO2 core-shell. It shows the strong Au signal at the centre of the core-shell and strong Ti and oxygen signals at the edge of the particle, revealing the core-shell structure. The line scanning elemental mapping of Au@N-TiO2 core-shell (Fig. 3) also shows the similar pattern as that of Au@TiO2 core-shell. But the presence of nitrogen signals all over the particle confirms the doping of nitrogen in TiO2 shell. From the FESEM analysis, it was observed that all Au@N-TiO2 core-shell NPs are spherical in shape as shown in Fig. S1. It was also observed that the loading of nitrogen does not alter the core-shell structure of the samples. The HRTEM image of Au@N-
Fig. 1. (a) TEM images of Au@ TiO2 core-shells, (b) TEM images of Au@N-TiO2 coreshells (c) HAADF-STEM image of Au@TiO2 core-shell (d) HAADF-STEM image of Au@N-TiO2 core-shell.
TiO2 core-shell NPs are shown in Fig. 4a. From the analysis of HRTEM image, it was found that the (111) lattice plane of Au NPs with d-spacing value of 0.23 nm, (200) and (101) lattice plane of anatase TiO2 with d-spacing value of 0.18 nm and 0.35 nm respectively are existed. The interface between Au core and TiO2 shell is also well distinguishable. The concentric diffraction rings in SAED spectra as shown in Fig. 4b, clearly reveals the polycrystalline nature of the particles. From the analysis of the SAED pattern of the Au@N-TiO2 core-shell NPs, the presence of (101), (004), (200), (105), (112), (220) and (320) planes of anatase TiO2, and (220) and (111) planes of Au NPs are confirmed. 3.2. Structural analysis The structural identification and phase composition of the prepared samples were investigated with the help of X-ray diffraction. The XRD pattern of all photocatalysts were recorded between the 2q angles from 20 to 80 and shown in Fig. 3. From Fig. 5a and b, it was found that, (101), (200), (211), (204), (220) and (215) peaks for the synthesized TiO2 are well indexed with JCPDS file No. 21-1272, indicating the anatase phase of TiO2. The peaks at 2q value at 29.54 , 39.67 and 43.31 are well indexed with JCPDS file No 21-1276, which confirmed the presence of rutile phase TiO2 in the samples. The sharp XRD peaks of all the samples indicate the well crystalline nature of all the photocatalysts. XRD peaks at 2q value of 44.13 , 64.64 and 77.74 in the samples Au@TiO2 and Au@N-TiO2 (Fig. 5c and d) are the characteristics peaks of Au NPs, which confirms the presence of Au NPs core in the photocatalysts (JCPDS file No. 01-1174). The broadening of the XRD peak at 2q value 38.43 in the samples Au@TiO2 and Au@N-TiO2 is due to the overlapping of (004) peak of anatase TiO2 and (111) peak of Au NPs. The average crystalline sizes of anatase TiO2 NPs (101 peak) was calculated by Scherer equation and found to be 26.1 nm. It was also observed that during the doping of nitrogen, the all crystalline phase of TiO2 did not alter. The relative percentage of rutile phases
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Fig. 2. Line scanning elemental mapping profile of Au@TiO2 core-shell NPs.
Fig. 3. Line scanning elemental mapping profile of Au@TiO2 core-shell NPs.
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Fig. 4. (a) HRTEM image of Au@N-TiO2 core-shell (b) SAED pattern of Au@N-TiO2 core-shell.
Fig. 5. X-ray diffraction patterns of (a) TiO2, (b) N-TiO2, (c) Au@TiO2 core-shell and (d) Au@N-TiO2 core-shell NPs.
in both bare TiO2 and Au@TiO2 core-shell NPs increased with doping of nitrogen as it was observed from the increase of intensity ratio between the (101) peak for anatase TiO2 and (110) peak of rutile TiO2. The all findings of XRD are well satisfied with that of the SAED analysis of the samples. 3.3. Optical properties analysis The optical properties of the samples were studied with the help of diffuse reflectance UVevisible spectra (DRS UVevisible spectra). Fig. 6 shows the DRS UVevisible spectra of the samples. From the figure it was found that the sharp decrease in reflectance between 320 and 380 nm is originated from the band gap energy of TiO2 (Fig. 6a). The lower reflectance of N-TiO2 in the visible region [Fig. 6b] is due to loading of nitrogen. The reflectance spectrum of Au@TiO2 and Au@N-TiO2 [Fig. 6c and d] slightly shifted towards left, is due to core-shell structure. The significantly enhanced absorption at 500-600 is due to LSPR of metallic gold nanoparticles. The strong absorbance of Au@TiO2 and Au@N-TiO2 in visible region is the reason for their enhanced catalytic activity. It was also observed that Au@N-TiO2 is more absorbed than that of Au@TiO2 in the visible region of the solar spectrum. It might be for the formation of an impurity bands above the valence band (VB) of TiO2 [41].
Fig. 6. DRS UVevisible reflectance spectra of (a) TiO2, (b) N-TiO2, (c) Au@TiO2 coreshell, and (d) Au@N-TiO2 core-shell NPs.
3.4. Photocatalytic activity analysis The photocatalytic activities of all prepared photocatalysts were evaluated for photocatalytic hydrogen evolution. The amount of hydrogen produced in the presence of bare TiO2, N-TiO2, Au@TiO2 and Au@N-TiO2 NPs are presented in Fig. 7. It was observed that the bare TiO2 NPs could evolve only 257 mmol of H2 in 4 h of reaction as shown in Fig. 5a. The hydrogen evolution capacity of TiO2 increases with doping of nitrogen on its surface as shown in Fig. 7b. The overall H2 production was enhanced in the case of Au@TiO2 coreshell NPs. It was observed that the amount of H2 evolved in case of Au@TiO2 was 710 mmol (Fig. 7c). But, the Au@N-TiO2 NPs exhibits more enhanced photocatalytic activity with 976 mmol of H2 (Fig. 7d). This enhancement of H2 evolution rate for Au@N-TiO2 is due to its higher capability to absorb in the visible region of the solar spectrum as shown in Fig. 6c. The LSPR property of Au core NPs is also responsible for the enhanced photocatalytic activity of Au@TiO2 and Au@N-TiO2 core-shell. The time course hydrogen evolutions capability for all the prepared samples were tested for 4 cycles and was found that the photocatalytic activity of the catalysts retained up to 4 cycles. In Table S1, we have compared the hydrogen evolution rate of N doped Au@TiO2 with similar reported TiO2 based composite photocatalysts and found that Au@N-TiO2 showed much higher evolution rate.
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Fig. 7. Hydrogen evolution amount changes of (a) TiO2, (b) N-TiO2 (c) Au@TiO2 core-shell and (d) Au@N-TiO2 core-shell NPs.
During photocatalysis, by absorbing photon the electron from valance band (VB) of TiO2 migrates to conduction band (CB) of TiO2. Thus a hole and an electron formed at the VB and CB of TiO2 respectively (Eq. (1)). These free electrons presented at the CB are responsible for the reduction for Hþ ions to hydrogen (Eq. (2)). So the concentration of free electrons at CB of TiO2 is proportional to the amount of hydrogen produced. The holes (hþ) generated at the VB of TiO2 during the photo separation of electrons and holes react with the sacrificial reagent (H2O þ CH3OH) to produce Hþ ions (Eqs. (3)e(6)) [42,43]. So the oxidation half reaction occurred at VB and reduction half reaction occurred at CB of TiO2. TiO2 þ hy / e (CB) þ hþ (VB)
(1)
4Hþ þ 4e- (CB) / 2H2
(2)
CH3OH þ hþ / CH2OH þ Hþ
(3)
CH2OH þ h / HCHO þ Hþ
(4)
H2O þ hþ / OH þ Hþ
(5)
HCHO þ OH þ hþ / COOH þ Hþ
(6)
So the overall reaction in VB of TiO2 is stated in equation (6). CH3OH þ H2O þ 4hþ / HCOOH þ 4Hþ
(7)
In addition the Au core NPs and doped nitrogen anion plays the important roles in decreasing the electron-hole recombination rate. The schematic diagram of photocatalysis and possible chemical reaction occurred during the photocatalysis on Au@N-TiO2 core-
Fig. 8. Schematic diagram of mechanism for hydrogen evolution on Au@N-TiO2 core-shell.
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shell NPs is presented in Fig. 8. In the case of Au@TiO2 and Au@NTiO2 core-shell NPs, LSPR effect of the Au core NPs plays a vital role in the enhancement of hydrogen evolution as compared with bare TiO2 and N-TiO2. The increased photo-absorption in visible light region as shown in Fig. 6c and d due to LSPR of Au NPs is responsible for enhanced photocatalytic hydrogen evolution. If visible light falls on the surface of TiO2, a Mott-Schottky junction forms at the interface between Au NPs core and TiO2 shell [44,45]. When the visible light irradiates on the Au NPs, the frequency of coherent oscillation of conductive electrons matches with that of the incident light, resulting in the propagation of surface wave at the interface between the TiO2 and the Au NPs. This leads to the interfacial charge transfer of activated electrons from the CB of TiO2 to the Au NPs and decreasing the electron-hole recombination rate on TiO2. This charge transfer makes the shifting of the net Fermi level towards more negative potential until they reach to the equilibrium. This negative shift of Fermi level leads to higher reduction of the electron-hole recombination rate [46]. This reduction of electron-hole recombination rate on TiO2 has a crucial effect for the enhancement of photocatalytic activity of the prepared Au@TiO2 and Au@N-TiO2 core-shell. Further in the case of Au@N-TiO2, an impurity band created in between VB and CB of TiO2 due to the mixing of N 2p and O 2p electrons by N doping, resulting in an increase of photo absorption efficiency in visible light region (Fig. 6d) due to reduction of band gap of TiO2 [47]. This enhanced photo absorption capability of nitrogen doped TiO2 in visible region helps to increase the activation rate of valance band electrons on its surface. 4. Conclusions Nitrogen has been successfully doped on the surface of Au@TiO2 core-shell NPs by microwave solvothermal process. From XRD, it was found that all prepared photocatalysts are polycrystalline in nature and both anatase and rutile phase of TiO2 were existed. The nitrogen doping has significantly increased the photocatalytic hydrogen evolution capacity of the synthesized photocatalysts. From the UVevisible absorbance spectra analysis it was found that the doping of nitrogen resulted in the formation of an impurity band in between VB and CB of TiO2, which caused the enhancement of photon absorption capability of the photocatalyst. Moreover the LSPR effect of Au NPs core helped in the reduction of electron-hole recombination on Au@N-TiO2 core-shell NPs photocatalysts. The combined effect of nitrogen doping and LSPR helped the enhancement of photocatalytic hydrogen evolution on the surface of Au@N-TiO2 core-shell photocatalysts. Acknowledgements This work was supported by 1) BK21 plus program from the Ministry of Education and Human-Resource Development, 2) National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) (BRL No. 2015042417, 2016R1A2B4014090) and 3) “Research Base Construction Fund Support Program” funded by Chonbuk National University in 2017. Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.jallcom.2018.08.277. References [1] Fujishima, K. Honda, Electrochemical photolysis of water at a semiconductor electrode, Nature 238 (1972) 37e38.
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