Introduction

The energy economy of the world mainly depends on the fossil fuels such as coal, natural gas, and petroleum products. Gradual depletion of fossil fuels and fast-growing energy demand has necessitated to develop alternative fuels, which should also be pollution-free, storable, and economical [1]. Hydrogen is considered as a promising alternative fuel for the future. It has higher energy content and high heating value compared to other fuels such as methane, methanol, gasoline, and diesel [2]. At present, hydrogen is mainly produced by steam reforming of methane or naphtha. Various alternative processes such as electrolysis, thermolysis, thermochemical reactions, photolysis, photoelectro-chemical, photocatalytic, and biochemical processes are being studied for the production of hydrogen. But only few of these methods are efficient and economically viable [3]. The production of hydrogen from water using solar radiation is one of the potential routes to achieve clean, low-cost, and eco-friendly fuel. Several oxide materials, such as TiO2, Fe2O3, ZnO, ZrO2, K4Nb6O17, K2La2Ti3O10, BaTi4O9, and Ta2O5, have been studied for the photodecomposition of water [4–8]. TiO2 has been reported to be an excellent photocatalyst for water-splitting reaction. Advantages of TiO2 include chemical inertness, photostability, and low cost. However, the activity of TiO2 is limited in UV region because of its wide band gap (3.2 eV). The visible light (λ > 400 nm) constitutes the major fraction of solar spectrum reaching the earth surface. For better utilization of the sunlight, it is essential to develop stable photocatalytic systems with suitable band gap to absorb most of the solar spectrum. In order to develop an efficient titania-based visible light active photocatalyst, various methods have been reported such as doping with metals and nonmetals, dye sensitization as well as coupling with other semiconductors [9]. The dopant impurities create additional energy levels within the band gap of titania acting either as donor or acceptor level. An extensive review has been done on visible light activity of titania by Pelaez et al. [10]. Photocatalytic activities of TiO2 doped with rare earth metals and nonmetals are mostly reported for the degradation of pollutants. Xu et al. [11] investigated various rare earth-doped TiO2 samples for the degradation of nitrite and observed an enhanced photoabsorption in the visible light region. Ce-doped TiO2 materials were also reported to show high activity under visible light irradiation for photocatalytic degradation of dye and phenol derivatives [12–14]. The N–TiO2 [15], and S–TiO2 [16] were studied for the photodegradations of methylene blue under UV/Visible light irradiation, while C–TiO2 [17] was reported to be active for the degradation of phenol. The titania co-doped with rare earth metals and nitrogen are also reported to show an enhanced activity for the decomposition of pollutants [18, 19]. Though doped TiO2 catalysts were extensively investigated for the degradation of pollutants, comparatively fewer studies have been reported for photocatalytic hydrogen production [20–22]. In this study, the photocatalytic activities of TiO2 co-doped with cerium and nonmetals were studied for hydrogen evolution from water–methanol mixture under visible light irradiation. The effect of various nonmetals such as B, C, N, and S on hydrogen production was studied. The physical and optical properties of the photocatalysts affecting the activity were investigated using various characterization techniques such as surface and pore analysis, XRD, UV–Vis, and photoluminescence spectroscopy.

Experimental

Preparation of photocatalyst

Cerium and nonmetal co-doped TiO2 samples were prepared by co-precipitation method. Titanium tetra isopropoxide (TTIP) and cerium nitrate hexahydrate were used as precursors for titania and ceria, respectively, and were obtained from Sigma-Aldrich. The boric acid, glucose, urea, and thiourea were used as sources for boron, carbon, nitrogen, and sulfur, respectively, and were procured from Merck. The isopropanol and ammonia solution were also procured from Merck.

Requisite amount of TTIP was mixed with isopropanol under continuous stirring condition for 10 min to form solution A. Solution B was prepared by dissolving the required amount of cerium nitrate hexahydrate and respective precursors of nonmetals in deionized water. The loadings in co-doped samples for Ce and nonmetals were 2.5 and 1 mol% of titanium, respectively. The loadings of Ce (x) and nonmetals (y) are defined as

$$x = \frac{{{\text{Ce}}({\text{mol}})}}{{{\text{Ti}}({\text{mol}})}} \times 100 = 2.5\;\text{mol} \% \quad \text{and} \quad y = \frac{{{\text{NM}}({\text{mol}})}}{{{\text{Ti}}({\text{mol}})}} \times 100 = 1\;\text{mol} \% .$$

Aqueous ammonia (25 wt%) and solution B were simultaneously added dropwise to the solution A with continuous stirring. Addition of ammonia was continued until complete precipitation occurred. The resultant mixture was stirred for 2 h at room temperature and was placed in a water bath at 363 K for overnight. The precipitate was filtered and dried in an oven at 373 K for 24 h. The as-prepared samples were calcined at 723 K for 3 h. In text, nonmetal co-doped samples are referred as Ce–NM–TiO2 (NM = B, C, N, S). For reference, cerium-doped titania were prepared by dissolving required amount of cerium nitrate hexahydrate in deionized water (solution B). The amount of Ce was 2.5 mol% of Ti, and sample is represented as Ce–TiO2 in text. The solution B was added to solution A along with aqueous ammonia as described earlier to obtain cerium-doped titania samples. Similar aging condition and calcination temperature were maintained for the preparation of Ce–TiO2 sample. Undoped titania was also prepared for reference by similar procedure without the addition of cerium or nonmetals and represented by TiO2 in text.

Characterization of photocatalyst

The surface areas of the samples were determined from N2 isotherms collected at 77 K using a Beckman Coulter SA™ 3100 analyzer. Prior to the experiments, the samples were degassed at 423 K for a period of 2 h. The pore size distribution of the photocatalysts was determined by Barrett–Joyner–Halenda (BJH) method. The X-ray diffraction (XRD) patterns were recorded on a Bruker D2 phaser X-ray diffractometer using Ni-filtered Cu Kα as radiation source (λ = 1.5406 Å) in the 2θ range from 20 to 80° at a scanning rate of 1°/min. A beam voltage and beam current of 40 kV and 40 mA were used, respectively. The diffuse reflectance UV–vis spectra of the photocatalysts were measured on a PerkinElmer Lambda 750 instrument with a 60-mm labsphere specular reflectance accessory at room temperature. The baseline correction was done using a calibrated sample of barium sulfate as reference. The scan parameter was set with slit size 2 nm and scan in the spectral range of 250–650 nm. The photoluminescence (PL) measurements were taken in a Thermo Spectronic (Aminco Bowman Series 2) instrument with a Xe lamp as the excitation source at room temperature. The powders were dispersed in ethanol, and the emission spectra were collected at an excitation wavelength of 325 nm. The same quantity was used for recording the PL spectra of all the samples. The entrance and exit slit widths were fixed as same for all the measurements. Field emission scanning electron microscopic analysis was done using instrument of ZEISS-Sigma. For FESEM analysis, the samples were dispersed in a solvent and deposited in an aluminum foil that was mounted on a sample holder for gold coating. The composition of the samples was determined by EDS analysis (energy-dispersive X-ray spectroscopy) equipped with SEM instrument (LEO 1430 VP).

Photocatalytic activity studies

The activities of the photocatalysts were examined using the experimental setup shown in Fig. 1. The reaction was typically carried out by adding 0.2 g catalyst to solution of water (25 ml) and methanol (1 ml) under stirred condition. Prior to irradiation, the reaction mixture was de-aerated with N2 gas (50 ml/min) for 30 min to completely remove the dissolved oxygen. Then, the reaction mixture was irradiated with a 500-W tungsten halogen lamp (Halonix, India), placed approximately 15 cm away from the reactor, as source of visible light. The emission spectrum of the lamp was measured in front of the reactor using Ocean optics USB4000 spectrometer as shown in Figure S1 in the supplementary information. The temperature of the reaction mixture was maintained at 307 K using a water circulation bath, which also acted as an IR filter. The evolved gas was collected in an inverted burette by water displacement method. The gas mixture was analyzed using a gas chromatograph (Varian, CP 3800) equipped with carbosieve SII column and thermal conductivity detector. After completion of the photocatalytic reaction, the liquid solution was analyzed for the detection of intermediates (formaldehyde and formic acid) by HPLC (SHIMADZU, C18 column). To confirm the photocatalytic activity of the catalysts, experiments were conducted first in the absence of light with catalysts and next in the presence of light without catalyst. In both cases, no hydrogen evolution was observed. The hydrogen evolution was observed only when the reaction was carried out in the presence of both light and catalyst. In all cases, same feed mixture was used. This confirmed the photocatalytic activity of the catalysts.

Fig. 1
figure 1

Schematic representation of experimental setup for photocatalytic water splitting

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Results and discussion

Characterization of prepared catalysts

The actual composition of the prepared photocatalysts was determined by EDS, and the results are shown in Table S1 of supplementary information. The actual cerium content in samples varied in the range of 2.39–2.49 mol%, and nonmetal content varied in the range of 0.8–0.97 mol% (Table S2 of supplementary information). Thus, the actual composition agreed well with the intended composition of the photocatalysts within experimental error.

The surface area and pore volumes of undoped TiO2, Ce–TiO2, Ce–NM–TiO2 (NM = B, C, N, S) samples are shown in Table 1. Surface area and pore volume of titania decreased with the addition of ceria. Structure modification and partial pore blockage in the presence of cerium may be responsible for decrease in surface area and pore volume in cerium-doped samples. The isotherm of Ce–TiO2 and corresponding pore size distribution are shown in Fig. 2a. The Ce–TiO2 sample exhibited a type IV isotherm with a H1-type hysteresis loop associated with open-ended cylindrical pores. The pore size distribution of Ce–TiO2 showed the presence of both mesopores and macropores with pore diameter in the range of 3–148 nm. Co-doping of titania with nonmetals along with cerium resulted in increase in surface area as shown in Table 1. The co-doped Ce–N–TiO2, Ce–C–TiO2, and Ce–B–TiO2 exhibited Type II isotherm with H3 hysteresis loop corresponding to slit-shaped pores (Fig. 2b). For sulfur co-doped sample, H1 hysteresis loop was observed, which indicated the presence of open cylindrical pores. Figure 2b shows that the pore size distribution varied significantly with nonmetals. For Ce–N–TiO2 and Ce–B–TiO2, narrow pore distribution was obtained, with 39 and 47 % pores being below 10 nm, respectively. The pore size distributions of Ce–C–TiO2 and Ce–S–TiO2 co-doped samples were much broader. The presence of significant amount of pores in the range of 20–50 nm may have contributed to their lower surface area. The surface area and pore size distribution results showed that the presence of nonmetal had significant effect on the stabilization of porous network. The maximum effect was observed for Ce–N–TiO2 with highest surface area.

Table1 Physicochemical and crystalline properties of undoped TiO2, Ce–TiO2, and Ce–NM–TiO2 (NM = B, C, N, S) samples
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Fig. 2
figure 2

N2 adsorption–desorption isotherms of Ce–TiO2 and Ce–NM–TiO2 (NM = B, C, N, S) samples (inset pore size distribution)

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The X-ray diffraction patterns of Ce–TiO2 and Ce–NM–TiO2 are shown in Fig. 3. All the samples, except Ce–N–TiO2, were dominated by anatase phase (2θ = 25.5°, 37°, 48°, 53.8°, 55.1°, 64°, JCPDS 21-1272). For Ce–N–TiO2, significant amount of rutile phase (2θ = 28°, 36.1°, 41.5°, JCPDS 21-1276) was observed. In general, the phase transformation of anatase to rutile occurs at temperature above 873 K and anatase phase is expected to be dominant in samples calcined at lower temperature of 723 K, used in this study. Accordingly, anatase phase was dominant in XRD profiles of all samples except Ce–N–TiO2. For Ce–N–TiO2 sample, the presence of significant amount of rutile phase suggests that nitrogen can catalyzes the anatase–rutile phase transformation significantly at lower temperature [23]. In addition, Ce–S–TiO2 co-doped sample was more amorphous in nature as observed from broad XRD peaks of low intensity compared to that of other nonmetal-doped samples. No characteristic peaks of CeO2 phases were observed in the XRD profiles, which suggest that the ceria was well dispersed in TiO2 matrix or were below detection limit. In addition, no peaks due to any Ce–Ti mixed oxides were observed in any of the samples.

Fig. 3
figure 3

XRD patterns of TiO2, Ce–TiO2, and Ce–NM–TiO2 (NM = B, C, N, S) samples

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On addition of Ce to TiO2, cerium ions can substitute Ti4+ sites or can occupy the interstitial sites with Ce–O–Ti linkages [24]. But this is expected to be associated with distortion in titania lattice due to larger size of Ce4+ (IR = 0.101 nm) and Ce3+ (IR = 0.111 nm) compared to Ti4+ (IR = 0.068 nm) ions. The lattice distortion values are shown in Table 1 and correspond to maximum strain observed in crystal lattice due to distortion by incorporation of dopant metals. The lattice distortion was calculated using the formula defined by Stokes and Wilson as ε = β/(4 tan θ), where β is the full width at half maximum of diffracted peaks and θ is the Bragg angle of the [h k l] reflection [25]. Maximum value of lattice distortion was obtained, as expected for Ce–S–TiO2 sample, since sulfur has largest ionic radii among the nonmetals used. The corresponding change in lattice parameters for tetragonal titania lattice is also included in Table 1. With the addition of Ce in TiO2, due to the lattice distortion, the lattice parameters increased compared to that of undoped titania. Co-addition of nonmetals resulted in further increase in lattice parameter values.

The FESEM images of undoped TiO2, Ce–TiO2, Ce–NM–TiO2 are shown in Fig. 4. The images showed a definite change in morphology of the samples when cerium and nitrogen were added to titania suggesting that the dopants played a prominent role in the development of material structure during co-precipitation. This resulted in different physical properties as was observed in Table 1. Particles of irregular shape and size were observed for undoped titania, whereas ceria-doped titania, prepared by co-precipitation, consisted of particles of regular size and shape. The particles were spherical in the range of 100–200 nm. The morphology again changed when titania was co-doped with cerium and nonmetals. All the co-doped Ce–NM–TiO2 samples were agglomerated in nature.

Fig. 4
figure 4

FESEM images of samples a TiO2, b Ce–TiO2, c Ce–N–TiO2, d Ce–S–TiO2, e Ce–B–TiO2, and f Ce–C–TiO2

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The UV–Visible absorption spectra of TiO2, Ce–TiO2, and Ce–NM–TiO2 are shown in Fig. 5. The Ce–TiO2 sample showed intense absorption bands in the visible light region ranging from 400 to 500 nm. The band gap energies of the samples were determined by extrapolating the rising part of the onset of the absorption edge to the x-axis (λ, nm). The values of λ was then used in the Planck’s Einstein equation to calculate the band gap, Eg = hc/λ, where Eg is band gap energy in eV, h is Planck’s constant in eV. s, c is the speed of light in m/s, and λ is the absorption wavelength in nm [26]. The calculated band gap energy values for undoped TiO2 and Ce–TiO2 were 3.11, and 2.81, respectively. The decreased band gap energy values of cerium-doped sample indicated the formation of new energy levels within the TiO2 band gap and resulted in redshift of the absorption edge. This shift can be attributed to the incorporation of Ce 4f levels into the TiO2 crystal structure just below the conduction band of TiO2 and thereby reducing the effective band gap [27]. Addition of nonmetal to cerium-doped TiO2 samples resulted in further shift in the band gap energy to longer wavelength. Significant reductions in the optical band gap energies from 3.11 eV for undoped TiO2 to about 2.88 eV for Ce–B–TiO2, 2.66 eV for Ce–C–TiO2, 2.29 eV for Ce–N–TiO2, and 2.24 eV for Ce–S–TiO2 were observed for co-doped samples. For the co-doped catalysts, the band gap energy was affected by the formation of additional impurity states within titania matrix by the interaction of p-orbitals of the nonmetal dopants (B, C, N, S) and 2p-orbitals of oxygen [28–31]. The relative position of the new states with respect to valence band of titania depends on the properties of dopants and its position within the matrix. Whether the nonmetal dopant would occupy anionic or cationic substitutional sites or interstitial site depends upon its size, chemical valence and electronegativity compared that of the host TiO2. The probability that a dopant will substitute oxygen depends on its electronegativity [32]. As difference in electronegativity values of dopant and oxygen becomes smaller, the probability that dopant will substitute oxygen becomes higher. Substitutional dopants form the new band states at higher energy level compared to interstitial dopant [33]. For boron co-doped catalyst, it is difficult to replace Ti4+ sites by B3+ due to the smaller ionic radius of B3+ (IR = 0.023 nm) compared to Ti4+ (IR = 0.068 nm). Therefore, boron may replace either an oxygen atom or incorporate in the interstitial position in TiO2 matrix. But due to the lower electronegativity of boron, its probability of occupying the interstitial sites is higher. Similarly, carbon can also substitute oxygen or occupy an interstitial site. In nitrogen co-doped sample, the substitution of oxygen sites is most probable due to their closer electronegativity values [34]. Substitutions of S in both anionic and cationic sites of TiO2 have been reported in the literature [35, 36]. However, anionic sulfur substitution by S2− (IR = 0.17 nm) might be difficult because of its larger ionic radius than O2− (IR = 0.122 nm). But the cationic substitution of Ti4+ (IR = 0.068 nm) by S6+ (IR = 0.029 nm) would be more favorable due to smaller size of latter. When TiO2 is doped with sulfur, either the mixing of S 3p states with valence band O 2p states or the formation of isolated p-orbitals of S above the valence band maximum of TiO2 contributes toward narrowing the band gap [37].

Fig. 5
figure 5

UV–Visible diffuse reflectance spectra of TiO2, Ce–TiO2, and Ce–NM–TiO2 (NM = B, C, N, S) samples

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The radiative recombination of electron–hole pairs in photocatalysts can be studied by the photoluminescence (PL) emission spectra. Figure 6 shows the room-temperature photoluminescence spectra of the TiO2, Ce–TiO2, and Ce–NM–TiO2 samples in the range of 350–600 nm. For all the samples, six emission peaks appeared at 360, 396, 450, 468, 481, and 492 nm wavelengths and the corresponding transition energy are 3.44, 3.13, 2.76, 2.65, 2.8, and 2.52 eV, respectively. In the literature, the peak in the range of 360–370 nm has been assigned to either anatase particle <10 nm [38] or to self-trapped excitons localized in TiO6 octahedral [39]. In this study, the emission peak at 360 nm can be ascribed to the latter as particle is much larger than 10 nm. The emission signal at 396 nm (3.13 eV) corresponds to the band gap transition of the anatase [38]. This agrees well with the band gap energy of titania calculated from UV–Vis spectra. The excitonic emissions in titania result due to the presence of defects and oxygen vacancies. The bands at 450 and 468 nm can be attributed to free excitons, and emission peaks at longer wavelength of 481 and 492 nm can be attributed to the bound excitons [40, 41]. The bound excitons result due to the presence of oxygen vacancies [40, 42, 43]. It can be observed from the Fig. 6 that Ce–TiO2 had lower PL intensity compared to TiO2. In Ce–TiO2, cerium dopant-induced energy level in TiO2 band structure, as discussed earlier, serves as an electron trap as represented by Eq. (1) resulting in non-radiative recombinations.

Fig. 6
figure 6

Photoluminescence spectra of TiO2, Ce–TiO2, and Ce–NM–TiO2 (NM = B, C, N, S) samples

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$${\text{Ce}}^{4 + } + {\text{e}}^{ - } \to {\text{Ce}}^{3 + }$$
(1)

This reduces the rate of radiative recombination of photogenerated electrons with holes in valence band of titania as shown in Fig. 6. The PL emission intensities of the cerium and nonmetal co-doped samples decreased in the order of TiO2 > Ce–TiO2 > Ce–B–TiO2 > Ce–C–TiO2 > Ce–N–TiO2 > Ce–S–TiO2. In the cerium and nonmetal co-doped samples, the oxygen vacancies generated by substituting nonmetal dopants can act as additional effective traps for photo-induced electrons, thereby reducing the radiative recombination rate of charge carriers further. The photogenerated electrons are trapped by oxygen vacancies by non-radiative combinations [42]. The decreasing emission intensity order can be attributed to increased oxygen vacancies resulted by increasing substitution effects from B to N as discussed in earlier section. In sulfur co-doped samples, the presence of isolated cationic S6+ dopant state may further act as surface trap states for photogenerated electrons, thereby resulting in a significant decrease in the recombination rate and lowering the PL emission.

Photocatalytic activity studies

The photocatalytic hydrogen production from water–methanol mixture for TiO2, Ce–TiO2, and Ce–NM–TiO2 (NM = B, C, N, S) catalysts is shown in Fig. 7. The activity of undoped TiO2 was very low but Ce–TiO2 showed significant photocatalytic activity and high hydrogen evolution. The high activity of cerium-doped TiO2 can be attributed to their ability of absorption of light in visible light region as shown in Fig. 5. Addition of nonmetals such as B, C, and N to Ce–TiO2 samples further enhanced hydrogen evolution activity. This may be attributed to higher surface area, enhanced visible light absorption as well as lower recombination rate of electron–hole pairs in co-doped samples. The highest activity of Ce–N–TiO2 among co-doped samples may be attributed to its higher surface area, higher visible light absorption, and lower recombination rate of electron–hole pairs compared to other nonmetals, B and C, doped samples. The activity of Ce–C–TiO2 should be higher than Ce–B–TiO2 due to higher absorption of visible light and lower recombination of photogenerated electron–hole pairs for former as shown by absorption spectra (Fig. 5) and PL spectra (Fig. 6), respectively. But slightly higher hydrogen evolution observed for Ce–B–TiO2 compared to that of Ce–C–TiO2 may be attributed to higher surface area of former providing more active sites. Much lower hydrogen production was observed for Ce–S–TiO2 sample, although it has enhanced absorption in the visible light region and efficient charge separation. The lower activity of Ce–S–TiO2 can be attributed to its very low surface area. Moreover, the S may be in isolated state and that may lead to lower mobility of the holes and limits the number of charge carriers reaching the catalyst surface. This may have also resulted in lower photocatalytic activity in S-doped samples.

Fig. 7
figure 7

Hydrogen evolution over TiO2, Ce–TiO2, and Ce–NM–TiO2 (NM = B, C, N, S) samples

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The apparent quantum efficiency of the photocatalysts was calculated using the method followed by Sasikala et al. [44]. The apparent quantum efficiency values of TiO2, Ce–TiO2, Ce–B–TiO2, Ce–C–TiO2, Ce–N–TiO2, and Ce–S–TiO2 were 1.6, 6.7, 10.2, 9.2, 17.2, and 2.2 %, respectively. The calculated values agreed with the reported apparent quantum efficiency range of 2–22 % for different photocatalysts [45]. For comparison, the photocatalytic activity of N–TiO2 was also measured. The apparent quantum efficiency of N–TiO2 (8.6 %) was slightly higher than that Ce–TiO2 (6.7 %) but the efficiency increased significantly for co-doped Ce–N–TiO2 to 17.2, suggesting that both doping components were contributory to photocatalytic activity, as discussed earlier, giving higher hydrogen production. The results are shown in Supplementary Figure S2.

The durability test of Ce–N–TiO2 for photocatalytic H2 production from water–methanol mixture was performed for 10 h irradiation period with hydrogen evolution being recorded at every 1 h as shown in Figure S3 of supplementary information. The total hydrogen evolution of 737 μmol was obtained after 10 h of irradiation time. The observed results showed that the Ce–N–TiO2 had high H2 evolution rate as well as desired durability.

Based on the observed band gap energies of co-doped samples, the position of the impurity states and probable transitions of electrons are schematically shown in Fig. 8. In Ce–TiO2, the photogenerated electrons are excited from the valence band to extended conduction band where electrons are trapped effectively by Ce4+/Ce3+ couple. This results in the accumulation of electrons in extended conduction band, which can serve as hydrogen formation site. On addition of nonmetals to cerium-doped titania, the excitation of electrons can also takes place from nonmetal-induced impurity levels resulting in the absorption of visible light. The reduction and oxidation reactions occurring in conduction band and valance band, respectively, can be represented as follows [46, 47].

Fig. 8
figure 8

Schematic representation of hydrogen production over cerium and nonmetal co-doped TiO2 photocatalysts

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$${\text{Photocatalyst}} + h\nu \to {\text{e}}^{ - } + h^{ + }$$
(2)

Reduction reaction

$$2{\text{e}}_{\text{cb}}^{ - } + 2{\text{H}}^{ + } \to {\text{H}}_{2}$$
(3)

Oxidation reactions:

$${\text{CH}}_{3} {\text{OH}} + 2h_{vb}^{ + } \to 2{\text{H}}^{ + } + {\text{HCHO}}$$
(4)
$${\text{HCHO}} + {\text{H}}_{2} {\text{O}} \to {\text{HCO}}_{2} {\text{H}} + {\text{H}}_{2}$$
(5)
$${\text{HCO}}_{2} {\text{H}} + h_{vb}^{ + } \to {\text{CO}}_{2} + {\text{H}}^{ + }$$
(6)

In addition to proposed hole-mediated path as shown in the above reaction scheme, methanol can also enhances hydrogen generation by reacting with hydroxyl radicals generated from photocatalytic water splitting [48]. The oxidation of methanol results in the formation of HCHO, HCOOH, and CO2. For oxidation of formaldehyde to formic acid, the extra oxygen atom is provided by water through hydroxyl radical. Intermediate products were only detected in liquid phase, and no intermediate products were detected in gaseous phase in the present study.

Conclusions

Cerium and nonmetal co-doped samples synthesized by co-precipitation method were found to be potential photocatalysts for hydrogen generation under visible light radiation. Doping of titania with cerium induced distinct redshift of the absorption edge in visible light region. Nonmetal modification further enhanced the visible light absorption and separation efficiency of photogenerated electron–hole pairs. The effective band gap energies of co-doped samples depended on the properties of nonmetals. The surface area and phase distribution were also modified in the presence of nonmetal. Highest hydrogen evolution rate of 206 μmol/h was observed for cerium–nitrogen co-doped sample. The higher photoactivty of Ce–N–TiO2 can be attributed to higher surface area, higher light absorption, and efficient charge separation.