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International Journal of Photoenergy
Volume 2012 (2012), Article ID 749135, 20 pages
Photoresponse of Visible Light Active CM-n-TiO2, HM-n-TiO2, CM-n-Fe2O3, and CM-p-WO3 towards Water Splitting Reaction
1Department of Marine Chemistry, Faculty of Marine Sciences, King Abdulaziz University, P.O. Box 80207, Jeddah 21589, Saudi Arabia
2Department of Chemistry and Biochemistry, Duquesne University, Pittsburgh, PA 15282, USA
Received 14 August 2011; Accepted 25 September 2011
Academic Editor: Shifu Chen
Copyright © 2012 Yasser A. Shaban and Shahed U. M. Khan. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Photoresponses of visible light active carbon modified titanium oxide (CM-n-TiO2), hydrogen modified titanium oxide (HM-n-TiO2), carbon modified iron oxide (CM-n-Fe2O3), carbon modified tungsten oxide (CM-p-WO3) towards water splitting reaction are reported in this article. Carbon and hydrogen in titanium oxide were found to be responsible for red shift from UV region to visible region which in turn enhanced the photoconversion efficiency by an order of magnitude for water splitting reaction. Photocurrent densities and photoconversion efficiencies of regular n-TiO2 and CM-n-TiO2 towards water splitting reaction under monochromatic light illumination from a xenon lamp and sunlight were compared and found in reasonable agreement. These oxides were characterized by photocurrent measurements, UV-Vis spectra, scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDS) and x-ray diffraction (XRD) studies and these results are also reported in this article.
Sunlight is the unlimited source of clean and renewable energy if it could be efficiently utilized to split water to hydrogen and oxygen. Numerous studies focused on photoelectrochemical water splitting reaction [1–36]. The enhancement of efficiency of a solar hydrogen production process is of considerable importance because high efficiency translates into lower costs. It is, therefore, important to search for a stable and low-cost material for efficiently harvesting the solar energy. To use semiconductors as possible photoelectrodes, their viability depends on their ability to absorb enough sunlight as well as their stability against photocorrosion. The electronic structure of a semiconductor plays a key role in its photoactivity. Unlike a conductor, a semiconductor consists of a valence band (VB) and the conduction band CB). Energy difference between these two levels is said to be the bandgap () .
For efficient H2 production using a visible-light-driven semiconductor (Figure 1), the bandgap should be less than 3.0 eV, but larger than 1.23 eV [38–41]. Moreover, the conduction band (CB) and valence band (VB) levels should satisfy the energy requirements set by the reduction and oxidation potentials for , respectively. For hydrogen production, the CB level should be more negative than hydrogen production level while the VB should be more positive than water oxidation level for efficient oxygen production from water by photocatalysis (Figure 1). Figure 2 shows the bandgap energies; CB and VB positions at pH 2 for different oxide materials that can be used as photoelectrodes. It shows that the CB levels of SrTiO3 and KTaO3 are above but the VB levels of these oxides are below redox energy level. This means that only the SrTiO3 and KTaO3 can photosplit water without external bias. The TiO2 needs at least 0.3 volt of external bias to photosplit water. However, most of these oxides have very high bandgap energy (>3.1 eV) and hence cannot absorb the visible light of solar spectrum.
1.1. Mechanism of Hydrogen Production from Water
Photoelectrolysis of water in a photoelectrochemical cell (PEC) involves several processes within photoelectrodes and at the photoelectrode/electrolyte interface, including the following.
Autoionization of Water Molecules
In an electrolyte solution, water molecules dissociates into and :
When a semiconductor electrode is illuminated by a light source or directly by solar radiation that can provide the electrode with photonic energy () greater than its bandgap energy, (), then an electron from the valence band will be excited into the conduction band, and correspondently, a positively charged hole will be left in the valence band:
Reduction of Hydrogen Ions
The electrons generated as a result of (2) are transferred over the external circuit to cathode, resulting in the reduction of hydrogen () ions to gaseous hydrogen: at cathode.
Oxidation of Hydroxyl Ions
The light-induced holes from (2), at the photoanode/electrolyte interface, oxidize the hydroxyl () ions to gaseous oxygen as at photoanode.
The overall reaction will be
1.2. Development of Photoelectrodes for Water Splitting
Among the photoelectrode materials, titanium dioxide (n-TiO2) was found to be the most promising because of its low cost, chemical inertness, nontoxicity, and photostability. However, its wide bandgap (3.0–3.2 eV) limits its photoresponse in the ultraviolet region which is only a small fraction (~5%) of the sun energy compared to visible light from 400 nm to 750 nm (~49%). Hence, any shift in the optical response of n-TiO2 from the UV to the visible spectral range will have a profound positive effect on the photocatalytic efficiency of the material. Several attempts were made to lower the bandgap of n-type titanium oxide (n-TiO2) by transition metal dopants [42, 43] but no noticeable change in bandgap energy of n-TiO2 was observed. The visible light absorption by the transition metal-doped n-TiO2 was found to be mainly due to d-d transition of electrons in the transition metal dopants but not due to bandgap lowering. The transition metal dopants acted adversely on photocatalytic activity of n-TiO2 because they acted as recombination centers for the photogenerated electron-hole pairs.
However, recent studies involved doping of n-TiO2 by carbon [1, 15–24, 44–49], nitrogen [50, 51], sulphur , also fabricating the carbon-modified (CM)-n-TiO2 in mesoporous [53, 54], and nanostructure [55, 56] forms. Photocatalytic activity of sulphur-doped n-TiO2 was found to diminish under UV and visible light illuminations due to catalytic poisoning induced by oxidation of sulfur to SO2 and . Photocatalytic activity of nitrogen-doped n-TiO2 was reported to be much lower than carbon-doped n-TiO2 .
It was found that carbon modification of n-TiO2 photocatalyst synthesised by thermal oxidation of Ti metal sheet in a natural gas flame lowered the bandgap energy of n-TiO2 to 2.32 eV and exhibited water splitting to hydrogen and oxygen with a photoconversion efficiency of 8.35%  under artificial light illumination from a Xenon lamp. This progress stimulated further investigation into carbon-modified n-TiO2 (CM-n-TiO2) as visible light active photocatalysts [43–49] and also as photoelectrodes [14–24, 55–59] for water-splitting reaction with enhanced photoconversion efficiency. Xu and Khan  also showed enhanced rate of water splitting at spray pyrolytically synthesised visible light active carbon-doped n-TiO2 thin films. Enhanced photoresponse for water splitting at nanocrystalline carbon-doped n-TiO2 thin films was also reported by Xu et al. . Argon was found to be the most effective calcining ambient to enhance the carbon doping of n-TiO2 films. The resultant carbon-modified n-TiO2 thin film calcined in Ar at the optimum temperature of 700°C contained 3.8 at % carbon and showed higher photoresponse in both UV and visible regions compared to those calcined in air at the optimum temperature of 500°C that contained 2.3 at % carbon. Carbon content in these samples calcined in presence of oxygen in air reduced due to removal of some carbon in the forms of CO and CO2. This wet process synthesis allowed molecular scale mixing of the carbon source with precursor of TiO2 for improved doping homogeneity and was capable of tuning the carbon content in the film.
Importantly, Mohaputra et al.  synthesised CM-n-TiO2 thin films by flame oxidation of Ti metal sheet and reported photoconversion efficiency of 8.5% for water splitting which is slightly higher than earlier results . Carbon doping was considered to be responsible for the lowering of the bandgap of n-TiO2 and consequent high photocatalytic activity under visible light illumination. Noworyta and Augustynski  observed shift in the spectral response (up to 425 nm) during water-splitting reaction for carbon-doped n-TiO2 film electrodes formed in the flame of a burner fed with various gas mixtures. Shankar et al.  investigated the effect of flame annealing on the spectral photoresponse of titania nanotubes and demonstrated the enhancement of the visible light absorption and the photocurrent for water splitting due to annealing in the flame. However, in their study, the flame annealing was carried out at 1020°C which is much higher than the optimum temperature of 850°C observed earlier . Such high temperature generally removes considerable amounts of carbon from the sample and thus reduces its photoresponse. Given the sensitivity of CM-n-TiO2 to flame temperature, Shankar et al.  and Xu et al.  prepared highly ordered n-TiO2 nanotube arrays by electrochemical anodization of Ti metal sheet followed by calcinations in an electric oven and further oxidation in natural gas flame at optimum temperature to incorporate carbon in it. Significant enhancement of the photoresponse of these carbon-modified (CM) n-TiO2 nanotube films for water splitting was also reported [58, 59]. Hahn et al.  prepared carbon-doped self-organized TiO2 nanotubes layers formed by electrochemical anodisation of Ti in an HF/Na2HPO4 electrolyte. The tubes were treated at 500°C under a mixed flux of N2 and acetylene (C2H2). Nakano et al.  prepared TiO2:C films by oxidative annealing of sputtered TiC films. They reported three bands having energies 0.86 eV, 1.30 eV, and 2.34 eV below the conduction band of carbon-doped n-TiO2. They attributed the 0.86 eV level to the intrinsic nature of TiO2, whereas the 1.30 eV and the 2.34 eV levels were newly introduced by C-doping and behaved as deep-level bands. In particular, the pronounced 2.34 eV band contributed to bandgap narrowing by mixing C 2p with the O 2p valence bands. Therefore, the 2.34 eV level played a significant role for the visible-light sensitivity in TiO2:C. Ren et al.  synthesised a visible light active TiO2 photocatalyst by carbon doping using glucose as carbon source. The preparation was performed by a hydrothermal method at temperature as low as 160°C. The carbon-doped n-TiO2 showed absorption in the 400–450 nm range with a red shift in the bandgap transition. Sun et al.  reported that carbon-doping-made In2O3 red shifted from UV to visible range up to 500 nm and 40% contribution from visible light was observed for the photoelectrochemical splitting of water to hydrogen. Cardenas et al.  reported marked decrease in resistivity of carbon-doped antimony sulfide (Sb2S3) thin films to 102 Ω cm compared to 108 Ω cm for undoped samples, and electrical resistivity was also possible to tune by controlling the carbon content (wt %) in the sample. The bandgap energy the carbon-doped Sb2S3 thin films was found to decrease to 1.7 eV from 2.57 eV for undoped sample. This result indicates that carbon doping of Sb2S3 is not only capable of bandgap lowering but can also enhance the conductivity of the sample.
In an important theoretical study, Nie and Sohlberg  reported the lowering of the bandgap of n-TiO2 to 2.32 eV (535 nm) due to carbon incorporation and predicted that the bandgap value of 1.58 eV may be possible to achieve by some complex carbon incorporation. Interestingly, it was found experimentally that enhanced carbon doping lowered the bandgap of n-TiO2 up to 1.45 eV . Wang and Lewis [66, 67] addressed theoretically the effects of carbon dopants concentration on the photoresponse of n-TiO2 in the visible-light region. They found that the substitutional and interstitial carbon dopants incorporated into TiO2 drastically affected the electronic structure of the material, thus improving its photoactivity. They predicted the low bandgap of 2.35 eV for carbon-doped TiO2 as it was experimentally observed earlier . The theoretical findings of Di Valentin et al.  revealed the presence of substitutional and interstitial carbon in carbon-modified (CM)-n-TiO2 (Figure 3) which were found to be responsible for the lowering of its main bandgap as well as generating a mid-gap band.
Among various oxide semiconductor photocatalysts, iron (III) oxide (Fe2O3) is a low-cost semiconductor with high stability. Iron (III) oxide has a bandgap of 2.0–2.3 eV; therefore, it can absorb solar radiation up to 620 nm, which covers ~37% of the photons of the solar spectrum of global AM 1.5 (1 sun) . However, challenges are set by its quite low photoresponse due to its high resistivity and consequent recombination of photogenerated carriers. To minimise these limitations, it has been reported that dopants, either n-type [4, 7, 26, 27] or p-type [28–30], improved the photoresponse of iron (III) oxide (Fe2O3) towards water splitting to hydrogen. Thin films of n-Fe2O3 were extensively studied [4, 7, 25–36]. For example, n-Fe2O3 films were synthesised by a sputtering method , by pressing powders of Fe2O3 [32–34], and by spray pyrolysis method [4, 7, 28–30, 32]. Silicon-doped Fe2O3 films synthesised by ultrasonic spray pyrolysis showed 1.0 mA cm−2 at +0.1 V versus AgCl  under solar-simulated light illumination at 1 sun, AM 1.5 (100 mW cm−2).
Tungsten trioxide (WO3) is also of great interest and has been investigated extensively due to its promising physical and chemical properties [70–73] The WO3 films are nontoxic and have relatively low price. Semiconducting tungsten oxide (WO3) is an interesting candidate for photocataylsis because of its relatively low bandgap energy (Eg = 2.6 eV), [74–76] resulting in possible utilization of 12% of the solar spectrum . Spectral response studies were carried out for n-type WO3 by different workers [78, 79]. For example, n-WO3 films were synthesized by conventional evaporation techniques [80, 81] and, more recently, with the sol-gel process [82, 83]. It is important to note that regular undoped tungsten oxide is an n-type semiconductor, but when it is carbon-doped or carbon-modified, it was found to be a p-type tungsten oxide. Carbon-modified p-type tungsten oxide (CM-p-WO3) films were synthesized by thermal flame oxidation method at several flame temperatures for different lengths of time .
In this paper, we summarize our studies on thin-film photoelectrodes of the following(i)visible light active carbon-modified titanium oxides (CM-n-TiO2);(ii)visible light active hydrogen modified n-type titanium oxide (HM-n-TiO2) thin films;(iii)carbon-modified iron oxides (CM-n-Fe2O3) thin films;(iv)visible light active carbon-modified p-type tungsten oxides (CM-p-WO3) thin film.
The results of photocurrent density measurements on these photoelectrodes for water splitting to H2 and O2 and the maximum photoconversion efficiencies under both Xenon lamp and natural sunlight of global AM 1.5 illuminations are also provided. Synthesized thin-film photoelectrodes are characterized in terms of bandgap energies, X-ray diffraction pattern, scanning electron micrograms, and carbon contents to correlate with their photoresponse towards water-splitting reaction.
2. Experimental Details and Results
2.1. Visible Light Active Carbon-Modified Titanium Oxides (CM-n-TiO2) Thin Films
Shaban and Khan  synthesized visible light active carbon-modified n-type titanium oxide (CM-n-TiO2) thin films by flame oxidation of Ti metal sheets both flat and grooved and also by using combination of spray pyrolysis and flame oxidation methods. Also un-doped reference n-TiO2 samples were synthesized in an electric oven for comparison. The flame was custom designed by Knight Co. The flame temperature was controlled by adjusting the flow rates of oxygen and natural gas.
2.1.1. Dependence of Photocurrent Density on Measured Electrode Potential for Titanium Oxides
Plots of photocurrent density (, mA cm−2) as a function of measured potential, (V/SCE) for oven-made n-TiO2 synthesized at 825°C for 16 min (sample 1); CM-n-TiO2 synthesized by only thermal flame oxidation of Ti at 825°C for 16 min (sample 2); grooved CM-n-TiO2 synthesized by only thermal flame oxidation of Ti (0.005 inch groove depth) at 820°C for 18 min (sample 3); CM-n-TiO2 synthesized by spray pyrolysis of 0.175 M TiCl4 in ethanol at substrate temperature of 460°C for 20 sec at carrier gas (oxygen) pressure of 20 psi followed by thermal flame oxidation at 825°C for 16 min (sample 4) are shown in Figure 4. Sample 1 shows the lowest photocurrent density of 1.6 mA cm−2 measured at –0.4 V/SCE. The observed low photocurrent density for sample 1 was due to absence of carbon as well as lower thickness in such electric oven-made samples. Sample 4 generated the highest photoresponse compared to samples, 1, 2, and 3. Photocurrent density, measured at –0.7 V/SCE, increased significantly from 9.17 mA cm−2 for sample 2 to 11.44 mA cm−2 for sample 3. The surface grooving of Ti metal increased the effective surface area of oxide layer in sample 3 and consequently generated higher photocurrent density. A sharp increase in photocurrent density to 14.68 mA cm−2 measured at –0.7 V/SCE for sample 4 can be attributed to increased thickness of the oxide layer synthesized by combination of spray pyrolysis and flame oxidation methods. This is because increased thickness allows absorption of more light to generate more electron-hole pairs (excitons).
2.1.2. Dependence of Photoconversion Efficiency on Applied Potential for CM-n-TiO2 Films
The calculation of total percent photoconversion efficiency () of light energy under white light illumination to chemical energy in the presence of an external applied potential () was carried out using the equation given earlier [1, 4] as where is the photocurrent density (mA cm−2), is the standard reversible potential (which is 1.23 V for water splitting reaction), is the power density of incident light (mW cm−2), is the absolute value of the applied potential which is obtained as  where (versus SCE) is the potential at which the photocurrent density was measured, (versus SCE) is the electrode potential at open circuit condition in the same electrolyte solution under the same illumination intensity at which the photocurrent density was measured. The values of (versus SCE) were found to be −0.847 V, −0.940 V, −0.942 V, −0.915 V for samples 1, 2, 3, and 4, respectively.
The corresponding maximum photoconversion efficiencies (see Figure 5) were found to be 1.2% at 0.447 V bias, 9.08% at 0.240 V bias, 11.31% at 0.242 V bias, and 14.04% at 0.215 V bias for samples 1, 2, 3, and 4, respectively, under illumination intensity of 100 mW cm−2 from a 150 watt Xenon lamp.
2.1.3. Monochromatic Photocurrent Density-Wavelength Dependence for CM-n-TiO2 Films
The plots of monochromatic photocurrent density, , as a function of wavelength of light, , for samples 1, 2, 3, and 4 are shown in Figure 6. The monochromatic photocurrent density, , was measured at measured potentials corresponding to applied potential at which maximum % was observed (see Figure 5) under white light illumination. The values of used were 0.477 V, 0.240 V 0.242 V, and 0.215 V for samples 1, 2, 3, and 4, respectively. Performing integration under the curve, total photocurrent densities of 1.25 mA cm−2, 8.95 mA cm−2, 11.30 mA cm−2, and 13.58 mA cm−2 were found for samples 1, 2, 3, and 4, respectively. These values are a little lower than the corresponding photocurrent density values under white light illumination from the Xenon lamp because of loss of some light passing through the monochromator.
Note that the observed photocurrent in the visible region for the CM-TiO2 samples cannot be due to artifact of a monochromator or due to second harmonic generation because if that would be the case the regular n-TiO2 would have shown photocurrent in the visible region. Figure 6 clearly shows no photocurrent for regular n-TiO2 in the wavelength region between 500 nm 750 nm. Recent combined sputtering and XPS studies showed that carbon incorporation was limited to few angstroms (15 to 20 ) deep. Hence, the amounts of visible light absorption were low as demonstrated by low photocurrent in the visible region between 500 nm to 750 nm (see Figure 6).
2.1.4. Photoconversion Efficiency from Monochromatic Photocurrent Density for CM-n-TiO2 Films
Alternatively, the percent total photoconversion efficiency () of light energy to chemical energy can be obtained by using the following equation [20, 21, 68, 70]: where is wavelength-dependent photocurrent density under monochromatic light illumination (mA cm−2 nm−1) and is the power density of incident wavelength-dependent monochromatic light (mW cm−2 nm−1). Note that the applied potential, , was also found to be wavelength-dependent and can be expressed as where is the electrode potential in volts at open-circuit conditions under monochromatic light illumination. However, = 250 nm for light from Xenon lamp and 300 nm for sunlight were used, and = 1239.85/ is the threshold wavelength corresponding to bandgap energy, , of the sample in electron volt unit.
Note that wavelength-dependent percent photoconversion efficiency, can be expressed as [20, 21], The plot of versus wavelength, , using (10) is given in Figure 7 where total intensity in the denominator of (10) was taken as 100 mW cm−2. Performing the integration (as given in (8)) from = 250 nm (for Xenon lamp white light) to nm or determining the area under the curve in Figure 7 obtained using (10), the total photoconversion efficiencies of 0.94%, 8.86%, 11.16%, and 13.79% were obtained for samples 1, 2, 3, and 4, respectively. Note that represents the maximum wavelength available in sunlight which is 4200 nm. These values are little lower but comparable to 1.2%, 9.08%, 11.31%, and 14.04% (when calculated using (6) under white light illumination of intensity 100 mW cm−2 from a 150 W Xenon lamp) for samples 1, 2, 3, and 4, respectively. These lower values can be attributed to loss of some light through the monochromator as well as the use of average value of instead of using its experimentally determined value at each wavelength, .
2.1.5. Bandgap Energy Determination for n-TiO2 and CM-n-TiO2 Films
The bandgap energy, , can be determined from the frequency of light, , dependent quantum efficiency, , using the following equation [4, 5, 7]: where is a constant, equals either 0.5 for allowed direct transition or 2 for allowed indirect transitions, is the photon energy of frequency . The wavelength, , dependent quantum efficiency, , or incident photon conversion efficiency, , can be expressed as [20, 21] where is the electronic charge and the incident photon flux, and the wavelength-dependent photon flux, , (cm−2 nm−1 s−1) was expressed as [20, 21] where is the Planck’s constant and is the velocity of light.
Figure 8 shows versus plot for samples 1, 2, 3, and 4, respectively.From the intercepts of the straight lines the bandgaps were obtained. It shows two indirect bandgap energies for all CM-n-TiO2 photoelectrodes. The first bandgap energy of 2.65 eV corresponds to lowering of original bandgap (samples 2, 3, and 4) and second bandgap with values of 1.6 eV and 1.4 eV can be attributed to mid-gap band generated by carbon doping as shown in the schematic diagram in Figure 9. Theoretical analysis [65, 66, 70] and experimental findings [20, 21] also showed multiple bandgaps in CM-n-TiO2. Oven-made n-TiO2 showed only one bandgap of 2.9 eV, and as a result, poor photoresponse was obtained. Note that the first bandgap energy values for all CM-n-TiO2 samples were found equal. The slopes of bandgap plots for sample 4 were found to be the highest, which is in accordance with the highest observed photoresponse generated by this sample.
2.1.6. Spectral Distribution of Light for Global AM 1.5 Natural Sunlight (1 Sun) and Xenon Lamp Light (100 mW cm−2)
It is important to compare the spectral distribution of light from both Xenon lamp and sunlight. Figure 10 compares the spectral distribution of these two lights. The integrated value of the intensity from 300 up to 800 nm for global sunlight AM 1.5 (1 sun = 100 mW cm−2 measured by NREL ) was found to be 59.00 mW cm−2 clearly indicating that the contribution from remaining light from 800 nm to 4100 nm present in global sunlight will be 41.00 mW cm−2 [= (100–59) mW cm−2]. Hence, total Xenon lamp light comparable to sunlight (1sun) will be 98 mW cm−2 [= (57 + 41) mW cm−2] where 57.00 mW cm−2 is the integrated value of intensity from 200 nm to 800 nm for Xenon lamp light.
Note that the spectral distribution of Xenon lamp light begins below 250 nm compared to global AM 1.5 sunlight that begins at 300 nm. Furthermore, the spectral distribution of Xenon lamp light is higher in the UV region, but in the visible region, it is lower than that of sunlight. Hence, one will expect comparable photocurrent density and photoconversion efficiencies for CM-n-TiO2 samples for water splitting under actual global sunlight illumination of AM 1.5 (1 sun) and the Xenon lamp illumination of intensity of 100 mW cm−2. However, relatively higher values for photocurrent density and photoconversion efficiency under Xenon lamp illumination will be expected compared to those under sunlight for CM-TiO2 due to higher spectral distribution of the former in the UV region than in the visible region (see Figure 10) and that the absorption coefficient of light for CM-TiO2 in UV region is much higher compared to that in the visible region. Also, only 1.5 to 2.0 nm deep carbon modified region in CM-n-TiO2 could not absorb enough light in the visible region.
2.1.7. Photocurrent Density and Photoconversion Efficiency for Titanium Oxides from Using Illumination Intensity, of Natural Global AM 1.5 Sunlight
Utilizing the values of the wavelength-dependent incident photon conversion efficiency or quantum efficiency of these samples, it is possible to determine the photocurrent density, , under natural global AM 1.5 sunlight illumination as [20, 21] where is the wavelength-dependent photon flux of incident global AM 1.5 sunlight which can be obtained using (13) in which photon power density of sunlight was used from the tabulated data given in National Renewable Energy Laboratory (NREL) website .
The plots of [data obtained from (14)] versus the wavelength of light, , for samples 1–4 are given in Figure 11. The integrated values of total current density under sunlight illumination were found to be 0.85 mA cm−2, 5.89 mA cm−2, 7.71 mA cm−2, and 12.27 mA cm−2 for sample 1, 2, 3, and 4, respectively.
The total photocurrent density, (total), under sunlight illumination can be expressed as [20, 21] The total photoconversion efficiency under natural sunlight illumination can be expressed as [20, 21] where the denominator was taken as 100 mW cm−2 for actual natural global AM 1.5 sunlight (1 sun).
Hence, using (15) and the denominator in (16) as 100 mW cm−2 for global AM 1.5 sunlight, one can express the simplified form of (16) as where = 1.23 volt was used for water-splitting reaction and an average value of the absolute value of applied potential, , was used for all wavelength, values.
Photoconversion efficiencies under sunlight illumination were obtained using (16) or (18). Figure 12 shows the calculated wavelength-dependent percent photoconversion efficiencies under sunlight illumination, , as a function of wavelength of incident light using (17). Performing the integration under the curve from 300 nm to , photoconversion efficiencies of 0.67%, 5.63%, 7.62, and 12.26% were found for samples 1, 2, 3, and 4, respectively. These results indicate that CM-n-TiO2 thin films synthesized by combination of spray pyrolysis and natural gas flame oxidation (sample 4) can be expected to split water with ≥10% even under actual sunlight illumination. The photoconversion efficiency values for these 4 samples under different illumination conditions are summarized in Table 1.
2.1.8. UV-Vis Spectra of n-TiO2 and CM-n-TiO2 Films
The UV-Vis absorbance of n-TiO2 and CM-n-TiO2 photoelectrodes is shown in Figure 13. The absorption spectra of CM-n-TiO2 samples 1, 2, 3, and 4 demonstrate a wide absorption in the UV and visible regions with a tail extending to near infrared region up to 800 nm. This indicates low bandgap energy values for these samples. The reference oven-made n-TiO2 (sample 1) shows absorption only up to 415 nm which corresponds to bandgap energy of 3.0 eV. The lower bandgaps of CM-n-TiO2 samples may be due to carbon incorporation during the flame oxidation of Ti metal sheet. Note that carbon modification also generates oxygen vacancy by partial reduction of TiO2 and this intern may be partially responsible for visible light activity of carbon-modified titanium oxide (CM-n-TiO2). The UV-Vis spectroscopic results show higher absorption in UV as well as in the visible regions for CM-n-TiO2 films synthesized by combination of spray pyrolysis and flame oxidation (sample 3) compared to sample 2. Also, these UV-Vis spectra are consistent with the higher photocurrent densities at CM-n-TiO2 thin-film electrodes prepared by the combination of two methods. Importantly, this observation conforms to the important hypothesis that the spray pyrolysis increased the thickness of the visible light active CM-n-TiO2 films. Consequently, these films absorbed more light.
However, the UV-Vis spectra (see Figure 13) do not match well with the monochromatic photocurrent density-wavelength dependence (see Figure 6). The UV-Vis spectra represent the excitation of electron from lower energy state (to generate holes in this state) to higher energy state by absorption of light. However, to have observed photocurrent, photogenerated holes will have to (1) transport to the interface and (2) undergo transition across the interface to available (thermally distributed) hole acceptor states (e.g., ) in solution. These two extra processes are responsible not having UV-Vis spectra to match with the monochromatic photocurrent density-wavelength-dependent plots.
The electrons that are excited to conduction band from the shallow mid-gap states in CM-n-TiO2 (see Figure 9) by light of around 500 nm reasonable amount of absorption were observed in the UV-Vis spectra but the holes generated in the mid gap band (see Figure 9) were lost due to very low availability of acceptor states of holes in solution (e.g., ion) and consequently generated negligible photocurrent density (see Figure 6). However, absorption in UV-Vis spectra around 500 nm is observed since such absorption does not need availability of acceptor state of holes in solution. Furthermore, the light around 500 nm (2.48 eV) having sub-bandgap energy cannot be absorbed by CM-n-TiO2 to generate electron-hole pairs in its valence band. However, the light around 600 nm to 700 nm can excite electrons from the valence band to mid-gap band states of CM-n-TiO2 (see Figure 9) and these electrons move to the counter electrode. The corresponding holes generated in the valence band undergo transition to available hole acceptor states in solution. Consequently, some photocurrents are observed in this wavelength region (see Figure 6). Furthermore, it should be noted that the carbon modification or doping was confined to surface region (2.0 nm deep) in CM-n-TiO2, the absorption of light in the visible region was found low.
2.1.9. Scanning Electron Microscopy (SEM) of CM-n-TiO2 Films
The surface characterization of a CM-n-TiO2 film (sample 2) was performed by employing a scanning electron microscope (SEM) and the image is shown in Figure 14. The surface looks extremely rough and nanocrystalline nanowalls hence provided much higher surface area to absorb incident light, multiple absorption, and reflection to absorb more light and consequently generated enhanced photoresponse. Generation of multiple electron-hole pairs (excitons) by absorption of single high-energy photon in UV-region in nanocrystals (Ncs) was observed and also explained theoretically in terms of impact ionization [86–92]. Furthermore, the microscopic rough surface could temporarily trap microscopic oxygen gas bubbles on rough surface that may have acted as tiny lens and spherical mirrors that helped to concentrate the incident light  on the electrode surface and hence augmented the photoresponse for water splitting by an order of magnitude.
2.1.10. X-Ray Diffraction (XRD) for Titanium Oxides
X-ray diffraction (XRD) measurements were carried out to characterize the structure of n-TiO2 and CM-n-TiO2 samples. Figure 15 shows the XRD patterns for oven-made n-TiO2 (S1); CM-n-TiO2 synthesized by only thermal flame oxidation (S2) and the CM-n-TiO2 synthesized by spray pyrolysis and thermal flame oxidation (S3).
The peaks of these XRD spectra of both CM-n-TiO2 photoelectrodes (S2 and S3) are consistent with the rutile structure. The XRD results demonstrate that the crystal structure of oven-made n-TiO2 (S1) is of mixture of anatase and rutile forms. It is important to note that some peaks of rutile structure are missing in both CM-n-TiO2 samples (S2 and S3) compared to those in oven-made n-TiO2 (S1). This indicates the marked influence of carbon modification on the structure of n-TiO2.
2.1.11. Energy-Dispersive Spectroscopy (EDS) for CM-n-TiO2 Films
Figure 16 shows the pattern of energy-dispersive spectroscopic (EDS) data for samples 1, 2, 3, and 4. The carbon contents increased significantly from 17.60 atom % for CM-n-TiO2 synthesized by only thermal flame oxidation to 23.23 atom % for CM-n-TiO2 synthesized by combination of spray pyrolysis and thermal flame oxidation (see Table 2 for detail).
2.2. Visible Light Active Hydrogen-Modified n-Type Titanium Oxide (HM-n-TiO2) Thin Films
Frites and Khan  reported a new method for the synthesis of hydrogen-modified (HM)-n-TiO2 thin films by thermal oxidation of Ti metal sheet (Alfa Co. 0.25 mm thick) to TiO2 in an electric oven followed by incorporation of hydrogen in it by electrochemical generation of hydrogen gas on it under cathodic polarization at −1.6 V versus Pt in an alkaline medium at room temperature and also provided its photoelectrochemical behavior during water-splitting reactions.
2.2.1. Photocurrent Density-Potential Dependence for HM-n-TiO2
The photoresponse of the HM-n- TiO2 was evaluated by measuring the rate of water-splitting reaction to hydrogen and oxygen in terms of photocurrent density, . The optimized electric oven-made n-TiO2 and HM-n-TiO2 photoelectrodes showed photocurrent densities of 0.2 mA cm−2 and 1.60 mA cm−2, respectively, at a measured potential of −0.4 V versus Pt at illumination intensity of 100 mWcm−2 from a 150 W Xenon lamp (Figure 17). This indicated an eightfold increase in photocurrent density for HM-n-TiO2 compared to oven-made n-TiO2 at the same measured electrode potential. The bandgap energy of HM-n-TiO2 was found to be 2.7 eV compared to 2.82 eV for electric oven-made n-TiO2 and a mid-gap band at 1.67 eV above the valence band was also observed. The HM-n-TiO2 thin-film photoelectrodes were characterized using photocurrent density under monochromatic light illumination and UV-Vis spectral measurements.
2.2.2. Monochromatic Photocurrent-Wavelength Dependence for HM-n-TiO2
The monochromatic photocurrent density, , was measured under wavelength, , dependent monochromatic light illumination from a Xenon lamp by using a Spectra Physics monochromator (Model 77250) at a measured potential of −0.1 V versus Pt (see Figure 18). Performing integration under the curve in the wavelength range between 250 nm and 775 nm in Figure 18, a total photocurrent density of 2.2 mA/cm2 was observed at HM-n-TiO2 electrode. This value is little higher within the experimental error than the corresponding photocurrent density values of 2.1 mA/cm2 for HM-n-TiO2 samples under white light illumination, at the same measured potential of −0.1 V versus Pt (see Figure 17). The monochromatic photocurrent density versus wavelength plots shows a clear shift of the photosensitivity of the HM-n-TiO2 to the visible region with a second peak around 630 nm compared to oven-made n-TiO2 which shows only one pick in the UV region only (see Figure 18).
2.2.3. Bandgap Energy Determination for HM-n-TiO2
The bandgap of HM-n-TiO2 becomes narrower from 3.0 eV for regular rutile TiO2 to 2.7 eV (Figure 19). Also, a mid-gap band was created within the bandgap of n-TiO2 due to hydrogen reduction; which is located 1.67 eV above the valence band for the hydrogen-modified titanium oxide.
2.3. Carbon-Modified Iron Oxides (CM-n-Fe2O3) Thin Films
Shaban and Khan  synthesized nanocrystalline carbon-modified iron (III) oxide (CM-n-Fe2O3) thin films by flame oxidation of Fe metal sheet at different temperatures (700–850°C) and oxidation time (3–15 min), using a custom-designed large area flame (Knight, model RN. 3.5 xa wc) under controlled oxygen and natural gas flows.
2.3.1. Photocurrent Density-Potential Dependence for Iron Oxides Photoelectrodes
Figure 20 shows the dependence of photocurrent density (, mA cm−2) for water splitting on measured electrode potentials (, V/SCE) at light intensity of 100 mW cm−2, for oven-made-n-Fe2O3 (sample A) and flame-made CM-n-Fe2O3 (sample B). Under illumination of light intensity of 100 mW cm−2, and at measured potential of −0.1 V/SCE, photocurrent density increased from 2.8 mA cm−2 for oven-made n-Fe2O3 to 7.83 mA cm−2 for carbon-modified n-CM-n-Fe2O3 synthesized by thermal flame oxidation. The electrode potentials at open circuit condition under illumination of light, , were found to be –0.498 V/SCE and −0.510 V/SCE for samples A and B, respectively.
2.3.2. Photoconversion Efficiency-Potential Dependence for Iron Oxides Photoelectrodes
Figure 21 shows the photoconversion efficiency, , as a function of applied potential, (V versus ), for samples A and B. Under illumination intensity of 100 m W cm−2, the maximum photoconversion efficiency for water splitting improved significantly from 3.61% (at a minimal applied potential of +0.5 V versus ) for sample A 6.5% (at a minimal applied potential of +0.402 V versus ) for sample B.
2.3.3. Monochromatic Photocurrent-Wavelength Dependence for Iron Oxides
The dependence of monochromatic photocurrent density, , versus the wavelength, , of light for CM-n-Fe2O3 (sample B) is shown in Figure 22. The monochromatic photocurrent density, , at measured potential of 0.0 V/SCE was measured under monochromatic light illumination from a 150 watt Xenon lamp by using a spectra physics monochromator (Model 77250). Performing integration under the curve, total photocurrent densities of 7.2 mA cm−2 were found for CM-n-Fe2O3. This value is less than that under white light illumination (7.83 mA cm−2). This is due to loss of light through the monochromator. It is also due to steady-state measurements of photocurrents under monochromatic light illumination. Note that the photocurrent densities under white light illumination were measured with a scan rate of 50 mV/sec.
2.3.4. Photoconversion Efficiency from Monochromatic Photocurrent Densities at Iron Oxide Electrodes
The plot of versus wavelength, , using (10) for CM-n-Fe2O3 is given in Figure 23 where total intensity in the denominator of (10) was used as 100 mW cm−2. Performing integration in (8) from = 250 nm to = 1240/Eg or determining the area under the curve in Figure 23 obtained using (10), a total photoconversion efficiency of 5.9% is also lower than that under white light illumination (6.5%) for the reasons mentioned in the above conditions.
2.3.5. Bandgap Energy Determination for Iron Oxides
Figure 24 shows versus for samples A and B, respectively. From the intercept of these plots, the bandgaps were found to be 2.3 eV and 1.95 eV for samples A and B, respectively. These bandgap values are in agreement with those obtained from UV-Vis spectra (see Figure 25). Note that is the wavelength, , dependent quantum efficiency as expressed in (12).
2.3.6. UV-Vis Spectra for Iron Oxides
Figure 25 compares the UV-Vis absorbance of flame-made CM-n-Fe2O3 and oven-made n-Fe2O3 photoelectrodes having maximum photoconversion efficiencies of 6.5% and 3.61%, respectively. Both electrodes were synthesized at the same optimum conditions of 850°C for 15 min. The UV-Vis spectroscopic results show a significant increase in the absorbance of light in the UV as well as in the visible regions for flame-made CM-n-Fe2O3 film compared to that of the oven-made n-Fe2O3. The UV-Vis spectra are consistent with the higher photocurrent densities of flame-made CM-n-Fe2O3 thin-film electrodes. The absorption spectra demonstrate a wide absorption in the UV and visible regions with a tail extending up to 600 nm which corresponds to a bandgap energy of 1.95 eV for flame-made CM-n-Fe2O3 and up to 545 nm which corresponds to bandgap energy of 2.3 eV for oven-made n-Fe2O3. The low bandgap energy value for flame-made CM-n-Fe2O3 can be attributed to carbon incorporation during the flame oxidation process of Fe metal sheet.
2.3.7. X-Ray Diffraction (XRD) for Iron Oxides
XRD patterns (Figure 26) show that in addition to Fe2O3, Fe3O4 and FeO are also present in both oven-made and flame-made thin films. Analysis of XRD confirms carbon incorporation in flame-made iron (III) oxide film (sample B) by the presence the peaks for iron carbide compounds in the forms of FeC, Fe2C, Fe3C, and FeCO3. None of these peaks for iron carbide compounds were observed in oven-made iron (III) oxide (sample A). It can be concluded that the flame oxidation process is responsible for carbon incorporation onto iron (III) oxide thin film.
2.3.8. Energy-Dispersive Spectroscopy (EDS) for Iron Oxides
Table 3 shows the EDS results of oven-made n-Fe2O3 (sample A) and flame-made CM-n-Fe2O3 (sample B) photoelectrodes. The presence of carbon in CM-n-Fe2O3 photoelectrode and its absence in the oven-made n-Fe2O3 were observed. The corresponding EDS spectra of oven-made n-Fe2O3 and flame-made CM-n-Fe2O3 samples are shown in Figures 27(a) and 27(b), respectively. Note that the regular oven-made n-Fe2O3 did not show any carbon peak.
2.3.9. Scanning Electron Microscopic (SEM) Images of Iron Oxides
Morphological differences were observed from the scanning electron microscopic (SEM) images (Figure 28) between (a) oven-made n-Fe2O3 film and (b) flame-made CM-n-Fe2O3 film. The surface of oven-made n-Fe2O3 shows nanowires but CM-n-Fe2O3 surface of sample B does not show nanowires even at higher magnification (200 nm compared to 1 micro meter used for oven-made sample a). The nanowires on oven-made n-Fe2O3 films (Figure 28(a)) helped to minimize the recombination rate of photogenerated carriers and consequently generated much higher photoresponse compared to its nanocrystalline thin films synthesized by spray pyrolysis methods earlier (4). The CM-n-Fe2O3 films show only the nanocrystals (Figure 28(b)).
2.4. Visible Light Active Carbon-Modified p-Type Tungsten Oxides (CM)-p-WO3 Thin Film
Shaban and Khan  synthesized carbon-modified p-type tungsten oxide (CM-p-WO3) photoelectrodes by flame oxidation of tungsten metal sheets (Alfa Co. 0.25 mm thick) at several temperatures for different lengths of time.
2.4.1. Photocurrent Density-Potential Dependence for Tungsten Oxides Photoelectrodes
The photocatalytic activity of CM-p-WO3 was evaluated by measuring the rate of water-splitting reaction to hydrogen and oxygen, which is proportional to photocurrent density, . Carbon-modified tungsten oxide photoelectrodes exhibited p-type photoresponse in acidic medium of 0.5 M H2SO4. The highest photocurrent density of 2.08 mA cm−2 was observed under illumination intensity of 80 mW cm−2 from a 150 Watt (Kratos Model LH 150/1) Xenon arc lamp at an optimal thermal oxidation temperature of 900°C for 15 min at applied potential of −0.4 V versus (Figure 29).
2.4.2. Photoconversion Efficiency-Potential Dependence for Tungsten Oxides Photoelectrodes
The optimized CM-p-WO3 photoelectrodes were found to generate the highest photoconversion efficiency of 2.16% for photosplitting of water (Figure 30).
2.4.3. Monochromatic Photocurrent Density-Wavelength Dependencefor CM-p-WO3 Electrodes
The dependence of monochromatic photocurrent density, , versus the wavelength, , of light for CM-p-WO3 thin-film electrodes, having photoconversion efficiency of 2.16% (synthesized by thermal oxidation of W metal at oxidation temperature of 900°C for 15 minutes), is shown in Figure 31.
Significant visible light absorption activity by carbon-modified p-WO3 can be noticed, where a maximum monochromatic photocurrent density value of 3.25 μA cm−2 at 425 nm was observed.
2.4.4. Dependence of Percent Incident Photon Conversion Efficiency () on Wavelength of Light,
Maximum of 42.7% at wavelength of 260 nm can be observed (Figure 32). However, in the visible region, the CM-p-WO3 shows incident photon conversion efficiency percent () in the range between 0.9% at 500 nm and 9.9% at 400 nm. These activities in the visible region can be attributed to carbon modification of WO3 during the flame oxidation process of tungsten metal.
2.4.5. Bandgap Energy
Figure 33 shows the plot for versus for thermally prepared CM-p-WO3 at oxidation temperature of 900°C for 15 min. From the intercept of the straight line, the bandgap is obtained. It can be observed that the bandgap energy became lower from 2.6 eV for regular n-WO3 (to 2.42 eV for of CM-p-WO3). This low bandgap energy can be attributed to carbon incorporation during the flame oxidation process of tungsten metal.
2.4.6. UV-Vis Spectra on CM-p-WO3
The UV-Vis absorption spectra show (Figure 34) a significant improvement in the absorbance of light in UV and visible regions for flame-made CM-p-WO3 films compared to oven-made n-WO3. The absorption spectra of flame-made p-WO3 show higher absorption in the visible region with a tail extending to 512 nm which corresponds to bandgap energy value of 2.42 eV.
The threshold of absorption for oven-made-n-WO3 is at 475 nm (2.6 eV) which is in agreement with a bandgap value of 2.6 eV for n-WO3 [74, 75]. The low bandgap energy value of 2.42 eV for CM-p-WO3 films can be attributed to the carbon incorporation during the flame oxidation process of tungsten metal sheet. This low bandgap energy values for CM-p-WO3 determined by UV-Vis spectra (Figure 34) agree well with the values obtained from the monochromatic photocurrent densities and the corresponding quantum efficiencies (Figure 33).
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