- Email: [email protected]
TiO2 nanocomposite as effective photocatalyst for water splitting
Accepted Manuscript N-doped Ni/C/TiO2 nanocomposite as effective photocatalyst for water splitting Nasser A.M. Barakat, Enas Ahmed, Mohamed T. Amen, Mohammad Ali Abdelkareem, A.A. Farghali PII: DOI: Reference:
S0167-577X(17)31361-7 http://dx.doi.org/10.1016/j.matlet.2017.09.009 MLBLUE 23123
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
4 July 2017 17 August 2017 5 September 2017
Please cite this article as: N.A.M. Barakat, E. Ahmed, M.T. Amen, M.A. Abdelkareem, A.A. Farghali, N-doped Ni/ C/TiO2 nanocomposite as effective photocatalyst for water splitting, Materials Letters (2017), doi: http://dx.doi.org/ 10.1016/j.matlet.2017.09.009
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
N-doped Ni/C/TiO2 nanocomposite as effective photocatalyst for water splitting Nasser A. M. Barakat1,2,*, Enas Ahmed3, Mohamed T. Amen1, Mohammad Ali Abdelkareem2,4, A.A. Farghali5 1
Organic Materials and Fiber Engineering Dept., and Bionanosytem Engineering Dept., Chonbuk National University, Jeonju 561-756, Republic of Korea. 2 Chemical Engineering Department, Minia University, El-Minia, Egypt. 3 Renewable Energy Science and Engineering Department, Faculty of Postgraduate Studies for Advanced Sciences, Beni-Suef University, Egypt. 4 Department of Sustainable and Renewable Energy Engineering, University of Sharjah, PO Box 27272, Sharjah, United Arab Emirates 5 Materials Science and Nanotechnology Department, Faculty of Postgraduate Studies for Advanced Sciences, Beni-Suef University, Egypt.
Corresponding
author:
Nasser
A. M.
Barakat,
[email protected], Tel:+82632702363
Fax:
+82632704248
Abstract: N-doped Ni/C/TiO2 nanocomposite is introduced as an effective photocatalyst for water splitting under visible light radiation. The proposed photocatalyst contains most of the effective co-catalysts enhancing the photocatalytic activity of the titanium oxide. The nanocomposite was prepared by sintering a vacuously dried sol-gel composed of polyvinylpyrrolidone, titanium isopropoxide and nickel acetate under nitrogen atmosphere at 700 oC for 3h. TEM, XRD and XPS analyses confirmed that the introduced catalyst is N-doped & TiO2-incorporated amorphous carbon sheets decorated by Ni nanoparticles. The introduced nanocomposite revealed distinct performance as photocatalyst toward water splitting reaction, numerically the generated hydrogen was 0.383 mmol.sec-1.gcat-1 ; 8.6 ml.sec-1.gcat-1. However, nickel content somewhat influences the catalytic performance; 15 wt% reveals the best performance. Keywords: Nanocomposites; Carbon materials; Water photosplitting
1
1. Introduction Environmentally and practically, hydrogen is the most convenient energy source to replace the fusel fuels after the expected near depletion. Accordingly, several techniques have been introduced to generate the hydrogen including fermentation of waste biomasses[1, 2], water electrolysis [3], water photosplitting [4], thermal production [5], etc. However, water photosplitting has attracted the most attention due to simplicity, high yield, and economical and environmental aspects. Moreover, if an efficient photocatalyst working under solar irradiation is utilized, the process can be considered a renewable energy technology [6, 7]. Water photosplitting is a challenging reaction because it is one of the most important reactions for solving energy and environmental problems. Overall, water splitting into H2 and O2 is accompanied by a largely positive change in the Gibbs free energy (∆GO = 237 kJ/mol) and is an up-hill reaction. In other words, a special characteristics catalyst is required to store the solar radiation in the form of chemical energy; hydrogen. Semiconductors having convenient band gap energies can be exploited as photocatalysts for water splitting. Typically, when light with energy larger than the band gap is incident on the catalyst, holes and electrons are generated in the valence and conduction bands, respectively. The photogenerated holes and electrons lead to form redox reactions similar to electrolysis which results in reduction of the water molecules by the electrons to form H2 and oxidation by the holes to form O2 [8]. Titanium oxide is commonly exploited as photocatalyst for water splitting under the UV irradiation. Besides the low hydrogen evolution, working under UV constraints the wide applications. Moreover, the very fast electrons/holes recombination phenomenon is a big dilemma facing the photocatalytic activity of the titania-based materials.
2
Accordingly, the
researchers tried to enhance the photocatalytic performance using doping by different materials. For instance, carbon-doped TiO2 shows good contribution in maximizing the photocleavage of water under white-light irradiation [9, 10]. Moreover, some reports indicated that nitrogen doping enhances the adsorption capacity of the active species as well as improves the photo characteristics under the visible light irradiation which consequently results in relatively high hydrogen evolution rate [4, 11, 12]. Incorporation of metals nanoparticles distinctly remedies the problem of the electrons/holes recombination problem due to collecting the excited electrons on the highly conductive metal nanoparticles which act as electron sinks [13, 14]. In this study, for the first time, a novel composite including titanium oxide with most of the effective co-catalysts is introduced. Typically, N-doped Ni/C/TiO2 nanocomposite could be prepared by one-pot procedure. The synthesized photocatalyst showed good performance toward water photosplitting reaction. 2. Experimental work Polyvinylpyrrolidone (PVP, Sigma) was used as a precursor of carbon and nitrogen while titanium isopropoxide (Ti(Iso), Sigma) and nickel acetate (NiAc, Alfa Aesar) were utilized as precursors for TiO2 and nickel, respectively. Absolute ethanol was exploited as a solvent. First, 3 g of PVP was dissolved in 12 g methanol overnight until getting clear viscous polymer solution. In another flask, 2 ml of Ti(Iso) were added to 4 ml ethanol and 4 ml acetic acid, the acid was added dropwisely until the solution became clear and homogeneous. To get composites having different content of Ni nanoparticles with respect to the Ti(Iso), 0.1, 0.2, 0.3 and 0.4 g of NiAc were dissolved individually in 3 ml ethanol. Finally, each NiAc solution was mixed with similar polymer and Ti(Iso)
3
solutions and vigorously stirred for 1h to get final sol-gels having 5, 10, 15 and 20% NiAc with respect to Ti(Iso). Later on, the prepared gels were dried under vacuum at 60 o
C for 1 day and sintered under nitrogen atmosphere at 700 oC for 3 h. Water
photosplitting experiments have been conducted under 2000 W mercury lamp. 50 mg of the catalyst was added to 25/75 ml methanol/water mixture. The hydrogen evaluation was estimated by collecting the evolved gases over water surface. Briefly, the catalyst and methanol/water solution was placed in a conical flask with aside opening connected by a rubber tube to a graduated cylinder filled by water. The number of moles of the produced hydrogen was estimated based on ideal equation of state at standard conditions. Results and discussion As it was aforementioned, several organic and inorganic precursors have been utilized to prepare the proposed composites. Due to calcination under an inert atmosphere, it is expected that the utilized polymer will be partially graphitized while, because of the very high activity of the titanium, the Ti(Iso) will be decomposed to a stable oxide form; TiO2. On the other hand, based on ours and others studies, sintering of NiAc under an inert atmosphere results in producing the pristine metal rather than the expected oxide form due to evolution of strong reducing gases (mainly CO and H2) [15-17]. To properly verify the aforementioned hypotheses, XRD analysis has been invoked; Fig. 1. As shown in the obtained pattern, the high intensity and wide peak at 2ϴ value of 25.29o corresponding to (101) crystal plane indicates formation of small grain size anatase (PDF# 21-1272). Based on Scherrer’s equation, the estimated grain sizes of the anatase in the prepared composites were 4.04, 4.96, 1.82 and 1.31 nm for the composites obtained from
4
precursors having 5, 10, 15 and 20 wt% NiAc, respectively. It is noteworthy mentioning that the observed peak cannot be assigned to graphite or rutile phase TiO2 which are identified by their main peaks at 26.6o (PDF# 26-1079) and 27.5o (PDF# 21-1276), respectively. It is noteworthy mentioning that detection of anatse rather than the expected rutile phase at this high calcination temperature can be attributed to presence of nickel which catalyze formation of this phase. Formation of pristine nickel can be claimed due to appearance of the representative peaks at 44.5o and 51.8o corresponding to (111) and (200) crystal plans, respectively; PDF# 04-850. The mechanism of pristine nickel formation has been explained in details in our previous studies [18, 19]. Fig. 2A displays normal TEM image for the obtained composite. Based on XRD results, the black nanoparticles attached the formed sheets can be assigned to nickel. On the other hand, as shown in Fig. 2B, anatase can be detected as small nanoparticles incorporated inside an amorphous graphite sheet matrix; the red arrows point to anatase. Theoretically, carbon and nitrogen contents in the polyvinylpyrrolidone ((C6H9NO)n) are 64.8 and 12.6 wt%, respectively[20, 21]. Accordingly, sintering of polymer under inert atmosphere might result in partial graphitization. Considering the comparatively low oxygen content, 14.4 wt%, it is expected that some nitrogen can be captured inside the formed graphite. TEM image (Fig. 2B) indicates amorphous material in the formed Ni-decorated sheets, this material can be assigned to carbon. To properly prove this claim, XPS analyses was invoked. As shown in Fig. 3, big amount of carbon (C1s peak) could be detected. Moreover, appearance the characterized N1s nitrogen peak at around 409 eV confirms presence of nitrogen. The quantitative results indicated that the investigated sample contains 4.78 and 77.42 At% nitrogen and carbon, respectively.
5
It is noteworthy mentioning that the detected nitrogen in the final composite is small compared to the amount presence in the utilized polymer which can be attributed to formation of some nitrous gases during the calcination process which results in losing some nitrogen. Accordingly, based on the invoked characterization techniques, it is safe to claim that the produced material is Ni-decorated & N-doped C/TiO2 nanocomposite. Fig. 4 displays the hydrogen evolution rate using the prepared composite. As shown, almost linear hydrogen generation with respect to the time is obtained. Moreover, the obtained hydrogen generation rate is relatively high compared to several reports[22]. Numerically, the hydrogen production rate is 0.33913, 0.37393, 0.38282 and 0.38309 mmol hydrogen (at STP)/sec.gcat. However, as shown the sample obtained from 15 wt% NiAc revealed slightly better results. As aforementioned, every co-catalyst used in the newly introduced composite showed distinct role in enhancement of the photocatalytic activity of TiO2 toward water splitting reaction. The slight advantage of the 15 wt% sample can be attributed to Ni/TiO2 ratio besides the distribution of the metal nanoparticles on the carbon support. However, as it was expected and proved in the organic pollutants photo degradation reactions [23] but, based to our best knowledge, was not discussed before in case of water splitting, the metallic nanoparticles have good contribution in the process enhancement. In other words, nickel nanoparticles attracted the exited electrons and inhibit the electrons/holes recombination process which results in improvement the water splitting process. Fig. 5 displays schematic diagram for the water splitting mechanism over the introduced nanocomposite. Conclusion
6
Calcination of a sol-gel composed of polyvinylpyrrolidone, titanium isopropoxide and nickel acetate under nitrogen atmosphere leads to produce N-doped & Ni-decorated & TiO2-incoporated amorphous graphite sheets. The prepared nanocomposite can be utilized as effective photocatalyst for water splitting reaction. Overall, nickel content has a slight impact in the catalyst performance however 15 wt% reveals the best performance. Acknowledgement The authors would like to thank Mr. Mohamed N. A. Barakat for his efforts in doing some experimental work.
Fig. 1 XRD patterns for the prepared samples.
Fig. 2 Normal; (A) and high resolution; (B) TEM images for the 15 % Ni sample.
7
Fig. 3 XPS analysis spectrum for the 15 % Ni sample.
Fig. 4 Produced hydrogen (at STP conditions) using the prepared composites.
Fig. 5 Schematic diagram for the water photosplitting mechanism. References 8
[1] Levin DB, Pitt L, Love M. Biohydrogen production: prospects and limitations to practical application. Int J Hydrogen Energ. 2004;29:173-85. [2] Han S-K, Shin H-S. Biohydrogen production by anaerobic fermentation of food waste. Int J Hydrogen Energ. 2004;29:569-77. [3] Zeng K, Zhang D. Recent progress in alkaline water electrolysis for hydrogen production and applications. Prog Energy Combust Sci. 2010;36:307-26. [4] Ni M, Leung MK, Leung DY, Sumathy K. A review and recent developments in photocatalytic water-splitting using TiO2 for hydrogen production. Renew Sust Energ Rev. 2007;11:401-25. [5] Grochala W, Edwards PP. Thermal decomposition of the non-interstitial hydrides for the storage and production of hydrogen. Chem Rev. 2004;104:1283-316. [6] Kotay SM, Das D. Biohydrogen as a renewable energy resource—prospects and potentials. Int J Hydrogen Energ. 2008;33:258-63. [7] Perkins C, Weimer AW. Solar‐thermal production of renewable hydrogen. AlChE J. 2009;55:286-93. [8] Kudo A. Photocatalyst materials for water splitting. Catalysis Surveys from Asia. 2003;7:31-8. [9] Park JH, Kim S, Bard AJ. Novel carbon-doped TiO2 nanotube arrays with high aspect ratios for efficient solar water splitting. Nano Lett. 2006;6:24-8. [10] Sheikh FA, Appiah-Ntiamoah R, Zargar MA, Chandradass J, Chung W-J, Kim H. Photocatalytic properties of Fe 2 O 3-modified rutile TiO 2 nanofibers formed by electrospinning technique. Mater Chem Phys. 2016;172:62-8. [11] In S, Orlov A, Berg R, García F, Pedrosa-Jimenez S, Tikhov MS, et al. Effective visible light-activated B-doped and B, N-codoped TiO2 photocatalysts. J Am Chem Soc. 2007;129:13790-1. [12] Hoang S, Guo S, Hahn NT, Bard AJ, Mullins CB. Visible light driven photoelectrochemical water oxidation on nitrogen-modified TiO2 nanowires. Nano Lett. 2011;12:26-32. [13] Kanjwal MA, Barakat NA, Sheikh FA, Khil MS, Kim HY. Functionalization of electrospun titanium oxide nanofibers with silver nanoparticles: strongly effective photocatalyst. International Journal of Applied Ceramic Technology. 2010;7. [14] Barakat NA, Kanjwal MA, Chronakis IS, Kim HY. Influence of temperature on the photodegradation process using Ag-doped TiO 2 nanostructures: negative impact with the nanofibers. J Mol Catal A: Chem. 2013;366:333-40. [15] Barakat NAM, Kim B, Kim HY. Production of Smooth and Pure Nickel Metal Nanofibers by the Electrospinning Technique: Nanofibers Possess Splendid Magnetic Properties. J Phys Chem C. 2008;113:531-6. [16] Barakat NAM, Motlak M, Elzatahry AA, Khalil KA, Abdelghani EAM. NixCo1−x alloy nanoparticle-doped carbon nanofibers as effective non-precious catalyst for ethanol oxidation. Int J Hydrogen Energ. 2014;39:305-16. [17] De Jesus JC, González I, Quevedo A, Puerta T. Thermal decomposition of nickel acetate tetrahydrate: an integrated study by TGA, QMS and XPS techniques. J Mol Catal A: Chem. 2005;228:283-91. [18] Barakat NA, Kim B, Kim HY. Production of smooth and pure nickel metal nanofibers by the electrospinning technique: nanofibers possess splendid magnetic properties. J Phys Chem C. 2008;113:531-6. 9
[19] Barakat NA, Abadir M, Shaheer Akhtar M, El-Newehy M, Shin Y-s, Yong Kim H. Synthesis and characterization of Pd-doped Co nanofibers as a multifunctional nanostructure. Mater Lett. 2012;85:120-3. [20] Sheikh FA, Cantu T, Macossay J, Kim H. Fabrication of poly (vinylidene fluoride)(PVDF) nanofibers containing nickel nanoparticles as future energy server materials. Science of advanced materials. 2011;3:216-22. [21] Sheikh FA, Macossay J, Kanjwal MA, Abdal-hay A, Tantry MA, Kim H. Titanium dioxide nanofibers and microparticles containing nickel nanoparticles. ISRN nanomaterials. 2012;2012. [22] Zarkadoulas A, Koutsouri E, Mitsopoulou CA. A perspective on solar energy conversion and water photosplitting by dithiolene complexes. Coord Chem Rev. 2012;256:2424-34. [23] Barakat NA, Kanjwal MA, Al-Deyab SS, Chronakis IS. Influences of silver-doping on the crystal structure, morphology and photocatalytic activity of TiO2 nanofibers. Materials Sciences and Applications. 2011;2:1188.
10
11
•
N-doped, Ni-decorated, TiO2-incoporated amorphous carbon sheets are introduced.
•
The prepared nanocomposite was synthesized by one-pot procedure
•
The nanocomposites reveal distinct photocatalytic activity toward water splitting
•
The photosplitting process are carried out under visible light radiation
12
S0167-577X(17)31361-7 http://dx.doi.org/10.1016/j.matlet.2017.09.009 MLBLUE 23123
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
4 July 2017 17 August 2017 5 September 2017
Please cite this article as: N.A.M. Barakat, E. Ahmed, M.T. Amen, M.A. Abdelkareem, A.A. Farghali, N-doped Ni/ C/TiO2 nanocomposite as effective photocatalyst for water splitting, Materials Letters (2017), doi: http://dx.doi.org/ 10.1016/j.matlet.2017.09.009
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
N-doped Ni/C/TiO2 nanocomposite as effective photocatalyst for water splitting Nasser A. M. Barakat1,2,*, Enas Ahmed3, Mohamed T. Amen1, Mohammad Ali Abdelkareem2,4, A.A. Farghali5 1
Organic Materials and Fiber Engineering Dept., and Bionanosytem Engineering Dept., Chonbuk National University, Jeonju 561-756, Republic of Korea. 2 Chemical Engineering Department, Minia University, El-Minia, Egypt. 3 Renewable Energy Science and Engineering Department, Faculty of Postgraduate Studies for Advanced Sciences, Beni-Suef University, Egypt. 4 Department of Sustainable and Renewable Energy Engineering, University of Sharjah, PO Box 27272, Sharjah, United Arab Emirates 5 Materials Science and Nanotechnology Department, Faculty of Postgraduate Studies for Advanced Sciences, Beni-Suef University, Egypt.
Corresponding
author:
Nasser
A. M.
Barakat,
[email protected], Tel:+82632702363
Fax:
+82632704248
Abstract: N-doped Ni/C/TiO2 nanocomposite is introduced as an effective photocatalyst for water splitting under visible light radiation. The proposed photocatalyst contains most of the effective co-catalysts enhancing the photocatalytic activity of the titanium oxide. The nanocomposite was prepared by sintering a vacuously dried sol-gel composed of polyvinylpyrrolidone, titanium isopropoxide and nickel acetate under nitrogen atmosphere at 700 oC for 3h. TEM, XRD and XPS analyses confirmed that the introduced catalyst is N-doped & TiO2-incorporated amorphous carbon sheets decorated by Ni nanoparticles. The introduced nanocomposite revealed distinct performance as photocatalyst toward water splitting reaction, numerically the generated hydrogen was 0.383 mmol.sec-1.gcat-1 ; 8.6 ml.sec-1.gcat-1. However, nickel content somewhat influences the catalytic performance; 15 wt% reveals the best performance. Keywords: Nanocomposites; Carbon materials; Water photosplitting
1
1. Introduction Environmentally and practically, hydrogen is the most convenient energy source to replace the fusel fuels after the expected near depletion. Accordingly, several techniques have been introduced to generate the hydrogen including fermentation of waste biomasses[1, 2], water electrolysis [3], water photosplitting [4], thermal production [5], etc. However, water photosplitting has attracted the most attention due to simplicity, high yield, and economical and environmental aspects. Moreover, if an efficient photocatalyst working under solar irradiation is utilized, the process can be considered a renewable energy technology [6, 7]. Water photosplitting is a challenging reaction because it is one of the most important reactions for solving energy and environmental problems. Overall, water splitting into H2 and O2 is accompanied by a largely positive change in the Gibbs free energy (∆GO = 237 kJ/mol) and is an up-hill reaction. In other words, a special characteristics catalyst is required to store the solar radiation in the form of chemical energy; hydrogen. Semiconductors having convenient band gap energies can be exploited as photocatalysts for water splitting. Typically, when light with energy larger than the band gap is incident on the catalyst, holes and electrons are generated in the valence and conduction bands, respectively. The photogenerated holes and electrons lead to form redox reactions similar to electrolysis which results in reduction of the water molecules by the electrons to form H2 and oxidation by the holes to form O2 [8]. Titanium oxide is commonly exploited as photocatalyst for water splitting under the UV irradiation. Besides the low hydrogen evolution, working under UV constraints the wide applications. Moreover, the very fast electrons/holes recombination phenomenon is a big dilemma facing the photocatalytic activity of the titania-based materials.
2
Accordingly, the
researchers tried to enhance the photocatalytic performance using doping by different materials. For instance, carbon-doped TiO2 shows good contribution in maximizing the photocleavage of water under white-light irradiation [9, 10]. Moreover, some reports indicated that nitrogen doping enhances the adsorption capacity of the active species as well as improves the photo characteristics under the visible light irradiation which consequently results in relatively high hydrogen evolution rate [4, 11, 12]. Incorporation of metals nanoparticles distinctly remedies the problem of the electrons/holes recombination problem due to collecting the excited electrons on the highly conductive metal nanoparticles which act as electron sinks [13, 14]. In this study, for the first time, a novel composite including titanium oxide with most of the effective co-catalysts is introduced. Typically, N-doped Ni/C/TiO2 nanocomposite could be prepared by one-pot procedure. The synthesized photocatalyst showed good performance toward water photosplitting reaction. 2. Experimental work Polyvinylpyrrolidone (PVP, Sigma) was used as a precursor of carbon and nitrogen while titanium isopropoxide (Ti(Iso), Sigma) and nickel acetate (NiAc, Alfa Aesar) were utilized as precursors for TiO2 and nickel, respectively. Absolute ethanol was exploited as a solvent. First, 3 g of PVP was dissolved in 12 g methanol overnight until getting clear viscous polymer solution. In another flask, 2 ml of Ti(Iso) were added to 4 ml ethanol and 4 ml acetic acid, the acid was added dropwisely until the solution became clear and homogeneous. To get composites having different content of Ni nanoparticles with respect to the Ti(Iso), 0.1, 0.2, 0.3 and 0.4 g of NiAc were dissolved individually in 3 ml ethanol. Finally, each NiAc solution was mixed with similar polymer and Ti(Iso)
3
solutions and vigorously stirred for 1h to get final sol-gels having 5, 10, 15 and 20% NiAc with respect to Ti(Iso). Later on, the prepared gels were dried under vacuum at 60 o
C for 1 day and sintered under nitrogen atmosphere at 700 oC for 3 h. Water
photosplitting experiments have been conducted under 2000 W mercury lamp. 50 mg of the catalyst was added to 25/75 ml methanol/water mixture. The hydrogen evaluation was estimated by collecting the evolved gases over water surface. Briefly, the catalyst and methanol/water solution was placed in a conical flask with aside opening connected by a rubber tube to a graduated cylinder filled by water. The number of moles of the produced hydrogen was estimated based on ideal equation of state at standard conditions. Results and discussion As it was aforementioned, several organic and inorganic precursors have been utilized to prepare the proposed composites. Due to calcination under an inert atmosphere, it is expected that the utilized polymer will be partially graphitized while, because of the very high activity of the titanium, the Ti(Iso) will be decomposed to a stable oxide form; TiO2. On the other hand, based on ours and others studies, sintering of NiAc under an inert atmosphere results in producing the pristine metal rather than the expected oxide form due to evolution of strong reducing gases (mainly CO and H2) [15-17]. To properly verify the aforementioned hypotheses, XRD analysis has been invoked; Fig. 1. As shown in the obtained pattern, the high intensity and wide peak at 2ϴ value of 25.29o corresponding to (101) crystal plane indicates formation of small grain size anatase (PDF# 21-1272). Based on Scherrer’s equation, the estimated grain sizes of the anatase in the prepared composites were 4.04, 4.96, 1.82 and 1.31 nm for the composites obtained from
4
precursors having 5, 10, 15 and 20 wt% NiAc, respectively. It is noteworthy mentioning that the observed peak cannot be assigned to graphite or rutile phase TiO2 which are identified by their main peaks at 26.6o (PDF# 26-1079) and 27.5o (PDF# 21-1276), respectively. It is noteworthy mentioning that detection of anatse rather than the expected rutile phase at this high calcination temperature can be attributed to presence of nickel which catalyze formation of this phase. Formation of pristine nickel can be claimed due to appearance of the representative peaks at 44.5o and 51.8o corresponding to (111) and (200) crystal plans, respectively; PDF# 04-850. The mechanism of pristine nickel formation has been explained in details in our previous studies [18, 19]. Fig. 2A displays normal TEM image for the obtained composite. Based on XRD results, the black nanoparticles attached the formed sheets can be assigned to nickel. On the other hand, as shown in Fig. 2B, anatase can be detected as small nanoparticles incorporated inside an amorphous graphite sheet matrix; the red arrows point to anatase. Theoretically, carbon and nitrogen contents in the polyvinylpyrrolidone ((C6H9NO)n) are 64.8 and 12.6 wt%, respectively[20, 21]. Accordingly, sintering of polymer under inert atmosphere might result in partial graphitization. Considering the comparatively low oxygen content, 14.4 wt%, it is expected that some nitrogen can be captured inside the formed graphite. TEM image (Fig. 2B) indicates amorphous material in the formed Ni-decorated sheets, this material can be assigned to carbon. To properly prove this claim, XPS analyses was invoked. As shown in Fig. 3, big amount of carbon (C1s peak) could be detected. Moreover, appearance the characterized N1s nitrogen peak at around 409 eV confirms presence of nitrogen. The quantitative results indicated that the investigated sample contains 4.78 and 77.42 At% nitrogen and carbon, respectively.
5
It is noteworthy mentioning that the detected nitrogen in the final composite is small compared to the amount presence in the utilized polymer which can be attributed to formation of some nitrous gases during the calcination process which results in losing some nitrogen. Accordingly, based on the invoked characterization techniques, it is safe to claim that the produced material is Ni-decorated & N-doped C/TiO2 nanocomposite. Fig. 4 displays the hydrogen evolution rate using the prepared composite. As shown, almost linear hydrogen generation with respect to the time is obtained. Moreover, the obtained hydrogen generation rate is relatively high compared to several reports[22]. Numerically, the hydrogen production rate is 0.33913, 0.37393, 0.38282 and 0.38309 mmol hydrogen (at STP)/sec.gcat. However, as shown the sample obtained from 15 wt% NiAc revealed slightly better results. As aforementioned, every co-catalyst used in the newly introduced composite showed distinct role in enhancement of the photocatalytic activity of TiO2 toward water splitting reaction. The slight advantage of the 15 wt% sample can be attributed to Ni/TiO2 ratio besides the distribution of the metal nanoparticles on the carbon support. However, as it was expected and proved in the organic pollutants photo degradation reactions [23] but, based to our best knowledge, was not discussed before in case of water splitting, the metallic nanoparticles have good contribution in the process enhancement. In other words, nickel nanoparticles attracted the exited electrons and inhibit the electrons/holes recombination process which results in improvement the water splitting process. Fig. 5 displays schematic diagram for the water splitting mechanism over the introduced nanocomposite. Conclusion
6
Calcination of a sol-gel composed of polyvinylpyrrolidone, titanium isopropoxide and nickel acetate under nitrogen atmosphere leads to produce N-doped & Ni-decorated & TiO2-incoporated amorphous graphite sheets. The prepared nanocomposite can be utilized as effective photocatalyst for water splitting reaction. Overall, nickel content has a slight impact in the catalyst performance however 15 wt% reveals the best performance. Acknowledgement The authors would like to thank Mr. Mohamed N. A. Barakat for his efforts in doing some experimental work.
Fig. 1 XRD patterns for the prepared samples.
Fig. 2 Normal; (A) and high resolution; (B) TEM images for the 15 % Ni sample.
7
Fig. 3 XPS analysis spectrum for the 15 % Ni sample.
Fig. 4 Produced hydrogen (at STP conditions) using the prepared composites.
Fig. 5 Schematic diagram for the water photosplitting mechanism. References 8
[1] Levin DB, Pitt L, Love M. Biohydrogen production: prospects and limitations to practical application. Int J Hydrogen Energ. 2004;29:173-85. [2] Han S-K, Shin H-S. Biohydrogen production by anaerobic fermentation of food waste. Int J Hydrogen Energ. 2004;29:569-77. [3] Zeng K, Zhang D. Recent progress in alkaline water electrolysis for hydrogen production and applications. Prog Energy Combust Sci. 2010;36:307-26. [4] Ni M, Leung MK, Leung DY, Sumathy K. A review and recent developments in photocatalytic water-splitting using TiO2 for hydrogen production. Renew Sust Energ Rev. 2007;11:401-25. [5] Grochala W, Edwards PP. Thermal decomposition of the non-interstitial hydrides for the storage and production of hydrogen. Chem Rev. 2004;104:1283-316. [6] Kotay SM, Das D. Biohydrogen as a renewable energy resource—prospects and potentials. Int J Hydrogen Energ. 2008;33:258-63. [7] Perkins C, Weimer AW. Solar‐thermal production of renewable hydrogen. AlChE J. 2009;55:286-93. [8] Kudo A. Photocatalyst materials for water splitting. Catalysis Surveys from Asia. 2003;7:31-8. [9] Park JH, Kim S, Bard AJ. Novel carbon-doped TiO2 nanotube arrays with high aspect ratios for efficient solar water splitting. Nano Lett. 2006;6:24-8. [10] Sheikh FA, Appiah-Ntiamoah R, Zargar MA, Chandradass J, Chung W-J, Kim H. Photocatalytic properties of Fe 2 O 3-modified rutile TiO 2 nanofibers formed by electrospinning technique. Mater Chem Phys. 2016;172:62-8. [11] In S, Orlov A, Berg R, García F, Pedrosa-Jimenez S, Tikhov MS, et al. Effective visible light-activated B-doped and B, N-codoped TiO2 photocatalysts. J Am Chem Soc. 2007;129:13790-1. [12] Hoang S, Guo S, Hahn NT, Bard AJ, Mullins CB. Visible light driven photoelectrochemical water oxidation on nitrogen-modified TiO2 nanowires. Nano Lett. 2011;12:26-32. [13] Kanjwal MA, Barakat NA, Sheikh FA, Khil MS, Kim HY. Functionalization of electrospun titanium oxide nanofibers with silver nanoparticles: strongly effective photocatalyst. International Journal of Applied Ceramic Technology. 2010;7. [14] Barakat NA, Kanjwal MA, Chronakis IS, Kim HY. Influence of temperature on the photodegradation process using Ag-doped TiO 2 nanostructures: negative impact with the nanofibers. J Mol Catal A: Chem. 2013;366:333-40. [15] Barakat NAM, Kim B, Kim HY. Production of Smooth and Pure Nickel Metal Nanofibers by the Electrospinning Technique: Nanofibers Possess Splendid Magnetic Properties. J Phys Chem C. 2008;113:531-6. [16] Barakat NAM, Motlak M, Elzatahry AA, Khalil KA, Abdelghani EAM. NixCo1−x alloy nanoparticle-doped carbon nanofibers as effective non-precious catalyst for ethanol oxidation. Int J Hydrogen Energ. 2014;39:305-16. [17] De Jesus JC, González I, Quevedo A, Puerta T. Thermal decomposition of nickel acetate tetrahydrate: an integrated study by TGA, QMS and XPS techniques. J Mol Catal A: Chem. 2005;228:283-91. [18] Barakat NA, Kim B, Kim HY. Production of smooth and pure nickel metal nanofibers by the electrospinning technique: nanofibers possess splendid magnetic properties. J Phys Chem C. 2008;113:531-6. 9
[19] Barakat NA, Abadir M, Shaheer Akhtar M, El-Newehy M, Shin Y-s, Yong Kim H. Synthesis and characterization of Pd-doped Co nanofibers as a multifunctional nanostructure. Mater Lett. 2012;85:120-3. [20] Sheikh FA, Cantu T, Macossay J, Kim H. Fabrication of poly (vinylidene fluoride)(PVDF) nanofibers containing nickel nanoparticles as future energy server materials. Science of advanced materials. 2011;3:216-22. [21] Sheikh FA, Macossay J, Kanjwal MA, Abdal-hay A, Tantry MA, Kim H. Titanium dioxide nanofibers and microparticles containing nickel nanoparticles. ISRN nanomaterials. 2012;2012. [22] Zarkadoulas A, Koutsouri E, Mitsopoulou CA. A perspective on solar energy conversion and water photosplitting by dithiolene complexes. Coord Chem Rev. 2012;256:2424-34. [23] Barakat NA, Kanjwal MA, Al-Deyab SS, Chronakis IS. Influences of silver-doping on the crystal structure, morphology and photocatalytic activity of TiO2 nanofibers. Materials Sciences and Applications. 2011;2:1188.
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N-doped, Ni-decorated, TiO2-incoporated amorphous carbon sheets are introduced.
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The prepared nanocomposite was synthesized by one-pot procedure
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The nanocomposites reveal distinct photocatalytic activity toward water splitting
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The photosplitting process are carried out under visible light radiation
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