The excellent photocatalytic performances of silver doped ZnO nanoparticles for hydrogen evolution
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Ag based ZnO photocatalyst materials were prepared by facile, quick and inexpensive combustion method followed by calcinations at 700 °C for 3 h. The resulted nanoparticles were characterized using XRD, UV–Vis DRS, SEM, BET, FTIR, XPS, EDX and PL spectroscopy. The XRD analysis of silver doped ZnO (SZO) nanoparticles confirmed hexagonal wurtzite structure of ZnO with minimum crystal size about 19 nm. The doping of Ag in ZnO crystal lattice successfully suppressed the crystal growth of ZnO nanoparticles evidenced by XRD results. The DRS and PL spectra studies revealed that Ag doped ZnO nanoparticles showed absorption in visible light region and charge recombination was efficiently suppressed respectively. The DRS spectra analysis indicated that photon energy band gap Eg for SZO nanoparticles were in the range of 3.27–3.33 eV and reduced with increase in Ag doping. As a novel photocatalyst, the photocatalytic activity for hydrogen evolution was tested by water splitting using ethanol as sacrificial agent under solar light irradiation. The optimal hydrogen evolution rate 805 µmol h−1 g−1 was shown by 6% SZO photocatalyst. This was nearly 54 times higher hydrogen evolution rate as compared to pure ZnO. The improved photocatalytic activity was attributed to small particle size, plasmon band, charge separation and increased surface area. This report clearly revealed the benefit of using Ag doped ZnO catalyst for higher photocatalytic hydrogen generation.
KeywordsZnO Combustion Silver FTIR UV–Vis DRS Hydrogen evolution
Hydrogen energy is at the top priority of any nation’s obligations and goals when discussing the clean and inexpensive energy agenda. Hydrogen energy promises a redly accessible source, harmless emissions, efficient (large energy per mass of fuel), non-toxic (a scarcity among frequent used fuel sources), more dominant (accomplished more in less) and a promising potential candidate. In spite of the fact that hydrogen is frequently and abundantly present everywhere, but never found alone and required to be separated from other species. Unlikely to fossil fuels that are easy to store and transport, hydrogen requires economy challenges and infrastructures for transfer as well as for commercial and industrial use. At last, mostly industrial methods for separating hydrogen still required the application of fossil fuels; it is ironical for that reducing our dependence on the non renewable sources we should depend on them.
Oxidation of hydrocarbons is reputed for such methods , but it generates CO2 and consumes sufficient energy. A very facile process to achieve hydrogen energy is through electrolysis . In this water is exposed to an electric current, which will split water into H2 and O2. The apparent drawback is the requirement for an extensive transport of energy. H2 can also be achieved from biomass gasification [3, 4, 5, 6], but application is restricted by the economical feasibility of the mechanism.
Application of light irradiation process to already existing processes can be sufficiently improved, resulting in H2 evolution from renewable sources. Naturally this requires the application of light energy to become really free of non-renewable sources [7, 8]. The required alternate hydrogen production sources believed to be photocatalytic water splitting and oxidation of hydrocarbons (photocatalytic reforming of biomass) [9, 10, 11]. The later is useful since H2 is generated with photocatalytic degradation of organic molecules existing in water at mild conditions, also participating to purification of commercial water. This process could attract even more attention, if natural renewable sources (such as solar energy, largely rich in South American and European regions) are used.
Titanium dioxide (TiO2) had been widely used in photo induced processes [12, 13]. The extensive use of this salt had increased the search for alternate attractive photocatalyst with enhanced properties . ZnO is a wide band gap semiconductor photocatalyst similar to that of titanium dioxide (TiO2). In addition ZnO is inexpensive and contains exceptional morphologic versatility . ZnO materials can be synthesized with different shapes like nanoparticles, Nanotubes, nanotetra pods, nanospheres, nanorods, nanoflowers etc. [16, 17, 18]. These synthesized materials are normally designated as tunable band gap semiconductors and can absorb in UV region of solar spectrum. The scientific community attempted to modify the energy band gap of TiO2 and ZnO due to their restricted activity under visible light irradiation, and this band gap modification will increase solar light absorption in visible range. Structural and morphological modifications were made to enable these materials as efficient photocatalysts. The application of TiO2 hybrids including carbons like carbon fibers, carbon Nanotubes, activated carbon, graphene etc. had been reported as efficient technique to increase the semiconductor photocatalytic activity under UV and visible light irradiation [19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30]. Dye photo-sensitization variations had also been reported. The photocatalytic activity of both TiO2 and ZnO has been enhanced with metal doping like Ag [31, 32, 33, 34, 35]. The silver had been selected for doping purpose due to its band fluctuation ability . Silver is a noble metal which shows several properties such as no photo-corrosion, anchored on surface and surface Plasmon band in visible range [37, 38]. Although, Ag doped TiO2 photocatalyst had been reported in the presence of ethanol as sacrificial agent [37, 38], but Ag doped ZnO photocatalyst had not been reported as photocatalyst for hydrogen generation. In this work, we reported Ag doped ZnO photocatalyst for photocatalytic hydrogen production which was synthesized by facile, quick, time saving and inexpensive combustion method. Moreover, synthesized photocatalysts had showed higher photocatalytic activity for hydrogen generation. The synthesized photocatalysts were tested for photocatalytic hydrogen evolution in the presence of water–ethanol solution under solar light irradiation.
2.1 Synthesis of ZnO materials
Combustion method was used to synthesize ZnO and silver doped ZnO nanoparticles AgxZn1−xO (x = 0, 1%, 2%, 4%, 6%, 8%) using Zinc Nitrate Hexahydrate [Zn (NO3)2·6(H2O)], silver nitrate (AgNO3) and glycine (NH2CH2COOH). All reagents were bought from Merck Pakistan with 99.90% purity. Zinc Nitrate Hexahydrate and silver nitrate were used as oxidant while Glycine was used as fuel for combustion process. In this study fuel to oxidant molar ratio (Ψ) was taken as 1.7 due to its optimal previous results . The required amount of all Ag doped ZnO nanoparticles as said above AgxZn1−xO were taken in beakers. The zinc nitrate Hexahydrate being hygroscopic absorbed moisture and turned into transparent slurry. The slurry was heated at hot plate with constant stirring to make homogeneous solution. The process of mixing continued for 1 h and then temperature of solution was increased to 280 °C with continuous stirring on hot plate. The color of the solution changed from white to red, red to yellow and yellow to black at high temperature of 280 °C. The solution was swallowed into foam like black gel and burst with evolution of heat and large amount of non-toxic gases and result in dry, loose and voluminous black nano powders with high porosity. The heat treatment was applied at 700 °C for 3 h to absorb any remained moisture in nano powders, which changed the color of nanoparticles to white. The ZnO doped with 1, 2, 4, 6 and 8% mol of silver were labeled as samples 1%SZO, 2%SZO, 4%SZO, 6%SZO, and 8%SZO respectively.
2.2 Characterization of ZnO and Ag–ZnO photocatalysts
Here D is particle size, k is shape factor with value 0.9, λ = 0.15406 nm is wavelength of Cu-Kα radiation source, β is full width at half maximum (FWHM) measured in radians and θ is diffraction angle. The scanning electron microscope (SEM), Hitachi S-4800 was applied to observe the morphology of nanoparticles. The compositional analysis of the synthesized nanoparticles was studied by energy dispersive X-ray spectroscopy. The optical characteristics of the synthesized samples were recorded using a PerkinElmer-lambda 5 UV–visible spectrophotometer and energy band gaps were measured by the Kubelka–Munk function. PL spectra were recorded at room temperature equipped with Crylas CW-276 nm Laser as an exciton source. The emission was recorded by a CCD camera coupled with grating mono-chromator laser and power was maintained in 2 mW. FTIR spectra were measured by JASCO-MFT 2000 apparatus. The interval of operating parameters was 4000–500 cm−1 with 32 scans along with mirror velocity of 0.6139 cm/s. Brunauer–Emmett–Teller specific surface area was measured by Gemini-2375, Shimadzu.
2.3 Photo activity
The photocatalytic hydrogen production was operated in doubled walls quartz photo-chemical reactor. A mixture containing 10 mg mass of the synthesized sample and 12 mL of ethanol solution with volume ratio (ethanol: water) of 1:5 was obtained through constant magnetic stirring. Continuous water circulating through the photo-chemical reactor was arranged with the help of thermostatic bath to maintain the room temperature. Argon (Ar) bubbling by deaerator was carried out for 30 min to wipe out any air traces before irradiation. A 300 W mercury xenon (Hg–Xe) lamp of PerkinElmer- CarMax PE-300 was used as irradiation source. Agilent 8620-GC chromatograph coupled with TCD with the help of Porapak Q (90/100 mesh) column was used to measure room temperature produced gases during experiments with the help of gas chromatograph, using Ar as carrier wave. A gas-tight syringe with optimum volume of 80 mL was used to measure the amount of gas evolved at time interval of 30 min. To test the reproducibility of the hydrogen evolution activity, each photocatalyst was tested three times.
3 Results and discussions
3.1 Photocatalysts characterization
Shows particle size, surface area and hydrogen evolution rate
Particle size (nm)
Surface area (m2 g−1)
Hydrogen evolution rate (µmol h−1 g−1)
Energy band gap (eV)
If Ag was substituted for Zn+2 then corresponding peak shift should be observed in XRD pattern or presence of peak corresponding to AgO or Ag2O phase in XRD pattern. Since XRD pattern showed no such peak shift, suggested that there exist no positional variation of peaks. XRD pattern indicated broadening of peaks due to presence of clusters. Similar observations had been reported for ZnO [42, 46]. The absence of AgO or Ag2O phase as mentioned above, showed the absence of Ag substitution in ZnO lattice . Previous reports told that Lupan et al.  had assigned analogous peaks both for AgO and Ag2O, though there was no evidence for the presence of these peaks in our case. Also peak shift had been reported in their work while we did not observe such peak shifts, hence our samples contained only silver. To conclude, SZO photocatalysts maintained hexagonal wurtzite structure, while particle size and intensity was reduced with Ag doping into ZnO lattice. Thus crystallinity was decreased due to creation of defects with Ag doping in ZnO system. Further XRD pattern did shown any peak shift with Ag doping, indicating that Ag atoms were grown on the surface of ZnO and there was no incorporation of Ag due to substitution or interstitial effects. The low particle size will be suitbale for maximum photocatalytic activity.
3.2 UV–Vis diffuse reflectance spectroscopy analysis
However, energy band gap for 6% SZO photocatalyst 3.27 eV, a blue shift of 0.06 eV due to Ag doping as compared to energy band gap of pure ZnO. However it is worth mentioning that Ag doping decreased the energy band gap absorption intensity, as shown by the presence of plasmon, suppressing the penetration intensity of incoming light, perhaps due to scattering. Thus it was concluded that 6% SZO photocatalyst had the least energy band gap and will be suitable for optimal photocatalytic hydrogen evolution.
3.3 X-ray photoelectron spectroscopy
3.4 Fourier-transform infrared spectra
3.5 Photoluminescence spectra
3.6 BET surface area
Brunauer–Emmett Teller (BET) surface area showed very small variation in surface area on Ag doping. The observed increased surface area with Ag doping revealed that Ag was producing legroom into ZnO nano crystals. The BET surface area increased obviously to 31 m2 g−1 after 8% Ag doping into ZnO. The results are shown in Table 1. The doping of Ag suppressed the growth of ZnO phase noticeably, resulted in a small increase in BET surface area, retain porosity and generated shallow type new pores. The retarded growth of ZnO may be attributed to combustion synthesis, which was a rapid method in which the product was synthesized with a few minutes at high temperature; the presence of Ag retarded the growth of ZnO. The slight increase in surface area for Ag doped ZnO photocatalyst was attributed to decrease in particle size with Ag doping.
3.7 Scanning electron microscopy
3.8 EDX analysis
3.9 I–V measurement
4 Photocatalytic hydrogen evolutions
In this paper synthesis of ZnO and Ag doped ZnO nanoparticles using combustion method and their photocatalytic activity for hydrogen evolution have been studied. The impact of various Ag doping contents on ZnO was investigated by XRD, DRS, PL, XPS, FTIR, SEM, BET surface area and EDX analysis. It can be found that content of Ag has a optimal value, which can suppress the electron hole pair recombination so as to increase the photocatalytic activity. The absorption edge of SZO nanoparticles in UV–Vis absorbance spectra had a pronounced red shift. High content of silver (8%SZO) can become the recombination centre for electrons and holes and is the cause of decreased photocatalytic activity. Hydrogen evolution rate of 6%SZO catalyst was increased to 805 µmol h−1 g−1 which was approximately 54 times greater than to pure ZnO due to the excellent effects of silver doping. Thus present results indicate that SZO photocatalysts are potential candidate for future practical for hydrogen evolution by water splitting.
Compliance with ethical standards
Conflict of interest
Authors declare that there are no conflict of interest statements.
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