Fuel 140 (2015) 267–274

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Novel microbial synthesis of Cu doped LaCoO3 photocatalyst and its highefficient hydrogen production from formaldehyde solution under visiblelight irradiation

http://dx.doi.org/10.1016/j.fuel.2014.09.1070016-2361/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +86 592 2188283; fax: +86 592 2184822.E-mail address: jials@xmu.edu.cn (L. Jia).

Liqing Wang, Qi Pang, Qianqian Song, Xinwei Pan, Lishan Jia ⇑Department of Chemical and Biochemical Engineering, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, Fujian, China

h i g h l i g h t s

� Cu doped LaCoO3 photocatalysts were prepared using microorganism Pichia pastoris GS115.� Cu and biomass residue display synergistic effect on enhancing the photocatalytic activity.� The highest H2 evolution rate reaches 1.13 mmol h�1 g�1.

a r t i c l e i n f o

Article history:Received 10 August 2014Received in revised form 25 September 2014Accepted 26 September 2014Available online 13 October 2014

Keywords:BiomassPerovskiteHydrogen productionVisible lightPhotocatalyst

a b s t r a c t

Cu doped LaCoO3 photocatalyst by microbial synthesis (M-LaCo0.7Cu0.3O3), using the extract of Pichiapastoris GS115, presents an outstanding photocatalytic performance for hydrogen production fromformaldehyde solution. The structure and physicochemical properties of the photocatalyst were character-ized by XRD, EDS, XPS, FTIR, Photoluminescence (PL), and UV–vis (DRS). The results indicate that copperdoping contributes to the formation of impurity level and appropriate oxygen vacancy. Differing from sep-arate citric acid complexation with metal ions, biomass intervention in preparation process can help adjustthe crystal structure and surface structure of catalyst, which makes the diffraction angle and unit cellchange and simultaneously modulates surface oxygen defects. Furthermore, some organic functionalgroups from biomass residue on the surface act as photosensitizer, so that the M-LaCo0.7Cu0.3O3 markedlyabsorbs the visible light and shows higher absorbance than LaCo0.7Cu0.3O3 and LaCoO3. Consequently,M-LaCo0.7Cu0.3O3 exhibits the highest rate of photocatalytic hydrogen production (1.13 mmol h�1 g�1).This research provides a new way for designing and developing visible-light active materials using biomass.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Photocatalytic degradation of organic pollutants in water andsimultaneous production of hydrogen not only realize environ-mental remediation, but also obtain clean and renewable sourcefor hydrogen fuel [1]. Formaldehyde as common pollutant can beused as electron donors during the photocatalytic reaction processto achieve a much higher hydrogen production rate. Since UV lightaccounts for only a small fraction (8%) of the sun’s energycompared to visible light (45%) [2], development of materials thathave small band gap to adsorb visible light is vitally important foreffective solar energy conversion [3–5].

Perovskite type (ABO3) photocatalysts such as LaCoO3 [6–8] andLaNiO3 [9,10] are favorable materials for photocatalysis, mainlyowing to that they are stable enough to form a solid solution withdiverse metal ions through A- or B-site substitution [11,12] fordesired physicochemical properties. Such doping could bring adefect impurity level in perovskite to narrow the band gap, andalso leads to a certain amount of oxygen vacancy that could retardthe recombination between photogenerated electrons and holeseffectively on the surface of the photocatalyst [13,14]. Copper hasbeen reported as an excellent doping element [15,16]. Recently,our research group has prepared Cu doped LaNiO3 photocatalystand greatly improved its photocatalytic activity by reduction treat-ment at high temperature. Nevertheless, the treatment conditionsare relative complicated and hard to control.

At present, biomass, which possesses some unique propertiescompared with conventional chemicals, has been widely applied

Fig. 1. (A) X-ray diffraction patterns of all samples. (a) LaCoO3, (b) LaCo0.8Cu0.2O3, (c) LaCo0.7Cu0.3O3, (d) LaCo0.6Cu0.4O3 and (e) M-LaCo0.7Cu0.3O3. (B) The corresponding XRDpatterns in the 33.05�–33.5� 2h range of (a, c and e).

Fig. 2. EDS image of M-LaCo0.7Cu0.3O3. The inset shows the corresponding elemental composition of M-LaCo0.7Cu0.3O3.

268 L. Wang et al. / Fuel 140 (2015) 267–274

in preparing hybrid inorganic/organic materials [17]. These materi-als are a novel kind of functional nanocomposites with enhancedoptical, thermal and mechanical properties [18,19]. Some examplesare given such as carbonized-bamboo-TiO2 for the removal of ben-zene [20], biosynthesis of Pd-TiO2 for hydrogen production [21],and iron- or nickel-based catalyst supported on calcined scallopshell [22]. And there are a lot of organic functional groups frommicroorganism that are beneficial for preparing catalysts likebiopolymer sodium alginate templated porous TiO2 [23] and palla-dium supported on Desulfovibrio desulfuricans [24]. However, thereare few reports on the use of biomass in perovskite and there is nodetailed research on biomass-modified perovskite used as photo-catalyst for degradation of formaldehyde in water up to now.

In this paper, instead of difficult reduction treatment, we usePichia pastoris GS115 as a complexing agent to synthesize Cu dopedLaCoO3 photocatalyst (M-LaCo0.7Cu0.3O3) for hydrogen productionfrom formaldehyde solution under visible light irradiation. Theobjective is to further investigate the role played by biomass inthe preparation of photocatalytic materials and the process of form-aldehyde solution into hydrogen. This work may pave a new andfacile pathway for microbial synthesis of new visible-light-activematerials.

2. Materials and methods

2.1. Materials

P. pastoris GS115 was prepared by cultivating cells at 30 �C and200 rpm for 48 h in a shake flask containing YPD (glucose, 20 g L�1;peptone, 20 g L�1; yeast extract, 10 g L�1) [25]. The bacterial sus-pension thereof was centrifuged and resultant precipitates wereseparated, washed twice with deionized water. It was then driedat 353 K under vacuum overnight, and ground in an agate mortar.Following, the dried P. pastoris GS115 powder was constantlystirred at 80 �C water bath for 48 h. The mixture was filtrated toget the extract (5.66 g L�1). La(NO3)3�nH2O, Co(NO3)3�6H2O,Cu(NO3)�3H2O, and citric acid (C6H8O7�3H2O) were obtained fromSinopharm Chemical Reagent Co., Ltd.

2.2. Photocatalyst synthesis procedure

Microbial synthesis of Cu doped perovskite (defined asM-LaCo0.7Cu0.3O3) proceeded as follow. La(NO3)3�nH2O, Co(NO3)3�6H2O, Cu(NO3)�3H2O and citric acid were dissolved in deionized

Fig. 3. O 1s core level spectra for (a) LaCoO3, (b) LaCo0.8Cu0.2O3, (c) LaCo0.7Cu0.3O3,(d) LaCo0.6Cu0.4O3 and (e) M-LaCo0.7Cu0.3O3.

L. Wang et al. / Fuel 140 (2015) 267–274 269

water under vigorous stirring at a molar ratio of 1:0.6 (metal cat-ions/citric acid). A certain amount of P. pastoris GS115 solution(2.33 wt% of citric acid) was added to the above solution. The mix-ture was polymerized under infrared irradiation for more than 12 hto give a gel-like product. The obtained sample was manuallyground, and then calcined at 400 �C for 2 h, and at 800 �C for 4 h.

2.3. Catalyst characterization

X-ray diffraction (XRD) patterns were obtained on a Panalyticalpert spathic powder diffractometer with Cu Ka radiation. Energydispersive spectrometer (EDS) analysis was taken with a LEO-1530electron microscope (Germany). X-ray photoelectron spectroscopy(XPS) analyses were performed on a PHI Quantum 2000 ScanningESCA microprobe with a monochromatized microfocused Al X-raysource. Fourier transform infrared spectroscopy was carried outusing Nicolet Avatar 330 FT-IR spectrophotometer (USA) with KBras dispersant. Photoluminescence spectrum (PL) was measured atroom temperature with a Hitachi F-7000 spectrophotometer byusing a Xe lamp as the excitation source. Ultraviolet–visible(UV–vis) diffuse reflection spectroscopy of the photocatalyst wasrecorded using a Varian Cary 5000 UV–vis spectrophotometer.

2.4. Photocatalytic H2 production

The photocatalytic reaction was evaluated in a self-made quartzinner-irradiation-type reaction vessel. The photocatalyst (0.1 g)

Table 1The ratio of adsorbed oxygen to surface lattice oxygen for all samples.

Sample Binding energy (eV)

Adsorbed oxygen Surface lattice

LaCoO3 528.9 531.4LaCo0.8Cu0.2O3 528.6 531.3LaCo0.7Cu0.3O3 528.6 531.4LaCo0.6Cu0.4O3 528.6 531.4M-LaCo0.7Cu0.3O3 528.4 531.2

was suspended in aqueous formaldehyde solution (HCHO 20 mL,H2O 140 mL) with a magnetic stirrer. Previous to irradiation, thesolution was continuously bubbled with N2 at a rate of60 mL min�1 for 30 min. Then the gas content was checked byGC to make sure that no oxygen was present. 2 mol/L NaNO2 solu-tion was introduced into the water jacket as an internal circulationcooling medium to eliminate UV-light (cut-off k < 400 nm). Irradi-ation was conducted by a 125 W xenon lamp and the reaction tem-perature was kept at 323 K. The gas evolved was gathered andanalyzed by GC (TCD, molecular sieve 5 Å column and Ar carrier).

3. Results and discussion

3.1. XRD and EDS measurement

Fig. 1A shows the crystal structures of photocatalysts. It exhibitsthat all the modified samples maintain a perovskite structure sim-ilar to LaCoO3, compared with the standard parameters given inJCPDS data cards (JCPDS 00-025-1060). However, it is easy to findthat the characteristic diffraction peaks shift to lower angles afterCu doping [26]. It can be concluded that a series of solid solutionin the system is formed. Fig. 1B presents the magnified view at2h angles ranging from 33.0� to 33.5�. In comparison to pureLaCo0.7Cu0.3O3, the peak of M-LaCo0.7Cu0.3O3 shifts slightly to ahigher 2h value, which is possibly due to the interaction betweenbiomass residue and metal ions.

Fig. 2 is EDS image of M-LaCo0.7Cu0.3O3. The atomic ratio is well-matched with the stoichiometric ratio of perovskite. And there isno existence of S, N, and P, but C with a relative high contentresulting from biomass residue.

3.2. XPS measurement

Fig. 3 presents the O 1s core level spectra measured fromLaCoO3, Cu doped LaCoO3 and M-LaCo0.7Cu0.3O3. The bindingenergy around 528.6 eV is correlated to surface lattice oxygen,and the peak around 531.3 eV is attributed to adsorbed oxygenspecies [27]. The amount of adsorbed oxygen is effected by oxygenvacancy which is beneficial for some reactions [28]. The ratio ofadsorbed oxygen to surface lattice oxygen is shown in Table 1and it can be seen that copper doping causes a decrease in theamount of lattice oxygen to gain more oxygen vacancy. The ratiovalue is 1.58 for LaCo0.7Cu0.3O3, and decreases to 1.40 forM-LaCo0.7Cu0.3O3. This result may be caused by the competitivecomplexing between biomass and chemical chelate agent (citricacid) with metal ions. Biomass could play a part in adjusting theratio of adsorbed oxygen to surface lattice oxygen. Relativelyappropriate oxygen vacancy can effectively restrain the recombi-nation between photogenerated electrons and holes, and thenimprove photocatalytic performance [29,30].

The Cu 2p spectra, Fig. 4A, consist of two peaks located at 933and 953 eV for Cu 2p3/2 and 2p1/2, respectively. Fig. 4B is thefitted XPS spectra in relevant regions of Cu doped LaCoO3 andM-LaCo0.7Cu0.3O3. We assign the signal of Cu 2p3/2 at Binding

The ratio of adsorbed oxygen to surface lattice oxygen

oxygen

0.900.971.581.241.40

Fig. 4. (A) Cu 2p3/2 and (B) the fitting Cu 2p3/2 of (a) LaCo0.8Cu0.2O3, (b) LaCo0.7Cu0.3O3, (c) LaCo0.6Cu0.4O3 and (d) M-LaCo0.7Cu0.3O3.

Fig. 5. The infrared spectra (FTIR) of (a) LaCo0.7Cu0.3O3 and (b) M-LaCo0.7Cu0.3O3. Fig. 6. TG curves of the precursor of (a) M-LaCo0.7Cu0.3O3, and (b) LaCo0.7Cu0.3O3.

270 L. Wang et al. / Fuel 140 (2015) 267–274

Energy = 932.8 eV (with shake-up satellite features in the 940–945 eV range) to Cu2+ ions, and that at BE = 934.6 eV to Cu3+ ions[31,32]. Biomass still keeps proper proportion of divalent and triva-lent copper, which helps to improve light absorption performance.

3.3. FTIR measurement

The effect of biomass synthesis is studied in Fig. 5. It is of inter-est to note that curves a and b are similar. Some differences can befound through careful observation. The band at 1120 cm�1 oncurve a may be assigned to CAC framework vibration, whichmoves to a lower frequency (1113 cm�1) with the intensitystrengthened on curve b [33]. Simultaneously, a set of new peakshave appeared on curve b. Peaks at 1040, 1062 and 1162 cm�1

are attributed to CAOAC stretching vibration [34,35]. And the peakat 1010 cm�1 corresponds to m(C@C) [36]. Obviously, on the

surface of M-LaCo0.7Cu0.3O3, there are a few organic functionalgroups originated from biomass residue, which may act as photo-sensitizer to improve the adsorption for visible light [37].

3.4. TG measurement

Fig. 6 displays the TG curves of the precursor of (a) M-LaCo0.7-

Cu0.3O3, and (b) LaCo0.7Cu0.3O3. Owing to the biomass complexa-tion with metal ions, after 800 �C heat treatment, there is still acertain amount of biomass existing in M-LaCo0.7Cu0.3O3.

3.5. PL measurement

Fig. 7A and B display Photoluminescence (PL) spectrum of mod-ified LaCoO3. Fig. 7A reveals the influence of copper doping to PLintensity which increase successively for LaCo0.7Cu0.3O3, LaCo0.6-

Fig. 7. (A) Photoluminescence emission spectra for (a) LaCo0.8Cu0.2O3, (b) LaCo0.7Cu0.3O3, and (c) LaCo0.6Cu0.4O3 (excitation wavelength 280 nm). (B) Photoluminescenceemission spectra for (a) LaCo0.7Cu0.3O3, and (b) M-LaCo0.7Cu0.3O3 (excitation wavelength 280 nm).

Fig. 8. The plots of [F(R)�hm]2 versus hm of (a) LaCoO3, (b) LaCo0.8Cu0.2O3, (c) LaCo0.7Cu0.3O3, (d) LaCo0.6Cu0.4O3, (e) M-LaCo0.7Cu0.3O3.

L. Wang et al. / Fuel 140 (2015) 267–274 271

Fig. 9. UV–vis DRS patterns of the samples.

272 L. Wang et al. / Fuel 140 (2015) 267–274

Cu0.4O3 and LaCo0.8Cu0.2O3. Seen from Fig. 7B, the intensity ofLaCo0.7Cu0.3O3 is stronger than that of M-LaCo0.7Cu0.3O3. Asreported, PL emission is the result of the recombination ofelectrons and holes, in other words, lower PL intensity equals toa lower recombination rate [38] and an excellent photocatalyticactivity. This can be ascribed to some organic functional groupsfrom biomass residue on the surface of M-LaCo0.7Cu0.3O3 as effi-cient separation of charge carriers.

3.6. DRS measurement

The band gap energy (Eg, eV) can be evaluated by correspondingabsorption spectra shown in Fig. 8. And the diffuse reflectancespectra of the prepared materials, as shown in Fig. 9, indicate thatthe M-LaCo0.7Cu0.3O3 markedly absorbs the light at the range ofvisible-light and has higher absorbance than LaCo0.7Cu0.3O3 andLaCoO3, which is due to the effect of biomass residues and Cu. InUV–vis region, positions of the absorption edges are determined

Fig. 10. Proposed formation mec

by the interception of the straight line fitted through the low-energy side of the curve [F(R)�hm]2 versus hm, where F(R) is the Kub-elka–Munk function and hm is the energy of the incident photon[39]. The band gap energy is 2.88 eV for LaCoO3, and notablelydecreases as Cu portion increases due to the formation of impuritylevel. It was calculated to be 2.69 eV, 2.54 eV, and 2.72 eV corre-sponding to LaCo0.8Cu0.2O3, LaCo0.7Cu0.3O3, and LaCo0.6Cu0.4O3,respectively. Moreover, that value for M-LaCo0.7Cu0.3O3 is the nar-rowest (2.38 eV), making it the best in the light harvesting processsince narrower band gap energy is conducive to enhance theadsorption for visible light.

3.7. Possible formation mechanism of M-LaCo0.7Cu0.3O3

Although the exact microbial synthesis mechanism ofM-LaCo0.7Cu0.3O3 using P. pastoris GS115 as a complexing agentis rather complicated, we here propose a possible mechanism sche-matically elucidated in Fig. 10. This mechanism is deduced fromthe above structure characterizations. We think that the extractof P. pastoris GS115 plays an important role in the formation ofM-LaCo0.7Cu0.3O3.

First, the addition of the extract of P. pastoris GS115 with citricacid at room temperature leads to the formation of metal ion sol. Inother words, both biomass and citric acid interacts with metal ionsas complexing agent for perovskite particles. As the calcination ofthe xerogel proceeds, the biomass residue still exists in the perov-skite nanoparticles due to the complexation. Biomass interventionin this process can help adjust the crystal structure and surfacestructure of catalyst.

3.8. Photocatalytic activity

Fig. 11A studies the effect of Cu doping amount by evaluatingthe hydrogen production from formaldehyde aqueous solutionunder visible light irradiation for 60 min. The hydrogen generationrate of LaCoO3 is only 0.027 mmol h�1 g�1. However, Cu dopedLaCoO3 [LaCo1�xCuxO3 (x = 0.2, 0.3, 0.4)] show excellent photocata-lytic performance. Moreover, as Cu doping amount increase, thehydrogen production first increases, then decreases (LaCo0.6Cu0.4O3

hanism of M-LaCo0.7Cu0.3O3.

Fig. 11. (A) and (B) Photocatalytic activity of different samples under xenon-lamp irradiation on the cut-off wavelength of 400 nm. (a) LaCoO3, (b) LaCo0.8Cu0.2O3, (c)LaCo0.7Cu0.3O3, (d) LaCo0.6Cu0.4O3 and (e) M-LaCo0.7Cu0.3O3.

Fig. 12. Time course of photocatalytic activity for H2 evolution overM-LaCo0.7Cu0.3O3 (irradiation time 270 min).

L. Wang et al. / Fuel 140 (2015) 267–274 273

is 0.21 mmol h�1 g�1). Thus, there is an optimum amount (LaCo0.7-

Cu0.3O3) to achieve the maximum rate of photocatalytic hydrogenproduction (0.31 mmol h�1 g�1). Nonetheless, the amount formicrobial synthesis of Cu doped LaCoO3 photocatalyst (M-LaCo0.7-

Cu0.3O3) increases to 1.13 mmol h�1 g�1, which is about 3.65 timesthat of LaCo0.7Cu0.3O3 (see Fig. 11B). Fig. 12 reveals the stability ofM-LaCo0.7Cu0.3O3. The evolved hydrogen amount increases withthe increment time of irradiation within 270 min and the rate dis-plays minor change. It is exciting to see that the total amountreaches 2.20 mmol g�1 in 270 min.

3.9. Reaction mechanism

The major oxidative and reductive process in the photodegrada-tion of formaldehyde solution in nitrogen atmosphere withM-LaCo0.7Cu0.3O3 can be written as follows:

Step 1:

k > 400 nm

M� LaCo0:7Cu0:3O3 ! e� þ hþ

H2Oþ hþ �������!photooxidationHþ þHO�

2Hþ þ 2e��������!photoreduction

H2

Step 2:

HCHOþ hþ þHO� �������!photooxidationHCOOHþHþ

Step 3:

HCOOH! HCOO� þHþ

HCOO� þ 2hþ �������!photooxidationCO2 þHþ

2Hþ þ 2e� �������!photoreductionH2

In this process, doped copper and biomass residue in M-LaCo0.7

Cu0.3O3 catalyst play a key role. First, copper doping contributes tothe formation of impurity level and appropriate oxygen vacancywhich can efficiently restrain the recombination between photo-generated electrons and holes. Second, biomass can help adjustthe crystal structure and surface structure of catalyst and someorganic functional groups from biomass residue which act as pho-tosensitizer can enhance its light-harvesting capacity. Simulta-neously, formaldehyde in solution acts as an electron donor andh+ scavenger, thereby suppressing the e�/h+ recombination andproducing H2. Thus M-LaCo0.7Cu0.3O3 exhibit good photocatalyticactivity.

4. Conclusion

We set forth a novel kind of copper doping perovskite photocat-alyst using the extract of P. pastoris GS115 (M-LaCo0.7Cu0.3O3) thatpresents excellent photocatalytic performance for hydrogen pro-duction from formaldehyde solution under visible light irradiation.Our study demonstrates that biomass as a complexing agentconduces to the formation M-LaCo0.7Cu0.3O3. Copper contributesto the formation of impurity level and appropriate oxygen vacancy.Furthermore, some organic functional groups from biomass residueon the surface of M-LaCo0.7Cu0.3O3 act as photosensitizer. Thisresearch will play a leading role and provide good reference for pre-paring a new type of visible-light active materials using microbe.

274 L. Wang et al. / Fuel 140 (2015) 267–274

Acknowledgments

This work is supported by general program of the NationalNatural Science Foundation of China (Grant No. 21176203). Theauthors thank Analysis and Testing Center of Xiamen Universityfor the analysis and observation work in this study.

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